S protein-reactive IgG and memory B cell production after human SARS-CoV-2 infection includes broad reactivity to the S2 subunit

Abstract

The high susceptibility of humans to SARS-CoV-2 infection, the cause of COVID-19, reflects the novelty of the virus and limited preexisting B cell immunity. IgG against the SARS-CoV-2 spike (S) protein, which carries the novel receptor binding domain (RBD), is absent or at low levels in unexposed individuals. To better understand the B cell response to SARS-CoV-2 infection, we asked whether virus-reactive memory B cells (MBCs) were present in unexposed subjects and whether MBC generation accompanied virus-specific IgG production in infected subjects. We analyzed sera and PBMCs from non-SARS-CoV-2-exposed healthy donors and COVID-19 convalescent subjects. Serum IgG levels specific for SARS-CoV-2 proteins (S, including the RBD and S2 subunit, and nucleocapsid [N]) and non-SARS-CoV-2 proteins were related to measurements of circulating IgG MBCs. Anti-RBD IgG was absent in unexposed subjects. Most unexposed subjects had anti-S2 IgG and a minority had anti-N IgG, but IgG MBCs with these specificities were not detected, perhaps reflecting low frequencies. Convalescent subjects had high levels of IgG against the RBD, S2, and N, together with large populations of RBD- and S2-reactive IgG MBCs. Notably, IgG titers against the S protein of the human coronavirus OC43 in convalescent subjects were higher than in unexposed subjects and correlated strongly with anti-S2 titers. Our findings indicate cross-reactive B cell responses against the S2 subunit that might enhance broad coronavirus protection. Importantly, our demonstration of MBC induction by SARS-CoV-2 infection suggests that a durable form of B cell immunity is maintained even if circulating antibody levels wane.IMPORTANCERecent rapid worldwide spread of SARS-CoV-2 has established a pandemic of potentially serious disease in the highly susceptible human population. Key questions are whether humans have preexisting immune memory that provides some protection against SARS-CoV-2 and whether SARS-CoV-2 infection generates lasting immune protection against reinfection. Our analysis focused on pre- and post-infection IgG and IgG memory B cells (MBCs) reactive to SARS-CoV-2 proteins. Most importantly, we demonstrate that infection generates both IgG and IgG MBCs against the novel receptor binding domain and the conserved S2 subunit of the SARS-CoV-2 spike protein. Thus, even if antibody levels wane, long-lived MBCs remain to mediate rapid antibody production. Our study also suggests that SARS-CoV-2 infection strengthens preexisting broad coronavirus protection through S2-reactive antibody and MBC formation.

COVID-19 survival associates with the immunoglobulin response to the SARS-CoV-2 spike receptor binding domain

“…Intriguingly, the observed heterogeneity across sera in the competition experiment was suggestive of an increased prevalence of cross-reactive HCoV-OC43 antibodies (i.e., those whose binding could be at least partially competed with SARS-CoV-2 antigen) in the small set of paucisymptomatic individuals with confirmed SARS-CoV-2 infection compared with symptomatic patients with COVID-19. These data are consistent with previously published findings in SARS-CoV-1-infected patients (46) and recent reports describing immune responses to SARS-CoV-2 already present before the pandemic (47)(48)(49)(50). They also suggest that immunological phenomena often referred to as antigenic sin (51), i.e., the expansion of a preexisting memory response against partially homologous antigens of a related pathogen, might play a role in the progression from SARS-CoV-2 infection to full blown COVID-19 disease.…”

Antibody and B cell responses to SARS-CoV-2 infection and vaccination

“…The S glycoprotein is proteolytically processed into two subunits: S1 (which contains RBD and the N-terminal domain [NTD]) and S2 (which mediates host-viral membrane fusion). S2 is the main target for pre-existing anti-S antibodies, consistent with its greater sequence conservation compared to S1, among HCoV species ( Anderson et al., 2021 ; Jaimes et al., 2020 ; Ng et al., 2020 ; Nguyen-Contant et al., 2020 ). While RBD is the main target for virus-neutralizing antibodies, responses to S epitopes outside of the RBD have been shown to have some neutralizing activity, for example, by binding to the S1 NTD, or by preventing protease cleavage or conformational changes required for entry into cells ( Chi et al., 2020 ; McCallum et al., 2021a ; Poh et al., 2020 ).…”

Humoral Immunity against SARS-CoV-2 and the Impact on COVID-19 Pathogenesis

“…These pre-existing antibodies were more common in children and adolescents (Ng et al, 2020), suggesting a possibility that higher HCoV (common cold coronaviruses) infection rates in children than adults correlate with relatively less severe symptoms in children with COVID-19 (Castagnoli et al, 2020). In contrast, other studies have reported limited pre-existing cross-reactive antibodies against SARS-CoV-2 in unexposed individuals (Nguyen-Contant et al, 2020;Poston et al, 2020;Song et al, 2020). Specifically, only S2-targeted IgG antibody was detected and had some neutralizing activity, whereas anti-RBD IgG antibody was absent in unexposed individuals, though they found pre-existing cross-reactive memory B cells that were activated upon SARS-CoV-2 infection.…”

“…Inversely, antibodies with neutralizing activity against SARS-CoV-2 isolated from COVID-19 convalescent individuals also cross-neutralize SARS-CoV and MERS-CoV . The degree of cross-reactivity between human coronaviruses (HCoVs) and SARS-CoV-2 has been widely studied but resulted in controversial findings (Anderson et al, 2021;Ng et al, 2020;Nguyen-Contant et al, 2020;Poston et al, 2020;. Using a flow cytometry-based method, SARS-CoV-2 spike protein-reactive antibodies have been detected in SARS-CoV-2-uninfected individuals (Ng et al, 2020).…”