Regulation of the expression of nitric oxide synthase by Leishmania mexicana amastigotes in murine dendritic cells

Biostimulated-sesame sprout extracts as potential agents against Leishmania mexicana

Leishmania mexicana is one of the causal agents of cutaneous leishmaniasis. Current antileishmanial chemotherapeutics have demonstrated adverse side effects; thus, alternative treatments are needed. In this study, we performed in silico and in vitro analyses of the leishmanicidal potential of the most abundant phenolic compounds identified in black sesame sprouts biostimulated with Bacillus clausii. The molecular docking analysis showed strong interactions (binding free energies between -6.5 and -9.5 kcal/mol) of sesaminol 2-O-triglucoside, pinoresinol dihexoside, isoverbascoside, and apigenin with the arginase, leishmanolysin, cysteine peptidase B, and pyruvate kinase leishmanial enzymes. Furthermore, almost all phenolic compounds interacted with the active site residues of L. mexicana enzymes. In vitro, the B. clausii-biostimulated sprout phenolic extracts and apigenin inhibited the growth of promastigotes with IC50 values of 0.08 mg gallic acid equivalent/mL and 6.42 μM (0.0017 mg/mL), respectively. Additionally, in the macrophage infection model, cells treated with B. clausii-biostimulated sprout phenolic extracts and infected with L. mexicana exhibited significantly (P < 0.05) reduced nitric oxide production and decreased parasite burden. Altogether, our study provides important data related to high efficacy and less toxic natural antileishmanial candidates against promastigotes of L. mexicana.

Leishmania mexicana centrin knockout parasites promote M1-polarizing metabolic changes

Leishmaniasis is a tropical disease prevalent in 90 countries. Presently, there is no approved vaccine for human use. We developed a live attenuated L. mexicana Cen-/-(LmexCen-/-) strain as a vaccine candidate that showed excellent efficacy, characterized by reduced Th2 and enhanced Th1 responses in C57BL/6 and BALB/c mice, respectively, compared to wild-type L. mexicana (LmexWT) infection. Toward understanding the immune mechanisms of protection, we applied untargeted mass spectrometric analysis to LmexCen-/- and LmexWT infections. Data showed enrichment of the pentose phosphate pathway (PPP) in ears immunized with LmexCen-/-versus naive and LmexWT infection. PPP promotes M1 polarization in macrophages, suggesting a switch to a pro-inflammatory phenotype following LmexCen-/- inoculation. Accordingly, PPP inhibition in macrophages infected with LmexCen-/- reduced the production of nitric oxide and interleukin (IL)-1β, hallmarks of classical activation. Overall, our study revealed the immune regulatory mechanisms that may be critical for the induction of protective immunity.

Inflammasome NLRP3 activation induced by Convulxin, a C-type lectin-like isolated from Crotalus durissus terrificus snake venom

Convulxin (CVX), a C-type lectin-like protein isolated from the venom of the snake species, Crotalus durissus terrificus, stimulates platelet aggregation by acting as a collagen receptor agonist for glycoprotein VI found in the platelets. The effect of CVX on platelets has been studied, but its effect on human peripheral blood mononuclear cells (PBMCs) remains unclear. Given the significance of PBMCs in inflammation, this study explored the effect of CVX on PBMCs, specifically regarding NLRP3 inflammasome activation by assessing cell viability, ability to induce cell proliferation, reactive oxygen species (ROS) and nitric oxide production, interleukin (IL)-2 and IL-10 secretion, NLRP3 complex activation, and the role of C-type lectin-like receptors (CTLRs) in these. CVX was not toxic to PBMCs at the investigated concentrations and did not increase PBMC growth or IL-2 release; however, CVX induced IL-10 release and ROS generation via monocyte activation. It also activated the NLRP3 complex, resulting in IL-1β induction. Furthermore, the interaction between CVX and Dectin-2, a CTLR, induced IL-10 production. CVX interaction with CTLR has been demonstrated by laminarin therapy. Because of the involvement of residues near the Dectin-2 carbohydrate-recognition site, the generation of ROS resulted in inflammasome activation and IL-1β secretion. Overall, this work helps elucidate the function of CVX in immune system cells.

Leishmania mexicana: Novel Insights of Immune Modulation through Amastigote Exosomes

Exosomes are extracellular microvesicles of endosomal origin (multivesicular bodies, MVBs) constitutively released by eukaryotic cells by fusion of MVBs to the plasma membrane. The exosomes from Leishmania parasites contain an array of parasite molecules such as virulence factors and survival messengers, capable of modulating the host immune response and thereby favoring the infection of the host. We here show that exosomes of L. mexicana amastigotes (aExo) contain the virulence proteins gp63 and PP2C. The incubation of aExo with bone marrow-derived macrophages (BMMs) infected with L. mexicana led to their internalization and were found to colocalize with the cellular tetraspanin CD63. Furthermore, aExo inhibited nitric oxide production of infected BMMs, permitting enhanced intracellular parasite survival. Expressions of antigen-presenting (major histocompatibility complex class I, MHC-I, and CD1d) and costimulatory (CD86 and PD-L1) molecules were modulated in a dose-dependent fashion. Whereas MHC-I, CD86 and PD-L1 expressions were diminished by exosomes, CD1d was enhanced. We conclude that aExo of L. mexicana are capable of decreasing microbicidal mechanisms of infected macrophages by inhibiting nitric oxide production, thereby enabling parasite survival. They also hamper the cellular immune response by diminishing MHC-I and CD86 on an important antigen-presenting cell, which potentially interferes with CD8 T cell activation. The enhanced CD1d expression in combination with reduction of PD-L1 on BMMs point to a potential shift of the activation route towards lipid presentations, yet the effectivity of this immune activation is not evident, since in the absence of costimulatory molecules, cellular anergy and tolerance would be expected.

Astronium fraxinifolium Schott Exerts Leishmanicidal Activity by Providing a Classically Polarized Profile in Infected Macrophages

BACKGROUND

Leishmania braziliensis is prevalent in Latin American countries, including Brazil. It causes cutaneous and mucocutaneous leishmaniasis, leading to high morbidity, and has a low cure rate. Treatment is based on pentavalent antimonials; nonetheless, there are problems related to high toxicity, high cost, and parasitic resistance. Discovery of new leishmanicidal drugs without these limitations and that stimulate the cellular immune response is necessary.

PURPOSE

The present work evaluates whether Astronium fraxinifolium Schott exerts leishmanicidal activity against L. braziliensis by providing a classically polarized profile in infected macrophages.

METHODS

For the evaluation of the A. fraxinifolium Schott leishmanicidal activity, amastigote cell death was demonstrated in infected RAW 267.4 macrophages treated with an ethanolic extract from the plant sapwood (EEAF). For the evaluation of the EEAF capacity in providing a classically polarized profile in infected macrophages, the following analyses were done: detection of LAMP-1 protein by the baculovirus technology, measurement of superoxide anion by the NBT testing, quantification of TNF-α, IL-12p40, IL-10, IL-4, and TGF-β by sandwich-type enzyme immune assays, and iNOS and COX-2 expression by RT-PCR technique.

RESULTS

The EEAF significantly reduced amastigote counts inside the cells. Vacuoles were visualized in infected and treated cells before and after May-Grünwald-Giemsa staining. A strong LAMP-1 protein fluorescence revealed phagosome maturation in infected cells treated with the EEAF. No production of superoxide was visualized in infected cells treated with the plant material. Nonetheless, high levels of TNF-α, IL-12p40, and IL-10 were found in cell supernatants, but reduced levels of TGF-β and no IL-4 production. We identified augmented mRNA expression for COX-2, but no expression of iNOS mRNA.

CONCLUSION

Our results demonstrated that A. fraxinifolium induced a classically polarized profile in infected macrophages but also provided a less harmful environment by stimulating the production of certain anti-inflammatory mediators, such as IL-10.

Differential Regulation of l-Arginine Metabolism through Arginase 1 during Infection with Leishmania mexicana Isolates Obtained from Patients with Localized and Diffuse Cutaneous Leishmaniasis

l-Arginine metabolism through arginase 1 (Arg-1) and inducible nitric oxide synthase (NOS2) constitutes a fundamental axis for the resolution or progression of leishmaniasis. Infection with Leishmania mexicana can cause two distinct clinical manifestations: localized cutaneous leishmaniasis (LCL) and diffuse cutaneous leishmaniasis (DCL). In this work, we analyzed in an in vivo model the capacity of two L. mexicana isolates, one obtained from a patient with LCL and the other from a patient with DCL, to regulate the metabolism of l-arginine through Arg-1 and NOS2. Susceptible BALB/c mice were infected with L. mexicana isolates from both clinical manifestations, and the evolution of the infection as well as protein presence and activity of Arg-1 and NOS2 were evaluated. The lesions of mice infected with the DCL isolate were bigger, had higher parasite loads, and showed greater protein presence and enzymatic activity of Arg-1 than the lesions of mice infected with the LCL isolate. In contrast, NOS2 protein synthesis was poorly or not induced in the lesions of mice infected with the LCL or DCL isolate. The immunochemistry analysis of the lesions allowed the identification of highly parasitized macrophages positive for Arg-1, while no staining for NOS2 was found. In addition, we observed in lesions of patients with DCL macrophages with higher parasite loads and stronger Arg-1 staining than those in lesions of patients with LCL. Our results suggest that L. mexicana isolates obtained from patients with LCL or DCL exhibit different virulence or pathogenicity degrees and differentially regulate l-arginine metabolism through Arg-1.

Tracking of quiescence in Leishmania by quantifying the expression of GFP in the ribosomal DNA locus

Under stressful conditions some microorganisms adopt a quiescent stage characterized by a reversible non or slow proliferative condition that allows their survival. This adaptation was only recently discovered in Leishmania. We developed an in vitro model and a biosensor to track quiescence at population and single cell levels. The biosensor is a GFP reporter gene integrated within the 18S rDNA locus, which allows monitoring the expression of 18S rRNA (rGFP expression). We showed that rGFP expression decreased significantly and rapidly during the transition from extracellular promastigotes to intracellular amastigotes and that it was coupled in vitro with a decrease in replication as measured by BrdU incorporation. rGFP expression was useful to track the reversibility of quiescence in live cells and showed for the first time the heterogeneity of physiological stages among the population of amastigotes in which shallow and deep quiescent stages may coexist. We also validated the use of rGFP expression as a biosensor in animal models of latent infection. Our models and biosensor should allow further characterization of quiescence at metabolic and molecular level.

Dihydrotestosterone enhances growth and infectivity of Leishmania Mexicana

A strong sex-associated susceptibility towards Leishmania has been reported in males, yet little is known on the effect of hormones in Leishmania physiopathogenicity. Due to the enhanced susceptibility of males to Leishmania mexicana infections, we were interested in analysing the effect exerted by the main androgen produced in males (DHT) on L. mexicana promastigotes. Thus, the aim of this study was to assess the regulation exerted by dihydrotestosterone (DHT) on L. mexicana replication, infectivity, survival and development of tissue lesions. Experiments included growth curves of L. mexicana promastigotes incubated with different doses of DHT, their infection rate, intracellular survival and lesion development in BALB/c mice. Our data show that DHT significantly enhances parasite replication, infection rate and survival in bone marrow-derived macrophages (BMMФ). Promastigotes in the presence of DHT produced significantly larger lesions in BALB/c earlobes. These results suggest that DHT probably plays a critical role during L. mexicana infections, and the higher susceptibility of males possibly relates to benefits gained by the parasite from host-derived hormones. Our data shed new light on the physiopathology of Leishmania infections and are the first attempt to understand the direct interaction between Leishmania and androgens, particularly DHT. Understanding this trans-regulation process employed by parasites to exploit host molecules sheds new light on L. mexicana physiopathogenesis and opens a possible field for studies on drug development.

Hydrophilic Acylated Surface Protein A (HASPA) of Leishmania donovani: Expression, Purification and Biophysico-Chemical Characterization

Hydrophilic acylated surface proteins (HASPs) are acidic surface proteins which get localized on the surface of Leishmania parasite during infective stages through a "non-classical" pathway. In this study, we report the heterologous expression and purification of Leishmania donovani HASPA (r-LdHASPA) in E. coli system and its partial characterization. The structural aspects of the purified protein were analyzed using CD spectroscopy and modeling studies which indicate that r-LdHASPA consists of random coils. Studies in mouse macrophage RAW264.7 cell lines indicate that r-LdHASPA enhances reactive oxygen species (ROS) production. Co-immunoprecipitation (IP) studies indicate that r-LdHASPA interacts with certain macrophage proteins which however could not be identified unambiguously. The present study provides key insights into the structural and functional aspects of an important Leishmania protein, HASPA, which we believe could be useful for further research on vaccine/drug development.

Differential phagocytosis of Leishmania mexicana promastigotes and amastigotes by monocyte-derived dendritic cells

Leishmania species are dimorphic protozoan parasites that live and replicate in the gut of sand flies as promastigotes or in mammalian hosts as amastigotes. Different immune cells, including DCs, and receptors differ in their involvement in phagocytosis of promastigotes and amastigotes and in recognition of different Leishmania species. In the case of L. mexicana, differences in phagocytosis of promastigotes and amastigotes by DCs and participation of C-type lectin receptors (CLRs) have not been established. In the present study, flow cytometry and confocal microscopy were used to investigate the phagocytosis by monocyte-derived dendritic cells (moDCs) of L. mexicana promastigotes and amastigotes in the presence or absence of immune serum during various periods of time. Blocking antibodies against mannose receptors and DC-SIGN were used to explore the participation of these receptors in the phagocytosis of L. mexicana by moDC. The major differences in interactions of L. mexicana promastigotes and amastigotes with moDC were found to occur within the first 3 hr, during which phagocytosis of promastigotes predominated as compared with opsonization of promastigotes and amastigotes. However, after 6 hr of incubation, opsonized promastigotes were preferentially phagocytosed as compared with unopsonized promastigotes and amastigotes and after 24 hr of incubation there were no differences in the phagocytosis of promastigotes and amastigotes. Finally, after 3 hr incubation, DC-SIGN was involved in the phagocytosis of promastigotes, but not of amastigotes.

The leishmanicidal activity of oleuropein is selectively regulated through inflammation- and oxidative stress-related genes

Background

Much research effort has been focused on investigating new compounds derived from low-cost sources, such as natural products, for treating leishmaniasis. Oleuropein derived from numerous plants, particularly from the olive tree, Olea europaea L. (Oleaceae), is a biophenol with many biological activities. Our previous findings showed that oleuropein exhibits leishmanicidal effects against three Leishmania spp. in vitro, and minimizes the parasite burden in L. donovani-infected BALB/c mice. The aim of the present study is to investigate the possible mechanism(s) that mediate this leishmanicidal activity.

Methods

We determined the efficacy of oleuropein in elevating ROS and NO production in L. donovani-infected J774A.1 macrophages and in explanted splenocytes and hepatocytes obtained from L. donovani-infected BALB/c mice. We also assessed the expression of genes that are related to inflammation, T-cell polarization and antioxidant defense, in splenocytes. Finally, we determined the ratios of specific IgG2a/IgG1 antibodies and DTH reactions in L. donovani-infected BALB/c mice treated with oleuropein.

Results

Oleuropein was able to elevate ROS production in both in vitro and in vivo models of visceral leishmaniasis and raised NO production in ex vivo cultures of splenocytes and hepatocytes. The extensive oxidative stress found in oleuropein-treated mice was obviated by the upregulation of the host’s antioxidant enzyme (mGCLC) and the simultaneous downregulation of the corresponding enzyme of the parasite (LdGCLC). Moreover, oleuropein was able to mount a significant Th1 polarization characterized by the expression of immune genes (IL-12β, IL-10, TGF-β1, IFN-γ) and transcription factors (Tbx21 and GATA3). Moreover, this immunomodulatory effect was also correlated with an inhibitory effect on IL-1β gene expression, rather than with the expression of IL-1α, IL-1rn and TNF-α. Furthermore, oleuropein-treated BALB/c mice mounted a delayed-type hypersensitivity (DTH) response and an elevated Leishmania-specific IgG2a/IgG1 ratio that clearly demonstrated an in vivo protective mechanism.

Conclusion

The ability of Oleuropein to promote a Th1 type immune response in L. donovani-infected BALB/c mice points towards the candidacy of this bioactive compound as an immunomodulatory agent that may complement therapeutic approaches to leishmaniasis.

Leishmania mexicana: promastigotes and amastigotes secrete protein phosphatases and this correlates with the production of inflammatory cytokines in macrophages

Phosphatase activity of Leishmania spp. has been shown to deregulate the signalling pathways of the host cell. We here show that Leishmania mexicana promastigotes and amastigotes secrete proteins with phosphatase activity to the culture medium, which was higher in the Promastigote Secretion Medium (PSM) as compared with the Amastigote Secretion Medium (ASM) and was not due to cell lysis, since parasite viability was not affected by the secretion process. The biochemical characterization showed that the phosphatase activity present in PSM was higher in dephosphorylating the peptide END (pY) INASL as compared with the peptide RRA (pT)VA. In contrast, the phosphatase activity in ASM showed little dephosphorylating capacity for both peptides. Inhibition assays demonstrated that the phosphatase activity of both PSM and ASM was sensible only to protein tyrosine phosphatases inhibitors. An antibody against a protein phosphatase 2C (PP2C) of Leishmania major cross-reacted with a 44·9 kDa molecule in different cellular fractions of L. mexicana promastigotes and amastigotes, however, in PSM and ASM, the antibody recognized a protein about 70 kDa. By electron microscopy, the PP2C was localized in the flagellar pocket of amastigotes. PSM and ASM induced the production of tumor necrosis factor alpha, IL-1β, IL-12p70 and IL-10 in human macrophages.

Leishmania mexicana amastigotes inhibit p38 and JNK and activate PI3K/AKT: role in the inhibition of apoptosis of dendritic cells

Leishmania mexicana is the causal agent of cutaneous leishmaniasis in Mexico. Dendritic cells (DC) are one of the host cells of Leishmania parasites. Intracellular microorganisms inhibit host cell apoptosis as a strategy to ensure their survival in infected cells. We have previously shown that Leishmania mexicana promastigotes and amastigotes inhibit camptothecin-induced apoptosis of monocyte-derived dendritic cells (moDC), but the mechanisms underlying the inhibition of apoptosis of DC by Leishmania have not been established. MAP kinases and PI3K participate in the process of apoptosis and are modulated by different species of Leishmania. As shown in this study, the infection of moDC with L. mexicana amastigotes diminished significantly the phosphorylation of the MAP kinases p38 and JNK. The inhibition of both kinases diminished significantly DNA fragmentation in moDC stimulated with camptothecin. On the other hand, L. mexicana amastigotes were able to activate the anti-apoptotic pathways PI3K and AKT. Our results indicate that L. mexicana amastigotes have the capacity to diminish MAP kinases activation and activate PI3K and AKT, which is probably one of the strategies employed by L. mexicana amastigotes to inhibit apoptosis in the infected moDC.

Leishmania mexicana promastigotes down regulate JNK and p-38 MAPK activation: Role in the inhibition of camptothecin-induced apoptosis of monocyte-derived dendritic cells

Dendritic cells (DC) are one of the principal host cells of the obligate intracellular parasite Leishmania. Inhibition of host cell apoptosis is a strategy employed by multiple pathogens to ensure their survival in the infected cell. We have previously shown that the infection of monocyte-derived dendritic cells (moDC) with Leishmania mexicana inhibits campthotecin-induced apoptosis. Nevertheless, the mechanisms involved in the inhibition of apoptosis of dendritic cells by Leishmania have not been established. Mitogen-activated protein kinases (MAPK) are key participants in the process of apoptosis and different species of Leishmania have been shown to regulate these kinases. In the present study, we analyzed the effect of L. mexicana promastigotes in the activation of JNK and p38 MAP kinase and their participation in the inhibition of apoptosis. The infection of moDC with L. mexicana promastigotes diminished significantly the phosphorylation of the MAP kinases JNK and p38. The inhibition of both kinases diminished DNA fragmentation, but in a major extent was the reduction of DNA fragmentation when JNK was inhibited. The capacity of L. mexicana promastigotes to diminish MAP kinases activation is probably one of the strategies employed to delay apoptosis induction in the infected moDC and may have implications for Leishmania pathogenesis by favoring the invasion of its host and the persistence of the parasite in the infected cells.

Leishmania amazonensis amastigotes highly express a tryparedoxin peroxidase isoform that increases parasite resistance to macrophage antimicrobial defenses and fosters parasite virulence

Professional phagocytes generate a myriad of antimicrobial molecules to kill invading microorganisms, of which nitrogen oxides are integral in controlling the obligate intracellular pathogen Leishmania. Although reactive nitrogen species produced by the inducible nitric oxide synthase (iNOS) can promote the clearance of intracellular parasites, some Leishmania species/stages are relatively resistant to iNOS-mediated antimicrobial activity. The underlying mechanism for this resistance remains largely uncharacterized. Here, we show that the amastigote form of L. amazonensis is hyper-resistant to the antimicrobial actions of cytokine-activated murine and human macrophages as compared to its promastigote counterpart. Amastigotes exhibit a marked ability to directly counter the cytotoxicity of peroxynitrite (ONOO-), a leishmanicidal oxidant that is generated during infection through the combined enzymatic activities of NADPH oxidase and iNOS. The enhanced antinitrosative defense of amastigotes correlates with the increased expression of a tryparedoxin peroxidase (TXNPx) isoform that is also upregulated in response to iNOS enzymatic activity within infected macrophages. Accordingly, ectopic over-expression of the TXNPx isoform by L. amazonensis promastigotes significantly enhances parasite resistance against ONOO- cytotoxicity. Moreover, TXNPx-overexpressing parasites exhibit greater intra-macrophage survival, and increased parasite growth and lesion development in a murine model of leishmaniasis. Our investigations indicate that TXNPx isoforms contribute to Leishmania's ability to adapt to and antagonize the hostile microenvironment of cytokine-activated macrophages, and provide a mechanistic explanation for persistent infection in experimental and human leishmaniasis.

Collective nitric oxide production provides tissue-wide immunity during Leishmania infection

Nitric oxide (NO) production is critical for the host defense against intracellular pathogens; however, it is unclear whether NO-dependent control of intracellular organisms depends on cell-intrinsic or cell-extrinsic activity of NO. For example, NO production by infected phagocytes may enable these cells to individually control their pathogen burden. Alternatively, the ability of NO to diffuse across cell membranes might be critical for infection control. Here, using a murine ear infection model, we found that, during infection with the intracellular parasite Leishmania major, expression of inducible NO synthase does not confer a cell-intrinsic ability to lower parasite content. We demonstrated that the diffusion of NO promotes equally effective parasite killing in NO-producing and bystander cells. Importantly, the collective production of NO by numerous phagocytes was necessary to reach an effective antimicrobial activity. We propose that, in contrast to a cell-autonomous mode of pathogen control, this cooperative mechanism generates an antimicrobial milieu that provides the basis for pathogen containment at the tissue level.

TgMAPK1 is a Toxoplasma gondii MAP kinase that hijacks host MKK3 signals to regulate virulence and interferon-γ-mediated nitric oxide production

The parasite Toxoplasma gondii controls tissue-specific nitric oxide (NO), thereby augmenting virulence and immunopathology through poorly-understood mechanisms. We now identify TgMAPK1, a Toxoplasma mitogen-activated protein kinase (MAPK), as a virulence factor regulating tissue-specific parasite burden by manipulating host interferon (IFN)-γ-mediated inducible nitric oxide synthase (iNOS). Toxoplasma with reduced TgMAPK1 expression (TgMAPK1(lo)) demonstrated that TgMAPK1 facilitates IFN-γ-driven p38 MAPK activation, reducing IFN-γ-generated NO in an MKK3-dependent manner, blunting IFN-γ-mediated parasite control. TgMAPK1(lo) infection in wild type mice produced ≥ten-fold lower parasite burden versus control parasites with normal TgMAPK1 expression (TgMAPK1(con)). Reduced parasite burdens persisted in IFN-γ KO mice, but equalized in normally iNOS-replete organs from iNOS KO mice. Parasite MAPKs are far less studied than other parasite kinases, but deserve additional attention as targets for immunotherapy and drug discovery.

FMS-like tyrosine kinase 3 ligand treatment of mice aggravates acute lung injury in response to Streptococcus pneumoniae: role of pneumolysin

FMS-like tyrosine kinase-3 ligand (Flt3L) is a dendritic cell (DC) growth and differentiation factor with potential in antitumor therapies and antibacterial immunization strategies. However, the effect of systemic Flt3L treatment on lung-protective immunity against bacterial infection is incompletely defined. Here, we examined the impact of deficient (in Flt3L knockout [KO] mice), normal (in wild-type [WT] mice), or increased Flt3L availability (in WT mice pretreated with Flt3L for 3, 5, or 7 days) on lung DC subset profiles and lung-protective immunity against the major lung-tropic pathogen, Streptococcus pneumoniae. Although in Flt3L-deficient mice the numbers of DCs positive for CD11b (CD11b(pos) DCs) and for CD103 (CD103(pos) DCs) were diminished, lung permeability, a marker of injury, was unaltered in response to S. pneumoniae. In contrast, WT mice pretreated with Flt3L particularly responded with increased numbers of CD11b(pos) DCs and with less pronounced numbers of CD103(pos) DCs and impaired bacterial clearance and with increased lung permeability following S. pneumoniae challenge. Notably, infection of Flt3L-pretreated mice with S. pneumoniae lacking the pore-forming toxin, pneumolysin (PLY), resulted in substantially less lung CD11b(pos) DCs activation and reduced lung permeability. Collectively, this study establishes that Flt3L treatment enhances the accumulation of proinflammatory activated lung CD11b(pos) DCs which contribute to acute lung injury in response to PLY released by S. pneumoniae.

Role of oxidative stress and apoptosis in the cellular response of murine macrophages uponLeishmaniainfection

Leishmaniaparasites are able to survive in the macrophage, one of the most hostile environments of the vertebrate host. The present study investigated howLeishmaniainfection influences these host cell defence mechanisms. Macrophages were infected with antimony-susceptible and -resistantLeishmaniastrains. Free radical production inLeishmania-infected macrophages was measured by electron paramagnetic resonance. Apoptosis was detected with fluorescence microscopy using Annexin-V FITC labelling and with Western blotting to detect caspase-3 cleavage. Independent of their drug susceptibility profile or species background, all studiedLeishmaniastrains induced a similar increase in free radical production in macrophages. O2●−production was significantly elevated during phagocytosis of the stationary phase promastigotes. Conversely, NO levels increased later in the infection and none of the strains induced capsase-3 cleavage.Leishmania donovaniinfection led to phosphatidylserine externalization only in RAW 264.7 cells. After an initial burst of O2●−during phagocytosis of promastigotes, amastigotes protect themselves by decreasing the O2●−production to the basal level. An increased NO production was observed 6 h after infection. Finally, induction of cell death is probably not essential in the survival of the parasite within the macrophage.

Characteristics of "Tip-DCs and MDSCs" and Their Potential Role in Leishmaniasis

Since the first description of dendritic cells (DCs) by Steinman and Cohn (1973), the myeloid lineage of leukocytes was investigated intensively. Nowadays it is obvious that myeloid cells, especially DCs, are crucial for the adaptive and innate immune response against intracellular pathogens such as Leishmania major parasites. Based on the overlapping expression of molecules that were commonly used to classify myeloid cells, it becomes difficult to denominate those cell types precisely. Of note, most of these markers used for myeloid cell identification are expressed on a broad range of myeloid cells, and should therefore be handled with care if used for subtyping of myeloid cells. In this mini-review we aim to discuss the relative impact of DCs that release TNF and nitric oxide (Tip-DCs) and myeloid cells with suppressive capacities (myeloid-derived suppressor cells, MDSCs) in infectious diseases such as experimental leishmaniasis. In our point of view it cannot be excluded that the novel subsets that were denominated as “Tip-DCs” and “MDSCs” might not be classical “subsets” but rather represent myeloid cells in a transient maturation stage expressing different genes, in response to the surrounding environment.

Leishmania promotes its own virulence by inducing expression of the host immune inhibitory ligand CD200

Leishmania parasites infect macrophages, cells normally involved in innate defense against pathogens. Leishmania amazonensis and Leishmania major cause severe or mild disease, respectively, consistent with each parasite's ability to survive within activated macrophages. The mechanisms underlying increased virulence of L. amazonensis are mostly unknown. We show that L. amazonensis promotes its own survival by inducing expression of CD200, an immunoregulatory molecule that inhibits macrophage activation. L. amazonensis does not form typical nonhealing lesions in CD200(-/-) mice and cannot replicate in CD200(-/-) macrophages, an effect reversed by exogenous administration of soluble CD200-Fc. The less virulent L. major does not induce CD200 expression and forms small, self-healing lesions in both wild-type and CD200(-/-) mice. Notably, CD200-Fc injection transforms the course of L. major infection to one resembling L. amazonensis, with large, nonhealing lesions. CD200-dependent iNOS inhibition allows parasite growth in macrophages, identifying a mechanism for the increased virulence of L. amazonensis.

Leishmania-macrophage interactions: insights into the redox biology

Leishmaniasis is a neglected tropical disease that affects about 350 million individuals worldwide. The protozoan parasite has a relatively simple life cycle with two principal stages: the flagellated mobile promastigote living in the gut of the sandfly vector and the intracellular amastigote within phagolysosomal vesicles of the vertebrate host macrophage. This review presents a state-of-the-art overview of the redox biology at the parasite-macrophage interface. Although Leishmania species are susceptible in vitro to exogenous superoxide radical, hydrogen peroxide, nitric oxide, and peroxynitrite, they manage to survive the endogenous oxidative burst during phagocytosis and the subsequent elevated nitric oxide production in the macrophage. The parasite adopts various defense mechanisms to cope with oxidative stress: the lipophosphoglycan membrane decreases superoxide radical production by inhibiting NADPH oxidase assembly and the parasite also protects itself by expressing antioxidant enzymes and proteins. Some of these enzymes could be considered potential drug targets because they are not expressed in mammals. In respect to antileishmanial therapy, the effects of current drugs on parasite-macrophage redox biology and its involvement in the development of drug resistance and treatment failure are presented.