In a recent study published in the Science Immunology, researchers identified a human monoclonal antibody (mAb), 87G7, with in vitro neutralizing activity against several severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern (VOCs), including Alpha, Beta, Gamma, Delta, and Omicron.
The SARS-CoV-2 evolution at the antigenic level continues to pose challenges in the development of clinical mAbs for the prevention and treatment of coronavirus disease 2019 (COVID-19).
In a clinical setting, the neutralization potential of an anti-SARS-CoV-2 mAb remains the first selection criteria. However, the next significant consideration is the potential of the mAb to cross-neutralize SARS-CoV-2 variants by targeting highly conserved sites on the spike (S) protein to alleviate the immune evading risk by future emerging variants.
About the study
In the present study, researchers explored the antibody repertoire of H2L2 transgenic mice following vaccination with the SARS-CoV-2 S ectodomain. These mice carried chimeric immunoglobulin (IgG) genes with human variable heavy and light chains and murine-constant region.
The team screened neutralizing activity of hybridoma supernatants against SARS-CoV-2 S pseudovirus carrying the S E484K mutation. It is noteworthy that mutation in this residue varies in VOCs with immune escape potential.
The team assessed the antibody interference with S-mediated receptor-binding activity to understand the neutralization mechanism of antibodies against SARS-CoV-2 and its VOCs. Additionally, they used enzyme-linked immunosorbent assay (ELISA), biolayer interferometry (BLI), cryo-electron microscopy (cryo-EM), and site-directed mutagenesis for their experiments.
The researchers used mice and Syrian hamsters (small animals) to evaluate the antibody-mediated protective efficacy against challenges with SARS-CoV-2 and its VOCs. Post euthanasia, the team quantified infectious virus in lung and nasal tissues on the cultured cells. Additionally, they used immunohistochemistry to study viral antigens and evaluated the pathology in lung and nasal turbinate tissues using histology.
Study results
The mAb, 87G7, neutralized Wuhan-Hu-1 S mediated cell entry with a half maximum inhibitory concentration (IC50) of 5.4 ng/ml. Additionally, it blocked several pseudoviruses harboring S proteins from Beta, Alpha, Delta, and Omicron sub-variant BA.1 with IC50 values ranging between 1.4 to 5.1 ng/ml.
Conversely, REGN10987 lost neutralization potency against Omicron, whereas REGN10933 showed a 20-fold and 350-fold loss in IC50 against Beta and Omicron, respectively. In the live virus neutralization assay, REGN10933 completely lost its neutralization activity against Omicron BA.1 sub-variant. Contrastingly, 87G7 potently neutralized Alpha, Beta, Gamma, Delta, and Omicron, sub-variants BA.1 and BA.2 VOCs with IC50 values ranging from 3.1 to 12.5 ng/ml. In addition, 87G7 neutralized Lambda and Mu variants with IC50 of 1.2 and 4.8 ng/ml, respectively.
ELISA showed that 87G7 was bound to the S receptor-binding domain (RBD), and BLI data revealed that 87G7 displayed bivalent binding to the S trimer. It displayed a strong affinity against monomeric S1 and trimeric S ectodomain. Similar to REGN10933, 87G7 blocked the binding of recombinant S trimer to angiotensin-converting enzyme 2 (ACE2), reaffirming its neutralization potential.
The three-dimensional (3D) classification of the data revealed that structurally the S ectodomain had all three RBDs in the open conformation with the 87G7 Fab fragment bound to the convex tip of the receptor-binding ridge (RBR). Moreover, the 87G7 epitope overlapped with the ACE2 binding site, preventing receptor engagement through steric hindrance.
The site-mutagenesis results showed that the F486A mutation blocked binding by REGN10933, whereas it did not alter the neutralization potency of REGN10987. Conversely, it strongly reduced 87G7 and S binding, as observed in an ELISA.
First, the researchers intraperitoneally injected K18-hACE2 transgenic mice with 87G7 (10 mg/kg body weight) or IgG1 isotype control. Next, they challenged these animals with 105 PFU of SARS-CoV-2 D614G strain or Alpha, Beta, Gamma, or Delta VOCs, 16 hours later.
87G7-treated animals remained protected against weight loss during the SARS-CoV-2 challenge. Compared to isotype-control treated animals, they showed reduced lung antigen levels on day five after the viral challenge. Moreover, live virus decreased in lung homogenates by one to three orders of magnitude compared to mice receiving the control antibody.
Intraperitoneal injection of 87G7 on day 1 after nasal challenge with D614G reduced weight loss up to 13% relative to 22% in the control group, lung antigen levels, and infectious SARS-CoV-2 titers in the lungs by two orders of magnitude relative to IgG1 isotype-treated mice.
Overall, the study data highlighted the prophylactic and therapeutic efficacy of 87G7 against the SARS-CoV-2 challenge and four VOCs in mice. In addition, the prophylactic treatment with 87G7 resulted in a substantial reduction in antigen expression in the nasal cavity and lungs of Syrian hamsters.
Conclusions
The current study findings could help in the development of sustainable mAb strategies against COVID-19 using combinations of broadly neutralizing clinical mAbs that more robustly combat SARS-CoV-2 antigenic diversity. Most importantly, the study identified 87G7 mAb of potential value for the treatment of individuals who are unable to make endogenous antibodies in response to vaccination or infection.
