Possibility of membrane modification as a mechanism of antimony resistance in Leishmania donovani

Integrating genomics and proteomics permits identification of immunodominant antigens associated with drug resistance in human visceral leishmaniasis in India

Resistance of human pathogens like Leishmania to drugs is a growing concern where the multidrug-resistant phenotype renders chemotherapy ineffective. The acquired resistance of Leishmania to antimony has promoted intense research on the mechanisms involved but the question has not been resolved yet. In this study we have explored host-pathogen- drug interactions leading to identification of pharmacological determinants of host macrophages that resist the sodium antimony gluconate (SAG) mediated intracellular parasite killing. mRNA profiling of mammalian host stage amastigotes of sodium antimony gluconate (SAG) 'sensitive' and 'resistant' parasite lines was carried out using Affymetrix GeneChip^® Human Genome U133 Plus 2.0 Array. Patient sera was used to identify immunogenic proteins by two-dimensional gel analysis (2DE) and mass spectrometric analysis (LC-MS/MS). Immunofluorescence microscopy confirmed the identities on 'sensitive' and 'resistant' parasite lines. A total of nine immunogenic proteins whose intensities changed significantly and consistently in multiple experiments were detected, suggesting that a cohort of proteins are altered in expression levels in the 'resistant' parasites. Global expression profiling using microarrays revealed this regulation was not reflected by changes in the levels of the cognate mRNAs. Following identification of proteins by mass spectrometry, one such regulated protein, enolase, was chosen for more detailed analysis. Immunofluorescence microscopy employing antisera against this enzyme confirmed that its level was differentially regulated in the 'resistant' isolate. We show that high serum level of immunoreactive protein is associated with 'resistant' phenotype. Differentially expressed proteins with immunomodulatory activities were found to be associated with the 'resistant phenotype'.

Cross-resistance of Leishmania infantum isolates to nitric oxide from patients refractory to antimony treatment, and greater tolerance to antileishmanial responses by macrophages

Visceral leishmaniasis is a life-threatening disease characterized by intense parasitism of the spleen, liver, and bone marrow. Antimonials have served as front-line antileishmanial therapeutics for decades, but the increasing failure rates under antimonial treatment have challenged the continued use of these drugs. Pentavalent antimonials are known to reinforce the killing mechanisms of macrophages, although the associated mechanism remains unclear. Here, for the first time, we determined whether Leishmania infantum strains isolated from patients refractory to antimony treatment (relapse cases) were cross-resistant to antimonials, liposomal amphotericin B, and/or nitric oxide, and also whether these strains modulate macrophage infection. We selected four clinical isolates from relapse cases and two clinical isolates from antimony-responsive patients (control group) for the present study. The L. infantum promastigotes from all four relapse cases were resistant to trivalent antimonial treatment and nitric oxide, while only one isolate was resistant to liposomal amphotericin B. We evaluated whether the resistant strains from relapse cases showed enhanced infectivity and amastigote survival in macrophages, or macrophage-killing mechanisms in macrophages activated by lipopolysaccharide plus interferon gamma. Infection indexes calculated using macrophages infected with isolates from relapse were higher than those observed with control strains that were stimulated independently. Macrophage infection was higher with L. infantum isolates from relapse cases and correlated with enhanced interleukin 1-β production but showed similar nitrite production. Our results demonstrate that L. infantum field isolates from relapse cases were resistant to antimonials and nitric oxide and that these parasites stimulated inflammatory cytokines and were resistant to macrophage-killing mechanisms, factors that may contribute to disease severity.

Comparative Fitness of a Parent Leishmania donovani Clinical Isolate and Its Experimentally Derived Paromomycin-Resistant Strain

Paromomycin has recently been introduced for the treatment of visceral leishmaniasis and emergence of drug resistance can only be appropriately judged upon its long term routine use in the field. Understanding alterations in parasite behavior linked to paromomycin-resistance may be essential to assess the propensity for emergence and spread of resistant strains. A standardized and integrated laboratory approach was adopted to define and assess parasite fitness of both promastigotes and amastigotes using an experimentally induced paromomycin-resistant Leishmania donovani strain and its paromomycin-susceptible parent wild-type clinical isolate. Primary focus was placed on parasite growth and virulence, two major components of parasite fitness. The combination of in vitro and in vivo approaches enabled detailed comparison of wild-type and resistant strains for which no differences could be demonstrated with regard to promastigote growth, metacyclogenesis, in vitro infectivity, multiplication in primary peritoneal mouse macrophages and infectivity for Balb/c mice upon infection with 2 x 10⁷ metacyclic promastigotes. Monitoring of in vitro intracellular amastigote multiplication revealed a consistent decrease in parasite burden over time for both wild-type and resistant parasites, an observation that was subsequently also confirmed in a larger set of L. donovani clinical isolates. Though the impact of these findings should be further explored, the study results suggest that the epidemiological implications of acquired paromomycin-resistance may remain minimal other than the loss of one of the last remaining drugs effective against visceral leishmaniasis.

The overexpression of genes of thiol metabolism contribute to drug resistance in clinical isolates of visceral leishmaniasis (kala azar) in India

Background

Visceral leishmaniasis (VL), also called Kala Azar (KA) or black fever in India, claims around 20,000 lives every year. Chemotherapy remains one of the most important tools in the control of VL. Current chemotherapy for Kala Azar in India relies on a rather limited arsenal of drugs including sodium antimony gluconate and amphotericin B in addition to the very expensive drug miltefosine. Pentavalent antimonials have been used for more than half a century in the therapy of leishmaniasis as it is relatively safe and inexpensive, however, the spread of resistance to this drug is forcing clinicians in India to abandon this treatment. Consequently, improvement of antimonial chemotherapy has become a major challenging area of study by leishmaniacs worldwide. The alarming emergence of resistance to the commonly used antleishmanial drug, sodium antimony gluconate, in India, has led us to elucidate the resistance mechanism(s) in clinical isolates. Studies on laboratory mutants have shown that resistance to antimonials is highly dependent on thiol levels. The parasite evades cytotoxic effects of antimonial therapy by enhanced efflux of drug upon conjugation with thiols, through overexpressed membrane proteins belonging to the superfamily of ABC transporters.

Methods

We have carried out functional studies to determine the activity of the efflux pumps in antimonial resistant clinical isolates collected from disease endemic areas in India and also carried out molecular characterization of thiol levels in these parasites.

Results

Overexpression of the gene coding for γ glutamylcysteine synthetase was observed in these resistant clinical isolates thereby establishing that thiols represent the key determinants of antimonial resistance. The SbIII/thiol conjugates can be sequestered by ABC transporter multidrug resistance protein A (MRPA) into intracellular organelles or can be directly pumped out by an uncharacterized transporter.

Conclusions

Our studies investigating antimonial resistance in different L. donovani clinical isolates suggest that over functioning of MRP plays a role in generation of antimony resistance phenotype in some L. donovani clinical isolates.

Antimony resistance in leishmania, focusing on experimental research

Leishmaniases are parasitic diseases that spread in many countries with a prevalence of 12 million cases. There are few available treatments and antimonials are still of major importance in the therapeutic strategies used in most endemic regions. However, resistance toward these compounds has recently emerged in areas where the replacement of these drugs is mainly limited by the cost of alternative molecules. In this paper, we reviewed the studies carried out on antimonial resistance in Leishmania. Several common limitations of these works are presented before prevalent approaches to evidence antimonial resistance are related. Afterwards, phenotypic determination of resistance is described, then confronted to clinical outcome. Finally, we detail molecular mechanisms and targets involved in resistance and already identified in vitro within selected mutant strains or in clinical isolates.

Bioinformatic Analysis of Leishmania donovani Long-Chain Fatty Acid-CoA Ligase as a Novel Drug Target

Fatty acyl-CoA synthetase (fatty acid: CoA ligase, AMP-forming; (EC 6.2.1.3)) catalyzes the formation of fatty acyl-CoA by a two-step process that proceeds through the hydrolysis of pyrophosphate. Fatty acyl-CoA represents bioactive compounds that are involved in protein transport, enzyme activation, protein acylation, cell signaling, and transcriptional control in addition to serving as substrates for beta oxidation and phospholipid biosynthesis. Fatty acyl-CoA synthetase occupies a pivotal role in cellular homeostasis, particularly in lipid metabolism. Our interest in fatty acyl-CoA synthetase stems from the identification of this enzyme, long-chain fatty acyl-CoA ligase (LCFA) by microarray analysis. We found this enzyme to be differentially expressed by Leishmania donovani amastigotes resistant to antimonial treatment. In the present study, we confirm the presence of long-chain fatty acyl-CoA ligase gene in the genome of clinical isolates of Leishmania donovani collected from the disease endemic area in India. We predict a molecular model for this enzyme for in silico docking studies using chemical library available in our institute. On the basis of the data presented in this work, we propose that long-chain fatty acyl-CoA ligase enzyme serves as an important protein and a potential target candidate for development of selective inhibitors against leishmaniasis.

Use of antimony in the treatment of leishmaniasis: current status and future directions

In the recent past the standard treatment of kala-azar involved the use of pentavalent antimonials Sb(V). Because of progressive rise in treatment failure to Sb(V) was limited its use in the treatment program in the Indian subcontinent. Until now the mechanism of action of Sb(V) is not very clear. Recent studies indicated that both parasite and hosts contribute to the antimony efflux mechanism. Interestingly, antimonials show strong immunostimulatory abilities as evident from the upregulation of transplantation antigens and enhanced T cell stimulating ability of normal antigen presenting cells when treated with Sb(V) in vitro. Recently, it has been shown that some of the peroxovanadium compounds have Sb(V)-resistance modifying ability in experimental infection with Sb(V) resistant Leishmania donovani isolates in murine model. Thus, vanadium compounds may be used in combination with Sb(V) in the treatment of Sb(V) resistance cases of kala-azar.

Leishmania donovani isolates with antimony-resistant but not -sensitive phenotype inhibit sodium antimony gluconate-induced dendritic cell activation

The inability of sodium antimony gluconate (SAG)-unresponsive kala-azar patients to clear Leishmania donovani (LD) infection despite SAG therapy is partly due to an ill-defined immune-dysfunction. Since dendritic cells (DCs) typically initiate anti-leishmanial immunity, a role for DCs in aberrant LD clearance was investigated. Accordingly, regulation of SAG-induced activation of murine DCs following infection with LD isolates exhibiting two distinct phenotypes such as antimony-resistant (Sb^RLD) and antimony-sensitive (Sb^SLD) was compared in vitro. Unlike Sb^SLD, infection of DCs with Sb^RLD induced more IL-10 production and inhibited SAG-induced secretion of proinflammatory cytokines, up-regulation of co-stimulatory molecules and leishmanicidal effects. Sb^RLD inhibited these effects of SAG by blocking activation of PI3K/AKT and NF-κB pathways. In contrast, Sb^SLD failed to block activation of SAG (20 µg/ml)-induced PI3K/AKT pathway; which continued to stimulate NF-κB signaling, induce leishmanicidal effects and promote DC activation. Notably, prolonged incubation of DCs with Sb^SLD also inhibited SAG (20 µg/ml)-induced activation of PI3K/AKT and NF-κB pathways and leishmanicidal effects, which was restored by increasing the dose of SAG to 40 µg/ml. In contrast, Sb^RLD inhibited these SAG-induced events regardless of duration of DC exposure to Sb^RLD or dose of SAG. Interestingly, the inhibitory effects of isogenic Sb^SLD expressing ATP-binding cassette (ABC) transporter MRPA on SAG-induced leishmanicidal effects mimicked that of Sb^RLD to some extent, although antimony resistance in clinical LD isolates is known to be multifactorial. Furthermore, NF-κB was found to transcriptionally regulate expression of murine γglutamylcysteine synthetase heavy-chain (mγGCS_hc) gene, presumably an important regulator of antimony resistance. Importantly, Sb^RLD but not Sb^SLD blocked SAG-induced mγGCS expression in DCs by preventing NF-κB binding to the mγGCS_hc promoter. Our findings demonstrate that Sb^RLD but not Sb^SLD prevents SAG-induced DC activation by suppressing a PI3K-dependent NF-κB pathway and provide the evidence for differential host-pathogen interaction mediated by Sb^RLD and Sb^SLD.

Kala-azar, a life-threatening parasitic disease caused by Leishmania donovani (LD), is widening its base in different parts of the world. Currently, there is no effective vaccine against kala-azar. The antimonial drugs like sodium antimony gluconate (SAG) have been the mainstay of therapy for this disease. Recently, due to the emergence of antimony-resistance in parasites, SAG often fails to cure kala-azar patients, which is compounding the disaster further. It is still unknown how infection with LD exhibiting antimony-resistant phenotype, in contrast to antimony-sensitive phenotype, is handled by the kala-azar patients upon SAG treatment. This demands an understanding of the nature of host immune responses against these two distinct categories of parasites. Accordingly, we compared the impact of infection with LD exhibiting antimony-resistant versus antimony-sensitive phenotype on dendritic cells (DCs). DCs upon activation/maturation initiate anti-leishmanial immunity. We showed that parasites with antimony-resistant but not antimony-sensitive phenotype prevented SAG-induced DC activation/maturation by blocking activation of NF-κB. The latter is a key signaling pathway regulating DC activation/maturation. Our studies for the first time provide both a cellular and molecular basis for differential response of host cells to parasite isolates with antimony-resistant and antimony-sensitive phenotype, which may influence the outcome of the disease.

Drug resistance in visceral leishmaniasis

Visceral leishmaniasis remains a public health problem worldwide. This illness was included by the World Health Organization in the list of neglected tropical diseases targeted for elimination by 2015. The widespread emergence of resistance to pentavalent antimonials in India where half cases occur globally and the unavailability of a vaccine in clinical use constitute major obstacles in achieving of this goal. The last decade new antileishmanials became available, including the oral agent miltefosine. However, in poor endemic countries their wide use was curtailed because of the high costs, and also due to concerns of toxicity and emergence of resistance. Various mechanisms of antileishmanial resistance were identified recently in field isolates. Their elucidation will boost the design of new drugs and the molecular surveillance of resistance. Combination regimens should be evaluated in large trials. Overall, the development of antileishmanials has been generally slow; new drugs are needed. In order to control visceral leishmaniasis worldwide, treatment advances should become affordable in the poorest countries, where they are needed most.

In vitro susceptibility of field isolates of Leishmania donovani to Miltefosine and amphotericin B: correlation with sodium antimony gluconate susceptibility and implications for treatment in areas of endemicity

Indian Leishmania donovani isolates (n = 19) from regional zones representing various levels of antimony resistance displayed significantly (P < 0.01) correlated results with respect to in vitro susceptibility to the antileishmanial drugs sodium antimony gluconate, amphotericin B, and Miltefosine, raising the possibility of cross-resistance mechanisms operating in the field isolates. The results of gene expression analysis of LdMT and LdRos3 were suggestive of alternate mechanisms of Miltefosine susceptibility in the isolates.

Proteophosphoglycan is differentially expressed in sodium stibogluconate-sensitive and resistant Indian clinical isolates of Leishmania donovani

Leishmania produce several types of mucin-like glycoproteins called proteophosphoglycans (PPGs) some of which are secreted while others are found on the surface of promastigotes and amastigotes. These proteins are thought to be important in the transmission, invasion and subsequent intracellular survival of parasites. The structure and function of PPGs are species and stage-specific in the case of L. major and L. mexicana, but no such information has hitherto been available for L. donovani. This study presents, for the first time, an initial characterization (localization) of PPG in sodium stibogluconate (SSG)-resistant and sensitive clinical isolates of L. donovani from Bihar (India) by confocal microscopy, flow cytometry and Western blotting using antibodies to L. major PPG. Confocal microscopy analysis revealed that both promastigotes and amastigotes possess PPG on their cell membrane and flagellar pocket membrane but its expression was variable in different isolates. The quantitative analysis by FACS and Western blotting showed that the expression and intensity of PPG bands was higher in SSG-resistant isolates. This study suggests the possibilities of involvement of PPG in drug-resistant mechanisms and of using PPG abundance as a marker for identifying drug-resistant clinical isolates in Indian kala azar.