Explore the vast range of titles available.

Bacterial invasion of host tissues triggers polymorphonuclear leukocytes to release DNA [neutrophil extracellular traps (NETs)], thereby immobilizing microbes for subsequent clearance by innate defenses including macrophage phagocytosis. We report here that Staphylococcus aureus escapes these defenses by converting NETs to deoxyadenosine, which triggers the caspase-3–mediated death of immune cells. Conversion of NETs to deoxyadenosine requires two enzymes, nuclease and adenosine synthase, that are secreted by S. aureus and are necessary for the exclusion of macrophages from staphylococcal abscesses. Thus, the pathogenesis of S. aureus infections has evolved to anticipate host defenses and to repurpose them for the destruction of the immune system.

The cell wall peptidoglycan of Gram-positive bacteria functions as a surface organelle for the transport and assembly of proteins that interact with the environment, in particular, the tissues of an infected host. Signal peptide-bearing precursor proteins are secreted across the plasma membrane of Gram-positive bacteria. Some precursors carry C-terminal sorting signals with unique sequence motifs that are cleaved by sortase enzymes and linked to the cell wall peptidoglycan of vegetative forms or spores. The sorting signals of pilin precursors are cleaved by pilus-specific sortases, which generate covalent bonds between proteins leading to the assembly of fimbrial structures. Other precursors harbour surface (S)-layer homology domains (SLH), which fold into a three-pronged spindle structure and bind secondary cell wall polysaccharides, thereby associating with the surface of specific Gram-positive microbes. Type VII secretion is a non-canonical secretion pathway for WXG100 family proteins in mycobacteria. Gram-positive bacteria also secrete WXG100 proteins and carry unique genes that either contribute to discrete steps in secretion or represent distinctive substrates for protein transport reactions.

Key Points Staphylococcus aureus evades innate and adaptive immune responses to cause localized or systemic infections in humans. Because the development of protective immunity is prevented, S. aureus infections reoccur even with antibiotic or surgical therapy. Mechanisms have been revealed whereby products secreted by S. aureus interfere with neutrophil chemotaxis, complement activation, opsonization and phagocytic killing of bacteria. Immune evasion determinants can interact with host factors from humans, but not with counterparts from other vertebrates, which presents a challenge for the development of animal models. The hallmark of S. aureus is the secretion of coagulases that associate with prothrombin to generate fibrin clots. Through the fibrinogen and fibrin binding attributes of staphylococcal surface proteins, the pathogen shields itself from host phagocytes, which is a prerequisite for abscess lesion formation in infected tissues. Macrophage access to these lesions is restricted by staphylococcal-induced degradation of neutrophil extracellular traps (NETs). Staphylococcal protein A (SpA) crosslinks B cell receptors and triggers proliferative expansion of V H 3 + B cells and the secretion of antibodies that fail to recognize S. aureus antigens. SpA blocks host antibody responses that are required for the establishment of protective immunity. T cell superantigens crosslink major histocompatibility class II molecules of antigen-presenting cells with the T cell receptor, promoting lymphocyte proliferation, anergy and the release of cytokines (a cytokine storm). Superantigens vary between S. aureus strains and activate distinct subsets of Vβ chain T cell receptors, endowing staphylococcal isolates with unique T cell-avoidance attributes. Genome sequencing of S. aureus isolates from humans or domesticated animals revealed that only some immune evasion genes are conserved among all strains. Even conserved genes display sequence polymorphisms, which presents a formidable challenge for the design of S. aureus vaccines. Panoplies of immune evasion factors endow staphylococcal strains with unique virulence attributes and with the ability for epidemic spread. During recurrent staphylococcal infections, Staphylococcus aureus uses several strategies to evade detection by the host immune system. In this Review, Schneewind and colleagues describe the staphylococcal factors that manipulate innate and adaptive immune responses through inhibition, modification and phagocyte destruction. Staphylococcus aureus , a bacterial commensal of the human nares and skin, is a frequent cause of soft tissue and bloodstream infections. A hallmark of staphylococcal infections is their frequent recurrence, even when treated with antibiotics and surgical intervention, which demonstrates the bacterium's ability to manipulate innate and adaptive immune responses. In this Review, we highlight how S. aureus virulence factors inhibit complement activation, block and destroy phagocytic cells and modify host B cell and T cell responses, and we discuss how these insights might be useful for the development of novel therapies against infections with antibiotic resistant strains such as methicillin-resistant S. aureus .

The bacterial pathogen Staphylococcus aureus seeds abscesses in host tissues to replicate at the center of these lesions, protected from host immune cells via a pseudocapsule. Using histochemical staining, we identified prothrombin and fibrin within abscesses and pseudocapsules. S. aureus secretes two clotting factors, coagulase (Coa) and von Willebrand factor binding protein (vWbp). We report here that Coa and vWbp together are required for the formation of abscesses. Coa and vWbp promote the non-proteolytic activation of prothrombin and cleavage of fibrinogen, reactions that are inhibited with specific antibody against each of these molecules. Coa and vWbp specific antibodies confer protection against abscess formation and S. aureus lethal bacteremia, suggesting that coagulases function as protective antigens for a staphylococcal vaccine.

Host cell death programs are fundamental processes that shape cellular homeostasis, embryonic development, and tissue regeneration. Death signaling and downstream host cell responses are not only critical to guide mammalian development, they often act as terminal responses to invading pathogens. Here, we briefly review and contrast how invading pathogens and specifically Staphylococcus aureus manipulate apoptotic, necroptotic, and pyroptotic cell death modes to establish infection. Rather than invading host cells, S. aureus subverts these cells to produce diffusible molecules that cause death of neighboring hematopoietic cells and thus shapes an immune environment conducive to persistence. The exploitation of cell death pathways by S. aureus is yet another virulence strategy that must be juxtaposed to mechanisms of immune evasion, autophagy escape, and tolerance to intracellular killing, and brings us closer to the true portrait of this pathogen for the design of effective therapeutics and intervention strategies.

Heritable defects in human B cell/antibody development are not associated with increased susceptibility to Staphylococcus aureus infection. Protein A (SpA), a surface molecule of S. aureus , binds the Fcγ domain of immunoglobulin (Ig) and cross-links the Fab domain of V H 3-type B cell receptors (IgM). Here we generated S. aureus spa variants harboring amino acid substitutions at four key residues in each of the five Ig-binding domains of SpA. Wild-type S. aureus required SpA binding to Ig to resist phagocytosis and SpA-mediated B cell receptor cross-linking to block antibody development in mice. The spa KKAA mutant, which cannot bind Ig or IgM, was phagocytosed and elicited B cell responses to key virulence antigens that protected animals against lethal S. aureus challenge. The immune evasive attributes of S. aureus SpA were abolished in µMT mice lacking mature B cells and antibodies. Thus, while wild-type S. aureus escapes host immune surveillance, the spa KKAA variant elicits adaptive responses that protect against recurrent infection. IMPORTANCE Staphylococcus aureus causes recurrent skin and bloodstream infections without eliciting immunity. Heritable defects in neutrophil and T cell function, but not B cell or antibody development, are associated with increased incidence of S. aureus infection, and efforts to develop antibody-based S. aureus vaccines have thus far been unsuccessful. We show here that the Fcγ and V H 3-type Fab binding activities of staphylococcal protein A (SpA) are essential for S. aureus escape from host immune surveillance in mice. The virulence attributes of SpA in mice required mature B cells and immunoglobulin. These results suggest that antibodies and B cells play a key role in the pathogenesis of staphylococcal infections and provide insights into the development of a vaccine against S. aureus . Staphylococcus aureus causes recurrent skin and bloodstream infections without eliciting immunity. Heritable defects in neutrophil and T cell function, but not B cell or antibody development, are associated with increased incidence of S. aureus infection, and efforts to develop antibody-based S. aureus vaccines have thus far been unsuccessful. We show here that the Fcγ and V H 3-type Fab binding activities of staphylococcal protein A (SpA) are essential for S. aureus escape from host immune surveillance in mice. The virulence attributes of SpA in mice required mature B cells and immunoglobulin. These results suggest that antibodies and B cells play a key role in the pathogenesis of staphylococcal infections and provide insights into the development of a vaccine against S. aureus .

Staphylococcus aureus is a human-adapted pathogen that replicates by asymptomatically colonizing its host. Nasal colonization occurs in the first weeks of life and persists in about 30% of the population. Using the mouse-adapted strain WU1 to model persistent colonization, we reported earlier that inoculation of bacteria lacking Staphylococcal protein A (SpA/Δ spa ) or neutralization of SpA through vaccination result in the slow decolonization of animals. Secretory (S)IgA is considered a first line of defense against pathogens at mucosal surfaces. Here, we use Ighasec -/- mutant mice to evaluate the contribution of SIgA towards decolonization. We observe that WU1 burdens are reduced in colonized Ighasec -/- mice compared to C57BL/6J animals. Both C57BL/6J and Ighasec -/- mice eliminate Δ spa bacteria, yet elimination occurs more rapidly in animals lacking IgA. SpA captures Fab-V H 3-type antibodies, including IgA, on the bacterial cell surface. We propose that this activity promotes colonization. Yet, we also find that antibody responses to the pathogen are altered when SpA and IgA are missing. Colonized C57BL/6J mice display a low serum IgG2c/IgG1 ratio towards staphylococcal antigens. This ratio is increased in animals colonized with Δ spa and is further enhanced in Ighasec -/- mice. We attribute the former to the loss of immune evasion activity in absence of SpA, and the latter to a host compensatory mechanism upon exposure to S. aureus . Importantly, the increased IgG2c/IgG1 ratio correlates with decolonization and enhanced killing of S. aureus . Similarly, we observe that decolonization induced by SpA-vaccination is accelerated in Ighasec -/- mice which display higher anti-SpA IgG2c titers as compared to C57BL/6J animals. Together, these findings suggest that S. aureus exploits SIgA in a SpA-dependent manner for colonization and in absence of IgA, serum opsonizing antibodies may promote bacterial clearance at mucosal surfaces.

Staphylococcus aureus infection is a frequent cause of sepsis in humans, a disease associated with high mortality and without specific intervention. When suspended in human or animal plasma, staphylococci are known to agglutinate, however the bacterial factors responsible for agglutination and their possible contribution to disease pathogenesis have not yet been revealed. Using a mouse model for S. aureus sepsis, we report here that staphylococcal agglutination in blood was associated with a lethal outcome of this disease. Three secreted products of staphylococci--coagulase (Coa), von Willebrand factor binding protein (vWbp) and clumping factor (ClfA)--were required for agglutination. Coa and vWbp activate prothrombin to cleave fibrinogen, whereas ClfA allowed staphylococci to associate with the resulting fibrin cables. All three virulence genes promoted the formation of thromboembolic lesions in heart tissues. S. aureus agglutination could be disrupted and the lethal outcome of sepsis could be prevented by combining dabigatran-etexilate treatment, which blocked Coa and vWbp activity, with antibodies specific for ClfA. Together these results suggest that the combined administration of direct thrombin inhibitors and ClfA-antibodies that block S. aureus agglutination with fibrin may be useful for the prevention of staphylococcal sepsis in humans.

Caspase-3 controls the apoptotic pathway, a form of programmed cell death designed to be immunologically silent. Polymorphisms leading to reduced caspase-3 activity are associated with variable effects on tumorigenesis and yet arise frequently. Staphylococcus aureus is a human commensal and a frequent cause of soft tissue and bloodstream infections. Successful commensalism and virulence can be explained by the secretion of a plethora of immune evasion factors. One such factor, AdsA, destroys phagocytic cells by exploiting the apoptotic pathway. However, human CASP3 variants with loss-of-function alleles shield phagocytes from AdsA-mediated killing. This finding raises the possibility that some caspase-3 alleles may arise from exposure to S. aureus and other human pathogens that exploit the apoptotic pathway for infection. Staphylococcus aureus is a deadly pathogen that causes fatal diseases in humans. During infection, S. aureus secretes nuclease (Nuc) and adenosine synthase A (AdsA) to generate cytotoxic deoxyadenosine (dAdo) from neutrophil extracellular traps which triggers noninflammatory apoptosis in macrophages. In this manner, replicating staphylococci escape phagocytic killing without alerting the immune system. Here, we show that mice lacking caspase-3 in immune cells exhibit increased resistance toward S. aureus . Caspase-3-deficient macrophages are resistant to staphylococcal dAdo and gain access to abscess lesions to promote bacterial clearance in infected animals. We identify specific single nucleotide polymorphisms in CASP3 as candidate human resistance alleles that protect macrophages from S. aureus -derived dAdo, raising the possibility that the allelic repertoire of caspase-3 may contribute to the outcome of S. aureus infections in humans. IMPORTANCE Caspase-3 controls the apoptotic pathway, a form of programmed cell death designed to be immunologically silent. Polymorphisms leading to reduced caspase-3 activity are associated with variable effects on tumorigenesis and yet arise frequently. Staphylococcus aureus is a human commensal and a frequent cause of soft tissue and bloodstream infections. Successful commensalism and virulence can be explained by the secretion of a plethora of immune evasion factors. One such factor, AdsA, destroys phagocytic cells by exploiting the apoptotic pathway. However, human CASP3 variants with loss-of-function alleles shield phagocytes from AdsA-mediated killing. This finding raises the possibility that some caspase-3 alleles may arise from exposure to S. aureus and other human pathogens that exploit the apoptotic pathway for infection.

Rickettsial diseases have long been diagnosed with serum antibodies cross-reactive against Proteus vulgaris (Weil–Felix reaction). Although Weil–Felix antibodies are associated with the development of immunity, their rickettsial target and contribution to disease pathogenesis are not established. Here, we developed a transposon for insertional mutagenesis of Rickettsia conorii, isolating variants defective for replication in cultured cells and in spotted fever pathogenesis. Mutations in the polysaccharide synthesis operon (pso) abolish lipopolysaccharide O-antigen synthesis and Weil–Felix serology and alter outer-membrane protein assembly. Unlike wild-type R. conorii, pso mutants cannot elicit bactericidal antibodies that bind O antigen. The pso operon is conserved among rickettsial pathogens, suggesting that bactericidal antibodies targeting O antigen may generate universal immunity that could be exploited to develop vaccines against rickettsial diseases.