Developmental modification of the lipophosphoglycan from Leishmania major promastigotes during metacyclogenesis

Gels and cells: the Leishmania biofilm as a space and place for parasite transmission

Leishmania make an abundant glycoprotein and proteophosphoglycan-rich gel, called the promastigote secretory gel, in the anterior midgut of their sand fly vector. This gel is a multi-faceted virulence factor which promotes the survival and transmission of the parasites between hosts. Here, we present the case that Leishmania parasites embedded in the promastigote secretory gel should be redefined as a biofilm as it shares striking similarities in biogenesis, form, and function with biofilms of other unicellular organisms. We believe that this reinterpretation will stimulate new hypotheses and avenues of research to improve our understanding of the developmental programme of Leishmania and the interaction these parasites and other kinetoplastids have with their insect hosts.

Phenotypical Differences between Leishmania (Leishmania) amazonensis PH8 and LV79 Strains May Impact Survival in Mammal Host and in Phlebotomine Sand Flies

We previously showed that L. (Leishmania) amazonensis promastigotes and amastigotes of the PH8 strain generated larger lesions in mice than LV79, and that lesion-derived amastigotes from the two strains differ in their proteomes. We recently reported that PH8 promastigotes are more phagocytized by macrophages. Promastigotes' membrane-enriched proteomes showed several differences, and samples of each strain clustered based on proteomes. In this paper, we show phenotypic differences between PH8 and LV79 promastigotes that may explain the higher virulence of PH8. We compared in vitro macrophage infections by day 4 (early) and day 6 (late stationary phase) cultures, resistance to complement, and LPG characteristics. PH8 promastigotes showed a higher infectivity and were more resistant to murine complement. LPG was different between the strains, which may influence the interaction with macrophages and survival to complement. We compared the infection of the permissive vector Lutzomyia longipalpis. PH8 was more abundant in the vector's gut 72 h after feeding, which is a moment where blood digestion is finished and the parasites are exposed to the gut environment. Our results indicate that PH8 promastigotes are more infective, more resistant to complement, and infect the permissive vector more efficiently. These data suggest that PH8 is probably better adapted to the sand fly and more prone to survive in the vertebrate host.

How to get away with murder: The multiple strategies employed by pathogenic protozoa to avoid complement killing

Parasitic protozoa are eukaryotic unicellular organisms that depend on a variety of living organisms and can develop intra- and extracellularly inside their hosts. In humans, these parasites cause diseases with a significant impact on public health, such as malaria, toxoplasmosis, Chagas disease, leishmaniasis and amebiasis. The ability of a parasite in establishing a successful infection depends on a series of intricate evolutionarily selected adaptations, which include the development of molecular and cellular strategies to evade the host immune system effector mechanisms. The complement system is one of the main effector mechanisms and the first humoral shield of hosts innate immunity against pathogens. For unicellular pathogens, such as protozoa, bacteria and fungi, the activation of the complement system may culminate in the elimination of the invader mainly via 1- the formation of a pore that depolarizes the plasma membrane of the parasite, causing cell lysis; 2- opsonization and killing by phagocytes; 3- increasing vascular permeability while also recruiting neutrophils to the site of activation. Numerous strategies to avoid complement activation have been reported for parasitic protozoa, such as 1- sequestration of complement system regulatory proteins produced by the host, 2- expression of complement system regulatory proteins, 3- proteolytic cleavage of different complement effector molecules, 4- formation of a physical glycolipid barrier that prevents deposition of complement molecules on the plasma membrane, and 5- removal, by endocytosis, of complement molecules bound to plasma membrane. In this review, we revisit the different strategies of blocking various stages of complement activation described for the main species of parasitic protozoa, present the most recent discoveries in the field and discuss new perspectives on yet neglected strategies and possible new evasion mechanisms.

The effect of the sugar metabolism on Leishmania infantum promastigotes inside the gut of Lutzomyia longipalpis: A sweet relationship?

It is well-known that Leishmania parasites can alter the behavior of the sand fly vector in order to increase their transmission potential. However, little is known about the contribution of the infecting host’s blood composition on subsequent sand fly infection and survival. This study focused on the host’s glucose metabolism and the insulin/insulin-like growth factor 1 (IGF-1) pathway as both metabolic processes are known to impact vector-parasite interactions of other protozoa and insect species. The focus of this study was inspired by the observation that the glycemic levels in the blood of infected Syrian golden hamsters inversely correlated to splenic and hepatic parasite burdens. To evaluate the biological impact of these findings on further transmission, Lutzomyia longipalpis sand flies were infected with blood that was artificially supplemented with different physiological concentrations of several monosaccharides, insulin or IGF-1. Normoglycemic levels resulted in transiently higher parasite loads and faster appearance of metacyclics, whereas higher carbohydrate and insulin/IGF-1 levels favored sand fly survival. Although the recorded effects were modest or transient of nature, these observations support the concept that the host blood biochemistry may affect Leishmania transmission and sand fly longevity.

Past research on the interaction between the Leishmania parasite and the sand fly vector has revealed that Leishmania is capable of changing vector behavior to favor transmission of parasites in the environment. Little is known about the impact of host blood composition on parasite development inside the vector and on vector survival. Here, we showed that parasite burdens in the spleen and the liver inversely correlated to the serum blood glucose levels of infected animals, which triggered us to further investigate the effect of blood monosaccharides, insulin and insulin-like growth factor 1 (IGF-1) on sand fly infection and survival. We demonstrated that normal serum glucose levels in the initial parasitized blood meal resulted in transiently higher parasite loads and a faster appearance of infectious parasites, whereas higher sugar and insulin/IGF-1 levels favored sand fly survival, which supports the concept that the host blood biochemistry may affect Leishmania transmission and sand fly longevity.

Insights into Leishmania Molecules and Their Potential Contribution to the Virulence of the Parasite

Neglected parasitic diseases affect millions of people worldwide, resulting in high morbidity and mortality. Among other parasitic diseases, leishmaniasis remains an important public health problem caused by the protozoa of the genus Leishmania, transmitted by the bite of the female sand fly. The disease has also been linked to tropical and subtropical regions, in addition to being an endemic disease in many areas around the world, including the Mediterranean basin and South America. Although recent years have witnessed marked advances in Leishmania-related research in various directions, many issues have yet to be elucidated. The intention of the present review is to give an overview of the major virulence factors contributing to the pathogenicity of the parasite. We aimed to provide a concise picture of the factors influencing the reaction of the parasite in its host that might help to develop novel chemotherapeutic and vaccine strategies.

Proteins involved in the biosynthesis of lipophosphoglycan in Leishmania: a comparative genomic and evolutionary analysis

BACKGROUND

Leishmania spp. are digenetic parasites capable of infecting humans and causing a range of diseases collectively known as leishmaniasis. The main mechanisms involved in the development and permanence of this pathology are linked to evasion of the immune response. Crosstalk between the immune system and particularities of each pathogenic species is associated with diverse disease manifestations. Lipophosphoglycan (LPG), one of the most important molecules present on the surface of Leishmania parasites, is divided into four regions with high molecular variability. Although LPG plays an important role in host-pathogen and vector-parasite interactions, the distribution and phylogenetic relatedness of the genes responsible for its synthesis remain poorly explored. The recent availability of full genomes and transcriptomes of Leishmania parasites offers an opportunity to leverage insight on how LPG-related genes are distributed and expressed by these pathogens.

RESULTS

Using a phylogenomics-based framework, we identified a catalog of genes involved in LPG biosynthesis across 22 species of Leishmania from the subgenera Viannia and Leishmania, as well as 5 non-Leishmania trypanosomatids. The evolutionary relationships of these genes across species were also evaluated. Nine genes related to the production of the glycosylphosphatidylinositol (GPI)-anchor were highly conserved among compared species, whereas 22 genes related to the synthesis of the repeat unit presented variable conservation. Extensive gain/loss events were verified, particularly in genes SCG1-4 and SCA1-2. These genes act, respectively, on the synthesis of the side chain attached to phosphoglycans and in the transfer of arabinose residues. Phylogenetic analyses disclosed evolutionary patterns reflective of differences in host specialization, geographic origin and disease manifestation.

CONCLUSIONS

The multiple gene gain/loss events identified by genomic data mining help to explain some of the observed intra- and interspecies variation in LPG structure. Collectively, our results provide a comprehensive catalog that details how LPG-related genes evolved in the Leishmania parasite specialization process.

Leishmania species-dependent functional duality of toll-like receptor 2

Toll-like receptors (TLRs) are a subset of pattern recognition receptors (PRR) in innate immunity and act as a connecting link between innate and adaptive immune systems. During Leishmania infection, the activation of TLRs influences the pathogen-specific immune responses, which may play a decisive role in determining the outcome of infection, toward elimination or survival of the pathogen. Antigen-presenting cells (APCs) of the innate immune system such as macrophages, dendritic cells (DCs), neutrophils, natural killer (NK) cells, and NKT cells express TLR2, which plays a crucial role in the parasite recognition and elicitation of immune responses in Leishmania infection. Depending on the infecting Leishmania species, the TLR2 pathways may result in a host-protective or a disease-exacerbating response. While Leishmania major and Leishmania donovani infections trigger TLR2-related host-protective and non-protective immune responses, Leishmania mexicana and Leishmania infantum infections are reported to elicit TLR2-mediated host-protective responses and Leishmania amazonensis and Leishmania braziliensis infections are reported to evoke a disease-exacerbating response. These findings illustrate that TLR2-related effector functions are diverse and may be exerted in a species- or strain-dependent manner. TLR2 agonists or antagonists may have therapeutic potentials to trigger the desired immune response during leishmaniasis. In this review, we discuss the TLR2-related immune responses during leishmaniasis and highlight the novel insights into the possible role of TLR2-driven resistance or susceptibility to Leishmania.

Phosphatidylcholine synthesis through cholinephosphate cytidylyltransferase is dispensable in Leishmania major

Phosphatidylcholine (PC) is a major cell membrane constituent and precursor of important second messengers. In Leishmania parasites, PC synthesis can occur via the choline branch of the Kennedy pathway, the N-methylation of phosphatidylethanolamine (PE), or the remodeling of exogenous phospholipids. To investigate the role of de novo PC synthesis in Leishmania major, we focused on the cholinephosphate cytidylyltransferase (CPCT) which catalyzes the formation of CDP-choline, a key intermediate in the choline branch of the Kennedy pathway. Without CPCT, L. major parasites cannot incorporate choline into PC, yet the CPCT-null mutants contain similar levels of PC and PE as wild type parasites. Loss of CPCT does not affect the growth of parasites in complete medium or their virulence in mice. These results suggest that other mechanisms of PC synthesis can compensate the loss of CPCT. Importantly, CPCT-null parasites exhibited severe growth defects when ethanolamine and exogenous lipids became limited or when they were co-cultured with certain bacteria that are known to be members of sandfly midgut microbiota. These findings suggest that Leishmania employ multiple PC synthesis pathways to utilize a diverse pool of nutrients, which may be crucial for their survival and development in the sandfly.

Metacyclogenesis of Leishmania (Viannia) guyanensis: a comprehensive study of the main transformation features in axenic culture and purification of metacyclic promastigotes by negative selection with Bauhinia purpurea lectin

Leishmania (Viannia) guyanensis is one species that causes cutaneous leishmaniasis in the New World. The incidence of infections with this parasite is probably underestimated and few studies exist on this species, despite its epidemiological importance. In particular, there are no studies concerning L. guyanensis metacyclogenesis and no technique for obtaining metacyclic promastigotes for this species is presently available. Here, we have studied L. guyanensis metacyclogenesis in axenic culture, describing the main changes that occur during this process, namely, in morphology and size, sensitivity to complement-mediated lysis, surface carbohydrates and infectivity to macrophages. We have shown that metacyclogenesis in L. guyanensis promastigotes is basically complete on the 4th day of culture, as determined by decreased body size, increased flagellum length, resistance to complement-mediated lysis and infectivity. We have also found that only a fraction of the parasites is agglutinated by Bauhinia purpurea lectin. The non-agglutinated parasites, which also peaked on the 4th day of culture, had all morphological traits typical of the metacyclic stage. This is the first report describing metacyclogenesis in L. guyanensis axenic promastigotes and a simple and efficient method for the purification of metacyclic forms. Furthermore, a model of human macrophage infection with L. guyanensis was established.

Reverse Epidemiology: An Experimental Framework to Drive Leishmania Biomarker Discovery in situ by Functional Genetic Screening Using Relevant Animal Models

Leishmania biomarker discovery remains an important challenge that needs to be revisited in light of our increasing knowledge on parasite-specific biology, notably its genome instability. In the absence of classical transcriptional regulation in these early-branching eukaryotes, fluctuations in transcript abundance can be generated by gene and chromosome amplifications, which have been linked to parasite phenotypic variability with respect to virulence, tissue tropism, and drug resistance. Conducting in vitro evolutionary experiments to study mechanisms of Leishmania environmental adaptation, we recently validated the link between parasite genetic amplification and fitness gain, thus defining gene and chromosome copy number variations (CNVs) as important Leishmania biomarkers. These experiments also demonstrated that long-term Leishmania culture adaptation can strongly interfere with epidemiologically relevant, genetic signals, which challenges current protocols for biomarker discovery, all of which rely on in vitro expansion of clinical isolates. Here we propose an experimental framework independent of long-term culture termed “reverse” epidemiology, which applies established protocols for functional genetic screening of cosmid-transfected parasites in animal models for the identification of clinically relevant genetic loci that then inform targeted field studies for their validation as Leishmania biomarkers.

Quantification of Intracellular Growth Inside Macrophages is a Fast and Reliable Method for Assessing the Virulence of Leishmania Parasites

The lifecycle of Leishmania, the causative agent of leishmaniasis, alternates between promastigote and amastigote stages inside the insect and vertebrate hosts, respectively. While pathogenic symptoms of leishmaniasis can vary widely, from benign cutaneous lesions to highly fatal visceral disease forms depending on the infective species, all Leishmania species reside inside host macrophages during the vertebrate stage of their lifecycle. Leishmania infectivity is therefore directly related to its ability to invade, survive and replicate within parasitophorous vacuoles (PVs) inside macrophages. Thus, assessing the parasite's ability to replicate intracellularly serves as a dependable method for determining virulence. Studying leishmaniasis development using animal models is time-consuming, tedious and often difficult, particularly with the pathogenically important visceral forms. We describe here a methodology to follow the intracellular development of Leishmania in bone marrow-derived macrophages (BMMs). Intracellular parasite numbers are determined at 24 h intervals for 72 - 96 h following infection. This method allows for a reliable determination of the effects of various genetic factors on Leishmania virulence. As an example, we show how a single allele deletion of the Leishmania Mitochondrial Iron Transporter gene (LMIT1) impairs the ability of the Leishmania amazonensis mutant strain LMIT1/ΔLmit1 to grow inside BMMs, reflecting a drastic reduction in virulence compared to wild-type. This assay also allows precise control of experimental conditions, which can be individually manipulated to analyze the influence of various factors (nutrients, reactive oxygen species, etc.) on the host-pathogen interaction. Therefore, the appropriate execution and quantification of BMM infection studies provide a non-invasive, rapid, economical, safe and reliable alternative to conventional animal model studies.

Genetic metabolic complementation establishes a requirement for GDP-fucose in Leishmania

To survive in its sand fly vector, the trypanosomatid protozoan parasite Leishmania first attaches to the midgut to avoid excretion, but eventually it must detach for transmission by the next bite. In Leishmania major strain Friedlin, this is controlled by modifications of the stage-specific adhesin lipophosphoglycan (LPG). During differentiation to infective metacyclics, d-arabinopyranose (d-Arap) caps the LPG side-chain galactose residues, blocking interaction with the midgut lectin PpGalec, thereby leading to parasite detachment and transmission. Previously, we characterized two closely related L. major genes (FKP40 and AFKP80) encoding bifunctional proteins with kinase/pyrophosphorylase activities required for salvage and conversion of l-fucose and/or d-Arap into the nucleotide-sugar substrates required by glycosyltransferases. Whereas only AFKP80 yielded GDP-d-Arap from exogenous d-Arap, both proteins were able to salvage l-fucose to GDP-fucose. We now show that Δafkp80⁻ null mutants ablated d-Arap modifications of LPG as predicted, whereas Δfkp40⁻ null mutants resembled wild type (WT). Fucoconjugates had not been reported previously in L. major, but unexpectedly, we were unable to generate fkp40⁻/afkp80⁻ double mutants, unless one of the A/FKPs was expressed ectopically. To test whether GDP-fucose itself was essential for Leishmania viability, we employed “genetic metabolite complementation.” First, the trypanosome de novo pathway enzymes GDP-mannose dehydratase (GMD) and GDP-fucose synthetase (GMER) were expressed ectopically; from these cells, the Δfkp40⁻/Δafkp80⁻ double mutant was now readily obtained. As expected, the Δfkp40⁻/Δafkp80⁻/+TbGMD-GMER line lacked the capacity to generate GDP-Arap, while synthesizing abundant GDP-fucose. These results establish a requirement for GDP-fucose for L. major viability and predict the existence of an essential fucoconjugate(s).

Developmental differentiation in Leishmania lifecycle progression: post-transcriptional control conducts the orchestra

The successful progression of Leishmania spp. through their lifecycle entails a series of differentiation processes; the proliferative procyclic promastigote forms become quiescent, human-infective metacyclic promastigotes during metacyclogenesis in the sandfly vector, which then differentiate into amastigotes during amastigogenesis in the mammalian host. The progression to these infective forms requires two components: environmental cues and a coordinated cellular response. Recent studies have shown that the Leishmania cellular transformation into mammalian-infective stages is triggered by broad changes in the absolute and relative RNA and protein levels. In this review, we will discuss the implications of Leishmania transcriptomic and proteomic fluctuations, which adapt the parasitic cell for survival.

Differential Impact of LPG-and PG-Deficient Leishmania major Mutants on the Immune Response of Human Dendritic Cells

Background

Leishmania major infection induces robust interleukin-12 (IL12) production in human dendritic cells (hDC), ultimately resulting in Th1-mediated immunity and clinical resolution. The surface of Leishmania parasites is covered in a dense glycocalyx consisting of primarily lipophosphoglycan (LPG) and other phosphoglycan-containing molecules (PGs), making these glycoconjugates the likely pathogen-associated molecular patterns (PAMPS) responsible for IL12 induction.

Methodology/Principal Findings

Here we explored the role of parasite glycoconjugates on the hDC IL12 response by generating L. major Friedlin V1 mutants defective in LPG alone, (FV1 lpg1-), or generally deficient for all PGs, (FV1 lpg2-). Infection with metacyclic, infective stage, L. major or purified LPG induced high levels of IL12B subunit gene transcripts in hDCs, which was abrogated with FV1 lpg1- infections. In contrast, hDC infections with FV1 lpg2- displayed increased IL12B expression, suggesting other PG-related/LPG2 dependent molecules may act to dampen the immune response. Global transcriptional profiling comparing WT, FV1 lpg1-, FV1 lpg2- infections revealed that FV1 lpg1- mutants entered hDCs in a silent fashion as indicated by repression of gene expression. Transcription factor binding site analysis suggests that LPG recognition by hDCs induces IL-12 in a signaling cascade resulting in Nuclear Factor κ B (NFκB) and Interferon Regulatory Factor (IRF) mediated transcription.

Conclusions/Significance

These data suggest that L. major LPG is a major PAMP recognized by hDC to induce IL12-mediated protective immunity and that there is a complex interplay between PG-baring Leishmania surface glycoconjugates that result in modulation of host cellular IL12.

Leishmaniasis is a group of parasitic diseases caused by intracellular protozoa belonging to the genus Leishmania, pathological manifestations ranging from self-healing cutaneous forms to severe visceral infections that result in death. These clinical outcomes are dictated by the Leishmania species initiating the infection and are influenced by early responses of host immune cells, which ultimately initiate an IL12 mediated immune response in resolving infections. Like the diseases themselves, the magnitude of IL12 induction in hDCs is Leishmania-species and strain specific, where species that elicit visceral disease do not induce IL12, while most cutaneous disease-causing L. major strains induce robust IL12 responses and confer life-long immunity. The molecular mechanisms that mediate the ability of these innate immune cells to discriminate between pathogens remain elusive and have been primarily investigated in murine model systems. Here we identified L. major LPG as a major PAMP that induces IL12 in hDCs. Elucidation of this critical component of human immunity to L. major has ramifications for leishmaniasis vaccine development.

In vitro metacyclogenesis of Leishmania (Viannia) braziliensis and Leishmania (Leishmania) amazonensis clinical field isolates, as evaluated by morphology, complement resistance, and infectivity to human macrophages

This study was designed to assess in vitro metacyclogenesis of Leishmania (Viannia) braziliensis and Leishmania (Leishmania) amazonensis clinical field isolates obtained from patient lesions (L. braziliensis IMG3 and PPS6m; L. amazonensis MAB6). Metacyclogenesis was evaluated by different criteria, namely, promastigote size (morphometric analysis and flow cytometry), surface modifications (loss of lectin or monoclonal antibody (mAb) binding, complement resistance), and infectivity to human macrophages. Growth curves were similar for all parasites evaluated. The various features analyzed were expressed in a high percentage of promastigotes at 6th and 10th days of culture and a low percentage at the 2nd day. However, in most isolates, these features, considered as markers of metacyclogenesis, seemed to develop with different time courses, since the percentages of metacyclic forms detected with each technique were usually different. Parasites from 6th or 10th day and those negatively selected with lectin or mAb similarly infected human macrophages. From all isolates analyzed, L. amazonensis PH8 and MAB6 showed the highest and the lowest levels of susceptibility, respectively, to leishmanicidal activity of IFN-γ/LPS-activated macrophages. Our results showed that by using different techniques to evaluate different aspects of metacyclogenesis (morphological and biochemical modifications) different percentages of metacyclic promastigotes can be detected in each isolate culture.

Leishmania lipophosphoglycan: how to establish structure-activity relationships for this highly complex and multifunctional glycoconjugate?

A key feature of many pathogenic microorganisms is the presence of a dense glycocalyx at their surface, composed of lipid-anchored glycoproteins and non-protein-bound polysaccharides. These surface glycolipids are important virulence factors for bacterial, fungal and protozoan pathogens. The highly complex glycoconjugate lipophosphoglycan (LPG) is one of the dominant surface macromolecules of the promastigote stage of all Leishmania parasitic species. LPG plays critical pleiotropic roles in parasite survival and infectivity in both the sandfly vector and the mammalian host. Here, we review the composition of the Leishmania glycocalyx, the chemical structure of LPG and what is currently known about its effects in the mammalian host, specifically. We will then discuss the current approaches employed to elucidate LPG functions. Finally, we will provide a viewpoint on future directions that this area of investigation could take to unravel in detail the biological activity of the specific molecular elements composing the structurally complex LPG.

The Leishmania major BBSome subunit BBS1 is essential for parasite virulence in the mammalian host

Bardet-Biedl syndrome (BBS) is a human genetic disorder with a spectrum of symptoms caused by primary cilium dysfunction. The disease is caused by mutations in one of at least 17 identified genes, of which seven encode subunits of the BBSome, a protein complex required for specific trafficking events to and from the primary cilium. The molecular mechanisms associated with BBSome function remain to be fully elucidated. Here, we generated null and complemented mutants of the BBSome subunit BBS1 in the protozoan parasite, Leishmania. In the absence of BBS1, extracellular parasites have no apparent defects in growth, flagellum assembly, motility or differentiation in vitro but there is accumulation of vacuole-like structures close to the flagellar pocket. Infectivity of these parasites for macrophages in vitro is reduced compared with wild-type controls but the null parasites retain the ability to differentiate to the intracellular amastigote stage. However, infectivity of BBS1 null parasites is severely compromised in a BALB/c mouse footpad model. We hypothesize that the absence of BBS1 in Leishmania leads to defects in specific trafficking events that affect parasite persistence in the host. This is the first report of an association between the BBSome complex and pathogen infectivity.

Differential surface deposition of complement proteins on logarithmic and stationary phase Leishmania chagasi promastigotes

Previous works demonstrated that various species of Leishmania promastigotes exhibit differential sensitivity to complement-mediated lysis (CML) during development. Upon exposure to normal human serum (NHS), cultures of Leishmania chagasi promastigotes recently isolated from infected hamsters (fewer than 5 in vitro passages) are CML-sensitive when in the logarithmic growth phase but become CML-resistant upon transition to the stationary culture phase. Visualization by light and electron microscopy revealed dramatic morphological differences between promastigotes from the 2 culture phases following exposure to NHS. Flow cytometric analysis demonstrated that surface deposition of the complement components C3, C5, and C9 correlated inversely with promastigote CML-resistance. The highest levels of complement protein surface accumulation were observed for logarithmic phase promastigotes, while stationary phase promastigotes adsorbed the least amount of complement proteins. Additionally, fluorescence microscopy revealed that C3 and C5 localized in a fairly uniform pattern to the plasma membrane of promastigotes from logarithmic phase cultures, while the staining of promastigotes from stationary phase cultures was indistinguishable from background. By Western blot analysis, high levels of the complement proteins C3, C5, and C9 were detected in the total lysates of NHS-exposed logarithmic phase L. chagasi promastigotes, relative to NHS-exposed stationary phase promastigotes; this finding indicates that the low levels of C3 and C5 seen on the surface of stationary phase promastigotes were not due to protein uptake/internalization. Together, these data demonstrate the differential deposition of complement proteins on the surfaces of logarithmic and stationary phase L. chagasi promastigotes. The data support a model wherein stationary phase L. chagasi promastigotes resist CML by limiting the deposition of C3 and its derivatives, which, in turn, limit surface levels of complement proteins (including C5 and C9) that form the lytic membrane attack complex.

Metabolic variation during development in culture of Leishmania donovani promastigotes

The genome sequencing of several Leishmania species has provided immense amounts of data and allowed the prediction of the metabolic pathways potentially operating. Subsequent genetic and proteomic studies have identified stage-specific proteins and putative virulence factors but many aspects of the metabolic adaptations of Leishmania remain to be elucidated. In this study, we have used an untargeted metabolomics approach to analyze changes in the metabolite profile as promastigotes of L. donovani develop during in vitro cultures from logarithmic to stationary phase. The results show that the metabolomes of promastigotes on days 3–6 of culture differ significantly from each other, consistent with there being distinct developmental changes. Most notable were the structural changes in glycerophospholipids and increase in the abundance of sphingolipids and glycerolipids as cells progress from logarithmic to stationary phase.

Leishmania infections are considered neglected tropical diseases as the parasites affect millions of people worldwide but there are limited research efforts aimed at obtaining vaccines and new drugs. Leishmania has a digenetic life cycle alternating between promastigote forms, which develop in the sand-fly, the vector of the disease, and an amastigote form, which grows in mammals after being bitten by an infected sand-fly. In vitro studies with the promastigote forms are routinely used to gain insights about the parasite's cell biology. Little is known about how the different promastigotes forms are metabolically adapted to their particular micro-environment in the host or how they are pre-adapted metabolically for infecting a mammal, thus we have undertaken a study of the metabolite profile of L. donovani promastigotes in order to gain an understanding of the changes that occur during promastigote development. The analysis has revealed that the changes in promastigotes' metabolome between days 3 and 6 take place in a progressive manner; however major differences were observed when comparing the promastigotes on days 3 and 6. An increase in lipid abundance as promastigote development occurred was notable and is likely to reflect remodelling of the parasite's surface in readiness for infecting a mammal.

Stage-specific adhesion of Leishmania promastigotes to sand fly midguts assessed using an improved comparative binding assay

BACKGROUND

The binding of Leishmania promastigotes to the midgut epithelium is regarded as an essential part of the life-cycle in the sand fly vector, enabling the parasites to persist beyond the initial blood meal phase and establish the infection. However, the precise nature of the promastigote stage(s) that mediate binding is not fully understood.

METHODOLOGY/PRINCIPAL FINDINGS

To address this issue we have developed an in vitro gut binding assay in which two promastigote populations are labelled with different fluorescent dyes and compete for binding to dissected sand fly midguts. Binding of procyclic, nectomonad, leptomonad and metacyclic promastigotes of Leishmania infantum and L. mexicana to the midguts of blood-fed, female Lutzomyia longipalpis was investigated. The results show that procyclic and metacyclic promastigotes do not bind to the midgut epithelium in significant numbers, whereas nectomonad and leptomonad promastigotes both bind strongly and in similar numbers. The assay was then used to compare the binding of a range of different parasite species (L. infantum, L. mexicana, L. braziliensis, L. major, L. tropica) to guts dissected from various sand flies (Lu. longipalpis, Phlebotomus papatasi, P. sergenti). The results of these comparisons were in many cases in line with expectations, the natural parasite binding most effectively to its natural vector, and no examples were found where a parasite was unable to bind to its natural vector. However, there were interesting exceptions: L. major and L. tropica being able to bind to Lu. longipalpis better than L. infantum; L. braziliensis was able to bind to P. papatasi as well as L. major; and significant binding of L. major to P. sergenti and L. tropica to P. papatasi was observed.

CONCLUSIONS/SIGNIFICANCE

The results demonstrate that Leishmania gut binding is strictly stage-dependent, is a property of those forms found in the middle phase of development (nectomonad and leptomonad forms), but is absent in the early blood meal and final stages (procyclic and metacyclic forms). Further they show that although gut binding may be necessary for parasite establishment, in several vector-parasite pairs the specificity of such in vitro binding alone is insufficient to explain overall vector specificity. Other significant barriers to development must exist in certain refractory Leishmania parasite-sand fly vector combinations. A re-appraisal of the specificity of the Leishmania-sand fly relationship is required.

Ascorbate peroxidase from Leishmania major controls the virulence of infective stage of promastigotes by regulating oxidative stress

Background

Peroxidase represents a heterogeneous group of distinct enzyme family that plays extremely diverse biological functions. Ascorbate peroxidase from Leishmania major (LmAPX) has been shown to be central to the redox defense system of Leishmania. To investigate further its exact physiological role in Leishmania, we attempted to create LmAPX -knockout mutants by gene replacement in L. major strains.

Methodology/Principal Findings

The null mutant cell culture contains a higher percentage of metacyclic and apoptotic cells compared to both wild type and LmAPX overexpressing cells. Flowcytometric analysis reveals the presence of a higher concentration of intracellular H₂O₂, indicative of increased oxidative stress in parasites lacking LmAPX. IC₅₀ value for exogenously added H₂O₂ shows that deletion of LmAPX in L. major renders the cell more susceptible to H₂O₂. Real time PCR studies demonstrate an elevated mRNA level of non-selenium glutathione peroxidase in LmAPX null mutant cell line, suggesting that these enzymes were induced to compensate the LmAPX enzyme. The null mutant cells exhibit hypervirulence after infection with macrophages as well as inoculation into BALB/c mice; in contrast, overexpressing cells show avirulence.

Conclusions/Significance

Collectively, these data provide strong evidence that LmAPX is an important factor for controlling parasite differentiation and survival within macrophages.

Leishmania major: disruption of signal peptidase type I and its consequences on survival, growth and infectivity

Leishmania major (L. major) signal peptidase type I (SPase I) is an endopeptidase encoded by a single-copy gene. In all organisms, SPase I is responsible for removing the signal peptide from secretory pre-proteins and releasing mature proteins to cellular or extra-cellular space. In this study, the role of SPase I in L. major is investigated by gene deletion using homologous recombination (HR). The null mutant of SPase I was not possible to create, suggesting that SPase I is an essential gene for parasite survival. The obtained heterozygote mutant by disrupting one allele of SPase I in L. major showed significantly reduced level of infectivity in bone marrow-derived macrophages. In addition, the heterozygote mutants are unable to cause cutaneous lesion in susceptible BALB/c mice. This is the first report showing that SPase I may have an important role in Leishmania infectivity, e.g. in differentiation and survival of amastigotes. Apparently, the SPase I expression is not essential for in vitro growth of the parasite.

Leishmania major lipophosphoglycan: discrepancy in Toll-like receptor signaling

Lipophosphoglycan (LPG) is structurally characterized by a series of phosphoglycan repeat units. Cellular LPG, isolated from promastigotes, has a very similar structure to culture supernatant LPG, but differs in the average number of phosphorylated oligosaccharide repeat units and in glycan composition. Comparison of these LPGs with capillary electrophoresis and immunoblotting indicate that these molecules are highly conserved structurally and composed of galactosylated Gal-Man repeats but their size and molecular weight are very different which is due to glycan portion. There are 30 and 20 repeat units in sLPG and mLPG, respectively. Both LPGs induced nitric oxide in macrophages cell line while sLPG had the higher stimulatory effect. In the presence of anti-TLR2 nitric oxide stimulated by LPG was reduced to control levels. In addition, in the presence of anti-TLR4, nitric oxide stimulated by LPGs was not affected. We propose that lipophosphoglycan induces nitric oxide production via TLR2 signaling pathway.

Leishmania dihydroxyacetonephosphate acyltransferase LmDAT is important for ether lipid biosynthesis but not for the integrity of detergent resistant membranes

Glycerolipid biosynthesis in Leishmania initiates with the acylation of glycerol-3-phosphate by a single glycerol-3-phosphate acyltransferase, LmGAT, or of dihydroxyacetonephosphate by a dihydroxyacetonephosphate acyltransferase, LmDAT. We previously reported that acylation of the precursor dihydroxyacetonephosphate rather than glycerol-3-phosphate is the physiologically relevant pathway for Leishmania parasites. We demonstrated that LmDAT is important for normal growth, survival during the stationary phase, and for virulence. Here, we assessed the role of LmDAT in glycerolipid metabolism and metacyclogenesis. LmDAT was found to be implicated in the biosynthesis of ether glycerolipids, including the ether lipid derived virulence factor lipophosphoglycan and glycosylphosphatidylinositol-anchored proteins. The null mutant produced longer lipophosphoglycan molecules that were not released in the medium, and augmented levels of glycosylphosphatidylinositol-anchored proteins. In addition, the integrity of detergent resistant membranes was not affected by the absence of the LmDAT gene. Further, our genetic analyses strongly suggest that LmDAT was synthetic lethal with the glycerol-3-phosphate acyltransferase encoding gene LmGAT, implying that Leishmania expresses only two acyltransferases that initiate the biosynthesis of its cellular glycerolipids. Last, despite the fact that LmDAT is important for virulence the null mutant still exhibited the typical characteristics of metacyclics.

Over-expression of Leishmania major MAP kinases reveals stage-specific induction of phosphotransferase activity

During the infectious cycle, protozoan parasites undergo various developmental transitions and switch virulence factors in response to extracellular signals in insect vectors and human hosts. Despite the importance of environmental sensing in parasite pathogenicity, little is known about the pathways that transduce extracellular signals into stage-specific gene expression. Here, we used a transgenic approach to gain insight into localisation and activity of three green fluorescence protein (GFP)-tagged Leishmania major mitogen-activated protein kinases, LmaMPK4, 7 and 10. The GFP-LmaMPKs in both L. major and Leishmania donovani transgenic lines showed predominant cytoplasmic localisation and the over-expression had no effect on promastigote morphology, growth and the ability to differentiate into stationary-phase metacyclics for L. major and axenic amastigotes for L. donovani. We isolated the GFP-tagged MPKs from parasite extracts and tested their phosphotransferase activity across various culture conditions. For all three GFP-LmaMPKs, kinase activity was low or absent in promastigote extracts but significantly increased in L. major promastigotes after exposure to pH 5.5 and 34 degrees C, and in axenic L. donovani amastigotes. Enhanced activity correlated with increased GFP-LmaMPK phosphorylation as judged by phospho-specific fluorescent staining of the immuno-precipitated kinases. We could extend these findings to the endogenous LmaMPK10, which accumulated in the phospho-protein fraction of axenic amastigotes but not promastigotes, and thus follows the stage-specific phosphorylation profile of episomally expressed GFP-LmaMPK10. These results provide evidence for the functional conservation of Leishmania MAP kinases in parasite environmental sensing and underscore the potential of transgenic approaches to gain insight into signaling events during the Leishmania life cycle.

Leishmania major: Reactive oxygen species and interferon gamma induction by soluble lipophosphoglycan of stationary phase promastigotes

Protozoan parasites of the genus Leishmania cause a number of important human diseases. One of the key determinants of parasite infectivity and survival is membrane glycoconjugate lipophosphoglycan (mLPG). In addition, it has been shown that mLPG could be used as a transmission blocking vaccine. Since culture supernatant of parasite promastigotes is a good source of LPG, we attempted to compare the immunological properties of culture supernatant and membrane LPG prepared from stationary phase promastigotes of Leishmania major. The purity of supernatant LPG (sLPG) and membrane LPG (mLPG) was determined by thin layer chromatography. The effect of sLPG and mLPG on the production of reactive oxygen species (ROS) was studied using PBMCs isolated from healthy individuals. In addition, induction of IL-12, IFN-gamma and IL-10 secretion in the presence of sLPG and mLPG was investigated. Reactive oxygen species in addition to IL-10 and IL-12 were induced by both sLPG and mLPG. However, IFN-gamma production was promoted only in response to sLPG suggesting its ability to promote Th1 response and implication in vaccine design.

Characterisation of a Leishmania mexicana knockout lacking guanosine diphosphate-mannose pyrophosphorylase

In eukaryotes, the enzyme GDP-mannose pyrophosphorylase (GDP-MP) is essential for the formation of GDP-mannose, the donor of activated mannose for all glycosylation reactions. Unlike other eukaryotes, where deletion of GDP-mannose pyrophosphorylase is lethal, deletion of this gene in Leishmania mexicana has no effect on viability, but leads to the generation of avirulent parasites. In this study, we show that the null mutants have a perturbed morphology and cytokinesis, retarded growth and increased adherence to the substratum where they form large colonies. The null mutants attach avidly to mouse macrophages, but unlike the wild type organisms, they do not bind to the complement receptor 3 and are slow to induce phagocytosis. Once internalised, they localise to the phagolysosome, but in contrast to wild type organisms which transform into the intracellular amastigote and establish in the macrophage, they are cleared by 24 h in culture and by 5 h in vivo. The null mutants are hypersensitive to human but not mouse complement and to temperature and acidic pH. Surprisingly, in view of the lack of several known host-protective antigens, injection of the mutant parasites into BALB/c mice confers significant and long lasting protection against infection, suggesting that these temperature sensitive mutants are an attractive candidate for a live attenuated vaccine.

Internal and surface subpopulations of the major surface protease (MSP) of Leishmania chagasi

Major surface protease (MSP) facilitates Leishmania promastigote evasion of complement-mediated lysis in the mammalian host and enhances host macrophage phagocytosis of the promastigotes. We previously showed that the steady-state abundance of MSP protein increases 14-fold during in vitro cultivation of L. chagasi promastigotes from logarithmic to stationary phase, despite the fact that the total amount of MSP mRNA does not increase. Furthermore, 10 major MSP isoforms are differentially expressed in different promastigote growth phases, and attenuation of parasites by long-term in vitro cultivation influences MSP isoform expression. Herein, we report that although about two-thirds of newly synthesized MSP becomes surface localized, the rest of the MSP does not reach the promastigote surface. This internal MSP is stable without detectable decrease in abundance up to 6 days after biosynthesis. Furthermore, surface-localized MSP is released at different rates from logarithmic and stationary phase virulent Leishmania promastigotes. These data are consistent with the hypothesis that the major mechanism regulating MSP abundance is the rate of loss of surface-localized MSP from the promastigote surface, and that internally localized MSP is very stable.

A role for insect galectins in parasite survival

Insect galectins are associated with embryonic development or immunity against pathogens. Here, we show that they can be exploited by parasites for survival in their insect hosts. PpGalec, a tandem repeat galectin expressed in the midgut of the sandfly Phlebotomus papatasi, is used by Leishmania major as a receptor for mediating specific binding to the insect midgut, an event crucial for parasite survival, and accounts for species-specific vector competence for the most widely distributed form of cutaneous leishmaniasis in the Old World. In addition, these studies demonstrate the feasibility of using midgut receptors for parasite ligands as target antigens for transmission-blocking vaccines.

The role of complement in the early phase of Leishmania (Leishmania) amazonensis infection in BALB/c mice

Complement-depleted and -non-depleted BALB/c mice were inoculated with Leishmania (Leishmania) amazonensis promastigotes into the hind footpad to study the role of the complement system in cutaneous leishmaniasis. Total serum complement activity was measured by hemolytic assay and C3 fragment deposit at the inoculation site was determined by direct immunofluorescence in the early period of infection, i.e., at 3, 24, 48 h and 7 days post-infection. The inflammatory reaction and the parasite burden were evaluated in the skin lesion at 7 and 30 days post-infection. Total serum complement activity decreased in the early phase of infection, from 3 to 24 h, in non-depleted mice compared to non-infected and non-depleted mice. C3 fragment deposit at the site of parasite inoculation was present throughout the period of infection in non-depleted mice. In contrast, no C3 fragment deposit was observed at the inoculation site in complement-depleted mice. Complement-depleted mice showed a significant decrease in the inflammatory response and a significant increase in the number of parasites (70.0 +/- 5.3 vs 5.3 +/- 1.5) at 7 days of infection (P<0.05). A higher number of parasites were also present at 30 days of infection at the inoculation site of complement-depleted mice (78.5 +/- 24.9 vs 6.3 +/- 5.7). These experiments indicate that complement has an important role at the beginning of experimental cutaneous leishmaniasis caused by L. (L.) amazonensis by controlling the number of parasites in the lesion.

The major surface protease (MSP or GP63) of Leishmania sp. Biosynthesis, regulation of expression, and function

Leishmania sp. are digenetic protozoa that cause an estimated 1.5-2 million new cases of leishmaniasis per year worldwide. Among the molecular factors that contribute to Leishmania sp. virulence and pathogenesis is the major surface protease, alternately called MSP, GP63, leishmanolysin, EC3.4.24.36, and PSP, which is the most abundant surface protein of leishmania promastigotes. Recent studies using gene knockout, antisense RNA and overexpression mutants have demonstrated a role for MSP in resistance of promastigotes to complement-mediated lysis and either a direct or indirect role in receptor-mediated uptake of leishmania. The MSP gene clusters in different Leishmania sp. include multiple distinct MSPs that tend to fall into three classes, which can be distinguished by their sequences and by their differential expression in parasite life stages. Regulated expression of MSP class gene products during the parasite life cycle occurs at several levels involving both mRNA and protein metabolism. In this review we summarize advances in MSP research over the past decade, including organization of the gene families, crystal structure of the protein, regulation of mRNA and protein expression, biosynthesis and possible functions. The MSPs exquisitely demonstrate the multiple levels of post-transcriptional gene regulation that occur in Leishmania sp. and other trypanosomatid protozoa.

Leishmania major LACK antigen is required for efficient vertebrate parasitization

The Leishmania major LACK antigen is a key target of the immune response in susceptible BALB/c mice and remains a viable vaccine candidate for human leishmaniasis. We describe the genomic organization of the four lack genes in the L. major diploid genome together with results of selected lack gene targeting. Parasites containing a single lack gene in either the upstream or downstream locus grew comparably to wild-type promastigotes in vitro, but failed to parasitize BALB/c mice efficiently, even in a T cell-deficient environment. The replication of single copy lack mutants as amastigotes was attenuated in macrophages in vitro, and parasites failed to increase in numbers in immunodeficient mice, despite their persistence over months. Complementation with an additional lack copy was sufficient to induce robust lesion development, which also occurred using parasites with two lack genes. Conversely, attempts to generate lack-null parasites failed, suggesting that LACK is required for parasite viability. These data suggest that LACK is critical for effective mammalian parasitization and thus represents a potential drug target for leishmaniasis.

Leishmania lipophosphoglycan (LPG) activates NK cells through toll-like receptor-2

Toll-like receptors (TLRs) mediate the cellular response to conserved molecular patterns shared by microorganisms. We report that TLR-2 on human NK cells is upregulated and stimulated by Leishmania major lipophosphoglycan (LPG), a phosphoglycan belonging to a family of unique Leishmania glycoconjugates. We found that purified L. major LPG upregulates both mRNA and the membrane expression of TLR-2 in NK cells. Additionally, IFN-gamma and TNF-alpha production and nuclear translocation of NF-kappaB was enhanced. The activation effect was more intense with LPG purified from infectious metacyclic parasites than from noninfectious procyclic Leishmania. Since the difference between the molecules derived from these two stages of the parasite growth cycle lies exclusively in the number of phosphosaccharide repeat domains and in the composition of glycan side chains that branch off these domains, we propose that TLR-2 possibly distinguishes between phosphorylated glycan repeats on LPG molecules. The effect of LPG on cytokine production and on membrane expression of TLR-2 could be blocked with F(ab')2 fragments of the mAb against LPG (WIC 79.3). Confocal microscopy demonstrated the co-localization of LPG and TLR-2 on the NK cell membrane. Binding of LPG to TLR-2 in NK cells was demonstrated by immunoprecipitations done with anti-TLR-2 and anti-LPG mAb followed by immunoblotting with anti-LPG and anti-TLR-2, respectively. Both antibodies recognized the immune complexes. These results suggest that NK cells are capable of recognition of, and activation by, Leishmania LPG through TLR-2, enabling them to participate autonomously in the innate immune system and thereby increasing the effective destruction of the parasite.

Identification of genes encoding arabinosyltransferases (SCA) mediating developmental modifications of lipophosphoglycan required for sand fly transmission of leishmania major

At key steps in the infectious cycle pathogens must adhere to target cells, but at other times detachment is required for transmission. During sand fly infections by the protozoan parasite Leishmania major, binding of replicating promastigotes is mediated by galactosyl side chain (scGal) modifications of phosphoglycan repeats of the major surface adhesin, lipophosphoglycan (LPG). Release is mediated by arabinosyl (Ara) capping of LPG scbetaGal residues upon differentiation to the infective metacyclic stage. We used intraspecific polymorphisms of LPG structure to develop a genetic strategy leading to the identification of two genes (SCA1/2) mediating scAra capping. These LPG side chain beta1,2-arabinosyltransferases (scbetaAraTs) exhibit canonical glycosyltransferase motifs, and their overexpression leads to elevated microsomal scbetaAraT activity. Although the level of scAra caps is maximal in metacyclic parasites, scbetaAraT activity is maximal in log phase cells. Because quantitative immunolocalization studies suggest this is not mediated by sequestration of SCA scbetaAraTs away from the Golgi apparatus during log phase, regulation of activated Ara precursors may control LPG arabinosylation in vivo. The SCA genes define a new family of eukaryotic betaAraTs and represent novel developmentally regulated LPG-modifying activities identified in Leishmania.

Functional identification of galactosyltransferases (SCGs) required for species-specific modifications of the lipophosphoglycan adhesin controlling Leishmania major-sand fly interactions

Lipophosphoglycan (LPG) is an abundant surface molecule that plays key roles in the infectious cycle of Leishmania major. The dominant feature of LPG is a polymer of phosphoglycan (PG) (6Galbeta1,4Manalpha1-PO(4)) repeating units. In L. major these are extensively substituted with Gal(beta1,3) side chains, which are required for binding to midgut lectins and survival. We utilized evolutionary polymorphisms in LPG structure and cross-species transfections to recover genes encoding the LPG side chain beta1,3-galactosyltransferases (betaGalTs). A dispersed family of six SCG genes was recovered, whose predicted proteins exhibited characteristics of eukaryotic GalTs. At least four of these proteins showed significant LPG side chain betaGalT activity; SCG3 exhibited initiating GalT activity whereas SCG2 showed both initiating and elongating GalT activity. However, the activity of SCG2 was context-dependent, being largely silent in its normal genomic milieu, and different strains show considerable variation in the extent of LPG galactosylation. Thus the L. major genome encodes a family of SCGs with varying specificity and activity, and we propose that strain-specific LPG galactosylation patterns reflect differences in their expression.

Role of peroxidoxins in Leishmania chagasi survival. Evidence of an enzymatic defense against nitrosative stress

The mechanisms by which Leishmania parasites survive exposure to highly reactive oxygen (ROS) and nitrogen (RNS) species within phagosomes of macrophages are not well known. Recently it has been shown that RNS alone is sufficient and necessary to control Leishmania donovani infection in mice (Murray, H. W., and Nathan, C. F. (1999) J. Exp. Med. 189, 741-746). No enzymatic defense against RNS has been discovered in Leishmania to date. We have previously isolated two peroxidoxins (LcPxn1 and LcPxn2) from Leishmania chagasi and showed that recombinant LcPxn1 protein was capable of detoxifying hydrogen peroxide, hydroperoxide, and hydroxyl radicals (Barr, S. D., and Gedamu, L. (2001) J. Biol. Chem. 276, 34279-34287). In further characterizing the physiological role of peroxidoxins in Leishmania survival, we show here that recombinant LcPxn1 protein can detoxify RNS in addition to ROS, whereas recombinant LcPxn2 protein can only detoxify hydrogen peroxide. LcPxn1 and LcPxn2 are localized to the cytoplasm, and overexpression of LcPxn1 in L. chagasi parasites enhanced survival when exposed to exogenous ROS and RNS and enhanced survival within U937 macrophage cells. Site-directed mutagenesis studies revealed that the conserved Cys-52 residue is essential for detoxifying hydrogen peroxide, t-butyl hydroperoxide, and hydroxyl radicals, whereas the conserved Cys-173 residue is essential for detoxifying t-butyl hydroperoxide and peroxynitrite. This is the first report of an enzymatic defense against RNS in Leishmania.

Molecular aspects of parasite-vector and vector-host interactions in leishmaniasis

Leishmania-sand fly interactions are reviewed in the context of the potential barriers to the complete development of the parasite that exist within the midgut environment of phlebotomine flies and the molecular adaptations that the parasite has evolved that permit the development of transmissible infections to proceed. Cell surface and secreted phosphoglycans protect the parasite from the proteolytic activities of the blood-fed midgut, mediate attachment to the gut wall in order to maintain infection during excretion of the bloodmeal, and contribute to the formation of a biological plug in the anterior gut that may promote transmission by bite. The importance of vector saliva in modulating the host response to transmitted parasites is also reviewed.

A lipophosphoglycan-independent method for isolation of infective Leishmania metacyclic promastigotes by density gradient centrifugation

At the end of their growth in the sand fly, Leishmania parasites differentiate into the infective metacyclic promastigote stage, which is transmitted to the mammalian host. Thus, in experimental studies of parasite infectivity toward animals or macrophages, the use of purified metacyclics is generally preferred. While metacyclics of several Leishmania species can be efficiently purified with the aid of lectins or monoclonal antibodies, which differentially exploit stage-specific differences in the structure of the abundant surface glycolipid lipophosphoglycan (LPG), such reagents are unavailable for most species and they are unsuitable for studies involving LPG-deficient mutants. Here we describe a simple density gradient centrifugation method, which allows the rapid purification of infective metacyclic parasites from both wild-type and LPG-deficient Leishmania major. The purified metacyclic promastigotes are authentic, as judged by criteria such as their morphology, expression of the metacyclic-specific gene SHERP, and ability to invade and replicate within macrophages in vitro. Preliminary studies suggest that this method is applicable to other Leishmania species including L. donovani.

Heterogeneity of wild Leishmania major isolates in experimental murine pathogenicity and specific immune response

Virulence variability was investigated by analyzing the experimental pathogenicity of 19 Leishmania major strains in susceptible BALB/c mice. Twelve strains were isolated from Tunisian patients with zoonotic cutaneous leishmaniasis; seven strains were isolated in Syria (n = 1), Saudi Arabia (n = 2), Jordan (n = 2), or Israel (n = 2). BALB/c mice were injected in the hind footpad with 2 x 10(6) amastigotes of the various isolates, and lesion progression was recorded weekly for 9 weeks. Interleukin-4 (IL-4) and gamma interferon (IFN-gamma) production of lymph node mononuclear cells activated in vitro with parasite antigens were evaluated 5 weeks after infection. We show that disease progression induced by different L. major isolates was largely heterogeneous although reproducible results were obtained when using the same isolate. Interestingly, isolates from the Middle East induced a more severe disease than did the majority of Tunisian isolates. Strains with the highest virulence tend to generate more IL-4 and less IFN-gamma in vitro at week 5 postinfection as well as higher levels of early IL-4 mRNA in the lymph node draining the inoculation site at 16 h postinfection. These results suggest that L. major isolates from the field may differ in virulence, which influences the course of the disease induced in mice and the type of immune response elicited by the infected host.

Glycoconjugate structures of parasitic protozoa

Glycoconjugates are abundant and ubiquitious on the surface of many protozoan parasites. Their tremendous diversity has implicated their critical importance in the life cycle of these organisms. This review highlights our current knowledge of the major glycoconjugates, with particular emphasis on their structures, of representative protozoan parasites, including Leishmania, Trypanosoma, Giardia, Plasmodia, and others.

Inducible resistance to oxidant stress in the protozoan Leishmania chagasi

Leishmania sp. protozoa are introduced into a mammalian skin by a sandfly vector, whereupon they encounter increased temperature and toxic oxidants generated during phagocytosis. We studied the effects of 37 degrees C "heat shock" or sublethal menadione, which generates superoxide and hydrogen peroxide, on Leishmania chagasi virulence. Both heat and menadione caused parasites to become more resistant to H(2)O(2)-mediated toxicity. Peroxide resistance was also induced as promastigotes developed in culture from logarithmic to their virulent stationary phase form. Peroxide resistance was not associated with an increase in reduced thiols (trypanothione and glutathione) or increased activity of ornithine decarboxylase, which is rate-limiting in trypanothione synthesis. Membrane lipophosphoglycan increased in size as parasites developed to stationary phase but not after environmental exposures. Instead, parasites underwent a heat shock response upon exposure to heat or sublethal menadione, detected by increased levels of HSP70. Transfection of promastigotes with L. chagasi HSP70 caused a heat-inducible increase in resistance to peroxide, implying it is involved in antioxidant defense. We conclude that leishmania have redundant mechanisms for resisting toxic oxidants. Some are induced during developmental change and others are induced in response to environmental stress.

Characterization of the glucosyltransferases that assemble the side chains of the Indian Leishmania donovani lipophosphoglycan

The life cycle of Leishmania parasites within its sand fly vector involves the development of extracellular promastigotes from a noninfective, "procyclic" stage into an infective, "metacyclic" stage that is adapted for transmission in the fly and survival in the mammalian host. Lipophosphoglycan (LPG), the predominant surface glycoconjugate in both procyclic and metacyclic stages, is a critical virulence determinant. LPG is a multidomain molecule; the structural polymorphisms among species lie in branching from the backbone 6Galbeta1,4Man(alpha1)-PO(4) repeat units and in the composition of the small oligosaccharide caps. We have recently demonstrated that the LPG from an Indian isolate of Leishmania donovani differs from a Sudanese strain by possessing one or two side chain beta(1,3)-linked glucose residues. We now have characterized the glucosyltransferase activities responsible for glucosylating the LPG. When incubated with UDP-[(3)H]glucose and Mn(2+), microsomal membranes from the Indian isolate transferred [(3)H]glucose to the repeat units of the exogenous acceptor Sudanese L. donovani LPG, which does not contain any side chain branching. Glucose addition was maximal at 28 degrees C, the optimal growth temperature of procyclic L. donovani. Consistent with the lack of side chain branching in its LPG, Sudanese L. donovani showed minimal glucosyltransferase activity. Indian metacyclic promastigotes, in contrast to procyclic promastigotes, express no glucose side chains off the repeat units. Therefore, we compared the relative activity of the glucosyltransferases in microsomes from procyclic and metacyclic promastigotes and observed approximately 80% less activity in the latter. These results provide evidence that the glucose side chain addition to LPG is developmentally regulated during the parasite's life cycle and that the glucosyltransferases of L. donovani are strain specific.

Glycoconjugates in Leishmania infectivity

Leishmaniasis is a major health problem to humans and is caused by one of the world's major pathogens, the Leishmania parasite. These protozoa have the remarkable ability to avoid destruction in hostile environments they encounter throughout their life cycle. That Leishmania parasites have adapted to not only survive, but to proliferate largely is due to the protection conferred by unique glycoconjugates that are either on the parasites' cell surface or secreted. Most of these specialized molecules are members of a family of phosphoglycans while others are a family of glycosylinositol phospholipids. Together they have been implicated in a surprisingly large number of functions for the parasites throughout their life cycle and, therefore, are key players in their pathogenesis. This review summarizes the biological roles of these glycoconjugates and how they are believed to contribute to Leishmania survival in destructive surroundings.

Phosphoglycosylation of a secreted acid phosphatase from Leishmania donovani

The secreted acid phosphatase (SAcP) of L.donovani is a heterogeneous glycoprotein that displays a wide array of N- and O-linked glycosylations. The O-linked sugars are of particular interest due to their similarity to the phosphoglycan structures of the major lipophosphoglycan surface antigen and released phosphoglycan (Turco et al., 1987; Greis et al., 1992). This study describes a structural analysis of the SAcP O-linked glycosylations using mass spectroscopy, amino acid sequencing, and enzymatic carbohydrate sequencing. Analysis of glycan chain lengths and peptide glycosylation site distribution was performed, revealing that the average O-linked structure was approximately 32 repeat units in length. Amino acid sequence analysis of glycosylated peptides showed that phosphoglycosylations did not occur randomly but were localized to specific serine residues within an array of degenerate serine/threonine-rich repeat sequences localized in the C-terminus. No evidence was obtained for modification of threonine residues. The observed pattern suggested that a consensus sequence may exist for localization of phosphoglycan structures.

A high-yield, enzymatic synthesis of GDP-D-[3H]arabinose and GDP-L-[3H]fucose

For assays involving glycosyltransferases or transporters, several GDP-sugars are either commercially unavailable or expensive. We describe an enzymatic synthesis of GDP-d-[3H]arabinosep and GDP-l-[3H]fucose that yields 66-95% nucleotide-sugar from the appropriate radiolabeled sugar in less than 30 min. The coupled reaction requires Mg2+, ATP, and GTP along with the appropriate radioactive monosaccharide, sugar-1-kinase, and pyrophosphorylase. The latter two activities are present in a cytosolic fraction of Crithidia fasciculata, which is easily grown at room temperature in simple culture medium without serum or added CO2. Addition of commercial yeast inorganic pyrophosphatase shifts the equilibrium of the pyrophosphorylase reaction toward nucleotide-sugar formation. To verify that these nucleotide-sugars are biologically active, we tested their ability to serve as substrates for glycosyltransferases. GDP-l-[3H]fucose functions as the donor substrate for recombinant human fucosyltransferase V, and GDP-d-[3H]arabinosep serves as the donor substrate for the arabinosyltransferase activities present in Leishmania major microsomes.

Targeted gene deletion of Leishmania major genes encoding developmental stage-specific leishmanolysin (GP63)

The major surface glycoprotein of Leishmania major is a zinc metalloproteinase of 63 kDa referred to as leishmanolysin or GP63, which is encoded by a family of seven genes. Targeted gene replacement was used to delete gp63 genes 1-6 encoding the highly expressed promastigote and constitutively expressed GP63. In the L. major homozygous mutants deficient in gp63 genes 1-6, there was no expression of GP63 as detected by reverse transcription-polymerase chain reaction (RT-PCR) or fluorescent staining in promastigotes from the procyclic stage (logarithmic growth phase). The remaining L. major gP63 gene 7 was shown to be developmentally regulated, as it was expressed exclusively in infectious metacyclic stage (late stationary growth phase) promastigotes and in lesion amastigotes. The gp63 genes 1-6-deficient mutants showed increased sensitivity to complement-mediated lysis. The sensitivity to lysis was greater in procyclics than in metacyclics when compared with the equivalent wild-type stages. Increased resistance of the mutant metacyclic promastigotes correlated with the expression of gp63 gene 7 and was restored to the same levels as wild-type promastigotes by transfection with gp63 gene 1. Thus, expression of GP63 is clearly involved in conferring resistance to complement-mediated lysis. The L. major GP63 1-6 mutants were capable of infecting mouse macrophages and differentiating into amastigotes. Similar levels of infection and subsequent intracellular survival were observed when mouse macrophages were infected in vitro with wild type, GP63 1-6 mutants and mutants transfected with gp63 gene 1. The GP63 1-6 mutants were capable of lesion formation in BALB/c mice and, thus, gp63 genes 1-6 do not play a role in the survival of the parasite within mouse macrophages. The role of gp63 genes 1-6 in parasite development within the sandfly vector was studied. GP63 1-6 mutants grew normally in the blood-engorged midgut of both Phlebotomus argentipes and P. papatasi However, both wild-type and mutant promastigotes were lost after 2 days' growth in P. papatasi. The complete developmental pathway in P. argentipes was observed for wild-type promastigotes, GP63 1-6 mutants and mutants transfected with gp63 gene 1. Normal stage differentiation from amastigotes to procyclics, to nectomonads, to haptomonads and to infectious metacyclics was observed. Thus, the highly expressed promastigote forms of GP63, encoded by gp63 genes 1-6, do not appear to be required for nutrient utilization in the bloodmeal during the early stages of development in the sandfly or for midgut attachment and further development. gp63 1-6 genes do, however, play a major protective role against complement-mediated lysis when promastigotes are introduced into the mammalian host.

Glycoprotein 46 mRNA abundance is post-transcriptionally regulated during development of Leishmania chagasi promastigotes to an infectious form

GP46 is an abundant glycoprotein of 46 kDa on the surface of the promastigote form of most Leishmania species. We show that the steady state level of GP46 mRNA increases >>30-fold as Leishmania chagasi promastigotes develop in vitro from a less infectious form during logarithmic growth to a highly infectious form in the stationary phase of cultivation. Nuclear run-on experiments demonstrate that this increase in GP46 mRNA abundance is regulated post-transcriptionally. Plasmids containing the 3'-untranslated regions (UTRs) and downstream intergenic regions (IRs) of two different GP46 genes fused immediately downstream of the beta-galactosidase coding region were transfected into L. chagasi, and beta-galactosidase activity and mRNA levels were examined. The presence of the 3'-UTR + IR of one GP46 gene (gp46A) resulted in a steady increase in beta-galactosidase activity and mRNA level as the transfected promastigotes developed from logarithmic to stationary phase. This differential effect parallels that of the 3'-UTRs + IRs of a family of genes for an unrelated Leishmania surface glycoprotein, GP63. Thus, post-transcriptional regulation of the genes for two different surface glycoproteins of Leishmania occurs via a similar mechanism.