Journal of Leukocyte Biology 44:212-217 (1988) © 1988 Alan R. Liss, Inc. Platelet Activating Factor Increases Expression of

The phospholipid inflammatory mediator platelet activating factor (PAF) has been shown to stimulate certain functions of polymorphonuclear leukocytes (PMN). However, the effect of PAF on surface complement receptors of PMN has not been described. Using monoclonal antibodies and flow cytometry, we have assessed the effects of PAF on surface expression of membrane receptors for C3bi (CR3) and C3b (CR1) in human PMN. PAF (optimal concentration of I x 108 M) increased CR3 190 % and CR1 174 % com-pared with unstimulated cells at 37#{176}C,while the PAF analogue lyso-PAF had no stimula-tory effect. Both CR3 and CR1 responses to PAF reached maximum levels at 15-30 mm. PAF effects were comparable to peak effects induced by LTB4 but less than induced by FMLP. A PAF receptor antagonist, SRI 63-441, blocked the increased complement receptor expression in a dose-dependent manner with maximal inhibition of 80-95 % at 5 x 106 M. Extracellular calcium had no effect on CR1 expression but slightly enhanced and EGTA partially inhibited the PAF-induced increase in CR3 expression. Simultaneous incubation with PAF and LTB4 enhanced CR3 and CR1 expression more than either agent alone. These findings indicate that PAF, alone and in combination with LTB4, can induce altered expression of complement receptors on the surface of PMN. This effect may enhance adhesion and phagocytosis by PMN at inflammatory reaction sites. Key words: PAF, leukocyte, activatio