Patent Publication Number: US-2022235119-A1

Title: Compositions and methods for treating a covid-19 infection

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 63/051,481, filed Jul. 14, 2020, which is expressly incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under grant numbers GM103712 and AI072726 awarded by the National Institutes of Health. The government has certain rights in the invention. 
    
    
     FIELD 
     The present disclosure relates to the field of treatment of a COVID-19 infection. 
     BACKGROUND 
     Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), which causes COVID-19, is a coronavirus closely related to SARS-CoV and Middle East Respiratory Syndrome (MERS) coronaviruses (A. C. Walls et al. (2020)). COVID-19 can manifest in adults as a severe interstitial pneumonia with hyperinflammation while severe respiratory manifestations are rare in children (L. Cristiani et al. (2020); M. Z. Tay, et al. (2020); N. Vabret et al. (2020)). Recently, however, multisystem inflammatory syndrome in children (MIS-C) has been recognized in patients that either tested positive for COVID-19 (by PCR or serology) or had epidemiological links to COVID-19 (S. Riphagen, et al. (2020); L. Verdoni et al. (2020); Z. Belhadjer et al. (2020)). These children present with a constellation of symptoms including hypotension, multiorgan involvement, and elevated inflammatory markers. After initial reports in UK (S. Riphagen, et al. (2020)), many cases have now been reported in Europe (L. Verdoni et al. (2020); Z. Belhadjer et al. (2020)), and New York (USA CDC). However, no such cases have been reported in China, Japan, or South Korea, which have also been severely impacted by the COVID-19 pandemic (ECDC). What is needed are compositions and methods for treating a COVID-19 infection, including MIS-C. The compositions and methods disclosed herein address these and other needs. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1 (A-B) shows binding of TCR to SARS-CoV-2 spike trimer near the “PRRA” insert region. Overall ( FIG. 1A ) and closeup ( FIG. 1B ) views of the complex and interfacial interactions. In  FIG. 1B , the spike monomers are colored white, ice blue, and spectrally from blue (N-terminal domain) to red, all displayed in surface representation. For better visualization, the spike trimer is oriented such that its receptor binding domains (RBDs) are at the bottom. TCR α- and β-chains are in red and cyan ribbons. In  FIG. 1B , the segment S 680 PPRAR 685  (SEQ ID NO:119) including the PRRA (SEQ ID NO:2) insert and highly conserved cleavage site R685 is shown in van der Waals representation (black labels) and nearby CDR residues of the TCRV β domain are labeled in blue/white. See additional information in  FIG. 5 . 
         FIG. 2 (A-D) shows sequence and structural properties of the insert “PRRA” motif.  FIGS. 2A-2B  show that SARS-CoV-2 encodes both a cleavage site (1) and neurotoxin motifs (J. P. Changeux, et al. (2020)) near the insertion PRRA (SEQ ID NO:2) that distinguishes it from SARS-CoV.  FIG. 2A  shows sequence alignment of SARS-CoV-2 and multiple SARS-CoV and Bat SARS-like CoV strains (A. C. Walls et al. (2020)) near the insertion PRRA (SEQ ID NO:2).  FIG. 2B  shows structural alignment of SARS-CoV-2 and SARS-CoV at the same region. The PRRARS (SEQ ID NO:6) motif is shown in red sticks.  FIG. 2C  shows sequence similarity between neurotoxin motifs and the close neighborhood of the PRRA insert, reported earlier (J. P. Changeux, et al. (2020)) as well as HIV-1 gp120 SAg motif (L. Bracci, et al. (1997)) in the last row.  FIG. 2D  shows SARS-CoV-2 S trimer composed of S1 subunits only. The protomers are colored orange, red and gray, and displayed in van der Waals format. The protruding motifs E661-R685 are highlighted in white, green, red, and blue representing the hydrophobic, hydrophilic, acidic, and basic residues. The sequences in  FIG. 2  are 
       
         
           
             
                 
                 
              
                 
                     
                   (SEQ ID NO: 43) 
                 
                 
                     
                   CASYQTQTNSPRRARSVASQSI, 
                 
                 
                     
                     
                 
                 
                     
                   (SEQ ID NO: 44) 
                 
                 
                     
                   CASYQTQTNS, 
                 
                 
                     
                     
                 
                 
                     
                   (SEQ ID NO: 45) 
                 
                 
                     
                   RSVASQSI, 
                 
                 
                     
                     
                 
                 
                     
                   (SEQ ID NO: 46) 
                 
                 
                     
                   CASYHTVSSL, 
                 
                 
                     
                     
                 
                 
                     
                   (SEQ ID NO: 47) 
                 
                 
                     
                   RSTSQKSI, 
                 
                 
                     
                     
                 
                 
                     
                   (SEQ ID NO: 48) 
                 
                 
                     
                   YQTQTNSPRRAR, 
                 
                 
                     
                     
                 
                 
                     
                   (SEQ ID NO: 49) 
                 
                 
                     
                   CDGFCSSRGKR, 
                 
                 
                     
                     
                 
                 
                     
                   (SEQ ID NO: 50) 
                 
                 
                     
                   CDAFCSSRGKV, 
                 
                 
                     
                     
                 
                 
                     
                   (SEQ ID NO: 51) 
                 
                 
                     
                   CDIFTNSRGKR, 
                 
                 
                     
                     
                 
                 
                     
                   (SEQ ID NO: 52) 
                 
                 
                     
                   CDAFCSIRGKR, 
                 
                 
                     
                   and 
                 
                 
                     
                     
                 
                 
                     
                   (SEQ ID NO: 53) 
                 
                 
                     
                   FNISTSIRGKV. 
                 
              
             
           
         
       
         FIG. 3 (A-E) shows that the “PRRA” insert in SARS-CoV-2 spike exhibits sequence and structure properties similar to those of bacterial superantigen SEB.  FIG. 3A  shows alignment of the superantigenic sequence of SEB (G. Arad et al. (2011)) against a homologous sequence of SARS-CoV-2 spike near the PRRA insert and corresponding SARS-CoV segment. Alignments are displayed for both forward (left) and reverse (right) ordering of the SEB sequence. Note the similarity between the former two, while the third (SARS-CoV) shows similarities to SARS-CoV-2, but not SEB, sequence.  FIG. 3B  shows structure of the superantigenic peptide (T150-D161) observed in the crystal structure of SEB (A. C. Papageorgiou, et al. (1998)) (PDB: 3SEB).  FIG. 3C  shows structural model for SARS-CoV-2 S palindromic motif E661-R685.  FIG. 3D  shows homologous region in SARS-CoV S exhibits totally distinctive structural features: a salt bridge, K152-E159 (in SEB) or R685-E661 (SARS-CoV-2), is absent in SARS-CoV spike; the former two are poly-basic (with three lysines and three arginines in the respective motifs), whereas SARS-CoV spike counterpart has one basic residue (R667) only; and the former two possess a scaffolding ASN, which is absent on SARS1.  FIG. 3E  shows structural alignment of CD28, the receptor binding SEB, onto TCRβ domain, in support of the adaptability of the putative SAg site to accommodate spike-TCRβ or SEB-CD28 interactions. The sequences in  FIG. 3  are YNKKKATVQELD (SEQ ID NO:54), TNSPRRARSVASQ (SEQ ID NO:55), SLLRSTSQK (SEQ ID NO:56), DLEQVTAKKKNY (SEQ ID NO:57), ECDIPIGAGICASYQTQTNSPRRAR (SEQ ID NO:58), and ECDIPIGAGICASYHTVSLLR (SEQ ID NO:59). 
         FIG. 4 (A-D) shows that the interfacial interactions between SARS-CoV-2 spike and αβTCR are further stabilized by the association of an ICAM-like motif with TCRVα domain.  FIG. 4A  shows interface between SARS-CoV-2 spike and TCR variable domains. Spike is shown in yellow; TCR Vα and Vβ are in magenta and cyan, respectively. The PRRARS (SEQ ID NO:6) insert is highlighted in red; The mutation site D839 identified in recent study (B. Korber et al., (2020)) is in green; SARS-CoV-2 counterpart of CD54-like motif identified for SARS-CoV spike (Y. Li et al., (2004)) is in orange. Residues involved in close interfacial contacts are shown in sticks, with nitrogen and oxygen atoms colored blue and red, respectively. Interactions between atom pairs separated by less than 2.5 Å are indicated by black dashed lines.  FIG. 4B  shows a close-up view of the interactions between the PRRARS (SEQ ID NO:6) insert/motif and TCR Vβ.  FIG. 4C  shows same for the D839 mutation site.  FIG. 4D  shows interactions between selected residues on ICAM-1-like motif (labeled, orange) TCRVα CDRs. 
         FIG. 5 (A-H) shows top-ranking binary complexes of SARS-CoV-2 spike with the T cell receptor (TCR) predicted by ClusPro.  FIGS. 5A-5B  show binding of TCR near the “PRRA” insert region.  FIGS. 5C-5D  show binding of TCR near the RBD of a subunit. The spike trimer subunits are colored red, orange, and gray. The PRRA insert region (E661 to R685) is shown in yellow. TCR α- and β-chains are shown in cyan and magenta.  FIGS. 5E-5F  show binding of TCR near the “PRRA” insert region of monomer 2 (dark red) and monomer 1 (gray) in the respective panels A and B, showing that the PRRA insert and its close vicinity presents a high-affinity binding site for TCR.  FIGS. 5G-5H  show binding of TCR near the RBD of a subunit, indicating that the RBD is an alternative high-affinity site. The spike trimer subunits are colored dark red, orange, and gray. The PRRA insert region (E661 to R685) is colored yellow. The TCR α- and β-chains are shown in magenta and cyan, respectively. See more details for the interaction between the PRRA insert region and TCR in  FIG. 1 . 
         FIG. 6 (A-B) shows motifs associated with superantigen, toxin, cytokine, and membrane surface proteins predicted for SARS-CoV spike and mapped onto SARS-CoV-2 spike sequence and structure.  FIG. 6A  shows sequence alignment of these motifs on SARS-CoV (upper rows) and SARS-CoV-2 spikes (lower rows), corresponding residue numbers (3rd column) and sequence identity (4th/last column). Superantigenic and toxic-like motifs are highlighted in pink. Residues that interact with TCR Vα are marked in red.  FIG. 6B  shows predicted motifs mapped onto the trimeric structure of SARS-CoV-2 spike, with one of its subunits colored in yellow. The motifs are colored red (superantigenic and toxic-like) or orange (others). Mutation sites reported in recent work (Zhan, et al. (2020), Korber, B. et al. (2020)) are shown in spheres. The mutation site D839Y/N/E is highlighted in green. The sequences in  FIG. 6  are 
       
         
           
             
                 
              
                 
                   (SEQ ID NO: 60) 
                 
                 
                   VIPFKDGIYFAATEKSNVVRGWVFGSTM, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 61) 
                 
                 
                   VLPFNDGVYFASTEKSNIIRGWIFGTTL, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 62) 
                 
                 
                   QTHTMIFDNAFNCTFEYISDAFSLDVS, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 63) 
                 
                 
                   ESEFRVYSSANNCTFEYVSQPFLMDLE, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 64) 
                 
                 
                   NITNFRAILTAFXSPAQDIWGTSA, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 65) 
                 
                 
                   NITRFQTLLALHRSYLTPGDSSSGWTAGA, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 66) 
                 
                 
                   YDENGTITDAVDCSQNPLAEKLKC,  
                 
                 
                     
                 
                 
                   (SEQ ID NO: 67) 
                 
                 
                   YNENGTITDAVDCALDPLSETKC, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 68) 
                 
                 
                   LKCSVKSFEIDKGIYQTSNFRVVPSGDVVRFPNITNLCPFGEVFNATK 
                 
                 
                     
                 
                 
                   FPSVY, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 69) 
                 
                 
                   TKCTLSKFTVEKGIYQTSNFRVQPTESIVRFPNITNLCPFGEVFNATR 
                 
                 
                     
                 
                 
                   FASVY, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 70) 
                 
                 
                   GCLIGAEHVDTSYECDIPIG, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 71) 
                 
                 
                   GCLIGAEHVNNSYECDIPIG, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 72) 
                 
                 
                   NTREVFAQVKQMYKTPTLKYFGGFNFSQILP, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 73) 
                 
                 
                   NTQEVFAQVKQIYKTPPIKDFGGFNFSQILP, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 74) 
                 
                 
                   EAEVQIDRLITGRLQSLQTYVTQQLIRAAEIRASANLAATKMSECVLG 
                 
                 
                     
                 
                 
                   QSKRVDFCGFPQAAPHGVVFLHVTYVPS, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 75) 
                 
                 
                   EAEVQIDRLITGRLQSLQTYVTQQLIRAAEIRASANLAATKMSECVLG 
                 
                 
                     
                 
                 
                   QSKRVDFCGFPQSAPHGVVFLHBTYVPA, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 76) 
                 
                 
                   LQPELDSFKEELDKYFKNHTSPDVDLGDISGINASVVNIQKEIDRLNE 
                 
                 
                     
                 
                 
                   VAKNLNESL, 
                 
                 
                   and 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 77) 
                 
                 
                   LQPELDSFKEELDKYFKNHTSPDVDLGDISGINASVVNIQKEIDRLNE 
                 
                 
                     
                 
                 
                   VAKNLNESL. 
                 
              
             
           
         
       
         FIG. 7 (A-B) shows modeled ternary complex formed by SARS CoV-2 spike, MHCII and αβTCR.  FIG. 7A  shows side view of the complex. The trimeric spike subunits are shown in the same style and color as in  FIG. 5 , as well as TCRα and TCRβ. The TCR retains a similar pose as in  FIG. 5B , now rotated by 180° along the z-axis. MHCII is displayed in green ribbon diagram.  FIG. 7B  shows top view of the interfacial contacts. Note that three spike residues reported to mutate in recent strains observed in European and western counties (Zhan, et al. (2020), Korber, B. et al. (2020)) (D614G, A831V and D839Y/N/E) are within 3-5 Å from either MHCII or TCR Vβ. 
         FIG. 8 (A-D) shows in silico mutagenesis analysis of SARS-CoV-2 spike protein residue D839Y/N/E.  FIG. 8A  shows a close-up view of the interaction between the wild type residue D839 in the spike and N30 of TCR Vβ.  FIGS. 8B-8D  show results obtained upon mutation to asparagine, glutamic acid, and tyrosine. The spike and TCR Vβ are shown in yellow and cyan, respectively. The mutation site is highlighted in green. Atomic interactions are indicated by black dashed lines along with their distances in Angstroms. 
         FIG. 9 (A-D) shows complex conformers formed upon binding of TCR onto SARS-CoV spike ( FIG. 9 (A-B)) and MERS-CoV spike ( FIG. 9 (C-D)), representative of the most probable clusters predicted by docking simulations. In  FIGS. 9A-9B , top two TCR binding poses predicted for SARS-CoV spike: one within the RBDs and the other within S2 subunits near the C-terminal domains. In  FIGS. 9C-9D , top two TCR binding poses predicted for MERS-CoV: one between the RBDs in the down conformation and the other on the RBD of the up conformation. The cleavage region in SARS-CoV (E647 to R667) and MERS-CoV (D726 to R751) are shown in yellow. TCR α- and β-chains are shown in magenta and cyan. Docking simulations were performed using ClusPro. 
         FIG. 10 (A-C) shows neurotoxin-like sequences in SARS-CoV-2 S RBD and their ability to bind TCRs.  FIG. 10A  shows comparison of bioactive/neurotoxin-like (green) and ICAM-1 like (orange) segments identified for SARS1 S and their SARS-CoV-2 S counterparts.  FIG. 10B  shows loci of two neurotoxin-like regions (enclosed in green circles) and one ICAM-1 region (orange circle, see  FIG. 4 ) conserved between the two CoVs, shown on one monomer (highlighted in yellow) of SARS-CoV-2 S.  FIG. 10C  shows binding poses of TCR on SARS-CoV-2 (left) and SARS1 (right) S proteins, making contacts with the indicated conserved neurotoxin motif. 
         FIG. 11  shows that Fab 6D3 can bind to the TMPRSS2 cleavage site as well as the superantigen motif of SARS-CoV-2. 
         FIG. 12 (A-C) shows skewing of TRBV-usage in severe/hyperinflammatory COVID-19 patients. 24 repertoires of severe/hyperinflammatory COVID-19 cases versus 42 repertoires of mild/moderate COVID-19 cases were analyzed with and without 23 repertoires of age-matched healthy donors (age-matched to severe/hyperinflammatory COVID-19 group).  FIG. 12A  shows principal component analysis (PCA) of TRBV-Usage. Principal components 1 and 2 are shown, percentage of axis contributions are given in parentheses. Statistical analysis was performed using MANOVA Pillai-Bartlett test.  FIG. 12B  shows TRBV-Usage. The fraction of individual TRBV genes per repertoire is shown as mean±SEM. TRBV genes are sorted to enriched fractions in severe/hyperinflammatory versus mild/moderate COVID-19 disease in ascending order from bottom to top. The top TRBVs enriched in severe/hyperinflammatory COVID-19 patients (TRBV5-6, TRBV14, TRBV13 and TRBV24-1) are enlarged in the inset.  FIG. 12C  shows PCA of TRBJ-Usage as described in  FIG. 12A . See also  FIG. 14 . 
         FIG. 13  shows position of SARS-CoV-2 S cross-reactive epitopes identified (Mateus J, et al. (2020)) in people who have not been exposed to SARS-CoV-2, which overlap with the neurotoxin-like fragment 299-355 identified here to have a strong affinity to bind TCRs. The positions of eight cross-reactive epitopes (15-mers each, with the starting amino acid shown in each case) that were recognized by CD4+ T cells are indicated by blue and red bars. In each case the corresponding reactivity strength (SFC/10 6  cells) and the number of donors (out of a total of 16) who showed this type of ‘memory’ response (presumably due to earlier human coronavirus infections) are written. Two of the epitopes were found in the docking simulations to bind TCRs (see  FIG. 10 ). Note that this is one of three neurotoxin-like regions on SARS-CoV-2 spike (see 3, 5, and 7 in  FIGS. 6 and 3, 5, and 7  in  FIG. 10A ). The other two regions also contained epitopes that were cross-reactive, but this one was distinguished by its high frequency (fraction of donors) and high strength (SFC). The sequence in  FIG. 13  is 
       
         
           
             
                 
              
                 
                   (SEQ ID NO: 78) 
                 
                 
                   TKCTLKSFTVEKGIYQTSNFRVQPTESIVRFPNITNLCPFGEVFNATR 
                 
                 
                     
                 
                 
                   FASVY 
                 
              
             
           
         
       
         FIG. 14  shows TRBJ-Usage of TRBV genes enriched in hyperinflammatory COVID-19. 24 Repertoires of severe/hyperinflammatory COVID-19 cases versus 23 repertoires of age-matched healthy donors were analyzed. The fraction of individual TRBVJ gene combinations of TRBVS-6, TRBV13, TRBV14 and TRBV24-1 and all 13 different TRBJ genes per repertoire is shown, lines indicating median with interquartile range. 
         FIG. 15  (A-F) shows binding of TCRs (with Vβ chains identical to those overrepresented in the TCR repertoire of patients with severe COVID-19) to the putative SAg site and complexation with MHC II.  FIG. 15 (A-C) shows that complexes predicted between SARS-CoV-2 spike SAg-like region and αβTCRs corresponding to the genes ( FIG. 15A ) TCRBV5-6; ( FIG. 15B ) TCRBV14; and ( FIG. 15C ) TCRBV24-1. The spike subunits are colored dark red, orange, and gray; the SAg-like region (E661 to R685) is colored yellow; and the neurotoxin motif (299-351) region is green. The TCR α- and β-chains are shown in magenta and cyan, respectively.  FIG. 15D  shows sequence alignment of the Vβ domain of the TCRs shown in  FIG. 1  (TCRB) and in  FIG. 15 (A-C), generated by Clustal Omega (Sievers F, et al. (2011)). The βTCR paratopes that bind to the SAg-like site on spike are indicated by the color-coded bars, and the CDRs are highlighted in green. Note that there is an additional segment, highlighted in green, which also includes residues making interfacial contacts with the SAg region of the spike, despite its sequence heterogeneity.  FIG. 15 (E-F) shows ternary complexes with MHCII predicted for the overrepresented TCRs, illustrated for two cases. The sequences in  FIG. 15  include 
       
         
           
             
                 
              
                 
                   (SEQ ID NO: 79) 
                 
                 
                   MVDGGITQSPKYLFRKEGQNVTLSCEQNLNHDAMYWYRQDPGQGLRLI 
                 
                 
                     
                 
                 
                   YYSQIVNDFQKGDIAEGYSVSREKKESFPLTVTSAQKNPTAFYLCASS 
                 
                 
                     
                 
                 
                   SRSSYEQYFGPGTRLTVTEDL, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 80) 
                 
                 
                   MAGVTQSPTHLIKTRGQQVTLRCSPKSGHDTVSWYQQALGQGPQFIFQ 
                 
                 
                     
                 
                 
                   YYEEEERQRGNFPDRFSGHQFPNYSSELNVNALLLGDSALYLCASSLG 
                 
                 
                     
                 
                 
                   EGRVDGYTFGSGRLTVVEDL, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 81) 
                 
                 
                   EAGVTQFPSHSVIEKGQTVTLRCDPISGHDNLYWYRRVMGKEIKFLLH 
                 
                 
                     
                 
                 
                   FVKESKQDESGMPNNRFLAERTGGTYSTLKVQPAELEDSGVYFCASSQ 
                 
                 
                     
                 
                 
                   DRDTQYFGPGTRLTVLEDL, 
                 
                 
                   and 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 82) 
                 
                 
                   ADVTQTPRNRITKTGKRIMLECSQTKGHDRMYWYRQDPGLGLRLIYYS 
                 
                 
                     
                 
                 
                   FDVKDINKGEISDGYSVSRQAQAKFSLSLESAIPNQTALYFCATSDES 
                 
                 
                     
                 
                 
                   YGYTFGSGTRLTVVEDL. 
                 
              
             
           
         
       
         FIG. 16 (A-C) shows SARS-CoV-2 Spike (S) glycoprotein structure, sequence alignment against other CoVs, and interaction sites observed in cryo-EM studies with neutralizing antibodies.  FIG. 16A  shows SARS-CoV-2 S trimer in the pre-fusion state. Protomers 1 and 2 are in white and light blue, respectively; and protomer 3, in spectral colors from blue (N-terminal domain, NTD; residue 1-305) to red (C-terminus), except for the  681 PRRA 684  (SEQ ID NO:2) insert in magenta. The insert is modeled using SWISS-MODEL (Waterhouse et al., 2018). Each protomer&#39;s RBD (residues 331-524) can assume up or down conformations in the respective receptor-bound and -unbound states.  FIG. 16B  shows sequence alignment of SARS-CoV-2 near the S1/S2 cleavage site against multiple bat and pangolin SARS-related strains, and other HCoVs, adjusted following previous studies (Coutard et al., 2020; Zhou et al., 2020b). Viruses belonging to the same lineage are shown by the same color shade; and HCoVs that encode furin-like cleavage sites are highlighted in bold fonts. Note that the polybasic insert PRRA (SEQ ID NO:2) of SARS-CoV-2 S is not found in closely related SARS-like CoVs but exists in MERS and HCoVs HKU1 and OC43. The furin-like cleavage site is indicated by the blue-shaded box.  FIG. 16C  shows side (left) and bottom (right) views of receptor (ACE2)- and antibody-binding sites observed in cryo-EM structures resolved for the S protein complexed with the ACE2 and/or various Abs. The S trimer is shown in cartoons with light blue protomer in RBD-up conformation, and gray and light orange protomers in RBD-down conformation. Binding sites for ACE2 and antibodies C105 (Barnes et al., 2020), 2-4 (Liu et al., 2020), 5309 (Pinto et al., 2020), H014 (Lv et al., 2020b), 4A8 (Chi et al., 2020), Ab23 (Cao et al., 2020b), and EY6A (Zhou et al., 2020a) are shown in space-filling surfaces in different colors (see the code in the inset). See Table 4 for additional information. The sequences in  FIG. 16  include CASYQTQT (SEQ ID NO:83), NSPRRARSVASQSI (SEQ ID NO:84), CASYHTVS (SEQ ID NO:85), RSVSSQAI (SEQ ID NO:86), VGTNSI (SEQ ID NO:87), CASYHTAS (SEQ ID NO:88), RNTGQKSI (SEQ ID NO:89), RSTSQKAI (SEQ ID NO:90), RSTGQKAI (SEQ ID NO:91), CIDYALPS (SEQ ID NO:92), SRRKRRGISSPYR (SEQ ID NO:93), CVDYSK (SEQ ID NO:94), NRRSRGAITTGY (SEQ ID NO:95), NRRSRGAITTGY (SEQ ID NO:96), CALPDTPSTLTPRSVRSVPGEMR (SEQ ID NO:97), CADGSIIAVQP (SEQ ID NO:98), RNVSYDSV (SEQ ID NO:99), CADGSLIPVRP (SEQ ID NO:100), RNSSDNGI (SEQ ID NO:101), and RSVASQSI (SEQ ID NO:45). 
         FIG. 17 (A-E) shows that SEB-associated mAb 6D3 binds the furin-cleavage site of SARS-CoV-2 S protein, interfering with the S1/S2 cleavage by furin or TMPRSS2.  FIG. 17A  shows binding pose of three SEB-neutralizing Abs (mAbs 6D3, 14G8, and 20B1) onto SEB. The diagram is generated by superposing the crystal structures (PDB IDs 4RGN and 4RGM) resolved for the complexes (Dutta et al., 2015). SEB is colored beige, with its SAg motif  150 TNKKKATVQELD 161  (SEQ ID NO:116) highlighted in blue space-filling.  FIG. 17B  shows close-up view of the tight interaction between the acidic residues E50, D52, and D55 of the 6D3 heavy chain and four basic residues of SEB. In  FIG. 17 (C-E),  FIG. 17C  shows interface between 6D3 and SEB SAg motif. Heavy and light chains of 6D3 are green and cyan, respectively. Overall ( FIG. 17D ) and close-up ( FIG. 17E ) views of the complex model for S protein and anti-SEB mAb 6D3. The interfacial interactions engage the arginines in the PRRA insert. SARS-CoV-2 S interfacial residues include 1210-Q218, N603-Q607, E654-Y660 and A688-I693, and the SAg motif residues Y674, T678-R683. 6D3 interfacial residues include A24-K33, E50, D52, S54, D55, Y57, N59, K74-T77, and A100-A104 in the heavy chain, and D1, 12, Q27, N31-F38, Y55, W56, and D97-Y100 in the light chain. The Spike-6D3 complex is generated in silico using the S structure modeled with one RBD up (PDB: 6VSB). 
         FIG. 18 (A-B) shows that glycosylation of SARS-CoV-2 spike at N603 does not block mAb 6D3 binding and may even assist in binding the mAb.  FIG. 18A  shows computationally-modeled SARS-CoV-2 glycosylated spike. The three monomers of the spike protein are shown in blue, red and orange surface representations, with their SAg region (residues 661 to 685) colored yellow. High-mannose N-glycans are shown in colored (small) spheres.  FIG. 18B  shows structural alignment of computationally modeled 6D3-spike protein complex onto the glycosylated spike. No spatial clash was observed. The inset shows a closeup view. 
         FIG. 19 (A-D) shows that monoclonal antibody 6D3 prevents SARS-CoV-2 infection. 6D3 or isotype control antibodies (at indicated concentrations) were incubated with virus (100 PFU/well) for 1 hour at room temperature before addition to Vero-E6 cells (5×10 3  cells/well). 48 hours post infection cells were fixed and stained for dsRNA or SARS-CoV-2 spike protein.  FIG. 19A  shows quantification of the percentage of infected cells per well by dsRNA staining.  FIG. 19B  shows representative fluorescence images of 6D3-mediated inhibition of virus infection (dsRNA).  FIG. 19C  shows quantification of the percentage of infected cells per well by spike staining.  FIG. 19D  shows representative fluorescence images of 6D3-mediated inhibition of virus infection (spike). Data were analyzed by t test (6D3 vs. isotype control) with multiple testing correction (FDR). Data is presented as mean +/− standard error of the mean. n=3 technical replicates. Data are representative of three independent experiments. *p&lt;0.05, **&lt;0.01, ***p&lt;0.001. See also  FIG. 21  for detailed results as a function of 6D3 concentration. 
         FIG. 20 (A-B) shows binding poses of human proteases TMPRSS2 and furin to SARS-CoV-2 S protein.  FIG. 20 (A-B) shows structural models for the SARS-CoV-2 S protein complexed with TMPRSS2 ( FIG. 20A ), and furin ( FIG. 20B ), obtained from docking simulations followed by refinements. An overview (left) and a zoomed in view (right) are shown in each case. The arginines in the S1/S2 loop P 681 RRARS 686 (SEQ ID NO:6) are shown in different shades of blue, and their interaction partner (acidic residues) in the proteases are shown in red. Spheres (right panels) highlight the R685↑S686 peptide bond. TMPRSS2 catalytic triad residues are S441 (yellow), H296 (green) and D345 (dark red). Their counterparts in furin are S368, H194 and D153. Note the short distance between the carbonyl carbon of R685 and the hydroxyl oxygen of S441 of TMPRSS2 (3.5 Å) or S368 of furin (3.1 Å). Black dashed lines show interfacial polar contacts and salt bridges, and those including the S1/S2 loop arginines are highlighted by ellipses. 
         FIG. 21 (A-B) shows that monoclonal antibody 6D3 prevents SARS-CoV-2 infection, related to  FIG. 19 . 6D3 or isotype control antibodies (at indicated concentrations) were incubated with virus (100 PFU/well) for 1 hour at room temperature before addition to Vero-E6 cells (5×10 3  cells/well). 48 hours post infection cells were fixed and stained for dsRNA or SARS-CoV-2 spike protein.  FIG. 21A  shows representative fluorescence images of 6D3 mediated inhibition of virus infection as measured by dsRNA staining.  FIG. 21B  shows representative fluorescence images of 6D3 mediated inhibition of virus infection as measured by SARS-CoV-2 spike staining. 
         FIG. 22 (A-B) shows binding of TMPRSS2 to the SARS-CoV-2 Spike (S) protein yields an ensemble of conformers including one where the protease binds to the PRRA (SEQ ID NO:2) insert, Related to  FIG. 20A .  FIG. 22A  shows overlay of models generated by the protein docking software ClusPro (Kozakov et al., 2017). The three subunits of the S protein are colored tan, orange and green; alternative poses of TMPRSS2 are shown in magenta.  FIG. 22B  shows Model 1 where TMPRSS2 catalytic residues are positioned in close proximity of the S1/S2 cleavage site. Three basic residues, R682, R683, and R685 from the S protein, are shown as van der Waals (vdW) balls in different shades of blue; the acidic residues of TMPRSS2 which form salt bridges with these three basic residues are displayed in red vdW balls with catalytic residue D345 in a darker red and catalytic serine residue S441 is shown as yellow vDW balls. Model 1, found to be most favorable energetically, is shown in  FIG. 20A . 
         FIG. 23  shows structural models generated for SARS-CoV-2 S protein complexed with furin, related to  FIG. 20B . Six models, labeled Model 1 to Model 6, representative of clusters formed by top-ranking conformers are displayed. In all models, the catalytic residues (D153, H194 and S368) of furin are in close proximity to the cleavage site 685RS686 of spike. The subunits from the S protein are colored tan, orange and green; and furin is in cyan cartoons. 
       Three basic residues R682, R683, and R685 from spike are shown in blue vdW representation; the acidic residues which form salt bridges with these three basic residues from spike are displayed in red vdW balls. Note that furin has multiple acidic residues that form intermolecular salt bridges in multiple poses: D153, D154, E236, D258, D264, D306, and E331, and the close proximity of the S1/S2 site is highly favorable, both energetically and entropically. Model 5, found to be most favorable energetically, is shown in  FIG. 20B . 
         FIG. 24  shows that antibody 6D3 and TMPRSS2 compete for the same binding site on SARS-CoV-2 spike protein, Related to  FIG. 17  and  FIG. 20 . The figure shows the overlay of the structural models generated for spike-TMPRSS2 and spike- 6D3 complexes, which illustrates how TMPRSS2 spatially overlaps with the variable domains (not seen, eclipsed by TMPRSS2) of 6D3. The diagram is generated by superposing the S protein of the complexes predicted in silico. Similar results were found for the spike-furin complex (not shown). The three S subunits are colored blue/violet, brick and dark orange, with the SAg region colored red (visible for the orange monomer only). 
         FIG. 25 (A-C) shows that polyacidic residues in the CDR2 of the mAbs 6D3 heavy chain play a major role in blocking the furin-like cleavage site of SARS-CoV-2 S protein.  FIG. 25A  shows multiple sequence alignment of the VH domain of anti-SEB Abs (6D3, 14G8 and 20B1) and anti-SARS-CoV-2 S Abs (see the names on the left column). The residue ranges of the three CDRs are: CDR1 (residues 25 to 32), CDR2 (51 to 58), and CDR3 (100 to 116) (Chi et al., 2020), as indicated by the blue bars.  FIG. 25B  shows overall and close-up views of the complex and interfacial interaction of the Spike protein complexed with 6D3 antibody. Note that three acidic residues from CDR2 interact with the basic residues R682, R683 and R685 of the S protein. The complex in  FIG. 25B  is generated in silico using SARS-CoV-2 S structure with all three RBDs in the down conformer (PDB id: 6VXX).  FIG. 25C  is same as  FIG. 25B , repeated for human cold virus HCoV-OC43 S protein. The complex in  FIG. 25C  is generated in silico using HCoV-OC43 S structure with all three RBDs in the down conformer (PDB id: 6NZK). HCoV-OC43 encodes a S1/S2 furin-like cleavage site at  754 RRAR↑G 758  (SEQ ID NO:120). Note that three acidic residues from CDR2 interact with R754, R755 and R757 in hCoV-OC43 S protein. The residues belonging to the Abs are labelled in lightface, those of the S protein in boldface in both  FIGS. 25B and 25C . The sequences in  FIGS. 25  include 
       
         
           
             
                 
              
                 
                   (SEQ ID NO: 102) 
                 
                 
                   QVQLQQSGAELVKPGASVKLSCKASGYTFTSHWMHWVKQRPGQGLEWI 
                 
                 
                     
                 
                 
                   GEIDPSDSYINYNQIFEGKATLTVDKSSTTAYLQLSSLTSEDSAVYYC 
                 
                 
                     
                 
                 
                   ARTAGLLAPMDYWGQ, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 103) 
                 
                 
                   EVQLVESGAEVKKPGASVKVSCKVSGYTLTELSMHWVRQAPGKGLEWM 
                 
                 
                     
                 
                 
                   GGFDPEDGETMYAQKFQGRVTMEDTSTDTAYMELSSLRSEDTAVYYCA 
                 
                 
                     
                 
                 
                   TSTAVAGTPDLFDYYYGMDVWGQ, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 104) 
                 
                 
                   QVQLVQSGAEVKKPGASVKVSCKASGYPFTSYGISWVRQAPGQGLEWM 
                 
                 
                     
                 
                 
                   GWISTYNGNTNYAQKFQGRVTMTTDTSTTTGYMELRRLRSDDTAVYYC 
                 
                 
                     
                 
                 
                   ARDYTRGAWXFGESLIGGFDNWGQ, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 105) 
                 
                 
                   QVQLVESGGGLIQPGGSLRLSCAASGFTVSSNYMSWVRQAPGKGLEWV 
                 
                 
                     
                 
                 
                   SVIYSGGSTYYADSVKGRFTISRDNSKNTLYLQMNSLRAEDTAVYYCA 
                 
                 
                     
                 
                 
                   RGEGWELPYDYWGQ, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 106) 
                 
                 
                   QVQLVQSGAEVKKPGASVKVSCKASGYTFTGYYMHWVRQAPGQGLEWM 
                 
                 
                     
                 
                 
                   GWINPNSGGTNYTQMFQGRVTMTRDTSISTAYMEVSRLRSDDTAVYYC 
                 
                 
                     
                 
                 
                   ARDRSWAVVYYYMDVWGK, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 107) 
                 
                 
                   QVQLVQSGSELKKPGASVKVSCKASGYTFTSYAMNWVRQAPGQGLEWM 
                 
                 
                     
                 
                 
                   GWINTNTGNPTYAQGFTGRFVFSLDTSVSTAYLQISSLKAEDTAVYYC 
                 
                 
                     
                 
                 
                   ARPQGGSSWYRDYYYGMDVWGQ, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 108) 
                 
                 
                   EVQLVESGGGVVQPGRSLRLSCAASAFTFSSYDMHWVRQAPGKGLEWV 
                 
                 
                     
                 
                 
                   AVISYDGSNKYYADSVKGRFTISRDNSKNTLYLQMNSLRAEDTAVYYC 
                 
                 
                     
                 
                 
                   AKDGGKLWVYYFDYWGQ, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 109) 
                 
                 
                   EVQLVQSGAEVKKPGATVKISCKVSGYSFSNYYIHWVKQAPGKSLEWI 
                 
                 
                     
                 
                 
                   GYIDPFNGGTSDNLKFKGAATLTADTSTDTAYMELSSLRSEDTAVYYC 
                 
                 
                     
                 
                 
                   ARSEYDPYYVMDYWGQ, 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 110) 
                 
                 
                   QIQLVQSGPELKKPGETVRISCKASGYIFTIAGIQWVQKMPGRGLRWI 
                 
                 
                     
                 
                 
                   GWINTHSGVPEYAEEFKGRFAFSLETSARTAYLQISNLKDEDTATYFC 
                 
                 
                     
                 
                 
                   ARIYYGNNGGVMDYWGQ, 
                 
                 
                   and 
                 
                 
                     
                 
                 
                   (SEQ ID NO: 111) 
                 
                 
                   EVNLIESGGDLVKPGGSLKLSCATSGFTFSAYGLSWVRQTPERRLEWV 
                 
                 
                     
                 
                 
                   ASISGGGSVYYPDSVKGRFTISRDTAGDILFLQMNSLRSEDSAIYYCV 
                 
                 
                     
                 
                 
                   RDLYGDYVGRYAYWGQ. 
                 
              
             
           
         
       
         FIG. 26 (A-D) shows examination of binding characteristics of SARS-CoV-2-neutralizing mAbs 4A8, related to  FIG. 20  and  FIG. 25 .  FIG. 26A  shows Cryo-EM structure (PDB: 7C2L) (Chi et al., 2020);  FIG. 26 (B-C) shows energetically most favorable conformers predicted for the S protein-4A8 complex. The former resembles the cryo-EM structure, involving the same segment, R245-G256, at the binding epitope of S. In the latter case, the viral SAg-like region which also overlaps with the S1/S2 cleavage site, serves as the 4A8-binding epitope.  FIG. 26D  shows competition between 4A8 and TMRPSS2 for binding to the S1/S2 cleavage site, based on the overlap between the binding poses of these two substrates. The diagram is generated by superposing the S protein of the two complexes predicted in silico. 
         FIG. 27  shows that anti-SEB mAb, 6D3, to cross-react with the SARS-CoV-2 spike. Live virus assays demonstrated that 6D3 effectively blocks SARS-CoV-2 viral entry. 6D3 binds the S1/S2 site, interfering with the proteolytic activity of TMPRSS2/furin. An acidic residue cluster at 6D3 VH enables strong binding to the polybasic S1/S2 site. 
       SUMMARY 
       Provided herein are methods of treating a COVID-19 infection in a subject, comprising administering to the subject an effective amount of a composition that reduces the superantigen character of SARS-CoV-2 Spike protein. In some embodiments, the compositions are mimetic peptides of the superantigen region. In some embodiments, the compositions are humanized antibodies such as humanized mAb 6D3 that bind to the superantigen region. 
       Accordingly, provided herein are methods of treating a COVID-19 infection in a subject, comprising administering to the subject an effective amount of one or more of a humanized mAb 6D3, a humanized mAb 14G8, and a functional fragment thereof. In some embodiments, the humanized mAb 6D3 comprises one or more of a V H  CDR amino acid sequence selected from the group consisting of SEQ ID NO:14, SEQ ID NO:15 and SEQ ID NO:16. In some embodiments, the humanized mAb 6D3 comprises one or more of a V L  CDR amino acid sequence selected from the group consisting of SEQ ID NO:18, SEQ ID NO:19 and SEQ ID NO:20. In some embodiments, the humanized mAb 6D3 comprises (a) a V H  domain having an amino acid sequence comprising SEQ ID NO:13, and (b) a V L  domain having an amino acid sequence comprising SEQ ID NO:17. In some embodiments, the humanized mAb 14G8 comprises one or more of a V H  CDR amino acid sequence selected from the group consisting of SEQ ID NO:36, SEQ ID NO:37 and SEQ ID NO:38. In some embodiments, the humanized mAb 14G8 comprises one or more of a V L  CDR amino acid sequence selected from the group consisting of SEQ ID NO:40, SEQ ID NO:41 and SEQ ID NO:42. In some embodiments, the humanized mAb 14G8 comprises (a) a V H  domain having an amino acid sequence comprising SEQ ID NO:35, and (b) a V L  domain having an amino acid sequence comprising SEQ ID NO:39. 
       Also included herein are methods of treating a COVID-19 infection in a subject, comprising administering to the subject an effective amount of one or more SARS-CoV-2 superantigenic (SAg) peptides, wherein the one or more peptides comprise SEQ ID NO:2 or SEQ ID NO:3. In some embodiments, the one or more peptides comprise SEQ ID NO:4. In some embodiments, the one or more peptides comprise SEQ ID NO:5. In some embodiments, the one or more peptides comprise SEQ ID NO:6. In some embodiments, the one or more peptides comprise SEQ ID NO:7. In some embodiments, the one or more peptides comprise SEQ ID NO:8. In some embodiments, the one or more peptides comprise SEQ ID NO:9. In some embodiments, the one or more peptides comprise SEQ ID NO:10. In some embodiments, the one or more peptides comprise SEQ ID NO:11. In some embodiments, the one or more peptides comprise SEQ ID NO:12. 
       In some embodiments of the above-described methods, the subject is a human. The human can be of any age, but in some embodiments, the human is a child and the treatment results in an amelioration of a multisystem inflammatory syndrome. In some embodiments, the treatment results in an amelioration of a pneumonia. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein is the surprising discovery that SARS-CoV-2 encodes a superantigen (SAg) motif near the S1/S2 cleavage site of its Spike protein. This region is highly similar in structure to the SEB SAg motif that interacts with both the TCR and CD28 (G. Arad et al. (2011)) and mediates toxic shock syndrome (TSS). Superantigens are highly potent T cell activators that can bind to MHC class II (MHCII) molecules and/or to TCRs of both CD4+ and CD8+ T cells. The ability of SAgs to bypass the antigen specificity of the TCRs results in broad activation of T cells and a cytokine storm, leading to toxic shock (H. Li, et al. (1999), T. Krakauer (2019)). Notably SAgs do not bind the major (antigenic) peptide binding groove of MHCII, but instead bind other regions as well as the αβTCRs, directly. While early studies showed that bacterial SAgs activate T cells by binding the β-chain of dimeric TCRs at their variable domain (V) (M. T. Scherer, et al. (1993), Y. W. Choi et al. (1989), J. D. Fraser, T. Proft (2008)), more recent studies revealed that they can bind to either α- or β-chains, or both (M. Saline et al. (2010)). As a SAg, SEB enables large-scale T cell activation and proliferation (T. Krakauer (2019)), resulting in massive production of pro-inflammatory cytokines including IFNγ, TNFα and IL-2 from T cells as well as IL-1 and TNFα from antigen presenting cells (APCs) (T. Krakauer (2019)). This cytokine storm leads to multi-organ tissue damage similar to what is now observed in MIS-C. 
     Accordingly, included in the present invention are methods of treating a COVID-19 infection in a subject, comprising administering to the subject an effective amount of a composition that that prevents the SARS-CoV-2 Spike protein from acting as a SAg. Included herein are compositions and methods for reducing an amount of SARS-CoV-2 Spike protein binding to a T cell receptor, an MHC molecule and/or CD28. In some embodiments, the compositions and methods include or employ a 6D3 antibody, mAb 6D3. In some embodiments, the compositions and methods include or employ a SARS-CoV-2 SAg peptide. 
     Terms used throughout this application are to be construed with ordinary and typical meaning to those of ordinary skill in the art. However, Applicants desire that the following terms be given the particular definition as provided below. 
     Terminology 
     As used in the specification and claims, the singular form “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “a cell” includes a plurality of cells, including mixtures thereof. 
     The terms “about” and “approximately” are defined as being “close to” as understood by one of ordinary skill in the art. In one non-limiting embodiment the terms are defined to be within 10%. In another non-limiting embodiment, the terms are defined to be within 5%. In still another non-limiting embodiment, the terms are defined to be within 1%. 
     The term “administering” refers to an administration that is oral, topical, intravenous, subcutaneous, transcutaneous, transdermal, intramuscular, intra-joint, parenteral, intra-arteriole, intradermal, intraventricular, intracranial, intraperitoneal, intralesional, intranasal, rectal, vaginal, by inhalation or via an implanted reservoir. The term “parenteral” includes subcutaneous, intravenous, intramuscular, intra-articular, intra-synovial, intrasternal, intrathecal, intrahepatic, intralesional, and intracranial injections or infusion techniques. 
     The term “antibody” is used in the broadest sense, and specifically covers monoclonal antibodies (including full length monoclonal antibodies), polyclonal antibodies, and multispecific antibodies (e.g., bispecific antibodies). Antibodies (Abs) and immunoglobulins (Igs) are glycoproteins having the same structural characteristics. While antibodies exhibit binding specificity to a specific target, immunoglobulins include both antibodies and other antibody-like molecules which lack target specificity. Native antibodies and immunoglobulins are usually heterotetrameric glycoproteins of about 150,000 daltons, composed of two identical light (L) chains and two identical heavy (H) chains. Each heavy chain has at one end a variable domain (V H ) followed by a number of constant domains. Each light chain has a variable domain at one end (V L ) and a constant domain at its other end. In some embodiments, the antibody is a human antibody. In some embodiments, the antibody is a humanized antibody. 
     The term “antibody fragment” refers to a portion of a full-length antibody, generally the target binding or variable region. Examples of antibody fragments include Fab, Fab′, F(ab′) 2  and Fv fragments. The phrase “functional fragment or analog” of an antibody is a compound having qualitative biological activity in common with a full-length antibody. For example a “functional fragment” refers to a fragment of mAb 6D3 that reduces binding of a SARS-CoV-2 Spike protein SAg region to a T cell receptor (TCR), an MHC molecule and/or CD28. As used herein, “functional fragment” with respect to antibodies, refers to Fv, F(ab) and F(ab′) 2  fragments. An “Fv” fragment is the minimum antibody fragment which contains a complete target recognition and binding site. This region consists of a dimer of one heavy and one light chain variable domain in a tight, non-covalent association (V H -V L  dimer). It is in this configuration that the three CDRs of each variable domain interact to define an target binding site on the surface of the V H -V L  dimer. Collectively, the six CDRs confer target binding specificity to the antibody. However, even a single variable domain (or half of an Fv comprising only three CDRs specific for a target) has the ability to recognize and bind target, although at a lower affinity than the entire binding site. “Single-chain Fv” or “sFv” antibody fragments comprise the V H  and V L  domains of an antibody, wherein these domains are present in a single polypeptide chain. Generally, the Fv polypeptide further comprises a polypeptide linker between the V H  and V L  domains which enables the sFv to form the desired structure for target binding. The Fab fragment contains the constant domain of the light chain and the first constant domain (CH1) of the heavy chain. Fab′ fragments differ from Fab fragments by the addition of a few residues at the carboxyl terminus of the heavy chain CH1 domain including one or more cysteines from the antibody hinge region. F(ab′) fragments are produced by cleavage of the disulfide bond at the hinge cysteines of the F(ab′) 2  pepsin digestion product. Additional chemical couplings of antibody fragments are known to those of ordinary skill in the art. 
     As used herein, the word “child” refers to a human between the ages of 1 day and 16 years. In some embodiments, the child is between the ages of 1 and 16 years, 3 and 13 years, 5 and 11 years, or 6 and 10 years. In some embodiments, the child is 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 or 16 years old. A “composition” is intended to include a combination of active agent and another compound or composition, inert (for example, a detectable agent or label) or active, such as an adjuvant. 
     As used herein, the term “comprising” is intended to mean that the compositions and methods include the recited elements, but not excluding others. “Consisting essentially of” when used to define compositions and methods, shall mean excluding other elements of any essential significance to the combination. Thus, a composition consisting essentially of the elements as defined herein would not exclude trace contaminants from the isolation and purification method and pharmaceutically acceptable carriers, such as phosphate buffered saline, preservatives, and the like. “Consisting of” shall mean excluding more than trace elements of other ingredients and substantial method steps for administering the compositions of this invention. Embodiments defined by each of these transition terms are within the scope of this invention. 
     A “control” is an alternative subject or sample used in an experiment for comparison purpose. A control can be “positive” or “negative.” 
     “COVID-19” refers herein to a disease or disorder caused in whole or in part by a severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). 
     As used herein, a “COVID-19 infection symptom” includes, but is not limited to, pneumonia, interstitial pneumonia, interstitial pneumonia with hyperinflammation, multisystem inflammatory syndrome in children (MIS-C), hypotension, sepsis, septic shock, multiple organ disfunction syndrome (MODS), and respiratory failure, dyspnea, respiratory frequency≥30/min, blood oxygen saturation (SpO2)≤93%, PaO2/FiO2 ratio or P/F [the ratio between the blood pressure of the oxygen (partial pressure of oxygen, PaO2) and the percentage of oxygen supplied (fraction of inspired oxygen, FiO2)]&lt;300, and/or lung infiltrates&gt;50% within 24 to 48 hours. In some embodiments, the COVID-19 infection symptom is MIS-C. 
     “Effective amount” encompasses, without limitation, an amount that can ameliorate, reverse, mitigate, and/or lessen a symptom or sign of a medical condition or disorder (e.g., a COVID-19 infection). Unless dictated otherwise, explicitly or by context, an “effective amount” is not limited to a minimal amount sufficient to reverse, mitigate, and/or lessen a condition. The severity of a disease or disorder, as well as the ability of a treatment to reverse, mitigate, and/or lessen the disease or disorder can be measured, without implying any limitation, by a biomarker or by a clinical parameter. In some instances, an amelioration, reversal, mitigation, and/or lessening is achieved following administration of multiple dosages of the composition to the subject over a period of days, weeks, or years. The effective amount will vary depending on the compound, such as a mAb 6D3 or COVID-19 SAg peptide, the disorder or conditions and its severity, the route of administration, time of administration, rate of excretion, drug combination, judgment of the treating physician, dosage form, and the age, weight, general health, sex and/or diet of the subject to be treated. In the context of the present methods, an effective amount or dose of a mAb 6D3 antibody or COVID-19 SAg peptide includes an amount that is sufficient to reverse, mitigate, and/or lessen a COVID-19 infection symptom. In some embodiments, an effective amount or dose of a mAb 6D3 or COVID-19 SAg peptide is an amount sufficient to ameliorate a COVID-19 infection symptom. In some embodiments, an effective amount or dose of a mAb 6D3 or COVID-19 SAg peptide is an amount sufficient to ameliorate a pneumonia. In some embodiments, an effective amount or dose of a mAb 6D3 or COVID-19 SAg peptide is an amount sufficient to ameliorate an interstitial pneumonia. In some embodiments, an effective amount or dose of a mAb 6D3 or COVID-19 SAg peptide is an amount sufficient to ameliorate hyperinflammation. In some embodiments, an effective amount or dose of a mAb 6D3 or COVID-19 SAg peptide is an amount sufficient to ameliorate a multisystem inflammatory syndrome in children (MIS-C). 
     “Homologs” are defined herein as two polynucleotides or two polypeptides that have identity or homology. Homologs include allelic variants, orthologs, and paralogs having the same relevant function (e.g., ability to bind to a COVID-19 SAg or ability to block binding of a COVID-19 SAg to a T cell receptor, a major histocompatibility (MHC) molecule and/or CD28). In some embodiments, homologs have about 99%, 98%, 97%, 96%, 95%, 94%, 93%, 92, 91% or 90% homology. In other embodiments, homologs have about 80% or about 85% homology. 
     “Humanized” forms of non-human (e.g. murine) antibodies are chimeric immunoglobulins, immunoglobulin chains or fragments thereof (such as Fv, Fab, Fab′, F(ab′) 2  or other target-binding subsequences of antibodies) which contain minimal sequence derived from non-human immunoglobulin. In general, the humanized antibody will comprise substantially all of at least one, and typically two, variable domains, in which all or substantially all of the CDR regions correspond to those of a non-human immunoglobulin and all or substantially all of the framework regions are those of a human immunoglobulin consensus sequence. The humanized antibody may also comprise at least a portion of an immunoglobulin constant region (Fc), typically that of a human immunoglobulin. 
     The term “identity” or “homology” shall be construed to mean the percentage of nucleotide bases or amino acid residues in the candidate sequence that are identical with the bases or residues of a corresponding sequence to which it is compared, after aligning the sequences and introducing gaps, if necessary to achieve the maximum percent identity for the entire sequence, and not considering any conservative substitutions as part of the sequence identity. Neither N- nor C-terminal extensions nor insertions shall be construed as reducing identity or homology. A polynucleotide or polynucleotide region (or a polypeptide or polypeptide region) that has a certain percentage (for example, 80%, 85%, 90%, or 95%) of “sequence identity” to another sequence means that, when aligned, that percentage of bases (or amino acids) are the same in comparing the two sequences. This alignment and the percent homology or sequence identity can be determined using software programs known in the art. In one embodiment, default parameters are used for alignment. In one embodiment a BLAST program is used with default parameters. In one embodiment, BLAST programs BLASTN and BLASTP are used with the following default parameters: Genetic code=standard; filter=none; strand=both; cutoff=60; expect=10; Matrix=BLOSUM62; Descriptions=50 sequences; sort by=HIGH SCORE; Databases=non-redundant, GenBank+EMBL+DDBJ+PDB+GenBank CDS translations+Swis sProtein+SPupdate+PIR. 
     The term “isolated” means separated from constituents, cellular and otherwise, in which the polynucleotide, peptide, polypeptide, protein, antibody, or fragments thereof, are normally associated with in nature. 
     The term “monoclonal antibody” or “mAb” as used herein refers to an antibody obtained from a substantially homogeneous population of antibodies, i.e., the individual antibodies within the population are identical except for possible naturally occurring mutations that may be present in a small subset of the antibody molecules. 
     The term “subject” is defined herein to include animals such as mammals, including, but not limited to, primates (e.g., humans), cows, sheep, goats, horses, dogs, cats, rabbits, rats, mice and the like. In some embodiments, the subject is a human. 
     The terms “treat,” “treating,” “treatment,” and grammatical variations thereof as used herein, include partially or completely delaying, alleviating, mitigating or reducing the intensity of one or more attendant symptoms of a disorder or condition and/or alleviating, mitigating or impeding one or more causes of a disorder or condition. In some instances, the terms “treat”, “treating”, “treatment” and grammatical variations thereof, include partially or completely reducing the severity of a COVID-19 infection, partially or completely reducing a multisystem inflammatory syndrome, or partially or completely reducing a pneumonia associated with a COVID-19 infection as compared with prior to treatment of the subject or as compared with the incidence of such symptom in a general or study population. 
     Compositions and Methods 
     As noted above, it is disclosed herein that SARS-CoV-2 encodes a superantigen (SAg) motif near the S1/S2 cleavage site of its Spike protein. As used herein, “Spike protein” or “S protein” refers to a polypeptide that mediates binding of a SARS-CoV virus to a cell and/or membrane fusion of the virus to a cell. The Spike protein contains an extracellular domain (EC) with two subunits, a receptor-binding subunit (S1) and a membrane-fusion subunit (S2). S1 contains two domains, an N-terminal domain (S1-NTD) and receptor binding domain (RBD), which play a key role in receptor recognition and binding. During host-virus membrane fusion, Spike protein is usually cleaved at the S1/S2 boundary by host proteases, releasing the spike fusion peptide, which is necessary for virus entry. 
     In some embodiments, the Spike protein is that identified in a publicly available database as follows: UniProtKB P0DTC2. In some embodiments, the Spike protein comprises the sequence of SEQ ID NO: 1, or a polypeptide sequence having at or greater than about 80%, about 85%, about 90%, about 95%, or about 98% homology with SEQ ID NO: 1, or a polypeptide comprising a portion of SEQ ID NO: 1. In some embodiments, the Spike protein is an isoform of SEQ ID NO:1. In some embodiments, the Spike protein is a ortholog of SEQ ID NO:1. The Spike protein of SEQ ID NO: 1 may represent an immature or pre-processed form of mature Spike protein, and accordingly, included herein are mature or processed portions of the Spike protein in SEQ ID NO: 1. In other embodiments, the Spike protein comprises the sequence of SEQ ID NO:1 modified by one or more of the following mutations: D614G, A831V and D839Y/N/E. 
     Accordingly, included herein are methods of treating a COVID-19 infection in a subject, comprising administering to the subject an effective amount of a composition that that reduces binding of a SARS-CoV-2 Spike protein SAg region to a T cell receptor (TCR), an MHC molecule and/or CD28. In some embodiments, binding to a TCR alpha chain is reduced. In other embodiments, binding to a TCR beta chain is reduced. In some embodiments, the MHC molecule is an MHC Class II. In some embodiments, the reduction in binding is about 100%, about 95%, about 90%, about 85%, about 80%, about 75%, about 70%, about 65%, about 60%, about 55%, or about 50% as compared to a control. 
     One SAg region of the SARS-CoV-2 Spike protein comprises amino acids PRRA (SEQ ID NO:2), which correspond or correlate with amino acid positions 681 to 684 of SEQ ID NO:1. In some embodiments, the SAg region comprises 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 amino acids and includes amino acids that correspond or correlate with positions 681 to 684 of SEQ ID NO:1. Another SAg region of the SARS-CoV-2 Spike protein comprises amino acids YNENGTITDAVDCALDPLSETKC (SEQ ID NO:3), which correspond or correlate with amino acid positions 279 to 301 of SEQ ID NO:1. 
     In some embodiments, the SAg region comprises amino acids TNSPRRAR (SEQ ID NO:4), which correspond or correlate with amino acid positions 678 to 685 of SEQ ID NO:1. In some embodiments, the SAg region comprises amino acids YQTQTNSPRRAR (SEQ ID NO:5), which correspond or correlate with amino acid positions 674 to 685 of SEQ ID NO:1. In some embodiments, the SAg region comprises amino acids PRRARS (SEQ ID NO:6), which correspond or correlate with amino acid positions 681 to 686 of SEQ ID NO:1. In some embodiments, the SAg region comprises amino acids PRRASVASQ (SEQ ID NO:7), which correspond or correlate with amino acid positions 681 to 690 of SEQ ID NO:1. In some embodiments, the SAg region comprises amino acids PRRASVASQSI (SEQ ID NO:8), which correspond or correlate with amino acid positions 681 to 692 of SEQ ID NO:1. In some embodiments, the SAg region comprises amino acids TNSPRRASVASQ (SEQ ID NO:9), which correspond or correlate with amino acid positions 678 to 690 of SEQ ID NO:1. In some embodiments, the SAg region comprises amino acids QTNSPRRARSVAS (SEQ ID NO:10), which correspond or correlate with amino acid positions 677 to 689 of SEQ ID NO:1. In some embodiments, the SAg region comprises amino acids ECDIPIGAGICASYQTQTNSPRRARSV (SEQ ID NO:11), which correspond or correlate with amino acid positions 661 to 687 of SEQ ID NO:1. In some embodiments, the SAg region comprises amino acids ECDIPIGAGICASYQTQTNSPRRAR (SEQ ID NO:12), which correspond or correlate with amino acid positions 661 to 685 of SEQ ID NO: 1. In some embodiments, the SAg region comprises amino acids YNENGTITDAVDCALDPLSETKC (SEQ ID NO:3), which correspond or correlate with amino acid positions 279 to 301 of SEQ ID NO:1. 
     The present invention also includes compositions such as peptide mimetics of the SARS-CoV-2 SAg region and methods for using those compositions in treating a COVID-19 infection in a subject. As used herein, the term “peptide mimetic” refers to an amino acid sequence that comprises or corresponds to a SARS-CoV-2 Spike protein SAg region. Accordingly, included herein are SARS-CoV-2 SAg peptides comprising amino acids PRRA (SEQ ID NO:2), which correspond or correlate with amino acid positions 681 to 684 of SEQ ID NO:1. In some embodiments, the SARS-CoV-2 SAg peptide comprises 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 amino acids and includes amino acids that correspond or correlate with positions 681 to 684 of SEQ ID NO:1. 
     In some embodiments, the SARS-CoV-2 SAg peptide comprises amino acids TNSPRRAR (SEQ ID NO:4), which correspond or correlate with amino acid positions 678 to 685 of SEQ ID NO:1. In some embodiments, the SARS-CoV-2 SAg peptide comprises amino acids YQTQTNSPRRAR (SEQ ID NO:5), which correspond or correlate with amino acid positions 674 to 685 of SEQ ID NO:1. In some embodiments, the SARS-CoV-2 SAg peptide comprises amino acids PRRARS (SEQ ID NO:6), which correspond or correlate with amino acid positions 681 to 686 of SEQ ID NO:1. In some embodiments, the SARS-CoV-2 SAg peptide comprises amino acids PRRASVASQ (SEQ ID NO:7), which correspond or correlate with amino acid positions 681 to 690 of SEQ ID NO:1. In some embodiments, the SARS-CoV-2 SAg peptide comprises amino acids PRRASVASQSI (SEQ ID NO:8), which correspond or correlate with amino acid positions 681 to 692 of SEQ ID NO:1. In some embodiments, the SARS-CoV-2 SAg peptide comprises amino acids TNSPRRASVASQ (SEQ ID NO:9), which correspond or correlate with amino acid positions 678 to 690 of SEQ ID NO:1. In some embodiments, the SAg peptide comprises amino acids QTNSPRRARSVAS (SEQ ID NO:10), which correspond or correlate with amino acid positions 677 to 689 of SEQ ID NO:1. In some embodiments, the SAg peptide comprises amino acids ECDIPIGAGICASYQTQTNSPRRARSV (SEQ ID NO:11), which correspond or correlate with amino acid positions 661 to 687 of SEQ ID NO:1. In some embodiments, the SARS-CoV-2 SAg peptide comprises amino acids ECDIPIGAGICASYQTQTNSPRRAR (SEQ ID NO:12), which correspond or correlate with amino acid positions 661 to 685 of SEQ ID NO:1. In some embodiments, the SARS-CoV-2 SAg peptide comprises amino acids YNENGTITDAVDCALDPLSETKC (SEQ ID NO:3), which correspond or correlate with amino acid positions 279 to 301 of SEQ ID NO:1. 
     It should be understood that the present invention includes methods of administering an effective amount of one or more SARS-CoV-2 SAg peptides for treating a COVID-19 infection in a subject. Provided are methods of treating a COVID-19 infection in a subject, comprising administering to the subject an effective amount of one or more SARS-CoV-2 superantigenic (SAg) peptides, wherein the one or more peptides are selected from the group consisting of SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11 and SEQ ID NO:12. In some embodiments, the method of treating a COVID-19 infection in a subject, comprises administering to the subject an effective amount of a peptide comprising SEQ ID NO:2. In some embodiments, the method of treating a COVID-19 infection in a subject, comprises administering to the subject an effective amount of a peptide comprising SEQ ID NO:3. In some embodiments, the method of treating a COVID-19 infection in a subject, comprises administering to the subject an effective amount of a peptide comprising SEQ ID NO:4. In some embodiments, the method of treating a COVID-19 infection in a subject, comprises administering to the subject an effective amount of a peptide comprising SEQ ID NO:5. In some embodiments, the method of treating a COVID-19 infection in a subject, comprises administering to the subject an effective amount of a peptide comprising SEQ ID NO:6. In some embodiments, the method of treating a COVID-19 infection in a subject, comprises administering to the subject an effective amount of a peptide comprising SEQ ID NO:7. In some embodiments, the method of treating a COVID-19 infection in a subject, comprises administering to the subject an effective amount of a peptide comprising SEQ ID NO:8. In some embodiments, the method of treating a COVID-19 infection in a subject, comprises administering to the subject an effective amount of a peptide comprising SEQ ID NO:9. In some embodiments, the method of treating a COVID-19 infection in a subject, comprises administering to the subject an effective amount of a peptide comprising SEQ ID NO:10. In some embodiments, the method of treating a COVID-19 infection in a subject, comprises administering to the subject an effective amount of a peptide comprising SEQ ID NO:11. In some embodiments, the method of treating a COVID-19 infection in a subject, comprises administering to the subject an effective amount of a peptide comprising SEQ ID NO:12. 
     Also included in the present invention are methods of treating a COVID-19 infection in a subject, comprising administering to the subject an effective amount of one or more of an mAb 6D3, an mAb 20B1, an mAb 14G8, a functional fragment thereof, or a humanized form thereof. These methods can be used separately or in conjunction with the methods of administering one or more SARS-CoV-2 SAg peptides also described herein. 
     The term “mAb 6D3” refers to a 6D3 antibody as described in Patent Application Publication US 2014/0234325 (U.S. app. Ser. No. 14/346,981) and/or Patent Application Publication US 2016/0039914 (U.S. app. Ser. No. 14/774/283), each of which is incorporated by reference herein in its entirety. In some embodiments, the mAb 6D3 comprises a V H  amino acid sequence of SEQ ID NO:13. In some embodiments, the mAb 6D3 comprises a V L  amino acid sequence of SEQ ID NO:17. In some embodiments, the mAb 6D3 comprises a CDR amino acid sequence of one or more of SEQ ID NO:14 (V H  CDR1), SEQ ID NO:15 (V H  CDR2) and SEQ ID NO:16 (V H  CDR3). In some embodiments, the mAb 6D3 comprises a CDR amino acid sequence of one or more of SEQ ID NO:18 (V L  CDR1), SEQ ID NO:19 (V L  CDR2) and SEQ ID NO:20 (V L  CDR3). In some embodiments, the mAb 6D3 comprises a CDR3 amino acid sequence of SEQ ID NO:16 and a CDR3 amino acid sequence of SEQ ID NO:20. 
     In some embodiments, the mAb 6D3 is humanized. A “humanized mAb 6D3” refers to a chimeric antibody that comprises two or more CDR amino acid sequences of an mAb 6D3 and one or more human antibody sequences. In some embodiments, the humanized mAb 6D3 comprises human framework regions. In some embodiments, the humanized mAb 6D3 further comprises a human Fc region. In some embodiments, the humanized mAb 6D3 comprises two or more murine CDRs selected from the group consisting of SEQ ID NO:14 (V H  CDR1), SEQ ID NO:15 (V H  CDR2), SEQ ID NO:16 (V H  CDR3), SEQ ID NO:18 (V L  CDR1), SEQ ID NO:19 (V L  CDR2) and SEQ ID NO:20 (V L  CDR3). In some embodiments, the humanized mAb 6D3 comprises three or more murine CDRs selected from the group consisting of SEQ ID NO:14 (V H  CDR1), SEQ ID NO:15 (V H  CDR2), SEQ ID NO:16 (V H  CDR3), SEQ ID NO:18 (V L  CDR1), SEQ ID NO:19 (V L  CDR2) and SEQ ID NO:20 (V L  CDR3). In some embodiments, the humanized mAb 6D3 comprises four or more murine CDRs selected from the group consisting of SEQ ID NO:14 (V H  CDR1), SEQ ID NO:15 (V H  CDR2), SEQ ID NO:16 (V H  CDR3), SEQ ID NO:18 (V L  CDR1), SEQ ID NO:19 (V L  CDR2) and SEQ ID NO:20 (V L  CDR3). In some embodiments, the humanized mAb 6D3 comprises five or more murine CDRs selected from the group consisting of SEQ ID NO:14 (V H  CDR1), SEQ ID NO:15 (V H  CDR2), SEQ ID NO:16 (V H  CDR3), SEQ ID NO:18 (V L  CDR1), SEQ ID NO:19 (V L  CDR2) and SEQ ID NO:20 (V L  CDR3). In some embodiments, the humanized mAb 6D3 comprises all of SEQ ID NO:14 (V H  CDR1), SEQ ID NO:15 (V H  CDR2), SEQ ID NO:16 (V H  CDR3), SEQ ID NO:18 (V L  CDR1), SEQ ID NO:19 (V L  CDR2) and SEQ ID NO:20 (V L  CDR3). 
     The term “mAb 20B1” refers to a 20B1 antibody as described in Patent Application Publication US 2014/0234325 (U.S. app. Ser. No. 14/346,981) and/or Patent Application Publication US 2016/0039914 (U.S. app. Ser. No. 14/774,283). In some embodiments, the mAb 20B1 comprises a V H  amino acid sequence of SEQ ID NO:21. In some embodiments, the mAb 20B1 comprises a V L  amino acid sequence of SEQ ID NO:25. In some embodiments, the mAb 20B1 comprises a CDR amino acid sequence of one or more of SEQ ID NO:22 or SEQ ID NO:29 (V H  CDR1), SEQ ID NO:23 or SEQ ID NO:30 (V H  CDR2)and SEQ ID NO:24 or SEQ ID NO:31 (V H  CDR3). In some embodiments, the mAb 20B1 comprises a CDR amino acid sequence of one or more of SEQ ID NO:26 or SEQ ID NO:32 (V L  CDR1), SEQ ID NO:27 or SEQ ID NO:33 (V L  CDR2) and SEQ ID NO:28 or SEQ ID NO:34 (V L  CDR3). In some embodiments, the mAb 20B1 comprises a CDR3 amino acid sequence of SEQ ID NO:24 or SEQ ID NO:31 and a CDR3 amino acid sequence of SEQ ID NO:28 or SEQ ID NO:34. 
     In some embodiments, the mAb 20B1 is humanized. A “humanized mAb 20B1” refers to a chimeric antibody that comprises two or more CDR amino acid sequences of an mAb 20B1 and one or more human antibody sequences. In some embodiments, the humanized mAb 20B1 comprises human framework regions. In some embodiments, the humanized mAb 20B1 further comprises a human Fc region. In some embodiments, the humanized mAb 20B1 comprises two or more murine CDRs selected from the group consisting of SEQ ID NO:22 or SEQ ID NO:29 (V H  CDR1), SEQ ID NO:23 or SEQ ID NO:30 (V H  CDR2), SEQ ID NO:24 or SEQ ID NO:31 (V H  CDR3), SEQ ID NO:26 or SEQ ID NO:32 (V L  CDR1), SEQ ID NO:27 or SEQ ID NO:33 (V L  CDR2) and SEQ ID NO:28 or SEQ ID NO:34 (V L  CDR3). In some embodiments, the humanized mAb 20B1 comprises three or more murine CDRs selected from the group consisting of SEQ ID NO:22 or SEQ ID NO:29 (V H  CDR1), SEQ ID NO:23 or SEQ ID NO:30 (V H  CDR2), SEQ ID NO:24 or SEQ ID NO:31 (V H  CDR3), SEQ ID NO:26 or SEQ ID NO:32 (V L  CDR1), SEQ ID NO:27 or SEQ ID NO:33 (V L  CDR2) and SEQ ID NO:28 or SEQ ID NO:34 (V L  CDR3). In some embodiments, the humanized mAb 20B1 comprises four or more murine CDRs selected from the group consisting of SEQ ID NO:22 or SEQ ID NO:29 (V H  CDR1), SEQ ID NO:23 or SEQ ID NO:30 (V H  CDR2), SEQ ID NO:24 or SEQ ID NO:31 (V H  CDR3), SEQ ID NO:26 or SEQ ID NO:32 (V L  CDR1), SEQ ID NO:27 or SEQ ID NO:33 (V L  CDR2) and SEQ ID NO:28 or SEQ ID NO:34 (V L  CDR3). In some embodiments, the humanized mAb 20B1 comprises five or more murine CDRs selected from the group consisting of SEQ ID NO:22 or SEQ ID NO:29 (V H  CDR1), SEQ ID NO:23 or SEQ ID NO:30 (V H  CDR2), SEQ ID NO:24 or SEQ ID NO:31 (V H  CDR3), SEQ ID NO:26 or SEQ ID NO:32 (V L  CDR1), SEQ ID NO:27 or SEQ ID NO:33 (V L  CDR2) and SEQ ID NO:28 or SEQ ID NO:34 (V L  CDR3). In some embodiments, the humanized mAb 20B1 comprises six murine CDRs selected from the group consisting of SEQ ID NO:22 or SEQ ID NO:29 (V H  CDR1), SEQ ID NO:23 or SEQ ID NO:30 (V H  CDR2), SEQ ID NO:24 or SEQ ID NO:31 (V H  CDR3), SEQ ID NO:26 or SEQ ID NO:32 (V L  CDR1), SEQ ID NO:27 or SEQ ID NO:33 (V L  CDR2) and SEQ ID NO:28 or SEQ ID NO:34 (V L  CDR3). 
     The term “mAb 14G8” refers to a 14G8 antibody as described in Patent Application Publication US 2014/0234325 (U.S. app. Ser. No. 14/346,981) and/or Patent Application Publication US 2016/0039914 (U.S. app. Ser. No. 14/774,283). In some embodiments, the mAb 14G8 comprises a V H  amino acid sequence of SEQ ID NO:35. In some embodiments, the mAb 14G8 comprises a V L  amino acid sequence of SEQ ID NO:39. In some embodiments, the mAb 14G8 comprises a CDR amino acid sequence of one or more of SEQ ID NO:36 (V H  CDR1), SEQ ID NO:37 (V H  CDR2) and SEQ ID NO:38 (V H  CDR3). In some embodiments, the mAb 14G8 comprises a CDR amino acid sequence of one or more of SEQ ID NO:40 (V L  CDR1), SEQ ID NO:41 (V L  CDR2) and SEQ ID NO:42 (V L  CDR3). In some embodiments, the mAb 14G8 comprises a CDR3 amino acid sequence of SEQ ID NO:38 and a CDR3 amino acid sequence of SEQ ID NO:42. 
     In some embodiments, the mAb 14G8 is humanized. A “humanized mAb 14G8” refers to a chimeric antibody that comprises two or more CDR amino acid sequences of an mAb 14G8 and one or more human antibody sequences. In some embodiments, the humanized mAb 14G8 comprises human framework regions. In some embodiments, the humanized mAb 14G8 further comprises a human Fc region. In some embodiments, the humanized mAb 14G8 comprises two or more murine CDRs selected from the group consisting of SEQ ID NO:36 (V H  CDR1), SEQ ID NO:37 (V H  CDR2), SEQ ID NO:38 (V H  CDR3), SEQ ID NO:40 (V L  CDR1), SEQ ID NO:41 (V L  CDR2), and SEQ ID NO:42 (V L  CDR3). In some embodiments, the humanized mAb 14G8 comprises three or more murine CDRs selected from the group consisting of SEQ ID NO:36 (V H  CDR1), SEQ ID NO:37 (V H  CDR2), SEQ ID NO:38 (V H  CDR3), SEQ ID NO:40 (V L  CDR1), SEQ ID NO:41 (V L  CDR2), and SEQ ID NO:42 (V L  CDR3). In some embodiments, the humanized mAb 14G8 comprises four or more murine CDRs selected from the group consisting of SEQ ID NO:36 (V H  CDR1), SEQ ID NO:37 (V H  CDR2), SEQ ID NO:38 (V H  CDR3), SEQ ID NO:40 (V L  CDR1), SEQ ID NO:41 (V L  CDR2), and SEQ ID NO:42 (V L  CDR3). In some embodiments, the humanized mAb 14G8 comprises five or more murine CDRs selected from the group consisting of SEQ ID NO:36 (V H  CDR1), SEQ ID NO:37 (V H  CDR2), SEQ ID NO:38 (V H  CDR3), SEQ ID NO:40 (V L  CDR1), SEQ ID NO:41 (V L  CDR2), and SEQ ID NO:42 (V L  CDR3). In some embodiments, the humanized mAb 14G8 comprises all of SEQ ID NO:36 (V H  CDR1), SEQ ID NO:37 (V H  CDR2), SEQ ID NO:38 (V H  CDR3), SEQ ID NO:40 (V L  CDR1), SEQ ID NO:41 (V L  CDR2), and SEQ ID NO:42 (V L  CDR3). 
     In one aspect, the disclosed methods can be employed 60, 59, 58, 57, 56, 55, 54, 53, 52, 51, 50, 49, 48, 47, 46, 45, 44, 43, 42, 41, 40, 39, 38, 37, 36, 35, 34, 33, 32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1 years;12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1 months; 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, or 3 days; 60, 48, 36, 30, 24, 18, 15, 12, 10, 9, 8, 7, 6, 5, 4, 3, or 2 hours prior to the onset of a COVID-19 symptom; or 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 75, 90, 105, 120 minutes; 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 15, 18, 24, 30, 36, 48, 60 hours; 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 45, 60, 90 or more days; 4, 5, 6, 7, 8, 9, 10, 11, 12 or more months; 60, 59, 58, 57, 56, 55, 54, 53, 52, 51, 50, 49, 48, 47, 46, 45, 44, 43, 42, 41, 40, 39, 38, 37, 36, 35, 34, 33, 32, 31, 30, 29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 years after the onset of COVID-19 symptom. In some embodiments, the disclosed methods can be employed prior to or following the administering of another anti-SARS-CoV-2 agent. 
     A SARS-CoV-2 SAg peptide and/or a humanized mAb 6D3 described herein can be administered to the subject via any route including oral, topical, intravenous, subcutaneous, transcutaneous, transdermal, intramuscular, intra-joint, parenteral, intra-arteriole, intradermal, intraventricular, intracranial, intraperitoneal, intralesional, intranasal, rectal, vaginal, by inhalation or via an implanted reservoir. The term “parenteral” includes subcutaneous, intravenous, intramuscular, intra-articular, intra-synovial, intrasternal, intrathecal, intrahepatic, intralesional, and intracranial injections or infusion techniques. 
     Dosing frequency for a SARS-CoV-2 SAg peptide and/or a humanized mAb 6D3 of any preceding aspects, includes, but is not limited to, at least once every year, once every two years, once every three years, once every four years, once every five years, once every six years, once every seven years, once every eight years, once every nine years, once every ten year, at least once every two months, once every three months, once every four months, once every five months, once every six months, once every seven months, once every eight months, once every nine months, once every ten months, once every eleven months, at least once every month, once every three weeks, once every two weeks, once a week, twice a week, three times a week, four times a week, five times a week, six times a week, daily, twice a day, three times a day, four times a day, or five times a day. Administration can also be continuous and adjusted to maintaining a level of the compound within any desired and specified range. 
     EXAMPLES 
     The following examples are set forth below to illustrate the compositions, methods, and results according to the disclosed subject matter. These examples are not intended to be inclusive of all aspects of the subject matter disclosed herein, but rather to illustrate representative methods and results. These examples are not intended to exclude equivalents and variations of the present invention which are apparent to one skilled in the art. 
     Example 1. Materials and Methods 
     Viruses. SARS-CoV-2 (P0DTC2) and SARS-CoV (CVHSA_P59594) spike models were generated using SWISS-MODEL (L. Bordoli, T. Schwede, (Springer, 2011)), based on the resolved spike glycoprotein structures of SARS-CoV-2 (D. Wrapp et al. (2020)) (PDB: 6VSB) and SARS-CoV (W. Song, et al. (2018)) (PDB: 6ACD).The missing loops in the crystal structures were built using libraries of backbone fragments (Y. Zhang, J. Skolnick, (2005)) or by constraint space de novo reconstruction of these backbone segments (M. C. Peitsch (1995)). Two mutants associated with European Covid-19 patients (B. Korber et al. (2020)) were constructed using CHARMM-GUI (S. Jo, T. Kim, et al. (2008)): one is the main strain mutant D614G and the other contains four mutations including Q239K, A831V, D614G and D839Y. These two SARS-CoV-2 spike mutants together with the SARS-CoV-2 (P0DTC2) originally taken from Wuhan were used to investigate the binding to αβTCR, and MHCII (PDB: 2XN9) (M. Saline et al. (2010)) using ClusPro (D. Kozakov et al. (2017)) and PRODIGY (L. C. Xue, et al. (2016)). 
     Generation of a binary complex between SARS-CoV-2 spike and T cell receptor (TCR). SARS-CoV-2 spike model in the prefusion state was generated using SwissModel (Bordoli, L. &amp; Schwede, T. (Springer, 2011)) based on the resolved cryo-EM structure (Protein Data Bank (PDB): 6VSB (Wrapp, D. et al. (2020)) for the spike glycoprotein where one of the receptor binding domains (RBDs) is in the up conformation. The structure of the T cell receptor (TCR) containing both TCRα and TCRβ chains was taken from the crystal structure of the ternary complex between human TCR, staphylococcal enterotoxin H (SEH) and human major histocompatibility complex class II (MHCII) molecule (Saline, M. et al. (2010)). Using protein-protein docking software ClusPro (Kozakov, D. et al. (2017)), a series of binary complexes were constructed in silico for SARS-CoV-2 spike and TCR. 30 clusters of conformations were obtained for spike-TCR binary complexes, upon clustering ˜1000 models generated by ClusPro. The clusters were rank-ordered by cluster size, as recommended (Kozakov, D. et al. (2017)). All models were analyzed which found that the majority (&gt;90%) showed that TCR bound to spike via its constant domain. Given that the constant domain is proximal to the cell membrane and TCR employs the variable domain for binding superantigens and/or antigen/MHC complexes (Saline, M. et al. (2010)), restraints were then added to the docking simulations to prevent the binding of TCR constant domain and filter out those conformers where the variable domain would bind to the spike. This led to 27 clusters (based on a set of 666 models) from ClusPro. Interestingly, 45% of models showed the binding of TCR near the region of “PRRA” insert and 46% of models showed the binding of TCR within multiple RBDs. Thus, two hot spots were identified for TCR binding within SARS-CoV-2 spike: one is near “PRRA” insert and the other within the RBD. Representative members belonging to the top-ranking clusters are presented in  FIG. 5 . 
     SARS-CoV-2 spike model in the prefusion state was generated using SwissModel (A. Waterhouse et al. (2018)) based on the resolved cryo-EM structure (Protein Data Bank (PDB): 6VSB) (Wrapp, D. et al. (2020)) for the S glycoprotein where one of the receptor binding domains (RBDs) is in the up conformation and the other two in the down conformation. The structure of the T cell receptor (TCR) containing both α- and β-chains was taken from the crystal structure (PDB: 2×N9) of the ternary complex resolved for human TCR, staphylococcal enterotoxin H (SEH) and human major histocompatibility complex class II (MHCII) molecule (Saline, M. et al. (2010)). Using protein-protein docking software ClusPro (Kozakov, D. et al. (2017)), a series of binary complexes were constructed in silico for SARS-CoV-2 spike and TCR. 30 clusters of conformations were obtained for spike-TCR binary complexes, upon clustering the ˜1000 models generated by ClusPro. The clusters were rank-ordered by cluster size, as recommended (Kozakov, D. et al. (2017)). We analyzed all models and found that a large fraction showed that TCR bound to spike via its constant domain. Given that the constant domain is proximal to the cell membrane and TCR employs the variable domain for binding superantigens (SAgs) and/or antigen/MHC complexes (Saline, M. et al. (2010)), restraints to the docking simulations were then added to filter out those conformers where the variable domain would bind to the spike. This led to 27 clusters (based on a set of 666 models) from ClusPro. Interestingly, in 45% of the generated models, the TCR was observed to bind to a spike epitope that contained the “PRRA” insert; and in 46% of models we observed an interaction between the TCR and one or two of the three RBDs. 
     Thus, two hot spots were identified for TCR binding within the SARS-CoV-2 spike: one overlapping with the “PRRA” insert and the other on the RBD surface. Representative members belonging to the top-ranking clusters are presented in  FIG. 5 .  FIGS. 5E and 5F  illustrate two cases where the TCR α- or β-chain tightly binds to the PRRA (SEQ ID NO:2) insert region (of monomers 2 (dark red) and monomer 1 (gray), respectively); and  FIGS. 5G-5H  illustrate two cases where the TCR binds to RBDs. 
     Generation of a binary complex between SARS-CoV spike and TCR. Further, SARS-CoV (SARS1) spike model in the prefusion state was generated using SwissModel (A. Waterhouse et al. (2018)) based on the cryo-EM structure resolved for SARS-CoV spike (PDB: 6ACD) (W. Song, et al. (2018)) where one of the RBDs is in the up conformation, and the other two in the down conformation. Following the same approach as we did for SARS-CoV-2 spike, a series of binary complexes were constructed in silico for SARS-CoV spike and TCR using ClusPro (Kozakov, D. et al. (2017)). Using the same filtering procedure, this led to 30 clusters (based on 686 models), among which 38% showed the binding of TCR to multiple RBDs (see  FIG. 9A ) similar to the behavior observed (in  FIGS. 5G-5H ) for SARS-CoV-2. Differently, 48% of the models showed the binding of TCR to two S2 subunits near the C-terminal domain of the trimers (see  FIG. 9B ). No significant binding of TCR near the S1/S2 cleavage site RS 668  of the SARS-CoV spike was observed. Note that the residues S 664 LLRS 668  (SEQ ID NO:117) of SARS-CoV spike, which are sequentially aligned to SARS-COV-2 spike T 678 SPRRARS 686  (SEQ ID NO:118) containing the “PRRA” insert (see  FIG. 3A ), lack the polybasic character of their counterpart SARS-COV-2. The lack of TCR binding to this region is consistent with the absence of this motif in SARS-CoV, which serves as a strong attractor in SARS-COV-2. 
     Generation of a binary complex between MERS-CoV spike and TCR. MERS-CoV spike model was generated using SwissModel (A. Waterhouse et al. (2018)) based on the cryo-EM structure resolved for MERS-CoV spike (W. Song, et al. (2018)) (PDB: 5×5F) in which one of RBDs is in the up conformation. 30 clusters (based on 588 models) were predicted by ClusPro (Kozakov, D. et al. (2017)). 56% of models led to TCR binding to the RBDs. Two representative poses from these most populated clusters are shown in  FIGS. 9C-9D , which are comparable to those observed in SARS-CoV-2 spike ( FIGS. 5G-5H ) and SARS-CoV spike ( FIG. 9A ). Simulations also indicated that TCR binds near the S1/S2 cleavage site region of MERS-CoV spike (segment D726-R751; counterpart of SARS-CoV-2 E661-R685 at the C-terminus of subunit S1). Note that at this region the PRRA (SEQ ID NO:2) insert of SARS-CoV-2 spike is replaced by MERS-CoV spike sequence PRSV. The region near PRSV shows a tendency to bind TCR but it is weaker than that of SARS-CoV-2 spike due to the lack of the critical residues (e.g. N679 and R683 in SARS-CoV-2 spike) that are involved in the interface the spike makes with the TCR. The lack of polybasic residues at this sequence motif, as well as counterparts of N679 and R683 of SARS-CoV-2 spike renders this structural region less attractive to TCRs, indicating that MERS-CoV did not harbor a superantigen-like motif near its S1/S2 cleavage site. 
     Examination of neurotoxin-like and other bioactive segments on SARS-CoV-2 spike.  FIG. 6  displays nine SARS-CoV segments that have been identified to be bioactive, neurotoxin-like or ICAM-like. In each case the 2nd row is the SARS-CoV-2 segment, identified by Li et al, 2004 and the 1st row is its SARS-CoV-2 spike counterpart. The last row lists their percent sequence identity. Note that the neurotoxin-like sequence #5, residues 299-351, contains several fragments (15-mers) that were recently shown (Mateus J, et al. (2020)) to stimulate T cell reactivity (illustrated in  FIG. 13 ). 
     Generation of a ternary complex between SARS-CoV-2 spike, TCR, and MHCII. Structure of the human MHCII was taken from the crystal structure of the ternary complex (Saline, M. et al. (2010)) (PDB: 2×N9) between human TCR, SEH and MHCII. First, docking simulations were performed to generate binary complexes between MHCII and SARS-CoV-2 spike. Six representative MHCII-spike binary complexes were selected to explore further docking of TCR to form a ternary complex. All predicted ternary complex models of MHCII-Spike-TCR were analyzed. Tertiary MHCII-Spike-TCR complex models were selected following three filtering criteria: (1) TCR either binds near “PRRA” insert region or the RBD; (2) the binding regions involve homologous superantigen or toxin binding motifs predicted for SARS-CoV ( FIG. 6 ); ( 3 ) MHCII and TCR are in close proximity. These filters led to the MHCII-Spike-TCR complex model illustrated in  FIG. 7A . The SARS-CoV-2 spike binding region harbors three residues that have been recently reported to have mutated in new strains from Europe and USA (49, 50) ( FIG. 7B ): D614G, A831V and D839Y/N/E). While the possible occurrence of other potential tertiary complexes was not excluded, especially those involving the RBDs, the complex shown in  FIG. 7  uniquely satisfied all three aforementioned criteria. 
     In silico mutagenesis of D839 of SARS-CoV-2 spike. D839 of the SARS-CoV-2 spike were mutated in silico to asparagine, glutamic acid and tyrosine in line with the aforementioned mutant D839Y/N/E observed in a new strain from Europe. To this aim, PyMOL mutagenesis tool (DeLano, W. L. (2002)) was used and the change in local conformation and energetics were evaluated in the complex formed with TCR. The most probable rotamers were selected and energetically minimized in the presence of the bound TCR (conformation shown in  FIG. 1 ) using OpenMM (Eastman, P. et al. (2017)). These were further subjected to short (1 ns) molecular dynamics (MD) simulations for equilibration and energy minimization under the AMBER14 ff14SB forcefield (J. A. Maier et al (2015)). Five independent runs were carried out for each mutant (Y, N, or E, at the position 839) as well as the wild type (D839) spike, to assess the statistical significance of the results for each case. Binding affinities (AG) and dissociation constants (Kd) were obtained for (i) the full complex (with the intact spike and entire TCR as interactors) or (ii) a single spike subunit and TCRVβ with the D839Y/N/E mutation on spike at 37° C. using PRODIGY server (Vangone, A. &amp; Bonvin, A. M. (2015), Xue, L. C., et al. (2016)). The results are presented in Table 3. 
     Analysis of NGS immunosequencing data from COVID-19 patients. Blood collection from 38 patients (42 samples) with mild/moderate COVID-19, and 8 patients (24 samples) with severe/hyperinflammatory COVID-19 was performed under institutional review board approval number 2020-039. The patients and controls, and their immune repertoires, were part of a previously published cohort (Schultheiss C, et al. (2020)). For details of NGS data acquisition, please refer to the earlier work (Schultheiss C, et al. (2020)). Only productive TRB rearrangements were used and all repertoires were normalized to 20,000 reads. For the analyses, we used R version 3.5.1 for plotting of TRBV and TRBJ gene usage as previously described (Simnica D, et al. (2019), Simnica D, et al. (2019)). Differences in principal component analysis were studied by Pillai-Bartlett test of MANOVA. To study TRBJ gene diversity, J genes were extracted if they were part of rearrangements containing TRBV rearrangements expanded in patients with hyperinflammatory COVID-19. Frequencies of J gene families were summarized per repertoire and plotted separately for each rearrangement. See  FIG. 14 . 
     Generation of complexes between SARS-CoV-2 spike, SAg-specific TCRs and MHC II. Four TCR Vβ genes (TRBV5-6, TRBV14, TRBV13 and TRBV24-1) were found to be overrepresented in severe/hyperinflammatory COVID-19 patients ( FIG. 12 ). The binding properties of the βTCRs encoded by those genes were investigated. To this aim, the amino acid sequences corresponding to these respective genes were extracted from the UniProtKB (The UniProt Consortium (2016)), and were used in FASTA format to search for the corresponding structures, if any, in the Protein Data Bank (PDB) (Berman H M, et al. (2000)), using SwissModel (Waterhouse A, et al. (2018)). Structural data was found in the PDB for TCRVβ chains of three of the genes, TRBV5-6 (UniProt id: A0A599), TRB (A0A5B0), and TRBV24-1 (A0A075B6N3). The respective PDB structures have PDB ids: 6ULR (Sim M J W, et al. (2020)), 2ESV (Hoare H L, et al. (2006)), and 6EH6 (Holland C J, et al. (2018)). These structures contain both α- and β-chains and. their Vβ domains have 95-100% sequence identity with the Vβ chains encoded by the respective TRVB genes. These PDB structures were used in docking simulations using the software ClusPro (Kozakov D, et al. (2017)) to examine their binding properties with respect to the SARS-CoV-2 spike. 30 clusters (obtained upon grouping ˜700 models) were generated for each of the TCRs complexed with the spike, and in each case there were 3 or more clusters where the TCR was bound to the SAg.  FIG. 15  panels A-C display representative conformers from these clusters.  FIG. 15D  displays the multiple sequence alignment generated for the TCR Vβ chains (with a few residues of the constant domain succeeding the CDR3). The binding paratopes are indicated by color-coded bars above the alignment. Simulations using the same protocol as the one adopted for generating  FIG. 15  showed that ternary complexes with MHCII were also energetically favorable for all three cases.  FIGS. 15E and 15F  illustrate the ternary complexes with MHCII for the TCRs corresponding to TRBV5-6 and TRBV14. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Binding affinities between the αβTCR and SARS-CoV-2 spike, 
               
               
                 for the wild type and mutant (D839Y/N/E) S glycoproteins 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Aspartic 
                 Tyrosine 
                 Glutamic 
                 Asparagine 
               
               
                   
                 Acid (D) 
                 (Y) 
                 Acid (E) 
                 (N) 
               
               
                   
                 ΔG 
                 ΔG 
                 ΔG 
                 ΔG 
               
               
                   
                 (kcal mol −1 ) 
                 (kcal mol −1 ) 
                 (kcal mol −1 ) 
                 (kcal mol −1 ) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Full complex 
                 −18.4 ± 0.2 
                 −19.3 ± 0.7 
                 −19.0 ± 1.3 
                 −19.0 ± 0.5 
               
               
                 S subunit - 
                 −13.3 ± 0.3 
                 −14.5 ± 0.3 
                 −13.7 ± 0.6 
                 −13.9 ± 0.5 
               
               
                 TCR Vβ 
               
               
                   
               
               
                 *Binding affinities (ΔG) were obtained at 37° C. using PRODIGY server. 
               
            
           
         
       
     
     Example 2. Introduction 
     MIS-C manifests as persistent fever and hyperinflammation with multi organ system involvement including cardiac, gastrointestinal, renal, hematologic, dermatologic and neurologic symptoms (S. Riphagen, et al. (2020); L. Verdoni et al. (2020); Z. Belhadjer et al. (2020)) which are highly reminiscent of toxic shock syndrome (TSS) (D. E. Low (2013), A. Cook, et al. (2020)). (Table 1), rather than Kawasaki disease due to marked demographic, clinical, and laboratory differences (L. Verdoni et al. (2020)). The similarities to TSS and the association of MIS-C with COVID-19 indicate that SARS-CoV-2 can possess superantigenic fragments that induce an inflammatory cascade and contribute to the hyperinflammation and cytokine storm features observed in severe adult COVID-19 cases (M. Z. Tay, et al. (2020), N. Vabret et al. (2020)). The question is: does SARS-CoV-2 S possess superantigenic fragments that can elicit such reactions upon binding proteins involved in the host cell cytotoxic adaptive immune response? Such a reaction was not observed in the SARS-CoV pandemic of 2003 (shortly SARS1). What is unique to SARS-CoV-2, and how recent mutations in SARS-CoV-2 S promotes such an increased virulence? 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Similarities between clinical and laboratory 
               
               
                 features of MIS-C and pediatric TSS 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 Pediatric 
               
               
                   
                 Clinical Features 
                 MIS-C a   
                 TSS b   
               
               
                   
                   
               
               
                   
                 High fever 
                 + 
                 + 
               
               
                   
                 Skin rash 
                 + 
                 + 
               
               
                   
                 Conjunctivitis 
                 + 
                 + 
               
               
                   
                 Oral mucosal involvement 
                 + 
                 + 
               
               
                   
                 Myalgia 
                 + 
                 + 
               
               
                   
                 Hypotension 
                 + 
                 + 
               
               
                   
                 Myocardial involvement (dysfunction) 
                 + 
                 + 
               
               
                   
                 Gastro-intestinal symptoms (vomiting, 
                 + 
                 + 
               
               
                   
                 diarrhea, abdominal pain) 
               
               
                   
                 Renal involvement 
                 + 
                 + 
               
               
                   
                 CNS symptoms, altered mental state 
                 + 
                 + 
               
               
                   
                 Headache 
                 + 
                 + 
               
               
                   
                 High CRP 
                 + 
                 + 
               
               
                   
                 High Ferritin 
                 + 
                 + 
               
               
                   
                 High IL-6 
                 + 
                 + 
               
               
                   
                 High D-dimers 
                 + 
                 + 
               
               
                   
                 High Procalcitonin 
                 + 
                 + 
               
               
                   
                 Lymphopenia 
                 + 
                 + 
               
               
                   
                 Reduced Platelet count 
                 + 
                 + 
               
               
                   
                 Increased Neutrophil count 
                 + 
                 + 
               
               
                   
                 Increased AST and ALST 
                 + 
                 + 
               
               
                   
                 High Pro-BNP 
                 + 
                 NA 
               
               
                   
                 High Troponin 
                 + 
                 NA 
               
               
                   
                 Isolation of TSS inducing bacteria 
                 − 
                 + 
               
               
                   
                 ( Staphylococcus  or  Streptococcus ) 
               
               
                   
                   
               
               
                   
                   a taken from refs (S. Riphagen, et al. (2020); L. Verdoni et al. (2020); Z. Belhadjer et al. (2020)); 
               
               
                   
                   b taken from refs (8-11). 
               
               
                   
                 + represents association with reported cases; NA: not available. 
               
            
           
         
       
     
     TSS can be caused by two types of superantigens (SAgs): bacterial or viral. Bacterial SAgs have been broadly studied. They include proteins secreted by  Staphylococcus aureus  and  Streptococcus pyogenes  that induce inflammatory cytokine gene induction and toxic shock. Typical examples are TSS toxin 1 (TSST1), and staphylococcal enterotoxins B (SEB) and H (SEH). They are highly potent T cell activators that can bind to MHC class II (MHCII) molecules and/or to TCRs of both CD4+ and CD8+ T cells. The ability of SAgs to bypass the antigen specificity of the TCRs results in broad activation of T cells and a cytokine storm, leading to toxic shock (H. Li, et al. (1999), T. Krakauer (2019)). Notably SAgs do not bind the major (antigenic) peptide binding groove of MHCII, but instead bind other regions as well as the αβTCRs, directly. While early studies showed that bacterial SAgs activate T cells by binding the β-chain of dimeric TCRs at their variable domain (V) (M. T. Scherer, et al. (1993), Y. W. Choi et al. (1989), J. D. Fraser, T. Proft (2008)), more recent studies revealed that they can bind to either α- or β-chains, or both (M. Saline et al. (2010)). The question is then, does SARS-CoV-2 S possess any superantigenic fragments/domains that can bind to the αβTCRs? 
     Here, computational modelling was used to determine whether the SARS-CoV-2 S possesses SAg activity. An insert present in SARS-CoV-2 S was found, which is absent from SARS1 and MERS, mediates high affinity, non-specific binding to the TCR. Notably, a motif of ˜20 amino acids enclosing this insert unique to SARS-CoV-2 among beta coronaviruses has sequence and structure features highly similar to those of the SEB toxin. Furthermore, this analysis shows that a SARS-CoV-2 S mutation detected in a European strain can enhance TCR binding, indicating such mutations can account for geographical differences in MIS-C occurrence. 
     Example 3. SARS-CoV-2 Spike Harbors a High Affinity Site for TCR β-Chain Binding, which Contains an Insertion, P 681 RRA 684  (SEQ ID NO:2), Unique to SARS2 
     First, whether SARS-CoV-2 S binds to the αβTCR was examined. To this aim, a SARS-CoV-2 S structural model was constructed based on the cryo-EM structure resolved for the spike glycoprotein (D. Wrapp et al. (2020)), and the X-ray structure of αβTCR resolved in a ternary complex was used with SEH and MHCII (M. Saline et al. (2010)), and a series of structural models were generated for possible SARS-CoV-2 S glycoprotein—TCR complex formation using ClusPro (D. Kozakov et al. (2017)). These simulations revealed two most probable TCR binding sites on each monomer of the S trimer: one on the receptor binding domain (RBD; residues R319-K529), and the other near the S 1/S2 cleavage site between the subunits S1 and S2. The former was also shared by SARS1 and MERS-CoV S, while the latter was unique (or strongly preferred) in SARS-CoV-2 S, as de-scribed in detail in the  FIG. 5  and  FIG. 9 . These simulations led to the binding pose presented in  FIG. 1A  as one of the most probable mechanisms of complex formation, as described in detail in  FIG. 5 . Therein, the TCR binds at the interface between the S1 and S2 subunits of the spike protein, near the S1/S2 cleavage site. A closeup view of the interface between the spike and TCRVβ domain ( FIG. 1B ) reveals several strong interatomic interactions, involving residues S680-R683 on the spike, and R70-E74 and [Q52, D56] on the respective CDRs 3 and 2 on Vβ. 
     The TCRVβ-binding epitope on SARS-CoV-2 S is centered around a sequence motif, P 681 RRA 684  (SEQ ID NO:2) (or shortly PRRA, hereafter), and its sequential and spatial neighbors. Comparison of SARS-CoV-2 S to other beta-coronavirus spike protein sequences showed (20) that SARS-CoV-2 is distinguished by the existence of this four-residue insertion, PRRA, preceding the furin cleavage site (R685-5686 peptide bond) between the subunits S1 and S2 of each protomer ( FIG. 2A ). Structural comparison of the trimeric S proteins between SARS-CoV and SARS-CoV-2 further shows their close structural similarity in general (except for the RBD which is engaged in specific interfacial interactions (D. Wrapp et al. (2020)), but the two spikes significantly differ near the PRRARS motif unique to SARS-CoV-2, which is exposed to the extracellular medium ( FIG. 2B ). 
     Notably the exposure of this motif and its close sequential neighbors is further accentuated in the S1 trimeric form ( FIG. 2D ) shed after cleavage by the human proteases (TMPRSS2 or furin) to enable the activation of the fusion trimer (composed of three S2 subunits). 
     Example 4. Further Examination of the Motif near PRRA Reveals Close Structural Similarity to the SEB Superantigen as well as Sequence Similarities to Neurotoxins and a Viral SAg 
     The insertion PRRA (SEQ ID NO:2) together with the sequentially preceding seven amino acids and succeeding Arg (fully conserved among β-coronaviruses) have been pointed out to form a motif, Y 674 QTQTNSPRRAR685 (SEQ ID NO:48), homologous to that of neurotoxins from  Ophiophagus  (cobra) and  Bungarus genera , as well as neurotoxin-like regions from three RABV strains (J. P. Changeux, et al. (2020)) ( FIG. 2C ). Further, the same segment bears close similarity to HIV-1 glycoprotein gp120 superantigenic motif F164-V164. 
     This close sequence similarity to both bacterial and viral SAgs, in support of the potential superantigenic character of the amino acid stretch Y674-R685 of SARS-CoV-2 S led to further analyze its local sequence and structural properties. This analysis led to an interesting sequence similarity between the partially overlapping fragment T678-Q690 of the spike and the SEB superantigenic peptide Y 150 NKKKATVQELD 161  (SEQ ID NO:112) ( FIG. 3A ). This dodecapeptide sequence within the SEB shows strong conservation among a broad range of staphylococcal and streptococcal SAgs (G. Arad et al. (2011), A. Popugailo, et al. (2019)). The sequentially aligned segment of SARS1 (5664-K672) bears minimal similarity to the SEB SAg ( FIG. 3A  left). What is even more interesting is that SARS-Cov-2 motif showed a palindromic behavior with respect to the superantigenic SEB sequence in the sense that a broader stretch, from E661 to R685, can be aligned to the superantigen peptide in the reverse direction as well ( FIG. 3A  right). This brings to the attention that the versatility and high propensity of the SARS-CoV-2 S TCRVβ-binding site residues to potentially act as a superantigenic fragment. 
     Significantly, the structures of the two peptides exhibit a remarkable similarity ( FIGS. 3B-3C ), including a salt bridge stabilizing each structural motif (E159-K152 in SEB and E661-R685 in SARS-CoV-2 S), the relative orientations of three positively charged residues (K152-K153-K154 in SEB and R682-R683-R685 in SARS-CoV-2 S), and an asparagine (N151 in SEB, N679 in SARS-CoV-2) completes this motif. All three features are absent in SARS1 S ( FIG. 3D ). A β-hairpin that apparently serves as a scaffold is conserved in all three spikes, and we observe a pair of cysteines that may potentially form a disulfide bond in SARS-Cov-2 and SARS1 spikes (C648-C657 and C662-C671, respectively). 
     This analysis overall indicates that the segment T 678 NSPRRAR 685  (SEQ ID NO:4) forms a putatively superantigenic core, consistently aligned against various bacterial or viral SAgs ( FIGS. 2C and 3A-3C ) with or without the participation of the adjoining amino acids. However, combined broader sequence and structure analysis in  FIG. 3A  (right) and  FIGS. 3B-3C , reveals an even more compelling feature: this putative SAg core is structurally consolidated by spatial proximity to a conserved acidic segment, E 661 CD 663 , which forms a highly stable salt bridge with the polybasic segment PRRAR (SEQ ID NO:114) of SARS-CoV-2 S, much in the same way as to the salt bridge observed in SEB (but not in SARS1 S) complemented by an asparagine shared between SARS-CoV-2 S and SEB (but not SARS1 S), and the SAg character can be conferred by this type of structural scaffolding. 
     The SEB superantigen peptide Y 150 NKKKATVQELD161 (SEQ ID NO:112) has been reported to bind CD28 (G. Arad et al. (2011)), a T cell receptor that provides co-stimulatory signals required for T cell activation and survival. CD28 and TCRV domains share the same (immunoglobulin) fold ( FIG. 3E ), and the binding mechanism shown in  FIG. 1B  can be adopted with minor rearrangements to interactions with other Ig-fold molecules including neutralizing antibodies. 
     Finally, because of the homologous superantigenic segment of SEB binding CD28, the potential binding of SARS-CoV-2 spike E661-R685 onto CD28 was also tested, considering the possibility that the target of SARS2 spike superantigenic segment is CD28. The simulations indicated that the same segment can equally bind to CD28, further supporting the strong propensity of the fragment to stimulate T cell activation. 
     Example 5. An ICAM-1 like Motif Shared between SARS1 and SARS-CoV-2 Spikes Interacts with TCRVa to Further Stabilize the S-TCR Complex 
     The existence of potential superantigenic, toxic or intercellular-adhesion molecule (ICAM)-like sequence fragments in SARS1 was thoroughly examined by Li et al. following the 2003 pandemic (Y. Li et al. (2004)). This led to the identification of the nine sequence stretches including three Botulinum neurotoxin type D or G precursors, and two motifs that have a high similarity with the intercellular adhesion molecule 1 (ICAM-1). Comparative analysis with SARS-CoV-2 spike sequence revealed that seven of these sequence motifs are conserved between SARS-CoV and SARS-CoV-2 (with &gt;68% sequence identity) ( FIG. 6 ). Among them, Y 279 NENGTITDAVDCALDPLSETKC 301  (SEQ ID NO:67), an ICAM-1 (CD54)-like motif, also participates in the association between the SARS-CoV-2 spike and the bound αβTCR (see  FIG. 4 ). 
     ICAM-1 involvement is critical to mediating immune and inflammatory responses. The observed interaction of the ICAM-1-like motif of SARS-CoV-2 S with TCRVα, in tandem with the interaction of the above discussed putative SAg motif (around the insert PRRA) with TCRVα, is to further strengthen the association of the virus with the T cell and the ensuing activation. Precisely, N280-E281-N282 and T286 belonging to the ICAM-like fragment closely interact with the TCRVα CDRs; mainly T286 (spike) makes close contacts with S94 (CDR3), E281 (spike) forms a hydrogen bond with T51 (CDR2), and N280 and N282 (spike) closely associate with R69 ( FIG. 4D ). 
     Example 6. A Rare Mutation, D839Y/E, Recently Observed in a SARS2 Strain from Europe Contributes to Stabilizing the Interaction with TCR 
     The SARS-CoV-2 spike binding region harbors three residues that have been recently reported to have mutated in new strains from Europe and USA (S. H. Zhan, et al. (2020), B. Korber et al. (2020)): D614G, A831V and D839Y/N/E). The former two may potentially interact with MHCII; while the latter (D839, European strain) is located close to TCRVβ and strongly interacts with N30; ( FIGS. 4A and 4C , and  FIGS. 7 and 8A ). Its substitution by glutamate in the mutant D839E increases the strength of the intermolecular (and thereby virus-T cell) association ( FIG. 8C ). Even stronger interactions between spike and TCRVβ are observed upon replacing D839 with a tyrosine as illustrated in  FIG. 8D : The interfacial interactions in this case are further stabilized by a hydrogen bond between Y839 and D32; an aromatic (polar-it) interaction between Y839 and N30; as well as electrostatic interactions with K73 and S97. Quantitative evaluation of the change in binding affinity between the spike and TCR upon mutating D839 to Y, E and N yields ΔΔG D→Y =−2.2 kcal mol −1 , ΔΔG D→E =−2.1 kcal mol −1 , and ΔΔG D→N =−1.3 kcal mol −1  respectively (see Table 2 for details). Thus, the D839Y/N/E mutations strengthen/support the above described association between the superantigenic PRRA-containing segment and the TCRVβ. The change in binding affinity between the spike and TCR upon mutating D839 to tyrosine is ΔΔG D→Y =−0.9±0.7 kcal/mol, indicating approximately 4-fold increase in binding affinity upon substituting the aspartic acid by a tyrosine at this position. The same qualitative effect is valid, but to a weaker extent in the mutations to asparagine or glutamic acid. See Table 3 therein for details on the method and results. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Binding affinities predicted for the interactions between the αβTCR 
               
               
                 and SARS-CoV-2 spike before/after the point mutation D839Y/N/E. 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Aspartic Acid (D) 
                 Tyrosine (Y) 
                 Glutamic Acid (E) 
                 Asparagine (N) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 ΔG 
                   
                 ΔG 
                   
                 ΔG 
                   
                 ΔG 
                   
               
               
                   
                 (kcal 
                 K d   
                 (kcal 
                 K d   
                 (kcal 
                 K d   
                 (kcal 
                 K d   
               
               
                   
                 mol −1 ) 
                 (nM) 
                 mol −1 ) 
                 (nM) 
                 mol −1 ) 
                 (nM) 
                 mol −1 ) 
                 (nM) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Full complex 
                 −11.0 
                 18 
                 −13.2 
                 0.46 
                 −13.1 
                 0.56 
                 −12.3 
                 2.3 
               
               
                 S subunit - 
                 −8.8 
                 580 
                 −10.3 
                 53 
                 −10.1 
                 80 
                 −9.5 
                 190 
               
               
                 TCR Vβ 
               
               
                   
               
               
                 * Binding affinities (ΔG) and dissociation constants (Kd) were obtained at 37° C. using PRODIGY server (Vangone, A. &amp; Bonvin, A. M. (2015), Xue, L. C., et al. (2016)). 
               
            
           
         
       
     
     Example 7. A Neurotoxin-Like Fragment at the RBD can also Bind αβTCR thus Further Enhancing the Immune Response 
     Further examination of the SARS-CoV-2 S segments sequentially homologous to the neurotoxinlike sequences identified (S. H. Zhan, et al. (2020)) for SARS1 S (rows highlighted in green in  FIGS. 7 and 10 ) pointed to two motifs conserved between the two CoVs: SARS-CoV-2 S residues 299-351 partially overlapping with the RBD and S2 subunit residues 777-807. The simulations in search of possible binding poses of TCR on the two CoVs indicated the RBD to be the 2nd highest affinity site (after the PRRA region) in SARS-CoV-2 S, and the 1st in SARS1 S.  FIG. 10C  illustrates such complexes. Thus, the exposure of a neurotoxin-like sequence on the RBD deserve attention as a possible source of CNS disorders in COVID-19 patients. 
     A recent study (Mateus J, et al. (2020)) detected significant T cell reactivity against 66 epitopes on the SARS-CoV-2 S glycoprotein in people who have not been exposed to the virus, inviting attention to memory response acquired upon exposure to human CoVs (HCoVs) such as common cold HCoV-OC43, -HKU1, -NL63, and -229E, which share sequence homology with SARS-CoV-2 genome. A total of 142 such cross-reactive epitopes were identified upon screening 474 peptides in the SARS-CoV-2 proteome (Mateus J, et al. (2020)). 
     The next experiment examined whether the neurotoxin-like regions identified here were among these cross-reactive epitopes. Notably, of the top-ranking four epitopes (ranked by T cell reactivity measured by spot-forming cells (SFC)/10 6  PBMCs), two (peptides 321-335 and 316-330) belong to the neurotoxin-like fragment T299-Y351, and one (236-250) to 234-262. In fact, the former was completely spanned by eight partially overlapping cross-reactive epitopes as illustrated in the  FIG. 13 , pointing to the distinctive ability of this region to trigger CD4 +  T cell response. These observations provide strong support to the predicted high affinity of this motif to bind TCR ( FIG. 10C ). Overall, this neurotoxin-like sequence T299-Y351 deserves attention as a source of CNS disorders in COVID-19 patients. 
     Among the 66 epitopes, we note 661-675, which lies within the SAg-like region E661-R685 ( FIG. 3 ), albeit at low reactivity and frequency. The insert PRRA (SEQ ID NO:2) among the cross-reactive epitopes is absent. This insert is unique to SARS-CoV-2 S among all SARS-related βCoVs, and cross-reactivity increases with sequence similarity to antigens/peptides to which the donors have been already exposed to. The sequence identity between SARS-CoV-2 and SARS1 is 40% in the portion 671-685 of the SAg-like region, and the percentage of cross-reactive peptides having 33-40% sequence identity is reported to be 1% (Mateus J, et al. (2020)). On the other hand, it is interesting to note that in a recent study (Sekine T, et al. (2020)) on epitopes that show strong T cell reactivity in convalescent patients who experienced severe COVID-19, an epitope (680-688) overlapping with the PRRA-containing part of SAg region is predicted to be one of the highest affinity epitopes binding to HLA. 
     Example 8. A Rare Mutation, D839Y/E, Recently Observed in a SARS2 Strain from Europe May Contribute to Stabilizing the Interaction with TCR 
     The SARS-CoV-2 S binding region harbors three residues that have been recently reported to have mutated in new strains from Europe and USA (Korber B, et al. (2020), Zhan S H, Deverman B E, &amp; Chan Y A (2020)): D614G, A831V and D839Y/N/E. The former two may potentially interact with MHCII based on a ternary model we generated for SARS-CoV-2 S, MHCII and TCR ( FIG. 7 ), while the latter (D839) is close to TCRVβ and strongly interacts with N30 ( FIGS. 4A and 4C , and  FIG. 8 ). The substitution of D839 by tyrosine strengthens the interactions between the spike and TCRVβ. The interfacial interactions in the D839Y mutant are stabilized by a hydrogen bond between Y839 and D32, an aromatic (polar-π) interaction between Y839 and N30, as well as electrostatic interactions with K73 and S97. The change in binding affinity between the spike and TCR upon mutating D839 to tyrosine is ΔΔG D→Y =−0.9±0.7 kcal/mol, indicating approximately 4-fold increase in binding affinity upon substituting the aspartic acid by a tyrosine at this position. The same qualitative effect is valid, but to a weaker extent in the mutations to asparagine or glutamic acid. See Table 3therein for details on the method and results. 
     Example 9. TCR Repertoire Analysis Shows TCRVβ Skewing and Junctional Diversity Indicating SAg Effect in Patients with Severe and Hyperinflammatory COVID-19 
     SAg binding to specific TCR Vβ chains results in Vβ skewing, such that T cells with specific Vβ chains and diverse antigen specificity dominate the TCR repertoire (Li H, Llera A, Malchiodi E L, &amp; Mariuzza R A (1999), Scherer M T (1993)). If the motif identified in SARS-CoV-2 S acts as a SAg, it can be reasoned that patients with mild/moderate COVID-19 disease courses and recovery without hyperinflammation show adaptive immune responses mediated by T cells recognizing SARS-CoV-2 epitopes in a CDR3-mediated fashion; whereas patients with severe/hyperinflammatory COVID-19 would show immune responses consistent with at least partial SAg recognition. NGS immunosequencing data from 38 patients (42 samples) with mild/moderate COVID-19 and 8 patients (24 samples) with severe, hyperinflammatory COVID-19, which were part of a previously studied cohort (Schultheiss C, et al. (2020)). Principal component analysis (PCA) of the TCR (3-chain variable gene (TRBV) repertoires corresponding to the two groups revealed that patients with mild/moderate COVID-19 course clustered apart from those with severe/hyperinflammatory COVID-19 ( FIG. 12A ). 
     Differential gene usage analysis showed that several TRBV genes were overrepresented in the severe/hyperinflammatory COVID-19 patient group ( FIG. 12B ). In contrast, PCA of J gene distribution showed much less skewing, indicating a selective pressure was preferentially exerted on the V gene distribution ( FIG. 12C ). To further investigate J gene diversity specifically for the V genes overrepresented in the severe/hyperinflammatory COVID-19 cases, all J genes rearranged with TRB V5-6, TRB V13, TRBV14 and TRB V24-1 were extracted from the repertoires of severe/hyperinflammatory COVID-19 patients and compared to J genes extracted from the age-matched healthy donors. This analysis showed very diverse TRBJ gene distribution, showing CDR3 diversity in the respective expanded rearrangements ( FIG. 14 ). 
     Together, these results show that patients with severe and hyperinflammatory COVID-19 show expansion of TCRs using distinct V genes, along with J gene/CDR3 diversity in these rearrangements, compatible with a SAg selection process. 
     Example 10. TCRs Corresponding to TRVB Genes Activated in Severe COVID-19 Patients can Bind to the SAg-Like Region of SARS-CoV-2 S 
     Finally, next experiment studied structurally resolved TCRs that contained Vβ chains encoded by the genes TRB V5-6, TRB V13, TRBV14 and TRBV24-1 enriched in severe/hyperinflammatory COVID-19 patients. Whether these TCRs could bind the SAg-like region E661-R685 of the SARS-CoV-2 S similarly to the TCR in  FIG. 1  was tested. The PDB search yielded αβPTCR structures corresponding to TRBV5-6, TRBV14 and TRBV24-1, i.e. TCRs whose Vβ chains were 95-100% identical to the protein product of these three genes. As shown in,  FIG. 15 , all three were verified to bind the SAg-like site with high-affinity, and to make interfacial interactions closely resembling those illustrated in  FIG. 1 . The models and simulations herein also indicated energetically favorable ternary complex formation between these TCRs, MHCII and spike. Overall, these simulations showed that these TCRs enriched in severe/inflammatory COVID-19 patients can bind the SARS-CoV-2 S at its SAg-like region and form ternary complexes with MHCII. 
     Example 11. Discussion 
     An understanding of the immunopathology leading to severe manifestations of COVID-19, in both adults and children, is of critical importance for effective management and treatment of the disease. MIS-C shows remarkable similarity to pediatric TSS (S. Riphagen, et al. (2020), L. Verdoni et al. (2020), Z. Belhadjer et al., (2020), D. E. Low, (2013), A. Cook, et al. (2020)). Using in silico modeling and analysis, it was found that SARS-CoV-2 encodes a superantigen motif near its S1/S2 cleavage site. This region is highly similar in structure to the SEB SAg motif that interacts with both the TCR and CD28 (G. Arad et al. (2011)) and mediates TSS. SEB enables large-scale T cell activation and proliferation, resulting in massive production of pro-inflammatory cytokines including IFNγ, TNFα and IL-2 from T cells as well as IL-1 and TNFα from APCs (T. Krakauer (2019)). This cytokine storm leads to multi-organ tissue damage similar to what is now observed in MIS-C. We therefore propose that MIS-C observed in COVID-19 patients may be mediated by superantigen activity of the SARS-CoV-2 S protein. Furthermore, these findings show that the hyperinflammation observed in severe cases of COVID-19 in adults can also be driven by the SAg-like activity of the S protein. Indeed, SAgs induce an inflammatory cytokine signature similar to that which predicts severity and death in COVID-19, including IL-6, TNFα, IL-8 and IL-1β (Krakauer T (2019), Del Valle DM, et al. (2020)). Moreover, the analysis of the T cell immune response in COVID-19 patients shows that those with more severe and hyperinflammatory clinical courses exhibit TCRVβ skewing consistent with SAg activity. 
     To date, MIS-C is mostly observed in Europe and East Coast of North America, and has not been described in Asia, despite sizeable outbreaks of COVID-19 (S. Riphagen, et al. (2020); L. Verdoni et al. (2020); Z. Belhadjer et al. (2020)) (CDC and ECDC). It is shown herein that a mutation at D839 found in a European strain of SARS-CoV-2 enhances the binding affinity of the SAg motif to the TCR. This can explain the geographical skewing of MIS-C to areas where the European strain is endemic, and identification of other strain-specific mutations helps predict where future outbreak of MIS-C may occur. 
     A study of SARS1 immunogenicity, conducted with a cohort of 128 individuals who have recovered from SARS1 (Li CK, et al. (2008)), showed that the SARS1 spike 18-mer D649-L666 (DIPIGAGICASYHTVSLL, SEQ ID NO:113) is one of the peptides most frequently recognized by T cells, among the screened 1,843 peptides that span the whole SARS1 CoV proteome (Table III in Li et al (Li CK, et al. (2008))). This segment coincides with the SARS1 S region E647-R667 that is sequentially (and structurally) homologous to our SARS-CoV-2 spike SAg-like motif E661-R685 ( FIG. 3A , bottom alignment). This provides a very strong support for the T cell stimulatory ability of the SAg motif, given that it shares 12/18 amino acids with that SARS1 18-mer. And the remaining amino acids (including the insert PRRA, not present in SARS1 S) would endow even stronger superantigenic properties by virtue of their close similarity to the aligned SEB fragment. 
     These findings indicate that immunomodulatory therapeutic options used for TSS can also be effective for MIS-C, including IVIG and steroids. Given structural similarities between SEB and the S protein SAg motif, cross-reactivity of these immunoglobins explains that at least in part the response of MIS-C cases to IVIG. Other FDA-approved anti-inflammatory drugs tested in models of SEB TSS can also be effective, including CTLA4-Ig which can inhibit CD28 co-stimulation (S. J. Whitfield et al. (2017)), and the mTOR inhibitor rapamycin (T. Krakauer, et al. (2010)), which is already in use for COVID-19. In addition, humanized monoclonal anti-SEB Abs have been described (E. A. Larkin, et al. (2010)) that can also be of therapeutic benefit in MIS-C patients. Notably, it has been shown in the mouse model of TSS that lethal SEB superantigen challenge can be prevented by short peptide mimetics of its superantigen motif (G. Arad et al. (2011)). Short peptide mimetics of SARS-CoV-2 spike superantigen region can be employed to prevent/attenuate inflammatory cytokine gene induction and toxic shock in MIS-C patients. 
     At present, the majority of antibody therapies under investigation are designed to target the SARS-CoV-2 receptor binding domains (RBDs) (M. Yuan et al., (2020), X. Chi et al. (2020)), and the simulations also indicated that RBD might potentially interact with TCRs. However, compared with RBDs, relatively fewer mutations are found in the SAg region of SARS-CoV-2; notably, the “PRRA” insert is unique to SARS-CoV-2 and retained among all of its isolates sequenced to date (S. H. Zhan, et al. (2020), B. Korber et al., (2020)). It is constructive to design antibodies or drugs targeting this SAg region, to not only block the cleavage essential to enabling viral entry (A. C. Walls et al. (2020), M. Hoffmann et al. (2020)) and modulate the SAg-induced inflammatory cytokine gene induction (T. Krakauer (2019)), but also block the cleavage essential to enabling viral entry (Walls A C, et al. (2020), Hoffmann M, et al. (2020)). Alternatively, combination therapies that target both the SAg-like region and the RBD can prove useful. 
     Fortunately, severe respiratory manifestations of COVID-19 in children as well as development of MIS-C are rare. This is due to trained immunity (L. Cristiani et al. (2020)) or cross-viral immunity to other coronavirus strains (A. Grifoni, et al. (2020)). T and B cells play an important role in the anti-viral response. CD4+ and CD8+ T cells from convalescent COVID-19 patients can recognize a range of SARS2 epitopes, and the S protein represents a major target (A. Grifoni, et al. (2020)). T cells from unexposed individuals can also respond to S protein epitopes from SARS-CoV-2, which supports the hypothesis of cross-viral immunity from other coronavirus strains (A. Grifoni, et al. (2020)). However, why only a fraction of infected children develop MIS-C is unclear. The present study shows that the mutation D839Y found in a European strain of SARS-CoV-2 enhances the binding affinity of the SAg motif to the TCR. This can explain the geographical skewing of MIS-C to areas where the European strain is endemic. A poor initial antibody response to the virus fails to neutralize the SAg, as recently shown in MIS-C patients (Weisberg SP, et al. (2020)), leading to immune enhancement following re-exposure. Certain HLA types are more permissive of binding SAg, and indeed HLA has been shown to play a role in COVID-19 susceptibility (Nguyen A, et al. (2020)). Of the nine cases initially reported in the UK, six were of Afro-Caribbean descent, which also suggests a potential genetic component to susceptibility (Riphagen S, et al. (2020)). In addition, approximately 80% of individuals over age 12 harbor anti-SEB antibodies (LeClaire R D &amp; Bavari S (2001), McGann V G, Rollins J B, &amp; Mason D W (1971)), which may provide protection against the SAg effects of SARS-CoV-2 S protein. The prevalence of preexisting anti-SEB antibodies can also contribute to the age distribution of severe COVID-19 cases in adults, as protective SEB titers fall in older adults after age 70. 
     Approximately a third or fewer of MIS-C patients tested positive for the SARS-CoV-2, but the majority (but not all) have serologic evidence of infection or a history of exposure to COVID-19 (S. Riphagen, et al. (2020); L. Verdoni et al. (2020); Z. Belhadjer et al. (2020)). This indicates that the SARS-CoV-2 SAg causes a delayed hyperinflammation response in certain children. SAgs have been implicated in autoimmunity by triggering self-reactive T cells (H. Li, et al. (1999)). Antibody-mediated enhancement upon re-exposure to the virus can also contribute to uncontrolled infection and inflammation (S. M. C. Tirado, K.-J. Yoon (2003)). Despite a negative nasopharyngeal PCR test, the virus can still be present in the gastrointestinal tract (Y. Xu et al. (2020)). MIS-C patients demonstrate unusually severe GI symptoms, abdominal pain, vomiting and diarrhea, in addition to severe myocardial dysfunction and cardiac shock (S. Riphagen, et al. (2020); L. Verdoni et al. (2020); Z. Belhadjer et al. (2020)) and such severe GI symptoms are also frequently associated with the SAg (A. Cook, et al. (2020)). In the case of SEB, cleavage and release of a specific fragment is responsible for induction of GI symptoms. The SARS-CoV-2 SAg-like structure shown herein can be similarly cleaved and underlie the GI symptoms observed in MIS-C patients. 
     It was also observed that a neurotoxin-like segment (T299-Y351) partially overlapping with the RBD exhibited a high affinity to bind TCRs. Notably, this region was recently observed to elicit strong and frequent T cell reactivity mediated by CD4+ T cells in donors who have not been exposed to SARS-CoV-2 (Mateus J, et al. (2020)). This invites attention to its ability to trigger neurotoxic immune response in individuals who have not been exposed to CoVs that contain sequentially homologous peptides. 
     In summary, disclosed herein are five major observations: (a) PRRAR and sequential neighbors interact with TCRVβ residues D56, R70 and E74 at the CDRs, and this association closely resembles that of SEB SAg with TCRVβ; (b) nearby D839 participates in this interaction and its mutation to tyrosine further strengthens the association with TCRVβ; (c) a sequence motif (N280-T286) typical of ICAM-1 further interacts with the TCRVα further stabilizing or enhancing the association between the viral spike and host cell TCR; and (d) a neurotoxin-like motif (T299-Y351) shows a high tendency to bind TCRs and trigger neurotoxic responses. This latter effect can be attenuated if the SARS-CoV-2-infected individual has been exposed HCoVs that contain homologous segments, as suggested (Mateus J, et al. (2020)) by a recent study; and (e) adult patients with severe/hyperinflammatory COVID-19 exhibit a skewed TCR Vβ repertoire distinguishing them from patients with mild/moderate COVID-19. Overall, these results from both computational modeling and NGS immunosequencing of TCRBs analysis of human samples indicate that strategies used for the treatment of SEB-mediated TSS or approaches to block the interaction of the S protein with TCRs can help reduce hyperinflammatory manifestations or (neuro)toxic effects of COVID-19 in both adults and children. 
     Example 12. A Monoclonal Antibody Against Staphylococcal Enterotoxin B Superantigen Inhibits SARS-CoV-2 Entry In Vitro 
     Introduction 
     Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) can cause severe interstitial pneumonia with hyperinflammation (Tay et al., 2020; Vabret et al., 2020), as well as many extrapulmonary manifestations (Gupta et al., 2020). A novel multisystem inflammatory syndrome (MIS), reported in both children (MIS-C) and adults (MIS-A), has been observed in patients that either tested positive for, or had epidemiological links to, COVID-19 (Belhadjer et al.; Cheung et al., 2020; Riphagen et al.; Verdoni et al.). MIS-C manifests as persistent fever and hyperinflammation with multi-organ involvement (Belhadjer et al.; Cheung et al., 2020; Riphagen et al.; Verdoni et al.). The clinical similarity between MIS-C/A and the toxic shock syndrome (TSS) caused by bacterial superantigens (SAgs) led to the hypothesis that SARS-CoV-2 might possess a SAg-like motif that triggers hyperinflammation (Cheng et al., 2020; Noval Rivas et al., 2020). Comparison with bacterial toxins indeed revealed a motif in the SARS-CoV-2 spike (S) protein, the sequence and structure of which highly resemble a segment of a bacterial SAg, staphylococcal enterotoxin B (SEB). SAg-like character of the S protein was further supported by T cell receptor (TCR) skewing typical of reaction to SAgs, which was observed in severe COVID-19 patients (Cheng et al., 2020). 
     The location of the SAg-like motif in the S protein is worthy of attention. SARS-CoV-2 S is a homotrimer, belonging to the family of human coronaviruses (HCoVs), which includes SARS-CoV and Middle East Respiratory Syndrome (MERS), as well as common cold HCoVs NL63, 229E, OC43 and HKU1 (Coutard et al., 2020; Cui et al., 2019; Forni et al., 2017). Each HCoV protomer is composed of two subunits, S1 and S2, playing different roles in viral infection. S1 contains the receptor-binding domain (RBD) that binds to the host cell receptor (human angiotensin converting enzyme 2 (ACE2) for SARS-CoV-2, SARS-CoV, and HCoV-NL63) (Benton et al., 2020; Hoffmann et al., 2020; Matsuyama et al., 2020; Shang et al., 2020; Walls et al., 2020; Wrapp et al., 2020; Yan et al., 2020); whereas S2 contains the fusion peptide required for viral entry (Coutard et al., 2020; Cui et al., 2019; Forni et al., 2017). The SAg-like motif (residues E661-R685) lies at the C-terminus of S1 (Cheng et al., 2020), at the boundary with S2. Membrane fusion requires two successive cleavages by host cell proteases, one at the S1/S2 interface (peptide bond R685↑S686), and the other at S2′ (R815↑S816) (Coutard et al., 2020; Hoffmann et al., 2020; Matsuyama et al., 2020; Shang et al., 2020; Walls et al., 2020; Wrapp et al., 2020; Yan et al., 2020). Thus, the SAg-like region overlaps with the S1/S2 cleavage site of the S protein ( FIGS. 16A -16B). 
     Another feature at the SAg-like region is a unique insertion,  681 PRRA 684  (SEQ ID NO:2), immediately neighboring the cleavage site R685↑S686 ( FIG. 16A ). Loss of these four residues in a mutant ΔPRRA has been recently shown to attenuate SARS-CoV-2 pathogenesis (Johnson et al., 2021). SARS-CoV-2 is the only member of the SARS-family of β-coronaviruses (βCoVs) that has such an insertion (see nine such members sequentially aligned in  FIG. 16B ; top 9 rows), despite its high sequence similarity with other members of this genus (&gt;80% sequence identity with SARS-CoV). MERS and common cold HKU1 and OC43 S proteins have a similar insertion at that position, despite their low (30-40%) overall sequence identity with respect to SARS-CoV-2 spike ( FIG. 16B ). The PRRA (SEQ ID NO:2) insert is highly flexible, and together with the adjacent arginine, the segment  681 PRRAR 685  (SEQ ID NO:114) forms a highly reactive site. It plays a role in recognizing and binding the host cell proteases transmembrane protease serine 2 (TMPRSS2) and furin, whose cleavage activity is essential to S protein priming (Hoffmann et al., 2020; Shang et al., 2020; Walls et al., 2020; Wrapp et al., 2020). Recent studies further showed the role of S1/S2 site in potentiating infectivity upon binding to the host cell co-receptor neuropilin-1 (Cantuti-Castelvetri et al., 2020; Daly et al., 2020); and our simulations revealed its propensity to bind TCRs (Cheng et al., 2020). 
     This polybasic site,  681 PRRAR 685  (SEQ ID NO:114), can thus serve as a target for SARS-CoV-2 S-neutralizing antibodies (Abs). Most SARS-CoV-2 S Abs under investigation target the RBD (and some, the N-terminal domain, NTD) (Cao et al., 2020b; Chi et al., 2020; Hansen et al., 2020; Pinto et al., 2020; Renn et al., 2020; Shi et al., 2020; Yuan et al., 2020).  FIG. 16C  illustrates the S protein epitopes (colored surfaces) that have been observed by cryo-EM to bind mAbs and ACE2 molecules. The Abs bind various poses/sites depending on the up or down states of the RBDs and their specific sequences (see Table 4). However, these cryo-EM studies were conducted with variants where the polybasic segment  682 RRAR 685  (SEQ ID NO:121) has been replaced by GSAS or SGAG (Barnes et al., 2020; Cao et al., 2020b; Chi et al., 2020; Liu et al., 2020; Lv et al., 2020b; Pinto et al., 2020; Zhou et al., 2020a; Zost et al., 2020). Therefore, the ability, if any, of wild type (wt) S protein to bind an Ab near the PRRA (SEQ ID NO:2) insert or the S1/S2 cleavage site can have eluded these experiments. Identification of alternative binding sites for neutralizing mAbs is now increasingly important with the need to design combination mAbs that target different sites, given the ability of newly emerging variants to potentially evade those Abs that target the RBD site (Andreano et al., 2020; Greaney et al., 2021; Kemp et al., 2020; McCarthy et al., 2020). 
     The present study focuses here on this polybasic site as a target for mAb binding. The recently detected sequence- and structure-similarity between the PRRA-insert-enclosing SAg-like motif and the bacterial toxin SEB indicated that previously generated anti-SEB monoclonal Abs (mAbs) can bind the viral SAg-like motif, and in particular the segment  682 RRAR 685  (SEQ ID NO:2), and can thus block access to the S1/S2 cleavage site. The in silico examination of the possible interactions of known anti-SEB mAbs (Dutta et al., 2015) with SARS-CoV-2 S revealed that SEB-specific mAb 6D3 has a high affinity for binding to the S1/S2 site. The models further show that the 6D3 binding site overlaps with those of TMPRSS2 and/or furin, indicating that 6D3 can impede viral entry. Experiments conducted with live viruses confirmed that 6D3 inhibited viral entry. Given that its binding site does not overlap with those of known Abs ( FIG. 16C ), 6D3 can be used in combination with other neutralizing Abs that target the RBD or other non-overlapping sites to increase efficacy. 
     Results 
     Anti-SEB antibody 6D3 is distinguished by its high affinity to bind SARS-CoV-2 S SAg-like region. As shown in the recent work (Cheng et al., 2020), the S residues E661-R685 that enclose the polybasic segment  681 PRRAR 685  (SEQ ID NO:114) are sequentially and structurally similar to the segment T150-D161 of SEB. Given this strong similarity, it was examined if mAbs specific for SEB (Dutta et al., 2015; Varshney et al., 2011) can neutralize SARS-CoV-2 S. The close proximity (or adjacency) of the SAg-like region to the cleavage bond R685↑S686 further indicated that an anti-SEB mAb that cross-reacts with SARS-CoV-2 can have the added potential to block the cleavage site essential to viral entry, apart from its ability to attenuate the SAg-mediated hyperinflammatory cytokine storm (Krakauer, 2019). 
     Three SEB-specific mAbs, 14G8, 6D3, and 20B1, have been generated as effective blockers of the SAg activity of SEB in an animal model of TSS (Varshney et al., 2011). Examination of their crystal structures shows that these mAbs bind different sites on SEB (Dutta et al., 2015), as illustrated in  FIG. 17A . Notably, only 6D3 targets the SEB polybasic segment T150-D161 (shown in dark blue space filling representation) that is the counterpart of the SARS-CoV-2 S SAg-like motif (Cheng et al., 2020). A closeup view shows the tight interaction between the acidic residues E50, D52 and D55 of 6D3 heavy chain and four basic residues of SEB ( FIG. 17B ). 
     Among these three SEB mAbs, 6D3 was the only one able to bind to the SARS-CoV-2 S SAg motif ( FIGS. 17C -17E), consistent with 6D3 binding to the precise SEB fragment that aligns with the spike SAg-like motif. The computational analysis indicated the 6D3 Ab to bind with an affinity of −14.2±2.3 kcal/mol (see Methods). Notably, acidic residues E50, D52 and D55 from the heavy chain of 6D3 were found to interact with polybasic insert PRRA (SEQ ID NO:2) in SARS-CoV-2 S, with R682 and R683 playing a central role. Yet, interfacial contacts were quite distributed, involving other SARS-CoV-2 S amino acids such as E654, N603 and N679 interacting with either the heavy or light chains of 6D3 ( FIG. 17E ). 
     Among those 6D3-interacting S residues, N603 has been identified as an N-linked glycan site by site-specific glycan analysis of SARS-CoV-2 S (Watanabe et al., 2020) ( FIG. 18A ). To investigate if the glycan sequons can interfere with 6D3 binding, the spike-6D3 complex model was aligned against the glycosylated spike (Woo et al., 2020). No steric overlap was observed between 6D3 and glycan sequons as illustrated in  FIG. 18B . Of note is that the N603-linked glycan even assists in the association of 6D3 with the specific binding epitope that overlaps with the SAg-like (and S1/S2 cleavage) site, rather than obstructing it. A tight interaction between N603 and Y57 on 6D3 heavy chain variable domain (VH) is observed, in addition to contacts between the glycan and 6D3 VH residues Y57-Y60 that further contribute to the stabilization of 6D3 binding. These results indicate that the anti-SEB mAb 6D3 shows high affinity binding to the SARS-CoV2 superantigen-like motif, therefore blocking its interaction with TCR, and attenuating the SAg-mediated T cell activation and cytokine release. 
     These results indicated that 6D3 can decrease the exposure of the cleavage site to the extracellular environment and interfere with SARS-CoV-2 viral entry upon competing with the host cell proteases TMPRSS2 and furin whose binding to the cleavage site, is essential to S protein priming for viral entry. Next, the investigation of the neutralizing effect of 6D3 was tested in live virus experiments. 
     Anti-SEB antibody, 6D3, inhibits SARS-CoV-2 infection in live virus assays. Here, whether the SEB-specific mAb 6D3 possessed any neutralizing efficacy vis-à-vis SARS-CoV-2 viral entry was investigated. To this end, the ability of 6D3 to inhibit SARS-CoV-2 infection was tested in an in vitro cell culture infection system. Antibodies were incubated with SARS-CoV-2 for 1 hour and then added to plated Vero-E6 cells. At 48 hours post infection, viral infection was analyzed by immunofluorescence using antibodies against dsRNA or SARS-CoV-2 S protein. ( FIG. 19  and  FIG. 21 ). It was found that 6D3 significantly inhibited viral infection, as measured by the percentage of dsRNA positive cells, at concentrations of 0.8, 4 and 20 μg/ml of antibody, with an IC50 of 5.63 μg/ml ( FIGS. 19A-19B  and  FIG. 21A ). Furthermore, in an independent set of experiments, it was found that 6D3 significantly inhibited viral infection, as measured by the percentage of spike positive cells, at concentrations of 4, 20 and 40 μg/ml of antibody, while there was a trend for inhibition at 0.16 and 0.8 μg/ml of antibody ( FIGS. 19C-19D  and  FIG. 21B ). 
     These results indicate that 6D3, can also block viral entry in a concentration-dependent manner, in addition to its high affinity binding to the SARS-CoV2 superantigen-like motif and potentially blocking its interaction with TCRs. Toward assessing whether 6D3 competitively binds the S1/S2 site in the presence of the proteases, the spike-binding mechanisms and affinities of TMPRSS2 and furin was explored and presented next. 
     TMPRSS2 and/or furin bind to the S1/S2 site in close association with the PRRA (SEQ ID NO:2) insert. The protease-binding characteristics of the S1/S2 site were analyzed to assess whether Abs that targets the PRRA (SEQ ID NO:2) site can also hinder the access of proteases. The S1/S2 site, also known as furin-cleavage site, typically contains eight central residues including the polybasic segment (here  680 SPRRAR↑SV 687 , SEQ ID NO:115), flanked by solvent-accessible residues on both sides (Tian et al., 2012). The resulting structural models generated for the interactions of TMPRSS2 and furin with the S protein are presented in the respective  FIGS. 20A  an  20 B, and more details are reported in the  FIGS. 22 and 23 . To generate these models, the available structural data was used (Dahms et al., 2016; Walls et al., 2020; Wrapp et al., 2020) for the proteins, as well as the docking software ClusPro (Kozakov et al., 2017) and protocols outlined in the STAR Methods. An ensemble of structural models were generated for each complex, and those conformers satisfying the criteria for S1/S2 cleavage, mainly positioning of catalytic residues within 3-7 Å atom-atom distance from the cleavage site, were selected for further refinement and energetic evaluation using PRODIGY (Xue et al., 2016). 
     TMPRSS2 catalytic residues (H296, D345 and S441) were observed to bind near  681 PRRARS 686  (SEQ ID NO:6) in 7.5% of the generated models ( FIG. 22B ); their binding affinities varied from −14.1 to −11.3 kcal/mol with an average of −12.7±2.0 kcal/mol.  FIG. 20A  displays the most energetically-favorable model where the three arginines in  681 PRRARS 686  (SEQ ID NO:6) penetrate in the catalytic cavity ( FIG. 20A , right): R682 forms a salt bridge with TMPRSS2 residue D435; R683 with catalytic aspartate D345, and R685 with TMPRSS2 E299, positioning the scissile bond (spheres) near catalytic residues S441 and H296. 
     In the case of furin binding, 70% of the structural models showed the catalytic residues (D153, H194 and 5368) stabilized in close proximity of  681 PRRARS 686  (SEQ ID NO:6) (see  FIG. 23 ), indicating that binding of furin to the cleavage site was entropically more favorable than that of TMPRSS2. The binding affinities varied from −16.4 to −11.8 kcal/mol with an average of −14.1±2.3 kcal/mol. The best pose with the catalytic residues facing the S1/S2 site, shown in  FIG. 20B  (and  FIG. 23A ), reveals the insertion of R682 and R683 into negatively charged pockets of furin to enable the cleavage of the SARS-CoV-2 S. 
     Overall, the analysis shows that TMPRSS2 or furin engage in tight intermolecular interactions, in which the basic residues R682 and R683 reach out to the catalytic site of either protease. Binding of either enzyme is accommodated by changes in the local conformations near the cleavage region. However, this analysis also shows that furin binds with higher potency and probability, compared to TMPRSS2. Most importantly, 6D3 and the proteases compete for the same binding site ( FIG. 24 ). Comparison with the binding affinity of 6D3 evaluated above shows that 6D3 has a spike-binding affinity comparable to that of furin, and stronger than TMPRSS2, indicating that it can effectively compete with those proteases, in agreement with the experimentally observed efficacy in reducing viral entry. 
     An acidic residue cluster at V H  CDR2 is the hallmark of Abs targeting the furin-like cleavage site. The study pointed to the distinctive ability 6D3 to bind to the S1/S2 cleavage site while other mAbs (in Table 4) did not show such a binding propensity. Which sequence/structure features distinguish 6D3 from others was investigated. Abs target viruses mainly through their three complementarity determining regions (CDR1-3) in the variable domains, especially in the heavy chains (Li et al., 2020).  FIG. 25A  compares the sequences of the VH chains of the SARS-CoV-2 S-associated mAbs, and three mAbs associated with SEB. CDR3s exhibit large sequence variation, in accord with their role in conferring specificity. However, the alignment reveals a unique feature that distinguishes 6D3 and another mAb, 4A8, from all other mAbs: mainly a poly-acidic cluster at their CDR2. Specifically, the 6D3 CDR2 possesses three acidic residues E50, D52, and D55, already noted above to enable binding to the precise cleavage-site on the S protein. Likewise, mAb 4A8 has four acidic residues D52, E54, D55 and D57 ( FIG. 25A ). 4A8 is known to bind the NTD of the spike (Chi et al., 2020; McCarthy et al., 2020) ( FIG. 26A ). The docking simulations also indicated that the particular S epitope and 4A8 paratope observed in the cryo-EM structure of the spike-4A8 complex were selected as the most favorable binding pose ( FIG. 26B ). However, the SAg-like motif E661-R685 was also found to be favorable, albeit with a weaker binding affinity ( FIG. 26C ) and could compete with human proteases for binding the same site ( FIG. 26D ). These simulations indicated a binding affinity of −13.4±2.4 kcal/mol for the NTD (experimentally observed and computationally most probable) site, consistent with the equilibrium dissociation constant (K d =2.14 nM, or corresponding ΔG=−12.3 kcal/mol) measured by biolayer interferometry for the spike-4A8 complex (Chi et al., 2020). 
     A poly-acidic CDR2 at the VH chain thus emerges as a hallmark of the mAbs that target the polybasic furin-like cleavage site. As shown in  FIG. 25B , these acidic residues facilitate Ab-spike complexation through salt bridges formed with the basic residues (R682, R683 and R685) in  680 SPRRARSV 687  (SEQ ID NO:115), the central component of typical furin-cleavage sites (Tian et al., 2012), thus attenuating, if not blocking, access of proteases. 
     As shown in  FIG. 16B , the polybasic insertion of SARS-CoV-2 is not shared by other SARS-family βCoVs, but found in common cold HCoVs HKU1 and OC43, and in MERS. Present findings strongly show that mAbs 6D3 can also target these other HCoVs that encode a furin-cleavage site. The binding properties of 6D3 to a structural model for HCoV-OC43 S was investigated based on the OC43 cryo-EM structure (PDB: 6NZK) (Tortorici et al., 2019). The highest affinity binding pose predicted by our simulations are presented in  FIG. 25C , indicating that 6D3 binds the S 1/S2 cleavage site of HCoV-OC43 S. Again, poly-acidic residues in CDR2 play a primary role in binding to the cleavage site of HCoV-OC43 S. These findings underscore the effectiveness and cross-reactivity of 6D3 that target the furin-cleavage site between the S protein of SARS-CoV-2 and other selected HCoVs. 
     DISCUSSION 
     A new strategy for combatting SARS-CoV-2: repurposing of antibodies that target the S1/S2 cleavage site. SARS-CoV-2 S is the main determinant of cell entry and the major target of neutralizing Abs (Cao et al., 2020b; Chi et al., 2020; Hansen et al., 2020; Pinto et al., 2020; Renn et al., 2020; Shi et al., 2020; Yuan et al., 2020). The majority of COVID-19 Ab therapies under investigation are designed to target the S protein RBD, while other potential neutralizing epitopes have also been found (Cao et al., 2020b; Chi et al., 2020; Hansen et al., 2020; Liu et al., 2020; Pinto et al., 2020; Renn et al., 2020; Shi et al., 2020; Yuan et al., 2020). Given the high glycosylation and antigenic variability of SARS-CoV-2 S (Graham et al., 2019), a combination of mAbs that target multiple sites and multiple conformations of SARS-CoV-2 S, is likely the most effective strategy. Besides blocking ACE2 binding, distinct neutralizing mechanisms have been proposed, including Ab-dependent cell cytotoxicity and phagocytosis (Pinto et al., 2020) and restraining the structural changes of SARS-CoV-2 Spike (Chi et al., 2020). 
     Proteolytic cleavage of SARS-CoV-2 S is the second critical step, succeeding ACE2 binding, in the life cycle of SARS-CoV-2. TMPRSS2 and furin inhibitors have been found to block cell entry of SARS-CoV-2 (Bestle et al., 2020; Hoffmann et al., 2020). The critical role of the furin-cleavage site in SARS-CoV-2 infectivity and Ab activity is also demonstrated in a recent study where the deletion APRRA reduced SARS-CoV-2 viral replication in a human respiratory and attenuated infectivity (Johnson et al., 2021). Unlike TMPRSS2, furin is a ubiquitous proprotein convertase and is required for normal development and function (Thomas, 2002) and its inhibition is not a viable strategy. But, design or repurposing of Abs that block the S1/S2 site is an attractive alternative solution that avoids effects on the (other) activities of TMPRSS2 and furin. 
     It is well known that the SARS-CoV-2 spike is heavily glycosylated, and the possible interference of glycans with Ab binding is a plausible consideration (Casalino et al., 2020). Notably, 6D3-binding did not give rise to steric clash with the N-linked glycan sequons near the S1/S2 site (e.g. N603, or N657/N658 as reported (Watanabe et al., 2020)). In addition SARS-CoV-2 S was predicted to be 0-glycosylated at S673, T678 and S686 near the S1/S2 cleavage site (Andersen et al., 2020), yet to be confirmed by experiments (Shajahan et al., 2020; Watanabe et al., 2020). 6D3 can target directly the S1/S2 site of SARS-CoV-2 S (as the host proteases do) without any shielding effect by glycans. On the contrary, the glycosylation near N603 can even assist in promoting its binding near the PRRA (SEQ ID NO:2) site. The modulating role of glycans on the structure and dynamics of the S glycoprotein has been pointed out to be a feature that could be harnessed in anti-SARS-CoV-2 vaccine development (Casalino et al., 2020). 
     The ability of the polybasic insert to bind antibodies may have escaped prior cryo-EM studies with mutant S protein. It has been a challenge to resolve The S1/S2 loop in cryo-EM studies of HCoV S proteins. First, pre-activation of HCoV S during protein preparation results in a mixture of cleaved and uncleaved spikes (Cai et al., 2020). Second, local conformational changes near the S1/S2 region may differ between cleaved and intact structures, as observed in influenza viruses (Steinhauer, 1999). Third, multiple conformations, if not a disordered state, may exist near that region, as indicated by microseconds simulations and ab initio modeling (Lemmin et al., 2020). Therefore, most of cryo-EM studies of SARS-CoV-2 S protein complexed with Abs have resorted to variants where the  682 RRAR 685  (SEQ ID NO:121) segment has been replaced by GSAS or SGAG (Barnes et al., 2020; Cao et al., 2020b; Chi et al., 2020; Liu et al., 2020; Lv et al., 2020b; Pinto et al., 2020; Zhou et al., 2020a; Zost et al., 2020) (Table 4). These ‘mutant spikes’ may have precluded the discovery of binding of Abs to the S1/S2 site. Molecular modeling and simulations provided insights into the interactions at this region, including those with proteases and other receptors (Cheng et al., 2020; Jaimes et al., 2020; Lemmin et al., 2020). Such modeling studies was utilized ( FIGS. 17C-17E, 18, 20 and 25 ), and performed live-virus experiments ( FIGS. 19  and  FIG. 21 ) to investigate the Abs that possibly interfere with the binding of TMPRSS2 and furin. An antibody (6D3) that can block the S1/S2 cleavage site in SARS-CoV-2 S protein was identified. It is noted that, due to the conformational variability of the S1/S2 loop, the actual binding site of 6D3 on SARS-CoV-2 S can be different from that predicted by computational models, and there can be multiple binding sites on S, as predicted for the Ab 4A8 ( FIG. 26 ). 
     6D3 is a repurposable anti-SEB mAb that targets the S1/S2 site and inhibits viral infection. 6D3 is an Ab originally discovered for neutralizing the superantigenic bacterial toxin SEB. it&#39;s the present study indicates its use as repurposable mAb against SARS-CoV-2 S protein, by virtue of its ability to bind a sequence motif shared between SEB and S protein. The recent study revealed the high similarity between SARS-CoV-2 S amino acids E661-R685 and SEB amino acids T150-D161, which can contribute to hyperinflammation and MIS-C/A pathogenesis through a SAg-induced immune activation (Cheng et al., 2020). This hypothesis was supported by the clinical and laboratory features observed in MIS-C and severe COVID-19 patients, which were similar to those of toxic shock syndrome (TSS) caused by bacterial toxins such as SEB (Cheng et al., 2020; Noval Rivas et al., 2020). Adult patients with severe Covid-19 (Cheng et al., 2020) as well as children with the multisystem inflammatory syndrome (MIS-C) (Porritt et al., 2020) displayed TCR skewing typical of SAg-induced immune responses. Among the three mAbs discovered against SEB, 6D3 was the only one specific to the region of interest ( FIGS. 17A-17B ), and computations and experiments indicated that this anti-SEB mAb can bind to the SARS-CoV-2 S protein. 
     Another feature was the fact that this SAg-like segment (that binds 6D3) overlapped with the furin-like cleavage site characteristic of SARS-CoV-2 (and MERS and HCOVs HKU1 and OC43; see  FIG. 16B ). Furin-cleavage sites usually involve ˜20 residues, eight of which play a central role (Tian et al., 2012). In the case of SARS-CoV-2, the segment  680 SPRRAR↑SV 687  (SEQ ID NO:115) of the S protein forms this central component. Simulations indeed showed strong interactions (salt bridges) formed between 6D3 V H  CDR2 (distinguished by a stretch of acidic residues) and the polybasic  682 RRAR 685  (SEQ ID NO:121) ( FIGS. 17C-17E and 25B ), and in vitro assays confirmed that 6D3 inhibited viral entry ( FIG. 19  and  FIG. 21 ). 
     By binding the viral spike protein, SARS-CoV-2 specific antibodies in the blood or mucosal surface could prevent the virus from binding to and infecting target cells. The antibody neutralization assay that we performed in cell culture simulates this scenario, where the specific mAb 6D3 incubated with SARS-CoV-2 binds and neutralizes the virus&#39;s ability to attach to the cell receptor and to initiate infection in vitro. Thus, mAb 6D3 can have a differentiating dual role in not only inhibiting viral entry but also blocking the SARS-CoV-2 superantigen-like motif-induced T cell activation, cytokine storm, and hyperinflammation. The next experimental steps assessing the in vivo effect of mAb 6D3 in relevant mouse models of SARS-CoV-2 infection are currently underway. 
     mAbs with a cluster of acidic residues at their VH CDR2 can mitigate viral infections caused by CoVs that contain furin-like cleavage sites. HCoVs include three highly pathogenic viruses, SARS-CoV-2, SARS-CoV and MERS, and four circulating endemic viruses (HCoV-NL63, HCoV-229E, HCoV-OC43 and HKU1) which cause mild to moderate upper respiratory diseases (Coutard et al., 2020; Cui et al., 2019; Forni et al., 2017). Many individuals who have not been exposed to SARS-CoV-2 possess SARS-CoV-2 Spike reactive T cells, due to cross-reaction of immune responses generated against other HCoV strains (Grifoni et al., 2020; Mateus et al., 2020). Cross-reactive antibodies between human βCoV strains have also been identified, including those between SARS and SARS-CoV-2 (Huang et al., 2020; Lv et al., 2020a). Indeed, SARS monoclonal antibody S309 can potently neutralize both SARS and SARS-CoV-2(Pinto et al., 2020). Furthermore, the effectiveness of IVIG (Belhadjer et al.; Riphagen et al.; Verdoni et al.), may, in part, be due to the presence of cross-reactive antibodies against other HCoV stains. These findings show designing wide spectrum Abs with cross-reactivity among HCoVs. The two Abs (6D3 and 4A8) identified in this study to present the suitable paratope for binding the PRRAR or similar polybasic inserts can block the S 1/S2 cleavage site in HCoVs that encode furin-like cleavage sites ( FIG. 25 ), providing additional benefit beyond those applicable to the current pandemic. The hallmark poly-acidic residues in the CDR2 of VH can be exploited as a benchmark to sort out mAbs that can target the SARS-CoV-2 furin cleavage site. 
     Alternative strategies targeting the S1/S2 site in the light of these repurposable mAbs. Based on the scaffold of 6D3 heavy chain, mini-proteins can be designed to target SARS-CoV-2, MERS, HCoV-OC43 or HKU1, to block CoV entry. Notably, designed de novo mini-proteins have been shown to block ACE2 binding, based on the scaffold of ACE2 (Cao et al., 2020a). Very recently, neuropilin-1 (NRP1) has been identified as a host factor for SARS-CoV-2 infection, bound to the  681 RRAR 685  (SEQ ID NO:121) segment (Daly et al., 2020). Remarkably, blockade of this interaction by RNAi or mAb against NRP1 significantly reduced in vitro SARS-CoV-2 cellular entry (Cantuti-Castelvetri et al., 2020; Daly et al., 2020). 6D3 can block the binding of NRP1. At present, no clinical treatments or prevention strategies are available for HCoVs (Cui et al., 2019). The present work leads to an improved understanding of coronavirus immunity, facilitating future studies to understand mechanisms of antibody recognition and neutralization, and help screen SARS-CoV-2 Abs for treatment of COVID-19. These findings also show designing therapeutic approaches using a combination of 6D3 and known neutralizing mAbs that bind the RBD, for treating severe COVID-19 and MIS-C/A patients and/or combatting the spread of the newly emerging variants. 
     METHOD DETAILS 
     In vitro viral inhibition assays. SARS-CoV-2 viral assays were performed in UCLA BSL3 high containment facility, following previous procedure(Garcia et al., 2020). SARS-CoV-2 Isolate USA-WA1/2020 was obtained from BEI Resources of National Institute of Allergy and Infectious Diseases (NIAID). Mouse Fab 6D3 (IgG2b) was generated as (Varshney et al., 2011). Vero-E6 cells were plated in 96-well plates (5×10 3  cells/well). 6D3 IgG2b or mouse IgG2b isotype control (Bio X Cell) were incubated with virus (100 PFU/well) for 1 hour at room temperature prior to addition to Vero-E6 cells. After 48 hours post-infection the cells were fixed with methanol for 30-60 minutes in −20° C. Cells were washed 3 times with PBS and permeabilized using blocking buffer (0.3% Triton X-100, 2% BSA, 5% Goat Serum, 5% Donkey Serum in 1×PBS) for 1 hour at room temperature. Subsequently, cells were incubated with mouse anti-dsRNA antibody (Absolute Antibody, 1:200) or anti-SARS-CoV-2 spike antibody (Sino Biological, 1:200) at 4° C. overnight. Cells were then washed 3 times with PBS and incubated with fluorescence conjugated secondary antibody: Goat anti-mouse IgG Secondary Antibody, Alexa Fluor 555 (Fisher Scientific, 1:1000) for 1 hour at room temperature. Nuclei were stained with DAPI (4′,6-Diamidino-2-Phenylindole, Dihydrochloride) (Life Technologies) at a dilution of 1:5000 in PBS for 10 minutes. Cells were analyzed by fluorescence microscopy. Images were obtained using a Biorevo BZ-X710 (Keyence) microscope and software. 
     Structural data for SARS-CoV-2, human TMPRSS2 and furin. SARS-CoV-2 (residues A27-D1146; UniProt ID: P0DTC2) spike models were generated using SWISS-MODEL (Waterhouse et al., 2018), based on the resolved SARS-CoV-2 Spike glycoprotein structures of SARS-CoV-2 in different conformational states (PDBs: 6VSB (Wrapp et al., 2020) and 6VXX (Walls et al., 2020)). The missing loops in the crystal structures, were built using the well-established libraries of backbone fragments (Zhang and Skolnick, 2005) and constraint space de novo reconstruction of the backbone segments (Peitsch, 1995). The catalytic domain of human TMPRSS2 (residues N146-D491; UniProt ID: O15393) was constructed using SWISS-MODEL (Waterhouse et al., 2018), based on the crystal structure of serine protease hepsin (PDB: 5CE1). A crystal structure of human furin (Y110-A408; P09958) was used as is (PDB: 5JMO) (Dahms et al., 2016). 
     Generation and assessment of SARS-CoV-2 Spike and protease complex models. To investigate priming of the S1/S2 site of SARS-CoV-2 Spike, protein-protein docking analysis was performed for TMPRSS2 or furin with SARS-CoV-2 Spike in the pre-fusion state. Using docking software ClusPro (Kozakov et al., 2017), a series of SARS-CoV-2 Spike and protease complexes were constructed in silico. SARS-CoV-2 Spike was set as receptor and protease as ligand. Residues in the proximity of the cleavage site from SARS-CoV-2 Spike (T676 to V687) were set as attractor sites of receptor, and the catalytic residues from TMPRSS2 (H296, D345 and S441) or furin (D153, H194 and 5368) were set as attractor sites for ligand. For each complex, 30 clusters of conformations were obtained, upon clustering ˜800 models generated by ClusPro. The clusters were rank-ordered by cluster size (Kozakov et al., 2017) as recommended, and representative members from top-ranking clusters were further examined and refined Mainly, protein-protein binding free energies were calculated using PRODIGY (Xue et al., 2016); and mutagenesis and sculpting wizards in PyMOL 2.3.0 (Open Source version) (DeLano, 2002) were used to interactively refine rotamers and interactions, respectively. 
     Monoclonal antibodies binding to SARS-CoV-2 Spike. SEB-associated monoclonal antibodies 14G8, 6D3 and 20B1 were taken from the crystal structures of SEB bound to two neutralizing Abs, 14G8 and 6D3 (PDB: 4RGN), and one neutralizing Ab, 20B1 (PDB: 4RGM). SARS-CoV-2 S-associated neutralizing Abs were taken from the crystal structures listed in Table 4. Ab-binding poses were predicted using protein-protein docking module in ClusPro (Kozakov et al., 2017) where SARS-CoV-2 spike was set as the receptor and 6D3 as the ligand. Computations repeated with the antibody mode of ClusPro confirmed the S1/S2 cleavage site to be most favorable binding site for mAb 6D3. All docking simulations were performed using ClusPro default parameters. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Antibody-bound complexes resolved by cryo-EM for SARS-CoV-2 spike mutants 
               
            
           
           
               
               
               
               
               
               
            
               
                 Binding 
                   
                 SARS-CoV-2 
                 Mutation 
                   
                   
               
               
                 domain 
                 Mechanism 
                 Ab 
                 at 
                 Epitope on 
               
               
                 (conformation) 
                 of action 
                 (PDB IDs) a   
                 “RRAR” 
                 SARS2 spike b   
                 Reference 
               
               
                   
               
               
                 RBD (up) 
                 sterically 
                 C105 (6XCN, 
                 SGAG 
                 D405, T415, G416, 
                 Barnes et 
               
               
                   
                 hinders 
                 6XCM) 
                   
                 K417, Y421, 
                 al., 
               
               
                   
                 ACE2 
                   
                   
                 Y453, F456, 
                 2020 
               
               
                   
                 binding 
                   
                   
                 R457, K458, 
               
               
                   
                   
                   
                   
                 N460, Y473, 
               
               
                   
                   
                   
                   
                 A475, G476, 
               
               
                   
                   
                   
                   
                 F486, N487, 
               
               
                   
                   
                   
                   
                 G502, Y505 
               
               
                 RBD (down) 
                 blocks 
                 2-4 (6XEY) 
                 GSAS 
                 Y449, Y453, 
                 Liu et al., 
               
               
                   
                 ACE2- 
                   
                   
                 L455, F456, 
                 2020 
               
               
                   
                 binding 
                   
                   
                 V483, E484, 
               
               
                   
                 interface of 
                   
                   
                 G485, F486, 
               
               
                   
                 RBD 
                   
                   
                 Y489, F490, 
               
               
                   
                   
                   
                   
                 L492, Q493, S494 
               
               
                 RBD 
                 Ab- 
                 S309 (6WPT, 
                 SGAG 
                 N334, L335, 
                 Pinto et 
               
               
                 (up/down) 
                 dependent 
                 6WPS) 
                   
                 P337, G339, 
                 al., 2020 
               
               
                 distinct from 
                 cell 
                   
                   
                 E340, N343, 
               
               
                 ACE2 binding 
                 cytotoxicity 
                   
                   
                 A344, T345, R346, 
               
               
                 sites 
                 and 
                   
                   
                 K356, R357, 
               
               
                   
                 phagocytosis 
                   
                   
                 S359, N360, 
               
               
                   
                   
                   
                   
                 C361, L441, N343 
               
               
                   
                   
                   
                   
                 glycan 
               
               
                 RBD (up) 
                 blocks 
                 H014 (7CAI, 
                 GSAS 
                 Y369, A372, S373, 
                 Lv et al., 
               
               
                   
                 ACE2 
                 7CAC, 
                   
                 F374, S375, T376, 
                 2020b 
               
               
                   
                 binding and 
                 7CAB, 
                   
                 F377, K378, 
               
               
                   
                 attachment 
                 7CAK, 7CAH) 
                   
                 C379, Y380, 
               
               
                   
                 to host cell 
                   
                   
                 V382, S383, 
               
               
                   
                   
                   
                   
                 P384, T385, 
               
               
                   
                   
                   
                   
                 D405, V407, 
               
               
                   
                   
                   
                   
                 R408, A411, 
               
               
                   
                   
                   
                   
                 P412, Q414, 
               
               
                   
                   
                   
                   
                 N437, V503 
               
               
                 NTD 
                 restrains S 
                 4A8 (7C2L) 
                 GSAS 
                 Y144, Y145, 
                 Chi et al., 
               
               
                 (up/down) 
                 protein 
                   
                   
                 H146, K147, 
                 2020 
               
               
                   
                 structural 
                   
                   
                 K150, W152, 
               
               
                   
                 changes 
                   
                   
                 H245, R246, 
               
               
                   
                   
                   
                   
                 S247, Y248, L249 
               
               
                 RBD/NTD 
                 blocks 
                 Ab23 (7BYR) 
                 GSAS 
                 G446, Y449, 
                 Cao et al., 
               
               
                 (down) 
                 ACE2 
                   
                   
                 E484, G485, 
                 2020b 
               
               
                   
                 binding 
                   
                   
                 F486, Y489, 
               
               
                   
                   
                   
                   
                 F490, L492, 
               
               
                   
                   
                   
                   
                 Q493, S494, 
               
               
                   
                   
                   
                   
                 G496, Q498 
               
               
                   
                   
                   
                   
                 N501, Y505, 
               
               
                   
                   
                   
                   
                 N165 glycan 
               
               
                 RBD 
                 blocks the 
                 EY6A 
                 GSAS 
                 Y369, F374, 
                 Zhou et 
               
               
                   
                 RBD 
                 (6ZDH) 
                   
                 S375, T376, F377, 
                 al., 2020a 
               
               
                   
                   
                   
                   
                 K378, C379, 
               
               
                   
                   
                   
                   
                 Y380, G381, 
               
               
                   
                   
                   
                   
                 V382, S383, 
               
               
                   
                   
                   
                   
                 P384, T385, 
               
               
                   
                   
                   
                   
                 K386, D389, 
               
               
                   
                   
                   
                   
                 L390, F392, P412, 
               
               
                   
                   
                   
                   
                 G413, D427, 
               
               
                   
                   
                   
                   
                 D428, F429, T430 
               
               
                   
               
               
                   a PDB IDs of the cryo-EM structures containing the indicated Ab are given in parentheses. 
               
               
                   b Epitope residues of SARS-CoV-2 within 4 Å distance of the antibody based on the first PDB ID listed in column 3. 
               
            
           
         
       
     
     Model refinement and binding affinity calculations. Selective protease-Spike and mAb-Spike complexes were further refined using the refinement protocol implemented in the webserver HADDOCK 2.4 (Van Zundert et al., 2016). Refinement was performed by MD energy minimization following the protocol and default parameters provided by the webserver. Binding free energies were evaluated using the inter-residue contact-based method accessible in the webserver PRODIGY (Xue et al., 2016). The standard deviations of binding free energy were estimated based on multiple binding poses taken from docking simulations and model refinement. 
     Sequence alignment. Multiple sequence alignment of the variable heavy chain domain of anti-SEB Abs (6D3, 14G8 and 20B1) and anti-SARS-CoV-2 S Abs were generated by Clustal Omega (Sievers et al., 2011). 
     Quantification and statistical analysis. For viral inhibition assays: Quantification of immunofluorescence images was performed manually, blinded to the conditions. Five images per well were quantified and the average calculated. n=3 technical replicates (wells) per condition. Data is presented as mean+/−standard error of the mean and is representative of three independent experiments. Data were analyzed by t test (6D3 vs. isotype control) with multiple testing correction (Benjamini, Krieger and Yekutieli FDR test) using GraphPad Prism software. No methods were used to test the assumptions of the statistical approach. Statistical analysis details are found in the methods description, results and figure captions. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Key resources table 
               
            
           
           
               
               
               
            
               
                 REAGENT or 
                   
                   
               
               
                 RESOURCE 
                 SOURCE 
                 IDENTIFIER 
               
               
                   
               
               
                 Antibodies 
                   
                   
               
               
                 6D3 IgG2b Stony Brook 
                 New York 
                 N/A 
               
               
                 University, 
               
               
                 InVivoMab mouse IgG2b 
                 BioXcell 
                 Cat#BE0086 
               
               
                 isotype control 
               
               
                 mouse anti-dsRNA [J2] 
                 Absolute 
                 Cat#Ab01299-2.0 
               
               
                 antibody 
                 Antibody 
               
               
                 Goat anti-Mouse IgG (H + L) 
                 Thermo Fisher 
                 Cat#A21422 
               
               
                 Cross-Adsorbed Secondary 
                 Scientific 
               
               
                 Antibody, Alexa Fluor 555 
               
               
                 Bacterial and Virus 
               
               
                 Strains 
               
               
                 SARS-CoV-2 
                 BEI Resources of 
                 Isolate USA-WA1/2020 
               
               
                   
                 National Institute 
               
               
                   
                 of Allergy and 
               
               
                   
                 Infectious 
               
               
                   
                 Diseases (NIAID) 
               
               
                 Chemicals, Peptides, and 
               
               
                 Recombinant Proteins 
               
               
                 DAPI (4′,6-Diamidino-2- 
                 Life Technologies 
                 Cat#D1306 
               
               
                 Phenylindole, 
               
               
                 Dihydrochloride) 
               
               
                 Penicillin-Streptomy cin 
                 Gibco 
                 Cat#15140122 
               
               
                 (10,000 U/mL) 
               
               
                 Deposited Data 
               
               
                 Antibody-Spike complexes 
                 Protein Data 
                 N/A 
               
               
                 see Table 4 
                 Bank (PDB) 
               
               
                 SARS-CoV-2 Spike with 
                 (Wrapp et al., 
                 PDB: 6VSB 
               
               
                 one chain in up state 
                 2020) 
               
               
                 SARS-CoV-2 Spike in 
                 (Walls et al., 
                 PDB: 6VXX 
               
               
                 down state 
                 2020) 
               
               
                 HCoV-OC43 Spike in down 
                 (Tortorici et al., 
                 PDB: 6NZK 
               
               
                 state 
                 2019) 
               
               
                 Furin 
                 (Dahms et al., 
                 PDB: 5JMO 
               
               
                   
                 2016) 
               
               
                 SARS-CoV-2 glycosylated 
                 (Woo et al., 2020) 
                 charmm-gui.org/?doc=archive&amp;lib=covid19 
               
               
                 Spike protein model 
               
               
                 Antibodies 6D3 and 14G8 
                 (Dutta et al., 
                 PDB: 4RGN 
               
               
                 bound to SEB 
                 2015) 
               
               
                 Antibody 20B1 bound to 
                 (Dutta et al., 
                 PDB: 4RGM 
               
               
                 SEB 
                 2015) 
               
               
                 Human TMPRSS2 
                 This paper 
                 zenodo.org/record/4667694#.YGz7DOhKhPZ 
               
               
                 homology model 
               
               
                 SARS-CoV-2 Spike bound 
                 This paper 
                 zenodo.org/record/4667694#.YGzUgOhKhPY 
               
               
                 to TMPRSS2 
               
               
                 SARS-CoV-2 Spike bound 
                 This paper 
                 zenodo.org/record/4667694#.YGzUgOhKhPY 
               
               
                 to furin 
               
               
                 Ab 6D3 bound to SARS- 
                 This paper 
                 zenodo.org/record/4667694#.YGzUgOhKhPY 
               
               
                 CoV-2 Spike in one up state 
               
               
                 Ab 6D3 bound to SARS- 
                 This paper 
                 zenodo.org/record/4667694#.YGzUgOhKhPY 
               
               
                 CoV-2 Spike in down state 
               
               
                 Ab 6D3 bound to HCoV- 
                 This paper 
                 zenodo.org/record/4667694#.YGzUgOhKhPY 
               
               
                 OC43 Spike in down state 
               
               
                 Experimental Models: 
               
               
                 Cell Lines 
               
               
                 Vero-E6 
                 ATCC 
                 Cat#CRL-1586 
               
               
                 Software and Algorithms 
               
               
                 ClusPro 
                 (Kozakov et al., 
                 cluspro.bu.edu/ 
               
               
                   
                 2017) 
               
               
                 SWISS-MODEL 
                 (Waterhouse et 
                 swissmodel.expasy.org/interactive 
               
               
                   
                 al., 2018) 
               
               
                 PRODIGY 
                 (Xue et al., 2016) 
                 bianca.science.uu.nl//prodigy/ 
               
               
                 PyMOL 
                 (DeLano, 2002) 
                 pymol.org/2/ 
               
               
                 HADDOCK 2. 4 
                 (Van Zundert et 
                 bianca.science.uu.nl/haddock2.4/ 
               
               
                   
                 al., 2016) 
               
               
                 Clustal Omega 
                 (Sievers et al., 
                 www.ebi.ac.uk/Tools/msa/clustalo/ 
               
               
                   
                 2011) 
               
               
                 Prism 
                 GraphPad 
                 www.graphpad.com/scientific-software/prism/ 
               
               
                 BZ-X700 Analysis Software 
                 Keyence 
                 keyence.com/landing/microscope/lp_fluorescence.jsp 
               
               
                 Other 
               
               
                 Eagle&#39;s Minimum Essential 
                 Corning 
                 Cat#10009CV 
               
               
                 Medium (MEM) 
               
               
                 Regular Fetal Bovine Serum 
                 Corning 
                 Cat#35010CV 
               
               
                 Goat Serum 
                 Cell Signaling 
                 Cat#5425S 
               
               
                 Normal Donkey Serum 
                 Jackson 
                 Cat#017-000-121 
               
               
                   
                 ImmunoResearch 
               
               
                 BZ-X710 Fluorescence 
                 Keyence 
                 Model#BZ-X710 
               
               
                 Microscope 
               
               
                   
               
            
           
         
       
     
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