Plasminogen is a critical host pathogenicity factor for group A streptococcal infection

Plasmin(ogen) acquisition by group A Streptococcus protects against C3b-mediated neutrophil killing

The globally significant human pathogen group A Streptococcus (GAS) sequesters the host protease plasmin to the cell surface during invasive disease initiation. Recent evidence has shown that localized plasmin activity prevents opsonization of several bacterial species by key components of the innate immune system in vitro. Here we demonstrate that plasmin at the GAS cell surface resulted in degradation of complement factor C3b, and that plasminogen acquisition is associated with a decrease in C3b opsonization and neutrophil-mediated killing in vitro. Furthermore, the ability to acquire cell surface plasmin(ogen) correlates directly with a decrease in C3b opsonization, neutrophil phagocytosis, and increased bacterial survival in a humanized plasminogen mouse model of infection. These findings demonstrate that localized plasmin(ogen) plays an important role in facilitating GAS escape from the host innate immune response and increases bacterial virulence in the early stages of infection.

Complement-mediated opsonization of invasive group A Streptococcus pyogenes strain AP53 is regulated by the bacterial two-component cluster of virulence responder/sensor (CovRS) system

Group A Streptococcus pyogenes (GAS) strain AP53 is a primary isolate from a patient with necrotizing fasciitis. These AP53 cells contain an inactivating mutation in the sensor component of the cluster of virulence (cov) responder (R)/sensor (S) two-component gene regulatory system (covRS), which enhances the virulence of the primary strain, AP53/covR(+)S(-). However, specific mechanisms by which the covRS system regulates the survival of GAS in humans are incomplete. Here, we show a key role for covRS in the regulation of opsonophagocytosis of AP53 by human neutrophils. AP53/covR(+)S(-) cells displayed potent binding of host complement inhibitors of C3 convertase, viz. Factor H (FH) and C4-binding protein (C4BP), which concomitantly led to minimal C3b deposition on AP53 cells, further showing that these plasma protein inhibitors are active on GAS cells. This resulted in weak killing of the bacteria by human neutrophils and a corresponding high death rate of mice after injection of these cells. After targeted allelic alteration of covS(-) to wild-type covS (covS(+)), a dramatic loss of FH and C4BP binding to the AP53/covR(+)S(+) cells was observed. This resulted in elevated C3b deposition on AP53/covR(+)S(+) cells, a high level of opsonophagocytosis by human neutrophils, and a very low death rate of mice infected with AP53/covR(+)S(+). We show that covRS is a critical transcriptional regulator of genes directing AP53 killing by neutrophils and regulates the levels of the receptors for FH and C4BP, which we identify as the products of the fba and enn genes, respectively.

A key role for the urokinase plasminogen activator (uPA) in invasive Group A streptococcal infection

Recruitment of the serine protease plasmin is central to the pathogenesis of many bacterial species, including Group A streptococcus (GAS), a leading cause of morbidity and mortality globally. A key process in invasive GAS disease is the ability to accumulate plasmin at the cell surface, however the role of host activators of plasminogen in this process is poorly understood. Here, we demonstrate for the first time that the urokinase-type plasminogen activator (uPA) contributes to plasmin recruitment and subsequent invasive disease initiation in vivo. In the absence of a source of host plasminogen activators, streptokinase (Ska) was required to facilitate cell surface plasmin acquisition by GAS. However, in the absence of Ska, host activators were sufficient to promote cell surface plasmin acquisition by GAS strain 5448 during incubation with plasminogen or human plasma. Furthermore, GAS were able mediate a significant increase in the activation of zymogen pro-uPA in human plasma. In order to assess the contribution of uPA to invasive GAS disease, a previously undescribed transgenic mouse model of infection was employed. Both C57/black 6J, and AlbPLG1 mice expressing the human plasminogen transgene, were significantly more susceptible to invasive GAS disease than uPA−/− mice. The observed decrease in virulence in uPA−/−mice was found to correlate directly with a decrease in bacterial dissemination and reduced cell surface plasmin accumulation by GAS. These findings have significant implications for our understanding of GAS pathogenesis, and research aimed at therapeutic targeting of plasminogen activation in invasive bacterial infections.

Subversion of the host fibrinolytic system by bacterial pathogens is recognised as a key process in severe disease initiation. Co-opting of plasmin by bacteria contributes to tissue destruction and bacterial dissemination, both hallmarks of invasive Group A streptococcal disease, and research aimed at therapeutic targeting of the nexus between group A streptococcus and the fibrinolytic system is increasing. The host plasminogen activator uPA is found at the surface of cells that contribute to epithelial and innate immune defense against bacterial infection, and may contribute to bacterial recruitment of plasmin, however, the role of uPA in group A streptococcal infection is not well characterised. Here, we describe for the first time the key role played by uPA in invasive group A streptococcal disease. The ability of this pathogen to cause severe infection, even in the absence of the bacterial plasminogen activator streptokinase, has significant implications for the development of therapeutics to control invasive bacterial infection.

Modulation of neutrophil NETosis: interplay between infectious agents and underlying host physiology

The ability of neutrophils and other leucocyte members of the innate immune system to expel their DNA into the extracellular environment in a controlled manner in order to trap and kill pathogenic microorganisms lead to a paradigm shift in our understanding of host microbe interactions. Surprisingly, the neutrophil extracellular trap (NET) cast by neutrophils is very wide and extends to the entrapment of viruses as well as multicellular eukaryotic parasites. Not unexpectedly, it has emerged that pathogenic microorganisms can employ a wide array of strategies to avoid ensnarement, including expression of DNAse enzymes that destroy the lattice backbone of NETs. Alternatively, they may use molecular mimicry to avoid detection or trigger events leading to the expression of immune modulatory cytokines such as IL-10, which dampen the NETotic response of neutrophils. In addition, the host microenvironment may contribute to the innate immune response by the production of lectin-like molecules that bind to bacteria and promote their entrapment on NETs. An example of this is the production of surfactant protein D by the lung epithelium. In addition, pregnancy provides a different challenge, as the mother needs to mount an effective response against pathogens, without harming her unborn child. An examination of these decoy and host response mechanisms may open the path for new therapies to treat pathologies mediated by overt NETosis.

The Bfp60 surface adhesin is an extracellular matrix and plasminogen protein interacting in Bacteroides fragilis

Plasminogen (Plg) is a highly abundant protein found in the plasma component of blood and is necessary for the degradation of fibrin, collagen, and other structural components of tissues. This fibrinolytic system is utilized by several pathogenic species of bacteria to manipulate the host plasminogen system and facilitate invasion of tissues during infection by modifying the activation of this process through the binding of Plg at their surface. Bacteroides fragilis is the most commonly isolated Gram-negative obligate anaerobe from human clinical infections, such as intra-abdominal abscesses and anaerobic bacteraemia. The ability of B. fragilis to convert plasminogen (Plg) into plasmin has been associated with an outer membrane protein named Bfp60. In this study, we characterized the function of Bfp60 protein in B. fragilis 638R by constructing the bfp60 defective strain and comparing its with that of the wild type regarding binding to laminin-1 (LMN-1) and activation of Plg into plasmin. Although the results showed in this study indicate that Bfp60 surface protein of B. fragilis is important for the recognition of LMN-1 and Plg activation, a significant slow activation of Plg into plasmin was observed in the mutant strain. For that reason, the possibility of another unidentified mechanism activating Plg is also present in B. fragilis cannot be discarded. The results demonstrate that Bfp60 protein is responsible for the recognition of laminin and Plg-plasmin activation. Although the importance of this protein is still unclear in the pathogenicity of the species, it is accepted that since other pathogenic bacteria use this mechanism to disseminate through the extracellular matrix during the infection, it should also contribute to the virulence of B. fragilis.

Characterization and protective immunogenicity of the SzM protein of Streptococcus zooepidemicus NC78 from a clonal outbreak of equine respiratory disease

Streptococcus zooepidemicus of Lancefield group C is a highly variable tonsillar and mucosal commensal that usually is associated with opportunistic infections of the respiratory tract of vertebrate hosts. More-virulent clones have caused epizootics of severe respiratory disease in dogs and horses. The virulence factors of these strains are poorly understood. The antiphagocytic protein SeM is a major virulence factor and protective antigen of Streptococcus equi, a clonal biovar of an ancestral S. zooepidemicus strain. Although the genome of S. zooepidemicus strain H70, an equine isolate, contains a partial homolog (szm) of sem, expression of the gene has not been documented. We have identified and characterized SzM from an encapsulated S. zooepidemicus strain from an epizootic of equine respiratory disease in New Caledonia. The SzM protein of strain NC78 (SzM(NC78)) has a predicted predominantly alpha-helical fibrillar structure with an LPSTG cell surface anchor motif and resistance to hot acid. A putative binding site for plasminogen is present in the B repeat region, the sequence of which shares homology with repeats of the plasminogen binding proteins of human group C and G streptococci. Equine plasminogen is activated in a dose-dependent manner by recombinant SzM(NC78). Only 23.20 and 25.46% DNA homology is shared with SeM proteins of S. equi strains CF32 and 4047, respectively, and homology ranges from 19.60 to 54.70% for SzM proteins of other S. zooepidemicus strains. As expected, SzM(NC78) reacted with convalescent-phase sera from horses with respiratory disease associated with strains of S. zooepidemicus. SzM(NC78) resembles SeM in binding equine fibrinogen and eliciting strong protective antibody responses in mice. Sera of vaccinated mice opsonized S. zooepidemicus strains NC78 and W60, the SzM protein of which shared partial amino acid homology with SzM(NC78). We conclude that SzM is a protective antigen of NC78; it was strongly reactive with serum antibodies from horses during recovery from S. zooepidemicus-associated respiratory disease.

Crossroads of coagulation and innate immunity: the case of deep vein thrombosis

Deep vein thrombosis (DVT) is a common condition characterized by the formation of an occlusive blood clot in the venous vascular system, potentially complicated by detachment and embolization of thrombi into the lung. Recent evidence from mouse models has shed light on the sequence of events and on the cellular (innate immune cells, platelets) and molecular (hematopoietic tissue factor, nucleic acids) components involved. In response to decreased blood flow, circulating neutrophils and monocytes adhere to the activated endothelium within hours. They initiate and propagate DVT by interacting with platelets and by the exposure and activation of circulating tissue factor and FXII. Intravascular blood coagulation is also induced by extracellular nucleosomes released mainly from activated neutrophils. Interestingly, these mechanisms are closely linked to an evolutionary conserved immune defense mechanism activated in response to infections. In this review, we will give an overview of DVT and the role of innate immune pathways supporting this process. While the latter are aimed at preserving tissue integrity and function, uncontrolled blood coagulation and activation of immune cells may result in pathological thrombus formation and vascular occlusion. Understanding the molecular and cellular players triggering occlusion of large veins, and their distinction from physiological hemostasis, is important for the development of strategies to prevent and treat DVT.

Murinization of internalin extends its receptor repertoire, altering Listeria monocytogenes cell tropism and host responses

Listeria monocytogenes (Lm) is an invasive foodborne pathogen that leads to severe central nervous system and maternal-fetal infections. Lm ability to actively cross the intestinal barrier is one of its key pathogenic properties. Lm crosses the intestinal epithelium upon the interaction of its surface protein internalin (InlA) with its host receptor E-cadherin (Ecad). InlA-Ecad interaction is species-specific, does not occur in wild-type mice, but does in transgenic mice expressing human Ecad and knock-in mice expressing humanized mouse Ecad. To study listeriosis in wild-type mice, InlA has been “murinized” to interact with mouse Ecad. Here, we demonstrate that, unexpectedly, murinized InlA (InlA^m) mediates not only Ecad-dependent internalization, but also N-cadherin-dependent internalization. Consequently, InlA^m-expressing Lm targets not only goblet cells expressing luminally-accessible Ecad, as does Lm in humanized mice, but also targets villous M cells, which express luminally-accessible N-cadherin. This aberrant Lm portal of entry results in enhanced innate immune responses and intestinal barrier damage, both of which are not observed in wild-type Lm-infected humanized mice. Murinization of InlA therefore not only extends the host range of Lm, but also broadens its receptor repertoire, providing Lm with artifactual pathogenic properties. These results challenge the relevance of using InlA^m-expressing Lm to study human listeriosis and in vivo host responses to this human pathogen.

Co-evolution of microbes with their hosts can select stringently specific host-microbe interactions at the cell, tissue and species levels. Listeria monocytogenes (Lm) is a foodborne pathogen that causes a deadly systemic infection in humans. Lm crosses the intestinal epithelium upon the interaction of its surface protein InlA with E-cadherin (Ecad). InlA-Ecad interaction is species-specific, does not occur in wild-type mice, but does in transgenic mice expressing human Ecad and knock-in mice expressing humanized mouse Ecad. To study listeriosis in wild-type mice, InlA has been “murinized” to interact with mouse Ecad. Here, we demonstrate that in addition to interacting with mouse Ecad, InlA^m also uses N-cadherin as a receptor, whereas InlA does not. This artifactual InlA^m-N-cadherin interaction promotes bacterial translocation across villous M cells, a cell type which is not targeted by InlA-expressing bacteria. This leads to intestinal inflammation and intestinal barrier damage, both of which are not seen in humans and humanized mouse models permissive to InlA-Ecad interaction. These results challenge the relevance of using InlA^m-expressing Lm as a model to study human listeriosis and host responses to this pathogen. They also illustrate that caution must be exercised before using “murinized” pathogens to study human infectious diseases.

Group B Streptococcus hijacks the host plasminogen system to promote brain endothelial cell invasion

Group B Streptococcus (GBS) is the leading cause of meningitis in neonates. We have previously shown that plasminogen, once recruited to the GBS cell surface and converted into plasmin by host-derived activators, leads to an enhancement of bacterial virulence. Here, we investigated whether plasmin(ogen) bound at the GBS surface contributes to blood-brain barrier penetration and invasion of the central nervous system. For that purpose, GBS strain NEM316 preincubated with or without plasminogen plus tissue type plasminogen activator was analyzed for the capacity to adhere to, invade and transmigrate the human brain microvascular endothelial cell (hBMEC) monolayer, and to penetrate the central nervous system using a neonatal mouse model. At earlier times of infection, plasmin(ogen)-treated GBS exhibited a significant increase in adherence to and invasion of hBMECs. Later, injury of hBMECs were observed with plasmin(ogen)-treated GBS that displayed a plasmin-like activity. The same results were obtained when hBMECs were incubated with whole human plasma and infected with untreated GBS. To confirm that the observed effects were due to the recruitment and activation of plasminogen on GBS surface, the bacteria were first incubated with epsilon-aminocaproic acid (εACA), an inhibitor of plasminogen binding, and thereafter with plasmin(ogen). A significant decrease in the hBMECs injury that was correlated with a decrease of the GBS surface proteolytic activity was observed. Furthermore, plasmin(ogen)-treated GBS infected more efficiently the brain of neonatal mice than the untreated bacteria, indicating that plasmin(ogen) bound to GBS surface may facilitate the traversal of the blood-brain barrier. A higher survival rate was observed in offspring born from εACA-treated mothers, compared to untreated mice, and no brain infection was detected in these neonates. Our findings suggest that capture of the host plasmin(ogen) by the GBS surface promotes the crossing of the blood-brain barrier and contributes to the establishment of meningitis.

Acquisition of the Sda1-encoding bacteriophage does not enhance virulence of the serotype M1 Streptococcus pyogenes strain SF370

The resurgence of invasive disease caused by Streptococcus pyogenes (group A Streptococcus [GAS]) in the past 30 years has paralleled the emergence and global dissemination of the highly virulent M1T1 clone. The GAS M1T1 clone has diverged from the ancestral M1 serotype by horizontal acquisition of two unique bacteriophages, encoding the potent DNase Sda1/SdaD2 and the superantigen SpeA, respectively. The phage-encoded DNase promotes escape from neutrophil extracellular traps and is linked to enhanced virulence of the M1T1 clone. In this study, we successfully used in vitro lysogenic conversion to transfer the Sda1-encoding phage from the M1T1 clonal strain 5448 to the nonclonal M1 isolate SF370 and determined the impact of this horizontal gene transfer event on virulence. Although Sda1 was expressed in SF370 lysogens, no capacity of the phage-converted strain to survive human neutrophil killing, switch to a hyperinvasive covRS mutant form, or cause invasive lethal infection in a humanized plasminogen mouse model was observed. This work suggests that the hypervirulence of the M1T1 clone is due to the unique synergic effect of the M1T1 clone bacteriophage-specific virulence factor Sda1 acting in concert with the M1T1 clone-specific genetic scaffold.

Fibrin facilitates both innate and T cell-mediated defense against Yersinia pestis

The Gram-negative bacterium Yersinia pestis causes plague, a rapidly progressing and often fatal disease. The formation of fibrin at sites of Y. pestis infection supports innate host defense against plague, perhaps by providing a nondiffusible spatial cue that promotes the accumulation of inflammatory cells expressing fibrin-binding integrins. This report demonstrates that fibrin is an essential component of T cell-mediated defense against plague but can be dispensable for Ab-mediated defense. Genetic or pharmacologic depletion of fibrin abrogated innate and T cell-mediated defense in mice challenged intranasally with Y. pestis. The fibrin-deficient mice displayed reduced survival, increased bacterial burden, and exacerbated hemorrhagic pathology. They also showed fewer neutrophils within infected lung tissue and reduced neutrophil viability at sites of liver infection. Depletion of neutrophils from wild-type mice weakened T cell-mediated defense against plague. The data suggest that T cells combat plague in conjunction with neutrophils, which require help from fibrin to withstand Y. pestis encounters and effectively clear bacteria.

A natural inactivating mutation in the CovS component of the CovRS regulatory operon in a pattern D Streptococcal pyogenes strain influences virulence-associated genes

A skin-tropic invasive group A Streptococcus pyogenes (GAS) strain, AP53, contains a natural inactivating mutation in the covS gene (covS(M)) of the two-component responder (CovR)/sensor (CovS) gene regulatory system. The effects of this mutation on specific GAS virulence determinants have been assessed, with emphasis on expression of the extracellular protease, streptococcal pyrogenic exotoxin B (SpeB), capsular hyaluronic acid, and proteins that allow host plasmin assembly on the bacterial surface, viz. a high affinity plasminogen (Pg)/plasmin receptor, Pg-binding group A streptococcal M protein (PAM), and the human Pg activator streptokinase. To further illuminate mechanisms of the functioning of CovRS in the virulence of AP53, two AP53 isogenic strains were generated, one in which the natural covS(M) gene was mutated to WT-covS (AP53/covS(WT)) and a strain that contained an inactivated covR gene (AP53/ΔcovR). Two additional strains that do not contain PAM, viz. WT-NS931 and NS931/covS(M), were also employed. SpeB was not measurably expressed in strains containing covR(WT)/covS(M), whereas in strains with natural or engineered covR(WT)/covS(WT), SpeB expression was highly up-regulated. Alternatively, capsule synthesis via the hasABC operon was enhanced in strain AP53/covS(M), whereas streptokinase expression was only slightly affected by the covS inactivation. PAM expression was not substantially influenced by the covS mutation, suggesting that covRS had minimal effects on the mga regulon that controls PAM expression. These results demonstrate that a covS inactivation results in virulence gene alterations and also suggest that the CovR phosphorylation needed for gene up- or down-regulation can occur by alternative pathways to CovS kinase.

PAR-1 contributes to the innate immune response during viral infection

Coagulation is a host defense system that limits the spread of pathogens. Coagulation proteases, such as thrombin, also activate cells by cleaving PARs. In this study, we analyzed the role of PAR-1 in coxsackievirus B3-induced (CVB3-induced) myocarditis and influenza A infection. CVB3-infected Par1(-/-) mice expressed reduced levels of IFN-β and CXCL10 during the early phase of infection compared with Par1(+/+) mice that resulted in higher viral loads and cardiac injury at day 8 after infection. Inhibition of either tissue factor or thrombin in WT mice also significantly increased CVB3 levels in the heart and cardiac injury compared with controls. BM transplantation experiments demonstrated that PAR-1 in nonhematopoietic cells protected mice from CVB3 infection. Transgenic mice overexpressing PAR-1 in cardiomyocytes had reduced CVB3-induced myocarditis. We found that cooperative signaling between PAR-1 and TLR3 in mouse cardiac fibroblasts enhanced activation of p38 and induction of IFN-β and CXCL10 expression. Par1(-/-) mice also had decreased CXCL10 expression and increased viral levels in the lung after influenza A infection compared with Par1(+/+) mice. Our results indicate that the tissue factor/thrombin/PAR-1 pathway enhances IFN-β expression and contributes to the innate immune response during single-stranded RNA viral infection.

Novel inhibitors of bacterial virulence: development of 5,6-dihydrobenzo[h]quinazolin-4(3H)-ones for the inhibition of group A streptococcal streptokinase expression

Resistance to antibiotics is an increasingly dire threat to human health that warrants the development of new modes of treating infection. We recently identified 1 (CCG-2979) as an inhibitor of the expression of streptokinase, a critical virulence factor in Group A Streptococcus that endows blood-borne bacteria with fibrinolytic capabilities. In this report, we describe the synthesis and biological evaluation of a series of novel 5,6-dihydrobenzo[h]quinazolin-4(3H)-one analogs of 1 undertaken with the goal of improving the modest potency of the lead. In addition to achieving an over 35-fold increase in potency, we identified structural modifications that improve the solubility and metabolic stability of the scaffold. The efficacy of two new compounds 12c (CCG-203592) and 12k (CCG-205363) against biofilm formation in Staphylococcus aureus represents a promising additional mode of action for this novel class of compounds.

Thrombosis as an intravascular effector of innate immunity

Thrombosis is the most frequent cause of mortality worldwide and is closely linked to haemostasis, which is the biological mechanism that stops bleeding after the injury of blood vessels. Indeed, both processes share the core pathways of blood coagulation and platelet activation. Here, we summarize recent work suggesting that thrombosis under certain circumstances has a major physiological role in immune defence, and we introduce the term immunothrombosis to describe this process. Immunothrombosis designates an innate immune response induced by the formation of thrombi inside blood vessels, in particular in microvessels. Immunothrombosis is supported by immune cells and by specific thrombosis-related molecules and generates an intravascular scaffold that facilitates the recognition, containment and destruction of pathogens, thereby protecting host integrity without inducing major collateral damage to the host. However, if uncontrolled, immunothrombosis is a major biological process fostering the pathologies associated with thrombosis.

Induction of brain microvascular endothelial cell urokinase expression by Cryptococcus neoformans facilitates blood-brain barrier invasion

The invasive ability of the blood-borne fungal pathogen Cryptococcus neoformans can be enhanced through interactions with host plasma components, such as plasminogen. Previously we showed by in vitro studies that plasminogen coats the surface of C. neoformans and is converted to the active serine protease, plasmin, by host plasminogen activators. Viable, but not formaldehyde- or sodium azide-killed, cryptococcal strains undergo brain microvascular endothelial cell-dependent plasminogen-to-plasmin activation, which results in enhanced, plasmin-dependent cryptococcal invasion of primary bovine brain microvascular endothelial cells and fungal ability to degrade plasmin substrates. In the present work, brain microvascular endothelial cells cultured with viable, but not killed, cryptococcal strains led to significant increases in both urokinase mRNA transcription and cell-associated urokinase protein expression. Soluble urokinase was also detected in conditioned medium from brain microvascular endothelial cells cultured with viable, but not killed, C. neoformans. Exposure of plasminogen pre-coated viable C. neoformans to conditioned medium from strain-matched brain microvascular endothelial cell-fungal co-cultures resulted in plasminogen-to-plasmin activation and plasmin-dependent cryptococcal invasion. siRNA-mediated silencing of urokinase gene expression or the use of specific inhibitors of urokinase activity abrogated both plasminogen-to-plasmin activation on C. neoformans and cryptococcal-brain microvascular endothelial cell invasion. Our results suggest that pathogen exploitation of the host urokinase-plasmin(ogen) system may contribute to C. neoformans virulence during invasive cryptococcosis.

Characterization of streptokinases from group A Streptococci reveals a strong functional relationship that supports the coinheritance of plasminogen-binding M protein and cluster 2b streptokinase

Group Astreptococcus (GAS) strains secrete the protein streptokinase (SK), which functions by activating host human plasminogen (hPg) to plasmin (hPm), thus providing a proteolytic framework for invasive GAS strains. The types of SK secreted by GAS have been grouped into two clusters (SK1 and SK2) and one subcluster (SK2a and SK2b). SKs from cluster 1 (SK1) and cluster 2b (SK2b) display significant evolutionary and functional differences, and attempts to relate these properties to GAS skin or pharynx tropism and invasiveness are of great interest. In this study, using four purified SKs from each cluster, new relationships between plasminogen-binding group A streptococcal M (PAM) protein and SK2b have been revealed. All SK1 proteins efficiently activated hPg, whereas all subclass SK2b proteins only weakly activated hPg in the absence of PAM. Surface plasmon resonance studies revealed that the lower affinity of SK2b to hPg served as the basis for the attenuated activation of hPg by SK2b. Binding of hPg to either human fibrinogen (hFg) or PAM greatly enhanced activation of hPg by SK2b but minimally influenced the already effective activation of hPg by SK1. Activation of hPg in the presence of GAS cells containing PAM demonstrated that PAM is the only factor on the surface of SK2b-expressing cells that enabled the direct activation of hPg by SK2b. As the binding of hPg to PAM is necessary for hPg activation by SK2b, this dependence explains the coinherant relationship between PAM and SK2b and the ability of these particular strains to generate the proteolytic activity that disrupts the innate barriers that limit invasiveness.

Novel inhibitors of Staphylococcus aureus virulence gene expression and biofilm formation

Staphylococcus aureus is a major human pathogen and one of the more prominent pathogens causing biofilm related infections in clinic. Antibiotic resistance in S. aureus such as methicillin resistance is approaching an epidemic level. Antibiotic resistance is widespread among major human pathogens and poses a serious problem for public health. Conventional antibiotics are either bacteriostatic or bacteriocidal, leading to strong selection for antibiotic resistant pathogens. An alternative approach of inhibiting pathogen virulence without inhibiting bacterial growth may minimize the selection pressure for resistance. In previous studies, we identified a chemical series of low molecular weight compounds capable of inhibiting group A streptococcus virulence following this alternative anti-microbial approach. In the current study, we demonstrated that two analogs of this class of novel anti-virulence compounds also inhibited virulence gene expression of S. aureus and exhibited an inhibitory effect on S. aureus biofilm formation. This class of anti-virulence compounds could be a starting point for development of novel anti-microbial agents against S. aureus.

Bacterial plasminogen receptors utilize host plasminogen system for effective invasion and dissemination

In order for invasive pathogens to migrate beyond the site of infection, host physiological barriers such as the extracellular matrix, the basement membrane, and encapsulating fibrin network must be degraded. To circumvent these impediments, proteolytic enzymes facilitate the dissemination of the microorganism. Recruitment of host proteases to the bacterial surface represents a particularly effective mechanism for enhancing invasiveness. Plasmin is a broad spectrum serine protease that degrades fibrin, extracellular matrices, and connective tissue. A large number of pathogens express plasminogen receptors which immobilize plasmin(ogen) on the bacterial surface. Surface-bound plasminogen is then activated by plasminogen activators to plasmin through limited proteolysis thus triggering the development of a proteolytic surface on the bacteria and eventually assisting the spread of bacteria. The host hemostatic system plays an important role in systemic infection. The interplay between hemostatic processes such as coagulation and fibrinolysis and the inflammatory response constitutes essential components of host defense and bacterial invasion. The goal of this paper is to highlight mechanisms whereby pathogenic bacteria, by engaging surface receptors, utilize and exploit the host plasminogen and fibrinolytic system for the successful dissemination within the host.

Bacterial plasminogen receptors: mediators of a multifaceted relationship

Multiple species of bacteria are able to sequester the host zymogen plasminogen to the cell surface. Once localised to the bacterial surface, plasminogen can act as a cofactor in adhesion, or, following activation to plasmin, provide a source of potent proteolytic activity. Numerous bacterial plasminogen receptors have been identified, and the mechanisms by which they interact with plasminogen are diverse. Here we provide an overview of bacterial plasminogen receptors and discuss the diverse role bacterial plasminogen acquisition plays in the relationship between bacteria and the host.

Tracing the evolutionary history of the pandemic group A streptococcal M1T1 clone

The past 50 years has witnessed the emergence of new viral and bacterial pathogens with global effect on human health. The hyperinvasive group A Streptococcus (GAS) M1T1 clone, first detected in the mid-1980s in the United States, has since disseminated worldwide and remains a major cause of severe invasive human infections. Although much is understood regarding the capacity of this pathogen to cause disease, much less is known of the precise evolutionary events selecting for its emergence. We used high-throughput technologies to sequence a World Health Organization strain collection of serotype M1 GAS and reconstructed its phylogeny based on the analysis of core genome single-nucleotide polymorphisms. We demonstrate that acquisition of a 36-kb genome segment from serotype M12 GAS and the bacteriophage-encoded DNase Sda1 led to increased virulence of the M1T1 precursor and occurred relatively early in the molecular evolutionary history of this strain. The more recent acquisition of the phage-encoded superantigen SpeA is likely to have provided selection advantage for the global dissemination of the M1T1 clone. This study provides an exemplar for the evolution and emergence of virulent clones from microbial populations existing commensally or causing only superficial infection.

Fibrinogen and factor XIII A-subunit genotypes interactively influence C-reactive protein levels during inflammation

OBJECTIVE

Fibrinogen is a target of autoimmune reactions in rheumatoid arthritis (RA). Fibrin(ogen) derivatives are involved in inflammatory processes and the generation of a stable fibrin network is necessary for sufficient inflammation control. As the density and stability of fibrin networks depend on complex interactions between factor XIIIA (F13A) and fibrinogen genotypes, the authors studied whether these genotypes were related to C-reactive protein (CRP) levels during acute-phase reactions.

METHODS

Association between α-fibrinogen (FGA), β-fibrinogen (FGB) and F13A genotypes with CRP levels was tested in two cohorts with longitudinal CRP measurements. Discovery and replication cohorts consisted of 288 RA (913 observations) and 636 non-RA patients (2541 observations), respectively.

RESULTS

Genotype FGB -455G>A (rs1800790) was associated with CRP elevations (≥ 10 mg/l) in both cohorts (RA, OR per allele 0.69, p=0.0007/P(adj)<0.015; non-RA, OR 0.70, p=0.0004/p(adj)<0.02; combined, OR 0.69, p<10(-5)/p(adj)=0.001). Genotype F13A 34VV (rs5985) was conditional for the association of FGB -455G>A with CRP as indicated by a clear restriction on F13A 34VV individuals and a highly significant heterogeneity between F13A 34VV and F13A 34L genotypes (p<10(-5), p(adj)=0.001). In both cohorts, mean CRP levels significantly declined with ascending numbers of FGB -455A alleles. Genotype FGA T312A (rs6050) exhibited opposite effects on CRP compared with FGB -455G>A. Again, this relation was dependent on F13A V34L genotype.

CONCLUSION

Novel genetic determinants of CRP completely unrelated to previously known CRP regulators were identified. Presumably, these haemostatic gene variants modulate inflammation by influencing fibrin crosslinking. These findings could give new perspectives on the genetic background of inflammation control.

Complement C3 is a novel plasma clot component with anti-fibrinolytic properties

BACKGROUND AND METHOD

Increased plasma clot density and prolonged lysis times are associated with cardiovascular disease. In this study, we employed a functional proteomics approach to identify novel clot components which may influence clot phenotypes.

RESULTS

Analysis of perfused, solubilised plasma clots identified inflammatory proteins, including complement C3, as novel clot components. Analysis of paired plasma and serum samples confirmed concentration-dependent incorporation of C3 into clots. Surface plasmon resonance indicated high-affinity binding interactions between C3 and fibrinogen and fibrin. Turbidimetric clotting and lysis assays indicated C3 impaired fibrinolysis in a concentration-dependent manner, both in vitro and ex vivo.

CONCLUSION

These data indicate functional interactions between complement C3 and fibrin leading to prolonged fibrinolysis. These interactions are physiologically relevant in the context of protection following injury and suggest a mechanistic link between increased plasma C3 concentration and acute cardiovascular thrombotic events.

Streptococcus uberis plasminogen activator (SUPA) activates human plasminogen through novel species-specific and fibrin-targeted mechanisms

Bacterial plasminogen (Pg) activators generate plasmin to degrade fibrin blood clots and other proteins that modulate the pathogenesis of infection, yet despite strong homology between mammalian Pgs, the activity of bacterial Pg activators is thought to be restricted to the Pg of their host mammalian species. Thus, we found that Streptococcus uberis Pg activator (SUPA), isolated from a Streptococcus species that infects cows but not humans, robustly activated bovine but not human Pg in purified systems and in plasma. Consistent with this, SUPA formed a higher avidity complex (118-fold) with bovine Pg than with human Pg and non-proteolytically activated bovine but not human Pg. Surprisingly, however, the presence of human fibrin overrides the species-restricted action of SUPA. First, human fibrin enhanced the binding avidity of SUPA for human Pg by 4-8-fold in the presence and absence of chloride ion (a negative regulator). Second, although SUPA did not protect plasmin from inactivation by α(2)-antiplasmin, fibrin did protect human plasmin, which formed a 31-fold higher avidity complex with SUPA than Pg. Third, fibrin significantly enhanced Pg activation by reducing the K(m) (4-fold) and improving the catalytic efficiency of the SUPA complex (6-fold). Taken together, these data suggest that indirect molecular interactions may override the species-restricted activity of bacterial Pg activators; this may affect the pathogenesis of infections or may be exploited to facilitate the design of new blood clot-dissolving drugs.

OspC is potent plasminogen receptor on surface of Borrelia burgdorferi

Host-derived proteases are crucial for the successful infection of vertebrates by several pathogens, including the Lyme disease spirochete bacterium, Borrelia burgdorferi. B. burgdorferi must traverse tissue barriers in the tick vector during transmission to the host and during dissemination within the host, and it must disrupt immune challenges to successfully complete its infectious cycle. It has been proposed that B. burgdorferi can accomplish these tasks without an endogenous extra-cytoplasmic protease by commandeering plasminogen, the highly abundant precursor of the vertebrate protease plasmin. However, the molecular mechanism by which B. burgdorferi immobilizes plasminogen to its surface remains obscure. The data presented here demonstrate that the outer surface protein C (OspC) of B. burgdorferi is a potent plasminogen receptor on the outer membrane of the bacterium. OspC-expressing spirochetes readily bind plasminogen, whereas only background levels of plasminogen are detectable on OspC-deficient strains. Furthermore, plasminogen binding by OspC-expressing spirochetes can be significantly reduced using anti-OspC antibodies. Co-immunofluorescence staining assays demonstrate that wild-type bacteria immobilize plasminogen only if they are actively expressing OspC regardless of the expression of other surface proteins. The co-localization of plasminogen and OspC on OspC-expressing spirochetes further implicates OspC as a biologically relevant plasminogen receptor on the surface of live B. burgdorferi.

Inhibitor of streptokinase gene expression improves survival after group A streptococcus infection in mice

The widespread occurrence of antibiotic resistance among human pathogens is a major public health problem. Conventional antibiotics typically target bacterial killing or growth inhibition, resulting in strong selection for the development of antibiotic resistance. Alternative therapeutic approaches targeting microbial pathogenicity without inhibiting growth might minimize selection for resistant organisms. Compounds inhibiting gene expression of streptokinase (SK), a critical group A streptococcal (GAS) virulence factor, were identified through a high-throughput, growth-based screen on a library of 55,000 small molecules. The lead compound [Center for Chemical Genomics 2979 (CCG-2979)] and an analog (CCG-102487) were confirmed to also inhibit the production of active SK protein. Microarray analysis of GAS grown in the presence of CCG-102487 showed down-regulation of a number of important virulence factors in addition to SK, suggesting disruption of a general virulence gene regulatory network. CCG-2979 and CCG-102487 both enhanced granulocyte phagocytosis and killing of GAS in an in vitro assay, and CCG-2979 also protected mice from GAS-induced mortality in vivo. These data suggest that the class of compounds represented by CCG-2979 may be of therapeutic value for the treatment of GAS and potentially other gram-positive infections in humans.

Full-genome dissection of an epidemic of severe invasive disease caused by a hypervirulent, recently emerged clone of group A Streptococcus

Group A Streptococcus (GAS) causes an exceptionally broad range of infections in humans, from relatively mild pharyngitis and skin infections to life-threatening necrotizing fasciitis and toxic shock syndrome. An epidemic of severe invasive human infections caused by type emm59 GAS, heretofore an exceedingly rare cause of disease, spread west to east across Canada over a 3-year period (2006 to 2008). By sequencing the genomes of 601 epidemic, historic, and other emm59 organisms, we discovered that a recently emerged, genetically distinct emm59 clone is responsible for the Canadian epidemic. Using near-real-time genome sequencing, we were able to show spread of the Canadian epidemic clone into the United States. The extensive genome data permitted us to identify patterns of geographic dissemination as well as links between emm59 subclonal lineages that cause infections. Mouse and nonhuman primate models of infection demonstrated that the emerged clone is unusually virulent. Transmission of epidemic emm59 strains may have occurred primarily by skin contact, as suggested by an experimental model of skin transmission. In addition, the emm59 strains had a significantly impaired ability to persist in human saliva and to colonize the oropharynx of mice, and seldom caused human pharyngitis. Our study contributes new information to the rapidly emerging field of molecular pathogenomics of bacterial epidemics and illustrates how full-genome data can be used to precisely illuminate the landscape of strain dissemination during a bacterial epidemic.

Tumor necrosis factor alpha modulates the dynamics of the plasminogen-mediated early interaction between Bifidobacterium animalis subsp. lactis and human enterocytes

The capacity to intervene with the host plasminogen system has recently been considered an important component in the interaction process between Bifidobacterium animalis subsp. lactis and the human host. However, its significance in the bifidobacterial microecology within the human gastrointestinal tract is still an open question. Here we demonstrate that human plasminogen favors the B. animalis subsp. lactis BI07 adhesion to HT29 cells. Prompting the HT29 cell capacity to activate plasminogen, tumor necrosis factor alpha (TNF-α) modulated the plasminogen-mediated bacterium-enterocyte interaction, reducing the bacterial adhesion to the enterocytes and enhancing migration to the luminal compartment.

Epidemiology and pathogenicity of zoonotic streptococci

Zoonotic infections caused by Streptococcus spp. have been neglected in spite of the fact that frequency and severity of outbreaks increased dramatically in recent years. This may be due to non-identification since respective species are often not considered in human medical diagnostic procedures. On the other hand, an expanding human population concomitant with an increasing demand for food and the increased number of companion animals favour conditions for host species adaptation of animal streptococci. This review aims to give an overview on streptococcal zoonoses with focus on epidemiology and pathogenicity of four major zoonotic species, Streptococcus canis, Streptococcus equi sub. zooepidemicus, Streptococcus iniae and Streptococcus suis.

Group a streptococcal vaccine candidates: potential for the development of a human vaccine

Currently there is no commercial Group A Streptococcus (GAS; S. pyogenes) vaccine available. The development of safe GAS vaccines is challenging, researchers are confronted with obstacles such as the occurrence of many unique serotypes (there are greater than 150 M types), antigenic variation within the same serotype, large variations in the geographical distribution of serotypes, and the production of antibodies cross-reactive with human tissue which can lead to host auto-immune disease. Cell wall anchored, cell membrane associated, secreted and anchorless proteins have all been targeted as GAS vaccine candidates. As GAS is an exclusively human pathogen, the quest for an efficacious vaccine is further complicated by the lack of an animal model which mimics human disease and can be consistently and reproducibly colonized by multiple GAS strains.

Modulation of the coagulation system during severe streptococcal disease

Haemostasis is maintained by a tightly regulated coagulation system that comprises platelets, procoagulant proteins, and anticoagulant proteins. During the local and systemic response to bacterial infection, the coagulation system becomes activated, and contributes to the pathophysiological response to infection. The significant human pathogen, Streptococcus pyogenes has multiple strategies to modulate coagulation. This can range from systemic activation of the intrinsic and extrinsic pathway of coagulation to local stimulation of fibrinolysis. Such diverse effects on this host system imply a finely tuned host-bacteria interaction. The molecular mechanisms that underlie this modulation of the coagulation system are discussed in this review.

Genetic Susceptibility to Fungal Infections in Humans

Most fungal infections in humans occur in the setting of iatrogenic immunosuppression or HIV infection. In the absence of these factors, fungi cause mild, self-limited infections that typically involve mucocutaneous surfaces. Hence, when persistent or recurrent mucocutaneous infections (chronic mucocutaneous candidiasis [CMC]) or invasive fungal infections (IFIs) develop in a "normal" host, they are indicative of genetic defects causing innate or adaptive immune dysfunction. In this review, recent developments concerning genetic and immunologic factors that affect the risk for IFIs and CMC are critically discussed.

Dengue virus-induced autoantibodies bind to plasminogen and enhance its activation

Dengue virus infection can lead to life-threatening dengue hemorrhagic fever (DHF) or dengue shock syndrome (DSS) in patients. Abnormal activation of the coagulation and fibrinolysis system is one of the hallmarks associated with DHF/DSS patients. However, the mechanisms that cause pathology in DHF/DSS patients are still unclear. Because conversion of plasminogen (Plg) to plasmin (Plm) is the first step in the activation of fibrinolysis, Abs against Plg found in DHF/DSS patients may be important. Therefore, to investigate the specificity, function, and possible origin of these Abs, we generated several Plg cross-reactive mAbs from DENV-immunized mice. An IgG mAb, 6H11, which recognizes an epitope associated with a dengue envelope protein, demonstrated a high level of cross-reactivity with Plg. The 6H11 Ab was further characterized with regard to its effect on Plg activation. Using Plm-specific chromogenic substrate S-2251, we found that mAb 6H11 demonstrated serine protease activity and could convert Plg directly to Plm. The serine protease activity of mAb 6H11 was further confirmed using serine protease chromogenic substrate S-2288. In addition, we found several Plg cross-reactive mAbs that could enhance urokinase-induced Plg activation. Lastly, mAb 6H11 could induce Plm activity and increase the level of D-dimer (a fibrin degradation product) in both human and mouse platelet-poor plasma. Taken together, these data suggest DENV-induced Plg cross-reactive Abs may enhance Plg conversion to Plm, which would be expected to contribute to hyperfibrinolysis in DHF/DSS patients.

Distinct single amino acid replacements in the control of virulence regulator protein differentially impact streptococcal pathogenesis

Sequencing of invasive strains of group A streptococci (GAS) has revealed a diverse array of single nucleotide polymorphisms in the gene encoding the control of virulence regulator (CovR) protein. However, there is limited information regarding the molecular mechanisms by which CovR single amino acid replacements impact GAS pathogenesis. The crystal structure of the CovR C-terminal DNA-binding domain was determined to 1.50 Å resolution and revealed a three-stranded β-sheet followed by a winged helix-turn-helix DNA binding motif. Modeling of the CovR protein-DNA complex indicated that CovR single amino acid replacements observed in clinical GAS isolates could directly alter protein-DNA interaction and impact protein structure. Isoallelic GAS strains that varied by a single amino acid replacement in the CovR DNA binding domain had significantly different transcriptomes compared to wild-type and to each other. Similarly, distinct recombinant CovR variants had differential binding affinity for DNA from the promoter regions of several virulence factor-encoding genes. Finally, mice that were challenged with GAS CovR isoallelic strains had significantly different survival times, which correlated with the transcriptome and protein-DNA binding studies. Taken together, these data provide structural and functional insights into the critical and distinct effects of variation in the CovR protein on GAS pathogenesis.

Group A Streptococcus (GAS) causes a variety of human infections including invasive disease that can often be deadly. GAS strains that cause serious infections may have alterations in the amino acid sequence of the control of virulence regulator (CovR) protein, but mechanisms by which changes in the CovR protein influence GAS disease are not understood. We determined the crystal structure of the CovR DNA binding region and found that alterations in the CovR protein observed in clinical, invasive GAS isolates are likely to disrupt CovR-DNA interaction and overall CovR structure. In accord with the structural data, CovR proteins with a single amino acid change had distinctly different binding affinities for various GAS virulence-factor encoding genes. Similarly, GAS strains that differed by only the presence of a single CovR amino acid change had distinct gene expression profiles. Finally, mice that were challenged with GAS strains that differed by only a single CovR amino acid replacement had significantly different survival times consistent with the idea that alterations in the CovR protein are a key determinant of clinical outcomes in GAS human infections. These findings provide mechanistic insights into how subtle genetic differences can profoundly impact the severity of bacterial infections.

Blood-brain barrier invasion by Cryptococcus neoformans is enhanced by functional interactions with plasmin

Cryptococcus neoformans can invade the central nervous system through diverse mechanisms. We examined a possible role for host plasma proteases in the neurotropic behaviour of this blood-borne fungal pathogen. Plasminogen is a plasma-enriched zymogen that can passively coat the surface of blood-borne pathogens and, upon conversion to the serine protease plasmin, facilitate pathogen dissemination by degrading vascular barriers. In this study, plasminogen-to-plasmin conversion on killed and viable hypoencapsulated strains of C. neoformans required the addition of plasminogen activator (PA), but this conversion occurred in the absence of supplemented PA when viable strains were cultured with brain microvascular endothelial cells (BMEC). Plasmin-coated C. neoformans showed an enhanced invasive ability in Matrigel invasion assays that was significantly augmented in the presence of BMEC. The invasive effect of plasmin required viable pathogen and correlated with rapid declines in BMEC barrier function. Plasmin-enhanced invasion was inhibited by aprotinin, carboxypeptidase B, the lysine analogue epsilon-aminocaproic acid, and by capsule development. C. neoformans caused plasminogen-independent declines in BMEC barrier function that were associated with pathogen-induced host damage; however, such declines were significantly delayed and less extensive than those observed with plasmin-coated pathogen. BMEC adhesion and damage by hypoencapsulated C. neoformans were diminished by capsule induction but unaltered by plasminogen and/or PA. We conclude that hypoencapsulated C. neoformans can invade BMEC by a plasmin-dependent mechanism, in vitro, and that small, or minimal, surface capsule expression during the blood-borne phase of cryptococcosis may promote virulence by means of plasmin(ogen) acquisition.

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.

Innate immunity and coagulation

Infection frequently elicits a coagulation response. Endotoxin triggers the formation of tissue factor initiating coagulation, down regulates anticoagulant mechanisms including the protein C pathway and heparin-like proteoglycans and up regulates plasminogen activator inhibitor. The overall physiological result of this is to promote coagulation through enhancing initiation, suppressing negative regulation and impairing fibrin removal. The response to infection also leads to tissue destruction. Nucleosomes and histones released from the injured cells trigger further inflammation, protection from the pathogen but further tissue injury leading to multi-organ failure. Such a complex response to infection presumably arises due to the role of coagulation in the control and clearance of the infectious agent.

Exploration of the host haemostatic system by group A streptococcus: implications in searching for novel antimicrobial therapies

The haemostatic system is heavily involved in the host response to infection. A number of host haemostatic factors, notably plasminogen and fibrinogen have been reported to bind and interact with various bacterial proteins. This review summarises the roles of host haemostatic factors such as plasminogen, factor V and fibrinogen in host defence against group A streptococcus infection and discusses the potential of targeting the host haemostatic system for therapeutic intervention against infectious diseases.

Genetic susceptibility to Aspergillus fumigatus infections

Invasive aspergillosis mostly caused by the opportunistic mould Aspergillus fumigatus is characterized by high morbidity and mortality in risk group patients. Several ethno-pathological factors promote the development and the course of this fungal infection like neutropenia, T-cell depletion, CD34-selected stem cell products, corticosteroid therapy, or cytomegalovirus infections. Furthermore, a growing number of defined single nucleotide polymorphisms affiliated to genes affecting the innate immune response has been described which genetically determine susceptibility to A. fumigatus. Thereby, it concerns a broad band ranging from genes encoding for cytokines or chemokines, their respective receptors to those of toll-like receptors including further genes involved in recognition and defence of pathogens by the innate immune system. Here, we summarize in detail the current knowledge about genetic markers correlated with invasive aspergillosis and their relevance for the developing and outcome of infections with A. fumigatus.

SCM, a novel M-like protein from Streptococcus canis, binds (mini)-plasminogen with high affinity and facilitates bacterial transmigration

Streptococcus canis is an important zoonotic pathogen capable of causing serious invasive diseases in domestic animals and humans. In the present paper we report the binding of human plasminogen to S. canis and the recruitment of proteolytically active plasmin on its surface. The binding receptor for plasminogen was identified as a novel M-like protein designated SCM (S. canis M-like protein). SPR (surface plasmon resonance) analyses, radioactive dot-blot analyses and heterologous expression on the surface of Streptococcus gordonii confirmed the plasminogen-binding capability of SCM. The binding domain was located within the N-terminus of SCM, which specifically bound to the C-terminal part of plasminogen (mini-plasminogen) comprising kringle domain 5 and the catalytic domain. In the presence of urokinase, SCM mediated plasminogen activation on the bacterial surface that was inhibited by serine protease inhibitors and lysine amino acid analogues. Surface-bound plasmin effectively degraded purified fibrinogen as well as fibrin clots, resulting in the dissolution of fibrin thrombi. Electron microscopic illustration and time-lapse imaging demonstrated bacterial transmigration through fibrinous thrombi. The present study has led, for the first time, to the identification of SCM as a novel receptor for (mini)-plasminogen mediating the fibrinolytic activity of S. canis.

Enolase: a key player in the metabolism and a probable virulence factor of trypanosomatid parasites-perspectives for its use as a therapeutic target

Glycolysis and glyconeogenesis play crucial roles in the ATP supply and synthesis of glycoconjugates, important for the viability and virulence, respectively, of the human-pathogenic stages of Trypanosoma brucei, Trypanosoma cruzi, and Leishmania spp. These pathways are, therefore, candidate targets for antiparasite drugs. The glycolytic/gluconeogenic enzyme enolase is generally highly conserved, with similar overall fold and identical catalytic residues in all organisms. Nonetheless, potentially important differences exist between the trypanosomatid and host enzymes, with three unique, reactive residues close to the active site of the former that might be exploited for the development of new drugs. In addition, enolase is found both in the secretome and in association with the surface of Leishmania spp. where it probably functions as plasminogen receptor, playing a role in the parasite's invasiveness and virulence, a function possibly also present in the other trypanosomatids. This location and possible function of enolase offer additional perspectives for both drug discovery and vaccination.

Specificity for human hemoglobin enhances Staphylococcus aureus infection

Iron is required for bacterial proliferation, and Staphylococcus aureus steals this metal from host hemoglobin during invasive infections. This process involves hemoglobin binding to the cell wall of S. aureus, heme extraction, passage through the cell envelope, and degradation to release free iron. Herein, we demonstrate an enhanced ability of S. aureus to bind hemoglobin derived from humans as compared to other mammals. Increased specificity for human hemoglobin (hHb) translates into an improved ability to acquire iron and is entirely dependent on the staphylococcal hemoglobin receptor IsdB. This feature affects host-pathogen interaction as demonstrated by the increased susceptibility of hHb-expressing mice to systemic staphylococcal infection. Interestingly, enhanced utilization of human hemoglobin is not a uniform property of all bacterial pathogens. These results suggest a step in the evolution of S. aureus to better colonize the human host and establish hHb-expressing mice as a model of S. aureus pathogenesis.

Clonorchis sinensis enolase: identification and biochemical characterization of a glycolytic enzyme from excretory/secretory products

Enolase plays a key role in energy metabolism and development of most organisms. We isolated a gene encoding enolase from Clonorchis sinensis (C. sinensis) adult cDNA library and expressed the recombinant protein in Escherichia coli. C. sinensis enolase (Csenolase) was identified as both an excretory/secretory product and a tegumental component of C. sinensis by western blot analysis. The transcriptional level of Csenolase was examined at adult worm, metacercaria, cercaria and egg of C. sinensis, and results showed that Csenolase is transcribed at the four life stages of C. sinensis while showing a significant higher expression level at the stage of adult worm. Immunohistochemical localization indicated that Csenolase was specifically deposited on the tegument of adult worm and cyst wall of metacercaria. Ligand blot assay revealed a specific characteristic of dose-dependent plasminogen-binding activity of Csenolase and kinetic parameters were explored using 2-phospho-D-glycerate (2-PGA) as the primary substrate by monitoring the conversion of nicotinamide-adenine dinucleotide (NADH) into nicotinamide adenine dinucleotide (NAD). In addition, Csenolase exhibited active enzyme activity in catalytic reactions while the anti-Csenolase serum inhibited the enzyme activity. In vitro incubation experiments revealed that Csenolase might play key roles in the growth of the parasites. In conclusion, Csenolase is an important glycolytic enzyme required for the development of C. sinensis, and may be a potential vaccine candidate and drug target against C. sinensis infection.

Virulence of serotype M3 Group A Streptococcus strains in wax worms (Galleria mellonella larvae)

Group A Streptococcus (GAS) causes human infections that range in severity from pharyngitis ("strep-throat") to necrotizing fasciitis ("flesh-eating disease").  To facilitate investigation of the molecular basis of host-pathogen interactions, infection models capable of rapidly screening for differences in GAS strain virulence are needed.  To this end, we developed a Galleria mellonella larvae (wax worm) model of invasive GAS infection and used it to compare the virulence of serotype M3 GAS strains.  We found that GAS causes severe tissue damage and kills wax worms in a dose-dependent manner.  The virulence of genetically distinct GAS strains was compared by Kaplan-Meier survival analysis and determining 50% lethal doses (LD 50).  Host-pathogen interactions were further characterized using quantitative culture, histopathology and TaqMan assays.  GAS strains known to be highly pathogenic in mice and monkeys caused significantly lower survival and had significantly lower LD 50s in wax worms than GAS strains associated with attenuated virulence or asymptomatic carriage.  Furthermore, isogenic inactivation of proven virulence factors resulted in a significantly increased LD 50 and decreased lesion size compared to the wild-type strain, a finding that also strongly correlates with animal studies.  Importantly, survival analysis and LD 50 determination in wax worms supported our hypothesis that a newly emerged GAS subclone that is epidemiologically associated with more human necrotizing fasciitis cases than its progenitor lineage has significantly increased virulence.  We conclude that GAS virulence in wax worms strongly correlates with the data obtained in vertebrate models, and thus, the Galleria mellonella larva is a useful host organism to study GAS pathogenesis.

Plasmin plays an essential role in amplification of psoriasiform skin inflammation in mice

Background

Although increased levels of plasminogen activators have been found in psoriatic lesions, the role of plasmin converted from plasminogen by plasminogen activators in pathogenesis of psoriasis has not been investigated.

Methodology/Principal Findings

Here we examined the contribution of plasmin to amplification of inflammation in patients with psoriasis. We found that plasminogen was diminished, but that the amount and activity of its converted product plasmin were markedly increased in psoriasis. Moreover, annexin II, a receptor for plasmin was dramatically increased in both dermis and epidermis in psoriasis. Plasmin at sites of inflammation was pro-inflammatory, eliciting production of inflammatory factors, including CC chemokine ligand 20 (CCL20) and interleukin-23 (IL-23), that was mediated by the nuclear factor-kappaB (NF-κB) signaling pathway and that had an essential role in the recruitment and activation of pathogenic C-C chemokine receptor type 6 (CCR6)⁺ T cells. Moreover, intradermal injection of plasmin or plasmin together with recombinant monocyte/macrophage chemotactic protein-1 (MCP-1) resulted in induction of psoriasiform skin inflammation around the injection sites with several aspects of human psoriasis in mice.

Conclusions/Significance

Plasmin converted from plasminogen by plasminogen activators plays an essential role in amplification of psoriasiform skin inflammation in mice, and targeting plasmin receptor - annexin II - may harbor therapeutic potential for the treatment of human psoriasis.