The Leishmania donovani lipophosphoglycan excludes the vesicular proton-ATPase from phagosomes by impairing the recruitment of synaptotagmin V

VAPA mediates lipid exchange between Leishmania amazonensis and host macrophages

Leishmania is a vacuolar pathogen that replicates within parasitophorous vacuoles inside host phagocytes. To promote its replication, Leishmania relies on a panoply of strategies to acquire macromolecules such as lipids from host macrophages. In this study, we have evaluated the role of VAPA, an endoplasmic reticulum-resident membrane protein involved in inter-organellar lipid transport, in macrophages infected with L. amazonensis. Following infection of bone marrow-derived macrophages with L. amazonensis metacyclic promastigotes, we observed that VAPA gradually associates with communal parasitophorous vacuoles. Knockdown of VAPA prevented the replication of L. amazonensis, which was accompanied by an impaired parasitophorous vacuole expansion. Using fluorescent ceramide, we established that VAPA is required for the transport of sphingolipids to the parasitophorous vacuoles and for its acquisition by L. amazonensis amastigotes. Proximity-ligation assays revealed that L. amazonensis hijacks VAPA by disrupting its interactions with the host cell lipid transfer proteins CERT and ORP1L. Finally, we found that VAPA is essential for the transfer of the Leishmania virulence glycolipid lipophosphoglycan from the parasitophorous vacuoles to the host cell endoplasmic reticulum. We propose that VAPA contributes to the ability of L. amazonensis to colonize macrophages by mediating bi-directional transfer of lipids essential for parasite replication and virulence between the parasitophorous vacuoles and the host cell endoplasmic reticulum.

Immune response to viscerotropic Leishmania: a comprehensive review

L. donovani and L. infantum infections are associated with a broad clinical spectrum, ranging from asymptomatic cases to visceral leishmaniasis (VL) with high mortality rates. Clinical manifestations such as post-kala-azar dermal leishmaniasis (PKDL) and visceral leishmaniasis-associated hemophagocytic lymphohistiocytosis-mimic (VL-associated HLH-mimic) further contribute to the diversity of clinical manifestations. These clinical variations are intricately influenced by the complex interplay between the host's immune response and the parasite's escape mechanisms. This narrative review aims to elucidate the underlying immunological mechanisms associated with each clinical manifestation, drawing from published literature within the last 5 years. Specific attention is directed toward viscerotropic Leishmania sinfection in patients with inborn errors of immunity and acquired immunodeficiencies. In VL, parasites exploit various immune evasion mechanisms, including immune checkpoints, leading to a predominantly anti-inflammatory environment that favors parasite survival. Conversely, nearly 70% of individuals are capable of mounting an effective pro-inflammatory immune response, forming granulomas that contain the parasites. Despite this, some patients may experience reactivation of the disease upon immunosuppression, challenging current understandings of parasite eradication. Individuals living with HIV and those with inborn errors of immunity present a more severe course of infection, often with higher relapse rates. Therefore, it is crucial to exclude both primary and acquired immune deficiencies in patients presenting disease relapse and VL-associated HLH-mimic. The distinction between VL and HLH can be challenging due to clinical similarities, suggesting that the nosological entity known as VL-associated HLH may represent a severe presentation of symptomatic VL and it should be considered more accurate referring to this condition as VL-associated HLH-mimic. Consequently, excluding VL in patients presenting with HLH is essential, as appropriate antimicrobial therapy can reverse immune dysregulation. A comprehensive understanding of the immune-host interaction underlying Leishmania infection is crucial for formulating effective treatment and preventive strategies to mitigate the disease burden.

Neutrophils versus Protozoan Parasites: Plasmodium, Trichomonas, Leishmania, Trypanosoma, and Entameoba

Neutrophils are the most abundant polymorphonuclear granular leukocytes in human blood and are an essential part of the innate immune system. Neutrophils are efficient cells that eliminate pathogenic bacteria and fungi, but their role in dealing with protozoan parasitic infections remains controversial. At sites of protozoan parasite infections, a large number of infiltrating neutrophils is observed, suggesting that neutrophils are important cells for controlling the infection. Yet, in most cases, there is also a strong inflammatory response that can provoke tissue damage. Diseases like malaria, trichomoniasis, leishmaniasis, Chagas disease, and amoebiasis affect millions of people globally. In this review, we summarize these protozoan diseases and describe the novel view on how neutrophils are involved in protection from these parasites. Also, we present recent evidence that neutrophils play a double role in these infections participating both in control of the parasite and in the pathogenesis of the disease.

The role of apoptosis and oxidative stress in the pathophysiology of Acanthamoeba spp. infection in the kidneys of hosts with different immunological status

BACKGROUND

Acanthamoeba spp. are opportunistic pathogens that cause inflammation, mostly in the brain, lungs and cornea. Recent reports indicate kidney dysfunction in hosts with systemic acanthamoebiasis. The aim of the study was to analyze the gene expression and protein concentration of NADPH oxidase 2 and 4 (NOX2 and NOX4, respectively) and nuclear erythroid 2-related factor (Nrf2) in the kidneys of hosts with systemic acanthamoebiasis. We also aimed to determine the protein and gene expressions of Bcl2, Bax, caspases 3 and 9.

METHODS

 Mice were divided into four groups based on their immunological status and Acanthamoeba sp. infection: A, immunocompetent Acanthamoeba sp.-infected mice; AS, immunosuppressed Acanthamoeba sp.- infected mice; C, immunocompetent uninfected mice; CS, immunosuppressed uninfected mice. NOX2, NOX4 and Nrf2 were analyzed by quantitative reverse transcription PCR (qRT-PCR) and ELISA methods, while pro-apoptotic and anti-apoptotic proteins (Bax and Bcl-2, respectively), Cas9, Cas3 were analyzed by qRT-PCR and western blot methods.  RESULTS: Increased gene expression and/or protein concentration of NOX2 and NOX4 were found in both immunocompetent and immunosuppressed mice infected with Acanthamoeba sp. (groups A and AS, respectively). Gene expression and/or protein concentration of Nrf2 were higher in group A than in control animals. Compared to control mice, in the AS group the expression of the Nrf2 gene was upregulated while the concentration of Nrf2 protein was decreased. Additionally in A group, higher gene and protein expression of Bcl-2, and lower gene as well as protein expression of Bax, caspases 3 and 9 were noted. In contrast, the AS group showed lower gene and protein expression of Bcl-2, and higher gene as well as protein expression of Bax, caspases 3 and 9.

CONCLUSIONS

This study is the first to address the mechanisms occurring in the kidneys of hosts infected with Acanthamoeba sp. The contact of Acanthamoeba sp. with the host cell surface and/or the oxidative burst caused by elevated levels of NOXs lead to an antioxidant response enhanced by the Nrf2 pathway. Acanthamoeba sp. have various strategies concerning apoptosis. In immunocompetent hosts, amoebae inhibit the apoptosis of kidney cells, and in immunosuppressed hosts, they lead to increased apoptosis by the intrinsic pathway and thus to a more severe course of the disease.

Emerging strategies and challenges of molecular therapeutics in antileishmanial drug development

Molecular therapy refers to targeted therapies based on molecules which have been intelligently directed towards specific biomolecular structures and include small molecule drugs, monoclonal antibodies, proteins and peptides, DNA or RNA-based strategies, targeted chemotherapy and nanomedicines. Molecular therapy is emerging as the most effective strategy to combat the present challenges of life-threatening visceral leishmaniasis, where the successful human vaccine is currently unavailable. Moreover, current chemotherapy-based strategies are associated with the issues of ineffective targeting, unavoidable toxicities, invasive therapies, prolonged treatment, high treatment costs and the development of drug-resistant strains. Thus, the rational approach to antileishmanial drug development primarily demands critical exploration and exploitation of biochemical differences between host and parasite biology, immunocharacteristics of parasite homing, and host-parasite interactions at the molecular/cellular level. Following this, the novel technology-based designing and development of host and/or parasite-targeted therapeutics having leishmanicidal and immunomodulatory activity is utmost essential to improve treatment efficacy. Thus, the present review is focused on immunological and molecular checkpoint targets in host-pathogen interaction, and molecular therapeutic prospects for Leishmania intervention, and the challenges ahead.

Zooming in on common immune evasion mechanisms of pathogens in phagolysosomes: potential broad-spectrum therapeutic targets against infectious diseases

The intracellular viral, bacterial, or parasitic pathogens evade the host immune challenges to propagate and cause fatal diseases. The microbes overpower host immunity at various levels including during entry into host cells, phagosome formation, phagosome maturation, phagosome-lysosome fusion forming phagolysosomes, acidification of phagolysosomes, and at times after escape into the cytosol. Phagolysosome is the final organelle in the phagocyte with sophisticated mechanisms to degrade the pathogens. The immune evasion strategies by the pathogens include the arrest of host cell apoptosis, decrease in reactive oxygen species, the elevation of Th2 anti-inflammatory response, avoidance of autophagy and antigen cross-presentation pathways, and escape from phagolysosomal killing. Since the phagolysosome organelle in relation to infection/cure is seldom discussed in the literature, we summarize here the common host as well as pathogen targets manipulated or utilized by the pathogens established in phagosomes and phagolysosomes, to hijack the host immune system for their benefit. These common molecules or pathways can be broad-spectrum therapeutic targets for drug development for intervention against infectious diseases caused by different intracellular pathogens.

Autophagy induced macrophages by α-alumina(α-AL2O3) conjugated cysteine peptidase, enhances the cytotoxic activity of CD8+ T lymphocytes against Leishmania major

Introduction

Induction of a protective immune response against Leishmania major requires the activation of both TH1 and CD8+ T lymphocytes. Because L. major is an intra-phagosomal parasite, its antigens do not have access to MHC-I. The present study aimed to evaluate the effect of cysteine peptidase A (CPA)/cysteine peptidase B (CPB) conjugated to α-AL2O3 on autophagy induction in L. major infected macrophages and subsequent activation of cytotoxic CD8+ T lymphocytes.

Methods

Recombinant CPA and CPB of L. major were produced in expression vectors and purified. Aldehyde functionalized α-AL2O3 were conjugated to hydrazine-modified CPA/CPB by a chemical bond was confirmed by Fourier-transform infrared spectroscopy (FTIR). The High efficient internalization of α-AL2O3 conjugated CPA/CPB to macrophages was confirmed using a fluorescence microscope and flowcytometry. Induction of the acidic autophagosome and LC3 conversion in macrophages was determined by acridine orange (AO) staining and western blot. Autophagy-activated macrophages were used for CD8+ T cell priming. Cytotoxic activity of the primed CD8+ T cell against L. major infected macrophages was measured using apoptosis assay.

Results

α-AL2O3 conjugated CPA/CPB enhances macrophages antigen uptake and increases acidic vacuole formation and LC-3I to LC-3II conversion. Co-culture of autophagy-activated macrophages with CD8+ T cells augmented CD8+ T cells priming and proliferation more than in other study groups. These primed CD8+ T cells induce significant apoptotic death of L. major infected macrophages compared with non-primed CD8+ T cells.

Conclusion

α-AL2O3 nanoparticles enhance the cross-presentation of L. major antigens to CD8+ T cells by inducing autophagy. This finding supports the positive role of autophagy and encourages the use of α-AL2O3 in vaccine design.

Macrophage Mitochondrial Biogenesis and Metabolic Reprogramming Induced by Leishmania donovani Require Lipophosphoglycan and Type I Interferon Signaling

Pathogen-specific rewiring of host cell metabolism creates the metabolically adapted microenvironment required for pathogen replication. Here, we investigated the mechanisms governing the modulation of macrophage mitochondrial properties by the vacuolar pathogen Leishmania. We report that induction of oxidative phosphorylation and mitochondrial biogenesis by Leishmania donovani requires the virulence glycolipid lipophosphoglycan, which stimulates the expression of key transcriptional regulators and structural genes associated with the electron transport chain. Leishmania-induced mitochondriogenesis also requires a lipophosphoglycan-independent pathway involving type I interferon (IFN) receptor signaling. The observation that pharmacological induction of mitochondrial biogenesis enables an avirulent lipophosphoglycan-defective L. donovani mutant to survive in macrophages supports the notion that mitochondrial biogenesis contributes to the creation of a metabolically adapted environment propitious to the colonization of host cells by the parasite. This study provides novel insight into the complex mechanism by which Leishmania metacyclic promastigotes alter host cell mitochondrial biogenesis and metabolism during the colonization process. IMPORTANCE To colonize host phagocytes, Leishmania metacyclic promastigotes subvert host defense mechanisms and create a specialized intracellular niche adapted to their replication. This is accomplished through the action of virulence factors, including the surface coat glycoconjugate lipophosphoglycan. In addition, Leishmania induces proliferation of host cell mitochondria as well as metabolic reprogramming of macrophages. These metabolic alterations are crucial to the colonization process of macrophages, as they may provide metabolites required for parasite growth. In this study, we describe a new key role for lipophosphoglycan in the stimulation of oxidative phosphorylation and mitochondrial biogenesis. We also demonstrate that host cell pattern recognition receptors Toll-like receptor 4 (TLR4) and endosomal TLRs mediate these Leishmania-induced alterations of host cell mitochondrial biology, which also require type I IFN signaling. These findings provide new insight into how Leishmania creates a metabolically adapted environment favorable to their replication.

Immunolocalization of Metabolite Transporter Proteins in a Model Cnidarian-Dinoflagellate Symbiosis

Bidirectional nutrient flow between partners is integral to the cnidarian-dinoflagellate endosymbiosis. However, our current knowledge of the transporter proteins that regulate nutrient and metabolite trafficking is nascent. Four transmembrane transporters that likely play an important role in interpartner nitrogen and carbon exchange were investigated with immunocytochemistry in the model sea anemone Exaiptasia diaphana ("Aiptasia"; strain NZ1): ammonium transporter 1 (AMT1), V-type proton ATPase (VHA), facilitated glucose transporter member 8 (GLUT8), and aquaporin-3 (AQP3). Anemones lacking symbionts were compared with those in symbiosis with either their typical, homologous dinoflagellate symbiont, Breviolum minutum, or the heterologous species, Durusdinium trenchii and Symbiodinium microadriaticum. AMT1 and VHA were only detected in symbiotic Aiptasia, irrespective of symbiont type. However, GLUT8 and AQP3 were detected in both symbiotic and aposymbiotic states. All transporters were localized to both the epidermis and gastrodermis, though localization patterns in host tissues were heavily influenced by symbiont identity, with S. microadriaticum-colonized anemones showing the most distinct patterns. These patterns suggested disruption of fixed carbon and inorganic nitrogen fluxes when in symbiosis with heterologous versus homologous symbionts. This study enhances our understanding of nutrient transport and host-symbiont integration, while providing a platform for further investigation of nutrient transporters and the host-symbiont interface in the cnidarian-dinoflagellate symbiosis. IMPORTANCE Coral reefs are in serious decline, in particular due to the thermally induced dysfunction of the cnidarian-dinoflagellate symbiosis that underlies their success. Yet our ability to react to this crisis is hindered by limited knowledge of how this symbiosis functions. Indeed, we still have much to learn about the cellular integration that determines whether a particular host-symbiont combination can persist, and hence whether corals might be able to adapt by acquiring new, more thermally resistant symbionts. Here, we employed immunocytochemistry to localize and quantify key nutrient transporters in tissues of the sea anemone Aiptasia, a globally adopted model system for this symbiosis, and compared the expression of these transporters when the host is colonized by native versus nonnative symbionts. We showed a clear link between transporter expression and symbiont identity, elucidating the cellular events that dictate symbiosis success, and we provide a methodological platform for further examination of cellular integration in this ecologically important symbiosis.

Leishmania infantum Defective in Lipophosphoglycan Biosynthesis Interferes With Activation of Human Neutrophils

Visceral leishmaniasis (VL) is often associated with hematologic manifestations that may interfere with neutrophil response. Lipophosphoglycan (LPG) is a major molecule on the surface of Leishmania promastigotes, which has been associated with several aspects of the parasite–vector–host interplay. Here, we investigated how LPG from Leishmania (L.) infantum, the principal etiological agent of VL in the New World, influences the initial establishment of infection during interaction with human neutrophils in an experimental setting in vitro. Human neutrophils obtained from peripheral blood samples were infected with either the wild-type L. infantum (WT) strain or LPG-deficient mutant (∆lpg1). In this setting, ∆lpg1 parasites displayed reduced viability compared to WT L. infantum; such finding was reverted in the complemented ∆lpg1+LPG1 parasites at 3- and 6-h post-infection. Confocal microscopy experiments indicated that this decreased survival was related to enhanced lysosomal fusion. In fact, LPG-deficient L. infantum parasites more frequently died inside neutrophil acidic compartments, a phenomenon that was reverted when host cells were treated with Wortmannin. We also observed an increase in the secretion of the neutrophil collagenase matrix metalloproteinase-8 (MMP-8) by cells infected with ∆lpg1 L. infantum compared to those that were infected with WT parasites. Furthermore, collagen I matrix degradation was found to be significantly increased in ∆lpg1 parasite-infected cells but not in WT-infected controls. Flow cytometry analysis revealed a substantial boost in production of reactive oxygen species (ROS) during infection with either WT or ∆lpg1 L. infantum. In addition, killing of ∆lpg1 parasites was shown to be more dependent on the ROS production than that of WT L. infantum. Notably, inhibition of the oxidative stress with Apocynin potentially fueled ∆lpg1 L. infantum fitness as it increased the intracellular parasite viability. Thus, our observations demonstrate that LPG may be a critical molecule fostering parasite survival in human neutrophils through a mechanism that involves cellular activation and generation of free radicals.

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

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

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.

Paradoxical immune response in leishmaniasis: the role of toll-like receptors in disease progression

Toll-like receptors (TLRs), members of pattern recognition receptors, are expressed on many cells of the innate immune system and their engagements with antigens regulates specific immune responses. TLRs signalling influences species-specific immune responses during Leishmania infection, thus, TLRs play a decisive role towards elimination or exacerbation of Leishmania infection. To date, there is no single therapeutic or prophylactic approach that fully effective against Leishmaniasis. An in-depth understanding of the mechanisms by which Leishmania species evade, or exploit host immune machinery could lead to the development of novel therapeutic approaches for the prevention and management of leishmaniasis. In this review, the role of TLRs in the induction of a paradoxical immune response in leishmaniasis was discussed. This review focuses on highlighting the novel interplay of TLR2/TLR9 driven resistance or susceptibility to 5 clinically important Leishmania species in human. The activation of TLR2/TLR9 can induce a diverse anti-Leishmania activities depending on the species of infecting Leishmania parasite. Infection with L. infantum and L. mexicana initiate TLR2/9 activation leading to host protective immune response while infection with L. major, L. donovani, and L. amazonensis trigger either a TLR2/9 related protective or non-protective immune responses. These findings suggest that TLR2 and TLR9 are targets worth pursuing either for modulation or blockage to trigger host protective immune response towards leishmaniasis.

The Glycosylphosphatidylinositol Anchor: A Linchpin for Cell Surface Versatility of Trypanosomatids

The use of glycosylphosphatidylinositol (GPI) to anchor proteins to the cell surface is widespread among eukaryotes. The GPI-anchor is covalently attached to the C-terminus of a protein and mediates the protein’s attachment to the outer leaflet of the lipid bilayer. GPI-anchored proteins have a wide range of functions, including acting as receptors, transporters, and adhesion molecules. In unicellular eukaryotic parasites, abundantly expressed GPI-anchored proteins are major virulence factors, which support infection and survival within distinct host environments. While, for example, the variant surface glycoprotein (VSG) is the major component of the cell surface of the bloodstream form of African trypanosomes, procyclin is the most abundant protein of the procyclic form which is found in the invertebrate host, the tsetse fly vector. Trypanosoma cruzi, on the other hand, expresses a variety of GPI-anchored molecules on their cell surface, such as mucins, that interact with their hosts. The latter is also true for Leishmania, which use GPI anchors to display, amongst others, lipophosphoglycans on their surface. Clearly, GPI-anchoring is a common feature in trypanosomatids and the fact that it has been maintained throughout eukaryote evolution indicates its adaptive value. Here, we explore and discuss GPI anchors as universal evolutionary building blocks that support the great variety of surface molecules of trypanosomatids.

Sec22b Regulates Inflammatory Responses by Controlling the Nuclear Translocation of NF-κB and the Secretion of Inflammatory Mediators

Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) regulate the vesicle transport machinery in phagocytic cells. Within the secretory pathway, Sec22b is an endoplasmic reticulum-Golgi intermediate compartment (ERGIC)-resident SNARE that controls phagosome maturation and function in macrophages and dendritic cells. The secretory pathway controls the release of cytokines and may also impact the secretion of NO, which is synthesized by the Golgi-active inducible NO synthase (iNOS). Whether ERGIC SNARE Sec22b controls NO and cytokine secretion is unknown. Using murine bone marrow-derived dendritic cells, we demonstrated that inducible NO synthase colocalizes with ERGIC/Golgi markers, notably Sec22b and its partner syntaxin 5, in the cytoplasm and at the phagosome. Pharmacological blockade of the secretory pathway hindered NO and cytokine release, and inhibited NF-κB translocation to the nucleus. Importantly, RNA interference-mediated silencing of Sec22b revealed that NO and cytokine production were abrogated at the protein and mRNA levels. This correlated with reduced nuclear translocation of NF-κB. We also found that Sec22b co-occurs with NF-κB in both the cytoplasm and nucleus, pointing to a role for this SNARE in the shuttling of NF-κB. Collectively, our data unveiled a novel function for the ERGIC/Golgi, and its resident SNARE Sec22b, in the production and release of inflammatory mediators.

Arginine sensing in intracellular parasitism of Leishmania

Protozoa of the genus Leishmania are intracellular parasites that cause human leishmaniasis, a disease spread mostly in the tropics and subtropics. Leishmania cycle between the midgut of female sand flies and phagolysosome of mammalian macrophages. During their life cycle they constantly encounter changing nutritional environments. To monitor the external concentration of essential nutrients, the invading parasites employ sensors that report on the availability of these nutrients; but to-date only a few sensing pathways have been identified in Leishmania. This review focuses on the Arginine Deprivation Response, which both extracellular and intracellular Leishmania utilize to monitor environmental arginine and adjust their arginine transporter (AAP3) levels accordingly.

The Paradox of a Phagosomal Lifestyle: How Innate Host Cell-Leishmania amazonensis Interactions Lead to a Progressive Chronic Disease

Intracellular phagosomal pathogens represent a formidable challenge for innate immune cells, as, paradoxically, these phagocytic cells can act as both host cells that support pathogen replication and, when properly activated, are the critical cells that mediate pathogen elimination. Infection by parasites of the Leishmania genus provides an excellent model organism to investigate this complex host-pathogen interaction. In this review we focus on the dynamics of Leishmania amazonensis infection and the host innate immune response, including the impact of the adaptive immune response on phagocytic host cell recruitment and activation. L. amazonensis infection represents an important public health problem in South America where, distinct from other Leishmania parasites, it has been associated with all three clinical forms of leishmaniasis in humans: cutaneous, muco-cutaneous and visceral. Experimental observations demonstrate that most experimental mouse strains are susceptible to L. amazonensis infection, including the C57BL/6 mouse, which is resistant to other species such as Leishmania major, Leishmania braziliensis and Leishmania infantum. In general, the CD4⁺ T helper (Th)1/Th2 paradigm does not sufficiently explain the progressive chronic disease established by L. amazonensis, as strong cell-mediated Th1 immunity, or a lack of Th2 immunity, does not provide protection as would be predicted. Recent findings in which the balance between Th1/Th2 immunity was found to influence permissive host cell availability via recruitment of inflammatory monocytes has also added to the complexity of the Th1/Th2 paradigm. In this review we discuss the roles played by innate cells starting from parasite recognition through to priming of the adaptive immune response. We highlight the relative importance of neutrophils, monocytes, dendritic cells and resident macrophages for the establishment and progressive nature of disease following L. amazonensis infection.

Interplay of Trypanosome Lytic Factor and innate immune cells in the resolution of cutaneous Leishmania infection

Trypanosome Lytic Factor (TLF) is a primate-specific high-density lipoprotein (HDL) complex that, through the cation channel-forming protein apolipoprotein L-1 (APOL1), provides innate immunity to select kinetoplastid parasites. The immunoprotective effects of TLF have been extensively investigated in the context of its interaction with the extracellular protozoan Trypanosoma brucei brucei, to which it confers sterile immunity. We previously showed that TLF could act against an intracellular pathogen Leishmania, and here we dissected the role of TLF and its synergy with host-immune cells. Leishmania major is transmitted by Phlebotomine sand flies, which deposit the parasite intradermally into mammalian hosts, where neutrophils are the predominant phagocytes recruited to the site of infection. Once in the host, the parasites are phagocytosed and shed their surface glycoconjugates during differentiation to the mammalian-resident amastigote stage. Our data show that mice producing TLF have reduced parasite burdens when infected intradermally with metacyclic promastigotes of L. major, the infective, fly-transmitted stage. This TLF-mediated reduction in parasite burden was lost in neutrophil-depleted mice, suggesting that early recruitment of neutrophils is required for TLF-mediated killing of L. major. In vitro we find that only metacyclic promastigotes co-incubated with TLF in an acidic milieu were lysed. However, amastigotes were not killed by TLF at any pH. These findings correlated with binding experiments, revealing that labeled TLF binds specifically to the surface of metacyclic promastigotes, but not to amastigotes. Metacyclic promastigotes of L. major deficient in the synthesis of surface glycoconjugates LPG and/or PPG (lpg1^- and lpg5A^-/lpg5B^- respectively) whose absence mimics the amastigote surface, were resistant to TLF-mediated lysis. We propose that TLF binds to the outer surface glycoconjugates of metacyclic promastigotes, whereupon it kills the parasite in the acidic phagosome of phagocytes. We hypothesize that resistance to TLF requires shedding of the surface glycoconjugates, which occurs several hours after phagocytosis by immune cells, creating a relatively short-lived but effective window for TLF to act against Leishmania.

Leishmaniasis, the disease caused by parasites of the genus Leishmania, can be divided into cutaneous, muco-cutaneous and visceral leishmaniasis depending on the parasite species and the clinical outcome of the disease. Of the three, cutaneous leishmaniasis is the most common form, which is usually characterized by a localized lesion due to the infection of immune cells, primarily dermal and lymph node-resident macrophages. The time between infection and lesion appearance ranges from weeks to years, while some individuals never develop lesions. The length of this subclinical stage of leishmaniasis depends on a variety of factors: parasite virulence, infectious dose, and the host immune response. Therefore, it remains crucial to develop our understanding of each component of the host-parasite interface and assess the role that each component plays in the clinical outcome. Here, we analyze the interaction between L. major, a cutaneous strain, and the host innate immune factor Trypanosome Lytic Factor (TLF), a sub-class of circulating High-Density Lipoprotein (HDL). TLF provides sterile immunity to most extracellular African Trypanosomes by osmotically lysing the parasites. Lysis is driven by the primate specific protein apolipoprotein L-1 (APOL1), a cation channel-forming protein that is activated by a series of pH-dependent conformational changes. APOL1 inserts into cellular membranes at acidic pH and forms a closed ion channel that subsequently opens when re-exposed to neutral pH, resulting in ion flux.

Using transgenic mice producing primate TLF, we show that both human and baboon TLFs ameliorate cutaneous Leishmania major infection and that this reduction in parasite burden correlates with: 1. infectious dose of metacyclic promastigotes 2. the concentration of circulating TLF in plasma and 3. early recruitment of neutrophils at the site of infection. Our results show that the acidification step is essential for TLF-mediated lysis of axenic metacyclic promastigotes of Leishmania in vitro. The susceptibility of metacyclic promastigotes to TLF-mediated lysis is governed by the surface glycoconjugates of Leishmania. We find that surface glycoconjugate-deficient Leishmania are resistant to TLF-mediated killing. Based on these data, we conclude that the shedding of surface glycoconjugates while transitioning from metacyclic promastigotes to amastigotes, results in parasite resistance to TLF-mediated lysis. Whether TLF is effective at killing metacyclic promastigotes of other experimentally tractable Leishmania sp., such as L. infantum and L. donovani, which have different surface glycoconjugate structures is yet to be tested. Our data raise the possibility that TLF may have lytic activity against a broader range of pathogens such as bacteria, viruses, fungi and parasites with surface glycoconjugates that transit through intracellular acidic compartments.

An overview of the fatty acid biosynthesis in the protozoan parasite Leishmania and its relevance as a drug target against leishmaniasis

Leishmaniasis is one of the fast-growing parasitic diseases worldwide. The treatment of this fatal disease presents a daunting challenge because of its adverse effects, necessity for long-term treatment regime, unavailability of functional drugs, emergence of drug resistance and the related expenditure. This calls for an urgent need for novel drugs and the evaluation of new targets. Proteins of the fatty acid biosynthetic pathway are validated as drug targets in pathogenic bacteria and certain viruses. Likewise, this pathway has been speculated as a suitable target against parasite infections. Fatty acid synthesis in parasites seems to be very complex and distinct from the counterpart mammalian host due to the presence of unique mechanisms for fatty acid biosynthesis and acquisition. In recent times, there have been few evidences of the existence of this pathway in the bloodstream form of some pathogens. The fatty acid biosynthesis thus presents a viable and attractive target for emerging therapeutics. Understanding the mechanisms underlying fatty acid metabolism is key to identifying a potential drug target. However, investigations in this direction are still limited and this article attempts to outline the existing knowledge, while highlighting the scope and relevance of the fatty acid biosynthetic pathway as a drug target. This review highlights the advances in the treatment of leishmaniasis, the importance of lipids in the pathogen, known facts about the fatty acid biosynthesis in Leishmania and how this pathway can be manipulated to combat leishmaniasis, suggesting novel drug targets.

Revisiting the Mechanisms of Immune Evasion Employed by Human Parasites

For the establishment of a successful infection, i.e., long-term parasitism and a complete life cycle, parasites use various diverse mechanisms and factors, which they may be inherently bestowed with, or may acquire from the natural vector biting the host at the infection prelude, or may take over from the infecting host, to outmaneuver, evade, overcome, and/or suppress the host immunity, both innately and adaptively. This narrative review summarizes the up-to-date strategies exploited by a number of representative human parasites (protozoa and helminths) to counteract the target host immune defense. The revisited information should be useful for designing diagnostics and therapeutics as well as vaccines against the respective parasitic infections.

Leishmania donovani Metacyclic Promastigotes Impair Phagosome Properties in Inflammatory Monocytes

Leishmaniasis, a debilitating disease with clinical manifestations ranging from self-healing ulcers to life-threatening visceral pathologies, is caused by protozoan parasites of the Leishmania genus. These professional vacuolar pathogens are transmitted by infected sand flies to mammalian hosts as metacyclic promastigotes and are rapidly internalized by various phagocyte populations. Classical monocytes are among the first myeloid cells to migrate to infection sites. Recent evidence shows that recruitment of these cells contributes to parasite burden and the establishment of chronic disease. However, the nature of Leishmania-inflammatory monocyte interactions during the early stages of host infection has not been well investigated. Here, we aimed to assess the impact of Leishmania donovani metacyclic promastigotes on antimicrobial responses within these cells. Our data showed that inflammatory monocytes are readily colonized by L. donovani metacyclic promastigotes, while infection with Escherichia coli is efficiently cleared. Upon internalization, metacyclic promastigotes inhibited superoxide production at the parasitophorous vacuole (PV) through a mechanism involving exclusion of NADPH oxidase subunits gp91^phox and p47^phox from the PV membrane. Moreover, we observed that unlike phagosomes enclosing zymosan particles, vacuoles containing parasites acidify poorly. Interestingly, whereas the parasite surface coat virulence glycolipid lipophosphoglycan (LPG) was responsible for the inhibition of PV acidification, impairment of the NADPH oxidase assembly was independent of LPG and GP63. Collectively, these observations indicate that permissiveness of inflammatory monocytes to L. donovani may thus be related to the ability of this parasite to impair the microbicidal properties of phagosomes.

Lysosome Sensing Is a Key Mechanism in Leishmania Intracellular Development

Phagolysosomes of macrophages are the niche where the parasitic protozoan Leishmania resides and causes human leishmaniasis. During infection, this organism encounters dramatic environmental changes. These include heat shock (from 26°C in the vector to 33°C or 37°C in the host, for cutaneous and visceral species, respectively) and acidic pH typical to the lysosome and nutrient availability. Leishmania cells developed ways to sense the lysosome-specific environment (acidic pH and body temperature) as means of recognition and, subsequently, initiation of differentiation into the intracellular form. Recent studies have indicated that protein kinase A plays a role as the gatekeeper that enables differentiation initiation. This review provides an update on the lysosome signaling pathway-mediated Leishmania intracellular development.

LmjF.36.3850, a novel hypothetical Leishmania major protein, contributes to the infection

Leishmania is a protozoan parasite that resides in mammalian macrophages and inflicts the disease known as leishmaniasis. Although prevalent in 88 countries, an anti-leishmanial vaccine remains elusive. While comparing the virulent and avirulent L. major transcriptomes by microarray, PCR and functional analyses for identifying a novel virulence-associated gene, we identified LmjF.36.3850, a hypothetical protein significantly less expressed in the avirulent parasite and without any known function. Motif search revealed that LmjF.36.3850 protein shared phosphorylation sites and other structural features with sucrose non-fermenting protein (Snf7) that shuttles virulence factors. LmjF.36.3850 was predicted to bind diacylglycerol (DAG) with energy value similar to PKCα and PKCβ, to which DAG is a cofactor. Indeed, 1-oleoyl-2-acetyl-sn-glycerol (OAG), a DAG analogue, enhanced the phosphorylation of PKCα and PKCβI. We cloned LmjF.36.3850 gene in a mammalian expression vector and primed susceptible BALB/c mice followed by challenge infection. We observed a higher parasite load, comparable antibody response and higher anti-inflammatory cytokines such as IL-4 and IL-10, while expression of major anti-leishmanial cytokine, IFN-γ, remained unchanged in LmjF.36.3850-vaccinated mice. CSA restimulated LN cells from vaccinated mice after challenge infection secreted comparable IL-4 and IL-10 but reduced IFN-γ, as compared to controls. These observations suggest a skewed Th2 response, diminished IFN-γ secreting Th1-T_EM cells and increased central and effector memory subtype of Th2, Th17 and Treg cells in the vaccinated mice. These data indicate that LmjF.36.3850 is a plausible virulence factor that enhances disease-promoting response, possibly by interfering with PKC activation and by eliciting disease-promoting T cells.

Linking membrane fluidity with defective antigen presentation in leishmaniasis

Hampering-surface presentation of immunogenic peptides by class I/II MHCs is a key strategy opted by several intracellular protozoan pathogens including Leishmania to escape CD8/CD4 mediated host-protective T-cell response. Although Leishmania parasites (LP) primarily hijack/inhibit host lysosomal/proteasomal pathways to hamper antigen-processing/presentation machinery, recent pieces of evidence have linked host-membrane fluidity as a major cause of defective antigen presentation in leishmaniasis. Increased membrane fluidity severely compromised peptide-MHC stability in the lipid raft regions, thereby abrogating T-cell mediated-signalling in the infected host. LP primarily achieves this by quenching host cholesterol, which acts as cementing material in maintaining the membrane fluidity. In this review, we have particularly focused on several strategies opted by LP to hijack-host cholesterol resulting in lipid droplets accumulation around leishmania-containing parasitophorous vacuole favouring intracellular survival of LP. In fact, LP infection can result in altered cholesterol and lipid metabolism in the infected host, thereby favouring the establishment and progression of the infection. From our analysis of two genome-wide transcriptomics data sets of LP infected host, we propose a possible molecular network that connects these interrelated events of altered lipid metabolism with eventual compromised antigen presentation, still existing as a gap in our current understanding of Leishmania infection.

Mannose-Decorated Dendritic Polyglycerol Nanocarriers Drive Antiparasitic Drugs To Leishmania infantum-Infected Macrophages

Macrophages are hosts for intracellular pathogens involved in numerous diseases including leishmaniasis. They express surface receptors that may be exploited for specific drug-targeting. Recently, we developed a PEGylated dendritic polyglycerol-based conjugate (PG–PEG) that colocalizes with intracellular parasite. We hereby study the effect of surface decoration with mannose units on the conjugates’ targeting ability toward leishmania intracellular parasites. Murine and human macrophages were exposed to fluorescently labeled mannosylated PG–PEG and uptake was quantified by flow cytometry analysis. Nanocarriers bearing five mannose units showed the highest uptake, which varied between 30 and 88% in the population in human and murine macrophages, respectively. The uptake was found to be dependent on phagocytosis and pinocytosis (80%), as well as clathrin-mediated endocytosis (79%). Confocal microscopy showed that mannosylated PG–PEGs target acidic compartments in macrophages. In addition, when both murine and human macrophages were infected and treated, colocalization between parasites and mannosylated nanoconjugates was observed. Leishmania-infected bone marrow-derived macrophages (BMM) showed avidity by mannosylated PG–PEG whereas non-infected macrophages rarely accumulated conjugates. Moreover, the antileishmanial activity of Amphotericin B was kept upon conjugation to mannosylated PG–PEG through a pH-labile linker. This study demonstrates that leishmania infected macrophages are selectively targeted by mannosylated PEGylated dendritic conjugates.

The macrophage microtubule network acts as a key cellular controller of the intracellular fate of Leishmania infantum

The parasitophorous vacuoles (PVs) that insulate Leishmania spp. in host macrophages are vacuolar compartments wherein promastigote forms differentiate into amastigote that are the replicative form of the parasite and are also more resistant to host responses. We revisited the biogenesis of tight-fitting PVs that insulate L. infantum in promastigote-infected macrophage-like RAW 264.7 cells by time-dependent confocal laser multidimensional imaging analysis. Pharmacological disassembly of the cellular microtubule network and silencing of the dynein gene led to an impaired interaction of L. infantum-containing phagosomes with late endosomes and lysosomes, resulting in the tight-fitting parasite-containing phagosomes never transforming into mature PVs. Analysis of the shape of the L. infantum parasite within PVs, showed that factors that impair promastigote-amastigote differentiation can also result in PVs whose maturation is arrested. These findings highlight the importance of the MT-dependent interaction of L. infantum-containing phagosomes with the host macrophage endolysosomal pathway to secure the intracellular fate of the parasite.

Kinetoplastid parasites of the genus Leishmania are responsible for a diverse spectrum of mammalian infectious diseases, the leishmaniases, including cutaneous, mucocutaneous, and mucosal pathologies. Infectious metacyclic promastigotes of infected female Phlebotomus sandflies are injected into the host at the site of the bite during the sandfly blood meal, after which they are internalized by host professional phagocytic neutrophils and macrophages. Leishmania infantum is an etiological agent of potentially fatal visceral pathology. This study molecularly dissects the maturation of L. infantum-containing phagosomes/parasitophorous vacuoles (PVs) in host macrophages. We reveal the requirement of vacuolar movement along macrophage microtubule tracks for the phagosome trafficking toward the endolysosomal pathway necessary for the development of the mature tight-fitting PV crucial for L. infantum survival and proliferation.

Guanylate Binding Proteins Restrict Leishmania donovani Growth in Nonphagocytic Cells Independent of Parasitophorous Vacuolar Targeting

Interferon (IFN)-inducible guanylate binding proteins (GBPs) play important roles in host defense against many intracellular pathogens that reside within pathogen-containing vacuoles (PVs). For instance, members of the GBP family translocate to PVs occupied by the protozoan pathogen Toxoplasma and facilitate PV disruption and lytic parasite killing. While the GBP defense program targeting Toxoplasma has been studied in some detail, the role of GBPs in host defense to other protozoan pathogens is poorly characterized. Here, we report a critical role for both mouse and human GBPs in the cell-autonomous immune response against the vector-borne parasite Leishmania donovani Although L. donovani can infect both phagocytic and nonphagocytic cells, it predominantly replicates inside professional phagocytes. The underlying basis for this cell type tropism is unclear. Here, we demonstrate that GBPs restrict growth of L. donovani in both mouse and human nonphagocytic cells. GBP-mediated restriction of L. donovani replication occurs via a noncanonical pathway that operates independent of detectable translocation of GBPs to L. donovan-containing vacuoles (LCVs). Instead of promoting the lytic destruction of PVs, as reported for GBP-mediated killing of Toxoplasma in phagocytic cells, GBPs facilitate the delivery of L. donovani into autolysosomal-marker-positive compartments in mouse embryonic fibroblasts as well as the human epithelial cell line A549. Together our results show that GBPs control a novel cell-autonomous host defense program, which renders nonphagocytic cells nonpermissible for efficient Leishmania replication.IMPORTANCE The obligate intracellular parasite Leishmania causes the disease leishmaniasis, which is transmitted to mammalian hosts, including humans, via the sandfly vector. Following the bite-induced breach of the skin barrier, Leishmania is known to live and replicate predominantly inside professional phagocytes. Although Leishmania is also able to infect nonphagocytic cells, nonphagocytic cells support limited parasitic replication for unknown reasons. In this study, we show that nonphagocytic cells possess an intrinsic property to restrict Leishmania growth. Our study defines a novel role for a family of host defense proteins, the guanylate binding proteins (GBPs), in antileishmanial immunity. Mechanistically, our data indicate that GBPs facilitate the delivery of Leishmania into antimicrobial autolysosomes, thereby enhancing parasite clearance in nonphagocytic cells. We propose that this GBP-dependent host defense program makes nonphagocytic cells an inhospitable host cell type for Leishmania growth.

Transcriptional analysis of THP-1 cells infected with Leishmania infantum indicates no activation of the inflammasome platform

Leishmaniasis is caused by intracellular parasites transmitted to vertebrates by sandfly bites. Clinical manifestations include cutaneous, mucosal or visceral involvement depending upon the host immune response and the parasite species. To assure their survival inside macrophages, these parasites developed a plethora of highly successful strategies to manipulate various immune system pathways. Considering that inflammasome activation is critical for the establishment of a protective immune response in many parasite infections, in this study we determined the transcriptome of THP-1 cells after infection with L. infantum, with a particular focus on the inflammasome components. To this end, the human cell line THP-1, previously differentiated into macrophages by PMA treatment, was infected with L. infantum promastigotes. Differentiated THP-1 cells were also stimulated with LPS to be used as a comparative parameter. The gene expression signature was determined 8 hours after by RNA-seq technique. Infected or uninfected THP-1 cells were stimulated with nigericin (NIG) to measure active caspase-1 and TNF-α, IL-6 and IL-1β levels in culture supernatants after 8, 24 and 48 hours. L. infantum triggered a gene expression pattern more similar to non-infected THP-1 cells and very distinct from LPS-stimulated cells. Some of the most up-regulated genes in L. infantum-infected cells were CDC20, CSF1, RPS6KA1, CD36, DUSP2, DUSP5, DUSP7 and TNFAIP3. Some up-regulated GO terms in infected cells included cell coagulation, regulation of MAPK cascade, response to peptide hormone stimulus, negative regulation of transcription from RNA polymerase II promoter and nerve growth factor receptor signaling pathway. Infection was not able to induce the expression of genes associated with the inflammasome signaling pathway. This finding was confirmed by the absence of caspase-1 activation and IL-1β production after 8, 24 and 48 hours of infection. Our results indicate that L. infantum was unable to activate the inflammasomes during the initial interaction with THP-1 cells.

Visceral leishmaniasis, caused by Leishmania infantum, is a disease that affects millions of people worldwide. The entry of microorganisms into the host is commonly associated with activation of a multiprotein platform called inflammasome whose assembly culminates in caspase-1 activation and IL-1β production. ILβ activates other cells and effector mechanisms leading to clearance of pathogens. However, the involvement of inflammasomes in the human infection with L. infantum is poorly known. To investigate the parasite-host interaction is fundamental to understand the immunopathogenesis of visceral leishmaniasis and to allow the development of new therapeutic strategies. In this study, we used RNA-seq, a tool that allowed to investigate the global gene expression of THP-1 cells, which is a macrophage-like human cell line, infected with L. infantum. By using computational analysis, this approach allowed us to evaluate the expression of genes that compose the inflammasomes pathway and other gene networks and signaling pathways triggered after infection. This analysis indicated that, unlike species causing cutaneous leishmaniasis, L. infantum did not induce the expression of genes of inflammasome pathways, nor caspase-1 activation or IL-1β production, possibly reflecting a parasite strategy to manipulate immune system and therefore, to allow its survival inside the cells.

Calcium Signaling Commands Phagosome Maturation Process

Phagosome-lysosome (P-L) fusion is one of the central immune-effector responses of host. It is known that phagosome maturation process is associated with numerous signaling cascades and among these, important role of calcium (Ca²⁺) signaling has been realized recently. Ca²⁺ plays key roles in actin rearrangement, activation of NADPH oxidase and protein kinase C (PKC). Involvement of Ca²⁺ in these cellular processes directs phagosomal maturation process. Some of the intracellular pathogens have acquired the strategies to modulate Ca²⁺ associated pathways to block P-L fusion process. In this review we have described the mechanism of Ca²⁺ signals that influence P-L fusion by controlling ROS, actin and PKC signaling cascades. We have also discussed the strategies implemented by the intracellular pathogens to manipulate Ca²⁺ signaling to consequently subvert P-L fusion. A detail study of factors associated in manipulating Ca²⁺ signaling may provide new insights for the development of therapeutic tools for more effective treatment options against infectious diseases.

SILAC-based quantitative proteomics reveals pleiotropic, phenotypic modulation in primary murine macrophages infected with the protozoan pathogen Leishmania donovani

Leishmaniases are major vector-borne tropical diseases responsible for great human morbidity and mortality, caused by protozoan, trypanosomatid parasites of the genus Leishmania. In the mammalian host, parasites survive and multiply within mononuclear phagocytes, especially macrophages. However, the underlying mechanisms by which Leishmania spp. affect their host are not fully understood. Herein, proteomic alterations of primary, bone marrow-derived BALB/c macrophages are documented after 72 h of infection with Leishmania donovani insect-stage promastigotes, applying a SILAC-based, quantitative proteomics approach. The protocol was optimised by combining strong anion exchange and gel electrophoresis fractionation that displayed similar depth of analysis (combined total of 6189 mouse proteins). Our analyses revealed 86 differentially modulated proteins (35 showing increased and 51 decreased abundance) in response to Leishmania donovani infection. The proteomics results were validated by analysing the abundance of selected proteins. Intracellular Leishmania donovani infection led to changes in various host cell biological processes, including primary metabolism and catabolic process, with a significant enrichment in lysosomal organisation. Overall, our analysis establishes the first proteome of bona fide primary macrophages infected ex vivo with Leishmania donovani, revealing new mechanisms acting at the host/pathogen interface. SIGNIFICANCE: Little is known on proteome changes that occur in primary macrophages after Leishmania donovani infection. This study describes a SILAC-based quantitative proteomics approach to characterise changes of bone marrow-derived macrophages infected with L. donovani promastigotes for 72 h. With the application of SILAC and the use of SAX and GEL fractionation methods, we have tested new routes for proteome quantification of primary macrophages. The protocols developed here can be applicable to other diseases and pathologies. Moreover, this study sheds important new light on the "proteomic reprogramming" of infected macrophages in response to L. donovani promastigotes that affects primary metabolism, cellular catabolic processes, and lysosomal/vacuole organisation. Thus, our study reveals key molecules and processes that act at the host/pathogen interface that may inform on new immuno- or chemotherapeutic interventions to combat leishmaniasis.

Protective or Detrimental? Understanding the Role of Host Immunity in Leishmaniasis

The intracellular protozoan parasites of the genus Leishmania are the causative agents of leishmaniasis, a vector-borne disease of major public health concern, estimated to affect 12 million people worldwide. The clinical manifestations of leishmaniasis are highly variable and can range from self-healing localized cutaneous lesions to life-threatening disseminated visceral disease. Once introduced into the skin by infected sandflies, Leishmania parasites interact with a variety of immune cells, such as neutrophils, monocytes, dendritic cells (DCs), and macrophages. The resolution of infection requires a finely tuned interplay between innate and adaptive immune cells, culminating with the activation of microbicidal functions and parasite clearance within host cells. However, several factors derived from the host, insect vector, and Leishmania spp., including the presence of a double-stranded RNA virus (LRV), can modulate the host immunity and influence the disease outcome. In this review, we discuss the immune mechanisms underlying the main forms of leishmaniasis, some of the factors involved with the establishment of infection and disease severity, and potential approaches for vaccine and drug development focused on host immunity.

Deciphering the Role Played by Autophagy in Leishmania Infection

In recent decades, studies have shown that, depending on parasite species and host background, autophagy can either favor infection or promote parasite clearance. To date, relatively few studies have attempted to assess the role played by autophagy in Leishmania infection. While it has been consistently shown that Leishmania spp. induce autophagy in a variety of cell types, published results regarding the effects of autophagic modulation on Leishmania survival are contradictory. The present review, after a short overview of the general aspects of autophagy, aims to summarize the current body of knowledge surrounding how Leishmania spp. adaptively interact with macrophages, the host cells mainly involved in controlling leishmaniasis. We then explore the scarce studies that have investigated interactions between these parasite species and the autophagic pathway, and finally present a critical perspective on how autophagy influences infection outcome.

Anti-Leishmanial Vaccines: Assumptions, Approaches, and Annulments

Leishmaniasis is a neglected protozoan parasitic disease that occurs in 88 countries but a vaccine is unavailable. Vaccination with live, killed, attenuated (physically or genetically) Leishmania have met with limited success, while peptide-, protein-, or DNA-based vaccines showed promise only in animal models. Here, we critically assess several technical issues in vaccination and expectation of a host-protective immune response. Several studies showed that antigen presentation during priming and triggering of the same cells in infected condition are not comparable. Altered proteolytic processing, antigen presentation, protease-susceptible sites, and intracellular expression of pathogenic proteins during Leishmania infection may vary dominant epitope selection, MHC-II/peptide affinity, and may deter the reactivation of desired antigen-specific T cells generated during priming. The robustness of the memory T cells and their functions remains a concern. Presentation of the antigens by Leishmania-infected macrophages to antigen-specific memory T cells may lead to change in the T cells' functional phenotype or anergy or apoptosis. Although cells may be activated, the peptides generated during infection may be different and cross-reactive to the priming peptides. Such altered peptide ligands may lead to suppression of otherwise active antigen-specific T cells. We critically assess these different immunological issues that led to the non-availability of a vaccine for human use.

Leishmania braziliensis: Strain-Specific Modulation of Phagosome Maturation

Leishmania (Viannia) braziliensis is responsible for the largest number of American tegumentary leishmaniasis (ATL) in Brazil. ATL can present several clinical forms including typical (TL) and atypical (AL) cutaneous and mucocutaneous (ML) lesions. To identify parasite and host factors potentially associated with these diverse clinical manifestations, we first surveyed the expression of two virulence-associated glycoconjugates, lipophosphoglycan (LPG) and the metalloprotease GP63 by a panel of promastigotes of Leishmania braziliensis (L. braziliensis) strains isolated from patients with different clinical manifestations of ATL and from the sand fly vector. We observed a diversity of expression patterns for both LPG and GP63, which may be related to strain-specific polymorphisms. Interestingly, we noted that GP63 activity varies from strain to strain, including the ability to cleave host cell molecules. We next evaluated the ability of promastigotes from these L. braziliensis strains to modulate phagolysosome biogenesis in bone marrow-derived macrophages (BMM), by assessing phagosomal recruitment of the lysosome-associated membrane protein 1 (LAMP-1) and intraphagosomal acidification. Whereas, three out of six L. braziliensis strains impaired the phagosomal recruitment of LAMP-1, only the ML strain inhibited phagosome acidification to the same extent as the L. donovani strain that was used as a positive control. While decreased phagosomal recruitment of LAMP-1 correlated with higher LPG levels, decreased phagosomal acidification correlated with higher GP63 levels. Finally, we observed that the ability to infect and replicate within host cells did not fully correlate with the inhibition of phagosome maturation. Collectively, our results revealed a diversity of strain-specific phenotypes among L. braziliensis isolates, consistent with the high genetic diversity within Leishmania populations.

The host cell secretory pathway mediates the export of Leishmania virulence factors out of the parasitophorous vacuole

To colonize phagocytes, Leishmania subverts microbicidal processes through components of its surface coat that include lipophosphoglycan and the GP63 metalloprotease. How these virulence glycoconjugates are shed, exit the parasitophorous vacuole (PV), and traffic within host cells is poorly understood. Here, we show that lipophosphoglycan and GP63 are released from the parasite surface following phagocytosis and redistribute to the endoplasmic reticulum (ER) of macrophages. Pharmacological disruption of the trafficking between the ER and the Golgi hindered the exit of these molecules from the PV and dampened the cleavage of host proteins by GP63. Silencing by RNA interference of the soluble N-ethylmaleimide-sensitive-factor attachment protein receptors Sec22b and syntaxin-5, which regulate ER-Golgi trafficking, identified these host proteins as components of the machinery that mediates the spreading of Leishmania effectors within host cells. Our findings unveil a mechanism whereby a vacuolar pathogen takes advantage of the host cell's secretory pathway to promote egress of virulence factors beyond the PV.

Leishmania promastigotes are internalized by phagocytes into a highly modified phagosome that promotes parasite growth and differentiation into the amastigote form. To survive in the phagosome, Leishmania employs surface-bound glycoconjugates such as the GP63 metalloprotease and lipophosphoglycan to subvert the phagosome’s microbicidal potential. In particular, GP63 cleaves host cell vesicle fusion molecules that regulate phagosomal processes ranging from antigen cross-presentation to cytokine secretion. Unlike apicomplexan parasites and bacteria, Leishmania does not inject its virulence-associated glycoconjugates across the phagosome membrane. We found that post-phagocytosis, Leishmania co-opts the host cell secretory pathway to promote the egress of its virulence factors out of the phagosome. Importantly, chemical and genetic inhibition of endoplasmic reticulum (ER) to Golgi transport hindered the redistribution of GP63 and lipophosphoglycan, thereby impeding the cleavage of GP63 target Synaptotagmin XI. Notably, knockdown ER/ERGIC-resident membrane fusion regulators Sec22b and syntaxin-5 revealed that these host molecules were essential to the phagosomal egress of Leishmania virulence factors. These findings provide new insight into how Leishmania sabotages the host cell endomembrane system for its own benefit.

Intraspecies Polymorphisms in the Lipophosphoglycan of L. braziliensis Differentially Modulate Macrophage Activation via TLR4

Lipophosphoglycan (LPG) is the major Leishmania surface glycoconjugate having importance during the host-parasite interface. Leishmania (Viannia) braziliensis displays a spectrum of clinical forms including: typical cutaneous leishmaniasis (TL), mucocutaneous (ML), and atypical lesions (AL). Those variations in the immunopathology may be a result of intraspecies polymorphisms in the parasite's virulence factors. In this context, we evaluated the role of LPG of strains originated from patients with different clinical manifestations and the sandfly vector. Six isolates of L. braziliensis were used: M2903, RR051 and RR418 (TL), RR410 (AL), M15991 (ML), and M8401 (vector). LPGs were extracted and purified by hydrophobic interaction. Peritoneal macrophages from C57BL/6 and respective knock-outs (TLR2^−/− and TLR-4^−/−) were primed with IFN-γ and exposed to different LPGs for nitric oxide (NO) and cytokine production (IL-1β, IL-6, IL-12, and TNF-α). LPGs differentially activated the production of NO and cytokines via TLR4. In order to ascertain if such functional variations were related to intraspecies polymorphisms in the LPG, the purified glycoconjugates were subjected to western blot with specific LPG antibodies (CA7AE and LT22). Based on antibody reactivity preliminary variations in the repeat units were detected. To confirm these findings, LPGs were depolymerized for purification of repeat units. After thin layer chromatography, intraspecies polymorphisms were confirmed especially in the type and/size of sugars branching-off the repeat units motif. In conclusion, different isolates of L. braziliensis from different clinical forms and hosts possess polymorphisms in their LPGs that functionally affected macrophage responses.

Leishmania major degrades murine CXCL1 - An immune evasion strategy

Leishmaniasis is a global health problem with an estimated report of 2 million new cases every year and more than 1 billion people at risk of contracting this disease in endemic areas. The innate immune system plays a central role in controlling L. major infection by initiating a signaling cascade that results in production of pro-inflammatory cytokines and recruitment of both innate and adaptive immune cells. Upon infection with L. major, CXCL1 is produced locally and plays an important role in the recruitment of neutrophils to the site of infection. Herein, we report that L. major specifically targets murine CXCL1 for degradation. The degradation of CXCL1 is not dependent on host factors as L. major can directly degrade recombinant CXCL1 in a cell-free system. Using mass spectrometry, we discovered that the L. major protease cleaves at the C-terminal end of murine CXCL1. Finally, our data suggest that L. major metalloproteases are involved in the direct cleavage and degradation of CXCL1, and a synthetic peptide spanning the CXCL1 cleavage site can be used to inhibit L. major metalloprotease activity. In conclusion, our study has identified an immune evasion strategy employed by L. major to evade innate immune responses in mice, likely reservoirs in the endemic areas, and further highlights that targeting these L. major metalloproteases may be important in controlling infection within the reservoir population and transmittance of the disease.

Our study discovered a highly specific role for L. major metalloprotease in cleaving and degrading murine CXCL1. Indeed, L. major metalloprotease did not cleave murine CXCL2 or human CXCL1, CXCL2 and CXCL8. CXCL1 is a critical chemokine required for neutrophil recruitment to the site of infection; thus, we propose that this metalloprotease may have evolved to evade immune responses specifically in the murine host. We have further identified that the C-terminal end on CXCL1 is targeted for cleavage by the L. major metalloprotease. Finally, this cleavage site information was used to design peptides that are able to inhibit CXCL1 degradation by L. major. Our study highlights an immune evasion strategy utilized by L. major to establish infection within a murine host.

Leishmania donovani Lipophosphoglycan Increases Macrophage-Dependent Chemotaxis of CXCR6-Expressing Cells via CXCL16 Induction

CXCL16 is a multifunctional chemokine that is highly expressed by macrophages and other immune cells in response to bacterial and viral pathogens; however, little is known regarding the role of CXCL16 during parasitic infections. The protozoan parasite Leishmania donovani is the causative agent of visceral leishmaniasis. Even though chemokine production is a host defense mechanism during infection, subversion of the host chemokine system constitutes a survival strategy adopted by the parasite. Here, we report that L. donovani promastigotes upregulate CXCL16 synthesis and secretion by bone marrow-derived macrophages (BMDM). In contrast to wild-type parasites, a strain deficient in the virulence factor lipophosphoglycan (LPG) failed to induce CXCL16 production. Consistent with this, cell treatment with purified L. donovani LPG augmented CXCL16 expression and secretion. Notably, the ability of BMDM to promote migration of cells expressing CXCR6, the cognate receptor of CXCL16, was augmented upon L. donovani infection in a CXCL16- and LPG-dependent manner. Mechanistically, CXCL16 induction by L. donovani required the activity of AKT and the mechanistic target of rapamycin (mTOR) but was independent of Toll-like receptor signaling. Collectively, these data provide evidence that CXCL16 is part of the inflammatory response elicited by L. donovani LPG in vitro Further investigation using CXCL16 knockout mice is required to determine whether this chemokine contributes to the pathogenesis of visceral leishmaniasis and to elucidate the underlying molecular mechanisms.

How to master the host immune system? Leishmania parasites have the solutions!

Infection by protozoan parasites of the genus Leishmania results in the development of leishmaniasis, an increasingly prevalent group of diseases affecting over 12 million people worldwide. Leishmaniasis can have very different outcomes ranging from cutaneous lesions, mucosal lesions to visceralization depending on the species of the infecting parasite and on the immune response developed by the host. As an obligate intracellular parasite, residing within macrophages, Leishmania evolved in strict contact with the host immune system, developing different mechanisms to evade or modulate the immune response. Various types of immune responses are observed during different Leishmania spp. infections, resulting in parasite clearance but also contributing to the pathogenesis, thus increasing the complexity of the course of the disease. Interestingly, depending on the type of leishmaniasis developed, opposite treatment strategies, which either boost or inhibit the inflammatory response, have shown efficacy. In this review, we summarize the contribution of different immune cell types to the development of the anti-leishmanial immune response and the parasite strategies to evade and modulate host immunity. Further, we discuss the involvement of co-infecting pathogens in the determination of the outcome of leishmaniasis and on the effectiveness of treatment and the implication of the immune response for treatment and vaccine development.

Synaptotagmin 5 regulates Ca2+-dependent Weibel-Palade body exocytosis in human endothelial cells

Elevations of intracellular free Ca²⁺ concentration ([Ca²⁺]_i) are a potent trigger for Weibel-Palade body (WPB) exocytosis and secretion of von Willebrand factor (VWF) from endothelial cells; however, the identity of WPB-associated Ca²⁺-sensors involved in transducing acute increases in [Ca²⁺]_i into granule exocytosis remains unknown. Here, we show that synaptotagmin 5 (SYT5) is expressed in human umbilical vein endothelial cells (HUVECs) and is recruited to WPBs to regulate Ca²⁺-driven WPB exocytosis. Western blot analysis of HUVECs identified SYT5 protein, and exogenously expressed SYT5-mEGFP localised almost exclusively to WPBs. shRNA-mediated knockdown of endogenous SYT5 (shSYT5) reduced the rate and extent of histamine-evoked WPB exocytosis and reduced secretion of the WPB cargo VWF-propeptide (VWFpp). The shSYT5-mediated reduction in histamine-evoked WPB exocytosis was prevented by expression of shRNA-resistant SYT5-mCherry. Overexpression of SYT5-EGFP increased the rate and extent of histamine-evoked WPB exocytosis, and increased secretion of VWFpp. Expression of a Ca²⁺-binding defective SYT5 mutant (SYT5-Asp197Ser-EGFP) mimicked depletion of endogenous SYT5. We identify SYT5 as a WPB-associated Ca²⁺ sensor regulating Ca²⁺-dependent secretion of stored mediators from vascular endothelial cells.

Harnessing Macrophages for Controlled-Release Drug Delivery: Lessons From Microbes

With the effectiveness of therapeutic agents ever decreasing and the increased incidence of multi-drug resistant pathogens, there is a clear need for administration of more potent, potentially more toxic, drugs. Alternatively, biopharmaceuticals may hold potential but require specialized protection from premature in vivo degradation. Thus, a paralleled need for specialized drug delivery systems has arisen. Although cell-mediated drug delivery is not a completely novel concept, the few applications described to date are not yet ready for in vivo application, for various reasons such as drug-induced carrier cell death, limited control over the site and timing of drug release and/or drug degradation by the host immune system. Here, we present our hypothesis for a new drug delivery system, which aims to negate these limitations. We propose transport of nanoparticle-encapsulated drugs inside autologous macrophages polarized to M1 phenotype for high mobility and treated to induce transient phagosome maturation arrest. In addition, we propose a significant shift of existing paradigms in the study of host-microbe interactions, in order to study microbial host immune evasion and dissemination patterns for their therapeutic utilization in the context of drug delivery. We describe a system in which microbial strategies may be adopted to facilitate absolute control over drug delivery, and without sacrificing the host carrier cells. We provide a comprehensive summary of the lessons we can learn from microbes in the context of drug delivery and discuss their feasibility for in vivo therapeutic application. We then describe our proposed "synthetic microbe drug delivery system" in detail. In our opinion, this multidisciplinary approach may hold the solution to effective, controlled drug delivery.

Glycoconjugates of Gram-negative bacteria and parasitic protozoa - are they similar in orchestrating the innate immune response?

Innate immunity is an evolutionarily ancient form of host defense that serves to limit infection. The invading microorganisms are detected by the innate immune system through germline-encoded PRRs. Different classes of PRRs, including TLRs and cytoplasmic receptors, recognize distinct microbial components known collectively as PAMPs. Ligation of PAMPs with receptors triggers intracellular signaling cascades, activating defense mechanisms. Despite the fact that Gram-negative bacteria and parasitic protozoa are phylogenetically distant organisms, they express glycoconjugates, namely bacterial LPS and protozoan GPI-anchored glycolipids, which share many structural and functional similarities. By activating/deactivating MAPK signaling and NF-κB, these ligands trigger general pro-/anti-inflammatory responses depending on the related patterns. They also use conservative strategies to subvert cell-autonomous defense systems of specialized immune cells. Signals triggered by Gram-negative bacteria and parasitic protozoa can interfere with host homeostasis and, depending on the type of microorganism, lead to hypersensitivity or silencing of the immune response. Activation of professional immune cells, through a ligand which triggers the opposite effect (antagonist versus agonist) appears to be a promising solution to restoring the immune balance.

The Capsule of Cryptococcus neoformans Modulates Phagosomal pH through Its Acid-Base Properties

Cryptococcus neoformans is the causative agent of cryptococcosis, a devastating fungal disease that affects thousands of individuals worldwide. This fungus has the capacity to survive inside phagocytic cells, which contributes to persistence of infection and dissemination. One of the major antimicrobial mechanisms of host phagocytes is to acidify the phagosomal compartment after ingestion of microbes. This study shows that the capsule of C. neoformans can interfere with full phagosomal acidification by serving as a buffer.

Phagosomal acidification is a critical cellular mechanism for the inhibition and killing of ingested microbes by phagocytic cells. The acidic environment activates microbicidal proteins and creates an unfavorable environment for the growth of many microbes. Consequently, numerous pathogenic microbes have developed strategies for countering phagosomal acidification through various mechanisms that include interference with phagosome maturation. The human-pathogenic fungus Cryptococcus neoformans resides in acidic phagosomes after macrophage ingestion that actually provides a favorable environment for replication, since the fungus replicates faster at acidic pH. We hypothesized that the glucuronic acid residues in the capsular polysaccharide had the capacity to affect phagosomal acidity through their acid-base properties. A ratiometric fluorescence comparison of imaged phagosomes containing C. neoformans to phagosomes containing beads showed that the latter were significantly more acidic. Similarly, phagosomes containing nonencapsulated C. neoformans cells were more acidic than those containing encapsulated cells. Acid-base titrations of isolated C. neoformans polysaccharide revealed that it behaves as a weak acid with maximal buffering capacity around pH 4 to 5. We interpret these results as indicating that the glucuronic acid residues in the C. neoformans capsular polysaccharide can buffer phagosomal acidification. Interference with phagosomal acidification represents a new function for the cryptococcal capsule in virulence and suggests the importance of considering the acid-base properties of microbial capsules in the host-microbe interaction for other microbes with charged residues in their capsules.

IMPORTANCECryptococcus neoformans is the causative agent of cryptococcosis, a devastating fungal disease that affects thousands of individuals worldwide. This fungus has the capacity to survive inside phagocytic cells, which contributes to persistence of infection and dissemination. One of the major antimicrobial mechanisms of host phagocytes is to acidify the phagosomal compartment after ingestion of microbes. This study shows that the capsule of C. neoformans can interfere with full phagosomal acidification by serving as a buffer.

PDL-1 Blockade Prevents T Cell Exhaustion, Inhibits Autophagy, and Promotes Clearance of Leishmania donovani

Leishmania donovani is a causative pathogen of potentially fatal visceral leishmaniasis (VL). Therapeutic agents are available; however, their use is limited because of high cost, serious side effects, and development of antimicrobial resistance. Protective immunity against VL depends on CD4⁺ Th1 cell-mediated immunity. Studies have shown that progression of VL is due to exhaustion of T cells; however, the mechanism involved is not clearly understood. Here, we examined the role of PD1/PDL-1 in the pathogenesis of VL by using a murine model of VL. Our data indicate that L. donovani is able to elicit initial expansion of gamma interferon-producing CD4⁺ Th1 and CD8⁺ T cells at day 7 postinfection (p.i.); however, the frequency of those cells and inflammatory response decreased at day 21 p.i., despite persistence of parasites. Persistent infection-induced expansion of interleukin-10⁺ FOXP3⁺ Treg and CD4⁺ and CD8⁺ T cells expressing PD1. Blocking of PDL-1 signaling in vivo resulted in restoration of protective type 1 responses by both CD4⁺ and CD8⁺ T cells, which resulted in a significant decrease in the parasite burden. Mechanistically, PDL-1 blocking inhibited autophagy, a cellular degradation process hijacked by Leishmania to acquire host cell nutrients for their survival. Inhibition of autophagy was marked by decreased lipidation of microtubule-associated protein 1 light chain 3, a marker of autophagosome formation, and P62 accumulation. Together, our findings show for the first time that anti-PDL-1 antibody is an effective therapeutic approach for restoration of effector arms of protective immunity against VL and subsequent parasite clearance.

Leishmania Hijacks Myeloid Cells for Immune Escape

Protozoan parasites of the Leishmania genus are the causative agents of leishmaniasis, a group of neglected tropical diseases whose clinical manifestations vary depending on the infectious Leishmania species but also on host factors. Recognition of the parasite by host myeloid immune cells is a key to trigger an effective Leishmania-specific immunity. However, the parasite is able to persist in host myeloid cells by evading, delaying and manipulating host immunity in order to escape host resistance and ensure its transmission. Neutrophils are first in infiltrating infection sites and could act either favoring or protecting against infection, depending on factors such as the genetic background of the host or the parasite species. Macrophages are the main host cells where the parasites grow and divide. However, macrophages are also the main effector population involved in parasite clearance. Parasite elimination by macrophages requires the priming and development of an effector Th1 adaptive immunity driven by specific subtypes of dendritic cells. Herein, we will provide a comprehensive outline of how myeloid cells regulate innate and adaptive immunity against Leishmania, and the mechanisms used by the parasites to promote their evasion and sabotage. Understanding the interactions between Leishmania and the host myeloid cells may lead to the development of new therapeutic approaches and improved vaccination to leishmaniases, an important worldwide health problem in which current therapeutic or preventive approaches are limited.

Leishmania infantum Lipophosphoglycan-Deficient Mutants: A Tool to Study Host Cell-Parasite Interplay

Lipophosphoglycan (LPG) is the major surface glycoconjugate of metacyclic Leishmania promastigotes and is associated with virulence in various species of this parasite. Here, we generated a LPG-deficient mutant of Leishmania infantum, the foremost etiologic agent of visceral leishmaniasis in Brazil. The L. infantum LPG-deficient mutant (Δlpg1) was obtained by homologous recombination and complemented via episomal expression of LPG1 (Δlpg1 + LPG1). Deletion of LPG1 had no observable effect on parasite morphology or on the presence of subcellular organelles, such as lipid droplets. While both wild-type and add-back parasites reached late phase in axenic cultures, the growth of Δlpg1 parasites was delayed. Additionally, the deletion of LPG1 impaired the outcome of infection in murine bone marrow-derived macrophages. Although no significant differences were observed in parasite load after 4 h of infection, survival of Δlpg1 parasites was significantly reduced at 72 h post-infection. Interestingly, L. infantum LPG-deficient mutants induced a strong NF-κB-dependent activation of the inducible nitric oxide synthase (iNOS) promoter compared to wild type and Δlpg1 + LPG1 parasites. In conclusion, the L. infantum Δlpg1 mutant constitutes a powerful tool to investigate the role(s) played by LPG in host cell-parasite interactions.

Function of Macrophage and Parasite Phosphatases in Leishmaniasis

The kinetoplastid protozoan parasites belonging to the genus Leishmania are the causative agents of different clinical forms of leishmaniasis, a vector-borne infectious disease with worldwide prevalence. The protective host immune response against Leishmania parasites relies on myeloid cells such as dendritic cells and macrophages in which upon stimulation by cytokines (e.g., interferon-γ) a complex network of signaling pathways is switched on leading to strong antimicrobial activities directed against the intracellular parasite stage. The regulation of these pathways classically depends on post-translational modifications of proteins, with phosphorylation events playing a cardinal role. Leishmania parasites deactivate their phagocytic host cells by inducing specific mammalian phosphatases that are capable to impede signaling. On the other hand, there is now also evidence that Leishmania spp. themselves express phosphatases that might target host cell molecules and thereby facilitate the intracellular survival of the parasite. This review will present an overview on the modulation of host phosphatases by Leishmania parasites as well as on the known families of Leishmania phosphatases and their possible function as virulence factors. A more detailed understanding of the role of phosphatases in Leishmania–host cell interactions might open new avenues for the treatment of non-healing, progressive forms of leishmaniasis.

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.

The Macrophage–Parasite Interface as a Chemotherapeutic Target in Leishmaniasis

Parasites of the genus Leishmania are intravacuolar pathogens that create compartments within their mammalian hosts where they can live, replicate and manipulate host immune responses. To generate these parasitophorous vacuoles, Leishmania diverts the default phagolysosomal biogenesis process, in part through the action of virulence factors on the host cell membrane fusion machinery. Components of this machinery essential to the biogenesis, maintenance and function of parasitophorous vacuoles may constitute attractive targets for the design of compounds that will disrupt the integrity of the Leishmania intracellular niche and interfere with parasite replication. Targeting components of the fusion machinery thus represents a promising avenue for the discovery of anti-leishmanial compounds that may not be plagued with problems associated with the development of resistance.

Phagosome proteomics to study Leishmania's intracellular niche in macrophages

Intracellular pathogens invade their host cells and replicate within specialized compartments. In turn, the host cell initiates a defensive response trying to kill the invasive agent. As a consequence, intracellular lifestyle implies morphological and physiological changes in both pathogen and host cell. Leishmania spp. are medically important intracellular protozoan parasites that are internalized by professional phagocytes such as macrophages, and reside within the parasitophorous vacuole inhibiting their microbicidal activity. Whereas the proteome of the extracellular promastigote form and the intracellular amastigote form have been extensively studied, the constituents of Leishmania's intracellular niche, an endolysosomal compartment, are not fully deciphered. In this review we discuss protocols to purify such compartments by means of an illustrating example to highlight generally relevant considerations and innovative aspects that allow purification of not only the intracellular parasites but also the phagosomes that harbor them and analyze the latter by gel free proteomics.