Purification and functional properties of soluble forms of membrane cofactor protein (CD46) of complement: Identification of forms increased in cancer patients' sera

Normal human sera contained 10-60 ng/ml of soluble membrane cofactor protein (MCP, CD46) whereas sera of >50% of the cancer patients contained >60 ng/ml. MCP purified by immunoaffinity chromatography from both normal and cancer patients' sera consisted of three bands of 56, 47 and 29 kDa on SDS-PAGE/immunoblotting. The upper two components were increased in cancer patient sera. The 56 and 47 kDa soluble forms served as a cofactor for factor I-mediated cleavage of C3b. MCP expressed on Chinese hamster ovary (CHO) cells protects host cells from human C3 deposition and complement-mediated cytolysis, especially by activation of the alternative pathway. In this same assay system, exogenously added soluble MCP also protected untransfected CHO cells; however, its potency was much less than that of the endogenous membrane form. For example, 8 μg/ml of soluble MCP was equal to 10⁴copies/cell of the expressed MCP. Recombinant soluble forms possessed similar activity to the naturally occurring soluble forms and high doses (>150μg) blocked Arthus-like reaction induced in guinea-pigs by anti-Forssman antibody. These data establish that soluble forms of MCP are present in human sera that possess cofactor activity and their concentrations, especially the 56 and 47 kDa forms, are increased in sera of cancer patients. High doses of the recombinant soluble forms may be therapeutically useful for suppressing inflammatory responses.