Modulation of the Alternative Pathway of Complement by Murine Factor H-Related Proteins

Factor H (FH) is a key alternative pathway regulator that controls complement activation both in the fluid phase and on specific cell surfaces, thus allowing the innate immune response to discriminate between self and foreign pathogens. However, the interrelationships between FH and a group of closely related molecules, designated the FH-related (FHR) proteins, are currently not well understood. Whereas some studies have suggested that human FHR proteins possess complement regulatory abilities, recent studies have shown that FHR proteins are potent deregulators. Furthermore, the roles of the FHR proteins have not been explored in any in vivo models of inflammatory disease. In this study, we report the cloning and expression of recombinant mouse FH and three FHR proteins (FHR proteins A-C). Results from functional assays show that FHR-A and FHR-B proteins antagonize the protective function of FH in sheep erythrocyte hemolytic assays and increase cell-surface C3b deposition on a mouse kidney proximal tubular cell line (TEC) and a human retinal pigment epithelial cell line (ARPE-19). We also report apparent K_D values for the binding interaction of mouse C3d with mouse FH (3.85 μM), FHR-A (136 nM), FHR-B (546 nM), and FHR-C (1.04 μM), which directly correlate with results from functional assays. Collectively, our work suggests that similar to their human counterparts, a subset of mouse FHR proteins have an important modulatory role in complement activation. Further work is warranted to define the in vivo context-dependent roles of these proteins and determine whether FHR proteins are suitable therapeutic targets for the treatment of complement-driven diseases.