Simultaneous quantification of anti-Spike IgG subclasses against various SARS-CoV-2 variants using a multiplex serological assay

Recent research indicates that the composition of serum anti-SARS-CoV-2 Spike IgG subclass antibodies changes upon repeated exposure to viral Spike. These antibody subclass dynamics were suggested to impact adaptive immune responses and protection against infection. To characterize the role of the IgG subclass composition in the immune response to SARS-CoV-2, assays that measure virus variant-specific anti-Spike IgG subclass concentrations are needed. Here, we describe an easy-to-use serological method for quantifying anti-Spike IgG subclass antibodies against various SARS-CoV-2 variants in multiplex from less than 10 µL of serum. Our assay enables antigen-independent quantification, making it easily adaptable to emerging variants or other pathogens. We validated our method with 20 sera collected from 10 infection-naïve individuals after the second and third COVID-19 mRNA vaccination. We observed increasing anti-Spike IgG2 and, especially, IgG4 concentrations after the third immunization. More recent Spike variants preserved this pattern with decreased levels of total anti-Spike IgG. Our results showed a strong positive correlation with those from an established flow cytometry-based approach. Our method might help deepen our understanding of immune responses to SARS-CoV-2 and, moreover, help gauge the individual's protection from severe disease.