Nonhealing Infection despite Th1 Polarization Produced by a Strain ofLeishmania majorin C57BL/6 Mice

The β-Carboline Harmine Has a Protective Immunomodulatory Role in Nonhealing Cutaneous Leishmaniasis

Cutaneous leishmaniasis affects 1 million people worldwide annually. Although conventional treatments primarily target the parasite, there is growing interest in host immune modulation. In this study, we investigated the impact of synthetic β-carboline harmine (ACB1801), previously shown to be immunoregulatory in cancer, on the pathology caused by a drug-resistant Leishmania major strain causing persistent cutaneous lesions. Exposure to ACB1801 in vitro had a modest impact on parasite burden within host macrophages. Moreover, it significantly increased major histocompatibility complex II and costimulatory molecule expression on infected dendritic cells, suggesting an enhanced immune response. In vivo, ACB1801 monotherapy led to a substantial reduction in lesion development and parasite burden in infected C57BL/6 mice, comparable with efficacy of amphotericin B. Transcriptomics analysis further supported ACB1801 immunomodulatory effects, revealing an enrichment of TNF-α, IFN-γ, and major histocompatibility complex II antigen presentation signatures in the draining lymph nodes of treated mice. Flow cytometry analysis confirmed an increased frequency (1.5×) of protective CD4+IFN-γ+TNF-α+ T cells and a decreased frequency (2×) in suppressive IL-10+FoxP3- T cells at the site of infection and in draining lymph nodes. In addition, ACB1801 downregulated the aryl hydrocarbon receptor signaling, known to enhance immunosuppressive cytokines. Thus, these results suggest a potential use for ACB1801 alone or in combination therapy for cutaneous leishmaniasis.

Antigen recognition reinforces regulatory T cell mediated Leishmania major persistence

Cutaneous Leishmania major infection elicits a rapid T cell response that is insufficient to clear residually infected cells, possibly due to the accumulation of regulatory T cells in healed skin. Here, we used Leishmania-specific TCR transgenic mice as a sensitive tool to characterize parasite-specific effector and immunosuppressive responses in vivo using two-photon microscopy. We show that Leishmania-specific Tregs displayed higher suppressive activity compared to polyclonal Tregs, that was mediated through IL-10 and not through disrupting cell-cell contacts or antigen presentation. In vivo expansion of endogenous Leishmania-specific Tregs resulted in disease reactivation that was also IL-10 dependent. Interestingly, lack of Treg expansion that recognized the immunodominant Leishmania peptide PEPCK was sufficient to restore robust effector Th1 responses and resulted in parasite control exclusively in male hosts. Our data suggest a stochastic model of Leishmania major persistence in skin, where cellular factors that control parasite numbers are counterbalanced by Leishmania-specific Tregs that facilitate parasite persistence.

A fine mapping of single nucleotide variants and haplotype analysis of IL13 gene in patients with Leishmania guyanensis-cutaneous leishmaniasis and plasma cytokines IL-4, IL-5, and IL-13

Introduction

Leishmaniasis continues to pose a substantial health burden in 97 countries worldwide. The progression and outcome of Leishmania infection are influenced by various factors, including the cytokine milieu, the skin microbiota at the infection site, the specific Leishmania species involved, the genetic background of the host, and the parasite load. In endemic regions to leishmaniasis, only a fraction of individuals infected actually develops the disease. Overexpression of IL-13 in naturally resistant C57BL/6 mice renders them susceptible to L. major infection. Haplotypes constructed from several single nucleotide variant (SNV) along a chromosome fragment may provide insight into any SNV near the fragment that may be genuinely associated with a phenotype in genetic association studies.

Methods

We investigated nine SNVs (SNV1rs1881457A>C, SNV2rs1295687C>G, SNV3rs2069744C>T, SNV4rs2069747C>T, SNV5rs20541A>G, SNV6rs1295685A>G, SNV7rs848A>C, SNV8rs2069750G >C, and SNV9rs847T>C) spanning the entire IL13 gene in patients with L. guyanensis cutaneous leishmaniasis (Lg-CL).

Results

Our analysis did not reveal any significant association between the SNVs and susceptibility/protection against Lg-CL development. However, haplotype analysis, excluding SNV4rs2069747 and SNV8rs2069750 due to low minor allele frequency, revealed that carriers of the haplotype CCCTAAC had a 93% reduced likelihood developing Lg-CL. Similarly, the haplotypes ACCCGCT (ORadj=0.02 [95% CI 0.00-0.07]; p-value, 6.0×10-19) and AGCTAAC (ORadj=0.00[95% CI 0.00-0.00]; p-value 2.7×10-12) appeared to provide protection against the development of Lg-CL. Conversely, carriers of haplotype ACCTGCC have 190% increased likelihood of developing Lg-CL (ORadj=2.9 [95%CI 1.68-5.2]; p-value, 2.5×10-6). Similarly, haplotype ACCCAAT (ORadj=2.7 [95%CI 1.5-4.7]; p-value, 3.2×10-5) and haplotype AGCCGCC are associated with susceptibility to the development of Lg-CL (ORadj=1.7[95%CI 1.04-2.8]; p-value, 0.01). In our investigation, we also found a correlation between the genotypes of rs2069744, rs20541, rs1295685, rs847, and rs848 and plasma IL-5 levels among Lg-Cl patients. Furthermore, rs20541 showed a correlation with plasma IL-13 levels among Lg-Cl patients, while rs2069744 and rs848 showed a correlation with plasma IL-4 levels among the same group.

Conclusions

Overall, our study identifies three haplotypes of IL13 associated with resistance to disease development and three haplotypes linked to susceptibility. These findings suggest the possibility of a variant outside the gene region that may contribute, in conjunction with other genes, to differences in susceptibility and partially to the pathology.

The cytokine/chemokine response in Leishmania/HIV infection and co-infection

HIV infection progressively weakens the immune system by infecting and destroying cells involved in host defense. Viral infection symptoms are generated and aggravated as immunosuppression progresses, triggered by the presence of opportunistic infections: among these is leishmaniasis, a disease caused by the intracellular parasite Leishmania. The incidence of this co-infection is growing progressively due to the geographic distribution overlap. Both pathogens infect monocytes/macrophages and dendritic cells, although they can also modulate the activity of other cells without co-infecting, such as T and B lymphocytes. Leishmania/HIV co-infection could be described as a system comprising modulations of cell surface molecule expression, production of soluble factors, and intracellular death activities, leading ultimately to the potentiation of infectivity, replication, and spread of both pathogens. This review describes the cytokine/chemokine response in Leishmania/HIV infection and co-infection, discussing how these molecules modulate the course of the disease and analyzing the therapeutic potential of targeting this network.

Cytokines and Signaling Networks Regulating Disease Outcomes in Leishmaniasis

Cytokines play crucial roles in commencing and coordinating the organized recruitment and activation of immune cells during infection. These molecular regulators play an important part in deciding the fate of disease outcomes in leishmaniasis, a parasitic disease of tropical and subtropical countries. T helper 1 (Th1) cell-mediated inflammatory cytokines usually play a host-protective role, while T helper 2 (Th2) cell activation produces an anti-inflammatory milieu necessary for parasite survival. It is noteworthy that in such a multifaceted disease, the role played by any particular cytokine cannot be generalized as either beneficial or detrimental. For example, a "host-favorable" cytokine in one form of the disease has been found to be "pathogen friendly" in another form of leishmaniasis. On the other hand, the complex signaling network regulating the production of cytokines is further complicated by the nature of the host as well as the presence of other cytokines in the milieu. The present review focuses on the differential roles played by cytokines and the intricate signaling network responsible for the regulation of such cytokines during infection by different species of Leishmania. While many more studies are required in the future to better understand the role of these molecules in both animal models and patient samples, current studies indicate that these molecules are potential candidates to be targeted for therapy against this deadly disease.

Eosinophils, but Not Type 2 Innate Lymphoid Cells, Are the Predominant Source of Interleukin 4 during the Innate Phase of Leishmania major Infection

Interleukin (IL)-4 plays a central role in the initiation of a type 2 T helper cell (Th2) response, which leads to non-healing and progressive infections with the protozoan parasite Leishmania (L.) major. Here, we tested the hypothesis that type 2 innate lymphoid cells (ILC2), which promote the development of Th2 cells, form an important source of IL-4 early after intradermal or subcutaneous L. major infection. Lineage-marker negative CD90.2⁺CD127⁺PD1⁻ ILC2 were readily detectable in the ear or foot skin, but hardly in the draining lymph nodes of both naïve and L. major-infected self-healing C57BL/6 and non-healing BALB/c mice and made up approximately 20% to 30% of all CD45⁺SiglecF⁻ cells. Dermal ILC2 of C57BL/6 mice expressed the inducible T cell-costimulator (ICOS, CD278), whereas BALB/C ILC2 were positive for the stem cell antigen (Sca)-1. Within the first 5 days of infection, the absolute numbers of ILC2 did not significantly change in the dermis, which is in line with the unaltered expression of cytokines activating (IL-18, IL-25, IL-33, TSLP) or inhibiting ILC2 (IL-27, IFN-γ). At day 5 to 6 post infection, we observed an upregulation of IL-4, but not of IL-5, IL-10 or IL-13 mRNA. Using IL-4-reporter (4get) mice, we found that the production of IL-4 by C57BL/6 or BALB/c mice was largely restricted to CD45⁺SiglecF⁺ cells of high granularity, i.e., eosinophils. From these data, we conclude that eosinophils, but not ILC2, are a major innate source of IL-4 at the skin site of L. major infection.

CD4 expression in effector T cells depends on DNA demethylation over a developmentally established stimulus-responsive element

The epigenetic patterns that are established during early thymic development might determine mature T cell physiology and function, but the molecular basis and topography of the genetic elements involved are not fully known. Here we show, using the Cd4 locus as a paradigm for early developmental programming, that DNA demethylation during thymic development licenses a novel stimulus-responsive element that is critical for the maintenance of Cd4 gene expression in effector T cells. We document the importance of maintaining high CD4 expression during parasitic infection and show that by driving transcription, this stimulus-responsive element allows for the maintenance of histone H3K4me3 levels during T cell replication, which is critical for preventing de novo DNA methylation at the Cd4 promoter. A failure to undergo epigenetic programming during development leads to gene silencing during effector T cell replication. Our study thus provides evidence of early developmental events shaping the functional fitness of mature effector T cells.

Aspirin-triggered resolvin D1 reduces parasitic cardiac load by decreasing inflammation in a murine model of early chronic Chagas disease

Background

Chagas disease, caused by the protozoan Trypanosoma cruzi, is endemic in Latin America and is widely distributed worldwide because of migration. In 30% of cases, after years of infection and in the absence of treatment, the disease progresses from an acute asymptomatic phase to a chronic inflammatory cardiomyopathy, leading to heart failure and death. An inadequate balance in the inflammatory response is involved in the progression of chronic Chagas cardiomyopathy. Current therapeutic strategies cannot prevent or reverse the heart damage caused by the parasite. Aspirin-triggered resolvin D1 (AT-RvD1) is a pro-resolving mediator of inflammation that acts through N-formyl peptide receptor 2 (FPR2). AT-RvD1 participates in the modification of cytokine production, inhibition of leukocyte recruitment and efferocytosis, macrophage switching to a nonphlogistic phenotype, and the promotion of healing, thus restoring organ function. In the present study, AT-RvD1 is proposed as a potential therapeutic agent to regulate the pro-inflammatory state during the early chronic phase of Chagas disease.

Methodology/Principal findings

C57BL/6 wild-type and FPR2 knock-out mice chronically infected with T. cruzi were treated for 20 days with 5 μg/kg/day AT-RvD1, 30 mg/kg/day benznidazole, or the combination of 5 μg/kg/day AT-RvD1 and 5 mg/kg/day benznidazole. At the end of treatment, changes in immune response, cardiac tissue damage, and parasite load were evaluated. The administration of AT-RvD1 in the early chronic phase of T. cruzi infection regulated the inflammatory response both at the systemic level and in the cardiac tissue, and it reduced cellular infiltrates, cardiomyocyte hypertrophy, fibrosis, and the parasite load in the heart tissue.

Conclusions/Significance

AT-RvD1 was shown to be an attractive therapeutic due to its regulatory effect on the inflammatory response at the cardiac level and its ability to reduce the parasite load during early chronic T. cruzi infection, thereby preventing the chronic cardiac damage induced by the parasite.

Chagas disease is prevalent in Latin America and is widely distributed worldwide due to migration. In 30% of patients, if the parasite is left untreated, the disease may progress from an acute symptomless phase to chronic myocardial inflammation, which can cause heart failure and death, years after the infection. Imbalances in the inflammatory response are related to this progression. Current treatments cannot prevent or reverse the cardiac damage inflicted by the parasite. Aspirin-triggered resolvin D1, also named AT-RvD1, can modify cellular and humoral inflammatory responses leading to the resolution of inflammation, thus promoting healing and restoring organ function. In this study, AT-RvD1, in an N-formyl peptide receptor 2 (FPR2)-dependent manner, was shown to regulate local and systemic inflammation and decrease cellular infiltration in the heart tissue of mice chronically infected with the parasite and reduce cardiac hypertrophy and fibrosis in the early stages of the chronic phase of the disease. Importantly, AT-RvD1 was able to decrease parasite load in the infected hearts. Thus, this research indicates that At-RvD1 treatment is a potential therapeutic strategy that offers an improvement on current drug therapies.

Variation in Leishmania chemokine suppression driven by diversification of the GP63 virulence factor

Leishmaniasis is a neglected tropical disease with diverse outcomes ranging from self-healing lesions, to progressive non-healing lesions, to metastatic spread and destruction of mucous membranes. Although resolution of cutaneous leishmaniasis is a classic example of type-1 immunity leading to self-healing lesions, an excess of type-1 related inflammation can contribute to immunopathology and metastatic spread. Leishmania genetic diversity can contribute to variation in polarization and robustness of the immune response through differences in both pathogen sensing by the host and immune evasion by the parasite. In this study, we observed a difference in parasite chemokine suppression between the Leishmania (L.) subgenus and the Viannia (V.) subgenus, which is associated with severe immune-mediated pathology such as mucocutaneous leishmaniasis. While Leishmania (L.) subgenus parasites utilize the virulence factor and metalloprotease glycoprotein-63 (gp63) to suppress the type-1 associated host chemokine CXCL10, L. (V.) panamensis did not suppress CXCL10. To understand the molecular basis for the inter-species variation in chemokine suppression, we used in silico modeling to identify a putative CXCL10-binding site on GP63. The putative CXCL10 binding site is in a region of gp63 under significant positive selection, and it varies from the L. major wild-type sequence in all gp63 alleles identified in the L. (V.) panamensis reference genome. Mutating wild-type L. (L.) major gp63 to the L. (V.) panamensis sequence at the putative binding site impaired cleavage of CXCL10 but not a non-specific protease substrate. Notably, Viannia clinical isolates confirmed that L. (V.) panamensis primarily encodes non-CXCL10-cleaving gp63 alleles. In contrast, L. (V.) braziliensis has an intermediate level of activity, consistent with this species having more equal proportions of both alleles. Our results demonstrate how parasite genetic diversity can contribute to variation in immune responses to Leishmania spp. infection that may play critical roles in the outcome of infection.

Leishmaniasis is a neglected tropical disease caused by Leishmania parasites and spread by the bites of infected sand flies. Most cases of leishmaniasis present as self-healing sores that are resolved by a balanced immune response. Other cases of leishmaniasis involve spread to sites distant from the original bite, including damage of the inner surfaces of the mouth and nose. These cases of leishmaniasis involve an excessive immune response. Leishmania parasites produce virulence factor proteins, such as GP63, to trick the immune system into mounting a weaker response. GP63 specifically degrades signaling proteins that attract and activate certain immune cells. Here, we demonstrate that Leishmania parasite species have evolved to differ in their ability to degrade signaling proteins. In Leishmania species known to cause more immune-mediated tissue damage, the GP63 virulence factor has evolved to not degrade specific immune signaling proteins, thus attracting, and activating more immune cells. Our results demonstrate how diversity among Leishmania parasite species can contribute to variation in immune responses that may play critical roles in the outcome of infection.

Visualizing the In Vivo Dynamics of Anti-Leishmania Immunity: Discoveries and Challenges

Intravital microscopy, such as 2-photon microscopy, is now a mainstay in immunological research to visually characterize immune cell dynamics during homeostasis and pathogen infections. This approach has been especially beneficial in describing the complex process of host immune responses to parasitic infections in vivo, such as Leishmania. Human-parasite co-evolution has endowed parasites with multiple strategies to subvert host immunity in order to establish chronic infections and ensure human-to-human transmission. While much focus has been placed on viral and bacterial infections, intravital microscopy studies during parasitic infections have been comparatively sparse. In this review, we will discuss how in vivo microscopy has provided important insights into the generation of innate and adaptive immunity in various organs during parasitic infections, with a primary focus on Leishmania. We highlight how microscopy-based approaches may be key to providing mechanistic insights into Leishmania persistence in vivo and to devise strategies for better parasite control.

Pleiotropic Effect of Hormone Insulin-Like Growth Factor-I in Immune Response and Pathogenesis in Leishmaniases

Leishmaniases are diseases caused by several Leishmania species, and many factors contribute to the development of the infection. Because the adaptive immune response does not fully explain the outcome of Leishmania infection and considering that the initial events are crucial in the establishment of the infection, we investigated one of the growth factors, the insulin-like growth factor-I (IGF-I), found in circulation and produced by different cells including macrophages and present in the skin where the parasite is inoculated. Here, we review the role of IGF-I in leishmaniasis experimental models and human patients. IGF-I induces the growth of different Leishmania species in vitro and alters the disease outcome increasing the parasite load and lesion size, especially in L. major- and L. amazonensis-infected mouse leishmaniasis. IGF-I affects the parasite interacting with the IGF-I receptor present on Leishmania. During Leishmania-macrophage interaction, IGF-I acts on the arginine metabolic pathway, resulting in polyamine production both in macrophages and Leishmania. IGF-I and cytokines interact with reciprocal influences on their expression. IL-4 is a hallmark of susceptibility to L. major in murine leishmaniasis, but we observed that IGF-I operates astoundingly as an effector element of the IL-4. Approaching human leishmaniasis, patients with mucosal, disseminated, and visceral diseases presented surprisingly low IGF-I serum levels, suggesting diverse effects than parasite growth. We observed that low IGF-I levels might contribute to the inflammatory response persistence and delayed lesion healing in human cutaneous leishmaniasis and the anemia development in visceral leishmaniasis. We must highlight the complexity of infection revealed depending on the Leishmania species and the parasite's developmental stages. Because IGF-I exerts pleiotropic effects on the biology of interaction and disease pathogenesis, IGF-I turns up as an attractive tool to explore biological and pathogenic processes underlying infection development. IGF-I pleiotropic effects open further the possibility of approaching IGF-I as a therapeutical target.

Host-Directed Therapies for Cutaneous Leishmaniasis

Cutaneous leishmaniasis exhibits a wide spectrum of clinical presentations from self-resolving infections to severe chronic disease. Anti-parasitic drugs are often ineffective in the most severe forms of the disease, and in some cases the magnitude of the disease can result from an uncontrolled inflammatory response rather than unrestrained parasite replication. In these patients, host-directed therapies offer a novel approach to improve clinical outcome. Importantly, there are many anti-inflammatory drugs with known safety and efficacy profiles that are currently used for other inflammatory diseases and are readily available to be used for leishmaniasis. However, since leishmaniasis consists of a wide range of clinical entities, mediated by a diverse group of leishmanial species, host-directed therapies will need to be tailored for specific types of leishmaniasis. There is now substantial evidence that host-directed therapies are likely to be beneficial beyond autoimmune diseases and cancer and thus should be an important component in the armamentarium to modulate the severity of cutaneous leishmaniasis.

The Impact of Neutrophil Recruitment to the Skin on the Pathology Induced by Leishmania Infection

Leishmania (L.) are obligate intracellular protozoan parasites that cause the leishmaniases, a spectrum of neglected infectious vector-borne diseases with a broad range of clinical manifestations ranging from local cutaneous, to visceral forms of the diseases. The parasites are deposited in the mammalian skin during the blood meal of an infected female phlebotomine sand fly. The skin is a complex organ acting as the first line of physical and immune defense against pathogens. Insults to skin integrity, such as that occurring during insect feeding, induces the local secretion of pro-inflammatory molecules generating the rapid recruitment of neutrophils. At the site of infection, skin keratinocytes play a first role in host defense contributing to the recruitment of inflammatory cells to the infected dermis, of which neutrophils are the first recruited cells. Although neutrophils efficiently kill various pathogens including Leishmania, several Leishmania species have developed mechanisms to survive in these cells. In addition, through their rapid release of cytokines, neutrophils modulate the skin microenvironment at the site of infection, a process shaping the subsequent development of the adaptive immune response. Neutrophils may also be recruited later on in unhealing forms of cutaneous leishmaniasis and to the spleen and liver in visceral forms of the disease. Here, we will review the mechanisms involved in neutrophil recruitment to the skin following Leishmania infection focusing on the role of keratinocytes in this process. We will also discuss the distinct involvement of neutrophils in the outcome of leishmaniasis.

Evaluation of a New Topical Treatment for the Control of Cutaneous Leishmaniasis

Leishmania major (L. major) causes cutaneous leishmaniasis in the Old World. The infection mostly induces a localized lesion restricted to the sand fly bite. The costs and the side effects of current treatments render imperative the development of new therapies that are affordable and easy to administrate. Topical treatment would be the ideal option for the treatment of cutaneous leishmaniasis. MF29 is a 3-haloacetamidobenzoate that was shown in vitro to inhibit tubulin assembly in Leishmania. Here, we tested a topical cream formulated with MF29. BALB/c mice were infected in the ear dermis with L. major metacyclic promastigotes and once the lesion appeared, mice were treated with different concentrations of MF29 and compared to the control group treated with the cream used as the vehicle. We observed that topical application of MF29 reduced the progression of the infection while control groups developed an unhealing lesion that became necrotic. The treatment decreased the type 2 immune response. Comparison with SinaAmphoLeish, another topical treatment, revealed that MF29 treatment once a day was sufficient to control lesion development, while application SinaAmphoLeish needed applications twice daily. Collectively, our data suggest that MF-29 topical application could be a promising topical treatment for cutaneous leishmaniasis.

Identification of a Protective Leishmania Antigen Dihydrolipoyl Dehydrogenase and Its Responding CD4+ T Cells at Clonal Level

There is currently no clinically effective vaccine against cutaneous leishmaniasis because of poor understanding of the Ags that elicit protective CD4⁺ T cell immunity. In this study, we identified a naturally processed peptide (DLD_63-79) that is derived from Leishmania dihydrolipoyl dehydrogenase (DLD) protein. DLD is conserved in all pathogenic Leishmania species, is expressed by both the promastigote and amastigote stages of the parasite, and elicits strong CD4⁺ T cell responses in mice infected with L. major We generated I-A^b-DLD_63-79 tetramer and identified DLD-specific CD4⁺ T cells at clonal level. Following L. major infection, DLD_63-79-specific CD4⁺ T cells massively expanded and produced effector cytokines (IFN-γ and TNF). This was followed by a gradual contraction, stable maintenance following lesion resolution, and display of memory (recall) response following secondary challenge. Vaccination with rDLD protein induced strong protection in mice against virulent L. major challenge. Identification of Ags that elicit protective immunity and their responding Ag-specific T cells are critical steps necessary for developing effective vaccines and vaccination strategies against infectious agents, including protozoan parasites.

Granulomas in parasitic diseases: the good and the bad

Parasitic diseases affect more than one billion people worldwide, and most of them are chronic conditions in which the treatment and prevention are difficult. The appearance of granulomas, defined as organized and compact structures of macrophages and other immune cells, during various parasitic diseases is frequent, since these structures will only form when individual immune cells do not control the invading agent. Th2-typering various parasitic diseases are frequent, since these structures will only form when individual immune cells do not control the invading agent. The characterization of granulomas in different parasitic diseases, as well as recent findings in this field, is discussed in this review, in order to understand the significance of the granuloma and its modulation in the host-parasite interaction and in the immune, pathological, and parasitological aspects of this interaction. The parasitic granulomatous diseases granulomatous amebic encephalitis, toxoplasmosis, leishmaniasis, neurocysticercosis, and schistosomiasis mansoni are discussed as well as the mechanistic and dynamical aspects of the infectious granulomas.

Leishmania Immunity: Advancing Immunotherapy and Vaccine Development

Parasitic diseases still constitute a major global health problem affecting billions of people around the world. These diseases are capable of becoming chronic and result in high morbidity and mortality. Worldwide, millions of people die each year from parasitic diseases, with the bulk of those deaths resulting from parasitic protozoan infections. Leishmaniasis, which is a disease caused by over 20 species of the protozoan parasite belonging to the genus Leishmania, is an important neglected disease. According to the World Health Organization (WHO), an estimated 12 million people are currently infected in about 98 countries and about 2 million new cases occur yearly, resulting in about 50,000 deaths each year. Current treatment methods for leishmaniasis are not very effective and often have significant side effects. In this review, we discussed host immunity to leishmaniasis, various treatment options currently being utilized, and the progress of both immunotherapy and vaccine development strategies used so far in leishmaniasis. We concluded with insights into what the future holds toward the fight against this debilitating parasitic disease.

The Null Hypothesis of IFN-γ and Monocyte Function in Leishmaniasis

The phagosomal pathogen Leishmania appears unaffected by deliberate changes in the early Th1/Th2 balance. In this issue, Carneiro et al. explain these paradoxical results by showing that manipulations affecting IFN-γ-mediated phagocyte activation are counteracted by effects on IFN-γ-dependent recruitment of CCR2⁺ monocytes permissive to parasite growth.

Pathological roles of MRP14 in anemia and splenomegaly during experimental visceral leishmaniasis

Myeloid-related protein 14 (MRP14) belongs to the S100 calcium-binding protein family and is expressed in neutrophils and inflammatory macrophages. Increase in the number of MRP14+ cells or serum level of MRP14 is associated with various diseases such as autoimmune diseases and infectious diseases, suggesting the involvement of the molecule in pathogenesis of those diseases. In this study, to examine the pathological involvement of MRP14 during cutaneous and visceral leishmaniasis, wild-type (WT) and MRP14 knockout (MRP14KO) mice were infected with Leishmania major and L. donovani. Increase in the number of MRP14+ cells at the infection sites in wild-type mice was commonly found in the skin during L. major infection as well as the spleen and liver during L. donovani infection. In contrast, the influence of MRP14 to the pathology seemed different between the two infections. MRP14 depletion exacerbated the lesion development and ulcer formation in L. major infection. On the other hand, the depletion improved anemia and splenomegaly but not hepatomegaly at 24 weeks of L. donovani infection. These results suggest that, distinct from its protective role in CL, MRP14 is involved in exacerbation of some symptoms during VL.

Inflammasomes and Leishmania: in good times or bad, in sickness or in health

The inflammasomes are multi-molecular platforms that are activated in host cell cytoplasm when the innate immune cells are infected with pathogens or exposed to damage signals. Many independent groups reported that Leishmania infection trigger activation of the NLRP3 inflammasome in macrophages for restriction of intracellular parasite replication. Accordingly, Leishmania can dampen NLRP3 activation as an evasion strategy. In vivo, the NLRP3 inflammasome can promote parasite clearance, but the failure to eliminate parasites in the tissues together with sustained inflammasome activation can promote IL-1β-mediated disease pathology. In this review, we discuss the recent data regarding activation of the NLRP3 inflammasome in response to Leishmania and the beneficial and detrimental effects of the inflammasome during development of Leishmaniasis.

CD4+ T Cell-Mediated Immunity against the Phagosomal Pathogen Leishmania: Implications for Vaccination

The generation of an efficacious vaccine that elicits protective CD4⁺ T cell-mediated immunity has been elusive. The lack of a vaccine against the Leishmania parasite is particularly perplexing as infected individuals acquire life-long immunity to reinfection. Experimental observations suggest that the relationship between immunological memory and protection against Leishmania is not straightforward and that a new paradigm is required to inform vaccine design. These observations include: (i) induction of Th1 memory is a component of protective immunity, but is not sufficient; (ii) memory T cells may be protective only if they generate circulating effector cells prior to, not after, challenge; and (iii) the low-dose/high-inflammation conditions of physiological vector transmission compromises vaccine efficacy. Understanding the implications of these observations is likely key to efficacious vaccination.

Experimental Cutaneous Leishmaniasis: Mouse Models for Resolution of Inflammation Versus Chronicity of Disease

Experimental cutaneous leishmaniasis of mice is a valuable model to study the immune response to the protozoan pathogen Leishmania and to define mechanisms of parasite control and resolution of inflammation as well as of parasite evasion and chronicity of disease. In addition, over many years Leishmania-infected mice have been successfully used to analyze the function of newly discovered immune cell types, transcription factors, cytokines, and effector mechanisms in vivo. In this chapter we present detailed protocols for the culture, propagation, and inoculation of Leishmania promastigotes, the monitoring of the course of cutaneous infection, the determination of the tissue parasite burden and for the phenotyping of the ensuing immune response. The focus lies on the L. major mouse model, but an overview on other established models of murine cutaneous leishmaniasis is also provided.

Changes in the immune system in experimental acanthamoebiasis in immunocompetent and immunosuppressed hosts

Background

Acanthamoebiasis is most often found in patients with immune deficiency, with infections facilitated by the intake of immunosuppressive drugs. The host immune response to Acanthamoeba spp. infection is poorly understood. Thus, in this study, we aimed to examine the course of Acanthamoeba spp. infection taking into account the host’s immunological status, including assessment of the hematological parameters, cytokine analysis, immunophenotypic changes in spleen populations, and histological spleen changes, which could help clarify some aspects of the immune response to acanthamoebiasis. In our experimental study, we used Acanthamoeba strain AM 22 isolated from the bronchoaspirate of a patient with acute myeloid leukaemia (AML) and atypical pneumonia symptoms.

Results

Acanthamoeba spp. affected the hematological parameters in immunocompetent and immunosuppressed mice and induced a change in spleen weight during infection. Moreover, analysis of anti-inflammatory (IL-4 and IL-10) and pro-inflammatory (IL-17A and IFN-γ) cytokines produced by splenocytes stimulated with concanavalin A demonstrated that Acanthamoeba spp. induced a selective Th1, Th2 and Th17 response at later stages of the infection in immunocompetent hosts. In the case of hosts with low immunity, Acanthamoeba elicited robust Th1 cell-mediated immunity without the participation of Th17. We observed suppression of CD8+ and CD4+ T lymphocytes and CD3+CD4-CD8- double-negative (DN) T lymphocyte populations in the beginning, and in the case of CD3+/CD4+/CD8+ double-positive (DP) T cells in the final phase of Acanthamoeba spp. infection in hosts with low immunity. Also, CD4+T lymphocytes and CD3+/CD4+ and CD3+/CD8+ lymphocyte counts during each stage of acanthamoebiasis were shown to be upregulated.

Conclusions

We demonstrated that analysis of the immune response and pathogenesis mechanisms of clinical isolates of Acanthamoeba spp. in an animal model not only has purely cognitive significance but above all, may help in the development of effective methods of pharmacological therapy especially in patients with low immunity.

Alternative to Chemotherapy-The Unmet Demand against Leishmaniasis

Leishmaniasis is a neglected protozoan disease that mainly affects the tropical as well as subtropical countries of the world. The primary option to control the disease still relies on chemotherapy. However, a hindrance to treatments owing to the emergence of drug-resistant parasites, enormous side effects of the drugs, their high cost, and requirement of long course hospitalization has added to the existing problems of leishmaniasis containment program. This review highlights the prospects of immunotherapy and/or immunochemotherapy to address the limitations for current treatment measures for leishmaniasis. In addition to the progress in alternate therapeutic strategies, the possibility and advances in developing preventive measures against the disease have been pointed. The review highlights our recent understandings of the protective immunology that can be exploited to develop an effective vaccine against leishmaniasis. Moreover, an update on the approaches that have evolved over the recent years are predominantly focused to overcome the current challenges in developing immunotherapeutic as well as prophylactic antileishmanial vaccines is discussed.

Mannose receptor high, M2 dermal macrophages mediate nonhealing Leishmania major infection in a Th1 immune environment

The origin and functional specialization of dermal macrophages in cutaneous infections have been little studied. In this paper, we show that a strain of Leishmania major (L. major Seidman [LmSd]) that produces nonhealing cutaneous lesions in conventionally resistant C57BL/6 mice was more efficiently taken up by M2-polarized bone marrow (BM)-derived macrophages (BMDMs) in vitro and by mannose receptor (MR)^hi dermal macrophages in vivo compared with a healing strain (L. major Friedlin V1). Both in steady and in T helper type 1 (Th1) cell-driven inflammatory states, the MR^hi dermal macrophages showed M2 characteristics. The dermal macrophages were radio resistant and not replaced by monocytes or adult BM-derived cells during infection, but were locally maintained by IL-4 and IL-10. Notably, the favored infection of M2 BMDMs by LmSd in vitro was MR dependent, and genetic deletion of MR or selective depletion of MR^hi dermal macrophages by anti-CSF-1 receptor antibody reversed the nonhealing phenotype. We conclude that embryonic-derived, MR^hi dermal macrophages are permissive for parasite growth even in a strong Th1-immune environment, and the preferential infection of these cells plays a crucial role in the severity of cutaneous disease.

Autoinducer-2 Facilitates Pseudomonas aeruginosa PAO1 Pathogenicity in Vitro and in Vivo

Bacterial communication systems, such as quorum sensing (QS), have provided new insights of alternative approaches in antimicrobial treatment. We recently reported that one QS signal, named as autoinducer-2 (AI-2), can affect the behaviors of Pseudomonas aeruginosa PAO1 in a dose-dependent manner. In this study, we aimed to investigate the effects of AI-2 on P. aeruginosa PAO1 biofilm formation and virulence factors production in vitro, and in vivo using a pulmonary infection mouse model. Exogenous AI-2 resulted in increased biofilms architecture, the number of viable cells, and the yield of pyocyanin and elastase virulence factors in wild type P. aeruginosa PAO1. However, no such effect was observed in P. aeruginosa lasR rhlR mutant strain. In vivo, the use of AI-2 significantly increased the mortality, lung bacterial count and histological lung damage of mice with acute P. aeruginosa PAO1 infection. Our data suggest that AI-2 promotes the formation of P. aeruginosa PAO1 biofilms and the production of virulence factors by interfering with P. aeruginosa QS systems, resulting in decreased host survival. AI-2 may be a therapeutic target for the clinical treatment of a co-infection of P. aeruginosa and AI-2 producing bacteria.

The circadian clock in immune cells controls the magnitude of Leishmania parasite infection

The intracellular parasite Leishmania uses neutrophils and macrophages as host cells upon infection. These immune cells harbour their own intrinsic circadian clocks, known to influence many aspects of their functions. Therefore, we tested whether the host circadian clocks regulate the magnitude of Leishmania major infection in mice. The extent of parasitic infection varied over 24 h in bone marrow-derived macrophages in vitro and in two different in vivo models, footpad and peritoneal cavity infection. In vivo this was paralleled by time of day-dependent neutrophil and macrophage infiltration to the infection site and rhythmic chemokine expression. Thus, rhythmic parasitic infection observed in vivo was likely initiated by the circadian expression of chemoattractants and the subsequent rhythmic infiltration of neutrophils and macrophages. Importantly, all rhythms were abolished in clock-deficient macrophages and when mice lacking the circadian clock in immune cells were infected. Therefore we demonstrated a critical role for the circadian clocks in immune cells in modulating the magnitude of Leishmania infection. To our knowledge this is the first report showing that the circadian clock controls infection by protozoan parasites in mammals. Understanding the timed regulation of host-parasite interactions will allow developing better prophylactic and therapeutic strategies to fight off vector-borne diseases.

Who needs the hotspot? The effect of temperature on the fish host immune response to Tetracapsuloides bryosalmonae the causative agent of proliferative kidney disease

Proliferative kidney disease (PKD) of salmonids, caused by Tetracapsuloides bryosalmonae may lead to high mortalities at elevated water temperatures. However, it has not yet been investigated how temperature affects the fish host immune response to T. bryosalmonae. We exposed YOY (young of the year) rainbow trout (Oncorhynchus mykiss) to T. bryosalmonae at two temperatures (12 °C and 15 °C) that reflect a realistic environmental scenario and could occur in the natural habitat of salmonids. We followed the development of the parasite, host pathology and immune response over seven weeks. We evaluated the composition and kinetics of the leukocytes and their major subgroups in the anterior and posterior kidney. We measured immune gene expression profiles associated with cell lineages and functional pathways in the anterior and posterior kidney. At 12 °C, both infection prevalence and pathogen load were markedly lower. While the immune response was characterized by subtle changes, mainly an increased amount of lymphocytes present in the kidney, elevated expression of Th1-like signature cytokines and strong upregulation of the natural killer cell enhancement factor, NKEF at week 6 P.E. At 15 °C the infection prevalence and pathogen burden were ominously greater. While the immune response as the disease progressed was associated with a Th2-like switch at week 6 P.E and a prominent B cell response, evidenced at the tissue, cell and transcript level. Our results highlight how a subtle, environmentally relevant difference in temperature resulted in diverse outcomes in terms of the immune response strategy, altering the type of interaction between a host and a parasite.

Efficacy of intranasal LaAg vaccine against Leishmania amazonensis infection in partially resistant C57Bl/6 mice

Background

We have previously demonstrated that intranasal vaccination of highly susceptible BALB/c mice with whole Leishmania amazonensis antigens (LaAg) leads to protection against murine cutaneous leishmaniasis. Here, we evaluate the response of partially resistant C57BL/6 mice to vaccination as a more representative experimental model of human cutaneous leishmaniasis.

Methods

C57BL/6 mice from different animal facilities were infected with L. amazonensis (Josefa strain) to establish the profile of infection. Intranasal vaccination was performed before the infection challenge with two doses of 10 μg of LaAg alone or associated with the adjuvant ADDAVAX® by instillation in the nostrils. The lesion progression was measured with a dial caliper and the parasite load by limited dilution assay in the acute and chronic phases of infection. Cytokines were quantified by ELISA in the homogenates of infected footpads.

Results

C57BL/6 mice from different animal facilities presented the same L. amazonensis infection profile, displaying a progressive acute phase followed by a controlled chronic phase. Parasites cultured in M199 and Schneider’s media were equally infective. Intranasal vaccination with LaAg led to milder acute and chronic phases of the disease. The mechanism of protection was associated with increased production of IFN-gamma in the infected tissue as measured in the acute phase. Association with the ADDAVAX® adjuvant did not improve the efficacy of intranasal LaAg vaccination. Rather, ADDAVAX® reduced vaccination efficacy.

Conclusion

This study demonstrates that the efficacy of adjuvant-free intranasal vaccination with LaAg is extendable to the more resistant C57Bl/6 mouse model of infection with L. amazonensis, and is thus not exclusive to the susceptible BALB/c model. These results imply that mucosal immunomodulation by LaAg leads to peripheral protection irrespective of the genetic background of the host.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1822-9) contains supplementary material, which is available to authorized users.

Pro- and anti-inflammatory cytokines in cutaneous leishmaniasis: a review

Cutaneous leishmaniasis (CL) is caused by different species of the genus Leishmania. Pro- and anti-inflammatory cytokines play different roles in resistance/susceptibility and the immunopathogenesis of Leishmania infection. The balance and dynamic changes in cytokines may control or predict clinical outcome. T helper 1 (Th1) inflammatory cytokines (especially interferon-γ, tumor necrosis factor-α and interleukin-12) are the crucial factors in the initiation of protective immunity against L. major infection, whereas T helper 2 cytokines including IL-5, IL-4, and IL-13 facilitate the persistence of parasites by downregulating the Th1 immune response. On the other hand, aggravation of inflammatory reactions leads to collateral tissue damage and formation of ulcer. For this reason, immunity system such as T regulatory cells produce regulatory cytokines such as transforming growth factor-β and IL-10 to inhibit possible injures caused by increased inflammatory responses in infection site. In this article, we review the role of pro- and anti-inflammatory cytokines in the immunoprotection and immunopathology of CL.

Cutaneous leishmaniasis: immune responses in protection and pathogenesis

Cutaneous leishmaniasis is a major public health problem and causes a range of diseases from self-healing infections to chronic disfiguring disease. Currently, there is no vaccine for leishmaniasis, and drug therapy is often ineffective. Since the discovery of CD4(+) T helper 1 (TH1) cells and TH2 cells 30 years ago, studies of cutaneous leishmaniasis in mice have answered basic immunological questions concerning the development and maintenance of CD4(+) T cell subsets. However, new strategies for controlling the human disease have not been forthcoming. Nevertheless, advances in our knowledge of the cells that participate in protection against Leishmania infection and the cells that mediate increased pathology have highlighted new approaches for vaccine development and immunotherapy. In this Review, we discuss the early events associated with infection, the CD4(+) T cells that mediate protective immunity and the pathological role that CD8(+) T cells can have in cutaneous leishmaniasis.

Memory T cells in cutaneous leishmaniasis

Leishmania causes a spectrum of diseases that range from self-healing to fatal infections. Control of leishmania is dependent upon generating CD4+ Th1 cells that produce IFNγ, leading to macrophage activation and killing of the intracellular parasites. Following resolution of the disease, short-lived effector T cells, as well as long-lived central memory T cells and skin resident memory T cells, are retained and able to mediate immunity to a secondary infection. However, there is no vaccine for leishmaniasis, and the drugs used to treat the disease can be toxic and ineffective. While a live infection generates immunity, a successful vaccine will depend upon generating memory T cells that can be maintained without the continued presence of parasites. Since both central memory and skin resident memory T cells are long-lived, they may be the appropriate targets for a leishmaniasis vaccine.

Possibilities and challenges for developing a successful vaccine for leishmaniasis

Leishmaniasis is a vector-borne disease caused by different species of protozoan parasites of the genus Leishmania. It is a major health problem yet neglected tropical diseases, with approximately 350 million people worldwide at risk and more than 1.5 million infections occurring each year. Leishmaniasis has different clinical manifestations, including visceral (VL or kala-azar), cutaneous (CL), mucocutaneous (MCL), diffuse cutaneous (DCL) and post kala-azar dermal leishmaniasis (PKDL). Currently, the only mean to treat and control leishmaniasis is by rational medications and vector control. However, the number of available drugs is limited and even these are either exorbitantly priced, have toxic side effects or prove ineffective due to the emergence of resistant strains. On the other hand, the vector control methods are not so efficient. Therefore, there is an urgent need for developing a safe, effective, and affordable vaccine for the prevention of leishmaniasis. Although in recent years a large body of researchers has concentrated their efforts on this issue, yet only three vaccine candidates have gone for clinical trial, until date. These are: (i) killed vaccine in Brazil for human immunotherapy; (ii) live attenuated vaccine for humans in Uzbekistan; and (iii) second-generation vaccine for dog prophylaxis in Brazil. Nevertheless, there are at least half a dozen vaccine candidates in the pipeline. One can expect that, in the near future, the understanding of the whole genome of Leishmania spp. will expand the vaccine discovery and strategies that may provide novel vaccines. The present review focuses on the development and the status of various vaccines and potential vaccine candidates against leishmaniasis.

Impact of reactive oxygen species (ROS) on the control of parasite loads and inflammation in Leishmania amazonensis infection

Background

Reactive oxygen species (ROS) protect the host against a large number of pathogenic microorganisms. ROS have different effects on parasites of the genus Leishmania: some parasites are susceptible to their action, while others seem to be resistant. The role of ROS in L. amazonensis infection in vivo has not been addressed to date.

Methods

In this study, C57BL/6 wild-type mice (WT) and mice genetically deficient in ROS production by phagocytes (gp91^phox−/−) were infected with metacyclic promastigotes of L. amazonensis to address the effect of ROS in parasite control. Inflammatory cytokines, parasite loads and myeloperoxidase (MPO) activity were evaluated. In parallel, in vitro infection of peritoneal macrophages was assessed to determine parasite killing, cytokine, NO and ROS production.

Results

In vitro results show induction of ROS production by infected peritoneal macrophages, but no effect in parasite killing. Also, ROS do not seem to be important to parasite killing in vivo, but they control lesion sizes at early stages of infection. IFN-γ, TNF-α and IL-10 production did not differ among mouse strains. Myeloperoxidase assay showed augmented neutrophils influx 6 h and 72 h post - infection in gp91^phox−/− mice, indicating a larger inflammatory response in gp91^phox−/− even at early time points. At later time points, neutrophil numbers in lesions correlated with lesion size: larger lesions in gp91^phox−/− at earlier times of infection corresponded to larger neutrophil infiltrates, while larger lesions in WT mice at the later points of infection also displayed larger numbers of neutrophils.

Conclusion

ROS do not seem to be important in L. amazonensis killing, but they regulate the inflammatory response probably by controlling neutrophils numbers in lesions.

Autoinducer-2 of Streptococcus mitis as a Target Molecule to Inhibit Pathogenic Multi-Species Biofilm Formation In Vitro and in an Endotracheal Intubation Rat Model

Streptococcus mitis (S. mitis) and Pseudomonas aeruginosa (P. aeruginosa) are typically found in the upper respiratory tract of infants. We previously found that P. aeruginosa and S. mitis were two of the most common bacteria in biofilms on newborns' endotracheal tubes (ETTs) and in their sputa and that S. mitis was able to produce autoinducer-2 (AI-2), whereas P. aeruginosa was not. Recently, we also found that exogenous AI-2 and S. mitis could influence the behaviors of P. aeruginosa. We hypothesized that S. mitis contributes to this interspecies interaction and that inhibition of AI-2 could result in inhibition of these effects. To test this hypothesis, we selected PAO1 as a representative model strain of P. aeruginosa and evaluated the effect of S. mitis as well as an AI-2 analog (D-ribose) on mono- and co-culture biofilms in both in vitro and in vivo models. In this context, S. mitis promoted PAO1 biofilm formation and pathogenicity. Dual-species (PAO1 and S. mitis) biofilms exhibited higher expression of quorum sensing genes than single-species (PAO1) biofilms did. Additionally, ETTs covered in dual-species biofilms increased the mortality rate and aggravated lung infection compared with ETTs covered in mono-species biofilms in an endotracheal intubation rat model, all of which was inhibited by D-ribose. Our results demonstrated that S. mitis AI-2 plays an important role in interspecies interactions with PAO1 and may be a target for inhibition of biofilm formation and infection in ventilator-associated pneumonia.

Fatal Leishmaniasis in the Absence of TNF Despite a Strong Th1 Response

Induction of inducible nitric oxide synthase in mononuclear phagocytes by IFN-γ and innate tumor necrosis factor (TNF) provide the basis for an effective immune response to the intracellular parasite Leishmania (L.) major. In previous experiments, we observed a fatal visceral form of leishmaniasis in L. major-infected C57BL/6 TNF^-/- mice. To further delineate the protective function of TNF and its receptor requirements, we comparatively assessed L. major-infected C57BL/6 mice that were either deficient for membrane and soluble TNF (Tnf^-/-), for soluble TNF alone (memTnf^Δ/Δ), or the TNF receptors type 1 (Tnfr1^-/-) or type 2 (Tnfr2^-/-). We detected locally and systemically increased levels of the cytokine IFN-γ in the absence of the TNF-TNFR1-signaling pathway. An analysis of transcription factors and cytokines revealed that activated Tnf^-/- CD4⁺ T cells displayed a highly active Th1 phenotype with a strong usage of the T cell receptor Vβ5.1/2. From these data we conclude that the fatal outcome of L. major infection in Tnf^-/- mice does not result from a skewed or deficient Th1 differentiation.

The Nlrp3 inflammasome, IL-1β, and neutrophil recruitment are required for susceptibility to a nonhealing strain of Leishmania major in C57BL/6 mice

Infection of C57BL/6 mice with most Leishmania major strains results in a healing lesion and clearance of parasites from the skin. Infection of C57BL/6 mice with the L. major Seidman strain (LmSd), isolated from a patient with chronic lesions, despite eliciting a strong Th1 response, results in a nonhealing lesion, poor parasite clearance, and complete destruction of the ear dermis. We show here that in comparison to a healing strain, LmSd elicited early upregulation of IL-1β mRNA and IL-1β-producing dermal cells and prominent neutrophil recruitment to the infected skin. Mice deficient in Nlrp3, apoptosis-associated speck-like protein containing a caspase recruitment domain, or caspase-1/11, or lacking IL-1β or IL-1 receptor signaling, developed healing lesions and cleared LmSd from the infection site. Mice resistant to LmSd had a stronger antigen-specific Th1 response. The possibility that IL-1β might act through neutrophil recruitment to locally suppress immunity was supported by the healing observed in neutropenic Genista mice. Secretion of mature IL-1β by LmSd-infected macrophages in vitro was dependent on activation of the Nlrp3 inflammasome and caspase-1. These data reveal that Nlrp3 inflammasome-dependent IL-1β, associated with localized neutrophil recruitment, plays a crucial role in the development of a nonhealing form of cutaneous leishmaniasis in conventionally resistant mice.

Differential Impact of LPG-and PG-Deficient Leishmania major Mutants on the Immune Response of Human Dendritic Cells

Background

Leishmania major infection induces robust interleukin-12 (IL12) production in human dendritic cells (hDC), ultimately resulting in Th1-mediated immunity and clinical resolution. The surface of Leishmania parasites is covered in a dense glycocalyx consisting of primarily lipophosphoglycan (LPG) and other phosphoglycan-containing molecules (PGs), making these glycoconjugates the likely pathogen-associated molecular patterns (PAMPS) responsible for IL12 induction.

Methodology/Principal Findings

Here we explored the role of parasite glycoconjugates on the hDC IL12 response by generating L. major Friedlin V1 mutants defective in LPG alone, (FV1 lpg1-), or generally deficient for all PGs, (FV1 lpg2-). Infection with metacyclic, infective stage, L. major or purified LPG induced high levels of IL12B subunit gene transcripts in hDCs, which was abrogated with FV1 lpg1- infections. In contrast, hDC infections with FV1 lpg2- displayed increased IL12B expression, suggesting other PG-related/LPG2 dependent molecules may act to dampen the immune response. Global transcriptional profiling comparing WT, FV1 lpg1-, FV1 lpg2- infections revealed that FV1 lpg1- mutants entered hDCs in a silent fashion as indicated by repression of gene expression. Transcription factor binding site analysis suggests that LPG recognition by hDCs induces IL-12 in a signaling cascade resulting in Nuclear Factor κ B (NFκB) and Interferon Regulatory Factor (IRF) mediated transcription.

Conclusions/Significance

These data suggest that L. major LPG is a major PAMP recognized by hDC to induce IL12-mediated protective immunity and that there is a complex interplay between PG-baring Leishmania surface glycoconjugates that result in modulation of host cellular IL12.

Leishmaniasis is a group of parasitic diseases caused by intracellular protozoa belonging to the genus Leishmania, pathological manifestations ranging from self-healing cutaneous forms to severe visceral infections that result in death. These clinical outcomes are dictated by the Leishmania species initiating the infection and are influenced by early responses of host immune cells, which ultimately initiate an IL12 mediated immune response in resolving infections. Like the diseases themselves, the magnitude of IL12 induction in hDCs is Leishmania-species and strain specific, where species that elicit visceral disease do not induce IL12, while most cutaneous disease-causing L. major strains induce robust IL12 responses and confer life-long immunity. The molecular mechanisms that mediate the ability of these innate immune cells to discriminate between pathogens remain elusive and have been primarily investigated in murine model systems. Here we identified L. major LPG as a major PAMP that induces IL12 in hDCs. Elucidation of this critical component of human immunity to L. major has ramifications for leishmaniasis vaccine development.

IRF-5-Mediated Inflammation Limits CD8+ T Cell Expansion by Inducing HIF-1α and Impairing Dendritic Cell Functions during Leishmania Infection

Inflammation is known to be necessary for promoting, sustaining, and tuning CD8⁺ T cell responses. Following experimental Leishmania donovani infection, the inflammatory response is mainly induced by the transcription factor IRF-5. IRF-5 is responsible for the activation of several genes encoding key pro-inflammatory cytokines, such as IL-6 and TNF. Here, we investigate the role of IRF-5-mediated inflammation in regulating antigen-specific CD8⁺ T cell responses during L. donovani infection. Our data demonstrate that the inflammatory response induced by IRF-5 limits CD8⁺ T cell expansion and induces HIF-1α in dendritic cells. Ablation of HIF-1α in CD11c⁺ cells resulted into a higher frequency of short-lived effector cells (SLEC), enhanced CD8⁺ T cell expansion, and increased IL-12 expression by splenic DCs. Moreover, mice with a targeted depletion of HIF-1α in CD11c⁺ cells had a significantly lower splenic parasite burden, suggesting that induction of HIF-1α may represent an immune evasive mechanism adopted by Leishmania parasites to establish persistent infections.

Inflammation is essential for inducing, sustaining, and regulating CD8⁺ T cell responses. The transcription factor IRF-5 is mainly responsible for initiating the inflammatory response following experimental Leishmani donovani infection. IRF-5 activates several genes encoding key pro-inflammatory cytokines, such as IL-6 and TNF. In this study, we investigate the role of IRF-5-mediated inflammation in regulating antigen-specific CD8⁺ T cell responses during L. donovani infection. Our data demonstrate that the inflammatory response induced by IRF-5 limits the expansion CD8⁺ T cell. This negative effect is mediated by the induction of HIF-1α in dendritic cells. Indeed, we observed a significant increase in CD8⁺ T cell expansion in mice lacking HIF-1α expression in dendritic cells. Moreover, these mice had a significantly lower parasite burden in the spleen, suggesting that induction of HIF-1α may represent an immune evasive mechanism adopted by Leishmania parasites to establish persistent infections.

Animal models for the analysis of immune responses to leishmaniasis

This unit focuses on the murine model of cutaneous leishmaniasis and models of visceral leishmaniasis in mice and hamsters. Each basic protocol describes the methods used to inoculate parasites and to evaluate infections with regard to lesion progression and visceralization, and quantification of parasite load.

Interleukin-10 from T cells, but not macrophages and granulocytes, is required for chronic disease in Leishmania mexicana infection

Chronic cutaneous disease of mice caused by the protozoan parasite Leishmania mexicana requires interleukin-10 (IL-10) and FcγRIII (an activating IgG receptor). Macrophages readily secrete IL-10 in response to IgG-coated amastigotes, making macrophages a prime candidate as the critical source of IL-10. However, indirect evidence suggested that macrophage IL-10 is not essential for chronic disease. I now show directly that mice lacking IL-10 from macrophages and granulocytes still have chronic disease, like wild-type C57BL/6 mice. However, T cell-derived IL-10 is required for chronic disease. CD4-cre IL-10flox/flox mice lack IL-10 from T cells (both CD4+ and CD8+) and heal their L. mexicana lesions, with parasite control. I had previously shown that depletion of CD25+ T cells had no effect on chronic disease, and thus, T cells other than CD25+ regulatory T (Treg) cells should be the important source of IL-10. Given that conventional T cells do not express FcγRs, there is likely to be an indirect pathway by which FcγRIII on some other cell engaged by IgG1-amastigote immune complexes induces IL-10 from T cells. Further work is needed to delineate these pathways.

Identifying vaccine targets for anti-leishmanial vaccine development

Leishmaniasis is a neglected tropical disease spread by an arthropod vector. It remains a significant health problem with an incidence of 0.2–0.4 million visceral leishmaniasis and 0.7–1.2 million cutaneous leishmaniasis cases each year. There are limitations associated with the current therapeutic regimens for leishmaniasis and the fact that after recovery from infection the host becomes immune to subsequent infection therefore, these factors force the feasibility of a vaccine for leishmaniasis. Publication of the genome sequence of Leishmania has paved a new way to understand the pathogenesis and host immunological status therefore providing a deep insight in the field of vaccine research. This review is an effort to study the antigenic targets in Leishmania to develop an anti-leishmanial vaccine.

An in vitro model of antibody-enhanced killing of the intracellular parasite Leishmania amazonensis

Footpad infection of C3HeB/FeJ mice with Leishmania amazonensis leads to chronic lesions accompanied by large parasite loads. Co-infecting these animals with L. major leads to induction of an effective Th1 immune response that can resolve these lesions. This cross-protection can be recapitulated in vitro by using immune cells from L. major-infected animals to effectively activate L. amazonensis-infected macrophages to kill the parasite. We have shown previously that the B cell population and their IgG2a antibodies are required for effective cross-protection. Here we demonstrate that, in contrast to L. major, killing L. amazonensis parasites is dependent upon FcRγ common-chain and NADPH oxidase-generated superoxide from infected macrophages. Superoxide production coincided with killing of L. amazonensis at five days post-activation, suggesting that opsonization of the parasites was not a likely mechanism of the antibody response. Therefore we tested the hypothesis that non-specific immune complexes could provide a mechanism of FcRγ common-chain/NADPH oxidase dependent parasite killing. Macrophage activation in response to soluble IgG2a immune complexes, IFN-γ and parasite antigen was effective in significantly reducing the percentage of macrophages infected with L. amazonensis. These results define a host protection mechanism effective during Leishmania infection and demonstrate for the first time a novel means by which IgG antibodies can enhance killing of an intracellular pathogen.

Tr-1-like CD4+CD25-CD127-/lowFOXP3- cells are the main source of interleukin 10 in patients with cutaneous leishmaniasis due to Leishmania braziliensis

CD4(+)CD25(+)FOXP3(+) regulatory T cells have long been shown to mediate susceptibility to Leishmania infection, mainly via interleukin 10 production. In this work, we showed that the main sources of interleukin 10 in peripheral blood mononuclear cells (PBMCs) from patients with cutaneous leishmaniasis due to Leishmania braziliensis are CD4(+)CD25(-)CD127(-/low)FOXP3(-) cells. Compared with uninfected controls, patients with CL had increased frequencies of circulating interleukin 10-producing CD4(+)CD25(-)CD127(-/low) cells, which efficiently suppressed tumor necrosis factor α production by the total PBMC population. Also, in CL lesions, interleukin 10 was mainly produced by CD4(+)CD25(-) cells, and interleukin 10 messenger RNA expression was associated with interleukin 27, interleukin 21, and interferon γ expression, rather than with FOXP3 or transforming growth factor β expressions. Active production of both interleukin 27 and interleukin 21, together with production of interferon γ and interleukin 10, was also detected in the lesions. Since these cytokines are associated with the differentiation and activity of Tr-1 cells, our results suggest that this cell population may play an important role in the immunomodulation of CL. Therefore, development of treatments that interfere with this pathway may lead to faster parasite elimination.

Immune responses during cutaneous and visceral leishmaniasis

SUMMARY 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.