Antiphagocytic activity of streptococcal M protein: selective binding of complement control protein factor H

A novel sialic acid binding site on factor H mediates serum resistance of sialylated Neisseria gonorrhoeae

Factor H (fH), a key alternative complement pathway regulator, is a cofactor for factor I-mediated cleavage of C3b. fH consists of 20 short consensus repeat (SCR) domains. Sialic acid binding domains have previously been localized to fH SCRs 6-10 and 13. To examine fH binding on a sialylated microbial surface, we grew Neisseria gonorrhoeae in the presence of 5'-cytidinemonophospho-N-acetylneuraminic acid, which sialylates lipooligosaccharide and converts to serum resistance gonococci previously sensitive to nonimmune serum killing. fH domains necessary for binding sialylated gonococci were determined by incubating organisms with recombinant human fH (rH) and nine mutant rH molecules (deletions spanning the entire fH molecule). rH and all mutant rH molecules that contained SCRs 16-20 bound to the sialylated strain; no mutant molecule bound to serum-sensitive nonsialylated organisms. Sialic acid was demonstrated to be the fH target by flow cytometry that showed a fourfold increase in fH binding that was reversed by neuraminidase-mediated cleavage of sialic acid off gonococci. Functional specificity of fH was confirmed by decreased total C3 binding and almost complete conversion to iC3b on sialylated gonococci. Sialic acid can therefore bind fH uniquely through SCRs 16-20. This blocks complement pathway activation for N. gonorrhoeae at the level of C3.

Expression of Two Different Antiphagocytic M Proteins by Streptococcus pyogenes of the OF+ Lineage

All clinical isolates of Streptococcus pyogenes (group A streptococcus) share the ability to resist phagocytosis and grow in human blood. In many strains, this property is due to the expression of a single antiphagocytic M protein, while other strains express more than one M-like molecule, of which the role in phagocytosis resistance is unclear. In particular, all S. pyogenes strains of the OF+ lineage, representing approximately half of all isolates, express two M-like proteins, Mrp and Emm, which are immunologically unrelated. These two proteins bind different ligands that have been implicated in phagocytosis resistance: Mrp binds fibrinogen and Emm binds the complement inhibitor C4BP. Using a clinical isolate of the common serotype 22, we created mutants affected in the mrp and emm genes and characterized them in phagocytosis experiments and by electron microscopy. A double mutant mrp−emm− showed strongly decreased resistance to phagocytosis, while mrp− and emm− single mutants grew well in blood. However, optimal growth required the expression of both Mrp and Emm. Experiments in which coagulation was inhibited using the specific thrombin inhibitor, hirudin, rather than heparin, indicated that Emm is more important than Mrp for resistance to phagocytosis. Tuftlike surface structures typical for S. pyogenes were still present in the mrp−emm− double mutant, but not in a mutant affected in the regulatory gene mga, indicating that the presence of these surface structures is not directly correlated to phagocytosis resistance. Our data imply that OF+ strains of S. pyogenes express two antiphagocytic M proteins with different ligand-binding properties.

Complement regulation

Because of its strong potential for generating inflammation and causing tissue destruction the complement system has to be kept strictly under control. Cells of the host need special protection against the cytolytic complement system. This paper will describe how inappropriate activation of complement in the fluid phase is prevented and how viable human blood cells defend themselves against being destroyed and cleared away by the complement system. Since disturbances in complement regulation occasionally result in disease a brief reference will be made to two of the syndromes caused by complement regulator deficiency, hereditary angioedema (HAE) and paroxysmal nocturnal hemoglobinuria (PNH).

Binding of complement subcomponent C1q to Streptococcus pyogenes: evidence for interactions with the M5 and FcRA76 proteins

Binding of C1q, the first component of the complement system, to some human pathogens has been earlier reported. In the present study, direct binding of C1q to group A streptococci (GAS) of various serotypes as well as some other Gram-positive and Gram-negative species was demonstrated. The interaction between C1q and GAS was investigated more in detail. In hot neutral extracts of a number of GAS strains two components of 64 and 52 kDa, respectively, bound C1q; alkaline and SDS extracts yielded the 52 kDa component as the main C1q-binding substance. Trypsin treatment of the SDS extracts of two GAS strains suggested the C1q-binding component(s) to be of protein nature. C1q-binding material purified from the SDS extract of an avirulent strain, type T27, was separated in 12% SDS-PAGE and probed in Western blot with human C1q and fibrinogen, conjugated to horse radish peroxidase (HRP) as well as rabbit IgG antibodies complexed to HRP (PAP system). The 52 kDa component was non-reactive with fibrinogen or rabbit IgG. However, C1q-binding components purified from the alkaline extracts of two M-positive strains revealed strong binding of either fibrinogen (type M5) or both fibrinogen and rabbit IgG (type M76); the molecular mass of these components. 55 kDa and 43-40 kDa, respectively, was in agreement with the reported molecular mass of the M5 and FcRA76 proteins. Our findings suggest that C1q may interact with GAS through certain M-family proteins as well as by a so far unidentified surface factor of protein nature occurring in most GAS strains. The involvement of M-family proteins, regarded as virulence factors of these organisms, may suggest the interaction of GAS with C1q as biologically important.

Mannan-specific immunoglobulin G antibodies in normal human serum mediate classical pathway initiation of C3 binding to Candida albicans

Candida albicans activates both the classical and alternative complement pathways. Previous studies found that immunoglobulin G (IgG) in normal human serum (NHS) mediates classical pathway initiation. The goal of this study was to determine the role of candidal mannan-specific human IgG antibodies in complement activation. Mannan was purified from the yeast cells, and naturally occurring antimannan IgG was isolated from pooled NHS or plasma samples by immunoaffinity chromatography. Early activation and binding of C3, characteristics of classical pathway activity, were abolished in yeast- or mannan-absorbed serum but could be restored to absorbed serum with added purified antimannan IgG in a dose-dependent manner. Microscopically, the immunofluorescence pattern of initial C3 binding was diffuse over the entire cell surface for yeast cells incubated in NHS or in mannan-absorbed NHS supplemented with antimannan IgG but was asynchronous and focal for yeast cells incubated in EGTA-treated or mannan-absorbed NHS. The antimannan IgG level in serum samples from 21 donors varied from 17 to 570 microg/ml of serum compared to 220 microg in pooled NHS samples. The rate of initial C3 binding to yeast cells correlated with the level of antimannan IgG in sera from different individuals (r2 = 0.94) and could be accelerated in sera containing lower amounts of antimannan IgG with exogenous antimannan IgG. These observations identify antimannan IgG as the initiator of classical pathway C3 deposition on C. albicans. Given the variability in the levels of antimannan antibodies in sera from different individuals, the presence or absence of these antibodies may be an important determinant of host resistance to disseminated candidiasis.

Bordetella pertussis binds the human complement regulator C4BP: role of filamentous hemagglutinin

C4BP (C4b-binding protein) is a high-molecular-weight plasma protein that inhibits the classical pathway of complement activation. Recent experiments have demonstrated that C4BP binds to many strains of the gram-positive bacterium Streptococcus pyogenes, a major respiratory tract pathogen. Binding to S. pyogenes was shown to be due to members of the M protein family, a group of surface proteins important for virulence. Here we report that human C4BP also binds to all clinical isolates of the gram-negative bacterium Bordetella pertussis, the etiologic agent of whooping cough. In addition, binding of C4BP was demonstrated for other Bordetella species that can cause disease in humans. Characterization of different B. pertussis mutants showed that the binding of C4BP is strongly dependent on the expression of the cell surface protein filamentous hemagglutinin, a well-known virulence factor. Inhibition experiments suggested that B. pertussis and S. pyogenes bind to the same region in C4BP. The finding that B. pertussis and S. pyogenes both have the ability to bind human C4BP suggests that these two unrelated respiratory tract pathogens may use a common mechanism during the establishment of an infection.

Streptococcal protein H forms soluble complement-activating complexes with IgG, but inhibits complement activation by IgG-coated targets

Protein H, a surface protein of Streptococcus pyogenes interacting with the constant Fc region of IgG, is known to be released from the streptococcal surface by a cysteine proteinase produced by the bacteria. Poststreptococcal glomerulonephritis and rheumatic fever are conditions in which immune complexes and autoimmune mechanisms have been suggested to play pathogenetic roles. The present study demonstrates that addition of protein H to human serum produces complement activation with dose-dependent cleavage of C3. The activation was IgG-dependent and the result of complexes formed between IgG and protein H. These complexes were size heterogeneous with molecular masses of 400 kDa to 1.4 MDa. Using complement-depleted serum reconstituted with complement proteins, the activation by protein H was found to be dependent of the classical, but independent of the alternative pathway of complement. In contrast to results of experiments based on soluble protein H.IgG complexes, complement activation was inhibited by protein H when IgG was immobilized on a surface. The interaction between C1q and immunoglobulins represents the first step in the activation of the classical pathway, and protein H efficiently inhibited the binding of C1q to IgG immobilized on polyacrylamide beads. Protein H reduced C3 deposition on the IgG-coated beads and inhibited immune hemolysis of IgG-sensitized erythrocytes. Finally, significantly less C3 was deposited on the surface of protein H-expressing wild-type streptococci than on the surface of isogenic mutant bacteria devoid of protein H. The results demonstrate that protein H.IgG complexes released from the streptococcal surface can produce complement breakdown at the sites of infection, whereas complement activation on bacterial surfaces is inhibited. This should have important implications for host-parasite relationships. In addition, soluble protein H.IgG complexes might contribute to immunological complications of streptococcal infections.

Human IgG binding ability of streptococcal M3 protein: its related complement activation-dependent M3 protein polymerization

We previously showed that M3 protein bound both fibrinogen and human serum albumin. Here, I report that M3 protein also has affinity for human immunoglobulin G. In contrast, M3 protein did not show affinity for polyclonal immunoglobulin G from other mammalian species (rabbit and goat). On the human immunoglobulin G molecule, the Fab domain was mainly responsible for the interaction with M3 protein, although the Fc region had a low degree of interaction with the M3 protein. Also, since the 35 kDa C-terminal fragment of M3 protein bound human immunoglobulin G, the binding site for human immunoglobulin G on M3 protein is present in this portion of the protein. The M3 protein-human immunoglobulin G complexes initiated complement activation via both classical and alternative pathways in normal human serum. When C3 was precipitated in the fluid phase with anti-C3 antibody and analyzed by SDS-PAGE under reducing conditions, M3 protein coprecipitated with the complexes and was polymerized. However, there was no polymerization of M3 protein when incubated with normal human serum treated with magnesium-ethyleneglycol-bis-(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid in the presence of M3 protein. Thus, this polymerization is mostly mediated via the classical activation pathway. It is probably helpful for the understanding of the antiphagocytic activity of M protein.

Localization by site-directed mutagenesis of the site in human complement factor H that binds to Streptococcus pyogenes M protein

M-protein receptors located on Streptococcus pyogenes cells are known to bind human plasma protein factor H. Human factor H is composed of 20 short consensus repeat (SCR) domains containing approximately 60 amino acids each. Factor H controls the activation of the alternative pathway of complement in plasma. We have scanned the entire human factor H molecule by site-directed deletion mutagenesis, expressed the recombinant proteins in insect cells using the baculovirus system, and measured the binding of different purified mutant proteins to three strains of S. pyogenes. These studies have revealed that recombinant factor H lacking SCR domains 6 to 10 does not bind to wild-type M+ S. pyogenes JRS4. Experiments performed with S. pyogenes JRS251, in which both C-repeat domains of M protein were deleted, demonstrated that all of the factor H mutant proteins bound weakly to these cells except those lacking the SCR region from domains 6 to 10. Neither human factor H nor any of the recombinant proteins bound to the M- strain JRS145. Our results indicate that the only binding site on human factor H that interacts with streptococcus M protein is located in SCR domains 6 to 10 of factor H and that regions of M protein outside the C-repeat domains are involved in binding factor H.

DNA sequencing and gene expression of the emm gene cluster in an M50 group A streptococcus strain virulent for mice

The strain B514, an M serotype 50 strain, is capable of causing a natural upper respiratory infection leading to death in mice, as reported by Hook et al. in 1960 (E. W. Hook, R. R. Wagner, and R. C. Lancefield, Am. J. Hyg. 72:111-119, 1960). Thus, this strain was of interest for use in developing an animal model for group A streptococcal colonization and disease. The emm gene cluster for this strain was examined by PCR mapping and found to contain three emm family genes and cluster pattern 5. PCR-generated fragments corresponding to the SF4 (mrp50), SF2 (emmL50), and SF3 (enn50) genes were cloned and the entire gene cluster was sequenced. The gene cluster has greater than 97% DNA identity to previously sequenced regions of the gene cluster of the M2 strain T2/44/RB4 if two small divergent regions that encode the mature amino terminus of the SF-2 and SF-3 gene products are not included. If expressed, the genes encode proteins which bind human immunoglobulin G (Mrp50 and EmmL50) or immunoglobulin A (Enn50). However, in isolates taken directly after passage in mice, the surface proteins arising from these genes were barely detectable. The transcription of each gene in the B514 strain was investigated by Northern (RNA) hybridization, and mRNA transcripts were detected and quantitated relative to those of the recA gene, a housekeeping gene. Transcription of all three emm family genes was found to be over 30-fold attenuated relative to transcription of the same genes in strain T2/44/RB4. This suggests that the positive regulator, Mga, either is not expressed in this strain or has a different requirement for activation; it also suggests that the capsule may be sufficient to inhibit phagocytosis under these circumstances.

Hyaluronate capsule and surface M protein in resistance to opsonization of group A streptococci

The major virulence determinant of group A streptococci is the ability to resist opsonization and phagocytic ingestion. The present studies were performed to compare the mechanisms of resistance to opsonization of type 18 and type 24 streptococci and to determine the relative roles of M protein-fibrinogen interaction and the hyaluronate capsule in preventing phagocytic ingestion and killing. By use of parent strains and acapsular transposon mutants in the presence and absence of fibrinogen, we show that type 18 and type 24 streptococci rely on somewhat different mechanisms for resistance to opsonization. Type 24 streptococci bound fibrinogen avidly to their surfaces, and encapsulated organisms were completely resistant to opsonization only in the presence of fibrinogen. In contrast, type 18 streptococci bound 10-fold less fibrinogen than type 24 streptococci and were fully resistant to phagocytosis only when they expressed capsule. The general structural characteristics of the amino-terminal halves of type 18 and type 24 M proteins differed in that type 18 M protein contained only one complete B repeat, whereas type 24 M protein contained five complete B repeats, a structural difference which could potentially be related to the differences in fibrinogen binding between the two serotypes. Immunofluorescence assays of complement deposition were used in combination with 125I-C3 binding assays to show that encapsulated type 24 streptococci were fully resistant to opsonization by C3 only in the presence of plasma. Encapsulated and unencapsulated type 18 streptococci were equally opsonized by C3 in either plasma or serum, yet only encapsulated organisms resisted phagocytic killing in blood. The results of this study indicate that opsonization by C3 does not necessarily lead to phagocytic ingestion and that the hyaluronate capsule and M proteins are variably important in resistance to different group A streptococci to opsonization and phagocytic killing.

Methylation analysis of a marsupial X-linked CpG island by bisulfite genomic sequencing

Paternal X chromosome inactivation occurs in rodent extraembryonic membranes and in all tissues of marsupials. Methylation of CpG islands occurs on the inactive X in eutherians and is considered to be a stabilizing mechanism. The only previous study of a marsupial X-linked CpG island was of the G6PD gene of the Virginia opossum, in which the paternally derived allele is not completely repressed. We have cloned the 5' end of the G6PD gene from an Australian marsupial, the common wallaroo, and sequenced the associated CpG island. The paternally derived G6PD allele is completely repressed in tissues of this species. Methylation analysis using HpaII and Cfol restriction enzymes and bisulfite genomic sequencing of 47 CpG dinucleotides in a 613-bp region reveals hypomethylation of male and female DNA from tissues, cultured fibroblasts (in which the paternal allele is partially expressed) and sperm. This suggests that methylation of CpG islands is not required for maintenance of X inactivation in marsupials even where repression of the paternal allele is complete.

Protein SIC, a novel extracellular protein of Streptococcus pyogenes interfering with complement function

The human pathogen Streptococcus pyogenes possesses a chromosomal region, the mga regulon, that contains co-regulated genes important to the virulence of these bacteria. A novel gene located in the mga regulon of a S. pyogenes strain of serotype M1 was cloned and sequenced. It translates into a protein of 305 amino acid residues, including a signal sequence of 32 amino acids and a central region consisting of three tandem repeats. The sequence represents a novel structure with no significant homology to any previously published sequence. The protein was purified from the streptococcal culture media where it is present in substantial amounts. Affinity chromatography of human plasma on Sepharose coupled with the protein specifically absorbed two plasma proteins which were identified as clusterin and histidine-rich glycoprotein (HRG). The interactions between the streptococcal protein and the plasma proteins were further characterized using purified clusterin and HRG. Inhibition experiments indicated that they have affinity for overlapping or closely located sites in the streptococcal protein. Both clusterin and HRG are regulators of the membrane attack complex (C5b-C9) of complement. When the streptococcal protein was added to serum, complement-mediated lysis of sensitized sheep erythrocytes and guinea pig erythrocytes was inhibited. In addition, the streptococcal protein was incorporated into C5b-C9 in serum, indicating the location of its action. The name, protein SIC, streptococcal inhibitor of complement-mediated lysis, is therefore suggested for this novel protein. The occurrence of protein SIC and its gene was investigated in a collection of S. pyogenes strains comprising 55 different M serotypes. Only M1 and M57 strains were positive in this screening, indicating that protein SIC could be a virulence determinant. Thus, during recent years, the M1 serotype has been connected with a world-wide increase of severe and toxic S. pyogenes infections.

Efficient destruction of human immunodeficiency virus in human serum by inhibiting the protective action of complement factor H and decay accelerating factor (DAF, CD55)

Activation of the human complement system leads to complement deposition on human immunodeficiency virus (HIV) and HIV-infected cells without causing efficient complement-mediated lysis. Even in the presence of HIV-specific antibodies, only a few particles are destroyed, demonstrating that HIV is intrinsically resistant to human complement. Here we report that, in addition to decay accelerating factor (DAF) being partially responsible, human complement factor H (CFH), a humoral negative regulator of complement activation, is most critical for this resistance. In the presence of HIV-specific antibodies, sera devoid of CFH (total genetic deficiency or normal human serum depleted of CFH by affinity chromatography) lysed free virus and HIV-infected but not uninfected cells. In the presence of CFH, lysis of HIV was only obtained when binding of CFH to gp41 was inhibited by a monoclonal antibody against a main CFH-binding site in gp41. Since CFH is an abundant protein in serum, and high local concentration of CFH can be obtained at the surface of HIV as the result of specific interactions of CFH with the HIV envelope, it is proposed that the resistance of HIV and HIV-infected cells against complement-mediated lysis in vivo is dependent on DAF and CFH and can be overcome by suppressing this protection. Neutralization of HIV may be achieved by antibodies against DAF and, more importantly, antibodies against CFH-binding sites on HIV envelope proteins.

Control of the complement system

The complement system has developed a remarkably simple but elegant manner of regulating itself. It has faced and successfully dealt with how to facilitate activation on a microbe while preventing the same on host tissue. It solved this problem primarily by creating a series of secreted and membrane-regulatory proteins that prevent two highly undesirable events: activation in the fluid phase (no target) and on host tissue (inappropriate target). Also, if not checked, even on an appropriate target, the system would go to exhaustion and have nothing left for the next microbe. Therefore, the complement enzymes have an intrinsic instability and the fluid-phase control proteins play a major role in limiting activation in time. The symmetry of the regulatory process between fluid phase and membrane inhibitors at the C4/C3 step of amplification and convertase formation as well as at the MAC steps are particularly striking features of the self/nonself discrimination system. The use of glycolipid anchored proteins on membranes to decay enzymes and block membrane insertion events is unlikely to be by chance. Finally, it is economical for the cofactor regulatory activity to produce derivatives of C3b that now specifically engage additional receptors. Likewise, C1-Inh leads to C1q remaining on the immune complex to interact with the C1q receptor. Thus the complement system is designed to allow rapid, efficient, unimpeded activation on an appropriate foreign target while regulatory proteins intervene to prevent three undesirable consequences of complement activation: excessive activation on a single target, fluid phase activation, and activation on self.

Gonococcal infection in a nonhuman host is determined by human complement C1q

Human C1q displayed a dose-dependent protection of gonococcal cells (GC) from the bactericidal effect of newborn rat serum. All rat pups injected with C1q-preincubated GC developed bacteremia, while none of the animals injected with GC only were infected. After clearance of bacteremia at day 6, live GC could still be recovered from tested organs, including the liver. Preincubation of GC with higher concentrations of C1q was associated with increased morbidity. In contrast to human serum as a source of C1q, rat, rabbit, and mouse sera did not increase the in vivo virulence of Neisseria gonorrhoeae. C1q-deficient human serum, heat-inactivated C1q or human serum, type IV collagen, and complement C3 were inefficient in inducing infection. Experimental infection by C1q-preincubated GC was inhibited by anti-C1q antibodies in a dose-dependent fashion, demonstrating a causal effect of C1q function. This report demonstrates the novel finding that human C1q, a component of the human immune system with a general function for elimination of infection, may increase GC virulence and result in the development of disseminated infection in a nonhuman host.

Mitogenicity of M5 protein extracted from Streptococcus pyogenes cells is due to streptococcal pyrogenic exotoxin C and mitogenic factor MF

M proteins of Streptococcus pyogenes are virulence factors which impede phagocytosis, bind to many plasma proteins, and induce formation of cross-reactive autoimmune antibodies. Recently, it has been reported that some M proteins, extracted with pepsin from streptococci (pep M), are superantigens. One of these, pep M5, was investigated in detail and was shown to stimulate human T cells bearing V beta 2, V beta 4, and V beta 8. In the present study, we extracted and purified M5 protein by different biochemical methods from two M type 5 group A streptococcal strains. The crude extracts were fractionated by affinity chromatography and ion-exchange chromatography. All fractions were tested in parallel for M protein by immunoblotting and for T-cell-stimulating activity. Although several crude preparations of M5 protein were associated with mitogenicity for V beta 2 and V beta 8 T cells, the M5 proteins, irrespective of the extraction method, could be purified to the extent that they were no longer mitogenic. The mitogenic activity was not destroyed during the purification procedures but was found in fractions separated from M protein. In these fractions, streptococcal pyrogenic exotoxin C and mitogenic factor MF could be detected by protein blotting and enzyme-linked immunosorbent assay. Moreover, anti-M protein sera did not inhibit the mitogenic activity of crude extracts, but antisera which contained anti-streptococcal pyrogenic exotoxin C antibodies showed inhibition. The inability of M5 protein to stimulate T cells was confirmed with recombinant pep M5 produced in Escherichia coli. Our data strongly suggest that the mitogenic activity in M protein preparations is caused by traces of streptococcal superantigens different from M protein.

Allosteric and temperature effects on the plasma protein binding by streptococcal M protein family members

Most group A streptococcal strains bind immunoglobulins (Ig) and fibrinogen to their cell walls. It is shown in this paper that the Ig-binding of three different strains was much weaker at 37 degrees C than at room temperature (20 degrees C), whereas the fibrinogen binding was unaffected by temperature. The binding properties and molecular sizes of two purified group A streptococcal cell surface proteins from the M protein family were studied at various temperatures, M1 protein with affinity for IgG, fibrinogen and albumin, and protein Sir22 with affinity for IgA and IgG. Both proteins appeared as monomers which bound all their ligands, including fibrinogen, very weakly at 37 degrees C, and as strongly binding dimers at 10 and 20 degrees C. Furthermore, the results demonstrated that the plasma protein binding of the bacterial proteins was allosterically regulated, i.e. the binding of a ligand to one site modulated the binding of a ligand to a second site. For example, the binding of albumin or IgG to purified M1 protein at 10 and 20 degrees C strongly enhanced the binding of fibrinogen at 37 degrees C. This indicates that the high affinity dimer form of the bacterial proteins can be stabilized at 37 degrees C, a possible explanation for the strong fibrinogen binding of whole bacteria. Finally, the sizes and binding properties of three M1 protein fragments were studied and the results indicated that the centrally located C-repeats, which are conserved among the members of the M protein family, are important for the formation of the high-affinity dimers of the bacterial proteins.

Purification and characterization of M3 protein expressed on the surface of group A streptococcal type 3 strain C203

Monoclonal antibodies (mAbs) have been produced by immunizing BALB/C mice with whole M+ bacteria in incomplete Freund adjuvant and the resulting mAbs for M3 protein have been selected by an indirect immuno-fluorescent technique using formaldehyde-fixed M+ and M- bacteria. Four mAbs reacted with a 65 kDa protein in an extract obtained from the cell wall of M+ bacteria after treatment with N-acetyl muramidase and lysozyme. The purified 65 kDa protein neutralized the phagocytic activity of rabbit anti-M3 antibody. The N-terminal amino acid sequence of the 65 kDa protein was identical with that of protein generated by the M3 gene which has been previously cloned and sequenced. The evidence indicates that the 65 kDa protein is M3 protein. The M3 protein bound not only human fibrinogen but also human serum albumin (HSA). When the M3 protein was purified by gel-filtration and ion-exchange chromatography in the absence of phenylmethyl sulfonyl fluoride (PMSF), four fragments (35 kDa, 32 kDa, 30 kDa, and 25 kDa) in addition to the intact molecule appeared. N-terminal amino acid sequence analysis showed that 35 kDa and 25 kDa fragments were ANAAD and DARSV, respectively, being identical at positions 1-5 and 198-202 to the M3 gene derived protein. Therefore, the 35 kDa and 25 kDa fragments, which were presumed to be cleavage products, may be derived from the C-terminal part and N-terminal part of the intact molecule, respectively. When the effect of purified M3 protein in the bactericidal activity of normal human blood in the presence of M- bacteria was investigated, the M3 protein was responsible for the organism's resistance to attack by phagocytic cells.

HIV glycoprotein 41 and complement factor H interact with each other and share functional as well as antigenic homology

We have shown that complement factor H (CFH) interacts with HIV-1 at the level of the sequence Env 105-119, contained in the C1 domain of gp120. CFH interaction with HIV was evident only after dissociation of the Env complex induced by exposure to sCD4. We hypothesized that CFH could act as a gp41 analog in the interaction with Env 105-119. A panel of partially overlapping, synthetic peptides reproducing the extracellular portion of gp41 was therefore used to compete the binding of CFH to Env 105-119. Three sets of peptides that competed this interaction were identified. These peptides defined a region of functional homology between the gp41 molecule and CFH (Env 580-600), and two regions of interaction (Env 620-640 and Env 650-670). In addition to this, a monoclonal antibody directed against peptide Env 580-600 and a polyclonal mouse antiserum raised against recombinant gp41 were shown to recognize CFH in Western blots and ELISA, respectively, also defining a region of antigenic homology between gp41 and CFH. These data provide evidence for interaction and molecular mimicry between an HIV structural protein and a negative regulator of the complement pathway. We show here that CFH can interact with both HIV Env proteins, suggesting a possible and efficient mechanism of downregulation of the complement cascade at the surface of infected cells.

Superantigens and pseudosuperantigens of gram-positive cocci

Superantigens use an elaborate and unique mechanism of T lymphocyte stimulation. Prototype superantigen are the pyrogenic exotoxins produced by Staphylococcus aureus and Streptococcus pyogenes. Many candidate proteins of bacterial, viral and protozoal origin have recently been reported to be superantigens. In most cases the evidence that these proteins are in fact superantigens is highly indirect. In this review the evidence that gram-positive cocci produce superantigens other than the pyrogenic exotoxins is critically discussed. Evidence in described demonstrating that the epidermolytic toxins of Staphylococcus aureus and the pyrogenic exotoxin B and M-proteins of Streptococcus pyrogenes are not superantigens. Criteria are described for acceptance of a candidate as a superantigen.

Listeria monocytogenes p60 supports host cell invasion by and in vivo survival of attenuated Salmonella typhimurium

The extracellular protein p60 is a major virulence factor of the intracellular bacterium Listeria monocytogenes. Its roles in pathogen survival in vivo and host cell invasion in vitro were studied. To this end, Salmonella typhimurium SL7207 was used as carrier for secreted p60-HlyA fusion protein by Escherichia coli HlyB and HlyD transport proteins. C57BL/6 mice infected intravenously with this strain suffered from increased bacterial numbers in livers and spleens compared with the p60-nonexpressing control strain, but only transiently. In vitro experiments showed that p60 promotes invasion of recombinant S. typhimurium SL7207 p60 into hepatocytes and resting macrophages independent from complement. Moreover, the uptake of wild-type L. monocytogenes EGD and L. monocytogenes BUG 8, an internalin-deficient strain, into hepatocytes was partially blocked by anti-p60 antibodies. The impaired invasion of dissociated bacterial chains of L. monocytogenes RIII, a p60 expression mutant, into hepatocytes and macrophages was partially restored by addition of p60- or p60-HlyA-enriched bacterial supernatants. These data suggest that the L. monocytogenes surface-associated proteins, p60 and internalin, act in concert to achieve optimal uptake into nonprofessional phagocytes and macrophages. Together, these experiments reveal a substantial impact of p60 on cell invasion and virulence and thus emphasize the importance of the intracellular habitat for survival of L. monocytogenes in the host.

Membrane cofactor protein (CD46) is a keratinocyte receptor for the M protein of the group A streptococcus

The pathogenic Gram-positive bacterium Streptococcus pyogenes (group A streptococcus) is the causative agent of numerous suppurative diseases of human skin. The M protein of S. pyogenes mediates the adherence of the bacterium to keratinocytes, the most numerous cell type in the epidermis. In this study, we have constructed and analyzed a series of mutant M proteins and have shown that the C repeat domain of the M molecule is responsible for cell recognition. The binding of factor H, a serum regulator of complement activation, to the C repeat region of M protein blocked bacterial adherence. Factor H is a member of a large family of complement regulatory proteins that share a homologous structural motif termed the short consensus repeat. Membrane cofactor protein (MCP), or CD46, is a short consensus repeat-containing protein found on the surface of keratinocytes, and purified MCP could competitively inhibit the adherence of S. pyogenes to these cells. Furthermore, the M protein was found to bind directly to MCP, whereas mutant M proteins that lacked the C repeat domain did not bind MCP, suggesting that recognition of MCP plays an important role in the ability of the streptococcus to adhere to keratinocytes.

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.

Expression of the Arp protein, a member of the M protein family, is not sufficient to inhibit phagocytosis of Streptococcus pyogenes

Many Streptococcus pyogenes immunoglobulin-binding proteins have structural similarities to the antiphagocytic M protein, including the well-known C repeats. One of these molecules is the immunoglobulin A-binding protein Arp, which is expressed by a serotype 4 strain for which no antiphagocytic M protein has yet been described. We expressed Arp4 in an S. pyogenes strain from which the structural gene for the M protein has been deleted and found that Arp4 is not sufficient to inhibit phagocytosis.

Location of the complement factor H binding site on streptococcal M6 protein

The surface M protein of group A streptococci binds factor H, a regulatory protein of the alternative complement pathway, which may contribute to the antiphagocytic activity of the M molecules. To locate the factor H binding domain in the alpha-helical coiled-coil structure of the M molecule, the M protein was cleaved with pepsin at pH 5.8, which separates the molecule approximately in half. Western blot (immunoblot), amino acid sequence, and mass spectrometric analyses revealed that factor H bound to a 14.6-kDa C-terminal fragment of the M molecule. Competitive inhibition of factor H binding to the 14.6-kDa fragment with M protein peptides localized the binding site to amino acids 256 to 292. This segment is located within the surface-exposed region of the M6 protein, identified as the C-repeat region, whose sequence is conserved among heterologous M and M-like molecules. These studies also identified a second pepsin-susceptible site with the sequence ELAK located within the cell wall-associated region of the M molecule.

Protective immunity to the group A Streptococcus may be only strain specific

M protein enables Group A streptococci to resist phagocytosis. Protective immunity is considered to be mediated by opsonic antibodies directed against this M protein. In recent studies we have shown that genetically distinct populations exist within an M-type. The question asked in this study was whether human and rabbit type specific M1 antibody was opsonic for all strains of M-type 1, irrespective of their restriction fragment length polymorphism type. When locating a blood donor from among our staff for use in the indirect bactericidal test, selective variation in opsonic ability was demonstrated by one person. Subsequent testing of 44 randomly selected human sera revealed that 11 (25%) had opsonic antibody. Of these 11, 6 opsonised all seven strains and 5 demonstrated selective opsonisation. We conclude that opsonic antibody is not necessarily type specific but may be strain specific.

Resistance of Enterococcus faecium to neutrophil-mediated phagocytosis

During a previous study of the opsonic requirements for neutrophil (polymorphonuclear leukocyte [PMN])-mediated killing of enterococci, we identified two strains of Enterococcus faecium (TX0015 and TX0016) that were resistant to PMN-mediated killing. To better define the mechanism of this resistance, we examined phagocytosis with a fluorescence assay and found that TX0016 was completely resistant to phagocytosis by PMNs; this finding was confirmed by electron microscopy. Examination of multiple strains of enterococci revealed that all 20 strains of Enterococcus faecalis tested were readily phagocytosed (mean, 18 intracellular organisms per PMN; range, 7 to 28). In contrast, only 13 (50%) of 26 strains of E. faecium tested were susceptible to phagocytosis (> or = 7 organisms per PMN); the other 13 strains showed < or = 3 organisms per PMN. Enterococcus casseliflavus ATCC 25788 and one strain of Enterococcus hirae were also resistant to phagocytosis, while two strains of Enterococcus durans, Enterococcus mundtii ATCC 43186, and one strain each of Enterococcus raffinosus and Enterococcus solitarius were readily phagocytosed. Exposure of E. faecium TX0016 to sodium periodate, but not to the protease trypsin or pronase or to phospholipase C, eliminated resistance to phagocytosis. Sialic acid, a common periodate-sensitive structure used by microorganisms to resist opsonization, could not be demonstrated in E. faecium TX0016 by the thiobarbituric acid method, nor was phagocytosis of TX0016 altered by neuraminidase treatment. This study suggests that there is a difference in susceptibility to phagocytosis by PMNs between different species of enterococci and that a carbohydrate-containing moiety which is not sialic acid may be involved in the resistance of E. faecium TX0016 to phagocytosis.

Cloning and sequencing of a Bordetella pertussis serum resistance locus

We have characterized a new virulence factor in Bordetella pertussis: serum resistance. Compared with Escherichia coli HB101, wild-type B. pertussis was relatively resistant to classical-pathway, complement-dependent killing by normal human serum. However, a mutant of B. pertussis (BPM2041) which is less virulent in mice and which has Tn5 lac inserted in a previously uncharacterized bvg-regulated gene was found to be at least 10-fold more susceptible to serum killing than the wild type. We have named this locus brk, for Bordetella resistance to killing. We have cloned and sequenced the brk locus, and it encodes two divergently transcribed open reading frames (ORFs), termed BrkA and BrkB. Both ORFs are necessary for serum resistance. Within the 300 bases which separate the two ORFs and upstream of each ORF are putative sites for BvgA binding. BrkA shows 29% identity to pertactin and has two RGD motifs in addition to a conserved proteolytic processing site and an outer membrane targeting signal. Like pertactin, BrkA is involved in adherence and invasion. Despite the similarities, a pertactin mutant was found to be not as sensitive to serum killing as the BrkA or BrkB mutants. BrkB is similar to ORFs in E. coli and Mycobacterium leprae and displays domains of homology to various transporters. On the basis of its hydropathy profile, BrkB is predicted to be a cytoplasmic membrane protein. By Southern blot, brk sequences were found in Bordetella bronchiseptica and Bordetella parapertussis but not in Bordetella avium.

Inhibition of C3 deposition on Streptococcus equi subsp. equi by M protein: a mechanism for survival in equine blood

The effect of the M protein of Streptococcus equi subsp. equi on complement deposition, complement degradation, and bacterial survival in equine whole blood was examined in vitro. Preincubation of bacteria with rabbit M protein-specific immunoglobulin G (IgG) inhibited the survival of the M+ strain in whole blood by 20-fold (P < 0.01). In addition, preincubation of bacteria with M protein-specific F(ab')2 fragments inhibited the survival of M+ cells in whole blood by 3.8-fold (P < 0.01). In the absence of specific antibody, an M+ strain (CF32) of S. equi subsp. equi survived 100-fold better in whole blood than an M- isolate (strain 19) (P < 0.01). Complement inactivation by cobra venom factor significantly enhanced the ability of the M- and M+ strains of S. equi subsp. equi to survive in whole blood, the latter in the presence or absence of M protein-specific IgG. The major opsonic forms of C3, C3b and iC3b, were present on both M- and M+ cells after opsonization in nonimmune plasma. However, colloidal gold staining indicated that the M- strain bound four times as much C3 as the M+ strain (P < 0.02) and that preincubation of the M+ strain with M protein-specific IgG or F(ab')2 fragments also enhanced the amount of C3 deposited by a factor of 4 (P < 0.02). Therefore, at least part of the M protein's ability to enhance bacterial survival in equine whole blood may be related to its ability to interfere with the deposition of equine complement on the bacterial surface.

M12 protein from Streptococcus pyogenes is a receptor for immunoglobulin G3 and human albumin

We previously showed that M12 protein from opacity factor-negative Streptococcus pyogenes (group A streptococci) CS24 is responsible for immunoglobulin G3 (IgG3) binding activity. Here, we report that this M protein binds human serum albumin (HSA). Deletion analysis showed that the C repeats are sufficient for binding HSA, although upstream regions may be required for optimal binding. Like protein G, IgG3 and HSA bind to independent domains in the M protein. Experiments showed that bound IgG3 did not inhibit HSA binding to the M protein. The interaction between M12 protein and HSA is specific. M12 protein does not bind chicken egg and bovine serum albumins. Alignments of C1 and C2 repeats of M12 protein to sequences at the carboxy termini of other M proteins and Ig receptors revealed highly homologous sequences in the FcRV, M5, M6, ML2.1, and M57 proteins, suggesting that all could bind HSA. As predicted from the alignment, M5 protein and M6+ streptococci bound HSA, whereas an isogenic M6- mutant did not bind HSA. Furthermore, M2 protein from an opacity factor-positive strain also bound HSA.

Streptococcus pyogenes type IIa IgG Fc receptor expression is co-ordinately regulated with M protein and streptococcal C5a peptidase

Streptococcus pyogenes is an important agent of human disease which expresses a variety of proteins and polysaccharides on its surface. Surface molecules M protein and streptococcal C5a peptidase (SCPA) are virulence factors which undergo concurrent phase variation and are under the co-ordinate control of the virR locus. Most opacity factor-positive (OF+) strains of S. pyogenes also express IgG Fc receptor proteins on their surface. These studies were initiated to determine whether the type IIa Fc receptor on the surface of S. pyogenes phase-varies with members of this regulatory circuit. Several methods were applied to M+ and M- variant strains to evaluate this question. (i) Immunoblot assays quantified Fc receptors on whole cells by using human IgG myeloma protein and receptor-specific antibody. M+ strains bound IgG and antibody specific for Fc protein, whereas M- strains did not. (ii) Enzyme-linked immunosorbent assays quantified Fc receptor antigen expression and showed that M+ strains produce more Fc receptor protein than their M- derivatives. (iii) Quantitative RNA dot blots showed that the message for the Fc receptor gene (fcrA) was reduced in M- strains. RNA from M+ strains hybridized to the fcrA probe at a greater dilution than that from their M- counterparts. (iv) Northern hybridization showed that the fcrA transcript is 1200 nucleotides in size and distinct from transcripts for M and SCPA proteins. These data are evidence for the co-ordinate transcriptional control of the Fc receptor, M protein, and SCPA and show that these proteins co-ordinately phase-vary within the same regulatory circuit.

Streptokinase activates plasminogen bound to human group C and G streptococci through M-like proteins

An ability to interact with plasminogen or plasmin could provide micro-organisms with a mechanism for invasion. Thus, group A, C and G streptococci secrete streptokinase which binds and activates plasminogen. Some streptococci also express surface structures which bind plasminogen without causing its activation. Plasminogen-binding surface proteins were extracted from one group C and one group G streptococcal isolate. Both proteins were found to bind plasmin, fibrinogen and serum albumin in addition to plasminogen. Gene fragments encoding the streptococcal proteins were amplified by PCR and were subsequently cloned and expressed in Escherichia coli. DNA sequence determination revealed for both genes open reading frames encoding proteins which contained repetitive domains and a carboxyl-terminal unrepeated region that were typical of M and M-like proteins. Though the amino-terminal regions of the group C and G streptococcal proteins demonstrated a rather high overall similarity between themselves, they were not similar to the variable regions of other M-like proteins with one exception: there was a 46% identity between the first 22 amino acids of the group G streptococcal protein and the corresponding sequence of PAM, the plasminogen-binding M-like protein of type M53 group A streptococci. Like the proteins extracted from the streptococci, the recombinant proteins bound plasminogen, fibrinogen and albumin. The three plasma proteins bound to separate sites on the streptococcal M-like proteins. Plasminogen bound by the group C and G streptococcal proteins was readily activated by streptokinase, providing evidence for a functional link between the secreted plasminogen-activator and proteins exposed on the bacterial surface.

Localization of immunoglobulin A-binding sites within M or M-like proteins of group A streptococci

Many strains of group A streptococci are capable of binding human immunoglobulin A (IgA) by a nonimmune mechanism. M or M-like proteins constitute a family of structurally diverse molecules which form surface fibrillae, and some of the M or M-like protein forms are responsible for the IgA-binding activity. In this report, the binding site for IgA is localized within two structurally distinct M or M-like proteins, ML2.2 and Arp4. Apart from those structural domains which are common to all M and M-like proteins, ML2.2 and Arp4 lack significant levels of amino acid sequence homology, with the exception of a short segment (ALXGENXDLR) located at residues 21 to 30 of the mature ML2.2 protein. Recombinant fusion polypeptides containing portions of the ML2.2 and Arp4 proteins were expressed in Escherichia coli and tested for binding of human myeloma IgA. A 58-residue polypeptide containing residues 14 to 71 of ML2.2 bound human IgA. The IgA-binding site of Arp4 could be localized to a 53-residue polypeptide containing residues 43 to 95, which encompasses the ALXGENXDLR consensus sequence of Arp4 positioned at residues 50 to 59. Site-specific mutagenesis at three codons within the ALXGENXDLR coding sequence of both the ML2.2 and Arp4 recombinant polypeptides leads to a loss in IgA-binding activity. Thus, the ALXGENXDLR consensus sequence is essential for the nonimmune binding of IgA by both ML2.2 and Arp4. However, the failure to bind IgA by polypeptides which partially overlap the 58- and 53-residue IgA-binding polypeptides of ML2.2 and Arp4, yet contain the ALXGENXDLR consensus sequence, strongly suggests that flanking regions are also critical for IgA binding. In summary, the results indicate that common functional domains bearing significant sequence homology are distributed within regions of M or M-like molecules that are otherwise highly divergent.

Role of YadA in resistance of Yersinia enterocolitica to phagocytosis by human polymorphonuclear leukocytes

Pathogenic Yersinia enterocolitica cells do not induce the chemiluminescence response of human polymorphonuclear leukocytes (PMNs). We tested the chemiluminescence response to Y. enterocolitica mutants affected in the known pYV-encoded factors. We did not detect any influence of the Yops in this phenomenon. By contrast, the presence of YadA correlated with a lack of chemiluminescence. The expression of YadA at the bacterial surface also reduced the phagocytosis by PMNs. Finally, we measured the survival of Y. enterocolitica cells confronted with PMNs by the classical plating method and by a new luminometry assay. We observed that YadA+ bacteria were not killed, while YadA- bacteria were killed. We conclude that the presence of YadA at the surface of Y. enterocolitica cells prevents phagocytosis and killing by PMNs. This conclusion is in good agreement with our recent observation that YadA protects Y. enterocolitica from opsonization by C3b.

M protein mediates streptococcal adhesion to HEp-2 cells

Streptococcus pyogenes adheres to human epithelial cells in vitro and in vivo. To identify adhesins, cell wall components were extracted from S. pyogenes M6 with alkali or by treatment with mutanolysin and lysozyme. HEp-2 cells were incubated with extracts of S. pyogenes M6 and then analyzed by Western blot (immunoblot) assays, using antibodies to S. pyogenes. Only one streptococcal component (62 kDa) was bound to HEp-2 cells and was identified serologically as M6 protein. Experiments with pepsin-cleaved fragments of M protein indicated that the binding site was located at the N-terminal half of the molecule. M protein was bound selectively to two trypsin-sensitive surface components, 97 and 205 kDa, of HEp-2 cells on nitrocellulose blots of sodium dodecyl sulfate-polyacrylamide gels. Tritium-labeled lipoteichoic acid bound to different HEp-2 cell components, 34 and 35 kDa, in a parallel experiment, indicating that lipoteichoic acid was not complexed with M protein and does not mediate M-protein binding. The four HEp-2 components were unrelated to fibronectin since they did not react with specific antibodies. An M-protein-deficient (M-) strain of streptococcus (JRS75), grown in chemically defined medium, showed 73% less adhesion activity to HEp-2 monolayers than an M+ strain (JRS4). Streptococcal adhesion was insensitive to competitive inhibition by selected monosaccharides. These results indicate that M protein binds directly to certain HEp-2 cell membrane components and mediates streptococcal adhesion.

Binding ability of complement receptor CR1 to C3 bound on the surface of M+ group A streptococci

A previous study demonstrated that although M+ bacteria bound C3, mainly C3b and iC3b, via the classical pathway of complement activation, they were not phagocytosed by polymorphonuclear leucocytes (PMN). To elucidate this mechanism, we attempted to distinguish between the possibilities that M+ bacteria are effectively adhering to PMN but are not being endocytosed, or that the C3 deposited on M+ bacteria is not able to interact with the complement receptors on PMN. In the present study, we studied the interaction of C3-coated M+ bacteria with complement receptor CR1, which was isolated from the stroma of human erythrocytes. We show that the isolated complement receptor CR1 can associate with C3-coated M+ bacteria as well as with C3-coated M- bacteria, and the C3 deposited on M+ bacteria is cleaved and releases a C3 fragment in the presence of factor I and liquid-phase CR1. These results suggest that the C3 bound on the surface of M+ bacteria is able to promote adherence to the complement receptor CR1 on PMN. We also studied the distribution of C3 deposited on M+ bacteria in normal human serum (NHS) or normal human plasma (NHP). By immunofluorescence, we show that the C3 bound to M+ bacteria in NHS was deposited uniformly over the surface of the bacteria. On the other hand, the C3 bound to M+ bacteria in NHP was deposited at both ends between adjacent daughter cocci. The results suggest that an additional factor contained in NHP is related to the enhancement of anti-phagocytic activity of M+ bacteria.

Short consensus repeat-3 domain of recombinant decay-accelerating factor is recognized by Escherichia coli recombinant Dr adhesin in a model of a cell-cell interaction

A bacterial pathogen that is important in both urinary tract and intestinal infections is Escherichia coli which expresses Dr or related adhesins. In this report, we present a model for testing cell-cell interaction, using both molecularly characterized laboratory cells that express recombinant molecules of human decay-accelerating factor (DAF), and recombinant bacterial Dr colonization factors. Dr adhesin ligand was identified as DAF (CD55), a membrane protein that protects autologous tissues from damage due to the complement system. Structure-function studies mapped the adhesin-binding site on the DAF molecule. A single-point substitution in the third short consensus repeat domain, Ser165 to Leu, corresponding to the Dra to Drb allelic polymorphism, caused complete abolition of adhesin binding to DAF.

Studies on binding of glycosaminoglycans to Streptococcus pyogenes by using 125I-heparan sulphate as a probe

Binding of 125I-heparan sulphate to the cell surface of Streptococcus pyogenes is mediated by proteins, that could be released from the streptococcal cell wall by using alkaline buffer. SDS-electrophoresis revealed two bands with molecular weights of 63 and 58 kDa. Binding of the 125I-labelled heparan sulphate probe to streptococci seems to be due to charge interactions, as the same probe was displaced by unlabelled heparan sulphate, other negatively charged molecules such as heparin, dextran sulphate, dermatan sulphate or by high ionic strength. The interaction was also strongly influenced by pH. The binding constant at pH 7.2 was estimated to be 9.8 x 10(6) mol/l, suggesting a moderate affinity. The presence of collagen of different types enhanced binding of 125I-labelled heparan sulphate to streptococci, whereas fibronectin and vitronectin had an inhibitory effect. The cooperation between heparan sulphate and collagen could be important for the adhesion of streptococci to connective tissue.

Strangles

The etiology, epizootiology, pathogenesis, and clinical presentation of strangles are described. Streptococcus equi, the causative organism, is highly host-adapted to Equidae and shows no antigenic variation. Protective immunity apparently is mediated by a combination of serum opsonic and nasopharyngeal mucosal humoral responses. Vaccines based on M protein or inactivated bacterial suspensions may reduce the clinical attack rate by 50%, a level of protection much lower than that produced during recovery from strangles.

Role of the YadA protein in prevention of opsonization of Yersinia enterocolitica by C3b molecules

When mixed with normal human serum, wild-type pathogenic Yersinia enterocolitica, previously incubated at 37 degrees C, fixed less C3b than its variant cured of the virulence plasmid pYV. Mutants unable to secrete the Yop proteins were still protected against C3b deposition. By contrast, mutants deficient in the production of outer membrane protein YadA fixed more C3b than their YadA+ parent. Gene yadA, cloned as a minimal polymerase chain reaction fragment and introduced in trans, complemented the mutations. Production of YadA by recombinant Escherichia coli LK111 also resulted in a reduction of the amount of C3b deposited on the bacterial surface. The reduction of C3b at the surface of Y. enterocolitica YadA+ compared with YadA- cells correlated with an increase of the amount of factor H fixed at the bacterial surface. The YadA monomer separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to a nitrocellulose membrane was able to bind factor H. We conclude that factor H bound to YadA reduces the C3b deposition on the bacterial surface, probably by a rapid inactivation of C3b.

An M protein with a single C repeat prevents phagocytosis of Streptococcus pyogenes: use of a temperature-sensitive shuttle vector to deliver homologous sequences to the chromosome of S. pyogenes

The major virulence factor of the important human pathogen Streptococcus pyogenes is the M protein, which prevents phagocytosis of the bacterium. In different strains of streptococci, there are over 80 serologically different M proteins and there are additional M-like proteins, some of which bind immunoglobulins. Although the sequence of the M molecules differs among different S. pyogenes strains, all M proteins, and some of the immunoglobulin-binding molecules, have at least two copies of the C repeat region. We describe construction of a deletion mutation in S. pyogenes, which has only one C repeat copy, and show that the mutant strain is still resistant to phagocytosis. The mutation was constructed in vitro and used to replace the resident emm allele in an S. pyogenes strain. To facilitate homologous recombination into the streptococcal chromosome, we adapted a shuttle vector which is temperature sensitive for replication in Gram-positive bacteria but not in Gram-negative hosts. This new method for delivery of a homologous DNA fragment to the S. pyogenes chromosome is efficient and reproducible and should be of general use.