Discussion
Numerous mAbs against the S protein of SARS-CoV-1 have been generated for research and diagnostic assay development. Some of these may be able to cross-react with the S protein of SARS-CoV-2 and serve as tools to aid research on this newly emerged virus. In this current study, an immunogenic domain in the S2 subunit of SARS-CoV-1 was found to be highly conserved in multiple strains of SARS-CoV-2 (Figure 1 and Table). Consistently, WB and IF analyses showed that four different mAbs generated using this SARS-CoV-1 domain were cross-reactive against the S protein of SARS-CoV-2 (Figures 2 and 3).
Recent cross-reactivity studies have evaluated SARS-CoV-1 neutralising antibodies that bind to the RBD-containing S1 subunit. Although both SARS-CoV-1 and SARS-CoV-2 use ACE2 as a receptor for viral entry [3,16], several SARS-CoV-1 RBD-directed mAbs did not cross-react with SARS-CoV-2 RBD [28,29]. Interestingly, CR3022, which was isolated from a SARS convalescent patient [22], showed cross-reactivity to SARS-CoV-2 RBD and recognises an epitope that does not overlap with the ACE2 binding site [28]. Among the four mAbs tested in this study, indirect ELISA showed that 1A9 binds strongest to the S protein of SARS-CoV-2 (Figure 3B). To determine if 1A9 is useful for detection of S protein in a sandwich ELISA, it was paired with CR3022 since 1A9 binds to S2 subunit while CR3022 binds to S1 subunit. As would be expected, these two antibodies can be paired to detect S protein from 15.6 ng/mL (Figure 4A). In addition, mAb 1A9 stained a considerable number of SARS-CoV-2-infected cells at 24 hours post-infection showing that it is sensitive enough to detect the expression of S protein during infection (Figure 4B). Thus, mAbs 1A9 will be useful for studying the kinetics of SARS-CoV-2 replication in vitro and development of diagnostic assays for COVID-19. It is noteworthy that cytotoxic T-lymphocyte (CTL) epitopes also reside at residues 884–891 and 1116–1123 within the S2 subunit of SARS-CoV-1 [30]. Interestingly, the latter CTL epitope overlaps with the epitope recognised by mAb 1A9 [21]. Hence, the S2 subunit may serve as an important antigen for inducing both humoral as well as cell-mediated immunity against SARS-CoV-1 and SARS-CoV-2.
To our knowledge, this is the first study showing that mAbs targeting the S2 domain of SARS-CoV-1 can cross-react with SARS-CoV-2 and this observation is consistent with the high sequence conservation in the S2 subunit. The ability of these antibodies, particularly 1A9, to detect SARS-CoV-2 S protein in indirect and sandwich ELISAs demonstrate their utility for detection of SARS-CoV-2 infections in a public health setting. Whether or not the current sensitivity of these antibodies are sufficient for robust detection of SARS-CoV-2 infections in a clinical setting and how they compare to existing PCR-based detection remains to be determined. Successful development of these antibodies into a point of care diagnostic kit will provide a complementary approach to existing detection methods. Besides the mAbs characterised here, several other mAbs have been reported to bind to epitopes in the S2 subunit of SARS-CoV-1 [31-33]. Thus, it will be important to determine if these mAbs can also cross-react with SARS-CoV-2.