PLATELET-ACTIVATING-FACTOR ABROGATES APOPTOSIS INDUCED BY CROSS-LINKING OF THE SURFACE IGM RECEPTOR IN A HUMAN B-LYMPHOBLASTOID CELL-LINE

B lymphocyte development is characterized by deletion, via apoptosis, of immature cells that are stimulated via the B cell receptor in the a bsence of a second signal. We have investigated whether platelet-activ ating factor (PAF), a potent B lymphocyte activator, can provide a com plementary signal with B cell receptor ligation to abrogate apoptosis. Cross-linking of the surface IgM on Ramos B lymphoblastoid cells usin g anti-IgM Abs (2 mu g/ml) caused programmed cell death in 34 +/- 5.4% of the cells. Coincubation of PAF (10(-7) M) with alpha lgM led to a significant decrease in apoptotic cells as measured by DNA laddering a nd TUNEL assay (13.8 +/- 3%). The effect of PAF was dose dependent (10 (-7)-10(-9) M) and was inhibited by the specific PAF receptor antagoni st, WEB 2170. PAF protected cells from the effect of alpha lgM for up to 1 h after it was added. alpha lgM-induced programmed cell death in Ramos cells was blocked by catalase and, therefore, is caused in part by the production of toxic hydroxyl radicals from hydrogen peroxide. W e investigated the action of PAF on markers of intracellular oxidation . H2O2 in low doses induced apoptosis, via production of OH . radicals , PAF inhibited H2O2-induced apoptosis in Ramos cells; it also attenua ted H2O2- and alpha lgM-mediated increases in hydroxyl radical (OH .) as measured by the oxidation of 2',7'-dichlorofluorescein diacetate to 2',7'-dichlorofluorescein and blocked the depletion of reduced glutat hione induced by alpha lgM. PAF maintained IgM secretion, which was gr eatly inhibited by incubation with alpha lgM alone. These data indicat e that PAF potentially provides an important cosignal to surface IgM-s timulated Ramos cells by inhibiting apoptosis. This is in part due to the activity of PAF in the oxidant/antioxidant pathway.