Amidst growing concerns on the use and potential benefits of convalescent plasma therapy, a research group in the United States has identified three human antibodies (Abs) that, when combined, demonstrate both robust viral suppressive properties against all tested severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Variants of Concern (VoC) in vitro and profound antiviral efficacy in vivo.
These recombinant Abs, identified initially from coronavirus disease 19 (COVID-19) convalescent patients’ plasma, bind to three non-overlapping epitopes on the receptor-binding domain (RBD), and each one exerts potent antiviral activity itself.
A pre-print version of the research paper is available on the bioRxiv* server while the article undergoes peer review.
Need for an effective Covid-19 therapy
COVID-19 pandemic continues to pose extraordinary health and economic challenges across the globe. Although many preventive vaccines have been launched, protection concerns still exist due to vaccine hesitancy, a rise in novel vaccine-resistant variants, immune insufficiency, or other deficits in the immune response.
Convalescent plasma therapy that provides passive immunization has been used worldwide to treat patients hospitalized with COVID-19 and prevent disease progression. However, uncertainty is mounting regarding plasma efficacy, as randomized controlled trials have provided mixed results regarding the survival benefits of convalescent plasma.
Mutations in the spike (S) protein of SARS-CoV-2 have resulted in more infectious or more virulent variant strains. Additionally, recent reports have demonstrated that Ab therapies containing single or two-Ab cocktails may generate escape mutants after a few passages.
Hence, the team emphasizes the development of a robust Ab cocktail containing at least three Abs to provide an efficacious antiviral measure for preventing the generation of viral escape variants.
Three-Ab cocktail, IMM-BCP-01, shows robust antiviral effect at higher initial viral loads
Using an unbiased interrogation of a library of patient-derived antiviral Abs, the team has identified three complementary Abs, IMM20190, IMM20184, and IMM20253, that have robust antiviral effects when combined.
Treatment with a single or two of this Ab combination led to robust viral clearance from lungs when administered to Syrian Golden hamsters 6 hours after the SARS-CoV-2 challenge. However, the most robust viral clearance was observed with the three-Ab cocktail.
IMM-BCP-01 cocktail also reduced the viral titer in the lungs of animals infected with a 10-fold higher initial viral load than reported in other studies. In addition, the IMM-BCP-01 cocktail comprising all three Abs at equimolar (1:1:1) ratios achieved the most consistent level of viral clearance.
Efficacious IgG levels in the blood were achieved with dose levels as low as 0.1 mg each and the data suggest that doses as low as 0.1 mg of each Ab (0.3 mg total dose) are efficacious in both the prophylactic and treatment settings.
The team also observed a clear dose-dependent response against both the reference (WA1/2020) and Beta (B.1.351) isolates in vivo, contrasting with the previous reports of plateaued response in the hamster infection model.
Ab cocktail synergistically neutralizes SARS-CoV-2 variants in a dose-dependent manner
IMM- BCP-01 was capable of neutralizing pseudoviruses corresponding to a range of VoC and Variants of Interest (VoI) in a dose-dependent manner. Interestingly, the three-Ab cocktail showed a modest increase in potency against Delta (B.1.617.2), Lambda (C. 37), and Epsilon (B.1.429) pseudoviruses, compared to reference spike pseudovirus.
IMM-BCP-01 also outperforms S309, the parental clone of VIR-7831 (clinically used for COVID-19 treatment, against both the Delta and a reference pseudovirus when tested in pseudovirus neutralization assays.
Noticeably, on testing with intact virus isolates, the team observed equivalent, or better potency of the IMM- BCP-01 cocktail against reference WA1/2020, BavPat (D614G), Alpha (B.1.1.7), Beta (B.1.351) and Gamma (P.1) variants as compared to the corresponding pseudovirus neutralization assay.
The three-Ab cocktail combination also induced a synergistic effect in a dose-dependent manner.
Antibody cocktail induces synergistic effector responses in vitro
Intact effector function is required for robust viral clearance in animal models of COVID-19. Using in vitro standard assays, the team observed that the IMM-BCP-01 Ab cocktail induces oligoclonal response to the spike protein that activates productive Fc-mediated effector functions such as phagocytosis and classical complement pathway activation antibody-dependent cellular cytotoxicity (ADCC). The team attributes this response to the intact IgG1 Fc domains on contributing Abs, and non-competitive binding to the RBD.
Efficacy across multiple SARS-CoV-2 variants is attributable to Ab binding to non-competing conserved epitopes
Epitope binning and alanine scanning identified unique, non-overlapping epitopes for each of the three antibodies that did not compete for binding sites.
One antibody exhibits a composite epitope blocking ACE2 binding. The second antibody bridges two Spike proteins. The third neutralizes the virus by binding to a conserved epitope outside the ACE2 binding site. IMM20190 interacts with 10 amino acid residues in the RBD, of which eight are conserved across all CDC VoCs. IMM20184 also binds to the amino acid residues in the core RBD completely conserved among all SARS-CoV-2 VOCs, including the Delta variant. Only two residues are critical for the binding of IMM20253, which are again conserved among all VOCs.
“IMM-BCP-01 is efficacious across the spectrum of variants known to date and based upon the conserved nature of the epitopes, it is predicted to be efficacious against future variants”, the team highlights.
ELISA-based receptor inhibition assay demonstrated that IMM20184 most potently inhibited ACE2 binding to all three RBD variants (reference [Wuhan/Washington], Alpha or the UK [B.1.1.7] and Beta or SA [B.1.351] tested, followed by IMM20190, and IMM20253. However, the inhibitory effect of the IMM-BCP-01 cocktail was more pronounced than the effect of each individual antibody.
“When combined, the IMM20190/IMM20184/IMM20253 cocktail consistently showed a robust antiviral potency in vivo, neutralized all VoC (including Delta B.1.617.2 and Delta Plus B.1.617.2.1) tested in pseudovirus and live virus neutralization assays and induced a potent Fc-mediated response, including activation of phagocytosis, and induction of the classical complement pathway”, the team concludes.
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.