Factor H

Trypanosoma cruzi Evades the Complement System as an Efficient Strategy to Survive in the Mammalian Host: The Specific Roles of Host/Parasite Molecules and Trypanosoma cruzi Calreticulin

American Trypanosomiasis is an important neglected reemerging tropical parasitism, infecting about 8 million people worldwide. Its agent, Trypanosoma cruzi, exhibits multiple mechanisms to evade the host immune response and infect host cells. An important immune evasion strategy of T. cruzi infective stages is its capacity to inhibit the complement system activation on the parasite surface, avoiding opsonizing, immune stimulating and lytic effects. Epimastigotes, the non-infective form of the parasite, present in triatomine arthropod vectors, are highly susceptible to complement-mediated lysis while trypomastigotes, the infective form, present in host bloodstream, are resistant. Thus T. cruzi susceptibility to complement varies depending on the parasite stage (amastigote, trypomastigotes or epimastigote) and on the T. cruzi strain. To avoid complement-mediated lysis, T. cruzi trypomastigotes express on the parasite surface a variety of complement regulatory proteins, such as glycoprotein 58/68 (gp58/68), T. cruzi complement regulatory protein (TcCRP), trypomastigote decay-accelerating factor (T-DAF), C2 receptor inhibitor trispanning (CRIT) and T. cruzi calreticulin (TcCRT). Alternatively, or concomitantly, the parasite captures components with complement regulatory activity from the host bloodstream, such as factor H (FH) and plasma membrane-derived vesicles (PMVs). All these proteins inhibit different steps of the classical (CP), alternative (AP) or lectin pathways (LP). Thus, TcCRP inhibits the CP C3 convertase assembling, gp58/68 inhibits the AP C3 convertase, T-DAF interferes with the CP and AP convertases assembling, TcCRT inhibits the CP and LP, CRIT confers ability to resist the CP and LP, FH is used by trypomastigotes to inhibit the AP convertases and PMVs inhibit the CP and LP C3 convertases. Many of these proteins have similar molecular inhibitory mechanisms. Our laboratory has contributed to elucidate the role of TcCRT in the host-parasite interplay. Thus, we have proposed that TcCRT is a pleiotropic molecule, present not only in the parasite endoplasmic reticulum, but also on the trypomastigote surface, participating in key processes to establish T. cruzi infection, such as inhibition of the complement system and serving as an important virulence factor. Additionally, TcCRT interaction with key complement components, participates as an anti-angiogenic and anti-tumor molecule, inhibiting at least in important part, tumor growth in infected animals.

Further structural insights into the binding of complement factor H by complement regulator-acquiring surface protein 1 (CspA) of Borrelia burgdorferi

Borrelia burgdorferi has evolved many mechanisms of evading the different immune systems across its range of reservoir hosts, including the capture and presentation of host complement regulators factor H and factor H-like protein-1 (FHL-1). Acquisition is mediated by a family of complement regulator-acquiring surface proteins (CRASPs), of which the atomic structure of CspA (BbCRASP-1) is known and shows the formation of a homodimeric species which is required for binding. Mutagenesis studies have mapped a putative factor H binding site to a cleft between the two subunits. Presented here is a new atomic structure of CspA which shows a degree of flexibility between the subunits which may be critical for factor H scavenging by increasing access to the binding interface and allows the possibility that the assembly can clamp around the bound complement regulators.

ErpC, a member of the complement regulator-acquiring family of surface proteins from Borrelia burgdorferi, possesses an architecture previously unseen in this protein family

Borrelia burgdorferi is a spirochete responsible for Lyme disease, the most commonly occurring vector-borne disease in Europe and North America. The bacterium utilizes a set of proteins, termed complement regulator-acquiring surface proteins (CRASPs), to aid evasion of the human complement system by recruiting and presenting complement regulator factor H on its surface in a manner that mimics host cells. Presented here is the atomic resolution structure of a member of this protein family, ErpC. The structure provides new insights into the mechanism of recruitment of factor H and other factor H-related proteins by acting as a molecular mimic of host glycosaminoglycans. It also describes the architecture of other CRASP proteins belonging to the OspE/F-related paralogous protein family and suggests that they have evolved to bind specific complement proteins, aiding survival of the bacterium in different hosts.

Contribution of the infection-associated complement regulator-acquiring surface protein 4 (ErpC) to complement resistance of Borrelia burgdorferi

Borrelia burgdorferi evades complement-mediated killing by interacting with complement regulators through distinct complement regulator-acquiring surface proteins (CRASPs). Here, we extend our analyses to the contribution of CRASP-4 in mediating complement resistance of B. burgdorferi and its interaction with human complement regulators. CRASP-4 (also known as ErpC) was immobilized onto magnetic beads and used to capture proteins from human serum. Following Western blotting, factor H (CFH), CFH-related protein 1 (CFHR1), CFHR2, and CFHR5 were identified as ligands of CRASP-4. To analyze the impact of native CRASP-4 on mediating survival of serum-sensitive cells in human serum, a B. garinii strain was generated that ectopically expresses CRASP-4. CRASP-4-producing bacteria bound CFHR1, CFHR2, and CFHR5 but not CFH. In addition, transformed spirochetes deposited significant amounts of lethal complement components on their surface and were susceptible to human serum, thus indicating that CRASP-4 plays a subordinate role in complement resistance of B. burgdorferi.

Complement control protein factor H: the good, the bad, and the inadequate

The complement system is an essential component of the innate immune system that participates in elimination of pathogens and altered host cells and comprises an essential link between the innate and adaptive immune system. Soluble and membrane-bound complement regulators protect cells and tissues from unintended complement-mediated injury. Complement factor H is a soluble complement regulator essential for controlling the alternative pathway in blood and on cell surfaces. Normal recognition of self-cell markers (i.e. polyanions) and C3b/C3d fragments is necessary for factor H function. Inadequate recognition of host cell surfaces by factor H due to mutations and polymorphisms have been associated with complement-mediated tissue damage and disease. On the other hand, unwanted recognition of pathogens and altered self-cells (i.e. cancer) by factor H is used as an immune evasion strategy. This review will focus on the current knowledge related to these versatile recognition properties of factor H.

Acquisition of factor H by a novel surface protein on group B Streptococcus promotes complement degradation

Binding of the host complement regulator, factor H (FH), by some pathogenic microbes constitutes an important virulence mechanism, whereby complement is broken down to help microbes survive in the host. Although it has been hypothesized for the past two decades that GBS type III binds FH via sialic acid present on its capsule, neither the binding of FH to GBS has been demonstrated nor the mechanism of interaction identified. We observed that FH bound to both wild-type and capsule or sialic acid-deficient GBS that were used as negative controls. Wild-type and acapsular GBS were incubated with serum or pure FH degraded almost 90% of C3b, suggesting that the GBS-bound FH maintained cofactor activity. In addition, dot-blot analysis showed approximately 5-10% of C5 and C9 formation, as compared to an Escherichia coli control, suggesting breakdown at the C3b level. Protease treatment of the bacteria completely abolished binding of FH. Using overlay assays and mass spectroscopic analysis, we identified the FH receptor as the streptococcal histidine triad (SHT) surface protein. The ability of binding FH to SHT was further confirmed by using recombinant SHT. This report describes the identification of the SHT as an FH-binding protein on the surface of GBS type III, revealing a novel mechanism by which the bacterium acquires FH to evade complement opsonization.

C-Reactive protein binds to apoptotic cells, protects the cells from assembly of the terminal complement components, and sustains an antiinflammatory innate immune response: implications for systemic autoimmunity

C-reactive protein (CRP) is a serum protein that is massively induced as part of the innate immune response to infection and tissue injury. As CRP has been detected in damaged tissues and is known to activate complement, we assessed whether apoptotic lymphocytes bound CRP and determined the effect of binding on innate immunity. CRP bound to apoptotic cells in a Ca(2+)-dependent manner and augmented the classical pathway of complement activation but protected the cells from assembly of the terminal complement components. Furthermore, CRP enhanced opsonization and phagocytosis of apoptotic cells by macrophages associated with the expression of the antiinflammatory cytokine transforming growth factor beta. The antiinflammatory effects of CRP required C1q and factor H and were not effective once cells had become necrotic. These observations demonstrate that CRP and the classical complement components act in concert to promote noninflammatory clearance of apoptotic cells and may help to explain how deficiencies of the classical pathway and certain pentraxins lead to impaired handling of apoptotic cells and increased necrosis with the likelihood of immune response to self.

Nephritogenic λ Light Chain Dimer: A Unique Human Miniautoantibody Against Complement Factor H

A unique monoclonal Ig λ light chain dimer (protein LOI) was isolated from the serum and urine of a patient with hypocomplementemic membranoproliferative glomerulonephritis. In vitro the λ light chain dimer efficiently activated the alternative pathway of complement (AP). When added to normal human serum, LOI temporarily enhanced AP hemolytic activity, but during a prolonged incubation the hemolytic activity was depleted. Protein LOI was found to bind to factor H, the main regulator molecule of AP. By binding to the short consensus repeat domain 3 of factor H, the dimer LOI blocked one of three interaction sites between H and C3b and thus inhibited the activity of H and induced an uncontrolled activation of the AP. Structural analysis showed that LOI belonged to the Vλ3a subgroup of λ light chains. The variable (V) region of LOI was most closely related to the predicted product of the Vλ3 germline gene Iglv3s2, although it contained several unique residues that in a tertiary homology model structure form an unusual ring of charged residues around a hydrophobic groove in the putative Ag binding site. This site fitted considerably well with a putative binding site in the molecular model of domain 3 of factor H containing a reciprocal ring of charged amino acids around a hydrophobic area. Apparently, functional blocking of factor H by the Ab fragment-like λ light chain dimer had initiated the development of a severe form of membranoproliferative glomerulonephritis. Thus, the λ light chain dimer LOI represents the first described pathogenic miniautoantibody in human disease.

The factor H protein family

The factor H gene family provides a prime example of a multidomain multifunctional protein family whose individual members are defined by conserved common structural elements and display diverse but often overlapping functions. The six identified members of this protein family represent secreted plasma proteins that are primarily synthesized in the liver. Here, we summarize the current understanding of the function of these proteins and suggest a common role in complement control. Factor H is the best characterized member and acts as a complement regulator. The protein displays cofactor activity for factor I in the degradation of the central complement component C3b, acts as a decay accelerating factor for the C3 convertase, C3bBb and is a competitor for factor B binding to C3b. Factor H is a multifunctional protein and displays functions outside the complement system: it binds to the cellular integrin receptor (CD11b/CD18), interacts with cell surface glycosaminoglycans and also binds to the surface of certain pathogenic microorganisms. In addition, factor H has several binding sites for the C3 protein. The factor H-like protein 1 (FHL-1) or reconectin shares the complement regulatory functions with factor H and interacts with heparin. The protein displays cell spreading activity and binds to the N-terminus of the streptococcal M protein. The function of the factor H-related proteins (FHR-1 to FHR-4) is currently under investigation. These proteins are differently distributed. Three proteins (FHR-1, FHR-2 and FHR-4) are constituents of lipoproteins, while FHR-3 interacts with heparin. Binding to C3b and C3d has been demonstrated for FHR-3 and FHR-4 and the two proteins display a cofactor related activity.

Regulation of complement activation by C-reactive protein

C-reactive protein (CRP) is an acute-phase serum protein and a mediator of innate immunity. CRP binds to microbial polysaccharides and to ligands exposed on damaged cells. Binding of CRP to these substrates activates the classical complement pathway leading to their uptake by phagocytic cells. Complement activation by CRP is restricted to C1, C4, C2 and C3 with little consumption of C5-9. Surface bound CRP reduces deposition of and generation of C5b-9 by the alternative pathway and deposition of C3b and lysis by the lectin pathway. These activities of CRP are the result of recruitment of factor H resulting in regulation of C3b on bacteria or erythrocytes. Evidence is presented for direct binding of H to CRP. H binding to CRP or C3b immobilized on microtiter wells was demonstrated by ELISA. Attachment of CRP to a surface was required for H binding. H binding to CRP was not inhibited by EDTA or phosphocholine, which inhibit ligand binding, but was inhibited by a 13 amino acid CRP peptide. The peptide sequence was identical to the region of CRP that showed the best alignment to H binding peptides from Streptococcus pyogenes (M6) and Neisseria gonorrhoeae (Por1A). The results suggest that CRP bound to a surface provides secondary binding sites for H resulting in greater regulation of alternative pathway amplification and C5 convertases. Complement activation by CRP may help limit the inflammatory response by providing opsonization with minimal generation of C5a and C5b-9.

Identification of the Second Heparin-Binding Domain in Human Complement Factor H

Complement factor H (fH) regulates activation of the alternative pathway of C, reducing the amount of C3b deposited on sialic acid-rich surfaces. Heparin binding has been used as a model for examining the sialic acid-binding characteristics of fH. We have previously shown that of the 20 short consensus repeat (SCR) modules of fH, SCR 7 contains an important heparin binding site, but other SCRs also play a role in heparin binding. To localize the other sites, we prepared recombinant truncated and SCR deletion mutants of fH and tested them by heparin-agarose affinity chromatography. The 5 C-terminal SCRs were found to contain a heparin binding site as an SCR 7 deletion mutant of the N terminal 15 SCRs did not bind heparin, but a construct consisting of SCRs 16–20 was shown to bind heparin. Double deletion of SCRs 7 and 20 from fH abrogated binding to heparin, indicating that SCR 20 contains a heparin binding site. This finding was confirmed with the observation that attachment of SCR 20 to a group of nonbinding SCRs produced a heparin-binding protein. A protein consisting of SCRs 19 and 20 did not bind heparin, whereas SCRs 18–20 did, indicating that, although SCR 20 contains a heparin binding site, at least two nonspecific adjacent SCRs are required. fH-related protein-3 (FHR-3) possesses an SCR homologous to SCR 7 of fH and bound heparin, whereas FHR-4, which lacks such an SCR, did not. Thus, fH contains two separate heparin binding sites, which are located in SCRs 7 and 20.

The C-terminus of factor H: monoclonal antibodies inhibit heparin binding and identify epitopes common to factor H and factor H-related proteins

We have generated monoclonal antibodies (mAbs) specific for the C-terminus of factor H that can be used as inhibitory antibodies for heparin binding and for the specific detection of factor H and factor H-related proteins (FHRs) in plasma and triacylglycerol-rich lipoproteins. Four distinct mAbs were established: IXF9 (IgG1), VD3 (IgG2a), VIG8 (IgG1) and IIC5 (IgG1). Each reacts specifically with FHR-1 and factor H (and also with FHR-2 in the case of VIG8), but none binds to the related FHR-3 and FHR-4 proteins nor to factor H-like protein 1. By the use of deletion mutants of factor H and by comparing the reactivity with FHR-1 and FHR-2, the binding epitopes of the mAbs were identified and localized to different short consensus repeats (SCRs): mAbs IXF9 and VD3 bind to related or even identical sites within SCR18 (factor H) and SCR3 (FHR-1) respectively. mAbs VIG8 and IIC5 bind to different epitopes located within SCRs 19 to 20 of factor H and SCRs 4 to 5 of FHR-1 respectively. Only mAb VIG8 reacts with the corresponding SCRs 3 to 4 of FHR-2. These antibodies are useful for the detection of the corresponding proteins in biological specimens such as fractions of lipoproteins. In addition, mAb VIG8 has the unique feature of inhibiting binding of factor H to heparin. Given the recent identification of a heparin- and a C3b-binding domain within the C-terminus of factor H, these mAbs should provide useful tools for functional analysis and for the precise localization of the domain(s) required for this interaction.

Mapping of the domains required for decay acceleration activity of the human factor H-like protein 1 and factor H

The human factor H-like protein 1 (FHL-1) is composed of seven repetitive elements (short consensus repeats; SCR) that are identical in sequence to the seven N-terminal SCR of complement factor H. We show that the FHL-1 protein has decay acceleration activity in that it can dissociate C3/C5-convertases bound to the surface of sheep red blood cells. The same activity was also determined for factor H. However, compared to FHL-1, factor H was more efficient in decay acceleration, as about 100-fold less protein was required for a 50% inhibition of activity. The domain required for decay accelerating activity of FHL-1 and factor H was mapped by the use of recombinant fragments. FHL-1 and a series of truncated forms of the protein were expressed in the baculovirus system. Recombinant FHL-1 and all mutants which include SCR 1-4 were functionally active. These four N-terminal SCR are essential and sufficient for activity, as deletion mutants which lack SCR 1 or SCR 4 showed no activity. These results demonstrate that FHL-1 and factor H have identical and overlapping regulatory functions in the complement system and that the domain required for this activity is located in the overlapping region of both proteins within the N-terminal four SCR.

Contribution of genes from the capsule gene complex (cps) to lipooligosaccharide biosynthesis and serum resistance in Neisseria meningitidis

Within the capsule gene complex (cps) of Neisseria meningitidis B a 5.5 kb DNA fragment encodes proteins with strong homologies to enzymes of the lipopolysaccharide biosynthetic pathway of Salmonella typhimurium and Escherichia coli, GalE, RfbB, RfbC and RfbD. A meningococcal galE mutant expressed a truncated lipooligosaccharide (LOS), which terminated at the glucose residue between inner and outer core, and a second galE gene present outside the cps cluster was found to be transcriptionally and functionally inactive and, thus, unable to complement this defect. Because of the defect in the outer core, the LOS of the galE-defective meningococcal mutant was not sialylated. In contrast, carbohydrate analysis of the LOS of an rfb-defective meningococcal mutant revealed no difference from the LOS of the wild-type strain, suggesting that the rfb genes are inactive. This was supported by Northern blot analysis, which showed that expression of the rfb gene products was transcriptionally regulated. The inability of the meningococcal galE mutant, which cannot sialylate the LOS, allowed us to investigate the significance of LOS sialylation in relation to the presence of the polysialic acid capsule. Sialylated LOS, but not the polysialic acid capsule, is necessary to confer complete serum resistance on the meningococcus by inhibition of the alternative complement pathway.

Effect of complement-protein-C3b density on the binding of complement factor H to surface-bound C3b

Various amounts of the activation fragment C3b of the complement (C) protein C3 were coupled to Sepharose 4B by catalysis with the C3 convertase of the alternative pathway of C. The binding of radioactively labelled C proteins B and H (= factor H) to the C3b-carrying particles was assayed. It was found that the relative binding of H, but not of B, fell rapidly with decreasing densities of solid-phase C3b, suggesting a sigmoidal relationship between C3b density and binding of H. To study the phenomenon in more detail, preformed C3b was coupled to activated thiopropyl-Sepharose 6B at various densities. By using this model system, it was shown that the binding of H/unit amount of C3b was positively correlated to C3b density up to a C3b concentration of about 0.5 mg/ml of gel, whereas binding of B was independent of C3b density. The results show that accumulation of high densities of C3b on a surface creates high-affinity binding sites for H. Because H has recently been shown to form dimers in solution, the interaction of dimeric H with neighbouring C3b molecules is a likely explanation for the phenomenon. The C3b density effect may be a regulatory mechanism keeping the activation of the alternative pathway of C on activating surface within reasonable limits.