The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) started emerging in humans in late 2019, and rapidly spread to a pandemic with millions of cases worldwide. SARS-CoV-2 infections cause coronavirus disease 2019 (COVID-19) with severe respiratory symptoms, but also pathological inflammation and multi-organ dysfunction, including acute respiratory distress syndrome, cardiovascular events, coagulopathies and neurological symptoms (Helms et al., 2020; Zhou et al., 2020; Zhu et al., 2020) . Some aspects of the diverse clinical manifestations may result from a hyperinflammatory response, as suggested by reduced mortality in hospitalized COVID-19 patients under dexamethasone therapy (Horby et al., 2020) .
Understanding the immune response to SARS-CoV-2 therefore is of utmost importance. Multiple recombinant SARS-CoV-2 mAbs from convalescent patients have been reported (Brouwer et al., 2020; Cao et al., 2020; Ju et al., 2020; Kreer et al., 2020; Robbiani et al., 2020; Rogers et al., 2020; Wec et al., 2020) . mAbs targeting the receptor-binding domain (RBD) of the viral spike protein S1 can compete with its binding to human angiotensin converting enzyme 2 (ACE2) and prevent viral entry and subsequent replication (Cao et al., 2020; Ju et al., 2020; Walls et al., 2020) . Potent virus neutralizing mAbs that were isolated from diverse variable immunoglobulin (Ig) genes typically carry low levels of somatic hypermutations (SHM). Several of these neutralizing mAbs selected for in vitro efficacy showed prophylactic or therapeutic potential in animal models (Cao et al., 2020; Liu et al., 2020; Rogers et al., 2020; Zost et al., 2020) . The low number of SHM suggests limited affinitymaturation in germinal centers compatible with an acute infection. Near-germline mAbs usually constitute the first line of defense to pathogens, but carry the risk of self-reactivity to autoantigens (Lerner, 2016; Liao et al., 2011; Zhou et al., 2007) . Although critical for the therapeutic use in humans, potential potential tissue-reactivity of near-germline SARS-CoV-2 antibodies has not been examined so far. 5 Here, we systematically selected 18 strongly neutralizing mAbs out of 598 antibodies from 10 COVID-19 patients by characterization of their biophysical properties, authentic SARS-CoV-2 neutralization, and exclusion of off-target binding to murine tissue. Additionally, we solved two crystal structures of neutralizing mAbs in complex with the RBD, showing antibody engagement with the ACE2 binding site from different approach angles. Finally, we selected mAb CV07-209 by its in vitro efficacy and the absence of tissue-reactivity for in vivo evaluation. Systemic application of CV07-209 in a hamster model of SARS-CoV-2 infection led to profound reduction of clinical, paraclinical and histopathological COVID-19 pathology, thereby reflecting its potential for translational application in patients with 
We first characterized the B cell response in COVID-19 using single-cell Ig gene sequencing of human mAbs (Fig. 1A) . From ten COVID-19 patients with serum antibodies to the S1 subunit of the SARS-CoV-2 spike protein (Fig. S1 , Supplementary Table ST1), we isolated two populations of single cells from peripheral blood mononuclear cells with fluorescence-activated cell sorting (FACS): CD19 + CD27 + CD38 + antibody secreting cells (ASC) reflecting the unbiased humoral immune response and SARS-CoV-2-S1-labeled CD19 + CD27 + memory B cells (S1-MBC) for characterization of antigenspecific responses ( Fig. S2A and S2B). We obtained 598 functional paired heavy and light chain Ig sequences (Supplementary Table ST2 ). Of 432 recombinantly expressed mAbs, 122 were reactive to SARS-CoV-2-S1 (S1+), with a frequency of 0.0-18.2% (median 7.1%) within ASC and 16.7-84.1%
(median 67.1%) within S1-MBC ( Fig. 1B and 1C ). Binding to S1 did not depend on affinity maturation as measured by the number of SHM (Fig. 1D ). Compared to mAbs not reactive to SARS-CoV-2-S1, S1+ mAbs had less SHM, but equal lengths for both their light and heavy chain complementaritydetermining region 3 (CDR3) (Fig. S2C, S2D and S2E ). Within the ASC and S1-MBC population, 45.0% and 90.2% of S1+ mAbs bound the RBD, respectively (Fig. S2F ). S1+ mAbs were enriched in certain Ig genes including VH1-2, VH3-53, VH3-66, VK1-33 and VL2-14 ( Fig. S3 ). We identified both clonally related antibody clones within patients and public and shared S1+ clonotypes from multiple patients (Fig. S4) . Some public or shared clonotypes had been previously reported, such as IGHV3-53 and IGHV3-66 (Supplementary Table ST3 ) (Cao et al., 2020; Yuan et al., 2020a) , while others were newly identified, such as IGHV3-11 (Fig. S4C ).
We next determined the mAbs with the highest capacity to neutralize SARS-CoV-2 in a plaque reduction neutralization tests (PRNT) using authentic virus (Munich isolate 984) (Wolfel et al., 2020) . 8 Many SARS-CoV-2 neutralizing mAbs carry few SHM or are in germline-configuration ( Fig. 1D ) (Ju et al., 2020; Kreer et al., 2020) . Such antibodies close to germline might be reactive to more than one target (Zhou et al., 2007) . Prompted by the abundance of near-germline SARS-CoV-2 antibodies and to exclude potential side-effects of mAb treatment, we next analyzed whether SARS-CoV-2 antibodies can bind to self-antigens. Therefore, we tested the binding of S1-mAbs to unfixed murine tissues. Surprisingly, four of the Top-18 potent SARS-CoV-2 neutralizing mAbs showed anatomically distinct tissue reactivities (Fig. 2, Supplementary Table ST4 ). CV07-200 intensively stained brain sections in the hippocampal formation, olfactory bulb, cerebral cortex and basal ganglia ( Fig. 2A) . CV07-222 also bound to brain tissue, as well as to smooth muscle (Fig. 2B ). CV07-255 and CV07-270 were reactive to smooth muscle from sections of lung, heart, kidney and colon, but not liver ( Fig. 2C and 2D, Supplementary Table ST4 ). None of the Top-18 mAbs bound to HEp-2 cells, cardiolipin or beta-2 microglobulin as established polyreactivity-related antigens (Jardine et al., 2016) (Fig. S8 ).
For further characterization using X-ray crystallography, we selected two neutralizing mAbs CV07-250 and CV07-270 based on differences in the number of SHM, extent of ACE2 competition and binding to murine tissue. CV07-250 (IC50=3.5 ng/ml) had 33 SHM (17/16 on heavy and light chain, respectively), strongly reduced ACE2 binding and showed no binding to murine tissue. In contrast, CV07-270 (IC50= 82.3 ng/ml) had only 2 SHM (2/0), did not reduce ACE2 binding in our assay, and showed binding to smooth muscle tissue. Using X-ray crystallography, we determined structures of CV07-250 and CV07-270 in complex with SARS-CoV-2 RBD to resolutions of 2.55 and 2.70 Å, respectively (Fig. 3, Supplementary Tables ST5 and ST6) .
The binding mode of CV07-250 to RBD is unusual in that it is dominated by the light chain ( Fig. 3A and 3D), whereas in CV07-270, the heavy chain dominates as frequently found in other antibodies 9 ( Fig. 3B and 3E ). Upon interaction with the RBD, CV07-250 has a buried surface area (BSA) of 399 Å 2 and 559 Å 2 on the heavy and light chains, respectively, compared to 714 Å 2 and 111 Å 2 in CV07-270. CV07-250 uses CDR H1, H3, L1, L3, and framework region 3 (LFR3) for RBD interaction (Fig. 3D and Fig. 4A, 4B and 4C) , whereas CV07-270 interacts with CDR H1, H3, L1, and L2 (Fig. 3E and Fig. 4D, 4E and 4F) .
The epitope of CV07-250 completely overlaps with the ACE2 binding site with a similar angle of approach as ACE2 (Fig. 3A , 3C, 4G and 4I). In contrast, the CV07-270 epitope only partially overlaps with the ACE2 binding site and the antibody approaches the RBD from a different angle compared to CV07-250 and ACE2 (Fig. 3B , 3C, 4H, 4I), explaining differences in ACE2 competition. Although CV07-250 and CV07-270 both contact 25 epitope residues, only seven residues are shared (G446/G447/E484/G485/Q493/S494/Q498). Furthermore, CV07-270 binds to a similar epitope as SARS-CoV-2 neutralizing antibody P2B-2F6 (Ju et al., 2020) with a similar angle of approach (Fig.   S9 ). In fact, 18 out of 20 residues in the P2B-2F6 epitope overlap with the CV07-270 epitope, although CV07-270 and P2B-2F6 are encoded by different germline genes for both heavy and light chains.
Interestingly, CV07-250 was isolated 19 days after symptom onset, but already acquired 33 SHM, the highest number among all S1+ MBCs (Fig. S2C ). Some non-germline amino acids are not directly involved in RBD binding, including all five SHMs on CDR H2 (Fig. S10 ). This observation suggests that CV07-250 could have been initially affinity-matured against a different antigen.
Next, we selected mAb CV07-209 for evaluation of in vivo efficacy based on its high capacity to neutralize SARS-CoV-2 and the absence of reactivity to mammalian tissue. We used the hamster model of COVID-19, as it is characterized by rapid weight loss and severe lung pathology 10 (Osterrieder et al., 2020) . In this experimental set-up, hamsters were intranasally infected with authentic SARS-CoV-2. Nine hamsters per group received either a prophylactic application of CV07-209 24 hours before viral challenge, or a therapeutic application of CV07-209 or control antibody mGO53 two hours after viral challenge (Fig. 5A) .
Hamsters under control mAb treatment lost 5.5±4.4% (mean±SD) of body weight, whereas those that received mAb CV07-209 as a therapeutic or prophylactic single dose gained 2.2±3.4% or 4.8±3.4% weight after 5 days post-infection (dpi), respectively. Mean body weights gradually converged in the animals followed up until 13 dpi, reflecting the recovery of control-treated hamsters from SARS-CoV-2 infection (Fig. 5B ).
To investigate the presence of SARS-CoV-2 in the lungs, we measured functional SARS-CoV-2 particles from lung tissue homogenates. Plaque forming units were below the detection threshold for all animals in the prophylactic and in 2 of 3 in the treatment group at 3 and 5 dpi ( Fig. 5C and 5D ). qPCR measurements of lung viral genomic RNA copies revealed a 4-5 and 3-4 log decrease at both time points, indicating a dramatic reduction of SARS-CoV-2 virus particles in both the prophylactic and therapeutic group. However, genomic viral RNA copy numbers from nasal washes and laryngeal swaps were similar between all groups ( Fig. 5C and 5D ).
Additionally, we performed histopathological analyses of infected hamsters. As expected, all lungs from control-treated animals sacrificed at 3 dpi revealed typical histopathological signs of necrosuppurative pneumonia with suppurative bronchitis, necrosis of bronchial epithelial cells and endothelialitis ( Fig. 6A ). At 5 dpi, control-treated animals showed marked bronchial hyperplasia, severe interstitial pneumonia with marked type II alveolar epithelial cell hyperplasia and endothelialitis ( Fig. 6D ). In contrast, animals receiving prophylactic treatment with CV07-209 showed no signs of pneumonia, bronchitis, necrosis of bronchial epithelial cells, or endothelialitis at 3 dpi. A mild interstitial pneumonia with mild type II alveolar epithelial cell hyperplasia became apparent 5 dpi.
Animals receiving therapeutic treatment with CV07-209 also showed a marked reduction of 11 histopathological signs of COVID-19 pathology, although at both time points one out of three animals showed mild bronchopulmonary pathology with signs of interstitial pneumonia and endothelialitis.
These qualitative findings were mirrored in the reduction of the bronchitis and edema scores (Fig. 6B,   6E and Supplementary Table ST7 ).
To confirm the absence of viral particles under CV07-209 treatment, we performed in-situ hybridization of viral RNA at 3 dpi. No viral RNA was detectable in the prophylactic group, whereas all animals in the control group and one in the therapeutic group revealed intensive staining of viral RNA in proximity of bronchial epithelial cells (Fig. 6C ). Taken together, these findings show that systemic application of SARS-CoV-2 neutralizing mAb CV07-209 protects hamsters from COVID-19 lung pathology and weight loss in both the prophylactic and the therapeutic setting.
Our results add several new aspects to the growing knowledge on the humoral immune response in SARS-CoV-2 infections. First, we solved the structures of two neutralizing mAbs binding to the RBD of SARS-CoV-2 at resolutions of 2.55 and 2.70 Å, allowing detailed characterization of the target epitopes and gaining insight into the mechanism of mAbs neutralizing SARS-CoV-2. SARS-CoV-2 mAbs can compete with ACE2 binding and exert neutralizing activity by inhibiting viral particle binding to host cells (Barnes et al., 2020; Brouwer et al., 2020; Cao et al., 2020; Ju et al., 2020; Kreer et al., 2020; Robbiani et al., 2020; Rogers et al., 2020; Wec et al., 2020) , a key mechanism previously identified in SARS-CoV neutralizing antibodies (Prabakaran et al., 2006; ter Meulen et al., 2006) .
Steric hindrance of mAbs blocking ACE2 binding to the RBD provides one mechanistic explanation of virus neutralization (Barnes et al., 2020; Cao et al., 2020; Wu et al., 2020) . CV07-250 clearly belongs to this category of antibodies, as its epitope lies within the ACE2 binding site and it approaches the RBD from a similar angle as ACE2. In contrast, the epitope of CV07-270 only partially overlaps with the ACE2 binding site and approaches the RBD ridge from a different angle. In line with these findings, competition of CV07-270 with ACE2 binding as detected by ELISA was very weak. Its mechanism of virus neutralization therefore remains elusive. Of note, there have been reports of neutralizing antibodies targeting epitopes distant to the ACE2 binding site (Chi et al., 2020) . Future research will need to clarify if additional mechanisms like triggering conformational changes in the spike protein upon antibody binding contribute to virus neutralization, as reported for SARS-CoV (Walls et al., 2019) .
Secondly, the majority of our SARS-CoV-2 mAbs are close to germline configuration, supporting previous studies (Kreer et al., 2020; Robbiani et al., 2020) . Given the increased probability of autoreactivity of near-germline antibodies, we examined reactivity with murine tissue and indeed found that a fraction of SARS-CoV-2 neutralizing antibodies also bound to with brain, lung, heart, kidney or gut expressed epitopes. Such reactivity with host antigens should ideally be prevented by 13 immunological tolerance mechanisms, but complete exclusion of such antibodies would generate "holes" in the antibody repertoire. In fact, HIV utilizes epitopes shared by its envelope and mammalian self-antigens, thus harnessing immunological tolerance to impair anti-HIV antibody responses (Yang et al., 2013) and impeding successful vaccination (Jardine et al., 2016) . To defy viral escape in HIV, but similarly COVID-19, anergic strongly self-reactive B cells likely enter germinal centers and undergo clonal redemption to mutate away from self-reactivity, while retaining HIV or SARS-CoV-2 binding (Reed et al., 2016) . Interestingly, longitudinal analysis of mAbs in COVID-19 showed that the number of SHM in SARS-CoV-2-neutralizing antibodies only marginally increased over time (Kreer et al., 2020) . This finding suggests that the self-reactivity observed in this study may not be limited to mAbs of the early humoral immune response in SARS-CoV-2 infections. Whether self-reactive antibodies could contribute to extra-pulmonary symptoms in COVID-19, awaits further studies and should be closely monitored also in vaccination trials.
Finally, we evaluated in detail the in vivo efficacy of the most potent neutralizing antibody CV07-209 in a Syrian hamster model. This model is characterized by a severe phenotype including weight loss and distinct lung pathology. Our results demonstrated that prophylaxis and treatment with a single dose of CV07-209 not only led to clinical improvement as shown by the absence of weight loss, but also to markedly reduced lung pathology. While the findings confirm the efficacy of prophylactic mAb administration reported by other groups in mice, hamsters and rhesus macaques (Cao et al., 2020; Liu et al., 2020; Rogers et al., 2020; Zost et al., 2020) , our work is the first to demonstrate the efficacy of post-exposure treatment in hamsters leading to viral clearance, clinical remission and prevention of lung injury. To our knowledge, data on post-exposure treatment so far were restricted to transgenic hACE2 mice (Cao et al., 2020 ) and a mouse model using adenovector delivery of human ACE2 before viral challenge (Liu et al., 2020) . Collectively, our results indicate that mAb treatment can be fine-tuned for exclusion of self-reactivity with mammalian tissues and that mAb administration can also be efficacious after the infection, which will be the prevailing setting in COVID-19 patients. Cao, Y., Su, B., Guo, X., Sun, W., Deng, Y., Bao, L., Zhu, Q., Zhang, X., Zheng, Y., Geng, C., et al. (2020) . Potent neutralizing antibodies against SARS-CoV-2 identified by high-throughput single-cell sequencing of convalescent patients' B cells. Cell.
Chi, X., Yan, R., Zhang, J., Zhang, G., Zhang, Y., Hao, M., Zhang, Z., Fan, P., Dong, Y., . Zost, S.J., Gilchuk, P., Case, J.B., Binshtein, E., Chen, R.E., Nkolola, J.P., Schafer, A., Reidy, J.X., Trivette, A., Nargi, R. S., et al. (2020) . Potently neutralizing and protective human antibodies against SARS-CoV-2. Nature.
We thank Stefanie Bandura, Matthias Sillmann and Doreen Brandl for excellent technical assistance, Christian Meisel for performing a cardiolipin ELISA and Martin Barner for assistance in generating the circos plot in Fig. S4B 
Related to this work the German Center for Neurodegenerative Diseases (DZNE) and the Charité -Universitätsmedizin have filed a patent application on which JK, SMR, HCK, ESS, VMC, MAM, DW, LES and HP are named as inventors.
Antibodies will be made available by the corresponding authors upon request under a Material Transfer Agreement (MTA) for non-commercial usage.
X-ray coordinates and structure factors are deposited at the RCSB Protein Data Bank under accession codes 6XKQ and 6XKP. The raw sequencing data associated with this manuscript will be deposited together with the analysis using BASE on Code Ocean upon publication and will be shared upon request during review. The software used for Ig sequence analysis is available on https://github.com/automatedSequencing/BASE.
(A) Diagram depicting the strategy for isolation of 18 potently neutralizing mAbs (Top-18). (B) Normalized binding to S1 of SARS-CoV-2 for mAbs isolated from antibody secreting cells (▼; blue = S1-binding, grey = not S1-binding). (OD=optical density in ELISA) (C) Normalized binding to S1 of SARS-CoV-2 for mAbs isolated from S1-stained memory B cells (▲; colors like in (B)) (D) S1-binding plotted against the number of somatic hypermutations (SHM) for all S1-reactive mAbs.
(E) Concentration-dependent binding of Top-18 SARS-CoV-2 mAbs to the RBD of S1 (mean±SD from two wells of one experiment). 
V H F100 V H N97 V H S95 V H D96 L455 F456 V H Y100b V L F32 V L Y91 Y449 G446 V L G68 V L S67 V H D31 V H T28 K444 R346 V H W100h V H W100k V L Y49 Y449 V H W100h V H I100d F490 E484 V H R100g
The patients have given written informed consent and analyses were approved by the Institutional Review Board of Charité -Universitätsmedizin Berlin, corporate member of Freie Universität Berlin, Humboldt-Universität Berlin, and Berlin Institute of Health, Berlin. All patients in this study were tested positive for SARS-CoV-2 infection by RT-PCR. Patient characteristics are described in Supplementary Table ST1 .
Recombinant SARS-CoV-2-S1 protein produced in HEK cells (Creative Diagnostics, DAGC091) was covalently labeled using CruzFluor647 (Santa Cruz Biotechnology, sc-362620) according to the manufacturer's instructions.
Using fluorescence-activated cell sorting we sorted viable single cells from freshly isolated peripheral blood mononuclear cells (PBMCs as 7AAD -CD19 + CD27 + CD38 + antibody-secreting cells (ASCs) or SARS-CoV2-S1-enriched 7AAD -CD19 + CD27 + memory B cells (MBCs) into 96-well PCR plates.
Staining was performed on ice for 25 minutes in PBS with 2 % FCS using the following antibodies: 7- (clone O323, BioLegend, 302809) and DAPI as viability dye. 
Screening for SARS-CoV-2-specific mAbs was done by using anti-SARS-CoV-2-S1 IgG ELISAs (EUROIMMUN Medizinische Labordiagnostika AG) according to the manufacturer's protocol. mAb containing cell culture supernatants were pre-diluted 1:5, patient sera 1:100. Optical density (OD) ratios were calculated by dividing the OD at 450 nm by the OD of the calibrator included in the kit. OD ratios of 0.5 were considered reactive.
Binding to the receptor-binding domain (RBD) of S1 was tested in an ELISA. To this end, a fusion protein (RBD-Fc) of the signal peptide of the NMDA receptor subunit GluN1, the RBD-SD1 part of 27 SARS-CoV2-S1 (amino acids 319-591) and the constant region of rabbit IgG1 heavy chain (Fc) was expressed in HEK293T cells and immobilized onto 96-well plates from cell culture supernatant via anti-rabbit IgG (Dianova, 111-035-045) antibodies. Then, human mAbs were applied and detected using horseradish peroxidase (HRP)-conjugated anti-human IgG (Dianova, 109-035-088) and the HRP substrate 1-step Ultra TMB (Thermo Fisher Scientific, Waltham, MA). All S1+ mAbs were screened at a human IgG concentration of 10 ng/ml to detect strong RBD binders and the ones negative at this concentration were re-evaluated for RBD reactivity using a 1:5 dilution of the cell plates was incubated with mAbs at 10 µg/ml for 15 minutes. Then, one volume of biotinylated mcAbs at 100 ng/ml was added and the mixture incubated for additional 15 minutes, followed by detection using HRP-conjugated streptavidin (Roche Diagnostics) and 1-step Ultra TMB. Background by the HRP-conjugated detection antibodies alone was subtracted from all absorbance values.
Antibodies which share same V and J gene on both Ig heavy and light chain are considered to be one clonotype. Such clonotypes are considered public if they are identified in different patients. After identification of public clonotypes, they were plotted in a Circos plot using the R package circlize (Gu et al., 2014) .
To identify the most potent SARS-CoV-2 neutralizing mAb, all 122 S1-reactive mAbs were screened for binding to RBD. 86 were defined as strongly binding to RBD (defined as detectable binding at 10 ng/ml in an RBD ELISA) and then assessed for neutralization of authentic SARS-CoV-2 at 25 and 250 ng/ml using mAb-containing cell culture supernatants. Antibodies were further selected (i) as the strongest neutralizing mAb of the respective donor and / or (ii) with an estimated IC50 of 25 ng/ml or below and / or (iii) with an estimated IC90 of 250 ng/ml or below. These were defined as the 18 most potent antibodies (Top-18) and expressed as purified antibodies for detailed biophysical characterization.
The antigen (SARS-CoV-2 S protein-RBD-mFc, Accrobiosystems) was reversibly immobilized on a 
To detect neutralizing activity of SARS-CoV-2-specific mAbs, plaque reduction neutralization tests (PRNT) were done as described before (Wolfel et al., 2020 Avicel solution in DMEM. After 3 days at 37°C, the supernatants were removed and the 24-well plates were fixed and inactivated using a 6% formaldehyde/PBS solution and stained with crystal violet. All dilutions were tested in duplicates. For PRNT-screening mAb dilutions of 25 and 250 ng of IgG/ml were assessed. IC50 was determined from non-linear regression models using Graph Pad Prism 8.
Recombinant spike protein-based immunofluorescence assays were done as previously described μl of the cell suspension was applied to each incubation field of a multitest cover slide (Dunn Labortechnik). The multitest cover slides were incubated for 6 hours before they were washed with PBS and fixed with ice-cold acetone/methanol (ratio 1:1) for 10 minutes. For the immunofluorescence test, the incubation fields were blocked with 5% non-fat dry milk in PBS/0.2% Tween for 60 minutes.
Purified mAbs were diluted in EUROIMMUN sample buffer to a concentration of 5 µg/ml and 30 μl of the dilution was applied per incubation field. After 1 hour at room temperature, cover slides were washed 3 times for 5 minutes with PBS/0.2% Tween. Secondary detection was done using a 1:200 dilution of a goat-anti human IgG-Alexa488 (Dianova). After 30 minutes at room temperature, slides were washed 3 times for 5 minutes and rinsed with water. Slides were mounted using DAPI prolonged mounting medium (FisherScientific).
The receptor binding domain (RBD; residues 319-541) of the SARS-CoV-2 spike (S) protein was expressed in High Five cells and purified using affinity and size exclusion chromatography as described previously (Yuan et al., 2020b) . 
Preparations of brain, lung, heart, liver, kidney and gut from 8-12 weeks old C57BL/6J mice were frozen in -50°C 2-methylbutane, cut on a cryostat in 20 µm sections and mounted on glass slides. For tissue reactivity screening according to established protocols (Kreye 2016 Brain), thawed unfixed tissue slices were rinsed with PBS then blocked with blocking solution (PBS supplemented with 2%
Bovine Serum Albumin (Roth) and 5% Normal Goat Serum (Abcam)) for 1 hour at room temperature before incubation of mAbs at 5 µg/ml overnight at 4°C. After three PBS washing steps, goat anti- (abcam, ab108410) was used.
HEp-2 cell reactivity was investigated using the NOVA Lite HEp-2 ANA Kit (Inova Diagnostics) according to the manufacturer's instructions using mAb containing culture supernatant (screening of all S1+ mAbs) or purified mAbs at 50 µg/ml (polyreactivity testing of CV07-200, CV07-209, CV07-222, CV07-255, CV07-270 and CV38-148) and examined under an inverted fluorescence microscope.
Purified mAbs were screened for reactivity against cardiolipin and beta-2 microglobulin at 50 µg/ml using routine laboratory ELISAs kindly performed by Christian Meisel (Labor Berlin).
The 
For histopathological examination and in situ hybridization (ISH) of lung tissues, the left lung lobe was carefully removed and immersed in fixative solution (4% formalin, pH 7.0) for 48 hours. Lungs were then embedded in paraffin and cut in 2 μm sections. For histopathology, slides were stained with hematoxylin and eosin (HE) followed by blinded microscopic evaluation by board-certified veterinary pathologists as previously described (Dietert et al., 2017; Osterrieder et al., 2020) . The following parameters were evaluated to assess three different scores: (1) the bronchitis score that includes severity of bronchial inflammation and epithelial cell necrosis of bronchi;
(2) the regeneration score combining the presence of hyperplasia of bronchial epithelial cells and alveolar type II epithelial cells;
and (3) Roti®-Mount Fluor-Care DAPI (4, 6-diaminidino-2-phenylindole; Carl Roth). An irrelevant probe for the detection of pneumolysin was used as a control for sequence-specific binding. HE-stained and ISH slides were analyzed and images taken using a BX41 microscope (Olympus) with a DP80
Microscope Digital Camera and the cellSens™ Imaging Software, Version 1.18 (Olympus).
To determine virus titers from 25 mg lung tissue, tissue homogenates were serially diluted and titrated SARS-CoV-2-S1-stained memory B cells (S1-MBC). Cells were pre-gated on live CD19 + B cells. (C) Comparison of somatic hypermutation (SHM) count within immunoglobulin V genes combined from heavy and light chains of S1-reactive (S1+, blue) and non-S1-reactive (S1-, grey) mAbs. Statistical significance was determined using a one-way ANOVA (F = 19.22) with posthoc Tukey's multiple comparisons tests.
(ASC: n = 20 S1+, n = 260 S1-; S1-MBC: n = 102 S1+, n = 50 S1-). All expressed mAbs are displayed. Each triangle represents a S1+
(blue) or S1-(grey) mAb, isolated from an ASC (pointing downwards) or a S1-MBC (pointing upwards). (D-E) Length comparison of complementarity-determining region (CDR) 3 amino acid sequences between S1+ and S1-mAbs within (D) heavy and (E) light chains.
Symbols and colors have the same meaning as in (C). (F) Frequency of RBD-binder (S1+RBD+) and non-RBD-binder (S1+RBD-) relative to all expressed mAbs (upper lanes) and relative to S1+ mAbs (lower lanes). Comparison of gene usage between SARS-CoV-2-S1-reactive (S1+) and non-reactive (S1-) mAbs is shown for immunoglobulin (A) variable heavy (IGHV), (B) variable kappa (IGKV) and (C) variable lambda (IGLV) genes. Bars depict percentage of gene usage of all expressed mAbs within each group.
(A) Pie charts represent clonal relationship of all expressed mAbs from each donor separately for antibody secreting cells (ASC) and S1stained memory B cells (S1-MBC). mAbs were considered S1-reactive (S1+) or non-S1-reactive (S1-) based on SARS-CoV-2-S1 ELISA measurements. Antibodies were considered to be clonally expanded when they were isolated from multiple cells. (B) Circos plot displays all isolated mAbs from ten donors. Interconnecting lines indicate relationship between mAbs that share the same V and J gene on both Ig heavy and light chain. Such public or shared clonotypes in which more than 50% of mAbs are S1-reactive are represented as colored lines.
Small black angles at the outer circle border indicate expanded clones within the respective donor. (C) Properties of public clonotypes from CV-X2 n=5 S1+ mAbs from expanded clones unique S1+ mAbs S1-mAbs from expanded clones unique S1-mAbs C A B 42 S1+ mAbs according to the colors used in (B) with sequence similarities between mAbs isolated from different donors, also within complementarity-determining region (CDR) three. IGHV, IGHJ IGKV, IGKJ, IGLV, IGLJ = V and J genes of immunoglobulin heavy, kappa, lambda chains; n.exp. = not expressed.
Binding kinetics of mAbs to RBD were modeled (black) from multi-cycle surface plasmon resonance (SPR) measurements (blue, purple, orange). Fitted monovalent analyte model is shown. For CV07-200, neither a bivalent nor a monovalent analyte model described the data accurately (no model is shown). Three out of the 18 selected mAbs for detailed characterization (Top-18) were not analyzed using multicycle-kinetics: CV07-270 was excluded as it interacted with the anti-mouse IgG reference surface on initial qualitative measurements.
CV07-255 and CV-X2-106 were not analyzed since they showed biphasic binding kinetics and relatively fast dissociation rates in initial qualitative measurements. Non-neutralizing CV03-191, a mAb not included in the Top-18 mAbs, was included in the multicycle experiments as it has the same clonotype as strongly neutralizing CV07-209 (Fig. S4C ). All measurements are performed by using a serial 2-fold dilution of mAbs on reversibly immobilized SARS-CoV-2-S1 RBD-mFc. 
(A) ELISA-based measurements of human ACE2 binding to SARS-CoV-2 RBD after pre-incubation with the indicated neutralizing mAbs.
Values are shown relative to antibody-free condition as mean+SD from three independent measurements. (B) Competition for RBD binding between combinations of potent neutralizing mAbs is illustrated as a heat map. Shades of green indicate the degree of competition for RBD binding of detection mAb in presence of 100-fold excess of competing mAb relative to non-competition conditions. Green squares indicate no competition. Values are shown as mean of two independent experiments. C V 0 5 -1 6 3 C V 0 7 -2 0 0 C V 0 7 -2 0 9 C V 0 7 -2 2 2 C V 0 7 -2 5 0 C V 0 7 -2 5 5 C V 0 7 -2 6 2 C V 0 7 -2 7 0 C V 0 7 -2 8 3 C V 0 7 -2 8 7 C V 0 7 -3 1 5 C V 3 8 -1 1 3 C V 3 8 -1 3 9 C V 3 8 -1 4 2 C V 3 8 -1 8 3 C V 3 8 -2 2 1 C V -X 1 -1 2 6 C V - CV-X2-106 5.6 n/a n/a n/a 17.6 19.9 none 