Dendritic cell-based immunotherapy combined with antimony-based chemotherapy cures established murine visceral leishmaniasis

Immunotherapy and immunochemotherapy in combating visceral leishmaniasis

Visceral leishmaniasis (VL), a vector-borne disease, is caused by an obligate intramacrophage, kinetoplastid protozoan parasite of the genus Leishmania. Globally, VL is construed of diversity and complexity concerned with high fatality in tropics, subtropics, and Mediterranean regions with ~50,000-90,000 new cases annually. Factors such as the unavailability of licensed vaccine(s), insubstantial measures to control vectors, and unrestrained surge of drug-resistant parasites and HIV-VL co-infections lead to difficulty in VL treatment and control. Furthermore, VL treatment, which encompasses several problems including limited efficacy, emanation of drug-resistant parasites, exorbitant therapy, and exigency of hospitalization until the completion of treatment, further exacerbates disease severity. Therefore, there is an urgent need for the development of safe and efficacious therapies to control and eliminate this devastating disease. In such a scenario, biotherapy/immunotherapy against VL can become an alternative strategy with limited side effects and no or nominal chance of drug resistance. An extensive understanding of pathogenesis and immunological events that ensue during VL infection is vital for the development of immunotherapeutic strategies against VL. Immunotherapy alone or in combination with standard anti-leishmanial chemotherapeutic agents (immunochemotherapy) has shown better therapeutic outcomes in preclinical studies. This review extensively addresses VL treatment with an emphasis on immunotherapy or immunochemotherapeutic strategies to improve therapeutic outcomes as an alternative to conventional chemotherapy.

Emerging strategies and challenges of molecular therapeutics in antileishmanial drug development

Molecular therapy refers to targeted therapies based on molecules which have been intelligently directed towards specific biomolecular structures and include small molecule drugs, monoclonal antibodies, proteins and peptides, DNA or RNA-based strategies, targeted chemotherapy and nanomedicines. Molecular therapy is emerging as the most effective strategy to combat the present challenges of life-threatening visceral leishmaniasis, where the successful human vaccine is currently unavailable. Moreover, current chemotherapy-based strategies are associated with the issues of ineffective targeting, unavoidable toxicities, invasive therapies, prolonged treatment, high treatment costs and the development of drug-resistant strains. Thus, the rational approach to antileishmanial drug development primarily demands critical exploration and exploitation of biochemical differences between host and parasite biology, immunocharacteristics of parasite homing, and host-parasite interactions at the molecular/cellular level. Following this, the novel technology-based designing and development of host and/or parasite-targeted therapeutics having leishmanicidal and immunomodulatory activity is utmost essential to improve treatment efficacy. Thus, the present review is focused on immunological and molecular checkpoint targets in host-pathogen interaction, and molecular therapeutic prospects for Leishmania intervention, and the challenges ahead.

Host-directed antileishmanial interventions: Harvesting unripe fruits to reach fruition

Leishmaniasis is an exemplary paradigm of immune evasion, fraught with the perils of limited clinical assistance, escalating costs of treatment and made worse with the lack of suitable vaccine. While drugs remain central to large-scale disease control, the growing emergence of parasite resistance necessitates the need for combination therapy involving host-directed immunological agents. Also, since prolonged disease progression is associated with strong immune suppression of the host, augmentation of host immunity via restoration of the immunoregulatory circuit involving antigen-presenting cells and T-cells, activation of macrophage function and/or CD4⁺ T helper 1 cell differentiation may serve as an ideal approach to resolve severe cases of leishmaniasis. As such, therapies that embody a synergistic approach that involve direct killing of the parasite in addition to elevating host immunity are likely to pave the way for widespread elimination of leishmaniasis in the future. With this review, we aim to recapitulate the various immunotherapeutic agents found to hold promise in antileishmanial treatment both in vitro and in vivo. These include parasite-specific antigens, dendritic cell-targeted therapy, recombinant inhibitors of various components intrinsic to immune cell signaling and agonists or antagonists to immune cells and cytokines. We also summarize their abilities to direct therapeutic skewing of the host cell-immune response and review their potential to combat the disease either alone, or as adjunct modalities.

Leishmaniasis: where are we and where are we heading?

Leishmaniasis is a zoonotic disease in humans caused by the bite of a parasite-infected sandfly. The disease, widely referred to as "poor man's disease," affects millions of people worldwide. The clinical manifestation of the disease depends upon the species of the parasite and ranges from physical disfigurement to death if left untreated. Here, we review the past, present, and future of leishmaniasis in detail. The life cycle of Leishmania sp., along with its epidemiology, is discussed, and in addition, the line of therapeutics available for treatment currently is examined. The current status of the disease is critically evaluated, keeping emerging threats like human immunodeficiency virus (HIV) coinfection and post kala-azar dermal leishmaniasis (PKDL) into consideration. In summary, the review proposes a dire need for new therapeutics and reassessment of the measures and policies concerning emerging threats. New strategies are essential to achieve the goal of leishmaniasis eradication in the next few decades.

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.

Can We Harness Immune Responses to Improve Drug Treatment in Leishmaniasis?

Leishmaniasis is a vector-borne parasitic disease that has been neglected in priority for control and eradication of malaria, tuberculosis, and HIV/AIDS. Collectively, over one seventh of the world’s population is at risk of being infected with 0.7–1.2 million new infections reported annually. Clinical manifestations range from self-healing cutaneous lesions to fatal visceral disease. The first anti-leishmanial drugs were introduced in the 1950′s and, despite several shortcomings, remain the mainstay for treatment. Regardless of this and the steady increase in infections over the years, particularly among populations of low economic status, research on leishmaniasis remains under funded. This review looks at the drugs currently in clinical use and how they interact with the host immune response. Employing chemoimmunotherapeutic approaches may be one viable alternative to improve the efficacy of novel/existing drugs and extend their lifespan in clinical use.

Combination of Mycobacterium indicus pranii and Heat-Induced Promastigotes Cures Drug-Resistant Leishmania Infection: Critical Role of Interleukin-6-Producing Classical Dendritic Cells

The major issues in available therapeutic modalities against leishmaniasis are cost, toxicity, and the emergence of drug resistance. The aim of this work was to develop a successful therapeutic adjuvant against drug-resistant Leishmania donovani infection by means of combining Mycobacterium indicus pranii with heat-induced promastigotes (HIP). One-month postinfected BALB/c mice were administered subcutaneously with M. indicus pranii (10⁸ cells) and HIP (100 μg) for 5 days. Spleens were harvested for flow cytometric and reverse transcriptase PCR analysis. The antileishmanial effect of the combination strategy was associated with induction of a disease-resolving Th1 and Th17 response with simultaneous downregulation of CD4⁺ CD25⁺ Foxp3⁺ (nTreg) cells and CD4⁺ CD25^- Foxp3^- (Tr1) cells in the spleen. The significant expansion of CD4⁺ T_CM (CD4⁺ CD44^hi CD11a^hi CD62L^hi) cells was a further interesting outcome of this therapeutic strategy in the context of long-term protection of hosts against secondary infection. Toll-like receptor 2 (TLR2) was also found instrumental in this antiparasitic therapy. Induced interleukin-6 (IL-6) production from expanded CD11c⁺ CD8α⁺ (cDC1) and CD11c⁺ CD11b⁺ (cDC2) dendritic cells (DCs) but not from the CD11b⁺ Ly6c⁺ inflammatory monocytes (iMOs), was found critical in the protective expansion of Th17 as evidenced by an in vivo IL-6 neutralization assay. It also promoted the hematopoietic conversion toward DC progenitors (pre-DCs) from common dendritic cell progenitors (CDPs), the immediate precursors, in bone marrow. This novel combinational strategy demonstrated that expansion of Th17 by IL-6 released from CD11c⁺ classical DCs is crucial, together with the conventional Th1 response, to control drug-resistant infection.

Reciprocal changes in CD11c+CD11b+ and CD11c+CD8α+ dendritic cell subsets determine protective or permissive immune response in murine experimental VL

CD11c⁺CD8α⁺ and CD11c⁺CD11b⁺ dendritic cells are two major subsets of murine splenic CD11c⁺ DCs which play a crucial role in T cell priming and shaping Th1/Th2 responses, but their role in the context of experimental visceral leishmaniasis (VL) is poorly understood. Herein, we showed that L. donovani infection in Balb/c mice preferentially decreased the population abundance of CD11c⁺CD11b⁺ DCs and increased relative abundance of splenic CD11c⁺CD8α ⁺DCs. During infection, splenic CD11c⁺CD11b⁺ DCs induced Th1 differentiation whereas CD11c⁺CD8α⁺ DCs promoted Th2 differentiation. Additionally, treatment of infected mice with miltefosine as experimental control exhibited host defense allowing the restoration of CD11c⁺CD11b⁺ population and decrease in CD11c⁺CD8α⁺ subset. Furthermore, reciprocal regulation of immune accessory surface molecules, Sema4A and OX40L critically determined Th1/Th2 response induced by these DC subsets during VL. L. donovani infection significantly induced OX40L expression and slightly downregulated SEMA 4A expression in CD11c⁺CD8α⁺ DCs whereas miltefosine treatment significantly downregulated OX40L expression along with pronounced upregulation of SEMA 4A expression in CD11c⁺CD11b⁺ DCs. SiRNA mediated knockdown of SEMA 4A markedly reduced CD11c⁺CD11b⁺ driven IFN-γ, TNF-α and IL-12 synthesis in miltefosine treated mice whereas functional blocking of OX40L decreased CD11c⁺CD8α⁺ induced IL-10, IL-4 and TGF-β synthesis in L. donovani infected group. Vaccination of Balb/c mice with antigen-pulsed + CpG-ODN-activated DC subsets revealed that only antigen-pulsed CD11c⁺CD11b⁺ DCs eliminated parasite load in visceral organ and restored protective Th1 cytokine response. Collectively, our results suggest that differential regulation of splenic CD11c⁺ subsets by L. donovani is essential for disease progression and specific subtypes may be exploited as prophylactic measures against visceral leishmaniasis.

Anti-Leishmanial Vaccines: Assumptions, Approaches, and Annulments

Leishmaniasis is a neglected protozoan parasitic disease that occurs in 88 countries but a vaccine is unavailable. Vaccination with live, killed, attenuated (physically or genetically) Leishmania have met with limited success, while peptide-, protein-, or DNA-based vaccines showed promise only in animal models. Here, we critically assess several technical issues in vaccination and expectation of a host-protective immune response. Several studies showed that antigen presentation during priming and triggering of the same cells in infected condition are not comparable. Altered proteolytic processing, antigen presentation, protease-susceptible sites, and intracellular expression of pathogenic proteins during Leishmania infection may vary dominant epitope selection, MHC-II/peptide affinity, and may deter the reactivation of desired antigen-specific T cells generated during priming. The robustness of the memory T cells and their functions remains a concern. Presentation of the antigens by Leishmania-infected macrophages to antigen-specific memory T cells may lead to change in the T cells' functional phenotype or anergy or apoptosis. Although cells may be activated, the peptides generated during infection may be different and cross-reactive to the priming peptides. Such altered peptide ligands may lead to suppression of otherwise active antigen-specific T cells. We critically assess these different immunological issues that led to the non-availability of a vaccine for human use.

Cytokines: Key Determinants of Resistance or Disease Progression in Visceral Leishmaniasis: Opportunities for Novel Diagnostics and Immunotherapy

Leishmaniasis is a parasitic disease of humans, highly prevalent in parts of the tropics, subtropics, and southern Europe. The disease mainly occurs in three different clinical forms namely cutaneous, mucocutaneous, and visceral leishmaniasis (VL). The VL affects several internal organs and is the deadliest form of the disease. Epidemiology and clinical manifestations of VL are variable based on the vector, parasite (e.g., species, strains, and antigen diversity), host (e.g., genetic background, nutrition, diversity in antigen presentation and immunity) and the environment (e.g., temperature, humidity, and hygiene). Chemotherapy of VL is limited to a few drugs which is expensive and associated with profound toxicity, and could become ineffective due to the parasites developing resistance. Till date, there are no licensed vaccines for humans against leishmaniasis. Recently, immunotherapy has become an attractive strategy as it is cost-effective, causes limited side-effects and do not suffer from the downside of pathogens developing resistance. Among various immunotherapeutic approaches, cytokines (produced by helper T-lymphocytes) based immunotherapy has received great attention especially for drug refractive cases of human VL. Therefore, a comprehensive knowledge on the molecular interactions of immune cells or components and on cytokines interplay in the host defense or pathogenesis is important to determine appropriate immunotherapies for leishmaniasis. Here, we summarized the current understanding of a wide-spectrum of cytokines and their interaction with immune cells that determine the clinical outcome of leishmaniasis. We have also highlighted opportunities for the development of novel diagnostics and intervention therapies for VL.

Induction of protective cellular immune responses against experimental visceral leishmaniasis mediated by dendritic cells pulsed with the N-terminal domain of Leishmania infantum elongation factor-2 and CpG oligodeoxynucleotides

Leishmania elongation factor 2 (EF-2) has been previously identified as a T_H1-stimulatory protein. In this study, we assayed the protective potential of the N-terminal domain of EF-2 (N-LiEF-2, 1-357 aa) that has been predicted to contain several overlapping MHC class I and II-restricted epitopes injected in the form of dendritic cell (DC)-based vaccine. Ex vivo pulsing of DCs with the recombinant N-LiEF-2 domain along with CpG oligodeoxynucleotides (ODNs) resulted in their functional differentiation. BALB/c vaccinated with CpG-triggered DCs pulsed with N-LiEF-2 were found to be the most immune-reactive in terms of induction of DTH responses, increased T cell proliferation and IL-2 production. Moreover, vaccination induced antigen-specific T_H1 type immune response as evidenced by increased IFN-γ and TNFα levels followed by a significant increase of nitrite (NO) and reactive oxygen species (ROS) in splenocyte cultures. Vaccinated mice showed a pronounced decrease in parasite load in spleen and liver when challenged with L. infantum, increased expression of Stat1 and Tbx21 mRNA transcripts versus reduced expression of Foxp3 transcripts and were able to produce significantly elevated levels of IL-2, IFN-γ and TNFα but not IL-10 compared to non-vaccinated mice. Both antigen and parasite-specific CD4⁺ T and CD8⁺ T cells contributed to the IFN-γ production indicating that both subtypes contribute to the resistance to infection and correlated with robust nitrite generation, critical in controlling Leishmania infection. Together, these findings demonstrated the immunogenic as well as protective potential of the N-terminal domain of Leishmania EF-2 when given with CpG-triggered DCs representing a basis for the development of rationalized vaccine against leishmaniasis.

Cholesterol-lowering drug, in combination with chromium chloride, induces early apoptotic signals in intracellular L. donovani amastigotes, leading to death

Leishmania establishes a successful parasitism by evading both oxidative and non-oxidative killing pathways, and its drug resistance against the currently available therapeutics demands for a safe and cheap drug. Since the parasite synthesizes ergosterol instead of cholesterol, using the same biochemical pathway and enzymes, an inhibitor of HMG-CoA-Reductase, Lovastatin, has been tried for its anti-Leishmanial effect. Lovastatin, being an inhibitor of HMG-CoA-Reductase, inhibits infection by cholesterol depletion, while chromium chloride complexes, at their higher concentrations, are reported to exhibit cytotoxicity. In intracellular amastigotes, cytotoxicity has been checked by assessing various manifestation of cell death, viz. DNA fragmentation, AnnexinV-FITC binding and JC-1 fluorescence ratio. Release of hydrogen peroxide (HPO) and nitric oxide (NO) has been assessed in live cell. Lovastatin and CrCl3.6H2O in combination has appeared to be ineffective on promastigotes but has induced cytotoxic effect on the intracellular amastigotes through up-regulation of cellular signalling mechanisms. CrCl ₃.6H2O stimulates generation of NO, leading to reduction of the number of intracellular amastigote, while Lovastatin shows HPO-mediated killing of the same, keeping the host cell unaffected. This novel therapeutic approach, involving two known safe compounds in suboptimal doses, may resolve human visceral Leishmaniasis.

Emerging therapeutic targets for treatment of leishmaniasis

INTRODUCTION

Parasitic diseases that pose a threat to human life include leishmaniasis - caused by protozoan parasite Leishmania species. Existing drugs have limitations due to deleterious side effects like teratogenicity, high cost and drug resistance. This calls for the need to have an insight into therapeutic aspects of disease. Areas covered: We have identified different drug targets via. molecular, imuunological, metabolic as well as by system biology approaches. We bring these promising drug targets into light so that they can be explored to their maximum. In an effort to bridge the gaps between existing knowledge and prospects of drug discovery, we have compiled interesting studies on drug targets, thereby paving the way for establishment of better therapeutic aspects. Expert opinion: Advancements in technology shed light on many unexplored pathways. Further probing of well established pathways led to the discovery of new drug targets. This review is a comprehensive report on current and emerging drug targets, with emphasis on several metabolic targets, organellar biochemistry, salvage pathways, epigenetics, kinome and more. Identification of new targets can contribute significantly towards strengthening the pipeline for disease elimination.

Nivolumab Enhances In Vitro Effector Functions of PD-1+ T-Lymphocytes and Leishmania-Infected Human Myeloid Cells in a Host Cell-Dependent Manner

Functional impairment of T-cells and a concomitant augmented expression of programmed death-1 (PD-1) have been observed in visceral leishmaniasis patients, as well as in experimental models for visceral and cutaneous leishmaniasis. The PD-1/PD-1-ligand (PD-1/PD-L) interaction negatively regulates T-cell effector functions, which are required for parasite control during leishmaniasis. The aim of this study was to elucidate the impact of the PD-1/PD-L axis in a human primary in vitro infection model of Leishmania major (Lm). Blocking the PD-1/PD-L interaction with nivolumab increased T-cell proliferation and release of the proinflammatory cytokines TNFα and IFNγ during the cocultivation of Lm-infected human monocyte-derived macrophages (hMDMs) or dendritic cells (hMDDC) with autologous PD-1⁺-lymphocytes. As a consequence Lm infection decreased, being the most pronounced in hMDDC, compared to proinflammatory hMDM1 and anti-inflammatory hMDM2. Focusing on hMDDC, we could partially reverse effects mediated by PD-1 blockade by neutralizing TNFα but not by neutralizing IFNγ. Furthermore, PD-1 blockade increased intracellular expression of perforin, granulysin, and granzymes in proliferating CD4⁺-T-cells, which might be implicated in reduction of Lm-infected cells. In all, our data describe an important role for the PD-1/PD-L axis upon Lm infection using a human primary cell system. These data contribute to a better understanding of the PD-1-induced T-cell impairment during disease and its influence on immune effector mechanisms to combat Lm infection.

Leishmaniasis and various immunotherapeutic approaches

Leishmaniasis is a vector-borne infectious disease caused by multiple Leishmania (L.) species with diverse clinical manifestations. There is currently no vaccine against any form of the disease approved in humans, and chemotherapy is the sole approach for treatment. Unfortunately, treatment options are limited to a small number of drugs, partly due to high cost and significant adverse effects. The other obstacle in leishmaniasis treatment is the potential for drug resistance, which has been observed in multiple endemic countries. Immunotherapy maybe another important avenue for controlling leishmaniasis and could help patients control the disease. There are different approaches for immunotherapy in different infectious diseases, generally with low-cost, limited side-effects and no possibility to developing resistance. In this paper, different immunotherapy approaches as alternatives to routine drug treatment will be reviewed against leishmaniasis.

Successful Therapy of Murine Visceral Leishmaniasis with Astrakurkurone, a Triterpene Isolated from the Mushroom Astraeus hygrometricus, Involves the Induction of Protective Cell-Mediated Immunity and TLR9

In our previous report, we showed that astrakurkurone, a triterpene isolated from the Indian mushroom Astraeus hygrometricus (Pers.) Morgan, induced reactive oxygen species, leading to apoptosis in Leishmania donovani promastigotes, and also was effective in inhibiting intracellular amastigotes at the 50% inhibitory concentration of 2.5 μg/ml. The aim of the present study is to characterize the associated immunomodulatory potentials and cellular activation provided by astrakurkurone, leading to effective antileishmanial activity in vitro and in vivo Astrakurkurone-mediated antileishmanial activity was evaluated by real-time PCR and flow cytometry. The involvement of Toll-like receptor 9 (TLR9) was studied by in vitro assay in the presence of a TLR9 agonist and antagonist and by in silico modeling of a three-dimensional structure of the ectodomain of TLR9 and its interaction with astrakurkurone. Astrakurkurone caused a significant increase in TLR9 expression of L. donovani-infected macrophages along with the activation of proinflammatory responses. The involvement of TLR9 in astrakurkurone-mediated amastigote killing has been evidenced from the fact that a TLR9 agonist (CpG, ODN 1826) in combination with astrakurkurone enhanced the amastigote killing, while a TLR9 antagonist (bafilomycin A1) alone or in combination with astrakurkurone curbed the amastigote killing, which could be further justified by in silico evidence of docking between mouse TLR9 and astrakurkurone. Astrakurkurone was found to reduce the parasite burden in vivo by inducing protective cytokines, gamma interferon and interleukin 17. Moreover, astrakurkurone was nontoxic toward peripheral blood mononuclear cells of immunocompromised patients with visceral leishmaniasis. Astrakurkurone, a nontoxic antileishmanial, enhances the immune efficiency of host cells, leading to parasite clearance in vitro and in vivo.

Leishmania infantum FML pulsed-dendritic cells induce a protective immune response in murine visceral leishmaniasis

AIM

To investigate the efficacy of FML loaded dendritic cells (DCs) in protection against visceral leishmaniasis.

MATERIALS & METHODS

Mice were immunized with FML- or soluble Leishmania antigen-loaded DCs as well as FML or soluble Leishmania antigen in saponin and challenged with parasite. The levels of cytokines before and after challenge were detected by ELISA. Parasite burden (total Leishman-Donovan unit) was determined after parasite challenge.

RESULTS

FML-saponin induced the highest IFN-γ/IL-4 ratio among vaccinated groups, though this ratio was higher in FML-loaded DCs group subsequent to challenge with Leishmania infantum. Moreover, the greatest reduction in parasite number was detected in mice vaccinated with FML-loaded DCs compared with phosphate-buffered saline-treated mice (p = 0.002).

CONCLUSION

FML-loaded DCs are one of the promising tools for protection against murine visceral leishmaniasis.

Protective effect of Croton caudatus Geisel leaf extract against experimental visceral leishmaniasis induces proinflammatory cytokines in vitro and in vivo

In the present state of overwhelming emergence of drug-unresponsive phenotypes of Leishmania donovani and persistent severe toxicity in conventional anti-leishmanial therapy, in search for novel leads, the aim of this study has been fixed to identify the active extract(s) of Croton caudatus Geisel. var. tomentosus Hook effective against the parasitic protozoans in vitro and in vivo. C. caudatus Geisel. is often used by Chakma and Hmar community, the local tribes of north-east India for medicinal and veterinary purposes. Among the five semi-purified extracts tested, C. caudatus leaves, extracted in hexane and subsequently semi-purified in a column packed with silica gel (70-130 µM; mesh size 60 A°) using ethyl acetate-hexane solvent (9:1), was found to be the most effective growth inhibitor (JDHex) against the Leishmania promastigotes and amastigotes. JDHex significantly altered the biochemical parameters (protein, lipid and carbohydrates) in promastigotes followed by the morphological changes, DNA condensation and subsequent apoptosis in L. donovani. In consequent steps, it has been also proved that JDHex reduced the replication of intracellular amastigotes with concomitant release of nitric oxide and pro-inflammatory cytokines, IL-12 and TNF-α in vitro. Significantly, the 50% inhibitory concentration of JDHex was estimated much lower against the intracellular amastigotes (2.5 µg/mL) in comparison to promastigotes (10 µg/mL). JDHex was also found efficient in reducing parasite burden in spleen and liver when treated in vivo and increased the intracellular IFN-γ and decreased the IL-10 in CD4+ T cells in splenocytes of orally treated animals. The results of this study support the importance in exploration of novel anti-leishmanial leads from C. caudatus Geisel. var. tomentosus Hook. against the L. donovani (MHOM/IN/83/AG83) infection. Partial chemical characterization of JDHex revealed the presence of terpenoids. However, the further chemical investigation of JDHex is warranted.

Immunotherapy and targeted therapies in treatment of visceral leishmaniasis: current status and future prospects

Visceral leishmaniasis (VL) is a vector-borne chronic infectious disease caused by the protozoan parasite Leishmania donovani or Leishmania infantum. VL is a serious public health problem, causing high morbidity and mortality in the developing world with an estimated 0.2-0.4 million new cases each year. In the absence of a vaccine, chemotherapy remains the favored option for disease control, but is limited by a narrow therapeutic index, significant toxicities, and frequently acquired resistance. Improved understanding of VL pathogenesis offers the development and deployment of immune based treatment options either alone or in combination with chemotherapy. Modulations of host immune response include the inhibition of molecular pathways that are crucial for parasite growth and maintenance; and stimulation of host effectors immune responses that restore the impaired effector functions. In this review, we highlight the challenges in treatment of VL with a particular emphasis on immunotherapy and targeted therapies to improve clinical outcomes.

Immunotherapy and Immunochemotherapy in Visceral Leishmaniasis: Promising Treatments for this Neglected Disease

Leishmaniasis has several clinical forms: self-healing or chronic cutaneous leishmaniasis or post-kala-azar dermal leishmaniasis; mucosal leishmaniasis; visceral leishmaniasis (VL), which is fatal if left untreated. The epidemiology and clinical features of VL vary greatly due to the interaction of multiple factors including parasite strains, vectors, host genetics, and the environment. Human immunodeficiency virus infection augments the severity of VL increasing the risk of developing active disease by 100–2320 times. An effective vaccine for humans is not yet available. Resistance to chemotherapy is a growing problem in many regions, and the costs associated with drug identification and development, make commercial production for leishmaniasis, unattractive. The toxicity of currently drugs, their long treatment course, and limited efficacy are significant concerns. For cutaneous disease, many studies have shown promising results with immunotherapy/immunochemotherapy, aimed to modulate and activate the immune response to obtain a therapeutic cure. Nowadays, the focus of many groups centers on treating canine VL by using vaccines and immunomodulators with or without chemotherapy. In human disease, the use of cytokines like interferon-γ associated with pentavalent antimonials demonstrated promising results in patients that did not respond to conventional treatment. In mice, immunomodulation based on monoclonal antibodies to remove endogenous immunosuppressive cytokines (interleukin-10) or block their receptors, antigen-pulsed syngeneic dendritic cells, or biological products like Pam3Cys (TLR ligand) has already been shown as a prospective treatment of the disease. This review addresses VL treatment, particularly immunotherapy and/or immunochemotherapy as an alternative to conventional drug treatment in experimental models, canine VL, and human disease.

Selective inhibition of Leishmania donovani by active extracts of wild mushrooms used by the tribal population of India: An in vitro exploration for new leads against parasitic protozoans

The study was intended at evaluating the anti-proliferating effect of mushrooms used in traditional folklore of Santal tribal population in India against Leishmania donovani (MHOM/IN/83/AG83). A total of eighteen extracts, three estracts from each mushroom [(80% ethanol extracted; Fa), (water-soluble polysaccharide fraction; Fb), (polyphenolic fraction; Fc)], from six wild mushrooms were obtained. These extracts were tested against the promastigotes and amastigotes for their antileishmanial capacity. Fa fractions (250 μg/mL) of Astraeus hygrometricus and Tricholoma giganteum significantly inhibited the growth of L. donovani promastigotes and interfered in lipid biosynthesis. Moreover, both fractions induced apoptosis in promastigotes. Water soluble Fb fractions of A. hygrometricus, Russula laurocerasi, Russula albonigra, Termitomyces eurhizus, Russula delica and polyphenolic Fc fraction of R. laurocerasi were found to inhibit the replication of intracellular amastigotes in macrophages dose dependently. Significantly, 50% inhibitory concentration of the active extracts against intracellular amastigotes induced release of nitric oxide and IL-12 in murine macrophages and dendritic cells assay and also found considerably non-toxic on murine splenocytes. Results of this study can be used as a basis for further phytochemical and pharmacological investigations in the effort for search of novel anti-leishmanial leads.

Targeting Leishmania major Antigens to Dendritic Cells In Vivo Induces Protective Immunity

Efficient vaccination against the parasite Leishmania major, the causative agent of human cutaneous leishmaniasis, requires development of type 1 T-helper (Th1) CD4⁺ T cell immunity. Because of their unique capacity to initiate and modulate immune responses, dendritic cells (DCs) are attractive targets for development of novel vaccines. In this study, for the first time, we investigated the capacity of a DC-targeted vaccine to induce protective responses against L. major. To this end, we genetically engineered the N-terminal portion of the stress-inducible 1 protein of L. major (LmSTI1a) into anti-DEC205/CD205 (DEC) monoclonal antibody (mAb) and thereby delivered the conjugated protein to DEC⁺ DCs in situ in the intact animal. Delivery of LmSTI1a to adjuvant-matured DCs increased the frequency of antigen-specific CD4⁺ T cells producing IFN-γ⁺, IL-2⁺, and TNF-α⁺ in two different strains of mice (C57BL/6 and Balb/c), while such responses were not observed with the same doses of a control Ig-LmSTI1a mAb without receptor affinity or with non-targeted LmSTI1a protein. Using a peptide library for LmSTI1a, we identified at least two distinct CD4⁺ T cell mimetopes in each MHC class II haplotype, consistent with the induction of broad immunity. When we compared T cell immune responses generated after targeting DCs with LmSTI1a or other L. major antigens, including LACK (Leishmania receptor for activated C kinase) and LeIF (Leishmania eukaryotic ribosomal elongation and initiation factor 4a), we found that LmSTI1a was superior for generation of IFN-γ-producing CD4⁺ T cells, which correlated with higher protection of susceptible Balb/c mice to a challenge with L. major. For the first time, this study demonstrates the potential of a DC-targeted vaccine as a novel approach for cutaneous leishmaniasis, an increasing public health concern that has no currently available effective treatment.

Vaccination with Leishmania histone H1-pulsed dendritic cells confers protection in murine visceral leishmaniasis

Visceral leishmaniasis is the most severe form of leishmaniases affecting millions of people worldwide often resulting in death despite optimal therapy. Thus, there is an urgent need for the development of effective anti-infective vaccine(s). In the present study, we evaluated the prophylactic value of bone marrow-derived dendritic cells (BM-DCs) pulsed with the Leishmania (L.) infantum histone H1. We developed fully mature BM-DCs characterized by enhanced capacity of IL-12 production after ex vivo pulsing with GST-LeishH1. Intravenous administration of these BM-DCs in naive BALB/c mice resulted in antigen-specific spleenocyte proliferation and IgG1 isotype antibody production and conferred protection against experimental challenge with L. infantum independently of CpG oligonucleotides (ODNs) co-administration. Protection was associated with a pronounced enhancement of parasite-specific IFNγ-producing cells and reduction of cells producing IL-10, whereas IL-4 production was comparable in protected and non-protected mice. The polarization of immune responses to Th1 type was further confirmed by the elevation of parasite-specific IgG2a/IgG1 ratio in protected mice. The above data indicate the immunostimulatory capacity of Leishmania histone H1 and further support its exploitation as a candidate protein for vaccine development against leishmaniasis.

Vaccine Development Against Leishmania donovani

Visceral leishmaniasis (VL) caused by Leishmania donovani and Leishmania infantum/chagasi represents the second most challenging infectious disease worldwide, leading to nearly 500,000 new cases and 60,000 deaths annually. Zoonotic VL caused by L. infantum is a re-emergent canid zoonoses which represents a complex epidemiological cycle in the New world where domestic dogs serve as a reservoir host responsible for potentially fatal human infection and where dog culling is the only measure for reservoir control. Life-long immunity to VL has motivated development of prophylactic vaccines against the disease but very few have progressed beyond the experimental stage. No licensed vaccine is available till date against any form of leishmaniasis. High toxicity and increasing resistance to the current chemotherapeutic regimens have further complicated the situation in VL endemic regions of the world. Advances in vaccinology, including recombinant proteins, novel antigen-delivery systems/adjuvants, heterologous prime-boost regimens and strategies for intracellular antigen presentation, have contributed to recent advances in vaccine development against VL. Attempts to develop an effective vaccine for use in domestic dogs in areas of canine VL should be pursued for preventing human infection. Studies in animal models and human patients have revealed the pathogenic mechanisms of disease progression and features of protective immunity. This review will summarize the accumulated knowledge of pathogenesis, immune response, and prerequisites for protective immunity against human VL. Authors will discuss promising vaccine candidates, their developmental status and future prospects in a quest for rational vaccine development against the disease. In addition, several challenges such as safety issues, renewed and coordinated commitment to basic research, preclinical studies and trial design will be addressed to overcome the problems faced in developing prophylactic strategies for protection against this lethal infection.

Leishmania donovani: CD2 biased immune response skews the SAG mediated therapy for a predominant Th1 response in experimental infection

We have evaluated the effect of combining CD2 with conventional antimonial (sb) therapy in protection in BALB/c mice infected with either drug sensitive or resistant strain of Leishmania donovani with 3×10(7) parasites via-intra-cardiac route. Mice were treated with anti CD2 adjunct SAG sub-cutaneously twice a week for 4 weeks. Assessment for measurement of weight, spleen size, anti-Leishmania antibody titer, T cell and anti-leishmanial macrophage function was carried out day 0, 10, 22 and 34 post treatments. The combination therapy was shown boosting significant proportion of T cells to express CD25 compared to SAG monotherapy. Although, the level of IFN-γ was not statistically different between combination vs monotherapy (p=0.298) but CD2 treatment even alone significantly influenced IFN-γ production than either SAG treatment (p=0.045) or with CD2 adjunct SAG treatment (p=0.005) in Ld-S strain as well as in Ld-R strain. The influence of CD2 adjunct treatment was also documented in anti-leishmanial functions in macrophages. As shown, the super-oxide generation began enhancing very early on day 10 after SAG treatment with CD2 during which SAG action was at minimum. Interestingly, the super-oxide generation ability remained intact in macrophage after treatment with immuno-chemotherapy even in mice infected with Leishmania resistant strain. Unlike SAG treatment, treatment of SAG with CD2 also led to production of nitric oxide and TNF-α, resulting in resulting in most effective clearance of L. donovani from infected macrophages. Our results indicate that CD2, which can boost up a protective Th1 response, might also be beneficial to enable SAG to induce Macrophages to produce Leishmanicidal molecules and hence control the infection in clinical situation like Kala-azar. Drug resistance is the major impedance for disease control but the encouraging results obtained after infecting mice with resistant strain of the parasite strongly imply that this drug can be effective even in treating resistant cases of Kala-azar.

Protective therapy with novel chromone derivative against Leishmania donovani infection induces Th1 response in vivo

BACKGROUND

Visceral leishmaniasis is a chronic protozoan disease caused by Leishmania donovani, an obligatory intracellular parasite that resides and multiplies within macrophages of the reticuloendothelial system. The aim of this study was to evaluate the efficacy of nine novel synthetic chromone derivatives as antileishmanial molecules in experimental murine visceral leishmaniasis.

METHODS

In vitro activity of the molecules (2, 5 and 10 μg/ml) was assessed against promastigotes of both pentavalent antimonial-responsive strain AG83 and pentavalent antimonial-resistant strain GE1F8R at days 2 (48 h), 4 (96 h) and 6 (144 h). The efficacy of the most efficient chromone derivative [C-(6-Methyl-4-oxo-4H-1-benzopyran-3-yl)-N-(p-tolyl) nitrone], designated here as NP1, was also tested against intracellular amastigotes in vitro and in vivo.

RESULTS

NP1, 5 μg/ml, inhibited the growth of AG83 and GE1F8R promastigotes by 98.57% (day 4) and 75.75% (day 6), respectively, and also inhibited the growth of intracellular amastigotes by 85% (day 3), compared to DMSO control. Treatment of L. donovani-infected mice with NP1 resulted in a 70% significant decrease in parasite load in the spleen in the 7th week after infection (5 mice in each group), with associated induction of interferon-γ synthesis by dose 2 (37.5 mg/kg body weight) compared to DMSO control. Dose 2 was found efficient over dose 1 (25 mg/kg body weight).

CONCLUSIONS

The novel synthetic chromone derivative is effective in the treatment of visceral leishmaniasis and induces the synthesis of interferon-γ in rodent models.

Development of Vaccines against Visceral Leishmaniasis

Leishmaniasis is a neglected disease resulting in a global morbidity of 2,090 thousand Disability-Adjusted Life Years and a mortality rate of approximately 60,000 per year. Among the three clinical forms of leishmaniasis (cutaneous, mucosal, and visceral), visceral leishmaniasis (VL) accounts for the majority of mortality, as if left untreated VL is almost always fatal. Caused by infection with Leishmania donovani or L. infantum, VL represents a serious public health problem in endemic regions and is rapidly emerging as an opportunistic infection in HIV patients. To date, no vaccine exists for VL or any other form of leishmaniasis. In endemic areas, the majority of those infected do not develop clinical symptoms and past infection leads to robust immunity against reinfection. Thus the development of vaccine for Leishmania is a realistic public health goal, and this paper summarizes advances in vaccination strategies against VL.

Endogenous IL-13 plays a crucial role in liver granuloma maturation during Leishmania donovani infection, independent of IL-4Rα-responsive macrophages and neutrophils

Previous studies comparing interleukin 4 receptor α (IL-4Rα)^-/- and interleukin 4 (IL-4)^-/- BALB/c mice have indicated that interleukin 13 (IL-13), whose receptor shares the IL-4Rα subunit with IL-4, plays a protective role during visceral leishmaniasis. We demonstrate that IL-13^-/- BALB/c mice were less able to control hepatic growth of Leishmania donovani compared with wild-type mice. This correlated with significantly retarded granuloma maturation in IL-13^-/- mice, defective interferon γ (IFN-γ) production, and elevated IL-4 and interleukin 10 (IL-10) levels. L.donovani–infected IL-13^-/- mice also responded poorly to sodium stibogluconate-mediated chemotherapy compared with wild-type BALB/c mice. Because murine lymphocytes do not have IL-13 receptors, we examined the ability of macrophage/neutrophil-specific IL-4Rα^-/- mice to control primary infection with L. donovani and to respond to chemotherapy. Macrophage/neutrophil-specific IL-4Rα^-/- mice were as resistant to leishmaniasis as wild-type mice, and chemotherapy retained its efficacy. Consequently, in L. donovani infected BALB/c mice, IL-13 promotes hepatic granuloma formation and controls parasite burdens independently of direct effects on macrophages/neutrophils.

Vaccination with TAT-antigen fusion protein induces protective, CD8(+) T cell-mediated immunity against Leishmania major

In murine leishmaniasis, healing is mediated by IFN-γ-producing CD4(+) and CD8(+) T cells. Thus, an efficacious vaccine should induce Th1 and Tc1 cells. Dendritic cells (DCs) pulsed with exogenous proteins primarily induce strong CD4-dependent immunity; induction of CD8 responses has proven to be difficult. We evaluated the immunogenicity of fusion proteins comprising the protein transduction domain of HIV-1 TAT and the Leishmania antigen LACK (Leishmania homolog of receptors for activated C kinase), as TAT-fusion proteins facilitate major histocompatibility complex class I-dependent antigen presentation. In vitro, TAT-LACK-pulsed DCs induced stronger proliferation of Leishmania-specific CD8(+) T cells compared with DCs incubated with LACK alone. Vaccination with TAT-LACK-pulsed DCs or fusion proteins plus adjuvant in vivo significantly improved disease outcome in Leishmania major-infected mice and was superior to vaccination with DCs treated with LACK alone. Vaccination with DC+TAT-LACK resulted in stronger proliferation of CD8(+) T cells when compared with immunization with DC+LACK. Upon depletion of CD4(+) or CD8(+) T cells, TAT-LACK-mediated protection was lost. TAT-LACK-pulsed IL-12p40-deficient DCs did not promote protection in vivo. In summary, these data show that TAT-fusion proteins are superior in activating Leishmania-specific Tc1 cells when compared with antigen alone and suggest that IL-12-dependent preferential induction of antigen-specific CD8(+) cells promotes significant protection against this important human pathogen.

Ubiquitin conjugation of open reading frame F DNA vaccine leads to enhanced cell-mediated immune response and induces protection against both antimony-susceptible and -resistant strains of Leishmania donovani

Resistance of Leishmania donovani to sodium antimony gluconate has become a critical issue in the current, prolonged epidemic in India. Hence, there is an urgent need for a vaccine that is protective against both antimony-susceptible and -resistant strains of L. donovani. The multigene LD1 locus located on chromosome 35 of Leishmania is amplified in approximately 15% of the isolates examined. The open reading frame F (ORFF), a potential vaccine candidate against visceral leishmaniasis, is part of the multigene LD1 locus. ORFF was expressed as a chimeric conjugate of ubiquitin to elicit an Ag-specific cell-mediated immune response. Analysis of the cellular immune responses of ubiquitin-conjugated ORFF (UBQ-ORFF) DNA-immunized, uninfected BALB/c mice demonstrated that the vaccine induced enhanced IFN-gamma-producing CD4(+) and CD8(+) T cells compared with nonubiquitinated ORFF DNA vaccine. Higher levels of IL-12 and IFN-gamma and the low levels of IL-4 and IL-10 further indicated that the immune responses with UBQ-ORFF were mediated toward the Th1 rather than Th2 type. Infection of immunized mice with either the antimony-susceptible (AG83) or -resistant (GE1F8R) L. donovani strain showed that UBQ-ORFF DNA vaccine induced higher protection when compared with ORFF DNA. UBQ-ORFF DNA-immunized and -infected mice showed a significant increase in IL-12 and IFN-gamma and significant down-regulation of IL-10. High levels of production of nitrite and superoxide, two macrophage-derived oxidants that are critical in controlling Leishmania infection, were observed in protected mice. The feasibility of using ubiquitinated-conjugated ORFF DNA vaccine as a promising immune enhancer for vaccination against both antimony-susceptible and -resistant strains of L. donovani is reported.

Designing therapies against experimental visceral leishmaniasis by modulating the membrane fluidity of antigen-presenting cells

The membrane fluidity of antigen-presenting cells (APCs) has a significant bearing on T-cell-stimulating ability and is dependent on the cholesterol content of the membrane. The relationship, if any, between membrane fluidity and defective cell-mediated immunity in visceral leishmaniasis has been investigated. Systemic administration of cholesterol by liposome delivery (cholesterol liposomes) in Leishmania donovani-infected hamsters was found to cure the infection. Splenic macrophages as a prototype of APCs in infected hamsters had decreased membrane cholesterol and an inability to drive T cells, which was corrected by cholesterol liposome treatment. The effect was cholesterol specific because liposomes made up of the analogue 4-cholesten-3-one provided almost no protection. Infection led to increases in interleukin-10 (IL-10), transforming growth factor beta, and IL-4 signals and concomitant decreases in gamma interferon (IFN-gamma), tumor necrosis factor alpha, and inducible NO synthase signals, which reverted upon cholesterol liposome treatment. The antileishmanial T-cell repertoire, whose expansion appeared to be associated with protection, was presumably type Th1, as shown by enhanced IFN-gamma signals and the predominance of the immunoglobulin G2 isotype. The protected group produced significantly more reactive oxygen species and NO than the infected groups, which culminated in killing of L. donovani parasites. Therefore, cholesterol liposome treatment may be yet another simple strategy to enhance the cell-mediated immune response to L. donovani infection. To our knowledge, this is the first report on the therapeutic effect of cholesterol liposomes in any form of the disease.

Radiation-induced apoptosis along with local and systemic cytokine elaboration is associated with DC plus radiotherapy-mediated renal cell tumor regression

Utilizing melanoma and sarcoma tumor models syngeneic to C57BL/6 mice, we previously reported the antitumor effects of intratumoral (i.t.) administration of dendritic cells (DC) combined with localized radiotherapy (RT). However, the mechanisms underlying the augmented therapeutic effects have yet to be fully defined. Using the BALB/c host, we explored in this study the capacity of RT to augment the therapeutic efficacy of DC in the syngeneic renal cell cancer, Renca. I.t. DC administration combined with RT inhibited tumor growth in a synergistic manner. This extends our previous findings using a different host strain and two histologically distinct tumor models. More importantly, we provide evidence in this report that RT induced significant apoptosis and necrosis in Renca tumor cells, which involved down-regulated expression of Bcl-2 and a concurrent up-regulated expression of Bax. We also found significantly elevated expression of TNFalpha in RT plus DC-treated Renca tumors. Furthermore, splenocytes isolated from DC plus RT-treated mice elaborated higher levels of IL-2, IL-4, IFNgamma and IgG, IgM in response to tumor cells compared with splenocytes from monotherapy-treated hosts. These data support the conclusion that radiotherapy enhanced DC vaccination by inducing tumor cell apoptosis in BABL/c host, and the significantly augmented therapeutic efficacy by RT+DC treatment was associated with an increased local production of TNFalpha as well as an amplified systemic antitumor responses conferred by the combined therapy. I.t. DC administration in concert with localized RT may represent a promising novel regimen for human cancer therapy.

Leishmaniavaccines: progress and problems

Leishmaniaare 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 12 million new cases each year. Current treatment is based on chemotherapy, which is difficult to administer, expensive and becoming ineffective due to the emergence of drug resistance. Leishmaniasis is considered one of a few parasitic diseases likely to be controllable by vaccination. The relatively uncomplicated leishmanial life cycle and the fact that recovery from infection renders the host resistant to subsequent infection indicate that a successful vaccine is feasible. Extensive evidence from studies in animal models indicates that solid protection can be achieved by immunisation with protein or DNA vaccines. However, to date no such vaccine is available despite substantial efforts by many laboratories. Advances in our understanding ofLeishmaniapathogenesis and generation of host protective immunity, together with the completedLeishmaniagenome sequence open new avenues for vaccine research. The major remaining challenges are the translation of data from animal models to human disease and the transition from the laboratory to the field. This review focuses on advances in anti-leishmania vaccine development over the recent years and examines current problems hampering vaccine development and implementation.

Distinct roles for IL-6 and IL-12p40 in mediating protection against Leishmania donovani and the expansion of IL-10+ CD4+ T cells

Adoptive dendritic cell (DC) immunotherapy provides a useful experimental tool to evaluate immunoregulation in vivo and has previously been successfully used to enhance host resistance in a variety of experimental models of leishmaniasis. Here, we used this approach to identify IL-6 and IL-12p40 as critical cytokines that cooperate to mediate host protection to Leishmania donovani but which act independently to regulate expansion of IL-10(+) CD4(+) T cells, shown here for the first time to be associated with this infection. Adoptive transfer of LPS-activated bone marrow-derived DC (BMDC) from wild-type mice was therapeutically beneficial and led to enhanced resistance as measured by spleen parasite burden. In contrast, IL-6- or IL-12p40-deficient BMDC had no protective benefit, indicating that production of both cytokines was essential for the therapeutic efficacy of DC. IL-10 production by CD25(-) FoxP3(-) IL-10(+) CD4(+) T cells is a strong correlate of disease progression, and BMDC from wild-type mice inhibited expansion of these cells. Strikingly, IL-12-deficient BMDC could also inhibit the expansion of this T cell population whereas IL-6-deficient BMDC could not, indicating that IL-6 played a key role in this aspect of DC function in vivo. Breadth of cytokine production is thus an important factor when considering strategies for DC-based interventions.

Antagonizing deactivating cytokines to enhance host defense and chemotherapy in experimental visceral leishmaniasis

In experimental visceral leishmaniasis, inhibition of interleukin 10 (IL-10) signaling enhances Th1-cell-associated responses, promoting gamma interferon (IFN-gamma) secretion, granuloma assembly, macrophage activation with substantial liver parasite killing, and synergy with pentavalent antimony (Sb) chemotherapy. To determine if inhibiting other suppressive cytokines has similar therapeutic potential, Leishmania donovani-infected BALB/c mice were injected with anti-IL-4 monoclonal antibody or receptor fusion antagonists of IL-13 or transforming growth factor beta (TGF-beta). Targeting IL-13 or TGF-beta enabled inhibition of L. donovani replication but little parasite killing; anti-IL-4 had no effect. None of the three antagonists promoted IFN-gamma production, granuloma maturation, or Sb efficacy. Excess IL-13 and TGF-beta exacerbated liver infection; however, effects were transient. Among IL-10, IL-4, IL-13, and TGF-beta, cytokines capable of disabling Th1-cell mechanisms (including those which support chemotherapy), IL-10 appears to be the appropriate target for therapeutic inhibition in visceral L. donovani infection.

Leishmania spp.: on the interactions they establish with antigen-presenting cells of their mammalian hosts

Identification of macrophages as host cells for the mammalian stage of Leishmania spp. traces back to about 40 years ago, but many questions concerning the ways these parasites establish themselves in these cells, which are endowed with potent innate microbicidal mechanisms, are still unanswered. It is known that microbicidal activities of macrophages can be enhanced or induced by effector T lymphocytes following the presentation of antigens via MHC class I or class II molecules expressed at the macrophage plasma membrane. However, Leishmania spp. have evolved mechanisms to evade or to interfere with antigen presentation processes, allowing parasites to partially resist these T cell-mediated immune responses. Recently, the presence of Leishmania amastigotes within dendritic cells has been reported suggesting that they could also be host cells for these parasites. Dendritic cells have been described as the only cells able to induce the activation of naive T lymphocytes. However, certain Leishmania species infect dendritic cells without inducing their maturation and impair the migration of these cells, which could delay the onset of the adaptive immune responses as both processes are required for naive T cell activation. This review examines how Leishmania spp. interact with these two cell types, macrophages and dendritic cells, and describes some of the strategies used by Leishmania spp. to survive in these inducible or constitutive antigen-presenting cells.

Interleukin 10 receptor blockade--pentavalent antimony treatment in experimental visceral leishmaniasis

Interleukin 10 (IL-10), a suppressive Th2 cell-type cytokine, promotes disease progression in experimental visceral leishmaniasis. To extend testing the therapeutic effects of applying IL-10 receptor (IL-10R) blockade with antileishmanial chemotherapy, BALB/c mice with established intracellular Leishmania donovani infection were injected once with anti-IL-10R mAb at the time low-dose, daily pentavalent antimony (Sb) therapy was initiated. In this treatment model, simultaneous administration of anti-IL-10R enhanced overall antileishmanial activity in the liver in an interferon-gamma-dependent fashion, and accelerated the kinetics of Sb-associated killing, induced a >10-fold Sb dose-sparing effect and shortened the required duration of Sb treatment. These results suggest the possibility of using mAb-induced IL-10R blockade to develop low-dose and/or short-course immunochemotherapeutic regimens in visceral leishmaniasis.

Dendritic cells in Leishmania infection

Dendritic cells (DCs) are key elements of the immune system, which function as sentinel in the periphery and alert T lymphocytes about the type of invading antigen and address their polarisation, in order to mount an efficacious immune response. Leishmania spp. are parasitic protozoa which may cause severe disease in humans and domestic animals. In this work, the main studies concerning the role of DCs in Leishmania infection are reviewed, in both the murine and human models. In particular, the importance of the genetic status of the hosts and of the different Leishmania species in modulating DC-mediated immune response is examined. In addition, different approaches of DC-based vaccination against experimental leishmaniasis, which could have important future applications, are summarised.

Myd88-dependent in vivo maturation of splenic dendritic cells induced by Leishmania donovani and other Leishmania species

The usual agent of visceral leishmaniasis in the Old World is Leishmania donovani, which typically produces systemic diseases in humans and mice. L. donovani has developed efficient strategies to infect and persist in macrophages from spleen and liver. Dendritic cells (DC) are sentinels of the immune system. Following recognition of evolutionary conserved microbial products, DC undergo a maturation process and activate antigen-specific naïve T cells. In the present report we provide new insights into how DC detect Leishmania in vivo. We demonstrate that in both C57BL/6 and BALB/c mice, systemic injection of L. donovani induced the migration of splenic DC from marginal zones to T-cell areas. During migration, DC upregulated the expression of major histocompatibility complex II and costimulatory receptors (such as CD40, CD80, and CD86). Leishmania-induced maturation requires live parasites and is not restricted to L. donovani, as L. braziliensis, L. major, and L. mexicana induced a similar process. Using a green fluorescent protein-expressing parasite, we demonstrate that DC undergoing maturation in vivo display no parasite internalization. We also show that L. donovani-induced DC maturation was partially abolished in MyD88-deficient mice. Taken together, our data suggest that Leishmania-induced DC maturation results from direct recognition of Leishmania by DC, and not from DC infection, and that MyD88-dependent receptors are implicated in this process.