ABSTRACT
The different effector functions of human IgG are closely associated with its four subtypes. Class switch towards IgG4 occurs after long-term antigen exposure, downregulates immune responses and is associated with several autoimmune diseases. Interestingly, significantly elevated IgG4 levels have recently been detected after more than two mRNA vaccinations. We here study the distribution of IgG subtypes in the context of Post-COVID syndrome. To this end, we analyzed serum samples from two cohorts of 64 patients after COVID and 64 convalescent COVID-19 patients. We found differences in the absolute levels of Spike protein-specific IgG subtypes for both cohorts. IgG1 was the most abundant subtype, followed by IgG3 and IgG2 and IgG4 in declining order. A significant difference was only detected for IgG2. When further analyzing the IgG4 levels reactive against the Spike protein receptor-binding domain (RBD) and the nucleocapsid-protein of SARS-CoV-2, a small but significant difference was detected for the RBD but not nucleocapsid proteins. Since the total IgG4 levels are very low, we do not expect a biologically relevant role in the development and progression of post-COVID syndrome. However, low IgG2 levels, as seen in the Post-COVID cohort, could contribute to the persistent presence of SARS-CoV-2 antigens, causing chronic inflammation in the setting of post-COVID.