A comprehensive approach to identification of surface-exposed, outer membrane-spanning proteins of Leptospira interrogans

Characterisation of common hypothetical surface peptides between protozoan parasites (Perkinsus olseni) originating from different geographical locations

Perkinsus olseni and P. marinus are classified as notifiable pathogens by the World Organisation for Animal Health and are known to cause perkinsosis in a variety of molluscs globally. Mass mortalities due to these parasites in farms and in the wild have been a recurrent issue. Diagnosis for these protozoans is currently done using Ray's fluid thioglycollate medium method followed by optical microscopy or molecular assays. Both require a high level of skill and are time-consuming. An immunoassay method would make the diagnosis of perkinsosis quicker and cheaper. The present study used mass spectrometry-based proteomics to investigate common hypothetical surface peptides between different geographical isolates of P. olseni, which could be used to develop immunoassays in the future. Two peptides were identified: POLS_08089, which is a 42.7 kDa peptide corresponding to the 60S ribosomal subunit protein L4; and POLS_15916, which is a conserved hypothetical protein of 55.6 kDa. The identification of peptides may allow the development of immunoassays through a more targeted approach.

Components Subcellular Localization: Cell Surface Exposure

Surface-exposed proteins of Gram-negative bacteria are represented by integral outer membrane β-barrel proteins and lipoproteins. There are no computational methods to predict surface-exposed lipoproteins, and therefore lipoprotein topology must be experimentally tested. This chapter describes several distinct but complementary methods for detection of surface-exposed proteins: cell surface protein labeling, accessibility to extracellular protease or antibodies, and SpyTag/SpyCatcher system.

Leptospiral cell wall hydrolase (LIC_10271) binding peptidoglycan, lipopolysaccharide, and laminin and the protein show LysM and M23 domains are co-existing in pathogenic species

Leptospirosis, a global reemerging zoonosis caused by the spirochete Leptospira, has severe human and veterinary implications. Cell wall hydrolase (LIC_10271) with LytM (peptidase M23) and LysM domains are found to be associated with various pathogenic bacteria. These domains regulate effects on extracellular matrix and biofilm components, which promote cell wall remodeling and pathogen dissemination in the host. In this study, we present the cloning, expression, purification, and characterization of LIC_10271. To determine the localization of LIC_10271 within the inner membrane of Leptospira, Triton X-114 subcellular fractionation and immunoblot studies were performed. Furthermore, r-LIC_10271 binds with peptidoglycan, lipopolysaccharide, and laminin in a dose-dependent manner. Analysis of the signal peptide, M23, and LysM domains revealed conservation primarily within the P1 group of Leptospira, which encompasses the most pathogenic species. Moreover, the presence of native-LIC_10271 in the inner membrane and the distribution of M23 and LysM domains across pathogenic strains indicates their potential involvement in the interaction between the host and Leptospira.

Leptospiral imelysin (LIC_10713) is secretory, immunogenic and binds to laminin, fibronectin, and collagen IV

Leptospirosis is a widespread zoonotic disease caused by pathogenic Leptospira. Early and accurate diagnosis is the prime step in managing the disease. Secretory proteins of Leptospira remain distinguished for diagnosis due to their availability as soluble proteins in the serum and their interaction with the host immune response due to their extracellular presence. This study presents the cloning, expression, purification, and characterization of imelysin or LruB (LIC_10713), a putative leptospiral protein. We report that the localization of imelysin showed its presence in the inner membrane and in the culture supernatant. The imelysin was upregulated under in vitro physiological conditions of infection. The LIC_10713 interacted significantly with laminin, fibronectin, collagen type I, and collagen type IV in a dose-dependent manner. Phylogenetic analysis showed that LIC_10713 is predominately found in the pathogenic species of Leptospira, and the GxHxxE motif of imelysin-like proteins is represented as the amino acid sequence G_WH_AIE. Also, immunoglobulins in leptospirosis-infected patients recognize recombinant-LIC_10713 with 100% specificity and 90.9% sensitivity. The secretion nature, abundance, upregulation, binding to ECM components, and immunogenicity determine LIC_10713 as an important molecule that can be used as an anti-leptospirosis measure. KEY POINTS: • The imelysin-like protein (LIC_10713) of Leptospira is a secretory protein • The protein LIC_10713 can bind ECM molecules • The LIC_10713 is mainly found in pathogenic leptospires • The anti-LIC_10713 antibody from human serum can detect the r-LIC_10713.

Characterization of Treponema denticola Major Surface Protein (Msp) by Deletion Analysis and Advanced Molecular Modeling

Treponema denticola, a keystone pathogen in periodontitis, is a model organism for studying Treponema physiology and host-microbe interactions. Its major surface protein Msp forms an oligomeric outer membrane complex that binds fibronectin, has cytotoxic pore-forming activity, and disrupts several intracellular processes in host cells. T. denticola msp is an ortholog of the Treponema pallidum tprA to -K gene family that includes tprK, whose remarkable in vivo hypervariability is proposed to contribute to T. pallidum immune evasion. We recently identified the primary Msp surface-exposed epitope and proposed a model of the Msp protein as a β-barrel protein similar to Gram-negative bacterial porins. Here, we report fine-scale Msp mutagenesis demonstrating that both the N and C termini as well as the centrally located Msp surface epitope are required for native Msp oligomer expression. Removal of as few as three C-terminal amino acids abrogated Msp detection on the T. denticola cell surface, and deletion of four residues resulted in complete loss of detectable Msp. Substitution of a FLAG tag for either residues 6 to 13 of mature Msp or an 8-residue portion of the central Msp surface epitope resulted in expression of full-length Msp but absence of the oligomer, suggesting roles for both domains in oligomer formation. Consistent with previously reported Msp N-glycosylation, proteinase K treatment of intact cells released a 25 kDa polypeptide containing the Msp surface epitope into culture supernatants. Molecular modeling of Msp using novel metagenome-derived multiple sequence alignment (MSA) algorithms supports the hypothesis that Msp is a large-diameter, trimeric outer membrane porin-like protein whose potential transport substrate remains to be identified. IMPORTANCE The Treponema denticola gene encoding its major surface protein (Msp) is an ortholog of the T. pallidum tprA to -K gene family that includes tprK, whose remarkable in vivo hypervariability is proposed to contribute to T. pallidum immune evasion. Using a combined strategy of fine-scale mutagenesis and advanced predictive molecular modeling, we characterized the Msp protein and present a high-confidence model of its structure as an oligomer embedded in the outer membrane. This work adds to knowledge of Msp-like proteins in oral treponemes and may contribute to understanding the evolutionary and potential functional relationships between T. denticola Msp and the orthologous T. pallidum Tpr proteins.

Insight into the Structure, Functions, and Dynamics of the Leptospira Outer Membrane Proteins with the Pathogenicity

Leptospirosis is a widespread zoonosis that frequently occurs in tropical and subtropical countries. Leptospira enters the host through wounds or mucous membranes and spreads to the whole body through the blood, causing systemic infection. Kidneys are the preferential site where Leptospira accumulates, especially in the renal interstitium and renal tubule epithelial cells. Clinical symptoms in humans include high fever, jaundice, renal failure, and severe multiple-organ failure (Weil’s syndrome). Surface-exposed antigens are located at the outermost layer of Leptospira and these potential virulence factors are likely involved in primary host-pathogen interactions, adhesion, and/or invasion. Using the knockout/knockdown techniques to the evaluation of pathogenicity in the virulence factor are the most direct and effective methods and many virulence factors are evaluated including lipopolysaccharides (LPS), Leptospira lipoprotein 32 (LipL32), Leptospira ompA domain protein 22 (Loa22), LipL41, LipL71, Leptospira immunoglobulin-like repeat A (LigA), LigB, and LipL21. In this review, we will discuss the structure, functions, and dynamics of these virulence factors and the roles of these virulence factors in Leptospira pathogenicity. In addition, a protein family with special Leucine-rich repeat (LRR) will also be discussed for their vital role in Leptospira pathogenicity. Finally, these surface-exposed antigens are discussed in the application of the diagnosis target for leptospirosis and compared with the serum microscope agglutination test (MAT), the gold standard for leptospirosis.

Revisiting the Development of Vaccines Against Pathogenic Leptospira: Innovative Approaches, Present Challenges, and Future Perspectives

Human vaccination against leptospirosis has been relatively unsuccessful in clinical applications despite an expressive amount of vaccine candidates has been tested over years of research. Pathogenic Leptospira encompass a great number of serovars, most of which do not cross-react, and there has been a lack of genetic tools for many years. These obstacles have hampered the understanding of the bacteria's biology and, consequently, the identification of an effective antigen. Thus far, many approaches have been used in an attempt to find a cost-effective and broad-spectrum protective antigen(s) against the disease. In this extensive review, we discuss several strategies that have been used to develop an effective vaccine against leptospirosis, starting with Leptospira-inactivated bacterin, proteins identified in the genome sequences of pathogenic Leptospira, including reverse vaccinology, plasmid DNA, live vaccines, chimeric multi-epitope, and toll- and nod-like receptors agonists. This overview should be able to guide scientists working in the field to select potential antigens and to choose the appropriate formulation to administer the candidates.

Surface proteomics and label-free quantification of Leptospira interrogans serovar Pomona

Leptospirosis is a re-emerging zoonosis with a global distribution. Surface-exposed outer membrane proteins (SE-OMPs) are crucial for bacterial-host interactions. SE-OMPs locate and expose their epitope on cell surface where is easily accessed by host molecules. This study aimed to screen for surface-exposed proteins and their abundance profile of pathogenic Leptospira interrogans serovar Pomona. Two complementary approaches, surface biotinylation and surface proteolytic shaving, followed by liquid chromatography tandem-mass spectrometry (LC-MS/MS) were employed to identify SE-OMPs of intact leptospires. For quantitative comparison, in-depth label-free analysis of SE-OMPs obtained from each method was performed using MaxQuant. The total number of proteins identified was 1,001 and 238 for surface biotinylation and proteinase K shaving, respectively. Among these, 39 were previously known SE-OMPs and 68 were predicted to be localized on the leptospiral surface. Based on MaxQuant analysis for relative quantification, six known SE-OMPs including EF- Tu, LipL21, LipL41, LipL46, Loa22, and OmpL36, and one predicted SE-OMPs, LipL71 were found in the 20 most abundant proteins, in which LipL41 was the highest abundant SE-OMP. Moreover, uncharacterized LIC14011 protein (LIP3228 ortholog in serovar Pomona) was identified as a novel predicted surface βb-OMP. High-abundance leptospiral SE-OMPs identified in this study may play roles in virulence and infection and are potential targets for development of vaccine or diagnostic tests for leptospirosis.

Identification of leptospiral protein antigens recognized by WC1+ γδ T cell subsets as target for development of recombinant vaccines

Pathogenic Leptospira species cause leptospirosis, a neglected zoonotic disease recognized as a global public health problem. It is also the cause of the most common cattle infection that results in major economic losses due to reproductive problems. γδ T cells play a role in the protective immune response in livestock species against Leptospira while human γδ T cells also respond to Leptospira. Thus, activation of γδ T cells has emerged as a potential component in the optimization of vaccine strategies. Bovine γδ T cells proliferate and produce IFN-γ in response to vaccination with inactivated leptospires and this response is mediated by a specific subpopulation of the WC1-bearing γδ T cells. WC1 molecules are members of the group B scavenger receptor cysteine rich (SRCR) superfamily and are composed of multiple SRCR domains, of which particular extracellular domains act as ligands for Leptospira. Since WC1 molecules function as both pattern recognition receptors and γδ TCR coreceptors, the WC1 system has been proposed as a novel target to engage γδ T cells. Here, we demonstrate the involvement of leptospiral protein antigens in the activation of WC1⁺ γδ T cells and identified two leptospiral outer membrane proteins able to interact directly with them. Interestingly, we show that the protein-specific γδ T cell response is composed of WC1.1⁺ and WC1.2⁺ subsets, although a greater number of WC1.1⁺ ???? T-cell respond. Identification of protein antigens will enhance our understanding of the role γδ T cells play in the leptospiral immune response and in recombinant vaccine development.

The Leptospiral General Secretory Protein D (GspD), a secretin, elicits complement-independent bactericidal antibody against diverse Leptospira species and serovars

Leptospirosis, the most common zoonotic infection worldwide, is a multi-system disorder affecting the kidney, liver, and lungs. Infections can be asymptomatic, self-limiting or progress to multi-organ system failure and pulmonary hemorrhage. The incidence of canine and human leptospirosis is steadily increasing worldwide. At least sixty-four Leptospira species and several hundred lipopolysaccharide-based serovars have been defined. Preventive vaccines are available for use in veterinary medicine and limited use in humans in some countries. All commercially available vaccines are bacterin formulations that consist of a combination of laboratory cultivated strains of different lipopolysaccharide serotypes. The development of a broadly protective subunit vaccine would represent a significant step forward in efforts to combat leptospirosis in humans, livestock, and companion animals worldwide. Here we investigate the potential of General secretory protein D (GspD; LIC11570), a secretin, to serve as a possible antigen in a multi-valent vaccine formulation. GspD is conserved, expressed in vitro, antigenic during infection and elicits antibody with complement independent bactericidal activity. Importantly, antibody to GspD is bactericidal against diverse Leptospira species of the P1 subclade. Epitope mapping localized the bactericidal epitopes to the N-terminal N0 domain of GspD. The data within support further exploration of GspD as a candidate for inclusion in a next generation multi-protein subunit vaccine.

Leptospiral Infection, Pathogenesis and Its Diagnosis-A Review

Leptospirosis is a perplexing conundrum for many. In the existing literature, the pathophysiological mechanisms pertaining to leptospirosis is still not understood in full. Considered as a neglected tropical zoonotic disease, leptospirosis is culminating as a serious problem worldwide, seemingly existing as co-infections with various other unrelated diseases, including dengue and malaria. Misdiagnosis is also common as non-specific symptoms are documented extensively in the literature. This can easily lead to death, as the severe form of leptospirosis (Weil's disease) manifests as a complex of systemic complications, especially renal failure. The virulence of Leptospira sp. is usually attributed to the outer membrane proteins, including LipL32. With an armament of virulence factors at their disposal, their ability to easily adhere, invade and replicate within cells calls for a swift refinement in research progress to establish their exact pathophysiological framework. As an effort to reconstitute the current knowledge on leptospirosis, the basis of leptospiral infection, including its risk factors, classification, morphology, transmission, pathogenesis, co-infections and clinical manifestations are highlighted in this review. The various diagnostic techniques are also outlined with emphasis on their respective pros and cons.

GspD, The Type II Secretion System Secretin of Leptospira, Protects Hamsters against Lethal Infection with a Virulent L. interrogans Isolate

The wide variety of pathogenic Leptospira serovars and the weak protection offered by the available vaccines encourage the search for protective immunogens against leptospirosis. We found that the secretin GspD of the type II secretion system (T2S) of Leptospira interrogans serovar Canicola was highly conserved amongst pathogenic serovars and was expressed in vivo during infection, as shown by immunohistochemistry. Convalescent sera of hamsters, dogs, and cows showed the presence of IgG antibodies, recognizing a recombinant version of this protein expressed in Escherichia coli (rGspDLC) in Western blot assays. In a pilot vaccination study, a group of eight hamsters was immunized on days zero and 14 with 50 µg of rGspDLC mixed with Freund’s incomplete adjuvant (FIA). On day 28 of the study, 1,000 LD50 (Lethal Dose 50%) of a virulent strain of Leptospira interrogans serovar Canicola (LOCaS46) were inoculated by an intraoral submucosal route (IOSM). Seventy-five percent protection against disease (p = 0.017573, Fisher’s exact test) and 50% protection against infection were observed in this group of vaccinated hamsters. In contrast, 85% of non-vaccinated hamsters died six to nine days after the challenge. These results suggest the potential usefulness of the T2S secretin GspD of Leptospira as a protective recombinant vaccine against leptospirosis.

Triton X-114 Fractionated Subcellular Proteome of Leptospira interrogans Shows Selective Enrichment of Pathogenic and Outer Membrane Proteins in the Detergent Fraction

The Triton X-114-based solubilization and temperature-dependent phase separation of proteins is used for subcellular fractionation where, aqueous, detergent, and pellet fractions represents cytoplasmic, outer membrane (OM), and inner membrane proteins, respectively. Mass spectrometry-based proteomic analysis of Triton X-114 fractions of proteomic analysis of Leptospira interrogans identified 2957 unique proteins distributed across the fractions. The results are compared with bioinformatics predictions on their subcellular localization and pathogenic nature. Analysis of the distribution of proteins across the Triton X-114 fractions with the predicted characteristics is performed based on "number" of unique type of proteins, and "quantity" which represents the amount of unique protein. The highest number of predicted outer membrane proteins (OMPs) and pathogenic proteins are found in aqueous and pellet fractions, whereas detergent fraction representing the OM has the highest quantity of OMPs and pathogenic proteins though lower in number than the aqueous and pellet fractions. This leaves the possibility of an upsurge in pathogenic proteins and OMPs on the OM under pathogenic conditions suggesting their potential use to combat leptospirosis. Further, the Triton X-114 subcellular fractions are more correlated to enrichment of pathogenic proteins predicted by MP3 software than predicted localization.

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.

Role of Supramolecule ErpY-Like Lipoprotein of Leptospira in Thrombin-Catalyzed Fibrin Clot Inhibition and Binding to Complement Factors H and I, and Its Diagnostic Potential

Leptospirosis is one of the most widespread zoonoses caused by pathogenic Leptospira spp. In this study, we report that the LIC11966/ErpY-like lipoprotein is a surface-exposed outer membrane protein exclusively present in pathogenic species of Leptospira The recombinant ErpY (rErpY)-like protein is recognized by the immunoglobulins of confirmed leptospirosis sera of diverse hosts (human, bovine, and canine), suggesting the expression of the native leptospiral surface protein during infection. Circular dichroism of pure rErpY-like protein showed the secondary structural integrity to be uncompromised during the purification process. Analysis of the rErpY-like protein by native polyacrylamide gel electrophoresis, chemical cross-linking, dynamic light scattering, and field emission transmission electron microscopy demonstrated it undergoes supramolecular assembly. The rErpY-like protein can bind to diverse host extracellular matrices, and it presented a saturable and strong binding affinity (dissociation constant [K_D ] of 70.45 ± 4.13 nM) to fibrinogen, a central host plasma component involved in blood clotting. In the presence of the rErpY-like supramolecule, thrombin-catalyzed fibrin clot formation is inhibited up to 7%, implying its role in inhibiting blood coagulation during Leptospira infection. In addition, binding of the rErpY-like supramolecule to complement factors H and I suggests the protein also contributes to Leptospira evading innate host defense during infection by inactivating alternative complement pathways. This study reveals that rErpY-like protein is functionally active in the supramolecular state and performs moonlighting activity under the given in vitro conditions.

One particle, two targets: A combined action of functionalised gold nanoparticles, against Pseudomonas fluorescens biofilms

Attempts to deal with the problem of detrimental biofilms using nanoparticle technologies have generally focussed on exploiting biocidal approaches. However, it is now recognised that biofilm matrix-components may be targets for the disruption or dispersion of biofilms. Here, we show that the functionalization of gold nanoparticles with the enzyme, proteinase-K (PK) led to both biocidal and matrix disruption effects within Pseudomonas fluorescens biofilms and released cells. This study highlights the potential mechanisms underpinning the properties of Proteinase-K functionalized gold nanoparticles. With the emergence of biocide-resistant biofilm-forming organisms, novel nanoparticle strategies may provide the ideal solution for disrupting and inactivating biofilm cells, thereby minimising the use of biocides or antibiotics.

Isolation and Identification of Putative Protein Substrates of the AAA+ Molecular Chaperone ClpB from the Pathogenic Spirochaete Leptospira interrogans

Bacterial ClpB is an ATP-dependent Hsp100 chaperone that reactivates aggregated proteins in cooperation with the DnaK chaperone system and promotes survival of bacteria under stress conditions. A large number of publications also indicate that ClpB supports the virulence of bacteria, including a pathogenic spirochaete Leptospira interrogans responsible for leptospirosis in both animals and humans. However, the exact role of ClpB in bacterial pathogenicity remains poorly characterized. It can be assumed that ClpB, due to its role as the molecular chaperone, mediates refolding of essential bacterial proteins, including the known virulence factors, which may become prone to aggregation under infection-induced stresses. In this study, we identified putative substrates of ClpB from L. interrogans (ClpB_Li). For this purpose, we used a proteomic approach combining the ClpB-Trap affinity pull-down assays, Liquid chromatography-tandem mass spectrometry (LC-MS-MS/MS), and bioinformatics analyses. Most of the identified proteins were enzymes predominantly associated with major metabolic pathways like the tricarboxylic acid (TCA) cycle, glycolysis–gluconeogenesis and amino acid and fatty acid metabolism. Based on our proteomic study, we suggest that ClpB can support the virulence of L.interrogans by protecting the conformational integrity and catalytic activity of multiple metabolic enzymes, thus maintaining energy homeostasis in pathogen cells.

Comparative proteome analysis reveals pathogen specific outer membrane proteins of Leptospira

Proteomes of pathogenic Leptospira interrogans and L. borgpetersenii and the saprophytic L. biflexa were filtered through computational tools to identify Outer Membrane Proteins (OMPs) that satisfy the required biophysical parameters for their presence on the outer membrane. A total of 133, 130, and 144 OMPs were identified in L. interrogans, L. borgpetersenii, and L. biflexa, respectively, which forms approximately 4% of proteomes. A holistic analysis of transporting and pathogenic characteristics of OMPs together with Clusters of Orthologous Groups (COGs) among the OMPs and their distribution across 3 species was made and put forward a set of 21 candidate OMPs specific to pathogenic leptospires. It is also found that proteins homologous to the candidate OMPs were also present in other pathogenic species of leptospires. Six OMPs from L. interrogans and 2 from L. borgpetersenii observed to have similar COGs while those were not found in any intermediate or saprophytic forms. These OMPs appears to have role in infection and pathogenesis and useful for anti-leptospiral strategies.

Catecholamine-Modulated Novel Surface-Exposed Adhesin LIC20035 of Leptospira spp. Binds Host Extracellular Matrix Components and Is Recognized by the Host during Infection

In this study, the effect of the host stress hormone catecholamine on Leptospira gene transcripts encoding outer membrane proteins was investigated. There was no impact of catecholamine supplementation on the in vitro growth pattern of Leptospira interrogans; however, 7 genes out of 41 were differentially transcribed, and the effect was reversed to the basal level in the presence of the antagonist propranolol. Comprehensive analysis of one of the differentially regulated proteins, LIC20035 (in serovar Copenhageni)/LB047 (in serovar Lai) (due to catecholamine supplementation), revealed immunogenicity and ability to adhere to host extracellular matrices. Protease accessibility assay and phase partition of integral membrane proteins of Leptospira showed LIC20035/LB047 to be an outer membrane surface-exposed protein. The recombinant LIC20035 protein can be serologically detected using human/bovine sera positive for leptospirosis. Moreover, the recombinant LIC20035 can bind to diverse host extracellular matrices, with a higher affinity toward collagen and chondroitin sulfate.IMPORTANCE Leptospirosis is a neglected tropical disease of global importance. This study aimed to identify outer membrane proteins of pathogenic Leptospira responding to host chemical signals like catecholamines, with the potential to serve as virulence factors, new serodiagnostic antigens, and vaccine candidates. This study mimicked the plausible means by which Leptospira during infection and hormonal stress intercepts host catecholamines to disseminate in host tissues.

Recent findings related to immune responses against leptospirosis and novel strategies to prevent infection

What are the new approaches and emerging ideas to prevent leptospirosis, a neglected bacterial re-emerging zoonotic disease? How do Leptospira interrogans escape the host defenses? We aim here to review and discuss the most recent literature that provides some answers to these questions, in particular data related to a better understanding of adaptive and innate immunity towards leptospires, and design of vaccines. This is an opinion paper, not a comprehensive review. We will try to highlight the new strategies and technologies boosting the search for drugs and vaccines. We will also address the bottlenecks and difficulties impairing the search for efficient vaccines and the many gaps in our knowledge of immunity against leptospirosis. Finally, we aim to delineate how Leptospira spp. escape the innate immune responses of Toll-Like receptors (TLR) and Nod-Like receptors (NLR). The rational use of TLR and NLR agonists as adjuvants could be key to design future vaccines against pathogenic leptospires.

Cell Surface Exposure

Surface-exposed proteins of Gram-negative bacteria are represented by integral outer membrane beta-barrel proteins and lipoproteins. No computational methods exist for predicting surface-exposed lipoproteins, and therefore lipoprotein topology must be experimentally tested. This chapter describes three distinct but complementary methods for the detection of surface-exposed proteins: cell surface protein labeling, accessibility to extracellular protease and antibodies.

Multifunctional and Redundant Roles of Leptospira interrogans Proteins in Bacterial-Adhesion and fibrin clotting inhibition

Pathogenic Leptopira is the etiological agent of leptospirosis, the most widespread zoonotic infection in the world. The disease represents a major public health problem, especially in tropical countries. The present work focused on two hypothetical proteins of unknown function, encoded by the genes LIC13059 and LIC10879, and predicted to be surface-exposed proteins. The genes were cloned and the proteins expressed using E. coli as a host system. We report that the recombinant proteins interacted with extracellular matrix (ECM) laminin, in a dose-dependent fashion and are novel potential adhesins. The recombinant proteins were called Lsa25.6 (rLIC13059) and Lsa16 (rLIC10879), for Leptospiral surface adhesins, followed by the respective molecular masses. The proteins attached to plasminogen (PLG), generating plasmin, in the presence of PLG-activator uPA. Both proteins bind to fibrinogen (Fg), but only Lsa25.6 inhibited fibrin clotting by thrombin-catalyzed reaction. Moreover, Lsa16 interacts with the mammalian cell receptor E-cadherin, and could contribute to bacterial attachment to epithelial cells. The proteins were recognized by confirmed leptospirosis serum samples, suggesting that they are expressed during infection. The corresponding leptospiral proteins are surface exposed based on proteinase K accessibility assay, being LIC10879 most probably exposed in its dimer form. The data of this study extend the spectrum of surface-exposed proteins of L. interrogans and indicate a possible role of the originally annotated hypothetical proteins in infection processes.

A Universal Vaccine against Leptospirosis: Are We Going in the Right Direction?

Leptospirosis is the most widespread zoonosis in the world and a neglected tropical disease estimated to cause severe infection in more than one million people worldwide every year that can be combated by effective immunization. However, no significant progress has been made on the leptospirosis vaccine since the advent of bacterins over 100 years. Although protective against lethal infection, particularly in animals, bacterin-induced immunity is considered short term, serovar restricted, and the vaccine can cause serious side effects. The urgent need for a new vaccine has motivated several research groups to evaluate the protective immune response induced by recombinant vaccines. Significant protection has been reported with several promising outer membrane proteins, including LipL32 and the leptospiral immunoglobulin-like proteins. However, efficacy was variable and failed to induce a cross-protective response or sterile immunity among vaccinated animals. As hundreds of draft genomes of all known Leptospira species are now available, this should aid novel target discovery through reverse vaccinology (RV) and pangenomic studies. The identification of surface-exposed vaccine candidates that are highly conserved among infectious Leptospira spp. is a requirement for the development of a cross-protective universal vaccine. However, the lack of immune correlates is a major drawback to the application of RV to Leptospira genomes. In addition, as the protective immune response against leptospirosis is not fully understood, the rational use of adjuvants tends to be a process of trial and error. In this perspective, we discuss current advances, the pitfalls, and possible solutions for the development of a universal leptospirosis vaccine.

Outer membrane lipoprotein biogenesis: Lol is not the end

Bacterial lipoproteins are lipid-anchored proteins that contain acyl groups covalently attached to the N-terminal cysteine residue of the mature protein. Lipoproteins are synthesized in precursor form with an N-terminal signal sequence (SS) that targets translocation across the cytoplasmic or inner membrane (IM). Lipid modification and SS processing take place at the periplasmic face of the IM. Outer membrane (OM) lipoproteins take the localization of lipoproteins (Lol) export pathway, which ends with the insertion of the N-terminal lipid moiety into the inner leaflet of the OM. For many lipoproteins, the biogenesis pathway ends here. We provide examples of lipoproteins that adopt complex topologies in the OM that include transmembrane and surface-exposed domains. Biogenesis of such lipoproteins requires additional steps beyond the Lol pathway. In at least one case, lipoprotein sequences reach the cell surface by being threaded through the lumen of a beta-barrel protein in an assembly reaction that requires the heteropentomeric Bam complex. The inability to predict surface exposure reinforces the importance of experimental verification of lipoprotein topology and we will discuss some of the methods used to study OM protein topology.

Proteomics-driven Antigen Discovery for Development of Vaccines Against Gonorrhea

Expanding efforts to develop preventive gonorrhea vaccines is critical because of the dire possibility of untreatable gonococcal infections. Reverse vaccinology, which includes genome and proteome mining, has proven very successful in the discovery of vaccine candidates against many pathogenic bacteria. However, progress with this approach for a gonorrhea vaccine remains in its infancy. Accordingly, we applied a comprehensive proteomic platform-isobaric tagging for absolute quantification coupled with two-dimensional liquid chromatography and mass spectrometry-to identify potential gonococcal vaccine antigens. Our previous analyses focused on cell envelopes and naturally released membrane vesicles derived from four different Neisseria gonorrhoeae strains. Here, we extended these studies to identify cell envelope proteins of N. gonorrhoeae that are ubiquitously expressed and specifically induced by physiologically relevant environmental stimuli: oxygen availability, iron deprivation, and the presence of human serum. Together, these studies enabled the identification of numerous potential gonorrhea vaccine targets. Initial characterization of five novel vaccine candidate antigens that were ubiquitously expressed under these different growth conditions demonstrated that homologs of BamA (NGO1801), LptD (NGO1715), and TamA (NGO1956), and two uncharacterized proteins, NGO2054 and NGO2139, were surface exposed, secreted via naturally released membrane vesicles, and elicited bactericidal antibodies that cross-reacted with a panel of temporally and geographically diverse isolates. In addition, analysis of polymorphisms at the nucleotide and amino acid levels showed that these vaccine candidates are highly conserved among N. gonorrhoeae strains. Finally, depletion of BamA caused a loss of N. gonorrhoeae viability, suggesting it may be an essential target. Together, our data strongly support the use of proteomics-driven discovery of potential vaccine targets as a sound approach for identifying promising gonococcal antigens.

Transcriptome Sequencing Reveals Wide Expression Reprogramming of Basal and Unknown Genes in Leptospira biflexa Biofilms

In this work, we describe the first transcriptome based on RNA-seq technology focused on studying transcriptional changes associated with biofilm growth in a member of the genus Leptospira. As many pathogenic species of this genus can survive inside the host but also persist in environmental water, mostly forming biofilms, identifying the molecular basis of this capacity can impact the understanding of how leptospires are able to fulfill a complete life cycle that alternates between adaptation to the host and adaptation to hostile external environmental conditions. We identified several genes and regulatory networks that can be the kickoff for deepening understanding of the molecular mechanisms involving bacterial persistence via biofilm formation; understanding this is important for the future development of tools for controlling leptospirosis.

The genus Leptospira is composed of pathogenic and saprophytic spirochetes. Pathogenic Leptospira is the etiological agent of leptospirosis, a globally spread neglected disease. A key ecological feature of some pathogenic species is their ability to survive both within and outside the host. For most leptospires, the ability to persist outside the host is associated with biofilm formation, a most important bacterial strategy to face and overcome hostile environmental conditions. The architecture and biochemistry of leptospiral biofilms are rather well understood; however, the genetic program underpinning biofilm formation remains mostly unknown. In this work, we used the saprophyte Leptospira biflexa as a model organism to assess over- and underrepresented transcripts during the biofilm state, using transcriptome sequencing (RNA-seq) technology. Our results showed that some basal biological processes like DNA replication and cell division are downregulated in the mature biofilm. Additionally, we identified significant expression reprogramming for genes involved in motility, sugar/lipid metabolism, and iron scavenging, as well as for outer membrane-encoding genes. A careful manual annotation process allowed us to assign molecular functions to many previously uncharacterized genes that are probably involved in biofilm metabolism. We also provided evidence for the presence of small regulatory RNAs in this species. Finally, coexpression networks were reconstructed to pinpoint functionally related gene clusters that may explain how biofilm maintenance is regulated. Beyond elucidating some genetic aspects of biofilm formation, this work reveals a number of pathways whose functional dissection may impact our understanding of leptospiral biology, in particular how these organisms adapt to environmental changes.

IMPORTANCE In this work, we describe the first transcriptome based on RNA-seq technology focused on studying transcriptional changes associated with biofilm growth in a member of the genus Leptospira. As many pathogenic species of this genus can survive inside the host but also persist in environmental water, mostly forming biofilms, identifying the molecular basis of this capacity can impact the understanding of how leptospires are able to fulfill a complete life cycle that alternates between adaptation to the host and adaptation to hostile external environmental conditions. We identified several genes and regulatory networks that can be the kickoff for deepening understanding of the molecular mechanisms involving bacterial persistence via biofilm formation; understanding this is important for the future development of tools for controlling leptospirosis.

Evaluation of the Leptospira interrogans Outer Membrane Protein OmpL37 as a Vaccine Candidate

The identification of potential vaccine candidates against leptospirosis remains a challenge. However, one such candidate is OmpL37, a potentially surface-exposed antigen that has the highest elastin-binding ability described to date, suggesting that it plays an important role in host colonization. In order to evaluate OmpL37's ability to induce a protective immune response, prime-boost, DNA and subunit vaccine strategies were tested in the hamster model of lethal leptospirosis. The humoral immune response was evaluated using an indirect ELISA test, and the cytokine profile in whole blood was determined by quantitative real-time PCR. Unlike the DNA vaccine, the administration of recombinant OmpL37 induced a strong IgG antibody response. When individually administrated, both formulations stimulated a TNF-α mediated inflammatory response. However, none of the OmpL37 formulations or vaccination strategies induced protective immunity. Further studies are required towards the identification of new vaccine targets against leptospirosis.

Life Stage-specific Proteomes of Legionella pneumophila Reveal a Highly Differential Abundance of Virulence-associated Dot/Icm effectors

Major differences in the transcriptional program underlying the phenotypic switch between exponential and post-exponential growth of Legionella pneumophila were formerly described characterizing important alterations in infection capacity. Additionally, a third state is known where the bacteria transform in a viable but nonculturable state under stress, such as starvation. We here describe phase-related proteomic changes in exponential phase (E), postexponential phase (PE) bacteria, and unculturable microcosms (UNC) containing viable but nonculturable state cells, and identify phase-specific proteins. We present data on different bacterial subproteomes of E and PE, such as soluble whole cell proteins, outer membrane-associated proteins, and extracellular proteins. In total, 1368 different proteins were identified, 922 were quantified and 397 showed differential abundance in E/PE. The quantified subproteomes of soluble whole cell proteins, outer membrane-associated proteins, and extracellular proteins; 841, 55, and 77 proteins, respectively, were visualized in Voronoi treemaps. 95 proteins were quantified exclusively in E, such as cell division proteins MreC, FtsN, FtsA, and ZipA; 33 exclusively in PE, such as motility-related proteins of flagellum biogenesis FlgE, FlgK, and FliA; and 9 exclusively in unculturable microcosms soluble whole cell proteins, such as hypothetical, as well as transport/binding-, and metabolism-related proteins. A high frequency of differentially abundant or phase-exclusive proteins was observed among the 91 quantified effectors of the major virulence-associated protein secretion system Dot/Icm (> 60%). 24 were E-exclusive, such as LepA/B, YlfA, MavG, Lpg2271, and 13 were PE-exclusive, such as RalF, VipD, Lem10. The growth phase-related specific abundance of a subset of Dot/Icm virulence effectors was confirmed by means of Western blotting. We therefore conclude that many effectors are predominantly abundant at either E or PE which suggests their phase specific function. The distinct temporal or spatial presence of such proteins might have important implications for functional assignments in the future or for use as life-stage specific markers for pathogen analysis.

The leptospiral outer membrane

The outer membrane (OM) is the front line of leptospiral interactions with their environment and the mammalian host. Unlike most invasive spirochetes, pathogenic leptospires must be able to survive in both free-living and host-adapted states. As organisms move from one set of environmental conditions to another, the OM must cope with a series of conflicting challenges. For example, the OM must be porous enough to allow nutrient uptake, yet robust enough to defend the cell against noxious substances. In the host, the OM presents a surface decorated with adhesins and receptors for attaching to, and acquiring, desirable host molecules such as the complement regulator, Factor H.Factor H. On the other hand, the OM must enable leptospires to evade detection by the host's immune system on their way from sites of invasion through the bloodstream to the protected niche of the proximal tubule. The picture that is emerging of the leptospiral OM is that, while it shares many of the characteristics of the OMs of spirochetes and Gram-negative bacteria, it is also unique and different in ways that make it of general interest to microbiologists. For example, unlike most other pathogenic spirochetes, the leptospiral OM is rich in lipopolysaccharide (LPS). Leptospiral LPS is similar to that of Gram-negative bacteria but has a number of unique structural features that may explain why it is not recognized by the LPS-specific Toll-like receptor 4 of humans. As in other spirochetes, lipoproteins are major components of the leptospiral OM, though their roles are poorly understood. The functions of transmembrane outer membrane proteins (OMPs) in many cases are better understood, thanks to homologies with their Gram-negative counterparts and the emergence of improved genetic techniques. This chapter will review recent discoveries involving the leptospiral OM and its role in leptospiral physiology and pathogenesis.

Genomics, proteomics, and genetics of leptospira

Recent advances in molecular genetics, such as the ability to construct defined mutants, have allowed the study of virulence factors and more generally the biology in Leptospira. However, pathogenic leptospires remain much less easily transformable than the saprophyte L. biflexa and further development and improvement of genetic tools are required. Here, we review tools that have been used to genetically manipulate Leptospira. We also describe the major advances achieved in both genomics and postgenomics technologies, including transcriptomics and proteomics.

Outer membrane proteome analysis of Indian strain of Pasteurella multocida serotype B:2 by MALDI-TOF/MS analysis

Identification of outer membrane proteins (OMPs) is important to understand the bacteria structure and function, host-pathogen interaction, development of novel vaccine candidates, and diagnostic antigens. But till now the key antigens of P. multocida B:2 isolate causing haemorrhagic septicaemia (HS) in animals are not clearly defined. In this study, P52 strain of P. multocida serotype B:2 was grown in vitro under iron-rich and iron-limited condition. The OMPs were extracted by sarkosyl method followed by SDS-PAGE and the proteins were identified by MALDI-TOF/MS analysis. In total, 22 proteins were identified, of which 7 were observed exclusively under iron-limited condition. Most of the high molecular weight proteins (TbpA, HgbA, HgbB, HasR, IroA, and HemR) identified in this study were involved in iron acquisition. Some hypothetical proteins (HP-KCU-10206, HP and AAUPMB 08244, HP AAUPMB 21592, HP AAUPMB 19766, AAUPMB 11295) were observed for the first time in this study which could be unique to serotype B:2. Further functional in vivo study of the proteins identified are required to explore the utility of these proteins in developing diagnostics and vaccine against HS.

Identification of cell-binding adhesins of Leptospira interrogans

Leptospirosis is a globally distributed bacterial infectious disease caused by pathogenic members of the genus Leptospira. Infection can lead to illness ranging from mild and non-specific to severe, with jaundice, kidney and liver dysfunction, and widespread endothelial damage. The adhesion of pathogenic Leptospira species (spp.), the causative agent of leptospirosis, to host tissue components is necessary for infection and pathogenesis. While it is well-established that extracellular matrix (ECM) components play a role in the interaction of the pathogen with host molecules, we have shown that pathogenic Leptospira interrogans binds to host cells more efficiently than to ECM components. Using in vitro phage display to select for phage clones that bind to EA.hy926 endothelial cells, we identified the putative lipoproteins LIC10508 and LIC13411, and the conserved hypothetical proteins LIC12341 and LIC11574, as candidate L. interrogans sv. Copenhageni st. Fiocruz L1-130 adhesins. Recombinant LIC11574, but not its L. biflexa homologue LBF1629, exhibited dose-dependent binding to both endothelial and epithelial cells. In addition, LIC11574 and LIC13411 bind to VE-cadherin, an endothelial cell receptor for L. interrogans. Extraction of bacteria with the non-ionic detergent Triton X-114 resulted in partitioning of the candidate adhesins to the detergent fraction, a likely indication that these proteins are outer membrane localized. All candidate adhesins were recognized by sera obtained from leptospirosis patients but not by sera from healthy individuals as assessed by western blot. This work has identified bacterial adhesins that are potentially involved in L. interrogans infection of the mammalian host, and through cadherin binding, may contribute to dissemination and vascular damage. Our findings may be of value in leptospirosis control and prevention, with the bacterial adhesins potentially serving as targets for development of diagnostics, therapeutics, and vaccines.

Application of zwitterionic detergent to the solubilization of Klebsiella pneumoniae outer membrane proteins for two-dimensional gel electrophoresis

Klebsiella pneumoniae is a frequent cause of nosocomial respiratory, urinary and gastrointestinal tract infections and septicemia with the multidrug-resistant K. pneumoniae being a major public health concern. Outer membrane proteins (OMPs) are important virulence factors responsible for the appropriate adaptation to the host environment. They constitute of the antigens being the first in contact with infected organism. However, K. pneumoniae strains are heavily capsulated and it is important to establish the OMPs isolation procedure prior to proteomics extensive studies. In this study we used Zwittergent Z 3-14® as a detergent to isolate the OMPs from K. pneumoniae cells and resolve them using two-dimensional electrophoresis (2-DE). As a result we identified 134 protein spots. The OMPs identified in this study are possible candidates for the development of a protein-based vaccine against K. pneumoniae infections.

Identification of novel surface-exposed proteins of Rickettsia rickettsii by affinity purification and proteomics

Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever, is the most pathogenic member among Rickettsia spp. Surface-exposed proteins (SEPs) of R. rickettsii may play important roles in its pathogenesis or immunity. In this study, R. rickettsii organisms were surface-labeled with sulfo-NHS-SS-biotin and the labeled proteins were affinity-purified with streptavidin. The isolated proteins were separated by two-dimensional electrophoresis, and 10 proteins were identified among 23 protein spots by electrospray ionization tandem mass spectrometry. Five (OmpA, OmpB, GroEL, GroES, and a DNA-binding protein) of the 10 proteins were previously characterized as surface proteins of R. rickettsii. Another 5 proteins (Adr1, Adr2, OmpW, Porin_4, and TolC) were first recognized as SEPs of R. rickettsii herein. The genes encoding the 5 novel SEPs were expressed in Escherichia coli cells, resulting in 5 recombinant SEPs (rSEPs), which were used to immunize mice. After challenge with viable R. rickettsii cells, the rickettsial load in the spleen, liver, or lung of mice immunized with rAdr2 and in the lungs of mice immunized with other rSEPs excluding rTolC was significantly lower than in mice that were mock-immunized with PBS. The in vitro neutralization test revealed that sera from mice immunized with rAdr1, rAdr2, or rOmpW reduced R. rickettsii adherence to and invasion of vascular endothelial cells. The immuno-electron microscopic assay clearly showed that the novel SEPs were located in the outer and/or inner membrane of R. rickettsii. Altogether, the 5 novel SEPs identified herein might be involved in the interaction of R. rickettsii with vascular endothelial cells, and all of them except TolC were protective antigens.

Production and characterization of a polyclonal antibody of anti-rLipL21-IgG against leptospira for early detection of acute leptospirosis

Leptospirosis is one of the zoonotic diseases in animals and humans throughout the world. LipL21 is one of the important surface-exposed lipoproteins in leptospires and the most effective cross protective immunogenic antigen. It is widely considered as a diagnostic marker for leptospirosis. In this study, we evaluated the serodiagnostic potential of LipL21 protein of Leptospira interrogans serovar Pomona. We have successfully amplified, cloned, and expressed LipL21 in E. coli and evaluated its specificity by immunoblotting. Purified recombinant LipL21 (rLipL21) was inoculated into rabbits for the production of polyclonal antibody. Characterization of the purified IgG antibody against rLipL21 was performed by cross reactivity assay. Only sera from leptospirosis patients and rabbit hyperimmune sera recognized rLipL21 while the nonleptospirosis control sera showed no reaction in immunoblotting. We confirmed that anti-rLipL21-IgG antibody cross reacted with and detected only pathogenic leptospiral species and it did not react with nonpathogenic leptospires and other bacterial species. Results observed showed that anti-rLipL21-IgG antibody has high specificity and sensitivity to leptospires. The findings indicated that the antibody could be used in a diagnostic assay for detection of leptospires or their proteins in the early phase of infection.

Interaction of Leptospira elongation factor Tu with plasminogen and complement factor H: a metabolic leptospiral protein with moonlighting activities

The elongation factor Tu (EF-Tu), an abundant bacterial protein involved in protein synthesis, has been shown to display moonlighting activities. Known to perform more than one function at different times or in different places, it is found in several subcellular locations in a single organism, and may serve as a virulence factor in a range of important human pathogens. Here we demonstrate that Leptospira EF-Tu is surface-exposed and performs additional roles as a cell-surface receptor for host plasma proteins. It binds plasminogen in a dose-dependent manner, and lysine residues are critical for this interaction. Bound plasminogen is converted to active plasmin, which, in turn, is able to cleave the natural substrates C3b and fibrinogen. Leptospira EF-Tu also acquires the complement regulator Factor H (FH). FH bound to immobilized EF-Tu displays cofactor activity, mediating C3b degradation by Factor I (FI). In this manner, EF-Tu may contribute to leptospiral tissue invasion and complement inactivation. To our knowledge, this is the first description of a leptospiral protein exhibiting moonlighting activities.

BB0347, from the lyme disease spirochete Borrelia burgdorferi, is surface exposed and interacts with the CS1 heparin-binding domain of human fibronectin

The causative agent of Lyme disease, Borrelia burgdorferi, codes for several known fibronectin-binding proteins. Fibronectin a common the target of diverse bacterial pathogens, and has been shown to be essential in allowing for the development of certain disease states. Another borrelial protein, BB0347, has sequence similarity with these other known fibronectin-binding proteins, and may be important in Lyme disease pathogenesis. Herein, we perform an initial characterization of BB0347 via the use of molecular and biochemical techniques. We found that BB0347 is expressed, produced, and presented on the outer surface of intact B. burgdorferi. We also demonstrate that BB0347 has the potential to be important in Lyme disease progression, and have begun to characterize the nature of the interaction between human fibronectin and this bacterial protein. Further work is needed to define the role of this protein in the borrelial infection process.

Detection of human leptospirosis as a cause of acute fever by capture ELISA using a Leptospira interrogans serovar Copenhageni (M20) derived antigen

BACKGROUND

Leptospirosis is a potentially lethal zoonosis mainly affecting low-resource tropical countries, including Peru and its neighbouring countries. Timely diagnosis of leptospirosis is critical but may be challenging in the regions where it is most prevalent. The serodiagnostic gold standard microagglutination test (MAT) may be technically prohibitive. Our objective in this study was to assess the sensitivity, specificity, and predictive value of an IgM antibody capture enzyme-linked immunoassay (MAC-ELISA) derived from the M20 strain of Leptospira interrogans serovar Copenhageni (M20) by comparison to MAT, which was used as the gold standard method of diagnosis.

METHODS

Acute and convalescent sera from participants participating in a passive febrile surveillance study in multiple regions of Peru were tested by both IgM MAC-ELISA and MAT. The sensitivity, specificity, positive and negative predictive value (PPV, NPV) of the MAC-ELISA assay for acute, convalescent and paired sera by comparison to MAT were calculated.

RESULTS

The sensitivity, specificity, PPV and NPV of the MAC-ELISA assay for acute sera were 92.3%, 56.0%, 35.3% and 96.6% respectively. For convalescent sera, the sensitivity, specificity, PPV and NPV of the MAC-ELISA assay were 93.3%, 51.5%, 63.6% and 89.5% respectively. For paired sera, the sensitivity, specificity, PPV and NPV of the MAC-ELISA assay were 93.6%, 37.5%, 59.2%, 85.7% respectively.

CONCLUSIONS

The M20 MAC-ELISA assay performed with a high sensitivity and low specificity in the acute phase of illness. Sensitivity was similar as compared with MAT in the convalescent phase and specificity remained low. Paired sera were the most sensitive but least specific by comparison to MAT serodiagnosis. NPV for acute, convalescent and paired sera was high. The limited specificity and high sensitivity of the MAC-ELISA IgM suggests that it would be most valuable to exclude leptospirosis in low-resource regions that lack immediate access to definitive reference laboratory techniques such as MAT.

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.

Leptospiral LruA is required for virulence and modulates an interaction with mammalian apolipoprotein AI

Leptospirosis is a worldwide zoonosis caused by spirochetes of the genus Leptospira. While understanding of pathogenesis remains limited, the development of mutagenesis in Leptospira has provided a powerful tool for identifying novel virulence factors. LruA is a lipoprotein that has been implicated in leptospiral uveitis as a target of the immune response. In this study, two lruA mutants, M754 and M765, generated by transposon mutagenesis from Leptospira interrogans serovar Manilae, were characterized. In M754, the transposon inserted in the middle of lruA, resulting in no detectable expression of LruA. In M765, the transposon inserted toward the 3' end of the gene, resulting in expression of a truncated protein. LruA was demonstrated to be on the cell surface in M765 and the wild type (WT). M754, but not M765, was attenuated in a hamster model of acute infection. A search for differential binding to human serum proteins identified a serum protein of around 30 kDa bound to the wild type and the LruA deletion mutant (M754), but not to the LruA truncation mutant (M765). Two-dimensional separation of proteins from leptospiral cells incubated with guinea pig serum identified the 28-kDa apolipoprotein A-I (ApoA-I) as a major mammalian serum protein that binds Leptospira in vitro. Interestingly, M754 (with no detectable LruA) bound more ApoA-I than did the LruA-expressing strains Manilae wild type and M765. Our data thus identify LruA as a surface-exposed leptospiral virulence factor that contributes to leptospiral pathogenesis, possibly by modulating cellular interactions with serum protein ApoA-I.

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.

LipL32 Is a Subsurface Lipoprotein of Leptospira interrogans: presentation of new data and reevaluation of previous studies

The agents of leptospirosis, a zoonosis with worldwide distribution, are pathogenic spirochetes belonging to the genus Leptospira. The leptospiral life cycle involves transmission via fresh water and colonization of the renal tubules of their reservoir hosts. Infection of accidental hosts, including humans, may result in life-threatening sequelae. Bacterial outer membrane proteins (OMPs), particularly those with surface-exposed regions, play crucial roles in pathogen virulence mechanisms and adaptation to environmental conditions, including those found in the mammalian host. Therefore, elucidation and characterization of the surface-exposed OMPs of Leptospira spp. is of great interest in the leptospirosis field. A thorough, multi-pronged approach for assessing surface exposure of leptospiral OMPs is essential. Herein, we present evidence for a sub-surface location for most or all of the major leptospiral lipoprotein, LipL32, based on surface immunofluorescence utilizing three different types of antibodies and four different permeabilization methods, as well as surface proteolysis of intact and lysed leptospires. We reevaluate prior evidence presented in support of LipL32 surface-exposure and present a novel perspective on a protein whose location has been misleading researchers, due in large part to its extraordinary abundance in leptospiral cells.

Leptospira and inflammation

Leptospirosis is an important zoonosis and has a worldwide impact on public health. This paper will discuss both the role of immunogenic and pathogenic molecules during leptospirosis infection and possible new targets for immunotherapy against leptospira components. Leptospira, possess a wide variety of mechanisms that allow them to evade the host immune system and cause infection. Many molecules contribute to the ability of Leptospira to adhere, invade, and colonize. The recent sequencing of the Leptospira genome has increased our knowledge about this pathogen. Although the virulence factors, molecular targets, mechanisms of inflammation, and signaling pathways triggered by leptospiral antigens have been studied, some questions are still unanswered. Toll-like receptors (TLRs) are the primary sensors of invading pathogens. TLRs recognize conserved microbial pattern molecules and activate signaling pathways that are pivotal to innate and adaptive immune responses. Recently, a new molecular target has emerged—the Na/K-ATPase—which may contribute to inflammatory and metabolic alteration in this syndrome. Na/K-ATPase is a target for specific fatty acids of host origin and for bacterial components such as the glycolipoprotein fraction (GLP) that may lead to inflammasome activation. We propose that in addition to TLRs, Na/K-ATPase may play a role in the innate response to leptospirosis infection.

Leptospiral outer membrane protein microarray, a novel approach to identification of host ligand-binding proteins

Leptospirosis is a zoonosis with worldwide distribution caused by pathogenic spirochetes belonging to the genus Leptospira. The leptospiral life cycle involves transmission via freshwater and colonization of the renal tubules of their reservoir hosts. Infection requires adherence to cell surfaces and extracellular matrix components of host tissues. These host-pathogen interactions involve outer membrane proteins (OMPs) expressed on the bacterial surface. In this study, we developed an Leptospira interrogans serovar Copenhageni strain Fiocruz L1-130 OMP microarray containing all predicted lipoproteins and transmembrane OMPs. A total of 401 leptospiral genes or their fragments were transcribed and translated in vitro and printed on nitrocellulose-coated glass slides. We investigated the potential of this protein microarray to screen for interactions between leptospiral OMPs and fibronectin (Fn). This approach resulted in the identification of the recently described fibronectin-binding protein, LIC10258 (MFn8, Lsa66), and 14 novel Fn-binding proteins, denoted Microarray Fn-binding proteins (MFns). We confirmed Fn binding of purified recombinant LIC11612 (MFn1), LIC10714 (MFn2), LIC11051 (MFn6), LIC11436 (MFn7), LIC10258 (MFn8, Lsa66), and LIC10537 (MFn9) by far-Western blot assays. Moreover, we obtained specific antibodies to MFn1, MFn7, MFn8 (Lsa66), and MFn9 and demonstrated that MFn1, MFn7, and MFn9 are expressed and surface exposed under in vitro growth conditions. Further, we demonstrated that MFn1, MFn4 (LIC12631, Sph2), and MFn7 enable leptospires to bind fibronectin when expressed in the saprophyte, Leptospira biflexa. Protein microarrays are valuable tools for high-throughput identification of novel host ligand-binding proteins that have the potential to play key roles in the virulence mechanisms of pathogens.

Leptospira interrogans catalase is required for resistance to H2O2 and for virulence

Pathogenic Leptospira spp. are likely to encounter higher concentrations of reactive oxygen species induced by the host innate immune response. In this study, we characterized Leptospira interrogans catalase (KatE), the only annotated catalase found within pathogenic Leptospira species, by assessing its role in resistance to H(2)O(2)-induced oxidative stress and during infection in hamsters. Pathogenic L. interrogans bacteria had a 50-fold-higher survival rate under H(2)O(2)-induced oxidative stress than did saprophytic L. biflexa bacteria, and this was predominantly catalase dependent. We also characterized KatE, the only annotated catalase found within pathogenic Leptospira species. Catalase assays performed with recombinant KatE confirmed specific catalase activity, while protein fractionation experiments localized KatE to the bacterial periplasmic space. The insertional inactivation of katE in pathogenic Leptospira bacteria drastically diminished leptospiral viability in the presence of extracellular H(2)O(2) and reduced virulence in an acute-infection model. Combined, these results suggest that L. interrogans KatE confers in vivo resistance to reactive oxygen species induced by the host innate immune response.

OmpL1 is an extracellular matrix- and plasminogen-interacting protein of Leptospira spp

Leptospirosis is a zoonosis with multisystem involvement caused by pathogenic strains of the genus Leptospira. OmpL1 is an outer membrane protein of Leptospira spp. that is expressed during infection. In this work, we investigated novel features of this protein. We describe that OmpL1 is a novel leptospiral extracellular matrix (ECM)-binding protein and a plasminogen (PLG) receptor. The recombinant protein was expressed in Escherichia coli BL21(DE3) Star/pLysS as inclusion bodies, refolded, and purified by metal-chelating chromatography. The protein presented a typical β-strand secondary structure, as evaluated by circular dichroism spectroscopy. The recombinant protein reacted with antibodies in serum samples from convalescent leptospirosis patients with a high specificity compared to serum samples from individuals with unrelated diseases. These data strengthen the usefulness of OmpL1 as a diagnostic marker of leptospirosis. The characterization of the immunogenicity of recombinant OmpL1 in inoculated BALB/c mice showed that the protein has the capacity to elicit humoral and cellular immune responses, as denoted by high antibody titers and the proliferation of lymphocytes. We demonstrate that OmpL1 has the ability to mediate attachment to laminin and plasma fibronectin, with K(D) (equilibrium dissociation constant) values of 2,099.93 ± 871.03 nM and 1,239.23 ± 506.85 nM, respectively. OmpL1 is also a PLG receptor, with a K(D) of 368.63 ± 121.23 nM, capable of generating enzymatically active plasmin. This is the first report that shows and characterizes OmpL1 as an ECM-interacting and a PLG-binding protein of Leptospira spp. that may play a role in bacterial pathogenesis when expressed during infection.