B-CELL RECEPTOR-INDUCED APOPTOSIS IN PRIMARY TRANSITIONAL MURINE B-CELLS - SIGNALING REQUIREMENTS AND MODULATION BY T-CELL HELP

Self-reactive immature B cells may be eliminated in the bone marrow (B M) after B cell receptor (BCR) engagement in a process known as negati ve selection. Immature B cells emigrating from the BM, the so-called t ransitional cells, remain sensitive to negative selection and are like ly to be important targets of tolerance towards peripheral antigens, T ransitional cells are deleted through apoptosis after BCR cross-linkin g in vitro. Using anti-Ig as a surrogate antigen, we determined the si gnaling requirements for the induction of apoptosis in transitional ce lls. Treatment with anti-Ig for only 20 min causes most cells to be ap optotic 16 h later. Furthermore, apoptosis of transitional cells is in duced with low doses of anti-Ig while mature cell proliferation requir es extended culture at 30-fold higher concentrations. For both populat ions of B cells, total surface Ig expression is equivalent, therefore indicating that the threshold of BCR signaling required to elicit thes e responses is different. T cell help can modulate B cell tolerance. H owever, specific help may not be available when apoptosis is triggered by a peripheral antigen. The opportunity to reverse apoptosis of tran sitional cells is surprisingly long. Even 8 h after anti-Ig treatment, IL-4 or anti-CD40 antibody can block apoptosis. The upper time limit of protection is concurrent with irreversibility of apoptosis as measu red by DNA fragmentation. These findings indicate that B cell negative selection is more easily triggered than activation, and that the indu ction of apoptosis and its reversal by T cell help can be events that occur in distinct microenvironments.