Modulation of interferon-gamma-induced macrophage activation by phosphotyrosine phosphatases inhibition. Effect on murine Leishmaniasis progression

Leishmania intercepts IFN-γR signaling at multiple levels in macrophages

IFN-γ, a type 2 interferon and a cytokine, is critical for both innate and adaptive immunity. IFN-γ binds to the IFN-γRs on the cell membrane of macrophages, signals through JAK1-STAT-1 pathway and induces IFN-γ-stimulated genes (ISGs). As Leishmania amastigotes reside and replicate within macrophages, IFN-γ mediated macrophage activation eventuate in Leishmania elimination. As befits the principle of parasitism, the impaired IFN-γ responsiveness in macrophages ensures Leishmania survival. IFN-γ responsiveness is a function of integrated molecular events at multiple levels in the cells that express IFN-γ receptors. In Leishmania-infected macrophages, reduced IFN-γRα expression, impaired IFN-γRα and IFN-γRβ hetero-dimerization due to altered membrane lipid composition, reduced JAK-1 and STAT-1 phosphorylation but increased STAT-1 degradation and impaired ISGs induction collectively determine the IFN-γ responsiveness and the efficacy of IFN-γ induced antileishmanial function of macrophages. Therefore, parasite load is not only decided by the levels of IFN-γ produced but also by the IFN-γ responsiveness. Indeed, in Leishmania-infected patients, IFN-γ is produced but IFN-γ signalling is downregulated. However, the molecular mechanisms of IFN-γ responsiveness remain unclear. Therefore, we review the current understanding of IFN-γ responsiveness of Leishmania-infected macrophages.

Temporary Shutdown of ERK1/2 Phosphorylation Is Associated With Activation of Adaptive Immune Cell Responses and Disease Progression During Leishmania amazonensis Infection in BALB/c Mice

Leishmania spp. infection outcomes are dependent on both host and parasite factors. Manipulation of host signaling pathways involved in the generation of immune responses is thought to be one of the most common mechanisms used by parasites for persistence within the host. Considering the diversity of pathologies caused by different Leishmania spp., it is plausible that significant differences may exist in the mechanisms of host cell manipulation by each parasite species, which may have implications when developing new vaccine or treatment strategies. Here we show that in L. braziliensis-infection in BALB/c mice, a model of resistance, activation of ERK1/2 coincides with the peak of inflammatory responses and resolution of tissue parasitism. In contrast, in the susceptibility model of L. amazonensis-infection, an early silent phase of infection is observed, detected solely by quantification of parasite loads. At this early stage, only basal levels of P-ERK1/2 are observed. Later, after a brief shutdown of ERK1/2 phosphorylation, disease progression is observed and is associated with increased inflammation, lesion size and tissue parasitism. Moreover, the short-term down-regulation of ERK1/2 activation affected significantly downstream inflammatory pathways and adaptive T cell responses. Administration of U0126, a MEK/ERK inhibitor, confirmed this phenomenon, since bigger lesions and higher parasite loads were seen in infected mice that received U0126. To investigate how kinetics of ERK1/2 activation could affect the disease progression, U0126 was administered to L. amazonensis-infected animals earlier than the P-ERK1/2 switch off time-point. This intervention resulted in anticipation of the same effects on inflammatory responses and susceptibility phenotype seen in the natural course of infection. Additionally, in vitro inhibition of ERK1/2 affected the phagocytosis of L. amazonensis by BMDMs. Collectively, our findings reveal distinct temporal patterns of activation of inflammatory responses in L. braziliensis and L. amazonensis in the same animal background and a pivotal role for a brief and specific shutdown of ERK1/2 activation at late stages of L. amazonensis infection. Since activation of inflammatory responses is a crucial aspect for the control of infectious processes, these findings may be important for the search of new and specific strategies of vaccines and treatment for tegumentary leishmaniasis.

Effect of Anadenanthera colubrina protease inhibitors as an antiinflamatory mediator

This work aimed to obtain and characterize protease inhibitors from A. colubrina leaves and evaluate their potential as inflammatory mediator and cell viability. The protein extract was analyzed and characterized by SDS-PAGE, RP-HPLC-PDA, MALDI-TOF/MS and Zeta potential. Bioassays were conducted in order to evaluate the cell viability in RAW 264.7, in vitro (NO and TNF-α production inhibition) and in vivo anti-inflammatory potential, inhibition rate of trypsin and hemagglutination activity from protein extract. The results revealed the presence of bands at 14, 21 and 30 kDa in SDS-PAGE, the RP-HPLC-PDA analysis showed peaks at 12, 13, 28 and 40 minutes and MALDI-TOF/MS showed peaks with 3.4, 4.7, 5.6, 9.4 and 11.2 kDa. The protein extracts presented enzymatic activity inhibition of trypsin (IC₅₀ 59.2 μgmL^-1), did not show any cytotoxicity to RAW264.7 cells, hemagglutination 8HU and insignificant reduction in NO and TNF-α production and reduced anti-inflammatory potential in vivo compared to dexamethasone.

M. tuberculosis-Initiated Human Mannose Receptor Signaling Regulates Macrophage Recognition and Vesicle Trafficking by FcRγ-Chain, Grb2, and SHP-1

Despite its prominent role as a C-type lectin (CTL) pattern recognition receptor, mannose receptor (MR, CD206)-specific signaling molecules and pathways are unknown. The MR is highly expressed on human macrophages, regulating endocytosis, phagocytosis, and immune responses and mediating Mycobacterium tuberculosis (M.tb) phagocytosis by human macrophages, thereby limiting phagosome-lysosome (P-L) fusion. We identified human MR-associated proteins using phosphorylated and non-phosphorylated MR cytoplasmic tail peptides. We found that MR binds FcRγ-chain, which is required for MR plasma membrane localization and M.tb cell association. Additionally, we discovered that MR-mediated M.tb association triggers immediate MR tyrosine residue phosphorylation and Grb2 recruitment, activating the Rac/Pak/Cdc-42 signaling cascade important for M.tb uptake. MR activation subsequently recruits SHP-1 to the M.tb-containing phagosome, where its activity limits PI(3)P generation at the phagosome and M.tb P-L fusion and promotes M.tb growth. In sum, we identify human MR signaling pathways that temporally regulate phagocytosis and P-L fusion during M.tb infection.

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.

Immune Checkpoint Targets for Host-Directed Therapy to Prevent and Treat Leishmaniasis

Leishmaniasis encompasses a group of diseases caused by protozoan parasites belonging to the genus Leishmania. These diseases range from life threatening visceral forms to self-healing cutaneous lesions, and each disease manifestations can progress to complications involving dissemination of parasites to skin or mucosal tissue. A feature of leishmaniasis is the key role host immune responses play in disease outcome. T cells are critical for controlling parasite growth. However, they can also contribute to disease onset and progression. For example, potent regulatory T cell responses can develop that suppress antiparasitic immunity. Alternatively, hyperactivated CD4+ or CD8+ T cells can be generated that cause damage to host tissues. There is no licensed human vaccine and drug treatment options are often limited and problematic. Hence, there is an urgent need for new strategies to improve the efficacy of current vaccine candidates and/or enhance both antiparasitic drug effectiveness and subsequent immunity in treated individuals. Here, we describe our current understanding about host immune responses contributing to disease protection and progression in the various forms of leishmaniasis. We also discuss how this knowledge may be used to develop new strategies for host-directed immune therapy to prevent or treat leishmaniasis. Given the major advances made in immune therapy in the cancer and autoimmune fields in recent years, there are significant opportunities to ride on the back of these successes in the infectious disease domain. Conversely, the rapid progress in our understanding about host immune responses during leishmaniasis is also providing opportunities to develop novel immunotherapy strategies that could have broad applications in diseases characterized by inflammation or immune dysfunction.

Protein Tyrosine Phosphatase Inhibition Prevents Experimental Cerebral Malaria by Precluding CXCR3 Expression on T Cells

Cerebral malaria induced by Plasmodium berghei ANKA infection is dependent on the sequestration of cytotoxic T cells within the brain and augmentation of the inflammatory response. Herein, we demonstrate that inhibition of protein tyrosine phosphatase (PTP) activity significantly attenuates T cell sequestration within the brain and prevents the development of neuropathology. Mechanistically, the initial upregulation of CXCR3 on splenic T cells upon T cell receptor stimulation was critically decreased through the reduction of T cell-intrinsic PTP activity. Furthermore, PTP inhibition markedly increased IL-10 production by splenic CD4⁺ T cells by enhancing the frequency of LAG3⁺CD49b⁺ type 1 regulatory cells. Overall, these findings demonstrate that modulation of PTP activity could possibly be utilized in the treatment of cerebral malaria and other CXCR3-mediated diseases.

Hepatocyte SHP-1 is a Critical Modulator of Inflammation During Endotoxemia

Liver hepatocytes (Hep) are known to be central players during the inflammatory response to systemic infection. Interestingly, the protein tyrosine phosphatases (PTP) SHP-1, has been recognized as a major regulator of inflammation; however their implication in the control of Hep-mediated inflammatory response is still unknown. To study its implication in the regulation of the Hep-mediated inflammatory response during endotoxemia, Cre-Lox mice with a Hep-specific Ptpn6 deletion (Ptpn6^H-KO) were injected with LPS. In contrast to the wild-type mice (Ptpn6^f/f) that started to die by 24 hrs post-inoculation, the Ptpn6^H-KO mice exhibited mortality by 6 hrs. In parallel, higher amounts of metabolic markers, pro-inflammatory mediators and circulating cytokines were detected in Ptpn6^H-KO mice. Primary Hep obtained from Ptpn6^H-KO, also showed increased secretion of pro-inflammatory cytokines and nitric oxide (NO) comparatively to its wild type (Ptpn6^f/f) counterpart. Pharmacological approaches to block TNF-α and NO production protected both the Ptpn6^f/f and the Ptpn6^H-KO mice against deadly LPS-mediated endotoxemia. Collectively, these results establish hepatocyte SHP-1 is a critical player regulating systemic inflammation. Our findings further suggest that SHP-1 activation could represent a new therapeutic avenue to better control inflammatory-related pathologies.

The role of PD-1 in regulation of macrophage apoptosis and its subversion by Leishmania donovani

Programmed death-1 receptor (PD-1) expressed in many immune cells is known to trigger T-cell exhaustion but the significance of macrophage-associated PD-1 in relevance to macrophage apoptosis is not known. This study is aimed to delineate whether PD-1 pathway has any role in eliciting macrophage apoptosis and, if so, then how the intra-macrophage parasite, Leishmania donovani modulates PD-1 pathway for protecting its niche. Resting macrophages when treated with H₂O₂ showed increased PD-1 expression and apoptosis, which was further enhanced on PD-1 agonist treatment. The administration of either PD-1 receptor or PD-1 ligand-blocking antibodies reversed the process thus documenting the involvement of PD-1 in macrophage apoptosis. On the contrary, L. donovani-infected macrophages showed decreased PD-1 expression concurrent with inhibition of apoptosis. The activation of PD-1 pathway was found to negatively regulate the phosphorylation of pro-survival AKT, which was reversed during infection. Infection-induced PD-1 downregulation led to the activation of AKT resulting in phosphorylation and subsequent inhibition of proapoptotic protein BAD. Strong association of SHP2 (a SH2-containing ubiquitously expressed tyrosine-specific protein phosphatase) with PD-1 along with AKT deactivation observed in H₂O₂-treated macrophages was reversed by L. donovani infection. Kinetic analysis coupled with inhibitor-based approach and knockdown experiments demonstrated that L. donovani infection actively downregulated the PD-1 by deactivating NFATc1 as revealed by its reduced nuclear translocation. The study thus elucidates the detailed mechanism of the role of PD-1 in macrophage apoptosis and its negative modulation by Leishmania for their intracellular survival.

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.

PKC/ROS-Mediated NLRP3 Inflammasome Activation Is Attenuated by Leishmania Zinc-Metalloprotease during Infection

Parasites of the Leishmania genus infect and survive within macrophages by inhibiting several microbicidal molecules, such as nitric oxide and pro-inflammatory cytokines. In this context, various species of Leishmania have been reported to inhibit or reduce the production of IL-1β both in vitro and in vivo. However, the mechanism whereby Leishmania parasites are able to affect IL-1β production and secretion by macrophages is still not fully understood. Dependent on the stimulus at hand, the maturation of IL-1β is facilitated by different inflammasome complexes. The NLRP3 inflammasome has been shown to be of pivotal importance in the detection of danger molecules such as inorganic crystals like asbestos, silica and malarial hemozoin, (HZ) as well as infectious agents. In the present work, we investigated whether Leishmania parasites modulate NLRP3 inflammasome activation. Using PMA-differentiated THP-1 cells, we demonstrate that Leishmania infection effectively inhibits macrophage IL-1β production upon stimulation. In this context, the expression and activity of the metalloprotease GP63 - a critical virulence factor expressed by all infectious Leishmania species - is a prerequisite for a Leishmania-mediated reduction of IL-1β secretion. Accordingly, L. mexicana, purified GP63 and GP63-containing exosomes, caused the inhibition of macrophage IL-1β production. Leishmania-dependent suppression of IL-1β secretion is accompanied by an inhibition of reactive oxygen species (ROS) production that has previously been shown to be associated with NLRP3 inflammasome activation. The observed loss of ROS production was due to an impaired PKC-mediated protein phosphorylation. Furthermore, ROS-independent inflammasome activation was inhibited, possibly due to an observed GP63-dependent cleavage of inflammasome and inflammasome-related proteins. Collectively for the first time, we herein provide evidence that the protozoan parasite Leishmania, through its surface metalloprotease GP63, can significantly inhibit NLRP3 inflammasome function and IL-1β production.

Leishmania parasites are the causative agent of leishmaniasis, a wide spread disease in tropical and subtropical areas. The microorganisms have been shown to be well-adapted to their hosts and are able to enter their target cells where they replicate themselves. To ensure these processes, Leishmania disrupts a multitude of cellular signals and protective mechanisms, which overall attenuates immune responses against the parasites. A key factor for inflammatory processes, also during infections, is IL-1β. As previous studies suggested a dysregulation of IL-1β levels after infection with Leishmania parasites, we herein investigated the underlying mechanisms. Our work reveals that Leishmania suppressing IL-1β production through its virulence factor GP63. Furthermore, our data suggests that the parasites can dampen the maturation of IL-1β after different stimuli. In this regard we established a role for the suppression of the kinase PKC and the generation of reactive oxygen species, as well as the cleavage of cellular proteins that are important for IL-1β-generation. Thus, we here present a novel aspect for how Leishmania parasites can counteract host protective mechanisms.

The genetics of Leishmania virulence

The ability of Leishmania parasites to infect and persist in the antigen-presenting cell population of their mammalian hosts is dependent on their ability to gain entry to their host and host cells, to survive the mammalian cell environment, and to suppress or evade the protective immune response mechanisms of their hosts. A multitude of genes and their products have been implicated in each of these virulence-enhancing strategies to date, and we present an overview of the nature and known function of such virulence genes.

Leishmanicidal activity of polysaccharides and their oxovanadium(IV/V) complexes

The parasites of the genus Leishmania cause a range of leishmaniasis diseases, whose treatment is impaired due to intramacrophage parasites living in the mammalian host. Immunostimulation has been considered an important strategy to leishmaniasis treatment. The immunomodulatory effects of the polysaccharides arabinogalactan (ARAGAL), galactomannan (GMPOLY), and xyloglucan (XGJ), as well as their oxovanadium (IV/V) complexes (ARAGAL:VO, GMPOLY:VO, and XGJ:VO) were evaluated on peritoneal macrophages. At 25 μg/mL of GMPOLY:VO and of XGJ:VO, and 10 μg/mL of ARAGAL:VO, nitric oxide (NO) production by the macrophages was not altered compared with the control group. All polymers increased the production of interleukins 1 beta and 6 (IL-1β and IL-6), but the oxovanadium complexes were more potent activators of these mediators. ARAGAL:VO 10 μg/mL, GMPOLY:VO and XGJ:VO 25 μg/mL led to an increase of 562%, 1054%, and 523% for IL-1β, respectively. For IL-6 at the same concentration, the levels increased by 539% and 794% for ARAGAL:VO and GMPOLY:VO, respectively. Polysaccharides and their oxovanadium complexes exhibited important leishmanicidal effects on amastigotes of Leishmania (L.) amazonensis. The native and complexed polymers reduced the growth of promastigote-form Leishmania by ∼60%. This effect was reached at concentrations 12 times lower than that observed for Glucantime (300 μg/mL promoted an inhibition of ∼60%). The 50% inhibitory concentration (IC50) values for the complexes were determined. XGJ:VO showed the lowest IC50 value (6.2 μg/mL; 0.07 μg/mL of vanadium), which for ARAGAL:VO was 6.5 μg/mL (0.21 μg/mL of vanadium) and 7.3 μg/mL (0.06 μg/mL of vanadium) for GMPOLY:VO. The upregulation of IL-1β and IL-6 release and downregulation of NO production by macrophages and the important leishmanicidal effect are essential to stablish their potential use against this pathology.

Impact of Leishmania mexicana infection on dendritic cell signaling and functions

Leishmania parasites have the ability to modify macrophage signaling pathways in order to survive and multiply within its mammalian host. They are also known to invade other cells including neutrophils, fibroblasts and dendritic cells (DCs). DCs have an important role in immunity as the link between innate and adaptive immunity, necessary for the development of an effective response; however, the impact of Leishmania mexicana infection on DCs has been poorly studied. Herein, we report that Leishmania infection rapidly induced DC protein tyrosine phosphatases activity, leading to MAP kinases inactivation. In line with this, L. mexicana was found to decrease the nuclear translocation of transcription factors such as AP-1 and NF-κB. Concomitantly, L. mexicana-infected DCs showed reduced expression of several surface antigen-presenting and co-stimulatory molecules upon LPS stimulation. Leishmania-induced interference on DC maturation was further reflected by their reduced capacity to present OVA antigen to OVA-specific T cells, as shown by abrogation of IL-2 production by the T cells. Collectively, our data revealed that DC infection by L. mexicana appears to affect the cellular and immunological mechanisms necessary for the development of an effective and protective immune response, therefore favouring the survival and propagation of the parasite within its host.

Parasites of the Leishmania genus have developed many strategies to survive inside their host. Initially, they were only considered capable of infecting macrophages; however, it has been observed that Leishmania is able to infect other cell types, such as fibroblast, neutrophils and dendritic cells (DCs). DCs are well known for their antigen-presentation capabilities, and they are considered as the fundamental bridge between the innate and adaptive immune responses. In this study, we attempted to elucidate the effect of L. mexicana promastigotes on DCs. Our results showed that L. mexicana inactivates signaling cascades responsible for the expression of immune effector molecules, such as cytokines, concomitantly with the activation of protein phosphatases in the host. Furthermore, we observed that promastigote-infected cells had lower expression of MHC and co-stimulatory molecules on their surface, as well as decreased antigen-presentation capacity. In conclusion, our study showed that Leishmania parasites are able to inactivate the immunological mechanisms of DCs, as they do in macrophages, in order to survive inside its host.

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.

Solute carrier 11A1 is expressed by innate lymphocytes and augments their activation

Solute carrier 11A1 (SLC11A1) is a divalent ion transporter formerly known as the natural resistance-associated macrophage protein (NRAMP1) and the Bcg/Lsh/Ity locus. SLC11A1 was thought to be exclusively expressed in monocyte/macrophages and to have roles in phagosome maturation and cell activation. We characterized the expression of SLC11A1 in the majority of human and bovine γδ T cells and NK cells and in human CD3(+)CD45RO(+) T cells. Consistent with a role for iron-dependent inhibition of protein tyrosine phosphatases, SLC11A1(+) lymphocytes were more prone to activation and retained tyrosine phosphorylation. Transfection of SLC11A1 into a human γδ T cell-like line rendered the cells more prone to activation. Nonadherent splenocytes from wild-type mice expressed significantly greater IFN-γ compared with cells from Sv/129 (SLC11A1(-/-)) mice. Our data suggest that SLC11A1 has a heretofore unknown role in activation of a large subset of innate lymphocytes that are critical sources of IFN-γ. SLC11A1(+) animals have enhanced innate IFN-γ expression in response to Salmonella infection compared with SLC11A1(-) mice, which include commonly used inbred laboratory mice. Expression of SLC11A1 in innate lymphocytes and its role in augmenting their activation may account for inconsistencies in studies of innate lymphocytes in different animal models.

Secreted virulence factors and immune evasion in visceral leishmaniasis

Evasion or subversion of host immune responses is a well-established paradigm in infection with visceralizing leishmania. In this review, we summarize current findings supporting a model in which leishmania target host regulatory molecules and pathways, such as the PTP SHP-1 and the PI3K/Akt signaling cascade, to prevent effective macrophage activation. Furthermore, we describe how virulence factors, secreted by leishmania, interfere with macrophage intracellular signaling. Finally, we discuss mechanisms of secretion and provide evidence that leishmania use a remarkably adept, exosome-based secretion mechanism to export and deliver effector molecules to host cells. In addition to representing a novel mechanism for trafficking of virulence factors across membranes, recent findings indicate that leishmania exosomes may have potential as vaccine candidates.

Killed but metabolically active Leishmania infantum as a novel whole-cell vaccine for visceral leishmaniasis

There are currently no effective vaccines for visceral leishmaniasis, the second most deadly parasitic infection in the world. Here, we describe a novel whole-cell vaccine approach using Leishmania infantum chagasi promastigotes treated with the psoralen compound amotosalen (S-59) and low doses of UV A radiation. This treatment generates permanent, covalent DNA cross-links within parasites and results in Leishmania organisms termed killed but metabolically active (KBMA). In this report, we characterize the in vitro growth characteristics of both KBMA L. major and KBMA L. infantum chagasi. Concentrations of S-59 that generate optimally attenuated parasites were identified. Like live L. infantum chagasi, KBMA L. infantum chagasi parasites were able to initially enter liver cells in vivo after intravenous infection. However, whereas live L. infantum chagasi infection leads to hepatosplenomegaly in mice after 6 months, KBMA L. infantum chagasi parasites were undetectable in the organs of mice at this time point. In vitro, KBMA L. infantum chagasi retained the ability to enter macrophages and induce nitric oxide production. These characteristics of KBMA L. infantum chagasi correlated with the ability to prophylactically protect mice via subcutaneous vaccination at levels similar to vaccination with live, virulent organisms. Splenocytes from mice vaccinated with either live L. infantum chagasi or KBMA L. infantum chagasi displayed similar cytokine patterns in vitro. These results suggest that KBMA technology is a potentially safe and effective novel vaccine strategy against the intracellular protozoan L. infantum chagasi. This approach may represent a new method for whole-cell vaccination against other complex intracellular pathogens.

Host cell signalling and leishmania mechanisms of evasion

Leishmania parasites are able to secure their survival and propagation within their host by altering signalling pathways involved in the ability of macrophages to kill pathogens or to engage adaptive immune system. An important step in this immune evasion process is the activation of host protein tyrosine phosphatase SHP-1 by Leishmania. SHP-1 has been shown to directly inactivate JAK2 and Erk1/2 and to play a role in the negative regulation of several transcription factors involved in macrophage activation. These signalling alterations contribute to the inactivation of critical macrophage functions (e.g., Nitric oxide, IL-12, and TNF-α). Additionally, to interfere with IFN-γ receptor signalling, Leishmania also alters several LPS-mediated responses. Recent findings from our laboratory revealed a pivotal role for SHP-1 in the inhibition of TLR-induced macrophage activation through binding to and inactivating IL-1-receptor-associated kinase 1 (IRAK-1). Furthermore, we identified the binding site as an evolutionarily conserved ITIM-like motif, which we named kinase tyrosine-based inhibitory motif (KTIM). Collectively, a better understanding of the evasion mechanisms utilized by Leishmania parasite could help to develop more efficient antileishmanial therapies in the near future.

TGF-β1 re-programs TLR4 signaling in L. donovani infection: enhancement of SHP-1 and ubiquitin-editing enzyme A20

Visceral leishmaniasis (VL), caused by Leishmania donovani, is a major health concern in India. It represents T-helper type 2 (Th2) bias of cytokines in active state and Th1 bias at cure. However, the role of the parasite in regulating Toll-like receptor (TLR)-mediated macrophage activation in VL patients remains elusive. In this report, we demonstrated that later stages of L. donovani infection rendered tolerance to macrophages, leading to incapability for the production of inflammatory cytokines like tumor necrosis factor (TNF)-α and interleukin (IL)-1β in response to TLR stimulation. Overexpression of transforming growth factor (TGF)-β(1), but not IL-10, resulted in suppressed lipopolysaccharide (LPS)-induced production of TNF-α and downregulation of TLR4 expression in L. donovani-infected macrophages. Recombinant human (rh)TGF-β(1) markedly enhanced tyrosine phosphatase (Src homology region 2 domain-containing phosphatase-1) activity, but inhibited IL-1 receptor-activated kinase (IRAK)-1 activation. Addition of neutralizing TGF-β(1) antibody reversed these effects, and thus suggesting the pivotal role of TGF-β(1) in promoting refractoriness for LPS in macrophages. Surprisingly, the use of a tyrosine phosphatase inhibitor (sodium orthovanadate, Na(3)VO(4)) promoted IRAK-1 activation, confirming the negative inhibitory role of tyrosine phosphatase in macrophage activation. Furthermore, rhTGF-β(1) induced tolerance in infected macrophages by reducing inhibitory protein (IκBα) degradation in a time-dependent manner. In addition, short interfering RNA studies proved that overexpression of A20 ubiquitin-editing protein complex induced inhibitory activity of TGF-β(1) on LPS-mediated nuclear factor-κB activation. Thus, these findings suggest that TGF-β(1) promotes overexpression of A20 through tyrosine phosphatase activity that ensures transient activation of inflammatory signaling pathways in macrophages in active L. donovani infection.

Inhibitory effects of decavanadate on several enzymes and Leishmania tarentolae in vitro

Multiple studies report apparent effects of vanadium on various systems in vivo and in vitro. Vanadium species may be possible deterrents for the growth of the Leishmania parasite, which causes the sometimes deadly diseases known as leishmaniasis. The current studies focus specifically on decavanadate V(10)O(28)(6-) (V10), which has a potential to be a potent effector for disease treatment. The X-ray structure of a new solvate salt of V10, namely (NH(4))(6)V(10)O(28)·5H(2)O, is also reported. Other vanadium complexes with imidazole carboxylate, anthranilate, or picolinate were also evaluated. The yellow-orange oxoanion, used as the (NH(4))(6)V(10)O(28)·6H(2)O salt, was tested (at 1-100 μM) directly with two strains of Leishmania tarentolae promastigotes in culture to evaluate the effect on cell viability. Vanadium coordination complexes are known effective inhibitors of phosphatases. Using the artificial phosphatase substrate para-nitrophenylphosphate in the presence of a bovine calf intestine alkaline phosphatase enzyme, V10 (from 5 to 100 μM) was shown to be a mixed inhibitor for this enzyme and decreased the activity of the other two phosphatases tested. The effect of V10 and the other vanadium complexes on the activity of phosphoglycerate mutase B (PGAM), an important enzyme in glycolysis and gluconeogenesis, was also evaluated. At 10 μM, V10 was the most potent inhibitor of PGAM, with an apparent reduction of about 50%. Taken together, we speculate that V10 could have a role in treating Leishmania diseases.

Restoration of IFNγR subunit assembly, IFNγ signaling and parasite clearance in Leishmania donovani infected macrophages: role of membrane cholesterol

Despite the presence of significant levels of systemic Interferon gamma (IFNγ), the host protective cytokine, Kala-azar patients display high parasite load with downregulated IFNγ signaling in Leishmania donovani (LD) infected macrophages (LD-MØs); the cause of such aberrant phenomenon is unknown. Here we reveal for the first time the mechanistic basis of impaired IFNγ signaling in parasitized murine macrophages. Our study clearly shows that in LD-MØs IFNγ receptor (IFNγR) expression and their ligand-affinity remained unaltered. The intracellular parasites did not pose any generalized defect in LD-MØs as IL-10 mediated signal transducer and activator of transcription 3 (STAT3) phosphorylation remained unaltered with respect to normal. Previously, we showed that LD-MØs are more fluid than normal MØs due to quenching of membrane cholesterol. The decreased rigidity in LD-MØs was not due to parasite derived lipophosphoglycan (LPG) because purified LPG failed to alter fluidity in normal MØs. IFNγR subunit 1 (IFNγR1) and subunit 2 (IFNγR2) colocalize in raft upon IFNγ stimulation of normal MØs, but this was absent in LD-MØs. Oddly enough, such association of IFNγR1 and IFNγR2 could be restored upon liposomal delivery of cholesterol as evident from the fluorescence resonance energy transfer (FRET) experiment and co-immunoprecipitation studies. Furthermore, liposomal cholesterol treatment together with IFNγ allowed reassociation of signaling assembly (phospho-JAK1, JAK2 and STAT1) in LD-MØs, appropriate signaling, and subsequent parasite killing. This effect was cholesterol specific because cholesterol analogue 4-cholestene-3-one failed to restore the response. The presence of cholesterol binding motifs [(L/V)-X(1-5)-Y-X(1-5)-(R/K)] in the transmembrane domain of IFNγR1 was also noted. The interaction of peptides representing this motif of IFNγR1 was studied with cholesterol-liposome and analogue-liposome with difference of two orders of magnitude in respective affinity (K(D): 4.27×10(-9) M versus 2.69×10(-7) M). These observations reinforce the importance of cholesterol in the regulation of function of IFNγR1 proteins. This study clearly demonstrates that during its intracellular life-cycle LD perturbs IFNγR1 and IFNγR2 assembly and subsequent ligand driven signaling by quenching MØ membrane cholesterol.

Uncoupling protein 2 negatively regulates mitochondrial reactive oxygen species generation and induces phosphatase-mediated anti-inflammatory response in experimental visceral leishmaniasis

To reside and multiply successfully within the host macrophages, Leishmania parasites impair the generation of reactive oxygen species (ROS), which are a major host defense mechanism against any invading pathogen. Mitochondrial uncoupling proteins are associated with mitochondrial ROS generation, which is the major contributor of total cellular ROS generation. In the present study we have demonstrated that Leishmania donovani infection is associated with strong upregulation of uncoupling protein 2 (UCP2), a negative regulator of mitochondrial ROS generation located at the inner membrane of mitochondria. Functional knockdown of macrophage UCP2 by small interfering RNA-mediated silencing was associated with increased mitochondrial ROS generation, lower parasite survival, and induction of marked proinflammatory cytokine response. Induction of proinflammatory cytokine response in UCP2 knocked-down cells was a direct consequence of p38 and ERK1/2 MAPK activation, which resulted from ROS-mediated inhibition of protein tyrosine phosphatases (PTPs). Administration of ROS quencher, N-acetyl-l-cysteine, abrogated PTP inhibition in UCP2 knocked-down infected cells, implying a role of ROS in inactivating PTP. Short hairpin RNA-mediated in vivo silencing of UCP2 resulted in decreased Src homology 2 domain-containing tyrosine phosphatase 1 and PTP-1B activity and host-protective proinflammatory cytokine response resulting in effective parasite clearance. To our knowledge, this study, for the first time, reveals the induction of host UCP2 expression during Leishmania infection to downregulate mitochondrial ROS generation, thereby possibly preventing ROS-mediated PTP inactivation to suppress macrophage defense mechanisms.

Use of antimony in the treatment of leishmaniasis: current status and future directions

In the recent past the standard treatment of kala-azar involved the use of pentavalent antimonials Sb(V). Because of progressive rise in treatment failure to Sb(V) was limited its use in the treatment program in the Indian subcontinent. Until now the mechanism of action of Sb(V) is not very clear. Recent studies indicated that both parasite and hosts contribute to the antimony efflux mechanism. Interestingly, antimonials show strong immunostimulatory abilities as evident from the upregulation of transplantation antigens and enhanced T cell stimulating ability of normal antigen presenting cells when treated with Sb(V) in vitro. Recently, it has been shown that some of the peroxovanadium compounds have Sb(V)-resistance modifying ability in experimental infection with Sb(V) resistant Leishmania donovani isolates in murine model. Thus, vanadium compounds may be used in combination with Sb(V) in the treatment of Sb(V) resistance cases of kala-azar.

Evasion of Host Defence by Leishmania donovani: Subversion of Signaling Pathways

Protozoan parasites of the genus Leishmania are responsible for causing a variety of human diseases known as leishmaniasis, which range from self-healing skin lesions to severe infection of visceral organs that are often fatal if left untreated. Leishmania donovani (L. donovani), the causative agent of visceral leishmaniasis, exemplifys a devious organism that has developed the ability to invade and replicate within host macrophage. In fact, the parasite has evolved strategies to interfere with a broad range of signaling processes in macrophage that includes Protein Kinase C, the JAK2/STAT1 cascade, and the MAP Kinase pathway. This paper focuses on how L. donovani modulates these signaling pathways that favour its survival and persistence in host cells.

Characterization of acid phosphatases from marine scuticociliate parasites and their activation by host's factors

Scuticociliates are histophagous marine parasites that cause mortality in fish. Acid phosphatases (AcPs) are considered virulence factors and they are used by different parasites to dephosphorylate host molecules. The aim of this work was to characterize the AcPs from 3 scuticociliate species, Uronema marinum, Miamiensis avidus and Parauronema virginianum, which parasitize marine finfish species. We identified AcP activity (pH 5.2) with differential cellular distribution in the 3 parasite species. Native gel electrophoresis of ciliate lysates revealed the presence of 1 high molecular weight AcP activity band in M. avidus (tartrate-sensitive), several low molecular weight AcPs in U. marinum and 1 low molecular weight band only in P. virginianum (tartrate-resistant). Scuticociliate AcP was inhibited by specific inhibitors of tyrosine protein phosphatases. AcP decreased upon starvation but rapid reactivation occurred following exposure to skin mucus. Groper (Polyprion oxygeneios) peripheral blood leucocytes (PBLs) and, to a lesser extent, red blood cells, also increased AcP activity. Protein tyrosine phosphatase PTP1b was primarily detected in the plasma membrane of M. avidus and ingestion of groper PBLs upregulated its expression. M. avidus recovered from experimentally infected groper had greater levels of PTP1b expression than the injected suspension. The present results highlight the importance of PTPs in histophagous parasites and their interaction with fish host's factors.

Trypanosomatid protein phosphatases

Protein phosphorylation is one of the most important post-translational modifications regulating various signaling processes in all known living organisms. In the cell, protein phosphatases and protein kinases play a dynamic antagonistic role, controlling the phosphorylation state of tyrosine (Tyr), serine (Ser) and threonine (Thr) side chains of proteins. The reversible phosphorylation modulates protein function, through initiating conformational changes, which influences protein complex formation, alteration of enzyme activity and changes in protein stability and subcellular localization. These molecular changes affect signaling cascades regulating the cell cycle, differentiation, cell-cell and cell-substrate interactions, cell motility, the immune response, ion-channel and transporter activities, gene transcription, mRNA translation, and basic metabolism. In addition to these processes, in unicellular parasites, like Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp., additional signaling pathways have evolved to enable the survival of parasites in the changing environment of the vector and host organism. In recent years the genome of five trypanosomatid genomes have been sequenced and annotated allowing complete definition of the composition of the trypanosomatid phosphatomes. The very diverse environments involved in the different stages of the kinetoplastids' life cycle might have played a role to develop a set of trypanosomatid-specific phosphatases in addition to orthologues of many higher eukaryote protein phosphatases present in the kinetoplastid phosphatomes. In spite of their well-described phosphatomes, few trypanosomatid protein phosphatases have been characterized and studied in vivo. The aim of this review is to give an up to date scope of the research, which has been carried out on trypanosomatid protein phosphatases.

Leishmania interferes with host cell signaling to devise a survival strategy

The protozoan parasite Leishmania spp. exists as extracellular promastigotes in its vector whereas it resides and replicates as amastigotes within the macrophages of its mammalian host. As a survival strategy, Leishmania modulates macrophage functions directly or indirectly. The direct interference includes prevention of oxidative burst and the effector functions that lead to its elimination. The indirect effects include the antigen presentation and modulation of T cell functions in such a way that the effector T cells help the parasite survive by macrophage deactivation. Most of these direct and indirect effects are regulated by host cell receptor signaling that occurs through cycles of phosphorylation and dephosphorylation in cascades of kinases and phosphatases. This review highlights how Leishmania selectively manipulates the different signaling pathways to ensure its survival.

Comparative study of the ability of Leishmania mexicana promastigotes and amastigotes to alter macrophage signaling and functions

Leishmania alternates between two morphologically different stages, promastigotes and amastigotes. While the majority of reports focused on how the promastigote form can alter macrophage (Mphi) signaling and function, fewer reports investigated signaling alterations mediated by amastigotes, and there is a lack of comparative studies. In this study, we performed a comparison between the ability of both forms of the parasite to alter Mphi signaling and functions. Here, we show that both promastigotes and amastigotes were able to rapidly activate host protein tyrosine phosphatases (PTPs), importantly the Src homology 2 domain-containing PTP (SHP-1). However, we found that PTP-1B is specifically activated by promastigote but not amastigote infection and that lmcpb(-/-) promastigotes were no longer able to activate PTP-1B. We also show a similarity in the way promastigotes and amastigotes inactivate the transcription factors (TFs) STAT-1alpha and AP-1, but we show differences in the modulation of NF-kappaB, with promastigotes cleaving the p65 subunit, generating a smaller p35 subunit, and amastigotes fully degrading the p65 subunit with no p35 production. Importantly, we show that the cysteine proteinase LmCPb plays a key role in the alteration of NF-kappaB, STAT-1alpha, and AP-1 by promastigote and amastigote infections, ultimately leading to the inability of these TFs to translocate to the nucleus in response to gamma interferon (IFN-gamma) stimulation and thus contributing to the ability of both parasite forms to effectively block IFN-gamma-mediated nitric oxide (NO) production in Mphis.

Regulation of macrophage nitric oxide production by the protein tyrosine phosphatase Src homology 2 domain phosphotyrosine phosphatase 1 (SHP-1)

Nitric oxide (NO) is a potent molecule involved in the cytotoxic effects mediated by macrophages (MØ) against microorganisms. We previously reported that Src homology 2 domain phosphotyrosine phosphatase 1 (SHP-1)-deficient cells generate a greater amount of NO than wild-type cells in response to interferon-gamma (IFN-gamma). We also reported that the Leishmania-induced MØ SHP-1 activity is needed for the survival of the parasite within phagocytes through the attenuation of NO-dependent and NO-independent mechanisms. In the present study, we investigated the role of SHP-1 in regulating key signalling molecules important in MØ NO generation. Janus tyrosine kinase 2 (JAK2), mitogen-activated extracellular signal-regulated protein kinase kinase (MEK), extracellular signal-regulated kinases 1 and 2 (Erk1/Erk2) mitogen-activated protein kinases, p38 and stress-activated mitogen-activated protein kinases/c-Jun NH(2)-terminal kinase (SAPK/JNK) were examined in immortalized bone marrow-derived MØ (BMDM) from both SHP-1-deficient motheaten mice (me-3) and their respective littermates (LM-1). The results indicated that Erk1/Erk2 and SAPK/JNK are the main kinases regulated by SHP-1 because the absence of SHP-1 caused an increase in their phosphorylation. Moreover, only Apigenin, the specific inhibitor of Erk1/Erk2, was able to block IFN-gamma-induced inducible nitric oxide synthase (iNOS) transcription and translation in me-3 cells. Transcription factor analyses revealed that in the absence of SHP-1, activator protein-1 (AP-1) was activated. The activation of AP-1, and not nuclear factor-kappaB (NF-kappaB) or signal transducer and activator of transcription-1 alpha (STAT-1 alpha), may explain the enhanced NO generation in SHP-1-deficient cells. These observations emphasize the involvement of the MAPKs Erk1/Erk2 and SAPK/JNK in NO generation via AP-1 activation. Collectively, our findings suggest that SHP-1 plays a pivotal role in the negative regulation of signalling events leading to iNOS expression and NO generation. Furthermore, our observations underline the importance of SHP-1-mediated negative regulation in maintaining NO homeostasis and thus preventing the abnormal generation of NO that can be detrimental to the host.

Protein tyrosine phosphatases regulate asthma development in a murine asthma model

Allergic asthma is a chronic inflammatory disease characterized by Th2-type inflammation. Although the cellular interactions are now well studied, the intracellular signaling involved in asthma development is still a developing field. Protein tyrosine kinases are one focus of such research and their inhibition shows improvement of asthmatic features. Interestingly, very little attention was given to protein tyrosine phosphatases (PTPs), the counterparts to protein tyrosine kinases, in the development of asthma. Previous studies from our laboratory showed that pharmacological inhibition of PTPs induced a transient Th1 response in the spleen. Therefore, we hypothesized that modulation of PTPs could influence asthma development. To assess PTP functions, we used the PTP inhibitor bis-peroxovanadium bpV(phen) in a murine model of asthma during either allergen sensitization or challenge. Inhibition of PTPs during allergen sensitization resulted in the reduction of key features of allergic asthma: serum IgE levels, lung tissue inflammation, eosinophilia, and airway hyperresponsiveness. Of utmost interest, PTP inhibition at allergen challenge resulted in a very similar improvement of asthmatic features. Of further importance, we observed that bpV(phen) treatment modulated cytokine expression in the spleen and, more specifically, favored Th1 cytokines while inhibiting Th2 cytokines. Collectively, we show for the first time that intact activity of PTPs is required for a complete induction of asthma in a mouse model. This clearly suggests that PTPs have a pivotal regulatory role in the development of asthmatic diseases, which opens the possibility of new therapeutic avenues.

Leishmania-induced IRAK-1 inactivation is mediated by SHP-1 interacting with an evolutionarily conserved KTIM motif

Parasites of the Leishmania genus can rapidly alter several macrophage (MØ) signalling pathways in order to tame down the innate immune response and inflammation, therefore favouring their survival and propagation within their mammalian host. Having recently reported that Leishmania and bacterial LPS generate a significantly stronger inflammatory response in animals and phagocytes functionally deficient for the Src homology 2 domain-containing protein tyrosine phosphatase (SHP-1), we hypothesized that Leishmania could exploit SHP-1 to inactivate key kinases involved in Toll-like receptor (TLR) signalling and innate immunity such as IL-1 receptor-associated kinase 1 (IRAK-1). Here we show that upon infection, SHP-1 rapidly binds to IRAK-1, completely inactivating its intrinsic kinase activity and any further LPS-mediated activation as well as MØ functions. We also demonstrate that the SHP-1/IRAK-1 interaction occurs via an evolutionarily conserved ITIM-like motif found in the kinase domain of IRAK-1, which we named KTIM (Kinase Tyrosyl-based Inhibitory Motif). This regulatory motif appeared in early vertebrates and is not found in any other IRAK family member. Our study additionally reveals that several other kinases (e.g. Erk1/2, IKKα/β) involved in downstream TLR signalling also bear KTIMs in their kinase domains and interact with SHP-1. We thus provide the first demonstration that a pathogen can exploit a host protein tyrosine phosphatase, namely SHP-1, to directly inactivate IRAK-1 through a generally conserved KTIM motif.

Leishmania developed several methods to seize control of macrophage signalling pathways in an effort to inactivate their killing abilities. One effective method utilized by the parasite is the activation of host protein tyrosine phosphatases, specifically SHP-1. This increased phosphatase activity contributes to the inactivation of signalling molecules involved in critical macrophage functions such as NO and cytokine production. Interestingly, the absence of SHP-1 results in stronger macrophage inflammatory responses to a bacterial cell wall component known as LPS, a molecule detected by macrophages through Toll-like receptors (TLRs). This observation suggested a role for SHP-1 in the regulation of TLR signalling. Our study reveals that upon Leishmania infection, SHP-1 is able to rapidly bind to and inactivate a critical kinase (IRAK-1) in this pathway. This regulatory binding was shown to be mediated by an evolutionarily conserved motif identified in the kinase. This motif was also present in other kinases involved in Toll signalling and therefore could represent a regulatory mechanism of relevance to many kinases. This work not only reports a unique mechanism by which Leishmania can avoid harmful TLR signalling, but also provides a platform on which extensive investigation on host evasion mechanisms and regulation of cellular kinases can be gained.

Leishmania major intracellular survival is not altered in SHP-1 deficient mev or CD45-/- mice

Protozoan parasites of the genus Leishmania escape from the immune response by interfering with signal transduction pathways of its host cell, the macrophage, thereby establishing permissive conditions for intracellular survival. Inhibition of macrophage activation after Leishmania infection has been suggested to require activation of the host cell phosphatase SHP-1. However, by utilizing infections of SHP-1 deficient (me(v)) and CD45 null mutant mice or macrophages, we provide evidence that intracellular survival of Leishmania major is not generally dependent on these cellular phosphatases.

Leishmania major abrogates gamma interferon-induced gene expression in human macrophages from a global perspective

Infection with Leishmania major triggers several pathways in the host cell that are crucial to initial infection as well as those that are used by Leishmania to enhance its replication and virulence. To identify the molecular events of the host cell in response to Leishmania, the global gene expression of the human monocytic cell line THP-1 either infected with Leishmania major in the presence and absence of gamma interferon (IFN-gamma) or in the presence of IFN-gamma alone was analyzed using high-density human oligonucleotide microarrays, followed by statistical analysis. The persistence of the parasite despite an extensive response to IFN-gamma, added 24 h after infection with L. major, suggests that L. major can survive in an IFN-gamma-enriched environment in vitro. Results demonstrate that L. major counteracts the IFN-gamma response in macrophages on a large scale. Expression of genes involved in the innate immune response, cell adhesion, proteasomal degradation, Toll-like receptor expression, a variety of signaling molecules, and matrix metalloproteinases was significantly modulated.

Role of protein tyrosine phosphatases in the regulation of interferon-γ-induced macrophage nitric oxide generation: implication of ERK pathway and AP-1 activation

NO is a potent molecule involved in the cytotoxic events mediated by macrophages (MØ) against microorganisms. We reported previously that inhibition of MØ protein tyrosine phosphatases (PTPs) mediates a protective effect against Leishmania infection, which was NO-dependent. Herein, we show that the PTP inhibitors of the peroxovanadium (pV) class, bpV(phen) and bpV(pic), can similarly increase murine MØ IFN-gamma-induced NO generation. Using various second messenger (JAK2, MEK, Erk1/Erk2, and p38) antagonists, we found that the Erk1/Erk2 pathway was the principal pathway submitted to regulation by PTPs in the context of IFN-gamma-driven MØ activation and increase in NO production. We observed that bpV(phen) increases inducible NO synthase (iNOS) expression, resulting in enhanced NO production, whereas the bpV(pic) increase of NO production does not seem to result from a modulation of iNOS expression. Transcription factors STAT-1alpha and NF-kappaB, recognized for their importance in NO generation, were not affected by the pV treatment. However, AP-1 was strongly activated by bpV(phen) but not by bpV(pic). Collectively, our results suggest that increased IFN-gamma-induced NO production, observed after bpV(phen) treatment, involves the activation of the transcription factor AP-1 by Erk1/Erk2- and stress-activated protein kinase/JNK-dependent transduction mechanisms.

Role of host protein tyrosine phosphatase SHP-1 in Leishmania donovani-induced inhibition of nitric oxide production

In order to survive within the macrophages of its host organism, the protozoan parasite Leishmania inhibits a number of critical, gamma interferon (IFN-gamma)-inducible, macrophage functions, including the generation of nitric oxide. We have previously shown that the protein tyrosine phosphatase SHP-1 (Src-homology 2 domain containing phosphatase-1) is activated during Leishmania infection and plays an important role in both the survival of Leishmania within cultured macrophages and disease progression in vivo by inhibiting nitric oxide production. Here we use a SHP-1-/- macrophage cell line derived from motheaten mice to address the mechanisms by which SHP-1 prevents IFN-gamma-dependent nitric oxide production during Leishmania donovani infection. We show that Leishmania inhibits nitric oxide production in response to IFN-gamma poorly in SHP-1-deficient macrophages. This correlates with the inability of Leishmania to alter JAK2 and mitogen-activated protein kinase extracellular signal-regulated kinase 1 and 2 (ERK1/2) phosphorylation and to prevent nuclear translocation of transcription factors NF-kappaB and AP-1, although the latter two to a lesser extent. Surprisingly, Leishmania inactivated the transcription factor STAT1 to a similar extent in SHP-1-deficient and wild-type macrophages, so STAT1 is not necessary for nitric oxide production by infected macrophages. Overall, this study demonstrates that induction of SHP-1 by Leishmania is vital for inhibition of nitric oxide generation and that this inhibition occurs through the inactivation of JAK2 and ERK1/2, and transcription factors NF-kappaB and AP-1.

Identification and Characterization of a Protein-tyrosine Phosphatase in Leishmania

Leishmania parasites are eukaryotic protozoans responsible for a variety of human diseases known as leishmaniasis, which ranges from skin lesions to fatal visceral infections. Leishmania is transmitted by the bite of an infected sandfly where it exists as promastigotes and, upon entry into a mammalian host, differentiates into amastigotes, which replicate exclusively in macro-phages. The biochemical pathways enabling Leishmania to differentiate and survive in the mammalian host are poorly defined. We have therefore examined the role of protein-tyrosine phosphorylation, which is essential in regulating cell function in higher eukaryotes. Using the recently completed Leishmania genome, we have identified and cloned a Leishmania protein-tyrosine phosphatase (PTP) gene (LPTP1) by virtue of its homology with the human protein-tyrosine phosphatase 1B gene (hPTP1B). The enzyme activity of recombinant LPTP1 was confirmed using a combination of PTP-specific substrates and inhibitors. We further demonstrate, by creating LPTP1 null mutants through gene targeting, that LPTP1 is necessary for survival as amastigotes in mice, but it is dispensable for survival as promastigotes in culture. Human PTPs, including the PTP1B enzyme, are actively pursued drug targets for a variety of diseases. The observations with the LPTP1 mutants in mice suggest that it may also represent a drug target against the mammalian amastigote stage. However, in silico structure analysis of LPTP1 revealed a striking similarity with hPTP1B in the active site suggesting that, although this is an attractive drug target, it may be difficult to develop an inhibitor specific for the Leishmania LPTP1.

Trypanosoma cruzi-Mediated IFN-γ-Inducible Nitric Oxide Output in Macrophages Is Regulated byiNOSmRNA Stability

Although the effects of activated macrophages (Muphi) on the intracellular parasite Trypanosoma cruzi are well documented, little is known about how host-Muphi functions are affected by this pathogen before activation. This study is aimed at assessing the capacity of T. cruzi infection to modulate J77.4 murine Muphi NO generation following IFN-gamma stimulation, and identifying mechanisms regulating this modulation. Results show that parasite infection potentiates Muphi to produce inducible NO synthase (iNOS) mRNA and protein as well as NO following IFN-gamma stimulation above IFN-gamma alone controls. This potentiation occurs through the concomitant activation of NF-kappaB, ERK1/ERK2 MAPK, and stress-activated protein kinase signaling pathways. Activation of the JAK/STAT pathway by IFN-gamma then leads to STAT1alpha translocation and the transcription of a stable iNOS mRNA species. A decreased rate of iNOS mRNA degradation results in elevated levels of iNOS protein and NO production. Maximal iNOS expression is likely achieved through NF-kappaB activation by T. cruzi, whereas iNOS mRNA stability results from ERK1/ERK2 MAPK and stress-activated protein kinase activation by the infection. Taken together, our data show that T. cruzi-infected Muphi NO generation is controlled at both pre- and posttranscriptional levels and relies on signaling pathway cross-talk. This is the first report of a parasite pathogen capable of heightening host mRNA stability.