DNase Sda1 Allows Invasive M1T1 Group A Streptococcus to Prevent TLR9-Dependent Recognition

Group A Streptococcus (GAS) has developed a broad arsenal of virulence factors that serve to circumvent host defense mechanisms. The virulence factor DNase Sda1 of the hyperinvasive M1T1 GAS clone degrades DNA-based neutrophil extracellular traps allowing GAS to escape extracellular killing. TLR9 is activated by unmethylated CpG-rich bacterial DNA and enhances innate immune resistance. We hypothesized that Sda1 degradation of bacterial DNA could alter TLR9-mediated recognition of GAS by host innate immune cells. We tested this hypothesis using a dual approach: loss and gain of function of DNase in isogenic GAS strains and presence and absence of TLR9 in the host. Either DNA degradation by Sda1 or host deficiency of TLR9 prevented GAS induced IFN-α and TNF-α secretion from murine macrophages and contributed to bacterial survival. Similarly, in a murine necrotizing fasciitis model, IFN-α and TNF-α levels were significantly decreased in wild type mice infected with GAS expressing Sda1, whereas no such Sda1-dependent effect was seen in a TLR9-deficient background. Thus GAS Sda1 suppressed both the TLR9-mediated innate immune response and macrophage bactericidal activity. Our results demonstrate a novel mechanism of bacterial innate immune evasion based on autodegradation of CpG-rich DNA by a bacterial DNase.     

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

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

Post-proteomic identification of a novel phage-encoded streptodornase, Sda1, in invasive M1T1 Streptococcus pyogenes

The M1T1 strain remains the most frequently isolated strain from group A streptococcal (GAS) infection cases worldwide. We previously reported that M1T1 differs from the fully sequenced M1 SF370 strain. To better understand the reason for the persistence and increased virulence of M1T1, we analysed its secreted proteome and identified two virulence proteins that are not present in the sequenced M1 SF370 strain: streptococcal pyrogenic exotoxin A (SpeA) and a streptodornase D (SdaD) homologue. In the present study, we determined the nucleotide sequence of the M1T1 streptodornase and found that its deduced amino acid sequence is highly similar to other streptococcal streptodornases, and is most closely related to the SdaD of GAS strain M49. M1T1 Sda shares two highly conserved domains with several DNases and putative DNases in streptococci; however, it possesses a unique C-terminal amino acid sequence. Thus, we named the protein Sda1, and we detected the presence of the sda1 gene in 16 M1T1 clinical isolates. The cloned and expressed Sda1 degrades both streptococcal and mammalian DNA at physiological pH. Amino acid similarity analyses of known GAS deoxyribonucleases suggest that Sda1 may be a chimeric protein created through recombination events. Moreover, a natural mutation that resulted in longer Sda1 and SdaD as compared to other GAS DNases was found to confer increased activity on the protein. Analysis of the sequences flanking sda1 determined that it is carried by a prophage or a prophage-like element inserted in the tRNA-Ser gene of M1T1 GAS. Ongoing studies in our laboratory aim to determine the contribution of Sda1 to the virulence of this globally disseminated M1T1 strain.     

The IL-8 protease SpyCEP/ScpC of group A Streptococcus promotes resistance to neutrophil killing

Interleukin-8 (IL-8) promotes neutrophil-mediated host defense through its chemoattractant and immunostimulatory activities. The Group A Streptococcus (GAS) protease SpyCEP (also called ScpC) cleaves IL-8, and SpyCEP expression is strongly upregulated in vivo in the M1T1 GAS strains associated with life-threatening systemic disease including necrotizing fasciitis. Coupling allelic replacement with heterologous gene expression, we show that SpyCEP is necessary and sufficient for IL-8 degradation. SpyCEP decreased IL-8-dependent neutrophil endothelial transmigration and bacterial killing, the latter by reducing neutrophil extracellular trap formation. The knockout mutant lacking SpyCEP was attenuated for virulence in murine infection models, and SpyCEP expression conferred protection to coinfecting bacteria. We also show that the zoonotic pathogen Streptococcus iniae possesses a functional homolog of SpyCEP (CepI) that cleaves IL-8, promotes neutrophil resistance, and contributes to virulence. By inactivating the multifunctional host defense peptide IL-8, the SpyCEP protease impairs neutrophil clearance mechanisms, contributing to the pathogenesis of invasive streptococcal infection.     

An endonuclease allows Streptococcus pneumoniae to escape from neutrophil extracellular traps

Streptococcus pneumoniae (pneumococcus) is the most common cause of community-acquired pneumonia, with high morbidity and mortality worldwide. A major feature of pneumococcal pneumonia is an abundant neutrophil infiltration . It was recently shown that activated neutrophils release neutrophil extracellular traps (NETs), which contain antimicrobial proteins bound to a DNA scaffold. NETs provide a high local concentration of antimicrobial components and bind, disarm, and kill microbes extracellularly. Here, we show that pneumococci are trapped but, unlike many other pathogens, not killed by NETs. NET trapping in the lungs, however, may allow the host to confine the infection, reducing the likelihood for the pathogen to spread into the bloodstream. DNases are expressed by many Gram-positive bacterial pathogens, but their role in virulence is not clear. Expression of a surface endonuclease encoded by endA is a common feature of many pneumococcal strains. We show that EndA allows pneumococci to degrade the DNA scaffold of NETs and escape. Furthermore, we demonstrate that escaping NETs promotes spreading of pneumococci from the upper airways to the lungs and from the lungs into the bloodstream during pneumonia.     

Group A Streptococcus Secreted Esterase Hydrolyzes Platelet-Activating Factor to Impede Neutrophil Recruitment and Facilitate Innate Immune Evasion

The innate immune system is the first line of host defense against invading organisms. Thus, pathogens have developed virulence mechanisms to evade the innate immune system. Here, we report a novel means for inhibition of neutrophil recruitment by Group A Streptococcus (GAS). Deletion of the secreted esterase gene (designated sse) in M1T1 GAS strains with (MGAS5005) and without (MGAS2221) a null covS mutation enhances neutrophil ingress to infection sites in the skin of mice. In trans expression of SsE in MGAS2221 reduces neutrophil recruitment and enhances skin invasion. The sse deletion mutant of MGAS5005 (Δsse ^MGAS5005) is more efficiently cleared from skin than the parent strain. SsE hydrolyzes the sn-2 ester bond of platelet-activating factor (PAF), converting biologically active PAF into inactive lyso-PAF. K_M and k _cat of SsE for hydrolysis of 2-thio-PAF were similar to those of the human plasma PAF acetylhydrolase. Treatment of PAF with SsE abolishes the capacity of PAF to induce activation and chemotaxis of human neutrophils. More importantly, PAF receptor-deficient mice significantly reduce neutrophil infiltration to the site of Δsse ^MGAS5005 infection. These findings identify the first secreted PAF acetylhydrolase of bacterial pathogens and support a novel GAS evasion mechanism that reduces phagocyte recruitment to sites of infection by inactivating PAF, providing a new paradigm for bacterial evasion of neutrophil responses.     

Severe invasive streptococcal infection by Streptococcus pyogenes and Streptococcus dysgalactiae subsp. equisimilis

Streptococcus pyogenes, a group A Streptococcus (GAS), has been recognized as the causative pathogen in patients with severe invasive streptococcal infection with or without necrotizing fasciitis. In recent epidemiological studies, Streptococcus dysgalactiae subsp. equisimilis (SDSE) has been isolated from severe invasive streptococcal infection. Complete genome sequence showed that SDSE is the closest bacterial species to GAS, with approximately 70% of genome coverage. SDSE, however, lacks several key virulence factors present in GAS, such as SPE‐B, the hyaluronan synthesis operon and active superantigen against human immune cells. A key event in the ability of GAS to cause severe invasive streptococcal infection was shown to be the acquisition of novel genetic traits such as phages. Strikingly, however, during severe invasive infection, GAS destroys its own covRS two‐component system, which negatively regulates many virulence factor genes, resulting in a hyper‐virulent phenotype. In contrast, this phenomenon has not been observed in SDSE. The present review describes the epidemiology of severe invasive streptococcal infection and the detailed pathogenic mechanisms of GAS and SDSE, emphasizing findings from their genome sequences and analyses of gene expression.     

Recruited macrophages control dissemination of group A Streptococcus from infected soft tissues

Group A Streptococcus (GAS) causes diverse infections in humans, ranging from mild to life-threatening invasive diseases, such as necrotizing fasciitis (NF), a rapidly progressing deep tissue infection. Despite prompt treatments, NF remains a significant cause of morbidity and mortality, even in previously healthy individuals. The early recruitment of leukocytes is crucial to the outcome of NF; however, although the role of polymorphonuclear neutrophils (PMNs) in host defense against NF is well established, the role of recruited macrophages remains poorly defined. Using a cutaneous murine model mimicking human NF, we found that mice deficient in TNF-α were highly susceptible to s.c. infections with GAS, and a paucity of macrophages, but not PMNs, was demonstrated. To test whether the effects of TNF-α on the outcome of infection are mediated by macrophages/monocytes, we systemically depleted C57BL/6 mice of monocytes by pharmacological and genetic approaches. Systemic monocyte depletion substantially increased bacterial dissemination from soft tissues without affecting the number of recruited PMNs or altering the bacterial loads in soft tissues. Enhanced GAS dissemination could be reverted by either i.v. injection of monocytes or s.c. administration of peritoneal macrophages. These experiments demonstrated that recruited macrophages play a key role in defense against the extracellular pathogen GAS by limiting its spread from soft tissues.     

Streptolysin O Promotes Group A Streptococcus Immune Evasion by Accelerated Macrophage Apoptosis

Group A Streptococcus (GAS) is a leading human bacterial pathogen capable of producing invasive infections even in previously healthy individuals. As frontline components of host innate defense, macrophages play a key role in control and clearance of GAS infections. We find GAS induces rapid, dose-dependent apoptosis of primary and cultured macrophages and neutrophils. The cell death pathway involves apoptotic caspases, is partly dependent on caspase-1, and requires GAS internalization by the phagocyte. Analysis of GAS virulence factor mutants, heterologous expression, and purified toxin studies identified the pore-forming cytolysin streptolysin O (SLO) as necessary and sufficient for the apoptosis-inducing phenotype. SLO-deficient GAS mutants induced less macrophage apoptosis in vitro and in vivo, allowed macrophage cytokine secretion, and were less virulent in a murine systemic infection model. Ultrastructural evidence of mitochondrial membrane remodeling, coupled with loss of mitochondrial depolarization and cytochrome c release, suggests a direct attack of the toxin initiates the intrinsic apoptosis pathway. A general caspase inhibitor blocked SLO-induced apoptosis and enhanced macrophage killing of GAS. We conclude that accelerated, caspase-dependent macrophage apoptosis induced by the pore-forming cytolysin SLO contributes to GAS immune evasion and virulence.Group A Streptococcus (GAS)⁴ is a leading human pathogen that annually infects hundreds of millions of people worldwide (1). The last 3 decades have witnessed a marked increase in severe, invasive forms of GAS infection, many attributable to a single globally disseminated clone of the M1T1 serotype (2). Invasive GAS infection defines a capacity of the pathogen to resist host innate defense mechanisms designed to prevent microbial spread beyond epithelial surfaces.Macrophages are critical host defense cells involved directly in bacterial clearance and also in alerting other immune system components to invading pathogens. Macrophage microbicidal activity is accomplished by phagocytic uptake coupled with the action of reactive oxygen species, enzymatic proteolysis, and cationic antimicrobial peptides; their role in amplification of the innate and adaptive immune responses is achieved through release of soluble factors such as cytokines and nitric oxide. Mice depleted of macrophages or treated with inhibitors of macrophage phagocytosis cannot clear GAS infections even at relatively low challenge doses (3), demonstrating the essential first line defense function of these immune cells against the pathogen.We sought to explore the interaction of the highly virulent GAS M1T1 clone with macrophages to better understand its propensity to produce invasive human infection. A prominent regulatory feature of macrophage biology in the context of infectious disease and inflammation is the process of apoptosis, mediated by caspase family proteases. Although a number of highly adapted intracellular bacterial pathogens, including Mycobacterium tuberculosis, Legionella pneumophila, and Brucella spp., have evolved mechanisms to block macrophage apoptosis and use the host cell as a vehicle for in vivo dissemination (4–6), a recent study of GAS M1T1 interactions with another host phagocytic cell type suggested a different outcome. In contrast to other prominent Gram-positive pathogens, including Staphylococcus aureus and Listeria monocytogenes, GAS induced an accelerated program of apoptosis in human neutrophils (7), although the specific virulence factor(s) involved, effects on caspase activation, and contribution to disease outcome were not studied.Here we report that GAS rapidly induces macrophage apoptosis through caspase-dependent pathways, promoted by release of cytochrome c and permeabilization of mitochondrial outer membranes. GAS-induced macrophage apoptosis is mediated by the cytolysin streptolysin O (SLO), which is both necessary and sufficient for the phenotype. SLO-mediated macrophage apoptosis leads to enhanced GAS survival, dampened cytokine responses, and increased virulence during systemic infection.     

Characterizing the role of tissue-type plasminogen activator in a mouse model of Group A streptococcal infection

Plasmin(ogen) acquisition is critical for invasive disease initiation by Streptococcus pyogenes (GAS). Host urokinase plasminogen activator (uPA) plays a role in mediating plasminogen activation for GAS dissemination, however the contribution of tissue-type plasminogen activator (tPA) to GAS virulence is unknown. Using novel tPA-deficient ALBPLG1 mice, our study revealed no difference in mouse survival, bacterial dissemination or the pathology of GAS infection in the absence of tPA in AlbPLG1/tPA^?/? mice compared to AlbPLG1 mice. This study suggests that tPA has a limited role in this humanized model of GAS infection, further highlighting the importance of its counterpart uPA in GAS disease.     

Virulence of Group A Streptococci Is Enhanced by Human Complement Inhibitors

Streptococcus pyogenes, also known as Group A Streptococcus (GAS), is an important human bacterial pathogen that can cause invasive infections. Once it colonizes its exclusively human host, GAS needs to surmount numerous innate immune defense mechanisms, including opsonization by complement and consequent phagocytosis. Several strains of GAS bind to human-specific complement inhibitors, C4b-binding protein (C4BP) and/or Factor H (FH), to curtail complement C3 (a critical opsonin) deposition. This results in diminished activation of phagocytes and clearance of GAS that may lead to the host being unable to limit the infection. Herein we describe the course of GAS infection in three human complement inhibitor transgenic (tg) mouse models that examined each inhibitor (human C4BP or FH) alone, or the two inhibitors together (C4BPxFH or ‘double’ tg). GAS infection with strains that bound C4BP and FH resulted in enhanced mortality in each of the three transgenic mouse models compared to infection in wild type mice. In addition, GAS manifested increased virulence in C4BPxFH mice: higher organism burdens and greater elevations of pro-inflammatory cytokines and they died earlier than single transgenic or wt controls. The effects of hu-C4BP and hu-FH were specific for GAS strains that bound these inhibitors because strains that did not bind the inhibitors showed reduced virulence in the ‘double’ tg mice compared to strains that did bind; mortality was also similar in wild-type and C4BPxFH mice infected by non-binding GAS. Our findings emphasize the importance of binding of complement inhibitors to GAS that results in impaired opsonization and phagocytic killing, which translates to enhanced virulence in a humanized whole animal model. This novel hu-C4BPxFH tg model may prove invaluable in studies of GAS pathogenesis and for developing vaccines and therapeutics that rely on human complement activation for efficacy.     

Neutrophil extracellular trap (NET)-mediated killing of Pseudomonas aeruginosa: evidence of acquired resistance within the CF airway, independent of CFTR

The inability of neutrophils to eradicate Pseudomonas aeruginosa within the cystic fibrosis (CF) airway eventually results in chronic infection by the bacteria in nearly 80 percent of patients. Phagocytic killing of P. aeruginosa by CF neutrophils is impaired due to decreased cystic fibrosis transmembrane conductance regulator (CFTR) function and virulence factors acquired by the bacteria. Recently, neutrophil extracellular traps (NETs), extracellular structures composed of neutrophil chromatin complexed with granule contents, were identified as an alternative mechanism of pathogen killing. The hypothesis that NET-mediated killing of P. aeruginosa is impaired in the context of the CF airway was tested. P. aeruginosa induced NET formation by neutrophils from healthy donors in a bacterial density dependent fashion. When maintained in suspension through continuous rotation, P. aeruginosa became physically associated with NETs. Under these conditions, NETs were the predominant mechanism of killing, across a wide range of bacterial densities. Peripheral blood neutrophils isolated from CF patients demonstrated no impairment in NET formation or function against P. aeruginosa. However, isogenic clinical isolates of P. aeruginosa obtained from CF patients early and later in the course of infection demonstrated an acquired capacity to withstand NET-mediated killing in 8 of 9 isolates tested. This resistance correlated with development of the mucoid phenotype, but was not a direct result of the excess alginate production that is characteristic of mucoidy. Together, these results demonstrate that neutrophils can kill P. aeruginosa via NETs, and in vitro this response is most effective under non-stationary conditions with a low ratio of bacteria to neutrophils. NET-mediated killing is independent of CFTR function or bacterial opsonization. Failure of this response in the context of the CF airway may occur, in part, due to an acquired resistance against NET-mediated killing by CF strains of P. aeruginosa.     

A Bacterial Pathogen Co-opts Host Plasmin to Resist Killing by Cathelicidin Antimicrobial Peptides

The bacterial pathogen Group A Streptococcus (GAS) colonizes epithelial and mucosal surfaces and can cause a broad spectrum of human disease. Through the secreted plasminogen activator streptokinase (Ska), GAS activates human plasminogen into plasmin and binds it to the bacterial surface. The resulting surface plasmin protease activity has been proposed to play a role in disrupting tissue barriers, promoting invasive spread of the bacterium. We investigated whether this surface protease activity could aid the immune evasion role through degradation of the key innate antimicrobial peptide LL-37, the human cathelicidin. Cleavage products of plasmin-degraded LL-37 were analyzed by matrix-assisted laser desorption ionization mass spectrometry. Ska-deficient GAS strains were generated by targeted allelic exchange mutagenesis and confirmed to lack surface plasmin activity after growth in human plasma or media supplemented with plasminogen and fibrinogen. Loss of surface plasmin activity left GAS unable to efficiently degrade LL-37 and increased bacterial susceptibility to killing by the antimicrobial peptide. When mice infected with GAS were simultaneously treated with the plasmin inhibitor aprotinin, a significant reduction in the size of necrotic skin lesions was observed. Together these data reveal a novel immune evasion strategy of the human pathogen: co-opting the activity of a host protease to evade peptide-based innate host defenses.     

T lymphocyte priming by neutrophil extracellular traps links innate and adaptive immune responses

Polymorphonuclear neutrophils constitute the first line of defense against infections. Among their strategies to eliminate pathogens they release neutrophil extracellular traps (NETs), being chromatin fibers decorated with antimicrobial proteins. NETs trap and kill pathogens very efficiently, thereby minimizing tissue damage. Furthermore, NETs modulate inflammatory responses by activating plasmacytoid dendritic cells. In this study, we show that NETs released by human neutrophils can directly prime T cells by reducing their activation threshold. NETs-mediated priming increases T cell responses to specific Ags and even to suboptimal stimuli, which would not induce a response in resting T cells. T cell priming mediated by NETs requires NETs/cell contact and TCR signaling, but unexpectedly we could not demonstrate a role of TLR9 in this mechanism. NETs-mediated T cell activation adds to the list of neutrophil functions and demonstrates a novel link between innate and adaptive immune responses.     

Neutrophil serine proteinases cleave bacterial flagellin, abrogating its host response-inducing activity

After bacterial infection, neutrophils dominate the cellular infiltrate. Their main function is assumed to be killing invading pathogens and resolving the inflammation they cause. Activated neutrophils are also known to release a variety of molecules, including the neutrophil serine proteinases, extracellularly. The release of these proteinases during inflammation creates a proteolytic environment where degradation of different molecules modulates the inflammatory response. Flagellin, the structural component of flagella on many bacterial species, is a virulence factor with a strong proinflammatory activity on epithelial cells and other cell types. In this study we show that both human and mouse neutrophil serine proteinases cleave flagellin from Pseudomonas aeruginosa and other bacterial species. More important, cleavage of P. aeruginosa flagellin by the neutrophil serine proteinases neutrophil elastase and cathepsin G resulted in loss of the biological activity of this virulence factor, as evidenced by the lack of innate host defense gene expression in human epithelial cells. The finding that flagellin is susceptible to cleavage by neutrophil serine proteinases suggests a novel role for these enzymes in the inflammatory response to infection. Not only can these enzymes kill bacteria, but they also degrade their virulence factors to halt the inflammatory response they trigger.     

PAD4参与中性粒细胞胞外诱捕网的抗菌作用

蛋白质精氨酸脱亚氨酶4(PAD4)是中性粒细胞胞外诱捕网(NET)介导的细菌杀灭途径中的重要免疫因子。PAD4催化组蛋白瓜氨酸化,促进细菌感染期中性粒细胞形成NET。PAD4/中性粒细胞在炎性因子或细菌作用下不能形成NET,PAD4/鼠相对于PAD4+/+鼠更易受细菌感染。