Regulation of C3 deposition on gp120 coated CD4 positive cells by decay accelerating factor and factor H

Mycoplasma hyopneumoniae evades complement activation by binding to factor H via elongation factor thermo unstable (EF-Tu)

Mycoplasmas persist in the host for a long time, suggesting that they possess mechanisms for immune evasion. Factor H is a negative regulator of the complement system, which binds to host cells to avoid unexpected complement activation. In this study, we revealed that many mycoplasmas, such as Mycoplasma hyopneumoniae, Mycoplasma hyorhinis, Mycoplasma hyosynoviae, Mycoplasma gallisepticum, Mycoplasma pneumoniae, Mycoplasma genitalium, Mycoplasma flocculare, and Mycoplasma bovis could hijack factor H such that they present themselves as a host tissue and thus escape from complement attack. Furthermore, the mechanism of recruiting factor H was identified in M. hyopneumoniae. M. hyopneumoniae binds factor H via factor H binding proteins, such as elongation factor thermo unstable (EF-Tu), P146, pyruvate dehydrogenase (acetyl-transferring) E1 component subunit alpha (PdhA), P46, Pyruvate dehydrogenase E1 component subunit beta (PdhB), glyceraldehyde-3-phosphate dehydrogenase (GAPDH), and three different hypothetical proteins. The binding of factor H by EF-Tu further contributes to decreased C3 deposition on the M. hyopneumoniae surface and ultimately blocks further complement activation. In fact, binding of factor H occurs in a multifactorial manner; factor H is not only exploited by M. hyopneumoniae via its regulator activity to help mycoplasmas escape from complement killing, but also increases M. hyopneumoniae adhesion to swine tracheal epithelial cells, partially through EF-Tu. Meanwhile, the high sequence identity among EF-Tu proteins in the above-mentioned mycoplasmas implied the universality of the mechanism. This is the first report that mycoplasmas can escape complement killing by binding to factor H.

Dengue virus and the complement alternative pathway

Dengue disease is an inflammatory-driven pathology, and complement overactivation is linked to disease severity and vascular leakage. Additionally, dysregulation of complement alternative pathway (AP) components has been described, such as upregulation of complement factor D and downregulation of complement factor H (FH), which activate and inhibit the AP, respectively. Thus, the pathology of severe dengue could in part result from AP dysfunction, even though complement and AP activation usually provide protection against viral infections. In dengue virus-infected macrophages and endothelial cells (ECs), the site of replication and target for vascular pathology, respectively, the AP is activated. The AP activation, reduced FH and vascular leakage seen in dengue disease in part parallels other complement AP pathologies associated with FH deficiency, such as atypical haemolytic uraemic syndrome (aHUS). aHUS can be therapeutically targeted with inhibitors of complement terminal activity, raising the idea that strategies such as inhibition of complement or delivery of FH or other complement regulatory components to EC may be beneficial to combat the vascular leakage seen in severe dengue.

Human factor H interacts selectively with Neisseria gonorrhoeae and results in species-specific complement evasion

Complement forms a key arm of innate immune defenses against gonococcal infection. Sialylation of gonococcal lipo-oligosaccharide, or expression of porin 1A (Por1A) protein, enables Neisseria gonorrhoeae to bind the alternative pathway complement inhibitor, factor H (fH), and evade killing by human complement. Using recombinant fH fragment-murine Fc fusion proteins, we localized two N. gonorrhoeae Por1A-binding regions in fH: one in complement control protein domain 6, the other in complement control proteins 18-20. The latter is similar to that reported previously for sialylated Por1B gonococci. Upon incubation with human serum, Por1A and sialylated Por1B strains bound full-length human fH (HufH) and fH-related protein 1. In addition, Por1A strains bound fH-like protein 1 weakly. Only HufH, but not fH from other primates, bound directly to gonococci. Consistent with direct HufH binding, unsialylated Por1A gonococci resisted killing only by human complement, but not complement from other primates, rodents or lagomorphs; adding HufH to these heterologous sera restored serum resistance. Lipo-oligosaccharide sialylation of N. gonorrhoeae resulted in classical pathway regulation as evidenced by decreased C4 binding in human, chimpanzee, and rhesus serum but was accompanied by serum resistance only in human and chimpanzee serum. Direct-binding specificity of HufH only to gonococci that prevents serum killing is restricted to humans and may in part explain species-specific restriction of natural gonococcal infection. Our findings may help to improve animal models for gonorrhea while also having implications in the choice of complement sources to evaluate neisserial vaccine candidates.

Functional significance of factor H binding to Neisseria meningitidis

Neisseria meningitidis is an important cause of septicemia and meningitis. To cause disease, the bacterium must successfully survive in the bloodstream where it has to avoid being killed by host innate immune mechanisms, particularly the complement system. A number of pathogenic microbes bind factor H (fH), the negative regulator of the alternative pathway of complement activation, to promote their survival in vivo. In this study, we show that N. meningitidis binds fH to its surface. Binding to serogroups A, B, and C N. meningitidis strains was detected by FACS and Far Western blot analysis, and occurred in the absence of other serum factors such as C3b. Unlike Neisseria gonorrhoeae, binding of fH to N. meningitidis was independent of sialic acid on the bacterium, either as a component of its LPS or its capsule. Characterization of the major fH binding partner demonstrated that it is a 33-kDa protein; examination of insertion mutants showed that porins A and B, outer membrane porins expressed by N. meningitidis, do not contribute significantly to fH binding. We examined the physiological consequences of fH bound to the bacterial surface. We found that fH retains its activity as a cofactor of factor I when bound to the bacterium and contributes to the ability of N. meningitidis to avoid complement-mediated killing in the presence of human serum. Therefore, the recruitment of fH provides another mechanism by which this important human pathogen evades host innate immunity.

A protein with characteristics of factor H is present on rodent platelets and functions as the immune adherence receptor

Complement-coated particles interact with specific immune adherence receptors (IAR). In primates, this function is served by complement receptor 1 (CR1) on erythrocytes. In contrast, rodent platelets bear IAR distinct from CR1, the identity of which was studied here. A 150-kDa C3b-binding protein was isolated from rat platelets, which had immunochemical and biochemical identity to plasma factor H. Immunofluorescence microscopy and flow cytometry demonstrated that factor H was present on the surface of rat and mouse platelets, which could be removed by treatment with neuraminidase. Sheep erythrocytes bearing C3b underwent immune adherence with rat and mouse platelets, which was blocked with anti-factor H F(ab')(2) antibodies, but not with antibodies binding to the complement regulator, Crry, on the platelet surface. By reverse transcription-polymerase chain reaction using rat platelet RNA and primers designed from mouse factor H, a 472-base pair product was generated that was identical in sequence to that produced from rat liver RNA. The translated protein product was 85% similar to mouse liver factor H. The 3'-nucleotide sequence from platelets predicted a soluble factor H protein. By Northern analysis, liver and platelets had identically sized factor H mRNA. Thus, rat and mouse platelets have a membrane protein with characteristics of factor H that is linked via sialic acid residues and functions as the IAR. Whether platelet factor H is acquired by passive adsorption from sera and/or is produced by platelets remains to be determined.

Multiple ligand binding sites on domain seven of human complement factor H

Foreign particles and damaged host cells can activate the complement system leading to their destruction by the host defense system. Factor H (fH) plays a vital role in restricting complement activation on host cells through interactions with polyanions such as heparin, while allowing activation to proceed on foreign surfaces. Complement activation by damaged host cells is also down regulated by fH, which is localized to injured areas through interactions with C-reactive protein (CRP). A number of pathogens have developed mechanisms by which they can also bind fH and thus exploit its protective properties. One such organism is Group A Streptococcus (GAS) which mediates fH binding via its surface expressed M-protein. fH consists of 20 conserved short consensus repeat (SCR) units and mutagenesis studies indicate that the seventh repeat is responsible for interactions with heparin, CRP and M-protein. We recently performed molecular modelling of fH SCR 7 and identified a cluster of positively charged residues on one face of the domain. By alanine replacement mutagenesis, we demonstrated that these residues are involved in heparin, CRP and M protein binding, which indicates that there is a common site within fH SCR 7 responsible for multiple ligand recognition.

The contrasting mechanisms of serum resistance of Neisseria gonorrhoeae and group B Neisseria meningitidis

Neisseria gonorrhoeae and Neisseria meningitidis have evolved intricate mechanisms to evade complement-mediated killing. Sialylation of gonococcal lipooligosaccharide (LOS) results in conversion of previously serum sensitive strains to unstable serum resistance, which is mediated by factor H binding. Porin (Por) is also instrumental in mediating stable serum resistance in gonococci. The 5th loop of certain gonococcal PorlAs binds factor H, which efficiently inactivates C3b to iC3b. Factor H glycan residues may be essential for factor H binding to certain Por1A strains. Por1A strains can also regulate the classical pathway by binding to C4b-binding protein (C4bp) probably via the 1st loop of the Por molecule. Certain serum resistant Por1 B strains can also regulate complement by binding C4bp through a loop other than loop 1. Purified C4b can inhibit binding of C4bp to Por 1B, but not Por1A, suggesting different binding sites on C4bp for the two Por types. Unlike serum resistant gonococci, resistant meningococci have abundant C3b on their surface, which is only partially processed to iC3b. The main mechanism of complement evasion by group B meningococci is inhibition of membrane attack complex (MAC) insertion by their polysaccharide capsule. LOS structure may act in concert with capsule to prevent MAC insertion. Meningococcal strains with Class 3 Por preferentially bind factor H, suggesting Class 3 Por acts as a receptor for factor H.

Binding of complement factor H to loop 5 of porin protein 1A: a molecular mechanism of serum resistance of nonsialylated Neisseria gonorrhoeae

Neisseria gonorrhoeae isolated from patients with disseminated infection are often of the porin (Por1A) serotype and resist killing by nonimmune normal human serum. The molecular basis of this resistance (termed stable serum resistance) in these strains has not been fully defined but is not related to sialylation of lipooligosaccharide. Here we demonstrate that Por1A bearing gonococcal strains bind more factor H, a critical downregulator of the alternative complement pathway, than their Por1B counterparts. This results in a sevenfold reduction in C3b, which is >75% converted to iC3b. Factor H binding to isogenic gonococcal strains that differed only in their porin serotype, confirmed that Por1A was the acceptor molecule for factor H. We identified a surface exposed region on the Por1A molecule that served as the binding site for factor H. We used gonococcal strains with hybrid Por1A/B molecules that differed in their surface exposed domains to localize the factor H binding site to loop 5 of Por1A. This was confirmed by inhibition of factor H binding using synthetic peptides corresponding to the putative exposed regions of the porin loops. The addition of Por1A loop 5 peptide in a serum bactericidal assay, which inhibited binding of factor H to the bacterial surface, permitted 50% killing of an otherwise completely serum resistant gonococcal strain. Collectively, these data provide a molecular basis to explain serum resistance of Por1A strains of N. gonorrhoeae.

Susceptibility to HIV infection and progression of AIDS in relation to variant alleles of mannose-binding lectin

BACKGROUND

Low serum concentrations of mannose-binding lectin (MBL) are associated with increased susceptibility to recurrent infection. Three variant alleles in the MBL gene (B, C, and D), cause low serum concentrations of the protein. We investigated whether variant alleles of MBL affect susceptibility to infection with HIV and progression of AIDS.

METHODS

Between 1983 and 1986, all men who attended two clinics in Copenhagen for HIV screening were invited to take part in our study. We investigated the prevalence of variant alleles of MBL (detected by PCR) and assessed the prognostic value of these alleles and the corresponding serum MBL concentrations (measured by ELISA) in 96 homosexual men with HIV infection and in two control groups (123 healthy adults and 36 HIV-negative homosexual men at high risk of HIV infection because of their sexual behaviour). Follow-up was for up to 10 years.

FINDINGS

Eight (8%) of the HIV-infected men were homozygous for the variant MBL alleles compared with one (0.8%) of the healthy controls (p = 0.005) and none of the high-risk homosexual controls (p = 0.05). We found no significant association between MBL genotype and time from first positive HIV test to progression of AIDS (p = 0.8). However, in the 61 HIV-infected men who developed AIDS, the median survival time was significantly shorter after the AIDS diagnosis for men who were carriers of the variant alleles (both homozygous and heterozygous) than for men homozygous for the normal MBL allele (11 [IQR 4-21] vs 18 months [9-44], p = 0.007). Among men who developed AIDS, there was a significant difference in survival time between those with serum MBL concentrations below the lower quartile, those within the IQR, and those above the upper quartile (p = 0.02). Multivariate analysis showed that men who developed AIDS and had low serum MBL concentrations had an increased rate of rapid death, independently of CD4 T-cell counts at AIDS diagnosis.

INTERPRETATION

Our findings suggest that homozygous carriers of variant MBL alleles are at increased risk of HIV infection, either directly or indirectly because of increased susceptibility to coinfections. These alleles are also associated with a significantly shorter survival time after a diagnosis of AIDS.