SIGNALING BY HEMOLYTICALLY INACTIVE C5B67, AN AGONIST OF POLYMORPHONUCLEAR LEUKOCYTES

The hemolytically inactive complement component complex C5b67, designa ted iC5b67, can signal human polymorphonuclear leukocytes (PMN) both a s a pertussis toxin-inhibitable agonist for chemotaxis and as an antag onist for C5a- and FMLP-stimulated chemotaxis and superoxide productio n, The signaling pathways utilized by iC5b67 have been further investi gated, In contrast to mastoparan, iC5b67 failed to directly activate G proteins to stimulate inositol phosphate formation in COS cells that had been transfected with G alpha(16). In COS cells co-transfected wit h both G alpha(16) and the C5a receptor, iC5b67 could neither activate phospholipase C nor inhibit C5a receptor-mediated activation of phosp holipase C, iC5b67 stimulated GTPase activity in a membrane-enriched f raction from PMN, These data support the hypothesis that iC5b67 signal s through a unique receptor, likely G protein linked, but distinct fro m the C5a receptor, iC5b67 was able to mobilize intracellular stores t o elicit increases in intracellular Ca2+, Based on the effects of herb imycin A, wortmannin, and chelerythrine on iC5b67-induced PMN chemotax is, iC5b67 signaling involved activation of tyrosine and phosphatidyli nositol 3-kinases, but not protein kinase C, Relevant to the capacity of iC5b67 to antagonize PMN superoxide production, iC5b67 induced rapi d and sustained increases in intracellular cAMP, which others have sho wn can inhibit superoxide formation, Although iC5b67 antagonizes C5a a nd FMLP receptor-mediated superoxide generation, iC5b67 had no effect on PMA-induced superoxide formation, The distinct agonist and antagoni st signaling pathways activated by iC5b67 in the PMN diverge soon afte r initial iC5b67 receptor-mediated transduction steps.