Polyanion-Induced Self-Association of Complement Factor H

Abstract

Factor H is the primary soluble regulator of activation of the alternative pathway of complement. It prevents activation of complement on host cells and tissues upon association with C3b and surface polyanions such as sialic acids, heparin, and other glycosaminoglycans. Here we show that interaction with polyanions causes self-association forming tetramers of the 155,000 Da glycosylated protein. Monomeric human factor H is an extended flexible protein that exhibits an apparent size of 330,000 Da, relative to globular standards, during gel filtration chromatography in the absence of polyanions. In the presence of dextran sulfate (5000 Da) or heparin an intermediate species of apparent m.w. 700,000 and a limit species of m.w. 1,400,000 were observed by gel filtration. Sedimentation equilibrium analysis by analytical ultracentrifugation indicated a monomer Mr of 163,000 in the absence of polyanions and a Mr of 607,000, corresponding to a tetramer, in the presence of less than a 2-fold molar excess of dextran sulfate. Increasing concentrations of dextran sulfate increased binding of factor H to zymosan-C3b 4.5-fold. This result was accompanied by an increase in both the decay accelerating and cofactor activity of factor H on these cells. An expressed fragment encompassing the C-terminal polyanion binding site (complement control protein domains 18–20) also exhibited polyanion-induced self-association, suggesting that the C-terminal ends of factor H mediate self-association. The results suggest that recognition of polyanionic markers on host cells and tissues by factor H, and the resulting regulation of complement activation, may involve formation of dimers and tetramers of factor H.

A fully recombinant human IgG1 Fc multimer (GL-2045) inhibits complement-mediated cytotoxicity and induces iC3b

“…These data are consistent with prior reports demonstrating that IVIG modulates C3bBb cleavage by factor H. 33 Similarly, pentraxin-3, an antibodylike protein, can both bind and sequester C1q and potentiate factor H function, allowing apoptotic cells to die a "silent death." 41 Recent structural data suggesting that factor H has the potential to form tetramers on polyanionic surfaces, resulting in a functional increase in both its decay accelerating and cofactor activities, 42 may explain the ability of the higher order multimer fractions of GL-2045 to enhance factor H activity.…”

Relative Role of Genetic Complement Abnormalities in Sporadic and Familial aHUS and Their Impact on Clinical Phenotype

“…Contrary to other reports (34,36), we found no difference in response to plasma infusion versus plasma exchange. The rationale behind using plasma exchange instead of infusion is that plasma exchange also removes mutant circulating molecules (37) and CFH autoantibodies and allows administration of higher volumes of plasma without the risk of fluid overload.…”

Functional analyses of complement convertases using C3 and C5-depleted sera

“…The majority of described human as well as microbial complement inhibitors target complement at the stage of convertases. Most abundant fluid phase inhibitors present in serum at concentrations of several hundreds micrograms per millilitre such as factor H (FH) [17] or C4b-bidning protein (C4BP) [18] are characterized by convertase decayacceleration activity, an ability to accelerate convertase disassembly, as well as cofactor activity, as they act as cofactors supporting cleavage by factor I (FI) of the activated complement components C3b and/or C4b necessary for convertase formation. Furthermore, all human cells express at least one membrane-bound inhibitor displaying decay-acceleration activity (CD35/CR1, CD55/DAF) or cofactor activity (CD35/CR1, CD46/MCP) [1].…”