Modulation of IgD and CD20 by Ligation of CD5 on Tonsillar B Cells

The CD5 molecule, pan T cell marker, has been known to be expressed on a minor population of B cells, termed B‐1 cells. However, the physiological function and pathological role of CD5+B (B‐1) cells remain to be fully elucidated in humans. In the present study, we aimed to clarify the significance of CD5 expression on the B lymphocytes in human tonsil. Using flow cytometric analysis by three‐colour immunofluorescence staining, we observed a majority of the cell surface CD5‐positive (sCD5+) B cells among the sIgD+ B‐cell population, as previously described. Contrary to our expectation, approximately half of the sIgD+/sCD5+ B cells expressed CD38 on their cell surface. Furthermore, a small number of sCD5+ were observed in the sIgD− B cell population. The addition of anti‐CD5 monoclonal antibody (MoAb) to the culture induced downmodulation of sCD20 and sIgD of the tonsillar B cells, resulting in an increase of sCD38−/sIgD− (memory) B cells during the 10 day culture periods in the CD40/l cell culture system. Our findings suggest that ligation of CD5 might transduce the signal to regulate B cell maturation.