Decreased C5b67-inhibitor activity in two families with hereditary functional deficiency of the eighth component of complement.

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Abstract
In addition to its role in hemolysis and host defense against Neisseria infection, the eighth component of human complement (C8) is one of several plasma proteins that are C5b67-inhibitors (C5b67-INH). The recent identification in our laboratory of two new families with hereditary deficiency of C8 provided an opportunity to study further the role of C8 as a C5b67-INH. Based on mixing and reconstitution experiments, the deficiency of C8 seemed to be due to a selective lack of the C8 beta-chain in one family and the C8 alpha-gamma subunit in the other family. Sera from individuals homozygous for the C8 abnormality were substantially deficient in C5b67-INH activity as well as totally deficient in hemolytic activity. Sera from control individuals possessed approximately 2500 C5b67-INH U/ml, whereas sera from the C8-deficient individuals had markedly depressed C5b67-INH activity, with a mean of only 428 U/ml. C5b67-INH activity was completely reconstituted in C8-deficient serum by the addition of purified human C8. We conclude that C8 constitutes the substantial majority of the C5b67-INH activity of normal human serum.