Comparison of surface recognition and C3b binding properties of mouse and human complement factor H.

Factor H (FH) is a central complement regulator both in plasma and on certain cellular and acellular surfaces that are in contact with plasma. Although FH deficiency has been shown to lead to similar diseases in man and mice (membranoproliferative glomerulonephritis or dense deposit disease) little is known about the similarity between the human and murine FH functions. We here characterize the interactions of murine FH (mFH) with C3b, glycosaminoglycans, and endothelial cells and compare these interactions with those of human FH (hFH). To achieve this we purified mFH and murine C3 from plasma, prepared murine C3b, and expressed recombinant mFH constructs containing domains 1-5 and 18-20 (mFH1-5 and mFH18-20). For comparisons, hFH, human C3b, and recombinant hFH1-5 and hFH18-20 were used. We demonstrate that mFH and mFH1-5 do act as cofactors for factor I-mediated cleavage of human C3b. Surface plasmon resonance analysis showed binding of mFH18-20 to murine C3b and weak binding to human C3b. The mFH18-20 construct bound to heparin in a manner comparable to hFH18-20. It was demonstrated by flow cytometry that mFH and mFH18-20 bind to human endothelial cells in a similar manner to hFH and hFH18-20. Taken together, locations of the key functions of mFH, i.e. complement regulation and surface recognition, are comparable to hFH. Recently, mutations in the carboxy-terminal end of hFH have been found to be associated with atypical hemolytic uremic syndrome (aHUS). Based on the results in this report it is conceptually attractive to establish a murine model for aHUS.

Complement factor H as a marker for detection of bladder cancer.

BACKGROUND: The BTA TRAK and BTA stat tests for bladder cancer use monoclonal antibodies (mAbs) X13.2 and X52.1 to detect factor H (FH)-related material in urine. The exact ligands remain unknown. METHODS: Western blot analyses of purified FH, recombinant factor H-related protein 1 (FHR-1), and serum and urine samples were used to identify the ligands of X13.2 and X52.1. Recombinant FH constructs were used to identify the target sites of X13.2 and X52.1. To analyze whether natural ligands of FH could compete with its recognition by the capture mAb X52.1, we used surface plasmon resonance analysis. The role of the ligands of X52.1 in the BTA TRAK assay was tested with use of purified proteins and FH-depleted samples. RESULTS: X13.2 bound to domain 3 of FH and FH-like protein 1, whereas X52.1 bound to domain 18 of FH and to FHR-1. Using specific FH depletion from a bladder cancer patient's urine and purified FH, we demonstrated that FH is the ligand recognized by the BTA TRAK test. By contrast, FHR-1 in urine reduced the FH-dependent test signal. CONCLUSIONS: FH is a tumor marker for bladder cancer. To reveal the presence of bladder cancer, the BTA TRAK assay detects FH, whereas FHR-1 is able to partly inhibit this detection. This indicates a special mechanism for a diagnostic immunoassay based on the combined effect of simultaneous positive and negative signals in a single sample.

Binding of complement factor H to endothelial cells is mediated by the carboxy-terminal glycosaminoglycan binding site.

Factor H (FH), the major fluid phase regulator of the alternative complement pathway, mediates protection of plasma-exposed host structures. It has recently been shown that short consensus repeats 19 to 20 of FH are mutational hot spots associated with atypical hemolytic uremic syndrome (aHUS), a disease with endothelial cell damage. Domain 20 of FH contains binding sites for heparin, C3b, and the cleavage product C3d. To study the role of these binding sites in target recognition, we performed site-directed mutagenesis in domain 20 and assayed the resulting recombinant proteins. The mutant FH15-20A (substitutions R1203E, R1206E, and R1210S) bound neither heparin nor endothelial cells. Similarly, an aHUS-derived mutant FH protein (E1172Stop, lacking domain 20) failed to bind endothelial cells and showed impaired binding to heparin. Binding of FH to endothelial cells was inhibited by heparin and a specific monoclonal antibody that inhibited heparin but not C3d binding, demonstrating that the heparin site on domains 19 to 20 mediates interaction of FH to endothelial cells. Binding of FH15-20 to heparin was inhibited by several cell surface- and basement membrane-associated glycosaminoglycans, suggesting that binding site specificity is not restricted to heparin. Thus, defective heparin/glycosaminoglycan-binding site on domains 19 to 20 of FH most probably mediates complement-induced endothelial cell damage in aHUS.

Hypercholesterolemia is a prerequisite for puromycin inducible damage in mouse kidney.

BACKGROUND: The mouse, as opposed to the rat, is relatively resistant to the experimental nephrosis induced by puromycin aminonucleoside. The reason for this species specificity is not known. Apolipoprotein E (apoE)-deficient mice were used to determine whether hypercholesterolemia plays a role in inducing proteinuria. METHODS: Thirty-two mice were divided into normal and high cholesterol diet groups and then divided further into four subgroups: puromycin, puromycin+probucol, probucol and control. Urinary albumin of these mice was analyzed by nephelometry. The lipid peroxidation (LPO) end products malonyldialdehyde (MDA) and 4-hydroxynonenal (4-HNE) were detected by immunohistochemistry, and the expression level of the glomerular slit diaphragm protein, nephrin, was studied by immunohistochemistry and real time RT-PCR. RESULTS: Overt proteinuria was induced by puromycin only in the apoE knockout mice ingesting the high cholesterol diet. The staining intensities of MDA and 4-HNE were stronger in the glomeruli of proteinuric mice compared to glomeruli of non-proteinuric mice. When serum cholesterol levels were reduced by probucol, proteinuria decreased and fewer LPO end products were seen immunohistochemically. Three and eight days after puromycin injection the level of nephrin mRNA in the kidneys of proteinuric mice decreased in comparison to the controls. Puromycin-treated mice kidneys demonstrated a clearly reduced reactivity to the nephrin antibodies. CONCLUSIONS: Hypercholesterolemia, possibly via LPO, is a prerequisite for puromycin-inducible glomerular damage in the mouse. Furthermore, nephrin protein and mRNA levels appear to be candidate markers of glomerular damage in the mouse.

Lysine-dependent multipoint binding of the Borrelia burgdorferi virulence factor outer surface protein E to the C terminus of factor H.

Serum resistance, an important virulence determinant of Borrelia burgdorferi sensu lato strains belonging to the Borrelia afzelii and B. burgdorferi sensu stricto genotypes, is related to binding of the complement inhibitor factor H to the spirochete surface protein outer surface protein E (OspE) and its homologues. In this study, we show that the C-terminal short consensus repeats 18-20 of both human and mouse factor H bind to OspE. Analogously, factor H-related protein 1, a distinct plasma protein with three short consensus repeat domains homologous to those in factor H, bound to OspE. Deleting 15-aa residues (region V) from the C terminus of the OspE paralog P21 (a 20.7-kDa OspE-paralogous surface lipoprotein in the B. burgdorferi sensu stricto 297 strain) abolished factor H binding. However, C-terminal peptides from OspE, P21, or OspEF-related protein P alone and the C-terminal deletion mutants of P21 inhibited factor H binding to OspE only partially when compared with full-length P21 or its N-terminal mutant. Alanine substitution of amino acids in peptides from the key binding regions of the OspE family indicated that several lysine residues are required for factor H binding. Thus, the borrelial OspE family proteins bind the C inhibitor factor H via multiple sites in a lysine-dependent manner. The C-terminal site V (Ala(151)-Lys(166)) is necessary, but not sufficient, for factor H binding in both rodents and humans. Identification of the necessary binding sites forms a basis for the development of vaccines that block the factor H-OspE interaction and thereby promote the killing of Borreliae.