Many group A streptococcal strains express two different immunoglobulin-binding proteins, encoded by closely linked genes: characterization of the proteins expressed by four strains of different M-type

Surface proteins of gram-positive bacteria and mechanisms of their targeting to the cell wall envelope

The cell wall envelope of gram-positive bacteria is a macromolecular, exoskeletal organelle that is assembled and turned over at designated sites. The cell wall also functions as a surface organelle that allows gram-positive pathogens to interact with their environment, in particular the tissues of the infected host. All of these functions require that surface proteins and enzymes be properly targeted to the cell wall envelope. Two basic mechanisms, cell wall sorting and targeting, have been identified. Cell well sorting is the covalent attachment of surface proteins to the peptidoglycan via a C-terminal sorting signal that contains a consensus LPXTG sequence. More than 100 proteins that possess cell wall-sorting signals, including the M proteins of Streptococcus pyogenes, protein A of Staphylococcus aureus, and several internalins of Listeria monocytogenes, have been identified. Cell wall targeting involves the noncovalent attachment of proteins to the cell surface via specialized binding domains. Several of these wall-binding domains appear to interact with secondary wall polymers that are associated with the peptidoglycan, for example teichoic acids and polysaccharides. Proteins that are targeted to the cell surface include muralytic enzymes such as autolysins, lysostaphin, and phage lytic enzymes. Other examples for targeted proteins are the surface S-layer proteins of bacilli and clostridia, as well as virulence factors required for the pathogenesis of L. monocytogenes (internalin B) and Streptococcus pneumoniae (PspA) infections. In this review we describe the mechanisms for both sorting and targeting of proteins to the envelope of gram-positive bacteria and review the functions of known surface proteins.

ELISPOT assay for Chlamydia-specific, antibody-producing cells correlated with conventional complement fixation and microimmunofluorescence

Chlamydia antigens cross-reactive with pneumoniae (TWAR), psittaci, and trachomatis strains were used to evaluate the ELISPOT assay for detecting antigen-specific, antibody-secreting cells (ASC). Human blood specimens from healthy and hospitalized persons were randomly collected and tested by coating the nitrocellulose membrane at the base of microtiter wells. Ficoll-separated mononuclear cells from blood specimens collected in EDTA were incubated in the wells with Iscove's growth medium in CO2 atmosphere at 37 degrees C. An IgG-specific conjugate labeled with biotin was used in an avidin-peroxidase chromogen system for indicating the areas (spots) of immunologically committed lymphocytes. Positive specimens had median levels of ASC above 8 per 10(6) cells (range 15-23 ASC/10(6) cells). Evidence that the ELISPOT is reliable, sensitive, and specific includes the following:(1) immunized animal and clinical human specimens in control experiments were selectively reactive in the presence of antigen, but negative without antigen, (2) serologically characterized reference sera demonstrated homologous rather than heterologous reactions with the antigens, (3) conventional complement fixation and microimmunofluorescence on serum fractions of clinical specimens correlated well (P < 0.02) with ELISPOT results that were both TWAR- and psittaci-positive, and (4) the array of specimens (from healthy donors, community hospitalized, and pulmonary service patients) selected for their increasing likelihood in that order for being positive due to illness was then confirmed and supported by their respectively increasing positivity rates (6, 15, and 25%) for TWAR/psittaci combined. The incidence of positive specimens for either TWAR or psittaci was greatest (23/54, 43%) in specimens from the hospitalized patients and least (8/33, 24%) in specimens from healthy individuals. These findings suggest that ELISPOT detects chalmydial antibody production at the cellular level. ELISPOT positivity thus indicates previous exposure and would favor earlier detection.

Influence of growth conditions on expression of immunoglobulin G binding in group A streptococci

Many group A streptococci (GAS) bind the Fc part of IgG. In the present work, the possible influence of growth time and incubation atmosphere on the expression of the IgG binding activity by GAS of various serotypes was studied. Among 13 GAS reference strains, two categories were distinguished on aerobic incubation at 37 degrees C, one expressing similar IgG binding activity at 6 h and 18 h (types M1, M4, M13, M15), and a second one which showed higher binding at 6 h than at 18 h (M9, M14, M22, M25, M48, M49). Only one strain (M36) bound less IgG at 6 h than at 18 h. Seven of the strains (M5, M6, M22, M25, M36, M48, M49) showed higher binding of IgG when grown in a 5% CO2 atmosphere than in air, whereas one strain (M14) showed a reverse pattern and in the remaining five strains, no influence was found. Protease activity was detected in the growth supernatant of most of the strains. For five selected strains, the time of appearance of supernatant protease activity coincided with a decay of surface IgG binding activity. Purified streptococcal cysteine protease was found to reduce or abolish the binding of IgG by each of three studied strains (M1, M13 and M15) and of type M1 or M15 purified IgG binding material. When tested in the stationary phase, a majority of 62 clinical GAS isolates belonging to 6 different M types showed high protease activity but low binding of IgG. We conclude that streptococcal IgG binding is often better expressed on growth in 5% CO2 atmosphere than in air. Furthermore, due to its sensitivity to streptococcal protease, the IgG binding activity is mostly higher during the logarithmic than during the stationary phase of growth.

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.

Protein F, a fibronectin-binding protein of Streptococcus pyogenes, also binds human fibrinogen: isolation of the protein and mapping of the binding region

During screening of a gene library of Streptococcus pyogenes type M15 for fibrinogen-binding material, a protein of approximately 100 kDa, encoded outside the vir region, was found. DNA sequencing revealed this component to be identical to protein F, a fibronectin-binding protein. Isolation of the recombinant protein, termed F15, was performed by the use of fibrinogen affinity chromatography. The affinity constant (Ka) of protein F15 for fibrinogen, 1.25 x 10(7) mol-1, was lower than that for fibronectin, 1.8 x 10(8) mol-1. The fibrinogen-binding domain was located in the N-terminal part of the molecule, while the fibronectin-binding domains, as previously determined, were in the C-terminal portion of protein F. To examine the amino acid sequence heterogeneity of protein F, the 5' part of the prtF gene, corresponding to the N-terminal variable region of the protein, was amplified by PCR from 12 strains of S. pyogenes belonging to six different M-types. Alignment of these nucleotide sequences indicated that the 5' portion of the prtF gene had probably undergone a number of intragenic recombination and horizontal gene transfer events, allowing a pattern of structural diversity of protein F observed earlier for some other streptococcal virulence factors. There was no strict correlation between M-type and nucleotide sequence of the variable region of the prtF gene and, compared to streptococcal M protein, the overall variation observed for protein F appeared more limited.

Two-domain motif for IgG-binding activity by group A streptococcal emm gene products

A biological role for the non-immune binding of human IgG by group A streptococci is evidenced by its strong association with a subpopulation of strains giving rise to tissue-specific infection. IgG-binding activity lies within many of the M and M-like surface proteins (encoded by emm genes), and several structurally distinct IgG-binding sites are known to exist. In this report, two adjacent IgG-binding domains, differing in their specificity for human IgG subclasses, are localized within the M-like protein, protein H. The putative coding regions for the two IgG-binding domains were mapped for 82 epidemiologically unrelated strains. Both coding regions are associated with phylogenetically distant emm genes, supporting a role for horizontal transfer and intergenomic recombination in the evolution of emm genes. In most instances, the two coding regions are tightly linked, suggesting that there exist strong selective pressures to maintain a two-domain binding motif. Both coding regions are found among all strains bearing emm gene markers associated with impetigo lesions as the principal tissue reservoir, but are absent from most strains that exhibit markers for a predominant nasopharyngeal reservoir. The data support the hypothesis that the pathogenic potential of an isolate is dictated, at least in part, by its unique array of multifunctional emm gene products.

Role of mga in growth phase regulation of virulence genes of the group A streptococcus

To determine whether growth phase affects the expression of mga and other virulence-associated genes in the group A streptococcus (GAS), total RNA was isolated from the serotype M6 GAS strain JRS4 at different phases of growth and transcript levels were quantitated by hybridization with radiolabeled DNA probes. Expression of mga (which encodes a multiple gene regulator) and the Mga-regulated genes emm (which encodes M protein) and scpA (which encodes a complement C5a peptidase) was found to be maximal in exponential phase and shut off as the bacteria entered stationary phase, while the housekeeping genes recA and rpsL showed constant transcript levels over the same period of growth. Expression of mga from a foreign phage promoter in a mga-deleted GAS strain (JRS519) altered the wild-type growth phase-dependent transcription profile seen for emm and scpA, as well as for mga. Therefore, the temporal control of mga expression requires its upstream promoter region, and the subsequent growth phase regulation of emm and scpA is Mga dependent. A number of putative virulence genes in JRS4 were shown not to require Mga for their expression, although several exhibited growth phase-dependent regulation that was similar to mga, i.e., slo (which encodes streptolysin O) and plr (encoding the plasmin receptor/glyceraldehyde-3-phosphate dehydrogenase). Still others showed a markedly different pattern of expression (the genes for the superantigen toxins MF and SpeC). These results suggest the existence of complex levels of global regulation sensitive to growth phase that directly control the expression of virulence genes and mga in GAS.

Streptococcal adhesion and colonization

Streptococci express arrays of adhesins on their cell surfaces that facilitate adherence to substrates present in their natural environment within the mammalian host. A consequence of such promiscuous binding ability is that streptococcal cells may adhere simultaneously to a spectrum of substrates, including salivary glycoproteins, extracellular matrix and serum components, host cells, and other microbial cells. The multiplicity of streptococcal adherence interactions accounts, at least in part, for their success in colonizing the oral and epithelial surfaces of humans. Adhesion facilitates colonization and may be a precursor to tissue invasion and immune modulation, events that presage the development of disease. Many of the streptococcal adhesins and virulence-related factors are cell-wall-associated proteins containing repeated sequence blocks of amino acids. Linear sequences, both within the blocks and within non-repetitive regions of the proteins, have been implicated in substrate binding. Sequences and functions of these proteins among the streptococci have become assorted through gene duplication and horizontal transfer between bacterial populations. Several adhesins identified and characterized through in vitro binding assays have been analyzed for in vivo expression and function by means of animal models used for colonization and virulence. Information on the molecular structure of adhesins as related to their in vivo function will allow for the rational design of novel acellular vaccines, recombinant antibodies, and adhesion agonists for the future control or prevention of streptococcal colonization and streptococcal diseases.

Experimental vaccination against group B streptococcus, an encapsulated bacterium, with highly purified preparations of cell surface proteins Rib and alpha

Encapsulated bacteria cause some of the most common diseases in humans. Although the polysaccharide capsules of these pathogens have attracted the most attention with regard to vaccine development, recent evidence suggests that bacterial surface proteins may also be used to confer protective immunity. We have analyzed this possibility in group B streptococcus (GBS), an encapsulated bacterium that is the major cause of invasive bacterial disease in the neonatal period. Previous work has shown that the majority of GBS strains causing invasive infections express the Rib protein, and that most strains lacking Rib express a protein designated alpha. Here we report that active immunization with highly purified preparations of Rib or alpha protected mice against lethal infection with strains expressing the corresponding protein. Vaccination with the Rib protein protected against two strains of capsular type III and two strains of type II, and vaccination with the alpha protein protected against one strain of type II and one strain of type Ib. The mice vaccinated with Rib or alpha showed a good immunoglobulin G response to the immunogen. These data suggest that a vaccine against GBS disease may be based on cell surface proteins and support the notion that proteins may be used for immunization against encapsulated bacteria.

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.

Structure of a group C streptococcal protein that binds to fibrinogen, albumin and immunoglobulin G via overlapping modules

Pathogenic streptococci express surface proteins that bind to host serum proteins. A novel multiple-ligand-binding protein has now been identified in a species belonging to serotype C streptococci. This protein binds to fibrinogen, albumin and IgG and was therefore designated FAI protein. The structure of the fai gene has been determined, and deletion analysis and expression of FAI fusion polypeptides revealed that the binding domain for fibrinogen and IgG is located within the nonrepetitive N-terminal half of the protein. A 93-amino acid peptide retained the ability to bind both proteins, whereas a 56-amino acid subpeptide only bound fibrinogen. IgG-binding activity required the complete fibrinogen-binding domain and an additional 37 amino acids C-terminal to it, and albumin-binding activity was only obtained with a polypeptide reflecting the complete surface-exposed region of FAI protein indicating that the binding sites for each ligand were located on overlapping modules. Signal sequence, C repeat region and C-terminus revealed high homology to group A streptococcal M proteins whereas the N-terminal region containing the fibrinogen/IgG-binding domains is completely different and exhibits no similarity to any other previously characterized protein. Thus FAI protein exhibits a framework structure that might have evolved in group C streptococci via fusion of unrelated sequences, thereby generating an albumin-binding domain in the functional context of multiple-ligand-binding activity.

Expression of both M protein and hyaluronic acid capsule by group A streptococcal strains results in a high virulence for chicken embryos

The human pathogenic microorganism Streptococcus pyogenes can resist against phagocytic attack of human granulocytes. Streptococcal M protein and hyaluronic acid were identified as virulence factors involved in this protection. So far, no experiments have been reported which describe the contribution of both components together in one system. We used the chicken embryo as an in vivo phagocytosis model to investigate the role of both components on the virulence of streptococci. For this, isogeneic mutants of group A streptococcal strains (GAS) which lack hyaluronic acid capsule (cap-) or M protein (M-) expression were used for infection and their virulence was compared with laboratory strains which had lost their ability to produce one or both virulence factors after long-time laboratory passages on blood agar. The experiments revealed that strains producing both M protein and hyaluronic capsule were highly virulent. Only 1-10 colony-forming units were enough to cause a 50% lethality of 12-day-old chicken embryos. Those strains lacking one of these components showed a significant decrease in virulence. Finally, strains which failed to express either hyaluronic acid or M protein showed an additional tenfold decrease in virulence. This indicates a partial contribution of both M protein and hyaluronic acid to the virulence of GAS in the chicken embryo.

Characterization of a type II'o group A streptococcal immunoglobulin-binding protein

The opacity factor positive M type 2 group A streptococcal isolate, A207, expresses a unique functional type II'o IgG-binding protein which reacts with all four human IgG subclasses and rabbit IgG. In order to determine the gene product or products responsible for this activity, three genes of the vir regulon from this isolate were cloned, expressed and analysed. The fcr A2 gene coded for a protein binding hyman IgG1, IgG2 and IgG4 but not IgG3. The enn2 gene coded for a protein reacting exclusively with human IgA, while the emmL2 gene product bound IgG1, IgG2, IgG3 and IgG4 as well as rabbit but not horse or pig IgG. The IgG3-binding activity of the EmmL2 protein was functionally indistinguishable from the Form 1 IgG3-binding activity present in heat extracts of group A isolate A207.

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 a 52-kilodalton immunoglobulin G-binding protein from Streptococcus suis capsular type 2

We previously reported that group D streptococci exhibited immunoglobulin G (IgG)-binding activity and that a 52-kDa IgG-binding protein was present in all Streptococcus suis strains examined (B. Serhir, R. Higgins, B. Foiry, and M. Jacques, J. Gen. Microbiol. 139:2953-2958, 1993). The objective of the present study was to purify and characterize this protein. Pig IgG were immobilized through their Fab fragments to ECH-Sepharose 4B, and the protein was purified by affinity chromatography. Electron microscopy observations of the purified material showed filamentous structures with a diameter of approximately 4 nm; these structures were not observed when the material was treated with either urea or ethanolamine. Electrophoretic and Western immunoblot analyses showed that the 52-kDa protein constituted the bulk of the recovered material. This protein was stained with either Coomassie brilliant blue or silver nitrate; it reacted with a large variety of mammalian IgG, human IgG (Fc) fragments, human IgA, and other human plasma proteins. The 52-kDa protein exhibited lower IgG-binding affinities than protein A and protein G. However, it was able to compete with protein A and protein G for binding to human IgG. In addition, it bound chicken IgG with high affinity. This last property differentiated the 52-kDa protein of S. suis from the six IgG-binding proteins described to date. The 52-kDa protein displayed similar affinities for untreated and deglycosylated pig IgG. The N-terminal amino acid sequence (SIITDVYAXEVLDSXGNPTLEV) revealed no homology with any bacterial proteins in the Swiss-Prot database. Its isoelectric point of approximately 4.6 and its amino acid composition, rich in aspartic and glutamic acids, showed that it had some similarities with other IgG-binding proteins. In this report, we have purified and characterized a 52-kDa IgG-binding protein from S. suis capsular type 2. Although this protein shares some similarities with other IgG- and/or IgA-binding proteins, it is unique in reacting with chicken IgG.

Characterization of a gene coding for a type IIo bacterial IgG-binding protein

Two antigenic classes of non-immune IgG-binding proteins can be expressed by group A streptococci. One antigenic group of proteins is recognized by an antibody prepared against the product of a cloned fcrA gene (anti-FcRA). In this study, the immunogen used to prepare the antibody that defines the second antigenic class was shown to be the product of the emm-like (emmL) gene of M serotype 55 group A isolate, A928. The emmL55 gene expressed in E. coli produced an M(r) approximately 58,000 molecule which bound human IgG1, IgG2, IgG3 and IgG4, as well as horse, rabbit and pig IgG in a non-immune fashion. These properties are characteristic of the previously described type IIo IgG-binding protein isolated from this strain. In addition, the recombinant protein was reactive with human serum albumin and fibrinogen. The emmL 55 gene sequence was analysed and found to have the organization and sequence characteristics of a typical class I emm-like gene.

Streptococcal cysteine proteinase releases biologically active fragments of streptococcal surface proteins

Streptococcus pyogenes are important pathogenic bacteria which produce an extracellular cysteine proteinase contributing to their virulence and pathogenicity. S. pyogenes also express surface molecules, M proteins, that are major virulence determinants due to their antiphagocytic property. In the present work live S. pyogenes bacteria of the M1 serotype were incubated with purified cysteine proteinase. Several peptides were solubilized, and analysis of their protein-binding properties and amino acid sequences revealed two internal fibrinogen-binding fragments of M1 protein (17 and 21 kDa, respectively), and a 36-kDa IgG-binding NH2-terminal fragment of protein H, an IgGFc-binding surface molecule. M protein also plays a role in streptococcal adherence, and removal of this and other surface proteins could promote bacterial dissemination, whereas the generation of soluble complexes between immunoglobulins and immunoglobulin-binding streptococcal surface proteins could be an etiological factor in the development of glomerulonephritis and rheumatic fever. Thus, in these serious complications to S. pyogenes infections immune complexes are found in affected organs. The cysteine proteinase also solubilized a 116-kDa internal fragment of C5a peptidase, another streptococcal surface protein. Activation of the complement system generates C5a, a peptide stimulating leukocyte chemotaxis. C5a-mediated granulocyte migration was blocked by the 116-kDa fragment. This mechanism, by which phagocytes could be prevented from reaching the site of infection, may also contribute to the pathogenicity and virulence of S. pyogenes.

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.

Sequencing of genes within the vir regulon of Streptococcus pyogenes type M15--an opacity factor-positive serotype with low opacity factor expression

Major virulence determinants of group A streptococci, such as M-protein, immunoglobulin Fc-receptors (FcRA, EmmL) and C5a peptidase, appear to be genetically co-regulated, their genes being located within a vir regulon. We studied the organization of these genes in a group A, type M15 strain of Streptococcus pyogenes, previously defined as OF-, by hybridization analysis of chromosomal DNA and of an S. pyogenes gene library in Escherichia coli, and by gene sequencing. Within the vir regulon, in addition to the virR and scpA genes, three so-called emm-related genes were found: fcrA, emmL and enn. Whereas IgG Fc-binding proteins were encoded by fcrA and emmL, the product of enn was not identified. The presence of three emm-related genes in this region is reminescent of vir regulon organization in OF+ rather than OF- strains as earlier defined by others. Furthermore, analysis of the deduced product of the emmL gene showed deletions and amino acid substitutions within the PGTS-rich domain and membrane anchor, which thus resembles corresponding products of OF+ rather than OF- strains. In view of these findings, the opacity factor (OF) activity of the strain was tested using growth supernatant, with negative outcome. However, a concentrated SDS cell extract revealed definite OF activity. One of two other type M15 reference strains also showed definite OF activity in SDS extracts. We therefore propose that type M15 strains belong to the OF+ category but often show low levels of expression of OF.

Cloning, sequencing, and expression of a fibronectin/fibrinogen-binding protein from group A streptococci

Lipoteichoic acid and several streptococcal proteins have been reported to bind fibronectin (Fn) or fibrinogen (Fgn), which may serve as host receptors. We searched for such proteins by screening a library of genes from M type 5 group A streptococci cloned into Escherichia coli. Lysates of clones were probed with biotinylated Fn and biotinylated Fgn. One clone expressed a 54-kDa protein that reacted with Fn and Fgn. The protein, termed FBP54, was purified and used to immunize rabbits. Anti-FBP54 serum reacted with purified, recombinant FBP54 and with a protein of similar electrophoretic mobility in extracts of M type 5, 6, and 24 streptococci. Anti-FBP54 serum also reacted with 5 of 15 strains of intact, live streptococci, suggesting that FBP54 may be a surface antigen. Southern blot analysis confirmed that the gene is found in group A streptococci but not in Staphylococcus aureus or E. coli. The cloned gene was sequenced and contained an open reading frame encoding a protein with a calculated molecular weight of 54,186. Partial amino acid sequencing of purified FBP54 confirmed that this open reading frame encoded the protein. As determined by utilizing fusion proteins containing truncated forms of FBP54, the primary Fn/Fgn-binding domain appears to be contained in residues 1 to 89. These data suggest that FBP54 may be a surface protein of streptococci that reacts with both Fn and Fgn and therefore may participate in the adhesion of group A streptococci to host cells.

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.

Streptococcus pyogenes type M12 protein shows selective binding to some human immunoglobulin G3 myeloma proteins

Purified, recombinant M12 protein from Streptococcus pyogenes CS24 has recently been demonstrated to bind human immunoglobulin G3 (IgG3). The binding site for IgG has been localized to an internal peptide encoded by a PvuII fragment of the gene emm12. We have investigated the ability of an isolated recombinant M12 protein consisting of the peptide encoded by the PvuII fragment to bind various monoclonal human IgG3 myeloma proteins representing a number of both Caucasian and Oriental IgG3 Gm(allotypic) phenotypes. Of nine Caucasian IgG3 myeloma proteins, only two bound strongly to the recombinant M12 protein in enzyme-linked immunosorbent assays. The allotypic phenotypes of the reactive proteins were IgG3m(b+)(g-) and IgG3m(b-)(g+). No binding was seen for seven IgG3 myeloma proteins of Oriental origin with G3m(st+)(u-)(b+)(g-), G3m(st-)(u+)(b+)(g-), G3m(st-)(u+)(b-)(g+), and G3m(st-)(u-)(b-)(g+) phenotypes. The binding of human IgG3 to M12 protein seems to be related to features other than its Gm allotypic markers. Selective reactivity of IgG3 myeloma proteins with M12 protein may provide another way to subclassify human IgG3 molecules. The biological significance of the selective reactivity is not known.

M-like, immunoglobulin-binding protein of Streptococcus pyogenes type M15

An M-like protein from Streptococcus pyogenes type M15 strain EF1949 (EMML15) was cloned in Escherichia coli and sequenced. Recombinant EMML15 protein revealed a unique binding pattern for human IgG subclasses not described previously. Comparative analysis of the EMML15 amino acid sequence with those of other M-like proteins of opacity factor positive (OF+) serotypes and protein H, and IgG receptor from OF- serotype M1, showed that IgG-binding proteins with common binding of IgG3 were closely related and distinct from streptococcal IgG receptors not binding IgG3. Thus, the Ig-binding proteins from S. pyogenes were subdivided into two main categories according to binding pattern, protein structure, and gene location.

M1 protein and protein H: IgGFc- and albumin-binding streptococcal surface proteins encoded by adjacent genes

M1 protein and Protein H are surface proteins simultaneously present at the surface of certain strains of Streptococcus pyogenes, important pathogenic bacteria in humans. The present study concerns the structure, protein-binding properties and relationship between these two molecules. The gene encoding M1 protein (emm1) was found immediately upstream of the Protein H gene (sph). Both genes were preceded by a promoter region. Comparison of the sequences revealed a high degree of similarity in the signal peptides, the C repeats located in the central parts of the molecules and in the C-terminal cell-wall-attached regions, whereas the N-terminal sequences showed no significant similarity. Protein H has affinity for the Fc region of IgG antibodies. Also M1 protein, isolated from streptococcal culture supernatants or from Escherichia coli expressing emm1, was found to bind human IgGFc. When tested against polyclonal IgG from eight other mammalian species, M1 protein and Protein H both showed affinity for baboon, rabbit and pig IgG. M1 protein also reacted with guinea-pig IgG, whereas both streptococcal proteins were negative in binding experiments with rat, mouse, bovine and horse IgG. The two proteins were also tested against other members of the immunoglobulin super family: human IgM, IgA, IgD, IgE, beta 2-microglobulin, and major histocompatibility complex (MHC) class-I and class-II antigens. M1 protein showed no affinity for any of these molecules whereas Protein H reacted with MHC class-II antigens. M1 protein is known to bind albumin and fibrinogen also. The binding sites for these two plasma proteins and for IgGFc were mapped to different sites on M1 protein. Thus albumin bound to the C repeats and IgGFc to a region (S) immediately N-terminal of the C repeats. Finally, fibrinogen bound further towards the N-terminus but close to the IgGFc-binding site. On the fibrinogen molecule, fragment D was found to mediate binding to M1 protein. The IgGFc-binding region of M1 protein showed no similarity to that of Protein H. Still, competitive binding experiments demonstrated that the two streptococcal proteins bound to overlapping sites on IgGFc.

Analysis of genes encoding two unique type IIa immunoglobulin G-binding proteins expressed by a single group A streptococcal isolate

An emm-like gene (emmL) and a fcrA gene from group A streptococcal strain 64/14 (emmL64/14 and fcrA64/14) were amplified by PCR and force cloned into the heat-inducible expression vector pJLA 602. The emmL gene encoded a recombinant protein that bound human IgG1, IgG2, and IgG4 in a nonimmune fashion. This is the reactivity profile of a type IIa IgG-binding protein. The emmL64/14 gene product was antigenically similar to the previously identified high-molecular-weight type IIa IgG-binding protein of strain 64/14 and had an N-terminal sequence identical to that of the wild-type protein. The fcrA gene also encoded a recombinant protein with type IIa functional activity. This protein was similar to the lower-molecular-weight type IIa IgG-binding protein previously isolated from strain 64/14 and was antigenically distinct from the higher-molecular-weight type IIa protein encoded by the emmL64/14 gene. The sequences for both genes including the intervening regions are presented. The emmL gene demonstrates significant homology to other class I emm and emmL genes expressed by opacity factor-negative group A streptococcal isolates. The fcrA gene was found to be homologous to other fcrA genes normally present in opacity factor-positive group A isolates. The sequence upstream of the fcrA gene and the intervening sequence between the end of the fcrA gene and the start of the emmL gene were similar to those reported for other fcrA genes.

Horizontal gene transfer in the evolution of group A streptococcal emm-like genes: gene mosaics and variation in Vir regulons

Most M type 5 group A streptococcal strains were found to contain a single emm-like gene between virR and scpA (the Vir regulon), but two distinct emm-like genes were identified in the Vir regulon of the M5 strain NCTC8193. The complete sequences of both of these genes were determined. One, called emm5.8193, was shown to be a minor variant of the previously described emm5 gene from strain Manfredo. The second, designated enn5.8193, expresses an IgG-binding protein when cloned in Escherichia coli. A comparison of enn5.8193 with emm-like gene sequences from other strains indicated that it has a mosaic structure, consisting of distinct segments originating from emm-like genes in different OF+ and OF- strains. These data provide the first clear evidence that the horizontal transfer of emm-like sequences between distinct strains contributes to the evolution of group A streptococcal emm-like genes and Vir regulons.

Susceptibility of chicken embryos to group A streptococci: correlation with fibrinogen binding

One problem in investigating group A streptococcal infections and virulence is the lack of appropriate in vivo models. In this study we introduce the chicken embryo model for determining virulence of Streptococcus pyogenes. We found that M protein positive strains, if administered intravenously, were highly virulent for 12-day-old chicken embryos. The LD50 of the strains tested could be correlated directly with the amount of cell wall exposed M protein, which has been determined by the capacity of streptococci to bind fibrinogen and by the ability of streptococci to survive in fresh normal human blood. The number of colony forming units (cfu) of M+ strains necessary to kill 50% of embryonated eggs was significantly lower (< 10(2) cfu) than for M-variants (> 10(4) cfu). Albumin and/or IgG binding streptococcal cells, which can also take place in proteins of the M protein family which do not bind to fibrinogen, did not show that clear correlation to the virulence in chicken embryos that did fibrinogen binding. Application of anti-streptococcal M protein antisera from chicken and rabbit reduced the lethality of the chicken embryos. In contrast, no correlation was found between lethality of chicken embryos and the in vitro production of erythrogenic toxins by the administered strains. Thus the results indicate that the presence of M-protein with its fibrinogen binding activity on the streptococcal cell surface is necessary for virulence of group A streptococci in the chicken embryo model.

Prevalence of IgA receptors in clinical isolates of Streptococcus pyogenes and Streptococcus agalactiae: serologic distinction between the receptors by blocking antibodies

Group A and B streptococci (Streptococcus pyogenes and Streptococcus agalactiae) are the only known bacterial pathogens expressing IgA Fc-receptors. However, the IgA binding proteins of the two species have been found genetically unrelated. In the present investigation the binding of human IgA among clinical isolates of group A and group B streptococci was studied and the respective IgA-binding epitopes were compared serologically. Surface binding of radiolabelled, monoclonal human IgA1 occurred in 38% of 115 unselected group A streptococcal isolates. Comparing four predominant T-types, IgA-binding was found in 77% and 85%, respectively, of types T4 and T28 strains but only in 5% and 25%, respectively, of T1 and T12 strains. In group B streptococci, 70% of 58 type Ib strains but only 2% of 399 strains of other serotypes bound IgA. Using rabbit immune sera raised to the two streptococcal species it was found that strains exhibiting IgA Fc-receptors often induced antibodies blocking the binding of IgA to bacteria. Furthermore, the blocking shown by an individual serum was restricted to the streptococcal group used for immunization showing that also the IgA-binding eptiopes in group A and B streptococci are conformationally distinct. Though infections with serotypes often binding IgA, compared to other types, are not known to differ, it is assumed that the non-immune binding of IgA might favour mucosal colonization of the organisms.

Antigenic diversity of IgA receptors in Streptococcus pyogenes

In a previous study, group A and group B streptococcal IgA receptors were shown to differ serologically, in agreement with their known structural unrelatedness. The present study was undertaken to serologically compare the IgA binding epitopes of group A streptococcal strains representing various serotypes by the use of antisera to this species. It was found that blocking antibodies occurred in antisera to IgA binding but not to non-binding strains and that binding of IgA to a streptococcal strain was generally blocked by antiserum to the homologous type. However, cross-testing of a panel of 11 IgA binding strains, representing various M and T serotypes, with 10 different antisera to group A streptococci, demonstrated that IgA receptors were inhibited to a highly variable degree and that inhibition patterns were unique for each type. Comparing solubilized IgA receptors of various strains in immunoblot experiments, a variation in the molecular mass, between approximately 35 and 45 kDa, emerged. The IgA binding epitopes, analogous to protective sites of streptococcal M-protein, thus exhibited hypervariability which may suggest that IgA binding also plays a key role for evading host immune defence mechanisms.

Protein rib: a novel group B streptococcal cell surface protein that confers protective immunity and is expressed by most strains causing invasive infections

The group B Streptococcus, an important cause of invasive infections in the neonate, is classified into four major serotypes (Ia, Ib, II, and III) based on the structure of the polysaccharide capsule. Since the capsule is a known virulence factor, it has been extensively studied, in particular in type III strains, which cause the majority of invasive infections. Two cell surface proteins, alpha and beta, have also been studied in detail since they confer protective immunity, but these proteins are usually not expressed by type III strains. We describe here a cell surface protein, designated protein Rib (resistance to proteases, immunity, group B), that confers protective immunity and is expressed by most strains of type III. Protein Rib was first identified as a distinct 95-kD protein in extracts of a type III strain, and was purified to homogeneity from that strain. Rabbit antiserum to protein Rib was used to demonstrate that it is expressed on the cell surface of 31 out of 33 type III strains, but only on 1 out of 25 strains representing the other three serotypes. Mouse protection tests showed that antiserum to protein Rib protects against lethal infection with three different strains expressing this antigen, including a strain representing a recently identified high virulence type III clone. Protein Rib is immunologically unrelated to the alpha and beta proteins, but shares several features with the alpha protein. Most importantly, the NH2-terminal amino acid sequences of the Rib and alpha proteins are identical at 6 out of 12 positions. In addition, both protein Rib and the alpha protein are relatively resistant to trypsin (and Rib is also resistant to pepsin) and both proteins vary greatly in size between different clinical isolates. Finally, both protein Rib and the alpha protein exhibit a regular ladderlike pattern in immunoblotting experiments, which may reflect a repetitive structure. Taken together, these data suggest that the Rib and alpha proteins are members of a family of proteins with related structure and function. Since protein Rib confers protective immunity, it may be valuable for the development of a protein vaccine against the group B Streptococcus, an encapsulated bacterium.

Three different types of organization of the vir regulon in group A streptococci

The DNA of group A streptococci (GAS) encodes several important virulence factors such as the antiphagocytic M protein, the Ig-Fc-binding M-related proteins (FcrA-like and EnnX-like) and the complement factor-inactivating C5a peptidase. The corresponding genes emm, fcrA, ennX, and scpA, respectively, were assumed to be located close together in the GAS genome. Additionally, emm and scpA have been found to be under the positive, coordinate control of the virR locus, which led to the designation "vir regulon" for the corresponding genomic segment. In order to map the vir regulons of many GAS serotypes and to analyse any correlation between the organization of vir regulons and circumscribed heterogeneities within the emm, virR, and scpA genes, an approach using several distinct sets of polymerase chain reaction (PCR) experiments was chosen. By examination of the genomic DNA of 42 GAS isolates from 36 different M serotypes three patterns of vir regulon topography were found. The first, designated "large vir regulon" (LVR), consists of virR--fcrA(-like)--emm--ennX(-like)--scpA. The second, designated "small vir regulon" (SVR), contains virR--emm--scpA, and the last, designated "unusual vir regulon" (UVR), resembles SVR but contains additional heterogeneous sequences between emm and scpA. The patterns correlate with heterogeneities at the 3' ends of the virR and scpA genes, with the M classification system and the occurrence of specific non-coding intervening sequences within the vir regulons. The potential impact of these patterns on models to account for generation of vir regulons is discussed.

Two major classes in the M protein family in group A streptococci

The M protein family of molecules in the group A streptococcus comprises a number of cell surface proteins that interact with the immune system of the host. One of the proteins in this family is the IgA receptor Arp4, which has C repeats similar to those that characterize the known M proteins. The streptococcal strain expressing Arp4 also expresses a second immunoglobulin-binding protein, Mrp4, which is shown here to be encoded by a gene located immediately upstream of the gene for Arp4. In addition to binding IgG, Mrp4 also binds fibrinogen, a property ascribed to M proteins. DNA sequence analysis demonstrated that the Mrp4 protein indeed is a member of the M protein family, but it was unexpectedly found to have a type of repeat that is identical to the A repeat described for FcRA76, a partially sequenced streptococcal Fc receptor. Purified FcRA76 was shown to bind fibrinogen and IgG, like Mrp4. These data show that the known molecules in the M protein family can be divided into two classes, A and C, according to the type of repeat region found. Hybridization studies with a panel of clinical isolates indicate that many streptococcal strains express class A and class C proteins, whereas some strains express only class C proteins. Class A molecules show amino-terminal sequence variation, like class C molecules, which suggests that proteins of both classes are targets for the immune response.