Enhanced Generation of O2– by Human Neutrophils via a Complement iC3b/Mac-1 Interaction

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Abstract
There is evidence for a tumor necrosis factor alpha (TNFα)-initiated and CD 11b/CD 18-dependent burst of superoxide anion (O2–) and hydrogen peroxide production by human polymorphonuclear leukocytes which are adherent to surfaces bearing a variety of proteins. In the current studies neutrophils were stimulated with opsonized (by fresh human serum) zymosan particles in the presence of cytochalasin B, to prevent internalization of particles and to simulate the interaction of neutrophils with protein-bearing surfaces. Under these conditions, the cells demonstrated 2.9-fold greater production of O2– when compared to nonopsonized zymosan particles. Heat inactivation or cobra venom factor treatment of human serum prior to opsonization resulted in 98% and 66% reductions, respectively, in O2– responses. C3 and factor B were required for this response, since sera deficient in either component caused 56 and 68% reductions, respectively, in O2– production. Sera deficient in Clq, C2, C4, C5, C6, C7 or C9 showed no defect in their ability to enhance O2– responses to zymosan particles. Monoclonal antibody to iC3b, but not monoclonal antibodies to C3c or C3d, caused a 29% reduction (p < 0.01) in O2– generation. Antibodies to CD18 (R15.7) or CD11b (CL44 and 60.1) reduced the incremental production of O2– by 76, 71 and 77%, respectively. Two antibodies directed against CD11a as well as the isotype-matched control (MOPC 21) were without effects. These data suggest that, in this model of neutrophil activation, the pathway for O2– generation is a Mac-1 (but not LFA-1)-dependent pathway and also requires iC3b. These findings may be relevant to complement-mediated, neutrophil-dependent vascular injury in vivo.

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