Identification of Leishmania donovani antigens stimulating cellular immune responses in exposed immune individuals

Potential biomarkers of immune protection in human leishmaniasis

Leishmaniasis is a vector-borne neglected tropical disease endemic in over 100 countries around the world. Available control measures are not always successful, therapeutic options are limited, and there is no vaccine available against human leishmaniasis, although several candidate antigens have been evaluated over the last decades. Plenty of studies have aimed to evaluate the immune response development and a diverse range of host immune factors have been described to be associated with protection or disease progression in leishmaniasis; however, to date, no comprehensive biomarker(s) have been identified as surrogate marker of protection or exacerbation, and lack of enough information remains a barrier for vaccine development. Most of the current understanding of the role of different markers of immune response in leishmaniasis has been collected from experimental animal models. Although the data generated from the animal models are crucial, it might not always be extrapolated to humans. Here, we briefly review the events during Leishmania invasion of host cells and the immune responses induced against Leishmania in animal models and humans and their potential role as a biomarker of protection against human leishmaniasis.

Gene deleted live attenuated Leishmania vaccine candidates against visceral leishmaniasis elicit pro-inflammatory cytokines response in human PBMCs

Currently no effective vaccine is available for human visceral leishmaniasis(VL) caused by Leishmania donovani. Previously, we showed that centrin1 and p27gene deleted live attenuated Leishmania parasites (LdCen1(-/-) and Ldp27(-/-)) are safe, immunogenic and protective in animal models. Here, to assess the correlates of protection, we evaluated immune responses induced by LdCen1(-/-) and Ldp27(-/-) in human blood samples obtained from healthy, healed VL (HVL), post kala-azar dermal leishmaniasis(PKDL) and VL subjects. Both parasites infected human macrophages, as effectively as the wild type parasites. Further, LdCen1(-/-) and Ldp27(-/-) strongly stimulated production of pro-inflammatory cytokines including, IL-12, IFN-γ, TNF-α, IL-2, IL-6 and IL-17 in the PBMCs obtained from individuals with a prior exposure to Leishmania (HVL and PKDL). There was no significant stimulation of anti-inflammatory cytokines (IL-4 and IL-10). Induction of Th1 biased immune responses was supported by a remarkable increase in IFN-γ secreting CD4(+) and CD8(+) T cells and IL-17 secreting CD4(+) cells in PBMCs from HVL cases with no increase in IL-10 secreting T cells. Hence, LdCen1(-/-) and Ldp27(-/-) are promising as live vaccine candidates against VL since they elicit strong protective immune response in human PBMCs from HVL, similar to the wild type parasite infection, mimicking a naturally acquired protection following cure.

Immunoprotective responses of T helper type 1 stimulatory protein-S-adenosyl-L-homocysteine hydrolase against experimental visceral leishmaniasis

It is well known that a patient in clinical remission of visceral leishmaniasis (VL) remains immune to reinfection, which provides a rationale for the feasibility of a vaccine against this deadly disease. In earlier studies, observation of significant cellular responses in treated Leishmania patients as well as in hamsters against leishmanial antigens from different fractions led to its further proteomic characterization, wherein S-adenosyl-L-homocysteine hydrolase (AdoHcy) was identified as a helper type 1 (Th1) stimulatory protein. The present study includes immunological characterization of this protein, its cellular responses [lymphoproliferation, nitric oxide (NO) production and cytokine responses] in treated Leishmania-infected hamsters and patients as well as prophylactic efficacy against Leishmania challenge in hamsters and the immune responses generated thereof. Significantly higher cellular responses were noticed against recombinant L. donovani S-adenosyl-L-homocysteine hydrolase (rLdAdoHcy) compared to soluble L. donovani antigen in treated samples. Moreover, stimulation of peripheral blood mononuclear cells with rLdAdoHcy up-regulated the levels of interferon (IFN)-γ, interleukin (IL)-12 and down-regulated IL-10. Furthermore, vaccination with rLdAdoHcy generated perceptible delayed-type hypersensitivity response and exerted considerably good prophylactic efficacy (∼70% inhibition) against L. donovani challenge. The efficacy was confirmed by the increased expression levels of inducible NO synthase and Th1-type cytokines, IFN-γ and IL-12 and down-regulation of IL-4, IL-10 and transforming growth factor (TGF)-β. The results indicate the potentiality of rLdAdoHcy protein as a suitable vaccine candidate against VL.

Comparative Analysis of Cellular Immune Responses in Treated Leishmania Patients and Hamsters against Recombinant Th1 Stimulatory Proteins of Leishmania donovani

Our prior studies demonstrated that cellular response of T helper 1 (Th1) type was generated by a soluble antigenic fraction (ranging from 89.9 to 97.1 kDa) of Leishmania donovani promastigote, in treated Leishmania patients as well as hamsters and showed significant prophylactic potential against experimental visceral leishmaniasis (VL). Eighteen Th1 stimulatory proteins were identified through proteomic analysis of this subfraction, out of which 15 were developed as recombinant proteins. In the present work, we have evaluated these 15 recombinant proteins simultaneously for their comparative cellular responses in treated Leishmania patients and hamsters. Six proteins viz. elongation factor-2, enolase, aldolase, triose phosphate isomerase, protein disulfide isomerase, and p45 emerged as most immunogenic as they produced a significant lymphoproliferative response, nitric oxide generation and Th1 cytokine response in PBMCs and lymphocytes of treated Leishmania patients and hamsters respectively. The results suggested that these proteins may be exploited for developing a successful poly-protein and/or poly-epitope vaccine against VL.

Characterization of glycolytic enzymes--rAldolase and rEnolase of Leishmania donovani, identified as Th1 stimulatory proteins, for their immunogenicity and immunoprophylactic efficacies against experimental visceral leishmaniasis

Th1 immune responses play an important role in controlling Visceral Leishmaniasis (VL) hence, Leishmania proteins stimulating T-cell responses in host, are thought to be good vaccine targets. Search of such antigens eliciting cellular responses in Peripheral blood mononuclear cells (PBMCs) from cured/exposed/Leishmania patients and hamsters led to the identification of two enzymes of glycolytic pathway in the soluble lysate of a clinical isolate of Leishmania donovani--Enolase (LdEno) and aldolase (LdAld) as potential Th1 stimulatory proteins. The present study deals with the molecular and immunological characterizations of LdEno and LdAld. The successfully cloned and purified recombinant proteins displayed strong ability to proliferate lymphocytes of cured hamsters' along with significant nitric-oxide production and generation of Th1-type cytokines (IFN-γ and IL-12) from stimulated PBMCs of cured/endemic VL patients. Assessment of their prophylactic potentials revealed ∼ 90% decrease in parasitic burden in rLdEno vaccinated hamsters against Leishmania challenge, strongly supported by an increase in mRNA expression levels of iNOS, IFN-γ, TNF-α and IL-12 transcripts along with extreme down-regulation of TGF-β, IL-4 and IL-10. However, animals vaccinated with rLdAld showed comparatively lesser prophylactic efficacy (∼ 65%) with inferior immunological response. Further, with a possible implication in vaccine design against VL, identification of potential T-cell epitopes of both the proteins was done using computational approach. Additionally, in-silico 3-D modelling of the proteins was done in order to explore the possibility of exploiting them as potential drug targets. The comparative molecular and immunological characterizations strongly suggest rLdEno as potential vaccine candidate against VL and supports the notion of its being effective T-cell stimulatory protein.

Anti-leishmanial and toxicity activities of some selected Iranian medicinal plants

Leishmaniasis is caused by protozoan parasites belonging to the genus Leishmania. Cutaneous leishmaniasis is the most common form of leishmaniasis in Iran. As there is not any vaccine for leishmaniasis, treatment is important to prevent the spreading of parasites. There is, therefore, a need to develop newer drugs from different sources. The aim of this study was to assess anti-leishmanial activity of the ethanolic extracts of 17 different medicinal plants against Leishmania major promastigotes and macrophage cell line J774. The selection of the hereby studied 17 plants was based on the existing information on their local ethnobotanic history. Plants were dried, powdered, and macerated in a hydroalcoholic solution. Resulting extracts have been assessed for in vitro anti-leishmanial and brine shrimp toxicity activities. Four plants, Caesalpinia gilliesii, Satureia hortensis, Carum copticum heirm, and Thymus migricus, displayed high anti-leishmanial activity (IC50, 9.76 ± 1.27, 15.625 ± 3.76, 15.625 ± 5.46, and 31.25 ± 15.44 μM, respectively) and were toxic against the J774 macrophage cell line at higher concentrations than those needed to inhibit the parasite cell growth (IC50, 45.13 ± 3.17, 100.44 ± 17.48, 43.76 ± 0.78, and 39.67 ± 3.29 μM, respectively). Glucantime as positive control inhibited the growth of L. major promastigotes with IC50 = 254 μg/ml on promastigotes (1 × 10(6)/100 μ/well) of a log phase culture, without affecting the growth of J774 macrophages. These data revealed that C. gilliesii, S. hortensis, C. copticum heirm, and T. migricus extracts contain active compounds, which could serve as alternative agents in the control of cutaneous leishmaniasis. The activity of these herbs against L. major promastigotes and macrophage cell line J774 was reported for the first time in our study.

Leishmania donovani: immunostimulatory cellular responses of membrane and soluble protein fractions of splenic amastigotes in cured patient and hamsters

Visceral leishmaniasis (VL), caused by the intracellular parasite Leishmania donovani, L. chagasi and L. infantum is characterized by defective cell-mediated immunity (CMI) and is usually fatal if not treated properly. An estimated 350 million people worldwide are at risk of acquiring infection with Leishmania parasites with approximately 500,000 cases of VL being reported each year. In the absence of an efficient and cost-effective antileishmanial drug, development of an appropriate long-lasting vaccine against VL is the need of the day. In VL, the development of a CMI, capable of mounting Th1-type of immune responses, play an important role as it correlate with recovery from and resistance to disease. Resolution of infection results in lifelong immunity against the disease which indicates towards the feasibility of a vaccine against the disease. Most of the vaccination studies in Leishmaniasis have been focused on promastigote- an infective stage of parasite with less exploration of pathogenic amastigote form, due to the cumbersome process of its purified isolation. In the present study, we have isolated and purified splenic amastigotes of L. donovani, following the traditional protocol with slight modification. These were fractionated into five membranous and soluble subfractions each i.e MAF1-5 and SAF1-5 and were subjected for evaluation of their ability to induce cellular responses. Out of five sub-fractions from each of membrane and soluble, only four viz. MAF2, MAF3, SAF2 and SAF3 were observed to stimulate remarkable lymphoproliferative, IFN-γ, IL-12 responses and Nitric Oxide production, in Leishmania-infected cured/exposed patients and hamsters. Results suggest the presence of Th-1 type immunostimulatory molecules in these sub-fractions which may further be exploited for developing a successful subunit vaccine from the less explored pathogenic stage against VL.

Immunity to visceral leishmaniasis using genetically defined live-attenuated parasites

Leishmaniasis is a protozoan parasitic disease endemic to the tropical and subtropical regions of the world, with three major clinical forms, self-healing cutaneous leishmaniasis (CL), mucocutaneous leishmaniasis (MCL), and visceral leishmaniasis (VL). Drug treatments are expensive and often result in the development of drug resistance. No vaccine is available against leishmaniasis. Subunit Leishmania vaccine immunization in animal models has shown some efficacy but little or none in humans. However, individuals who recover from natural infection are protected from reinfection and develop life-long protection, suggesting that infection may be a prerequisite for immunological memory. Thus, genetically altered live-attenuated parasites with controlled infectivity could achieve such memory. In this paper, we discuss development and characteristics of genetically altered, live-attenuated Leishmania donovani parasites and their possible use as vaccine candidates against VL. In addition, we discuss the challenges and other considerations in the use of live-attenuated parasites.

Prophylactic efficacy of high-molecular-weight antigenic fractions of a recent clinical isolate of Leishmania donovani against visceral leishmaniasis

T-cell mediated immune responses are key determinants to the natural course of infection caused by intracellular parasites such as Leishmania. Thus, T-cell activating proteins of these microbes continue to generate active interest particularly in view of their possible role in the design and development of newer and more effective vaccines. We have recently reported the presence of T-cell immunostimulatory antigens with the high-molecular-weight (MW) fractions (134-64.2 kDa) of whole Leishmania donovani antigen (strain 2001), which stimulated variable amounts of IFN-gamma, IL-12 and IL-10 in exposed immune individuals. The present study was undertaken to further evaluate these high-MW antigenic fractions (MW range >100-60 kDa) for potential protective efficacy. The high-MW region of the parasite was resolved into five antigenic fractions (Prep A-E) using continuous elution gel electrophoresis. Prior to in vivo protection studies in hamsters, these fractions were used to evaluate in vitro cellular responses in eight Leishmania-exposed individuals and treated cured hamsters. The protective efficacy of prep (A + B), C, D and E in combination with BCG was evaluated in inbred hamsters using standard immunization protocol. Proliferative responses were seen in all eight of eight exposed individuals to prep D [median stimulation index (SI): 5.2 (range 3.9-7.1)] and E [median SI: 5.6 (range 4.4-8.2)], five of eight individuals to prep B and prep C and three of eight to prep A [median SI: 0.2 (range 0.1-7.2)]. The median proliferative responses to prep D and prep E were significantly higher than to fraction prep A; (P < 0.05) but not to prep B and prep C. However, prep A-E induced equivalent levels of IFN-gamma, IL-10 and IL-12 cytokines. Fractions D and E also exhibited marked parasite inhibition in spleen (52.5% and 73.7%) and liver (65% and 80.2%) as compared with prep (A + B) (23% in spleen and 24% in liver) and prep C (38% in spleen and 24% in liver). Prep D and prep E vaccinated animals showed higher in vitro stimulatory responses (mean SI: 6.6 and 8.8) and nitric oxide (NO) induction (mean NO levels: 6.4 and 10.7 mug/ml) against whole cell extract as compared with other groups. The protection also correlated with presence of suppressed Leishmania-specific IgG levels in prep D and prep E immunized hamsters. These studies indicate the presence of immunostimulatory and protective molecules in 60-80 kDa region of L. donovani, which may be further exploited for developing a subunit vaccine.