Patent Publication Number: US-2021188939-A1

Title: Methods and compositions for treating and protecting against porcine reproductive and respiratory syndrome virus

Description:
GOVERNMENT LICENSE RIGHTS 
     This invention was made with government support under USDA-NIFA 2017-67015-26774 awarded by The U.S. Department of Agriculture. The government has certain rights in the invention. 
    
    
     FIELD 
     The present disclosure relates to compositions for treating and protecting against porcine reproductive and respiratory syndrome virus and methods of using these compositions. Additionally, the disclosure provides for methods of detecting the compositions. 
     BACKGROUND 
     Initially described in the USA in 1987 as the “mystery swine disease”, porcine reproductive and respiratory syndrome (PRRS) is now endemic in many swine-producing countries. The principal impacts of infection are reproductive failure, poor growth performance, and respiratory disease in young pigs. The causative agent, porcine reproductive and respiratory syndrome virus (PRRSV), was first isolated and sequenced in The Netherlands in 1991 and named Lelystad virus. Soon afterwards, VR-2332 was isolated and sequenced in North America. North American (NA) and European (EU) PRRSV isolates represent two distinct genotypes, which share only about 70% identity at the nucleotide level. Recently, the nomenclature was modified to designate the prototype EU isolates as PRRSV-1 and NA isolates as PRRSV-2. 
     PRRSV is a small, enveloped positive-sense single-stranded RNA virus from the order Nidovirales, family Arteriviridae, genus  Arterivirus  and has a restricted host and cell tropism for cells of the monocyte-macrophage lineage. 
     The main receptor for PRRSV on macrophages is CD163, a 130 kDa PRRSV-I transmembrane protein. The gene contains 17 exons, which code for a peptide signal sequence followed by nine scavenger receptor cysteine-rich (SRCR) domains, with SRCR6 and 7 separated by a proline-serine-threonine rich (PST) polypeptide of approximately 35 amino acids. Following the SRCR domains, a short, 16 amino acid PST linker domain (PSTII) connects SRCR5 with a transmembrane domain and an intracellular cytoplasmic tail. Currently, only SRCR2, SRCR3 and SRCR5 have been found to be involved in biological processes. A 13-amino-acid motif within SRCR2 mediates erythroblast binding and interacts with both Gram-positive and Gram-negative bacteria. In addition, hemoglobin-haptoglobin (Hb-Hp) complexes are internalized after binding to SRCR3. In 2003, CD163 was described as a putative receptor for African swine fever virus (ASFV). This conclusion was based on the observation that infected macrophages possess a mature CD163-positive phenotype combined with the capacity of anti-CD163 monoclonal antibodies to reduce ASFV infection of primary alveolar macrophages in culture. However, genetically modified pigs lacking CD163 are susceptible to infection by the ASFV Georgia/07 isolate. In 2007 Calvert et al. described CD163 as a receptor for PRRSV. Transfection with CD163 cDNAs from simian, human, canine, and mouse origins rendered a variety of non-permissive cell lines fully permissive to PRRSV-2 PRRSV infection, including the production of progeny virus. Subsequently, in 2010 Van Gorp et al. mapped the individual SRCR domains involved in determining permissiveness of a PRRSV-1 genotype virus. The experimental design was based on the transfection of non-permissive HEK293T cells with cDNAs possessing various SRCR domain deletions and/or substitutions with domain homologs from human CD163-like 1 protein (hCD163L1). Cells transfected with CD163 cDNA possessing a swap between SRCR5 and the hCD163LI homolog, SRCR5, were not permissive after infection with a genotype 1 virus (PRRSV-1), identifying SRCR5 as the domain involved in PRRSV infection by PRRSV-1 viruses. Recently, we showed that genetically modified (GM) pigs lacking CD163 expression on macrophages fail to support infection with both genotype 1 and 2 (PRRSV-2) viruses. Furthermore, the substitution of CD163 SRCR5 with the hCD163L1 homolog, SRCR8, conferred resistance of macrophages and pigs to infection with PRRSV-1 viruses confirming the results of Van Gorp et al., 2010. However, pigs retain the ability to support replication after infection with PRRSV-2 viruses. Together these results showed that PRRSV-1 and PRRSV-2 viruses possess distinct differences in the recognition of the CD163 protein. 
     The importance of SRCR5 for infection with PRRSV-2 viruses was demonstrated by two recent reports. In 2016, Ma et al. carried out structure-based studies, which located an arginine residue at position 561 (Arg561) within SRCR5 as important for PRRSV infection. More recently, Burkard et al. showed that porcine alveolar macrophages (PAMs) or peripheral blood monocyte-derived macrophages (PMMs) from GM pigs possessing a complete deletion of SRCR5 are resistant to both PRRSV-1 and PRRSV-2 viruses. 
     SUMMARY 
     The overall goal was to identify the minimum changes in CD163 sufficient to make cells resistant to infection with a genotype 2 PRRSV. The study identified domains in CD163, which are involved in infection by PRRSV-2 PRRSV. The results demonstrate a requirement for SRCR5 and PSTII as necessary for infection. Within PSTII, the deletion of 12 amino acids encoded by exon 13 prevents infection without affecting the expression of CD163 on the surface of the cell. The deletion of SRCR8 and 9 domains have a lesser effect on infection. The mapping of peptide domains within SRCR5 show that the insertion of proline-arginine (PR) after amino acids 8, 54 and 99, inhibit infection. Furthermore, the deletion of individual disulfide bonds by the replacement of cysteine residues with alanines also inhibits infection. And finally, the deletion of, or amino acid substitution within the SRCR4-5 interdomain sequence, AHRK (SEQ ID NO. 64), also blocks infection. Together, these mutant constructs provide a means to express CD163 receptors that are resistant to PRRSV infection. 
     The CD163 constructs used to study mutations within CD163 are described in  FIGS. 1, 2, and 3 .  FIG. 1  shows the effect of the domain deletions in CD163 on the infection of HEK cells. The peptide sequence referred to in  FIG. 1  is from GenBank No. AJ311716. The experimental system used to study infection incorporated a plasmid that contained CD163 fused to EGFP. After transfection into PRRSV non-permissive HEK cells, the expression of CD163 is apparent by the presence of green fluorescence. The PRRSV-2 virus used in this study, PRRSV-RFP, expressed red fluorescence in infected cells. Therefore, after infection of transfected HEK cells, the presence of red fluorescence in a green cell is a positive result. The truncated CD163 cDNA fragments shown in  FIG. 1  were prepared by PCR amplification of the pcDNA template using the GoTaqGreen® Master Mix (Promega) according to manufacturer&#39;s instructions. PCR conditions included 95° C. for 2 minutes, followed by 30 cycles of 94° C. for 30 seconds, 65° C. for 30 seconds, and 72° C. for 2 minutes, and a final extension at 72° C. for 10 minutes. The PCR primers for constructs shown in  FIG. 1  as well as some other constructs are listed in Table 1. The PCR products were cloned into pCR° 2.1-TOPO® vector (Invitrogen) and transformed into One Shot® TOP10 chemically competent E. coli cells (Invitrogen). The purified plasmids were double-digested with KpnI and XbaI and cloned into the KpnI-XbaI sites of the pcDNA3.1-EGFP vector. Plasmids were transfected into HEK cells using FuGENE® HD reagent (Promega) according to manufacturer&#39;s instructions and viewed for the presence of EGFP expression under a fluorescence microscope. The resulting N-terminally truncated CD163 proteins are illustrated in  FIG. 1A  (constructs B through G). 
     Constructs that possessed deletions in domains from the C-terminal end of CD163 incorporated primers that possessed PacI restriction sites, which are listed in Table 1. Deletions were made using a long PCR protocol designed to amplify the desired CD163 fragment along with the entire pcDNA3.1-EGFP plasmid. PCR amplification was performed using LongAmp® Taq DNA Polymerase (New England Biolabs Inc). PCR conditions included 94° C. for 30 s, followed by 30 cycles of 94° C. for 30 s, 65° C. for 1 min, and 65° C. for 8 min, followed by a final extension at 65° C. for 10 min. The PCR products were cut with PacI and the plasmid re-circularized by ligation with Anza™ T4 DNA Ligase Master Mix (Invitrogen), and then transfected into HEK cells. The resulting CD163 deletion constructs retained intact transmembrane and cytoplasmic domains along with an added PacI site. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Primers for amplification of CD163 constructs shown in FIG. 1 (underlined 
               
               
                 nucleotides identify restriction sites) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                 B 
                 FW:  GGTACC ATGGGATCTGATTTAGAGATGAGG-SEQ ID NO. 25 
               
               
                   
                 RV:  TCTAGA TTGTACTTCAGAGTGGTC TCC-SEQ ID NO. 26 
               
               
                   
               
               
                 C 
                 FW:  GGTACC ATGGGAGCAGACCTGAAACTG-SEQ ID NO. 27 
               
               
                   
                 RV:  TCTAGA TTGTACTTCAGAGTGGTC TCC-SEQ ID NO. 26 
               
               
                   
               
               
                 D 
                 FW:  GGTACC ATGCACAGGAAACCCAGGC-SEQ ID NO. 28 
               
               
                   
                 RV:  TCTAGA TTGTACTTCAGAGTGGTC TCC-SEQ ID NO. 26 
               
               
                   
               
               
                 E 
                 FW:  GGTACC ATGTACACACAAATCCGC-SEQ ID NO. 29 
               
               
                   
                 RV:  TCTAGA TTGTACTTCAGAGTGGTC TCC-SEQ ID NO. 26 
               
               
                   
               
               
                 F 
                 FW:  GGTACC ATGAGTGGTCAACTTCGCCTG-SEQ ID NO. 30 
               
               
                   
                 RV:  TCTAGA TTGTACTTCAGAGTGGTC TCC-SEQ ID NO. 26 
               
               
                   
               
               
                 G 
                 FW:  GGTACC ATGAAAATAAGACTTCAAGAAGGAAACACT-SEQ ID NO. 31 
               
               
                   
                 RV:  TCTAGA TTGTACTTCAGAGTGGTC TCC-SEQ ID NO. 26 
               
               
                   
               
               
                 H 
                 FW:  GGTACC ATGGGAAAAGACAAGGAG-SEQ ID NO. 32 
               
               
                   
                 RV:  TCTAGA TTCTGAGCACGTCACAGC-SEQ ID NO. 33 
               
               
                   
               
               
                 I 
                 FW: ATTA TTAATTAA GTTTGTTGCACTTGCAATCTTTGGGGT-SEQ ID NO. 34 
               
               
                   
                 RV: ATCA TTAATTAA ATTTAAGCAAATCACTCCAGCATCCTCAG-SEQ ID NO. 35 
               
               
                   
               
               
                 J 
                 FW: ATTA TTAATTAA GTTTGTTGCACTTGCAATCTTTGGGGT-SEQ ID NO. 34 
               
               
                   
                 RV: ATCG TTAATTAA TCTTGAGCAGACTACGCCG-SEQ ID NO. 36 
               
               
                   
               
               
                 K 
                 FW: ATTA TTAATTAA GTTTGTTGCACTTGCAATCTTTGGGGT-SEQ ID NO. 34 
               
               
                   
                 RV: CAGT TTAATTAA CTCTGAGCAGATGACTCCTGC-SEQ ID NO. 37 
               
               
                   
               
               
                 L 
                 FW: CACT TTAATTAA GTACACACAAATCCGCTTGGTGAATG-SEQ ID NO. 38 
               
               
                   
                 RV: CATA TTAATTAA GGCTGAGCAGGTAATTTTGGCTTC-SEQ ID NO. 39 
               
               
                   
               
               
                 M 
                 FW: ATTA TTAATTAA GATTGCAAAGAGCCGAGAATCCCTACATG-SEQ ID NO. 40 
               
               
                   
                 RV: ATCA TTAATTAA ATTTAAGCAAATCACTCCAGCATCCTCAG-SEQ ID NO. 41 
               
               
                   
               
               
                 N 
                 FW: ATTA TTAATTAA GATTGCAAAGAGCCGAGAATCCCTACATG-SEQ ID NO. 40 
               
               
                   
                 RV: ATCG TTAATTAA TCTTGAGCAGACTACGCCG-SEQ ID NO. 42 
               
               
                   
               
               
                 O 
                 FW: ATTA TTAATTAA GATTGCAAAGAGCCGAGAATCCCTACATG-SEQ ID NO. 40 
               
               
                   
                 RV: CAGT TTAATTAA CTCTGAGCAGATGACTCCTGC-SEQ ID NO. 37 
               
               
                   
               
               
                 P 
                 FW: CACA CCGCGG CTTTTGTTGCACTTGCAATCTTTGGGGTCATTCTGT-SEQ ID NO. 21 
               
               
                   
                 RV: CCCA CCGCGG CTGTGGCATGTAGGGATTCTCGGCTCTTT-SEQ ID NO. 22 
               
               
                   
               
               
                 Q 
                 FW: CACA CCGCGG GTCGCTCATCTTTTGTTGCACTT GCAATCTTT-SEQ ID NO. 23 
               
               
                   
                 RV: AACA CCGCGG CTGAGCACGTCACAGCAGCATCCT-SEQ ID NO. 24 
               
               
                   
               
               
                 C7 to A 
                 FW: CAAC CCGCGG CCTGACGGGACAGCCAGCCAC-SEQ ID NO. 55 
               
               
                   
                 RV: ACAA CCGCGG TGCTACTGGGCAGAGTGAAAGGTGGGACTC-SEQ ID NO. 56 
               
               
                   
               
            
           
         
       
     
     The CD163-EGFP deletion constructs transfected into HEK cells are illustrated in  FIG. 1   a.  All constructs showed EGFP fluorescence within 24 hrs after transfection of plasmids. Western blots stained with anti-GFP confirmed that each construct migrated according to the predicted size ( FIG. 1 b   ). HEK cells transfected with the full-length CD163-EGFP fusion plasmid, Construct A, served as a positive control for infection with the P129-RFP virus. A plasmid expressing a soluble form of CD163-EGFP, Construct H, was included as a negative infection control. The N-terminal truncation mutants are identified as constructs B-G in  FIG. 1A . Constructs A-D, which contained SRCR5 were positive for infection. The remaining N-terminal deletion mutants, constructs E-G, which lacked SRCR5, were negative for infection. The results for the C-terminal deletions lacking PSTII, constructs I through K, were all negative for infection, including the constructs J and K, which retained the SRCR5 domain. Constructs N and O, which were identical to J and K, except for the addition of the PSTII domain, were positive for infection. When taken together, all of the constructs in Figure la lacking SRCR5 or PSTII were negative for infection. These results indicate that the presence of SRCR5 and/or PSTII is required for infection. 
     The CD163 PSTII partial and complete deletion constructs were amplified by incorporating primers that possessed SacII restriction sites. The SacII sites were placed in the frame that would result in desired deletion along with the insertion or substitution of alanines. The primers used for amplification are listed in Table 1. The 16 amino acid PSTII domain (SEQ ID NO. 57) can be divided into two regions. The exon 14 portion consists of the four amino acids, GRSS (SEQ ID NO. 21-22) (see  FIG. 2 a   ). The remainder of exon 14 includes the transmembrane domain and a portion of the cytoplasmic tail. The substitution of GRSS (SEQ ID NO. 21-22) in SEQ ID NO. 62 with three alanines (Construct P) (SEQ ID NO. 77) resulted in only a small reduction in infection. The removal of 10 amino acids within the exon 13 (SEQ ID NO. 23-24) (SEQ ID NO. 78) region completely blocked infection. The results in  FIG. 2 b    showed that all constructs were expressed on the surface of transfected HEK cells. Therefore, the deletion of the region of CD163 encoded by exon 13 is sufficient to prevent infection. 
     The role of individual mutations in SRCR5 in the permissiveness of CD163 for PRRSV infection is illustrated in  FIG. 3 . The peptide sequence number is above the sequence with the CD163 peptide sequence coordinates in parentheses. The arrows show the location and direction of beta sheets and the dotted lines, the location of alpha helices. The asterisks show the location of SacII (proline-arginine) insertions. Primers containing SacII restriction site insertions were used to PCR amplify the CD163 plasmid (see Table 2). The restriction sites were placed in the same coding frame as the CD163 peptide sequence. The resulting PCR products were cut with SacII and then re-ligated to restore the circular plasmid. The six base pair SacII restriction site, CCG CGG (SEQ ID NO. 59), when placed in frame, codes for a proline-arginine dipeptide. The purpose for inserting a proline-arginine is to disrupt protein secondary structure. In addition, arginine contains a charged R-group. When placed in the right position within domain 5, the insertion of the dipeptide would be sufficient to disrupt the primary or secondary structure of domain 5 and prevent recognition by PRRSV. The presence of a SacII site provides a convenient means to test for the presence of the genomic modification. Recombinant plasmids were transfected into HEK cells. The infection results for the different SacII restriction site insertions are presented in Table 2. The results showed a wide variation in the infection levels of the different constructs. For the purpose of comparison, the relative number of infected cells was compared to HEK cells transfected with the wild type (WT—unmodified) CD163 (SEQ ID NO. 60). As can be seen, all of the tested constructs were resistant to PRRSV infection in comparison to the unmodified CD163 (control). Constructs, such as PR-15, PR-62, PR-78 and PR-89 showed a level of infection that was slightly less than the positive control. In contrast, other constructs showed greatly reduced levels of infection. Three constructs, PR-9, PR-55 and PR-100, showed only a few infected cells. These constructs are considered the most “resistant” to PRRSV infection. When inserted into the genome of a pig, these insertions should constitute and confer a significant degree of resistance to PRRSV. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Primer sequence for the preparation of constructs in FIG. 3 (SacII restriction 
               
               
                 site is underlined). 
               
            
           
           
               
               
               
            
               
                 SRCR5 
                   
                 HEK 
               
               
                 Construct 
                 PCR Primers 
                 Infection 
               
               
                   
               
               
                 None *1   
                 Unmodified CD163-SEQ ID NO. 60 
                 ++++ *2   
               
               
                   
               
               
                 PR-9 
                 F-ATTACCGCGGCCCTGCTCTGGTCGTGTTG-SEQ ID NO. 3 
                 +/- 
               
               
                   
                 R-ATTACCGCGGAATGTCCCCTCCAACCAGCC-SEQ ID NO. 4 
                   
               
               
                   
               
               
                 PR-15 
                 F-CACCCCGCGGGAAGTACAACATGGAGACACGTGGGG-SEQ ID NO. 43 
                 +++ 
               
               
                   
                 R-ACCACCGCGGAACACGACCAGAGCAGGGAATGTC-SEQ ID NO. 44 
                   
               
               
                   
               
               
                 PR-22 
                 F-ATTACCGCGGTGGGGCACCGTCTGTGATTC-SEQ ID NO. 5 
                 + 
               
               
                   
                 R-GTGACCGCGGCGTGTCTCCATGTTGTACTTCAAC-SEQ ID NO. 6 
                   
               
               
                   
               
               
                 PR-32 
                 F-TATACCGCGGCTGGAGGCGGCCAGCGT-SEQ ID NO. 7 
                 + 
               
               
                   
                 R-CGCTCCGCGGAGAGAAGTCAGAATCACAGACGGTGC-SEQ ID NO. 8 
                   
               
               
                   
               
               
                 PR-38 
                 F-GATACCGCGGCTGTGCAGGGAACTACAGTGCGGCACT-SEQ ID NO. 45 
                 ++ 
               
               
                   
                 R-TATACCGCGGCACGCTGGCCGCCTCCAGAGAGAA-SEQ ID NO. 46 
                   
               
               
                   
               
               
                 PR-42 
                 F-ATCCCCGCGGCTACAGTGCGGCACTGTGGTTTCC-SEQ ID NO. 9 
                 ++ 
               
               
                   
                 R-ATCACCGCGGTTCCCTGCACAGCACGCTGGC-SEQ ID NO. 10 
                   
               
               
                   
               
               
                 PR-55 
                 F-CAACCCGCGGCACTTTGGAGAAGGAAGTGGACAGATCTGGGC-SEQ ID NO. 11 
                 +/- 
               
               
                   
                 R-ACACCCGCGGAGCTCCCCCCAGGAGGGAAACCAC-SEQ ID NO. 12 
                   
               
               
                   
               
               
                 PR-62 
                 F-CAACCCGCGGCAGATCTGGGCTGAAGAATTCCAGTGT-SEQ ID NO. 53 
                 +++ 
               
               
                   
                 R-CACACCGCGGTCCACTTCCTTCTCCAAAGTGAGCTCCC-SEQ ID NO. 54 
                   
               
               
                   
               
               
                 PR-67 
                 F-CACCCCGCGGGAATTCCAGTGTGAGGGGCACGAG-SEQ ID NO. 13 
                 + 
               
               
                   
                 R-ACCCCCGCGGTTCAGCCCAGATCTGTCCACTTCC-SEQ ID NO. 14 
                   
               
               
                   
               
               
                 PR-78 
                 F-AAGGCCGCGGTCACTCTGCCCAGTAGCACCCC-SEQ ID NO. 15 
                 +++ 
               
               
                   
                 R-CACACCGCGGAAGGTGGGACTCGTGCCCCTCACA-SEQ ID NO. 16 
                   
               
               
                   
               
               
                 PR-89 
                 F-CCGACCGCGGACATGTAGCCACAGCAGGGACGTC-SEQ ID NO. 17 
                 +++ 
               
               
                   
                 R-TATACCGCGGCCCGTCAGGGCGGGGTGC-SEQ ID NO. 18 
                   
               
               
                   
               
               
                 PR-100 
                 F-CGCGCCGCGGTGCTCAAGATACACACAAATCCGC-SEQ ID NO. 19 
                 +/- 
               
               
                   
                 R-CAACCCGCGGGACTACGCCGACGTCCCTGC-SEQ ID NO. 20 
               
               
                   
               
               
                   *1 The number shows the nucleotide number with amino acid position in parentheses. 
               
               
                   *2 Key: ++++, infection of multiple cells with foci containing clusters of infecte cells; +++, fewer number of infected cells, but still possessing foci containing clusters of infected cells; ++, several single cells infected with virus; + a small number of cells showing infection; +/- only one or two infected cells in the well. 
               
            
           
         
       
     
     The predicted locations of the PR-9, PR-55 and PR-100 mutations on the surface of the SRCR5 structure are shown in  FIG. 4 a   . A space-filling model showed that the three mutations were located on a single face of the SRCR5 polypeptide, in close proximity to the LBP and the loop 5-6 region previously described by Van Gorp et al., 2010 and Graversen et al., 2002 as potential CD163-PRRSV contact regions. Even though the PR-9 and PR-100 mutations are located at the ends of the polypeptide sequence, they are predicted to be located in close proximity to each other in the polypeptide structure (see  FIG. 4 a   ). The ribbon structure in  FIG. 4 b    showed that the PR-9 insertion between Ile-8 and Pro-9 is predicted to create a short alpha helical structure, located just prior to the beginning of the β1 strand. One effect of the insertion is the re-orientation of the Pro-9 to the backside of the polypeptide face (see  FIG. 4 a   ). The PR-55 insertion, located between Ala-54 and His-55, is predicted to create a kink within the loop structure between β4 and β5. The result is the disappearance of Ala-54 from the surface of the polypeptide (see  FIG. 4 a   ). And finally, the PR-100 mutation, between Val-99 and Cys-100, is predicted to result in the early termination of the β7 strand. The space-filling model predicts that the net effect of the insertion is a separation of the valine and cysteine residues. Together, the modeling information shows that the PR insertions are located on an exposed region of SRCR5. The PR insertions are predicted to have a specific and localized effect on SRCR5 structure. The PR-9 and PR-100 mutations, even though located near the ends of the peptide sequence are located in close proximity to each other within the SRCR5 three dimensional protein structure. 
     The PRRSV-resistant properties of the PR-9, PR-55, and PR-100 mutations were further investigated by comparing titration end-points and growth curves for HEK cells transfected with the different mutants. The results for three independent titration experiments are presented in Table 3. HEK cells transfected with the wild type CD163 plasmid served as a positive control. The results for three independent experiments showed that the initial mean percentage of infected WT CD163 cells was 59.3+/−3.6, with a titration end-point of 10 −3  for all three experiments. In contrast, the infection of HEK cells expressing PR-9, PR-55 and PR-100 CD163 constructs showed only a few infected cells. The mean percent infection ranged between 0.8 and 3.0%. The titration end-point for PR-9 and PR-100 was 10 −2 . The titration end-point for PR-55 was 10 −1 . These results show that the mutations lessen the ability of PRRSV to interact with CD163. Furthermore, the data showed that the PR-55 mutation produced the greatest negative effect on PRRSV infection. 
     The results for the titration experiments were supported by growth curves (see  FIG. 5 ). For the growth curve experiment, samples from transfected HEK cells were collected every 12 hrs. After washing, the residual virus remaining in the well after infection was 4 log 10  TCID 50 /ml for all constructs. The concentration of virus in the HEK cells transfected with the wild type CD163 peaked at 7 log 10  TCID 50 /ml, which represents a 3 log increase in virus concentration. In contrast, virus infection of the PR mutants showed no increase in virus concentration over time. Furthermore, virus concentrations continued to decay over time, reaching undetectable levels by 36 hrs. The results indicate that expression of the mutant CD163 proteins resulted in no net increase in virus infection, thus confirming the high resistant properties of PR-9, PR-55 and PR-100. 
     As illustrated in  FIG. 3 , SRCR5 is predicted to possess four disulfide bonds formed by the eight cysteine residues located at amino acid positions 10, 26, 39, 44, 70, 80, 90 and 100. For these experiments, individual disulfide bonds were removed by substituting one of the partner cysteines with an alanine. Alanine to cysteines substitutions were made for C1, C3, C5 and C7. For C1, C3 and C5, the codons for two existing proline-arginine dipeptides, located at positions 1 and 84 of SRCR5, were changed to SacII sites. The CD163-EGFP plasmid was cut with SacII and the intervening DNA sequence replaced with a synthesized fragment (IDT, Inc.) possessing SacII restriction sites on the ends. The proper orientation of the cDNA insert was confirmed by DNA sequencing. Since C7 is located near the PR-84, the alanine substitution was made by amplifying the plasmid using a unique primer pair possessing SacII restriction sites and the codon for the alanine substitution (see Table 1). The results showed that all alanine mutations conferred resistance to transfected HEK cells infected with PRRSV-RFP. The reduction in infection (percent infected cells) was greater than 20 fold. 
     In some forms of the present disclosure, exon 13 of CD163 is disrupted by mutation to the sequence thereof. Preferably, the mutation is in frame such that the mutation&#39;s effects are only to exon 13. In one embodiment, the mutation is a deletion of at least a portion of exon 13. Such a portion can encompass just a single codon encoding for a single amino acid. In other embodiments, such a portion can encompass 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, or 16 amino acids. In other embodiments, the mutation includes a deletion, as described above, accompanied with an insertion of the same, or a different number of codons for the same or a different number of amino acids. Such a deletion and accompanying insertion can be termed a “substitution”. Thus, mutations which incorporate a deletion also confer resistance to infection by PRRSV, as described herein. 
     In some forms of the present disclosure, peptides are inserted into CD163 as described above. In other forms, dipeptides (other than those disclosed above), tripeptides, tetrapeptides, or other oligopeptides are inserted into CD163, as described above. Preferably, the insertion is in-frame and utilizes the least number of peptides as needed to disrupt the secondary protein structure of CD163 such that it is more resistant to (in comparison to CD163 that does not have an insertion therein) or is no longer permissive to PRRSV infection. The restriction site can also be placed in a different reading frame, creating a different effect on primary and secondary protein structure, such as the insertion of an alanine. In some preferred forms, the peptide or oligopeptide includes a charged amino acid. Preferably, the charged amino acid is selected from the group consisting of arginine, lysine, aspartic acid, and glutamic acid. In other preferred forms the peptides or oligopeptide includes a hydrophobic amino acid. In some particularly preferred forms of the disclosure, a charged amino acid is combined with at least one hydrophobic amino acid. Preferably, the hydrophobic amino acid is selected from the group consisting of alanine, isoleucine, leucine, phenylalanine, valine, proline, and glycine. When oligopeptides larger than dipeptides are utilized, use of one or more charged amino acids or one or more hydrophobic amino acids is preferred. In other preferred forms, one or more charged amino acids and one or more hydrophobic amino acids are utilized. The remaining amino acids can be any natural or non-natural amino acid. 
     In other preferred forms, the location for insertion of peptides or oligopeptides is selected based on the proximity to beta sheets and/or alpha helices. In some forms, the insertion site is within 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 amino acids of the beginning of the beta sheet or alpha helix. This can be either prior to the beginning of the beta sheet or alpha helix, within or after the sequence of the beta sheet or alpha helix. In other forms, the insertion site is within 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 amino acids of the beginning or end of a disulfide bond. 
     The term “resistant” refers to a comparison of the permissiveness of cells to PRRSV infection. A cell that is more resistant to PRRSV infection allows less PRRSV infection compared to an unmodified cell. Preferably, cells modified in accordance with the present disclosure are 5% more resistant, preferably 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, or more resistant to PRRSV infection than unmodified cells. Similarly, a group of cells modified as described herein is more resistant when a fewer number of cells of the group allow infection with PRRSV compared to an unmodified group of cells. Preferably, a group of cells modified in accordance with the present disclosure are 5% more resistant, preferably 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, or more resistant to PRRSV infection than a group of unmodified cells. 
     Conversely, the term “reducing the susceptibility” refers to a comparison of the susceptibility of cells to PRRSV infection. A cell that is less susceptible to PRRSV infection allows less PRRSV infection compared to an unmodified cell. Preferably, cells modified in accordance with the present disclosure are 5% less susceptible, preferably 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, or less susceptible to PRRSV infection than unmodified cells. Similarly, a group of cells modified as described herein is less susceptible when a fewer number of cells of the group allow infection with PRRSV compared to an unmodified group of cells. Preferably, a group of cells modified in accordance with the present disclosure are 5% less susceptible, preferably 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99%, or less susceptible to PRRSV infection than a group of unmodified cells. 
     As noted above, in one preferred form, the peptide or oligopeptide also functions as a detectable sequence that confirms the presence of the modification. The presence of a SacII restriction site, to insert a dipeptide or other peptide sequence not resulting in an amino acid change, also functions as a confirmatory diagnostic. When the restriction site is detected, one of skill in the art knows that the modification is present. One of skill in the art will be able to select other such peptides and oligopeptides that function in a similar fashion. Alternatively, the peptides or oligopeptides utilized in accordance with this disclosure can be tagged or coupled with a detectable moiety that confirms the presence of the modification. 
     Another aspect of the present disclosure is providing a method of reducing the susceptibility of a cell to infection by PRRSV comprising the steps of introducing a modification to the cell&#39;s genome, wherein the reduction of susceptibility is in comparison to a cell that has not been modified, and wherein the modification is selected from the group consisting of: a) deleting a portion of exon 13; b) deleting at least one of the SRCR8 and SRCR9 domains; c) inserting coding sequences into the genome; d) deleting at least one disulfide bond; e) deleting at least a portion of, or amino acid substitution within, theSRCR4-5 interdomain sequence; f) deleting at least a portion of the SRCR5 domain; g) deleting at least a portion of the PSTII region; h) substituting a coding sequence in the genome; and i) any combination thereof. In some preferred forms, the modification of exon 13 includes the deletion of at least 10 amino acids. In other preferred forms, the modification of exon 13 includes the complete deletion of exon 13. In some preferred forms, the modification of deleting at least one of the SRCR8 and SRCR9 domains further includes the insertion of proline-arginine residues into the deleted domains. In still other preferred forms, the modification of inserting coding sequences into the genome includes inserting proline-arginine residues. In still other preferred forms, these coding sequences are inserted into the SRCR domain 5. In some preferred forms, the proline arginine residues are inserted after the 8 th  or the 54 th , or the 99th amino acid in SRCR5. In some forms of deleting at least one disulfide bond, the modification includes deleting a plurality of disulfide bonds. In some preferred forms, the cysteine of at least one of the deleted disulfide bonds is replaced with alanine. In some preferred forms, the disulfide bonds are located in the SRCR5 region. Preferably, a cell with modified genome as described above is at least 50% less susceptible than an unmodified cell. 
    
    
     
       DRAWINGS 
         FIG. 1A  illustrates the deletion mutants used in the transfection of HEK293T cells wherein ovals and squares identify the SRCR and PST domains, respectively; 
         FIG. 1B  is a photograph of the Western blots results using anti-GFP antibody for the detection of the CD163-EGFP fusion protein; 
         FIG. 1C  is a schematic of the result for PRRSV infection of transfected cells wherein (+++), similar to results for wild type CD163 including numerous large clusters of infected cells; (++), several small clusters of infected cells; (+), multiple single infected cells, but no clusters; (+/−), a few scattered infected cells; (−), no detectable infected cells. 
         FIG. 2A  illustrates the peptide sequences for the constructs shown in  FIG. 2A , (SEQ ID NO. 21, 22, 23 and 24), respectively, wherein the peptide sequence shown in bold letters (SEQ ID NO. 57), flanked by SEQ ID NOs. 61 and 62 is the region covered by the PSTII domain. SEQ ID NOs. 77 and 78 represent the Exon 14 mutation and Exon 13 deletion, respectively. The infection results are described in  FIG. 1C ; 
         FIG. 2B  is a graph illustrating the surface expression of CD163 for the same constructs shown in  FIG. 2A . 
         FIG. 3  is an illustration of SEQ ID NO. 58 showing the location of PR insertions in SRCR5 of porcine CD163 wherein the SRCR5 peptide sequence is from GenBank No. AJ311716. The peptide sequence position number is above the sequence with the CD163 coordinates in parentheses. The arrows show the location and direction of beta sheets and the dotted lines show the location of alpha helices. The asterisks show the location of the proline-arginine insertions. Below each construct is the result for infection of transfected HEK293T cells, which is also described in the description for  FIG. 1C . 
         FIG. 4A  is an illustration of the location of PR insertion mutations in SRCR5. This figure shows the location of the PR insertion mutations (red areas) on the surface of the space filling model. The inserted PR dipeptides are shown in green. 
         FIG. 4B  shows the ribbon structure of SRCR5 and the effect of the individual PR mutations on the protein structure. The structures are based on the X-ray crystallography data deposited in RCSB Protein Data Bank (PDB code SJFB) and viewed using UCSF Chimera. 
         FIG. 5  is a graph of the growth curves for HEK293T cells transfected with wild type and mutant CD163 constructs. HEK cells were transfected with different mutant constructs and, after 24 hrs, were infected with PRRSV-RFP. Two hours post-infection, the cells were washed and media was collected every 12 hrs. The TCID 50  was calculated by titration of viruses on MARC-145 cells. Results are shown for a single experiment. 
         FIG. 6  is a schematic depicting the location of SacII insertion sites in domain 5 of porcine CD163 (unmodified CD163) (SEQ ID NO. 60) in comparison to the location of the disulfide bonds. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the present teachings, application, of uses. Thus, variations that do not depart from the gist of that which is described are intended to be within the scope of the teachings. Such variations are not to be regarded as a departure from the spirit and scope of the teachings. 
     EXAMPLE 1 
     Materials and Methods 
     Virus and cells. The type 2 PRRSV was originally derived from an infectious clone that expressed a red fluorescent protein (RFP). Virus was propagated on MARC-145 cells in media containing 7% FBS, Pen-Strep (80 U/ml and 80 μg/ml, respectively), 3 μg/ml Fungizone, and 25 mM HEPES. For virus titration, the samples were serially diluted 1:10 and propagated on MARC-145 cells. Dilutions were performed in quadruplicate. The titration end-point was determined as the last well showing CPE. The log 10  50% tissue culture infectious dose (TCID 50 )/ml was calculated according to the method of Reed and Muench. 
     Construction of CD163 deletion mutants. The backbone for the preparation of the mutated CD163 constructs was a porcine CD163 cDNA (GenBank Acc No. EU016226) cloned into a pcDNA3.1-EGFP expression vector. The truncated CD163 cDNA fragments were prepared by PCR amplification of the pcDNA template using the GoTaqGreen® Master Mix (Promega) according to manufacturer&#39;s instructions. PCR conditions included 95° C. for 2 min, followed by 30 cycles of 94° C. for 30 s, 65° C. for 30 s, and 72° C. for 2 min, and a final extension at 72° C. for 10 min. The PCR primers are listed in Table 1. The PCR products were cloned into pCR®2.1-TOPO® vector (Invitrogen) and transformed into One Shot® TOP10 chemically competent  E. coli  cells (Invitrogen). The purified plasmids were double-digested with KpnI and XbaI and cloned into the KpnI-XbaI sites of the pcDNA3.1-EGFP vector. Plasmids were transfected into HEK cells using FuGENE® HD reagent (Promega) according to manufacturer&#39;s instructions and viewed for the presence of EGFP expression under a fluorescence microscope. The resulting N-terminally truncated proteins are illustrated in  FIG. 1A  (constructs B through H). 
     Constructs that possessed deletion domains from the C-terminal end of CD163 incorporated primers with PacI restriction sites, which are listed in Table 1. Deletions were made using a long PCR protocol designed to amplify the desired CD163 fragment along with the entire pcDNA3.1-EGFP plasmid. PCR amplification was performed using LongAmp® Taq DNA Polymerase (New England Biolabs Inc). PCR conditions included 94° C. for 30 seconds, followed by 30 cycles of 94° C. for 30 seconds, 65° C. for 1 minute, and 65° C. for 8 minutes, followed by a final extension at 65° C. for 10 minutes. The PCR products were cut with PacI and the plasmid re-circularized by ligation with Anza™ T4 DNA Ligase Master Mix (Invitrogen), and then transfected into HEK cells. The resulting CD163 deletion constructs retained intact transmembrane and cytoplasmic domains along with an added PacI site (see  FIG. 1 —constructs I through O). The CD163 PSTII partial and complete deletion constructs incorporated primers that possessed SacII restriction sites. The SacII sites were placed in a reading frame to code for alanines. 
     Insertion mutagenesis of SRCR5. Proline-arginine (PR) insertions into SRCR5 were achieved by inserting SacII sites into the CD163 cDNA plasmid. The method for insertion of SacII sites was the same as described for the preparation of the C-terminal deletion mutants. The intact pcDNA3.1 CD163-EGFP was used as a template and the entire plasmid amplified using LongAmp® Taq DNA Polymerase and the same amplification conditions described above for the C-terminal truncations. The primers used for amplification are listed in Table 2. 
     Western blot analysis. Cell monolayers on a 24-well plate were washed once with cold PBS followed by the addition of 300 μl NP-40 lysis buffer to each well (Invitrogen). Cells were removed with a cell scraper and transferred to a microcentrifuge tube. After a 30 minute incubation on ice under constant agitation, the supernatant was removed and proteins separated on a 10% SDS-PAGE gel. Proteins were transferred to a 0.45 μm PVDF membrane: the SDS-PAGE gel was rinsed in PBS-Tween 20 (PBST) and the PVDF membrane was submerged in methanol for membrane activation, followed by rinsing in double-distilled water for 10 minutes. Prior to assembly, the gel, filter paper (0.83 mm) and PVDF membrane were soaked for 20 minutes in 1× transfer buffer (0.03 M glycine, 0.04 M tris base, 0.04% SDS, double-distilled water and 20% methanol). The blot apparatus was assembled and electrophoretic transfer performed on a Mini Trans-Blot® Electrophoretic Transfer Cell (BIO-RAD) following the manufacturer&#39;s instructions. After transfer, the membrane was blocked overnight in PBS with 5% non-fat dry milk (PBS-NFDM) at 4° C. The CD163-EGFP fusion proteins were detected with horseradish peroxidase (HRP)-conjugated goat anti-GFP antibody (R&amp;D Systems), diluted 1:1000 in PBS-NFDM for 1 hour at room temperature. Peroxidase activity was visualized using a CN/DAB Substrate Kit (Thermo Scientific Pierce) according to the kit instructions. The peroxidase reaction was stopped by rinsing the membrane with double-distilled water. 
     Flow cytometry for surface expression of CD163. At 72 hours after transfection with CD163 plasmids, HEK cells were washed twice with PBS and detached with TrypLE™ Express (Thermofisher Scientific) according to manufacturer&#39;s instructions. Cells were adjusted to a concentration of approximately 2×10 7 /ml in PBS with 5% mouse serum (PBS-MS) and 100 μl placed in 12 mm×75 mm polystyrene flow cytometry tubes. Cells were pelleted by centrifugation and re-suspended in 100 μl of mouse anti-porcine CD163 mAb at a concentration of 10 μg/ml in PBS-MS (Clone: 2A10/11, AbD Serotec). After a 30 minute incubation on ice, the cells were washed twice with PBS containing 1% fetal bovine serum (FBS; MIDSCI Co.) and incubated with 1:100 dilution of rabbit anti-mouse conjugated with allophycocyanin (APC; 0.5 mg/ml, Jackson ImmunoResearch), for 30 minutes on ice. Samples were washed twice with PBS containing 1% FBS and brought to a final volume of 300-500 μl and analyzed on the BD LSR Fortessa X-20 Flow Cytometer (BD Biosciences) with FCS Express 5 software (De Novo Software). A minimum of 10,000 cells were analyzed for each sample. 
     Computer prediction of PR insertions in SRCR5. Coordinates for the CD163 SRCR5 peptide sequence were accessed through the RCSB Protein Data Bank (PDB code SJFB; deposited by Ma et al., 2017). The structural predictions for the proline-arginine (PR) insertion mutations were generated using the PyMOL molecular graphics and modeling system. The I-TASSER V5.1 simulator was used for the refinement of the predicted conformational changes. A C-score value was used to determine the accuracy of the simulation. C-scores between 1.0 and 2.0 were considered accurate. For simulations that gave multiple model structures, the prediction with the greatest C-score value was chosen. The generated structures were loaded into the open source molecular visual program, UCSF Chimera, for analysis. 
     Removal of disulfide bonds in SRCR5. As illustrated in  FIGS. 3 and 6 , there are eight cysteines that form four disulfide bonds in SRCR5 (Ma et al., 2017). Individual disulfide bonds were interrupted by deleting one of the participating cysteines. Unique restriction sites were placed in the pcDNA3.1 CD163-EGFP plasmid by changing the codons for two existing proline-arginine dipeptides, located at positions 1 and 84 of SRCR5, to SacII restriction sites. The nucleotide changes did not affect the SRCR5 polypeptide sequence. The CD163-EGFP plasmid was cut with SacII and intervening DNA sequence replaced with synthesized fragments (IDT) possessing SacII sites on the ends. Four DNA fragments were synthesized, which included cysteine to alanine substitutions at C1, C3, C5 or C7 (see  FIG. 6 ). The final CD163-EGFP plasmids were transfected into HEK cells and infected with P129-RFP. 
     Results 
     Infection of HEK cells transfected with CD163 domain deletion mutants. The CD163-EGFP deletion constructs transfected into HEK cells are described in  FIG. 1A . All constructs showed EGFP fluorescence within 24 hrs after transfection of plasmids. Western blots stained with anti-GFP antibody confirmed that each construct migrated according to the predicted size ( FIG. 1B ). HEK cells transfected with the full-length CD163-EGFP fusion plasmid, Construct A, served as a positive control for infection with the PRRSV-RFP virus. A plasmid expressing a soluble form of CD163-EGFP, Construct H, was included as a negative infection control. The N-terminal truncation mutants are identified as constructs B-G. Constructs A-D, which contained SRCR5 were positive for infection. The remaining N-terminal deletion mutants, constructs E-G, which lacked SRCR5, were negative for infection. The results for the C-terminal deletions lacking PSTII, constructs I through K, were all negative for infection, including the constructs J and K, which retained the SRCR5 domain. Constructs N and O, which were identical to J and K, except for the addition of the PSTII domain were positive for infection. When taken together, all of the constructs in  FIG. 1  lacking SRCR5 or PSTII were negative for infection. 
     The 16 amino acid PSTII domain can be divided into two regions. The exon 14 portion consists of the four amino acids, GRSS (SEQ ID NO. 21-22) (see  FIG. 2A ). The remainder of exon 14 includes the transmembrane domain and a portion of the cytoplasmic tail. The substitution of GRSS (SEQ ID NO. 23-24) with three alanines (Construct P) resulted in only a small reduction in infection. The removal of the remaining 12 amino acids within PSTII blocked infection. The results in  FIG. 2B  showed that all constructs were expressed on the surface of transfected HEK cells. Therefore, the deletion of the region of CD163 encoded by exon 13 is sufficient to prevent infection. When taken together, the results from the experiments in  FIGS. 1 and 2  demonstrate that type 1 and type 2 PRRSV possess similar requirements for SRCR5 and PSTII domains. 
     The effect of PR insertions in SRCR5 on PRRSV infection. The approach for the characterization of polypeptide sequences within SRCR5 involved in the recognition of CD163 incorporated the disruption of primary and/or secondary polypeptide structure through the insertion of single proline-arginine (PR) dipeptides. SRCR5 already possesses two naturally occurring PR dipeptides, located at positions 1 and 84 (see  FIG. 3 ). The primers for inserting the SacII sites, listed in Table 2, were designed to place a PR dipeptide at about every 10 amino acids along the SRCR5 polypeptide sequence (see  FIG. 3 ). The results after infection with PRRSV-RFP identified three mutations, PR-9, PR-55, and PR-100, which produced the greatest reduction of RFP-positive cells. In addition, the deletion of, or amino acid substitution within, the SRCR4-5 interdomain peptide sequence, AHRK, also affected virus infection. 
     The predicted locations of the PR-9, PR-55 and PR-100 mutations on the surface of the SRCR5 structure are shown in  FIG. 4A . A space-filling model showed that the three mutations were located on a single face of the SRCR5 polypeptide, in close proximity to the Ligand-binding pocket (LBP) and the loop 5-6 region previously described by Van Gorp et al., 2010 and Graversen et al., 2002. Even though the PR-9 and PR-100 mutations are located at the ends of the polypeptide sequence, they are predicted to be in close proximity to each other in the polypeptide structure (see  FIG. 4A ). The ribbon structure in  FIG. 4B  showed that the PR-9 insertion between Ile-8 and Pro-9 is predicted to create a short alpha helical structure, located just prior to the beginning of the (31 strand. One effect of the insertion is the re-orientation of the Pro-9 to the backside of the polypeptide face (see  FIG. 4A ). The PR-55 insertion, located between Ala-54 and His-55, is predicted to create a kink within the loop structure between β4 and β5. The result is the disappearance of Ala-54 from the surface of the polypeptide (see  FIG. 4A ). And finally, the PR-100 mutation, between Val-99 and Cys-100, is predicted to result in the early termination of the β7 strand. The space-filling model predicts that the net effect of the insertion is a separation of the valine and cysteine residues. Together, these data show that the PR insertions are predicted to produce only small localized changes in the overall SRCR5 structure. 
     A more careful analysis of the effect of the PR-9, PR-55 and PR-100 insertions on infection was performed by comparing titration end-points and growth curves for HEK cells transfected with the different mutants. The results for three independent titration experiments are presented in Table 3. HEK cells transfected with the wild type CD163 plasmid served as a positive control. The results for three independent experiments showed that the initial mean percentage of infected WT CD163 cells was 59.3+/−3.6, with a titration end-point of 10 −3  for all three experiments. In contrast, the infection of HEK cells expressing PR-9, PR-55 and PR-100 CD163 constructs showed only a few infected cells. The mean percent infection ranged between 0.8 and 3.0%. The titration end-point for PR-9 and PR-100 was 10 −2 . The titration endpoint for PR-55 was 10 −1 . 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Percent infection of HEK293T cells transfected with different  
               
               
                 PR dipeptide insertion constructs* 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Exp. 1 
                 Exp. 2 
                 Exp. 3 
                 Mean ± SD 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 10 −1  Dilution of Virus 
                   
                   
                   
                   
               
               
                 WT 
                 55.3 
                 62.3 
                 60.4 
                 59.3 ± 3.6  
               
               
                 PR-9 
                 2.1 
                 3.0 
                 1.6 
                 2.2 ± 0.7 
               
               
                 PR-55 
                 1.7 
                 1.6 
                 1.4 
                 1.6 ± 0.2 
               
               
                 PR-100 
                 0.8 
                 1.1 
                 1.2 
                 1.0 ± 0.2 
               
               
                 10 −2  Dilution of Virus 
                   
                   
                   
                   
               
               
                 WT 
                 28.5 
                 28.5 
                 30.6 
                 29.2 ± 1.2  
               
               
                 PR-9 
                 0.7 
                 1.0 
                 0.6 
                 0.8 ± .2  
               
               
                 PR-55 
                 0.0 
                 0.0 
                 0.0 
                 0 
               
               
                 PR-100 
                 0.4 
                 0.5 
                 0.4 
                  .4 ± 0.1 
               
               
                 10 −3  Dilution of Virus 
                   
                   
                   
                   
               
               
                 WT 
                 3.2 
                 4.5 
                 6.6 
                 4.8 ± 1.7 
               
               
                 PR-55 
                 0 
                 0 
                 0 
                 0 
               
               
                 PR-100 
                 0 
                 0 
                 0 
                 0 
               
               
                 10 −4  Dilution of Virus 
                   
                   
                   
                   
               
               
                 WT 
                 0 
                 1.3 
                 1.4 
                 1.4 
               
               
                 10 −5  Dilution of Virus 
                   
                   
                   
                   
               
               
                 WT 
                 0 
                 0 
                 0 
                 0 
               
               
                   
               
               
                 *Cells were infected with different dilutions of PRRSV-RFP virus (starting concentrations = 6.5 Log 10 TCID 50 /ml). Results are shown as percent infection of CD163-positive HEK293T cells at 72 hrs after infection. Each experiment was performed on a single 24 well plate. Transfection efficiency, as determined by the percentage of green fluorescent cells for the CD163-EGFP plasmid constructs was between 60 and 70% (data not shown) 
               
            
           
         
       
     
     The results for the titration experiments were supported by growth curves (see  FIG. 5 ). For the growth curve experiment, samples from transfected HEK cells were collected every 12 hrs. After washing, the residual virus remaining in the well after infection was 4 log 10  TCID 50 /ml for all constructs. The concentration of virus in the HEK cells transfected with the wild type CD163 peaked at 7 log 10  TCID 50 /ml, which represents a 3 log increase in virus concentration. In contrast, virus infection of the PR mutants showed no increase in virus concentration over time. Furthermore, virus concentrations continued to decay over time, reaching undetectable levels by 36 hrs. These data show that the PR-9, 55, and 100 mutations in SRCR5 of CD163 severely impair the ability of PRRSV to infect cells. 
     The effects of modifications in the SRCR4-5 interdomain peptide sequence, AHRK, on infection. The peptide sequence, alanine-histidine-arginine-lysine (AHRK) (SEQ ID NO. 64) is located between SRCR4 (KITCS (SEQ ID NO. 63)) and SRCR5 (PRLVG (SEQ ID NO. 76)) domains. Table 3 shows the effect of making deletions, insertions and amino acid substitutions in the AHRK (SEQ ID NO. 64) peptide sequence. All constructs shown in Table 4 possess the intact CD163, with on mutation within the AHRK (SEQ ID NO. 64) peptide sequence. For example, CD163 Construct No. 2, which possesses a complete deletion of AHRK (SEQ ID NO. 64) produced a dramatic reduction in infection. The substitution of AHRK (SEQ ID NO. 64) with AAAA (SEQ ID NO. 65) (Construct No. 3) retained the four amino acids, but still produced a reduction in infection. Further substitution of AHRK (SEQ ID NO. 64) with AAAK (SEQ ID NO. 66) (Construct No. 4), AARA (SEQ ID NO. 68) (Construct No. 6), or AKKK (SEQ ID NO. 72) (Construct No. 10), resulted in reduced infection. Examples of how the insertion of an amino acid can affect infection are found in the results for constructs no. 12 and 13. Together, these results demonstrate that amino acid deletions, insertions and substitutions within the SRCR4-5 interdomain region affect the ability of CD163 to function as a receptor for PRRSV. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Peptide sequence modifications in the SRCR4-5 that affect infection 
               
               
                 Peptide Sequence 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                 SEQ ID 
                 Inter- 
                 SEQ 
                   
                 SEQ ID 
                   
               
               
                 No. 
                 SRCR4 
                 NO. 
                 domain 
                 ID NO. 
                 SRCR5 
                 NO. 
                 Infection* 
               
               
                   
               
               
                  1 
                 KITCS 
                 63 
                 AHRK 
                 64 
                 PRLVG 
                 76 
                 +++ 
               
               
                  2 
                 . . . . . 
                   
                 - - - - 
                   
                 . . . . . 
                   
                 +/− 
               
               
                  3 
                 . . . . . 
                   
                 . AAA 
                 65 
                 . . . . . 
                   
                 +/− 
               
               
                  4 
                 . . . . . 
                   
                 . AA . 
                 66 
                 . . . . . 
                   
                 +/− 
               
               
                  5 
                 . . . . . 
                   
                 . . AA 
                 67 
                 . . . . . 
                   
                 +++ 
               
               
                  6 
                 . . . . . 
                   
                 . A . A 
                 68 
                 . . . . . 
                   
                 +/− 
               
               
                  7 
                 . . . . . 
                   
                 . A . . 
                 69 
                 . . . . . 
                   
                 +++ 
               
               
                  8 
                 . . . . . 
                   
                 . . . R 
                 70 
                 . . . . . 
                   
                 +++ 
               
               
                  9 
                 . . . . . 
                   
                 . . K . 
                 71 
                 . . . . . 
                   
                 ++ 
               
               
                 10 
                 . . . . . 
                   
                 . KK . 
                 72 
                 . . . . . 
                   
                 +/− 
               
               
                 11 
                 . . . . . 
                   
                 D . . . 
                 73 
                 . . . . . 
                   
                 +++ 
               
               
                 12 
                 . . . . . 
                   
                 AHRAK 
                 74 
                 . . . . . 
                   
                 +/− 
               
               
                 13 
                 . . . . . 
                   
                 AHARK 
                 75 
                 . . . . . 
                   
                 + 
               
               
                   
               
               
                 *Key: 
               
               
                 ++++, infection of multiple cells with foci containing clusters of infected cells; 
               
               
                 +++, fewer number of infected cells, but still possessing foci containing clusters of infected cells; 
               
               
                 ++, several single cells infected with virus; 
               
               
                 + a small number of cells showing infection; 
               
               
                 +/− only one or two infected cells in the well. 
               
            
           
         
       
     
     The effect of the removal of disulfide bonds in SRCR5 of CD163 on infection. As illustrated in  FIG. 6 , SRCR5 is predicted to possess four disulfide bonds formed by the eight cysteine residues located at amino acid positions 10, 26, 39, 44, 70, 80, 90 and 100. For these experiments, SacII sites were inserted into the SRCR5 region of CD163 in such as manner as to not affect the underlying peptide sequences. DNA segments were synthesized that possessed SacII sites of the DNA strand on the ends along with cysteine to alanine substitutions at positions, 10, 39, 70 or 90. The alanine substitutions were designed to interrupt each disulfide bond, one at a time. The results showed that all alanine mutations conferred resistance to transfected HEK cells infected with PRRS-RFP. The reduction in infection (percent infected cells) was greater than 20 fold. (See Table 5) 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Effect of disulfide bonds on type 2 PRRSV infection 
               
            
           
           
               
               
               
               
            
               
                   
                 Mutation 
                 Disulfide Bond 
                 %* 
               
               
                   
                   
               
               
                   
                 None 
                 NA 
                 61.2 
               
               
                   
                 Cys-10 
                 C10-C44 
                  0.8 
               
               
                   
                 Cys-26 
                 C26-90 
                  1.0 
               
               
                   
                 Cys-39 
                 C39-C100 
                  1.2 
               
               
                   
                 Cys-70 
                 C70-80 
                  1.3 
               
               
                   
                   
               
               
                   
                 *Percent infection calculated as the number of red cells (infected) divided by the number of green cells (CD163 expressing) at the 72 hrs after infection with PRRSV-REP. The percentage of EGFP-expressing cells was between 60-70 for all constructs. 
               
            
           
         
       
     
     Discussion 
     The genetic modification of pigs that lack CD163 expression on macrophages are completely resistant to infection with PRRSV-1 and PRRSV-2. For example, CD163-modified pigs that possess a substitution of SRCR5 with a human CD163-like SRCR8 homolog domain are resistant to infection with PRRSV-1 isolates but retain the capacity to be productively infected with PRRSV-isolates. To better understand the difference in how PRRSV-1 and PRRSV-2 viruses recognized PRRSV, experiments were performed to determine the effect of domain deletions in CD163 on transfected HEK cells infected with a PRRSV-2 virus. The results showed that deletions in SRCR5 or PSTII conferred resistance to infection with PRRSV-RFP. The deletion of other domains, such as SRCR7, 8 and 9, had a lesser effect on infection. In terms of the individual domains involved in PRRSV infection, there appears to be no significant difference between PRRSV-1 and PRRSV-2 viruses. The requirement for SRCR5 was recently confirmed by demonstrating that macrophages from genetically modified pigs possessing a deletion in SRCR5 are resistant to both PRRSV-1 and PRRSV-2 viruses. Therefore, these data indicate that the difference between PRRSV-1 and PRRSV-2 viruses in the recognition of CD163 locates within SRCR5. 
     The purpose of this study was to investigate the regions within SRCR5 and PSTII that are important to recognition of PRRSV. The goal is to construct CD163 receptors that are resistant to PRRSV, while retaining overall CD163 function. The approach for identifying regions in SRCR5 involved in recognition by PRRSV was the insertion of single PR dipeptides along the entire SRCR5 peptide sequence. Three PR mutations, inserted after amino acids 8 (PR-9), 54 (PR-55), or 99 (PR-100) of the SRCR5 sequence produced the greatest impact on the infection of CD163-transfected HEK cells (see Table 3). Reduced infection was supported by the structural changes imposed on SRCR5 by each mutation. Even though the mechanism for reduced infection is not known, one possibility is that the interruption of the primary and/or secondary protein structure is sufficient to interrupt the interaction between SRCR5 and corresponding amino acids on a corresponding PRRSV protein. Very little is known regarding the PRRSV proteins that form interactions with CD163. The fact that both PSTII and SRCR5 are spatially separated from each other suggests that multiple envelope proteins form multiple contacts with CD163. The best available model for the interaction between CD163 and PRRSV involves the heterotrimer formed by GP2, GP3 and GP4. In addition, the surface of the virion is populated by GP5-M homodimers. Additional viral glycoproteins, such as GP3 and the GP5-M heterodimer, may further stabilize the virus-receptor complex. After infection of transfected HEK cells possessing the PR-9, PR-55 and PR-100 mutations, a small number of infected cells remained. The persistence of a similar background level for all three mutants indicates that more than one region in SRCR5 participates in PRRSV infection.