Probing the Molecular Mechanism of Hypericin-Induced Parasite Death Provides Insight into the Role of Spermidine beyond Redox Metabolism in Leishmania donovani

Hypericin, a natural compound from Hypericum perforatum (St. John''s wort), has been identified as a specific inhibitor of Leishmania donovani spermidine synthase (LdSS) using integrated computational and biochemical approaches. Hypericin showed in vitro inhibition of recombinant LdSS enzyme activity. The in vivo estimation of spermidine levels in Leishmania promastigotes after hypericin treatment showed significant decreases in the spermidine pools of the parasites, indicating target specificity of the inhibitor molecule. The inhibitor, hypericin, showed significant antileishmanial activity, and the mode of death showed necrosis-like features. Further, decreased trypanothione levels and increased glutathione levels with elevated reactive oxygen species (ROS) levels were observed after hypericin treatment. Supplementation with trypanothione in the medium with hypericin treatment restored in vivo trypanothione levels and ROS levels but could not prevent necrosis-like death of the parasites. However, supplementation with spermidine in the medium with hypericin treatment restored in vivo spermidine levels and parasite death was prevented to a large extent. The data overall suggest that the parasite death due to spermidine starvation as a result of LdSS inhibition is not related to elevated levels of reactive oxygen species. This suggests the involvement of spermidine in processes other than redox metabolism in Leishmania parasites. Moreover, the work provides a novel scaffold, i.e., hypericin, as a potent antileishmanial molecule.     

4-Amino Bis-Pyridinium Derivatives as Novel Antileishmanial Agents

The antileishmanial activity of a series of bis-pyridinium derivatives that are analogues of pentamidine have been investigated, and all compounds assayed were found to display activity against promastigotes and intracellular amastigotes of Leishmania donovani and Leishmania major, with 50% effective concentrations (EC₅₀s) lower than 1 μM in most cases. The majority of compounds showed similar behavior in both Leishmania species, being slightly more active against L. major amastigotes. However, compound VGP-106 {1,1′-(biphenyl-4,4′-diylmethylene)bis[4-(4-bromo-N-methylanilino)pyridinium] dibromide} exhibited significantly higher activity against L. donovani amastigotes (EC₅₀, 0.86 ± 0.46 μM) with a lower toxicity in THP-1 cells (EC₅₀, 206.54 ± 9.89 μM). As such, VGP-106 was chosen as a representative compound to further elucidate the mode of action of this family of inhibitors in promastigote forms of L. donovani. We have determined that uptake of VGP-106 in Leishmania is a temperature-independent process, suggesting that the compound crosses the parasite membrane by diffusion. Transmission electron microscopy analysis showed a severe mitochondrial swelling in parasites treated with compound VGP-106, which induces hyperpolarization of the mitochondrial membrane potential and a significant decrease of intracellular free ATP levels due to the inhibition of ATP synthesis. Additionally, we have confirmed that VGP-106 induces mitochondrial ROS production and an increase in intracellular Ca²⁺ levels. All these molecular events can activate the apoptotic process in Leishmania; however, propidium iodide assays gave no indication of DNA fragmentation. These results underline the potency of compound VGP-106, which may represent a new avenue for the development of novel antileishmanial compounds.     

Identification of Metal Dithiocarbamates as a Novel Class of Antileishmanial Agents

Dithiocarbamates have emerged as potent carbonic anhydrase (CA) inhibitors in recent years. Given that CAs are important players in cellular metabolism, the objective of this work was to exploit the CA-inhibitory property of dithiocarbamates as a chemotherapeutic weapon against the Leishmania parasite. We report here strong antileishmanial activity of three hitherto unexplored metal dithiocarbamates, maneb, zineb, and propineb. They inhibited CA activity in Leishmania major promastigotes at submicromolar concentrations and resulted in a dose-dependent inhibition of parasite growth. Treatment with maneb, zineb, and propineb caused morphological deformities of the parasite and Leishmania cell death with 50% lethal dose (LD₅₀) values of 0.56 μM, 0.61 μM, and 0.27 μM, respectively. These compounds were even more effective against parasites growing in acidic medium, in which their LD₅₀ values were severalfold lower. Intracellular acidosis leading to apoptotic and necrotic death of L. major promastigotes was found to be the basis of their leishmanicidal activity. Maneb, zineb, and propineb also efficiently reduced the intracellular parasite burden, suggesting that amastigote forms of the parasite are also susceptible to these metal dithiocarbamates. Interestingly, mammalian cells were unaffected by these compounds even at concentrations which are severalfold higher than their antileishmanial LD₅₀s). Our data thus establish maneb, zineb, and propineb as a new class of antileishmanial compounds having broad therapeutic indices.     

Green Synthesis of Silver and Titanium Dioxide Nanoparticles Using Euphorbia prostrata Extract Shows Shift from Apoptosis to G0/G1 Arrest followed by Necrotic Cell Death in Leishmania donovani

The aim of the present study was to synthesize silver (Ag) and titanium dioxide (TiO₂) nanoparticles (NPs) using green synthesis from aqueous leaf extract of Euphorbia prostrata as antileishmanial agents and to explore the underlying molecular mechanism of induced cell death. In vitro antileishmanial activity of synthesized NPs was tested against promastigotes of Leishmania donovani by alamarBlue and propidium iodide uptake assays. Antileishmanial activity of synthesized NPs on intracellular amastigotes was assessed by Giemsa staining. The leishmanicidal effect of synthesized Ag NPs was further confirmed by DNA fragmentation assay and by cell cycle progression and transmission electron microscopy (TEM) of the treated parasites. TEM analysis of the synthesized Ag NPs showed a spherical shape with an average size of 12.82 ± 2.50 nm, and in comparison to synthesized TiO₂ NPs, synthesized Ag NPs were found to be most active against Leishmania parasites after 24 h exposure, with 50% inhibitory concentrations (IC₅₀) of 14.94 μg/ml and 3.89 μg/ml in promastigotes and intracellular amastigotes, respectively. A significant increase in G₀/G₁ phase of the cell cycle with a subsequent decrease in S (synthesis) and G₂/M phases compared to controls was observed. The growth-inhibitory effect of synthesized Ag NPs was attributed to increased length of S phase. A decreased reactive oxygen species level was also observed, which could be responsible for the caspase-independent shift from apoptosis (G₀/G₁ arrest) to massive necrosis. High-molecular-weight DNA fragmentation as a positive consequence of necrotic cell death was also visualized. We also report that the unique trypanothione/trypanothione reductase (TR) system of Leishmania cells was significantly inhibited by synthesized Ag NPs. The green-synthesized Ag NPs may provide promising leads for the development of cost-effective and safer alternative treatment against visceral leishmaniasis.     

Statins and Voriconazole Induce Programmed Cell Death in Acanthamoeba castellanii

Members of the genus Acanthamoeba are facultative pathogens of humans, causing a sight-threatening keratitis and a life-threatening encephalitis. In order to treat those infections properly, it is necessary to target the treatment not only to the trophozoite but also to the cyst. Furthermore, it may be advantageous to avoid parasite killing by necrosis, which may induce local inflammation. We must also avoid toxicity of host tissue. Many drugs which target eukaryotes are known to induce programmed cell death (PCD), but this process is poorly characterized in Acanthamoeba. Here, we study the processes of programmed cell death in Acanthamoeba, induced by several drugs, such as statins and voriconazole. We tested atorvastatin, fluvastatin, simvastatin, and voriconazole at the 50% inhibitory concentrations (IC₅₀s) and IC₉₀s that we have previously established. In order to evaluate this phenomenon, we investigated the DNA fragmentation, one of the main characteristics of PCD, with quantitative and qualitative techniques. Also, the changes related to phosphatidylserine exposure on the external cell membrane and cell permeability were studied. Finally, because caspases are key to PCD pathways, caspase activity was evaluated in Acanthamoeba. All the drugs assayed in this study induced PCD in Acanthamoeba. To the best of our knowledge, this is the first study where PCD induced by drugs is described quantitatively and qualitatively in Acanthamoeba.     

Alkyl Galactofuranosides Strongly Interact with Leishmania donovani Membrane and Provide Antileishmanial Activity

We investigated the in vitro effects of four alkyl-galactofuranoside derivatives, i.e., octyl-β-d-galactofuranoside (compound 1), 6-amino-β-d-galactofuranoside (compound 2), 6-N-acetamido-β-d-galactofuranoside (compound 3), and 6-azido-β-d-galactofuranoside (compound 4), on Leishmania donovani. Their mechanism of action was explored using electron paramagnetic resonance spectroscopy (EPR) and nuclear magnetic resonance (NMR), and ultrastructural alterations were analyzed by transmission electron microscopy (TEM). Compound 1 showed the most promising effects by inhibiting promastigote growth at a 50% inhibitory concentration (IC₅₀) of 8.96 ± 2.5 μM. All compounds exhibit low toxicity toward human macrophages. Compound 1 had a higher selectivity index than the molecule used for comparison, i.e., miltefosine (159.7 versus 37.9, respectively). EPR showed that compound 1 significantly reduced membrane fluidity compared to control promastigotes and to compound 3. The furanose ring was shown to support this effect, since the isomer galactopyranose had no effect on parasite membrane fluidity or growth. NMR showed a direct interaction of all compounds (greatest with compound 1, followed by compounds 2, 3, and 4, in descending order) with the promastigote membrane and with octyl-galactopyranose and octanol, providing evidence that the n-octyl chain was primarily involved in anchoring with the parasite membrane, followed by the putative crucial role of the furanose ring in the antileishmanial activity. A morphological analysis of compound 1-treated promastigotes by TEM revealed profound alterations in the parasite membrane and organelles, but this was not the case with compound 3. Quantification of annexin V binding by flow cytometry confirmed that compound 1 induced apoptosis in >90% of promastigotes. The effect of compound 1 was also assessed on intramacrophagic amastigotes and showed a reduction in amastigote growth associated with an increase of reactive oxygen species (ROS) production, thus validating its promising effect.     

Elucidation of Cellular Mechanisms Involved in Experimental Paromomycin Resistance in Leishmania donovani

Leishmania donovani is the causative agent of the potentially fatal disease visceral leishmaniasis (VL). Chemotherapeutic options available to treat VL are limited and often face parasite resistance, inconsistent efficacy, and toxic side effects. Paromomycin (PMM) was recently introduced to treat VL as a monotherapy and in combination therapy. It is vital to understand the mechanisms of PMM resistance to safeguard the drug. In the present study, we utilized experimentally generated PMM-resistant L. donovani to elucidate the mechanisms of resistance and parasite biology. We found increased membrane fluidity accompanied by decreased intracellular drug accumulation in the PMM-resistant parasites. There were marked increases in gene expression of ATP-binding cassette (ABC) transporters (MDR1 and MRPA) and protein phosphatase 2A that evince increased drug efflux. Further, evaluation of parasite tolerance toward host leishmanicidal mechanisms revealed PMM-resistant parasites as being more tolerant to nitrosative stress at the promastigote and amastigote stages. The PMM-resistant parasites also predicted a better survival capacity, as indicated by resistance to complement-mediated lysis and increased stimulation of host interleukin-10 (IL-10) expression. The susceptibilities of PMM-resistant isolates to other antileishmanial agents (sodium antimony gluconate and miltefosine) remained unchanged. The data implicated the roles of altered membrane fluidity, decreased drug accumulation, increased expression of ABC transporters, and greater tolerance of parasites to host defense mechanisms in conferring PMM resistance in Leishmania.     

From Drug Screening to Target Deconvolution: a Target-Based Drug Discovery Pipeline Using Leishmania Casein Kinase 1 Isoform 2 To Identify Compounds with Antileishmanial Activity

Existing therapies for leishmaniases present significant limitations, such as toxic side effects, and are rendered inefficient by parasite resistance. It is of utmost importance to develop novel drugs targeting Leishmania that take these two limitations into consideration. We thus chose a target-based approach using an exoprotein kinase, Leishmania casein kinase 1.2 (LmCK1.2) that was recently shown to be essential for intracellular parasite survival and infectivity. We developed a four-step pipeline to identify novel selective antileishmanial compounds. In step 1, we screened 5,018 compounds from kinase-biased libraries with Leishmania and mammalian CK1 in order to identify hit compounds and assess their specificity. For step 2, we selected 88 compounds among those with the lowest 50% inhibitory concentration to test their biological activity on host-free parasites using a resazurin reduction assay and on intramacrophagic amastigotes using a high content phenotypic assay. Only 75 compounds showed antileishmanial activity and were retained for step 3 to evaluate their toxicity against mouse macrophages and human cell lines. The four compounds that displayed a selectivity index above 10 were then assessed for their affinity to LmCK1.2 using a target deconvolution strategy in step 4. Finally, we retained two compounds, PP2 and compound 42, for which LmCK1.2 seems to be the primary target. Using this four-step pipeline, we identify from several thousand molecules, two lead compounds with a selective antileishmanial activity.     

Studies on the Antileishmanial Mechanism of Action of the Arylimidamide DB766: Azole Interactions and Role of CYP5122A1

Arylimidamides (AIAs) are inspired by diamidine antimicrobials but show superior activity against intracellular parasites. The AIA DB766 {2,5-bis[2-(2-i-propoxy)-4-(2-pyridylimino)aminophenyl]furan hydrochloride} displays outstanding potency against intracellular Leishmania parasites and is effective in murine and hamster models of visceral leishmaniasis when given orally, but its mechanism of action is unknown. In this study, through the use of continuous DB766 pressure, we raised Leishmania donovani axenic amastigotes that displayed 12-fold resistance to this compound. These DB766-resistant (DB766R) parasites were 2-fold more sensitive to miltefosine than wild-type organisms and were hypersensitive to the sterol 14α-demethylase (CYP51) inhibitors ketoconazole and posaconazole (2,000-fold more sensitive and over 12,000-fold more sensitive than the wild type, respectively). Western blot analysis of DB766R parasites indicated that while expression of CYP51 is slightly increased in these organisms, expression of CYP5122A1, a recently identified cytochrome P450 associated with ergosterol metabolism in Leishmania, is dramatically reduced in DB766R parasites. In vitro susceptibility assays demonstrated that CYP5122A1 half-knockout L. donovani promastigotes were significantly less susceptible to DB766 and more susceptible to ketoconazole than their wild-type counterparts, consistent with observations in DB766R parasites. Further, DB766-posaconazole combinations displayed synergistic activity in both axenic and intracellular L. donovani amastigotes. Taken together, these studies implicate CYP5122A1 in the antileishmanial action of the AIAs and suggest that DB766-azole combinations are potential candidates for the development of synergistic antileishmanial therapy.     

Tafenoquine,an Antiplasmodial 8-Aminoquinoline,Targets Leishmania Respiratory Complex III and Induces Apoptosis

Tafenoquine (TFQ), an 8-aminoquinoline analogue of primaquine, which is currently under clinical trial (phase IIb/III) for the treatment and prevention of malaria, may represent an alternative treatment for leishmaniasis. In this work, we have studied the mechanism of action of TFQ against Leishmania parasites. TFQ impaired the overall bioenergetic metabolism of Leishmania promastigotes, causing a rapid drop in intracellular ATP levels without affecting plasma membrane permeability. TFQ induced mitochondrial dysfunction through the inhibition of cytochrome c reductase (respiratory complex III) with a decrease in the oxygen consumption rate and depolarization of mitochondrial membrane potential. This was accompanied by ROS production, elevation of intracellular Ca²⁺ levels and concomitant nuclear DNA fragmentation. We conclude that TFQ targets Leishmania mitochondria, leading to an apoptosis-like death process.Leishmaniasis includes a wide variety of clinical manifestations caused by the protozoan parasite Leishmania. Visceral leishmaniasis is the most severe form of the disease and is usually fatal if not treated (http://www.who.int/leishmaniasis/burden/en/). In the absence of a reliable vaccine, leishmaniasis treatment relies exclusively on chemotherapy. Resistance to organic pentavalent antimonials (until recently considered to be the standard treatment) in northeast India (4), together with the severe side effects associated with their use, has led to the use of alternative treatments based on the incorporation of drugs such as amphotericin B, miltefosine, and paromomycin into the arsenal of antileishmanial drugs (8). Nevertheless, the limited number of active drugs has prompted the WHO to recommend a combined therapy in order to extend the life expectancy of these compounds.Among the new drugs under development, sitamaquine (WR6026; GlaxoSmithKline), an 8-aminoquinoline, currently under phase IIb clinical trials, represents a promising drug for the oral treatment of leishmaniasis (35). In addition, another 8-aminoquinolines have been synthesized and evaluated for their leishmanicidal activity (29, 36). However, the leishmanicidal mechanism of 8-aminoquinolines is still unknown. Sitamaquine, for example, accumulates in the acidocalcisomes, but this organelle has been ruled out as its final target (17). The collapse of mitochondrial potential in digitonized Leishmania donovani promastigotes has also been reported (39). Tafenoquine (TFQ), formerly known as WR238605, is an analogue of primaquine with much lower toxicity than the parental drug. It has demonstrated significant leishmanicidal activity in the mouse experimental model (41) and may represent an alternative treatment for leishmaniasis.In the present study, we have shown that TFQ inhibits the mitochondrial cytochrome c reductase of Leishmania promastigotes. This inhibition causes a drop in the intracellular ATP levels of the parasite and the loss of mitochondrial membrane potential. TFQ induces ROS production and deregulation of Ca²⁺ homeostasis, followed by nicking and fragmentation of DNA in Leishmania promastigotes leading to an apoptosis-like death. Our results provide the first insight into the mechanistic lethal pathway of an 8-aminoquinoline in Leishmania. This information may be useful for the design of more specific and less toxic compounds against leishmaniasis.     

Comparison of a High-Throughput High-Content Intracellular Leishmania donovani Assay with an Axenic Amastigote Assay

Visceral leishmaniasis is a neglected tropical disease with significant health impact. The current treatments are poor, and there is an urgent need to develop new drugs. Primary screening assays used for drug discovery campaigns have typically used free-living forms of the Leishmania parasite to allow for high-throughput screening. Such screens do not necessarily reflect the physiological situation, as the disease-causing stage of the parasite resides inside human host cells. Assessing the drug sensitivity of intracellular parasites on scale has recently become feasible with the advent of high-content screening methods. We describe here a 384-well microscopy-based intramacrophage Leishmania donovani assay and compare it to an axenic amastigote system. A panel of eight reference compounds was tested in both systems, as well as a human counterscreen cell line, and our findings show that for most clinically used compounds both axenic and intramacrophage assays report very similar results. A set of 15,659 diverse compounds was also screened using both systems. This resulted in the identification of seven new antileishmanial compounds and revealed a high false-positive rate for the axenic assay. We conclude that the intramacrophage assay is more suited as a primary hit-discovery platform than the current form of axenic assay, and we discuss how modifications to the axenic assay may render it more suitable for hit-discovery.     

Identification of Synthetic and Natural Host Defense Peptides with Leishmanicidal Activity

Leishmania parasites are a major public health problem worldwide. Effective treatment of leishmaniasis is hampered by the high incidence of adverse effects to traditional drug therapy and the emergence of resistance to current therapeutics. A vaccine is currently not available. Host defense peptides have been investigated as novel therapeutic agents against a wide range of pathogens. Here we demonstrate that the antimicrobial peptide LL-37 and the three synthetic peptides E6, L-1018, and RI-1018 exhibit leishmanicidal activity against promastigotes and intramacrophage amastigotes of Leishmania donovani and Leishmania major. We also report that the Leishmania protease/virulence factor GP63 confers protection to Leishmania from the cytolytic properties of all l-form peptides (E6, L-1018, and LL-37) but not the d-form peptide RI-1018. The results suggest that RI-1018, E6, and LL-37 are promising peptides to develop further into components for antileishmanial therapy.     

Apoptotic Marker Expression in the Absence of Cell Death in Staurosporine-Treated Leishmania donovani

The protozoan parasite Leishmania donovani undergoes several developmental transitions in its insect and vertebrate hosts that are induced by environmental changes. The roles of protein kinases in these adaptive differentiation steps and their potential as targets for antiparasitic intervention are only poorly characterized. Here, we used the generic protein kinase inhibitor staurosporine to gain insight into how interference with phosphotransferase activities affects the viability, growth, and motility of L. donovani promastigotes in vitro. Unlike the nonkinase drugs miltefosine and amphotericin B, staurosporine strongly reduced parasite biosynthetic activity and had a cytostatic rather than a cytotoxic effect. Despite the induction of a number of classical apoptotic markers, including caspase-like activity and surface binding of annexin V, we determined that, on the basis of cellular integrity, staurosporine did not cause cell death but caused cell cycle arrest and abrogated parasite motility. In contrast, targeted inhibition of the parasite casein kinase 1 (CK1) protein family by use of the CK1-specific inhibitor D4476 resulted in cell death. Thus, pleiotropic inhibition of L. donovani protein kinases and possibly other ATP-binding proteins by staurosporine dissociates apoptotic marker expression from cell death, which underscores the relevance of specific rather than broad kinase inhibitors for antiparasitic drug development.     

N-Butyl-[1-(4-Methoxy)Phenyl-9H-β-Carboline]-3-Carboxamide Prevents Cytokinesis in Leishmania amazonensis

Leishmaniasis, a complex of diseases caused by protozoa of the genus Leishmania, is endemic in 98 countries, affecting approximately 12 million people worldwide. Current treatments for leishmaniasis have many disadvantages, such as toxicity, high costs, and prolonged treatment, making the development of new treatment alternatives highly relevant. Several studies have verified the antileishmanial activity of β-carboline compounds. In the present study, we investigated the in vitro antileishmanial activity of N-butyl-[1-(4-methoxy)phenyl-9H-β-carboline]-3-carboxamide (β-CB) against Leishmania amazonensis. The compound was active against promastigote, axenic amastigote, and intracellular amastigote forms of L. amazonensis, exhibiting high selectivity for the parasite. Moreover, β-CB did not exhibit hemolytic or mutagenic potential. Promastigotes treated with the alkaloid presented rounding of the body cell, cell membrane projections, an increase in the number of promastigotes presenting two flagella, and parasites of abnormal phenotype, with three or more flagella and/or nuclei. Furthermore, we observed an increase in the subpopulation of cells in the G₂/M stage of the cell cycle. Altogether, these results suggest that β-CB likely prevents cytokinesis, although it does not interfere with the duplication of cell structures. We also verified an increase in O₂^·− production and the accumulation of lipid storage bodies. Cell membrane integrity was maintained, in addition to the absence of phosphatidylserine externalization, DNA fragmentation, and autophagosomes. Although the possibility of an apoptotic process cannot be discarded, β-CB likely exerts its antileishmanial activity through a cytostatic effect, thus preventing cellular proliferation.     

MAPK1 of Leishmania donovani Modulates Antimony Susceptibility by Downregulating P-Glycoprotein Efflux Pumps

Emergence of resistance to pentavalent antimonials has become a severe obstacle in the treatment of visceral leishmaniasis (VL) in the Indian subcontinent. Mitogen-activated protein kinases (MAPKs) are well-known mediators of signal transduction of eukaryotes, regulating important processes, like proliferation, differentiation, stress response, and apoptosis. In Leishmania, MAPK1 has been shown to be consistently downregulated in antimony-resistant field isolates, suggesting that it has a role in antimony resistance. The present work investigates the molecular mechanism of MAPK1 in antimony resistance in Leishmania donovani. The L. donovani MAPK1 (LdMAPK1) single-allele replacement mutants exhibited increased resistance to Sb(III) (5.57-fold) compared to wild-type promastigotes, while overexpressing parasites became much more susceptible to antimony. The LdMAPK1-mediated drug sensitivity was directly related to antimony-induced apoptotic death of the parasite, as was evidenced by a 4- to 5-fold decrease in cell death parameters in deletion mutants and a 2- to 3-fold increase in MAPK1-overexpressing cells. LdMAPK1-underexpressing parasites also exhibited increased P-glycoprotein (P-gp)-mediated efflux pump activity, while a significant decrease in pump activity was observed in overexpressing cells. This change in efflux pump activity was directly related to expression levels of P-gp in all cell lines. However, episomal complementation of the gene restored normal growth, drug sensitivity, P-gp expression, and efflux pump activity. The data indicate that LdMAPK1 negatively regulates the expression of P-glycoprotein-type efflux pumps in the parasite. The decrease in efflux pump activity with an increase in LdMAPK1 expression may result in increased antimony accumulation in the parasite, making it more vulnerable to the drug.     

Inhibition of Phagolysosomal Biogenesis by the Leishmania Lipophosphoglycan

Whereas amastigotes of the protozoan parasite Leishmania proliferate inside acidic phagolysosomal vacuoles of the macrophage, vacuoles induced by Leishmania donovani promastigotes during initiation of infection are poorly characterized. Here, evidence is presented that interaction of these parasitophorous vacuoles with endocytic organelles is very limited. In contrast, vacuoles formed around L. donovani mutants lacking the cell surface lipophosphoglycan (LPG) fuse extensively with endosomes and lysosomes. The role of LPG repeating units in the inhibition of phagosome–endosome fusion was demonstrated using two different approaches. First, genetic complementation of the LPG-defective C3PO mutant restored its ability to inhibit phagosome–endosome fusion to a degree similar to that of wild-type promastigotes. Second, opsonization of C3PO mutant cells with purified L. donovani LPG also conferred to this mutant the ability to inhibit phagosome–endosome fusion. Inasmuch as LPG is essential for infecting macrophages, these results suggest that inhibition of phagolysosomal biogenesis by LPG repeating units represents an intramacrophage survival strategy used by promastigotes to establish infection.     

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.     

Behavior of Visceral Leishmania donovani in an Experimentally Induced T Helper Cell 2 (Th2)-Associated Response Model

Although implicated in the clinical expression of human visceral leishmaniasis, a disease-exacerbating T helper cell 2 (Th2)-associated immune response involving interleukin-4 (IL-4) and/ or IL-10 is not readily detectable in experimental visceral infection. To overcome this obstacle to analyzing visceral Leishmania donovani in this relevant immunopathogenetic environment, we sought a model in which a Th2 response induces noncuring infection. Four initial approaches were tested primarily in BALB/c mice which control intracellular L. donovani via an IL-12– and interferon-γ (IFN-γ)–dependent Th1 mechanism: (a) modifying the cytokine milieu when the parasite is first encountered (treatment with exogenous IL-4 or anti–IL-12), (b) providing sustained endogenous exposure to a Th2 cytokine (infection of IL-4 transgenic mice), (c) increasing the parasite challenge inoculum, and (d) injecting heat-killed L. major promastigotes (HKLMP) to induce a cross-reactive Th2 response to live L. donovani. Only the last approach generated a functional Th2-type response that induced disease exacerbation accompanied by inhibition of tissue granuloma assembly. In HKLMP-primed BALB/c mice, prophylaxis with anti–IL-4, anti–IL-10, or exogenous IL-12 (but not IFN-γ) readily restored resistance. In primed mice with established visceral infection, treatment with either IL-12 or IFN-γ also successfully induced antileishmanial activity. The results in this model (a) suggest that rather than acting alone, IL-4 and IL-10 may act in concert to prevent acquisition of resistance to L. donovani, (b) reemphasize the capacity of IL-12 to reverse early Th2-related effects, and (c) demonstrate that Th1 cytokines (IL-12, IFN-γ) have therapeutic action in an established systemic infection despite the presence of a disease-exacerbating Th2-type response.     

Thiopurine derivatives containing triazole and steroid: Synthesis, antimalarial and antileishmanial activities

A series of novel 6-thiopurine derivates containing 1,2,3-triazole were synthesized and their in vivo antimalarial activity and in vitro antileishmanial activity were examined. The compounds 10, 11, 12 and 14 presented higher values of inhibition of parasite multiplication than chloroquine. For antileishmanial activity, the compound 14 showed activity against the three species of Leishmania tested. None of compounds showed cytotoxicity against mammalian cells.