Transcriptional responses of Leptospira interrogans to host innate immunity: significant changes in metabolism, oxygen tolerance, and outer membrane

Characterization of the Bat proteins in the oxidative stress response of Leptospira biflexa

Background

Leptospires lack many of the homologs for oxidative defense present in other bacteria, but do encode homologs of the Bacteriodes aerotolerance (Bat) proteins, which have been proposed to fulfill this function. Bat homologs have been identified in all families of the phylum Spirochaetes, yet a specific function for these proteins has not been experimentally demonstrated.

Results

We investigated the contribution of the Bat proteins in the model organism Leptospira biflexa for their potential contributions to growth rate, morphology and protection against oxidative challenges. A genetically engineered mutant strain in which all bat ORFs were deleted did not exhibit altered growth rate or morphology, relative to the wild-type strain. Nor could we demonstrate a protective role for the Bat proteins in coping with various oxidative stresses. Further, pre-exposing L. biflexa to sublethal levels of reactive oxygen species did not appear to induce a general oxidative stress response, in contrast to what has been shown in other bacterial species. Differential proteomic analysis of the wild-type and mutant strains detected changes in the abundance of a single protein only – HtpG, which is encoded by the gene immediately downstream of the bat loci.

Conclusion

The data presented here do not support a protective role for the Leptospira Bat proteins in directly coping with oxidative stress as previously proposed. L. biflexa is relatively sensitive to reactive oxygen species such as superoxide and H₂O₂, suggesting that this spirochete lacks a strong, protective defense against oxidative damage despite being a strict aerobe.

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.

Modeling seasonal leptospirosis transmission and its association with rainfall and temperature in Thailand using time-series and ARIMAX analyses

OBJECTIVE

To study the number of leptospirosis cases in relations to the seasonal pattern, and its association with climate factors.

METHODS

Time series analysis was used to study the time variations in the number of leptospirosis cases. The Autoregressive Integrated Moving Average (ARIMA) model was used in data curve fitting and predicting the next leptospirosis cases.

RESULTS

We found that the amount of rainfall was correlated to leptospirosis cases in both regions of interest, namely the northern and northeastern region of Thailand, while the temperature played a role in the northeastern region only. The use of multivariate ARIMA (ARIMAX) model showed that factoring in rainfall (with an 8 months lag) yields the best model for the northern region while the model, which factors in rainfall (with a 10 months lag) and temperature (with an 8 months lag) was the best for the northeastern region.

CONCLUSION

The models are able to show the trend in leptospirosis cases and closely fit the recorded data in both regions. The models can also be used to predict the next seasonal peak quite accurately.

Leptospiral hemolysins induce proinflammatory cytokines through Toll-like receptor 2-and 4-mediated JNK and NF-κB signaling pathways

BACKGROUND

Infection with pathogenic Leptospira species causes serious systemic inflammation in patients. Although a few leptospiral proinflammatory molecules have been identified, Leptospira likely encodes other unidentified strong inflammation stimulators. The pathogenic L. interrogans genome encodes numerous putative hemolysin genes. Since hemolysins from other bacteria can cause inflammatory reactions, we hypothesized that leptospiral hemolysins may function as proinflammatory stimulators that contribute to the strong inflammation associated with Leptospira infection.

METHODOLOGY/PRINCIPAL FINDINGS

We first used cytokine protein microarrays for systematic analysis of serum cytokine profiles in leptospirosis patients and leptospire-infected mice. We found that IL-1β, IL-6 and TNF-α were the main proinflammatory cytokines in the sera of both the patients and the mice. We then analyzed eight putative hemolysins in L. interrogans strain Lai. The results showed that five of them, Sph1, Sph2, Sph3, HlpA and TlyA were secreted and had hemolytic activity. More importantly, these five hemolysins induced the strong production of IL-1β, IL-6 and TNF-α in human and mouse macrophages (although a bit lower in the latter). Furthermore, blockade of TLR2 or TLR4 with either antibodies or inhibitors of the NF-κB or JNK signaling pathways significantly reduced the production of hemolysin-induced IL-1β, IL-6 and TNF-α. Macrophages isolated from TLR2-, TLR4-or double TLR2-and 4-deficient mice also confirmed that the leptospiral hemolysins that induce proinflammatory cytokines are both TLR2-and TLR4-dependent.

CONCLUSIONS/SIGNIFICANCE

Our findings demonstrate that L. interrogans secretes many hemolysins that function as powerful inducers of proinflammatory cytokines through both TLR2-and TLR4-dependent JNK and NF-κB pathways.

Differential in vivo gene expression of major Leptospira proteins in resistant or susceptible animal models

Transcripts of Leptospira 16S rRNA, FlaB, LigB, LipL21, LipL32, LipL36, LipL41, and OmpL37 were quantified in the blood of susceptible (hamsters) and resistant (mice) animal models of leptospirosis. We first validated adequate reference genes and then evaluated expression patterns in vivo compared to in vitro cultures. LipL32 expression was downregulated in vivo and differentially regulated in resistant and susceptible animals. FlaB expression was also repressed in mice but not in hamsters. In contrast, LigB and OmpL37 were upregulated in vivo. Thus, we demonstrated that a virulent strain of Leptospira differentially adapts its gene expression in the blood of infected animals.

Positive regulation of Leptospira interrogans kdp expression by KdpE as Demonstrated with a novel β-galactosidase reporter in Leptospira biflexa

Leptospirosis is a potentially deadly zoonotic disease that afflicts humans and animals. Leptospira interrogans, the predominant agent of leptospirosis, encounters diverse conditions as it proceeds through its life cycle, which includes stages inside and outside the host. Unfortunately, the number of genetic tools available for examining the regulation of gene expression in L. interrogans is limited. Consequently, little is known about the genetic circuits that control gene expression in Leptospira. To better understand the regulation of leptospiral gene expression, the L. interrogans kdp locus, encoding homologs of the P-type ATPase KdpABC potassium transporter with their KdpD sensors and KdpE response regulators, was selected for analysis. We showed that a kdpE mutation in L. interrogans prevented the increase in kdpABC mRNA levels observed in the wild-type L. interrogans strain when external potassium levels were low. To confirm that KdpE was a positive regulator of kdpABC transcription, we developed a novel approach for constructing chromosomal genetic fusions to the endogenous bgaL (β-galactosidase) gene of the nonpathogen Leptospira biflexa. We demonstrated positive regulation of a kdpA'-bgaL fusion in L. biflexa by the L. interrogans KdpE response regulator. A control lipL32'-bgaL fusion was not regulated by KdpE. These results demonstrate the utility of genetic fusions to the bgaL gene of L. biflexa for examining leptospiral gene regulation.

Inhibition effect produced by dominant negative mutant fusion protein PreS2-TLM-ScFv-HBcDN on HBV replication in vitro

A mammalian expression vector comprised of the PreS2-TLM (translocation motif), a single-chain variable fragment (ScFv) that binds to hepatitis B surface antigen (HBsAg) and the EGFP gene was constructed. A stably transformed cell line that could express and secrete the fusion protein (PreS2-TLM-ScFv-EGFP) was established. HBsAg-positive HepG2.2.15 cells and HepG2 and HeLa cells were incubated with the supernatant of the transformed cell line cultures for evaluating the cellular permeability of PreS2-TLM-ScFv-EGFP. The location of the fusion protein PreS2-TLM-ScFv-EGFP in HepG2.2.15 cells was observed with immunofluorescence staining. EGFP was next replaced by a dominant negative mutant of the hepatitis B virus core gene (HBcDN) for producing fusion protein PreS2-TLM-ScFv-HBcDN, which was detected by western blot. The supernatant containing fusion protein PreS2-TLM-ScFv-HBcDN was added to the cultures of HepG2.2.15 cells, and the packaged hepatitis B virus (HBV) pregenomic RNA expression levels in the cells were measured using qRT-PCR. The results of the in vitro study indicated that the packaged HBV pregenomic RNA expression levels in HepG2.2.15 cells significantly decreased when these cells were exposed to the supernatant at the dose of 25% for 24, 48 and 72 h, or at the dose of 12.5% for 72 h.

Antigen-specific T cells and cytokines detection as useful tool for understanding immunity against zoonotic infections

Zoonoses include a broad range of diseases, that are becoming of great interest, due to the climate changing, that cause the adaptation of vectors to new niches and environments. Host immune responses play a crucial role in determining the outcome of infections, as documented by expansion of antigen-specific T cells during several zoonotic infections. Thus, understanding of the contribution of antigen-specific T-cell subsets in the host immune response is a powerful tool to evaluate the different immunological mechanisms involved in zoonotic infections and for the development of effective vaccines. In this paper we discuss the role of T cells in some eukaryotic and prokaryotic infectious models.

Gene expression profiles of immune mediators and histopathological findings in animal models of leptospirosis: comparison between susceptible hamsters and resistant mice

Leptospirosis is a widespread zoonosis characterized by multiple organ failure and variable host susceptibility toward pathogenic Leptospira strains. In this study, we put the role of inflammatory mediators in parallel with bacterial burdens and organ lesions by comparing a susceptible animal model, the hamster, and a resistant one, the Oncins France 1 (OF1) mouse, both infected with virulent Leptospira interrogans serovar Icterohaemorrhagiae strain Verdun. Histological observations evidenced edema, congestion, hemorrhage, and inflammatory infiltration in the organs of hamsters, in contrast to limited changes in mice. Using reverse transcription-quantitative PCR techniques, we showed that the relative Leptospira burden progressively increased in hamster tissues, while a rapid clearance was observed in mouse tissues. The early regulation of the proinflammatory mediators interleukin-1β (IL-1β), IL-6, tumor necrosis factor alpha, and cyclo-oxygenase-2 and the chemokines gamma interferon-inducible protein 10 kDa/CXCL10 and macrophage inflammatory protein-1α/CCL3 in mouse tissues contrasted with their delayed and massive overexpression in hamster tissues. Conversely, the induction of the anti-inflammatory cytokine IL-10 was faster in the resistant than in the susceptible animal model. The role of these cytokines in the pathophysiology of leptospirosis and the implications of their differential regulation in the development of this disease are discussed.

Development of transcriptional fusions to assess Leptospira interrogans promoter activity

Background

Leptospirosis is a zoonotic infectious disease that affects both humans and animals. The existing genetic tools for Leptospira spp. have improved our understanding of the biology of this spirochete as well as the interaction of pathogenic leptospires with the mammalian host. However, new tools are necessary to provide novel and useful information to the field.

Methodology and Principal Findings

A series of promoter-probe vectors carrying a reporter gene encoding green fluorescent protein (GFP) were constructed for use in L. biflexa. They were tested by constructing transcriptional fusions between the lipL41, Leptospiral Immunoglobulin-like A (ligA) and Sphingomielynase 2 (sph2) promoters from L. interrogans and the reporter gene. ligA and sph2 promoters were the most active, in comparison to the lipL41 promoter and the non-induced controls. The results obtained are in agreement with LigA expression from the L. interrogans Fiocruz L1-130 strain.

Conclusions

The novel vectors facilitated the in vitro evaluation of L. interrogans promoter activity under defined growth conditions which simulate the mammalian host environment. The fluorescence and rt-PCR data obtained closely reflected transcriptional regulation of the promoters, thus demonstrating the suitability of these vectors for assessing promoter activity in L. biflexa.

Pathogenesis of leptospirosis: the influence of genomics

Leptospirosis is the most widespread zoonosis worldwide and is caused by serovars of pathogenic Leptospira species. The understanding of leptospiral pathogenesis lags far behind that of many other bacterial pathogens. Current research is thus directed at identification of leptospiral virulence factors. Saprophytic Leptospira species are environmental organisms that never cause disease. Comparative genomics of pathogens and saprophytes has allowed the identification of more than 900 genes unique to either Leptospira interrogans or Leptospira borgpetersenii; these genes potentially encode virulence-associated proteins. However, genes of unknown function are over-represented in this subset of pathogen-specific genes, accounting for 80% and 60% of open reading frames, respectively. This finding, together with the absence of virulence factor homologues among the proteins of known function, suggests that Leptospira possesses unique virulence mechanisms. Whole genome microarray studies have identified genes whose expression is differentially regulated under a range of simulated in vivo conditions, such as physiological temperature and osmolarity, low iron levels, and the presence of serum. The subset of genes identified by these studies is likely to include virulence factors. However, most such genes encode proteins of unknown function, consistent with the hypothesis that leptospiral virulence genes do not have homologues in other bacterial species. The recent development of mutagenesis systems for pathogenic Leptospira spp. has allowed the screening of defined mutants for attenuation of virulence in animal infection models and has identified definitively for the first time a range of virulence factors, including lipopolysaccharide, flagella, heme oxygenase, and the OmpA-family protein, Loa22. Interestingly, inactivation of a number of genes hypothesised to encode virulence factors based on in vitro virulence-associated properties did not result in attenuation of virulence, suggesting a degree of functional redundancy in leptospiral pathogenic mechanisms.