INTRACELLULAR SIGNALING EVENTS IN CD77-MEDIATED APOPTOSIS OF BURKITTS-LYMPHOMA CELLS

In the hematopoietic system CD77, a glycolipid surface antigen, is res tricted to group I Burkitt's lymphoma (BL) cell lines and a subset of germinal center B lymphocytes. Recently, we have reported that recombi nant B subunits of Verotoxin, which specifically binds to CD77, induce programmed cell death of CD77(+) BL cells. Here, we show that an anti -CD77 monoclonal antibody (38.13) immobilized on tissue culture dishes also induces apoptosis, and we have explored the signal transducing e vents leading to this cell death. We show that ligation of CD77 antige n causes an increase of the intracellular Ca2+ concentration owing to an influx of extracellular Ca2+ through calcium channels. Chelation of extracellular Ca2+ with EGTA partially prevents anti-CD77-induced apo ptosis, indicating that this process is probably Ca2+ dependent. We sh ow that the cross-linking of CD77 provokes an increase of intracellula r cAMP levels followed by cAMP-dependent protein kinase activation. We report that BL cells produce ceramide when they are exposed to 38.13 but, unexpectedly, without a concomitant decrease in sphingomyelin or CD77 content. Finally, we provide evidence that C2-ceramide, calcium i onophore, and forskolin (which increases intracellular levels of cAMP) independently induce apoptosis of CD77(+) BL cells and, moreover, tha t C2-ceramide and forskolin strongly synergize to cause cell death. Th e possible role of CD77-mediated apoptosis in the B cell selection tha t occurs in germinal centers is discussed. (C) 1997 by the American So ciety of Hematology.