Immune complex independent activation of complement, C1s secreted from hamster embryo malignant fibroblasts, Nil2C2 in serum free culture medium.

Antibody independent activation of complement C1s was examined by immunoblot analysis using an antibody against a synthetic peptide of hamster C1s L chain. Approx. 50% of C1s secreted from hamster embryo malignant fibroblasts Nil2C2 was functionally active in its two-chain form in the serum free culture medium. In contrast, no active C1s was found in a culture medium of hamster embryo fibroblasts (HEF). Active C1s was detectable, however, in the culture medium after HEF became a cell line. The immune complex independent activation of C1s was also observed in rat cell lines but not in secondary rat embryo fibroblasts. C1s in a membrane fraction of Nil2C2 was a proenzyme form and was not activated by incubation of the membrane itself suggesting that C1s was activated after secretion. The activation of C1s was not inhibited by human C1 inhibitor (C1-INH), benzamidine or soy bean trypsin inhibitor (SBTI) but was inhibited by leupeptin, nitrophenyl guanidinobenzoate and DFP. Our results suggest that C1s is activated either by a serine proteinase(s) other than those reported to cleave C1s or by an activator which directly stimulates autoactivation of C1s.