Expression of a hybrid complement regulatory protein, membrane cofactor protein decay accelerating factor on Chinese hamster ovary. Comparison of its regulatory effect with those of decay accelerating factor and membrane cofactor protein.

C activation on the cell surface is supposedly regulated by membrane cofactor protein (MCP) and decay accelerating factor (DAF). These are complementary in function: MCP acts as a cofactor in factor I-mediated C3b and C4b inactivation, thus preventing the assembly of C3 convertases, whereas DAF accelerates spontaneous decay of the assembled C3 convertase. In this report, a hybrid MCP-DAF was expressed on Chinese hamster ovary cells by transfecting cDNA, and its regulatory activity was compared with those of MCP and DAF transfectants and with a transfectant expressing both MCP and DAF (MCP + DAF). The C3 deposition on sensitized CHO cells through activation of the classical pathway was blocked to a different degree with these transfectants, the order being MCP + DAF > DAF > hybrid MCP-DAF > MCP. Likewise, the C3 deposition via the alternative pathway was blocked efficiently in the order hybrid > MCP + DAF > MCP. The C-mediated cytolysis of CHO cells virtually reflected the degree of C3 fragment deposition. The MCP-DAF transfectant acquired additive protective activity against alternative pathway-mediated C3 deposition and cytolysis but was less potent in circumventing classical pathway attack than cells that expressed DAF alone or DAF + MCP. Hybrid MCP-DAF may be useful for alleviating C-mediated cell damage, especially via the alternative pathway.