Critical Role of the C-Terminal Domains of Factor H in Regulating Complement Activation at Cell Surfaces

Abstract

The plasma protein factor H primarily controls the activation of the alternative pathway of complement. The C-terminal of factor H is known to be involved in protection of host cells from complement attack. In the present study, we show that domains 19–20 alone are capable of discriminating between host-like and complement-activating cells. Furthermore, although factor H possesses three binding sites for C3b, binding to cell-bound C3b can be almost completely inhibited by the single site located in domains 19–20. All of the regulatory activities of factor H are expressed by the N-terminal four domains, but these activities toward cell-bound C3b are inhibited by isolated recombinant domains 19–20 (rH 19–20). Direct competition with the N-terminal site is unlikely to explain this because regulation of fluid phase C3b is unaffected by domains 19–20. Finally, we show that addition of isolated rH 19–20 to normal human serum leads to aggressive complement-mediated lysis of normally nonactivating sheep erythrocytes and moderate lysis of human erythrocytes, which possess membrane-bound regulators of complement. Taken together, the results highlight the importance of the cell surface protective functions exhibited by factor H compared with other complement regulatory proteins. The results may also explain why atypical hemolytic uremic syndrome patients with mutations affecting domains 19–20 can maintain complement homeostasis in plasma while their complement system attacks erythrocytes, platelets, endothelial cells, and kidney tissue.

Mutations of Factor H Impair Regulation of Surface-bound C3b by Three Mechanisms in Atypical Hemolytic Uremic Syndrome

“…The C terminus of FH is crucial in self-cell protection as demonstrated by the severity of the aHUS cases and also in a recent mouse model of aHUS where domains 16 -20 had been deleted (30,31). Histopathology of aHUS in these mice had all the characteristics of human aHUS being concordant with the similarity of binding sites for C3b, heparin, and human umbilical vein endothelial cells between human and mouse FH domains 18 -20 (32).…”

Complement Factor H Binds at Two Independent Sites to C-reactive Protein in Acute Phase Concentrations*

“…SCR-6/8 was expressed and purified following previously established procedures (34). 6 Human CRP was isolated, purified, and characterized as described previously (27,35). For AUC, samples were extensively dialyzed into TBS buffers (Trisbuffered saline; 10 mM Tris, 2 mM CaCl 2 , pH 8.0) containing 50 mM NaCl or 137 mM NaCl.…”

“…FH also competes with factor B in binding to C3b to form the alternative pathway C3 convertase C3bBb (3) and accelerates the decay of the C3 convertase C3bBb (2,4). FH makes the initial contact with host cells through the C-terminal SCR-20 domain; this is followed by N-terminal regulatory activity (5,6). Polymorphisms and mutations in FH are associated with age-related macular degeneration (AMD) and atypical hemolytic uremic syndrome (aHUS), suggesting that impaired control of complement activation in the retina and the kidney endothelium, respectively, is involved in these diseases.…”

Structure of the N-terminal Region of Complement Factor H and Conformational Implications of Disease-linked Sequence Variations

“…To obtain a semiquantitative estimation of FH1-3 activity, a concentration titration cofactor assay was performed using the above assay conditions on full-length FH and FH1-3 at a series of molar-equivalent concentrations in a total reaction volume of 15 l and incubated for 10 min at 37°C. The decay acceleration assay was performed on FH1-3, FHlike-1, and FH as previously described (20).…”