How Group a Streptococcus Circumvents Host Phagocyte Defenses

REVIEW

How group A Streptococcus circumvents host phagocyte defenses

Laura A Kwinn & Group A Streptococcus (GAS) is a Gram-positive bacterium associated with a variety of Victor Nizet† mucosal and invasive human infections. GAS systemic disease reflects the diverse abilities of †Author for correspondence this pathogen to avoid eradication by phagocytic defenses of the innate immune system. Division of Pediatric Here we review how GAS can avoid phagocyte engagement, inhibit complement and Pharmacology & Drug Discovery, University of antibody functions required for opsonization, impair phagocytotic uptake mechanisms, California, San Diego School promote phagocyte lysis or apoptosis, and resist specific effectors of phagocyte killing such of Medicine, Skaggs School of as antimicrobial peptides and reactive oxygen species. Understanding the molecular basis Pharmacy & Pharmaceutical of GAS phagocyte resistance may reveal novel therapeutic targets for treatment and Sciences, 9500 Gilman prevention of invasive human infections. Drive, Mail Code 0687, La Jolla, CA 92093-0687, USA Group A Streptococcus (GAS; Streptococcus pyo- (e.g., N-formyl peptides), the strongest and most Tel.: +1 858 534 7408; genes) is a Gram-positive bacterium associated specific stimuli are host-derived. GAS has Fax: +1 858 534 5611; with a wide spectrum of disease conditions in the evolved mechanisms to interfere with two of the [email protected] human host. While the majority of GAS disease is most potent molecules promoting neutrophil limited to superficial sites such as the pharyngeal recruitment, the CXC chemokine interleukin mucosa (‘strep throat’) or skin (impetigo), the (IL)-8 and the complement-derived anaphylo- organism is also a leading agent of invasive infec- toxin C5a. In this way the kinetics of the innate tions, including the life-threatening conditions of immune response to GAS infection are delayed, necrotizing fasciitis and toxic shock syndrome. favoring bacterial survival. The annual burden of invasive GAS infection is IL-8 is a multifunctional protein involved in estimated at over 650,000 cases and 150,000 the migration of neutrophils out of the blood- deaths worldwide [1], reflecting the global dissem- stream and towards the site of infection. Not ination of strains of enhanced virulence potential, only does IL-8 act a potent chemoattractant [3], including a highly prevalent clone of the M1T1 it can also be found tethered to the luminal sur- serotype [2]. The propensity of GAS to produce face of the microvasculature where it provides a systemic infection in otherwise healthy children stop signal to rolling neutrophils [4,5]. GAS pro- and adults defines a capacity of the pathogen to duce a protease (ScpC, also known as SpyCEP) resist host innate immune clearance mechanisms that specifically cleaves the C terminus of IL-8, that normally function to prevent microbial leading to functional inactivation of the chemo- dissemination beyond epithelial surfaces. kine [6]. ScpC also cleaves the murine CXC Phagocytic cells such as neutrophils and macro- chemokines KC and MIP-2 [7]. Loss of ScpC phages represent a critical element of innate expression dramatically reduces GAS virulence immunity against invasive bacterial infection. The in the mouse necrotizing fasciitis model, reflect- general effectiveness of these cells in host defense ing increased neutrophil influx to the site of bespeaks specialized functions in directed migra- infection [7]. tion, microbial uptake and production of a variety C5a is an 11-kD fragment of the comple- of bactericidal effector molecules. In this review, ment cascade with multiple inflammatory prop- we examine the multiple virulence factors of GAS erties, including the recruitment of neutrophils capable of interfering with the host phagocyte and stimulation of their bactericidal capacity defense system, placing a particular emphasis on against GAS [8]. However, GAS express an recent discoveries established through molecular endopeptidase, ScpA, which cleaves human Keywords: complement, group A Streptococcus, innate genetic analysis of the pathogen. C5a between His-67 and Lys-68, residues immunity, macrophage, within the critical recognition site for leukocyte neutrophil, phagocytosis, Impairment of phagocyte recruitment surface receptors [9]. The anchorless surface Streptococcus pyogenes, virulence factor Circulating leukocytes respond to chemotactic dehydrogenase (SDH) is also shed from the signals to leave the vasculature and migrate to GAS surface whereupon it binds and inactivates part of the site of infection. While chemoattractants human C5a [10], providing another impediment include products from the bacteria cell wall to host neutrophil chemotaxis.

Escape from neutrophil C4BP interferes with the assembly of the extracellular traps membrane-bound C3 convertase of the classical It has recently been appreciated that, apart from pathway [15]. GAS is able to selectively acquire their phagocytic function, neutrophils can effi- host C4BP from human serum through the ciently capture and kill microbes in the extra- action of the hypervariable regions of several cellular space. This process involves neutrophil M-protein family members, thereby inhibiting extrusion of a matrix of DNA and histones classical pathway activation [16,17]. Monoclonal known as neutrophil extracellular traps (NETs), antibody mapping studies reveal the point of which ensnare bacteria and subject them to interaction of GAS M proteins with C4BP over- microbicidal effectors including the granule pro- laps with potential C4b binding sites [18]; how- teases elastase and myeloperoxidase [11]. With ever, the multi-armed structure of C4BP allows it chromatin representing the principal scaffold of to retains its ability to act as a cofactor for C4b NETs, the contribution of several GAS DNAse degradation even when bound to M protein. A enzymes to pathogenesis has come under exami- strong correlation can be established between nation. A GAS strain with mutations in three C4BP acquisition on the GAS surface and eva- encoded DNAses is significantly attenuated in sion of phagocytosis, highlighting the importance murine skin and systemic infection models as of this innate immune resistance mechanism [19]. well as pharyngeal infection of cynomolgus The lack of consensus sequence motifs among macaques [12]. Among these was the highly C4BP binding M-protein hypervariable regions active bacteriophage-encoded DNAse Sda1, reflects an interesting capacity for sequence diver- present in the secreted proteome of the virulent gence across M types while maintaining highly M1T1 GAS clone associated with severe, inva- specific ligand-binding functions [20]. sive infections [13]. Sumby and collegues demon- FH and the variant splice form of its coding strated significantly increased extracellular gene, FH-like protein (FHL)-1, are central fluid- killing of isogenic DNAse mutant GAS by phase regulators of the alternative complement human neutrophils using conditions known to pathway, functioning to accelerate the decay of promote NETs [12], and subsequent targeted the C3 convertase (C3bBb) and acting as mutagenesis and heterologous expression of cofactors for factor I-mediated degradation of Sda1 revealed that the enzyme is necessary and C3b [21]. M protein has long been known to sufficient for promoting GAS NET degradation restrict deposition of C3b on the GAS surface, a and resistance to neutrophil killing in vitro and function that can be correlated to resistance to in vivo [14]. Moreover, pharmacological inhibi- phagocytosis [22]. Many GAS M proteins were tion of Sda1 DNAse activity preserved host found capable of binding FH and FHL-1 pro- NET function and reduced the severity of GAS teins through their conserved C-repeat region skin infection [14]. and/or hypervariable N-terminal regions [23,24]. However, the overall significance of M protein Interference with complement function binding to FH and FHL-1 to complement resist- Following activation of the classical or alternative ance remains a matter of debate. Affinities for complement pathways, opsonization of foreign FH and FHL-1 vary widely by M-type – M18 microbes occurs through deposition of C3b and possesses the highest affinity, while M1 and M3 its cleavage fragment iC3b on their surface. proteins, representing strains commonly associ- Complement receptors (CR) on neutrophils and ated with invasive disease, show little if any bind- macrophages engage the bound C3b (CR1) or ing [25]. Recently, the M5 protein was shown to iC3b (CR3 and CR4) to facilitate phagocytosis. bind FH and FHL-1 at its N terminus, but the Since the complement system is capable of effi- bacteria resisted phagocytosis equally well regard- cient self-amplification, potential host cell dam- less of the inclusion or exclusion of this N-termi- age is mitigated by the counter-regulatory nal binding domain [26], corroborating similar proteins C4b-binding protein (C4BP) and observations using an M6 serotype strain [27]. In factor H (FH) that dampen the activity level of M1 strains, the M protein is dispensible for the classical and alternative pathways, respec- FH/FHL-1 binding; instead, the surface- tively. GAS exhibits the capacity to acquire anchored protein Fba mediates binding to these C4BP and FH, and these phenotypes have been complement regulatory factors. Fba promotes explored for potential roles in impeding host M1 GAS survival in human whole blood and complement activation or preventing efficient prevents deposition of C3b on the bacterial cell opsonophagocytosis. surface [28].

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Deposition of complement on GAS occurs via of its N-terminal domain has generated more the classical pathway even under nonimmune than 100 known serovariants among which lack conditions, but can be blocked by the ability of of cross-protection is commonplace. M proteins of certain GAS serotypes to bind The effector function of Ig may be thwarted fibrinogen, which reduces the amount of classical when the pathogen binds its Fc region, effec- pathway C3 convertase on the bacterial surface. tively decorating the bacterial surface with the The M-related protein Mrp, expressed by more host molecule in a ‘backwards’, nonopsonic ori- than half of GAS strains, also recruits fibrinogen entation. The surface-expressed GAS fibronectin- to the bacterial surface in a fashion that impairs binding protein (Sfb)I binds the Fc region of the complement deposition [29]. Binding of fibrino- Fc region of IgG, preventing phagocytosis of IgG- gen and fellow plasma protein albumin to the coated red-blood cells (RBCs) and Ab-dependent B- and C-repeats of GAS M protein plays an cell cytotoxicity by macrophages [33]. Several M important role in determining the location of protein types and related family members also opsonic and nonopsonic epitopes [30]. show capacity for binding the Fc domains of IgG The terminal complement pathway generates and/or IgA [34–36]. Protein H, a GAS surface pro- the membrane attack complex C5–9, which can tein structurally related to M protein, interacts disrupt bacterial cell membranes leading to hypo- with the Fc region of IgG and inhibits IgG- osmotic lysis; although this may not be the case dependent complement activation on the bacte- in Gram-positive bacteria due to the thickness of rial cell surface [37]. However, it should be noted the peptidoglycan layer in the cell wall. M1 and that there is scant evidence that Fc binding pro- M57 serotypes of GAS release the protein serum teins of GAS or other bacteria can specifically inhibitor of complement (SIC) that couples with bind antibacterial IgG, without first becoming clusterin and histidine-rich glycoprotein, two saturated with abundant nonimmune IgG. serum regulators of membrane-attack complex The broad spectrum GAS cysteine protease activity, consequently inhibiting complement- SpeB has been shown to cleave IgG, -A, -M, -D mediated cell lysis in an erythrocyte model [31]. and -E antibodies in vitro [38]. This degradation While preventing uptake of C567 onto the cell occurs even when IgG is specifically bound to a surface is a measurable property of SIC [32], the bacterial antigen via its Fab regions; yet when the protein has several additional activities that are IgG Fc region is bound to GAS surface proteins likely more important contributors to GAS in a nonopsonic fashion, it is then spared from phagocyte resistance (see below). SpeB proteolysis [39]. Mac-1/IdeS, a second GAS cysteine protease [40], cleaves IgG in vitro and Interference with in vivo, targeting the lower Fc region [41,42]. antibody-mediated opsonization Mac-1/IdeS further exhibits homology to the Immunoglobulins (Igs) generated against specific alpha-subunit of a leukocyte B2-integrin, bacterial epitopes provide a second effective form CD11b, which binds to FcγRIIIB (CD16) on of opsonization, promoting engagement and the surface of neutrophils inhibiting phago- uptake by host phagocytes expressing surface cytosis and activation of the oxidative burst [43]. receptors for the Ig Fc domain. GAS confounds Subsequently, the closely related Mac-2 protein this branch of host innate defense by a variety of was found to bind both Fcγ RII and III recep- means, including molecular mimicry, antigenic tors, likewise serving to competitively inhibit diversity, and specific proteins that degrade Ig mol- host phagocyte recognition of IgG on the bacte- ecules, bind them in a nonopsonic fashion, or rial surface [41]. Like Mac-1/IdeS, Mac-2 exhibits interfere with their recognition by phagocyte Fc IgG endopeptidase activity [44]. Finally, the receptors. In contrast to the serotype-specific secreted GAS protein EndoS hydrolyzes the polysaccharide capsules of group B Streptococcus chitobiose core of the asparagine-linked glycan and Streptococcus pneumoniae, which represent pri- on IgG, preventing recognition of IgG by phago- mary targets of protective immunity, the invariant cyte Fc receptors, blocking complement activa- GAS capsule consists of a homopolymer of tion through the classical pathway and impairing hyaluronic acid, identical to a major constituent of opsonophagocytosis [38,45]. the mammalian extracellular matrix. This effective mimicry provides the bacterium a protective cloak Avoidance of phagocytic uptake not recognizable as a foreign antigen. And while Beyond interference with complement and anti- the M protein on the GAS cell surface can serve as body-mediated opsonization, GAS employs sev- a target of protective immunity, hypervariability eral strategies to resist its uptake into phagocytes. futurefuture sciencescience groupgroup www.futuremedicine.com 77 REVIEW – Kwinn & Nizet

Although the GAS hyaluronic acid capsule does factors in the pathogenesis of invasive GAS infec- not block C3 deposition on the bacterial surface, tion [52,56,57]. The antiphagocytic functions of its antiphagocytic function is supported by sev- SLO may be more complex than direct cytolysis, as eral lines of evidence. Capsule-deficient GAS in an epithelial cell model SLO is seen to allow generated by targeted mutagenesis of the has bio- GAS to avoid lysosomal localization [58]. SLO also synthetic operon or through hyaluronidase treat- serves to deliver an NADase toxin to the host cell ment become susceptible to phagocytic clearance cytoplasm in a process known as cytolysin-medi- and less virulent in animal challenge models [46]. ated translocation [59]; the NADase activity Conversely, GAS variants with increased encap- depletes the host cell energy stores [60]. Finally, sulation are generated by animal passage and M protein released from the GAS surface can also mucoid strains are linked epidemiologically to be conceptualized as a toxin, forming complexes greater invasive disease potential [47]. The with fibrinogen that bind to β-integrins on host hyaluronic acid capsule appears to restrict access neutrophils, provoking the release of heparin bind- of phagocytes to a variety of opsonins on the ing protein and inflammatory changes leading to bacterial surface [48]. vascular leakage and severe disease [61]. GAS also possess the capacity to utilize various Upon phagocytosis, GAS mediate a program host matrix proteins to shield their surface of accelerated neutrophil apoptosis that can be and/or promote formation of bacterial aggre- correlated to enhanced phagocyte resistance rela- gates whose particle size may exceed the uptake tive to a variety of other common human patho- capacity of host phagocytes. For example, the gens [62]. Although the GAS virulence factors SfbI protein can bind fibronectin that can in involved in the neutrophil apoptosis differentia- turn recruit collagen, leading to matrix deposition program and their cellular targets remain to tion on and between bacteria and the develop- be elucidated, epithelial cell models suggest GAS ment of large aggregates [49]. Under physiological can induce a unique apoptosis pathway based on conditions, the B- and C-repeat regions of GAS caspase-9 release, mitochondrial dysfunction and M protein can bind fibrinogen and albumin, calcium regulation [63,64]. Additional evidence thus masking them from antibody binding [30]. links GAS-induced macrophage apoptosis to GAS have been shown to aggregate and form activation of matrix metalloproteases by the intratissue microcolonies or biofilms on cysteine protease SpeB [65]. uncoated polystyrene surfaces or those coated with fibronectin or collagen [50], likely restricting Resistance to effectors of phagocyte access. Finally, the multifunction phagocyte killing secreted SIC protein can colocalize with the After phagocytic uptake of the target bacteria, F-actin binding domain of ezrin that links the neutrophils and macrophages deploy an array of phagocyte cytoskeleton to the plasma mem- bactericidal mechanisms including vacuole acidi- brane. This interference can be viewed as a fication, generation of reactive oxygen and nitro- mechanism to impair the biophysical events gen species, and production of cationic required or phagocytic uptake, as evidenced by molecules including antimicrobial peptides the enhanced internalization of SIC-deficient (cathelicidins and defensins), myeloperoxidase mutants by human neutrophils [51]. and lysozyme [66]. Recently it has been shown that GAS can escape from the phagosome into Cytotoxicity & phagocyte apoptosis the cytoplasm of neutrophils [67]. Since viable GAS elaborate a variety of potent cytotoxins, GAS can be isolated from inside host phagocytic and another important mechanism for innate cells both in vitro and in vivo [68], the traditional immune resistance appears to involve triggering interpretation of GAS as an ‘extracellular’ bacte- the death of the phagocytic cell types before bac- rial pathogen is undergoing re-evaluation. Con- terial killing can be accomplished. The pore- sequently, increased attention has been focused forming GAS β-hemolysin streptolysin S (SLS) on the molecular basis of GAS survival within exerts cytocidal activity on host neutrophils and phagocytes. M protein was found to inhibit the thereby promotes GAS resistance to phagocytic fusion of azurophilic granules with the phago- killing [52,53]. The structurally unrelated choles- some and other membrane trafficking events terol-binding cytolysin streptolysin O (SLO) is required for phagosome maturation [69,70]. also toxic to human neutrophils and impairs Lacking catalase or carotenoid pigment such their phagocytic capacity [54,55]. Consequently, as those expressed by Staphylococcus aureus, GAS both the SLS and SLO toxins are key virulence has generally been considered susceptible to host

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oxidative burst killing. However, a recent study The two-component GAS global regulator revealed that GAS expression of GpoA gluta- CovR/S (for control of virulence) modulates the thione peroxidase allows the organism to adapt expression of several GAS virulence factors to oxidative stress and contributes to virulence including the hyaluronic acid capsule, cysteine in several animal models of pyogenic GAS protease SpeB and the cytotoxin SLS [79]. infection [71]. Another mechanism of neutrophil CovR/S is thought to play a role in GAS adapta- intracellular killing involves the action of cathe- tion to multiple stresses, including heat, acid and licidin antimicrobial peptides, as demonstrated hyperosomolarity stress [80]. Recent data indicate by the increased susceptibility of cathelicidin- that genetic mutations in the covR/S locus can be deficient mice to invasive GAS infection [72]. correlated with a phenotypic switch from One mechanism by which GAS resists cathelici- mucosal to invasive forms of GAS infection. In din killing is through incorporating positively comparing the transcriptional profile of GAS charged residues into its cell wall lipoteichoic isolates from pharyngeal versus systemic infec- acid, leading to electrostatic repulsion of the tion, two distinct patterns emerged [81]. The cationic antimicrobial peptide. In this fashion, switch from the pharyngeal to the invasive pat- D-alanylation of teichoic acids mediated by the tern could be recapitulated by mouse passage, dlt operon promotes GAS resistance to catheli- and was traced to mutations in either CovR or cidins and to neutrophil killing [73]. The human CovS that lead to increased expression of cathelicidin LL-37 and neutrophil α-defensins hyaluronic acid capsule, IL-8 protease ScpC, can be bound and inactivated by GAS protein protein SIC, DNAse Sda1, together with SIC [74]. Finally, the secreted cysteine protease decreased expression of the cysteine protease, SpeB is trapped on the GAS surface by SpeB [81]. Loss of SpeB preserved M protein and a2-macroglobulin bound to the bacterial surface streptokinase on the GAS surface, which allows protein GRAB; the retained SpeB is capable of accumulation and activation of the host protease degrading LL-37 and protecting the bacteria plasmin, facilitating spread of the organism to against its antimicrobial action [75]. the deep tissues [82]. A fascinating recent study by Graham and col- Regulation of GAS virulence phenotypes legues has examined the transcriptional profile of Expression of GAS virulence phenotypes is GAS during the course of necrotizing soft-tissue under the control of a complex set of global tran- infection and revealed a coordinated pattern of scriptional regulators. These comprise two-com- regulatory changes promoting increased expres- ponent sensor kinase/response regulators, stand- sion of virulence factors involved in innate alone response regulators and alternative sigma immune resistance and host tissue damage [83]. factors. Together, these frameworks coordinate Upregulation of the Mga, IhK/Irr and Fas- GAS response pathways, including those func- BCA/X [84] regulators together with downregula- tions to subvert host phagocyte clearance. For tion of negative transcription control factors such example, genes positively regulated by Mga as CovR/S and PerR [71] in sum result in the (mutli-gene regulator) include those encoding M robust in vivo expression of SIC, SLS, DNAse, and M-like proteins responsible for impairing IL-8 protease, M protein, SodA, hyaluronic acid opsonization and promoting GAS neutrophil capsule and other factors that can block phago- survival, the C5a peptidase (ScpA) targeting cytic uptake, promote GAS resistance to phago- neutrophil chemokines, and the multifunctional cyte intracellular killing mechanisms or prove antiphagocytic SIC protein [76]. Gene expression lethally toxic to the phagocytes. analysis of phagocytosed GAS uncovered a two- component response regulator Ihk/Irr critical for Conclusions pathogen survival [77]. Ihk/Irr influences expres- GAS derives its scientific name S. pyogenes from sion of 20% of the GAS genome, notably the Latin for ‘pus-generating’, consistent with including genes involved in cell wall formation neutrophilic infiltrates observed at the site of and peptidoglycan synthesis. Elimination of the acute GAS infection. The rising prevalence of response regulator gene (irr) yielded a GAS deep-seated, invasive GAS infections corrobo- mutant that was unable to resist killing by rates the sophisticated suite of defense mecha- cationic antimicrobial peptides and reactive oxy- nisms the pathogen has evolved to avoid gen species, and that was severely attenuated for clearance by the host phagocyte response. As virulence in the mouse necrotizing skin infection summarized in (Figure 1), these GAS virulence model [78]. traits interfere at multiple points, from initial futurefuture sciencescience groupgroup www.futuremedicine.com 79 REVIEW – Kwinn & Nizet

Figure 1. Several virulence mechanisms by which the pathogen Group A Streptococcus resists host phagocyte defenses.

Neutrophil lysis Interference with Chemokine complement degradation deposition ScpA (C5a) Breakdown ScpC (IL-8) of NETs SLO SLS C4BP C3b Impairment of phagocyte Fibrinogen Sda1 ?? Factor H uptake Group A Streptococcus (DNAse) M protein

Mga Ez SIC CovR/S FcγR Hyaluronic Transcriptional acid Ihk/Irr regulation of Mac-1/2 capsule phagocyte C3b + defense factors + Cloaki f FasBCA/X + opsonins + Others DltABCD Resistance to C3b antimicrobial peptides M protein SIC M-like proteins Sfb1 Fibronectin M protein Fibrinogen Protein H Albumin Sfb1 SpeB

Nonopsonic binding or degradation SpeB of immunoglobulins EndoS Mac1/IdsE

C4BP: C4b-binding protein; NET: Neutrophil extracellular trap; SIC: Serum inhibitor of complement; SLS: Streptolysin S; SLO: Streptolysin O.

neutrophil recruitment, to the processes of Future perspective opsonization, to bacterial entrapment and Most bacterial pathogens associated with signifi- uptake, and to intracellular effectors of bacterial cant human infection also typically exist in the killing; in several cases a single GAS molecule transient microflora of healthy individuals in the impairs multiple host defense mechanisms (e.g., context of asymptomatic colonization. Experi- the M protein). In certain strains of GAS, for mental analysis of GAS interaction with phago- example the globally disseminated M1T1 clone, cytic cells of the innate immune system accumulation of a larger repertoire of virulence represents a useful paradigm for discovery and factors (e.g., SIC and DNAse Sda1) poses a par- understanding of the underlying mechanisms ticular challenge to innate host defenses, and dictating the development or prevention of seri- finds corroboration in epidemiological associa- ous bacterial infection. Much work remains to be tions to severe disease, including necrotizing fas- done – of the many GAS factors reviewed in this ciitis and toxic shock syndrome. Moreover, paper, only a subset have been definitely proven mutations in the global transcriptional regulatory to contribute to innate immune subversion networks governing expression of GAS virulence in vivo, while others have been shown to pro- genes may suddenly alter the pathogen-phago- mote GAS phagocyte resistance in ex vivo sys- cyte equation, resulting in quantum shift toward tems, and still others have simply been shown to enhanced invasive disease potential. interact in vitro with host effector molecules in a

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fashion that could be predicted to promote the development has progressed slowly, complicated pathogen’s survival. Future investigations will by the lack of an immunogenic capsular poly- begin to define variations in host phagocyte saccharide, great antigenic diversity of the surface defense dictated by genetic polymorphism or M protein and caution regarding the association other factors such as concomitant viral infection of immunological cross-reactivity with the post- that may render individual patients particularly infectious complication of rheumatic heart dis- vulnerable. Although GAS remain universally sus- ease. Improved understanding of the molecular ceptible to β-lactam antibiotics including penicil- basis of GAS phagocyte resistance may provide lin, considerable morbidity and mortality opportunities for therapeutic intervention, associated with invasive GAS disease reflects the wherein novel pharmacologic agents act not to kill pathogen’s potential for rapid tissue spread and the bacterium directly, but rather to render it triggering organ system dysfunction. GAS vaccine susceptible to our normal innate defenses.

Executive summary Human infections caused by Group A Streptococcus • Group A Streptococcus (GAS) is a major pathogen producing a wide spectrum of superficial and invasive human infections. • Invasive GAS infections, including necrotizing fasciitis and toxic shock syndrome, strike up to 650,000 people annually with a mortality of 10–30%. • The propensity of GAS to produce invasive infection in previously healthy individuals reflects several virulence determinants capable of interfering with innate phagocytic defense mechanisms. GAS factors help the pathogen avoid engagement by phagocytes • The complement-derived chemoattractant peptide C5a is cleaved by the GAS peptidase ScpA. • The CXC chemokine interleukin-8 is degraded by the GAS serine protease ScpC. • GAS DNAses prevent capture in DNA-based neutrophil extracellular traps. GAS molecules inhibit complement & antibody function • The GAS hyaluronic acid capsule is nonimmunogenic, mimicking a common human matrix component, and cloaks opsonins deposited on the bacterial surface from phagocyte recognition. • M and M-like proteins and Sfb1 recruit host matrix proteins (fibronectin, fibrinogen and collagen) to form a protective coating against opsonin recognition. • M protein binds complement regulator C4b-binding protein to limit C3b deposition. • The protein streptococcal inhibitor of complement (SIC) interferes with formation of the C5–9 terminal complement membrane attack complex. • M protein and Sfb1 nonopsonically bind immunoglobulin via the Fc domain. • Cysteine protease SpeB degrades immunoglobulins. GAS impairs phagocytotic uptake mechanisms • EndoS hydrolyses the IgG glycans involved in Fcγ receptor recognition. • Mac-1 and -2 bind to neutrophil Fc receptors, inhibiting recognition of IgG on bacterial surface. • Protein SIC impairs actin cytoskeletal arrangements required for bacterial uptake. GAS promote phagocyte lysis and apoptosis • The pore-forming streptolysin S and streptolysin O are cytotoxic to neutrophils and macrophages. • GAS induces an accelerated apoptosis program in human neutrophils. GAS resists specific effectors of phagocyte killing • D-alanylation of lipoteichoic acid, SpeB proteolysis and SIC binding interfere with host cationic antimicrobial peptide function. • GAS escape the phagosome, and M protein can block azurophilic granule:phagosome fusion. • GpoA glutathione peroxidase allows GAS to adapt to oxidative stress. Transcriptional control of GAS phagocyte resistance phenotypes • GAS tightly regulates the expression of these phagocyte resistance factors through an interacting web of transcriptional regulators including CovR/S, Mga and Ihk/Irr. Future perspective • Understanding the molecular basis of GAS phagocyte resistance may reveal novel therapeutic targets for treatment and prevention of invasive human infections.

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