Proteophosphoglycan secreted by Leishmania mexicana amastigotes causes vacuole formation in macrophages

Leishmania species: A narrative review on surface proteins with structural aspects involved in host-pathogen interaction

In tropical and subtropical regions of the world, leishmaniasis is endemic and causes a range of clinical symptoms in people, from severe tegumentary forms (such as cutaneous, mucocutaneous, and diffuse leishmaniasis) to lethal visceral forms. The protozoan parasite of the genus Leishmania causes leishmaniasis, which is still a significant public health issue, according to the World Health Organization 2022. The public's worry about the neglected tropical disease is growing as new foci of the illness arise, which are exacerbated by alterations in behavior, changes in the environment, and an enlarged range of sand fly vectors. Leishmania research has advanced significantly during the past three decades in a few different avenues. Despite several studies on Leishmania, many issues, such as illness control, parasite resistance, parasite clearance, etc., remain unresolved. The key virulence variables that play a role in the pathogenicity-host-pathogen relationship of the parasite are comprehensively discussed in this paper. The important Leishmania virulence factors, such as Kinetoplastid Membrane Protein-11 (KMP-11), Leishmanolysin (GP63), Proteophosphoglycan (PPG), Lipophosphoglycan (LPG), Glycosylinositol Phospholipids (GIPL), and others, have an impact on the pathophysiology of the disease and enable the parasite to spread the infection. Leishmania infection may arise from virulence factors; they are treatable with medications or vaccinations more promptly and might greatly shorten the duration of treatment. Additionally, our research sought to present a modeled structure of a few putative virulence factors that might aid in the development of new chemotherapeutic approaches for the treatment of leishmaniasis. The predicted virulence protein's structure is utilized to design novel drugs, therapeutic targets, and immunizations for considerable advantage from a higher understanding of the host immune response.

Non-Vesicular Lipid Transport Machinery in Leishmania donovani: Functional Implications in Host-Parasite Interaction

Eukaryotic cells have distinct membrane-enclosed organelles, each with a unique biochemical signature and specialized function. The unique identity of each organelle is greatly governed by the asymmetric distribution and regulated intracellular movement of two important biomolecules, lipids, and proteins. Non-vesicular lipid transport mediated by lipid-transfer proteins (LTPs) plays essential roles in intra-cellular lipid trafficking and cellular lipid homeostasis, while vesicular transport regulates protein trafficking. A comparative analysis of non-vesicular lipid transport machinery in protists could enhance our understanding of parasitism and basis of eukaryotic evolution. Leishmania donovani, the trypanosomatid parasite, greatly depends on receptor-ligand mediated signalling pathways for cellular differentiation, nutrient uptake, secretion of virulence factors, and pathogenesis. Lipids, despite being important signalling molecules, have intracellular transport mechanisms that are largely unexplored in L. donovani. We have identified a repertoire of sixteen (16) potential lipid transfer protein (LTP) homologs based on a domain-based search on TriTrypDB coupled with bioinformatics analyses, which signifies the presence of well-organized lipid transport machinery in this parasite. We emphasized here their evolutionary uniqueness and conservation and discussed their potential implications for parasite biology with regards to future therapeutic targets against visceral leishmaniasis.

Leishmania Vesicle-Depleted Exoproteome: What, Why, and How?

Leishmaniasis, a vector-borne parasitic protozoan disease, is among the most important neglected tropical diseases. In the absence of vaccines, disease management is challenging. The available chemotherapy is suboptimal, and there are growing concerns about the emergence of drug resistance. Thus, a better understanding of parasite biology is essential to generate new strategies for disease control. In this context, in vitro parasite exoproteome characterization enabled the identification of proteins involved in parasite survival, pathogenesis, and other biologically relevant processes. After 2005, with the availability of genomic information, these studies became increasingly feasible and revealed the true complexity of the parasite exoproteome. After the discovery of Leishmania extracellular vesicles (EVs), most exoproteome studies shifted to the characterization of EVs. The non-EV portion of the exoproteome, named the vesicle-depleted exoproteome (VDE), has been mostly ignored even if it accounts for a significant portion of the total exoproteome proteins. Herein, we summarize the importance of total exoproteome studies followed by a special emphasis on the available information and the biological relevance of the VDE. Finally, we report on how VDE can be studied and disclose how it might contribute to providing biologically relevant targets for diagnosis, drug, and vaccine development.

Making the Most of Its Short Reads: A Bioinformatics Workflow for Analysing the Short-Read-Only Data of Leishmania orientalis (Formerly Named Leishmania siamensis) Isolate PCM2 in Thailand

Simple Summary

Leishmaniasis is a parasitic disease caused by flagellated protozoa of the genus Leishmania. Multiple genome sequencing platforms have been employed to complete Leishmania genomes at the expense of high cost. This study proposes an integrative bioinformatic workflow for assembling only the short-read data of Leishmania orientalis isolate PCM2 from Thailand and produce an acceptable-quality genome for further genomic analysis. This workflow gives extensive information required for identifying strain-specific markers and virulence-associated genes useful for drug and vaccine development before a more exhaustive and expensive investigation.

Abstract

Background: Leishmania orientalis (formerly named Leishmania siamensis) has been neglected for years in Thailand. The genomic study of L. orientalis has gained much attention recently after the release of the first high-quality reference genome of the isolate LSCM4. The integrative approach of multiple sequencing platforms for whole-genome sequencing has proven effective at the expense of considerably expensive costs. This study presents a preliminary bioinformatic workflow including the use of multi-step de novo assembly coupled with the reference-based assembly method to produce high-quality genomic drafts from the short-read Illumina sequence data of L. orientalis isolate PCM2. Results: The integrating multi-step de novo assembly by MEGAHIT and SPAdes with the reference-based method using the L. enriettii genome and salvaging the unmapped reads resulted in the 30.27 Mb genomic draft of L. orientalis isolate PCM2 with 3367 contigs and 8887 predicted genes. The results from the integrated approach showed the best integrity, coverage, and contig alignment when compared to the genome of L. orientalis isolate LSCM4 collected from the northern province of Thailand. Similar patterns of gene ratios and frequency were observed from the GO biological process annotation. Fifty GO terms were assigned to the assembled genomes, and 23 of these (accounting for 61.6% of the annotated genes) showed higher gene counts and ratios when results from our workflow were compared to those of the LSCM4 isolate. Conclusions: These results indicated that our proposed bioinformatic workflow produced an acceptable-quality genome of L. orientalis strain PCM2 for functional genomic analysis, maximising the usage of the short-read data. This workflow would give extensive information required for identifying strain-specific markers and virulence-associated genes useful for drug and vaccine development before a more exhaustive and expensive investigation.

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.

Host Organelle Hijackers: a similar modus operandi for Toxoplasma gondii and Chlamydia trachomatis: co-infection model as a tool to investigate pathogenesis

The bacterium Chlamydia trachomatis and the protozoan parasite Toxoplasma gondii are the causative agents of chlamydiosis and toxoplasmosis in humans, respectively. Both microorganisms are obligate intracellular pathogens and notorious for extensively modifying the cytoskeletal architecture and the endomembrane system of their host cells to establish productive infections. This review highlights the similar tactics developed by these two pathogens to manipulate their host cell despite their genetic unrelatedness. Using an in vitro cell culture model whereby single fibroblasts are infected by C. trachomatis and T. gondii simultaneously, thus setting up an intracellular competition, we demonstrate that the solutions to the problem of intracellular survival deployed by the parasite and the bacterium may represent an example of convergent evolution, driven by the necessity to acquire nutrients in a hostile environment.

Proteinases as virulence factors in Leishmania spp. infection in mammals

Leishmania parasites cause human tegumentary and visceral infections that are commonly referred to as leishmaniasis. Despite the high incidence and prevalence of cases, leishmaniasis has been a neglected disease because it mainly affects developing countries. The data obtained from the analysis of patients' biological samples and from assays with animal models confirm the involvement of an array of the parasite's components in its survival inside the mammalian host. These components are classified as virulence factors. In this review, we focus on studies that have explored the role of proteinases as virulence factors that promote parasite survival and immune modulation in the mammalian host. Additionally, the direct involvement of proteinases from the host in lesion evolution is analyzed. The gathered data shows that both parasite and host proteinases are involved in the clinical manifestation of leishmaniasis. It is interesting to note that although the majority of the classes of proteinases are present in Leishmania spp., only cysteine-proteinases, metalloproteinases and, to a lesser scale, serine-proteinases have been adequately studied. Members from these classes have been implicated in tissue invasion, survival in macrophages and immune modulation by parasites. This review reinforces the importance of the parasite proteinases, which are interesting candidates for new chemo or immunotherapies, in the clinical manifestations of leishmaniasis.

The diverse and dynamic nature of Leishmania parasitophorous vacuoles studied by multidimensional imaging

An important area in the cell biology of intracellular parasitism is the customization of parasitophorous vacuoles (PVs) by prokaryotic or eukaryotic intracellular microorganisms. We were curious to compare PV biogenesis in primary mouse bone marrow-derived macrophages exposed to carefully prepared amastigotes of either Leishmania major or L. amazonensis. While tight-fitting PVs are housing one or two L. major amastigotes, giant PVs are housing many L. amazonensis amastigotes. In this study, using multidimensional imaging of live cells, we compare and characterize the PV biogenesis/remodeling of macrophages i) hosting amastigotes of either L. major or L. amazonensis and ii) loaded with Lysotracker, a lysosomotropic fluorescent probe. Three dynamic features of Leishmania amastigote-hosting PVs are documented: they range from i) entry of Lysotracker transients within tight-fitting, fission-prone L. major amastigote-housing PVs; ii) the decrease in the number of macrophage acidic vesicles during the L. major PV fission or L. amazonensis PV enlargement; to iii) the L. amazonensis PV remodeling after homotypic fusion. The high content information of multidimensional images allowed the updating of our understanding of the Leishmania species-specific differences in PV biogenesis/remodeling and could be useful for the study of other intracellular microorganisms.

Leishmania parasites lodge in host cells within phagolysosome-like structures called parasitophorous vacuoles (PVs). Depending on the species, amastigote forms can be individually hosted within small, tight-fitting PVs or grouped within loose, spacious PVs. Using multidimensional live cell imaging, we examined the biogenesis of the two PV phenotypes in macrophages exposed to L. major (a representative of the tight PV phenotype) or L. amazonensis (an example of the loose PV phenotype) amastigotes. L. major PVs undergo fission as parasites divide; we demonstrate that in the course of fission there are transients of the lysosomotropic fluorescent probe Lysotracker. In contrast, during the course of amastigote population size expansion, L. amazonensis PVs do accumulate Lysotracker while increasing in diameter and volume. The large PVs fuse together, and the products of fusion undergo size and shape remodeling. The biogenesis/remodeling of the two types of Leishmania PVs is accompanied by a reduction in the number of macrophage acidic vesicles. The present imaging study adds new morphometric information to the cell biology of Leishmania amastigote intracellular parasitism.

Fusion between Leishmania amazonensis and Leishmania major parasitophorous vacuoles: live imaging of coinfected macrophages

Protozoan parasites of the genus Leishmania alternate between flagellated, elongated extracellular promastigotes found in insect vectors, and round-shaped amastigotes enclosed in phagolysosome-like Parasitophorous Vacuoles (PVs) of infected mammalian host cells. Leishmania amazonensis amastigotes occupy large PVs which may contain many parasites; in contrast, single amastigotes of Leishmania major lodge in small, tight PVs, which undergo fission as parasites divide. To determine if PVs of these Leishmania species can fuse with each other, mouse macrophages in culture were infected with non-fluorescent L. amazonensis amastigotes and, 48 h later, superinfected with fluorescent L. major amastigotes or promastigotes. Fusion was investigated by time-lapse image acquisition of living cells and inferred from the colocalization of parasites of the two species in the same PVs. Survival, multiplication and differentiation of parasites that did or did not share the same vacuoles were also investigated. Fusion of PVs containing L. amazonensis and L. major amastigotes was not found. However, PVs containing L. major promastigotes did fuse with pre-established L. amazonensis PVs. In these chimeric vacuoles, L. major promastigotes remained motile and multiplied, but did not differentiate into amastigotes. In contrast, in doubly infected cells, within their own, unfused PVs metacyclic-enriched L. major promastigotes, but not log phase promastigotes - which were destroyed - differentiated into proliferating amastigotes. The results indicate that PVs, presumably customized by L. major amastigotes or promastigotes, differ in their ability to fuse with L. amazonensis PVs. Additionally, a species-specific PV was required for L. major destruction or differentiation – a requirement for which mechanisms remain unknown. The observations reported in this paper should be useful in further studies of the interactions between PVs to different species of Leishmania parasites, and of the mechanisms involved in the recognition and fusion of PVs.

Many non-viral intracellular pathogens lodge within cell vesicles known as “parasitophorous vacuoles” (PVs), which exhibit a variety of pathogen-dependent functional and compositional phenotypes. PVs of the protozoan Leishmania are similar to the digestive organelles known as phagolysosomes. We asked if, in phagocytes infected with two different Leishmania species, would the two parasites be found in the same or in separate vacuoles? Of the species chosen, Leishmania amazonensis develops within large vacuoles which shelter many parasites; in contrast, Leishmania major lodges in small PVs containing one or two parasites. In the present experiments, the species and their life-cycle stages (extracellular promastigotes, and intracellular amastigotes) were distinguished by means of fluorescent markers, and the intracellular localization of the parasites was examined in living cells. We report here that, whereas L. major amastigotes remained within their individual vacuoles, L. major promastigotes were delivered to L. amazonensis vacuoles, in which they survived and multiplied but were unable to differentiate into amastigotes. A species-specific vacuole was thus required for L. major differentiation. The model should be useful in cellular and molecular studies of the biology of these parasites and of their parasitophorous vacuoles.

Macrophage and Leishmania: An Unacceptable Coexistence

Research over the past year has revealed several significant and interesting advances in the biology of macrophage, key cells responsible in body's host defense against invading pathogens and in immune responses. Perturbation of macrophage surface with different bacterial pathogens leads to activate general signal transduction pathways of macrophages, including activation of NADPH oxidase, nitric oxide synthase, and so on. However, in this review, the results of macrophage interactions only with Leishmania parasites, which harbors the host macrophages, are discussed. It appears that interference in transduction of regulatory signals during leishmanial invasion lead to an inadequate leishmanicidal response. In this connection, information concerning regulation of MHC molecules and other current events related to macrophage function after invasion by the parasites are also discussed.

Leishmania major: genome analysis for identification of putative adhesin-like and other surface proteins

The three Tritryps, the pathogenic protozoa, Leishmania major, Trypanosoma brucei and Trypanosoma cruzi use surface molecules among others to evolve strategies for evading the immune system and for their survival in the host systems. Since only 36% of the protein coding genes in L. major genome have a putative function ascribed to them, we undertook a genome analysis of L. major genome for identification of adhesin-like and other surface proteins from amongst these hypothetical sequences. Our analysis resulted in the identification of a total of 194 hits, 120 of which had a predicted transmembrane region, 56 had both a transmembrane and signal peptide region, 1 sequence had only a predicted signal peptide region whereas 17 sequences had neither of the two. Six protein sequences could be assigned a putative adhesin-like domain region based on the analysis. Hopefully future detailed experimental studies will elucidate more vividly the role of these hits in Leishmania pathogenesis.

Maturation of the Coxiella burnetii parasitophorous vacuole requires bacterial protein synthesis but not replication

This study examined whether protein synthesis and replication are required for maturation and fusogenicity of the lysosomal-like, large and spacious parasitophorous vacuole (PV) of Coxiella burnetii, an obligate intracellular bacterium. Large and spacious PV with multiple non-replicating C. burnetii were observed by phase microscopy in Vero cells infected at a multiplicity of infection of ten and treated with a bacteriostatic concentration of nalidixic acid or carbenicillin, antimicrobics that inhibit DNA and cell wall biosynthesis respectively. Conversely, large and spacious PV were not observed in cells treated with a bacteriostatic concentration of the protein synthesis inhibitor chloramphenicol. Rather, fluorescence microscopy of individual cells revealed multiple, acidic PV harbouring a single organism tightly bounded by a LAMP-1 positive vacuolar membrane. These vacuoles homotypically fused to form a large and spacious PV upon removal of the drug. Chloramphenicol also inhibited trafficking of latex beads to large and spacious PV and caused mature PV to collapse. Collectively, these results demonstrate that C. burnetii protein synthesis, but not replication, is required for fusion between nascent C. burnetii PV and latex bead phagosomes, and also for formation and maintenance of large and spacious, replicative PV. However, transit of nascent PV through the endocytic pathway to ultimately acquire lysosomal markers appears to occur irrespective of Coxiella protein synthesis.

Glycoinositol phospholipids from Trypanosoma cruzi transmit signals to the cells of the host immune system through both ceramide and glycan chains

Chagas' disease is a chronic disease affecting millions of people in Latin America. The cell surface of Trypanosoma cruzi, the etiological agent, is covered by a glycocalyx whose components play important roles in parasite survival and infectivity. The most abundant surface component is a glycolipid (glycoinositol phospholipid, GIPL) related in structure to glycosylphosphatidyl inositol anchors. In this review, we describe the biological effects of highly purified native GIPLs and their glycan or lipid moities on cells of the host immune system.

Glycoinositolphospholipids from Trypanosoma cruzi interfere with macrophages and dendritic cell responses

To investigate the possible effects of glycoinositolphospholipid (GIPL) from Trypanosoma cruzi on human antigen presenting cells, we tested their effects on lipopolysaccharide (LPS)-stimulated human macrophages and dendritic cells (DC). Human macrophages or DC were incubated with GIPL (50 microg/ml) and LPS (500 pg/ml) and tumor necrosis factor alpha (TNF-alpha), interleukin 8 (IL-8), IL-10, and IL-12p40 levels in supernatants were analyzed by enzyme-linked immunosorbent assay. TNF-alpha, IL-10, and IL-12 secretion were significantly decreased by GIPL both in macrophages and DC. In contrast, GIPL did not alter IL-8 production. We also analyzed the expression of CD80, CD86, HLA-DR, CD40, and CD57 on the macrophage surface after stimulation with LPS in the presence or absence of T. cruzi GIPL. GIPL led to a down-regulation in the expression of all tested molecules. We additionally examined the influence of T. cruzi GIPL on the response of human DC to LPS. LPS-induced HLA-DR, CD83, and CD86 up-regulation was significantly inhibited by GIPL. A slight down-regulation in CD80 and CD40 expression on DC surfaces in the presence of GIPL was also noticed. Similarly, GIPL led to down-modulation of CD83, CD80, CD86, and HLA-DR surface expression and TNF-alpha and IL-10 production when DC were stimulated by CD40L. The ceramide portion of GIPL was responsible for most of the activity exhibited by the whole molecule. Considering the important role of the immune response in determining the fate of the host-parasite relationship, the immunoregulatory activities of T. cruzi GIPL are potentially important for parasite evasion and then pathogenesis of infection with protozoan parasites.

Biogenesis ofLeishmania-harbouring parasitophorous vacuoles following phagocytosis of the metacyclic promastigote or amastigote stages of the parasites

Protozoan parasites Leishmania alternate between a flagellated promastigote form and an amastigote form. In their mammalian hosts, Leishmania survive and multiply in macrophages. Both forms can be internalized by these host cells at different stages of the infectious process and eventually establish themselves within parasitophorous vacuoles exhibiting phagolysosomal properties. To determine whether the biogenesis of these organelles differs according to the parasitic stage used to initiate infection, we compared their formation kinetics after phagocytosis of either metacyclic promastigotes or amastigotes of L. amazonensis or of L. major by mouse bone-marrow-derived macrophages pre-exposed or not to IFN-γ. After 10 minutes of contact, an accumulation of F-actin was observed around the promastigotes and amatigotes undergoing phagocytosis or those that had already been internalized. This accumulation was transient and rapidly disappeared at later times. At 30 minutes, most of the promastigotes were located in long, narrow organelles that were exactly the same shape as the parasites. The latter were elongated with their cell bodies near to the macrophage nucleus and their flagella towards the periphery. This suggests that promastigote phagocytosis mainly occurs in a polarized manner, with the cell body entering the macrophages first. Most, if not all, of the phagocytosed promastigotes were located in organelles that rapidly acquired phagolysosomal properties. At 30 minutes, lamp-1, macrosialin, cathepsins B and D were detected in 70-98% of these compartments and about 70% of them were surrounded by rab7p. These late endosome/lysosome `markers' were recruited through fusion with late endocytic compartments. Indeed, when late endosomes/lysosomes were loaded with fluorescein dextran, 81-98% of the promastigote-harbouring compartments contained the endocytic tracer 30 minutes after infection. Electron microscopy of infected macrophages previously loaded with peroxidase confirmed that the phagosomes rapidly fused with late endocytic compartments. When the amastigote stage of L. amazonensiswas used to initiate infection, the kinetics of acquisition of the different late endosome/lysosome `markers' by the phagosomes were similar to those measured after infection with metacyclics. However, more rab7p+-phagosomes were observed at early time points (e.g. 90% were rab7p+ at 30 minutes). The early endosome `markers', EEA1 and the transferrin receptor, were hardly detected in parasite-containing compartments regardless of the parasitic stage used to infect macrophages and the time after infection. In conclusion, both metacyclic- and amastigote-containing phagosomes fuse with late endosomes/lysosomes within 30 minutes. However, with L. amazonensis, the time required for the formation of the huge parasitophorous vacuoles, which are characteristic of this species, was much shorter after infection with amastigotes than after infection with metacyclic promastigotes. This indicates that the initial fusions with late endosomes/lysosomes are followed by a stage-specific sequence of events.

Parasitic adaptive mechanisms in infection by leishmania

Leishmania are a resilient group of intracellular parasites that infect macrophages. The resultant complex of diseases, or leishmaniases, caused by the parasites affect over twelve million people worldwide. Leishmania have developed unique adaptive mechanisms to ensure their survival in the harsh environments faced throughout their life cycle. These parasites must not only contend with the hostile digestive conditions found within the sand fly vector, but they must also avoid destruction by the host immune system while in the bloodstream, before entering the macrophage. To do so, Leishmania express unique lipophosphoglycan (LPG) molecules and the metalloprotease gp63, among other proteins, on their cell surface. To enter the macrophage, Leishmania utilizes a variety of cellular receptors to mediate endocytosis. Once inside the macrophage, Leishmania is protected from phagolysosome degradation by a variety of adaptations to inhibit cellular defense mechanisms. These include the inhibition of phagosome-endosome fusion, hydrolytic enzymes, cell signaling pathways, nitric oxide production, and cytokine production. While other parasites can also infect macrophages, Leishmania is distinctive in that it not only relies on its own defenses to survive and reproduce within the macrophage phagolysosome, but Leishmania also manipulates the host immune response in order to protect itself and to gain entry into the cell. These unique adaptive mechanisms help promote Leishmania survival.

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.

Phosphoglycan repeat-deficient Leishmania mexicana parasites remain infectious to macrophages and mice

The human pathogen Leishmania synthesizes phosphoglycans (PGs) formed by variably modified phosphodisaccharide [6-Galbeta1-4Manalpha1-PO(4)] repeats and mannooligosaccharide phosphate [(Manalpha1-2)(0-5)Manalpha1-PO(4)] caps that occur lipid-bound on lipophosphoglycan, protein-bound on proteophosphoglycans, and as an unlinked form. PG repeat synthesis has been described as essential for survival and development of Leishmania throughout their life cycle, including for virulence to the mammalian host. In this study, this proposal was investigated in Leishmania mexicana using a spontaneous mutant that was fortuitously isolated from an infected mouse, and by generating a lmexlpg2 gene deletion mutant (Deltalmexlpg2), that lacks a Golgi GDP-Man transporter. The spontaneous mutant lacks PG repeats but synthesizes normal levels of mannooligosaccharide phosphate caps, whereas the Deltalmexlpg2 mutant is deficient in PG repeat synthesis and down-regulates cap expression. In contrast to expectations, both L. mexicana mutants not only retain their ability to bind to macrophages, but are also indistinguishable from wild type parasites with respect to colonization of and multiplication within host cells. Moreover, in mouse infection studies, the spontaneous L. mexicana repeat-deficient mutant and the Deltalmexlpg2 mutant showed no significant difference to a wild type strain with respect to the severity of disease caused by these parasites. Therefore, at least in Leishmania mexicana, PG repeat synthesis is not an absolute requirement for virulence.

The immunologically protective P-4 antigen of Leishmania amastigotes. A developmentally regulated single strand-specific nuclease associated with the endoplasmic reticulum

The purified membrane-associated Leishmania pifanoi amastigote protein P-4 has been shown to induce protective immunity against infection and to elicit preferentially a T helper 1-like response in peripheral blood mononuclear cells of patients with American cutaneous leishmaniasis. As this molecule is potentially important for future vaccine studies, the L. pifanoi gene encoding the P-4 membrane protein was cloned and sequenced. Southern blot analyses indicate the presence of six tandemly arrayed copies of the P-4 gene in L. pifanoi; homologues of the P-4 gene are found in all other species of the genus Leishmania examined. DNA-derived protein sequence data indicated an identity to the P1 zinc-dependent nuclease of Penicillium citrinum (20.8%) and the C-terminal domain of the 3' nucleotidase of Leishmania donovani (33.7%). Consistent with these sequence analyses, purified L. pifanoi P-4 protein possesses single strand nuclease (DNA and RNA) and phosphomonoesterase activity, with a preference for UMP > TMP > AMP >> CMP. Double-labeling immunofluorescence microscopic analyses employing anti-binding protein antibodies revealed that the P-4 protein is localized in the endoplasmic reticulum of the amastigote. Northern blot analyses indicated that the gene is selectively expressed in the intracellular amastigote stage (mammalian host) but not in the promastigote stage (insect) of the parasite. Based upon its subcellular localization and single-stranded specific nuclease activity, possible roles of the P-4 nuclease in the amastigote in RNA stability (gene expression) or DNA repair are discussed.

Proteophosphoglycans of Leishmania

Proteophosphoglycans are an expanding family of highly glycosylated Leishmania proteins with many unusual and some unique structural features. The novel protein-glycan linkage in proteophosphoglycans - phosphoglycosylation of Ser by lipophosphoglycan-like structures - emerges as a major form of protein glycosylation in Leishmania. Here, Thomas Ilg reviews the chemical structure, the ultrastructure, the genes and the potential functions of different members of this novel family of parasite glycoproteins.

Proteophosphoglycans of Leishmania mexicana. Identification, purification, structural and ultrastructural characterization of the secreted promastigote proteophosphoglycan pPPG2, a stage-specific glycoisoform of amastigote aPPG

Protozoan parasites of the genus Leishmania secrete a range of proteophosphoglycans that appear to be important for successful colonization of the sandfly and for virulence in the mammalian host. A hallmark of these molecules is extensive phosphoglycosylation by phosphoglycan chains via the unusual linkage Manalpha1-PO(4)-Ser. In this study we have identified and purified to apparent homogeneity a novel proteophosphoglycan (pPPG2) which is secreted by Leishmania mexicana promastigotes (sandfly stage). Amino acid analysis and immunoblots using polypeptide-specific antisera suggest that pPPG2 shares a common protein backbone with a proteophosphoglycan (aPPG) secreted by Leishmania mexicana amastigotes (mammalian stage). Both pPPG2 and aPPG show a similar degree of Ser phosphoglycosylation (50. 5 mol% vs. 44.6 mol%), but the structure of their phosphoglycan chains is developmentally regulated: in contrast to aPPG which displays unique, complex and highly branched glycan chains [Ilg, Craik, Currie, Multhaup, and Bacic (1998) J. Biol. Chem. 273, 13509-13523], pPPG2 contains short unbranched structures consisting of >60 mol% neutral glycans, most likely (Manalpha1-2)(0-5)Man and Galbeta1-4Man, as well as about 40 mol% monophosphorylated glycans of the proposed structures PO(4)-6Galbeta1-4Man and PO(4)-6(Glcbeta1-3)Galbeta1-4Man. The major differences between pPPG2 and aPPG with respect to their apparent molecular mass, their ultrastructure and their proteinase sensitivity are most likely a consequence of this stage-specific glycosylation of their common protein backbone.

Proteophosphoglycans of Leishmania mexicana. Molecular cloning and characterization of the Leishmania mexicana ppg2 gene encoding the proteophosphoglycans aPPG and pPPG2 that are secreted by amastigotes and promastigotes

Intracellular amastigotes of the pathogenic protozoon Leishmania mexicana secrete an extensively phosphoglycosylated proteophosphoglycan (aPPG) into the phagolysosome of mammalian host macrophages, that appears to fulfil important functions for the parasites. Promastigotes (the sandfly vector forms) of the same species secrete a proteophosphoglycan with identical protein backbone but exhibiting stage-specific phosphoglycosylation patterns [Klein, Göpfert, Goehring, Stierhof and Ilg (1999) Biochem. J. 344, 775-786]. In this study we report the cloning of the novel repeat-containing proteophosphoglycan gene ppg2 by antibody screening of a Leishmania mexicana amastigote cDNA expression library. ppg2 is equally expressed in promastigotes and amastigotes at the mRNA level. Targeted gene replacement of both alleles of the single copy gene ppg2 results in the loss of pPPG2 expression in promastigotes. Antisera against Escherichia coli-expressed ppg2 recognize the deglycosylated forms of aPPG as well as pPPG2. These results confirm that ppg2 encodes the protein backbones of aPPG and pPPG2. An unusual finding is that ppg2 exhibits two stable allelic forms, ppg2a and ppg2b. Their main difference lies in the number of central 72 bp DNA repeats (7 versus 8). ppg2a and ppg2b encode polypeptide chains of 574 and 598 amino acids, respectively, that show no homology to known proteins. The novel 24 amino acid Ser-rich peptide repeats encoded by the 72 bp DNA repeats are targets for Ser phosphoglycosylation in Leishmania mexicana.

Cell biology of Leishmania

Leishmania are digenetic protozoa which inhabit two highly specific hosts, the sandfly, where they grow as motile flagellated promastigotes in the gut, and the mammalian macrophage, where they survive and grow intracellularly as non-flagellated amastigotes in the phagolysosome. Leishmaniasis is the outcome of an evolutionary 'arms race' between the host's immune system and the parasite's evasion mechanisms, which ensure survival and transmission in the population. The diverse spectrum of patterns and severity of disease reflect the varying contributions of parasite virulence factors and host responses, some of which act in a host protective manner while others exacerbate disease. This chapter describes the interaction of the Leishmania with their hosts, with emphasis on the molecules and mechanisms evolved by the parasites to avoid, subvert or exploit the environments in the sandfly and the macrophage, and to move from one to the other.

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.

Proteophosphoglycan, a major secreted product of intracellular Leishmania mexicana amastigotes, is a poor B-cell antigen and does not elicit a specific conventional CD4+ T-cell response

Secreted and surface-exposed antigens of intracellular pathogens are thought to provide target structures for detection by the host immune system. The major secreted product of intracellular Leishmania mexicana amastigotes, a proteophosphoglycan (aPPG), is known to contribute to the establishment of the parasitophorous vacuole and is able to activate complement. aPPG belongs to a novel class of serine- and threonine-rich Leishmania proteins that are extensively modified by phosphodiester-linked phosphooligosaccharides and terminal mannooligosaccharides. Here we show that mice chronically infected with L. mexicana generally do not produce antibodies or Th cells specific for aPPG. Similarly, antibody titers are very low in mice vaccinated with aPPG, and specific CD4+ T cells are undetectable. Comparative analyses of other Leishmania glycoconjugates indicate that L. mexicana-specific carbohydrate structures are poorly immunogenic in mice and that the proteophosphoglycan aPPG behaved immunologically like a carbohydrate. The latter observation is explained by the lack of induction of aPPG-specific CD4+ T cells. In contrast, recombinant aPPG peptides stimulate CD4+ T-cell responses and high titers of specific antibodies are found in the sera of mice vaccinated with these peptides. Native aPPG is highly resistant to proteinases and apparently cannot be degraded by macrophages. It is concluded that conventional CD4+ T cells against the polypeptide backbone of aPPG are not induced because the molecule resists antigen processing due to its extensive and complex carbohydrate modification. The complex glycan chains of aPPG, which exhibit important biological functions for the parasite, may therefore also have evolved to evade detection by the immune system of the host organism.

Leishmania major proteophosphoglycan is expressed by amastigotes and has an immunomodulatory effect on macrophage function

Proteophosphoglycan (PPG) is a newly described mucin-like glycoprotein found on the surface of Leishmania major promastigotes and secreted in the culture supernatant. We show here that antigenically similar PPGs are present in several Leishmania species. PPG could also be detected on the surface of amastigotes and in small, parasite-free vesicles in infected macrophages. Because of the similarity of its carbohydrate chains to lipophosphoglycan, a parasite receptor for host macrophages, PPG was tested for binding to macrophages. PPG bound to macrophages and was internalized in a time-dependent manner. PPG inhibited the production of tumor necrosis factor-alpha and synergized with interferon-gamma to stimulate the production of nitric oxide by macrophages. PPG may contribute to the binding of Leishmania to host cells and may play a role in modulating the biology of the infected macrophage at the early stage of infection.

Characterization of a novel GDP-mannose:Serine-protein mannose-1-phosphotransferase from Leishmania mexicana

Protozoan parasites of the genus Leishmania secrete a number of glycoproteins and mucin-like proteoglycans that appear to be important parasite virulence factors. We have previously proposed that the polypeptide backbones of these molecules are extensively modified with a complex array of phosphoglycan chains that are linked to Ser/Thr-rich domains via a common Manalpha1-PO4-Ser linkage (Ilg, T., Overath, P., Ferguson, M. A. J., Rutherford, T., Campbell, D. G., and McConville, M. J. (1994) J. Biol. Chem. 269, 24073-24081). In this study, we show that Leishmania mexicana promastigotes contain a peptide-specific mannose-1-phosphotransferase (pep-MPT) activity that adds Manalpha1-P to serine residues in a range of defined peptides. The presence and location of the Manalpha1-PO4-Ser linkage in these peptides were determined by electrospray ionization mass spectrometry and chemical and enzymatic treatments. The pep-MPT activity was solubilized in non-ionic detergents, was dependent on Mn2+, utilized GDP-Man as the mannose donor, and was expressed in all developmental stages of the parasite. The pep-MPT activity was maximal against peptides containing Ser/Thr-rich domains of the endogenous acceptors and, based on competition assays with oligosaccharide acceptors, was distinct from other leishmanial MPTs involved in the initiation and elongation of lipid-linked phosphoglycan chains. In subcellular fractionation experiments, pep-MPT was resolved from the endoplasmic reticulum marker BiP, but had an overlapping distribution with the cis-Golgi marker Rab1. Although Man-PO4 residues in the mature secreted glycoproteins are extensively modified with mannose oligosaccharides and phosphoglycan chains, similar modifications were not added to peptide-linked Man-PO4 residues in the in vitro assays. Similarly, Man-PO4 residues on endogenous polypeptide acceptors were also poorly extended, although the elongating enzymes were still active, suggesting that the pep-MPT activity and elongating enzymes may be present in separate subcellular compartments.

Proapoptotic Activity of a Trypanosoma cruzi Ceramide-Containing Glycolipid Turned on in Host Macrophages by IFN-γ

The effects of glycoinositolphospholipid (GIPL), from the pathogenic protozoan Trypanosoma cruzi, and its isolated glycan and lipid (dihydroceramide) components, were investigated in J774 cells and primary macrophages. Isolated GIPL ceramide, but not intact GIPL or its glycan, induced intense fluid phase endocytosis when added exogenously. In the presence of the cytokine IFN-γ, GIPL ceramide induced marked apoptosis in J774 cells and macrophages, independent of nitric oxide secretion. When cells were preincubated with the GIPL-derived glycan chain, addition of intact GIPL induced macrophage apoptosis in the presence of IFN-γ. Synthetic C2-dihydroceramide also induced apoptosis in the presence of IFN-γ. Induction of apoptosis in T. cruzi-infected macrophages by GIPL ceramide plus IFN-γ led to increased parasite release compared with IFN-γ treatment alone. Viable parasites released comprised both infective trypomastigote and spheromastigote forms. These results identify a novel pathway by which T. cruzi glycosylphosphatidylinositol family molecules affect host macrophages, with implications for the infectious process.

The biogenesis and properties of the parasitophorous vacuoles that harbour Leishmania in murine macrophages

Leishmania are protozoan parasites that, as amastigotes, live in the macrophages of mammalian hosts within compartments called parasitophorous vacuoles. These organelles share features with late endosomes/lysosomes and are also involved in the trafficking of several major histocompatibility complex (MHC)-encoded molecules. Improved knowledge of the parasitophorous vacuoles may help clarify how these protozoa persist in their hosts.

Structure of a filamentous phosphoglycoprotein polymer: the secreted acid phosphatase of Leishmania mexicana

The insect stage of the protozoan parasite Leishmania mexicana secretes a filamentous acid phosphatase (secreted acid phosphatase, SAP), a polymeric phosphoglycoprotein. The wild-type (wt) SAP filament is a copolymer composed of two related gene products SAP1 and SAP2, which are identical in the enzymatically active NH2-terminal domain and the COOH-terminal domain, but differ in the length of a highly glycosylated Ser/Thr-rich repeat region (32 amino acids and 383 amino acids, respectively) which is located between these domains. When expressed separately, full length SAP1, SAP2, or the NH2-terminal domain alone, are able to assemble into filaments. The Ser/Thr-rich region is the exclusive target for a novel type of O-glycosylation via phosphoserines. By using glycerol spraying/low-angle rotary metal shadowing and labelling with monoclonal antibodies it is demonstrated that the repetitive region adopts an extended conformation forming side arms which project radially from the filament core and terminate with the COOH-terminal domain. The length of the side arms of SAP1 and SAP2 (20 nm and 90 nm, respectively) corresponds to the predicted length of the Ser/Thr-rich repeat region of SAP1 and SAP2. Mass determination by scanning electron microscopy (STEM) shows that one morphologically defined globular particle of the filament core is a polypeptide dimer. We propose a model for the filament core, in which the globular NH2-terminal SAP domains form one strand composed of polypeptide dimers or two tightly associated strands of monomers which may twist into a double helix, similar to actin filaments. The highly O-glycosylated side arms project from the filament core conferring an overall bottle-brush-like appearance. The L. mexicana SAP is compared to SAPs secreted by the closely related species L. amazonensis and L. donovani.

Stage-specific proteophosphoglycan from Leishmania mexicana amastigotes. Structural characterization of novel mono-, di-, and triphosphorylated phosphodiester-linked oligosaccharides

Intracellular amastigotes of the protozoan parasite Leishmania mexicana secrete a macromolecular proteophosphoglycan (aPPG) into the phagolysosome of their host cell, the mammalian macrophage. The structures of aPPG glycans were analyzed by a combination of high pH anion exchange high pressure liquid chromatography, gas chromatography-mass spectrometry, enzymatic digestions, electrospray-mass spectrometry as well as 1H and 31P NMR spectroscopy. Some glycans are identical to oligosaccharides known from Leishmania mexicana promastigote lipophosphoglycan and secreted acid phosphatase. However, the majority of the aPPG glycans represent amastigote stage-specific and novel structures. These include neutral glycans ([Glcbeta1-3]1-2Galbeta1-4Man, Galbeta1-3Galbeta1-4Man, Galbeta1-3Glcbeta1-3Galbeta1-4Man), several monophosphorylated glycans containing the conserved phosphodisaccharide backbone (R-3-[PO4-6-Gal]beta1-4Man) but carrying stage-specific modifications (R = Galbeta1-, [Glcbeta1-3]1-2Glcbeta1-), and monophosphorylated aPPG tri- and tetrasaccharides that are uniquely phosphorylated on the terminal hexose (PO4-6-Glcbeta1-3Galbeta1-4Man, PO4-6-Glcbeta1-3Glcbeta1-3Galbeta1-4Man, PO4-6-Galbeta1-3Glcbeta1-3Galbeta1-4Man). In addition aPPG contains highly unusual di- and triphosphorylated glycans whose major species are PO4-6-Glcbeta1-3Glcbeta1-3[PO4-6-Gal]beta1-4Man, PO4-6-Galbeta1-3Glcbeta1-3[PO4-6-Gal]beta1-4Man, PO4-6-Galbeta1-3Glcbeta1-3Glcbeta1-3[PO4-6-Gal]beta1-+ ++4Man, PO4-6-Glcbeta1-3[PO4-6-Glc]beta1-3[PO4-6-Gal]beta1-4Man, PO4-6-Galbeta1-3[PO4-6-Glc]beta1-3Glcbeta1-3[PO4-6-Gal]beta1 -4Man, and PO4-6-Glcbeta1-3[PO4-6-Glc]beta1-3Glcbeta1-3[PO4-6-Gal]beta1 -4Man. These glycans are linked together by the conserved phosphodiester R-Manalpha1-PO4-6-Gal-R or the novel phosphodiester R-Manalpha1-PO4-6-Glc-R and are connected to Ser(P) of the protein backbone most likely via the linkage R-Manalpha1-PO4-Ser. The variety of stage-specific glycan structures in Leishmania mexicana aPPG suggests the presence of developmentally regulated amastigote glycosyltransferases which may be potential anti-parasite drug targets.

Secreted proteophosphoglycan of Leishmania mexicana amastigotes activates complement by triggering the mannan binding lectin pathway

Cutaneous lesions induced by infection of mice with the protozoan parasite, Leishmania mexicana, contain abundant amounts of a high molecular mass proteophosphoglycan (PPG), which is secreted by the amastigote stage residing in phagolysosomes of macrophages and can then be released into the tissue upon rupture of the infected cells. Amastigote PPG forms sausage-shaped but soluble particles and belongs to a novel class of serine-rich proteins that are extensively O-glycosylated by phosphooligosaccharides capped by mannooligosaccharides. The purified molecule is shown here to efficiently activate complement (C) and deplete hemolytic activity of normal serum and may prevent the opsonization of L. mexicana amastigotes. Complement activation is Ca2+ dependent but does not depend on antibodies or the complement component C1. PPG binds to serum mannan binding protein (MBP), thus activating the MBP-associated serine protease, P100. Subsequently, the C cascade is triggered through C4 leading to covalent modification probably of carbohydrate hydroxyls of PPG by C3 fragments. Thus, PPG is able to activate C via the mannan binding lectin pathway which is unusual for secreted, soluble products of microbial origin. The proteophosphoglycan-induced complement activation is postulated to contribute to the lesion development and pathology caused by the parasite.