Differences in the aromatic domain of homologous streptococcal fibronectin-binding proteins trigger different cell invasion mechanisms and survival rates.

Group A streptococci (GAS, Streptococcus pyogenes) and Group G streptococci (GGS, Streptococcus dysgalactiae ssp. equisimilis) adhere to and invade host cells by binding to fibronectin. The fibronectin-binding protein SfbI from GAS acts as an invasin by using a caveolae-mediated mechanism. In the present study we have identified a fibronectin-binding protein, GfbA, from GGS, which functions as an adhesin and invasin. Although there is a high degree of similarity in the C-terminal sequence of SfbI and GfbA, the invasion mechanisms are different. Unlike caveolae-mediated invasion by SfbI-expressing GAS, the GfbA-expressing GGS isolate trigger cytoskeleton rearrangements. Heterologous expression of GfbA on the surface of a commensal Streptococcus gordonii and purified recombinant protein also triggered actin rearrangements. Expression of a truncated GfbA (lacking the aromatic domain) and chimeric GfbA/SfbI protein (replacing the aromatic domain of SfbI with the GfbA aromatic domain) on S. gordonii or recombinant proteins alone showed that the aromatic domain of GfbA is responsible for different invasion mechanisms. This is the first evidence for a biological function of the aromatic domain of fibronectin-binding proteins. Furthermore, we show that streptococci invading via cytoskeleton rearrangements and intracellular trafficking along the classical endocytic pathway are less persistence than streptococci entering via caveolae.

Biological functions of GCS3, a novel plasminogen-binding protein of Streptococcus dysgalactiae ssp. equisimilis.

Increasing awareness of the relevance of Streptococcus dysgalactiae ssp. equisimilis as a human pathogen motivates the analysis of its pathomechanisms. One of the mechanisms that increases infectivity and dissemination of several streptococcal species is the recruitment and subsequent activation of host plasminogen on the streptococcal surface. This study identified GCS3 as a novel plasminogen-binding M protein of S. dysgalactiae ssp. equisimilis and revealed a difference in the mode of binding as compared to the plasminogen-binding protein PAM of S. pyogenes. In contrast to PAM, GCS3 did not bind to the kringle 1-3 region of plasminogen. Despite this difference, GCS3 exerts the same function of recruiting plasminogen to the streptococcal surface, which can be activated by streptokinase and host plasminogen activators to serve as a spreading factor. Moreover, we demonstrate a role of GCS3 in plasminogen-dependent streptococcal adherence to human pharyngeal cells (cell line Detroit 562) that indicates an additional function of the protein as an adhesin in the oral cavity.

Identification of active variants of PARF in human pathogenic group C and group G streptococci leads to an amended description of its consensus motif.

Certain streptococcal M proteins bind collagen via an octapeptide motif that is located in their hypervariable N-terminal region. The interaction with this extracellular matrix protein enhances adhesion to the host tissue and thereby facilitates infection. Moreover, it has the side effect of eliciting collagen autoimmune responses, a phenomenon which is also observed in patients with acute rheumatic fever. Therefore, the octapeptide motif was named peptide associated with rheumatic fever (PARF). Only a comprehensive characterization of the collagen-binding M proteins and their collagen-binding motifs will allow the investigation of their associations with certain streptococcal infections and their sequelae. Therefore, a collection of Streptococcus dysgalactiae equisimilis strains that were isolated from infected humans was examined, in order to identify collagen-binding proteins and motifs. Strains that bound collagen independent of a hyaluronic acid capsule belonged to 7 distinct types of the emm gene, which codes for the M protein (emm types). Only one of these emm types was previously described as collagen-binding. Five possessed a PARF sequence. The other 2 emm types stC2sk.0 and stG2574 had PARF-like motifs that diverged from the previously described consensus sequence AXYLZZLN but were able to induce collagen autoimmunity when injected into mice. The results led to the amended PARF consensus (A/E/T)XYLXXLN. Moreover, they demonstrate a predictive power regarding collagen-binding and elicitation of collagen autoimmunity, indicating that PARF may be one of the markers for strains that cause collagen-dependent acute rheumatic fever.

Crucial role of the CB3-region of collagen IV in PARF-induced acute rheumatic fever.

Acute rheumatic fever (ARF) and rheumatic heart disease are serious autoimmune sequelae to infections with Streptococcus pyogenes. Streptococcal M-proteins have been implicated in ARF pathogenesis. Their interaction with collagen type IV (CIV) is a triggering step that induces generation of collagen-specific auto-antibodies. Electron microscopy of the protein complex between M-protein type 3 (M3-protein) and CIV identified two prominent binding sites of which one is situated in the CB3-region of CIV. In a radioactive binding assay, M3-protein expressing S. pyogenes and S. gordonii bound the CB3-fragment. Detailed analysis of the interactions by surface plasmon resonance measurements and site directed mutagenesis revealed high affinity interactions with dissociation constants in the nanomolar range that depend on the recently described collagen binding motif of streptococcal M-proteins. Because of its role in the induction of disease-related collagen autoimmunity the motif is referred to as "peptide associated with rheumatic fever" (PARF). Both, sera of mice immunized with M3-protein as well as sera from patients with ARF contained anti-CB3 auto-antibodies, indicating their contribution to ARF pathogenesis. The identification of the CB3-region as a binding partner for PARF directs the further approaches to understand the unusual autoimmune pathogenesis of PARF-dependent ARF and forms a molecular basis for a diagnostic test that detects rheumatogenic streptococci.

Evaluation of the potential of animal streptococcal isolates belonging to serogroups C and G to elicit acute rheumatic fever.

OBJECTIVE: To determine whether groups C and G streptococci (GCS-GGS) isolated from animals have rheumatogenic traits associated with human GCS-GGS isolates, particularly the potential of the bacteria to interact with human collagen type IV (collagen-IV), known to be targeted during acute rheumatic fever (ARF). SAMPLE POPULATION: 64 GCS and GGS bacterial strains isolated from infected animals. PROCEDURES: Bacteria were analyzed for their ability to bind and aggregate collagen-IV and for the presence of collagen binding factors, such as the hyaluronic acid capsule, cne gene, and emm gene. RESULTS: Collagen-IV binding ability was detected in 19% (n = 12) of the isolates studied. Of the collagen-IV binding strains, 5 expressed hyaluronic acid capsule. Furthermore, emm was detected in the genome of 1 isolate, whereas all remaining collagen-IV binding isolates possessed the cne gene. Of the collagen binding factors investigated, the hyaluronic capsule was the only factor for which collagen-IV interaction could be detected. Investigation of the potential of these strains to aggregate collagen-IV revealed that animal isolates had a nonaggregating phenotype. CONCLUSIONS AND CLINICAL RELEVANCE: Despite efficiently binding collagen-IV via hyaluronic acid, animal isolates lacked the ability to initiate aggregation of this protein. Because collagen-IV aggregation is associated with all collagen-IV-binding rheumatogenic strains, this suggested a lack of rheumatogenic potential among animal-derived GCS and GGS and, therefore, a low chance of acquiring ARF through animal contact.

DNase Sda1 provides selection pressure for a switch to invasive group A streptococcal infection.

Most invasive bacterial infections are caused by species that more commonly colonize the human host with minimal symptoms. Although phenotypic or genetic correlates underlying a bacterium's shift to enhanced virulence have been studied, the in vivo selection pressures governing such shifts are poorly understood. The globally disseminated M1T1 clone of group A Streptococcus (GAS) is linked with the rare but life-threatening syndromes of necrotizing fasciitis and toxic shock syndrome. Mutations in the GAS control of virulence regulatory sensor kinase (covRS) operon are associated with severe invasive disease, abolishing expression of a broad-spectrum cysteine protease (SpeB) and allowing the recruitment and activation of host plasminogen on the bacterial surface. Here we describe how bacteriophage-encoded GAS DNase (Sda1), which facilitates the pathogen's escape from neutrophil extracellular traps, serves as a selective force for covRS mutation. The results provide a paradigm whereby natural selection exerted by the innate immune system generates hypervirulent bacterial variants with increased risk of systemic dissemination.

Upregulation of capsule enables Streptococcus pyogenes to evade immune recognition by antigen-specific antibodies directed to the G-related alpha2-macroglobulin-binding protein GRAB located on the bacterial surface.

One of the major problems associated with the development of a vaccine against Streptococcus pyogenes is the ability of this pathogen to escape recognition by antibodies directed against conserved surface-associated determinants and to establish infection in the setting of an acquired immune response. Identification of the mechanism(s) used by S. pyogenes to avoid recognition by antigen-specific antibodies and escape killing in blood was the focus of this study. We showed here that S. pyogenes was capable of surviving in human blood containing high levels of antibodies directed against the G-related alpha2-macroglobulin-binding protein GRAB, a highly conserved bacterial surface protein. S. pyogenes upregulated the hyaluronic acid capsule production during incubation with human blood, suggesting that the capsule may structurally minimize antibody access to protein GRAB. This hypothesis was confirmed by the ability of anti-GRAB antibodies to promote opsonophagocytosis of a capsule-deficient mutant of S. pyogenes but not of the encapsulated wild-type strain. Capsule upregulation and protection of S. pyogenes from opsonophagocytosis in the presence of anti-GRAB antibodies was also observed in a murine model of streptococcal infection. Thus, masking of surface immunogenic determinants by the hyaluronic acid capsule may constitute a novel mechanism of S. pyogenes for evasion of antigen-specific antibodies.

Identification of a streptococcal octapeptide motif involved in acute rheumatic fever.

Acute rheumatic fever is a serious autoimmune sequela of pharyngitis caused by certain group A streptococci. One mechanism applied by streptococcal strains capable of causing acute rheumatic fever is formation of an autoantigenic complex with human collagen IV. In some geographic regions with a high incidence of acute rheumatic fever pharyngeal carriage of group C and group G streptococci prevails. Examination of such strains revealed the presence of M-like surface proteins that bind human collagen. Using a peptide array and recombinant proteins with targeted amino acid substitutions, we could demonstrate that formation of collagen complexes during streptococcal infections depends on an octapeptide motif, which is present in collagen binding M and M-like proteins of different beta-hemolytic streptococcal species. Mice immunized with streptococcal proteins that contain the collagen binding octapeptide motif developed high serum titers of anti-collagen antibodies. In sera of rheumatic fever patients such a collagen autoimmune response was accompanied by specific reactivity against the collagen-binding proteins, linking the observed effect to clinical cases. Taken together, the data demonstrate that the identified octapeptide motif through its action on collagen plays a crucial role in the pathogenesis of rheumatic fever. Eradication of streptococci that express proteins with the collagen binding motif appears advisable for controlling rheumatic fever.

P159 is a proteolytically processed, surface adhesin of Mycoplasma hyopneumoniae: defined domains of P159 bind heparin and promote adherence to eukaryote cells.

Mycoplasma hyopneumoniae, the causative agent of porcine enzootic pneumonia, colonizes the respiratory cilia of affected swine causing significant economic losses to swine production worldwide. Heparin is known to inhibit adherence of M. hyopneumoniae to porcine respiratory epithelial cilia. M. hyopneumoniae cells bind heparin but the identity of the heparin-binding proteins is limited. Proteomic analysis of M. hyopneumoniae lysates identified 27 kDa (P27), 110 kDa (P110) and 52 kDa (P52) proteins representing different regions of a 159 kDa (P159) protein derived from mhp494. These cleavage fragments were surface located and present at all growth stages. Following purification of four recombinant proteins spanning P159 (F1P159, F2P159, F3P159 and F4P159), only F3P159 and F4P159 bound heparin in a dose-dependent manner (K(d) values 142.37 +/- 22.01 nM; 75.37 +/- 7.34 nM respectively). Scanning electron microscopic studies showed M. hyopneumoniae bound intimately to porcine kidney epithelial-like cells (PK15 cells) but these processes were inhibited by excess heparin and F4P159. Similarly, latex beads coated with F2P159 and F4P159 adhered to and entered PK15 cells, but heparin, F2P159 and F4P159 was inhibitory. These findings indicate that P159 is a post-translationally cleaved, glycosaminoglycan-binding adhesin of M. hyopneumoniae.

Rheumatic fever-associated Streptococcus pyogenes isolates aggregate collagen.

Acute rheumatic fever is a serious autoimmune sequel of Streptococcus pyogenes infection. This study shows that serotype M3 and M18 S. pyogenes isolated during outbreaks of rheumatic fever have the unique capability to bind and aggregate human basement membrane collagen type IV. M3 protein is identified as collagen-binding factor of M3 streptococci, whereas M18 isolates bind collagen through a hyaluronic acid capsule, revealing a novel function for M3 protein and capsule. Following in vivo mouse passage, conversion of a nonencapsulated and collagen-binding negative M1 S. pyogenes into an encapsulated, collagen-binding strain further supports the crucial role of capsule in mediating collagen binding. Collagen binding represents a novel colonization mechanism, as it is demonstrated that S. pyogenes bind to collagen matrix in vitro and in vivo. Moreover, immunization of mice with purified recombinant M3 protein led to the generation of anti-collagen type IV antibodies. Finally, sera from acute rheumatic fever patients had significantly increased titers of anti-collagen type IV antibodies as compared with healthy controls. These findings may suggest a link between the potential of rheumatogenic S. pyogenes isolates to bind collagen, and the presence of collagen-reactive autoantibodies in the serum of rheumatic fever patients, which may form a basis for post-streptococcal rheumatic disease. These anti-collagen antibodies may form a basis for poststreptococcal rheumatic disease.

Streptococcus pyogenes recruits collagen via surface-bound fibronectin: a novel colonization and immune evasion mechanism.

This study aimed to characterize matrix assembly mechanisms on the surface of the human pathogen Streptococcus pyogenes. Among 125 S. pyogenes isolates, 61% were able to recruit collagen type IV via surface-bound fibronectin. Streptococcus gordonii expressing the fibronectin-binding repeat domain of S. pyogenes SfbI protein was equally potent in recruiting collagen, indicating that this domain was sufficient to promote fibronectin-mediated collagen recruitment. Electron microscopic analysis of streptococci revealed that fibronectin-mediated collagen recruitment led to matrix deposition on and between streptococcal cells, which induced the formation of large bacterial aggregates. Furthermore, collagen-recruiting streptococci were able to colonize collagen fibres and were protected from adhering to human polymorphonuclear cells in the presence of opsonizing antibodies. Fibronectin-mediated collagen recruitment thus represents a novel aggregation, colonization and immune evasion mechanism of S. pyogenes.