Inhibition of the alternative pathway of human complement <i>in vitro</i> by a natural microbial product, complestatin.

Abstract

Abstract Complestatin, a natural product of the microbe Streptomyces lavendulae, gave 50% inhibition of lysis of rabbit E by normal human serum diluted in Mg-EGTA at a concentration of 6 µg/ml. The interaction of cell-bound C3b with B and D to form the amplification convertase, C3b,Bb, was inhibited in a dose-related fashion by complestatin and the 50% inhibitory dose of 6.5 µg/ml was not influenced by the presence of properdin (P). Increasing concentrations of Mg++ did not reverse the inhibitory action of complestatin. The decay of the preformed, P-stabilized amplification convertase on sheep E was not altered by complestatin. Three lines of evidence indicated that complestatin was inhibitory through a direct interaction with B. Complestatin was 20 times more inhibitory in the absence of D̄ as assessed by formation of functional C3b,B than when D̄ was present so as to generate C3b,Bb, with or without P. Complestatin blocked the Mg++-dependent uptake by cell-bound C3b of 126I-B in a dose-dependent manner. Pretreatment of B with complestatin and removal of unbound inhibitor by gel filtration rendered the protein inactive in both the hemolytic and C3b-binding assays. The function of the inhibited B could be restored by dilution of the inhibitor-B mixture, suggesting that complestatin inhibits convertase formation by reversibly binding to B.