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 p rotein (MCP, CD46) whereas sera of >50% of the cancer patients contain ed >60 ng/ml, MCP purified by immunoaffinity chromatography from both normal and cancer patients' sera consisted of three bands of 56, 47 an d 29 kDa on SDS-PAGE/immunoblotting. The upper two components were inc reased 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 de position and complement-mediated cytolysis, especially by activation o f the alternative pathway, In this same assay system, exogenously adde d soluble MCP also protected untransfected CHO cells; however, its pot ency was much less than that of the endogenous membrane form. For exam ple, 8 mu g/ml of soluble MCP was equal to 10(4) copies/cell of the ex pressed MCP. Recombinant soluble forms possessed similar activity to t he naturally occurring soluble forms and high doses (>150 mu g) blocke d Arthus-like reaction induced in guinea-pigs by anti-Forssman antibod y, These data establish that soluble forms of MCP are present in human sera that possess cofactor activity and their concentrations, especia lly the 56 and 47 kDa forms, are increased in sera of cancer patients, High doses of the recombinant soluble forms may be therapeutically us eful for suppressing inflammatory responses.