Characterisation of SARS-CoV-2 Membrane Protein-Specific Antibodies as an Additional Serological Target Following COVID-19 Infections Identified Through a High Content Microscopy-Based Platform

The prevalence and strength of antibody responses to SARS-CoV-2 proteins other than N and S, which may be of use as additional serological markers, is still unclear. To address this, we developed a novel high content microscopy (HCM) based method to assay humoral immune responses against full length StrepTagged SARS-CoV-2 proteins expressed in mammalian cells. We found that 85%(166/196) of unvaccinated individuals with RT-PCR confirmed SARS-CoV-2 infections and 74% (31/42) of individuals infected after being vaccinated developed detectable IgG against the structural protein M, which is higher than previous estimates. In comparison to N antibodies, M IgG displayed greater specificity in pre-pandemic negative control sera and, as a potential serological testing strategy in the era of mass SARS-CoV-2 vaccinations using S, enhanced serological sensitivity when used alongside testing for N IgG in serum samples from unvaccinated SARS-CoV-2 infected individuals (N sensitivity 93%, N+M sensitivity 95%) and in cases of infections following vaccination(N sensitivity 76%, N+M sensitivity 79%). In addition, profiling of N, S and M antibody levels in individual patients over time demonstrated that anti-M IgG is more durable than anti-N IgG in many cases. These findings raise the possibility of using M as a widespread serological marker alongside N and S to assay for antibody responses in both vaccinated and unvaccinated individuals and highlight the potential to use HCM as a rapidly deployable method to identify the most immunogenic targets of newly emergent pathogens in the early stages of any future pandemics.