Leishmania donovani-infected macrophages: characterization of the parasitophorous vacuole and potential role of this organelle in antigen presentation

A Targeted and Protease-Activated Genetically Encoded Melittin-Containing Particle for the Treatment of Cutaneous and Visceral Leishmaniasis

Intracellular infections are difficult to treat, as pathogens can take advantage of intracellular hiding, evade the immune system, and persist and multiply in host cells. One such intracellular parasite, Leishmania, is the causative agent of leishmaniasis, a neglected tropical disease (NTD), which disproportionately affects the world's most economically disadvantaged. Existing treatments have relied mostly on chemotherapeutic compounds that are becoming increasingly ineffective due to drug resistance, while the development of new therapeutics has been challenging due to the variety of clinical manifestations caused by different Leishmania species. The antimicrobial peptide melittin has been shown to be effective in vitro against a broad spectrum of Leishmania, including species that cause the most common form, cutaneous leishmaniasis, and the most deadly, visceral leishmaniasis. However, melittin's high hemolytic and cytotoxic activity toward host cells has limited its potential for clinical translation. Herein, we report a design strategy for producing a melittin-containing antileishmanial agent that not only enhances melittin's leishmanicidal potency but also abrogates its hemolytic and cytotoxic activity. This therapeutic construct can be directly produced in bacteria, significantly reducing its production cost critical for a NTD therapeutic. The designed melittin-containing fusion crystal incorporates a bioresponsive cathepsin linker that enables it to specifically release melittin in the phagolysosome of infected macrophages. Significantly, this targeted approach has been demonstrated to be efficacious in treating macrophages infected with L. amazonensis and L. donovani in cell-based models and in the corresponding cutaneous and visceral mouse models.

Global Remodeling of Host Proteome in Response to Leishmania Infection

The protozoan parasite Leishmania possesses an intrinsic ability to modulate a multitude of pathways in the host, toward aiding its own proliferation. In response, the host reprograms its cellular, immunological, and metabolic machinery to evade the parasite's lethal impact. Besides inducing various antioxidant signaling pathways to counter the elevated stress response proteins like heme oxygenase-1 (HO-1), Leishmania also attempts to delay host cell apoptosis by promoting anti-apoptotic proteins like Bcl-2. The downstream modulation of apoptotic proteins is regulated by effector pathways, including the PI3K/Akt survival pathway, the mitogen-activated protein kinases (MAPKs) signaling pathway, and STAT phosphorylation. In addition, Leishmania assists in its infection in a time-dependent manner by modulating the level of various proteins of autophagic machinery. Immune effector cells, such as mast cells and neutrophils, entrap and kill the pathogen by secreting various granular proteins. In contrast, the host macrophages exert their leishmanicidal effect by secreting various cytokines, such as IL-2, IL-12, etc. An interplay of various signaling pathways occurs in an organized network that is highly specific to both pathogen and host species. This Review analyzes the modulation of expression of proteins, including the cytokines, providing a realistic approach toward understanding the pathophysiology of disease and predicting some prominent markers for disease intervention and vaccine support strategies.

VAMP3 and VAMP8 regulate the development and functionality of parasitophorous vacuoles housing Leishmania amazonensis

To colonize mammalian phagocytic cells, the parasite Leishmania remodels phagosomes into parasitophorous vacuoles that can be either tight-fitting individual or communal. The molecular and cellular bases underlying the biogenesis and functionality of these two types of vacuoles are poorly understood. In this study, we investigated the contribution of host cell Soluble N-ethylmaleimide-sensitive-factor Attachment protein REceptor proteins to the expansion and functionality of communal vacuoles as well as on the replication of the parasite. The differential recruitment patterns of Soluble N-ethylmaleimide-sensitive-factor Attachment protein REceptor to communal vacuoles harboring L. amazonensis and to individual vacuoles housing L. major led us to further investigate the roles of VAMP3 and VAMP8 in the interaction of Leishmania with its host cell. We show that whereas VAMP8 contributes to optimal expansion of communal vacuoles, VAMP3 negatively regulates L. amazonensis replication, vacuole size, as well as antigen cross-presentation. In contrast, neither proteins has an impact on the fate of L. major. Collectively, our data support a role for both VAMP3 and VAMP8 in the development and functionality of L. amazonensis-harboring communal parasitophorous vacuoles.

Redundant and regulatory roles for Toll-like receptors in Leishmania infection

Toll-like receptors (TLRs) are germline-encoded, non-clonal innate immune receptors, which are often the first receptors to recognize the molecular patterns on pathogens. Therefore, the immune response initiated by TLRs has far-reaching consequences on the outcome of an infection. As soon as the cell surface TLRs and other receptors recognize a pathogen, the pathogen is phagocytosed. Inclusion of TLRs in the phagosome results in quicker phagosomal maturation and stronger adaptive immune response, as TLRs influence co-stimulatory molecule expression and determinant selection by major histocompatibility complex (MHC) class II and MHC class I for cross-presentation. The signals delivered by the TCR-peptide-MHC complex and co-stimulatory molecules are indispensable for optimal T cell activation. In addition, the cytokines induced by TLRs can skew the differentiation of activated T cells to different effector T cell subsets. However, the potential of TLRs to influence adaptive immune response into different patterns is severely restricted by multiple factors: gross specificity for the molecular patterns, lack of receptor rearrangements, sharing of limited number of adaptors that assemble signalling complexes and redundancy in ligand recognition. These features of apparent redundancy and regulation in the functioning of TLRs characterize them as important and probable contributory factors in the resistance or susceptibility to an infection.

Leishmania donovani resides in modified early endosomes by upregulating Rab5a expression via the downregulation of miR-494

Several intracellular pathogens arrest the phagosome maturation in the host cells to avoid transport to lysosomes. In contrast, the Leishmania containing parasitophorous vacuole (PV) is shown to recruit lysosomal markers and thus Leishmania is postulated to be residing in the phagolysosomes in macrophages. Here, we report that Leishmania donovani specifically upregulates the expression of Rab5a by degrading c-Jun via their metalloprotease gp63 to downregulate the expression of miR-494 in THP-1 differentiated human macrophages. Our results also show that miR-494 negatively regulates the expression of Rab5a in cells. Subsequently, L. donovani recruits and retains Rab5a and EEA1 on PV to reside in early endosomes and inhibits transport to lysosomes in human macrophages. Similarly, we have also observed that Leishmania PV also recruits Rab5a by upregulating its expression in human PBMC differentiated macrophages. However, the parasite modulates the endosome by recruiting Lamp1 and inactive pro-CathepsinD on PV via the overexpression of Rab5a in infected cells. Furthermore, siRNA knockdown of Rab5a or overexpression of miR-494 in human macrophages significantly inhibits the survival of the parasites. These results provide the first mechanistic insights of parasite-mediated remodeling of endo-lysosomal trafficking to reside in a specialized early endocytic compartment.

Apoptosis-like cell death in Leishmania donovani treated with KalsomeTM10, a new liposomal amphotericin B

OBJECTIVE

The present study aimed to elucidate the cell death mechanism in Leishmania donovani upon treatment with KalsomeTM10, a new liposomal amphotericin B.

METHODOLOGY/PRINCIPAL FINDINGS

We studied morphological alterations in promastigotes through phase contrast and scanning electron microscopy. Phosphatidylserine (PS) exposure, loss of mitochondrial membrane potential and disruption of mitochondrial integrity was determined by flow cytometry using annexinV-FITC, JC-1 and mitotraker, respectively. For analysing oxidative stress, generation of H2O2 (bioluminescence kit) and mitochondrial superoxide O2- (mitosox) were measured. DNA fragmentation was evaluated using terminal deoxyribonucleotidyl transferase mediated dUTP nick-end labelling (TUNEL) and DNA laddering assay. We found that KalsomeTM10 is more effective then Ambisome against the promastigote as well as intracellular amastigote forms. The mechanistic study showed that KalsomeTM10 induced several morphological alterations in promastigotes typical of apoptosis. KalsomeTM10 treatment showed a dose- and time-dependent exposure of PS in promastigotes. Further, study on mitochondrial pathway revealed loss of mitochondrial membrane potential as well as disruption in mitochondrial integrity with depletion of intracellular pool of ATP. KalsomeTM10 treated promastigotes showed increased ROS production, diminished GSH levels and increased caspase-like activity. DNA fragmentation and cell cycle arrest was observed in KalsomeTM10 treated promastigotes. Apoptotic DNA fragmentation was also observed in KalsomeTM10 treated intracellular amastigotes. KalsomeTM10 induced generation of ROS and nitric oxide leads to the killing of the intracellular parasites. Moreover, endocytosis is indispensable for KalsomeTM10 mediated anti-leishmanial effect in host macrophage.

CONCLUSIONS

KalsomeTM10 induces apoptotic-like cell death in L. donovani parasites to exhibit its anti-leishmanial function.

Hemoglobin guided nanocarrier for specific delivery of amphotericin B to Leishmania infected macrophage

Leishmania donovani being an intracellular parasite poses many challenges against the attempted chemotherapy. After the resistance towards the first line of antileishmanial drug, Amphotericin B has been the treatment of choice against visceral leishmaniasis, a fatal tropical disease. However, unfavorable toxicity profile, severe side effects, prolonged parenteral administration procedure limits the use of Amphotericin B. Lack of available specific delivery system also makes this drug unsafe for the prolonged use. In this current study, a chitosan-chondroitin sulfate based nanodelivery vehicle has been introduced. Hemoglobin has been attached on the surface of the delivery system for specifically targeting the leishmania infected macrophage taking the advantage of Leishmania being highly auxotrophic for heme. This cheap and biodegradable delivery vehicle has improved the toxicity profile and lowered LD50 value of the drug significantly compared to traditional way of its direct administration.

Attenuated Leishmania induce pro-inflammatory mediators and influence leishmanicidal activity by p38 MAPK dependent phagosome maturation in Leishmania donovani co-infected macrophages

Promastigote form of Leishmania, an intracellular pathogen, delays phagosome maturation and resides inside macrophages. But till date limited study has been done to manipulate the phagosomal machinery of macrophages to restrict Leishmania growth. Attenuated Leishmania strain exposed RAW 264.7 cells showed a respiratory burst and enhanced production of pro-inflammatory mediators. The augmentation of pro-inflammatory activity is mostly attributed to p38 MAPK and p44/42 MAPK. In our study, these activated macrophages are found to induce phagosome maturation when infected with pathogenic Leishmania donovani. Increased co-localization of carboxyfluorescein succinimidyl ester labeled pathogenic L. donovani with Lysosome was found. Moreover, increased co-localization was observed between pathogenic L. donovani and late phagosomal markers viz. Rab7, Lysosomal Associated Membrane Protein 1, Cathepsin D, Rab9, and V-ATPase which indicate phagosome maturation. It was also observed that inhibition of V-type ATPase caused significant hindrance in attenuated Leishmania induced phagosome maturation. Finally, it was confirmed that p38 MAPK is the key player in acidification and maturation of phagosome in attenuated Leishmania strain pre-exposed macrophages. To our knowledge, this study for the first time reported an approach to induce phagosome maturation in L. donovani infected macrophages which could potentiate short-term prophylactic response in future.

Leishmania hijacking of the macrophage intracellular compartments

Leishmania spp., transmitted to humans by the bite of the sandfly vector, are responsible for the three major forms of leishmaniasis, cutaneous, diffuse mucocutaneous and visceral. Leishmania spp. interact with membrane receptors of neutrophils and macrophages. In macrophages, the parasite is internalized within a parasitophorous vacuole and engages in a particular intracellular lifestyle in which the flagellated, motile Leishmania promastigote metacyclic form differentiates into non-motile, metacyclic amastigote form. This phenomenon is induced by Leishmania-triggered events leading to the fusion of the parasitophorous vacuole with vesicular members of the host cell endocytic pathway including recycling endosomes, late endosomes and the endoplasmic reticulum. Maturation of the parasitophorous vacuole leads to the intracellular proliferation of the Leishmania amastigote forms by acquisition of host cell nutrients while escaping host defense responses.

Screening strategies to identify new chemical diversity for drug development to treat kinetoplastid infections

The Drugs for Neglected Diseases initiative (DNDi) has defined and implemented an early discovery strategy over the last few years, in fitting with its virtual R&D business model. This strategy relies on a medium- to high-throughput phenotypic assay platform to expedite the screening of compound libraries accessed through its collaborations with partners from the pharmaceutical industry. We review the pragmatic approaches used to select compound libraries for screening against kinetoplastids, taking into account screening capacity. The advantages, limitations and current achievements in identifying new quality series for further development into preclinical candidates are critically discussed, together with attractive new approaches currently under investigation.

Identification of Drosophila gene products required for phagocytosis of Leishmania donovani

The identity and function of host factors required for efficient phagocytosis and intracellular maintenance of the protozoan parasite Leishmania donovani are poorly understood. Utilising the phagocytic capability of Drosophila S2 cells, together with available tools for modulating gene expression by RNAi, we have developed an experimental system in which to identify host proteins of this type on a genome-wide scale. We have shown that L. donovani amastigotes can be phagocytosed by S2 cells, in which they replicate and are maintained in a compartment with features characteristic of mammalian phagolysosomes. Screening with dsRNAs from 1920 conserved metazoan genes has identified transcripts that, when reduced in expression, cause either increased or decreased phagocytosis. Focussing on genes in the latter class, RNAi-mediated knockdown of the small GTPase Rab5, the prenylated SNARE protein YKT6, one sub-unit of serine palmitoyltransferase (spt2/lace), the Rac1-associated protein Sra1 and the actin cytoskeleton regulatory protein, SCAR, all lead to a significant reduction in parasite phagocytosis. A role for the lace mammalian homologue in amastigote uptake by mammalian macrophages has been verified using the serine palmitoyltransferase inhibitor, myriocin. These observations suggest that this experimental approach has the potential to identify a large number of host effectors required for efficient parasite uptake and maintenance.

Targeting host syntaxin-5 preferentially blocks Leishmania parasitophorous vacuole development in infected cells and limits experimental Leishmania infections

Our previous observations established a role for syntaxin-5 in the development of Leishmania parasitophorous vacuoles (LPVs). In this study, we took advantage of the recent identification of Retro-2, a small organic molecule that can cause the redistribution of syntaxin-5; we show herein that Retro-2 blocks LPV development within 2 hours of adding it to cells infected with Leishmania amazonensis. In infected cells incubated for 48 hours with Retro-2, LPV development was significantly limited; furthermore, infected cells harbored four to five times fewer parasites than infected cells incubated in vehicle alone. In vivo studies revealed that Retro-2 curbed experimental L. amazonensis infections in a dose-dependent manner. Retro-2 did not have any appreciable effect on the host cell physiological characteristics; furthermore, it had no apparent toxicity in experimental animals. An unexpected, but welcome, finding was that Retro-2 inhibited the replication of Leishmania parasites in axenic cultures. This study is significant because it identifies an endoplasmic reticulum/Golgi SNARE as a potential target for the control of Leishmania infections; moreover, it suggests that small organic molecules can be identified that can selectively disrupt the vesicle fusion machinery that promotes the development of pathogen-containing compartments without exerting toxic effects on the host.

Imaging of the host/parasite interplay in cutaneous leishmaniasis

An understanding of host-parasite interplay is essential for the development of therapeutics and vaccines. Immunoparasitologists have learned a great deal from 'conventional'in vitro and in vivo approaches, but recent developments in imaging technologies have provided us (immunologists and parasitologists) with the ability to ask new and exciting questions about the dynamic nature of the parasite-immune system interface. These studies are providing us with new insights into the mechanisms involved in the initiation of a Leishmania infection and the consequent induction and regulation of the immune response. Here, we review some of the recent developments and discuss how these observations can be further developed to understand the immunology of cutaneous Leishmania infection in vivo.

Multiple levels of gene regulation mediate differentiation of the intracellular pathogen Leishmania

For many years, mRNA abundance has been used as the surrogate measure of gene expression in biological systems. However, recent genome-scale analyses in both bacteria and eukaryotes have revealed that mRNA levels correlate with steady-state protein abundance for only 50-70% of genes, indicating that translation and post-translation processes also play important roles in determining gene expression. What is not yet clear is whether dynamic processes such as cell cycle progression, differentiation, or response to environmental changes change the relationship between mRNA and protein abundance. Here, we describe a systems approach to interrogate promastigote-to-amastigote differentiation in the obligatory intracellular parasitic protozoan Leishmania donovani. Our results indicate that regulation of mRNA levels plays a major role early in the differentiation process, while translation and post-translational regulation are more important in the latter part. In addition, it appears that the differentiation signal causes a transient global increase in the rate of protein synthesis, which is subsequently down-regulated by phosphorylation of α-subunit of translation initiation factor 2. Thus, Leishmania dynamically changes the relationship between mRNA and protein abundance as it adapts to new environmental circumstances. It is likely that similar mechanisms play a more important role than previously recognized in regulation of gene expression in other organisms.

Innate killing of Leishmania donovani by macrophages of the splenic marginal zone requires IRF-7

Highly phagocytic macrophages line the marginal zone (MZ) of the spleen and the lymph node subcapsular sinus. Although these macrophages have been attributed with a variety of functions, including the uptake and clearance of blood and lymph-borne pathogens, little is known about the effector mechanisms they employ after pathogen uptake. Here, we have combined gene expression profiling and RNAi using a stromal macrophage cell line with in situ analysis of the leishmanicidal activity of marginal zone macrophages (MZM) and marginal metallophilic macrophages (MMM) in wild type and gene targeted mice. Our data demonstrate a critical role for interferon regulatory factor-7 (IRF-7) in regulating the killing of intracellular Leishmania donovani by these specialised splenic macrophage sub-populations. This study, therefore, identifies a new role for IRF-7 as a regulator of innate microbicidal activity against this, and perhaps other, non-viral intracellular pathogens. This study also highlights the importance of selecting appropriate macrophage populations when studying pathogen interactions with this functionally diverse lineage of cells.

Leishmania donovani lacking the Golgi GDP-Man transporter LPG2 exhibit attenuated virulence in mammalian hosts

Surface phosophoglycans such as lipophosphoglycan (LPG) or proteophosphoglycan (PPG) and glycosylinositol phospholipids (GIPLs) modulate essential interactions between Leishmania and mammalian macrophages. Phosphoglycan synthesis depends on the Golgi GDP-mannose transporter encoded by LPG2. LPG2-null (lpg2(-)) Leishmania major cannot establish macrophage infections or induce acute pathology, whereas lpg2(-)Leishmania mexicana retain virulence. lpg2(-)Leishmania donovani has been reported to survive poorly in cultured macrophages but in vivo survival has not been explored. Herein we discovered that, similar to lpg2(-)L. major, lpg2(-)L. donovani promastigotes exhibited diminished virulence in mice, but persisted at consistently low levels. lpg2(-)L. donovani promastigotes could not establish infection in macrophages and could not transiently inhibit phagolysosomal fusion. Furthermore, lpg2(-) promastigotes of L. major, L. donovani and L. mexicana were highly susceptible to complement-mediated lysis. We conclude that phosphoglycan assembly and expression mediated by L. donovani LPG2 are important for promastigote and amastigote virulence, unlike L. mexicana but similar to L. major.

N-(2-hydroxypropyl)methacrylamide-amphotericin B (HPMA-AmB) copolymer conjugates as antileishmanial agents

Leishmaniasis is a major health problem in many parts of the world, caused by various species of Leishmania. Amastigotes are the clinically relevant form of the parasite in the human host and reside in the parasitophorous vacuole within macrophages. Polymer-drug conjugates have been used for lysosomotropic drug delivery and have already shown potential in anticancer and antileishmanial chemotherapy. We synthesised N-(2-hydroxypropyl)methacrylamide-amphotericin B (HPMA-AmB) copolymer conjugates in which the AmB was attached to the polymer through a degradable GlyPheLeuGly linker. Antileishmanial activity was assessed in vitro against intracellular amastigotes in host macrophages [murine peritoneal exudate macrophages (PEMs), murine bone marrow-derived macrophages (BMMs) and differentiated THP-1 cells]. The most potent copolymers had 50% effective concentration (EC(50)) values of 0.03 microg/mL AmB equivalent against Leishmania donovani amastigotes in PEMs and BMMs and an EC(50) of 0.57 microg/mL AmB equivalent against L. donovani in THP-1 cells. This activity was comparable with free AmB (EC(50)=0.03-0.07 microg/mL against L. donovani in PEMs and BMMs and 0.24-0.42 microg/mL against amastigotes in THP-1 cells) and Fungizone (EC(50)=0.04-0.07 microg/mL against amastigotes in PEMs). Conjugates also showed potent in vivo activity with ca. 50% inhibition of parasite burden at 1mg/kg body weight.

Is the in situ inflammatory reaction an important tool to understand the cellular immune response in American tegumentary leishmaniasis?

BACKGROUND

The study of American tegumentary leishmaniasis (ATL) lesions might contribute to the understanding of the dynamics of the infection.

OBJECTIVES

To examine the cellular infiltrate of cutaneous ATL lesions and to compare these results with the detection of the parasites and clinical data.

METHODS

Lesions of 19 patients with ATL were evaluated through immunohistochemical analysis.

RESULTS

The lesions presented an inflammatory reaction mainly consisting of T cells and macrophages. Analysis of the expression of nitric oxide synthase type 2 (NOS2) showed that its intensity was directly correlated with the number of CD3+ T cells. We also observed an association between high NOS2 expression and low quantity of parasites, highlighting the importance of NOS2 in the elimination of parasites.

CONCLUSIONS

The present results suggest that (i) the inflammatory process is intense in cutaneous ATL lesions and maintains a similar activity for several months; (ii) the dynamics of cell infiltration change during this period, with a gradual decrease in CD8+ T cells, probably correlated with a reduction in the parasite number; (iii) neutrophils may participate in the inflammatory process even during later stages of infection; (iv) the relative increase in the number of CD4+ T cells associated with the onset of fibrosis may suggest a participation of these cells in the control of the inflammatory process; and (v) late lesions with tendency for healing usually show focal inflammation. The study of healing lesions might contribute to the understanding of the late steps of the control of the inflammatory process in ATL lesions.

The development of effector T cell subsets in murine Leishmania major infection

Leishmania major infection has proven an exceptional model for CD4+ subset development in inbred mice. Most strains contain infection coincident with the appearance of T helper 1 (Th1) cells that produce gamma-interferon (IFN-gamma) required for macrophage activation. In contrast, mice on the BALB background are unable to control infection due to the development of Th2 cells that produce counter-regulatory cytokines, particularly interleukin 4 (IL-4), capable of abrogating the effects of IFN-gamma. Selective gene disruption studies in mice have illustrated critical components of the host response to L. major. Mice deficient in beta 2 microglobulin, which have no major histocompatibility complex (MHC) class I or CD8+ T cells, control infection as well as wild-type mice, whereas mice deficient in MHC class II (and CD4+ T cells) suffer fatal infection. Mice with disruption of the gene coding IFN-gamma are also incapable of containing infection, reflecting absolute requirements for this cytokine. A number of interventions have been demonstrated to abrogate Th2 cell development in BALB mice, enabling these mice to control infection. Each of these--IL-12, anti-IL-4, anti-IL-2, anti-CD4 and CTLA4-Ig--has in common the capacity to make IL-4 rate limiting at the time of CD4+ cell priming.

The ultrastructure of the parasitophorous vacuole formed by Leishmania major

Protozoan parasites of Leishmania spp. invade macrophages as promastigotes and differentiate into replicative amastigotes within parasitophorous vacuoles. Infection of inbred strains of mice with Leishmania major is a well-studied model of the mammalian immune response to Leishmania species, but the ultrastructure and biochemical properties of the parasitophorous vacuole occupied by this parasite have been best characterized for other species of Leishmania. We examined the parasitophorous vacuole occupied by L. major in lymph nodes of infected mice and in bone marrow-derived macrophages infected in vitro. At all time points after infection, single L. major amastigotes were wrapped tightly by host membrane, suggesting that amastigotes segregate into separate vacuoles during replication. This small, individual vacuole contrasts sharply with the large, communal vacuoles occupied by Leishmania amazonensis. An extensive survey of the literature revealed that the single vacuoles occupied by L. major are characteristic of those formed by Old World species of Leishmania, while New World species of Leishmania form large vacuoles occupied by many amastigotes.

Balancing immunity and pathology in visceral leishmaniasis

Experimental visceral leishmaniasis (VL) caused by infection with Leishmania donovani results in the development of organ-specific immunity in the two main target tissues of infection, the spleen and the liver. The liver is the site of an acute resolving infection associated with the development of inflammatory granulomas around infected Kupffer cells, and resistance to reinfection. Paradoxically, the spleen is an initial site for the generation of cell-mediated immune responses, but ultimately becomes a site of parasite persistence with associated immunopathological changes. These include splenomegaly and a breakdown in tissue architecture that is postulated to contribute to the immunocompromized status of the host. The progressive development of splenic pathology is largely associated with high levels of TNF and interleukin (IL)-10. Follicular dendritic cell (DC) networks are lost, whereas TNF mediates the destruction of marginal zone macrophages and gp38(+) stromal cells, and IL-10 promotes impaired DC migration into T-cell areas with consequent ineffective T-cell priming. Splenic stromal cell function is also altered, promoting the selective development of IL-10-producing DC with immunoregulatory properties. Ultimately, a fine immunological balance determines responses that effectively promote parasite clearance in the liver and those that promote pathology in the spleen, and future investigation aims to separate these responses to offer further means of parasite control in chronically infected VL patients.

Phagocytosis of Leishmania donovani amastigotes is Rac1 dependent and occurs in the absence of NADPH oxidase activation

Macrophages produce little superoxide during phagocytosis of Leishmania donovani amastigotes. In this study, we characterized molecular events associated with L. donovani amastigotes uptake by mouse macrophages, to further define the mechanisms by which they are internalized without triggering superoxide production. Using transient transfections, we first showed that internalization of L. donovani amastigotes is mediated by the GTPases Rac1 and Arf6, of which Rac1 is recruited and retained on parasite-containing phagosomes. Next, we showed that, whereas internalization of amastigotes induced no superoxide release, co-internalization of serum-opsonized zymozan and amastigotes resulted in superoxide production. Furthermore, in co-internalization experiments, we detected superoxide production in over 95% of phagosomes containing IgG-opsonized SRBC compared to 5% of amastigote-harboring phagosomes. These results suggest that amastigotes evade the ability of macrophages to produce superoxide during phagocytosis. Consistently, we observed that amastigotes induced barely detectable phosphorylation of the NADPH oxidase component p47phox, leading to a defective phagosomal recruitment of p67phox and p47phox. Finally, we showed that amastigotes disrupt phagosomal lipid raft integrity, potentially interfering with NADPH oxidase assembly. Collectively, our results indicate that internalization of L. donovani amastigotes is a Rac1- and Arf6-dependent process that occurs in the absence of significant NADPH oxidase activation.

Biogenesis of Leishmania major-harboring vacuoles in murine dendritic cells

In mammalian hosts, Leishmania sp. parasites are obligatory intracellular organisms that invade macrophages and dendritic cells (DC), where they reside in endocytic organelles termed parasitophorous vacuoles (PV). Most of the present knowledge of the characteristics of PV harboring Leishmania sp. is derived from studies with infected macrophages. Since DC play a key role in host resistance to leishmaniasis, there is a need to understand the properties and biogenesis of PV in Leishmania sp.-infected DC. Therefore, we determined the acquisition of endosomal and lysosomal molecules by Leishmania major-containing compartments in DC at different maturation stages, using fluorescence labeling and confocal microscopy. The results show that newly formed phagosomes in DC rapidly develop into late endosomal compartments. However, the small GTPase Rab7, which regulates late fusion processes, was found only in PV of mature bone marrow-derived DC (BMDC); it was absent in immature BMDC, suggesting an arrest of their PV biogenesis at the stage of late endosomes. Indeed, fusion assays with endocytic tracers demonstrated that the fusion activity of L. major-harboring PV toward lysosomes is higher in mature BMDC than in immature BMDC. The inhibition of PV-lysosome fusion in DC is dependent upon the viability and life cycle stage of the parasite, because live promastigotes blocked the fusion almost completely, whereas killed organisms and amastigotes induced a considerable level of fusion activity. The differences in the fusion competences of immature and mature DC may be relevant for their distinct functional activities in the uptake, transport, and presentation of parasite antigens.

Subversion of host cell signalling by the protozoan parasite Leishmania

The protozoa Leishmania spp. are obligate intracellular parasites that inhabit the macrophages of their host. Since macrophages are specialized for the identification and destruction of invading pathogens, both directly and by triggering an innate immune response, Leishmania have evolved a number of mechanisms for suppressing some critical macrophage activities. In this review, we discuss how various species of Leishmania distort the host macrophage's own signalling pathways to repress the expression of various cytokines and microbicidal molecules (nitric oxide and reactive oxygen species), and antigen presentation. In particular, we describe how MAP Kinase and JAK/STAT cascades are repressed, and intracellular Ca2+ and the activities of protein tyrosine phosphatases, in particular SHP-1, are elevated.

Subversion mechanisms by which Leishmania parasites can escape the host immune response: a signaling point of view

The obligate intracellular parasite Leishmania must survive the antimicrobial activities of its host cell, the macrophage, and prevent activation of an effective immune response. In order to do this, it has developed numerous highly successful strategies for manipulating activities, including antigen presentation, nitric oxide and oxygen radical generation, and cytokine production. This is generally the result of interactions between Leishmania cell surface molecules, particularly gp63 and LPG, and less well identified macrophage surface receptors, causing the distortion of specific intracellular signaling cascades. We describe some of the signaling pathways and intermediates that are repressed in infected cells, including JAK/STAT, Ca(2+)-dependent protein kinase C (PKC) isoforms, and mitogen-activated protein kinases (especially ERK1/2), and proteasome-mediated transcription factor degradation. We also discuss protein tyrosine phosphatases (particularly SHP-1), intracellular Ca2+, Ca(2+)-independent PKC, ceramide, and the suppressors of cytokine signaling family of repressors, which are all reported to be activated following infection, and the role of parasite-secreted cysteine proteases.

Leishmania donovani engages in regulatory interference by targeting macrophage protein tyrosine phosphatase SHP-1

Protozoan parasites of the genus leishmania are obligate intracellular parasites of monocytes and macrophages. These pathogens have evolved to invade the mammalian immune system and typically survive for long periods of time. Leishmania have developed a variety of remarkable strategies to prevent their elimination by both innate and acquired immune effector mechanisms. One particular strategy of interest involves manipulation of host cell regulatory pathways so as to prevent macrophage activation required for efficient microbicidal activity. These interference mechanisms are the main focus of this review. Several lines of evidence have been developed to show that the Src homology-2 domain containing tyrosine phosphatase-1 (SHP-1) becomes activated in leishmania-infected cells and that this contributes to disease pathogenesis. Recent studies aimed at understanding the mechanism responsible for the change in activation state of SHP-1 led to the identification of leishmania EF-1alpha as an SHP-1 binding protein and SHP-1 activator. This was a surprising finding given that this ubiquitous and highly conserved protein plays an essential role in protein translation in both prokaryotic and eukaryotic cells. The role of leishmania EF-1alpha as an SHP-1 activator and its contribution to pathogenesis are reviewed with particular attention to the properties that distinguish it from host EF-1alpha.

Contribution of electron and confocal microscopy in the study of Leishmania-macrophage interactions

Promastigotes of the protozoan parasite genus Leishmania are inoculated into a mammalian host when an infected sand fly takes a bloodmeal. Following their opsonization by complement, promastigotes are phagocytosed by macrophages. There, promastigotes differentiate into amastigotes, the form of the parasite that replicates in the phagolysosomal compartments of host macrophages. Although the mechanisms by which promastigotes survive the microbicidal consequence of phagocytosis remain, for the most part, to be elucidated, evidence indicates that glycoconjugates play a role in this process. One such glycoconjugate is lipophosphoglycan, an abundant promastigote surface glycolipid. Using quantitative electron and confocal laser scanning microscopy approaches, evidence was provided that L. donovani promastigotes inhibit phagolysosome biogenesis in a lipophosphoglycan-dependent manner. This inhibition correlates with an accumulation of periphagosomal F-actin, which may potentially form a physical barrier that prevents L. donovani promastigote-containing phagosomes from interacting with endocytic vacuoles. Inhibition of phagosome maturation may constitute a strategy to provide an environment propitious to the promastigote-to-amastigote differentiation.

Leishmania-induced inhibition of macrophage antigen presentation analyzed at the single-cell level

A number of studies have previously examined the capacity of intracellular Leishmania parasites to modulate the capacity of macrophages to process and present Ags to MHC class II-restricted CD4(+) T cells. However, the bulk culture approaches used for assessing T cell activation make interpretation of some of these studies difficult. To gain a more precise understanding of the interaction between Leishmania-infected macrophages and effector T cells, we have analyzed various parameters of T cell activation in individual macrophage-T cell conjugates. Leishmania-infected macrophages efficiently stimulate Ag-independent as well as Ag-dependent, TCR-mediated capping of cortical F-actin in DO.11 T cells. However, infected macrophages are less efficient at promoting the sustained TCR signaling necessary for reorientation of the T cell microtubule organizing center and for IFN-gamma production. A reduced ability to activate these T cell responses was not due to altered levels of surface-expressed MHC class II-peptide complexes. This study represents the first direct single-cell analysis of the impact of intracellular infection on the interaction of macrophages with T cells and serves to emphasize the subtle influence Leishmania has on APC function.

The chlamydial inclusion: escape from the endocytic pathway

Chlamydiae, bacterial obligate intracellular pathogens, are the etiologic agents of several human diseases. A large part of the chlamydial intracellular survival strategy involves the formation of a unique organelle called the inclusion that provides a protected site within which they replicate. The chlamydial inclusion is effectively isolated from endocytic pathways but is fusogenic with a subset of exocytic vesicles that deliver sphingomyelin from the Golgi apparatus to the plasma membrane. A combination of host and parasite functions contribute to the biogenesis of this compartment. Establishment of the mature inclusion is accompanied by the insertion of multiple chlamydial proteins, suggesting that chlamydiae actively modify the inclusion to define its interactions with the eukaryotic host cell. Despite being sequestered within a membrane-bound vacuole, chlamydiae clearly communicate with and manipulate the host cell from within this privileged intracellular niche.

Leishmania EF-1alpha activates the Src homology 2 domain containing tyrosine phosphatase SHP-1 leading to macrophage deactivation

The human leishmaniasis are persistent infections of macrophages caused by protozoa of the genus Leishmania. The chronic nature of these infections is in part related to induction of macrophage deactivation, linked to activation of the Src homology 2 domain containing tyrosine phosphatase-1 (SHP-1) in infected cells. To investigate the mechanism of SHP-1 activation, lysates of Leishmania donovani promastigotes were subjected to SHP-1 affinity chromatography and proteins bound to the matrix were sequenced by mass spectrometry. This resulted in the identification of Leishmania elongation factor-1alpha (EF-1alpha) as a SHP-1-binding protein. Purified Leishmania EF-1alpha, but not host cell EF-1alpha, bound directly to SHP-1 in vitro leading to its activation. Three independent lines of evidence indicated that Leishmania EF-1alpha may be exported from the phagosome thereby enabling targeting of host SHP-1. First, cytosolic fractions prepared from macrophages infected with [(35)S]methionine-labeled organisms contained Leishmania EF-1alpha. Second, confocal, fluorescence microscopy using Leishmania-specific antisera detected Leishmania EF-1alpha in the cytosol of infected cells. Third, co-immunoprecipitation showed that Leishmania EF-1alpha was associated with SHP-1 in vivo in infected cells. Finally, introduction of purified Leishmania EF-1alpha, but not the corresponding host protein into macrophages activated SHP-1 and blocked the induction of inducible nitric-oxide synthase expression in response to interferon-gamma. Thus, Leishmania EF-1alpha is identified as a novel SHP-1-binding and activating protein that recapitulates the deactivated phenotype of infected macrophages.

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.

A microbial strategy to multiply in macrophages: the pregnant pause

Humans live in harmony with much of the microbial world, thanks to a sophisticated immune system. As the first line of defense, macrophages engulf, digest, and display foreign material, then recruit specialists to eliminate potential threats. Yet infiltrators exist: certain fungi, viruses, parasites, and bacteria thrive within sentinel macrophages. By scrutinizing the life styles of these shrewd microbes, we can deduce how macrophages routinely mount an effective immune response. The bimorphic life cycles of three pathogens have dramatic consequences for phagosome traffic. In the transmissible state, Leishmania spp., Coxiella burnetii, and Legionella pneumophila block phagosome maturation; after a pregnant pause, replicative forms emerge and thrive in lysosomes.

Toxoplasma gondii and MHC-restricted antigen presentation: on degradation, transport and modulation

Resistance against Toxoplasma gondii, an obligate intracellular protozoan parasite surrounded by a parasitophorous vacuolar membrane, is mediated by the cellular arm of the immune system, namely CD8+ and CD4+ T cells. Thus, priming and activation of these cells by presentation of antigenic peptides in the context of major histocompatibility complex class I and class II molecules have to take place. This is despite the fact that the vacuolar membrane avoids fusion with the endocytic compartment and acts like a molecular sieve, restricting passive diffusion of larger molecules. This raises several cell biological and immunological questions which will be discussed in this review in the context of our current knowledge about major histocompatibility complex-restricted antigen presentation in other systems: (1) By which pathways are parasite-derived antigens presented to T cells? (2) Has the parasite evolved mechanisms to interfere with major histocompatibility complex-restricted antigen presentation in order to avoid immune recognition? (3) To what extent and by which mechanism is antigenic material, originating from the parasite, able to pass through the vacuolar membrane into the cytosol of the infected cell and is it then accessible to the antigen presentation machinery of the infected cell? (4) What are the actual antigen-presenting cells which prime specific T cells in lymphoid organs? An understanding of these mechanisms will not only provide new insights into the pathogenesis of Toxoplasma gondii and possibly other intravacuolar parasites, but will also improve vaccination strategies.

Interaction of Mycobacterium avium-containing phagosomes with the antigen presentation pathway

Pathogenic mycobacteria infect macrophages where they replicate in phagosomes that minimize contact with late endosomal/lysosomal compartments. Loading of Ags to MHC class II molecules occurs in specialized compartments with late endosomal characteristics. This points to a sequestration of mycobacteria-containing phagosomes from the sites where Ags meet MHC class II molecules. Indeed, in resting macrophages MHC class II levels decreased strongly in phagosomes containing M. avium during a 4-day infection. Phagosomal MHC class II of early (4 h) infections was partly surface-derived and associated with peptide. Activation of host macrophages led to the appearance of H2-M, a chaperon of Ag loading, and to a strong increase in MHC class II molecules in phagosomes of acute (1 day) infections. Comparison with the kinetics of MHC class II acquisition by IgG-coated bead-containing phagosomes suggests that the arrest in phagosome maturation by mycobacteria limits the intersection of mycobacteria-containing phagosomes with the intracellular trafficking pathways of Ag-presenting molecules.

Macrophage subsets harbouring Leishmania donovani in spleens of infected BALB/c mice: localization and characterization

The purpose of the current study was to characterize parasite-containing cells located in spleens of BALB/c mice infected with Leishmania donovani. In particular, expression of MHC class II molecules by these cells was examined to determine whether they could potentially act as cells capable of immunostimulating Leishmania-reactive CD4+ T lymphocytes. To this end, an immunohistological analysis of spleens taken at various time points after infection was undertaken. Using this approach, we observed, in the red pulp, the formation of small cellular infliltrates containing heavily infected macrophages that could be stained with the monoclonal antibodies MOMA-2 and FA/11. All of them expressed high levels of MHC class II molecules. Parasites were also detected in the white pulp, especially in MOMA-2+, FA/11+ and MHC class II+ macrophages of the periarteriolar lymphocyte sheath and in MOMA-2+ marginal zone macrophages. Infected cells were further characterized by fluorescence microscopy after their enrichment by adherence. All infected mononuclear cells recovered by this procedure could be stained with MOMA-2 and FA/11 and thus very probably belonged to the mononuclear phagocyte lineage. Furthermore, all of them strongly expressed both MHC class II as well as H-2M molecules, regardless of the time points after infection. Analysis of the parasitophorous vacuoles (PV) by confocal microscopy showed that these compartments were surrounded by a membrane enriched in lysosomal glycoproteins lamp-1 and lamp-2, in macrosialin (a membrane protein of prelysosomes recognized by FA/11) and in MOMA-2 antigen. About 80% of the PV also had MHC class II and H-2M molecules on their membrane. Altogether, these data indicate that in the spleens of L. donovani-infected mice, a high percentage of amastigotes are located in macrophages expressing MHC class II molecules and that they live in PV exhibiting properties similar to those of PV detected in mouse bone marrow-derived macrophages exposed to a low dose of interferon gamma (IFN-gamma) and infected in vitro.

Leishmania-host-cell interaction: complexities and alternative views

Leishmania are protozoan parasites that infect various mammalian species, including humans. It is generally thought that random attachment of the flagellated promastigotes to mononuclear phagocytes initiates their uptake via circumferential pseudopods. Intracellularly, the promastigotes become located in phagolysosomes in which they transform to and survive as 'aflagellated' amastigotes that hide their shortened flagellum within the flagellar pocket. Unrestricted replication of these amastigotes is assumed to cause the eventual burst of the host cell, thereby releasing the infectious parasites. Here, Mike Rittig and Christian Bogdan review a large body of literature containing potentially important but poorly appreciated findings, which together with recent results, argue for Leishmania-host-cell interactions that are much more complex than generally thought.

Leishmania promastigotes require lipophosphoglycan to actively modulate the fusion properties of phagosomes at an early step of phagocytosis

The lipophosphoglycan (LPG) of Leishmania promastigotes plays key roles in parasite survival in both insect and mammalian hosts. Evidence suggests that LPG decreases phagosome fusion properties at the onset of infection in macrophages. The mechanisms of action of this molecule are, however, poorly understood. In the present study, we used a panoply of Leishmania mutants displaying modified LPG structures to determine more precisely how LPG modulates phagosome-endosome fusion. Using an in vivo fusion assay measuring, at the electron microscope, the transfer of solute materials from endosomes to phagosomes, we provided further evidence that the repeating Gal(beta1,4)Man(alpha1-PO4) units of LPG are responsible for the alteration in phagosome fusion. The inhibitory effect of LPG on phagosome fusion was shown to be more potent towards late endocytic organelles and lysosomes than early endosomes, explaining how Leishmania promastigotes can avoid degradation in hydrolase-enriched compartments. The involvement of other repeating unit-containing molecules, including the secreted acid phosphatase, in the inhibition process was ruled out, as an LPG-defective mutant (Ipg1-) which secretes repeating unit-containing glycoconjugates was present in highly fusogenic phagosomes. In L. major, oligosaccharide side-chains of LPG did not contribute to the inhibition process, as Spock, an L. major mutant lacking LPG side-chains, blocked fusion to the same extent as wild-type parasites. Finally, dead parasites internalized from the culture medium were not as efficient as live parasites in altering phagosome-endosome fusion, despite the presence of LPG. However, the killing of parasites with vital dyes after their sequestration in phagosomes had no effect on the fusion properties of this organelle. Collectively, these results suggest that living promastigotes displaying full-length cell surface LPG can actively influence macrophages at an early stage of phagocytosis to generate phagosomes with poor fusogenic properties.

Phagosome dynamics and function

Phagocytosis of microorganisms and other particles is mediated most efficiently by receptors such as Fc-receptors (FcR) and complement-receptors (C3R). Interaction between these receptors and ligands on the particle results in signal transduction events that lead to actin polymerisation and phagosome formation. The phagosome then undergoes a maturation process whereby it transforms into a phagolysosome. Phagosome maturation depends on interactions (fusion events) with early and late endosomes as well as with lysosomes. The fusion processes are regulated by small GTP-binding proteins and other proteins that are also involved in fusion processes in the endocytic pathway. Although most phagocytosed microorganisms are killed in the lysosome, some pathogens have developed survival strategies and are able to live in the harsh conditions in the phagolysosome or interfere with the maturation process and thereby evade destruction by acid hydrolases.

Redirection of host vesicle trafficking pathways by intracellular parasites

Bacterial and protozoon intracellular parasites have evolved diverse mechanisms for evasion of host cellular defenses associated with adaptations for survival in distinct intracellular compartments. As the reagents identifying discrete steps in vesicle maturation and trafficking have become increasingly available, it has become clear that the vacuoles occupied by intracellular parasites are much more diverse than had been previously appreciated. Many parasites induce selective fusion competence with the vacuoles they occupy, without affecting vesicular trafficking elsewhere in the cell. A likely means of controlling vesicular interactions is modification of the parasitophorous vacuole membrane by the insertion of parasite-specific proteins. A rapidly expanding class of bacterial proteins that modify the vacuolar membrane are the chlamydial inclusion membrane proteins. Although the functions of most of these proteins remain to be defined, the majority are expressed early in the infectious process, suggesting that modification of the vacuole is critical to the outcome of the host-parasite interaction.

Yersinia pseudotuberculosis blocks the phagosomal acidification of B10.A mouse macrophages through the inhibition of vacuolar H(+)-ATPase activity

Yersinia pseudotuberculosis survived and multiplied in the phagosomes of B10.A mouse peritoneal macrophages. As one of the possible mechanisms for the bacteria's survival in the phagosomes, we demonstrated that live Y. pseudotuberculosis inhibited the phagosomal acidification; pH within phagosomes containing the live Y. pseudotuberculosis remained at about 6.0, whereas pH within phagosomes containing the dead Y. pseudotuberculosis fell to about 4. 5. This ability to inhibit intraphagosomal acidification was also shared by mutants lacking the 42 Md virulence plasmid, indicating that it is chromosomally encoded. The phagosomes containing dead bacteria raised the pH to 6.2 after the treatment of their macrophages with an inhibitor (bafilomycin A1) specific for V-ATPase. Although the amount of V-ATPase in the A and B subunits on the phagosomes was not significantly different between the live and dead bacteria infection, the phagosomes containing live bacteria had a 10-fold smaller V-ATPase activity than those containing the dead bacteria. These results indicated that the inhibition of phagosomal acidification by Y. pseudotuberculosis infection was due to the attenuation of V-ATPase activity, and not due to the exclusion of V-ATPase subunits from the phagosome membrane as found in Mycobacterium avium.

H-2M molecules, like MHC class II molecules, are targeted to parasitophorous vacuoles of Leishmania-infected macrophages and internalized by amastigotes of L. amazonensis and L. mexicana

In their amastigote stage, Leishmania are obligatory intracellular parasites of mammalian macrophages, residing and multiplying within phagolysosomal compartments called parasitophorous vacuoles (PV). These organelles have properties similar to those described for the MHC class II compartments of antigen-presenting cells, sites where peptide-class II molecule complexes are formed before their expression at the cell surface. After infection with Leishmania amazonensis or L. mexicana, endocytosis and degradation of class II molecules by intracellular amastigotes have also been described, suggesting that these parasites have evolved mechanisms to escape the potentially hazardous antigen-presentation process. To determine whether these events extend to other molecules of the antigen-presentation machinery, we have now studied the fate of the MHC molecule H-2M in mouse macrophages infected with Leishmania amastigotes. At least for certain class II alleles, H-2M is an essential cofactor, which catalyses the release of the invariant chain-derived CLIP peptide from the peptide-binding groove of class II molecules and facilitates the binding of antigenic peptides. H-2M was detected in PV of mouse macrophages infected with various Leishmania species including L. amazonensis, L. mexicana, L. major and L. donovani. PV thus contain all the molecules required for the formation of peptide-class II molecule complexes and especially of complexes with parasite peptides. The present data indicate, however, that if this process occurs, it does not lead to a clear increase of SDS-stable compact (alpha)(beta) dimers of class II. In PV that contained L. amazonensis or L. mexicana, both class II and H-2M molecules often colocalized at the level where amastigotes bind to the PV membrane, suggesting that these molecules are physically associated, directly or indirectly, and possibly interact with parasite components. Furthermore, as class II molecules, H-2M molecules were internalized by amastigotes of these Leishmania species and reached parasite compartments that also contained class II molecules. Immunostaining of H-2M within parasites was increased by treatment of infected macrophages with the cysteine protease inhibitors Z-Phe-AlaCHN2 or Z-Phe-PheCHN2 or by incubation of the parasites with the same inhibitors before infection. These data thus support the idea that amastigotes of certain Leishmania species capture and degrade some of the molecules required for antigen presentation. To examine whether endocytosis of class II molecules by the parasites occurs through interactions with parasite components involving their peptide-binding groove, we made use of the fact that a large fraction of the class II molecules of H-2M(alpha) knock-out H-2(b) mice are occupied by the peptide CLIP and are unable to bind other peptides. We found that, in Leishmania-infected macrophages of these mutant mice, class II-CLIP complexes reached PV and were internalized by amastigotes. These results thus prove that endocytosis of class II molecules by amastigotes (1) is H-2M-independent and (2) does not necessarily involve the peptide-binding pocket of these molecules. Altogether, these data are compatible with an endocytic mechanism based on general properties shared by classical and non-classical class II molecules.

Impaired recruitment of the small GTPase rab7 correlates with the inhibition of phagosome maturation by Leishmania donovani promastigotes

We have shown recently that one of the survival strategies used by Leishmania donovani promastigotes during the establishment of infection in macrophages consists in inhibiting phagosome-endosome fusion. This inhibition requires the expression of lipophosphoglycan (LPG), the predominant surface glycoconjugate of promastigotes, as parasites expressing truncated forms of LPG reside in phagosomes that fuse extensively with endocytic organelles. In the present study, we developed a single-organelle fluorescence analysis approach to study and analyse the intracellular trafficking of 'fusogenic' and 'low-fusogenic' phagosomes induced by an LPG repeating unit-defective mutant (Ipg2 KO) or by wild-type L. donovani promastigotes respectively. The results obtained indicate that phagosomes containing mutant parasites fuse extensively with endocytic organelles and transform into phagolysosomes by losing the early endosome markers EEA1 and transferrin receptor, and acquiring the late endocytic and lysosomal markers rab7 and LAMP1. In contrast, a majority of 'low-fusogenic' phagosomes containing wild-type L. donovani promastigotes do not acquire rab7, wheres they acquire LAMP1 with slower kinetics. These results suggest that L. donovani parasites use LPG to restrict phagosome-endosome fusion at the onset of infection in order to prevent phagosome maturation. This is likely to permit the transformation of hydrolase-sensitive promastigotes into hydrolase-resistant amastigotes within a hospitable vacuole not displaying the harsh environment of phagolysosomes.

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.

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.

Localisation of Nramp1 in macrophages: modulation with activation and infection

The murine natural resistance-associated macrophage protein, Nramp1, has multiple pleiotropic effects on macrophage activation and regulates survival of intracellular pathogens including Leishmania, Salmonella and Mycobacterium species. Nramp1 acts as an iron transporter, but precisely how this relates to macrophage activation and/or pathogen survival remains unclear. To gain insight into function, anti-Nramp1 monoclonal and polyclonal antibodies are used here to localise Nramp1 following activation and infection. Confocal microscope analysis in uninfected macrophages demonstrates that both the mutant (infection-susceptible) and wild-type (infection-resistant) forms of the protein localise to the membranes of intracellular vesicular compartments. Gold labelling and electron microscopy defines these compartments more precisely as electron-lucent late endosomal and electron-dense lysosomal compartments, with Nramp1 colocalizing with Lamp1 and cathepsins D and L in both compartments, with macrosialin in late endosomes, and with BSA-5 nm gold in pre-loaded lysosomes. Nramp1 is upregulated with interferon-(gamma) and lipopolysaccaride treatment, coinciding with an increase in labelling in lysosomes relative to late endosomes and apparent dispersion of Nramp1-positive vesicles from a perinuclear location towards the periphery of the cytoplasm along the microtubular network. In both control and activated macrophages, expression of the protein is 3- to 4-fold higher in wild-type compared to mutant macrophages. In Leishmania major-infected macrophages, Nramp1 is observed in the membrane of the pathogen-containing phagosomes, which retain a perinuclear localization in resting macrophages. In Mycobacterium avium-infected resting and activated macrophages, Nramp1-positive vesicles migrated to converge, but not always fuse, with pathogen-containing phagosomes. The Nramp1 protein is thus located where it can have a direct influence on phagosome fusion and the microenvironment of the pathogen, as well as in the more general regulation of endosomal/lysosomal function in macrophages.

Genetic control of immune response to recombinant antigens carried by an attenuated Salmonella typhimurium vaccine strain: Nramp1 influences T-helper subset responses and protection against leishmanial challenge

Attenuated strains of Salmonella typhimurium have been widely used as vehicles for delivery and expression of vaccine antigens in murine models of infectious disease. In mice, early bacterial replication following infection with S. typhimurium is controlled by the gene (Nramp1, formerly Ity/Lsh/Bcg) encoding the natural-resistance-associated macrophage protein (Nramp1). Nramp1 regulates macrophage activation and has multiple pleiotropic effects, including regulation of tumor necrosis factor alpha, interleukin 1beta (IL-1beta), and major histocompatibility complex class II molecules, all of which influence antigen processing and presentation. Nramp1 also has a direct effect on antigen processing, possibly by regulating the activity of proteases in the late endosomal compartment. Hence, there are multiple ways (regulation of bacterial load or recombinant antigen dose, class II molecule expression, costimulatory or adjuvant activity, and antigen processing) that Nramp1 might influence responses to recombinant salmonella vaccines. To test the hypothesis that Nramp1 influences responses to vaccination, congenic mouse strains have been used to analyze immune responses to recombinant antigens (tetanus toxoid antigen and leishmanial gp63) carried by live attenuated S. typhimurium aroA aroD mutants. Results show that congenic mice carrying the wild-type (S. typhimurium resistance) Nramp1 allele mount a predominantly T-helper-1 (IL-2 and gamma interferon) response to vaccination and show enhanced resolution of lesions following challenge infection with Leishmania major. In contrast, mice carrying mutant (S. typhimurium susceptibility) Nramp1 mount a T-helper-2 (immunoglobulin E and IL-4) response and show exacerbated lesion growth upon challenge.