ENGAGEMENT OF THE ANTIGEN-RECEPTOR ON IMMATURE MURINE B-LYMPHOCYTES RESULTS IN DEATH BY APOPTOSIS

During their development B lymphocytes pass through a maturational sta ge in which encounter with Ag leads to tolerance rather than activatio n. At least four mechanisms for achieving B cell tolerance have been r eported: deletion, anergy, receptor editing, and competition for folli cular niches. Although turnover rates for immature B cells in the adul t mouse bone marrow and several transgenic model systems suggest that a major process contributing to negative selection of B cells is delet ion, a detailed study of the negative effect of Ag-receptor engagement on primary, immature B cell survival has never been undertaken. We ha ve utilized an in vitro culture system to determine whether cross-link ing sIgM on tolerance-susceptible sIgM(+)IgD(-) B cells results in del etion by apoptosis. In contrast to the effect of sIgM cross-linking on mature splenic B cells, treatment of immature, bone marrow-derived B cells results in significant levels of apoptotic death. Ag receptor-me diated apoptosis is detectable by 14 h after sIgM engagement. Moreover , IL-4 and cycloheximide, which have previously been shown to prevent B cell tolerance induction, specifically block the sIgM-induced apopto sis observed in the immature B cells. Similarly, immature B cells from the neonatal spleen are also susceptible to apoptosis after sIgM cros s-linking, although they manifest somewhat higher levels of unstimulat ed apoptosis as compared with bone marrow-derived B cells. These studi es are the first detailed demonstration of Ag receptor-mediated apopto sis of primary immature stage B lymphocytes.