Antibody-secreting cells with a phenotype of Ki-67^low, CD138^high, CD31^high, and CD38^high secrete nonspecific IgM during primary hepatitis a virus infection

Although studies investigating the nature of Ab-secreting cells (ASCs) during acute infection with influenza or dengue virus found that the ASC response was dominated by virus-specific IgG secretion, the Ag specificity and phenotype of ASCs during primary acute viral infection were not identified. To this end, we investigated the nature of ASCs in direct ex vivo assays from patients with acute hepatitis A caused by primary infection with hepatitis Avirus (HAV).We found that the frequency of CD27^highCD38 ^high ASCs was markedly increased in the peripheral blood during the acute phase of HAV infection. Moreover, substantial numbers of ASCs were non-HAV-specific and dominantly secreted IgM. We detected HAV-specific ASCs by staining with fluorochrome-tagged HAV-VP1 protein. As compared with HAV-specific ASCs, non-HAV-specific ASCs were Ki-67^lowCD138 ^highCD31^highCD38^high, demonstrating that non-HAV-specific ASCs had a bone marrow plasma cell-like phenotype whereas HAV-specific ASCs had a phenotype typical of circulating plasmablasts. These data suggest that non-HAV-specific ASCs might be mobilized plasma cells from the bone marrow or the spleen, whereas HAV-specific ASCs were newly generated plasmablasts. In this study, we provide evidence that pre-existing plasma cells are released into the circulation and contribute to Ag-nonspecific secretion of IgM during primary HAV infection.