In a recent study posted to the bioRxiv* pre-print server, researchers investigated the potency of trimeric human nephrocystin SH3, TriSb92, a domain-derived antibody, against a conserved region in the receptor-binding domain (RBD) of the spike protein in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern (VOCs).
The currently emerging variants of SARS-CoV-2 are capable of escaping the vaccine-induced neutralizing antibodies and therefore, a prophylactic treatment complementing the action of the vaccine is essential to fight the current coronavirus disease 2019 (COVID-19) pandemic. The primary entry point of SARS-CoV-2 is the nasal cavity, and the epithelium of the nasal cavity is rich with ACE2 receptors critical for SARS-CoV-2 replication. Some animal studies have also shown promising results for intranasal antibodies administration.
Screening for a potent sherpabody
In the present study, the researchers screened a large antibody-mimetic phage library with approximately 1,011 unique sherpabodies against the RBD of spike glycoprotein of the Wuhan-Hu-1 strain. The large library of unique sherpabodies was reduced to 15 through affinity selection and phage amplification. These 15 unique sherpabodies were further tested with phase enzyme-linked immunoassay (phage-ELISA) for binding to RBD-mFc, control mFc, or a monoclonal antibody and all 15 sherpabodies displayed strong and specific RBD binding. Among the 15 sherpabodies, clone number 92 (Sb92) displayed the strongest RBD binding affinity to SARS-CoV-1 and was chosen for further development. The semi-quantitative antigen capture-ELISA for RBD-binding affinity of Sb92 displayed an affinity of 30nM.
Neutralization of Wuhan-Hu-1 and SARS-CoV-2 variants with Sb92
The researchers evaluated the potency of Sb92 as a neutralizing agent in the Beta VOC and Wuhan-Hu-1-based pseudovirus model on the luciferase lentiviral vectors infecting ACE2-expressing HEK293T cells in a Spike-dependent manner and found that Sb92 had a high neutralizing capacity with half-maximal inhibitory concentrations (IC50) at 7 nm against the Beta VOC and 28 nM against the Wuhan-Hu-1.
The researchers constructed a multimerized derivative of Sb92 with three tandem copies as TriSb92 connected by flexible 15-mer Gly-Ser linkers which resulted in increased potency of Sb92 by 200-fold for neutralizing these pseudoviruses with IC50 values of 150 pM for Wuhan-Hu-1 and 50 pM for Beta VOC. Interestingly, the most recently found VOCs Delta and Omicron were also neutralized by TriSb92 at IC50 values of 35 pM and 180 pM, respectively.
Further, the researchers tested TriSb92 in VeroE6 cells using clinical SARS-CoV-2 variant isolates and found that TriSb92 can completely neutralize all SARS-CoV-2 variants wat very low or subnanomolar concentrations. The end-point dilution values for different VOCs were 1.6 nM, 0.8 nM, 1.2 nM, 0.6 nM, 2.4 nM, and 6.3 nM for Alpha (B.1.1.7), Beta (B.1.351), Delta (B.1.617.2), Mu (B.1.6219), Kappa (B.1.617.1) and Omicron (B1.1.529), respectively.
Intranasal administration of TriSb92 in mice inhibits SARS-CoV-2
The study on the prophylactic efficiency of TriSb92 was conducted on mice where the mice received a modest dose of 25 µg/ nostril intranasally and were later exposed to the SARS-CoV-2 Beta (B.1.351) variant. At 2 days post-infection the euthanization of mice revealed that mice who received TriSb92 intranasally were protected against SARS-CoV-2 infection.
Another similar study on mice with a lower dose of 2.5 µg/nostril administered intranasally an hour before SARS-CoV-2 exposure also displayed no evidence of viral antigen expression in the lungs. Likewise, TriSb92 was administered 8 hours before the SARS-CoV-2 exposure revealed that 3 out of 4 mice had no viral antigen, but one animal did show low levels of viral RNA in bronchiolar epithelial cells.
TriSb92 targets a conserved region in the RBD
The structural and mechanistic insight of the TriSb92 revealed that it has a highly conserved region distal from ACE2 binding. The epitope of TriSb92 is distinct from therapeutic monoclonal antibodies which primarily targets the ACE2 binding site and therefore, are vulnerable to amino acid change occurring in this region.
Taken together, these findings indicated that TriSb92 is a potent inhibitor of SARS-CoV-2 infection targeting a conserved site in the RBD of the spike, which is insensitive for immune escape mutations found in the relevant VOCs, including Omicron. The animal model demonstrated that intranasal administration of TriSb92 at relatively low doses can provide impressive prophylactic protection against SARS-CoV-2 infection. Therefore, TriSb92 can be used as a novel inhibitor against the current Omicron VOC.
bioRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.