Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Complement Factor H”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 91 records · Page 5Linked to original sources
PubMedDate not supplied

Regulatory system of guinea-pig complement C3b: tests for compatibility of guinea-pig factors H and I with human factors.

Two proteins that are involved in cleavage of methylamine-treated C3 of guinea-pig origin (C3(MA)gp) have been isolated from guinea-pig serum. One of them functioned as a cofactor of human factor I (Ihu) for cleavage of C3(MA)gp and its molecular size was 150 kDa. The other was functionally pure and able to cleave C3(MA)gp together with human factor H (Hhu). They appear to be analogous to human factors H and I in the guinea-pig and will be referred to as Hgp and Igp. Methylamine-treated human C3 [C3(MA)hu] was not a compatible substrate for Hgp or Igp: little cleavage of C3(MA)hu was observed if human factor H (Hhu) or I was substituted with the guinea-pig counterpart. C3(MA)gp, on the other hand, served as a substrate, though less efficiently, for Hhu and Ihu. Human C4b-binding protein (C4bp) and membrane cofactor protein (MCP) as well as Hhu could participate in cleavage of C3(MA)gp by Igp or Ihu. In these assays, C3(MA)gp was degraded again less efficiently than C3(MA)hu. Interestingly, human C3b/C4b receptor (CR1) mediated factor I-dependent cleavage of C3(MA)hu and C3(MA)gp to a similar extent regardless the sources of factor I. These results suggest that factor I-dependent C3b regulatory system is species-specific except in the case of CR1, which may function as a cofactor irrespective of species.

Animals↗
PubMed1995

The human factor H-related gene 2 (FHR2): structure and linkage to the coagulation factor XIIIb gene.

The human factor H-related gene 2 (FHR2) encodes a serum protein structurally and immunologically related to complement factor H. We describe the isolation and genomic organization of the human FHR2 gene from a yeast artificial chromosome library. The FHR2 gene is organized in five exons and spans about 7 kilobases (kb) of human genomic DNA. A comparison with the corresponding cDNA sequence (clone DDESK59) shows that the analyzed FHR2 gene has a deleted region within exon 4. A new splice acceptor site created in the truncated exon indicates that the analyzed gene could be translated to a truncated protein. Further, we demonstrate that the genes for FHR2 and beta subunit of coagulation factor XIII are located in the same 165 kb YAC DNA. Thus, the three structurally related genes FXIIIb, FHR2, and factor H are linked on human chromosome 1 in the regulators of complement activation (RCA) gene cluster. The physical linkage of the FHR2 and the factor H genes provides additional evidence for a close relatedness of complement factor H and the factor H-related proteins. The linkage and the almost exclusive organization in short consensus repeat-containing domains indicates a close evolutionary relationship of the FXIIIb, FHR2, and factor H genes.

Amino Acid Sequence↗
PubMed1992

Expression of complement components of the alternative pathway by glioma cell lines.

Glioma cell lines express proteins of the complement alternative pathway, namely C3, factor B, factor H, and factor I. Secretion of these proteins was shown by a sensitive and specific ELISA. C3 and factor H were rapidly secreted by glioma cell line CB193 and reached a concentration of 140 ng/ml/10(6) cells after 72 h of culture. Factor B and factor I were secreted at a lower rate and reached concentrations of 25 and 15 ng/ml/10(6) cells, respectively. Western blot and immunoprecipitation experiments showed that secreted proteins were identical to the corresponding plasma proteins. For factor H, besides the well known 150-kDa species, an additional polypeptide of 45 kDa with factor H immunoreactivity was observed. This species corresponded to the N-terminal truncated form found in plasma. In preliminary experiments, we observed control of these syntheses by cytokines. IL-1 beta significantly increased C3 secretion, with no effect on factor H. Secretion of factor H was enhanced by IFN-gamma. These results show that a glioma cell line could be a useful tool to study complement biosynthesis by glial cells in humans.

Complement C3↗
PubMed1983

Monoclonal antibodies against the complement control protein factor H (beta 1 H).

Two mouse monoclonal antibodies against the human complement control protein, Factor H (beta 1H), are described. The antibodies are both IgG - gamma 1 - subclass and are directed against different epitopes on the human Factor H molecule. One of the antibodies, MRC OX 24, increases the cofactor activity of Factor H in Factor I-mediated cleavage of soluble C3b. The second antibody, MRC OX 23, which has no effect alone, reduces the increase in cofactor activity observed in the presence of the first antibody. However, MRC OX 24 inhibits the binding of 125I-labelled Factor H to surface-bound C3b (EAC3b). Again MRC OX 23 alone does not have an effect but decreases the inhibition in 125I-labelled Factor H binding to EAC3b observed with MRC OX 24. These studies show clearly that the interaction of Factor H with soluble C3b is different to its interaction with surface-bound C3b. In an indirect immunoprecipitation system using these monoclonal antibodies, single-chain molecules of 150 000 mol.wt. are specifically precipitated from human serum and also from the sera of other primates - rhesus monkey, cynomolgus monkey, and African green monkey. There was no precipitation from sera of cow, pig, sheep, chick, or rabbit. Using a radioimmunoassay with radiolabelled monoclonal MRC OX 23, the concentration of Factor H in human plasma was determined.

Animals↗
PubMed1992

In vitro biosynthesis of complement factor I by human endothelial cells.

We have studied the secretion of the complement regulatory protein factor I by human umbilical vein endothelial cells (HUVEC). Northern and Western blot analysis and biosynthetic labeling experiments indicate that HUVEC secrete factor I at very low levels in basal conditions and that this secretion is significantly enhanced by interferon-gamma. Analysis of the proteolytic inactivation of C3b by HUVEC supernatants show that factor I is secreted in a functional form and can promote the specific proteolytic inactivation of C3b to iC3b. Together with previous studies establishing the secretion of complement factor H by HUVEC, this work demonstrates that the endothelial cell is able to secrete in its environment two complement regulatory proteins, factor I and factor H, which can mediate the degradation of C3b to iC3b. The secretion of factor I by HUVEC provides a useful in vitro model to analyze the modulation of this secretion and may be relevant to the local deposition of iC3b at the surface of the endothelium during the inflammatory reaction.

Cells, Cultured↗
PubMed1983

Activation of complement via the alternative pathway.

Activation of complement via the alternative pathway represents one means of natural resistance to infection because it is capable of neutralizing a wide variety of potential pathogens in the total absence of antibody. The pathway involves six serum proteins and possesses a unique amplification system capable of depositing large numbers of C3b molecules on the surfaces of activating particles. C3b deposition enhances phagocytosis and results in activation of the membrane attack pathway of complement. C3b attachment is covalent, arising from a reaction between an intramolecular thiolester bond in nascent C3b and nucleophiles such as hydroxyl groups on surface carbohydrates. The reactions that initiate C3b attachment are not specific interactions like those initiating other biological cascade systems, but involve slow, spontaneous hydrolysis of the thiolester bond in C3 and subsequent random deposition of C3b onto all nearby surfaces. Once bound, C3b is capable of discriminating between host-derived cells and activating particles. Recognition is evidenced by a lower affinity between activator-bound C3b and the complement control protein factor H. Measurements of the association constant between unbound, soluble C3b and factor H suggest that activator-bound C3b recognizes structures on activators that inhibit factor H binding.

Complement C3↗
PubMed1989

Factor C3f is a spasmogenic fragment released from C3b by factors I and H: the heptadeca-peptide C3f was synthesized and characterized.

C3f, a heptadeca-peptide having the amino acid sequence of NH2-Ser-Ser-Lys-Ile-Thr-His-Arg-Ile-His-Trp-Glu-Ser-Ala-Ser-Leu-Leu-Arg- COOH, is liberated during the catabolic degradation of C3b in serum. The amino acid sequence of C3f is known both from the cDNA-derived structure of C3 and from protein analysis after isolation of the natural factor. C3f was synthesized by solid phase peptide synthesis. Both natural and synthetic C3f had identical retention times by RP-18 high performance liquid chromatography (HPLC) analysis and the respective amino acid compositions agreed with the expected theoretical values. C3f, but not des-Arg-C3f, was weakly spasmogenic inducing contraction of guinea pig ileum at a level of 5-10 x 10(-6) M. Since C3f and C3a were cross-tachyphylactic, it was concluded that these two spasmogens compete for the same receptors. Both C3f and des-Arg-C3f at concns of 1-4 x 10(-4) M enhanced vascular permeability in guinea pig skin. These observations further suggest that C3f functionally resembles C3a anaphylatoxin. Formation of C3f in human serum following CVF activation of C3 could be demonstrated by radioimmunoassay (RIA). Digestion of C3f with purified human serum carboxypeptidase N produced C3f-desArg. These observations suggest that when serum complement protein C3 undergoes conversion to C3b, further degradation by Factors H and I readily generates C3f. C3f is a weak spasmogen that functions like C3a anaphylatoxin and C3f-desArg is a major metabolite in serum.

Amino Acid Sequence↗
PubMed1995

Role of the conserved C-repeat region of the M protein of Streptococcus pyogenes.

The surface-located M protein functions to protect Streptococcus pyogenes (the group A streptococcus) from phagocytosis by polymorphonuclear leukocytes. It has been suggested that this protection results from the ability of M protein to bind factor H, a serum protein that can inhibit the activation of complement. Among different serological variants of M protein, the C-repeat domain is highly conserved and is exposed on the bacterial surface. This domain has been implicated in binding to complement factor H and in M-protein-mediated adherence of streptococci to human keratinocytes in the cutaneous epithelium. In this study, we constructed an S. pyogenes mutant strain which expresses an M6 protein from which the entire C-repeat domain was deleted. As predicted, this mutant did not adhere well to human keratinocytes and was unable to bind to factor H. Unexpectedly, the mutant was able to survive and multiply in human blood. Therefore, while the binding of factor H and the facilitation of adherence to keratinocytes appear to involve recognition of the C-repeat domain, a region of the M-protein molecule distinct from the C-repeat domain confers upon S. pyogenes its ability to resist phagocytosis.

Alleles↗
PubMed1991

Secondary structure of a complement control protein module by two-dimensional 1H NMR.

The complement control protein (CCP) module (also known as the short consensus repeat) is a consensus sequence of about 60 amino acid residues which is thought to fold independently. It occurs over 140 times in more than 20 extracellular mosaic proteins including 12 proteins of the complement cascade. An isolated CCP module, the 16th repeat from human complement factor H, has been expressed in a yeast vector and shown to fold with the same pattern of disulfide bond formation as is seen in the native protein. Two-dimensional 600-MHz 1H NMR spectra of this module have been recorded at pH 3.3 and 6.0 and analyzed to permit determination of secondary structure in solution. The CCP module comprises two predominantly extended segments (Glu1-His13 and Ala17-Glu27), two segments of double-stranded antiparallel beta-sheet (Gly14-Val16 paired with Tyr31-Cys33 and Gly38-Asp40 paired with Ser57-Ile59), and a short piece of triple-stranded beta-sheet (Glu27-Thr30, Ile44-Leu48, and Lys51-Ser53). Turns occur at Asp22, Gly36, and Glu50, while Gly41-Ala43 appear to form a looped-out segment or bulge. This structure is compared with a secondary structure prediction made on the basis of an alignment scheme of 101 sequences for CCP modules [Perkins, S. J., Haris, P. I., Sim, R. B., & Chapman, D. (1988) Biochemistry 27, 4004-4012]--the experimentally determined secondary structure bears an overall resemblance to the predicted one but differs in the number and position of turns. Some of those amino acid residues which are highly conserved throughout the range of CCP modules appear to play a role in stabilizing the global fold.

Amino Acid Sequence↗
PubMed1995

Activation of the endothelium by IL-1 alpha and glucocorticoids results in major increase of complement C3 and factor B production and generation of C3a.

Constitutive secretion of complement C3 and factor B by the endothelial cell (EC) is lowered by therapeutic concentrations of glucocorticoids such as hydrocortisone or dexamethasone, whereas regulatory protein factor H production is increased by these hormones. In contrast, the proinflammatory cytokine IL-1 alpha has a stimulatory effect on C3 and factor B secretion by the endothelium and an inhibitory effect on factor H secretion. In this study, we examined the combined effect of IL-1 alpha and glucocorticoids on C3 and factor B expression by the endothelial cell. When dexamethasone or hydrocortisone were added to IL-1 alpha, significant potentialization of IL-1 alpha-induced stimulation of C3 and factor B production was observed, occurring at various concentrations of either stimuli. Dose-response experiments indicate that, in vitro, optimal concentrations are in the range of 10(-7) to 10(-5) M for dexamethasone and 50-200 U for IL-1 alpha. In contrast, dexamethasone counteracts, in an additive way, the inhibitory effect of IL-1 alpha on regulatory complement protein factor H production by EC. Such a potentialization between glucocorticoids and IL-1 alpha was not observed for another marker of endothelial activation, IL-1 alpha-induced stimulation of coagulation tissue factor expression. The association of glucocorticoids and IL-1 alpha therefore appears to be a specific and major stimulus for the secretion of complement C3 and factor B, two acute-phase proteins, by the endothelium. As a result of the in vitro endothelium stimulation by glucocorticoids and IL-1 alpha, C3a is generated in the vicinity of the endothelial cell. This study further suggests that complement activation, with its deleterious consequences, may result from the stimulation of endothelium in situations where high levels of IL-1 alpha and endogenous glucocorticoids coexist, such as in septic shock.

Cells, Cultured↗
PubMed1985

Regulation of immunoglobulin secretion by factor H of human complement.

As human B lymphocytes and macrophages carry surface receptors for Factor H (B1H), we investigated the possibility that this complement component regulates their function. Factor H inhibits immunoglobulin secretion by peripheral mononuclear cells (MNC) stimulated with pokeweed mitogen if present at the initiation of the cultures and at concentrations greater than 50 micrograms/ml. Factor H also inhibited stimulation and differentiation of purified B cells into immunoglobulin-secreting cells by Epstein-Barr virus (EBV). The inhibitory effect of Factor H was abrogated if anti-Factor H antibody was present in the cultures. EBV-transformed B-cell lines secreted less immunoglobulin if Factor H was present in the culture for at least 4 days. Culture of MNC with Factor H did not lead to the generation of suppressor T cells or macrophages. In contrast, Factor H did not cause proliferation of human peripheral total MNC or enriched T-cell or B-cell subpopulations. Also, Factor H did not inhibit the proliferation of MNC in response to several mitogens and antigens. Our results strongly indicate that Factor H is able to block human B-cell differentiation in vitro without blocking the proliferative ability of the cells. Factor H seems to act directly on the B cells through its receptor on their surface, since it inhibited T-dependent and T-independent B-cell differentiation but generated no suppressor cells.

Antibody-Producing Cells↗
PubMed1985

Prevention of immune precipitation by purified components of the alternative pathway.

The role of the alternative pathway in complement-mediated prevention of immune precipitation has been investigated by the use of BSA-anti-BSA immune complex (IC) purified components. For immune precipitation to be prevented all six alternative pathway components (C3, factors B D, P, H and I) were required. In the absence of one or both of the control proteins H and I, excessive fluid phase turnover of C3 occurred with precipitation of IC. Kinetic studies showed that in the presence of the control proteins, an initial phase of precipitation occurred, and was followed by a phase of resolubilization of IC. When the efficiency of classical and alternative pathways in the prevention of immune precipitation was compared it was found that the classical pathway proteins were more effective than the alternative pathway components. A reaction mixture containing the components of both pathways was no better than the classical pathway protein alone. 125I-C3 was bound to IC which had been rendered soluble in the presence of classical or alternative pathway components. A molar ratio of two molecules C3b:five molecules IgG was calculated. Other complement components which were bound to IC which had been formed in the presence of serum were C1q, C4, C2, C3, C5, P and H. Factors B and I were not detected. Our findings suggest that the alternative pathway is of secondary importance to the classical pathway in the prevention of immune precipitation.

Antigen-Antibody Complex↗
PubMed1987

Decay accelerating factor (DAF) peptide sequences share homology with a consensus sequence found in the superfamily of structurally related complement proteins and other proteins including haptoglobin, factor XIII, beta 2-glycoprotein I, and the IL-2 receptor.

Amino acid sequence data derived from tryptic peptides of the decay accelerating factor indicate that this complement regulatory protein contains a sequence with homology to the superfamily of structurally related complement proteins, including the C4 binding protein, factor H, complement receptor type 1, complement receptor type 2, Ba, C1r, and to their non-complement relatives, including beta 2-glycoprotein I, factor XIIIb, the alpha 1 chain of haptoglobin, and the interleukin 2 receptor. Identifying DAF as a member of the superfamily of structurally related complement proteins provides evidence that DAF may contain a functionally important C4b and C3b binding domain.

Amino Acid Sequence↗
PubMed1984

C-reactive protein inhibits pneumococcal activation of the alternative pathway by increasing the interaction between factor H and C3b.

We previously studied two alternative pathway activators, Streptococcus pneumoniae and positively charged liposomes, which react with C-reactive protein (CRP). Binding of CRP to these surfaces initiates classical pathway but blocks alternative pathway activation. In this study, we investigated the mechanism of this inhibition using S. pneumoniae, R36a. R36a were pretreated with CRP (CRP-R36a) or buffer and incubated with C2-deficient human serum to which 125I-labeled C3 had been added. The amount of specific 125I-C3 binding was decreased from 8200 mol/CFU on R36a to 2200 mol/CFU on CRP-R36a. In contrast, when the same experiment was performed with purified factors B, D, P, and C3, in the absence of regulatory proteins, specific 125I-C3 uptake was slightly lower on R36a (6100 mol/CFU) than on CRP-R36a (8100 mol/CFU). The ability of the fixed C3b to inactivate factor B in the presence of factor D was equivalent on the two surfaces. The binding of the regulatory factor H to C3b fixed to R36a and CRP-R36a was compared by using purified 125I-labeled factor H. The ratio of factor H bound to C3 bound was twofold greater on CRP-R36a than on R36a. This increase was found by using C2-deficient serum or purified factors B, D, P, and C3 to fix C3b to the surfaces. The ability of CRP to inhibit C3 binding to R36a was restored by the addition of factors H and I to factors B, D, P, and C3. These results indicate that CRP inhibits alternative pathway activation by increasing regulation of bound C3.

Binding, Competitive↗
PubMed1987

Anti-idiotypic antibodies in antisera against human C3 and factor H and their application in the enrichment of antibodies specific for H-binding domains of C3.

Antisera separately raised against C3 and factor H should contain antibodies against the binding domains by which these factors interact. Because these interacting domains are likely to be sterically complementary to each other, antibodies specific for these domains should also be sterically complementary or of anti-idiotypic specificity. To test this hypothesis, rabbit anti-human C3 IgG antibodies which bound to Sepharose-coupled rabbit anti-factor H IgG (anti-H-binding anti-C3) were separated by affinity chromatography. In a control experiment, the anti-human factor H column was found to bind 10 times more anti-C3 antibodies than a Sepharose column coupled with nonimmune rabbit IgG. The binding specificity of the control eluate was indistinguishable from the original anti-C3 preparation, whereas the anti-H-binding anti-C3 was shown to be enriched in regard to antibodies directed against the 42 kd fragment of the C3 alpha-chain of C3c, as demonstrated by immunoblotting of electrophoretically separated polypeptides of C3c and C3d in polyacrylamide gels. In competition binding studies, factor H was 25 times more potent as inhibitor of the anti-H-binding anti-C3 than of the original anti-C3 preparation. This similarity in binding specificity also paralleled a functional similarity in that anti-H-binding anti-C3 in the presence of factor I effected a 20% degradation of the alpha'-chain of C3b into fragments of 65, 45, and 42 kd which are the normal degradation products in the presence of factors I and H. These results suggest that the affinity procedure described resulted in the enrichment of anti-C3 antibodies with selective specificities for factor H-binding domains of C3, from a polyclonal antibody source.

Antibody Specificity↗
PubMed1984

C3b covalently bound to IgG demonstrates a reduced rate of inactivation by factors H and I.

We have prepared C3b covalently linked to IgG via a hydroxylamine-sensitive bond between the C3b alpha' chain and sites predominantly, but not exclusively, located in the IgG heavy chain. This C3b species displays relative resistance to inactivation by factors H and I when compared with free C3b. This resistance appears to be due entirely to reduced affinity of C3b-IgG for factor H. Resistance to inactivation is not conferred on C3b by binding to another serum glycoprotein of similar size, ceruloplasmin, and may be a special property of IgG. C3b-IgG demonstrates an enhanced capacity to consume serum C3 relative to C3b. These alterations of the behavior of C3b when bound to IgG may in part explain the augmentation of alternative pathway activity by IgG. In addition, IgG-induced protection of C3b might influence both complement-mediated killing and phagocytosis of bacteria, as well as modify the in vivo handling of IgG-containing soluble immune complexes.

Ceruloplasmin↗
PubMed1985

Factor I co-factor activity of CR1 overcomes the protective effect of IgG on covalently bound C3b residues.

We have shown previously that C3b resides in a protected site when it is covalently bound to IgG (C3b-IgG). Such C3b displays a reduced affinity for factor H, with consequent enhanced survival in the presence of factors H and I and increased capacity for promoting alternative pathway consumption of C3. Because erythrocyte CR1 may be a major co-factor for factor I-mediated inactivation of immune complex-borne C3b in blood, we have examined the effect of covalently bound IgG on the C3b-CR1 interaction. Binding of monomeric C3b and C3b-IgG to human erythrocyte CR1 demonstrates identical ionic strength dependence for both species. Identical numbers of binding sites with indistinguishable affinities are detected by both ligands. Cleavage of the alpha'-chain of C3b and the alpha'-heavy chain of C3b-IgG proceeds at the same rate when erythrocyte CR1 serves as co-factor for factor I. Unlike factor H, CR1 supports a second cleavage of fluid-phase iC3b alpha'1 chain (free or bound to IgG) that generates C3c and a 33,000 m.w. fragment, which bears antigenic markers characteristic of C3g. Inactivation of C3b and C3b-IgG by CR1 and factor I also occurs at physiologic ionic strength, but proceeds very slowly relative to rates attainable with sub-physiologic inputs of factor H. CR1 does not recognize IgG-bound C3b as being in a protected site but, because of low binding affinity at physiologic ionic strength, is probably highly dependent on multivalent ligand-receptor interactions to efficiently exert its co-factor functions. Thus, inactivation of C3b-IgG heterodimers or small immune complexes bearing limited numbers of C3b residues may remain largely factor H-dependent in vivo, with resultant enhanced C3b survival.

Antigen-Antibody Complex↗
PubMed1986

Murine protein H is comprised of 20 repeating units, 61 amino acids in length.

A cDNA library constructed from size-selected (greater than 28 S) poly(A)+ RNA isolated from the livers of C57B10. WR mice was screened by using a 249-base-pair (bp) cDNA fragment encoding 83 amino acid residues of human protein H as a probe. Of 120,000 transformants screened, 30 hybridized with this cDNA probe. Ten positives were colony-purified, and the largest plasmid cDNA insert, MH8 (4.4 kb), was sequenced by the dideoxy chain termination method. MH8 contained the complete coding sequence for the precursor of murine complement protein factor H (3702 bp), 100 bp of 5'-untranslated sequence, 448 bp of 3'-untranslated sequence, and a polyadenylylated tail of undetermined length. Murine pre-protein H was deduced to consist of an 18-amino acid signal peptide and 1216 residues of H-protein sequence. Murine H was composed of 20 repetitive units, each about 61 amino acid residues in length. Similar repetitive units are present in the C4b binding protein, the C3b-receptor (CR1), complement factor B and C2, and in beta 2-glycoprotein I and the interleukin 2 receptor. This finding suggests a common evolutionary origin for regions of these proteins.

Amino Acid Sequence↗