Leishmania Amastigote Target Antigens: The Challenge of a Stealthy Intracellular Parasite

Oxidative Stress-Mediated Overexpression of Uracil DNA Glycosylase in Leishmania donovani Confers Tolerance against Antileishmanial Drugs

Leishmania donovani is an intracellular protozoan parasite that causes endemic tropical disease visceral leishmaniasis (VL). Present drugs used against this fatal disease are facing resistance and toxicity issues. Survival of leishmania inside the host cells depends on the parasite's capacity to cope up with highly oxidative environment. Base excision repair (BER) pathway in L. donovani remains unexplored. We studied uracil DNA glycosylase (UNG), the key enzyme involved in BER pathway, and found that the glycosylase activity of recombinant LdUNG (Leishmania donovani UNG) expressed in E. coli is in sync with the activity of the parasite lysate under different reaction conditions. Overexpression of UNG in the parasite enhances its tolerance towards various agents which produce reactive oxygen species (ROS) and shows a higher infectivity in macrophages. Surprisingly, exposure of parasite to amphotericin B and sodium antimony gluconate upregulates the expression of UNG. Further, we found that the drug resistant parasites isolated from VL patients show higher expression of UNG. Mechanisms of action of some currently used drugs include accumulation of ROS. Our findings strongly suggest that targeting LdUNG would be an attractive therapeutic strategy as well as potential measure to tackle the problem of drug resistance in the treatment of leishmaniasis.

Antigen presenting cells targeting and stimulation potential of lipoteichoic acid functionalized lipo-polymerosome: a chemo-immunotherapeutic approach against intracellular infectious disease

Antigen presenting cells (APC) are well-recognized therapeutic targets for intracellular infectious diseases, including visceral leishmaniasis. These targets have raised concerns regarding their potential for drug delivery due to overexpression of a variety of receptors for pathogen associated molecular pathways after infection. Since, lipoteichoic acid (LTA), a surface glycolipid of Gram-positive bacteria responsible for recognition of bacteria by APC receptors that also regulate their activation for pro-inflammatory cytokine secretion, provides additive and significant protection against parasite. Here, we report the nanoarchitechture of APC focused LTA functionalized amphotericin B encapsulated lipo-polymerosome (LTA-AmB-L-Psome) delivery system mediated by self-assembly of synthesized glycol chitosan-stearic acid copolymer (GC-SA) and cholesterol lipid, which can activate and target the chemotherapeutic agents to Leishmania parasite resident APC. Greater J774A and RAW264.7 macrophage internalization of FITC tagged LTA-AmB-L-Psome compared to core AmB-L-Psome was observed by FACSCalibur cytometer assessment. This was further confirmed by higher accumulation in macrophage rich liver, lung and spleen during biodistribution study. The LTA-AmB-L-Psome overcame encapsulated drug toxicity and significantly increased parasite growth inhibition beyond commercial AmB treatment in both in vitro (macrophage-amastigote system; IC50, 0.082 ± 0.009 μg/mL) and in vivo (Leishmania donovani infected hamsters; 89.25 ± 6.44% parasite inhibition) models. Moreover, LTA-AmB-L-Psome stimulated the production of protective cytokines like interferon-γ (IFN-γ), interleukin-12 (IL-12), tumor necrosis factor-α (TNF-α), and inducible nitric oxide synthase and nitric oxide with down-regulation of disease susceptible cytokines, like transforming growth factor-β (TGF-β), IL-10, and IL-4. These data demonstrate the potential use of LTA-functionalized lipo-polymerosome as a biocompatible lucrative nanotherapeutic platform for overcoming toxicity and improving drug efficacy along with induction of robust APC immune responses for effective therapeutics of intracellular diseases.

Immunological consequences of stress-related proteins – cytosolic tryparedoxin peroxidase and chaperonin TCP20 – identified in splenic amastigotes ofLeishmania donovanias Th1 stimulatory, in experimental visceral leishmaniasis

In earlier studies, proteomic characterization of splenic amastigote fractions from clinical isolates ofLeishmania donovani, exhibiting significant cellular responses in curedLeishmaniasubjects, led to the identification of cytosolic tryparedoxin peroxidase (LdcTryP) and chaperonin-TCP20 (LdTCP20) as Th1-stimulatory proteins. Both the proteins, particularly LdTCP20 for the first time, were successfully cloned, overexpressed, purified and were found to be localized in the cytosol of purified splenic amastigotes. When evaluated against lymphocytes of curedLeishmania-infected hamsters, the purified recombinant proteins (rLdcTryP and rLdTCP20) induced their proliferations as well as nitric oxide production. Similarly, these proteins also generated Th1-type cytokines (IFN-γ/IL-12) from stimulated PBMCs of cured/endemicLeishmaniapatients. Further, vaccination with rLdcTryP elicited noticeable delayed-type hypersensitivity response and offered considerably good prophylactic efficacy (~78% inhibition) againstL. donovanichallenge in hamsters, which was well supported by the increased mRNA expression of Th1 and Th2 cytokines. However, animals vaccinated with rLdTCP20 exhibited comparatively lesser prophylactic efficacy (~55%) with inferior immunological response. The results indicate the potentiality of rLdcTryP protein, between the two, as a suitable anti-leishmanial vaccine. Since, rLdTCP20 is also an important target, for optimization, further attempts towards determination of immunodominant regions for designing fusion peptides may be taken up.

Canine leishmaniosis. Modulation of macrophage/lymphocyte interactions by L. infantum

Canine leishmaniosis, caused by Leishmania infantum, is a systemic disease with variable clinical signs and a progressive evolution. This disease is characterized by impaired T cell-mediated immune response, which has been associated with disease chronicity and high mortality. Protective immunity against leishmaniosis is thought to be mediated by T cell and cytokine production. The T cell activation requires a primary signal delivered by the major histocompatibility complex (MHC) molecules present on the surface of antigen presenting cells, and a non-specific signal generated by co-stimulatory molecules. To characterize canine immune responses in the presence of L. infantum parasites or their antigens, in vitro cell cultures of canine macrophages and lymphocytes were established, and the macrophages presenting MHC class II molecules were evaluated as well as the expression of IL-12 and CD80-86 co-stimulatory molecules and nitric oxide production. The results showed for the first time the up-regulation of MHC class II molecules on the surface in canine peripheral blood monocyte-derived macrophages during L. infantum infection in the presence of lymphocytes. In addition, a lack of co-stimulatory expression and a reduced release of nitric oxide were observed, suggesting a loss of T cell function and consequently an inactivation of the macrophage oxidative burst which, in turn, favors the survival of Leishmania. These results constitute a new contribution for the understanding of the interactions between L. infantum and the canine immune system.

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.

Computational analysis of cysteine proteases (Clan CA, Family Cl) of Leishmania major to find potential epitopic regions

Leishmania is associated with a broad spectrum of diseases, ranging from simple cutaneous to invasive visceral leishmaniasis. Here, the sequences of ten cysteine proteases of types A, B and C of Leishmania major were obtained from GeneDB database. Prediction of MHC class I epitopes of these cysteine proteases was performed by NetCTL program version 1.2. In addition, by using BcePred server, different structural properties of the proteins were predicted to find out their potential B cell epitopes. According to this computational analysis, nine regions were predicted as B cell epitopes. The results provide useful information for designing peptide-based vaccines.

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.

In the belly of the beast: subversion of macrophage proinflammatory signalling cascades during Toxoplasma gondii infection

Macrophages (MØ) are used as the intracellular niche by several bacterial and protozoan microorganisms. Such microbial pathogens adopt diverse strategies to avoid MØ microbicidal effects. Recent insights into the Toxoplasma gondii-MØ interaction reveal novel ways that intracellular parasites subvert MØ function. In contrast to some microbial pathogens, Toxoplasma infection is not silent but induces rapid activation of transcription factors such as STAT-1 and NFkappaB. However, the parasite blocks nuclear translocation of both factors, and MØ cannot produce IL-12 or TNF-alpha when subsequently triggered with lipopolysaccharide. The nuclear import blockade is lifted 24 h after infection, but cells remain actively suppressed in TNF-alpha production. Nevertheless, IL-12 synthesis is initiated at this later time point. Toxoplasma gondii-induced production of this cytokine occurs through both MyD88- and CCR5-dependent pathways. The balance of cytokine subversion and stimulation during infection probably results from the parasite's need to simultaneously avoid immune elimination and trigger immunity to prevent host death.

In vitro cultivation and characterization of axenic amastigotes of Leishmania

The establishment of axenic cultures of the amastigote stage of Leishmania is important to understand the mechanisms regulating the differentiation, survival and pathogenicity of the parasite with a view to develop and identify molecular and chemotherapeutic targets. Recent developments in axenic culture and the characterization of amastigotes of different species of Leishmania are discussed.

Leishmania-host-cell interaction: complexities and alternative views

Leishmania are protozoan parasites that infect various mammalian species, including humans. It is generally thought that random attachment of the flagellated promastigotes to mononuclear phagocytes initiates their uptake via circumferential pseudopods. Intracellularly, the promastigotes become located in phagolysosomes in which they transform to and survive as 'aflagellated' amastigotes that hide their shortened flagellum within the flagellar pocket. Unrestricted replication of these amastigotes is assumed to cause the eventual burst of the host cell, thereby releasing the infectious parasites. Here, Mike Rittig and Christian Bogdan review a large body of literature containing potentially important but poorly appreciated findings, which together with recent results, argue for Leishmania-host-cell interactions that are much more complex than generally thought.

Presentation of the Leishmania antigen LACK by infected macrophages is dependent upon the virulence of the phagocytosed parasites

We have previously demonstrated that murine macrophages (Mphi) infected with Leishmania promastigotes, in contrast to Mphi infected with the amastigote stage of these parasites, are able to present the Leishmania antigen LACK (Leishmania homologue of receptors for activated C kinase) to specific, I-Ad-restricted T cell hybrids and to the T cell clone 9.1-2. These T cells react with the LACK (158-173) peptide, which is immunodominant in BALB/c mice. Here, we show that the level of stimulation of the LACK-specific T cell hybridoma OD12 by promastigote-infected Mphi is clearly dependent upon the differentiation state of the internalized parasites. Thus, shortly after infection with log-phase or stationary-phase promastigotes of L. major or of L. amazonensis, Mphi strongly activated OD12. The activity was transient and rapidly lost. However, under the same conditions, activation of OD12 by Mphi infected with metacyclic promastigotes of L. major or of L. amazonensis was barely detectable. At the extreme, Mphi infected with amastigotes were incapable to stimulate OD12. Thus, the presentation of LACK by infected Mphi correlates with the degree of virulence of the phagocytosed parasites, the less virulent being the best for the generation/expression of LACK (158-173)-I-Ad complexes. While the intracellular killing of the parasites appears to be an important condition for the presentation of LACK, it is not the only requisite. The partial or total destruction of intracellular L. amazonensis amastigotes does not allow the presentation of LACK to OD12. A preferential interaction of LACK (158-173) with recycling rather than newly synthesized MHC class II molecules does not explain the transient presentation of LACK by Mphi infected with log-phase or stationary-phase promastigotes because brefeldin A strongly inhibited the presentation of LACK to OD12. Taken together, these results suggest that virulent stages of Leishmania, namely metacyclics and amastigotes, have evolved strategies to avoid or minimize their recognition by CD4+ T lymphocytes.

How do protozoan parasites survive inside macrophages?

During infections with intracellular microbes, macrophages have two roles. On the one hand, they are important effector cells for the control and killing of intracellular bacteria and protozoan parasites by oxidative and non-oxidative mechanisms. On the other hand, macrophages may also serve as long-term host cells that facilitate the replication and survival of the pathogens, for example, by protecting them against toxic components of the extracellular milieu. In this review, Christian Bogdan and Martin Röllinghoff summarize some of the more recently discovered mechanisms by which intracellular protozoan parasites, such as Leishmania spp, Trypanosoma cruzi and Toxoplasma gondii, manage to exploit macrophages as safe target cells.

Roles of cysteine proteinases of trypanosomes and Leishmania in host-parasite interactions

Trypanosomes and Leishmania contain an abundance of stage-regulated cysteine proteinases encoded by several gene families. Analysis of parasites rendered defective in cysteine proteinase function, either through genetic manipulation or through the use of specific inhibitors, has revealed roles for the enzymes in parasite virulence, in modulation of the host's immune response and in parasite differentiation.