Anti CD5 Sustains the Proliferative Response of IgM‐Activated Human CD5+B Cells

The CD5 molecule is expressed by most T cells but it is present on a minor B cell subset. Whilst several studies have provided information on the physiological role of T cell CD5, the functional role of CD5 on B lymphocytes remains unclear. To address this question, tonsillar CD5 + B cells were sorted by dual‐colour fluorescence and FACS. Sorted cells were stimulated with polyclonal anti‐IgM antibodies (Ab), and monoclonal (MoAb) F(ab') 2 fragments of anti‐CD5. Proliferative responses were evaluated by enumeration of Ki‐67 positive cells using quantitative flow cytometry. Co‐stimulation with anti‐CD5 MoAb for 3 days did not affect the anti‐IgM and IL2‐induced proliferation of CD5 + B cells. This was seen under conditions where the anti‐CD5 was soluble, adsorbed to the microwells or cross‐linked by anti‐mouse antibodies. Fewer CD25 + cells were detected, however, in the presence of anti‐CD5. In contrast, the proliferative response of CD5 + B cells prestimulated for 3 days with IL‐2 and anti‐IgM, was sustained in a further 3‐day culture period when anti‐CD5 was added. It is concluded that CD5 occupancy might provide an additional signal to activated CD5 + B cells favouring their proliferation and differentiation into autoantibody secreting cells.