The Fc-mediated effector functions of a potent SARS-CoV-2 neutralizing antibody, SC31, isolated from an early convalescent COVID-19 patient, are essential for the optimal therapeutic efficacy of the antibody

Abstract

Although SARS-CoV-2-neutralizing antibodies are promising therapeutics against COVID-19, little is known about their mechanism(s) of action or effective dosing windows. We report the generation and development of SC31, a potent SARS-CoV-2 neutralizing antibody, isolated from a convalescent patient. Antibody-mediated neutralization occurs via an epitope within the receptor-binding domain of the SARS-CoV-2 Spike protein. SC31 exhibited potent anti-SARS-CoV-2 activities in multiple animal models. In SARS-CoV-2 infected K18-human ACE2 transgenic mice, treatment with SC31 greatly reduced viral loads and attenuated pro-inflammatory responses linked to the severity of COVID-19. Importantly, a comparison of the efficacies of SC31 and its Fc-null LALA variant revealed that the optimal therapeutic efficacy of SC31 requires Fc-mediated effector functions that promote IFNγ-driven anti-viral immune responses, in addition to its neutralization ability. A dose-dependent efficacy of SC31 was observed down to 5mg/kg when administered before viral-induced lung inflammatory responses. In addition, antibody-dependent enhancement was not observed even when infected mice were treated with SC31 at sub-therapeutic doses. In SARS-CoV-2-infected hamsters, SC31 treatment significantly prevented weight loss, reduced viral loads, and attenuated the histopathology of the lungs. In rhesus macaques, the therapeutic potential of SC31 was evidenced through the reduction of viral loads in both upper and lower respiratory tracts to undetectable levels. Together, the results of our preclinical studies demonstrated the therapeutic efficacy of SC31 in three different models and its potential as a COVID-19 therapeutic candidate.

Boosting of Cross-Reactive Antibodies to Endemic Coronaviruses by SARS-CoV-2 Infection but not Vaccination with Stabilized Spike

“…To this end, a subset of monoclonal antibodies isolated from SARS-CoV patients that were able to cross-react with, but not neutralize SARS-CoV-2 were able to confer protection in a mouse model 42 . Indeed, both passive transfer experiments of monoclonal antibodies [43][44][45][46][47] and evaluation of polyclonal antibodies raised in the context of vaccination or infection [48][49][50] have shown that effector mechanisms contribute to antiviral activity in vivo. This evidence extends beyond correlative observations 45,48 to include mechanistic 1 evidence of in vivo contributions via Fc sequence engineering to knockout or enhance these activities 44,46 , as well as on the basis of depletion of effector cells 46 .…”

Fc-Independent Protection from SARS-CoV-2 Infection by Recombinant Human Monoclonal Antibodies

“…Despite the interest in understanding the role of Fc in the in-vivo neutralization of SARS-CoV-2, only limited studies have addressed this point so far. Two studies have performed a side-by-side comparison of the in-vivo activity of a mutated Fc (LALA or LALA-PG) versus a fully active antibody [ 27 , 28 ]. Both studies have found that treatment outcome, when given post-exposure (2–24 h post-infection), was markedly affected if Fc-mediated activity was abrogated.…”

“…It was demonstrated that when given prophylactically, the protective activity of the mutated antibodies was minorly affected [ 25 , 26 , 27 ]. However, delaying the treatment, even for a short period (2–24 h post infection) markedly reduced viral clearance and the therapeutic effect conferred by these Fc-null antibodies [ 26 , 27 , 28 ]. The results of these studies may imply that once virus replication has begun, direct antibody neutralization is insufficient and other mechanisms that are based on Fc-activation are needed.…”

A Bacterial Cell-Based Assay To Study SARS-CoV-2 Protein-Protein Interactions

“…Mutated in both the Alpha and the Beta variants, residue N501 is localized at the binding interface with ACE2 ( 34 , 65 ), and many reports have suggested that the N501Y substitution increases the affinity of the RBD for ACE2 ( 23 , 48 , 66 – 70 ; see reference 71 for a discrepant prediction), potentially explaining the elevated infectivity of the Alpha variant. In a study in which the effects of all possible RBD amino acid substitutions were examined using a yeast surface display platform, Starr et al identified N501Y as one of the substitutions causing the highest gain in ACE2-binding affinity ( 23 ).…”