ACQUISITION OF SURFACE IGD FOILS TO PROTECT FROM TOLERANCE-INDUCTION - BOTH SURFACE IGM-MEDIATED AND SURFACE IGD-MEDIATED SIGNALS INDUCE APOPTOSIS OF IMMATURE MURINE B-LYMPHOCYTES

While mature splenic B cells are surface (s)IgM(+) and sIgD(+), immatu re, tolerance-susceptible B cells express sIgM and varying levels of s IgD, Differential expression of sIgD on tolerance-susceptible and resi stant B cells suggests that sIgM and sIgD may transmit qualitatively d ifferent signals, Alternatively, tolerance sensitivity may be associat ed with intrinsic differences in sIg signaling, regardless of the isot ype engaged, Here, we have exploited a stage of B cell development at which immature, tolerance-sensitive B cells co-express sIgD and sIgM, Using these immature stage B cells to evaluate isotypic differences in the ability to induce activation and deletion, we have found that nei ther ligation of sIgD nor sIgM is capable of inducing proliferation, M oreover, in contrast to mature B cells, both sIgD and sIgM induce apop tosis by immature stage B cells, Importantly, cross-linking sIgD does not protect immature B cells from sIgM-induced apoptosis, Thus, the di fferences in tolerance susceptibility of immature and mature B cells m ust be due to intrinsic developmental rather than isotypic differences in Ag receptor signal transduction.