Innate response activator (IRA) B cells reside in human tonsils and internalize bacteria in vitro

Innate response activator (IRA) B cells have been described in mice as a subset of B-1a B cells that produce granulocyte/macrophage colony-stimulating factor (GM-CSF) and have been found in the spleen upon activation. In humans, identification, tissue localization and functionality of these lymphocytes are poorly understood. We hypothesized that IRA B cells could reside in human palatine tonsils, which are a first line of defense from infection of the upper respiratory tract. In the present work, we used flow cytometry and confocal microscopy to identify and characterize human IRA (hIRA) B cells in tonsils. We show that CD19(+)CD20(+)GM-CSF+ B cells are present in the tonsils of all the subjects studied at a frequency ranging between similar to 0.2% and similar to 0.4% of the conventional CD19(+)CD20(+)GM-CSF- B cells. These cells reside within the B cell follicles, are mostly IgM(+)IgD(+), express CD5 and show phagocytic activity. Our results support a role for hIRA B cells in the effector immune response to infections in tonsils.