Anti-phagocytic mechanisms of Streptococcus pyogenes: binding of fibrinogen to M-related protein

A key attribute of invasive Streptococcus pyogenes is their ability to resist phagocytosis and multiply in human blood. M-related protein (Mrp) is a major anti-phagocytic factor but the mechanism whereby it helps streptococci to evade phagocytosis has not been demonstrated. We investigated phagocytosis resistance in a strain of serotype M4 by inactivating the mrp gene and also the emm, enn, sof and sfbX genes and by analysing the effect on streptococcal growth in blood and on complement deposition on the bacterial surface. Inactivation of enn4 and sfbX4 had little impact on growth in blood, but ablation of mrp4, emm4 or sof4 reduced streptococcal growth in human blood, confirming that Mrp and Emm are required for optimal resistance to phagocytosis and providing the first indication that Sof may be an anti-phagocytic factor. Moreover, antisera against Mrp4, Emm4 and Sof4 promoted the killing of S. pyogenes, but anti-SfbX serum had no effect. Growth of S. pyogenes in blood was dependent on the presence of fibrinogen and in the absence of fibrinogen there was a twofold increase in complement deposition. Inactivation of mrp4 resulted in a loss of fibrinogen-binding and caused a twofold increase in the binding of C3b that was inhibited by Mg-EGTA. Mrp contained two fibrinogen-binding sites, one of which is within a highly conserved region. These findings indicate that Mrp-fibrinogen interactions prevent surface deposition of complement via the classical pathway, thereby contributing to the ability of these streptococci to resist phagocytosis. This may be a common mechanism for evasion of phagocytosis because Mrp is expressed by approximately half of the clinical isolates of S. pyogenes.     

Non-Immune Binding of Human IgG to M-Related Proteins Confers Resistance to Phagocytosis of Group A Streptococci in Blood

The non-immune binding of immunoglobulins by bacteria is thought to contribute to the pathogenesis of infections. M-related proteins (Mrp) are group A streptococcal (GAS) receptors for immunoglobulins, but it is not known if this binding has any impact on virulence. To further investigate the binding of immunoglobulins to Mrp, we engineered mutants of an M type 4 strain of GAS by inactivating the genes for mrp, emm, enn, sof, and sfbX and tested these mutants in IgG-binding assays. Inactivation of mrp dramatically decreased the binding of human IgG, whereas inactivation of emm, enn, sof, and sfbx had only minor effects, indicating that Mrp is a major IgG-binding protein. Binding of human immunoglobulins to a purified, recombinant form of Mrp indicated that it selectively binds to the Fc domain of human IgG, but not IgA or IgM and that it preferentially bound subclasses IgG₁>IgG₄>IgG₂>IgG₃. Recombinant proteins encompassing different regions of Mrp were engineered and used to map its IgG-binding domain to its A-repeat region and a recombinant protein with 3 A-repeats was a better inhibitor of IgG binding than one with a single A-repeat. A GAS mutant expressing Mrp with an in-frame deletion of DNA encoding the A-repeats had a dramatically reduced ability to bind human IgG and to grow in human blood. Mrp exhibited host specificity in binding IgG; human IgG was the best inhibitor of the binding of IgG followed by pig, horse, monkey, and rabbit IgG. IgG from goat, mouse, rat, cow, donkey, chicken, and guinea pig were poor inhibitors of binding. These findings indicate that Mrp preferentially binds human IgG and that this binding contributes to the ability of GAS to resist phagocytosis and may be a factor in the restriction of GAS infections to the human host.     

Binding of complement regulators factor H and C4b binding protein to group A streptococcal strains isolated from tonsillar tissue and blood

Group A streptococcus (GAS) is the most common pathogen causing bacterial pharyngitis. We isolated streptococcal strains from tonsils removed from patients with tonsillar disease (n=202) and studied their ability to bind the complement regulators factor H (FH) and C4b binding protein (C4BP) using 125 I-labeled proteins. Blood isolates of GAS (n=10) were obtained from patients with bacteraemia. Streptococci were isolated from 21% of the tonsillitis patients. The emm and T types of the GAS strains were determined. Of the 26 GAS strains studied, only six could bind FH and/or C4BP above the threshold levels. The fraction of the offered radioactive protein bound ranged between 6-12% for FH and 19-56% for C4BP. The clinical course of the tonsillar disease was not related to the binding of FH or C4BP by GAS. The binding strains were mostly of the T4M4 or T28M28 type. From the invasive strains (n=10), three bound FH (binding level: 8-11%) and two C4BP (36-39%). The binding correlated only partially to M-protein (emm) type suggesting that the binding was not exclusively due to M-protein. The results indicate that complement regulator binding by GAS is only partially related to pathogenicity and not a universal property of all group A streptococci.     

Bordetella pertussis lipopolysaccharide resists the bactericidal effects of pulmonary surfactant protein A

Surfactant protein A (SP-A) plays an important role in the innate immune defense of the respiratory tract. SP-A binds to lipid A of bacterial LPS, induces aggregation, destabilizes bacterial membranes, and promotes phagocytosis by neutrophils and macrophages. In this study, SP-A interaction with wild-type and mutant LPS of Bordetella pertussis, the causative agent of whooping cough, was examined. B. pertussis LPS has a branched core structure with a nonrepeating trisaccharide, rather than a long-chain repeating O-Ag. SP-A did not bind, aggregate, nor permeabilize wild-type B. pertussis. LPS mutants lacking even one of the sugars in the terminal trisaccharide were bound and aggregated by SP-A. SP-A enhanced phagocytosis by human monocytes of LPS mutants that were able to bind SP-A, but not wild-type bacteria. SP-A enhanced phagocytosis by human neutrophils of LPS-mutant strains, but only in the absence of functional adenylate cyclase toxin, a B. pertussis toxin that has been shown to depress neutrophil activity. We conclude that the LPS of wild-type B. pertussis shields the bacteria from SP-A-mediated clearance, possibly by sterically limiting access to the lipid A region.     

高毒力A组链球菌致病机制研究进展

A组链球菌(Group A Streptococcus,GAS)常导致咽炎和皮肤感染,也能引起严重侵袭性感染.根据其表面M蛋白编码基因emm可将GAS分为200多型,严重侵袭性感染多由高毒力株引起,以emm1、emm3、emm12、emm28和emm89型常见.研究发现高毒力GAS株中CovRS基因突变可导致细菌逃逸固...     

Scavenger receptor gp340 aggregates group A streptococci by binding pili

Group A streptococci (GAS) are the most frequent cause of bacterial pharyngitis. The first obstacle to GAS colonization of the pharynx is saliva. As well as forming a physical barrier, saliva contains components of innate and acquired immunity. Previous work has shown that saliva induces bacterial aggregation, which may serve as a clearance mechanism. As the aggregation of some oral streptococci in saliva is mediated by long proteinaceous appendages, we hypothesized that pili of GAS might behave similarly. Wild-type GAS M1 strain SF370 aggregated in saliva, while pilus-defective mutants did not. Similarly, heterologous expression of diverse GAS pili on the surface of Lactococcus lactis induced aggregation in saliva, while control strains were unaffected. Further studies revealed that aggregating bacteria bound salivary component gp340. Purified gp340 aggregated wild-type GAS and L. lactis expressing GAS pili, but not control strains. GAS pilus-defective mutants were abrogated in gp340 binding and aggregation. Furthermore, gp340-mediated aggregation reduced bacterial adhesion to human epithelial cells, suggesting a role in host defence.     

Streptococcus pyogenes expressing M and M-like surface proteins are phagocytosed but survive inside human neutrophils

Strains of the Gram-positive human pathogen Streptococcus pyogenes (group A streptococcus) that express surface-associated M or M-like proteins survive and grow in non-immune fresh human blood. This is generally accepted to be caused by an antiphagocytic property of these proteins. However, in most previous studies, an inhibition of the internalization of the bacteria into host cells has not been studied or not directly demonstrated. Therefore, in the present paper, we used flow cytometry, fluorescence microscopy and electron microscopy to study phagocytosis by human neutrophils of wild-type S. pyogenes and strains deficient in expression of M protein and/or the M-like protein H. The results demonstrate that all strains of S. pyogenes tested, including the wild-type AP1 strain, induce actin polymerization and are efficiently phagocytosed by human neutrophils. In addition, using classical bactericidal assays, we show that the wild-type AP1 strain can survive inside neutrophils, whereas mutant strains are rapidly killed. We conclude that the ability of virulent S. pyogenes to survive and multiply in whole blood is most likely not possible to explain only by an antiphagocytic effect of bacterial surface components. Instead, our data suggest that bacterial evasion of host defences occurs intracellularly and that survival inside human neutrophils may contribute to the pathogenesis of S. pyogenes and the recurrence of S. pyogenes infections.     

Group A streptococcal phagocytosis resistance is independent of complement factor H and factor H-like protein 1 binding

Factor H (FH) and factor H-like protein 1 (FHL-1) regulate complement activation through the alternative pathway. Several extracellular bacterial pathogens, prime targets for the complement system, bind FH and FHL-1, thereby acquiring a potential mechanism for minimizing complement deposition on their surface. For group A streptococci (GAS), surface-bound antiphagocytic M proteins mediate the interaction. To study the role of the FH-FHL-1 interaction for complement deposition and opsonophagocytosis of GAS, we first constructed a set of truncated M5 protein variants and expressed them on the surface of a homologous M-negative GAS strain. Binding experiments with the resulting strains demonstrated that the major FH-FHL-1 binding is located in a 42-amino-acid region within the N-terminal third of M5. Measurement of bacteria-bound complement factor C3 after incubation in plasma showed that the presence of this region had little impact upon complement deposition through the alternative pathway. Moreover, streptococci expressing M5 proteins lacking the major FH and FHL-1 binding sequence resisted phagocytosis in human blood as efficiently as bacteria expressing the wild-type protein. Consequently, the data suggest that the binding of the regulators of the alternative pathway is of limited importance for GAS phagocytosis resistance.     

Effects of nonpolar mutations in each of the seven Bacillus subtilis mrp genes suggest complex interactions among the gene products in support of Na(+) and alkali but not cholate resistance

The Bacillus subtilis mrp (multiple resistance and pH) operon supports Na(+) and alkali resistance via an Na(+)/H(+) antiport, as well as cholate efflux and resistance. Among the individual mutants with nonpolar mutations in each of the seven mrp genes, only the mrpF mutant exhibited cholate sensitivity and a cholate efflux defect that were complemented by expression of the deleted gene in trans. Expression of mrpF in the mrp null (VKN1) strain also restored cholate transport and increased Na(+) efflux, indicating that MrpF does not require even low levels of other mrp gene expression for its own function. In contrast to MrpF, MrpA function had earlier seemed to depend upon at least modest expression of other mrp genes, i.e., mrpA restored Na(+) resistance and efflux to strain VK6 (a polar mrpA mutant which expresses low levels of mrpB to -G) but not to the null strain VKN1. In a wild-type background, each nonpolar mutation in individual mrp genes caused profound Na(+) sensitivity at both pH 7.0 and 8.3. The mrpA and mrpD mutants were particularly sensitive to alkaline pH even without added Na(+). While transport assays in membrane vesicles from selected strains indicated that MrpA-dependent antiport can occur by a secondary, proton motive force-dependent mechanism, the requirement for multiple mrp gene products suggests that there are features of energization, function, or stabilization that differ from typical secondary membrane transporters. Northern analyses indicated regulatory relationships among mrp genes as well. All the mrp mutants, especially the mrpA, -B, -D, -E, and -G mutants, had elevated levels of mrp RNA relative to the wild type. Expression of an upstream gene, maeN, that encodes an Na(+)/malate symporter, was coordinately regulated with mrp, although it is not part of the operon.     

Decreased outer membrane permeability protects mycobacteria from killing by ubiquitin-derived peptides

Ubiquitin-derived peptides are bactericidal in vitro and contribute to the mycobactericidal activity of the lysosome. To further define interactions of ubiquitin-derived peptides with mycobacteria, we screened for mutants with increased resistance to the bactericidal activity of the synthetic ubiquitin-derived peptide Ub2. The four Ub2-resistant Mycobacterium smegmatis mutants were also resistant to the bactericidal action of other antimicrobial peptides and macrophages. Two mutants were in the mspA gene encoding the main M. smegmatis porin. Using a translocation-deficient MspA point mutant, we showed that susceptibility of M. smegmatis to Ub2 was independent of MspA channel activity. Instead, the M. smegmatis Ub2-resistant mutants shared a common phenotype of decreased cell wall permeability compared with wild-type bacteria. Expression of mspA rendered Mycobacterium tuberculosis CDC1551 more susceptible both to ubiquitin-derived peptides in vitro and to lysosomal killing in macrophages. Finally, biochemical assays designed to assess membrane integrity indicated that Ub2 treatment impairs membrane function of M. smegmatis and M. tuberculosis cells . The M. smegmatis Ub2-resistant mutants were more resistant than wild-type M. smegmatis to this damage. We conclude that Ub2 targets mycobacterial membranes and that reduced membrane permeability provides mycobacteria intrinsic resistance against antimicrobial compounds including bactericidal ubiquitin-derived peptides.     

Expression of different group A streptococcal M proteins in an isogenic background demonstrates diversity in adherence to and invasion of eukaryotic cells

The M protein of group A streptococcus (GAS) is considered to be a major virulence factor because it renders GAS resistant to phagocytosis and allows bacterial growth in human blood. There are more than 80 known serotypes of M proteins, and protective opsonic antibodies produced during disease in humans are serotype specific. M proteins also mediate bacterial adherence to epithelial cells of skin and pharynx. GAS strains vary in the genomic organization of the mga regulon, which contains the genes encoding M and M-like proteins and other virulence factors. This diversity of organization makes it difficult to assess virulence of M proteins of different serotypes, unless they can be expressed in an isogenic background. Here, we express M proteins of different serotypes in the M protein- and protein F1-deficient GAS strain, SAM2, which also lacks M-like proteins. Genes encoding M proteins of different serotypes (emmXs) have been integrated into the SAM2 chromosome in frame with the emm6.1 promoter and its mga regulon, resulting in similar levels of emmX expression. Although SAM2 exhibits a very low level of adherence to and invasion of HEp-2 and HaCaT cells, a SAM2-derived strain expressing M6 protein adheres to and invades both cell types. In contrast, the isogenic strain expressing M18 protein adheres to both cell types, but invades with a very low efficiency. A strain expressing M3 protein adheres to both types of cells, but its invasion of HEp-2 cells is serum dependent. A GAS strain expressing M6 protein does not compete with the isogenic strain expressing M18 protein for adherence to or invasion of HaCaT cells. We conclude that M proteins of different serotypes recognize different repertoires of receptors on the surfaces of eukaryotic cells.     

Outer membrane protein A deficient Escherichia coli activates neutrophils to produce superoxide and shows increased susceptibility to antibacterial peptides

The outer membrane protein A (OmpA) of Gram-negative bacteria has been ascribed multiple functions including maintenance of structural membrane integrity and porin activity. OmpA has also been implicated in various host defense processes in that it contributes to bacterial serum resistance and activates certain immune cells. Recently, OmpA was shown to be the molecular target for neutrophil elastase (NE), and Escherichia coli mutants lacking OmpA were resistant to the bactericidal effects of NE. In addition to NE, neutrophils use a variety of other antibacterial effector molecules such as oxygen radicals and bactericidal peptides or proteins. The aim of this study was to investigate the role of E. coli OmpA regarding susceptibility to other neutrophil-derived defense systems. We found that OmpA-deficient (OmpA(-)), but not wild-type isogenic, E. coli activated human neutrophils to produce oxygen radicals intracellularly. This activation was found to require an intact neutrophil cytoskeleton but was independent of bacterial phagocytosis. Furthermore, we found that the OmpA(-) strain was more susceptible to membrane-acting bactericidal peptides than the wild-type strain, although the susceptibility to different oxygen radicals was independent of the presence of OmpA. Taken together, these data suggest an important role for OmpA in the context of bacteria vs. host interactions.     

Nucleotide substitutions and small-scale insertion produce size and antigenic variation in group A streptococcal M1 protein

The presence of M protein on the surface of group A streptococci (GAS) confers the ability of the cell to resist phagocytosis in the absence of type-specific antibodies. It undergoes antigenic variation with more than 80 different serotypes having been defined. We have sequenced the M protein gene (emm1.1) from strain CS190 and present evidence that individual nucleotide substitutions are responsible for sequence variation in the N-terminal non-repeat region of emm1.1 and these substitutions have altered antibody recognition of opsonic epitopes. The N-terminal non-repeat domains of two other closely related strains, 71-155 and 76-088, were found to have sequence identical to emm1.1 with the addition of a 21 bp insert. This study provides the first evidence that nucleotide substitutions and small insertions are responsible for size and antigenic variation in the N terminal non-repeat domain of the M protein of GAS.     

Icm/Dot-dependent upregulation of phagocytosis by Legionella pneumophila

Legionella pneumophila is the causative agent of Legionnaires' disease, a severe pneumonia. Dependent on the icm/dot loci, L. pneumophila survives and replicates in macrophages and amoebae within a specialized phagosome that does not fuse with lysosomes. Here, we report that phagocytosis of wild-type L. pneumophila is more efficient than uptake of icm/dot mutants. Compared with the wild-type strain JR32, about 10 times fewer icm/dot mutant bacteria were recovered from HL-60 macrophages in a gentamicin protection assay. The defect in phagocytosis of the mutants could be complemented by supplying the corresponding genes on a plasmid. Using fluorescence microscopy and green fluorescent protein (GFP)-expressing strains, 10-20 times fewer icm/dot mutant bacteria were found to be internalized by HL-60 cells and human monocyte-derived macrophages (HMMPhi). Compared with icm/dot mutants, wild-type L. pneumophila infected two to three times more macrophages and yielded a population of highly infected host cells (15-70 bacteria per macrophage) that was not observed with icm/dot mutant strains. Wild-type and icmT mutant bacteria were found to adhere similarly and compete for binding to HMMPhi. In addition, wild-type L. pneumophila was also phagocytosed more efficiently by Acanthamoeba castellanii, indicating that the process is independent of adherence receptor(s). Wild-type L. pneumophila enhanced phagocytosis of an icmT mutant strain in a synchronous co-infection, suggesting that increased phagocytosis results from (a) secreted effector(s) acting in trans.     

Phagocytosis of enteric Campylobacter by human and murine granulocytes

The phagocytosis of enteric Campylobacter strains by murine and human granulocytes was studied in vitro. The number of fluorescein isothiocyanate-labelled bacteria per granulocyte was determined microscopically. The phagocytic index is strain-dependent, ranging from 0.05 to 0.4 bacteria per granulocyte. Human granulocytes phagocytose Campylobacter sp. with a twofold higher effectivity than murine cells. Opsonization with immune sera increased phagocytosis 11.6-fold; flagella-defective mutants were phagocytosed without opsonization with 3.3-fold higher effectivity than the isogenic mother strain. Stimulation and phagocytosis of granulocytes by 14 clinical isolates of Campylobacter sp. was monitored by measuring the oxidative burst and phagocytosis. Stimulation of granulocytes varied from 0.4 to 1.8 (relative units) and phagocytosis ranged from 0.03 to 0.68 bacteria per granulocyte. No statistically significant correlation among bio- or serovars and the degree of stimulation and phagocytosis was observed.     

Human fibrinogen bound to Streptococcus pyogenes M protein inhibits complement deposition via the classical pathway

Human fibrinogen (Fg) binds to surface proteins expressed by many pathogenic bacteria and has been implicated in different host-pathogen interactions, but the role of bound Fg remains unclear. Here, we analyse the role of Fg bound to Streptococcus pyogenes M protein, a major virulence factor that confers resistance to phagocytosis. Studies of the M5 system showed that a chromosomal mutant lacking the Fg-binding region was completely unable to resist phagocytosis, indicating that bound Fg plays a key role in virulence. Deposition of complement on S. pyogenes occurred via the classical pathway even under non-immune conditions, but was blocked by M5-bound Fg, which reduced the amount of classical pathway C3 convertase on the bacterial surface. This property of M protein-bound Fg may explain its role in phagocytosis resistance. Previous studies have shown that many M proteins do not bind Fg, but interfere with complement deposition and phagocytosis by recruiting human C4b-binding protein (C4BP), an inhibitor of the classical pathway. Thus, all M proteins may share ability to recruit a human plasma protein, Fg or C4BP, which inhibits complement deposition via the classical pathway. Our data identify a novel function for surface-bound Fg and allow us to propose a unifying mechanism by which M proteins interfere with innate immunity.     

Molecular evolution of a multigene family in group A streptococci

The emm genes are members of a gene family in group A streptococci (GAS) that encode for antiphagocytic cell-surface proteins and/or immunoglobulin-binding proteins. Previously sequenced genes in this family have been named "emm," "fcrA," "enn," "arp," "protH," and "mrp"; herein they will be referred to as the "emm gene family." The genes in the emm family are located in a cluster occupying 3-6 kb between the genes mry and scpA on the chromosome of Streptococcus pyogenes. Most GAS strains contain one to three tandemly arranged copies of emm-family genes in the cluster, but the alleles within the cluster vary among different strains. Phylogenetic analysis of the conserved sequences at the 3' end of these genes differentiates all known members of this family into four evolutionarily distinct emm subfamilies. As a starting point to analyze how the different subfamilies are related evolutionarily, the structure of the emm chromosomal region was mapped in a number of diverse GAS strains by using subfamily-specific primers in the polymerase chain reaction. Nine distinct chromosomal patterns of the genes in the emm gene cluster were found. These nine chromosomal patterns support a model for the evolution of the emm gene family in which gene duplication followed by sequence divergence resulted in the generation of four major-gene subfamilies in this locus.