Plasmablast response during acute SARS-CoV-2 infection

Plasmablasts are a population of short-lived B cells that appear in the circulation shortly after vaccination and during acute infection. Plasmablasts are formed from resting B lymphocytes, from which they differ in their ability to secrete antibodies, making them similar to plasma cells. Plasmablasts are terminally differentiated cells that can form at various nodes and branches of the B cell response. The plasmablast response is an indicator of the success of vaccination and also helps in predicting antibody levels after recovery or vaccination. However, the definition and classification of plasmablasts faces great experimental and theoretical difficulties. The aim of the work was to determine the characteristics of the plasmablast response during acute SARS-CoV-2 infection. The study included patients (n = 28) with a severe form of COVID-19. Blood sampling was carried out once on the 10–18th day from the moment of hospitalization. B cells were isolated by immunomagnetic separation. Cells were phenotyped using flow cytometry. Secretion of IgM and IgG was determined by ELISpot method. B cell subsets were isolated using a cell sorter. Patients with COVID-19 had an approximately fourfold increase in total plasmablast levels compared to healthy donors. An even more pronounced excess over the negative control was observed for RBD-specific plasmablasts. In terms of their composition, plasmablasts were one third IgM⁺ cells. This distribution between B-cell BCR receptor isotypes was consistent with the primary nature of the immune response in COVID-19. Approximately a third of plasmablasts carried the CD138 antigen. CD138 marker is characteristic of the late stage of plasmablast maturation and is also found on plasma cells. The CD27⁺CD38⁺ population was divided according to the expression of the CD138 antigen. Using the ELISpot method, we have shown that a significant portion of circulating plasmablasts are antibody-secreting cells. Among circulating plasmablasts, both early and late plasmablasts can be distinguished, which are characterized by the absence of a surface BCR, but which carry the CD138 antigen. Determining how plasmablasts relate to other B cell populations is of paramount importance for the development of new treatments for COVID-19 and for the creation of promising vaccines against SARS-CoV-2 infection.