Cutting Edge: Cellular Fas-Associated Death Domain-Like IL-1-Converting Enzyme-Inhibitory Protein Protects Germinal Center B Cells from Apoptosis During Germinal Center Reactions

During germinal center (GC) reactions, follicular dendritic cells are believed to select memory B lymphocytes by switching off apoptosis in the successfully binding B cells. The cellular signals involved in this process are largely unknown. Here, we show that GC B lymphocytes have a long isoform of the cellular homologue of the viral Fas-associated death domain-like IL-1-converting enzyme-like inhibitory protein (cFLIPL), which is capable of inhibiting death receptor-induced caspase activation. In isolated GC B cells, cFLIPL decays rapidly even without Fas ligation, and this results in activation of caspase activity and apoptosis. Contact with follicular dendritic cells prevents cFLIPL degradation and blocks all signs of apoptosis, even in the presence of anti-Fas Abs. cFLIPL expression is sustained by CD40 ligation as well, suggesting that at least at some stage of the GC reaction activated T cells may help selected B cells to leave the follicular dendritic cell network without becoming apoptotic.