MEMBRANE IGM-STIMULATED HUMAN B-LYMPHOCYTES SUCCUMB TO ACTIVATION-RELATED APOPTOSIS AT A G(1)-]S TRANSITION - INFLUENCE OF LIGAND AFFINITY AND VALENCY

Culture of human B lymphocytes with polyclonally activating surrogates for type II T-cell-independent antigen, i.e., anti-IgM mAb and anti-I gM:dextran, resulted in both membrane IgM (mIgM)-triggered S/G(2)/M en try and apoptosis. Although high ligand valency could compensate for l ow affinity, and high affinity could compensate for low valency, in ac hieving mIgM-triggered apoptosis, the phenomenon was most pronounced w hen the soluble ''antigen'' had both high binding site affinity and va lency. Most of the mIgM-triggered apoptosis may represent B cells whic h progress into G(1) but fail to receive a sufficient level of continu ous mIgM-mediated signaling during G(1) for passage through a G(1) --> S phase restriction point(s). This was supported by the findings that (a) a lesser proportion of mig-triggered cells enter S phase than G(1 ); (b) maximal mIgM-triggered apoptosis was noted at 48-72 h of cultur e and surrounding activated cell clusters; (c) mIgM-triggered apoptosi s was not inhibited by pharmacologic blockers of S phase; and (d) a hi gh proportion of viable mIgM-triggered B cell blasts in G(1) succumb t o apoptosis rather than enter S phase, if high-affinity multivalent li gand is washed from the cultures. In addition to quantitative aspects of initial receptor engagement, the potential for a protracted period of recurrent mIgM signaling events may in fluence whether apoptosis or cell cycle progression is the functional outcome of B cell encounter with a multivalent antigen. (C) 1998 Academic Press.