Biosynthesis of the first component of complement by human fibroblasts

Abstract

Hemolytic activity corresponding to that of complement component 1 (C1) was synthesized and secreted by all 9 human fibroblast cell lines examined. No activity was found in the culture media of a variety of other human cell lines. The C1 hemolytic activity secreted by the fibroblast lines behaved in an identical manner, in most respects, with that of the C1 hemolytic activity of human serum. The C1 hemolytic activity secreted by fibroblasts was less susceptible to inhibition by rabbit fragment F(ab'')2 anti-human C1q than was the C1 haemolytic activity of human serum. Biosynthesis of fibroblast C1 hemolytic activity was inhibited by the presence of cycloheximide and regained on its removal. Incorporation of radioactivity into proteins secreted by the fibroblasts and release of C1 hemolytic activity by the fibroblasts increased in a linear manner until several days after the cultures reached a state of confluent growth. Radioactivity was incorporated into C1q, C1r and C18, as judged by the formation of specific immunoprecipitates and by absorption with immune aggregates. The immunoprecipitates formed by using antisera against C1r and C1s were run on polyacrylamide gels in sodium dodecyl sulfate; this provided physiochemical evidence for the biosynthesis of these subcomponents de novo. The results obtained with immunoprecipitates formed by using anti-C1q suggest that C1q which is isolated by conventional techniques of protein fractionation from fresh serum.