FC RECEPTOR OFF-SIGNAL IN THE B-CELL INVOLVES APOPTOSIS

By linking surface Ig to the FcR Fc gamma RII on the mouse B lymphocyt e surface, whole anti-Ig has been shown to block cell cycle entry and subsequent Ab production, a phenomenon called the ''Fc receptor off-si gnal.'' IL-4 or blocking Ab to Fc gamma RII, present with whole anti-I g, restores cell cycle progress to the levels observed with F(ab')(2) anti-Ig. The current study demonstrates that under ''off-signal'' cond itions with whole anti-Ig, the early entry of B cells into apoptosis w as accelerated relative to medium alone or equimolar F(ab')(2) anti-Ig . All reagents tested which opposed the whole-anti-Ig-induced blockade of B cell cycle entry also protected B cells from apoptosis (IL-4, PM A, dextran sulfate, and the monoclonal anti-Fc gamma RII 2.4G2). This protective effect was most evident at 4 to 12 h, waning at later times . Low dose cycloheximide partially protected B cells from whole anti-I g-induced apoptosis, but acted as a survival factor, failing to advanc e B cells from G(0) phase or stimulate thymidine incorporation. Additi ve early apoptosis-associated membrane changes were transiently seen w hen whole anti-Ig was combined with other apoptosis-accelerating agent s (trifluoperazine, staurosporine, dexamethasone, ionomycin, high-dose cycloheximide), but hypodiploid nuclei did not show this effect. B ce lls from bcl-2 transgenic mice showed less apoptosis when cultured wit h whole anti-Ig, or with any of the other agents tested. At 4 h the bc l-2-associated reduction in hypodiploid nuclei was greater than the re duction in membrane unpacking, but by 16 h this difference was much le ss. These results suggest that acceleration of apoptosis contributes t o the inhibition of proliferation induced by whole anti-Ig.