Unimpaired function of human phagocytes in the presence of phagocytosis-resistant group A streptococci

Augmented adherence and internalization of group A Streptococcus pyogenes to influenza A virus infected MDCK cells

Respiratory tract infections are one of the leading causes of morbidity and mortality. There is considerable epidemiologic evidence that infection with respiratory viruses increases the incidence and severity of secondary bacterial complications. However, very limited number of studies were concerned with the mechanism behind such synergy. In this context, our study aimed to explore the interaction between Group A Streptococcus pyogenes (GAS) and Influenza A virus (IAV). Our results revealed that the GAS adherence and internalization into Madin-Darby canine kidney (MDCK) cells markedly increased after IAV infection. When M6 protein defective mutant of GAS was used, the virus enhanced adherence and internalization was nearly abolished indicating the involvement of M protein binding sites on the MDCK cell surface. Interestingly, the modulation of some O-linked glycolproteins as well as sialic acid, mucin and fibrinogen-like residues on the surface of MDCK cells contributed to augmented bacterial adherence and/or internalization. In the same way, qRT-PCR experiments showed an overexpression of the membrane associated mucin (MUC1) on the surface of the MDCK cells after IAV infection. Altogether, the present study revealed that IAV infection augments the adherence and internalization of GAS to MDCK cells via modulation of membrane associated O-linked glycoproteins, fibrinogen, sialic acid residues and the mucin, MUC1 on the surface of MDCK cell.

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

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

Group A streptococci bind to mucin and human pharyngeal cells through sialic acid-containing receptors

The first step in the colonization of group A streptococci (Streptococcus pyogenes) is adherence to pharyngeal epithelial cells. Prior to adherence to their target tissue, the first barrier that the streptococci encounter is the mucous layer of the respiratory tract. The present study was undertaken to characterize the interaction between mucin, the major glycoprotein component of mucus, and streptococci. We report here that S. pyogenes is able to bind to bovine submaxillary mucin in solid-phase microtiter plate assays. Western blots probed with (125)I-labeled mucin and a panel of monoclonal antibodies revealed that the streptococcal M protein is one of two cell wall-associated proteins responsible for this binding. The binding was further localized to the N-terminal portion of the M molecule. Further analysis revealed that the M protein binds to the sialic acid moieties on mucin, and this interaction seems to be based on M-protein conformation rather than specific amino acid sequences. We found that sialic acid also plays a critical role in the adherence of an M6 streptococcal strain to the Detroit 562 human pharyngeal cell line and have identified alpha2-6-linked sialic acid as an important sialylated linkage for M-protein recognition. Western blot analysis of extracted pharyngeal cell membrane proteins identified three potential sialic acid-containing receptors for the M protein. The results are the first to show that sialic acid not only is involved in the binding of the streptococci to mucin but also plays an important role in adherence of group A streptococci to the pharyngeal cell surface.

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.

Mucosal and systemic immune responses to a recombinant protein expressed on the surface of the oral commensal bacterium Streptococcus gordonii after oral colonization

To circumvent the need to engineer pathogenic microorganisms as live vaccine-delivery vehicles, a system was developed which allowed for the stable expression of a wide range of protein antigens on the surface of Gram-positive commensal bacteria. The human oral commensal Streptococcus gordonii was engineered to surface express a 204-amino acid allergen from hornet venom (Ag5.2) as a fusion with the anchor region of the M6 protein of Streptococcus pyogenes. The immunogenicity of the M6-Ag5.2 fusion protein was assessed in mice inoculated orally and intranasally with a single dose of recombinant bacteria, resulting in the colonization of the oral/pharyngeal mucosa for 10-11 weeks. A significant increase of Ag5.2-specific IgA with relation to the total IgA was detected in saliva and lung lavages when compared with mice colonized with wild-type S. gordonii. A systemic IgG response to Ag5.2 was also induced after oral colonization. Thus, recombinant Gram-positive commensal bacteria may be a safe and effective way of inducing a local and systemic immune response.

Role of fibrinogen in complement inhibition by streptococcal M protein

M protein, the major virulence factor of group A streptococci, has antiopsonic activity in that it inhibits activation of the alternative complement pathway on the streptococcal surface. Two properties of M protein have been claimed to account for the inhibitory activity, namely, (i) its binding affinity for complement factor H, which is an inhibitor of alternative pathway activation, and (ii) its high binding affinity for fibrinogen. We have recently shown that fibrinogen, like M protein, inhibits alternative pathway activation by possessing binding affinity for factor H. Here we report that fibrinogen effectively competes with factor H for binding to M protein but retains its own binding affinity for factor H. The presence of fibrinogen did not significantly affect alternative pathway inhibition on the streptococcal surface.

The amino-terminal region of group A streptococcal M protein determines its molecular state of assembly and function

Group A streptococcal M protein, a major virulence factor, is an alpha-helical coiled-coil dimer on the surface of the bacteria. Limited proteolysis of type 57 streptococcus with pepsin released two fragments of the M57 molecule, with apparent molecular weights of 32,000 and 27,000 on SDS-PAGE. However, on gel filtration under nondenaturing conditions, each of these proteins eluted as two distinct molecular forms. The two forms corresponded to their dimeric and monomeric state as compared to the gel filtration characteristics of known dimeric coiled-coil proteins. The results of sedimentation equilibrium measurements were consistent with this, but further indicated that the "dimeric form" consisted of a dimer in rapid equilibrium with its monomer, whereas the "monomeric form" does not dimerize. The monomeric form was the predominant species for the 27 kD species, whereas the dimeric form predominated for the 32 kD species. Sequence analysis revealed the 27 kD species to be a truncated derivative of the 32 kD PepM57 species, lacking the N-terminal nonheptad region of the M57 molecule. These data strongly suggested that the N-terminal nonheptad region of PepM57 is important in determining the molecular state of the molecule. Consistent with this, PepM49, another nephritis-associated serotype, which lacks the nonheptad N-terminal region, also eluted as a monomer on gel filtration under nondenaturing conditions. Furthermore, removal of the N-terminal nonheptad segment of the dimeric PepM6 protein converted it into a monomeric form. The dimeric molecular form of both the 32 kD PepM57 and the 27 kD PepM57 did not represent a stable state of assembly, and were susceptible to conversion to the corresponding monomeric molecular forms by simple treatments, such as lyophilization. The 27 kD PepM57 exhibited a greater propensity than the 32 kD species to exist in the monomeric form. The 32 kD species contained the opsonic epitope of the M57 molecule, whereas the 27 kD species lacked the same. This is consistent with the previous reports on the importance of the N-terminal region of M protein for its opsonic activity. Together, these results strongly suggest that, in addition to its importance for the biological function, the N-terminal region of the M protein plays a dominant role in determining the molecular state of the M molecule, as well as its stability.

Streptococcal M protein: molecular design and biological behavior

M protein is a major virulence determinant for the group A streptococcus by virtue of its ability to allow the organism to resist phagocytosis. Common in eucaryotes, the fibrillar coiled-coil design for the M molecule may prove to be a common motif for surface proteins in gram-positive organisms. This type of structure offers the organism several distinct advantages, ranging from antigenic variation to multiple functional domains. The close resemblance of this molecular design to that of certain mammalian proteins could help explain on a molecular level the formation of epitopes responsible for serological cross-reactions between microbial and mammalian proteins. Many of the approaches described in the elucidation of the M-protein structure may be applied for characterizing similar molecules in other microbial systems.

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

Isolated complement components were used to study the regulation of the alternative complement pathway C3 convertase (EC 3.4.21.47), also called C3b,Bb, on M protein-carrying (M+) and M protein-lacking (M-) streptococci. Neither M- nor M+ streptococci directly affected the formation or dissociation of the surface-bound C3b,Bb or the inactivation of surface-bound C3b by factor I. However, the activity of the serum control protein of the alternative complement pathway, factor H, in controlling streptococcus-bound C3b and C3b,Bb was 6-8 times stronger on M+ organisms than on M- organisms. Furthermore, M+ streptococci of different serotypes and purified streptococcal M6 protein were shown to selectively bind factor H, the dissociation constants ranging from 4.5 X 10(-6) M to 6 X 10(-7) M. We conclude that the antiphagocytic activity of streptococcal M protein may be due to complement inhibition mediated by the binding of factor H. Binding of a regulatory protein appears to be a previously unrecognized route by which a pathogen is able to evade alternative pathway activation.

Expression of M type 12 protein by a group A streptococcus exhibits phaselike variation: evidence for coregulation of colony opacity determinants and M protein

Three major categories of colony opacity were observed for natural variants of the M type 12 (M12) group A streptococcus strain CS24. Colony opacity variants that switched between two alternative categories at significantly high frequencies were identified and are referred to as switching between more opaque (Op+) and less opaque (Op-) phenotypes. Twenty lineages of such variants were derived for analysis and were assessed for resistance to phagocytosis, acid-extractable M12 antigen, and M12 mRNA, criteria which define the M protein-positive phenotype (M+). Transition from the M+ to the M protein-negative phenotype (M-) correlated with a change from Op+ to Op-. Reversion to the Op+ phenotype was accompanied by reversion to the M+ state in all variants except one and occurred at a higher frequency than the forward M+ to M- switch. These data demonstrate the existence of M12 protein phaselike switching in the group A streptococcus strain CS24. The discovery of an Op+ M- revertant confirmed that colony opacity and M protein can be expressed independently and are distinct gene products. We suggest that coregulation of colony opacity and M protein expression accounts for their association among descendents of strain CS24. Southern blot hybridization analyses of digested genomic DNA from 27 M- variants and 15 M+ revertants were performed with DNA probes containing M12 protein and adjacent upstream sequences. DNA deletions were identified only in two stable M- variants, approximately 1.3 and 1.4 kilobases upstream from the M12 gene, respectively, whereas all unstable M- variants lacked detectable rearrangements. This suggests that deletions within or adjacent to the structural gene are unlikely to be responsible for the reversible switch in M protein expression. However, the association with the stable M- phenotype and the location of these deletions, as well as two other deletions, approximately 0.5 kilobase upstream from the M12 promoter in two previously described variants of strain CS24 suggests that a second gene product is required for full expression of M12 protein synthesis in this strain.

Difference in the structural features of streptococcal M proteins from nephritogenic and rheumatogenic serotypes

The association of only certain M protein serotypes of group A streptococci with acute glomerulonephritis is very well recognized. Structural information on the M protein, a dimeric alpha-helical coiled-coil molecule, has come so far from three rheumatogenic serotypes, 5, 6, and 24. However, M proteins from the nephritogenic serotypes have not been well characterized. In the present study, we have isolated a biologically active 20,000 Mr pepsin fragment of type 49 M protein (PepM49), a nephritogenic serotype, and purified it to homogeneity using DEAE-Sephadex and gel filtration. The amino acid composition of PepM49 is similar to those of the rheumatogenic M protein serotypes PepM5, PepM6, and PepM24. However, the sequence of the NH2-terminal 60 residues of PepM49 shows little homology to any of these M protein serotypes, although the latter have significant homology among themselves. Nevertheless, PepM49 exhibits a strong heptad periodicity in its nonpolar residues, suggesting its overall conformational similarity with the other M molecules. During the course of the present studies, Moravek et al. (17) reported the NH2-terminal sequence of another M protein serotype, PepM1, which also does not exhibit much homology with the PepM5, PepM6, and PepM24 proteins. Our analysis of this sequence revealed that the PepM1 protein also exhibits a heptad periodicity of the nonpolar amino acids. A closer examination has revealed that the pattern of heptad periodicity in PepM49 and PepM1 proteins is more regular and more similar to each other than has been previously seen for the PepM5, PepM6, and PepM24 proteins. PepM1 is also a nephritogenic serotype. Taken together, these findings indicate an underlying conservation of the tertiary structure of the various M protein serotypes, despite the complexity in their antigenic variation and suggest that the nephritogenic M protein serotypes M1 and M49 may be further apart evolutionarily from the rheumatogenic serotypes 5, 6, and 24. The distinct differences in the structural features of the PepM1 and PepM49 proteins relative to the PepM5, PepM6, and PepM24 proteins are also suggestive of a correlation with the earlier broader classification of the group A streptococci into rheumatogenic and nephritogenic serotypes.