Study confirms no preexisting B-cell immunity against SARS-CoV-2 in pre-pandemic samples

Several individuals with no prior history of exposure to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) have demonstrated T-cell immunity due to prior exposure to endemic human coronaviruses (HCoVs). These observations have led researchers to investigate the extent of B-cell immunity in these unexposed individuals, which may contribute to determining their susceptibility to SARS-CoV-2 infection and subsequent severity of coronavirus disease 2019 (COVID-19).

A recent iScience study finds that there is no considerable B-cell immunity against SARS-CoV-2 in unexposed healthy individuals.

Study: No substantial preexisting B cell immunity against SARS-CoV-2 in healthy adults. Image Credit: Kateryna Kon / Shutterstock.com

Preexisting immunity and course of the disease

SARS-CoV-2 infection can cause a range of clinical manifestations, from asymptomatic infection to severe life-threatening disease that can be accompanied by acute respiratory distress syndrome (ARDS), multi-organ failure, and death. Disease severity is the amalgamation of several contributing factors; however, it is primarily dictated by immune responses.

Thus, if an individual possesses preexisting immunity against SARS-CoV-2, it could lead to a milder disease. Moreover, preexisting background immunity could effectively mount a rapid immune response against SARS-CoV-2, thus limiting disease severity. Alternatively, preexisting immunity could negatively affect the clinical course of COVID-19 through antibody-dependent enhancement (ADE) or original antigenic sin (OAS), which can cause disease enhancement.

Several studies have demonstrated preexisting T-cell immunity against SARS-CoV-2 in unexposed individuals. T-cell reactivity has been observed against the spike (S), nucleocapsid (N), and nonstructural proteins NSP7 and NSP13.

In particular, T-cell reactivity was detected against the S proteins that are highly similar to the common-cold-causing HCoVs. Thus, prior exposure to endemic HCoVs can confer preexisting T-cell immunity against SARS-CoV-2.

B-cell immunity can either be germline-encoded in the naive B-cells or may originate due to cross-reactivity against related HCoVs. The existence of near-germline B-cell receptor (BCR) sequences with similarities to anti-SARS-CoV-2 antibodies has already been demonstrated using pre-pandemic blood samples.

The data regarding the presence of cross-reactive antibodies in unexposed individuals and their correlation to disease severity are contradictory. Moreover, most of the studies have examined enriched or secreted antibody fractions of unexposed individuals. Thus, there remains a need to comprehensively analyze BCR sequences and characterize recombinant monoclonal antibodies.

Anti-SARS-CoV-2 antibodies in pre-pandemic blood samples

Some anti-SARS-CoV-2 antibodies closely resemble near-germline BCR sequences. Researchers have therefore been interested in determining how this translates to antibody responses and whether unexposed individuals have preexisting anti-SARS-CoV-2 antibodies.

In the current study, the investigators analyzed pre-pandemic blood samples from 150 adults for binding and neutralizing activity against SARS-CoV-2. Blood plasma samples were analyzed for binding to the full trimeric SARS-CoV-2 S ectodomain (S1/S2) or the S1 subunit (S1) using enzyme-linked immunosorbent assay (ELISA). There was no or minimal binding of antibodies to S protein, whereas few samples showed notable reactivity.

The binding activity against full-length S proteins was also assessed by flow cytometry. None of the samples showed binding to full-length S protein. An occasional binding activity was observed that was attributed to unpurified plasma samples which might yield nonspecific reactivity.

To reduce nonspecific signals, polyclonal immunoglobulin G (IgG) antigens were purified from samples and analyzed for binding to trimeric S proteins. None of the samples showed binding to S protein; however, all showed binding to endemic betacoronaviruses, HKU1, and OC43. Thus, betacoronavirus-specific IgGs do not cross-react with SARS-CoV-2 S proteins.

The neutralizing activity of plasma samples or polyclonal IgGs was tested against pseudotyped SARS-CoV-2 (PSV) and/or wildtype SARS-CoV-2. Two plasma and six polyclonal IgG samples showed weak neutralization of 50–60%. However, this neutralizing activity could not be confirmed through serial dilutions of the samples.

Taken together, pre-pandemic blood samples showed no evidence of anti-SARS-CoV-2 antibodies.

SARS-CoV-2-specific B cell immunity in pre-pandemic blood samples

To identify SARS-CoV-2-specific B-cells, B-cells were sorted using S protein as bait.

Peripheral blood mononuclear cells (PBMCs) from 40 pre-pandemic samples, as well as samples from COVID-19 convalescent donors, were assessed. The frequencies of SARS-CoV-2-specific B-cells from pre-pandemic blood samples were significantly lower than those from COVID-19 convalescent donors.

Antibody sequences of 8,174 putative SARS-CoV-2-reactive B-cells were generated and tested. None of these antibodies demonstrated significant binding or neutralizing activity against SARS-CoV-2. Thus, pre-pandemic samples from healthy adults also did not have high-reactive B-cells against SARS-CoV-2.

Monoclonal antibodies from pre-pandemic blood samples

Functional testing of 200 antibody candidates from 36 donors was also performed. Antibodies were selected based on sequence similarity to 920 anti-SARS-CoV-2 antibodies.

A total of 158 monoclonal antibodies were produced for functional testing. The binding activity of these antibodies to the S protein and cross-reactivity to HKU1 and OC43 S proteins was analyzed by ELISA.

The 158 monoclonal antibodies showed no binding or cross-reactivity against any of the S proteins tested. Furthermore, none of these antibodies showed neutralization activity against SARS-CoV-2 pseudovirus. These findings demonstrate that putative SARS-CoV-2-specific B-cells from pre-pandemic samples did not have any SARS-CoV-2-reactive BCRs.

Conclusions

The current study conclusively demonstrates that unexposed individuals do not have competent preexisting antibodies and B-cell immunity against SARS-CoV-2.