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

Exposure to live saprophytic Leptospira before challenge with a pathogenic serovar prevents severe leptospirosis and promotes kidney homeostasis

Previous studies demonstrated that Leptospira biflexa , a saprophytic species, triggers innate immune responses in the host during early infection. This raised the question of whether these responses could suppress a subsequent challenge with pathogenic Leptospira . We inoculated C3H/HeJ mice with a single or a double dose of L. biflexa before challenge with a pathogenic serovar, L. interrogans serovar Copenhageni FioCruz (LIC). Pre-challenge exposure to L. biflexa did not prevent LIC dissemination and colonization of the kidney. However, it rescued weight loss and mouse survival thereby mitigating disease severity. Unexpectedly, there was correlation between rescue of overall health (weight gain, higher survival, lower kidney fibrosis marker ColA1) and higher shedding of LIC in urine. This stood in contrast to the L. biflexa unexposed LIC challenged control. Immune responses were dominated by increased frequency of effector T helper (CD4+) cells in spleen, as well as significant increases in serologic IgG2a. Our findings suggest that exposure to live saprophytic Leptospira primes the host to develop Th1 biased immune responses that prevent severe disease induced by a subsequent challenge with a pathogenic species. Thus, mice exposed to live saprophytic Leptospira before facing a pathogenic serovar may withstand infection with far better outcomes. Furthermore, a status of homeostasis may have been reached after kidney colonization that helps LIC complete its enzootic cycle.

Significance

Previous evidence of host innate immunity induced by live saprophytic Leptospira in mice led us to posit that these responses might mitigate leptospirosis severity upon a subsequent challenge with a pathogenic serovar. In this study, we validated our hypothesis. This is important for development of novel strategies to control leptospirosis and for understanding the epidemiologic risk factors of this and other infectious diseases transmitted by direct contact between pathogen and host. Unexpectedly, these studies also show that there is a correlation between kidney health after L. interrogans infection (less fibrosis marker ColA1) and higher shedding of this spirochete in urine. This suggests that a status of homeostasis may be reached after kidney colonization by L. interrogans that helps the spirochete fulfill its enzootic cycle.

Outer membrane vesicles as nanovaccine candidates against pathogenic Leptospira in experimental Guinea pig model

Leptospira interrogans serovar Hardjo is a long slender bacterium of size 0.1-0.3 μm × 5-50 μm. It is one of the major causes of bovine leptospirosis and is of economical importance because of the reproductive failure, still birth, abortion, and reduced productivity in cattle. It is also a zoonotic disease-causing infection in humans characterized by headaches, fever, chills, sweats and myalgia, lethargy, aching joints, pulmonary haemorrhages, and death in severe cases. Control of the disease involves antibiotic therapy, management and vaccination, of which immunization is the cheapest and effective means of disease prevention. The present study was developed to isolate and characterize the outer membrane vesicles of Leptospira interrogans serovar Hardjo and to evaluate their vaccine potential in guinea pig model. The OMVs were isolated from the culture by sonication and ultracentrifugation. In transmission electron microscopy, the isolated OMVs appeared as small spherical structures of 50-200 nm size. In Western blot and indirect ELISA, antibodies specific to OMVs were observed as indicative of a good humoral immune response elicited by L. interrogans serovar Hardjo OMV. The OMV-based Leptospira vaccine was able to prevent kidney lesions and renal colonization compared to the control and bacterin vaccinated group as proven by histopathology and PCR.

Humoral immune response and changes in peritoneal cell populations in rats immunized against two Leptospira serovars; serovar patoc and serovar pyrogenes

BACKGROUND

Leptospirosis is a zoonotic disease caused by Leptospira species. Variations in lipopolysaccharide (LPS) structure in Leptospira are known to be associated with the serovar diversity and antigenicity. Development of immunodiagnostics for early detection of leptospirosis based on immune responses against different pathogenic antigens as well as development of vaccines are important. Hence, this study has assessed the immune response generated against leptospiral LPS and whole antigen preparations of pathogenic and saprophytic Leptospira and specific changes in peritoneal cells was also studied to elucidate the cellular responses associated with immune response of Wistar rats.

METHODS

During the study, immune response induced by two types of Leptospira antigen preparations of two selected serovars was compared. Changes in the specific peritoneal cell subpopulations following immunizations of rats were analyzed using flow cytometry.

RESULTS

Of the two antigen preparations tested, the LPS extract induced a higher IgM immune response as opposed to the sonicated antigen preparation. Of the two serovars tested, L. interrogans serovar Pyrogenes had induced a higher IgM response compared to that by L. biflexa serovar Patoc. Considering the IgG titers, equivalent responses were observed with all four antigen preparations. Significant increases in lymphocytes were observed following immunization with LPS of both serovars. Interestingly, the B2 cell percentages increased significantly during the immunization period. Further, significant correlations were observed with both IgM and IgG responses and percentage of B2 cells in the peritoneal cavity (PC).

CONCLUSION

LPS extract of L. interrogans serovar Pyrogenes induced higher IgM response while the IgG response was equivalent among the four antigen preparations tested. Significant increase of B2 cell percentage in the peritoneal cavity during the immunization reflects the accumulation of B2 cells in the PC which may play considerable role in generating humoral response against Leptospira antigens.

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.

Challenges for the development of a universal vaccine against leptospirosis revealed by the evaluation of 22 vaccine candidates

Leptospirosis is a neglected disease of man and animals that affects nearly half a million people annually and causes considerable economic losses. Current human vaccines are inactivated whole-cell preparations (bacterins) of Leptospira spp. that provide strong homologous protection yet fail to induce a cross-protective immune response. Yearly boosters are required, and serious side-effects are frequently reported so the vaccine is licensed for use in humans in only a handful of countries. Novel universal vaccines require identification of conserved surface-exposed epitopes of leptospiral antigens. Outer membrane β-barrel proteins (βb-OMPs) meet these requirements and have been successfully used as vaccines for other diseases. We report the evaluation of 22 constructs containing protein fragments from 33 leptospiral βb-OMPs, previously identified by reverse and structural vaccinology and cell-surface immunoprecipitation. Three-dimensional structures for each leptospiral βb-OMP were predicted by I-TASSER. The surface-exposed epitopes were predicted using NetMHCII 2.2 and BepiPred 2.0. Recombinant constructs containing regions from one or more βb-OMPs were cloned and expressed in Escherichia coli. IMAC-purified recombinant proteins were adsorbed to an aluminium hydroxide adjuvant to produce the vaccine formulations. Hamsters (4-6 weeks old) were vaccinated with 2 doses containing 50 – 125 μg of recombinant protein, with a 14-day interval between doses. Immunoprotection was evaluated in the hamster model of leptospirosis against a homologous challenge (10 – 20× ED₅₀) with L. interrogans serogroup Icterohaemorrhagiae serovar Copenhageni strain Fiocruz L1-130. Of the vaccine formulations, 20/22 were immunogenic and induced significant humoral immune responses (IgG) prior to challenge. Four constructs induced significant protection (100%, P < 0.001) and sterilizing immunity in two independent experiments, however, this was not reproducible in subsequent evaluations (0 – 33.3% protection, P > 0.05). The lack of reproducibility seen in these challenge experiments and in other reports in the literature, together with the lack of immune correlates and commercially available reagents to characterize the immune response, suggest that the hamster may not be the ideal model for evaluation of leptospirosis vaccines and highlight the need for evaluation of alternative models, such as the mouse.

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.

Transient Presence of Live Leptospira interrogans in Murine Testes

Analysis of Leptospira dissemination and colonization of sex organs in rodents is of significant value as it queries the possibility of mammal-to-mammal venereal transmission. The aim of our study was to evaluate the presence and viability of Leptospira interrogans in testes of mice using models of infection that we previously developed. Using sublethal and lethal doses of bioluminescent strains of L. interrogans serovars Manilae and Copenhageni, we visualized the presence of leptospires in testes of C57BL/6 mice as early as 30 min and up to days 3-4 postinfection. This was confirmed by qPCR for the Copenhageni serovar after lethal infection of C3H/HeJ mice. In this model, no histopathological changes were noticed in testis. We further studied persistence of serovar Copenhageni in C3H/HeJ testes after lethal and sublethal infection, with different doses of leptospires. No viable leptospires were recovered from testes of lethally infected mice. However, we found live culturable Leptospira in testes of 19/19 (100%) sublethally infected mice at the acute phase but not at 15 days postinfection, which corresponds to the chronic phase of renal colonization. The data suggest that colonization of testes with live and potentially infectious leptospires is transient and limited to the spirochetemic phase of infection. Further studies are necessary to evaluate if presence of Leptospira in testes of mice leads to excretion in semen and to venereal transmission to female mice. IMPORTANCE Analysis of venereal transmission of Leptospira is important to determine if direct animal to animal transmission occurs, which could impact measures to prevent and treat leptospirosis. The goal of this study was to determine if live Leptospira colonize mouse testes. We found that colonization of mouse testes with live Leptospira was transient and limited to the acute spirochetemic phase of infection and that transient colonization of the testes was insufficient to cause histopathological changes.

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.

Biannual and Quarterly Comparison Analysis of Agglutinating Antibody Kinetics on a Subcohort of Individuals Exposed to Leptospira interrogans in Salvador, Brazil

Introduction

Leptospirosis is a zoonosis with a worldwide spread that leads to clinical manifestations ranging from asymptomatic infection to a life-threatening disease. The immune response is predominantly humoral mediated limited to the infecting serovar. Individuals living in an area endemic for leptospirosis are often exposed to an environment contaminated with leptospires and there is a paucity of information on naturally acquired immunity. In the present study, we evaluated the kinetics of agglutinating antibodies in individuals from an endemic area for leptospirosis in Salvador, Brazil comparing two different intersample collection times.

Methods

Between 2017–2018, we carried out a biannual prospective cohort with 2,086 individuals living in an endemic area for leptospirosis in Salvador, Brazil. To compare agglutinating antibody kinetics using microscopic agglutination test (MAT) with different collection times, a subcohort of 72 individuals with quarterly follow-up was carried out in parallel.

Results

The results revealed that using a shorter time for intersample collection led to the detection of a higher number of infections and reinfection events. Furthermore, we observed a higher rate of titer decay indicating partial and short protection. However, there was no indication of major changes in risk factors for the disease.

Conclusions

We evaluated antibody kinetics among residents of an endemic area for leptospirosis comparing two sample collection times. The constant exposure to the contaminated environment increases the risk for leptospirosis infection with reinfection events being more common than expected. This indicates that the burden of leptospirosis might be underestimated by serological surveys, and further studies are necessary to better characterize the humoral response after infection.

Enzyme immunoassays (EIA) for serodiagnosis of human leptospirosis: specific IgG3/IgG1 isotyping may further inform diagnosis of acute disease

The laborious microscopic agglutination test (MAT) is the gold standard serologic test for laboratory diagnosis of leptospirosis. We developed EIA based serologic assays using recombinant proteins (rLigA, rLigB, rLipL32) and whole-cell extracts from eight Leptospira serovars as antigen and assessed the diagnostic performance of the new assay within each class, against MAT positive (MAT+) human sera panels from Portugal/PT (n = 143) and Angola/AO (n = 100). We found that a combination of recombinant proteins rLigA, rLigB and rLipL32 correctly identified antigen-specific IgG from patients with clinical and laboratory confirmed leptospirosis (MAT+) with 92% sensitivity and ~ 97% specificity (AUC 0.974) in serum from the provinces of Luanda (LDA) and Huambo (HBO) in Angola. A combination of whole cell extracts of L. interrogans sv Copenhageni (LiC), L. kirschneri Mozdok (LkM), L. borgpetersenii Arborea (LbA) and L. biflexa Patoc (LbP) accurately identified patients with clinical and laboratory confirmed leptospirosis (MAT+) with 100% sensitivity and ~ 98% specificity for all provinces of Angola and Portugal (AUC: 0.997 for AO/LDA/HBO, 1.000 for AO/HLA, 0.999 for PT/AZ and 1.000 for PT/LIS). Interestingly, we found that MAT+ IgG+ serum from Angola had a significantly higher presence of IgD and that IgG3/IgG1 isotypes were significantly increased in the MAT+ IgG+ serum from Portugal. Given that IgM/IgD class and IgG3/IgG1 specific isotypes are produced in the earliest course of infection, immunoglobulin G isotyping may be used to inform diagnosis of acute leptospirosis. The speed, ease of use and accuracy of EIA tests make them excellent alternatives to the laborious and expensive MAT for screening acute infection in areas where circulating serovars of pathogenic Leptospira are well defined.

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.

Protective efficacy of whole-cell inactivated Leptospira vaccines made using virulent or avirulent strains in a hamster model

Whole-cell inactivated vaccines remain the only licensed vaccines used to control human and animal leptospirosis worldwide. Although they are protective against lethal infections, the efficacy of these vaccines has been divergent. The manufacturing process often involves the use of standard bacterial strains subjected to serial in vitro passages, with a risk of loss of virulence, and may affect the immunogenicity and consequently decrease protection. Thus, the objective of this study was to perform a comparative analysis of the efficacy of in-house bacterins produced with standard (avirulent) and virulent strains. Hamsters were immunized with killed bacteria produced using avirulent and virulent strains of L. interrogans serovars Copenhageni and Canicola. Vaccine efficacy was determined in terms of protection against lethal homologous or heterologous challenges. The results showed that immunization with both avirulent and virulent Canicola strains resulted in 100% protection against homologous challenge. Conversely, Copenhageni bacterins produced using an avirulent strain conferred only 25-37.5% protection against homologous challenge (P > 0.05), while virulent Copenhageni bacterin conferred 100% protection (P < 0.001). A single vaccine dose was sufficient to induce protection, and administration of a prime boost significantly reduced the bacterial load in the kidneys and improved the humoral immune response to the virulent Copenhageni strain. These findings suggest that the maintenance of virulent strains in bacterin formulations is essential for improving the immunogenicity and efficacy of leptospirosis vaccines.