Lsa30, a novel adhesin of Leptospira interrogans binds human plasminogen and the complement regulator C4bp

Leptospiral adhesins: from identification to future perspectives

Leptospirosis is a significant zoonosis worldwide, with disease severity ranging from a mild non-specific illness to multi-organ dysfunction and hemorrhage. The disease is caused by pathogenic bacteria of the genus Leptospira, which are classified into pathogenic and saprophytic clades. Bacterial binding to host molecules and cells, coordinated by adhesin proteins, is an important step in pathogenesis. While many leptospiral adhesins have been identified, the vast majority have not been characterized in vivo. Herein, we present an overview of the current methodologies and successes in identifying adhesins in Leptospira, including known biological roles in vivo. We will also identify and discuss potential areas for future research.

Human leptospirosis: In search for a better vaccine

Leptospirosis is a neglected disease caused by bacteria of the genus Leptospira and is more prevalent in tropical and subtropical countries. This pathogen infects humans and other animals, responsible for the most widespread zoonosis in the world, estimated to be responsible for 60 000 deaths and 1 million cases per year. To date, commercial vaccines against human leptospirosis are available only in some countries such as Japan, China, Cuba and France. These vaccines prepared with inactivated Leptospira (bacterins) induce a short-term and serovar-specific immune response, with strong adverse side effects. To circumvent these limitations, several research groups are investigating new experimental vaccines in order to ensure that they are safe, efficient, and protect against several pathogenic Leptospira serovars, inducing sterilizing immunity. Most of these protocols use attenuated cultures, preparations after LPS removal, recombinant proteins or DNA from pathogenic Leptospira spp. The aim of this review was to highlight several promising vaccine candidates, considering their immunogenicity, presence in different pathogenic Leptospira serovars, their role in virulence or immune evasion and other factors.

In silico screening and epitope mapping of leptospiral outer membrane protein-Lsa46

Leptospirosis is one of the neglected diseases caused by the spirochete, Leptospira interrogans. Leptospiral surface adhesion (Lsa) proteins are surface exposed outer membrane proteins present in the pathogen. It acts as laminin and plasminogen binding proteins which enable them to infect host cells. The major target for the development of vaccine in the current era focuses on surface exposed outer membrane proteins, as they can induce strong and fast immune response in hosts. Therefore, the present study mapped the potential epitopes of the Leptospiral outer membrane proteins, mainly the surface adhesion proteins. Protein sequence analysis of Lsa proteins was done by in silico methods. The primary protein sequence analysis revealed Lsa46 as a suitable target which can be a potent Leptospiral vaccine candidate. Its structure was modelled by threading based method in I-TASSER server and validated by Ramachandran plot. The predicted epitope's interactions with human IgG, IgM(Fab) and T-cell receptor TCR(αβ) were performed by molecular docking studies using Biovia Discovery studio 2018. One of the predicted B-cell epitopes and the IgG showed desirable binding interactions, while four of the predicted B-cell epitopes and T-cell epitopes showed desirable binding interactions with IgM and TCR respectively. The molecular dynamic simulation studies carried out with the molecular docked complexes gave minimized energies indicating stable interactions. The structural analysis of the entire simulated complex showed a stable nature except for one of the Epitope-IgM complex. Further the binding free energy calculation of eight receptor-ligand complex predicted them energetically stable. The results of the study help in elucidating the structural and functional characterization of Lsa46 for epitope-based vaccine design.Communicated by Ramaswamy H. Sarma.

Weil's Disease-Immunopathogenesis, Multiple Organ Failure, and Potential Role of Gut Microbiota

Leptospirosis is an important zoonotic disease, causing about 60,000 deaths annually. In this review, we have described in detail the immunopathogenesis of leptospirosis, the influence of cytokines, genetic susceptibility on the course of the disease, and the evasion of the immune response. These data are combined with information about immunological and pathomorphological changes in the kidneys, liver, and lungs, which are most affected by Weil's disease. The review also suggests a possible role of the gut microbiota in the clinical course of leptospirosis, the main mechanisms of the influence of gut dysbiosis on damage in the liver, kidneys, and lungs through several axes, i.e., gut-liver, gut-kidney, and gut-lungs. Modulation of gut microbiota by probiotics and/or fecal microbiota transplantation in leptospirosis may become an important area of scientific research.

Biofilm formation and social interaction of Leptospira in natural and artificial environments

In the recent decades, there has been increased interest in the study on social interactions of pathogenic bacteria and biofilm-forming microbes. Leptospira is a zoonotic pathogen that causes human leptospirosis. Biofilm formation by pathogenic and saprophytic Leptospira has been documented in various biotic and abiotic environments. Biofilm supports cell growth and protects them from a variety of environmental stress. Pathogenic bacterial biofilm might increase the virulence and pathogenesis. However, research on the social behaviour and biofilm production by Leptospira is limited. This review discusses the interplay between the different species in the biofilm formation of saprophytic and pathogenic Leptospira and potential future applications.

ErpY-like Lipoprotein of Leptospira Outsmarts Host Complement Regulation by Acquiring Complement Regulators, Activating Alternative Pathways, and Intervening in the Membrane Attack Complex

The survival of pathogenic Leptospira in the host depends on its proficiency to circumvent the immune response. These pathogens evade the complement system in serum by enticing and amassing the serum complement regulators onto their surface. ErpY-like lipoprotein, a surface-exposed protein of Leptospira spp., is conserved in the pathogenic Leptospira serovars. The recombinant form of this protein interacts with multiple extracellular matrix (ECM) components and serum proteins such as soluble complement regulators factor H (FH) and factor I (FI). Here, we document that the supplementation of rErpY-like protein (10 μg/mL) in human serum inhibits complement-mediated bacterial cell lysis and augments the viability of Escherichia coli and saprophytic Leptospira biflexa by more than two-fold. Complement regulators FH and FI, when bound to rErpY-like protein, preserve their respective cofactor and protease activity and cleave the complement component C3b. The supplementation of rErpY-like protein (40 μg/mL) in serum ensued in an ∼90% reduction of membrane attack complex (C5b-9/MAC) deposition through the alternative pathway (AP) of complement activation. However, rErpY-like protein could moderately reduce (∼16%) MAC deposition in serum through the classical pathway (CP). In addition, the rErpY-like protein solely initiated the AP, suggesting its role in the rapid consumption and depletion of the complement components. Blocking the pathogenic Leptospira interrogans surface with anti-rErpY-like antibodies resulted in an increase in MAC formation on the bacterial surface, indicating a specific role of the ErpY-like lipoprotein in complement-mediated immune evasion. This study underscores the role of the ErpY-like lipoprotein of Leptospira in complement evasion.

Screening of Surface-Exposed Lipoproteins of Leptospira Involved in Modulation of Host Innate Immune Response

Leptospira, a zoonotic pathogen, is capable of causing both chronic and acute infection in a susceptible host. Surface-exposed lipoproteins play a major role in modulating the host immune response by activating the innate cells like macrophages and dendritic cells or evading complement attack and killing by phagocytes like neutrophils to favor pathogenesis and establish infection. In this study, we screened some surface-exposed lipoproteins known to be involved in pathogenesis to assess their possible role in immune modulation (innate immune activation or evasion). Surface proteins of the Len family (LenB, LenD, and LenE), Lsa30, Loa22, and Lipl21 were purified in recombinant form and then tested for their ability to activate macrophages of the different host (mouse, human, and bovine). These proteins were tested for binding with complement regulators like Factor H (FH), C4 Binding Protein (C4BP), and host protease Plasminogen (PLG) and also as nucleases to access their possible role in innate immune evasion. Our results show that, of various proteins tested, Loa22 induced strong innate activation and Lsa30 was least stimulatory, as evident from the production of pro-inflammatory cytokines (interleukin-6 and tumor necrosis factor–α) and expression of surface markers [CD80, CD86, and major histocompatibility complex class II (MHCII)]. All the tested proteins were able to bind to FH, C4BP, and PLG; however, Loa22 showed strong binding to PLG correlating to plasmin activity. All the proteins except Loa22 showed nuclease activity, albeit with a requirement of different metal ions. The nuclease activity of these proteins correlated to in vitro degradation of neutrophil extracellular trap (NET). In conclusion, our results indicate that these surface proteins are involved in innate immune modulation and may play a critical role in assisting the bacteria in invading and colonizing the host tissue for persistent infection.

Deciphering the Role of Leptospira Surface Protein LigA in Modulating the Host Innate Immune Response

Leptospira, a zoonotic pathogen, is known to infect various hosts and can establish persistent infection. This remarkable ability of bacteria is attributed to its potential to modulate (activate or evade) the host immune response by exploiting its surface proteins. We have identified and characterized the domain of the variable region of Leptospira immunoglobulin-like protein A (LAV) involved in immune modulation. The 11^th domain (A₁₁) of the variable region of LigA (LAV) induces a strong TLR4 dependent innate response leading to subsequent induction of humoral and cellular immune responses in mice. A₁₁ is also involved in acquiring complement regulator FH and binds to host protease Plasminogen (PLG), there by mediating functional activity to escape from complement-mediated killing. The deletion of A₁₁ domain significantly impaired TLR4 signaling and subsequent reduction in the innate and adaptive immune response. It also inhibited the binding of FH and PLG thereby mediating killing of bacteria. Our study discovered an unprecedented role of LAV as a nuclease capable of degrading Neutrophil Extracellular Traps (NETs). This nuclease activity was primarily mediated by A₁₁. These results highlighted the moonlighting function of LigA and demonstrated that a single domain of a surface protein is involved in modulating the host innate immune defenses, which might allow the persistence of Leptospira in different hosts for a long term without clearance.

A Review on Host-Leptospira Interactions: What We Know and Future Expectations

Leptospirosis is a widespread zoonosis caused by pathogenic Leptospira spp. It is considered a neglected infectious disease of human and veterinary concern. Our group has been investigating proteins annotated as hypothetical, predicted to be located on the leptospiral surface. Because of their location, these proteins may have the ability to interact with various host components, which could allow establishment of the infection. These proteins act as adherence factors by binding to host receptor molecules, such as the extracellular matrix (ECM) components laminin and glycosaminoglycans to help bacterial colonization. Leptospira also interacts with the host fibrinolytic system, which has been demonstrated to be a powerful tool for invasion mechanisms. The interaction with fibrinogen and thrombin has been shown to reduce fibrin clot formation. Additionally, the degradation of coagulation cascade components by secreted proteases or by acquired surface plasmin could also play a role in reducing clot formation, hence facilitating dissemination during infection. Interaction with host complement system regulators also plays a role in helping bacteria to evade the immune system, facilitating invasion. Interaction of Leptospira to cell receptors, such as cadherins, can contribute to investigate molecules that participate in virulence. To achieve a better understanding of the host-pathogen interaction, leptospiral mutagenesis tools have been developed and explored. This work presents several proteins that mediate binding to components of the ECM, plasma, components of the complement system and cells, to gather research achievements that can be helpful in better understanding the mechanisms of leptospiral-host interactions and discuss genetic manipulation for Leptospira spp. aimed at protein function validation.

The interaction of two novel putative proteins of Leptospira interrogans with E-cadherin, plasminogen and complement components with potential role in bacterial infection

Leptospirosis is a worldwide zoonosis caused by pathogenic species of Leptospira. Leptospires are able to adhere to exposed extracellular matrix in injured tissues and, once in the bloodstream, can survive the attack of the immune system and spread to colonize target organs. In this work, we report that two novel putative proteins, coded by the genes LIC11711 and LIC12587 of L. interrogans serovar Copenhageni are conserved among pathogenic strains, and probably exposed in the bacterial surface. Soluble recombinant proteins were expressed in Escherichia coli, purified and characterized. Both recombinant proteins bound to laminin and E-cadherin, suggesting an initial adhesion function in host epithelial cells. The recombinant protein LIC11711 (rLIC11711) was able to capture plasminogen (PLG) from normal human serum and convert to enzymatically active plasmin (PLA), in the presence of PLG activator. rLIC12587 (recombinant protein LIC12587) displayed a dose dependent and saturable interaction with components C7, C8, and C9 of the complement system, reducing the bactericidal effect of the complement. Binding to C9 may have consequences such as C9 polymerization inhibition, interfering with the membrane attack complex formation. Blocking LIC11711 and LIC12587 on bacterial cells by the respective antiserum reduced leptospiral cell viability when exposed to normal human serum (NHS). Both recombinant proteins could be recognized by serum samples of confirmed leptospirosis, but not of unrelated diseases, suggesting that the native proteins are immunogenic and expressed during leptospirosis. Taken together, our data suggest that these proteins may have a role in leptospiral pathogenesis, participating in immune evasion strategies.

Strategies used by Leptospira spirochetes to evade the host complement system

Leptospires are highly invasive spirochetes equipped with efficient strategies for dissemination in the host. The Leptospira genus currently comprises 64 species divided into two major clades: the saprophytes composed of nonpathogenic, free-living organisms, and the pathogens encompassing all the species that cause mild or severe infections in humans and animals. While saprophytes are highly susceptible to the lytic action of the complement system, pathogenic (virulent) strains have evolved virulence strategies that allow efficient colonization of a variety of hosts and target organs, including mechanisms to circumvent hosts' innate and acquired immune responses. Pathogenic Leptospira avoid complement-mediated killing by recruiting host complement regulatory proteins and by targeting complement proteins using own and host-expressed proteases. This review outlines the role of complement in eradicating saprophytic Leptospira and the stratagems adopted by pathogenic Leptospira to maneuver the host complement system for their benefit.

Identification of a novel protein in the genome sequences of Leptospira interrogans with the ability to interact with host's components

BACKGROUND

Leptospirosis is an infectious disease that affects humans and animals worldwide. The etiological agents of this disease are the pathogenic species of the genus Leptospira. The mechanisms involved in the leptospiral pathogenesis are not full understood. The elucidation of novel mediators of host-pathogen interaction is important in the detection of virulence factors involved in the pathogenesis of leptospirosis.

OBJECTIVE

This work focused on identification and characterization of a hypothetical protein of Leptospira encoded by the gene LIC10920.

METHODS

The protein of unknown function was predicted to be surface exposed. Therefore, the LIC10920 gene was cloned and the protein expressed in Escherichia coli BL21 (DE3) Star pLysS strain. The recombinant protein was purified by metal affinity chromatography and evaluated with leptospirosis human serum samples. The interaction with host components was also performed.

RESULTS

The recombinant protein was recognized by antibodies present in leptopsirosis human serum, suggesting its expression during infection. Immunofluorescence and intact bacteria assays indicated that the bacterial protein is surface-exposed. The recombinant protein interacted with human laminin, in a dose-dependent and saturable manner and was named Lsa24.9, for Leptospiral surface adhesin, followed by its molecular mass. Lsa24.9 also binds plasminogen (PLG) in a dose-dependent and saturable fashion, fulfilling receptor ligand interaction. Moreover, Lsa24.9 has the ability to acquire PLG from normal human serum, exhibiting similar profile as observed with the human purified component. PLG bound Lsa24.9 was able of generating plasmin, which could increase the proteolytic power of the bacteria.

CONCLUSIONS

This novel leptospiral protein may function as an adhesin at the colonization steps and may help the invasion process by plasmin generation at the bacterial cell surface.

Characterization of a novel protein of Leptospira interrogans exhibiting plasminogen, vitronectin and complement binding properties

Leptospirosis is a severe zoonosis caused by pathogenic species of the genus Leptospira. This work focuses on a hypothetical protein of unknown function, encoded by the gene LIC13259, and predicted to be a surface protein, widely distributed among pathogenic leptospiral strain. The gene was amplified from L. interrogans serovar Copenhageni, strain Fiocruz L1-130, cloned and the protein expressed using Escherichia coli as a host system. Immunofluorescence assay showed that the protein is surface-exposed. The recombinant protein LIC13259 (rLIC13259) has the ability to interact with the extracellular matrix (ECM) laminin, in a dose-dependent manner but saturation was not reach. The rLIC13259 protein is a plasminogen (PLG)-binding protein, generating plasmin, in the presence of urokinase PLG-activator uPA. The recombinant protein is able to mediate the binding to human purified terminal complement route vitronectin, C7, C8 and C9, and to recruit and interact with these components from normal human serum (NHS). These interactions are dose-dependent on NHS increased concentration. The binding of rLIC13259 to C8 and vitronectin was slight and pronounced inhibited in the presence of increasing heparin concentration, respectively, suggesting that the interaction with vitronectin occurs via heparin domain. Most interesting, the interaction of rLIC13259 with C9 protein was capable of preventing C9 polymerization, suggesting that the membrane attack complex (MAC) formation was inhibited. Thus, we tentatively assign the coding sequence (CDS) LIC13259, previously annotated as unknown function, as a novel protein that may play an important role in the host's invasion and immune evasion processes, contributing to the establishment of the leptospiral infection.

Leptospirosis: Molecular trial path and immunopathogenesis correlated with dengue, malaria and mimetic hemorrhagic infections

Immuno-pathogenesis of leptospirosis can be recounted well by following its trail path from entry to exit, while inducing disastrous damages in various tissues of the host. Dysregulated, inappropriate and excessive immune responses are unanimously blamed in fatal leptospirosis. The inherent abilities of the pathogen and inabilities of the host were debated targeting the severity of the disease. Hemorrhagic manifestation through various mechanisms leading to a fatal end is observed when this disease is unattended. The similar vascular destructions and hemorrhage manifestations are noted in infections with different microbes in endemic areas. The simultaneous infection in a host with more than one pathogen or parasite is referred as the coinfection. Notably, common endemic infections such as leptospirosis, dengue, chikungunya, and malaria, harbor favorable environments to flourish in similar climates, which is aggregated with stagnated water and aggravated with the poor personal and environmental hygiene of the inhabitants. These factors aid the spread of pathogens and parasites to humans and potential vectors, eventually leading to outbreaks of public health relevance. Malaria, dengue and chikungunya need mosquitoes as vectors, in contrast with leptospirosis, which directly invades human, although the environmental bacterial load is maintained through other mammals, such as rodents. The more complicating issue is that infections by different pathogens exhibiting similar symptoms but require different treatment management. The current review explores different pathogens expressing specific surface proteins and their ability to bind with array of host proteins with or without immune response to enter into the host tissues and their ability to evade the host immune responses to invade and their affinity to certain tissues leading to the common squeal of hemorrhage. Furthermore, at the host level, the increased susceptibility and inability of the host to arrest the pathogens' and parasites' spread in different tissues, various cytokines accumulated to eradicate the microorganisms and their cellular interactions, the antibody dependent defense and the susceptibility of individual organs bringing the manifestation of the diseases were explored. Lastly, we provided a discussion on the immune trail path of pathogenesis from entry to exit to narrate the similarities and dissimilarities among various hemorrhagic fevers mentioned above, in order to outline future possibilities of prevention, diagnosis, and treatment of coinfections, with special reference to endemic areas.

Characterization of two new putative adhesins of Leptospira interrogans

We here report the characterization of two novel proteins encoded by the genes LIC11122 and LIC12287, identified in the genome sequences of Leptospira interrogans, annotated, respectively, as a putative sigma factor and a hypothetical protein. The CDSs LIC11122 and LIC12287 have signal peptide SPII and SPI and are predicted to be located mainly at the cytoplasmic membrane of the bacteria. The genes were cloned and the proteins expressed using Escherichia coli. Proteinase K digestion showed that both proteins are surface exposed. Evaluation of interaction of recombinant proteins with extracellular matrix components revealed that they are laminin binding and they were called Lsa19 (LIC11122) and Lsa14 (LIC12287), for Leptospiral-surface adhesin of 19 and 14 kDa, respectively. The bindings were dose-dependent on protein concentration, reaching saturation, fulfilling the ligand-binding criteria. Reactivity of the recombinant proteins with leptospirosis human sera has shown that Lsa19 and, to a lesser extent, Lsa14, are recognized by antibodies, suggesting that, most probably, Lsa19 is expressed during infection. The proteins interact with plasminogen and generate plasmin in the presence of urokinase-type plasminogen activator. Plasmin generation in Leptospira has been associated with tissue penetration and immune evasion strategies. The presence of a sigma factor on the cell surface playing a secondary role, probably mediating host -pathogen interaction, suggests that LIC11122 is a moonlighting protein candidate. Although the biological significance of these putative adhesins will require the generation of mutants, our data suggest that Lsa19 is a potential candidate for future evaluation of its role in adhesion/colonization activities during L. interrogans infection.

The role of Lsa23 to mediate the interaction of Leptospira interrogans with the terminal complement components pathway

Leptospirosis is a severe worldwide zoonotic disease caused by pathogenic Leptospira spp. It has been demonstrated that pathogenic leptospires are resistant to the bactericidal activity of normal human serum while saprophytic strains are susceptible. Pathogenic strains have the ability to bind soluble complement regulators and these activities are thought to contribute to bacterial immune evasion. One strategy used by some pathogens to evade the complement cascade, which is not well explored, is to block the terminal pathway. We have, thus, examined whether leptospires are able to interact with components of the terminal complement pathway. ELISA screening using anti-leptospires serum has shown that the pathogenic, virulent strain L. interrogans L1-130 can bind to immobilized human C8 (1 μg). However, virulent and saprophyte L. biflexa strains showed the ability to interact with C8 and C9, when these components were employed at physiological concentration (50 μg/mL), but the virulent strain seemed more competent. Lsa23, a putative leptospiral adhesin only present in pathogenic strains, interacts with C8 and C9 in a dose-dependent mode, suggesting that this protein could mediate the binding of virulent Leptospira with these components. To our knowledge, this is the first work reporting the binding of Leptospira to C8 and C9 terminal complement components, suggesting that the inhibition of this pathway is part of the strategy used by leptospires to evade the innate immunity.

Mammalian cell entry (Mce) protein of Leptospira interrogans binds extracellular matrix components, plasminogen and β2 integrin

A severe re-emergingzoonosis, leptospirosis, is caused by pathogenic spirochetes of the genus Leptospira. Several studies have identified leptospiral surface proteins with the ability to bind ECM and plasma components, which could mediate adhesion and invasion through the hosts. It has been shown that Mce of pathogenic Leptospira spp. is an RGD (Arg-Gly-Asp)-motif-dependent virulence factor, responsible for infection of cells and animals. In the present article, we decided to further study the repertoire of the Mce activities in leptospiral biological properties. We report that the recombinant Mce is a broad-spectrum ECM-binding protein, capable of interacting with laminin, cellular and plasma fibronectin and collagen IV. Dose--r-esponse interaction was observed for all the components, fulfilling ligand--receptor requirements. Mce is a PLG binding protein capable to recruit this component from NHS, generating PLA in the presence of PLG activator. Binding of Mce was also observed with the leukocyte cell receptors αLβ2 [(CD11a/CD18)-LFA-1] and αMβ2 [(CD11b/CD18)-Mac-1], suggesting the involvement of this protein in the host immune response. Indeed, virulent Leptospira L1-130 was capable of binding both integrins, whereas culture-attenuated M-20 strain only bind to αMβ2 [(CD11b/CD18)-Mac-1]. To the best of our knowledge, this is the first work to describe that Mce surface protein could mediate the attachment of Leptospira interrogans to human cell receptors αLβ2(CD11a/CD18) and αMβ2(CD11b/CD18).

Complement Immune Evasion by Spirochetes

The complement system plays an important role in the innate and acquired immune response against pathogens. A sophisticated network of activating and regulating proteins allows the distinction between intact and damaged host and non-host surfaces such as bacteria and other parasites. Non-host structures trigger the alternative pathway which may lead to their elimination by phagocytosis or cell lysis. In addition, complement proteins such as C1q, mannose binding lectin (MBL), and ficolins act as pathogen pattern-recognition molecules. Biological functions such as opsonization, activation of B lymphocytes and production of antibodies, degranulation of mast cells and basophils, and cell lysis that are important for elimination of microorganisms are dependent on complement activation. However, several pathogens including spirochetes have developed several specialized mechanisms to evade the complement system, thereby contributing to survival in the host. In this review, we give a brief overview of complement activation and regulation, and discuss in detail the strategies used by spirochetes from the genera Borrelia, Leptospira, and Treponema to overcome complement activation.

Complement Evasion by Pathogenic Leptospira

Leptospirosis is a neglected infectious disease caused by spirochetes from the genus Leptospira. Pathogenic microorganisms, notably those which reach the blood circulation such as Leptospira, have evolved multiple strategies to escape the host complement system, which is important for innate and acquired immunity. Leptospira avoid complement-mediated killing through: (i) recruitment of host complement regulators; (ii) acquisition of host proteases that cleave complement proteins on the bacterial surface; and, (iii) secretion of proteases that inactivate complement proteins in the Leptospira surroundings. The recruitment of host soluble complement regulatory proteins includes the acquisition of Factor H (FH) and FH-like-1 (alternative pathway), C4b-binding protein (C4BP) (classical and lectin pathways), and vitronectin (Vn) (terminal pathway). Once bound to the leptospiral surface, FH and C4BP retain cofactor activity of Factor I in the cleavage of C3b and C4b, respectively. Vn acquisition by leptospires may result in terminal pathway inhibition by blocking C9 polymerization. The second evasion mechanism lies in plasminogen (PLG) binding to the leptospiral surface. In the presence of host activators, PLG is converted to enzymatically active plasmin, which is able to degrade C3b, C4b, and C5 at the surface of the pathogen. A third strategy used by leptospires to escape from complement system is the active secretion of proteases. Pathogenic, but not saprophytic leptospires, are able to secrete metalloproteases that cleave C3 (central complement molecule), Factor B (alternative pathway), and C4 and C2 (classical and lectin pathways). The purpose of this review is to fully explore these complement evasion mechanisms, which act together to favor Leptospira survival and multiplication in the host.

Multifunctional and Redundant Roles of Borrelia burgdorferi Outer Surface Proteins in Tissue Adhesion, Colonization, and Complement Evasion

Borrelia burgdorferi is the causative agent of Lyme disease in the U.S., with at least 25,000 cases reported to the CDC each year. B. burgdorferi is thought to enter and exit the bloodstream to achieve rapid dissemination to distal tissue sites during infection. Travel through the bloodstream requires evasion of immune surveillance and pathogen clearance in the host, a process at which B. burgdorferi is adept. B. burgdorferi encodes greater than 19 adhesive outer surface proteins many of which have been found to bind to host cells or components of the extracellular matrix. Several others bind to host complement regulatory factors, in vitro. Production of many of these adhesive proteins is tightly regulated by environmental cues, and some have been shown to aid in vascular interactions and tissue colonization, as well as survival in the blood, in vivo. Recent work has described multifaceted and redundant roles of B. burgdorferi outer surface proteins in complement component interactions and tissue targeted adhesion and colonization, distinct from their previously identified in vitro binding capabilities. Recent insights into the multifunctional roles of previously well-characterized outer surface proteins such as BBK32, DbpA, CspA, and OspC have changed the way we think about the surface proteome of these organisms during the tick-mammal life cycle. With the combination of new and old in vivo models and in vitro techniques, the field has identified distinct ligand binding domains on BBK32 and DbpA that afford tissue colonization or blood survival to B. burgdorferi. In this review, we describe the multifunctional and redundant roles of many adhesive outer surface proteins of B. burgdorferi in tissue adhesion, colonization, and bloodstream survival that, together, promote the survival of Borrelia spp. throughout maintenance in their multi-host lifestyle.

Evaluation of two novel leptospiral proteins for their interaction with human host components

Pathogenic species of the genus Leptospira are the etiological agents of leptospirosis, the most widespread zoonosis. Mechanisms involved in leptospiral pathogenesis are not well understood. By data mining the genome sequences of Leptospira interrogans we have identified two proteins predicted to be surface exposed, LIC10821 and LIC10064. Immunofluorescence and proteinase K assays confirmed that the proteins are exposed. Reactivity of the recombinant proteins with human sera has shown that rLIC10821, but not rLIC10064, is recognized by antibodies in confirmed leptospirosis serum samples, suggesting its expression during infection. The rLIC10821 was able to bind laminin, in a dose-dependent fashion, and was called Lsa37 (leptospiral surface adhesin of 37 kDa). Studies with human plasma components demonstrated that rLIC10821 interacts with plasminogen (PLG) and fibrinogen (Fg). The binding of Lsa37 with PLG generates plasmin when PLG activator was added. Fibrin clotting reduction was observed in a thrombin-catalyzed reaction, when Fg was incubated with Lsa37, suggesting that this protein may interfere in the coagulation cascade during the disease. Although LIC10064 protein is more abundant than the corresponding Lsa37, binding activity with all the components tested was not detected. Thus, Lsa37 is a novel versatile adhesin that may mediate Leptospira-host interactions.

What Makes a Bacterial Species Pathogenic?:Comparative Genomic Analysis of the Genus Leptospira

Leptospirosis, caused by spirochetes of the genus Leptospira, is a globally widespread, neglected and emerging zoonotic disease. While whole genome analysis of individual pathogenic, intermediately pathogenic and saprophytic Leptospira species has been reported, comprehensive cross-species genomic comparison of all known species of infectious and non-infectious Leptospira, with the goal of identifying genes related to pathogenesis and mammalian host adaptation, remains a key gap in the field. Infectious Leptospira, comprised of pathogenic and intermediately pathogenic Leptospira, evolutionarily diverged from non-infectious, saprophytic Leptospira, as demonstrated by the following computational biology analyses: 1) the definitive taxonomy and evolutionary relatedness among all known Leptospira species; 2) genomically-predicted metabolic reconstructions that indicate novel adaptation of infectious Leptospira to mammals, including sialic acid biosynthesis, pathogen-specific porphyrin metabolism and the first-time demonstration of cobalamin (B12) autotrophy as a bacterial virulence factor; 3) CRISPR/Cas systems demonstrated only to be present in pathogenic Leptospira, suggesting a potential mechanism for this clade's refractoriness to gene targeting; 4) finding Leptospira pathogen-specific specialized protein secretion systems; 5) novel virulence-related genes/gene families such as the Virulence Modifying (VM) (PF07598 paralogs) proteins and pathogen-specific adhesins; 6) discovery of novel, pathogen-specific protein modification and secretion mechanisms including unique lipoprotein signal peptide motifs, Sec-independent twin arginine protein secretion motifs, and the absence of certain canonical signal recognition particle proteins from all Leptospira; and 7) and demonstration of infectious Leptospira-specific signal-responsive gene expression, motility and chemotaxis systems. By identifying large scale changes in infectious (pathogenic and intermediately pathogenic) vs. non-infectious Leptospira, this work provides new insights into the evolution of a genus of bacterial pathogens. This work will be a comprehensive roadmap for understanding leptospirosis pathogenesis. More generally, it provides new insights into mechanisms by which bacterial pathogens adapt to mammalian hosts.

Leptospira spp.: Novel insights into host-pathogen interactions

Leptospirosis is a widespread zoonosis caused by pathogenic Leptospira spp. It is an important infectious disease that affects humans and animals. The disease causes economic losses as it affects livestock, with decreased milk production and death. Our group is investigating the genome sequences of L. interrogans targeting surface-exposed proteins because, due to their location, these proteins are capable to interact with several host components that could allow establishment of the infection. These interactions may involve adhesion of the bacteria to extracellular matrix (ECM) components and, hence, help bacterial colonization. The bacteria could also react with the host fibrinolytic system and/or with the coagulation cascade components, such as, plasminogen (PLG) and fibrinogen (Fg), respectively. The binding with the first system generates plasmin (PLA), increasing the proteolytic power of the bacteria, while the second interferes with clotting in a thrombin-catalyzed reaction, which may promote hemorrhage foci and increase bacterial dissemination. Interaction with the complement system negative regulators may help bacteria to evade the host immune system, facilitating the invasion. This work compiles the main described leptospiral proteins that could act as adhesins, as PLG and fibrinogen receptors and as complement regulator binding proteins. We present models in which we suggest possible mechanisms of how leptospires might colonize and invade host tissues, causing the disease. Understanding leptospiral pathogenesis will help to identify antigen candidates that would contribute to the development of more effective vaccines and diagnostic tests.

C4b-binding protein: The good, the bad and the deadly. Novel functions of an old friend

C4b-binding protein (C4BP) is best known as a potent soluble inhibitor of the classical and lectin pathways of the complement system. This large 500 kDa multimeric plasma glycoprotein is expressed mainly in the liver but also in lung and pancreas. It consists of several identical 75 kDa α-chains and often also one 40 kDa β-chain, both of which are mainly composed of complement control protein (CCP) domains. Structure-function studies revealed that one crucial binding site responsible for inhibition of complement is located to CCP1-3 of the α-chain. Binding of anticoagulant protein S to the CCP1 of the β-chain provides C4BP with the ability to strongly bind apoptotic and necrotic cells in order to prevent inflammation arising from activation of complement by these cells. Further, C4BP interacts strongly with various types of amyloid and enhances fibrillation of islet amyloid polypeptide secreted from pancreatic beta cells, which may attenuate pro-inflammatory and cytotoxic effects of this amyloid. Full deficiency of C4BP has not been identified but non-synonymous alterations in its sequence have been found in haemolytic uremic syndrome and recurrent pregnancy loss. Furthermore, C4BP is bound by several bacterial pathogens, notably Streptococcus pyogenes, which due to inhibition of complement and enhancement of bacterial adhesion to endothelial cells provides these bacteria with a survival advantage in the host. Thus, depending on the context, C4BP has a protective or detrimental role in the organism.

Leptospira interrogans Lsa23 protein recruits plasminogen, factor H and C4BP from normal human serum and mediates C3b and C4b degradation

It has been reported that pathogenic Leptospira are resistant to normal human serum (NHS) due to their ability to evade the complement immune system by interacting with factor H (FH) and C4b-binding protein (C4BP) regulators. Moreover, plasmin generation on the leptospiral surface diminishes C3b and IgG deposition, decreasing opsonophagocytosis by immune competent cells. We have previously reported that Lsa23 (LIC11360) is a multipurpose protein capable of binding purified extracellular matrix molecules, FH, C4BP and plasminogen (PLG)/plasmin in the presence of PLG activators. In this work, we provide further evidence that Lsa23 is located at the bacterial surface by using immunofluorescence microscopy. We show that Lsa23 has the ability to acquire FH, C4BP and PLG from NHS, and use these interactions to evade innate immunity. The binding with the complement regulators FH and C4BP preserves factor I (FI) activity, leading to C3b and C4b degradation products, respectively. C3b and C4b alpha-chain cleavage was also observed when Lsa23 bound to PLG generating plasmin, an effect blocked by the protease inhibitor aprotinin. Lsa23 also inhibited lytic activity by NHS mediated by both classical and alternative complement pathways. Thus, Lsa23 has the ability to block both pathways of the complement system, and may help pathogenic Leptospira to escape complement-mediated clearance in human hosts. Indeed, NHS treated with Lsa23 confers a partial serum resistance phenotype to Leptospira biflexa, whereas blocking this protein with anti-Lsa23 renders pathogenic L. interrogans more susceptible to complement-mediated killing. Thus, Lsa23 is a multifunctional protein involved in many pathways, featuring C4b cleavage by plasmin, knowledge that may help in the development of preventive approaches to intervene with human complement escape by this versatile pathogen.

Protective Immunity and Reduced Renal Colonization Induced by Vaccines Containing Recombinant Leptospira interrogans Outer Membrane Proteins and Flagellin Adjuvant

Leptospirosis is a global zoonotic disease caused by different Leptospira species, such as Leptospira interrogans, that colonize the renal tubules of wild and domestic animals. Thus far, attempts to develop effective leptospirosis vaccines, both for humans and animals, have failed to induce immune responses capable of conferring protection and simultaneously preventing renal colonization. In this study, we evaluated the protective immunity induced by subunit vaccines containing seven different recombinant Leptospira interrogans outer membrane proteins, including the carboxy-terminal portion of the immunoglobulinlike protein A (LigA(C)) and six novel antigens, combined with aluminum hydroxide (alum) or Salmonella flagellin (FliC) as adjuvants. Hamsters vaccinated with the different formulations elicited high antigen-specific antibody titers. Immunization with LigA(C), either with alum or flagellin, conferred protective immunity but did not prevent renal colonization. Similarly, animals immunized with LigA(C) or LigA(C) coadministered with six leptospiral proteins with alum adjuvant conferred protection but did not reduce renal colonization. In contrast, immunizing animals with the pool of seven antigens in combination with flagellin conferred protection and significantly reduced renal colonization by the pathogen. The present study emphasizes the relevance of antigen composition and added adjuvant in the efficacy of antileptospirosis subunit vaccines and shows the complex relationship between immune responses and renal colonization by the pathogen.

A Novel Interaction between Complement Inhibitor C4b-binding Protein and Plasminogen That Enhances Plasminogen Activation

The complement, coagulation, and fibrinolytic systems are crucial for the maintenance of tissue homeostasis. To date numerous interactions and cross-talks have been identified between these cascades. In line with this, here we propose a novel, hitherto unknown interaction between the complement inhibitor C4b-binding protein (C4BP) and plasminogen of the fibrinolytic pathway. Binding of C4BP to Streptococcus pneumoniae is a known virulence mechanism of this pathogen and it was increased in the presence of plasminogen. Interestingly, the acute phase variant of C4BP lacking the β-chain and protein S binds plasminogen much stronger than the main isoform containing the β-chain and protein S. Indeed, the complement control protein (CCP) 8 domain of C4BP, which would otherwise be sterically hindered by the β-chain, primarily mediates this interaction. Moreover, the lysine-binding sites in plasminogen kringle domains facilitate the C4BP-plasminogen interaction. Furthermore, C4BP readily forms complexes with plasminogen in fluid phase and such complexes are present in human serum and plasma. Importantly, whereas the presence of plasminogen did not affect the factor I cofactor activity of C4BP, the activation of plasminogen by urokinase-type plasminogen activator to active plasmin was significantly augmented in the presence of C4BP. Taken together, our data demonstrate a novel interaction between two proteins of the complement and fibrinolytic system. Most complexes might be formed during the acute phase of inflammation and have an effect on the homeostasis at the site of injury or acute inflammation.

Interaction of spirochetes with the host fibrinolytic system and potential roles in pathogenesis

The pathogenic spirochetes Borrelia burgdorferi, B. hermsii, B. recurrentis, Treponema denticola and Leptospira spp. are the etiologic agents of Lyme disease, relapsing fever, periodontitis and leptospirosis, respectively. Lyme borreliosis is a multi-systemic disorder and the most prevalent tick-borne disease in the northern hemisphere. Tick-borne relapsing fever is persistent in endemic areas worldwide, representing a significant burden in some African regions. Periodontal disease, a chronic inflammatory disorder that often leads to tooth loss, is caused by several potential pathogens found in the oral cavity including T. denticola. Leptospirosis is considered the most widespread zoonosis, and the predominant human disease in tropical, undeveloped regions. What these diseases have in common is that they are a significant burden to healthcare costs in the absence of prophylactic measures. This review addresses the interaction of these spirochetes with the fibrinolytic system, plasminogen (Plg) binding to the surface of bacteria and the generation of plasmin (Pla) on their surface. The consequences on host-pathogen interactions when the spirochetes are endowed with this proteolytic activity are discussed on the basis of the results reported in the literature. Spirochetes equipped with Pla activity have been shown to degrade extracellular matrix (ECM) components, in addition to digesting fibrin, facilitating bacterial invasion and dissemination. Pla generation triggers the induction of matrix metalloproteases (MMPs) in a cascade of events that enhances the proteolytic capacity of the spirochetes. These activities in concert with the interference exerted by the Plg/Pla on the complement system - helping the bacteria to evade the immune system - should illuminate our understanding of the mechanisms involved in host infection.

Exploitation of plasmin(ogen) by bacterial pathogens of veterinary significance

The plasminogen (Plg) system plays an important homeostatic role in the degradation of fibrin clots, extracellular matrices and tissue barriers important for cellular migration, as well as the promotion of neurotransmitter release. Plg circulates in plasma at physiologically high concentrations (150-200μg ml(-1)) as an inactive proenzyme. Proteins enriched in lysine and other positively charged residues (histidine and arginine) as well as glycosaminoglycans and gangliosides bind Plg. The binding interaction initiates a structural adjustment to the bound Plg that facilitates cleavage by proteases (plasminogen activators tPA and uPA) that activate Plg to the active serine protease plasmin. Both pathogenic and commensal bacteria capture Plg onto their cell surface and promote its conversion to plasmin. Many microbial Plg-binding proteins have been described underpinning the importance this process plays in how bacteria interact with their hosts. Bacteria exploit the proteolytic capabilities of plasmin by (i) targeting the mammalian fibrinolytic system and degrading fibrin clots, (ii) remodeling the extracellular matrix and generating bioactive cleavage fragments of the ECM that influence signaling pathways, (iii) activating matrix metalloproteinases that assist in the destruction of tissue barriers and promote microbial metastasis and (iv) destroying immune effector molecules. There has been little focus on the exploitation of the fibrinolytic system by veterinary pathogens. Here we describe several pathogens of veterinary significance that possess adhesins that bind plasmin(ogen) onto their cell surface and promote its activation to plasmin. Cumulative data suggests that these attributes provide pathogenic and commensal bacteria with a means to colonize and persist within the host environment.

Evaluation of cell binding activities of Leptospira ECM adhesins

Pathogenic spirochetes of the genus Leptospira are the causative agents of leptospirosis, a zoonotic infection that occurs globally. The bacteria colonize the renal proximal tubules of many animals and are shed in the urine. Contact with the urine, or with water contaminated with the urine of infected animals can cause infection of new host animals, including humans. Mechanisms of colonization of the proximal tubule and other tissues are not known, but specific interactions between bacterial adhesins and host substrates are likely to be critical in this process. Several extracellular matrix (ECM) adhesins have been previously identified, but more recently, it has been shown that Leptospira bind more efficiently to cells than ECM. In this work, recombinant forms of five putative Leptospira ECM adhesins, namely LipL32, Loa22, OmpL1, p31/LipL45, and LenA were evaluated for binding to cells as well as an expanded variety of ECM components. Reproducible and significant adhesin activity was demonstrated only for OmpL1, which bound to both mammalian cell lines tested and to glycosaminoglycans (GAGs). While determination of biologically significant bacterial adhesion activity will require generation of site-directed mutant strains, our results suggest that OmpL1 is a strong candidate for future evaluation regarding the roles of the adhesin activity of the protein during L. interrogans infection.

Leptospirosis is the most widespread zoonotic infection in the world and represents a major public health problem, especially in tropical climates. The processes by which some Leptospira species cause infection, disease, and colonization of carrier animals remains poorly understood. Specific binding of Leptospira molecules and host molecules are likely important for infection and colonization. To identify Leptospira molecules that mediate attachment to host substrates, prior studies have evaluated Leptospira membrane proteins for binding to extracellular matrix (ECM) proteins. More recent data, however, show that Leptospira bind more efficiently to cells than to ECM. In search of adhesins mediating the latter activity, our study evaluated the direct cell binding activity of recombinant forms of a group of previously reported Leptospira ECM adhesins. Only one of these proteins, OmpL1, demonstrated reproducible direct cell binding activity. Further work will focus on identification of the mammalian receptor for OmpL1 and determining the biological significance of this activity during infection.

Features of two new proteins with OmpA-like domains identified in the genome sequences of Leptospira interrogans

Leptospirosis is an acute febrile disease caused by pathogenic spirochetes of the genus Leptospira. It is considered an important re-emerging infectious disease that affects humans worldwide. The knowledge about the mechanisms by which pathogenic leptospires invade and colonize the host remains limited since very few virulence factors contributing to the pathogenesis of the disease have been identified. Here, we report the identification and characterization of two new leptospiral proteins with OmpA-like domains. The recombinant proteins, which exhibit extracellular matrix-binding properties, are called Lsa46 - LIC13479 and Lsa77 - LIC10050 (Leptospiral surface adhesins of 46 and 77 kDa, respectively). Attachment of Lsa46 and Lsa77 to laminin was specific, dose dependent and saturable, with KD values of 24.3 ± 17.0 and 53.0 ± 17.5 nM, respectively. Lsa46 and Lsa77 also bind plasma fibronectin, and both adhesins are plasminogen (PLG)-interacting proteins, capable of generating plasmin (PLA) and as such, increase the proteolytic ability of leptospires. The proteins corresponding to Lsa46 and Lsa77 are present in virulent L. interrogans L1-130 and in saprophyte L. biflexa Patoc 1 strains, as detected by immunofluorescence. The adhesins are recognized by human leptospirosis serum samples at the onset and convalescent phases of the disease, suggesting that they are expressed during infection. Taken together, our data could offer valuable information to the understanding of leptospiral pathogenesis.

Analysis of Paracoccidioides secreted proteins reveals fructose 1,6-bisphosphate aldolase as a plasminogen-binding protein

Background

Despite being important thermal dimorphic fungi causing Paracoccidioidomycosis, the pathogenic mechanisms that underlie the genus Paracoccidioides remain largely unknown. Microbial pathogens express molecules that can interact with human plasminogen, a protein from blood plasma, which presents fibrinolytic activity when activated into plasmin. Additionally, plasmin exhibits the ability of degrading extracellular matrix components, favoring the pathogen spread to deeper tissues. Previous work from our group demonstrated that Paracoccidioides presents enolase, as a protein able to bind and activate plasminogen, increasing the fibrinolytic activity of the pathogen, and the potential for adhesion and invasion of the fungus to host cells. By using proteomic analysis, we aimed to identify other proteins of Paracoccidioides with the ability of binding to plasminogen.

Results

In the present study, we employed proteomic analysis of the secretome, in order to identify plasminogen-binding proteins of Paracoccidioides, Pb01. Fifteen proteins were present in the fungal secretome, presenting the ability to bind to plasminogen. Those proteins are probable targets of the fungus interaction with the host; thus, they could contribute to the invasiveness of the fungus. For validation tests, we selected the protein fructose 1,6-bisphosphate aldolase (FBA), described in other pathogens as a plasminogen-binding protein. The protein FBA at the fungus surface and the recombinant FBA (rFBA) bound human plasminogen and promoted its conversion to plasmin, potentially increasing the fibrinolytic capacity of the fungus, as demonstrated in fibrin degradation assays. The addition of rFBA or anti-rFBA antibodies was capable of reducing the interaction between macrophages and Paracoccidioides, possibly by blocking the binding sites for FBA. These data reveal the possible participation of the FBA in the processes of cell adhesion and tissue invasion/dissemination of Paracoccidioides.

Conclusions

These data indicate that Paracoccidioides is a pathogen that has several plasminogen-binding proteins that likely play important roles in pathogen-host interaction. In this context, FBA is a protein that might be involved somehow in the processes of invasion and spread of the fungus during infection.

Novel Leptospira interrogans protein Lsa32 is expressed during infection and binds laminin and plasminogen

Pathogenic Leptospira is the aetiological agent of leptospirosis, a life-threatening disease of human and veterinary concern. The quest for novel antigens that could mediate host-pathogen interactions is being pursued. Owing to their location, these antigens have the potential to elicit numerous activities, including immune response and adhesion. This study focuses on a hypothetical protein of Leptospira, encoded by the gene LIC11089, and its three derived fragments: the N-terminal, intermediate and C terminus regions. The gene coding for the full-length protein and fragments was cloned and expressed in Escherichia coli BL21(SI) strain by using the expression vector pAE. The recombinant protein and fragments tagged with hexahistidine at the N terminus were purified by metal affinity chromatography. The leptospiral full-length protein, named Lsa32 (leptospiral surface adhesin, 32 kDa), adheres to laminin, with the C terminus region being responsible for this interaction. Lsa32 binds to plasminogen in a dose-dependent fashion, generating plasmin when an activator is provided. Moreover, antibodies present in leptospirosis serum samples were able to recognize Lsa32. Lsa32 is most likely a new surface protein of Leptospira, as revealed by proteinase K susceptibility. Altogether, our data suggest that this multifaceted protein is expressed during infection and may play a role in host-L. interrogans interactions.

The molecular basis of leptospiral pathogenesis

The mechanisms of disease pathogenesis in leptospirosis are poorly defined. Recent developments in the application of genetic tools in the study of Leptospira have advanced our understanding by allowing the assessment of mutants in animal models. As a result, a small number of essential virulence factors have been identified, though most do not have a clearly defined function. Significant advances have also been made in the in vitro characterization of leptospiral interaction with host structures, including extracellular matrix proteins (such as laminin, elastin, fibronectin, collagens), proteins related to hemostasis (fibrinogen, plasmin), and soluble mediators of complement resistance (factor H, C4b-binding protein), although none of these in vitro findings has been translated to the host animal. Binding to host structures may permit colonization of the host, prevention of blood clotting may contribute to hemorrhage, while interaction with complement resistance mediators may contribute to survival in serum. While not a classical intracellular pathogen, the interaction of leptospires and phagocytic cells appears complex, with bacteria surviving uptake and promoting apoptosis; mutants relating to these processes (such as cell invasion and oxidative stress resistance) are attenuated in vivo. Another feature of leptospiral biology is the high degree of functional redundancy and the surprising lack of attenuation of mutants in what appear to be certain virulence factors, such as LipL32 and LigB. While many advances have been made, there remains a lack of understanding of how Leptospira causes tissue pathology. It is likely that leptospires have many novel pathogenesis mechanisms that are yet to be identified.

Plasminogen activation by staphylokinase enhances local spreading of S. aureus in skin infections

BACKGROUND

Staphylococcus aureus (S. aureus) is a frequent cause of skin and soft tissue infections. A unique feature of S. aureus is the combined presence of coagulases that trigger fibrin formation and of the plasminogen activator staphylokinase (SAK). Whereas the importance of fibrin generation for S. aureus virulence has been established, the role of SAK remains unclear. We studied the role of plasminogen activation by SAK in a skin infection model in mice and evaluated the impact of alpha-2-antiplasmin (α2AP) deficiency on the spreading and proteolytic activity of S. aureus skin infections. The species-selectivity of SAK was overcome by adenoviral expression of human plasminogen. Bacterial spread and density was assessed non-invasively by imaging the bioluminescence of S. aureus Xen36.

RESULTS

SAK-mediated plasmin activity increased the local invasiveness of S. aureus, leading to larger lesions with skin disruption as well as decreased bacterial clearance by the host. Even though fibrin and bacterial surfaces protected SAK-mediated plasmin activity from inhibition by α2AP, the deficiency of α2AP resulted in increased bacterial spreading. SAK-mediated plasmin also induced secondary activation of gelatinases, shown both in vitro and in lesions from the in vivo model.

CONCLUSION

SAK contributes to the phenotype of S. aureus skin infections by enhancing bacterial spreading as a result of fibrinolytic and proteolytic activation.

Intermediate and C-terminal regions of leptospiral adhesin Lsa66 are responsible for binding with plasminogen and extracellular matrix components

Leptospirosis, a worldwide zoonotic infection, is an important human and veterinary health problem. We have previously identified a leptospiral multipurpose adhesin, Lsa66, capable of binding extracellular matrix (ECM) components and plasminogen (PLG). In this work, we report the cloning, expression, purification and characterization of three fragments derived from the full-length Lsa66: N-terminal, intermediate and C-terminal regions. We employed Escherichia coli BL21-SI as expression cells. The recombinant fragments tagged with N-terminal His6 were purified by metal-charged chromatography to major protein bands that were recognized by anti-His-tag mAbs. The recombinant fragments were evaluated for their capacity to attach to ECM components and to PLG. The intermediate region bound to laminin, plasma fibronectin and PLG. Laminin also bound to the C-terminal region. Antibodies in leptospirosis-positive serum samples recognized Lsa66, being the immune epitopes located at the N-terminal and intermediate fragments. The data confirm that Lsa66 is expressed during infection and that this protein might have a role in bacterial infection.

Leptospira interrogans binds to cadherins

Leptospirosis, caused by pathogenic species of Leptospira, is the most widespread zoonosis and has emerged as a major public health problem worldwide. The adhesion of pathogenic Leptospira to host cells, and to extracellular matrix (ECM) components, is likely to be necessary for the ability of leptospires to penetrate, disseminate and persist in mammalian host tissues. Previous work demonstrated that pathogenic L. interrogans binds to host cells more efficiently than to ECM. Using two independent screening methods, mass spectrometry and protein arrays, members of the cadherin family were identified as potential L. interrogans receptors on mammalian host surfaces. We focused our investigation on vascular endothelial (VE)-cadherin, which is widely expressed on endothelia and is primarily responsible for endothelial cell-cell adhesion. Monolayers of EA.hy926 and HMEC-1 endothelial cells produce VE-cadherin, bind L. interrogans in vitro, and are disrupted upon incubation with the bacteria, which may reflect the endothelial damage seen in vivo. Dose-dependent and saturable binding of L. interrogans to the purified VE-cadherin receptor was demonstrated and pretreatment of purified receptor or endothelial cells with function-blocking antibody against VE-cadherin significantly inhibited bacterial attachment. The contribution of VE-cadherin to leptospiral adherence to host endothelial cell surfaces is biologically significant because VE-cadherin plays an important role in maintaining the barrier properties of the vasculature. Attachment of L. interrogans to the vasculature via VE-cadherin may result in vascular damage, facilitating the escape of the pathogen from the bloodstream into different tissues during disseminated infection, and may contribute to the hemorrhagic manifestations of leptospirosis. This work is first to describe a mammalian cell surface protein as a receptor for L. interrogans.

Functional and immunological evaluation of two novel proteins of Leptospira spp

This work shows the production and characterization of two novel putative lipoproteins encoded by the genes LIC10645 and LIC10731 identified in the genome sequences of Leptospira interrogans. In silico conservation analysis indicated that the proteins are well conserved among pathogenic leptospiral serovars and species. Recombinant proteins were obtained in Escherichia coli BL21(DE3) Star pLysS strain, purified by metal-affinity chromatography, and used for characterization and immunological evaluations. Recombinant proteins were capable of eliciting a combination of humoral and cellular immune responses in animal models, and could be recognized by antibodies present in human serum samples. The recombinant proteins Lsa44 and Lsa45 were able to bind laminin, and were named Lsa44 and Lsa45 for leptospiral surface adhesins of 44 and 45 kDa, respectively. The attachment to laminin was dose-responsive with K D values of 108.21 and 250.38 nM for Lsa44 and Lsa45, respectively. Moreover, these proteins interact with plasminogen (PLG) with K D values of 53.56 and 36.80 nM, respectively. PLG bound to the recombinant proteins could be converted to plasmin (PLA) in the presence of an activator. Cellular localization assays suggested that the Lsa44 and Lsa45 were surface-exposed. These are versatile proteins capable of interacting with laminin and PLG/PLA, and hence could mediate bacterial adhesion and contribute to tissue penetration.

High-temperature protein G is an essential virulence factor of Leptospira interrogans

Leptospira interrogans is a global zoonotic pathogen and is the causative agent of leptospirosis, an endemic disease of humans and animals worldwide. There is limited understanding of leptospiral pathogenesis; therefore, further elucidation of the mechanisms involved would aid in vaccine development and the prevention of infection. HtpG (high-temperature protein G) is the bacterial homolog to the highly conserved molecular chaperone Hsp90 and is important in the stress responses of many bacteria. The specific role of HtpG, especially in bacterial pathogenesis, remains largely unknown. Through the use of an L. interrogans htpG transposon insertion mutant, this study demonstrates that L. interrogans HtpG is essential for virulence in the hamster model of acute leptospirosis. Complementation of the htpG mutant completely restored virulence. Surprisingly, the htpG mutant did not appear to show sensitivity to heat or oxidative stress, phenotypes common in htpG mutants in other bacterial species. Furthermore, the mutant did not show increased sensitivity to serum complement, reduced survival within macrophages, or altered protein or lipopolysaccharide expression. The underlying cause for attenuation thus remains unknown, but HtpG is a novel leptospiral virulence factor and one of only a very small number identified to date.

Leptospiral extracellular matrix adhesins as mediators of pathogen-host interactions

Leptospirosis is been considered an important infectious disease that affects humans and animals worldwide. This review summarizes our current knowledge of bacterial attachment to extracellular matrix (ECM) components and discusses the possible role of these interactions for leptospiral pathogenesis. Leptospiral proteins show different binding specificity for ECM molecules: some are exclusive laminin-binding proteins (Lsa24/LfhA/LenA, Lsa27), while others have broader spectrum binding profiles (LigB, Lsa21, LipL53). These proteins may play a primary role in the colonization of host tissues. Moreover, there are multifunctional proteins that exhibit binding activities toward a number of target proteins including plasminogen/plasmin and regulators of the complement system, and as such, might also act in bacterial dissemination and immune evasion processes. Many ECM-interacting proteins are recognized by human leptospirosis serum samples indicating their expression during infection. This compilation of data should enhance our understanding of the molecular mechanisms of leptospiral pathogenesis.

Responses of murine and human macrophages to leptospiral infection: a study using comparative array analysis

Leptospirosis is a re-emerging tropical infectious disease caused by pathogenic Leptospira spp. The different host innate immune responses are partially related to the different severities of leptospirosis. In this study, we employed transcriptomics and cytokine arrays to comparatively calculate the responses of murine peritoneal macrophages (MPMs) and human peripheral blood monocytes (HBMs) to leptospiral infection. We uncovered a series of different expression profiles of these two immune cells. The percentages of regulated genes in several biological processes of MPMs, such as antigen processing and presentation, membrane potential regulation, and the innate immune response, etc., were much greater than those of HBMs (>2-fold). In MPMs and HBMs, the caspase-8 and Fas-associated protein with death domain (FADD)-like apoptosis regulator genes were significantly up-regulated, which supported previous results that the caspase-8 and caspase-3 pathways play an important role in macrophage apoptosis during leptospiral infection. In addition, the key component of the complement pathway, C3, was only up-regulated in MPMs. Furthermore, several cytokines, e.g. interleukin 10 (IL-10) and tumor necrosis factor alpha (TNF-alpha), were differentially expressed at both mRNA and protein levels in MPMs and HBMs. Some of the differential expressions were proved to be pathogenic Leptospira-specific regulations at mRNA level or protein level. Though it is still unclear why some animals are resistant and others are susceptible to leptospiral infection, this comparative study based on transcriptomics and cytokine arrays partially uncovered the differences of murine resistance and human susceptibility to leptospirosis. Taken together, these findings will facilitate further molecular studies on the innate immune response to leptospiral infection.

Although pathogenic Leptospira is not an obligate intracellular pathogen, recent studies have shown that phagocytosis and innate immunity play important roles in leptospirosis. The Leptospira-macrophage interaction is a common model used to elucidate the initial response in leptospiral infection. Our previous research has shown that there is little difference in the transcriptomics of pathogenic Leptospira infecting murine or human macrophage cell lines. Contrarily, in this study, we observed significant differences of murine and human primary macrophages infected by L. interrogans as shown in several processes, such as antigen processing and presentation, Toll-like receptor signaling pathway and innate immune response, complement and coagulation cascades, expression of major cytokines and chemokines, etc. These results suggested that different immune responses explain the major disparities in the murine and human Leptospira-macrophage infection models. This study added to the former leptospiral transcriptomics research on the Leptospira-macrophage interaction model and laid a foundation for further investigation in the pathogenesis of leptospirosis.

Adhesins of Leptospira interrogans mediate the interaction to fibrinogen and inhibit fibrin clot formation in vitro

We report in this work that Leptospira strains, virulent L. interrogans serovar Copenhageni, attenuated L. interrogans serovar Copenhageni and saprophytic L. biflexa serovar Patoc are capable of binding fibrinogen (Fg). The interaction of leptospires with Fg inhibits thrombin- induced fibrin clot formation that may affect the haemostatic equilibrium. Additionally, we show that plasminogen (PLG)/plasmin (PLA) generation on the surface of Leptospira causes degradation of human Fg. The data suggest that PLA-coated leptospires were capable to employ their proteolytic activity to decrease one substrate of the coagulation cascade. We also present six leptospiral adhesins and PLG- interacting proteins, rLIC12238, Lsa33, Lsa30, OmpL1, rLIC11360 and rLIC11975, as novel Fg-binding proteins. The recombinant proteins interact with Fg in a dose-dependent and saturable fashion when increasing protein concentration was set to react to a fix human Fg concentration. The calculated dissociation equilibrium constants (K_D) of these reactions ranged from 733.3±276.8 to 128±89.9 nM for rLIC12238 and Lsa33, respectively. The interaction of recombinant proteins with human Fg resulted in inhibition of fibrin clot by thrombin-catalyzed reaction, suggesting that these versatile proteins could mediate Fg interaction in Leptospira. Our data reveal for the first time the inhibition of fibrin clot by Leptospira spp. and presents adhesins that could mediate these interactions. Decreasing fibrin clot would cause an imbalance of the coagulation cascade that may facilitate bleeding and help bacteria dissemination

Leptospirosis is probably the most widespread zoonosis in the world. Caused by spirochaetes of the genus Leptospira, it has greater incidence in tropical and subtropical regions. The disease has become prevalent in cities with sanitation problems and a large population of urban rodent reservoirs, which contaminate the environment through their urine. Understanding the mechanisms involved in pathogenesis of leptospirosis should contribute to new strategies that would help fight the disease. We show in this work that Leptospira strains, virulent, attenuated or saprophytic are capable of binding fibrinogen (Fg). The interaction of leptospires with Fg inhibits the formation of fibrin clot that may result of an imbalance in the haemostatic equilibrium. In addition, we show that plasminogen (PLG)/plasmin (PLA) generation on the surface of leptospires can lead to Fg degradation, showing evidence of possible route of fibrinolysis in leptospirosis. We also present six leptospiral proteins, as novel Fg-binding proteins, capable of inhibiting fibrin clot formation by thrombin-catalyzed reaction, suggesting that in Leptospira these multifunctional proteins could mediate Fg interaction. Our data suggest possible mechanisms that leptospires could employ to affect the coagulation cascade and fibrinolytic system that might lead to bacteria spreading.

Characterization of three novel adhesins of Leptospira interrogans

We report cloning, expression, purification, and characterization of three predicted leptospiral membrane proteins (LIC11360, LIC11009, and LIC11975). In silico analysis and proteinase K accessibility data suggest that these proteins might be surface exposed. We show that proteins encoded by LIC11360, LIC11009 and LIC11975 genes interact with laminin in a dose-dependent and saturable manner. The proteins are referred to as leptospiral surface adhesions 23, 26, and 36 (Lsa23, Lsa26, and Lsa36), respectively. These proteins also bind plasminogen and generate active plasmin. Attachment of Lsa23 and Lsa36 to fibronectin occurs through the involvement of the 30-kDa and 70-kDa heparin-binding domains of the ligand. Dose-dependent, specific-binding of Lsa23 to the complement regulator C4BP and to a lesser extent, to factor H, suggests that this protein may interfere with the complement cascade pathways. Leptospira spp. may use these interactions as possible mechanisms during the establishment 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.

Pathogenic, diagnostic and vaccine potential of leptospiral outer membrane proteins (OMPs)

Pathogenic Leptospira species are important human and animal pathogen that causes leptospirosis, with more than half a million cases reported annually but little is known regarding the true incidence of leptospirosis due to the limitations in diagnosis. Proteins embedded in the outer membrane are found to be prime drug targets due to its key role as receptors for cellular communication and gatekeepers for iron and substrate transport across cell membranes. The major key issues to be addressed to overcome the disease burden of leptospirosis are: need to identify the genes that turn on in vivo; development of rapid diagnostic methods to facilitate the early diagnosis and to develop a universal vaccine. Recent whole genome sequencing of Leptospira species and development of in silico analysis tools have led to the identification of a large number of leptospiral virulence genes, metabolic pathways and surface protein secretion systems that represent potential new targets for the development of anti-leptospiral drug, vaccine and diagnostic strategies. This review surveys the different types of outer membrane proteins (OMPs) of Leptospira and combines all the novel features of OMPs reported till date and put forth some views for future research.

Adhesin activity of Leptospira interrogans lipoprotein identified by in vivo and in vitro shotgun phage display

Leptospira interrogans causes leptospirosis, one of the most common zoonotic diseases in the world. This pathogenic spirochete is able to bind to extracellular matrix, to express virulent factors and to cause host death. Until now, there is no effective human vaccine for the disease. Shotgun phage display genomic libraries of L. interrogans were constructed and used for in vivo biopanning in hamsters and screened for ligands able to bind to LLC-PK1 epithelial cells. In both panning procedures, clones coding for the putative lipoprotein LIC12976 were identified and, in order to confirm its adhesin activity, a recombinant protein was produced in Escherichia coli and showed to interact with A31 fibroblasts, LLC-PK1 and Vero epithelial cells in vitro. Moreover, rLIC12976 was shown to bind to laminin, indicating an adhesin function. This protein was also detected in extracts of L. interrogans from different serovars and it was found to be conserved among pathogenic leptospires. Further, the protein was tested as vaccine candidate and immunization of hamsters with LIC12976 did not confer protection against a lethal challenge with the homologous L. interrogans serovar Copenhageni. Nevertheless, LIC12976 seems to act as an adhesin, and may be important for the host-pathogen interaction, so that its study can contribute to the understanding of the virulence mechanisms in pathogenic leptospires.