FAS LIGATION INDUCES APOPTOSIS OF CD40-ACTIVATED HUMAN B-LYMPHOCYTES

Since CD40/CD40 ligand (CD40Lig) interactions are essential in vivo fo r the generation of germinal center B cells that express Fas (Apo-1/CD 95), we explored whether CD40 engagement may modulate Fas expression a nd function on human B lymphocytes. Resting tonsil B cells, isolated b y density gradient centrifugation, express either absent or low levels of Fas. They could be induced to promptly express Fas after ligation of their CD40, however, using either a recombinant human CD40Lig or a cross-linked anti-CD40 mAb. In contrast, engagement of the B cell anti gen receptor by immobilized anti-kappa and -lambda antibodies did not turn on Fas expression. Addition of anti-Fas mAb CH11 inhibited the la ter phases of CD40-induced B cell growth as a result of apoptotic cell death. Furthermore, Fas ligation inhibited proliferation and Ig secre tion of CD40-activated B cells in response to recombinant cytokines su ch as interleukin (IL)-2, IL-4, and IL-10, as well as a cytokine-rich supernatant of phytohemagglutinin-activated T cells, indicating that n one of those B cell tropic factors were able to prevent the Fas-induce d death. Taken together, the present results show that engagement of C D40 antigen on B cells induces Fas expression and sensitizes them to F as-mediated apoptosis. The delayed functional response to Fas ligation after CD40 activation may represent a way to limit the size of a spec ific B cell clone that is generated during T-B cell interactions.