Inhibition of activation of the classical pathway of complement by human neutrophil defensins

Defensins are small, cationic antimicrobial peptides that are present in the azurophilic granules of neutrophils. Earlier studies have shown that defensins may influence complement activation by specific interaction with activated C1, Clq, and CI-inhibitor. In the present study, we show that the defensin human neutrophil peptide-1 (HNP-1) is able to inhibit activation of the classical complement pathway by inhibition of Clq hemolytic activity. The binding site for HNP-1 on Clq is most likely located on the collagen-like stalks, as a clear, dose-dependent binding of HNP-1 to either intact Clq or to the collagen-like stalks of Clq was demonstrated using enzyme-linked immunosorbent assay (ELISA). Besides binding of HNP-1 to Clq, also a limited binding to C1 and to a mixture of Clr and Cls was observed, whereas no binding to C1-inhibitor was found. Because binding of HNP-1 to C1-inhibitor has been suggested in earlier studies, we also assessed the binding of HNP-1 to mixtures of C1-inhibitor with either C1r/C1s or C1. No binding was found. Using a competition ELISA. it was found that HNP-1, but not protamine, inhibited binding of biotin-labeled HNP-1 to Clq in a dose-dependent fashion. In the fluid phase, preincubation of HNP-1 with Clq resulted in complex formation of HNP-1 and Clq and generation of stable complexes. In conclusion, HNP-1 is able to bind to Clq in the fluid phase and inhibits the classical complement pathway. This mechanism may be involved in the control of an inflammatory response in vivo. (C) 1998 by The American Society of Hematology.