Phosphatidylserine exposure on the surface of Leishmania amazonensis amastigotes modulates in vivo infection and dendritic cell function

Leishmania donovani induces CD300a expression to dampen effector properties of CD11c+ dendritic and antigen activated CD8+ T cells

CD8⁺ T cells are an important regiment of adaptive immunity that play a decisive role in elimination of many species of Leishmania parasite from the host. In visceral leishmaniasis, caused by L. donovani, the loss of CD8⁺ T cells function has been found associated with augmented pathogenesis. The factors determining CD8⁺ T cells activation and function against Leishmania antigens are largely unknown. In this study, we investigated the role of an immune inhibitory receptor, CD300a, on the effector properties of dendritic cells and CD8⁺ T cells. We observed that the Leishmania regulates the effectors function of CD8⁺ T cells by increasing CD300a expression on CD11c⁺ dendritic cells. The abrogation of CD300a signaling in parasites infected animals induced CD8⁺ T cell abilities to produce IFN-γ, TNF-α and also helped them to acquire desired multifunctionality. The CD300a receptor blocking also enhanced the number of CD8⁺ T cells memory phenotypes at the early days of infection, suggesting its potential beneficial role in vaccine induced immunity. We also observed significantly enhanced levels of pro-inflammatory cytokines in the spleen of CD300a blocked infected animals with concomitant reduced spleen parasite load. Additionally, the abrogation of CD300a signals in the infected animals helped in establishing Th1 type protective humoral immunity with significantly elevated levels of IgG2a antibodies. Since CD8⁺ T cells are an important determinant of vaccine induced immunity against leishmaniasis, the findings corroborate the potential of CD300a in vaccine induced immunity and thus require further attention.

Leishmania amazonensis sabotages host cell SUMOylation for intracellular survival

Leishmania parasites use elaborate virulence mechanisms to invade and thrive in macrophages. These virulence mechanisms inhibit host cell defense responses and generate a specialized replicative niche, the parasitophorous vacuole. In this work, we performed a genome-wide RNAi screen in Drosophila macrophage-like cells to identify the host factors necessary for Leishmania amazonensis infection. This screen identified 52 conserved genes required specifically for parasite entry, including several components of the SUMOylation machinery. Further studies in mammalian macrophages found that L. amazonensis infection inhibited SUMOylation within infected macrophages and this inhibition enhanced parasitophorous vacuole growth and parasite proliferation through modulation of multiple genes especially ATP6V0D2, which in turn affects CD36 expression and cholesterol levels. Together, these data suggest that parasites actively sabotage host SUMOylation and alter host transcription to improve their intracellular niche and enhance their replication.

Bidirectional cytokine-microRNA control: A novel immunoregulatory framework in leishmaniasis

As effector innate immune cells and as a host to the protozoan parasite Leishmania, macrophages play a dual role in antileishmanial immunoregulation. The 2 key players in this immunoregulation are the macrophage-expressed microRNAs (miRNAs) and the macrophage-secreted cytokines. miRNAs, as small noncoding RNAs, play vital roles in macrophage functions including cytokines and chemokines production. In the reverse direction, Leishmania-regulated cytokines alter miRNAs expression to regulate the antileishmanial functions of macrophages. The miRNA patterns vary with the time and stage of infection. The cytokine-regulated macrophage miRNAs not only help parasite elimination or persistence but also regulate cytokine production from macrophages. Based on these observations, we propose a novel immunoregulatory framework as a scientific rationale for antileishmanial therapy.

Four new cycloartane-type triterpenoids from the leaves of Combretum mellifluum Eichler: assessment of their antioxidant and antileishmanial activities

The beneficial pharmacological actions including antioxidant effects as an antileishmanial, antibacterial, antifungal, antidiabetic, anti-inflammatory, antitumor, antiviral, and analgesic of compounds isolated from Combretum mellifluum Eichler (Combretaceae) are well established. The aim of the present study was to determine the phytochemistry as well as assess the antioxidant and antileishmanial activities of the leaves from Combretum mellifluum Eichler (Combretaceae). Analysis of ethanolic extract resulted in isolation and identification of two epimeric mixtures of four previously unknown cycloartane-type triterpenoids, methyl quadrangularate M and methyl 24-epiquadrangularate M, and 2α,3β,24β-trihydroxy-cycloart-25-ene and 2α, 3β, 24α-trihydroxy-cycloart-25-ene, and eight known compounds. Their structures were using one-dimensional nuclear magnetic resonance (1D NMR), 2D NMR and high-resolution electrospray ionization mass spectroscopy (HRESIMS) analysis. Further, the extract and fractions were tested for antioxidant potential. The ethyl acetate and aqueous fractions demonstrated the highest antioxidant activity against 2,2-dipheny-1-picrylhydrazl (DPPH) free radicals, which correlated directly with total flavonoid content. All extracts and fractions from C. mellifluum Eichler were assessed for antileishmanial activity. The supernatant fraction exhibited highest potential, inhibiting the growth of Leishmania amazonensis with IC₅₀ value 31.29 μg/ml. Our findings provide information on the chemical composition of C. mellifluum and the potential beneficial therapeutic usefulness as an antioxidant agent in various diseases.

The metamorphosis of neutrophil transcriptional program during Leishmania infection

The role of neutrophils in the course of Leishmania infection remains controversial, displaying tremendous variability depending on the species of parasite, stage of infection, host genetic background, and methodological discrepancies among studies. Although neutrophils have long been categorized as short-lived cells with limited capacity to express proteins de novo, recent advances have revealed significant plasticity in neutrophil transcriptional programs and intrapopulation heterogeneity, which can be regulated by both intrinsic and extrinsic factors that together determine the profile of neutrophil effector response. In this review, we focus on the current understanding of neutrophil transcriptional plasticity, neutrotime, evidence of Leishmania-mediated alterations in neutrophil transcriptome leading to the rise of subpopulations, and finally, functional implications of those findings to the course of Leishmania infection.

CD300a Receptor Blocking Enhances Early Clearance of Leishmania donovani From Its Mammalian Host Through Modulation of Effector Functions of Phagocytic and Antigen Experienced T Cells

The parasites of the genus Leishmania survive and proliferate in the host phagocytic cells by taking control over their microbicidal functions. The parasite also promotes differentiation of antigen-specific anti-inflammatory cytokines producing effector T cells, which eventually results in disease pathogenesis. The mechanisms that parasites employ to dominate host adaptive immunity are largely unknown. For the first time, we report that L. donovani, which causes visceral leishmaniasis in the Indian subcontinent, upregulates the expression of an immune inhibitory receptor i.e., CD300a on antigen presenting and phagocytic cells to dampen their effector functions. The blocking of CD300a signals in leishmania antigens activated macrophages and dendritic cells enhanced the production of nitric oxide, pro-inflammatory cytokines along with MHCI/II genes expression, and reduced parasitic uptake. Further, the abrogation of CD300a signals in Leishmania infected mice benefited antigen-experienced, i.e., CD4⁺CD44⁺ and CD8⁺CD44⁺ T cells to acquire more pro-inflammatory cytokines producing phenotypes and helped in the early clearance of parasites from their visceral organs. The CD300a receptor blocking also enhanced the conversion of CD4⁺ T effectors cells to their memory phenotypes i.e., CCR7^high CD62L^high up to 1.6 and 1.9 fold after 14 and 21 days post-infection, respectively. These findings implicate that CD300a is an important determinant of host phagocytic cells functions and T cells differentiation against Leishmania antigens.

Apoptotic mimicry as a strategy for the establishment of parasitic infections: parasite- and host-derived phosphatidylserine as key molecule

The establishment of parasitic infection is dependent on the development of efficient strategies to evade the host defense mechanisms. Phosphatidylserine (PS) molecules are pivotal for apoptotic cell recognition and clearance by professional phagocytes. Moreover, PS receptors are able to trigger anti-inflammatory and immunosuppressive responses by phagocytes, either by coupled enzymes or through the induction of regulatory cytokine secretion. These PS-dependent events are exploited by parasites in a mechanism called apoptotic mimicry. Generally, apoptotic mimicry refers to the effects of PS recognition for the initiation and maintenance of pathogenic infections. However, in this context, PS molecules can be recognized on the surface of the infectious agent or in the surface of apoptotic host debris, leading to the respective denomination of classical and non-classical apoptotic mimicry. In this review, we discuss the role of PS in the pathogenesis of several human infections caused by protozoan parasites.

CD4+ T Cell-Dependent Macrophage Activation Modulates Sustained PS Exposure on Intracellular Amastigotes of Leishmania amazonensis

Leishmania amazonensis amastigotes can make use of surface-exposed phosphatidylserine (PS) molecules to promote infection and non-classical activation of macrophages (MΦ), leading to uncontrolled intracellular proliferation of the parasites. This mechanism was quoted as apoptotic mimicry. Moreover, the amount of PS molecules exposed on the surface of amastigotes correlates with the susceptibility of the host. In this study, we tested whether host cellular responses influence PS expression on intracellular amastigotes. We found that the level of PS exposure on intracellular amastigotes was modulated by CD4⁺ T cell and MΦ activation status in vitro and in vivo. L. amazonensis infection generated a Th1/Th2-mixed cytokine profile, providing the optimal MΦ stimulation that favored PS exposure on intracellular amastigotes. Maintenance of PS exposed on the parasite was dependent on low, but sustained, levels of nitric oxide and polyamine production. Amastigotes obtained from lymphopenic nude mice did not expose PS on their surface, and adoptive transfer of CD4⁺ T cells reversed this phenotype. In addition, histopathological analysis of mice treated with anti-PS antibodies showed increased inflammation and similarities to nude mouse lesions. Collectively, our data confirm the role of pathogenic CD4⁺ T cells for disease progression and point to PS as a critical parasite strategy to subvert host immune responses.

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

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

LC3-associated phagocytosis in microbial pathogenesis

Phagocytosis is essential for uptake and elimination of pathogenic microorganisms. Autophagy is a highly conserved mechanism for incorporation of cellular constituents to replenish nutrients by degradation. Recently, parts of the autophagy machinery - above all microtubule-associated protein 1 light chain 3 (LC3) - were found to be specifically recruited to phagosomal membranes resulting in phagosome-lysosome fusion and efficient degradation of internalized cargo in a process termed LC3-associated phagocytosis (LAP). Many pathogenic bacterial, fungal and parasitic microorganisms reside within LAP-targeted single-membrane phagosomes or vacuoles after infection of host cells. In this minireview we describe the state of knowledge on the interaction of pathogens with LAP or LAP-like pathways and report on various pathogens that have evolved strategies to circumvent degradation in LAP compartments.

Cell Removal: Efferocytosis

In metazoans, removal of cells in situ is involved in larval maturation, metamorphosis, and embryonic development. In adults, such cell removal plays a role in the homeostatic maintenance of cell numbers and tissue integrity as well as in the response to cell injury and damage. This removal involves uptake of the whole or fragmented target cells into phagocytes. Depending on the organism, these latter may be near-neighbor tissue cells and/or professional phagocytes such as, in vertebrates, members of the myeloid family of cells, especially macrophages. The uptake processes appear to involve specialized and highly conserved recognition ligands and receptors, intracellular signaling in the phagocytes, and mechanisms for ingestion. The recognition of cells destined for this form of removal is critical and, significantly, is distinguished for the most part from the recognition of foreign materials and organisms by the innate and adaptive immune systems. In keeping with the key role of cell removal in maintaining tissue homeostasis, constant cell removal is normally silent, i.e., does not initiate a local tissue reaction. This article discusses these complex and wide-ranging processes in general terms as well as the implications when these processes are disrupted in inflammation, immunity, and disease.

Chlamydia trachomatis Cellular Exit Alters Interactions with Host Dendritic Cells

The strategies utilized by pathogens to exit host cells are an area of pathogenesis which has received surprisingly little attention, considering the necessity of this step for infections to propagate. Even less is known about how exit through these pathways affects downstream host-pathogen interactions and the generation of an immune response. Chlamydia trachomatis exits host epithelial cells through two equally active mechanisms: lysis and extrusion. Studies have characterized the outcome of interactions between host innate immune cells, such as dendritic cells and macrophages, and free, extracellular Chlamydia bacteria, such as those resulting from lysis. Exit via extrusion generates a distinct, host-membrane-bound compartment of Chlamydia separate from the original infected cell. In this study, we assessed the effect of containment within extrusions upon the interaction between Chlamydia and host dendritic cells. Extrusion dramatically affected the outcome of Chlamydia-dendritic cell interactions for both the bacterium and the host cell. Dendritic cells rapidly underwent apoptosis in response to engulfment of an extrusion, while uptake of an equivalent dose of free Chlamydia had no such effect. Containment within an extrusion also prolonged bacterial survival within dendritic cells and altered the initial innate immune signaling by the dendritic cell.

Leishmania, the phagosome, and host responses: The journey of a parasite

Leishmania is the eukaryotic parasite responsible for leishmaniases, a spectrum of diseases that puts at risk roughly 350millions of people in 98 countries according to the Drugs for Neglected Diseases initiative (DNDi). This parasite has a complex life cycle composed of two distinct stages, the promastigote form found in the female sand-fly vector and the amastigote form that replicates in the mammalian host (Teixeira et al., 2013) [1]. To survive, the parasite interacts with its host immune system at multiple levels. In this review, we discuss the nature of those interactions, how they affect the host immune system, and how they affect parasite survival from the very beginning of the life cycle in the vector to its dissemination within the mammalian host.

Imaging and therapeutic applications of zinc(ii)-dipicolylamine molecular probes for anionic biomembranes

This feature article describes the development of synthetic zinc(ii)-dipicolylamine (ZnDPA) receptors as selective targeting agents for anionic membranes in cell culture and living subjects. There is a strong connection between anionic cell surface charge and disease, and ZnDPA probes have been employed extensively for molecular imaging and targeted therapeutics. Fluorescence and nuclear imaging applications include detection of diseases such as cancer, neurodegeneration, arthritis, and microbial infection, and also quantification of cell death caused by therapy. Therapeutic applications include selective targeting of cytotoxic agents and drug delivery systems, photodynamic inactivation, and modulation of the immune system. The article concludes with a summary of expected future directions.

Zinc(II)-Dipicolylamine Coordination Complexes as Targeting and Chemotherapeutic Agents for Leishmania major

Cutaneous leishmaniasis is a neglected tropical disease that causes painful lesions and severe disfigurement. Modern treatment relies on a few chemotherapeutics with serious limitations, and there is a need for more effective alternatives. This study describes the selective targeting of zinc(II)-dipicolylamine (ZnDPA) coordination complexes toward Leishmania major, one of the species responsible for cutaneous leishmaniasis. Fluorescence microscopy of L. major promastigotes treated with a fluorescently labeled ZnDPA probe indicated rapid accumulation of the probe within the axenic promastigote cytosol. The antileishmanial activities of eight ZnDPA complexes were measured using an in vitro assay. All tested complexes exhibited selective toxicity against L. major axenic promastigotes, with 50% effective concentration values in the range of 12.7 to 0.3 μM. Similar toxicity was observed against intracellular amastigotes, but there was almost no effect on the viability of mammalian cells, including mouse peritoneal macrophages. In vivo treatment efficacy studies used fluorescence imaging to noninvasively monitor changes in the red fluorescence produced by an infection of mCherry-L. major in a mouse model. A ZnDPA treatment regimen reduced the parasite burden nearly as well as the reference care agent, potassium antimony(III) tartrate, and with less necrosis in the local host tissue. The results demonstrate that ZnDPA coordination complexes are a promising new class of antileishmanial agents with potential for clinical translation.

Bacterial imaging and photodynamic inactivation using zinc(II)-dipicolylamine BODIPY conjugates

Targeted imaging and antimicrobial photodynamic inactivation (PDI) are emerging methods for detecting and eradicating pathogenic microorganisms. This study describes two structurally related optical probes that are conjugates of a zinc(II)-dipicolylamine targeting unit and a BODIPY chromophore. One probe is a microbial targeted fluorescent imaging agent, mSeek, and the other is an oxygen photosensitizing analogue, mDestroy. The conjugates exhibited high fluorescence quantum yield and singlet oxygen production, respectively. Fluorescence imaging and detection studies examined four bacterial strains: E. coli, S. aureus, K. pneumonia, and B. thuringiensis vegetative cells and purified spores. The fluorescent probe, mSeek, is not phototoxic and enabled detection of all tested bacteria at concentrations of ∼100 CFU mL(-1) for B. thuringiensis spores, ∼1000 CFU mL(-1) for S. aureus and ∼10,000 CFU mL(-1) for E. coli. The photosensitizer analogue, mDestroy, inactivated 99-99.99% of bacterial samples and selectively killed bacterial cells in the presence of mammalian cells. However, mDestroy was ineffective against B. thuringiensis spores. Together, the results demonstrate a new two-probe strategy to optimize PDI of bacterial infection/contamination.

The Apoptotic Role of Metacaspase in Toxoplasma gondii

Toxoplasma gondii is a major opportunistic pathogen that spreads in a range of animal species and human beings. Quite a few characterizations of apoptosis have been identified in T. gondii treated with apoptosis inducers, but the molecular mechanisms of the pathway are not clearly understood. Metacaspases are caspase-like cysteine proteases that can be found in plants, fungi, and protozoa in which caspases are absent. Metacaspases are multifunctional proteases involved in apoptosis-like cell death, insoluble protein aggregate clearance, and cell proliferation. To investigate whether T. gondii metacaspase (TgMCA) is involved in the apoptosis of the parasites, we generated TgMCA mutant strains. Western blot analysis indicated that the autoproteolytic processing of TgMCA was the same as that for metacaspases of some other species. Indirect immunofluorescence assay (IFA) showed that TgMCA was dispersed throughout the cytoplasm and relocated to the nucleus when the parasites were exposed to the extracellular environment, which indicated the execution of its function in the nucleus. The number of apoptosis parasites was significantly diminished in the TgMCA knockout strain and increased in the TgMCA overexpression strain after treatment with extracellular buffer, as determined by the terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay. The lack of TgMCA did not affect the parasite propagation in vitro and virulence in vivo, suggesting that it is probably redundant in parasite propagation. But overexpression of TgMCA reduced the intracellular parasites growth in vitro. The TgMCA knockout strain showed more viability in extracellular buffer compared to the parental and overexpression lines. In this study, we demonstrated that TgMCA contributes to the apoptosis of T. gondii.

CD300f associates with IL-4 receptor α and amplifies IL-4-induced immune cell responses

IL-4 receptor (R) α, the common receptor chain for IL-4 and IL-13, is a critical component in IL-4- and IL-13-mediated signaling and subsequent effector functions such as those observed in type 2 inflammatory responses. Nonetheless, the existence of intrinsic pathways capable of amplifying IL-4Rα-induced responses remains unknown. In this study, we identified the myeloid-associated Ig receptor CD300f as an IL-4-induced molecule in macrophages. Subsequent analyses demonstrated that CD300f was colocalized and physically associated with IL-4Rα. Using Cd300f(-/-) cells and receptor cross-linking experiments, we established that CD300f amplified IL-4Rα-induced responses by augmenting IL-4/IL-13-induced signaling, mediator release, and priming. Consistently, IL-4- and aeroallergen-treated Cd300f(-/-) mice displayed decreased IgE production, chemokine expression, and inflammatory cell recruitment. Impaired responses in Cd300f(-/-) mice were not due to the inability to generate a proper Th2 response, because IL-4/IL-13 levels were markedly increased in allergen-challenged Cd300f(-/-) mice, a finding that is consistent with decreased cytokine consumption. Finally, CD300f expression was increased in monocytes and eosinophils obtained from allergic rhinitis patients. Collectively, our data highlight a previously unidentified role for CD300f in IL-4Rα-induced immune cell responses. These data provide new insights into the molecular mechanisms governing IL-4Rα-induced responses, and may provide new therapeutic tools to target IL-4 in allergy and asthma.

Disease severity and mortality can be independently regulated in a mouse model of experimental graft versus host disease

Graft versus host disease (GVHD) is the major limitation of allogeneic hematopoietic stem cell transplantation (HSCT) presenting high mortality and morbidity rates. However, the exact cause of death is not completely understood and does not correlate with specific clinical and histological parameters of disease. Here we show, by using a semi-allogeneic mouse model of GVHD, that mortality and morbidity can be experimentally separated. We injected bone marrow-derived dendritic cells (BMDC) from NOD2/CARD15-deficient donors into semi-allogeneic irradiated chimaeras and observed that recipients were protected from death. However, no protection was observed regarding clinical or pathological scores up to 20 days after transplantation. Protection from death was associated with decreased bacterial translocation, faster hematologic recovery and epithelial integrity maintenance despite mononuclear infiltration at day 20 post-GVHD induction with no skew towards different T helper phenotypes. The protected mice recovered from aGVHD and progressively reached scores compatible with healthy animals. Altogether, our data indicate that severity and mortality can be separate events providing a model to study transplant-related mortality.

Deception and manipulation: the arms of leishmania, a successful parasite

Leishmania spp. are intracellular parasitic protozoa responsible for a group of neglected tropical diseases, endemic in 98 countries around the world, called leishmaniasis. These parasites have a complex digenetic life cycle requiring a susceptible vertebrate host and a permissive insect vector, which allow their transmission. The clinical manifestations associated with leishmaniasis depend on complex interactions between the parasite and the host immune system. Consequently, leishmaniasis can be manifested as a self-healing cutaneous affliction or a visceral pathology, being the last one fatal in 85–90% of untreated cases. As a result of a long host–parasite co-evolutionary process, Leishmania spp. developed different immunomodulatory strategies that are essential for the establishment of infection. Only through deception and manipulation of the immune system, Leishmania spp. can complete its life cycle and survive. The understanding of the mechanisms associated with immune evasion and disease progression is essential for the development of novel therapies and vaccine approaches. Here, we revise how the parasite manipulates cell death and immune responses to survive and thrive in the shadow of the immune system.