Cell biology of host-parasite membrane interactions in leishmaniasis

Review of Leishmaniasis Treatment: Can We See the Forest through the Trees?

There are three known clinical syndromes of leishmaniasis: cutaneous (CL), mucocutaneous (MCL), and visceral disease (VL). In MCL and VL, treatment must be systemic (either oral or intravenous), while CL treatment options vary and include observation-only localized/topical treatment, oral medications, or parenteral drugs. Leishmaniasis treatment is difficult, with several factors to be considered. First, the efficacy of treatments varies among different species of parasites prevalent in different areas on the globe, with each species having a unique clinical presentation and resistance profile. Furthermore, leishmaniasis is a neglected tropical disease (NTD), resulting in a lack of evidence-based knowledge regarding treatment. Therefore, physicians often rely on case reports or case series studies, in the absence of randomized controlled trials (RCT), to assess treatment efficacy. Second, defining cure, especially in CL and MCL, may be difficult, as death of the parasite can be achieved in most cases, while the aesthetic result (e.g., scars) is hard to predict. This is a result of the biological nature of the disease, often diagnosed late in the course of disease (with possible keloid formation, etc.). Third, physicians must consider treatment ease of use and the safety profile of possible treatments. Thus, topical or oral treatments (for CL) are desirable and promote adherence. Fourth, the cost of the treatment is an important consideration. In this review, we aim to describe the diverse treatment options for different clinical manifestations of leishmaniasis. For each currently available treatment, we will discuss the various considerations mentioned above (efficacy, ease of use, safety, and cost).

Host–Pathogen Interaction in Leishmaniasis: Immune Response and Vaccination Strategies

Leishmaniasis is a zoonotic and vector-borne infectious disease that is caused by the genus Leishmania belonging to the trypanosomatid family. The protozoan parasite has a digenetic life cycle involving a mammalian host and an insect vector. Leishmaniasisis is a worldwide public health problem falling under the neglected tropical disease category, with over 90 endemic countries, and approximately 1 million new cases and 20,000 deaths annually. Leishmania infection can progress toward the development of species–specific pathologic disorders, ranging in severity from self-healing cutaneous lesions to disseminating muco-cutaneous and fatal visceral manifestations. The severity and the outcome of leishmaniasis is determined by the parasite’s antigenic epitope characteristics, the vector physiology, and most importantly, the immune response and immune status of the host. This review examines the nature of host–pathogen interaction in leishmaniasis, innate and adaptive immune responses, and various strategies that have been employed for vaccine development.

A comprehensive analysis of LACK (Leishmania homologue of receptors for activated C kinase) in the context of Visceral Leishmaniasis

The Leishmania homologue of activated C kinase (LACK) a known T cell epitope from soluble Leishmania antigens (SLA) that confers protection against Leishmania challenge. This antigen has been found to be highly conserved among Leishmania strains. LACK has been shown to be protective against L. donovani challenge. A comprehensive analysis of several LACK sequences was completed. The analysis shows a high level of conservation, lower variability and higher antigenicity in specific portions of the LACK protein. This information provides insights for the potential consideration of LACK as a putative candidate in the context of visceral Leishmaniasis vaccine target.

Delta-aminolevulinate-induced host-parasite porphyric disparity for selective photolysis of transgenic Leishmania in the phagolysosomes of mononuclear phagocytes: a potential novel platform for vaccine delivery

Leishmania double transfectants (DTs) expressing the 2nd and 3rd enzymes in the heme biosynthetic pathway were previously reported to show neogenesis of uroporphyrin I (URO) when induced with delta-aminolevulinate (ALA), the product of the 1st enzyme in the pathway. The ensuing accumulation of URO in DT promastigotes rendered them light excitable to produce reactive oxygen species (ROS), resulting in their cytolysis. Evidence is presented showing that the DTs retained wild-type infectivity to their host cells and that the intraphagolysosomal/parasitophorous vacuolar (PV) DTs remained ALA inducible for uroporphyrinogenesis/photolysis. Exposure of DT-infected cells to ALA was noted by fluorescence microscopy to result in host-parasite differential porphyrinogenesis: porphyrin fluorescence emerged first in the host cells and then in the intra-PV amastigotes. DT-infected and control cells differed qualitatively and quantitatively in their porphyrin species, consistent with the expected multi- and monoporphyrinogenic specificities of the host cells and the DTs, respectively. After ALA removal, the neogenic porphyrins were rapidly lost from the host cells but persisted as URO in the intra-PV DTs. These DTs were thus extremely light sensitive and were lysed selectively by illumination under nonstringent conditions in the relatively ROS-resistant phagolysosomes. Photolysis of the intra-PV DTs returned the distribution of major histocompatibility complex (MHC) class II molecules and the global gene expression profiles of host cells to their preinfection patterns and, when transfected with ovalbumin, released this antigen for copresentation with MHC class I molecules. These Leishmania mutants thus have considerable potential as a novel model of a universal vaccine carrier for photodynamic immunotherapy/immunoprophylaxis.

Intracellular targeting specificity of novel phthalocyanines assessed in a host-parasite model for developing potential photodynamic medicine

Photodynamic therapy, unlikely to elicit drug-resistance, deserves attention as a strategy to counter this outstanding problem common to the chemotherapy of all diseases. Previously, we have broadened the applicability of this modality to photodynamic vaccination by exploiting the unusual properties of the trypanosomatid protozoa, Leishmania, i.e., their innate ability of homing to the phagolysosomes of the antigen-presenting cells and their selective photolysis therein, using transgenic mutants endogenously inducible for porphyrin accumulation. Here, we extended the utility of this host-parasite model for in vitro photodynamic therapy and vaccination by exploring exogenously supplied photosensitizers. Seventeen novel phthalocyanines (Pcs) were screened in vitro for their photolytic activity against cultured Leishmania. Pcs rendered cationic and soluble (csPcs) for cellular uptake were phototoxic to both parasite and host cells, i.e., macrophages and dendritic cells. The csPcs that targeted to mitochondria were more photolytic than those restricted to the endocytic compartments. Treatment of infected cells with endocytic csPcs resulted in their accumulation in Leishmania-containing phagolysosomes, indicative of reaching their target for photodynamic therapy, although their parasite versus host specificity is limited to a narrow range of csPc concentrations. In contrast, Leishmania pre-loaded with csPc were selectively photolyzed intracellularly, leaving host cells viable. Pre-illumination of such csPc-loaded Leishmania did not hinder their infectivity, but ensured their intracellular lysis. Ovalbumin (OVA) so delivered by photo-inactivated OVA transfectants to mouse macrophages and dendritic cells were co-presented with MHC Class I molecules by these antigen presenting cells to activate OVA epitope-specific CD8+T cells. The in vitro evidence presented here demonstrates for the first time not only the potential of endocytic csPcs for effective photodynamic therapy against Leishmania but also their utility in photo-inactivation of Leishmania to produce a safe carrier to express and deliver a defined antigen with enhanced cell-mediated immunity.

A gp63 based vaccine candidate against Visceral Leishmaniasis

Visceral leishmaniasis is a macrophage associated disorder which leads to a profound decrease in the natural immunotherapeutic potential of the infected subjects to combat the disease. The major surface glycoprotein gp63 has been found to be a significant vaccine candidate against visceral leishmaniasis. The current study addresses the levels of similarity and identity in the gp63 obtained from different species of Leishmania viz donovoni, chagasi and infantum linked to the cause of visceral leishmaniasis. The results from BLAST, Phylogram and Cladogram studies indicate significant identity, similarity and conservation of important residues in the protein which lead us to conclude that a common gp63 based vaccine can be used as a therapeutical tool against visceral leishmaniasis caused by different species strains of leishmania.

Biological and biochemical characterization of tunicamycin-resistant Leishmania mexicana: mechanism of drug resistance and virulence

A parasitic protozoan, Leishmania mexicana amazonensis, was previously made resistant to tunicamycin (J.A. Kink and K.-P. Chang, Proc. Natl. Acad. Sci. USA 84:1253-1257, 1987). In the present study, six different tunicamycin-resistant variants were biologically and biochemically compared with their parental wild type to further delineate the mechanism of tunicamycin resistance and that of their virulence observed. In contrast to their parental wild type, all tunicamycin-resistant variants were found to grow and differentiate in tunicamycin-containing medium. The 50% lethal doses of tunicamycin for variants resistant to 10 or 80 micrograms of tunicamycin per ml were 20- and 100-fold higher, respectively, than that of the wild type. Specific activity of the microsomal N-acetylglucosamine-1-phosphate transferase was 4- to 12-fold higher in the tunicamycin-resistant cells than in their parental wild type and tunicamycin-sensitive revertants. The level of the enzyme activity is proportional to the degree of drug resistance. Inhibition kinetics studies showed that the enzyme from all groups was equally sensitive to the drug, with a 50% effective concentration of 1 to 1.3 micrograms of tunicamycin per ml. Thus, tunicamycin resistance of the variants is caused primarily by an increased level of their enzyme without alteration of its structure. Protein glycosylation determined by the incorporation of 2-D-[3H]mannose was about twofold higher in the tunicamycin-resistant variants than in their parental wild type. The increased glycosyltransferase activity in the latter apparently renders their protein glycosylation insensitive to the inhibition by tunicamycin. A major membrane glycoprotein of 63 kilodaltons (gp63) on the leishmania surface was found to be about threefold higher in the tunicamycin-resistant variants than in the wild type, as determined by immunoprecipitation with a monoclonal antibody specific for this antigen. Tunicamycin treatment of the wild type and tunicamycin-resistant variants caused changes in the electrophoretic mobility of this molecule, indicating a higher degree of its glycosylation in the latter cells. The tunicamycin-resistant variants parasitized macrophages in vitro more effectively than did the wild type, accounting for their virulence seen in mice. Thus, a high level of the glycosyltransferase enables the tunicamycin-resistant cells not only to overcome the inhibitory effect of tunicamycin on protein glycosylation but also to express their virulence, possibly by regulating N glycosylation of leishmanial proteins critical for leishmanias to establish intracellular parasitism.

Tunicamycin-resistant Leishmania mexicana amazonensis: expression of virulence associated with an increased activity of N-acetylglucosaminyltransferase and amplification of its presumptive gene

Tunicamycin at 10 micrograms/ml inhibits the growth and infectivity of the parasitic protozoan Leishmania mexicana amazonensis. Tunicamycin-resistant variants of this parasite were produced by gradual acclimatization of cells to increasing concentrations of the drug up to 80 micrograms/ml and a single-step selection of ethyl methanesulfonate-pretreated or differentiating leishmanias with the drug at 10 micrograms/ml. Prolonged exposure to the drug increases stability of drug resistance of those resistant to 10 micrograms/ml. Tunicamycin-resistant cells contain amplified DNA, which hybridizes in proportion to the cells' degree of drug resistance with Alg 7, a cloned DNA probe apparently encoding yeast N-acetylglucosaminyltransferase. This enzyme from all variants remained sensitive to inhibition by tunicamycin, but its specific activity was up to 15-fold higher than that of the wild type. Thus, amplification of the gene encoding this enzyme appears to result in its overproduction in the variants, accounting for their resistance to tunicamycin. The tunicamycin-resistant cells are more virulent to mice than their parental wild type. Thus, leishmanial virulence may be related to amplification or expression of gene(s) encoding enzymes involved in the regulation of N-glycosylation of parasite proteins.

The effect of tunicamycin on Leishmania brasiliensis. Glycosylation and the cell surface components

Culture conditions of Leishmania cells were developed to allow the study of the effect of tunicamycin (TM) on glycosylation and on the cell surface components. Leishmania incorporate [14C]-mannose and [35S]-methionine in vitro. The incorporation of [14C]-mannose is linear for 150 min and is inhibited by TM (2 micrograms/ml) in a time dependent effect which reaches a plateau of 45% inhibition at 36 h. Under the same experimental conditions [35S]-methionine incorporation into protein is slightly affected. This is reflected by an almost identical polypeptide pattern for TM treated and non-treated cells when analyzed on SDS-PAGE. On the contrary, strong differences were detected on the labeled compounds analyzed on SDS-PAGE followed by autoradiography when the precursor used was [14C]-mannose. A shift in the electrophoretic mobility of most of the glycopeptides synthesized in the presence of TM was observed, which is also reflected in the structure of the main Leishmania cell surface components. The findings are discussed in the light of biological implications.

The Leishmania receptor for macrophages is a lipid-containing glycoconjugate

The glycoconjugate of Leishmania major recognized by the monoclonal antibody WIC-79.3 exists in two forms. The cellular form associated with the promastigote is a population of amphipathic molecules consistent with membrane insertion. In contrast, the extracellular form mainly consists of hydrophilic molecules, and probably arises by cleavage of the cellular form by an endogenous phospholipase. The hydrophilic population of extracellular glycoconjugate molecules binds specifically to macrophages but not to T or B lymphoid cells. Binding of the glycoconjugate and also intact promastigotes to macrophages in vitro is specifically inhibited by Fab fragments of WIC-79.3. These data indicate that the L. major glycoconjugate is the parasite receptor for macrophages, and hence the molecule directly involved in the initiation of infection.