Scientists explore effectiveness of neutralizing antibodies against SARS-CoV-2 Omicron variant.

In a recent study published in the Journal of Experimental Medicine, researchers assessed the impact of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-neutralizing antibodies on the SARS-CoV-2 Omicron variant.

Researchers around the world have explored the development of vaccines and therapies to prevent and treat coronavirus disease 2019 (COVID-19). Various studies are also investigating the effectiveness of SARS-CoV-2-neutralizing antibodies-based immunotherapies, which has led to the usage of several monoclonal antibodies (mAbs) against COVID-19. 

About the study

In the present study, researchers reported the functional and molecular characteristics of human SARS-CoV-2 spike (S) mAbs derived from immunoglobulin G (IgG) and IgA memory B cells.

The team estimated the seroreactivity of IgG and IgM in convalescent persons who had tested COVID-19 positive in the first wave of infections to the SARS-CoV-2 Wuhan trimeric spike (tri-S) and receptor-binding domain (RBD) proteins via enzyme-linked immunosorbent assay (ELISA). The researchers also assessed the neutralizing activity of the IgG and IgA antibodies found in the purified serum samples against the SARS-CoV-2 Wuhan strain using an in vitro pseudoneutralization assay.

Furthermore, the team selected convalescent persons to obtain peripheral blood IgG and IgA memory B cells which were stained using fluorescently labeled tri-S and RBD. SARS-CoV-2 tri-S was used as bait in order to capture singular SARS-CoV-2-reactive B cells via cytometric sorting. From the SARS-CoV-2 tri-S IgG and IgA memory B cells isolated, the team developed unique human mAbs from selected B cells as recombinant IgG1 antibodies. Subsequently, ELISA and binding analyses based on flow cytometry were performed.

Results

The study results showed that the serological samples obtained from convalescent individuals who had a history of SARS-CoV-2 infection during the first wave of the pandemic exhibited high titers of anti-tri-S IgGs, primarily IgG1. These titers also included antibodies that cross-reacted against the Middle East respiratory syndrome-related coronavirus (MERS-CoV) tri-S protein. The team also observed high concentrations of serum anti-RBD IgGs, which were associated with anti-tri-S antibody titers.

A correlation was found between the SARS-CoV-2 seroreactivity of the IgG and the IgA antibodies. Serum IgG and IgG antibodies obtained from patients having the highest anti-SARS-CoV-2 tri-S antibody titers displayed robust ELISA binding to the SARS-CoV-2 Wuhan nucleocapsid (N), RBD, and the tri-S, S1, and S2 subunits. These antibodies also had a cross-reaction against recombinant S proteins belonging to other beta-CoV such as SARS-CoV-1 and MERS-CoV.

The in vitro pseudoneutralization assay showed that the 50% inhibitory concentrations (IC₅₀) of the purified IgA antibodies were lower than that of IgG antibodies. Moreover, the IC₅₀ values of the IgA antibodies were substantially inversely correlated to the respective binding levels exhibited by the SARS-CoV-2 RBD and S1 proteins.

Binding analyses showed that 76% of the purified mAbs selectively bound to the SARS-CoV-2 spike protein while RBD-binding cells comprised 11% tri-S IgA and 17% tri-S IgG B cells. Furthermore, anti-RBD IgA titers were inversely correlated with the IC₅₀ neutralization values for IgAs and directly proportional to the blood RBD IgA B-cell frequencies. The team also noted that some antibody clones were present in several COVID-19 convalescent individuals, suggesting the existence of inter-individual convergence of anti-SARS-CoV-2 antibody responses.

Epitope mapping analyses performed by ELISA using recombinant proteins revealed that almost 59% of the anti-S mAbs efficiently bind to the S2 subunit, 17% to the N-terminal domain (NTD), 1% to the S1 connecting domain (CD), and 7% to other regions present in the SARS-CoV-2 S protein. Approximately 0.99% of the total anti-S antibodies that targeted S2 recognized the tri-S protein but did not bind to the spike-covering linear peptides, indicating that several SARS-CoV-2 spike memory B-cell antibodies targeted conformational epitopes.

Notably, the antibodies Cv2.1169 and Cv2.3194 exhibited a reduced neutralization efficiency against the SARS-CoV-2 Omicron BA.1 sublineage as compared to the SARS-CoV-2 Delta variant. On the other hand, Cv2.1169 and Cv2.3194 displayed comparatively stronger RBD-binding against the Omicron BA.2 sublineage than BA.1. The two antibodies showed efficient blocking of BA.2 RBD-binding to angiotensin-converting enzyme-2 (ACE-2). 

Overall, the study findings showed that antibodies Cv2.1169 and Cv2.3194 exhibited robust neutralizing activity against SARS-CoV-2 Omicron BA.1 and BA.2 and were found to be the most potent cross-neutralizer used clinically against COVID-19. The researchers believe that these antibodies could serve as potential candidates in the development of prophylactic as well as therapeutic approaches against SARS-CoV-2 infections.