Differential TLR recognition of leptospiral lipid A and lipopolysaccharide in murine and human cells

Differential TLR2 and TLR4 mediated inflammatory and apoptotic responses in asymptomatic and symptomatic Leptospira interrogans infections in canine uterine tissue

Leptospirosis is major zoonotic disease with global implications, affecting both domestic animals and humans. It is caused by Leptospira interrogans (L. interrogans), which can damage multiple organs, including the kidneys, liver, testes, and uterus. Despite this, L. interrogans can also persist asymptomatically in tissues, akin to nonpathogenic strains. The mechanisms driving asymptomatic infections remain poorly understood. This study investigated the role of L. interrogans in asymptomatic infection within the uterine tissue of canines, focusing on the differential expression of Toll-like receptors (TLRs)2 and 4 and their roles in inflammatory and apoptotic pathways. We hypothesized that TLR2 and TLR4 coexpression is crucial for eliciting inflammation and apoptosis, whereas TLR4 alone might be insufficient. Our findings revealed that in symptomatic infections, both TLR2 and TLR4 are coexpressed, leading to markedly elevated levels of the proinflammatory cytokines IL-10, IL-1β, TNF-α, and IL-6. This enhanced inflammatory response is further evidenced by increased CD4 expression, indicating robust T helper cell activation. In contrast, asymptomatic infections are characterized by exclusive TLR4 expression, with inflammatory markers remaining at baseline levels. Additionally, we observed that L. interrogans induces apoptosis in symptomatic animals through TLR2 and TLR4 mediated activation of Caspase 8 and Caspase 3. These findings illustrate that L. interrogans drives both inflammation and apoptosis via the combination of TLR2 and TLR4 actions. When only TLR4 is activated, the immune response is insufficient, resulting in an asymptomatic disease course. This study provides novel insights into the differential roles of TLR receptors in leptospirosis, offering potential directions for targeted therapeutic strategies.

Role of Toll-like receptor 2 during infection of Leptospira spp: A systematic review

The involvement of Toll-like receptor 2 (TLR2) in leptospirosis is poorly understood. Our systematic review examined its role across in-vitro, in-vivo, ex-vivo, and human studies. Original articles published in English up to January 2024, exploring the role of TLR2 during leptospirosis, were selected from databases including PubMed, Web of Science, Scopus, Trip, and Google Scholar. Cochrane guidelines and Preferred Reporting Items for Systematic Reviews and Meta-Analyses were followed by this systematic review. The National Institute of Health Quality Assessment tool, Systematic Review Centre for Laboratory Animal Experimentation risk of bias tool, and Office of Health Assessment and Translation extended tool were used to assess the risk of bias of the studies. Out of 2458 studies retrieved, 35 were selected for the systematic review. These comprised 3 human, 17 in-vitro, 5 in-vivo, 3 ex-vivo, and 7 studies with combined experimental models. We assessed the direct TLR2 expression and indirect TLR2 involvement via the secretion/mRNA expression of immune effectors during leptospirosis. Notably, we observed the secretion/mRNA expression of several cytokines (IL6, IL8, IL-1β, TNFα, IFNγ, IL10, CCL2/MCP-1, CCL10, COX2, CXCL1/KC, CXCL2/MIP2) and immune effectors (hBD2, iNOS, Fibronectin, Oxygen, and Nitrogen reactive species) as key aspects of host TLR2 responses during leptospirosis. Even though increased TLR2 expression in in-vivo and in-vitro studies was evident, human studies reported mixed results showing that the postulated effect of TLR2 response based on other studies may not be valid for human leptospirosis. Besides the role of TLR2 in response to leptospirosis, the involvement of TLR4 and TLR5 was identified in in-vitro and in-vivo studies. TLR2 expression is inconclusive during human leptospirosis and further studies are needed to examine the immune effector regulation, through TLR2 for mitigating the harmful effects and promoting effective immune responses.

In silico prediction and experimental evaluation of LIP3228 of pathogenic Leptospira as a potential subunit vaccine target against leptospirosis

A protein subunit vaccine comprising conserved surface-exposed outer membrane proteins (SE-OMPs) is considered a promising platform for leptospirosis vaccine. The search for novel vaccine candidates that confer high protective efficacy against leptospirosis is ongoing. The LIP3228 protein was previously identified as a conserved and abundant SE-OMP with the potential to serve as an effective vaccine candidate. However, it is crucial to explore the immunological properties of this vaccine antigen before proceeding with animal experiments. This study aimed to assess the immunological characteristics of LIP3228 through in silico prediction and to validate its immunogenicity and protective efficacy in a hamster model of acute leptospirosis. The LIP3228 vaccine candidate was predicted in silico to be immunogenic, with strong binding B-cell and T-cell epitopes. In the immune simulator, it demonstrated stable interactions with Toll-like receptors 2 (TLR2) and 4 (TLR4) and induced immune responses, potentially stimulating host immune responses in vivo. The animal experiment showed that immunization with recombinant LIP3228 protein, formulated with AddaVax adjuvant, induced high and specific IgG responses in hamsters, with IgG2 being the predominant subclass. Although no significant improvement in survival was observed compared to the negative control after a homologous challenge with virulent leptospires, the vaccinated hamsters showed a reduction in histopathological changes and severity of lesions in target organs compared to unvaccinated hamsters. These results suggest that the immunoinformatic prediction is effective in predicting immunogenicity but not protective efficacy. Therefore, LIP3228 could be considered a potential vaccine candidate for mitigating severe tissue damage. These findings may have significant implications for the development of subunit vaccines in the future.

Innovative Strategies to Study the Pathogenesis of Elusive Spirochetes and Difficulties Managing the Chronic Infections They Cause

The major human spirochetal pathogens (Leptospira, Borrelia, and Treponema) are difficult to diagnose and lack vaccines to prevent infections. Infection by these spirochetes does not generate general protective immunity, allowing reinfection by different strains to occur. These stealth pathogens have uncommon physiology, pathogenesis, and clinical presentations and possess unique immune evasion mechanisms to facilitate their host adaptation and persistence. Collectively, host-spirochete interactions orchestrate systemic infections in a manner distinct from organ- and tissue-specific diseases caused by many bacterial pathogens. Difficulties in growing and genetic manipulation of infectious spirochetes have hindered the full understanding of their virulence factors despite decades to centuries of research. This article highlights the current understanding of the intricacies of spirochetal pathogenesis and diseases. Our comprehensive review of the progress versus gaps in knowledge lays a foundation for researchers to direct their studies toward the development of effective diagnostics and vaccines to protect patients from serious, chronic spirochetal diseases.

Leptospiral lipopolysaccharide dampens inflammation through upregulation of autophagy adaptor p62 and NRF2 signaling in macrophages

Leptospira interrogans are pathogenic bacteria responsible for leptospirosis, a worldwide zoonosis. All vertebrates can be infected, and some species like humans are susceptible to the disease whereas rodents such as mice are resistant and become asymptomatic renal carriers. Leptospires are stealth bacteria that are known to escape several immune recognition pathways and resist killing mechanisms. We recently published that leptospires may survive intracellularly in and exit macrophages, avoiding xenophagy, a pathogen-targeting form of autophagy. Interestingly, the latter is one of the antimicrobial mechanisms often highjacked by bacteria to evade the host immune response. In this study we explored whether leptospires subvert the key molecular players of autophagy to facilitate infection. We showed in macrophages that leptospires triggered a specific accumulation of autophagy-adaptor p62 in puncta-like structures, without altering autophagic flux. We demonstrated that Leptospira-induced p62 accumulation is a passive mechanism depending on the leptospiral virulence factor LPS signaling via TLR4/TLR2. p62 is a central pleiotropic protein, also mediating cell stress and death, via the translocation of transcription factors. We demonstrated that Leptospira-driven accumulation of p62 induced the translocation of transcription factor NRF2, a key player in the anti-oxidant response. However, NRF2 translocation upon Leptospira infection did not result as expected in antioxydant response, but dampened the production of inflammatory mediators such as iNOS/NO, TNF and IL6. Overall, these findings highlight a novel passive bacterial mechanism linked to LPS and p62/NRF2 signaling that decreases inflammation and contributes to the stealthiness of leptospires.

Exploring the complex interplay: gut microbiome, stress, and leptospirosis

Leptospirosis, a re-emerging zoonotic disease, remains a significant global health concern, especially amid floods and disasters such as the Kakhovka Dam destruction. As is known, the stress that occurs in the conditions of military conflicts among civilian and military personnel significantly affects susceptibility to infectious diseases and possibly even influences their course. This review aims to explore how the gut microbiome and stress mediators (such as catecholamines and corticosteroids) might impact the leptospirosis disease course. The review opens new horizons for research by elucidating the connections between the gut microbiome, stress, and leptospirosis.

Levels of Cytokines in Leptospirosis Patients with Different Serovars and rfb Locus

Leptospirosis has a wide spectrum of clinical manifestations ranging from mild to severe disease. The cytokine response is considered one of the key drivers for this varying manifestation. The different cytokine response observed in patients with leptospirosis could be due to the variation of infecting serovars. Since the rfb locus codes for the lipopolysaccharide synthesis of the bacterial cell wall, which also determines the serovar, this locus may play a role in driving a specific cytokine response in the host. We investigated 12 commonly used cytokine profiles in serum samples of culture, microscopic agglutination test (MAT), or polymerase chain reaction (PCR)-positive patients with leptospirosis. The sequences of the rfb locus in culture-positive samples were generated from whole genome sequencing and serovar status was drawn from original data published. Isolated cultures were subjected to whole genome sequencing using the PacBio RS II system, and the resulting data were used to determine the species. The recovered genomic data were annotated with the Rapid Annotation using Subsystem Technology (RAST) subsystem, and the rfb locus was extracted. The cytokine analysis was carried out using the Qiagen human ELISA kit. Eighteen samples were found to be positive by culture, while the other 7 samples were positive by PCR or MAT. Infections from Leptospira interrogans serovar Autumnalis (5), Pyrogens (3), Icterohaemorrhagiae (1) Leptospira borgpetersenii (all 7 samples clustered in same clonal group with serovar status not determined), Leptospira weilii (1 with serovar status not determined), and Leptospira kirschneri serovar Grippotyphosa (1) were included in the analysis. Three patients [infected with Leptospira interrogansserovar Autumnalis (2) and Pyrogens (1)] and 2 MAT-positive patients (highest titer against serovar Bratislava of L.interrognas) were reported to have severe clinical manifestations, while the rest had mild to moderate symptoms. Although the serum cytokine concentration of patients with severe clinical manifestation was comparatively higher, a statistically significant difference was observed only for interleukin (IL)-1β (P < 0.05). IL-10/tumor necrosis factor-alpha (TNF-α) ratio was high in patients with severe complications. In general, patients infected with L. interrogans showed higher concentration of cytokines compared to L. borgpetersenii.

The dangerous liaisons in innate immunity involving recombinant proteins and endotoxins: Examples from the literature and the Leptospira field

Endotoxins, also known as lipopolysaccharides (LPS), are essential components of cell walls of diderm bacteria such as Escherichia coli. LPS are microbe-associated molecular patterns that can activate pattern recognition receptors. While trying to investigate the interactions between proteins and host innate immunity, some studies using recombinant proteins expressed in E. coli reported interaction and activation of immune cells. Here, we set out to provide information on endotoxins that are highly toxic to humans and bind to numerous molecules, including recombinant proteins. We begin by outlining the history of the discovery of endotoxins, their receptors and the associated signaling pathways that confer extreme sensitivity to immune cells, acting alone or in synergy with other microbe-associated molecular patterns. We list the various places where endotoxins have been found. Additionally, we warn against the risk of data misinterpretation due to endotoxin contamination in recombinant proteins, which is difficult to estimate with the Limulus amebocyte lysate assay, and cannot be completely neutralized (e.g., treatment with polymyxin B or heating). We further illustrate our point with examples of recombinant heat-shock proteins and viral proteins from severe acute respiratory syndrome coronavirus 2, dengue and HIV, for which endotoxin contamination has eventually been shown to be responsible for the inflammatory roles previously ascribed. We also critically appraised studies on recombinant Leptospira proteins regarding their putative inflammatory roles. Finally, to avoid these issues, we propose alternatives to express recombinant proteins in nonmicrobial systems. Microbiologists wishing to undertake innate immunity studies with their favorite pathogens should be aware of these difficulties.

Leptospira Lipid A Is a Potent Adjuvant That Induces Sterilizing Immunity against Leptospirosis

Leptospirosis is a globally significant zoonotic disease. The current inactivated vaccine offers protection against specific serovars but does not provide complete immunity. Various surface antigens, such as Leptospira immunoglobulin-like proteins (LigA and LigB), have been identified as potential subunit vaccine candidates. However, these antigens require potent adjuvants for effectiveness. Bacterial lipopolysaccharides (LPSs), including lipid A, are a well-known immunostimulant, and clinical adjuvants often contain monophosphoryl lipid A (MPLA). Being less endotoxic, we investigated the adjuvant properties of lipid A isolated from L. interrogans serovar Pomona (PLA) in activating innate immunity and enhancing antigen-specific adaptive immune responses. PLA activated macrophages to a similar degree as MPLA, albeit at a higher dose, suggesting that it is less potent in stimulation than MPLA. Mice immunized with a variable portion of LigA (LAV) combined with alum and PLA (LAV-alum-PLA) exhibited significantly higher levels of LAV-specific humoral and cellular immune responses compared to alum alone but similar to those induced by alum-MPLA. The adjuvant activity of PLA resembles that of MPLA and is primarily achieved through the increased recruitment, activation, and uptake of antigens by innate immune cells. Furthermore, like MPLA, PLA formulation establishes a long-lasting memory response. Notably, PLA demonstrated superior potency than MPLA formulation and provided sterilizing immunity against the leptospirosis in a hamster model. Overall, our study sheds light on the adjuvant properties of Leptospira lipid A and offers promising avenues for developing LPS-based vaccines against this devastating zoonotic disease.

The human Toll-like receptor 2 (TLR2) response during pathogenic Leptospira infection

Background

Human innate immune responses are triggered through the interaction of human pattern recognition receptors and pathogen-associated molecular patterns. The role of toll-like receptor2 (TLR2) in mice innate immune response to leptospirosis is well established, while human studies are limited. The present study aimed to determine the TLR2 response among confirmed cases of leptospirosis.

Methodology/Principle findings

The study has two components. Clinically suspected patients of leptospirosis were confirmed using a previously validated qPCR assay. Total RNA was extracted from patients' RNA-stabilized whole blood samples. Human TLR2 gene expression (RT-qPCR) analysis was carried out using an exon-exon spanning primer pair, using CFX Maestro™ software. The first set of patient samples was used to calculate the Relative Normalized Expression (ΔΔCq value) of the TLR2 gene in comparison to a healthy control sample and normalized by the reference gene GAPDH (Glyceraldehyde-3-phosphate dehydrogenase). Secondly, recruited patient samples were subjected to TLR2 gene expression analysis and compared to healthy controls and normalized by the reference genes Beta-2-microglobulin(B2M), Hypoxanthine phosphoribosyltransferase 1 (HPRT 1).In the initial cohort of 64 confirmed leptospirosis cases, 18 were selected for human TLR2 gene expression analysis based on criteria of leptospiremia and RNA yield. Within this group, one individual exhibited a down-regulation of TLR2 gene (Expression/ΔΔCq=0.01352), whereas the remaining subjects presented no significant change in gene expression. In a subsequent cohort of 23 confirmed cases, 13 were chosen for similar analysis. Among these, three patients demonstrated down-regulation of TLR2 gene expression, with Expression/ΔΔCq values of 0.86574, 0.47200, and 0.28579, respectively. No TLR2 gene expression was noted in the other patients within this second group.

Conclusions

Our investigation into the acute phase of leptospirosis using human clinical samples has revealed a downregulation of TLR2 gene expression. This observation contrasts to the upregulation commonly reported in the majority of in-vitro and in-vivo studies of Leptospira infection. These preliminary findings prompt a need for further research to explore the mechanisms underlying TLR2's role in the pathogenesis of leptospirosis, which may differ in clinical settings compared to laboratory models.

Author Summary

The human immune system employs pattern recognition receptors like toll-like receptor 2 (TLR2) to detect and combat infections such as leptospirosis. While TLR2's role is well-documented in mice, its function in the human response to leptospirosis remains unclear. Our study evaluated TLR2 activity in patients with confirmed leptospirosis. We conducted a genetic analysis of blood samples from these patients, comparing TLR2 gene activity against healthy individuals, with standard reference genes for accuracy. Contrary to expectations and existing laboratory data, we observed a decrease in TLR2 activity in some patients. This suggests that human TLR2 responses in actual infections may diverge from established laboratory models. These findings indicate a need for further study to understand the human immune response to leptospirosis, which may significantly differ from that observed in controlled experimental settings.

Differential modulation of innate immune response by lipopolysaccharide of Leptospira

Leptospirosis is a worldwide zoonosis caused by pathogenic Leptospira spp. having more than 300 serovars. These serovars can infect a variety of hosts, some being asymptomatic carriers and others showing varied symptoms of mild to severe infection. Since lipopolysaccharide (LPS) is the major antigen which defines serovar specificity, this different course of infection may be attributed to a differential innate response against this antigen. Previous studies have shown that Leptospira LPS is less endotoxic. However, it is unclear whether there is a difference in the ability of LPS isolated from different serovars to modulate the innate response. In this study, we purified LPS from three widely prevalent pathogenic serovars, i.e. Icterohaemorrhagiae strain RGA, Pomona, Hardjo, and from non-pathogenic L. biflexa serovar semeranga strain Potac 1 collectively termed as L-LPS and tested their ability to modulate innate response in macrophages from both resistant (mice) and susceptible (human and bovine) hosts. L-LPS induced differential response being more proinflammatory in mouse and less proinflammatory in human and bovine macrophages but overall less immunostimulatory than E. coli LPS (E-LPS). Irrespective of serovar, this response was TLR2-dependent in humans, whereas TLR4-dependent/CD14-independent in mouse using MyD88 adapter and signalling through P38 and ERK-dependent MAP kinase pathway. L-LPS-activated macrophages were able to phagocytose Leptospira and this effect was significantly higher or more pronounced when the macrophages were stimulated with L-LPS from the corresponding serovar. L-LPS activated both canonical and non-canonical inflammasome, producing IL-1β without inducing pyroptosis. Further, L-LPS induced both TNF-mediated early and NO-mediated late apoptosis. Altogether, these results indicate that L-LPS induces a differential innate response that is quite distinct from that induced by E-LPS and may be attributed to the structural differences and its atypical nature.

Differential expression of six cytokines in Rattus rattus exposed to leptospirosis: A comprehensive transcriptomic analysis

BACKGROUND

Rattus rattus are the main carriers of various zoonotic diseases including leptospirosis. Regrettably, information underlying the cytokine response of wild R. rattus upon leptospirosis infection is lacking. This study aims to understand the immune response presented by specifically the kidney and liver of R. rattus during leptospirosis infection.

METHODOLOGY

High-throughput RNA-Sequencing technology was employed to discover the transcriptome alterations in the kidney and liver of R. rattus during natural infection. Both kidney and liver tissues from the healthy and infected rats were sequenced using the BGISEQ-500 sequencing platform. The GO and KEGG databases were utilized to functionally annotate the differentially expressed transcripts of the selected cytokines; TNF-α, IL-1β, IL-6, IL-10, MIP-1α, and IFN-γ.

RESULTS

A higher number of upregulated genes were signified in the kidney as compared to the liver during infection. Among the six selected cytokines, Interleukin-6 was found to be expressed during the early stage in the liver of R. rattus, while all the other six genes were upregulated during the late stage of leptospirosis in the kidney of R. rattus. The GO of the annotated genes was classified under inflammatory response and cellular response to lipopolysaccharide, while the KEGG pathway indicated cytokine-cytokine receptor interaction and the Toll-like receptor (TLR) signalling pathway. The upshots of this study correlated the different phases of cytokine response in different organs of R. rattus during leptospirosis infection.

CONCLUSION

Overall, these studies formulate a conceptual framework based on host and pathogen relationships of leptospirosis transmission patterns and the discovery of biomarkers in tracking the early stages of leptospires colonization.

Lipopolysaccharide and Glycolipoprotein Coordinately Triggered Necroptosis Contributes to the Pathogenesis of Leptospira Infection in Mice

Leptospirosis is a recurring but neglected zoonotic disease caused by pathogenic Leptospira. The explicit underlying mechanism of necroptosis and its role in Leptospira infection have not yet been elucidated. Here we reported that leptospiral pathogen-associated molecular patterns, lipopolysaccharide, and glycolipoprotein activate the necroptotic RIPK1-RIPK3-MLKL cascade through the TLR4 signaling pathway in mouse macrophages. Using the murine acute leptospirosis model, we reveal that abolition of necroptosis exhibited significantly improved outcomes in acute phases, with enhanced eradication of Leptospira from liver, mild clinical symptoms, and decreased cytokine production. RIPK3 was also found to exert a necroptosis-independent function in CXCL1 production and neutrophil recruitment, with the consequence of improved Leptospira control. These findings improve our understanding of the mechanism of Leptospira-macrophage interactions, indicating potential therapeutic values by targeting necroptosis signaling pathways.

Lipid A structural diversity among members of the genus Leptospira

Lipid A is the hydrophobic component of bacterial lipopolysaccharide and an activator of the host immune system. Bacteria modify their lipid A structure to adapt to the surrounding environment and, in some cases, to evade recognition by host immune cells. In this study, lipid A structural diversity within the Leptospira genus was explored. The individual Leptospira species have dramatically different pathogenic potential that ranges from non-infectious to life-threatening disease (leptospirosis). Ten distinct lipid A profiles, denoted L1-L10, were discovered across 31 Leptospira reference species, laying a foundation for lipid A-based molecular typing. Tandem MS analysis revealed structural features of Leptospira membrane lipids that might alter recognition of its lipid A by the host innate immune receptors. Results of this study will aid development of strategies to improve diagnosis and surveillance of leptospirosis, as well as guide functional studies on Leptospira lipid A activity.

Leptospira interrogans Prevents Macrophage Cell Death and Pyroptotic IL-1β Release through Its Atypical Lipopolysaccharide

Leptospira interrogans are bacteria that can infect all vertebrates and are responsible for leptospirosis, a neglected zoonosis. Some hosts, such as humans, are susceptible to the disease, whereas mice are resistant and get chronically colonized. Although leptospires escape recognition by some immune receptors, they activate the NOD-like receptor pyrin 3-inflammasome and trigger IL-1β secretion. Classically, IL-1β secretion is associated with lytic inflammatory cell death called pyroptosis, resulting from cytosolic LPS binding to inflammatory caspases, such as caspase 11. Interestingly, we showed that L. interrogans and Leptospira biflexa do not trigger cell death in either murine, human, hamster, or bovine macrophages, escaping both pyroptosis and apoptosis. We showed, in murine cells, that the mild IL-1β secretion induced by leptospires occurred through nonlytic caspase 8-dependent gasdermin D pore formation and not through activation of caspase 11/noncanonical inflammasome. Strikingly, we demonstrated a potent antagonistic effect of pathogenic L. interrogans and their atypical LPS on spontaneous and Escherichia coli LPS-induced cell death. Indeed, LPS of L. interrogans efficiently prevents caspase 11 dimerization and subsequent massive gasdermin D cleavage. Finally, we showed that pyroptosis escape by leptospires prevents massive IL-1β release, and we consistently found no major role of IL-1R in controlling experimental leptospirosis in vivo. Overall, to our knowledge, our findings described a novel mechanism by which leptospires dampen inflammation, thus potentially contributing to their stealthiness.

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.

Formulated Phospholipids as Non-Canonical TLR4 Agonists

Immunogenic agents known as adjuvants play a critical role in many vaccine formulations. Adjuvants often signal through Toll-like receptor (TLR) pathways, including formulations in licensed vaccines that target TLR4. While TLR4 is predominantly known for responding to lipopolysaccharide (LPS), a component of Gram-negative bacterial membranes, it has been shown to be a receptor for a number of molecular structures, including phospholipids. Therefore, phospholipid-based pharmaceutical formulations might have off-target effects by signaling through TLR4, confounding interpretation of pharmaceutical bioactivity. In this study we examined the individual components of a clinical stage oil-in-water vaccine adjuvant emulsion (referred to as a stable emulsion or SE) and their ability to signal through murine and human TLR4s. We found that the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) activated TLR4 and elicited many of the same immune phenotypes as canonical TLR4 agonists. This pathway was dependent on the saturation, size, and headgroup of the phospholipid. Interestingly, DMPC effects on human cells were evident but overall appeared less impactful than emulsion oil composition. Considering the prevalence of DMPC and other phospholipids used across the pharmaceutical space, these findings may contextualize off-target innate immune responses that could impact preclinical and clinical development.

Putative Pathogenic Genes of Leptospira interrogans and Leptospira weilii Isolated from Patients with Acute Febrile Illness

Leptospirosis is an important worldwide tropical disease caused by pathogenic Leptospira spp. The determination of virulence genes is important, as it influences patients' clinical manifestations and clinical outcomes. This case report focused on detecting the pathogenic genes of Leptospira in association with the clinical manifestations of patients at the Hospital Universiti Sains Malaysia, Malaysia, who presented with acute febrile illness. Two cases were found and, to the best of our knowledge, these were the first two cases in Malaysia in which patients presented with febrile illness were associated with successful Leptospira isolation from clinical samples. Both clinical isolates were identified by 16S rRNA sequencing as Leptospira weilii and Leptospira interrogans, respectively, and they were classified as pathogenic Leptospira by the presence of different pathogenic genes, based on a polymerase chain reaction (PCR) amplification of targeted genes. This report emphasizes that different infecting Leptospira species and the presence of different virulence factors cause a slight difference in clinical manifestations and laboratory findings of leptospirosis. Genomic sequencing and annotation revealed the detection of classical leptospiral virulence factor genes that were otherwise missed using PCR for detection of Leptospira weilii genome B208.

Host and Species-Specificities of Pattern Recognition Receptors Upon Infection With Leptospira interrogans

Leptospirosis is a zoonotic infectious disease affecting all vertebrates. It is caused by species of the genus Leptospira, among which are the highly pathogenic L. interrogans. Different mammals can be either resistant or susceptible to the disease which can present a large variety of symptoms. Humans are mostly asymptomatic after infection but can have in some cases symptoms varying from a flu-like syndrome to more severe forms such as Weil's disease, potentially leading to multiorgan failure and death. Similarly, cattle, pigs, and horses can suffer from acute forms of the disease, including morbidity, abortion, and uveitis. On the other hand, mice and rats are resistant to leptospirosis despite chronical colonization of the kidneys, excreting leptospires in urine and contributing to the transmission of the bacteria. To this date, the immune mechanisms that determine the severity of the infection and that confer susceptibility to leptospirosis remain enigmatic. To our interest, differential immune sensing of leptospires through the activation of or escape from pattern recognition receptors (PRRs) by microbe-associated molecular patterns (MAMPs) has recently been described. In this review, we will summarize these findings that suggest that in various hosts, leptospires differentially escape recognition by some Toll-like and NOD-like receptors, including TLR4, TLR5, and NOD1, although TLR2 and NLRP3 responses are conserved independently of the host. Overall, we hypothesize that these innate immune mechanisms could play a role in determining host susceptibility to leptospirosis and suggest a central, yet complex, role for TLR4.

Hematogenous dissemination of pathogenic and non-pathogenic Leptospira in a short-term murine model of infection

Leptospirosis is an emerging zoonosis caused by pathogenic Leptospira spp. Because rodents are natural hosts of Leptospira, rodent models of pathogenesis have been limited, but are valuable to understand infection in reservoir animals even in the absence of disease. Mouse models of infection provide advantages due to genetic tractability, so developing murine models of Leptospira infection is crucial for further understanding the biology of this organism. Previously our laboratory developed a short-term murine model of Borrelia burgdorferi hematogenous dissemination to investigate the role of adhesion proteins on bacterial survival and dissemination within a host. Here we adapt this model to Leptospira. C3H/HeJ mice are anesthetized, inoculated intravenously, and then bacteria are allowed to circulate for up to twenty-four hours. Mice are euthanized, perfused with saline, and tissues are harvested for culture and DNA purification. Bacterial burdens are determined by quantitative PCR. Reproducible burdens of bacteria were found in tissues upon inoculation with pathogens and non-pathogens, demonstrating the utility of this model to probe different Leptospira species and strains. Pathogenic L. interrogans has a significantly higher burden in blood, liver, kidney, and bladder at one-hour post-inoculation when compared to non-pathogenic L. biflexa. Colonization of the kidney is essential to the life cycle of pathogenic Leptospira in nature. Measurable burdens of non-pathogenic L. biflexa were found in numerous organs and live leptospires were recovered from blood samples for at least three hours post-inoculation, contrary to the previous belief that non-pathogenic leptospires are rapidly cleared. This short-term murine model of Leptospira hematogenous dissemination will allow for the interrogation of virulence factors potentially important for tissue colonization and evasion of host defenses, and represents a novel animal model for investigating determinants of Leptospira infection.

Alive Pathogenic and Saprophytic Leptospires Enter and Exit Human and Mouse Macrophages With No Intracellular Replication

Leptospira interrogans are pathogenic bacteria responsible for leptospirosis, a zoonosis impacting 1 million people per year worldwide. Leptospires can infect all vertebrates, but not all hosts develop similar symptoms. Human and cattle may suffer from mild to acute illnesses and are therefore considered as sensitive to leptospirosis. In contrast, mice and rats remain asymptomatic upon infection, although they get chronically colonized in their kidneys. Upon infection, leptospires are stealth pathogens that partially escape the recognition by the host innate immune system. Although leptospires are mainly extracellular bacteria, it was suggested that they could also replicate within macrophages. However, contradictory data in the current literature led us to reevaluate these findings. Using a gentamicin-protection assay coupled to high-content (HC) microscopy, we observed that leptospires were internalized in vivo upon peritoneal infection of C57BL/6J mice. Additionally, three different serotypes of pathogenic L. interrogans and the saprophytic L. biflexa actively infected both human (PMA differentiated) THP1 and mouse RAW264.7 macrophage cell lines. Next, we assessed the intracellular fate of leptospires using bioluminescent strains, and we observed a drastic reduction in the leptospiral intracellular load between 3 h and 6 h post-infection, suggesting that leptospires do not replicate within these cells. Surprisingly, the classical macrophage microbicidal mechanisms (phagocytosis, autophagy, TLR-mediated ROS, and RNS production) were not responsible for the observed decrease. Finally, we demonstrated that the reduction in the intracellular load was associated with an increase of the bacteria in the supernatant, suggesting that leptospires exit both human and murine macrophages. Overall, our study reevaluated the intracellular fate of leptospires and favors an active entrance followed by a rapid exit, suggesting that leptospires do not have an intracellular lifestyle in macrophages.

Pathogenic Leptospires Limit Dendritic Cell Activation Through Avoidance of TLR4 and TRIF Signaling

Leptospira interrogans is a bacterial species responsible for leptospirosis, a neglected worldwide zoonosis. Mice and rats are resistant and can become asymptomatic carriers, whereas humans and some other mammals may develop severe forms of leptospirosis. Uncommon among spirochetes, leptospires contain lipopolysaccharide (LPS) in their outer membrane. LPS is highly immunogenic and forms the basis for a large number of serovars. Vaccination with inactivated leptospires elicits a protective immunity, restricted to serovars with related LPS. This protection that lasts in mice, is not long lasting in humans and requires annual boosts. Leptospires are stealth pathogens that evade the complement system and some pattern recognition receptors from the Toll-like (TLR) and Nod-Like families, therefore limiting antibacterial defense. In macrophages, leptospires totally escape recognition by human TLR4, and escape the TRIF arm of the mouse TLR4 pathway. However, very little is known about the recognition and processing of leptospires by dendritic cells (DCs), although they are crucial cells linking innate and adaptive immunity. Here we tested the activation of primary DCs derived from human monocytes (MO-DCs) and mouse bone marrow (BM-DCs) 24h after stimulation with saprophytic or different pathogenic virulent or avirulent L. interrogans. We measured by flow cytometry the expression of DC-SIGN, a lectin involved in T-cell activation, co-stimulation molecules and MHC-II markers, and pro- and anti-inflammatory cytokines by ELISA. We found that exposure to leptospires, live or heat-killed, activated dendritic cells. However, pathogenic L. interrogans, especially from the Icterohaemorraghiae Verdun strain, triggered less marker upregulation and less cytokine production than the saprophytic Leptospira biflexa. In addition, we showed a better activation with avirulent leptospires, when compared to the virulent parental strains in murine BM-DCs. We did not observe this difference in human MO-DCs, suggesting a role for TLR4 in DC stimulation. Accordingly, using BM-DCs from transgenic deficient mice, we showed that virulent Icterohaemorraghiae and Manilae serovars dampened DC activation, at least partly, through the TLR4 and TRIF pathways. This work shows a novel bacterial immune evasion mechanism to limit DC activation and further illustrates the role of the leptospiral LPS as a virulence factor.

Bovine Leptospirosis Due to Persistent Renal Carriage of Leptospira borgpetersenii Serovar Tarassovi

Leptospirosis is a global zoonotic disease that causes significant morbidity and mortality in human and animal populations. Leptospira interrogans is a leading cause of human disease, and L. borgpetersenii is a leading cause of animal disease. Cattle are reservoir hosts of L. borgpetersenii serovar Hardjo, which is transmitted via urine, semen, and uterine discharges resulting in abortion and poor reproductive performance. Bovine bacterin vaccines can only protect against those serovars included in vaccine formulations and typically include serovar Hardjo among others. Genotyping and serotyping represent two different and unique methods for classifying leptospires that do not always correlate well; comprehensive characterization using either method requires recovery of isolates from infected animals. In this study, we report for the first time, isolation of L. borgpetersenii serovar Tarassovi from the urine of a dairy cow in the U.S. The classification of the isolate, designated strain MN900, was confirmed by whole-genome sequencing, serotyping with reference antisera and monoclonal antibodies, Matrix Assisted Laser Desorption/Ionization (MALDI), and immunoblotting with reference antisera. Strain MN900 was excreted in urine samples for 18 weeks even as the cow was seronegative for serovar Tarassovi. Strain MN900 has an unusual morphology since it is not as motile as other leptospires and lacks hooked ends. Serovar Tarassovi is not included in U.S. bacterin vaccines. These results demonstrate the importance of culture and concomitant genotyping and serotyping to accurately classify leptospires, and as required to design efficacious vaccine and diagnostic strategies to not only limit animal disease but reduce zoonotic risk.

Activation of Canine, Mouse and Human TLR2 and TLR4 by Inactivated Leptospira Vaccine Strains

Canine Leptospira vaccines contain inactivated strains of pathogenic Leptospira, the causative agents of leptospirosis. For an effective response to vaccination, activation of the innate immune system via pattern recognition receptors such as TLRs is crucial. However, it is not known which TLRs are activated by Leptospira in dogs. To investigate the involvement of canine TLR2, TLR4, and TLR5 in the recognition of Leptospira, we stimulated canine moDC and reporter cells expressing canine TLR2 with either whole-inactivated bacteria or purified LPS of Leptospira strains, representing the serogroups generally used in canine leptospirosis vaccines. Using the endotoxin neutralizing reagent polymyxin B and TLR4 antagonist RS-LPS, we demonstrate that Leptospira LPS and canine TLR4 are involved in IL-1β production as well as in the uptake of inactivated Leptospira in canine moDC. Furthermore, polymyxin B only partially inhibited IL-1β production induced by inactivated Leptospira, suggesting that next to TLR4, also other TLRs may be involved. The observed activation of canine TLR2-expressing reporter cells by inactivated Leptospira strains indicates that TLR2 could be one of these TLRs. Next, we analyzed TLR2 and TLR4 activating capabilities by the same Leptospira strains using human and mouse TLR-expressing reporter cells. Inactivated Leptospira and leptospiral LPS activated not only mouse, but also human TLR4 and this activation was shown to be LPS dependent in both cases. Additionally, inactivated Leptospira activated mouse and human TLR2-expressing reporter cell lines. In our study, we could not identify significant species differences in the recognition of Leptospira by TLR2 and TLR4 between dog, human and mouse. Lastly, we show that these inactivated Leptospira strains are recognized by both mouse and human TLR5 reporter cells only after exposure to additional heat-treatment. Unfortunately, we were not able to confirm this in the canine system. Our data show that TLR2 and TLR4 are involved in the recognition of Leptospira strains used in the production of canine Leptospira vaccines. This study contributes to the understanding of Leptospira-induced innate immune responses in dogs, humans, and mice. Future studies are needed to further explore the role of canine TLR2, TLR4 and TLR5 in the induction of vaccine-mediated immunity against Leptospira.

Xenophagy of invasive bacteria is differentially activated and modulated via a TLR-TRAF6-Beclin1 axis in echinoderms

In marine environments, organisms are confronted with numerous microbial challenges, although the differential regulation of xenophagy in response to different pathogenic bacterial species remains relatively unknown. Here, we addressed this issue using Apostichopus japonicus as a model. We identified 39 conserved autophagy-related genes by genome-wide screening, which provided a molecular basis for autophagy regulation in sea cucumbers. Furthermore, xenophagy of two Gram-negative bacteria, Vibrio splendidus and Escherichia coli, but not a Gram-positive bacteria, Micrococcus luteus, was observed in different autophagy assays. Surprisingly, a significantly higher autophagy capacity was found in the E. coli–challenged group than in the V. splendidus–challenged group. To confirm these findings, two different lipopolysaccharides, LPS^{V. splendidus} and LPS^E. coli, were isolated; we found that these LPS species differentially activated coelomocyte xenophagy. To explore the molecular mechanism mediating differential levels of xenophagy, we used an siRNA knockdown assay and confirmed that LPS^{V. splendidus}-mediated xenophagy was dependent on an AjTLR3-mediated pathway, whereas LPS^E. coli-mediated xenophagy was dependent on AjToll. Moreover, the activation of different AjTLRs resulted in AjTRAF6 ubiquitination and subsequent activation of K63-linked ubiquitination of AjBeclin1. Inversely, the LPS^{V. splendidus}-induced AjTLR3 pathway simultaneously activated the expression of AjA20, which reduced the extent of K63-linked ubiquitination of AjBeclin1 and impaired the induction of autophagy; however, this finding was no t evident with LPS^E. coli. Our present results provide the first evidence showing that xenophagy could be differentially induced by different bacterial species to yield differential autophagy levels in echinoderms.

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.

Inflammatory Signatures of Pathogenic and Non-Pathogenic Leptospira Infection in Susceptible C3H-HeJ Mice

The exact global impact of leptospirosis is unknown due to inadequate surveillance systems in place in most low-income countries. In this study, we analyzed the differences in mouse inflammatory signatures involved in pathogenic versus non-pathogenic Leptospira recognition at 24h and 72h post infection. Injection of C3H-HeJ mice with non-pathogenic L. biflexa increased circulation of a few chemokines (5/21, 24%) without secretion of cytokines in blood that resulted in engagement of resident macrophages, dendritic cells, neutrophils and NK cells without engagement of T cells. In contrast, pathogenic L. interrogans induced circulation of a much higher panel of chemokines (18/21, 86%) and pro- and anti-inflammatory cytokines (11/19, 58%) in blood with a resulting signaling cascade leading to engagement of macrophages, dendritic cells, monocytes, NK cells and T cells without engagement of neutrophils. Although neutrophils do not appear to be engaged, a considerable number of chemokines that recruit other granulocytes such as eosinophils and basophils were also increased at 72h post infection with L. interrogans. Overall, the data suggest that prevention of dissemination of L. biflexa is associated with an early engagement of the innate immune response characterized by upregulation of a few chemokines that results in an efficacious phagocytic response without an overwhelming increase of pro-inflammatory cytokines. However, when macrophages fail to clear a pathogenic serovar such as L. interrogans, the adaptive response (T cells) is engaged to help out, but the resulting chemo-cytokine storm mediates a robust but non-resolving inflammatory response to pathogenic Leptospira that results in dissemination, kidney colonization, pathology and disease.

Anti-Leptospira immunoglobulin profiling in mice reveals strain specific IgG and persistent IgM responses associated with virulence and renal colonization

Leptospira interrogans is a pathogenic spirochete responsible for leptospirosis, a neglected, zoonotic reemerging disease. Humans are sensitive hosts and may develop severe disease. Some animal species, such as rats and mice can become asymptomatic renal carriers. More than 350 leptospiral serovars have been identified, classified on the basis of the antibody response directed against the lipopolysaccharide (LPS). Similarly to whole inactivated bacteria used as human vaccines, this response is believed to confer only short-term, serogroup-specific protection. The immune response of hosts against leptospires has not been thoroughly studied, which complicates the testing of vaccine candidates. In this work, we studied the immunoglobulin (Ig) profiles in mice infected with L. interrogans over time to determine whether this humoral response confers long-term protection after homologous challenge six months post-infection. Groups of mice were injected intraperitoneally with 2×10⁷ leptospires of one of three pathogenic serovars (Manilae, Copenhageni or Icterohaemorrhagiae), attenuated mutants or heat-killed bacteria. Leptospira-specific immunoglobulin (IgA, IgM, IgG and 4 subclasses) produced in the first weeks up to 6 months post-infection were measured by ELISA. Strikingly, we found sustained high levels of IgM in mice infected with the pathogenic Manilae and Copenhageni strains, both colonizing the kidney. In contrast, the Icterohaemorrhagiae strain did not lead to kidney colonization, even at high dose, and triggered a classical IgM response that peaked at day 8 post-infection and disappeared. The virulent Manilae and Copenhageni serovars elicited high levels and similar profiles of IgG subclasses in contrast to Icterohaemorrhagiae strains that stimulated weaker antibody responses. Inactivated heat-killed Manilae strains elicited very low responses. However, all mice pre-injected with leptospires challenged with high doses of homologous bacteria did not develop acute leptospirosis, and all antibody responses were boosted after challenge. Furthermore, we showed that 2 months post-challenge, mice pre-infected with the attenuated M895 Manilae LPS mutant or heat-killed bacterin were completely protected against renal colonization. In conclusion, we observed a sustained IgM response potentially associated with chronic leptospiral renal infection. We also demonstrated in mice different profiles of protective and cross-reactive antibodies after L. interrogans infection, depending on the serovar and virulence of strains.

Leptospira interrogans is a pathogenic spirochete responsible for leptospirosis, a neglected zoonotic reemerging disease. The immune response of hosts against these bacteria has not been thoroughly studied. Here, we studied over 6 months the antibody profiles in mice infected with L. interrogans and determined whether this humoral response confers long-term protection after homologous challenge six months after primary infection. Groups of mice were infected intraperitoneally with 2×10⁷ bacteria of one of three different pathogenic serovars (Manilae, Copenhageni and Icterohaemorrhagiae) and some corresponding attenuated avirulent mutants. We measured by ELISA each type of Leptospira-specific immunoglobulin (Ig) (IgA, IgM, IgG and 4 subclasses) produced in the first weeks up to 6 months post-infection and studied their cross-reactivities among serovars. We showed different profiles of antibody response after L. interrogans challenge in mice, depending on the serovar and virulence of strains. However, all infected mice, including the ones harboring low antibody levels, like mice vaccinated with an inactivated, heat-killed strain, were protected against leptospirosis after challenge. Notably, we also showed an unusual sustained IgM response associated with chronic leptospiral colonization. Altogether, this long-term immune protection is different from what is known in humans and warrants further investigation.

Biochemical analysis of leptospiral LPS explained the difference between pathogenic and non-pathogenic serogroups

Lipopolysaccharide (LPS) is the major surface antigen of Leptospira. In this study, the genes involved in the LPS biosynthesis were analyzed and compared by bioinformatics tools. Also, the chemical composition analysis of leptospiral lipopolysaccharides (LPS) extracted from 5 pathogenic serovars like Autumnalis, Australis, Ballum, Grippotyphosa, Pomona, and the nonpathogenic serovar Andamana was performed. Methods used were Limulus amebocyte lysate assay (LAL), gas chromatography-mass spectrometry (GC-MS), fourier transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance spectroscopy (NMR). LAL assay showed a significantly higher level of endotoxicity among pathogenic serovars (~0.490 EU/mL) than that of nonpathogenic Andamana (~0.102 EU/mL). FAMES analysis showed the presence of palmitic acid (C16:0), hydroxy lauric acid (3-OH-C12:0), and oleic acid (C18:0). Palmitoleic acid (C16: 1), and 3- hydroxy palmitate (3-OH-C16:0) was detected only in pathogenic serovars. In contrast myristoleic acid (C14:1) and stearic acid (C18:0) were present in Andamana. FTIR analysis revealed C-O-C stretch of esters, 3°ROH functional groups and carbohydrate vibration range were similar among pathogenic serovars. The NMR analysis reveals similarity for 6 deoxy sugars and methyl groups of Autumnalis, Australis, and Ballum. Further, the presence of palmitoleic acid and 3-hydroxy palmitate may be the significant pathogen-associated predisposing factor. This mediates high osmolarity glycerol (HOG) mediated stress response in leptospiral LPS mediated pathogenesis.

Cord Blood-Based Approach to Assess Candidate Vaccine Adjuvants Designed for Neonates and Infants

Neonates and infants are particularly susceptible to infections, for which outcomes tend to be severe. Vaccination is a key strategy for preventing infectious diseases, but the protective immunity achieved through vaccination typically is weaker in infants than in healthy adults. One possible explanation for the poor acquisition of vaccine-induced immunity in infants is that their innate immune response, represented by toll-like receptors, is immature. The current system for developing pediatric vaccines relies on the confirmation of their safety and effectiveness in studies involving the use of mature animals or adult humans. However, creating vaccines for neonates and infants requires an understanding of their uniquely immature innate immunity. Here we review current knowledge regarding the innate immune system of neonates and infants and challenges in developing vaccine adjuvants for those children through analyses of cord blood.

Role of TLR4 in Persistent Leptospira interrogans Infection: A Comparative In Vivo Study in Mice

Toll-Like Receptor (TLR) 4, the LPS receptor, plays a central role in the control of leptospirosis and absence of TLR4 results in lethal infection in mice. Because human TLR4 does not sense the atypical leptospiral-LPS, we hypothesized that TLR4/MD-2 humanized transgenic mice (huTLR4) may be more susceptible to leptospirosis than wild-type mice, and thus may constitute a model of acute human leptospirosis. We infected huTLR4 mice, which express human TLR4 but not murine TLR4, with a high dose of L. interrogans serovar Copenhageni FioCruz (Leptospira) in comparison to C57BL/6J wild-type (WT) and, as a control, a congenic strain in which the tlr4 coding sequences are deleted (muTLR4^Lps-del). We show that the huTLR4 gene is fully functional in the murine background. We found that dissemination of Leptospira in blood, shedding in urine, colonization of the kidney and overall kinetics of leptospirosis progression is equivalent between WT and huTLR4 C57BL/6J mice. Furthermore, inflammation of the kidney appeared to be subdued in huTLR4 compared to WT mice in that we observed less infiltrates of mononuclear lymphocytes, less innate immune markers and no relevant differences in fibrosis markers. Thus, huTLR4 mice showed less inflammation and kidney pathology, and are not more susceptible to leptospirosis than WT mice. This study is significant as it indicates that one intact TLR4 gene, be it mouse or human, is necessary to control acute leptospirosis.

A Mouse Model of Sublethal Leptospirosis: Protocols for Infection with Leptospira Through Natural Transmission Routes, for Monitoring Clinical and Molecular Scores of Disease, and for Evaluation of the Host Immune Response

Leptospirosis is a zoonotic disease caused by pathogenic Leptospira species that are maintained in sylvatic and domestic environments by transmission among rodents and other carriers. Humans become infected after contact of breached skin or mucosa with contaminated water or soil. Understanding persistent or sublethal infection in a host is critical for controlling human risk of exposure to pathogenic Leptospira. Animal models that recapitulate disease progression after infection via natural transmission routes are more appropriate for validation of vaccines and therapeutics. Furthermore, the ability to measure shedding of live Leptospira in urine of reservoir and carrier hosts can be used to develop new diagnostic assays and sensors to evaluate human risk of exposure. We developed inbred mouse models of Leptospirosis, that bypass survival as a criterion, in which we can analyze both pathogen and host factors affecting sublethal infection (<1 month), including shedding of Leptospira in urine. Mice are infected with pathogenic Leptospira using a physiologic route, and the clinical, histological, and molecular scores of disease are measured. Furthermore, the host immune response to Leptospira is evaluated. This mouse model also provides a tool in which to test fundamental hypotheses related to host-pathogen interactions and the immune mechanisms engaged in protective and pathogenic immune responses. © 2020 Wiley Periodicals LLC Basic Protocol 1: Culture and maintenance of virulent Leptospira Basic Protocol 2: Infection of mice through a physiologic route and collection of clinical scores and biological samples Basic Protocol 3: Analysis of pathogenesis after Leptospira infection.

The pre-activated immune response induced by LPS protects host from leptospirosis

Leptospirosis is an important global zoonosis caused by pathogenic Leptospira. It is estimated that more than 1 million people are infected by Leptospira each year, and the death toll is about 60,000. Some studies showed that delayed immune response was associated with severe leptospirosis, and TLR4 was very important in the control of leptospirosis. In this study, we aimed to explore the effect of the classical activator (LPS) of TLR4 on leptospirosis in susceptible and resistant hosts. The results showed that LPS pretreatment increased the survival rate of hamsters to 80%. And LPS pre-treatment also significantly reduced the leptospiral load and alleviated the pathological injury in organs of hamsters and mice. The result detected by ELISA in mice showed that the levels of TNF-α and IL-1β were increased in the LPS-treated group compared to the control group before infection. However, two days after infection, the level of cytokines in LPS group was down-regulated compared with that in control group. In addition, in vitro results showed that LPS pre-treatment enhanced the phagocytosis and bactericidal ability of macrophages on Leptospira. Collectively, our results indicated that the pre-activated immune response induced by LPS enhanced the ability of host against leptospirosis.

Phagocyte Escape of Leptospira: The Role of TLRs and NLRs

The spirochetal bacteria Leptospira spp. are causative agents of leptospirosis, a globally neglected and reemerging zoonotic disease. Infection with these pathogens may lead to an acute and potentially fatal disease but also to chronic asymptomatic renal colonization. Both forms of disease demonstrate the ability of leptospires to evade the immune response of their hosts. In this review, we aim first to recapitulate the knowledge and explore the controversial data about the opsonization, recognition, intracellular survival, and killing of leptospires by scavenger cells, including platelets, neutrophils, macrophages, and dendritic cells. Second, we will summarize the known specificities of the recognition or escape of leptospire components (the so-called microbial-associated molecular patterns; MAMPs) by the pattern recognition receptors (PRRs) of the Toll-like and NOD-like families. These PRRs are expressed by phagocytes, and their stimulation by MAMPs triggers pro-inflammatory cytokine and chemokine production and bactericidal responses, such as antimicrobial peptide secretion and reactive oxygen species production. Finally, we will highlight recent studies suggesting that boosting or restoring phagocytic functions by treatments using agonists of the Toll-like or NOD receptors represents a novel prophylactic strategy and describe other potential therapeutic or vaccine strategies to combat leptospirosis.

AAA+ Molecular Chaperone ClpB in Leptospira interrogans: Its Role and Significance in Leptospiral Virulence and Pathogenesis of Leptospirosis

Bacterial ClpB is an ATP-dependent disaggregase that belongs to the Hsp100/Clp subfamily of the AAA+ ATPases and cooperates with the DnaK chaperone system in the reactivation of aggregated proteins, as well as promotes bacterial survival under adverse environmental conditions, including thermal and oxidative stresses. In addition, extensive evidence indicates that ClpB supports the virulence of numerous bacteria, including pathogenic spirochaete Leptospira interrogans responsible for leptospirosis in animals and humans. However, the specific function of ClpB in leptospiral virulence still remains to be fully elucidated. Interestingly, ClpB was predicted as one of the L. interrogans hub proteins interacting with human proteins, and pathogen-host protein interactions are fundamental for successful invasion of the host immune system by bacteria. The aim of this review is to discuss the most important aspects of ClpB's function in L. interrogans, including contribution of ClpB to leptospiral virulence and pathogenesis of leptospirosis, a zoonotic disease with a significant impact on public health worldwide.

Investigating the Immunological and Biological Equilibrium of Reservoir Hosts and Pathogenic Leptospira: Balancing the Solution to an Acute Problem?

Leptospirosis is a devastating zoonotic disease affecting people and animals across the globe. Pathogenic leptospires are excreted in urine of reservoir hosts which directly or indirectly leads to continued disease transmission, via contact with mucous membranes or a breach of the skin barrier of another host. Human fatalities approach 60,000 deaths per annum; though most vertebrates are susceptible to leptospirosis, complex interactions between host species and serovars of Leptospira can yield disease phenotypes that vary from asymptomatic shedding in reservoir hosts, to multi-organ failure in incidental hosts. Clinical symptoms of acute leptospirosis reflect the diverse range of pathogenic species and serovars that cause infection, the level of exposure, and the relationship of the pathogen with the given host. However, in all cases, pathogenic Leptospira are excreted into the environment via urine from reservoir hosts which are uniformly recognized as asymptomatic carriers. Therefore, the reservoir host serves as the cornerstone of persistent disease transmission. Although bacterin vaccines can be used to abate renal carriage and excretion in domestic animal species, there is an urgent need to advance our understanding of immune-mediated host–pathogen interactions that facilitate persistent asymptomatic carriage. This review summarizes the current understanding of host–pathogen interactions in the reservoir host and prioritizes research to unravel mechanisms that allow for colonization but not destruction of the host. This information is required to understand, and ultimately control, the transmission of pathogenic Leptospira.

Leptospiral LPS escapes mouse TLR4 internalization and TRIF‑associated antimicrobial responses through O antigen and associated lipoproteins

Leptospirosis is a worldwide re-emerging zoonosis caused by pathogenic Leptospira spp. All vertebrate species can be infected; humans are sensitive hosts whereas other species, such as rodents, may become long-term renal carrier reservoirs. Upon infection, innate immune responses are initiated by recognition of Microbial Associated Molecular Patterns (MAMPs) by Pattern Recognition Receptors (PRRs). Among MAMPs, the lipopolysaccharide (LPS) is recognized by the Toll-Like-Receptor 4 (TLR4) and activates both the MyD88-dependent pathway at the plasma membrane and the TRIF-dependent pathway after TLR4 internalization. We previously showed that leptospiral LPS is not recognized by the human-TLR4, whereas it signals through mouse-TLR4 (mTLR4), which mediates mouse resistance to acute leptospirosis. However, although resistant, mice are known to be chronically infected by leptospires. Interestingly, the leptospiral LPS has low endotoxicity in mouse cells and is an agonist of TLR2, the sensor for bacterial lipoproteins. Here, we investigated the signaling properties of the leptospiral LPS in mouse macrophages. Using confocal microscopy and flow cytometry, we showed that the LPS of L. interrogans did not induce internalization of mTLR4, unlike the LPS of Escherichia coli. Consequently, the LPS failed to induce the production of the TRIF-dependent nitric oxide and RANTES, both important antimicrobial responses. Using shorter LPS and LPS devoid of TLR2 activity, we further found this mTLR4-TRIF escape to be dependent on both the co-purifying lipoproteins and the full-length O antigen. Furthermore, our data suggest that the O antigen could alter the binding of the leptospiral LPS to the co-receptor CD14 that is essential for TLR4-TRIF activation. Overall, we describe here a novel leptospiral immune escape mechanism from mouse macrophages and hypothesize that the LPS altered signaling could contribute to the stealthiness and chronicity of the leptospires in mice.

Leptospira interrogans is a bacterial pathogen, responsible for leptospirosis, a worldwide neglected reemerging disease. L. interrogans may cause an acute severe disease in humans, whereas rodents and other animals asymptomatically carry the leptospires in their kidneys. They can therefore excrete live bacteria in urine and contaminate the environment. Leptospires are stealth pathogens known to escape the innate immune defenses of their hosts. They are covered in lipopolysaccharide (LPS), a bacterial motif recognized in mammals through the Toll-like receptor 4 (TLR4), which triggers two different signaling pathways. We showed previously that pathogenic leptospires fully escape TLR4 recognition in humans. Here we focused on the LPS signaling in mice that are, although resistant to acute leptospirosis, chronically infected. We showed in mouse cells that the leptospiral LPS triggers only one arm of the TLR4 pathway and escapes the other, hence avoiding production of antimicrobial compounds. Removing the lipoproteins that always co-purify with the leptospiral LPS, or using shorter LPS, restores the stimulation of both pathways. This suggests a novel escape mechanism linked to the LPS and involving lipoproteins that could be instrumental for leptospires to escape the mouse defense and to allow for their chronic renal colonization.

Escape of TLR5 Recognition by Leptospira spp.: A Rationale for Atypical Endoflagella

Leptospira (L.) interrogans are invasive bacteria responsible for leptospirosis, a worldwide zoonosis. They possess two periplasmic endoflagellae that allow their motility. L. interrogans are stealth pathogens that escape the innate immune recognition of the NOD-like receptors NOD1/2, and the human Toll-like receptor (TLR)4, which senses peptidoglycan and lipopolysaccharide (LPS), respectively. TLR5 is another receptor of bacterial cell wall components, recognizing flagellin subunits. To study the contribution of TLR5 in the host defense against leptospires, we infected WT and TLR5 deficient mice with pathogenic L. interrogans and tracked the infection by in vivo live imaging of bioluminescent bacteria or by qPCR. We did not identify any protective or inflammatory role of murine TLR5 for controlling pathogenic Leptospira. Likewise, subsequent in vitro experiments showed that infections with different live strains of L. interrogans and L. biflexa did not trigger TLR5 signaling. However, unexpectedly, heat-killed bacteria stimulated human and bovine TLR5, but did not, or barely induced stimulation via murine TLR5. Abolition of TLR5 recognition required extensive boiling time of the bacteria or proteinase K treatment, showing an unusual high stability of the leptospiral flagellins. Interestingly, after using antimicrobial peptides to destabilize live leptospires, we detected TLR5 activity, suggesting that TLR5 could participate in the fight against leptospires in humans or cattle. Using different Leptospira strains with mutations in the flagellin proteins, we further showed that neither FlaA nor Fcp participated in the recognition by TLR5, suggesting a role for the FlaB. FlaB have structural homology to Salmonella FliC, and possess conserved residues important for TLR5 activation, as shown by in silico analyses. Accordingly, we found that leptospires regulate the expression of FlaB mRNA according to the growth phase in vitro, and that infection with L. interrogans in hamsters and in mice downregulated the expression of the FlaB, but not the FlaA subunits. Altogether, in contrast to different bacteria that modify their flagellin sequences to escape TLR5 recognition, our study suggests that the peculiar central localization and stability of the FlaB monomers in the periplasmic endoflagellae, associated with the downregulation of FlaB subunits in hosts, constitute an efficient strategy of leptospires to escape the TLR5 recognition and the induced immune response.

Crawling Motility on the Host Tissue Surfaces Is Associated With the Pathogenicity of the Zoonotic Spirochete Leptospira

Bacterial motility is crucial for many pathogenic species in the process of invasion and/or dissemination. The spirochete bacteria Leptospira spp. cause symptoms, such as hemorrhage, jaundice, and nephritis, in diverse mammals including humans. Although loss-of-motility attenuate the spirochete's virulence, the mechanism of the motility-dependent pathogenicity is unknown. Here, focusing on that Leptospira spp. swim in liquid and crawl on solid surfaces, we investigated the spirochetal dynamics on the host tissues by infecting cultured kidney cells from various species with pathogenic and non-pathogenic leptospires. We found that, in the case of the pathogenic leptospires, a larger fraction of bacteria attached to the host cells and persistently traveled long distances using the crawling mechanism. Our results associate the kinetics and kinematic features of the spirochetal pathogens with their virulence.

MicroRNAs Regulated by the LPS/TLR2 Immune Axis as Bona Fide Biomarkers for Diagnosis of Acute Leptospirosis

Here, we used miRNAs that are differentially regulated by the LPS/TLR2 immune axis to devise a miRNA-based diagnosis for leptospirosis. The study established the role of the circulating stable miRNAs (miR-21-5p, miR-144-3p, and miR-let-7b-5p) as an early diagnostic marker for leptospirosis. These miRNAs can be used to diagnose acute leptospirosis and also to differentiate leptospiral infection from other bacterial and spirochetal infections, as proved by the use of human clinical samples. Thus, our findings indicate that miRNAs can play a crucial role in the diagnosis of infectious diseases, like leptospirosis, that are generally misdiagnosed.

Leptospirosis remains a significant human health issue due to its systemic complications. Therefore, biomarkers that are more effective are urgently needed for the early diagnosis of leptospirosis. MicroRNAs (miRNAs) are evolutionarily conserved regulatory RNAs that have shown the potential to be used as biomarkers for diagnosis, prognosis, and therapy of infectious diseases. In this study, we performed an unbiased screen using the miRNome miRNA array to identify circulating miRNAs with the potential to serve as authentic biomarkers for early diagnosis of leptospirosis. Because leptospiral lipopolysaccharide (LPS) is the predominant leptospiral antigen and plays a vital role in immunological and biological activities, we used LPS treated and untreated in vitro (THP1 cells) and in vivo (BALB/c mice) surrogate models to identify the LPS-specific miRNAs. Differential expression analysis revealed 18 miRNAs to be associated strongly with LPS stimulation in THP1 cells. Of these, three (miR-let-7b-5p, miR-144-3p, and miR-21-5p) were observed to be present at increased levels in vivo. The identified miRNAs were validated for their biomarker potential using serum samples from leptospirosis-negative patients and patients with confirmed cases of leptospirosis. Identified miRNAs were able to discriminate the acute leptospiral infection from other febrile diseases with a test sensitivity and specificity of 93.2% and 88.19%, respectively. Gene functional enrichment and protein-protein interaction (PPI) network analysis revealed that the identified miRNAs play important roles in disease signal transduction, signaling by interleukins, the stress-activated protein kinase signaling cascade, the mitogen-activated protein kinase (MAPK) signaling pathway, and the cellular response to a transforming growth factor β (TGF-β) stimulus with a notable interconnection between these biological processes.

IMPORTANCE Here, we used miRNAs that are differentially regulated by the LPS/TLR2 immune axis to devise a miRNA-based diagnosis for leptospirosis. The study established the role of the circulating stable miRNAs (miR-21-5p, miR-144-3p, and miR-let-7b-5p) as an early diagnostic marker for leptospirosis. These miRNAs can be used to diagnose acute leptospirosis and also to differentiate leptospiral infection from other bacterial and spirochetal infections, as proved by the use of human clinical samples. Thus, our findings indicate that miRNAs can play a crucial role in the diagnosis of infectious diseases, like leptospirosis, that are generally misdiagnosed.

Evidence of Pathogen-Induced Immunogenetic Selection across the Large Geographic Range of a Wild Seabird

Over evolutionary time, pathogen challenge shapes the immune phenotype of the host to better respond to an incipient threat. The extent and direction of this selection pressure depend on the local pathogen composition, which is in turn determined by biotic and abiotic features of the environment. However, little is known about adaptation to local pathogen threats in wild animals. The Gentoo penguin (Pygoscelis papua) is a species complex that lends itself to the study of immune adaptation because of its circumpolar distribution over a large latitudinal range, with little or no admixture between different clades. In this study, we examine the diversity in a key family of innate immune genes-the Toll-like receptors (TLRs)-across the range of the Gentoo penguin. The three TLRs that we investigated present varying levels of diversity, with TLR4 and TLR5 greatly exceeding the diversity of TLR7. We present evidence of positive selection in TLR4 and TLR5, which points to pathogen-driven adaptation to the local pathogen milieu. Finally, we demonstrate that two positively selected cosegregating sites in TLR5 are sufficient to alter the responsiveness of the receptor to its bacterial ligand, flagellin. Taken together, these results suggest that Gentoo penguins have experienced distinct pathogen-driven selection pressures in different environments, which may be important given the role of the Gentoo penguin as a sentinel species in some of the world's most rapidly changing environments.

Leptospira Detection in Cats in Spain by Serology and Molecular Techniques

Leptospirosis is the most neglected widespread zoonosis worldwide. In Spain, leptospirosis reports in people and animals have increased lately. Cats can become infected with Leptospira, as well as be chronic carriers. The aim of this study was to determine serological antibody prevalence against Leptospira sp., blood DNA, and shedding of DNA from pathogenic Leptospira species in the urine of cats in Spain. Microagglutination tests (MAT) and blood and urine TaqMan real-time polymerase chain reaction (PCR) were performed. Leptospira antibodies were detected in 10/244 cats; with 4.1% positive results (95% confidence interval (CI): 2.1-7.18%). Titers ranged from 1:20 to 1:320 (serovars Ballum; Bataviae; Bratislava; Cynopteri; Grippotyphosa Mandemakers; Grippotyphosa Moskva; Pomona; and Proechimys). The most common serovar was Cynopteri. Blood samples from 1/89 cats amplified for Leptospira DNA (1.12%; 95% CI: 0.05-5.41%). Urine samples from 4/232 cats amplified for Leptospira DNA (1.72%; 95% CI: 0.55-4.10%). In conclusion free-roaming cats in Spain can shed pathogenic Leptospira DNA in their urine and may be a source of human infection. Serovars not previously described in cats in Spain were detected; suggesting the presence of at least 4 different species of pathogenic leptospires in the country (L. borgpetersenii; L. interrogans; L. kirschneri; and L. noguchii).

Increased inflammation with crude E. coli LPS protects against acute leptospirosis in hamsters

Leptospirosis is a worldwide zoonotic disease that causes acute kidney injury, liver disease, bleeding disorders, and even death. Treatment of the disease is largely dependent on the use of antibiotics, but recent studies on pathogenesis of leptospirosis have shown that immunomodulation may also be an effective treatment for this disease. Since the delay in inflammation correlates with higher pathogenicity of leptospira, we studied the effect of inducing inflammation on leptospirosis by using TLR4 activator LPS. In accordance with our hypothesis, treatment with LPS protected against leptospirosis by enhancing the inflammatory response in hamsters. The gene expression levels of TLR2, TLR4, NLRP3 and inflammatory factors were higher in LPS-treated group during leptospira infection in hamsters. Although the levels of NO and iNOS were higher in LPS-treated group than in Leptospira-infected group, the protective effect induced by LPS is iNOS-independent. Treatment with LPS induced higher anti-leptospira IgG level than infection with leptospira alone. Then, expressions of costimulatory molecules and maturation markers were analysed. The data showed that treatment with LPS enhanced the expression of CD40, CD80 and CD86. Our results indicate that increased inflammation induced by LPS derived from Escherichia coli (E. coli) protects against leptospirosis in hamsters.

The route of infection with Leptospira interrogans serovar Copenhageni affects the kinetics of bacterial dissemination and kidney colonization

The goal of this study was to characterize how natural routes of infection affect the kinetics of pathogenic Leptospira dissemination to blood and kidney. C3H/HeJ mice were sublethally infected with L. interrogans serovar Copenhageni FioCruz L1-130 (Leptospira) through exposure of a dermis wound and through oral and nasal mucosa, in comparison to uninfected mice and to mice infected via standard intraperitoneal inoculation. In striking contrast to oral mucosa inoculation, transdermal and nasal mucosa infections led to weight loss, renal colonization and inflammation, as previously observed for conjunctival and intraperitoneal infections. However, the timing at which Leptospira gained access to blood, as well as Leptospira' colonization of the kidney and shedding in urine, differed from intraperitoneal infection. Furthermore, a comparative analysis of transcription of pro-inflammatory mediators in kidney and total immunoglobulin isotyping in serum from infected mice, showed increased innate immune response markers (KC, MIP-2, TNF-α) and lower Th1 associated IFN-γ in kidney, as well as lower Th1 associated IgG2a in mice infected through the nasal mucosa as compared to intraperitoneal infection. We conclude that the route of infection affects the timing at which Leptospira gains access to blood for dissemination, as well as the dynamics of colonization and inflammation of the kidney.

Purification of LPS from Leptospira

Leptospira species are one of the few spirochetes to possess a lipopolysaccharide (LPS) embedded in their outer membrane. Two protocols are currently available to extract and/or purify the leptospiral lipopolysaccharides: the rapid proteinase K method and the classical hot water/phenol extraction. The first method allows to get a quick overview of the LPS O antigen structure, whereas the second method is fitted to study the immunological properties of the leptospiral LPS. These two methods will be detailed in this chapter. Methodologies to assess the quality of the purification, such as the modified silver staining coloration, will also be reviewed. Both advantages and limitations of the different analyses will be described.

Leptospira and Leptospirosis

Leptospira spp. are morphologically and phylogenetically unique bacteria that are ubiquitous in the environment. Pathogenic species are the agents of leptospirosis, an emerging zoonotic disease transmitted to both human and animals from the environment contaminated with the urine of reservoir animals. The taxonomy of Leptospira has recently undergone extensive revisions with the use of whole-genome sequences, thus expanding considerably the number of species.

Gene expression is associated with virulence in murine macrophages infected with Leptospira spp

Leptospira genus contains species that affect human health with varying degrees of pathogenicity. In this context, we aimed to evaluate the differences in the modulation of host gene expression by strains of Leptospira varying in virulence. Our data showed a high number of differentially expressed transcripts in murine macrophages following 6h of infection. Leptospira infection modulated a set of genes independently of their degree of virulence. However, pathway analysis indicated that Apoptosis, ATM Signaling, and Cell Cycle: G2/M DNA Damage Checkpoint Regulation were exclusively regulated following infection with the virulent strain. Taken together, results demonstrated that species and virulence play a role during host response to Leptospira spp in murine macrophages, which could contribute to understanding the pathogenesis of leptospirosis.

Raised levels of Il-6, Il-17a, and Il-22 in fatal leptospirosis

Clinical manifestations of leptospirosis range from mild, common cold-like illness, to a life-threatening condition. The host immune response has been hypothesized to play a major role in leptospirosis outcome. Increased levels of inflammatory mediators, such as cytokines, may promote tissue damage that lead to increased disease severity. The question is whether cytokines levels may predict the outcome of leptospirosis and guide patient management. This study aimed to assess the association between Th1-, Th2-, and Th17-related cytokines with the clinical outcome of patients with leptospirosis. Different cytokine levels were measured in fifty-two plasma samples of hospitalized patients diagnosed with leptospirosis in Malaysia (January 2016-December 2017). Patients were divided into two separate categories: survived (n = 40) and fatal outcome (n = 12). Nineteen plasma samples from healthy individuals were obtained as controls. Cytokine quantification was performed using Simple Plex™ assays from ProteinSimple (San Jose, CA, USA). Measurements were done in triplicate and statistical analysis was performed using GraphPad software and SPSS v20. IL-6 (p = 0.033), IL-17A (p = 0.022), and IL-22 (p = 0.046) were significantly elevated in fatal cases. IL-17A concentration (OR 1.115; 95% CI 1.010-1.231) appeared to be an independent predictor of fatality of leptospirosis. Significantly higher levels of TNF-α (p ≤ 0.0001), IL-6 (p ≤ 0.0001), IL-10 (p ≤ 0.0001), IL-12 (p ≤ 0.0001), IL17A (p ≤ 0.0001), and IL-18 (p ≤ 0.0001) were observed among leptospirosis patients in comparison with healthy controls. Our study shows that certain cytokine levels may serve as possible prognostic biomarkers in leptospirosis patients.

An overview of human leptospirosis vaccine design and future perspectives

Introduction: It's been 20 years since the first report of a recombinant vaccine that protected against leptospirosis. Since then, numerous recombinant vaccines have been evaluated; however, no recombinant vaccine candidate has advanced to clinical trials. With the ever-increasing burden of leptospirosis, there is an urgent need for a universal vaccine against leptospirosis.Areas covered: This review covers the most promising vaccine candidates that induced significant, reproducible, protection and how advances in the field of bioinformatics has led to the discovery of hundreds of novel protein targets. The authors also discuss the most recent findings regarding the innate immune response and host-pathogen interactions and their impact on the discovery of novel vaccine candidates. In addition, the authors have identified what they believe are the most challenging problems for the discovery and development of a universal vaccine and their potential solutions.Expert opinion: A universal vaccine for leptospirosis will likely only be achieved using a recombinant vaccine as the bacterins are of limited use due to the lack of a cross-protective immune response. Although there are hundreds of novel targets, due to the lack of immune correlates and the need for more research into the basic microbiology of Leptospira spp., a universal vaccine is 10-15 years away.

The preventable efficacy of β-glucan against leptospirosis

Leptospirosis, caused by pathogenic Leptospira species, has emerged as an important neglected zoonotic disease. Few studies have reported the preventable effects of immunoregulators, except for antibiotics, against leptospirosis. Generally, immunostimulatory agents are considered effective for enhancing innate immune responses. Many studies have found that beta-glucan (β-glucan) could be a potent and valuable immunostimulant for improving immune responses and controlling diseases. In this study, we investigated the preventable role of β-glucan against Leptospira infection in hamsters. First, β-glucan was administered 24 h prior to, during and after infection. The results showed that β-glucan increased the survival rate to 100%, alleviated tissue injury, and decreased leptospire loads in target organs. Additionally, we found using quantitative real-time PCR that application of β-glucan significantly enhanced the expression of Toll-like receptor (TLR) 2, interleukin (IL)-1β and iNOS at 2 dpi (days post infection) and reduced the increase of TLR2, IL-1β and iNOS induced by Leptospira at 5 dpi. Furthermore, to induce memory immunity, β-glucan was administered 5 days prior to infection. β-Glucan also significantly increased the survival rates and ameliorated pathological damage to organs. Moreover, we demonstrated that β-glucan-trained macrophages exhibited elevated expression of proinflammatory cytokines (IL-1β and IL-6) in vitro, indicating that β-glucan induces an enhanced inflammatory response against Leptospira infection. These results indicate that administration of β-glucan and other immunostimulants could be potential valuable options for the control of Leptospira infection.

Leptospirosis, an important emerging neglected zoonotic disease, is caused by Leptospira and affects humans as well as animals. Due to the emergence of bacterial resistance to antibiotics, the development of alternatives to antibiotics has become an inevitable requirement in this new situation. Immunoregulators act as biological response regulators that do not induce toxicity, side effects, or resistance and can enhance, regulate, and restore nonspecific immunity to a host's immune response. β-Glucan, an immunostimulant, increased the survival rate, alleviated tissue injury, and decreased the abundance of leptospires in target organs. β-Glucan enhanced the inflammatory response, which was associated with enhanced prevention hamsters from Leptospira infection. Our findings also demonstrated that β-glucan-induced trained immunity protected against Leptospira infection. These results contributed to an explanation for the preventable mechanism against Leptospira infection and revealed that β-glucan and even other immunostimulants could be potent and valuable agents for controlling Leptospira infection.