The influence of early neutrophil-Leishmania interactions on the host immune response to infection

STAT6 Mediates Footpad Immunopathology in the Absence of IL-12p40 Following Infection of Susceptible BALB/c Mice With Leishmania major

Leishmania major (L. major) parasites are intracellular parasites belong to the Trypanosomatidae family and are the causative agent of cutaneous leishmaniasis. This disease affects approximately 1.5 million per year worldwide and there is currently no prophylactic vaccine available. L. major is transmitted by the bite of an infected sandfly and has been considered for decades now as a mouse model of choice to identify the factors implicated in T helper (Th)1 and Th2 polarization due to the natural resistance and susceptibility to infection of C57BL/6 and BALB/c mice, respectively. In this study, we refine the role of IL-12p40 cytokine, which is implicated the development of a protective Th1 response, and STAT6, a transcription factor involved in the signaling via detrimental interleukin (IL)-4 and IL-13 associated Th2 cytokines during L. major infection in the BALB/c model. In the absence of STAT6 and IL-12p40 signaling, double knockout (DKO) susceptible BALB/c mice displayed reduced footpad swelling and ulcerative lesion compared to IL-12p40^−/− mice upon L. major infection. Hence, they expressed slower upregulation of keratinocyte markers implicated in the inhibition of wound healing, such as keratin 6a (Krt6a) and Krt16. This coincides with the presence of neutrophils displaying an altered phenotype characterized by a lower expression of surface markers Ly6C, CD11b, and Ly6G. These neutrophils exhibited very lower levels of apoptosis similarly to neutrophils present in resistant STAT6^−/− mice. Interestingly, the reduced footpad swelling in DKO mice is associated with a high footpad parasite level similar to susceptible IL-12p40^−/− mice. In conclusion, this study demonstrate that in the absence of both STAT6 and IL-12p40 signaling, L. major-infected mice display smaller and less ulcerated lesions, which does, however, not correlate with reduced parasite load. In addition, the presence of neutrophils with an altered phenotype is associated with reduced apoptosis and delayed immunopathologies, demonstrating the detrimental role of STAT6 in infected susceptible BALB/c mice.

Infection of Human Monocytes with Leishmania infantum Strains Induces a Downmodulated Response when Compared with Infection with Leishmania braziliensis

Human infection with different species of Leishmania leads to distinct clinical manifestations, ranging from relatively mild cutaneous (Leishmania braziliensis) to severe visceral (Leishmania infantum) forms of leishmaniasis. Here, we asked whether in vitro infection of human monocytes by Leishmania strains responsible for distinct clinical manifestations leads to early changes in immunological characteristics and ability of the host cells to control Leishmania. We evaluated the expression of toll-like receptors and MHC class II molecules, cytokines, and Leishmania control by human monocytes following short-term infection with L. braziliensis (M2904), a reference strain of L. infantum (BH46), and a wild strain of L. infantum (wild). The induction of TLR2, TLR9, and HLA-DR were all lower in L. infantum when compared with L. braziliensis-infected cells. Moreover, L. infantum-infected monocytes (both strains) produced lower TNF-alpha and a lower TNF-alpha/IL-10 ratio, resulting in a weaker inflammatory profile and a 100-fold less effective control of Leishmania than cells infected with L. braziliensis. Our results show that L. infantum strains fail to induce a strong inflammatory response, less activation, and less control of Leishmania from human monocytes, when compared with that induced by L. braziliensis infection. This functional profile may help explain the distinct clinical course observed in patients infected with the different Leishmania species.

Gut Microbes Egested during Bites of Infected Sand Flies Augment Severity of Leishmaniasis via Inflammasome-Derived IL-1β

Leishmania donovani parasites are the cause of visceral leishmaniasis and are transmitted by bites from phlebotomine sand flies. A prominent feature of vector-transmitted Leishmania is the persistence of neutrophils at bite sites, where they protect captured parasites, leading to enhanced disease. Here, we demonstrate that gut microbes from the sand fly are egested into host skin alongside Leishmania parasites. The egested microbes trigger the inflammasome, leading to a rapid production of interleukin-1β (IL-1β), which sustains neutrophil infiltration. Reducing midgut microbiota by pretreatment of Leishmania-infected sand flies with antibiotics or neutralizing the effect of IL-1β in bitten mice abrogates neutrophil recruitment. These early events are associated with impairment of parasite visceralization, indicating that both gut microbiota and IL-1β are important for the establishment of Leishmania infections. Considering that arthropods harbor a rich microbiota, its potential egestion after bites may be a shared mechanism that contributes to severity of vector-borne disease.

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.

Heme Drives Oxidative Stress-Associated Cell Death in Human Neutrophils Infected with Leishmania infantum

Free heme is an inflammatory molecule capable of inducing migration and activation of neutrophils. Here, we examine the heme-driven oxidative stress-associated cell death mechanisms in human neutrophils infected with Leishmania infantum, an etiologic agent of visceral leishmaniasis (VL). We first performed exploratory analyses in a population of well characterized treatment-naïve VL patients as well as uninfected controls, who were part of previously reported studies. We noted a positive correlation between serum concentrations of heme with heme oxygenase-1 (HO-1) and lactate deydrogenase, as well as, a negative correlation between heme values and peripheral blood neutrophils counts. Moreover, in vitro infection with L. infantum in the presence of heme enhanced parasite burden in neutrophils, while increasing the production of reactive oxygen species and release of neutrophilic enzymes. Additional experiments demonstrated that treatment of infected neutrophils with ferrous iron (Fe⁺²), a key component of the heme molecule, resulted in increased parasite survival without affecting neutrophil activation status. Furthermore, stimulation of infected neutrophils with heme triggered substantial increases in HO-1 mRNA expression as well as in superoxide dismutase-1 enzymatic activity. Heme, but not Fe⁺², induced oxidative stress-associated cell death. These findings indicate that heme promotes intracellular L. infantum survival via activation of neutrophil function and oxidative stress. This study opens new perspectives for the understanding of immunopathogenic mechanisms involving neutrophils in VL.

NOX2-Derived Reactive Oxygen Species Control Inflammation during Leishmania amazonensis Infection by Mediating Infection-Induced Neutrophil Apoptosis

Reactive oxygen species (ROS) produced by NADPH phagocyte oxidase isoform (NOX2) are critical for the elimination of intracellular pathogens in many infections. Despite their importance, the role of ROS following infection with the eukaryotic pathogen Leishmania has not been fully elucidated. We addressed the role of ROS in C57BL/6 mice following intradermal infection with Leishmania amazonensis. Despite equivalent parasite loads compared with wild-type (WT) mice, mice deficient in ROS production by NOX2 due to the absence of the gp91 subunit (gp91^phox-/-) had significantly more severe pathology in the later stages of infection. Pathology in gp91^phox-/- mice was not associated with alterations in CD4⁺ T cell-mediated immunity but was preceded by enhanced neutrophil accumulation at the dermal infection site. Ex vivo analysis of infected versus uninfected neutrophils revealed a deficiency in infection-driven apoptosis in gp91^phox-/- mice versus WT mice. gp91^phox-/- mice presented with higher percentages of healthy or necrotic neutrophils but lower percentages of apoptotic neutrophils at early and chronic time points. In vitro infection of gp91^phox-/- versus WT neutrophils also revealed reduced apoptosis and CD95 expression but increased necrosis in infected cells at 10 h postinfection. Provision of exogenous ROS in the form of H₂O₂ reversed the necrotic phenotype and restored CD95 expression on infected gp91^phox-/- neutrophils. Although ROS production is typically viewed as a proinflammatory event, our observations identify the importance of ROS in mediating appropriate neutrophil apoptosis and the importance of apoptosis in inflammation and pathology during chronic infection.

Survival Mechanisms Used by Some Leishmania Species to Escape Neutrophil Killing

Neutrophils are the most abundant leukocytes in human blood. Upon microbial infection, they are massively and rapidly recruited from the circulation to sites of infection where they efficiently kill pathogens. To this end, neutrophils possess a variety of weapons that can be mobilized and become effective within hours following infection. However, several microbes including some Leishmania spp. have evolved a variety of mechanisms to escape neutrophil killing using these cells as a basis to better invade the host. In addition, neutrophils are also present in unhealing cutaneous lesions where their role remains to be defined. Here, we will review recent progress in the field and discuss the different strategies applied by some Leishmania parasites to escape from being killed by neutrophils and as recently described for Leishmania mexicana, even replicate within these cells. Subversion of neutrophil killing functions by Leishmania is a strategy that allows parasite spreading in the host with a consequent deleterious impact, transforming the primary protective role of neutrophils into a deleterious one.

Oliveira CN, de Lorena VMB, de Paiva-Cavalcanti M. The Equivocal Role of Th17 Cells and Neutrophils on Immunopathogenesis of Leishmaniasis

Advances in the understanding of leishmaniasis progression indicate that cellular interactions more complex than the Th1/Th2 paradigm define the course of infection. Th17 cells are a crucial modulator of adaptive immunity against Leishmania parasites acting mainly on neutrophil recruitment and playing a dual role at the site of infection. This review describes the roles of both these cell types in linking innate defense responses to the establishment of specific immunity. We focus on the Th17-neutrophil interaction as a crucial component of anti-Leishmania immunity, and the clinical evolution of cutaneous or visceral leishmaniasis. To date, information obtained through experimental models and patient evaluations suggests that the influence of the presence of interleukin (IL)-17 (the main cytokine produced by Th17 cells) and neutrophils during Leishmania infections is strictly dependent on the tissue (skin or liver/spleen) and parasite species. Also, the time at which neutrophils are recruited, and the persistence of IL-17 in the infection microenvironment, may also be significant. A clearer understanding of these interactions will enable better measurement of the influence of IL-17 and its regulators, and contribute to the identification of disease/resistance biomarkers.

Canine neutrophils activate effector mechanisms in response to Leishmania infantum

Canine leishmaniosis caused by L. infantum is a severe zoonotic disease. Although macrophages are the definitive host cells, neutrophils are the first cells to encounter the parasite soon after its inoculation in the dermis by the phlebotomine vector. To study the interaction of dog neutrophils and L. infantum promastigotes, blood neutrophils were isolated from healthy donors and the infection was established in vitro. In the majority of the dogs, L. infantum was efficiently phagocytized by neutrophils, and oxidative (superoxide production) and non-oxidative (neutrophil elastase exocytosis) intracellular effector mechanisms were activated, but the release of neutrophil extracellular traps was minimized. Furthermore, promastigotes and culture supernatants induced neutrophil migration, but the prior contact with Leishmania inhibits chemotaxis, which might contribute to neutrophil retention at the inoculation site. Neutrophil-parasite interaction resulted in a decrease in parasite viability, although some intracellular promastigotes survive and maintain their proliferative capacity. These findings indicate that dog neutrophils are competent effector cells able to control the initial L. infantum infection. However, some parasites evade intracellular effector mechanisms and can be transferred to the definitive host cell, the macrophage, contributing to the development of canine leishmaniosis.

The circadian clock in immune cells controls the magnitude of Leishmania parasite infection

The intracellular parasite Leishmania uses neutrophils and macrophages as host cells upon infection. These immune cells harbour their own intrinsic circadian clocks, known to influence many aspects of their functions. Therefore, we tested whether the host circadian clocks regulate the magnitude of Leishmania major infection in mice. The extent of parasitic infection varied over 24 h in bone marrow-derived macrophages in vitro and in two different in vivo models, footpad and peritoneal cavity infection. In vivo this was paralleled by time of day-dependent neutrophil and macrophage infiltration to the infection site and rhythmic chemokine expression. Thus, rhythmic parasitic infection observed in vivo was likely initiated by the circadian expression of chemoattractants and the subsequent rhythmic infiltration of neutrophils and macrophages. Importantly, all rhythms were abolished in clock-deficient macrophages and when mice lacking the circadian clock in immune cells were infected. Therefore we demonstrated a critical role for the circadian clocks in immune cells in modulating the magnitude of Leishmania infection. To our knowledge this is the first report showing that the circadian clock controls infection by protozoan parasites in mammals. Understanding the timed regulation of host-parasite interactions will allow developing better prophylactic and therapeutic strategies to fight off vector-borne diseases.

Myeloid Cell Phenotypes in Susceptibility and Resistance to Helminth Parasite Infections

Many major tropical diseases are caused by long-lived helminth parasites that are able to survive by modulation of the host immune system, including the innate compartment of myeloid cells. In particular, dendritic cells and macrophages show markedly altered phenotypes during parasite infections. In addition, many specialized subsets such as eosinophils and basophils expand dramatically in response to these pathogens. The changes in phenotype and function, and their effects on both immunity to infection and reactivity to bystander antigens such as allergens, are discussed.

Cutaneous Manifestations of Human and Murine Leishmaniasis

The leishmaniases are diseases caused by pathogenic protozoan parasites of the genus Leishmania. Infections are initiated when a sand fly vector inoculates Leishmania parasites into the skin of a mammalian host. Leishmania causes a spectrum of inflammatory cutaneous disease manifestations. The type of cutaneous pathology is determined in part by the infecting Leishmania species, but also by a combination of inflammatory and anti-inflammatory host immune response factors resulting in different clinical outcomes. This review discusses the distinct cutaneous syndromes described in humans, and current knowledge of the inflammatory responses associated with divergent cutaneous pathologic responses to different Leishmania species. The contribution of key hematopoietic cells in experimental cutaneous leishmaniasis in mouse models are also reviewed and compared with those observed during human infection. We hypothesize that local skin events influence the ensuing adaptive immune response to Leishmania spp. infections, and that the balance between inflammatory and regulatory factors induced by infection are critical for determining cutaneous pathology and outcome of infection.

A secondary wave of neutrophil infiltration causes necrosis and ulceration in lesions of experimental American cutaneous leishmaniasis

We evaluated the importance of neutrophils in the development of chronic lesions caused by L. Viannia spp. using the hamster as experimental model of American Cutaneous Leishmaniasis (ACL). Neutrophils infiltrated the lesion within the first six hours post-infection. Inhibition of this early infiltration using a polyclonal antibody or cyclophosphamide was associated with transient parasite control but the protective effect vanished when lesions became clinically apparent. At lesion onset (approximately 10 days p.i.), there was an increased proportion of both uninfected and infected macrophages, and subsequently a second wave of neutrophils infiltrated the lesion (after 19 days p.i.) This second neutrophil infiltration was associated with lesion necrosis and ulceration (R² = 0.75) and maximum parasite burden. Intradermal delivery of N-formylmethionyl-leucyl-phenylalanine (fMLP), aimed to increase neutrophil infiltration, resulted in larger lesions with marked necrosis and higher parasite burden than in mock treated groups (p<0.001 each). In contrast, reduced neutrophil infiltration via cyclophosphamide-mediated depletion led to more benign lesions and lower parasite loads compared to controls (p<0.001 each). Neutrophils of the second wave expressed significantly lower GM-CSF, reactive oxygen species and nitric oxide than those of the first wave, suggesting that they had less efficient anti-leishmania activity. However, there was increased inflammatory cytokines and expression of neutrophil proteases (myeloperoxidase, cathepsin G and elastase) in lesions during the second wave of neutrophil infiltration compared with the levels reached during the first wave (6h p.i.). This suggests that augmented neutrophil proteases and inflammatory cytokines during the secondary wave of neutrophils could contribute to skin inflammation, ulceration and necrosis in ACL. The overall results indicate that neutrophils were unable to clear the infection in this model, and that the second wave of neutrophils played an important role in the severity of ACL.

Leishmania infection: painful or painless?

The complex life cycle and immunopathological features underpinning the interaction of Leishmania parasites and their mammalian hosts poses frequent poorly explored and inconclusively resolved questions. The altered nociceptive signals over the course of leishmaniasis remain an intriguing issue for nociceptive and parasitology researchers. Experimental investigations have utilized behavioral, morphological, and neuro-immune approaches in the study of experimental cutaneous leishmaniasis (CL). The data generated indicates new venues for the study of the pathological characteristics of nociceptive processing in this parasitic disease. Leishmania-induced pain may be easily observed in mice and rats. However, nociceptive data is more complex in human investigations, including the occurrence of painless lesions in mucocutaneous and cutaneous leishmaniasis. Data from recent decades indicate that humans can also be affected by pain-related symptoms, often distinct from the region of body infection. The molecular and cellular mechanisms underlying such variable nociceptive states in humans during the course of leishmaniasis are an active area of research. The present article reviews nociception in leishmaniasis, including in experimental models of CL and clinical reports.

Phenotypic and functional characteristics of HLA-DR+ neutrophils in Brazilians with cutaneous leishmaniasis

The protozoan Leishmania braziliensis causes cutaneous leishmaniasis (CL) in endemic regions. In murine models, neutrophils (PMNs) are recruited to the site of infection soon after parasite inoculation. However, the roles of neutrophils during chronic infection and in human disease remain undefined. We hypothesized that neutrophils help maintain a systemic inflammatory state in subjects with CL. Lesion biopsies from all patients with CL tested contained neutrophils expressing HLA-DR, a molecule thought to be restricted to professional antigen-presenting cells. Although CL is a localized disease, a subset of patients with CL also had circulating neutrophils expressing HLA-DR and the costimulatory molecules CD80, CD86, and CD40. PMNs isolated from a low-density leukocyte blood fraction (LD-PMNs) contained a higher percentage of HLA-DR⁺ PMNs than did normal-density PMNs. In vitro coculture experiments suggested LD-PMNs do not suppress T cell responses, differentiating them from MDSCs. Flow-sorted HLA-DR⁺ PMNs morphologically resembled conventional PMNs, and they exhibited functional properties of PMNs. Compared with conventional PMNs, HLA-DR⁺ PMNs showed increased activation, degranulation, DHR123 oxidation, and phagocytic capacity. A few HLA-DR⁺ PMNs were observed in healthy subjects, and that proportion could be increased by incubation in either inflammatory cytokines or in plasma from a patient with CL. This was accompanied by an increase in PMN hladrb1 mRNA, suggesting a possible connection between neutrophil "priming" and up-regulation of HLA-DR. These data suggest that PMNs that are primed for activation and that also express surface markers of antigen-presenting cells emerge in the circulation and infected tissue lesions of patients with CL.

Pro- and anti-inflammatory cytokines in cutaneous leishmaniasis: a review

Cutaneous leishmaniasis (CL) is caused by different species of the genus Leishmania. Pro- and anti-inflammatory cytokines play different roles in resistance/susceptibility and the immunopathogenesis of Leishmania infection. The balance and dynamic changes in cytokines may control or predict clinical outcome. T helper 1 (Th1) inflammatory cytokines (especially interferon-γ, tumor necrosis factor-α and interleukin-12) are the crucial factors in the initiation of protective immunity against L. major infection, whereas T helper 2 cytokines including IL-5, IL-4, and IL-13 facilitate the persistence of parasites by downregulating the Th1 immune response. On the other hand, aggravation of inflammatory reactions leads to collateral tissue damage and formation of ulcer. For this reason, immunity system such as T regulatory cells produce regulatory cytokines such as transforming growth factor-β and IL-10 to inhibit possible injures caused by increased inflammatory responses in infection site. In this article, we review the role of pro- and anti-inflammatory cytokines in the immunoprotection and immunopathology of CL.

Differences in immune responses against Leishmania induced by infection and by immunization with killed parasite antigen: implications for vaccine discovery

The leishmaniases are a group of diseases caused by different species of the protozoan genus Leishmania and transmitted by sand fly vectors. They are a major public health problem in almost all continents. There is no effective control of leishmaniasis and its geographical distribution is expanding in many countries. Great effort has been made by many scientists to develop a vaccine against leishmaniasis, but, so far, there is still no effective vaccine against the disease. The only way to generate protective immunity against leishmaniasis in humans is leishmanization, consisting of the inoculation of live virulent Leishmania as a means to acquire long-lasting immunity against subsequent infections. At present, all that we know about human immune responses to Leishmania induced by immunization with killed parasite antigens came from studies with first generation candidate vaccines (killed promastigote extracts). In the few occasions that the T cell-mediated immune responses to Leishmania induced by infection and immunization with killed parasite antigens were compared, important differences were found both in humans and in animals. This review discusses these differences and their relevance to the development of a vaccine against leishmaniasis, the major problems involved in this task, the recent prospects for the selection of candidate antigens and the use of attenuated Leishmania as live vaccines.

Crosstalk between purinergic receptors and lipid mediators in leishmaniasis

Leishmaniasis is a neglected tropical disease affecting millions of people around the world caused by organisms of the genus Leishmania. Parasite escape mechanisms of the immune system confer the possibility of resistance and dissemination of the disease. A group of molecules that has become a target for Leishmania survival strategies are lipid mediators. Among them, leukotriene B₄ (LTB₄) has been described as a pro-inflammatory molecule capable of activating cells of the immune system to combat Leishmania. In an opposite way, prostaglandin E₂ (PGE₂) is a lipid mediator described as a deactivator of macrophages and neutrophils. The balance of these two molecules can be generated by extracellular nucleotides, such as adenosine 5'-triphosphate (ATP) and adenosine (Ado), which activate the purinergic receptors system. Herein, we discuss the role of extracellular nucleotides and the resulting balance of LTB₄ and PGE₂ in Leishmania fate, survival or death.

Leishmania exosomes and other virulence factors: Impact on innate immune response and macrophage functions

Leishmania parasites are the causative agents of the leishmaniases, a collection of vector-borne diseases that range from simple cutaneous to fatal visceral forms. Employing potent immune modulation mechanisms, Leishmania is able to render the host macrophage inactive and persist inside its phagolysosome. In the last few years, the role of exosomes in Leishmania-host interactions has been increasingly investigated. For instance, it was reported that Leishmania exosome release is augmented following temperature shift, a condition mimicking parasite's entry into its mammalian host. Leishmania exosomes were found to strongly affect macrophage cell signaling and functions, similarly to whole parasites. Importantly, these vesicles were shown to be pro-inflammatory, capable to recruit neutrophils at their inoculation site exacerbating the pathology. In this review, we provide the most recent insights on the role of exosomes and other virulence factors, especially the surface protease GP63, in Leishmania-host interactions, deepening our knowledge on leishmaniasis and paving the way for the development of new therapeutics.

Role of pro-inflammatory cytokine IL-17 in Leishmania pathogenesis and in protective immunity by Leishmania vaccines

The clinical outcome of Leishmania pathogenesis ranges from active skin lesions to fatal visceral dissemination and severely impaired T cell immunity. It is well established that a strong Th1 immune response is protective against cutaneous forms of the disease, however a mixed Th1/Th2 response is most commonly observed against visceral infections as evident from previous studies. Aside from Th1/Th2 cytokines, the pro-inflammatory IL-17 cytokine family plays an important role in the clearance of intracellular pathogens. In Leishmania induced skin lesions, IL-17 produced by Th17 cells is shown to exacerbate the disease, suggesting a role in pathogenesis. However, a protective role for IL-17 is indicated by the expansion of IL-17 producing cells in vaccine-induced immunity. In human visceral leishmaniasis (VL) it has been demonstrated that IL-17 and IL-22 are associated with protection against re-exposure to Leishmania, which further suggests the involvement of IL-17 in vaccine induced protective immunity. Although there is no vaccine against any form of leishmaniasis, the development of genetically modified live attenuated parasites as vaccine candidates prove to be promising, as they successfully induce a robust protective immune response in various animal models. However, the role of IL-17 producing cells and Th17 cells in response to these vaccine candidates remains unexplored. In this article, we review the role of IL-17 in Leishmania pathogenesis and the potential impact on vaccine induced immunity, with a special focus on live attenuated Leishmania parasites.

Twin Attributes of Tyrosyl-tRNA Synthetase of Leishmania donovani: A HOUSEKEEPING PROTEIN TRANSLATION ENZYME AND A MIMIC OF HOST CHEMOKINE

Aminoacyl-tRNA synthetases (aaRSs) are housekeeping enzymes essential for protein synthesis. Apart from their parent aminoacylation activity, several aaRSs perform non-canonical functions in diverse biological processes. The present study explores the twin attributes of Leishmania tyrosyl-tRNA synthetase (LdTyrRS) namely, aminoacylation, and as a mimic of host CXC chemokine. Leishmania donovani is a protozoan parasite. Its genome encodes a single copy of tyrosyl-tRNA synthetase. We first tested the canonical aminoacylation role of LdTyrRS. The recombinant protein was expressed, and its kinetic parameters were determined by aminoacylation assay. To study the physiological role of LdTyrRS in Leishmania, gene deletion mutations were attempted via targeted gene replacement. The heterozygous mutants showed slower growth kinetics and exhibited attenuated virulence. LdTyrRS appears to be an essential gene as the chromosomal null mutants did not survive. Our data also highlights the non-canonical function of L. donovani tyrosyl-tRNA synthetase. We show that LdTyrRS protein is present in the cytoplasm and exits from the parasite cytoplasm into the extracellular medium. The released LdTyrRS functions as a neutrophil chemoattractant. We further show that LdTyrRS specifically binds to host macrophages with its ELR (Glu-Leu-Arg) peptide motif. The ELR-CXCR2 receptor interaction mediates this binding. This interaction triggers enhanced secretion of the proinflammatory cytokines TNF-α and IL-6 by host macrophages. Our data indicates a possible immunomodulating role of LdTyrRS in Leishmania infection. This study provides a platform to explore LdTyrRS as a potential target for drug development.

Leishmania amazonensis Engages CD36 to Drive Parasitophorous Vacuole Maturation

Leishmania amastigotes manipulate the activity of macrophages to favor their own success. However, very little is known about the role of innate recognition and signaling triggered by amastigotes in this host-parasite interaction. In this work we developed a new infection model in adult Drosophila to take advantage of its superior genetic resources to identify novel host factors limiting Leishmania amazonensis infection. The model is based on the capacity of macrophage-like cells, plasmatocytes, to phagocytose and control the proliferation of parasites injected into adult flies. Using this model, we screened a collection of RNAi-expressing flies for anti-Leishmania defense factors. Notably, we found three CD36-like scavenger receptors that were important for defending against Leishmania infection. Mechanistic studies in mouse macrophages showed that CD36 accumulates specifically at sites where the parasite contacts the parasitophorous vacuole membrane. Furthermore, CD36-deficient macrophages were defective in the formation of the large parasitophorous vacuole typical of L. amazonensis infection, a phenotype caused by inefficient fusion with late endosomes and/or lysosomes. These data identify an unprecedented role for CD36 in the biogenesis of the parasitophorous vacuole and further highlight the utility of Drosophila as a model system for dissecting innate immune responses to infection.

Neutrophil extracellular traps release induced by Leishmania: role of PI3Kγ, ERK, PI3Kσ, PKC, and [Ca2+]

Upon in vitro stimulation, neutrophils undergo a cell death named netosis. This process is characterized by extracellular release of chromatin scaffold associated with granular and cytoplasmic proteins, which together, ensnare and kill microbes. We have previously described that interaction of Leishmania amazonensis with human neutrophils leads to the release of neutrophil extracellular traps, which trap and kill the parasite. However, the signaling leading to Leishmania induced netosis is still unknown. Thus, we sought to evaluate signaling events that drive L. amazonensis induced neutrophil extracellular trap release from human neutrophils. Here, we found that PI3K, independently of protein kinase B, has a role in parasite-induced netosis. We also described that the main isoforms involved are PI3Kγ and PI3Kδ, which work in reactive oxygen species-dependent and -independent ways, respectively. We demonstrated that activation of ERK downstream of PI3Kγ is important to trigger reactive oxygen species-dependent, parasite-induced netosis. Pharmacological inhibition of protein kinase C also significantly decreased parasite-induced neutrophil extracellular trap release. Intracellular calcium, regulated by PI3Kδ, represents an alternative reactive oxygen species-independent pathway of netosis stimulated by L. amazonensis Finally, intracellular calcium mobilization and reactive oxygen species generation are the major regulators of parasite-induced netosis. Our results contribute to a better understanding of the signaling behind netosis induced by interactions between Leishmania and neutrophils.

The site of the bite: Leishmania interaction with macrophages, neutrophils and the extracellular matrix in the dermis

Leishmania spp., the causative agents of leishmaniasis, are intracellular parasites, transmitted to humans via the bite of their sand fly vectors. Once inoculated, the promastigotes are exposed to the dermis, which is composed of extracellular matrix (ECM), growth factors and its resident cells. Promastigote forms are phagocytosed by macrophages recruited to the site of the sand fly bite, either directly or after interaction with neutrophils. Since Leishmania is an intracellular parasite, its interaction with the host ECM has been neglected as well as the immediate steps after the sand fly bite. However, promastigotes must overcome the obstacles presented by the dermis ECM in order to establish the infection. Thus, the study of the interaction between Leishmania promastigotes and ECM components as well as the earliest stages of infection are important steps to understand the establishment of the disease, and could contribute in the future to new drug developments towards leishmaniasis.

Leishmania Infection Engages Non-Receptor Protein Kinases Differentially to Persist in Infected Hosts

Protein kinases play important roles in the regulation of cellular activities. In cells infected by pathogens, there is an increasing appreciation that dysregulated expression of protein kinases promotes the success of intracellular infections. In Leishmania-infected cells, expression and activation of protein kinases, such as the mitogen-activated protein kinases, kinases in the PI3-kinase signaling pathway, and kinases in the NF-κB-signaling pathway, are modulated in some manner. Several recent reviews have discussed our current understanding of the roles of these kinases in Leishmania infections. Apart from the kinases in the pathways enumerated above, there are other host cell protein kinases that are activated during the Leishmania infection of mammalian cells whose roles also appear to be significant. This review discusses recent observations on the Abl family of protein kinases and the protein kinase regulated by RNA in Leishmania infections.

Macrophage cell death in microbial infections

Macrophages can respond to microbial infections with programmed cell death. The major cell death pathways of apoptosis, pyroptosis and necroptosis are tightly regulated to ensure adequate immune reactions to virulent and persistent invaders. Macrophage death eliminates the replicative niche of intracellular pathogens and induces immune attack. Not surprisingly, successful pathogens have evolved strategies to modulate macrophage cell death pathways to enable microbial survival and replication. Uncontrolled macrophage death can also lead to tissue damage, which may augment bacterial dissemination and pathology. In this review, we highlight how pathogens hijack macrophage cell death signals to promote microbial survival and immune evasion.

New approaches from nanomedicine for treating leishmaniasis

This review summarizes the recent progress in nanomedicine for the treatment of leishmaniasis.

Permissive and protective roles for neutrophils in leishmaniasis

Leishmania parasites are the causative agents of leishmaniasis, a neglected tropical disease that causes substantial morbidity and considerable mortality in many developing areas of the world. Recent estimates suggest that roughly 10 million people suffer from cutaneous leishmaniasis (CL), and approximately 76,000 are afflicted with visceral leishmaniasis (VL), which is universally fatal without treatment. Efforts to develop therapeutics and vaccines have been greatly hampered by an incomplete understanding of the parasite's biology and a lack of clear protective correlates that must be met in order to achieve immunity. Although parasites grow and divide preferentially in macrophages, a number of other cell types interact with and internalize Leishmania parasites, including monocytes, dendritic cells and neutrophils. Neutrophils appear to be especially important shortly after parasites are introduced into the skin, and may serve a dual protective and permissive role during the establishment of infection. Curiously, neutrophil recruitment to the site of infection appears to continue into the chronic phase of disease, which may persist for many years. The immunological impact of these cells during chronic leishmaniasis is unclear at this time. In this review we discuss the ways in which neutrophils have been observed to prevent and promote the establishment of infection, examine the role of anti-neutrophil antibodies in mouse models of leishmaniasis and consider recent findings that neutrophils may play a previously unrecognized role in influencing chronic parasite persistence.

Pimentel MI, Rosandiski Lyra M, Schubach ADO, Conceição-Silva F. Are Neutrophil Extracellular Traps Playing a Role in the Parasite Control in Active American Tegumentary Leishmaniasis Lesions?

Neutrophil extracellular traps (NETs) have been described as a network of extracellular fibers composed by DNA, histones and various proteins/enzymes. Studies have demonstrated that NETs could be responsible for the trapping and elimination of a variety of infectious agents. In order to verify the presence of NETs in American tegumentary leishmaniasis (ATL) and their relationship with the presence of amastigotes we evaluated active cutaneous lesions of 35 patients before treatment by the detection of parasites, neutrophils (neutrophil elastase) and histones through immunohistochemistry and confocal immunofluorescence. Intact neutrophils could be detected in all ATL lesions. NETs were present in 27 patients (median 1.1; range from 0.1 to 23.5/mm²) with lesion duration ranging from one to seven months. NETs were in close proximity with neutrophils (r = 0.586; p = 0.0001) and amastigotes (r = 0.710; p = 0.0001). Two patterns of NET formation were detected: small homogeneously distributed networks observed in all lesions; and large structures that could be visualized at a lower magnification in lesions presenting at least 20% of neutrophils. Lesions presenting the larger NET formation showed high parasite detection. A correlation between NET size and the number of intact amastigotes was observed (p=0.02). As we detected an association between NET and amastigotes, our results suggest that neutrophil migration and NET formation could be stimulated and maintained by stimuli derived from the parasite burden/parasite antigen in the extracellular environment. The observation of areas containing only antigens not intermingled with NETs (elastase and histone) suggests that the involvement of these structures in the control of parasite burden is a dynamic process in which the formation of NETs is exhausted with the destruction of the parasites. Since NETs were also associated with granulomas, this trapping would favor the activity of macrophages in order to control the parasite burden.

Transgenic expression of CXCR3 on T cells enhances susceptibility to cutaneous Leishmania major infection by inhibiting monocyte maturation and promoting a Th2 response

Cutaneous leishmaniasis, caused mainly by Leishmania major, an obligate intracellular parasite, is a disfiguring disease characterized by large skin lesions and is transmitted by a sand fly vector. We previously showed that the chemokine receptor CXCR3 plays a critical role in mediating resistance to cutaneous leishmaniasis caused by Leishmania major. Furthermore, T cells from L. major-susceptible BALB/c but not L. major-resistant C57BL/6 mice fail to efficiently upregulate CXCR3 upon activation. We therefore examined whether transgenic expression of CXCR3 on T cells would enhance resistance to L. major infection in susceptible BALB/c mice. We generated BALB/c and C57BL/6 transgenic mice, which constitutively overexpressed CXCR3 under a CD2 promoter, and then examined the outcomes with L. major infection. Contrary to our hypothesis, transgenic expression of CXCR3 (CXCR3(Tg)) on T cells of BALB/c mice resulted in increased lesion sizes and parasite burdens compared to wild-type (WT) littermates after L. major infection. Restimulated lymph node cells from L. major-infected BALB/c-CXCR3(Tg) mice produced more interleukin-4 (IL-4) and IL-10 and less gamma interferon (IFN-γ). Cells in draining lymph nodes from BALB/c-CXCR3(Tg) mice showed enhanced Th2 and reduced Th1 cell accumulation associated with increased neutrophils and inflammatory monocytes. However, monocytes displayed an immature phenotype which correlated with increased parasite burdens. Interestingly, transgenic expression of CXCR3 on T cells did not impact the outcome of L. major infection in C57BL/6 mice, which mounted a predominantly Th1 response and spontaneously resolved their infection similar to WT littermates. Our findings demonstrate that transgenic expression of CXCR3 on T cells increases susceptibility of BALB/c mice to L. major.

Microbiota-derived compounds drive steady-state granulopoiesis via MyD88/TICAM signaling

Neutropenia is probably the strongest known predisposition to infection with otherwise harmless environmental or microbiota-derived species. Because initial swarming of neutrophils at the site of infection occurs within minutes, rather than the hours required to induce "emergency granulopoiesis," the relevance of having high numbers of these cells available at any one time is obvious. We observed that germ-free (GF) animals show delayed clearance of an apathogenic bacterium after systemic challenge. In this article, we show that the size of the bone marrow myeloid cell pool correlates strongly with the complexity of the intestinal microbiota. The effect of colonization can be recapitulated by transferring sterile heat-treated serum from colonized mice into GF wild-type mice. TLR signaling was essential for microbiota-driven myelopoiesis, as microbiota colonization or transferring serum from colonized animals had no effect in GF MyD88(-/-)TICAM1(-/-) mice. Amplification of myelopoiesis occurred in the absence of microbiota-specific IgG production. Thus, very low concentrations of microbial Ags and TLR ligands, well below the threshold required for induction of adaptive immunity, sets the bone marrow myeloid cell pool size. Coevolution of mammals with their microbiota has probably led to a reliance on microbiota-derived signals to provide tonic stimulation to the systemic innate immune system and to maintain vigilance to infection. This suggests that microbiota changes observed in dysbiosis, obesity, or antibiotic therapy may affect the cross talk between hematopoiesis and the microbiota, potentially exacerbating inflammatory or infectious states in the host.

A single-centre, open-label, controlled, randomized clinical trial to assess the preventive efficacy of a domperidone-based treatment programme against clinical canine leishmaniasis in a high prevalence area

The innate immune response acting immediately after initial infection with Leishmania parasites is known to play a relevant role in prevention against clinical progression of the disease. Domperidone is a dopamine D2 receptor antagonist that has shown to enhance the innate cell-mediated immune response. The aim of this study was to assess the preventive efficacy of a domperidone-based treatment programme against clinical canine leishmaniasis (CanL) in a high prevalence area. The study was performed with 90 healthy, seronegative dogs of different sex, age, weight and breed from a single veterinary clinic located in Valencia (Spain). Dogs were randomly allocated into two groups. Dogs in one group (domperidone-treated group; n=44) were administered an oral suspension of domperidone at 0.5 mg/kg bw/day during 30 consecutive days, every 4 months. Dogs in the other group (negative control group; n=46) were left untreated. A 21-month follow-up period was implemented covering two seasonal phases of the sand fly vector. During this period all animals underwent periodic clinical examinations and blood samplings for anti-Leishmania serological testing. Dogs seropositive for Leishmania (IFAT antibody titre≥1:80) plus at least one clinical sign consistent with CanL (indicative of active infection and incipient disease progression) were categorized as a 'prevention failure'. These dogs were withdrawn from the study after confirming the infection by direct observation of the parasite in smears of lymph nodes and/or bone marrow aspirates. The cumulative percentage of 'prevention failure' after 12 months was significantly lower in the domperidone-treated group than in the negative control group (7% versus 35%, p=0.003). Differences between groups persisted after 21 months (11% versus 48%, p<0.001). The prevention rate provided by domperidone was 80% during the first 12 months and 77% throughout the complete 21-month follow-up period, with odds ratios of 7.3 (p=0.001) and 7.15 (p<0.001), respectively, this indicating that the risk for domperidone-treated dogs to develop the clinical disease is quite 7 times lower than for dogs left untreated. The results of this study demonstrate that the implementation of a strategic domperidone-based treatment programme consisting in quarterly repeated 30-day treatments with domperidone effectively reduces the risk to develop clinical CanL in areas with high prevalence of the disease.

3'-nucleotidase/nuclease activity allows Leishmania parasites to escape killing by neutrophil extracellular traps

Leishmaniasis is a widespread neglected tropical disease caused by parasites of the Leishmania genus. These parasites express the enzyme 3'-nucleotidase/nuclease (3'NT/NU), which has been described to be involved in parasite nutrition and infection. Bacteria that express nucleases escape the toxic effects of neutrophil extracellular traps (NETs). Hence, we investigated the role of 3'NT/NU in Leishmania survival of NET-mediated killing. Promastigotes of Leishmania infantum were cultured in high-phosphate (HP) or low-phosphate (LP) medium to modulate nuclease activity. We compared the survival of the two different groups of Leishmania during interaction with human neutrophils, assessing the role of neutrophil extracellular traps. As previously reported, we detected higher nuclease activity in parasites cultured in LP medium. Both LP and HP promastigotes were capable of inducing the release of neutrophil extracellular traps from human neutrophils in a dose- and time-dependent manner. LP parasites had 2.4 times more survival than HP promastigotes. NET disruption was prevented by the treatment of the parasites with ammonium tetrathiomolybdate (TTM), a 3'NT/NU inhibitor. Inhibition of 3'NT/NU by 3'-AMP, 5'-GMP, or TTM decreased promastigote survival upon interaction with neutrophils. Our results show that Leishmania infantum induces NET release and that promastigotes can escape NET-mediated killing by 3'-nucleotidase/nuclease activity, thus ascribing a new function to this enzyme.

Lundep, a sand fly salivary endonuclease increases Leishmania parasite survival in neutrophils and inhibits XIIa contact activation in human plasma

Neutrophils are the host's first line of defense against infections, and their extracellular traps (NET) were recently shown to kill Leishmania parasites. Here we report a NET-destroying molecule (Lundep) from the salivary glands of Lutzomyia longipalpis. Previous analysis of the sialotranscriptome of Lu. longipalpis showed the potential presence of an endonuclease. Indeed, not only was the cloned cDNA (Lundep) shown to encode a highly active ss- and dsDNAse, but also the same activity was demonstrated to be secreted by salivary glands of female Lu. longipalpis. Lundep hydrolyzes both ss- and dsDNA with little sequence specificity with a calculated DNase activity of 300000 Kunitz units per mg of protein. Disruption of PMA (phorbol 12 myristate 13 acetate)- or parasite-induced NETs by treatment with recombinant Lundep or salivary gland homogenates increases parasite survival in neutrophils. Furthermore, co-injection of recombinant Lundep with metacyclic promastigotes significantly exacerbates Leishmania infection in mice when compared with PBS alone or inactive (mutagenized) Lundep. We hypothesize that Lundep helps the parasite to establish an infection by allowing it to escape from the leishmanicidal activity of NETs early after inoculation. Lundep may also assist blood meal intake by lowering the local viscosity caused by the release of host DNA and as an anticoagulant by inhibiting the intrinsic pathway of coagulation.

Salivary components from disease vectors help the arthropod to acquire blood. Here we show that an arthropod vector salivary enzyme affects the innate immune system of the host—mainly the destruction of neutrophil traps—allowing the Leishmania parasite to evade the host immune response and to cause an infection. This work highlights the relevance of vector salivary components in parasite transmission and further suggests the inclusion of these proteins as components for an anti-Leishmania vaccine. Importantly, because salivary proteins are always present at the site of natural transmission, this work further encourages the testing of vaccine candidates using the natural route of transmission—the bites of an arthropod vector—instead of current practices based solely on needle injection of parasites.

Impact of neutrophil-secreted myeloid related proteins 8 and 14 (MRP 8/14) on leishmaniasis progression

The myeloid-related proteins (MRPs) 8/14 are small proteins mainly produced by neutrophils, which have been reported to induce NO production in macrophages. On the other hand, Leishmania survives and multiplies within phagocytes by inactivating several of their microbicidal functions. Whereas MRPs are rapidly released during the innate immune response, their role in the regulation of Leishmaniasis is still unknown. In vitro experiments revealed that Leishmania infection alters MRP-induced signaling, leading to inhibition of macrophage functions (NO, TNF-α). In contrast, MRP-primed cells showed normal signaling activation and NO production in response to Leishmania infection. Using a murine air-pouch model, we observed that infection with L. major induced leukocyte recruitment and MRP secretion comparable to LPS-treated mice. Depletion of MRPs significantly reduced these inflammatory events and augmented both parasite load and footpad swelling during the first 8 weeks post-infection, as also observed in MRP KO mice. On the contrary, mouse treatment with recombinant MRPs (rMRPs) had the opposite effect. Collectively, our results suggest that rapid secretion of MRPs by neutrophils at the site of infection may protect uninfected macrophages and favor a more efficient innate inflammatory response against Leishmania infection. In summary, our study reveals the critical role played by MRPs in the regulation of Leishmania infection and how this pathogen can subvert its action.

Parasites of the Leishmania genus have developed multiple mechanisms to subvert the immune response. Among these mechanisms are the activation of host phosphatases and inactivation of cell signaling pathways, which in turn activate the immune response. On the other hand, it has been observed that the Myeloid Related Proteins (MRPs) 8 and 14 are potent activators of some components of the immune response. In this study, we evaluated the effect of MRPs 8 and 14 on the progression of cutaneous Leishmaniasis. To do so, we used immortalized macrophages and stimulated them with MRPs before or after infection with L. major. We observed that stimulating macrophages with MRPs prior to infection induced NO and TNF-α production, as well as phosphorylation of MAPKs and nuclear translocation of transcription factors NF-κB and AP-1. However, when MRP stimulation was performed after infection, these effects where subverted. Moreover, using a murine model of cutaneous infection, we observed that depletion of MRPs caused increased parasite burden and bigger lesions. On the contrary, injection of recombinant MRPs directly into the lesion, considerably reduced lesion size and parasite burden. Our study suggests that MRPs could have a potential therapeutic use in the control of Leishmania infection.

Leishmania amazonensis amastigotes trigger neutrophil activation but resist neutrophil microbicidal mechanisms

Neutrophils are the first cells to infiltrate to the site of Leishmania promastigote infection, and these cells help to reduce parasite burden shortly after infection is initiated. Several clinical reports indicate that neutrophil recruitment is sustained over the course of leishmaniasis, and amastigote-laden neutrophils have been isolated from chronically infected patients and experimentally infected animals. The goal of this study was to compare how thioglycolate-elicited murine neutrophils respond to L. amazonensis metacyclic promastigotes and amastigotes derived from axenic cultures or from the lesions of infected mice. Neutrophils efficiently internalized both amastigote and promastigote forms of the parasite, and phagocytosis was enhanced in lipopolysaccharide (LPS)-activated neutrophils or when parasites were opsonized in serum from infected mice. Parasite uptake resulted in neutrophil activation, oxidative burst, and accelerated neutrophil death. While promastigotes triggered the release of tumor necrosis factor alpha (TNF-α), uptake of amastigotes preferentially resulted in the secretion of interleukin-10 (IL-10) from neutrophils. Finally, the majority of promastigotes were killed by neutrophils, while axenic culture- and lesion-derived amastigotes were highly resistant to neutrophil microbicidal mechanisms. This study indicates that neutrophils exhibit distinct responses to promastigote and amastigote infection. Our findings have important implications for determining the impact of sustained neutrophil recruitment and amastigote-neutrophil interactions during the late phase of cutaneous leishmaniasis.

CD8(+) granzyme B(+)-mediated tissue injury vs. CD4(+)IFNγ(+)-mediated parasite killing in human cutaneous leishmaniasis

A protective or deleterious role of CD8⁺T cells in human cutaneous leishmaniasis (CL) has been debated. The present report explores the participation of CD8⁺T cells in disease pathogenesis as well as in parasite killing. CD8⁺T cells accumulated in CL lesions as suggested by a higher frequency of CD8⁺CD45RO⁺T cells and CD8⁺CLA⁺T cells compared with peripheral blood mononuclear cells. Upon Leishmania braziliensis restimulation, most of the CD8⁺T cells from the lesion expressed cytolytic markers, CD107a and granzyme B. Granzyme B expression in CL lesions positively correlated with lesion size and percentage of TUNEL-positive cells. We also observed a significantly higher percentage of TUNEL-positive cells and granzyme B expression in the biopsies of patients showing a more intense necrotic process. Furthermore, coculture of infected macrophages and CD8⁺T lymphocytes resulted in the release of granzyme B, and the use of granzyme B inhibitor, as well as z-VAD, Fas:Fc, or anti-IFN-γ, had no effect upon parasite killing. However, coculture of infected macrophages with CD4⁺T cells strongly increased parasite killing, which was completely reversed by anti-IFN-γ. Our results reveal a dichotomy in human CL: CD8⁺ granzyme B⁺T cells mediate tissue injury, whereas CD4⁺IFN-γ⁺T cells mediate parasite killing.