Identification of C 3 / 3 Chain as the Human Serum Eosinophil Cytotoxicity Inhibitor

An eosinophil cytotoxicity inhibitor (ECI) was purified from serum of a human subject with severe allergic dermatitis. Molecular weight of the isolated polypeptide (75,000) and its NHzterminal amino acid sequence identified it as the 13 chain of the C3 complement component (apparently free, but perhaps attached to very small fragments of the c~ chain). Free/3 chain, prepared from normal plasma by reduction of C3, inhibited both eosinophil cytotoxicity and neutrophil adherence functions, with half-maximal activity at "~250 ng/ml. Apparently free C3 13 chain was detected in certain human biological fluids associated with inflammation; the presence of C3/3 chain correlated with ECI activity. This evidence demonstrates a potential role for free C3/3 chain as a suppressor of eosinophil and neutrophil functions in inflammation.