The dynamics of granuloma formation in experimental visceral leishmaniasis

Kupffer cell and recruited macrophage heterogeneity orchestrate granuloma maturation and hepatic immunity in visceral leishmaniasis

In murine models of visceral leishmaniasis (VL), the parasitization of resident Kupffer cells (resKCs) drives early Leishmania infantum growth in the liver, leading to granuloma formation and subsequent parasite control. Using the chronic VL model, we demonstrate that polyclonal resKCs redistributed to form granulomas outside the sinusoids, creating an open sinusoidal niche that was gradually repopulated by monocyte-derived KCs (moKCs) acquiring a tissue specific, homeostatic profile. Early-stage granulomas predominantly consisted of CLEC4F+KCs. In contrast, late-stage granulomas led to remodeling of the sinusoidal network and contained monocyte-derived macrophages (momacs) along with KCs that downregulated CLEC4F, with both populations expressing iNOS and pro-inflammatory chemokines. During late-stage infection, parasites were largely confined to CLEC4F-KCs. Reduced monocyte recruitment and increased resKCs proliferation in infected Ccr2-/- mice impaired parasite control. These findings show that the ontogenic heterogeneity of granuloma macrophages is closely linked to granuloma maturation and the development of hepatic immunity in VL.

Kupffer cell and recruited macrophage heterogeneity orchestrate granuloma maturation and hepatic immunity in visceral leishmaniasis

In murine models of visceral leishmaniasis (VL), parasitization of resident Kupffer cells (resKCs) is responsible for early growth of Leishmania infantum in the liver, which leads to granuloma formation and eventual parasite control. We employed the chronic VL model, and revealed an open niche established by KCs death and their migration outside of the sinusoids, resulting in their gradual replacement by monocyte-derived KCs (moKCs). While early granulomas were composed of resKCs, late granulomas were found outside of the sinusoids and contained resKC-derived macrophages, and monocyte-derived macrophages (momacs). ResKCs and moKCs within the sinusoids were identified as homeostatic/regulatory cells, while resKC-derived macrophages and momacs within late granulomas were pro-inflammatory. Despite the infection being largely confined to the resKC-derived macrophages, in the absence of monocyte recruitment, parasite control was strongly compromised. Macrophage heterogeneity, involving migration and reprogramming of resKCs, along with recruitment of monocyte-derived cells, is a hallmark of granuloma maturation and hepatic immunity in VL.

The role of CD4+ T cells in visceral leishmaniasis; new and emerging roles for NKG7 and TGFβ

Visceral leishmaniasis is a potentially devastating neglected tropical disease caused by the protozoan parasites Leishmania donovani and L. infantum (chagasi). These parasites reside in tissue macrophages and survive by deploying a number of mechanisms aimed at subverting the host immune response. CD4+ T cells play an important role in controlling Leishmania parasites by providing help in the form of pro-inflammatory cytokines to activate microbiocidal pathways in infected macrophages. However, because these cytokines can also cause tissue damage if over-produced, regulatory immune responses develop, and the balance between pro-inflammatory and regulatory CD4+ T cells responses determines the outcomes of infection. Past studies have identified important roles for pro-inflammatory cytokines such as IFNγ and TNF, as well as regulatory co-inhibitory receptors and the potent anti-inflammatory cytokine IL-10. More recently, other immunoregulatory molecules have been identified that play important roles in CD4+ T cell responses during VL. In this review, we will discuss recent findings about two of these molecules; the NK cell granule protein Nkg7 and the anti-inflammatory cytokine TGFβ, and describe how they impact CD4+ T cell functions and immune responses during visceral leishmaniasis.

Leishmania blood parasite dynamics during and after treatment of visceral leishmaniasis in Eastern Africa: A pharmacokinetic-pharmacodynamic model

BACKGROUND

With the current treatment options for visceral leishmaniasis (VL), recrudescence of the parasite is seen in a proportion of patients. Understanding parasite dynamics is crucial to improving treatment efficacy and predicting patient relapse in cases of VL. This study aimed to characterize the kinetics of circulating Leishmania parasites in the blood, during and after different antileishmanial therapies, and to find predictors for clinical relapse of disease.

METHODS

Data from three clinical trials, in which Eastern African VL patients received various antileishmanial regimens, were combined in this study. Leishmania kinetoplast DNA was quantified in whole blood with real-time quantitative PCR (qPCR) before, during, and up to six months after treatment. An integrated population pharmacokinetic-pharmacodynamic model was developed using non-linear mixed effects modelling.

RESULTS

Parasite proliferation was best described by an exponential growth model, with an in vivo parasite doubling time of 7.8 days (RSE 12%). Parasite killing by fexinidazole, liposomal amphotericin B, sodium stibogluconate, and miltefosine was best described by linear models directly relating drug concentrations to the parasite elimination rate. After treatment, parasite growth was assumed to be suppressed by the host immune system, described by an Emax model driven by the time after treatment. No predictors for the high variability in onset and magnitude of the immune response could be identified. Model-based individual predictions of blood parasite load on Day 28 and Day 56 after start of treatment were predictive for clinical relapse of disease.

CONCLUSION

This semi-mechanistic pharmacokinetic-pharmacodynamic model adequately captured the blood parasite dynamics during and after treatment, and revealed that high blood parasite loads on Day 28 and Day 56 after start of treatment are an early indication for VL relapse, which could be a useful biomarker to assess treatment efficacy of a treatment regimen in a clinical trial setting.

Leishmania (L.) infantum chagasi isolated from skin lesions of patients affected by non-ulcerated cutaneous leishmaniasis lead to visceral lesion in hamsters

In Central America, Leishmania (L.) infantum chagasi infection causes visceral leishmaniasis (VL) and non-ulcerated cutaneous leishmaniasis (NUCL). The aim of the present study was to evaluate the course of an experimental infection in hamsters caused by L. (L.) infantum chagasi isolated from patients affected by NUCL compared with a strain isolated from a patient with VL. Stationary phase parasites in culture were inoculated through subcutaneous and intraperitoneal routes in hamsters. Following the post-infection times, a histopathological study, parasite load and cytokine determination in skin from the cutaneous inoculation site and viscera were performed. Animals subcutaneously infected with the different strains did not develop macroscopic lesions at the inoculation site, and the histopathological changes in the dermis were very slight. Regarding the histopathological study of the viscera, we observed the portal mononuclear inflammatory infiltrate, the presence of nodules in the hepatic parenchyma and the proliferation of macrophages in the spleen, which increased over the infection course. Overall, the parasite load in the liver and spleen and in the total IgG titres in the sera of infected hamster showed an increase with the time of infection, regardless of the route of inoculation. Regarding cellular immunity, we did not observe an increase or decrease in pro- and anti-inflammatory cytokines compared to the healthy control, except for IL-10, which was evident in the infected animals. The data showed that strains isolated from NUCL cause visceral lesions in the hamsters regardless of the route of inoculation, and they were similar to parasites isolated from VL humans.

NLRP12-expressing dendritic cells mediate both dissemination of infection and adaptive immune responses in visceral leishmaniasis

The NLR protein NLRP12 contributes to innate immunity, but the mechanism remains elusive. Infection of Nlrp12 -/- or wild-type mice with Leishmania infantum led to aberrant parasite tropism. Parasites replicated to higher levels in livers of Nlrp12 -/- mice than in the livers of WT mice and failed to disseminate to spleens. Most retained liver parasites resided in dendritic cells (DCs), with correspondingly fewer infected DCs in spleens. Furthermore, Nlrp12 -/- DCs expressed lower CCR7 than WT DCs, failed to migrate toward CCL19 or CCL21 in chemotaxis assays, and migrated poorly to draining lymph nodes after sterile inflammation. Leishmania-infected Nlpr12 -/- DCs were significantly less effective at transporting parasites to lymph nodes than WT DCs. Consistently, adaptive immune responses were also impaired in infected Nlrp12 -/- mice. We hypothesize that Nlrp12-expressing DCs are required for efficient dissemination and immune clearance of L. infantum from the site of initial infection. This is at least partly due to the defective expression of CCR7.

Leishmania infection-derived extracellular vesicles drive transcription of genes involved in M2 polarization

Although it is known that the composition of extracellular vesicles (EVs) is determined by the characteristics of the cell and its environment, the effects of intracellular infection on EV composition and functions are not well understood. We had previously shown that cultured macrophages infected with Leishmania parasites release EVs (LiEVs) containing parasite-derived molecules. In this study we show that LdVash, a molecule previously identified in LiEVs from L. donovani infected RAW264.7 macrophages, is widely distributed in the liver of L. donovani infected mice. This result shows for the first time that parasite molecules are released in EVs and distributed in infected tissues where they can be endocytosed by cells in the liver, including macrophages that significantly increase numbers as the infection progresses. To evaluate the potential impact of LiEVs on macrophage functions, we show that primary peritoneal exudate macrophages (PECs) express transcripts of signature molecules of M2 macrophages such as arginase 1, IL-10, and IL-4R when incubated with LiEVs. In comparative studies that illustrate how intracellular pathogens control the composition and functions of EVs released from macrophages, we show that EVs from RAW264.7 macrophages infected with Salmonella Typhimurium activate PECs to express transcripts of signature molecules of M1 macrophages such as iNOS, TNF alpha, and IFN-gamma and not M2 signature molecules. Finally, in contrast to the polarized responses observed in in vitro studies of macrophages, both M1 and M2 signature molecules are detected in L. donovani infected livers, although they exhibit differences in their spatial distribution in infected tissues. In conclusion, EVs produced by macrophages during Leishmania infection lead to the gene expression consistent with M2 polarization. In contrast, the EVs produced during S. Typhimurium infection stimulated the transcription of genes associated with M1 polarization.

T Lymphocyte Exhaustion During Human and Experimental Visceral Leishmaniasis

A key point of immunity against protozoan Leishmania parasites is the development of an optimal T cell response, which includes a low apoptotic rate, high proliferative activity and polyfunctionality. During acute infection, antigen-specific T cells recognize the pathogen resulting in pathogen control but not elimination, promoting the development and the maintenance of a population of circulating effector cells that mount rapid response quickly after re-exposure to the parasite. However, in the case of visceral disease, the functionality of specific T cells is lost during chronic infection, resulting in inferior effector functions, poor response to specific restimulation, and suboptimal homeostatic proliferation, a term referred to as T cell exhaustion. Multiple factors, including parasite load, infection duration and host immunity, affect T lymphocyte exhaustion. These factors contribute to antigen persistence by promoting inhibitory receptor expression and sustained production of soluble mediators, influencing suppressive cell function and the release of endogenous molecules into chronically inflamed tissue. Together, these signals encourage several changes, reprogramming cells into a quiescent state, which reflects disease progression to more severe forms, and development of acquired resistance to conventional drugs to treat the disease. These points are discussed in this review.

Preclinical validation of a live attenuated dermotropic Leishmania vaccine against vector transmitted fatal visceral leishmaniasis

Visceral Leishmaniasis (VL), a potentially fatal disease is caused by Leishmania donovani parasites with no vaccine available. Here we produced a dermotropic live attenuated centrin gene deleted Leishmania major (LmCen^−/−) vaccine under Good Laboratory Practices and demonstrated that a single intradermal injection confers robust and durable protection against lethal VL transmitted naturally via bites of L. donovani-infected sand flies and prevents mortality. Surprisingly, immunogenicity characteristics of LmCen^−/− parasites revealed activation of common immune pathways like L. major wild type parasites. Spleen cells from LmCen^−/− immunized and L. donovani challenged hamsters produced significantly higher Th1-associated cytokines including IFN-γ, TNF-α, and reduced expression of the anti-inflammatory cytokines like IL-10, IL-21, compared to non-immunized challenged animals. PBMCs, isolated from healthy people from non-endemic region, upon LmCen^−/− infection also induced more IFN-γ compared to IL-10, consistent with our immunogenicity data in LmCen^−/− immunized hamsters. This study demonstrates that the LmCen^−/− parasites are safe and efficacious against VL and is a strong candidate vaccine to be tested in a human clinical trial.

Karmakar et al produced a dermotropic, live attenuated centrin gene-deleted Leishmania major (LmCen^−/−) vaccine against Visceral Leishmaniasis (VL). They demonstrated in hamsters that a single intradermal injection confers robust and durable protection against lethal VL that is transmitted naturally via bites of L. donovani-infected sand flies.

The Inflammatory Effects of Dietary Lipids Regulate Growth of Parasites during Visceral Leishmaniasis

Visceral leishmaniasis is a potentially fatal disease caused by the protozoon Leishmania donovani or L. infantum (Li). Although previous studies revealed that high lipid intake reduces parasite burdens in Leishmania donovani-infected mice, the specific contributions of dietary lipids to Li-associated pathogenesis are not known. To address this, we evaluated parasite growth, liver pathology, and transcriptomic signatures in Li-infected BALB/c mice fed either a control, high-fat, high-cholesterol, or high-fat–high-cholesterol diet. Using quantitative PCR (qPCR), we observed significantly reduced liver parasite burdens in mice fed the high-fat–high-cholesterol diet compared to mice fed the control diet. In contrast to the liver, parasite expansion occurred earlier in the spleens of mice fed the experimental diets. Histological examination revealed an intense inflammatory cell infiltrate in livers predominantly composed of neutrophils caused by the high-fat–high-cholesterol diet specifically. After 8 weeks of infection (12 weeks of diet), Illumina microarrays revealed significantly increased expression of transcripts belonging to immune- and angiogenesis-related pathways in livers of both uninfected and Li-infected mice fed the high-fat–high-cholesterol diet. These data suggest that increased fat and cholesterol intake prior to Li infection leads to a hepatic inflammatory environment and thus reduces the parasite burden in the liver. Defining inflammatory signatures as well as pathology in the liver may reveal opportunities to modify the therapeutic approach to Li infection.

IMPORTANCE Leishmaniasis is a spectrum of diseases caused by Leishmania species protozoa that is most common in warm climates, coinciding with impoverished regions. Visceral leishmaniasis is a potentially fatal disease in which parasites infect reticuloendothelial organs and cause progressive wasting and immunocompromise. The distribution and demographics of visceral leishmaniasis have changed over recent years, coinciding with modernizing societies and the increased availability of Western diets rich in lipid content. We report here that increased dietary fat and cholesterol intake affected disease pathogenesis by increasing inflammation and reducing localized parasite burdens in the liver. These diet-induced changes in disease pathogenesis might explain in part the changing epidemiology of visceral leishmaniasis. A relationship between diet and inflammatory responses may occur in leishmaniasis and other microbial or immune-mediated diseases, possibly revealing opportunities to modify the therapeutic approach to microbial infections.

Immune induction by adjuvanted Leishmania donovani vaccines against the visceral leishmaniasis in BALB/c mice

Visceral leishmaniasis (VL) is a neglected tropical disease caused by Leishmania donovani or Leishmania infantum. Currently, the patients are treated with chemotherapeutic drugs; however, their toxicity limits their use. It would be desirable to develop a vaccine against this infection. In this study, we assessed the efficacy of different vaccine formulations at variable time points. Heat-killed (HK) antigen of Leishmania donovani was adjuvanted with two adjuvants (AddaVax and Montanide ISA 201) and three immunizations at a gap of 2 weeks (wk) were given to BALB/c mice. After 2 weeks of the last booster, mice were given challenge infection and sacrificed before challenge and after 4wk, 8wk, and 12 wk post-challenge. Significant protective immunity was observed in all the immunized animals and it was indicated by the notable rise in delayed-type hypersensitivity (DTH) response, remarkably declined parasite burden, a significant increase in the levels of interferon-gamma (IFN-γ), interleukin-12, interleukin-17 (Th1 cytokines), and IgG2a in contrast to infected control mice. Montanide ISA 201 with HK antigen provided maximum protection followed by AddaVax with HK and then HK alone. These findings elaborate on the importance of the tested adjuvants in the vaccine formulations against murine visceral leishmaniasis.

Macrophages as host, effector and immunoregulatory cells in leishmaniasis: Impact of tissue micro-environment and metabolism

Leishmania are protozoan parasites that predominantly reside in myeloid cells within their mammalian hosts. Monocytes and macrophages play a central role in the pathogenesis of all forms of leishmaniasis, including cutaneous and visceral leishmaniasis. The present review will highlight the diverse roles of macrophages in leishmaniasis as initial replicative niche, antimicrobial effectors, immunoregulators and as safe hideaway for parasites persisting after clinical cure. These multiplex activities are either ascribed to defined subpopulations of macrophages (e.g., Ly6C^highCCR2⁺ inflammatory monocytes/monocyte-derived dendritic cells) or result from different activation statuses of tissue macrophages (e.g., macrophages carrying markers of of classical [M1] or alternative activation [M2]). The latter are shaped by immune- and stromal cell-derived cytokines (e.g., IFN-γ, IL-4, IL-10, TGF-β), micro milieu factors (e.g., hypoxia, tonicity, amino acid availability), host cell-derived enzymes, secretory products and metabolites (e.g., heme oxygenase-1, arginase 1, indoleamine 2,3-dioxygenase, NOS2/NO, NOX2/ROS, lipids) as well as by parasite products (e.g., leishmanolysin/gp63, lipophosphoglycan). Exciting avenues of current research address the transcriptional, epigenetic and translational reprogramming of macrophages in a Leishmania species- and tissue context-dependent manner.

Arginase and its mechanisms in Leishmania persistence

Leishmaniasis is a neglected infectious disease with clinical presentations ranging from asymptomatic or mild symptoms to chronic infection and eventual death. The mechanisms of disease susceptibility and pathology have been extensively studied, but there are no steadfast rules regarding leishmaniasis. A Th1 response is usually associated with infection control, while a predominant Th2 response is detrimental to the patient. In this scenario, the enzymes arginase and inducible nitric oxide synthase represent two possible pathways of immune response. While the former contributes to parasite replication, the latter is crucial for its control. In the present review, we collected study results that associate arginase expression in patients and in experimental models with disease susceptibility/chronicity and show some proposed mechanisms that explain the role of arginase in maintaining Leishmania infection, including polyamine and thiol synthesis, tissue-resident macrophage (TRM) proliferation and activation and T-cell suppression and exhaustion.

Interleukin-27 Functional Duality Balances Leishmania Infectivity and Pathogenesis

IL-27 is a cytokine that exerts diverse effects on the cells of innate and adaptive immune systems. Chiefly expressed in macrophages and dendritic cells during the early phase of Leishmania infection, IL-27 contributes to the protection against L. major infection but suppresses the protective Th1 response against L. donovani, L. infantum, L. amazonensis and L. braziliensis infections, suggesting its functional duality. During the late stage of Leishmania infection, IL-27 limits the immunopathogenic reactions and tissue damages. Herein, we analyze the mechanism of the functional duality of IL-27 in the resistance or susceptibility to Leishmania infection, prompting IL-27 for anti-Leishmanial therapy.

Establishment, optimisation and quantitation of a bioluminescent murine infection model of visceral leishmaniasis for systematic vaccine screening

Visceral leishmaniasis is an infectious parasitic disease caused by the protozoan parasites Leishmania donovani and Leishmania infantum. The drugs currently used to treat visceral leishmaniasis suffer from toxicity and the emergence of parasite resistance, and so a better solution would be the development of an effective subunit vaccine; however, no approved vaccine currently exists. The comparative testing of a large number of vaccine candidates requires a quantitative and reproducible experimental murine infection model, but the parameters that influence infection pathology have not been systematically determined. To address this, we have established an infection model using a transgenic luciferase-expressing L. donovani parasite and longitudinally quantified the infections using in vivo bioluminescent imaging within individual mice. We examined the effects of varying the infection route, the site of adjuvant formulation administration, and standardised the parasite preparation and dose. We observed that the increase in parasite load within the liver during the first few weeks of infection was directly proportional to the parasite number in the initial inoculum. Finally, we show that immunity can be induced in pre-exposed animals that have resolved an initial infection. This murine infection model provides a platform for systematic subunit vaccine testing against visceral leishmaniasis.

Nonclinical data supporting orphan medicinal product designations in the area of rare infectious diseases

This review provides an overview of nonclinical in vivo models that can be used to support orphan designation in selected rare infectious diseases in Europe, with the aim to inform and stimulate the planning of nonclinical development in this area of often neglected diseases.

A molecular signature for CD8+ T cells from visceral leishmaniasis patients

CD8⁺ T-cell function is compromised in chronic diseases such as visceral leishmaniasis (VL). However, little is known about the changes in gene expression that cause CD8+ T-cell dysfunction during VL. We used targeted transcriptional profiling of peripheral blood CD8⁺ T cells from VL patients pre- and post-anti-parasitic drug treatment, and compared them with the same cell population from healthy endemic controls to assess their activation, differentiation and functional status during disease. We found a predominance of downregulated immune genes in CD8⁺ T cells from VL patients. However, genes encoding several notable immune checkpoint molecules, including LAG-3, TIM-3 and CTLA-4, cytolytic molecules, such as granzymes A, B and H and perforin, as well as SOCS3, STAT1, JAK2 and JAK3 cytokine signalling genes were found to be increasingly expressed by VL patient CD8⁺ T cells. Additional studies confirmed increased expression of the inhibitory receptors LAG3 and TIM3 on VL patient CD8⁺ T cells, thereby identifying these molecules as potential targets to improve antigen-specific CD8⁺ T-cell responses during disease.

Identification of HLA-I restricted epitopes in six vaccine candidates of Leishmania tropica using immunoinformatics and molecular dynamics simulation approaches

In spite of numerous studies on vaccination for various species of Leishmania, research on the development of an effective vaccine for L. tropica is very scarce. In silico epitope prediction is a new way to survey the best vaccine candidates. Here, we predicted the best epitopes of six L. tropica antigens with vaccine capability against this pathogen, using highly frequent HLA-I alleles. Based on the frequent HLA alleles, the protein sequences were screened individually using four different MHC prediction applications, namely SYFPEITHI, ProPredI, BIMAS, and IEDB. Several in silico assays including clustering, human similarity exclusion, epitope conservancy prediction, investigating in experimental records, immunogenicity prediction, and prediction of population coverage were performed to narrow the results and to find the best epitopes. The selected epitopes and their restricted HLA-I alleles were docked and the final epitopes with the lowest binding energy (the highest binding affinity) were chosen. Finally, the stability and the binding properties of the best epitope-HLA-I combinations were analyzed using molecular dynamics simulation studies. We found ten potential peptides with strong binding affinity to highly frequent HLA-I alleles that can be further evaluated as vaccine targets against L. tropica.

Oral activity of the antimalarial endoperoxide 6-(1,2,6,7-tetraoxaspiro[7.11]nonadec-4-yl)hexan-1-ol (N-251) against Leishmania donovani complex

Visceral leishmaniasis (VL) is a major problem worldwide and causes significant morbidity and mortality. Existing drugs against VL have limitations, including their invasive means of administration long duration of treatment regimens. There are also concerns regarding increasing treatment relapses as well as the identification of resistant clinical strains with the use of miltefosine, the sole oral drug for VL. There is, therefore, an urgent need for new alternative oral drugs for VL. In the present study, we show the leishmanicidal effect of a novel, oral antimalarial endoperoxide N-251. In our In vitro studies, N-251 selectively and specifically killed Leishmania donovani D10 amastigotes with no accompanying toxicity toward the host cells. In addition, N-251 exhibited comparable activities against promastigotes of L. donovani D10, as well as other L. donovani complex parasites, suggesting a wide spectrum of activity. Furthermore, even after a progressive infection was established in mice, N-251 significantly eliminated amastigotes when administered orally. Finally, N-251 suppressed granuloma formation in mice liver through parasite death. These findings indicate the therapeutic effect of N-251 as an oral drug, hence suggest N-251 to be a promising lead compound for the development of a new oral chemotherapy against VL.

Histopathological and immunohistochemical characterisation of hepatic granulomas in Leishmania donovani-infected BALB/c mice: a time-course study

BACKGROUND

Visceral leishmaniasis (VL) is a neglected tropical disease (NTD), caused by the intracellular protozoan parasites Leishmania donovani and Leishmania infantum. Symptomatic VL is considered fatal when left untreated. At present, there is no effective vaccine licensed for human use and available chemotherapies have limitations. Understanding the local immune mechanisms required for the control of infection is a key factor for developing effective vaccines and therapeutics.

METHODS

We have investigated the development of the typical granulomatous lesions in the liver in experimental VL over time, together with the local immune responses. BALB/c mice were infected intravenously with a dose of 2 × 10⁷ L. donovani amastigotes (MHOM/ET/67/HU3) and sacrificed at 15, 35 and 63 days post-infection (dpi). Histopathology and immunohistochemical techniques were used for the detection of Leishmania antigen, selected cell types including B and T lymphocytes, macrophages and neutrophils (CD45R-B220+, CD3+, F4/80+ and Ly-6G+) and iNOS.

RESULTS

Granulomatous lesions were identified as early as 15 dpi in the livers of all infected animals. Three categories were used to classify liver granulomas (immature, mature and clear). Clear granulomas were exclusively detected from 35 dpi onwards. Kupffer cells (F4/80+) were predominant in immature granulomas, regardless of the dpi. Nonetheless, the highest expression was found 63 dpi. Positive staining for iNOS was mainly observed in the cytoplasm of fused Kupffer cells and the highest expression observed at 35 dpi. T cells (CD3+) and B cells (CD45R-B220+) were predominant in more advanced granuloma stages, probably related to the establishment of acquired immunity. Neutrophils (Ly-6G+) were predominantly observed in mature granulomas with the highest expression at 15 dpi. Neutrophils were lower in numbers compared to other cell types, particularly at later time points.

CONCLUSIONS

Our results reflect the role of macrophages during the early stage of infection and the establishment of a lymphocytic response to control the infection in more advanced stages.

Mixed Formulation of Conventional and Pegylated Meglumine Antimoniate-Containing Liposomes Reduces Inflammatory Process and Parasite Burden in Leishmania infantum-Infected BALB/c Mice

Pentavalent antimonial has been the first choice treatment for visceral leishmaniasis; however, it has several side effects that leads to low adherence to treatment. Liposome-encapsulated meglumine antimoniate (MA) arises as an important strategy for chemotherapy enhancement. We evaluated the immunopathological changes using the mixture of conventional and pegylated liposomes with MA. The mice were infected with Leishmania infantum and a single-dose treatment regimen. Comparison was made with groups treated with saline, empty liposomes, free MA, and a liposomal formulation of MA (Lipo MA). Histopathological analyses demonstrated that animals treated with Lipo MA showed a significant decrease in the inflammatory process and the absence of granulomas. The in vitro stimulation of splenocytes showed a significant increase of gamma interferon (IFN-γ) produced by CD8⁺ T cells and a decrease in interleukin-10 (IL-10) produced by CD4⁺ and CD8⁺ T cells in the Lipo MA. Furthermore, the Lipo MA group showed an increase in the IFN-γ/IL-10 ratio in both CD4⁺ and CD8⁺ T cell subsets. According to the parasite load evaluation using quantitative PCR, the Lipo MA group showed no L. infantum DNA in the spleen (0.0%) and 41.4% in the liver. In addition, we detected a low positive correlation between parasitism and histopathology findings (inflammatory process and granuloma formation). Thus, our results confirmed that Lipo MA is a promising antileishmanial formulation able to reduce the inflammatory response and induce a type 1 immune response, accompanied by a significant reduction of the parasite burden into hepatic and splenic compartments in treated animals.

Galectin-1 Impairs the Generation of Anti-Parasitic Th1 Cell Responses in the Liver during Experimental Visceral Leishmaniasis

Many infectious diseases are characterized by the development of immunoregulatory pathways that contribute to pathogen persistence and associated disease symptoms. In diseases caused by intracellular parasites, such as visceral leishmaniasis (VL), various immune modulators have the capacity to negatively impact protective CD4⁺ T cell functions. Galectin-1 is widely expressed on immune cells and has previously been shown to suppress inflammatory responses and promote the development of CD4⁺ T cells with immunoregulatory characteristics. Here, we investigated the role of galectin-1 in experimental VL caused by infection of C57BL/6 mice with Leishmania donovani. Mice lacking galectin-1 expression exhibited enhanced tissue-specific control of parasite growth in the liver, associated with an augmented Th1 cell response. However, unlike reports in other experimental models, we found little role for galectin-1 in the generation of IL-10-producing Th1 (Tr1) cells, and instead report that galectin-1 suppressed hepatic Th1 cell development. Furthermore, we found relatively early effects of galectin-1 deficiency on parasite growth, suggesting involvement of innate immune cells. However, experiments investigating the impact of galectin-1 deficiency on dendritic cells indicated that they were not responsible for the phenotypes observed in galectin-1-deficient mice. Instead, studies examining galectin-1 expression by CD4⁺ T cells supported a T cell intrinsic role for galectin-1 in the suppression of hepatic Th1 cell development during experimental VL. Together, our findings provide new information on the roles of galectin-1 during parasitic infection and indicate an important role for this molecule in tissue-specific Th1 cell development, but not CD4⁺ T cell IL-10 production.

Vaccination with poly(D,L-lactide-co-glycolide) nanoparticles loaded with soluble Leishmania antigens and modified with a TNFα-mimicking peptide or monophosphoryl lipid A confers protection against experimental visceral leishmaniasis

Visceral leishmaniasis (VL) persists as a major public health problem, and since the existing chemotherapy is far from satisfactory, development of an effective vaccine emerges as the most appropriate strategy for confronting VL. The development of an effective vaccine relies on the selection of the appropriate antigen and also the right adjuvant and/or delivery vehicle. In the present study, the protective efficacy of poly(D,L-lactide-co-glycolide) (PLGA) nanoparticles (NPs), which were surface-modified with a TNFα-mimicking eight-amino-acid peptide (p8) and further functionalized by encapsulating soluble Leishmania infantum antigens (sLiAg) and monophosphoryl lipid A (MPLA), a TLR4 ligand, was evaluated against challenge with L. infantum parasites in BALB/c mice. Vaccination with these multifunctionalized PLGA nanoformulations conferred significant protection against parasite infection in vaccinated mice. In particular, vaccination with PLGA-sLiAg-MPLA or p8-PLGA-sLiAg NPs resulted in almost complete elimination of the parasite in the spleen for up to 4 months post-challenge. Parasite burden reduction was accompanied by antigen-specific humoral and cellular immune responses. Specifically, injection with PLGA-sLiAg-MPLA raised exclusively anti-sLiAg IgG1 antibodies post-vaccination, while in p8-PLGA-sLiAg-vaccinated mice, no antibody production was detected. However, 4 months post-challenge, in mice vaccinated with all the multifunctionalized NPs, antibody class switching towards IgG2a subtype was observed. The study of cellular immune responses revealed the increased proliferation capacity of spleen cells against sLiAg, consisting of IFNγ-producing CD4⁺ and CD8⁺ T cells. Importantly, the activation of CD8⁺ T cells was exclusively attributed to vaccination with PLGA NPs surface-modified with the p8 peptide. Moreover, characterization of cytokine production in vaccinated-infected mice revealed that protection was accompanied by significant increase of IFNγ and lower levels of IL-4 and IL-10 in protected mice when compared to control infected group. Conclusively, the above nanoformulations hold promise for future vaccination strategies against VL.

Gamma Interferon-Regulated Chemokines in Leishmania donovani Infection in the Liver

In the livers of C57BL/6 mice, gamma interferon (IFN-γ) controls intracellular Leishmania donovani infection and the efficacy of antimony (Sb) chemotherapy. Since both responses usually correlate with granulomatous inflammation, we tested six prominently expressed, IFN-γ-regulated chemokines-CXCL9, CXCL10, CXCL13, CXCL16, CCL2, and CCL5-for their roles in (i) mononuclear cell recruitment and granuloma assembly and maturation, (ii) initial control of infection and self-cure, and (iii) responsiveness to Sb treatment. Together, the results for the L. donovani-infected livers of chemokine-deficient mice (CXCR6^-/- mice were used as CXCL16-deficient surrogates) indicated that individual IFN-γ-induced chemokines have diverse affects and (i) may be entirely dispensable (CXCL13, CXCL16), (ii) may promote (CXCL10, CCL2, CCL5) or downregulate (CXCL9) initial granuloma assembly, (iii) may enhance (CCL2, CCL5) or hinder (CXCL10) early parasite control, (iv) may promote granuloma maturation (CCL2, CCL5), (v) may exert a granuloma-independent action that enables self-cure (CCL5), and (vi) may have no role in responsiveness to chemotherapy. Despite the near absence of tissue inflammation in early-stage infection, parasite replication could be controlled (in CXCL10^-/- mice) and Sb was fully active (in CXCL10^-/-, CCL2^-/-, and CCL5^-/- mice). These results characterize chemokine action in the response to L. donovani and also reemphasize that (i) recruited mononuclear cells and granulomas are not required to control infection or respond to Sb chemotherapy, (ii) granuloma assembly, control of infection, and Sb's efficacy are not invariably linked expressions of the same T cell-dependent, cytokine-mediated antileishmanial mechanism, and (iii) granulomas are not necessarily hallmarks of protective antileishmanial immunity.

Bone marrow-derived and resident liver macrophages display unique transcriptomic signatures but similar biological functions

BACKGROUND & AIMS

Kupffer cells (KCs), the resident tissue macrophages of the liver, play a crucial role in the clearance of pathogens and other particulate materials that reach the systemic circulation. Recent studies have identified KCs as a yolk sac-derived resident macrophage population that is replenished independently of monocytes in the steady state. Although it is now established that following local tissue injury, bone marrow derived monocytes may infiltrate the tissue and differentiate into macrophages, the extent to which newly differentiated macrophages functionally resemble the KCs they have replaced has not been extensively studied.

METHODS

We studied the two populations of KCs using intravital microscopy, morphometric analysis and gene expression profiling. An ion homeostasis gene signature, including genes associated with scavenger receptor function and extracellular matrix deposition, allowed discrimination between these two KC sub-types.

RESULTS

Bone marrow derived "KCs" accumulating as a result of genotoxic injury, resemble but are not identical to their yolk sac counterparts. Reflecting the differential expression of scavenger receptors, yolk sac-derived KCs were more effective at accumulating acetylated low density lipoprotein, whereas surprisingly, they were poorer than bone marrow-derived KCs when assessed for uptake of a range of bacterial pathogens. The two KC populations were almost indistinguishable in regard to i) response to lipopolysaccharide challenge, ii) phagocytosis of effete red blood cells and iii) their ability to contain infection and direct granuloma formation against Leishmania donovani, a KC-tropic intracellular parasite.

CONCLUSIONS

Bone marrow-derived KCs differentiate locally to resemble yolk sac-derived KC in most but not all respects, with implications for models of infectious diseases, liver injury and bone marrow transplantation. In addition, the gene signature we describe adds to the tools available for distinguishing KC subpopulations based on their ontology.

LAY SUMMARY

Liver macrophages play a major role in the control of infections in the liver and in the pathology associated with chronic liver diseases. It was recently shown that liver macrophages can have two different origins, however, the extent to which these populations are functionally distinct remains to be fully addressed. Our study demonstrates that whilst liver macrophages share many features in common, regardless of their origin, some subtle differences in function exist.

DATA REPOSITORY

Gene expression data are available from the European Bioinformatics Institute ArrayExpress data repository (accession number E-MTAB-4954).

CD4+ Recent Thymic Emigrants Are Recruited into Granulomas during Leishmania donovani Infection but Have Limited Capacity for Cytokine Production

Recent thymic emigrants (RTEs) represent a source of antigen-naïve T cells that enter the periphery throughout life. However, whether RTEs contribute to the control of chronic parasitic infection and how their potential might be harnessed by therapeutic intervention is currently unclear. Here, we show that CD4⁺ recent thymic emigrants emerging into the periphery of mice with ongoing Leishmania donovani infection undergo partial activation and are recruited to sites of granulomatous inflammation. However, CD4⁺ RTEs displayed severely restricted differentiation either into IFNγ⁺ or IFNγ⁺TNFα⁺ effectors, or into IL-10-producing regulatory T cells. Effector cell differentiation in the chronically infected host was not promoted by adoptive transfer of activated dendritic cells or by allowing extended periods of post-thymic differentiation in the periphery. Nevertheless, CD4⁺ RTEs from infected mice retained the capacity to transfer protection into lymphopenic RAG2^-/- mice. Taken together, our data indicate that RTEs emerging into a chronically inflamed environment are not recruited into the effector pool, but retain the capacity for subsequent differentiation into host protective T cells when placed in a disease-free environment.

Leishmania infantum antigens modulate memory cell subsets of liver resident T lymphocyte

In the recent years, the liver has been recognized as an important immune organ with major regulatory functions and immune memory, adding to the well-described vital metabolic functions. There are evidences from experimental infections performed with visceral Leishmania species that immune responses to parasite infection can be organ-specific. The liver is the compartment of acute resolving infection, with minimal tissue damage and resistance to reinfection, whereas the spleen is the compartment of parasite persistence. Control of hepatic infection in mice requires a coordinated immune response that involves the development of inflammatory granulomas. It is also described that the liver harbors populations of resident lymphocytes, which may exhibit memory characteristics. Therefore, the present study aims to address the role of the liver as an immune memory organ in the context of Leishmania infantum infection, by characterizing phenotypically resident liver T lymphocytes. The dynamics of memory T cells in L. infantum infected BALB/c mice and the effect of anti-leishmanial treatment in the differentiation of memory cell subsets were analyzed. The potential of recognition, differentiation and selection of memory lymphocytes by three L. infantum recombinant proteins were also explored. L. infantum infection generates effector and central memory T cells, but the cells did not expand when recalled, demonstrating a possible parasite silencing effect. The treatment with a leishmanicidal drug (antimoniate meglumine) increases the levels of memory and effector T cells, eliciting a more robust hepatic immune response. L. infantum parasites with a decreased sensitivity to the leishmanicidal drug favor the expansion of memory CD8⁺ T cell subset, but inhibit the proliferation of CD8⁺ T effector cells, possibly assuring their own survival. The recombinant proteins LirCyp1 and LirSOD are strongly recognized by memory cells of treated mice, indicating that these proteins might be used in a prophylactic or therapeutic vaccine formulation. Thus, L. infantum released antigens induce the development of immune memory subsets in the liver resident T cell population that specifically recognized parasite antigens, including recombinant proteins.

Regulation of immunity during visceral Leishmania infection

Unicellular eukaryotes of the genus Leishmania are collectively responsible for a heterogeneous group of diseases known as leishmaniasis. The visceral form of leishmaniasis, caused by L. donovani or L. infantum, is a devastating condition, claiming 20,000 to 40,000 lives annually, with particular incidence in some of the poorest regions of the world. Immunity to Leishmania depends on the development of protective type I immune responses capable of activating infected phagocytes to kill intracellular amastigotes. However, despite the induction of protective responses, disease progresses due to a multitude of factors that impede an optimal response. These include the action of suppressive cytokines, exhaustion of specific T cells, loss of lymphoid tissue architecture and a defective humoral response. We will review how these responses are orchestrated during the course of infection, including both early and chronic stages, focusing on the spleen and the liver, which are the main target organs of visceral Leishmania in the host. A comprehensive understanding of the immune events that occur during visceral Leishmania infection is crucial for the implementation of immunotherapeutic approaches that complement the current anti-Leishmania chemotherapy and the development of effective vaccines to prevent disease.

Lessons from other diseases: granulomatous inflammation in leishmaniasis

The Leishmania granuloma shares some, though not all, properties with that formed following mycobacterial infection. As a simplified, noncaseating granuloma composed of relatively few and largely mononuclear cell populations, it provides a tractable model system to investigate intra-granuloma cellular dynamics, immune regulation, and antimicrobial resistance. Here, the occurrence of granulomatous pathology across the spectrum of leishmaniasis, in humans and animal reservoir hosts, is first described. However, this review focuses on the process of hepatic granuloma formation as studied in rodent models of visceral leishmaniasis, starting from the initial infection of Kupffer cells to the involution of the granuloma after pathogen clearance. It describes how the application of intravital imaging and the use of computational modeling have changed some of our thoughts on granuloma function, and illustrates how host-directed therapies have been used to manipulate granuloma form and function for therapeutic benefit. Where appropriate, lessons that may be equally applicable across the spectrum of granulomatous diseases are highlighted.

Eosinophils and mast cells in leishmaniasis

Leishmania spp. are parasitic protozoa endemic in tropical and subtropical regions and the causative agent of leishmaniasis, a collection of syndromes whose clinical manifestations vary according to host and pathogen factors. Leishmania spp. are inoculated into the mammalian host by the bite of an infected sand fly, whereupon they are taken up by phagocytosis, convert into the replicative amastigote stage within macrophages, reproduce, spread to new macrophages and cause disease manifestations. A curative response against leishmaniasis depends in the classical activation of macrophages and the IL-12-dependent onset of an adaptive type 1 response characterized by the production of IFN-γ. Emerging evidence suggests that neutrophils, dendritic cells and other immune cells can serve as either temporary or stable hosts for Leishmania spp. Furthermore, it is becoming apparent that the initial interactions of the parasite with resident or early recruited immune cells can shape both the macrophage response and the type of adaptive immune response being induced. In this review, we compile a growing number of studies demonstrating how the earliest interactions of Leishmania spp. with eosinophils and mast cells influence the macrophage response to infection and the development of the adaptive immune response, hence, determining the ultimate outcome of infection.

Evaluation of the immunoprophylactic potential of a killed vaccine candidate in combination with different adjuvants against murine visceral leishmaniasis

Despite a large number of field trials, till date no prophylactic antileishmanial vaccine exists for human use. Killed antigen formulations offer the advantage of being safe but they have limited immunogenicity. Recent research has documented that efforts to develop effective Leishmania vaccine have been limited due to the lack of an appropriate adjuvant. Addition of adjuvants to vaccines boosts and directs the immunogenicity of antigens. So, the present study was done to evaluate the effectiveness of four adjuvants i.e. alum, saponin, cationic liposomes and monophosphoryl lipid-A in combination with Autoclaved Leishmania donovani (ALD) antigen against murine visceral leishmaniasis (VL). BALB/c mice were immunized thrice with respective vaccine formulation. Two weeks after last booster, challenge infection was given. Mice were sacrificed 15 days after last immunization and on 30, 60 and 90 post infection/challenge days. A considerable protective efficacy was shown by all vaccine formulations. It was evident from significant reduction in parasite load, profound delayed type hypersensitivity responses (DTH), increased IgG2a titres and high levels of Th1 cytokines (IFN-γ, IL-12) as compared to the infected controls. However, level of protection varied with the type of adjuvant used. Maximum protection was achieved with the use of liposome encapsulated ALD antigen and it was closely followed by group immunized with ALD+MPL-A. Significant results were also obtained with ALD+saponin, ALD+alum and ALD antigen (alone) but the protective efficacy was reduced as compared to other immunized groups. The present study reveals greater efficacy of two vaccine formulations i.e. ALD+liposome and ALD+MPL-A against murine VL.

Immune responses during cutaneous and visceral leishmaniasis

Leishmania are protozoan parasites spread by a sandfly insect vector and causing a spectrum of diseases collectively known as leishmaniasis. The disease is a significant health problem in many parts of the world, resulting in an estimated 1·3 million new cases and 30 000 deaths annually. Current treatment is based on chemotherapy, which is difficult to administer, expensive and becoming ineffective in several endemic regions. To date there is no vaccine against leishmaniasis, although extensive evidence from studies in animal models indicates that solid protection can be achieved upon immunization. This review focuses on immune responses to Leishmania in both cutaneous and visceral forms of the disease, pointing to the complexity of the immune response and to a range of evasive mechanisms utilized by the parasite to bypass those responses. The amalgam of innate and acquired immunity combined with the paucity of data on the human immune response is one of the major problems currently hampering vaccine development and implementation.

Early induction of interleukin-10 limits antigen-specific CD4⁺ T cell expansion, function, and secondary recall responses during persistent phagosomal infection

Diverse pathogens have evolved to survive and replicate in the endosomes or phagosomes of the host cells and establish persistent infection. Ehrlichiae are Gram-negative, intracellular bacteria that are transmitted by ticks. Ehrlichiae reside in the endosomes of the host phagocytic or endothelial cells and establish persistent infection in their vertebrate reservoir hosts. CD4(+) T cells play a critical role in protection against phagosomal infections. In the present study, we investigated the expansion, maintenance, and functional status of antigen-specific CD4(+) T cells during persistent Ehrlichia muris infection in wild-type and interleukin-10 (IL-10)-deficient mice. Our study indicated that early induction of IL-10 led to reduced inflammatory responses and impaired bacterial clearance during persistent Ehrlichia infection. Notably, we demonstrated that the functional production of gamma interferon (IFN-γ) by antigen-specific CD4(+) T cells maintained during a persistent phagosomal infection progressively deteriorates. The functional loss of IFN-γ production by antigen-specific CD4(+) T cells was reversed in the absence of IL-10. Furthermore, we demonstrated that transient blockade of IL-10 receptor during the T cell priming phase early in infection was sufficient to enhance the magnitude and the functional capacity of antigen-specific effector and memory CD4(+) T cells, which translated into an enhanced recall response. Our findings provide new insights into the functional status of antigen-specific CD4(+) T cells maintained during persistent phagosomal infection. The study supports the concept that a better understanding of the factors that influence the priming and differentiation of CD4(+) T cells may provide a basis to induce a protective immune response against persistent infections.

Immune regulation during chronic visceral leishmaniasis

Visceral leishmaniasis is a chronic parasitic disease associated with severe immune dysfunction. Treatment options are limited to relatively toxic drugs, and there is no vaccine for humans available. Hence, there is an urgent need to better understand immune responses following infection with Leishmania species by studying animal models of disease and clinical samples from patients. Here, we review recent discoveries in these areas and highlight shortcomings in our knowledge that need to be addressed if better treatment options are to be developed and effective vaccines designed.

CD4+ T Cells: guardians of the phagosome

CD4(+) T cells are key cells of the adaptive immune system that use T cell antigen receptors to recognize peptides that are generated in endosomes or phagosomes and displayed on the host cell surface bound to major histocompatibility complex molecules. These T cells participate in immune responses that protect hosts from microbes such as Mycobacterium tuberculosis, Cryptococcus neoformans, Leishmania major, and Salmonella enterica, which have evolved to live in the phagosomes of macrophages and dendritic cells. Here, we review studies indicating that CD4(+) T cells control phagosomal infections asymptomatically in most individuals by secreting cytokines that activate the microbicidal activities of infected phagocytes but in a way that inhibits the pathogen but does not eliminate it. Indeed, we make the case that localized, controlled, persistent infection is necessary to maintain large numbers of CD4(+) effector T cells in a state of activation needed to eradicate systemic and more pathogenic forms of the infection. Finally, we posit that current vaccines for phagosomal infections fail because they do not produce this "periodic reminder" form of CD4(+) T cell-mediated immune control.

Tissue requirements for establishing long-term CD4+ T cell-mediated immunity following Leishmania donovani infection

Organ-specific immunity is a feature of many infectious diseases, including visceral leishmaniasis caused by Leishmania donovani. Experimental visceral leishmaniasis in genetically susceptible mice is characterized by an acute, resolving infection in the liver and chronic infection in the spleen. CD4+ T cell responses are critical for the establishment and maintenance of hepatic immunity in this disease model, but their role in chronically infected spleens remains unclear. In this study, we show that dendritic cells are critical for CD4+ T cell activation and expansion in all tissue sites examined. We found that FTY720-mediated blockade of T cell trafficking early in infection prevented Ag-specific CD4+ T cells from appearing in lymph nodes, but not the spleen and liver, suggesting that early CD4+ T cell priming does not occur in liver-draining lymph nodes. Extended treatment with FTY720 over the first month of infection increased parasite burdens, although this associated with blockade of lymphocyte egress from secondary lymphoid tissue, as well as with more generalized splenic lymphopenia. Importantly, we demonstrate that CD4+ T cells are required for the establishment and maintenance of antiparasitic immunity in the liver, as well as for immune surveillance and suppression of parasite outgrowth in chronically infected spleens. Finally, although early CD4+ T cell priming appeared to occur most effectively in the spleen, we unexpectedly revealed that protective CD4+ T cell-mediated hepatic immunity could be generated in the complete absence of all secondary lymphoid tissues.

A2 and other visceralizing proteins of Leishmania: role in pathogenesis and application for vaccine development

Visceral leishmaniasis is a re-emergent disease and a significant cause of morbidity worldwide. Amongst the more than 20 Leishmania species, Leishmania donovani, Leishmania infantum and more rarely Leishmania amazonensis are associated with visceral leishmaniasis. A major question in leishmaniasis research is how these species migrate to and infect visceral organs whereas other species such as Leishmania major and Leishmania braziliensis remain in the skin, causing tegumentary leishmaniasis. Here we present the more recent advances and approaches towards the identification of species-specific visceralizing factors of Leishmania, such as the A2 protein, leading to a better understanding of parasite biology. We also discuss their potential use for the development of a vaccine for visceral leishmaniasis.

Hyperlipidemia offers protection against Leishmania donovani infection: role of membrane cholesterol

Leishmania donovani (LD), the causative agent of visceral leishmaniasis (VL), extracts membrane cholesterol from macrophages and disrupts lipid rafts, leading to their inability to stimulate T cells. Restoration of membrane cholesterol by liposomal delivery corrects the above defects and offers protection in infected hamsters. To reinforce further the protective role of cholesterol in VL, mice were either provided a high-cholesterol (atherogenic) diet or underwent statin treatment. Subsequent LD infection showed that an atherogenic diet is associated with protection, whereas hypocholesterolemia due to statin treatment confers susceptibility to the infection. This observation was validated in apolipoprotein E knockout mice (AE) mice that displayed intrinsic hypercholesterolemia with hepatic granuloma, production of host-protective cytokines, and expansion of antileishmanial CD8(+)IFN- γ (+) and CD8(+)IFN- γ (+)TNF- α (+) T cells in contrast to the wild-type C57BL/6 (BL/6) mice when infected with LD. Normal macrophages from AE mice (N-AE-MΦ) showed 3-fold higher membrane cholesterol coupled with increased fluorescence anisotropy (FA) compared with wild-type macrophage (N-BL/6-MΦ). Characterization of in vitro LD-infected AE macrophage (LD-AE-MΦ) revealed intact raft architecture and ability to stimulate T cells, which were compromised in LD-BL/6-MΦ. This study clearly indicates that hypercholesterolemia, induced intrinsically or extrinsically, can control the pathogenesis of VL by modulating immune repertoire in favor of the host.

Making an anti-amastigote vaccine for visceral leishmaniasis: rational, update and perspectives

Visceral leishmaniasis is a major health problem in Latina America, as well as the Mediterranean region of Europe and Asia. We aimed to develop a vaccine against visceral leishmaniasis targeting the intracellular amastigotes, which is the parasite stage that persists throughout infections with Leishmania parasites. With this in mind, we identified an amastigote specific antigen (A2) that contains an immunogenic epitope for CD4+ T helper (Th) cells and multiple repetitive units encoding CD8+ cytotoxic T lymphocyte (CTL) epitopes. Vaccine formulations containing the recombinant A2 associated with saponin, alum and IL-12 or expressed by attenuated adenovirus were shown to be protective in mice, dogs and nonhuman-primates. We are currently identifying novel amastigote specific immunogenic proteins that could be aggregated to A2 to further improve the level of vaccine-induced cell-mediated immunity and protection against visceral leishmaniasis.

B cell: T cell interactions occur within hepatic granulomas during experimental visceral leishmaniasis

Hepatic resistance to Leishmania donovani infection in mice is associated with the development of granulomas, in which a variety of lymphoid and non-lymphoid populations accumulate. Although previous studies have identified B cells in hepatic granulomas and functional studies in B cell-deficient mice have suggested a role for B cells in the control of experimental visceral leishmaniasis, little is known about the behaviour of B cells in the granuloma microenvironment. Here, we first compared the hepatic B cell population in infected mice, where ≈60% of B cells are located within granulomas, with that of naïve mice. In infected mice, there was a small increase in mIgM(lo)mIgD(+) mature B2 cells, but no enrichment of B cells with regulatory phenotype or function compared to the naïve hepatic B cell population, as assessed by CD1d and CD5 expression and by IL-10 production. Using 2-photon microscopy to quantify the entire intra-granuloma B cell population, in conjunction with the adoptive transfer of polyclonal and HEL-specific BCR-transgenic B cells isolated from L. donovani-infected mice, we demonstrated that B cells accumulate in granulomas over time in an antigen-independent manner. Intra-vital dynamic imaging was used to demonstrate that within the polyclonal B cell population obtained from L. donovani-infected mice, the frequency of B cells that made multiple long contacts with endogenous T cells was greater than that observed using HEL-specific B cells obtained from the same inflammatory environment. These data indicate, therefore, that a subset of this polyclonal B cell population is capable of making cognate interactions with T cells within this unique environment, and provide the first insights into the dynamics of B cells within an inflammatory site.

Critical roles for LIGHT and its receptors in generating T cell-mediated immunity during Leishmania donovani infection

LIGHT (TNFSF14) is a member of the TNF superfamily involved in inflammation and defence against infection. LIGHT signals via two cell-bound receptors; herpes virus entry mediator (HVEM) and lymphotoxin-beta receptor (LTβR). We found that LIGHT is critical for control of hepatic parasite growth in mice with visceral leishmaniasis (VL) caused by infection with the protozoan parasite Leishmania donovani. LIGHT-HVEM signalling is essential for early dendritic cell IL-12/IL-23p40 production, and the generation of IFNγ- and TNF-producing T cells that control hepatic infection. However, we also discovered that LIGHT-LTβR interactions suppress anti-parasitic immunity in the liver in the first 7 days of infection by mechanisms that restrict both CD4⁺ T cell function and TNF-dependent microbicidal mechanisms. Thus, we have identified distinct roles for LIGHT in infection, and show that manipulation of interactions between LIGHT and its receptors may be used for therapeutic advantage.

Visceral leishmaniasis (VL) is a potentially fatal human disease caused by the intracellular protozoan parasites Leishmania donovani and L. infantum (chagasi). Parasites infect macrophages throughout the viscera, though the spleen and liver are the major sites of disease. VL is responsible for significant morbidity and mortality in the developing world, particularly in India, Sudan, Nepal, Bangladesh and Brazil. Because of the intrusive techniques required to analyse tissue in VL patients, our current understanding of the host immune response during VL largely derives from studies performed in genetically susceptible mice. We have discovered that mice which are unable to produce a cytokine called LIGHT have poor control of L. donovani infection in the liver, compared with wild-type control animals. In addition, we demonstrated that LIGHT has distinct roles during VL, depending on which of its two major cell-bound receptors it engages. Finally, we identified an antibody that stimulates the lymphotoxin β receptor (one of the LIGHT receptors), that can stimulate anti-parasitic activity during an established infection, thereby identifying this receptor as a therapeutic target during disease.

Immunotherapy as a strategy for treatment of leishmaniasis: a review of the literature

Leishmaniasis occurs as a spectrum of clinical syndromes divided into cutaneous, mucocutaneous and visceral forms. The epidemiology and clinical features are highly variable owing to the interplay of many factors ranging from parasite species and strains, vectors, host genetics and environment. Currently, there is no effective licensed vaccine for use in humans against leishmaniasis. Most traditional and low-cost treatment options, particularly in poor and endemic areas, are toxic with many adverse reactions and they require a long course of administration. The use of more effective, less toxic drugs is limited because total treatment cost is very high (expensive) and there are fears of development of drug resistance. Recent studies indicate that certain strategies aimed at modulating the host immune response (collectively called immunotherapy) could result in prophylactic and/or therapeutic cure of leishmaniasis under both laboratory and field conditions. In this review, we focus on treatment of leishmaniasis with a particular emphasis on immunotherapy/immunochemotherapy as an alternative to conventional drug treatment.

In vivo imaging of transgenic Leishmania parasites in a live host

Distinct species of Leishmania, a protozoan parasite of the family Trypanosomatidae, typically cause different human disease manifestations. The most common forms of disease are visceral leishmaniasis (VL) and cutaneous leishmaniasis (CL). Mouse models of leishmaniasis are widely used, but quantification of parasite burdens during murine disease requires mice to be euthanized at various times after infection. Parasite loads are then measured either by microscopy, limiting dilution assay, or qPCR amplification of parasite DNA. The in vivo imaging system (IVIS) has an integrated software package that allows the detection of a bioluminescent signal associated with cells in living organisms. Both to minimize animal usage and to follow infection longitudinally in individuals, in vivo models for imaging Leishmania spp. causing VL or CL were established. Parasites were engineered to express luciferase, and these were introduced into mice either intradermally or intravenously. Quantitative measurements of the luciferase driving bioluminescence of the transgenic Leishmania parasites within the mouse were made using IVIS. Individual mice can be imaged multiple times during longitudinal studies, allowing us to assess the inter-animal variation in the initial experimental parasite inocula, and to assess the multiplication of parasites in mouse tissues. Parasites are detected with high sensitivity in cutaneous locations. Although it is very likely that the signal (photons/second/parasite) is lower in deeper visceral organs than the skin, but quantitative comparisons of signals in superficial versus deep sites have not been done. It is possible that parasite numbers between body sites cannot be directly compared, although parasite loads in the same tissues can be compared between mice. Examples of one visceralizing species (L. infantum chagasi) and one species causing cutaneous leishmaniasis (L. mexicana) are shown. The IVIS procedure can be used for monitoring and analyzing small animal models of a wide variety of Leishmania species causing the different forms of human leishmaniasis.

Dynamic imaging of experimental Leishmania donovani-induced hepatic granulomas detects Kupffer cell-restricted antigen presentation to antigen-specific CD8 T cells

Kupffer cells (KCs) represent the major phagocytic population within the liver and provide an intracellular niche for the survival of a number of important human pathogens. Although KCs have been extensively studied in vitro, little is known of their in vivo response to infection and their capacity to directly interact with antigen-specific CD8⁺ T cells. Here, using a combination of approaches including whole mount and thin section confocal microscopy, adoptive cell transfer and intra-vital 2-photon microscopy, we demonstrate that KCs represent the only detectable population of mononuclear phagocytes within granulomas induced by Leishmania donovani infection that are capable of presenting parasite-derived peptide to effector CD8⁺ T cells. This restriction of antigen presentation to KCs within the Leishmania granuloma has important implications for the identification of new candidate vaccine antigens and for the design of novel immuno-therapeutic interventions.

Leishmania donovani is a protozoan parasite that causes severe disease in humans with associated pathology in the spleen and liver. In experimental models of L. donovani infection, the hepatic response to infection is characterised by the presence of a focal mononuclear cell-rich inflammatory response (a granuloma) surrounding cells infected with intracellular amastigotes. Granulomas provide focus to the ensuing immune response, helping to contain parasite dissemination and providing the major effector site responsible for parasites elimination from the liver. Although granulomas are believed to form around infected resident liver macrophages (Kupffer cells), the role of these cells in intra-granuloma antigen presentation is currently unknown. As CD8⁺ T cells have been shown to play an important role in hepatic resistance to L. donovani following natural infection, vaccination and during immunotherapy, we asked which cells within the granuloma microenvironment serve as targets for antigen recognition by effector CD8⁺ T cells. Here we provide evidence that the heavily infected mononuclear cell core of the granuloma is composed almost entirely of Kupffer cells, many having migrated from the surrounding sinusoids. Furthermore, by intra-vital 2-photon microscopy, we show that only Kupffer cells laden with intracellular amastigotes are able to form long-lasting antigen-specific interactions with CD8⁺ T cells within the granuloma microenvironment. These data have important implications for the understanding of how granulomas function to limit infection and may have important implications for the development of vaccines to Leishmania that are designed to induce CD8⁺ T cell responses.

Therapeutic glucocorticoid-induced TNF receptor-mediated amplification of CD4+ T cell responses enhances antiparasitic immunity

Chronic infectious diseases and cancers are often associated with suboptimal effector T cell responses. Enhancement of T cell costimulatory signals has been extensively studied for cancer immunotherapy but not so for the treatment of infectious disease. The few previous attempts at this strategy using infection models have lacked cellular specificity, with major immunoregulatory mechanisms or innate immune cells also being targeted. In this study, we examined the potential of promoting T cell responses via the glucocorticoid-induced TNF receptor (GITR) family-related protein in a murine model of visceral leishmaniasis. GITR stimulation during established infection markedly improved antiparasitic immunity. This required CD4(+) T cells, TNF, and IFN-gamma, but crucially, was independent of regulatory T (Treg) cells. GITR stimulation enhanced CD4(+) T cell expansion without modulating Treg cell function or protecting conventional CD4(+) T cells from Treg cell suppression. GITR stimulation substantially improved the efficacy of a first-line visceral leishmaniasis drug against both acute hepatic infection and chronic infection in the spleen, demonstrating its potential to improve clinical outcomes. This study identifies a novel strategy to therapeutically enhance CD4(+) T cell-mediated antiparasitic immunity and, importantly, achieves this goal without impairment of Treg cell function.