Human neutrophil aggregation was induced by highly purified human C3a and chemically synthetic COOH-terminal peptides of C3a (C3a-8R; Ala-Ala-Ala-Leu-Gly-Leu-Ala-Arg) in a dose-dependent manner and was 40 of human C5a-induced aggregation at each optimal concentration. In contrast to C5a and formyl-Met-Leu-Phe (f-MLP), C3a and C3a-8R showed little chemotactic activity. Specific desensitization of neutrophil aggregation was observed with C3a, C3a-8R, C5a and f-MLP, but not with C3a-des-Arg-7R, indicating that the human neutrophil has C3a-specific binding sites which are different from C5a and f-MLP receptors. An additive effect on aggregation was observed at suboptimal concentrations of C5a (1 × 10––/sup>8M) and C3a (1 × 10––6M) or C3a-8R (1 × 10––5M). These studies suggest that a subpopulation of human neutrophils have specific binding sites for C3a and C3a may work cooperatively with C5a during the process of neutrophil activation by increasing aggregation and lysosomal
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