Cooperation between decay-accelerating factor and membrane cofactor protein in protecting cells from autologous complement attack

Decay-accelerating factor (DAF or CD55) and membrane cofactor protein (MCP or CD46) function intrinsically in the membranes of self cells to prevent a ctivation of autologous complement on their surfaces. How these two regulat ory proteins cooperate on self-cell surfaces to inhibit autologous compleme nt attack is unknown. In this study, a GPI-anchored form of MCP was generat ed. The ability of this recombinant protein and that of naturally GPI-ancho red DAF to incorporate into cell membranes then was exploited to examine th e combined functions of DAF and MCP in regulating complement intermediates assembled from purified alternative pathway components on rabbit erythrocyt es. Quantitative studies with complement-coated rabbit erythrocyte intermed iates constituted with each protein individually or the two proteins togeth er demonstrated that DAF and MCP synergize the actions of each other in pre venting C3b deposition on the cell surface. Further analyses showed that MC P's ability to catalyze the factor I-mediated cleavage of cell-bound C3b is inhibited in the presence of factors B and D and is restored when DAF is i ncorporated into the cells. Thus, the activities of DAF and MCP, when prese nt together, are greater than the sum of the two proteins individually, and DAF is required for MCP to catalyze the cleavage of cell-bound C3b in the presence of excess factors B and D. These data are relevant to xenotranspla ntation, pharmacological inhibition of complement in inflammatory diseases, and evasion of tumor cells from humoral immune responses.