Role of fibrinogen in complement inhibition by streptococcal M protein

Complement and coagulation crosstalk - Factor H in the spotlight

The immune complement and the coagulation systems are blood-based proteolytic cascades that are activated by pathway-specific triggers, based on protein-protein interactions and enzymatic cleavage reactions. Activation of these systems is finely balanced and controlled through specific regulatory mechanisms. The complement and coagulation systems are generally viewed as distinct, but have common evolutionary origins, and several interactions between these homologous systems have been reported. This complement and coagulation crosstalk can affect activation, amplification and regulatory functions in both systems. In this review, we summarize the literature on coagulation factors contributing to complement alternative pathway activation and regulation and highlight molecular interactions of the complement alternative pathway regulator factor H with several coagulation factors. We propose a mechanism where factor H interactions with coagulation factors may contribute to both complement and coagulation activation and regulation within the haemostatic system and fibrin clot microenvironment and introduce the emerging role of factor H as a modulator of coagulation. Finally, we discuss the potential impact of these protein interactions in diseases associated with factor H dysregulation or deficiency as well as evidence of coagulation dysfunction.

Vaccine Approaches To Protect against Group A Streptococcal Pharyngitis

Streptococcal pharyngitis (or strep throat) is a common childhood disease affecting millions of children each year, but it is one of the only childhood diseases for which a vaccine does not exist. While for decades the development of a vaccine has been the center of attention in many laboratories worldwide, with some successes, no corporate development has yet to be initiated. The reason for this probably lies in our inability to conclusively identify the streptococcal molecule or molecules responsible for the heart cross-reactive antibodies observed in the serum of rheumatic fever patients. Without this specific knowledge, any streptococcal vaccine antigen is suspect and thus not the target for a billion-dollar investment, despite the fact that the exact role of cross-reactive antibodies in rheumatic fever is still questionable. This article will describe the development of several approaches to protect against Streptococcus pyogenes infections over the past several decades.

Surface Proteins on Gram-Positive Bacteria

Surface proteins are critical for the survival of gram-positive bacteria both in the environment and to establish an infection. Depending on the organism, their surface proteins are evolutionarily tailored to interact with specific ligands on their target surface, be it inanimate or animate. Most surface molecules on these organisms are covalently anchored to the peptidoglycan through an LPxTG motif found at the C-terminus. These surface molecules are generally modular with multiple binding or enzymatic domains designed for a specific survival function. For example, some molecules will bind serum proteins like fibronectin or fibrinogen in one domain and have a separate function in another domain. In addition, enzymes such as those responsible for the production of ATP may be generally found on some bacterial surfaces, but when or how they are used in the life of these bacteria is currently unknown. While surface proteins are required for pathogenicity but not viability, targeting the expression of these molecules on the bacterial surface would prevent infection but not death of the organism. Given that the number of different surface proteins could be in the range of two to three dozen, each with two or three separate functional domains (with hundreds to thousands of each protein on a given organism), exemplifies the complexity that exists on the bacterial surface. Because of their number, we could not adequately describe the characteristics of all surface proteins in this chapter. However, since the streptococcal M protein was one of the first gram-positive surface protein to be completely sequenced, and perhaps one of the best studied, we will use M protein as a model for surface proteins in general, pointing out differences with other surface molecules when necessary.

Binding host proteins to the M protein contributes to the mortality associated with influenza-Streptococcus pyogenes superinfections

The mortality associated with influenza A virus (IAV) is often due to the development of secondary bacterial infections known as superinfections. The group A streptococcus (GAS) is a relatively uncommon cause of IAV superinfections, but the mortality of these infections is high. We used a murine model to determine whether the surface-localized GAS M protein contributes to the outcome of IAV-GAS superinfections. A comparison between wild-type GAS and an M protein mutant strain (emm3) showed that the M3 protein was essential to virulence. To determine whether the binding, or recruitment, of host proteins to the bacterial surface contributed to virulence, GAS was suspended with BALF collected from mice that had recovered from a sub-lethal infection with IAV. Following intranasal inoculation of naïve mice, the mortality associated with the wild-type strain, but not the emm3 mutant strain, was greater compared to mice inoculated with GAS suspended with either BALF from uninfected mice or PBS. Further analyses showed that both albumin and fibrinogen (Fg) were more abundant in the respiratory tract 8 days after IAV infection, that M3 bound both proteins to the bacterial surface, and that suspension of GAS with either protein increased GAS virulence in the absence of antecedent IAV infection. Overall, the results showed that M3 is essential to the virulence of GAS in an IAV superinfection and suggested that increased abundance of albumin and Fg in the respiratory tract following IAV infection enhanced host susceptibility to secondary GAS infection.

The Transcriptional Regulator CpsY Is Important for Innate Immune Evasion in Streptococcus pyogenes

As an exclusively human pathogen, Streptococcus pyogenes (the group A streptococcus [GAS]) has specifically adapted to evade host innate immunity and survive in multiple tissue niches, including blood. GAS can overcome the metabolic constraints of the blood environment and expresses various immunomodulatory factors necessary for survival and immune cell resistance. Here we present our investigation of one such factor, the predicted LysR family transcriptional regulator CpsY. The encoding gene, cpsY, was initially identified as being required for GAS survival in a transposon-site hybridization (TraSH) screen in whole human blood. CpsY is homologous with transcriptional regulators of Streptococcus mutans (MetR), Streptococcus iniae (CpsY), and Streptococcus agalactiae (MtaR) that regulate methionine transport, amino acid metabolism, resistance to neutrophil-mediated killing, and survival in vivo Our investigation indicated that CpsY is involved in GAS resistance to innate immune cells of its human host. However, GAS CpsY does not manifest the in vitro phenotypes of its homologs in other streptococcal species. GAS CpsY appears to regulate a small set of genes that is markedly different from the regulons of its homologs. The differential expression of these genes depends on the growth medium, and CpsY modestly influences their expression. The GAS CpsY regulon includes known virulence factors (mntE, speB, spd, nga [spn], prtS [SpyCEP], and sse) and cell surface-associated factors of GAS (emm1, mur1.2, sibA [cdhA], and M5005_Spy0500). Intriguingly, the loss of CpsY in GAS does not result in virulence defects in murine models of infection, suggesting that CpsY function in immune evasion is specific to the human host.

A versatile assay to determine bacterial and host factors contributing to opsonophagocytotic killing in hirudin-anticoagulated whole blood

Most bacteria entering the bloodstream will be eliminated through complement activation on the bacterial surface and opsonophagocytosis. However, when these protective innate immune systems do not work optimally, or when bacteria are equipped with immune evasion mechanisms that prevent killing, this can lead to serious infections such as bacteremia and meningitis, which is associated with high morbidity and mortality. In order to study the complement evasion mechanisms of bacteria and the capacity of human blood to opsonize and kill bacteria, we developed a versatile whole blood killing assay wherein both phagocyte function and complement activity can easily be monitored and modulated. In this assay we use a selective thrombin inhibitor hirudin to fully preserve complement activity of whole blood. This assay allows controlled analysis of the requirements for active complement by replacing or heat-inactivating plasma, phagocyte function and bacterial immune evasion mechanisms that contribute to survival in human blood.

Biotypes and ScM types of isolates of Streptococcus canis from diseased and healthy cats

Lancefield group G Streptococcus canis is a component of the normal urogenital and pharyngeal flora of the cat. It is also frequently implicated in epizootics of severe disease in closed cat colonies and animal shelters. Given the importance of S canis as a feline pathogen and relative lack of published information on characteristics potentially associated with virulence, the authors have compared isolates from healthy and diseased cats in New York and California using fermentation profiles (biotype) and ScM sequences. With few exceptions, isolates associated with disease were biotype 1. Four alleles of scm were identified of which type 1 dominated in diseased cats. Type 4 allelic variants were found only in healthy cats and all but one were biotype 2. Type 2 and 3 alleles showed extensive N-terminal variation suggesting a plasminogen-binding site as found on the type 1 allele was absent. Cat antisera to ScM were opsonobactericidal, and these potentially protective antibodies increased during convalescence.

The Group A Streptococcus serotype M2 pilus plays a role in host cell adhesion and immune evasion

Group A Streptococcus (GAS), or Streptococcus pyogenes, is a human pathogen that causes diseases ranging from skin and soft tissue infections to severe invasive diseases, such as toxic shock syndrome. Each GAS strain carries a particular pilus type encoded in the variable fibronectin-binding, collagen-binding, T antigen (FCT) genomic region. Here, we describe the functional analysis of the serotype M2 pilus encoded in the FCT-6 region. We found that, in contrast to other investigated GAS pili, the ancillary pilin 1 lacks adhesive properties. Instead, the backbone pilin is important for host cell adhesion and binds several host factors, including fibronectin and fibrinogen. Using a panel of recombinant pilus proteins, GAS gene deletion mutants and Lactococcus lactis gain-of-function mutants we show that, unlike other GAS pili, the FCT-6 pilus also contributes to immune evasion. This was demonstrated by a delay in blood clotting, increased intracellular survival of the bacteria in macrophages, higher bacterial survival rates in human whole blood and greater virulence in a Galleria mellonella infection model in the presence of fully assembled FCT-6 pili.

Staphylococcus aureus Aggregation and Coagulation Mechanisms, and Their Function in Host-Pathogen Interactions

The human commensal bacterium Staphylococcus aureus can cause a wide range of infections ranging from skin and soft tissue infections to invasive diseases like septicemia, endocarditis, and pneumonia. Muticellular organization almost certainly contributes to S. aureus pathogenesis mechanisms. While there has been considerable focus on biofilm formation and its role in colonizing prosthetic joints and indwelling devices, less attention has been paid to nonsurface-attached group behavior like aggregation and clumping. S. aureus is unique in its ability to coagulate blood, and it also produces multiple fibrinogen-binding proteins that facilitate clumping. Formation of clumps, which are large, tightly packed groups of cells held together by fibrin(ogen), has been demonstrated to be important for S. aureus virulence and immune evasion. Clumps of cells are able to avoid detection by the host's immune system due to a fibrin(ogen) coat that acts as a shield, and the size of the clumps facilitates evasion of phagocytosis. In addition, clumping could be an important early step in establishing infections that involve tight clusters of cells embedded in host matrix proteins, such as soft tissue abscesses and endocarditis. In this review, we discuss clumping mechanisms and regulation, as well as what is known about how clumping contributes to immune evasion.

Post-infectious group A streptococcal autoimmune syndromes and the heart

There is a pressing need to reduce the high global disease burden of rheumatic heart disease (RHD) and its harbinger, acute rheumatic fever (ARF). ARF is a classical example of an autoimmune syndrome and is of particular immunological interest because it follows a known antecedent infection with group A streptococcus (GAS). However, the poorly understood immunopathology of these post-infectious diseases means that, compared to much progress in other immune-mediated diseases, we still lack useful biomarkers, new therapies or an effective vaccine in ARF and RHD. Here, we summarise recent literature on the complex interaction between GAS and the human host that culminates in ARF and the subsequent development of RHD. We contrast ARF with other post-infectious streptococcal immune syndromes - post-streptococcal glomerulonephritis (PSGN) and the still controversial paediatric autoimmune neuropsychiatric disorders associated with streptococcal infections (PANDAS), in order to highlight the potential significance of variations in the host immune response to GAS. We discuss a model for the pathogenesis of ARF and RHD in terms of current immunological concepts and the potential for application of in depth "omics" technologies to these ancient scourges.

Mutual exclusivity of hyaluronan and hyaluronidase in invasive group A Streptococcus

A recent analysis of group A Streptococcus (GAS) invasive infections in Australia has shown a predominance of M4 GAS, a serotype recently reported to lack the antiphagocytic hyaluronic acid (HA) capsule. Here, we use molecular genetics and bioinformatics techniques to characterize 17 clinical M4 isolates associated with invasive disease in children during this recent epidemiology. All M4 isolates lacked HA capsule, and whole genome sequence analysis of two isolates revealed the complete absence of the hasABC capsule biosynthesis operon. Conversely, M4 isolates possess a functional HA-degrading hyaluronate lyase (HylA) enzyme that is rendered nonfunctional in other GAS through a point mutation. Transformation with a plasmid expressing hasABC restored partial encapsulation in wild-type (WT) M4 GAS, and full encapsulation in an isogenic M4 mutant lacking HylA. However, partial encapsulation reduced binding to human complement regulatory protein C4BP, did not enhance survival in whole human blood, and did not increase virulence of WT M4 GAS in a mouse model of systemic infection. Bioinformatics analysis found no hasABC homologs in closely related species, suggesting that this operon was a recent acquisition. These data showcase a mutually exclusive interaction of HA capsule and active HylA among strains of this leading human pathogen.

The role of coagulation/fibrinolysis during Streptococcus pyogenes infection

The hemostatic system comprises platelet aggregation, coagulation and fibrinolysis and is a host defense mechanism that protects the integrity of the vascular system after tissue injury. During bacterial infections, the coagulation system cooperates with the inflammatory system to eliminate the invading pathogens. However, pathogenic bacteria have frequently evolved mechanisms to exploit the hemostatic system components for their own benefit. Streptococcus pyogenes, also known as Group A Streptococcus, provides a remarkable example of the extraordinary capacity of pathogens to exploit the host hemostatic system to support microbial survival and dissemination. The coagulation cascade comprises the contact system (also known as the intrinsic pathway) and the tissue factor pathway (also known as the extrinsic pathway), both leading to fibrin formation. During the early phase of S. pyogenes infection, the activation of the contact system eventually leads to bacterial entrapment within a fibrin clot, where S. pyogenes is immobilized and killed. However, entrapped S. pyogenes can circumvent the antimicrobial effect of the clot by sequestering host plasminogen on the bacterial cell surface that, after conversion into its active proteolytic form, plasmin, degrades the fibrin network and facilitates the liberation of S. pyogenes from the clot. Furthermore, the surface-localized fibrinolytic activity also cleaves a variety of extracellular matrix proteins, thereby enabling S. pyogenes to migrate across barriers and disseminate within the host. This review summarizes the knowledge gained during the last two decades on the role of coagulation/fibrinolysis in host defense against S. pyogenes as well as the strategies developed by this pathogen to evade and exploit these host mechanisms for its own benefit.

Disease manifestations and pathogenic mechanisms of Group A Streptococcus

Streptococcus pyogenes, also known as group A Streptococcus (GAS), causes mild human infections such as pharyngitis and impetigo and serious infections such as necrotizing fasciitis and streptococcal toxic shock syndrome. Furthermore, repeated GAS infections may trigger autoimmune diseases, including acute poststreptococcal glomerulonephritis, acute rheumatic fever, and rheumatic heart disease. Combined, these diseases account for over half a million deaths per year globally. Genomic and molecular analyses have now characterized a large number of GAS virulence determinants, many of which exhibit overlap and redundancy in the processes of adhesion and colonization, innate immune resistance, and the capacity to facilitate tissue barrier degradation and spread within the human host. This improved understanding of the contribution of individual virulence determinants to the disease process has led to the formulation of models of GAS disease progression, which may lead to better treatment and intervention strategies. While GAS remains sensitive to all penicillins and cephalosporins, rising resistance to other antibiotics used in disease treatment is an increasing worldwide concern. Several GAS vaccine formulations that elicit protective immunity in animal models have shown promise in nonhuman primate and early-stage human trials. The development of a safe and efficacious commercial human vaccine for the prophylaxis of GAS disease remains a high priority.

Disease manifestations and pathogenic mechanisms of Group A Streptococcus

Streptococcus pyogenes, also known as group A Streptococcus (GAS), causes mild human infections such as pharyngitis and impetigo and serious infections such as necrotizing fasciitis and streptococcal toxic shock syndrome. Furthermore, repeated GAS infections may trigger autoimmune diseases, including acute poststreptococcal glomerulonephritis, acute rheumatic fever, and rheumatic heart disease. Combined, these diseases account for over half a million deaths per year globally. Genomic and molecular analyses have now characterized a large number of GAS virulence determinants, many of which exhibit overlap and redundancy in the processes of adhesion and colonization, innate immune resistance, and the capacity to facilitate tissue barrier degradation and spread within the human host. This improved understanding of the contribution of individual virulence determinants to the disease process has led to the formulation of models of GAS disease progression, which may lead to better treatment and intervention strategies. While GAS remains sensitive to all penicillins and cephalosporins, rising resistance to other antibiotics used in disease treatment is an increasing worldwide concern. Several GAS vaccine formulations that elicit protective immunity in animal models have shown promise in nonhuman primate and early-stage human trials. The development of a safe and efficacious commercial human vaccine for the prophylaxis of GAS disease remains a high priority.

Factor H binds to the hypervariable region of many Streptococcus pyogenes M proteins but does not promote phagocytosis resistance or acute virulence

Many pathogens express a surface protein that binds the human complement regulator factor H (FH), as first described for Streptococcus pyogenes and the antiphagocytic M6 protein. It is commonly assumed that FH recruited to an M protein enhances virulence by protecting the bacteria against complement deposition and phagocytosis, but the role of FH-binding in S. pyogenes pathogenesis has remained unclear and controversial. Here, we studied seven purified M proteins for ability to bind FH and found that FH binds to the M5, M6 and M18 proteins but not the M1, M3, M4 and M22 proteins. Extensive immunochemical analysis indicated that FH binds solely to the hypervariable region (HVR) of an M protein, suggesting that selection has favored the ability of certain HVRs to bind FH. These FH-binding HVRs could be studied as isolated polypeptides that retain ability to bind FH, implying that an FH-binding HVR represents a distinct ligand-binding domain. The isolated HVRs specifically interacted with FH among all human serum proteins, interacted with the same region in FH and showed species specificity, but exhibited little or no antigenic cross-reactivity. Although these findings suggested that FH recruited to an M protein promotes virulence, studies in transgenic mice did not demonstrate a role for bound FH during acute infection. Moreover, phagocytosis tests indicated that ability to bind FH is neither sufficient nor necessary for S. pyogenes to resist killing in whole human blood. While these data shed new light on the HVR of M proteins, they suggest that FH-binding may affect S. pyogenes virulence by mechanisms not assessed in currently used model systems.

The human complement system may be rapidly activated upon infection and thereby plays a key role in innate immunity. However, activation must be tightly controlled, to avoid attack on self tissues. A key component of this control system is the plasma protein factor H (FH). Many pathogens bind FH, as first described for Streptococcus pyogenes, and it has been proposed that the surface-localized M protein of this bacterium “hijacks” FH to escape phagocytosis. However, it remains unclear whether FH-binding to M protein indeed protects S. pyogenes against phagocytosis and promotes bacterial growth in vivo. Here, we demonstrate that FH binds to some but not all M proteins and solely binds to the hypervariable region (HVR), a part of M protein important for virulence. Nevertheless, several lines of evidence, including studies with transgenic mice, indicated that FH-binding ability did not contribute to acute virulence or phagocytosis resistance. These data shed new light on the HVR of M proteins but underline the difficulty in determining the in vivo role of a ligand-binding region. Binding of FH may contribute to S. pyogenes virulence by mechanisms not assessed in currently used models.

Cooperative plasminogen recruitment to the surface of Streptococcus canis via M protein and enolase enhances bacterial survival

Streptococcus canis is a zoonotic pathogen capable of causing serious invasive diseases in domestic animals and humans. Surface-exposed M proteins and metabolic enzymes have been characterized as major virulence determinants in various streptococcal species. Recently, we have identified SCM, the M-like protein of S. canis, as the major receptor for miniplasminogen localized on the bacterial surface. The present study now characterizes the glycolytic enzyme enolase as an additional surface-exposed plasminogen-binding protein. According to its zoonotic properties, purified S. canis enolase binds to both human and canine plasminogen and facilitates degradation of aggregated fibrin matrices after activation with host-derived urokinase-type plasminogen activator (uPA). Unlike SCM, which binds to the C terminus of human plasminogen, the S. canis enolase interacts N terminally with the first four kringle domains of plasminogen, representing angiostatin. Radioactive binding analyses confirmed cooperative plasminogen recruitment to both surface-exposed enolase and SCM. Furthermore, despite the lack of surface protease activity via SpeB in S. canis, SCM is released and reassociated homophilically to surface-anchored SCM and heterophilically to surface-bound plasminogen. In addition to plasminogen-mediated antiphagocytic activity, reassociation of SCM to the bacterial surface significantly enhanced bacterial survival in phagocytosis analyses using human neutrophils.

IMPORTANCE

Streptococcal infections are a major issue in medical microbiology due to the increasing spread of antibiotic resistances and the limited availability of efficient vaccines. Surface-exposed glycolytic enzymes and M proteins have been characterized as major virulence factors mediating pathogen-host interaction. Since streptococcal infection mechanisms exert a subset of multicombinatorial processes, the investigation of synergistic activities mediated via different virulence factors has become a high priority. Our data clearly demonstrate that plasminogen recruitment to the Streptococcus canis surface via SCM and enolase in combination with SCM reassociation enhances bacterial survival by protecting against phagocytic killing. These data propose a new cooperative mechanism for prevention of phagocytic killing based on the synergistic activity of homophilic and heterophilic SCM binding in the presence of human plasminogen.

Invasive group a streptococcal disease: epidemiology, pathogenesis and management

Invasive group A streptococcal infections are uncommon, although serious, infections with high case fatality rates. Periodic resurgences in invasive group A streptococcal infections in industrialized countries have been reported from the 1980s onwards, with current estimates of incidence in these countries of approximately 3–4 per 100000 population. Infants, pregnant women and the elderly are at increased risk of invasive group A streptococcal infection. The group A streptococcus has an array of virulence factors that underpin its invasive capacity and, in approximately 10% of cases, super-antigen toxins produced by the bacteria stimulate a large proportion of T cells, leading to streptococcal toxic shock syndrome. Given the rapid clinical progression, effective management of invasive group A streptococcal infections hinges on early recognition of the disease and prompt initiation of supportive care (often intensive care) together with antibacterial therapy. In cases of toxic shock syndrome, it is often difficult to distinguish between streptococcal and staphylococcal infection before cultures become available and so antibacterial choice must include coverage of both of these organisms. In addition, clindamycin is an important adjunctive antibacterial because of its anti-toxin effects and excellent tissue penetration. Early institution of intravenous immunoglobulin therapy should be considered in cases of toxic shock syndrome and severe invasive infection, including necrotizing fasciitis. Early surgical debridement of necrotic tissue is also an important part of management in cases of necrotizing fasciitis.

Invasive group a streptococcal disease: epidemiology, pathogenesis and management

Invasive group A streptococcal infections are uncommon, although serious, infections with high case fatality rates. Periodic resurgences in invasive group A streptococcal infections in industrialized countries have been reported from the 1980s onwards, with current estimates of incidence in these countries of approximately 3-4 per 100 000 population. Infants, pregnant women and the elderly are at increased risk of invasive group A streptococcal infection. The group A streptococcus has an array of virulence factors that underpin its invasive capacity and, in approximately 10% of cases, superantigen toxins produced by the bacteria stimulate a large proportion of T cells, leading to streptococcal toxic shock syndrome. Given the rapid clinical progression, effective management of invasive group A streptococcal infections hinges on early recognition of the disease and prompt initiation of supportive care (often intensive care) together with antibacterial therapy. In cases of toxic shock syndrome, it is often difficult to distinguish between streptococcal and staphylococcal infection before cultures become available and so antibacterial choice must include coverage of both of these organisms. In addition, clindamycin is an important adjunctive antibacterial because of its anti-toxin effects and excellent tissue penetration. Early institution of intravenous immunoglobulin therapy should be considered in cases of toxic shock syndrome and severe invasive infection, including necrotizing fasciitis. Early surgical debridement of necrotic tissue is also an important part of management in cases of necrotizing fasciitis.

Molecular insight into invasive group A streptococcal disease

Streptococcus pyogenes is also known as group A Streptococcus (GAS) and is an important human pathogen that causes considerable morbidity and mortality worldwide. The GAS serotype M1T1 clone is the most frequently isolated serotype from life-threatening invasive (at a sterile site) infections, such as streptococcal toxic shock-like syndrome and necrotizing fasciitis. Here, we describe the virulence factors and newly discovered molecular events that mediate the in vivo changes from non-invasive GAS serotype M1T1 to the invasive phenotype, and review the invasive-disease trigger for non-M1 GAS. Understanding the molecular basis and mechanism of initiation for streptococcal invasive disease may expedite the discovery of novel therapeutic targets for the treatment and control of severe invasive GAS diseases.

M protein and hyaluronic acid capsule are essential for in vivo selection of covRS mutations characteristic of invasive serotype M1T1 group A Streptococcus

The initiation of hyperinvasive disease in group A Streptococcus (GAS) serotype M1T1 occurs by mutation within the covRS two-component regulon (named covRS for control of virulence regulatory sensor kinase), which promotes resistance to neutrophil-mediated killing through the upregulation of bacteriophage-encoded Sda1 DNase. To determine whether other virulence factors contribute to this phase-switching phenomenon, we studied a panel of 10 isogenic GAS serotype M1T1 virulence gene knockout mutants. While loss of several individual virulence factors did not prevent GAS covRS switching in vivo, we found that M1 protein and hyaluronic acid capsule are indispensable for the switching phenotype, a phenomenon previously attributed uniquely to the Sda1 DNase. We demonstrate that like M1 protein and Sda1, capsule expression enhances survival of GAS serotype M1T1 within neutrophil extracellular traps. Furthermore, capsule shares with M1 protein a role in GAS resistance to human cathelicidin antimicrobial peptide LL-37. We conclude that a quorum of GAS serotype M1T1 virulence genes with cooperative roles in resistance to neutrophil extracellular killing is essential for the switch to a hyperinvasive phenotype in vivo.

The pathogen group A Streptococcus (GAS) causes a wide range of human infections ranging from the superficial “strep throat” to potentially life-threatening conditions, such as necrotizing fasciitis, also known as “flesh-eating disease.” A marked increase in the number of cases of severe invasive GAS infection during the last 30 years has been traced to the emergence and spread of a single clone of the M1T1 serotype. Recent studies have shown that GAS serotype M1T1 bacteria undergo a genetic “switch” in vivo to a hypervirulent state that allows dissemination into the bloodstream. The present study was undertaken to identify specific GAS serotype M1T1 virulence factors required for this switch to hypervirulence. The surface-anchored GAS M1 protein and hyaluronic acid capsule are found to be essential for the switching phenotype, and a novel role for capsule in GAS resistance to host defense peptides and neutrophil extracellular killing is revealed.

Streptococcal M proteins and their role as virulence determinants

Group A streptococci (GAS, Streptococcus pyogenes) are exclusive human pathogens that have been extensively studied for many decades. The spectrum of diseases caused by these bacteria ranges from uncomplicated and superficial to severe and invasive infections. In order to give rise to these complications, GAS have evolved a number of surface-bound and secreted virulence factors, of which the M proteins are probably the best characterized. Evidence has emerged that M proteins are multifunctional pathogenic determinants, and over the years many interactions between M proteins and the human host have been reported. The present review article aims to present a state-of-the-art overview of the most important virulence mechanisms employed by M proteins to trigger disease.

The streptococcal M protein: a highly versatile molecule

Interaction of the M-protein of group A Streptococcus (GAS) with its numerous host binding partners might assist the bacteria in evading host immune responses. Although the extensive diversity of this protein has been highlighted by different GAS typing schemes, most of the structural and functional information has been obtained from a limited number of types. Increasing numbers of epidemiological, clinical and biological reports suggest that the structure and function of the M protein is less conserved than previously thought. This review focuses on the known interactions between M proteins and host ligand proteins, emphasizing that our understanding of this well-studied molecule is fragmented.

Inactivation of DltA modulates virulence factor expression in Streptococcus pyogenes

BACKGROUND

D-alanylated lipoteichoic acid is a virtually ubiquitous component of gram-positive cell walls. Mutations in the dltABCD operon of numerous species exhibit pleiotropic effects, including reduced virulence, which has been attributed to increased binding of cationic antimicrobial peptides to the more negatively charged cell surface. In this study, we have further investigated the effects that mutating dltA has on virulence factor expression in Streptococcus pyogenes.

METHODOLOGY/PRINCIPAL FINDINGS

Isogenic Delta dltA mutants had previously been created in two distinct M1T1 isolates of S. pyogenes. Immunoblots, flow cytometry, and immunofluorescence were used to quantitate M protein levels in these strains, as well as to assess their ability to bind complement. Bacteria were tested for their ability to interact with human PMN and to grow in whole human blood. Message levels for emm, sic, and various regulatory elements were assessed by quantitative RT-PCR. Cell walls of Delta dltA mutants contained much less M protein than cell walls of parent strains and this correlated with reduced levels of emm transcripts, increased deposition of complement, increased association of bacteria with polymorphonuclear leukocytes, and reduced bacterial growth in whole human blood. Transcription of at least one other gene of the mga regulon, sic, which encodes a protein that inactivates antimicrobial peptides, was also dramatically reduced in Delta dltA mutants. Concomitantly, ccpA and rofA were unaffected, while rgg and arcA were up-regulated.

CONCLUSIONS/SIGNIFICANCE

This study has identified a novel mechanism for the reduced virulence of dltA mutants of Streptococcus pyogenes in which gene regulatory networks somehow sense and respond to the loss of DltA and lack of D-alanine esterification of lipoteichoic acid. The mechanism remains to be determined, but the data indicate that the status of D-alanine-lipoteichoic acid can significantly influence the expression of at least some streptococcal virulence factors and provide further impetus to targeting the dlt operon of gram-positive pathogens in the search for novel antimicrobial compounds.

Protein F1 and Streptococcus pyogenes resistance to phagocytosis

Streptococcus pyogenes is a major cause of pharyngitis in humans and encodes several fibronectin-binding proteins. M protein and protein F1 (PrtF1/SfbI) are differentially regulated by CO(2) and O(2), respectively, and both mediate the invasion of epithelial cells. This study examined whether PrtF1/SfbI shares other properties with M protein. Expression of the PrtF1/SfbI protein by an M-negative mutant conferred resistance to phagocytosis and partial inhibition of C3 deposition on the S. pyogenes surface.

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.

Opsonophagocytosis-inhibiting mac protein of group a streptococcus: identification and characteristics of two genetic complexes

Recently, it was reported that a streptococcal Mac protein (designated Mac(5005)) made by serotype M1 group A Streptococcus (GAS) is a homologue of human CD11b that inhibits opsonophagocytosis and killing of GAS by human polymorphonuclear leukocytes (PMNs) (B. Lei, F. R. DeLeo, N. P. Hoe, M. R. Graham, S. M. Mackie, R. L. Cole, M. Liu, H. R. Hill, D. E. Low, M. J. Federle, J. R. Scott, and J. M. Musser, Nat. Med. 7:1298-1305, 2001). To study mac variation and expression of the Mac protein, the gene in 67 GAS strains representing 36 distinct M protein serotypes was sequenced. Two distinct genetic complexes were identified, and they were designated complex I and complex II. Mac variants in each of the two complexes were closely related, but complex I and complex II variants differed on average at 50.66 +/- 5.8 amino acid residues, most of which were located in the middle one-third of the protein. Complex I Mac variants have greater homology with CD11b than complex II variants. GAS strains belonging to serotypes M1 and M3, the most abundant M protein serotypes responsible for human infections in many case series, have complex I Mac variants. The mac gene was cloned from representative strains assigned to complexes I and II, and the Mac proteins were purified to apparent homogeneity. Both Mac variants had immunoglobulin G (IgG)-endopeptidase activity. In contrast to Mac(5005) (complex I), Mac(8345) (complex II) underwent autooxidation of its cysteine residues, resulting in the loss of IgG-endopeptidase activity. A Mac(5005) Cys94Ala site-specific mutant protein was unable to cleave IgG but retained the ability to inhibit IgG-mediated phagocytosis by human PMNs. Thus, the IgG-endopeptidase activity was not essential for the key biological function of Mac(5005). Although Mac(5005) and Mac(8345) each have an Arg-Gly-Asp (RGD) motif, the proteins differed in their interactions with human integrins alpha(v)beta(3) and alpha(IIb)beta(3). Binding of Mac(5005) to integrins alpha(v)beta(3) and alpha(IIb)beta(3) was mediated primarily by the RGD motif in Mac(5005), whereas binding of Mac(8345) involved the RGD motif and a region in the middle one-third of the molecule whose sequence is different in Mac(8345) and Mac(5005). Taken together, the data add to the emerging theme in GAS pathogenesis that allelic variation in virulence genes contributes to fundamental differences in host-pathogen interactions among strains.

Acquisition of regulators of complement activation by Streptococcus pyogenes serotype M1

Opsonization of bacteria by complement proteins is an important component of the immune response. The pathogenic bacterium Streptococcus pyogenes has evolved multiple mechanisms for the evasion of complement-mediated opsonization. One mechanism involves the binding of human regulators of complement activation such as factor H (FH) and FH-like protein 1 (FHL-1). Acquisition of these regulatory proteins can limit deposition of the opsonin C3b on bacteria, thus decreasing the pathogen's susceptibility to phagocytosis. Binding of complement regulatory proteins by S. pyogenes has previously been attributed to the streptococcal M and M-like proteins. Here, we report that the S. pyogenes cell surface protein Fba can mediate binding of FH and FHL-1. We constructed mutant derivatives of S. pyogenes that lack Fba, M1 protein, or both proteins and assayed the strains for FH binding, susceptibility to phagocytosis, and C3 deposition. Fba expression was found to be sufficient for binding of purified FH as well as for binding of FH and FHL-1 from human plasma. Plasma adsorption experiments also revealed that M1(+) Fba(+) streptococci preferentially bind FHL-1, whereas M1(-) Fba(+) streptococci have similar affinities for FH and FHL-1. Fba was found to contribute to the survival of streptococci incubated with human blood and to inhibit C3 deposition on bacterial cells. Streptococci harvested from log-phase cultures readily bound FH, but binding was greatly reduced for bacteria obtained from stationary-phase cultures. Bacteria cultured in the presence of the protease inhibitor E64 maintained FH binding activity in stationary phase, suggesting that Fba is removed from the cell surface via proteolysis. Western analyses confirmed that E64 stabilizes cell surface expression of Fba. These data indicate that Fba is an antiopsonic, antiphagocytic protein that may be regulated by cell surface proteolysis.

Functional blocking of Staphylococcus aureus adhesins following growth in ex vivo media

Defining the role of Staphylococcus aureus adhesins in disease pathogenesis may depend on the use of bacteria grown in culture media that more closely reflect the human milieu than conventional broth. This study examined the functional effect on S. aureus adhesins following growth in an ex vivo medium containing a complex mixture of human proteins (used peritoneal dialysate) relative to growth in Todd-Hewitt broth. The adherence of S. aureus, cultured in dialysate, to fibronectin and fibrinogen was markedly reduced despite the expresion of full-length ClfA, ClfB, and fibronectin-binding proteins. Growth in dialysate resulted in the acquisition of a surface coat, as visualized by transmission electron microscopy, which was shown to contain fibronectin, fibrinogen, and immunoglobulins. Adherence of S. aureus to fibrinogen following growth in dialysate was significantly reduced by expression of protein A but was restored following growth in immunoglobulin-depleted dialysate. We conclude that bacterial adherence to solid-phase protein is critically dependent on the culture medium, that S. aureus adhesins may become saturated with target protein prior to contact with solid surfaces, and that there is an interaction between fibrinogen-binding proteins and immunoglobulin bound to protein A following contact with host proteins. These findings have important implications for future studies of S. aureus adhesins.

Evasion of human innate and acquired immunity by a bacterial homolog of CD11b that inhibits opsonophagocytosis

Microbial pathogens must evade the human immune system to survive, disseminate and cause disease. By proteome analysis of the bacterium Group A Streptococcus (GAS), we identified a secreted protein with homology to the alpha-subunit of Mac-1, a leukocyte beta2 integrin required for innate immunity to invading microbes. The GAS Mac-1-like protein (Mac) was secreted by most pathogenic strains, produced in log-phase and controlled by the covR-covS two-component gene regulatory system, which also regulates transcription of other GAS virulence factors. Patients with GAS infection had titers of antibody specific to Mac that correlated with the course of disease, demonstrating that Mac was produced in vivo. Mac bound to CD16 (FcgammaRIIIB) on the surface of human polymorphonuclear leukocytes and inhibited opsonophagocytosis and production of reactive oxygen species, which resulted in significantly decreased pathogen killing. Thus, by mimicking a host-cell receptor required for an innate immune response, the GAS Mac protein inhibits professional phagocyte function by a novel strategy that enhances pathogen survival, establishment of infection and dissemination.

Pathogenesis of group A streptococcal infections

Group A streptococci are model extracellular gram-positive pathogens responsible for pharyngitis, impetigo, rheumatic fever, and acute glomerulonephritis. A resurgence of invasive streptococcal diseases and rheumatic fever has appeared in outbreaks over the past 10 years, with a predominant M1 serotype as well as others identified with the outbreaks. emm (M protein) gene sequencing has changed serotyping, and new virulence genes and new virulence regulatory networks have been defined. The emm gene superfamily has expanded to include antiphagocytic molecules and immunoglobulin-binding proteins with common structural features. At least nine superantigens have been characterized, all of which may contribute to toxic streptococcal syndrome. An emerging theme is the dichotomy between skin and throat strains in their epidemiology and genetic makeup. Eleven adhesins have been reported, and surface plasmin-binding proteins have been defined. The strong resistance of the group A streptococcus to phagocytosis is related to factor H and fibrinogen binding by M protein and to disarming complement component C5a by the C5a peptidase. Molecular mimicry appears to play a role in autoimmune mechanisms involved in rheumatic fever, while nephritis strain-associated proteins may lead to immune-mediated acute glomerulonephritis. Vaccine strategies have focused on recombinant M protein and C5a peptidase vaccines, and mucosal vaccine delivery systems are under investigation.

M-like proteins of Streptococcus dysgalactiae

Streptococcus dysgalactiae is one of the most important bacterial species isolated from bovine mastitis. To identify potential virulence factors of this species we prepared chromosomal DNA from strain 8215 and constructed a phage display library. By affinity selection of the library against fibrinogen (Fg), we isolated and characterized a gene, called demA, encoding a protein with the molecular mass of approximately 58 kDa, called DemA, displaying both plasma protein binding properties and sequence similarities with the M and M-like proteins of other streptococcal species. Purified recombinant DemA protein was found to completely inhibit Fg-binding to cells of S. dysgalactiae. A continued sequence analysis revealed that the demA gene was preceded by an open reading frame (dmgA) coding for a putative protein, called DmgA, with high similarities to the Mga proteins of Streptococcus pyogenes. By additional cloning, the corresponding dmgA and demA genes from another strain, called Epi9, were isolated and analyzed. These genes, called dmgB and demB, respectively, revealed a high degree of similarity to the corresponding genes in strain 8215. Increased binding of Fg by cells of strain Epi9, grown in an atmosphere with 10% CO(2), was correlated to an enhanced transcription of the demB gene as shown in a Northern blot. Strain 8215 did not respond to CO(2), which could be explained by a nonfunctional dmgA gene due to insertion of an insertion sequence element. Based on sequence similarities of the described proteins to Mga, M, and M-like proteins and the response to elevated level of CO(2), we suggest that the dmg and dem genes are members of a regulon similar to the described mga regulon in S. pyogenes, which encodes several virulence factors in this species.

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.

Role of the Hypervariable Region in Streptococcal M Proteins: Binding of a Human Complement Inhibitor

Antigenic variation allows pathogenic microorganisms to evade the immune system of the infected host. The variable structure must play an important role in pathogenesis, but its function is in most cases unknown. Here, we identify a function for the surface-exposed hypervariable region of streptococcal M5 protein, a virulence factor that inhibits phagocytosis. The hypervariable region of M5 was found to bind the human complement inhibitor FHL-1 (factor H-like protein 1), a 42-kDa plasma protein. Plasma absorption experiments with M5-expressing bacteria showed that the interaction with FHL-1 occurs also under physiologic conditions. Studies of another extensively characterized M protein, M6, indicated that this protein also has a binding site for FHL-1 in the hypervariable region. The complement-inhibitory function of FHL-1 was retained after binding to streptococci, suggesting that bound FHL-1 protects bacteria against complement attack. All available data now indicate that FHL-1, or another human complement inhibitor, binds to the hypervariable region of M proteins. These findings provide insights into the forces that drive antigenic variation and may explain why the hypervariable region of M protein is essential for phagocytosis resistance. Moreover, these data add to a growing body of evidence that human complement inhibitors are major targets for pathogenic microorganisms.

M protein of the group A Streptococcus binds to the seventh short consensus repeat of human complement factor H

Streptococcus pyogenes evades complement by binding the complement-regulatory protein factor H (fH) via the central conserved C-repeat region of M protein. However, the corresponding binding region within fH has not previously been precisely localized. fH is composed of 20 conserved modules called short consensus repeats (SCRs), each of which contains approximately 60 amino acids. A series of fH truncated and deletion mutants were prepared, and their interaction with M6 protein was examined. The M protein binding site was initially localized to SCRs 6 to 15 as demonstrated by ligand dot blotting, chemical cross-linking, and enzyme-linked immunosorbent assay. SCR 7 was then shown to contain the M protein binding site, as a construct consisting of the first seven SCRs bound M protein but a construct containing the first six SCRs did not bind. In addition, deletion of SCR 7 from full-length fH abolished binding to M protein. SCR 7 is known to contain a heparin binding domain, and binding of fH to M6 protein was almost totally inhibited in the presence of 400 U of heparin per ml. These results localize the M6 protein binding site of fH to SCR 7 and indicate that it is in close proximity to the heparin binding site.

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.

Streptococcus pyogenes type 5 M protein is an antigen, not a superantigen, for human T cells

M proteins are coiled-coil dimers expressed on group A streptococcal cell surfaces. They have an important role in host antistreptococcal immunity and in poststreptococcal autoimmune sequelae. Controversy has arisen regarding whether type 5 M proteins are superantigenic for human T cells. To investigate this, we have produced and tested M5 in the form of two novel recombinant proteins. We found no evidence of superantigenicity using either recombinant whole M5 protein (rM5) or recombinant pep M5 protein (rpepM5) to activate peripheral blood mononuclear cells (PBMC) from healthy adult volunteers. Short-term, rM5-specific T-cell lines from different subjects were uniformly self-APC restricted and showed no consistent pattern of TCR V beta usage. A synthetic peptide of M5 residues 217-237 was found to contain epitope(s) recognized by some rM5-specific human T cells. PBMC responses to rM5 and rpepM5 in 3- and 7-day proliferation assays were characteristic of antigenic rather than superantigenic stimulation. We conclude that type 5 M protein activates human T cells as a conventional antigen.

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.

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.

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.

Phenotypic characterization of Streptococcus dysgalactiae isolates from bovine mastitis by their binding to host derived proteins

The binding of 80 Streptococcus dysgalactiae mastitis isolates from 51 farms to plasma and connective tissue proteins fibronectin (29 kDa N-terminal fragment), vitronectin, collagen type I, fibrinogen, alpha 2-macroglobulin, IgG, and albumin was studied. All isolates boiund the bovine 29 kDa fibronectin fragment and the binding of bovine fibrinogen, caprine albumin, bovine alpha 2-macroglobulin-trypsin complexes and caprine IgG was also very frequent (92.5, 92.5, 72.5% and 87.5%, respectively). Binding to human vitronectin was observed in 55% of the isolates, whereas only 20% of the isolates bound human type I collagen. None of the isolates bound native alpha 2-macroglobulin. Nearly all isolates (91%) bound more than 3 ligands. The bacterial binding sites for these proteins (termed here receptors) occurred in different combinations of which the combination fibronectin-, albumin-, fibrinogen-, vitronectin-, alpha 2-macroglobulin- and IgG-receptor was the most common. More than one isolate was obtained from 10 farms. The isolates from 5 farms showed close similarity of binding profiles within the farm, indicating that they were of similar origin and suggesting that the binding characteristics were relatively stable. Wider variation among the isolates obtained from other 5 farms was detected. The different isolates of the same farm origin varied mostly in the binding of albumin, IgG and fibrinogen. Interestingly, a difference in the number of receptors between isolates from two different sampling areas was observed. The binding profiles offer a new phenotypic method for epidemiological studies and may also when combined with genetical studies provide more insight both into the role of the bacterial plasma and connective tissue protein receptors in the infection process and the regulation of receptor expression.

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.

Processing of viable group A streptococci leads to major histocompatibility complex class II presentation of T cell epitopes from the major protective antigen

We have previously mapped major histocompatibility complex (MHC) class II-restricted T cell epitopes of the surface M protein of type 5 group A streptococci (M5) and show here that two out of four epitopes investigated were efficiently processed during incubation of viable streptococci with spleen cells for presentation to M5-specific murine T cell clones. Viable streptococci were processed more efficiently than heat-killed bacteria suggesting that secreted virulence factors of streptococci do not obstruct processing of streptococcal antigens in the dose range used. Epitopes from different regions of M5 could be ranked according to the efficiency with which they were processed, which may contribute to their relative immunodominance. It was further demonstrated that T cell clones specific for M5 308-319, an epitope from the M type conserved carboxy-terminal half of M5, cross-reacted between M5, M6 and M12, but not M49, streptococci. Helper T cell epitopes which are shared between streptococcal M types and are presented by MHC class II molecules on antigen-presenting cells after processing of viable streptococci could be particularly useful in the design of multivalent streptococcal vaccines.

Cloning and characterization of a gene for a 19 kDa fibrinogen-binding protein from Staphylococcus aureus

Staphylococcus aureus has been shown to interact specifically with fibrinogen. Three different extracellular fibrinogen-binding proteins, two of which have coagulase activity, are produced by S. aureus strain Newman. The role of these fibrinogen-binding proteins during staphylococcal colonization and infection has not yet been fully elucidated. Here we describe the cloning, sequencing and expression of a gene for a 19 kDa fibrinogen-binding protein. This gene, called fib, encodes a 165-amino-acid polypeptide, including a 29-amino-acid signal sequence. The recombinant protein, which has an estimated molecular mass of 15.9 kDa, bound fibrinogen and was recognized by a polyclonal antiserum against the native Fib protein. Homologies between the Fib protein and the fibrinogen-binding domain of coagulase suggest that amino acids within this domain are involved in the binding to fibrinogen.

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.