SARS-CoV-2 Membrane protein-specific antibodies from critically ill COVID-19-infected individuals are potent stimulators of NK cell activation

Introduction. The M glycoprotein is the most abundant structural protein of the SARS-CoV-2 particle and is one of the key components for virion assembly and morphogenesis. It is a major source of peptide antigens driving T-cell responses and previous studies have shown that most patients make antibodies against its N-terminal domain.Nevertheless, no studies of the function of M-specific antibodies have been reported. Materials and Methods. We expressed the first 23 amino-acids of the M protein as a GST-fusion protein in E.coli. Sera from 37 COVID-19 patients (20 of them critically ill, admitted to ICU; 17 of them with mild disease) were assayed in a standard ELISA assay to detect reactive antibodies. The neutralization capacity of M-specific antibodies was assessed in neutralization experiments against SARS-CoV-2 virus using polyclonal antibodies purified by affinity chromatography from patient sera, while the ability of the antibodies in patients’ sera to activate Natural Killer cells was measured in an ELISA-based assay in which antigen-bound antibodies were incubated with PBMC and NK cell activation was detected by flow cytometry. Results. We could detect M-specific antibodies in most of the patients, and they could discriminate between infected and non-infected individuals with around 90% efficacy. No significant difference in the quantity of the antibody response was found between mild and critical patients. No polyclonal M-specific antibodies showed neutralization activity in the in vitro system used. Nevertheless, when PBMC from healthy donors were incubated with patients’ M- specific antibodies bound to the GST-M fusion protein in an ELISA plate, NK cells were significantly activated, as measured by LAMP1+ degranulation and cytokine (MIP1β) production. Conclusion. We show that most SARS-CoV-2 infected individuals produce specific antibodies against a surfaceexposed epitope at the N-terminus of M glycoprotein. Although these antibodies do not appear to neutralize the virus efficiently, they are able to mediate Fc/FcγR interactions to drive the activation of NK cells.