Modulation of expression of the antigen identified by FMC7 upon human B-lymphocyte activation: evidence for differences between activation in vivo and in vitro.

The antigen detected by the monoclonal antibody (mAb) FMC7 is expressed by a proportion of B lymphocytes, and previous information, based on leukaemias, cell lines and some normal cell studies, suggested that FMC7 reacts with relatively differentiated B cells. In this study, it is shown that cells in the high-density 'resting' fraction of tonsil B lymphocytes have a lower expression of FMC7 than cells in the low-density 'activated' fraction. However, activation of 'resting' B cells with anti-immunoglobulin (anti-IgM) together with IL-4 resulted in a decrease in FMC7 reactivity, whilst recognized markers of activation showed up-regulation, as expected. Activated cells which were cultured for an extra 3 days to allow return to a 'resting' stage showed an increase in FMC7 expression. Culture of activated B cells with low molecular weight B-cell growth factor (LMW-BCGF), tumour necrosis factor (TNF) or interleukin-6 (IL-6), did not lead to further changes in FMC7 reactivity, but stimulation with phorbol ester had an effect similar to that of anti-IgM + IL-4. These results suggest that FMC7 is a marker of differentiation rather than activation, and identifies a relatively mature subfraction of the pool of functionally mature B cells. Consistent with this interpretation, FMC7 and surface IgD tended to be expressed by reciprocal B-cell subpopulations.