CD2 Ligation Abrogates Antigen-Independent Apoptosis in B Cells

We have described recently the prevention of apoptosis by CD2–soluble CD48 interaction on antigen B cell receptor occupancy. Here, we show that CD2 ligation is also able to interfere with B cell receptor-independent apoptosis pathways such as spontaneous death in spleen B cells or serum deprivation and hydrogen peroxide exposure in the BAL-17 cell line. In all cases, CD2 ligation induces a signal that prevents the downregulation of Bcl-2 expression. The specific CD2 signal pathway involved in this phenomenon is still unknown. As reported, CD2 did not appear to induce Ca2+mobilization, phosphatidylinositol turnover, or PKC translocation in B cells. Nevertheless, we show that CD2 receptor ligation is coupled to the tyrosine phosphorylation pathway in B cells. These observations indicate that CD2 is functionally able to trigger at least an early signal that could play a role in apoptosis blockage B cells in addition to the adhesion one. The results suggest the participation of cellular membrane receptors other that CD40 in apoptosis rescue, not only in the antigen-dependent but also in the antigen-independent phases of B cell lymphopoiesis.