SummaryThe cleavage of C3 is a critical step for complement (C) activation in the classical and alternative pathways. This reaction is controUed by the regulators of C activation protein family. Membrane cofactor protein (MCP) is a cofactor for the factor I-mediated inactivation of C3b and C4b. As a widely distributed membrane protein, MCP may protect host cells from inadvertent C activation. Human MCP has recently been shown to protect transfected rodent cells from human C-mediated lysis. In this report the relationship of MCP expression to C3b deposition and cytoprotection was examined using NIH/3T3 cells transfected with human MCP and exposed to human serum as a source of C and naturally occurring anti-mouse antibody. MCP inhibited C3b deposition in a dose-dependent fashion and inhibited lysis of the mouse cells expressing it. MCP did not inhibit lysis on bystander cells. These results demonstrate the protective role of MCP, at the cellular level, by an intrinsic mechanism.T he human C system is an independent immune system that can separate self from nonself and function as a potent effector system (1). C components are spontaneously and continuously deposited on self tissue in small amounts (2), and in larger quantities during inflammatory reactions. Membrane cofactor protein (MCP; 1 CD46) and decay-accderating factor (DAF; CD55) are membrane glycoproteins that protect host cells from damage by the C system (reviewed in references 3 and 4). MCP is a cofactor for the factor I-mediated cleavage of C3b and C4b, while DAF accelerates decay dissociation of the C3 and C5 convertases. Both proteins are widely distributed on hematopoietic, endothelial, and epithdial cells. They are also highly expressed on certain malignant cell lines (3-7) and reproductive tissues (8).DAF is a glycolipid-anchored protein and functions intrinsically; that is, it protects only the cell on which it is expressed (9). The same mechanism has been postulated for MCP (3,10,11). Although experiments to date are consistent with this interpretation (11-15), the question has not been directly tested.Oglesby et al. (14), Atkinson et al. (15), and Lublin and Abbreviations used in this paper: DAF, decay-accderating factor; MCP, membrane cofactor protein; NHS, normal human serum.Coyne (13) have used nonhuman cell lines transfected with the human MCP or DAF gene to demonstrate the capacity of these proteins to protect cells from C-mediated damage. Seya et al. (12) have also shown that human T cell lines expressing MCP but no other C regulatory protein became susceptible to lysis by the alternative pathway if the activity of MCP was blocked by a mAb. To further assess the mechanism of the inhibitory effect of MCP on C activation, mouse fibroblasts were transfected with human MCP and then exposed to human serum. Only those cells on which MCP was expressed were protected from lysis by human C. This intrinsic mechanism of cytoprotection by MCP and the quantitative aspects of its activity could be of interest to tumor, transplant, and reproductive imm...

Human factor B is required for the initiation and propagation of the complement alternative pathway. It also participates in the amplification of the complement classical pathway. Alone, factor B is a zymogen with little known biochemical activity, but in the context of the alternative pathway convertases, the factor B serine protease is activated in a process that first involves the association with C3b and subsequently the cleavage of factor B into two fragments, Ba and Bb. Ba, the NH2-terminal fragment, is composed mainly of three tandem short consensus repeats, globular domains found in other complement proteins. It dissociates from the convertase during assembly, leaving the active C3 convertase, C3bBb. Previous reports suggest that the Ba region may be instrumental in convertase assembly. This hypothesis was tested using site-directed mutagenesis of recombinant factor B and monoclonal antibody epitope mapping to evaluate the relative importance of specific short consensus repeat amino acid residues. Three sites of interest were identified. Site 1 is a stretch of 19 contiguous amino acids in short consensus repeat 1 that form the epitope of a monoclonal antibody that effectively blocks factor B function. Site 2, composed of 6 contiguous amino acids in short consensus repeat 2, and site 3, consisting of 7 contiguous amino acids in short consensus repeat 3, were defined by mutations that reduce factor B hemolytic activity to 3% or less. Further analyses indicated that sites 2 and 3 contribute to factor B-C3b interactions.

The behaviour of the complement system during human reproduction is now the focus of much scientific attention. The presence of antisperm antibodies in the reproductive tracts of some infertile individuals, and of complement in cervical and ovarian follicular fluid, suggests that complement-mediated damage of spermatozoa is involved in some cases of infertility. Further, deposition of maternal IgG and of complement in the extrafetal tissues indicates that complement activation occurs within the fetoplacental unit. Recently, three complement-regulatory proteins--decay-accelerating factor, membrane cofactor protein and CD59--have been detected on spermatozoa and in the extrafetal tissues. It is likely that these inhibitors are essential for normal reproductive function. This article reviews current understanding of the interaction of the complement system with cells and tissues involved in reproduction, with emphasis on the nature and function of the controlling proteins.

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Mouse cells expressing the human complement regulatory proteins decay accelerating factor (DAF) or membrane cofactor protein (MCP) were produced both by hybridoma technology and by transfection with the appropriate cDNAs. The expression of either or both of these products protected the mouse cell from lysis by human (though not rabbit) complement in the presence of naturally occurring human anti-mouse antibody. This effect could be abrogated by the addition of monoclonal antibody against DAF or MCP. These data suggested that the production of animals transgenic for human complement regulatory proteins should in principle be similarly protected from hyperacute xenograft rejection.