Microfluidic Antibody Affinity Profiling Reveals the Role of Memory Reactivation and Cross-Reactivity in the Defense Against SARS-CoV-2

Recent efforts in understanding the course andseverity of SARS-CoV-2 infections have highlighted bothpotentially beneficial and detrimental effects of cross-reactiveantibodies derived from memory immunity. Specifically, due to asignificant degree of sequence similarity between SARS-CoV-2 andother members of the coronavirus family, memory B-cells thatemerged from previous infections with endemic human coronavi-ruses (HCoVs) could be reactivated upon encountering the newlyemerged SARS-CoV-2, thus prompting the production of cross-reactive antibodies. Determining the affinity and concentration ofthese potentially cross-reactive antibodies to the new SARS-CoV-2antigens is therefore particularly important when assessing bothexisting immunity against common HCoVs and adverse effects like antibody-dependent enhancement (ADE) in COVID-19.However, these two fundamental parameters cannot easily be disentangled by surface-based assays like enzyme-linkedimmunosorbent assays (ELISAs), which are routinely used to assess cross-reactivity. Here, we have used microfluidic antibodyaffinity profiling (MAAP) to quantitatively evaluate the humoral immune response in COVID-19 convalescent patients bydetermining both antibody affinity and concentration against spike antigens of SARS-CoV-2 directly in nine convalescent COVID-19patient and three pre-pandemic sera that were seropositive for common HCoVs. All 12 sera contained low concentrations of high-affinity antibodies against spike antigens of HCoV-NL63 and HCoV-HKU1, indicative of past exposure to these pathogens, whilethe affinity against the SARS-CoV-2 spike protein was lower. These results suggest that cross-reactivity as a consequence of memoryreactivation upon an acute SARS-CoV-2 infection may not be a significant factor in generating immunity against SARS-CoV-2.