A new model of self-resolving leptospirosis in mice infected with a strain of Leptospira interrogans serovar Autumnalis harboring LPS signaling only through TLR4

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.

Evaluating the Antimicrobial and Anti-Hemolytic Activity of Synthesized Pseudopeptide against Leptospiral Species: In Silico and In Vitro Approach

Bacterial infections are one of the leading causes of morbidity, mortality, and healthcare complications in patients. Leptospirosis is found to be the most prevalent, re-emergent, and neglected tropical zoonotic disease worldwide. The adaptation to various environmental conditions has made Leptospira acquire a large genome (~4.6 Mb) and a complex outer membrane, making it unique among bacteria that mimic the symptoms of jaundice and hemorrhage. Sph2 is another important virulence factor that enhances hemolytic sphingomyelinase-capable of moving inside mitochondria-which increases the ROS level and decreases the mitochondrial membrane potential, thereby leading to cell apoptosis. In the present study, 25 suspected bovine serum samples were subjected to the Microscopic Agglutination Test (MAT) across the Mysuru region. Different samples, such as urine, serum, and aborted materials from the confirmed MAT-positive animals, were used for isolation and genomic detection by conventional PCR targeting virulence gene, Lipl32, using specific primers. Further, in vitro and in silico studies were performed on isolated cultures to assess the anti-leptospiral, anti-hemolytic, and sphingomyelinase enzyme inhibition using novel pseudopeptides. The microdilution technique (MDT) and dark field microscope (DFM) assays revealed that at a concentration of 62.5 μg/mL, the pseudopeptide inhibited 100% of the growth of Leptospira spp., suggesting its efficiency in the treatment of leptospirosis. The flow cytometry analyses show the potency of the pseudopeptide against sphingomyelinase enzymes using human umbilical vein endothelial cells (HUVECs). Thus, the present study demonstrated the efficacy of the pseudopeptide in the inhibition of the growth of Leptospira, and therefore, this can be used as an alternative drug for the treatment of leptospirosis.

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.

Scavenger receptor A1 participates in uptake of Leptospira interrogans serovar Autumnalis strain 56606v and inflammation in mouse macrophages

Leptospirosis, caused by pathogenic Leptospira species, has emerged as a widespread zoonotic disease worldwide. Macrophages mediate the elimination of pathogens through phagocytosis and cytokine production. Scavenger receptor A1 (SR-A1), one of the critical receptors mediating this process, plays a complicated role in innate immunity. However, the role of SR-A1 in the immune response against pathogenic Leptospira invasion is unknown. In the present study, we found that SR-A1 is an important nonopsonic phagocytic receptor on murine macrophages for Leptospira. However, intraperitoneal injection of leptospires into WT mice presented with more apparent jaundice, subcutaneous hemorrhaging, and higher bacteria burdens in blood and tissues than that of SR-A1^-/- mice. Exacerbated cytokine and inflammatory mediator levels were also observed in WT mice and higher recruited macrophages in the liver than those of SR-A1^-/- mice. Our findings collectively reveal that although beneficial in the uptake of Leptospira by macrophage, SR-A1 might be exploited by Leptospira to modulate inflammatory activation and increase the susceptibility of infection in the host. These results provide our new insights into the innate immune response during early infection by L. interrogans.

Transcriptomic signatures of exacerbated progression in leptospirosis subclinical chronic kidney disease with secondary nephrotoxic injury

High-incidence regions of leptospirosis caused by Leptospira spp. coincide with chronic kidney disease. This study investigated whether asymptomatic leptospirosis is an emerging culprit that predisposes to progressive chronic kidney disease when superimposed on secondary nephrotoxic injury. Kidney histology/function and whole transcriptomic profiles were evaluated for Leptospira-infected C57/BL6 mice with adenine-induced kidney injury. The extent of tubulointerstitial kidney lesions and expression of inflammation/fibrosis genes in infected mice with low-dose (0.1%) adenine, particularly in high-dose (0.2%) adenine-fed superimposed on Leptospira-infected mice, were significantly increased compared with mice following infection or adenine diet alone, and the findings are consistent with renal transcriptome analysis. Pathway enrichment findings showed that integrin-β- and fibronectin-encoding genes had distinct expression within the integrin-linked kinase-signaling pathway, which were upregulated in 0.2% adenine-fed Leptospira-infected mice but not in 0.2% adenine-fed mice, indicating that background subclinical Leptospiral infection indeed enhanced subsequent secondary nephrotoxic kidney injury and potential pathogenic molecules associated with secondary nephrotoxic leptospirosis. Comparative analysis of gene expression patterns with unilateral ureteric obstruction-induced mouse renal fibrosis and patients with chronic kidney disease showed that differentially expressed orthologous genes such as hemoglobin-α₂, PDZ-binding kinase, and DNA topoisomerase II-α were identified in infected mice fed with low-dose and high-dose adenine, respectively, revealing differentially expressed signatures identical to those found in the datasets and may serve as markers of aggravated kidney progression. This study indicates that background subclinical leptospirosis, when subjected to various degrees of subsequent secondary nephrotoxic injury, may predispose to exacerbated fibrosis, mimicking the pathophysiological process of progressive chronic kidney disease.NEW & NOTEWORTHY Leptospira-infected mice followed by secondary nephrotoxic injury exacerbated immune/inflammatory responses and renal fibrosis. Comparison with the murine model revealed candidates involved in the progression of renal fibrosis in chronic kidney disease (CKD). Comparative transcriptome study suggests that secondary nephrotoxic injury in Leptospira-infected mice recapitulates the gene expression signatures found in CKD patients. This study indicates that secondary nephrotoxic injury may exacerbate CKD in chronic Leptospira infection implicating in the progression of CKD of unknown etiology.

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.

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.

Influence of breed on the clinical and hemato-biochemical parameters in sheep experimentally infected with Leptospira sp

Early diagnosis of leptospirosis may aid in a favorable prognosis in infected animals, but there are few reports of clinical and hematochemical changes in the ovine species, nor whether the breed exerts any influence on the response to infection. Thus, the objective of the present study was to evaluate the clinical, biochemical and hematological alterations in Santa Inês and crossbred ewes challenged with Leptospira interrogans serogroup Pomona serovar Kennewicki. Twenty-four sheep were used in this experiment, 12 crossbred (group A) and 12 Santa Inês (group B). In each group, sheep were conjunctivally and intraperitoneally challenged. During 60 days post-infection the ewes were evaluated for the presentation of clinical signs and the blood was collected for hemogram and serum biochemistry. Concentration of urea and creatinine; serum aspartate aminotransferase activity (AST); gamma-glutamyltransferase (GGT); total protein and albumin; total bilirubin, direct and indirect were analyzed. The urine of these sheep was collected for urinalysis. Only two Santa Inês sheep showed blood in the urine. Clinical signs implicated in Leptospira sp. infection were not identified. Some sheep had anemia, especially crossbred. However, anemia may be attributed to the more effective cellular response that has been identified in crossbred animals. Only one animal presented leukocytosis with neutrophilia, while 11 presented atypical leukopenia, especially those of the Santa Inês breed. There was a decrease in total protein and albumin, as well as the increase in gammaglutamyltranspeptidase (GGT), especially in Santa Inês sheep. The level of aspartate aminotransferase (AST) remained within the normal range for the species. A significant increase (p < 0.05) in conjugated bilirubin levels in challenged animals was detected. Only one sheep intraperitoneally challenged presented a high level of urea in the blood, but the creatinine level remained within the normal range. The intraperitoneal route was responsible for more significant changes (p < 0.05) in the hemogram and biochemistry when compared to the conjunctival route. The results indicate that crossbred sheep have a more efficient cellular response than Santa Inês sheep, which may confer a greater resistance to infection. Clinical signs are not good parameters to follow the development of leptospirosis in crossbred and Santa Inês breed. Hematological and biochemical analyzes were useful in the detection of anemia and possible liver changes caused by leptospirosis. The intraperitoneal route was able to cause more conclusive alterations of the analyzed parameters, however, it is possible that the alterations caused by the conjunctival route reproduce in a more faithful way what happens in a natural situation of infection.

Cytokine Profile in Early Infection by Leptospira interrogans in A/J Mice

Leptospirosis is considered a neglected disease with an estimated more than one million cases every year. Since rodents are at the same time the main reservoir and generally asymptomatic to Leptospira infection, understanding why some animal species are resistant and others are susceptible to this infection would shed some light in how to control this important zoonosis. The innate immune response against Leptospira is mainly dependent on phagocytosis and activation of the Complement System. In this context, cytokines may drive the early control of infection and the adaptive response. Since the Complement System is important to eliminate leptospires in vivo, we investigated if Complement C5 in A/J mice would modulate the cytokine production during infection by Leptospira interrogans serovar Kennewicki type Pomona Fromm (LPF). Thus, our aim was to investigate the systemic levels of pro- and anti-inflammatory cytokines during Leptospira infection in the blood, liver, lung, and kidney on the third and sixth days of infection in A/J C5^+/+ and A/J C5^-/- mice. Blood levels of TNF-α, IL-6, IFN-γ, and MCP-1 reached a peak on the third day. Although both mouse strains developed splenomegaly, similar histopathological alterations in the liver and the lung, levels of pro- and anti-inflammatory cytokines were different. A/J C5^+/+ mice had higher levels of liver IL-10, IL-1β, IL-12p40, and IL-12p70 and kidney IL-1β, IL-12p40, and IL-12p70 on the sixth day of infection when compared to A/J C5^-/- mice. Our results showed that in A/J genetic background, the Complement component C5 modulates a cytokine profile in the liver and kidney of infected mice, which may play a role in the control of disease progression.

Toll-like receptor 4: A promising therapeutic target for pneumonia caused by Gram-negative bacteria

Gram‐negative bacteria (GNB) emerge as important pathogens causing pulmonary infection, which can develop into sepsis due to bacterial resistance to antibiotics. GNB pneumonia poses a huge social and economic burden all over the world. During GNB infection in the lung, Toll‐like receptor 4 (TLR4) can form a complex with MD2 and CD14 after recognizing lipopolysaccharide of GNB, initiate the MyD88‐ and TRIF‐dependent signalling pathways and stimulate host non‐specific immune response. In this review, we summarize recent progress in our understanding of the role of TLR4 in GNB pneumonia. The latest experimental results, especially in TLR4 knockout animals, suggest a promising potential of targeting TLR4 signalling pathway for the treatment of GNB pneumonia. Furthermore, we highlight the benefits of Traditional Chinese Medicine as novel candidates for the therapy of GNB pneumonia due to the modulation of TLR4 signalling pathway. Finally, we discuss the promise and challenge in the development of TLR4‐based drugs for GNB pneumonia.

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

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