Leishmania exploits host cAMP/EPAC/calcineurin signaling to induce an IL-33-mediated anti-inflammatory environment for the establishment of infection

Host anti-inflammatory responses are critical for the progression of visceral leishmaniasis, and the pleiotropic cytokine interleukin (IL)-33 was found to be upregulated in infection. Here, we documented that IL-33 induction is a consequence of elevated cAMP-mediated exchange protein activated by cAMP (EPAC)/calcineurin-dependent signaling and essential for the sustenance of infection. Leishmania donovani-infected macrophages showed upregulation of IL-33 and its neutralization resulted in decreased parasite survival and increased inflammatory responses. Infection-induced cAMP was involved in IL-33 production and of its downstream effectors PKA and EPAC, only the latter was responsible for elevated IL-33 level. EPAC initiated Rap-dependent phospholipase C activation, which triggered the release of intracellular calcium followed by calcium/calmodulin complex formation. Screening of calmodulin-dependent enzymes affirmed involvement of the phosphatase calcineurin in cAMP/EPAC/calcium/calmodulin signaling-induced IL-33 production and parasite survival. Activated calcineurin ensured nuclear localization of the transcription factors, nuclear factor of activated T cell 1 and hypoxia-inducible factor 1 alpha required for IL-33 transcription, and we further confirmed this by chromatin immunoprecipitation assay. Administering specific inhibitors of nuclear factor of activated T cell 1 and hypoxia-inducible factor 1 alpha in BALB/c mouse model of visceral leishmaniasis decreased liver and spleen parasite burden along with reduction in IL-33 level. Splenocyte supernatants of inhibitor-treated infected mice further documented an increase in tumor necrosis factor alpha and IL-12 level with simultaneous decrease of IL-10, thereby indicating an overall disease-escalating effect of IL-33. Thus, this study demonstrates that cAMP/EPAC/calcineurin signaling is crucial for the activation of IL-33 and in effect creates anti-inflammatory responses, essential for infection.

PD-1 negatively tunes macrophage immune activation by turning off JNK and STAT1 signaling: Exploited by Leishmania for its intra-macrophage survival

The anti-inflammatory role of the programmed death-1 receptor (PD-1) is well appreciated. However, the mechanism of how PD-1 signaling inhibits the pro-inflammatory cytokine responses in macrophages, which is further exploited by Leishmania to foster their intracellular survival, was unknown. We found that among three major MAP kinases regulating immune activation, PD-1 signaling decreased only JNK phosphorylation without perturbing p38 and ERK. Inflammatory transcription factor STAT1 was also inhibited by PD-1. Association studies documented that SHP, the downstream phosphatase of PD-1, is directly responsible for the decreased phosphorylation of JNK and STAT1. JNK and STAT1 deactivation led to Elk-1/c-Fos inhibition, which significantly decreased IL-12 and TNF-α levels. Further investigation revealed c-Fos deactivation ultimately rendered transcription factor AP1 inactive and facilitating parasite-favorable anti-inflammatory environment.

Translationally Controlled Tumor Protein-Mediated Stabilization of Host Antiapoptotic Protein MCL-1 Is Critical for Establishment of Infection by Intramacrophage Parasite Leishmania donovani

In the early phase of infection, the intramacrophage pathogen Leishmania donovani protects its niche with the help of the antiapoptotic protein myeloid cell leukemia-1 (MCL-1). Whether Leishmania could exploit MCL-1, an extremely labile protein, at the late phase is still unclear. A steady translational level of MCL-1 observed up to 48 h postinfection and increased caspase-3 activity in MCL-1-silenced infected macrophages documented its importance in the late hours of infection. The transcript level of MCL-1 showed a sharp decline at 6 h postinfection, and persistent MCL-1 expression in cyclohexamide-treated cells negates the possibility of de novo protein synthesis, thereby suggesting infection-induced stability. Increased ubiquitination, a prerequisite for proteasomal degradation of MCL-1, was also found to be absent in the late hours of infection. Lack of interaction with its specific E3 ubiquitin ligase MULE (MCL-1 ubiquitin ligase E3) and specific deubiquitinase USP9X prompted us to search for blockade of the ubiquitin-binding site in MCL-1. To this end, TCTP (translationally controlled tumor protein), a well-known binding partner of MCL-1 and antiapoptotic regulator, was found to be strongly associated with MCL-1 during infection. Phosphorylation of TCTP, a requirement for MCL-1 binding, was also increased in infected macrophages. Knockdown of TCTP decreased MCL-1 expression and short hairpin RNA-mediated silencing of TCTP in an infected mouse model of visceral leishmaniasis showed decreased parasite burden and induction of liver cell apoptosis. Collectively, our investigation revealed a key mechanism of how L. donovani exploits TCTP to establish infection within the host.

The Pathogenicity and Virulence of Leishmania - interplay of virulence factors with host defenses

Leishmaniasis is a group of disease caused by the intracellular protozoan parasite of the genus Leishmania. Infection by different species of Leishmania results in various host immune responses, which usually lead to parasite clearance and may also contribute to pathogenesis and, hence, increasing the complexity of the disease. Interestingly, the parasite tends to reside within the unfriendly environment of the macrophages and has evolved various survival strategies to evade or modulate host immune defense. This can be attributed to the array of virulence factors of the vicious parasite, which target important host functioning and machineries. This review encompasses a holistic overview of leishmanial virulence factors, their role in assisting parasite-mediated evasion of host defense weaponries, and modulating epigenetic landscapes of host immune regulatory genes. Furthermore, the review also discusses the diagnostic potential of various leishmanial virulence factors and the advent of immunomodulators as futuristic antileishmanial drug therapy.

Delineating infection strategies of Leishmania donovani secretory proteins in Human through host-pathogen protein Interactome prediction

Interactions of Leishmania donovani secretory virulence factors with the host proteins and their interplay during the infection process in humans is poorly studied in Visceral Leishmaniasis. Lack of a holistic study of pathway level de-regulations caused due to these virulence factors leads to a poor understanding of the parasite strategies to subvert the host immune responses, secure its survival inside the host and further the spread of infection to the visceral organs. In this study, we propose a computational workflow to predict host-pathogen protein interactome of L.donovani secretory virulence factors with human proteins combining sequence-based Interolog mapping and structure-based Domain Interaction mapping techniques. We further employ graph theoretical approaches and shortest path methods to analyze the interactome. Our study deciphers the infection paths involving some unique and understudied disease-associated signaling pathways influencing the cellular phenotypic responses in the host. Our statistical analysis based in silico knockout study unveils for the first time UBC, 1433Z and HS90A mediator proteins as potential immunomodulatory candidates through which the virulence factors employ the infection paths. These identified pathways and novel mediator proteins can be effectively used as possible targets to control and modulate the infection process further aiding in the treatment of Visceral Leishmaniasis.

Oral combination of eugenol oleate and miltefosine induce immune response during experimental visceral leishmaniasis through nitric oxide generation with advanced cytokine demand

Conventional therapy of visceral leishmaniasis (VL) remains challenging with the pitfall of toxicity, drug resistance, and expensive. Hence, urgent need for an alternative approach is essential. In this study, we evaluated the potential of combination therapy with eugenol oleate and miltefosine in Leishmania donovani infected macrophages and in the BALB/c mouse model. The interactions between eugenol oleate and miltefosine were found to be additive against promastigotes and amastigotes with xΣFIC 1.13 and 0.68, respectively. Significantly (p < 0.001) decreased arginase activity, increased nitrite generation, improved pro-inflammatory cytokines, and phosphorylated p38MAPK were observed after combination therapy with eugenol oleate and miltefosine. >80% parasite clearance in splenic and hepatic tissue with concomitant nitrite generation, and anti-VL cytokines productions were observed after orally administered miltefosine (5 mg/kg body weight) and eugenol oleate (15 mg/kg body weight) in L. donovani-infected BALB/c mice. Altogether, this study suggested the possibility of an oral combination of miltefosine with eugenol oleate against visceral leishmaniasis.

Leishmania donovani Targets Host Transcription Factor NRF2 To Activate Antioxidant Enzyme HO-1 and Transcriptional Repressor ATF3 for Establishing Infection

We showed previously that antioxidant enzyme heme oxygenase 1 (HO-1) is critical for Leishmania survival in visceral leishmaniasis. HO-1 inhibits host oxidative burst and inflammatory cytokine production, leading to parasite persistence. In the present study, screening of reported HO-1 transcription factors revealed that infection upregulated (4.1-fold compared to control [P < 0.001]) nuclear factor erythroid 2 (NFE2)-related factor 2 (NRF2). Silencing of NRF2 reduced both HO-1 expression and parasite survival. Investigation revealed that infection-induced transient reactive oxygen species (ROS) production dissociated NRF2 from its inhibitor KEAP1 and enabled phosphorylation-dependent nuclear translocation. Both NRF2 and HO-1 silencing in infection increased production of proinflammatory cytokines. But the level was greater in NRF2-silenced cells than in HO-1-silenced ones, suggesting the presence of other targets of NRF2. Another stress responsive transcription factor ATF3 is also induced (4.6-fold compared to control [P < 0.001]) by NRF2 during infection. Silencing of ATF3 reduced parasite survival (59.3% decrease compared to control [P < 0.001]) and increased proinflammatory cytokines. Infection-induced ATF3 recruited HDAC1 into the promoter sites of tumor necrosis factor alpha (TNF-α) and interleukin 12b (IL-12b) genes. Resulting deacetylated histones prevented NF-κB promoter binding, thereby reducing transcription of inflammatory cytokines. Administering the NRF2 inhibitor trigonelline hydrochloride to infected BALB/c mice resulted in reduced HO-1 and ATF3 expression, decreased spleen and liver parasite burdens, and increased proinflammatory cytokine levels. These results suggest that Leishmania upregulates NRF2 to activate both HO-1 and ATF3 for disease progression.

Probing the molecular mechanism of aggressive infection by antimony resistant Leishmania donovani

The disease visceral leishmaniasis (VL) or kala azar is caused by the protozoan parasite, Leishmania donovani (LD). For many decades the pentavalent antimonial drugs countered the successive epidemics of the disease in the Indian sub-continent and elsewhere. With time, antimony resistant LD (LD^R) developed and the drug in turn lost its efficacy. Infection of mammals with LD^R gives rise to aggressive infection as compared to its sensitive counterpart (LD^S) coupled with higher surge of IL-10 and TGF-β. The IL-10 causes upregulation of multidrug resistant protein-1 which causes efflux of antimonials from LD^R infected cells. This is believed to be a key mechanism of antimony resistance. MicroRNAs (miRNAs) are tiny post-transcriptional regulators of gene expression in mammalian cells and in macrophage play a pivotal role in controlling the expression of cytokines involved in infection process. Therefore, a change in miRNA profiles of macrophages infected with LD^S or LD^R could explain the differential cytokine response observed. Interestingly, the outcome of LD infection is also governed by the critical balance of pro- and anti-inflammatory cytokines which is inturn regulated by miRNA-Ago2 or miRNP complex and its antagonist RNA binding protein HuR. Here Ago2 plays the fulcrum whose phosphorylation and de-phosphorylation dictates the process; which in turn is controlled by PP2A and HuR. LD^S and LD^R upregulate PP2A and downregulate HuR at different magnitude leading to various levels of anti-inflammatory to proinflammatory cytokine production and resulting pathology in the host. While ectopic HuR expression alone is sufficient to clear LD^S infection, simultaneous upregulation of HuR and inhibition of PP2A is required to inhibit LD^R mediated infection. Therefore, tampering with miRNA pathway could be a new strategy to control infection caused by LD^R parasite.

Increased host ATP efflux and its conversion to extracellular adenosine is crucial for establishing Leishmania infection

Intracellular survival of Leishmania donovani demands rapid production of host ATP for its sustenance. However, a gradual decrease in intracellular ATP in spite of increased glycolysis suggests ATP efflux during infection. Accordingly, upon infection, we show here that ATP is exported and the major exporter was pannexin-1, leading to raised extracellular ATP levels. Extracellular ATP shows a gradual decrease after the initial increase, and analysis of cell surface ATP-degrading enzymes revealed induction of the ectonucleotidases CD39 and CD73. Ectonucleotidase-mediated ATP degradation leads to increased extracellular adenosine (eADO), and inhibition of CD39 and CD73 in infected cells decreased adenosine concentration and parasite survival, documenting the importance of adenosine in infection. Inhibiting adenosine uptake by cells did not affect parasite survival, suggesting that eADO exerts its effect through receptor-mediated signalling. We also show that Leishmania induces the expression of adenosine receptors A2AR and A2BR, both of which are important for anti-inflammatory responses. Treating infected BALB/c mice with CD39 and CD73 inhibitors resulted in decreased parasite burden and increased host-favourable cytokine production. Collectively, these observations indicate that infection-induced ATP is exported, and after conversion into adenosine, propagates infection via receptor-mediated signalling.

MicroRNA exporter HuR clears the internalized pathogens by promoting pro-inflammatory response in infected macrophages

HuR is a miRNA derepressor protein that can act as miRNA sponge for specific miRNAs to negate their action on target mRNAs. Here we have identified how HuR, by inducing extracellular vesicles‐mediated export of miRNAs, ensures robust derepression of miRNA‐repressed cytokines essential for strong pro‐inflammatory response in activated mammalian macrophages. Leishmania donovani, the causative agent of visceral leishmaniasis, on the contrary alters immune response of the host macrophage by a variety of complex mechanisms to promote anti‐inflammatory response essential for the survival of the parasite. We have found that during Leishmania infection, the pathogen targets HuR to promote onset of anti‐inflammatory response in mammalian macrophages. In infected macrophages, Leishmania also upregulate protein phosphatase 2A that acts on Ago2 protein to keep it in dephosphorylated and miRNA‐associated form. This causes robust repression of the miRNA‐targeted pro‐inflammatory cytokines to establish an anti‐inflammatory response in infected macrophages. HuR has an inhibitory effect on protein phosphatase 2A expression, and mathematical modelling of macrophage activation process supports antagonistic miRNA‐modulatory roles of HuR and protein phosphatase 2A which mutually balances immune response in macrophage by targeting miRNA function. Supporting this model, ectopic expression of the protein HuR and simultaneous inhibition of protein phosphatase 2A induce strong pro‐inflammatory response in the host macrophage to prevent the virulent antimonial drug‐sensitive or drug‐resistant form of L. donovani infection. Thus, HuR can act as a balancing factor of immune responses to curtail the macrophage infection process by the protozoan parasite.

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

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

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

Modulation of TLR4 Sialylation Mediated by a Sialidase Neu1 and Impairment of Its Signaling in Leishmania donovani Infected Macrophages

Altered sialylation is generally maintained by a fine balance between sialidases and sialyltransferases, which plays an essential role during disease pathogenesis. TLR4 is a membrane-bound highly sialylated glycoprotein predominantly having α2,3-linked sialic acids. It is one of the most important client molecules in the anti-leishmanial innate immune arm. Here, we initiated a comprehensive study on the modulation of TLR4 sialylation in Leishmania donovani (L. d)-infected macrophages by a mammalian sialidase/neuraminidase-1 (Neu1) having substrate specificity toward α2,3-linked sialic acids. We observed reduced membrane-associated Neu1 with its decreased enzyme activity in infected macrophages. Moreover, we demonstrated reduced association of Neu1 with TLR4 leading to enhanced sialylation of TLR4 in these infected cells. Conversely, Neu1 over expression exhibited enhanced association of TLR4 with Neu1 leading to reduced sialylation which possibly linked to increased association of TLR4 with its downstream adaptor protein, MyD88. This, in turn, activated downstream MAP kinase signaling pathway, with enhanced nuclear translocation of NFκB that resulted in increased genetic and protein levels expression of Th1 cytokines and effector molecule nitric oxide secretion which ultimately leads to reduced parasite burden in macrophages. This was further validated by Neu1 silencing in infected macrophages which reversed such a situation. Such events strongly confirm the importance of Neu1 in modulation of TLR4 sialylation during parasite infection resulting in impairment of innate immune response. Furthermore, decreased membrane-bound Neu1 in infected macrophages could be attributed to its reduced tyrosine-phosphorylation as well as diminished association with cathepsin A. Both these phenomenon possibly play significant roles in inhibiting translocation of the sialidase from cytosol to membrane. Taken together, our study first time demonstrated impaired translocation of cytosolic Neu1 to the membrane of L. donovani-infected macrophages due to impaired phosphorylation of this enzyme. This novel finding establishes a link between enhanced α2,3-linked sialic acids on TLR4 and reduced membrane-bound Neu1 which plays a significant role for inhibiting downstream signaling to establish successful infection in the host cells.

Label-Free Proteomic Analysis Reveals Parasite-Specific Protein Alterations in Macrophages Following Leishmania amazonensis, Leishmania major, or Leishmania infantum Infection

Leishmania is an obligate intracellular parasite known to modulate the host cell to survive and proliferate. However, the complexity of host-parasite interactions remains unclear. Also, the outcome of the disease has been recognized to be species-specific and dependent on the host's immune responses. Proteomics has emerged as a powerful tool to investigate the host-pathogen interface, allowing us to deepen our knowledge about infectious diseases. Quantification of the relative amount of proteins in a sample can be achieved using label-free proteomics, and for the first time, we have used it to quantify Leishmania-specific protein alterations in macrophages. Protein extracts were obtained and digested, and peptides were identified and quantified using nano-LC coupled with tandem mass spectrometry analyses. Protein expression was validated by Western blot analysis. Integrated Proteomics Pipeline was used for peptide/protein identification and for quantification and data processing. Ingenuity Pathway Analysis was used for network analysis. In this work, we investigated how this intracellular parasite modulates protein expression on a host macrophage by comparing three different Leishmania species- L. amazonensis, one of the causative agents of cutaneous disease in the Amazon region; L. major, another causative agent of cutaneous leishmaniasis in Africa, the Middle East, China, and India; L. infantum, the causative agent of visceral leishmaniasis affecting humans and dogs in Latin America-and lipopolysaccharide stimulated macrophages as an in vitro inflammation model. Our results revealed that Leishmania infection downregulates apoptosis pathways while upregulating the activation of phagocytes/leukocytes and lipid accumulation.

Crosstalk of PD-1 signaling with the SIRT1/FOXO-1 axis during the progression of visceral leishmaniasis

Previously, we documented the role of the programmed death-1 (PD-1, also known as PDCD1) pathway in macrophage apoptosis and the downregulation of this signaling during infection by the intra-macrophage parasite Leishmania donovani However, we also found that, during the late phase of infection, PD-1 expression was significantly increased without activating host cell apoptosis; here we show that inhibition of PD-1 led to markedly decreased parasite survival, along with increased production of TNFα, IL-12, reactive oxygen species (ROS) and nitric oxide (NO). Increased PD-1 led to inactivation of AKT proteins resulting in nuclear sequestration of FOXO-1. Transfecting infected cells with constitutively active FOXO-1 (CA-FOXO) led to increased cell death, thereby suggesting that nuclear FOXO-1 might be inactivated. Infection significantly induced the expression of SIRT1, which inactivated FOXO-1 through deacetylation, and its knockdown led to increased apoptosis. SIRT1 knockdown also significantly decreased parasite survival along with increased production of TNFα, ROS and NO. Administration of the SIRT1 inhibitor sirtinol (10 mg/kg body weight) in infected mice decreased spleen parasite burden and a synergistic effect was found with PD-1 inhibitor. Collectively, our study shows that Leishmania utilizes the SIRT1/FOXO-1 axis for differentially regulating PD-1 signaling and, although they are interconnected, both pathways independently contribute to intracellular parasite survival.This article has an associated First Person interview with the first author of the paper.

Cytokines: Key Determinants of Resistance or Disease Progression in Visceral Leishmaniasis: Opportunities for Novel Diagnostics and Immunotherapy

Leishmaniasis is a parasitic disease of humans, highly prevalent in parts of the tropics, subtropics, and southern Europe. The disease mainly occurs in three different clinical forms namely cutaneous, mucocutaneous, and visceral leishmaniasis (VL). The VL affects several internal organs and is the deadliest form of the disease. Epidemiology and clinical manifestations of VL are variable based on the vector, parasite (e.g., species, strains, and antigen diversity), host (e.g., genetic background, nutrition, diversity in antigen presentation and immunity) and the environment (e.g., temperature, humidity, and hygiene). Chemotherapy of VL is limited to a few drugs which is expensive and associated with profound toxicity, and could become ineffective due to the parasites developing resistance. Till date, there are no licensed vaccines for humans against leishmaniasis. Recently, immunotherapy has become an attractive strategy as it is cost-effective, causes limited side-effects and do not suffer from the downside of pathogens developing resistance. Among various immunotherapeutic approaches, cytokines (produced by helper T-lymphocytes) based immunotherapy has received great attention especially for drug refractive cases of human VL. Therefore, a comprehensive knowledge on the molecular interactions of immune cells or components and on cytokines interplay in the host defense or pathogenesis is important to determine appropriate immunotherapies for leishmaniasis. Here, we summarized the current understanding of a wide-spectrum of cytokines and their interaction with immune cells that determine the clinical outcome of leishmaniasis. We have also highlighted opportunities for the development of novel diagnostics and intervention therapies for VL.

Leishmania donovani Exploits Macrophage Heme Oxygenase-1 To Neutralize Oxidative Burst and TLR Signaling-Dependent Host Defense

Suppression of host oxidative burst is essential for survival of the intracellular parasite Leishmania donovani Screening of macrophage antioxidant enzymes during infection revealed marked upregulation of the heme-degrading enzyme, heme oxygenase-1 (HO-1). Moreover, HO-1-silenced RAW macrophages depicted increased superoxide production and decreased parasite survival. HO-1 induction decreased cellular heme content, thereby inhibiting the heme-dependent maturation of gp91phox, a catalytic component of major reactive oxygen species-producing enzyme NAD(P)H oxidase. Decreased gp91phox expression resulted in reduced stability of p22phox, another component of the catalytic center of NAD(P)H oxidase. Replenishing infected cells with exogenous heme reversed these effects and restored NAD(P)H oxidase activity. Persistent HO-1 expression at late hour of infection prompted us to investigate its effect on other host defense parameters, and inhibition study revealed a reciprocal relationship of HO-1 with host proinflammatory responses. Among all the HO-1-mediated heme degradation products (CO, Fe, and biliverdin), only CO documented potent anti-inflammatory effects. Quenching of CO during infection increased the production of disease-resolving cytokines IL-12 and TNF-α. Coimmunoprecipitation experiments revealed that CO inhibited the interaction of TLR4 with MyD88 and TIR domain-containing adapter-inducing IFN-β, thereby dampening the activation of NF-κB and IFN regulatory factor 3-mediated production of proinflammatory cytokines. Administration of HO-1 inhibitor tin protoporphyrin IX dichloride in infected BALB/c mice led to a decrease in liver and spleen parasite burden along with increased production of IL-12 and TNF-α. These results suggest that HO-1 on one hand inhibits reactive oxygen species generation and on the other hand downregulates host favorable cytokine responses, thereby facilitating intramacrophage parasite survival.

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.

Leishmania infantum lipophosphoglycan induced-Prostaglandin E2 production in association with PPAR-γ expression via activation of Toll like receptors-1 and 2

Lipophosphoglycan (LPG) is a key virulence factor expressed on the surfaces of Leishmania promastigotes. Although LPG is known to activate macrophages, the underlying mechanisms resulting in the production of prostaglandin E₂ (PGE₂) via signaling pathways remain unknown. Here, the inflammatory response arising from stimulation by Leishmania infantum LPG and/or its lipid and glycan motifs was evaluated with regard to PGE₂ induction. Intact LPG, but not its glycan and lipid moieties, induced a range of proinflammatory responses, including PGE₂ and nitric oxide (NO) release, increased lipid droplet formation, and iNOS and COX2 expression. LPG also induced ERK-1/2 and JNK phosphorylation in macrophages, in addition to the release of PGE₂, MCP-1, IL-6, TNF-α and IL-12p70, but not IL-10. Pharmacological inhibition of ERK1/2 and PKC affected PGE₂ and cytokine production. Moreover, treatment with rosiglitazone, an agonist of peroxisome proliferator-activated receptor gamma (PPAR-γ), also modulated the release of PGE₂ and other proinflammatory mediators. Finally, we determined that LPG-induced PPAR-γ signaling occurred via TLR1/2. Taken together, these results reinforce the role played by L. infantum-derived LPG in the proinflammatory response seen in Leishmania infection.

Leishmania donovani inhibits inflammasome-dependent macrophage activation by exploiting the negative regulatory proteins A20 and UCP2

In visceral leishmaniasis, we found that the antileishmanial drug Amp B produces a higher level of IL-1β over the infected control. Moreover, administering anti-IL-1β antibody to infected Amp B-treated mice showed significantly less parasite clearance. Investigation revealed that Leishmania inhibits stimuli-induced expression of a multiprotein signaling platform, NLRP3 inflammasome, which in turn inhibits caspase-1 activation mediated maturation of IL-1β from its pro form. Attenuation of NLRP3 and pro-IL-1β in infection was found to result from decreased NF-κB activity. Transfecting infected cells with constitutively active NF-κB plasmid increased NLRP3 and pro-IL-1β expression but did not increase mature IL-1β, suggesting that IL-1β maturation requires a second signal, which was found to be reactive oxygen species (ROS). Decreased NF-κB was attributed to increased expression of A20, a negative regulator of NF-κB signaling. Silencing A20 in infected cells restored NLRP3 and pro-IL-1β expression, but also increased matured IL-1β, implying an NF-κB-independent A20-modulated IL-1β maturation. Macrophage ROS is primarily regulated by mitochondrial uncoupling protein 2 (UCP2), and UCP2-silenced infected cells showed an increased IL-1β level. Short hairpin RNA-mediated knockdown of A20 and UCP2 in infected mice independently documented decreased liver and spleen parasite burden and increased IL-1β production. These results suggest that Leishmania exploits A20 and UCP2 to impair inflammasome activation for disease propagation.-Gupta, A. K., Ghosh, K., Palit, S., Barua, J., Das, P. K., Ukil, A. Leishmania donovani inhibits inflammasome-dependent macrophage activation by exploiting the negative regulatory proteins A20 and UCP2.

Mitogen-activated protein kinase phosphatase 1 (MKP-1) in macrophage biology and cardiovascular disease. A redox-regulated master controller of monocyte function and macrophage phenotype

MAPK pathways play a critical role in the activation of monocytes and macrophages by pathogens, signaling molecules and environmental cues and in the regulation of macrophage function and plasticity. MAPK phosphatase 1 (MKP-1) has emerged as the main counter-regulator of MAPK signaling in monocytes and macrophages. Loss of MKP-1 in monocytes and macrophages in response to metabolic stress leads to dysregulation of monocyte adhesion and migration, and gives rise to dysfunctional, proatherogenic monocyte-derived macrophages. Here we review the properties of this redox-regulated dual-specificity MAPK phosphatase and the role of MKP-1 in monocyte and macrophage biology and cardiovascular diseases.

In silico analysis and in vitro evaluation of immunogenic and immunomodulatory properties of promiscuous peptides derived from Leishmania infantum eukaryotic initiation factor

It is generally considered as imperative the ability to control leishmaniasis through the development of a protective vaccine capable of inducing long-lasting and protective cell-mediated immune responses. In this current study, we demonstrated potential epitopes that bind to H2 MHC class I and II molecules by conducting the in silico analysis of Leishmania infantum eukaryotic Initiation Factor (LieIF) protein, using online available algorithms. Moreover, we synthesized five peptides (16-18 amino acids long) which are part of the N-terminal portion of LieIF and contain promising MHC class I and II-restricted epitopes and afterwards, their predicted immunogenicity was evaluated in vitro by monitoring peptide-specific T-cell responses. Additionally, the immunomodulatory properties of these peptides were investigated in vitro by exploring their potential of inducing phenotypic maturation and functional differentiation of murine Bone-Marrow derived Dendritic Cells (BM-DCs). It was revealed by our data that all the synthetic peptides predicted for H2 alleles; present the property of immunogenicity. Among the synthetic peptides which contained T-cell epitopes, the peptide 52-68 aa (LieIF_2) exhibited immunomodulatory properties with the larger potential. LieIF_2-pulsed BM-DCs up-regulated the expression of the co-stimulatory surface molecules CD80 and CD86, as well as the production of the proinflammatory cytokine TNF-α and of the Th1-polarizing cytokines IL-12 and IFN-γ. The aforementioned data suggest that selected parts of LieIF could be used to develop innovative subunit protective vaccines able to induce effective immunity mediated by MHC class I-restricted as well as class II-restricted T-cell responses.

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 donovani inhibits macrophage apoptosis and pro-inflammatory response through AKT-mediated regulation of β-catenin and FOXO-1

In order to establish infection, intra-macrophage parasite Leishmania donovani needs to inhibit host defense parameters like inflammatory cytokine production and apoptosis. In the present study, we demonstrate that the parasite achieves both by exploiting a single host regulator AKT for modulating its downstream transcription factors, β-catenin and FOXO-1. L. donovani-infected RAW264.7 and bone marrow-derived macrophages (BMDM) treated with AKT inhibitor or dominant negative AKT constructs showed decreased anti-inflammatory cytokine production and increased host cell apoptosis resulting in reduced parasite survival. Infection-induced activated AKT triggered phosphorylation-mediated deactivation of its downstream target, GSK-3β. Inactivated GSK-3β, in turn, could no longer sequester cytosolic β-catenin, an anti-apoptotic transcriptional regulator, as evidenced from its nuclear translocation during infection. Constitutively active GSK-3β-transfected L. donovani-infected cells mimicked the effects of AKT inhibition and siRNA-mediated silencing of β-catenin led to disruption of mitochondrial potential along with increased caspase-3 activity and IL-12 production leading to decreased parasite survival. In addition to activating anti-apoptotic β-catenin, phospho-AKT inhibits activation of FOXO-1, a pro-apoptotic transcriptional regulator. Nuclear retention of FOXO-1, inhibited during infection, was reversed when infected cells were transfected with dominant negative AKT constructs. Overexpression of FOXO-1 in infected macrophages not only documented increased apoptosis but promoted enhanced TLR4 expression and NF-κB activity along with an increase in IL-1β and decrease in IL-10 secretion. In vivo administration of AKT inhibitor significantly decreased liver and spleen parasite burden and switched cytokine balance in favor of host. In contrast, GSK-3β inhibitor did not result in any significant change in infectivity parameters. Collectively our findings revealed that L. donovani triggered AKT activation to regulate GSK-3β/β-catenin/FOXO-1 axis, thus ensuring inhibition of both host cell apoptosis and immune response essential for its intra-macrophage survival.

Leishmania donovani Utilize Sialic Acids for Binding and Phagocytosis in the Macrophages through Selective Utilization of Siglecs and Impair the Innate Immune Arm

Background

Leishmania donovani, belonging to a unicellular protozoan parasite, display the differential level of linkage-specific sialic acids on their surface. Sialic acids binding immunoglobulin-like lectins (siglecs) are a class of membrane-bound receptors present in the haematopoetic cell lineages interact with the linkage-specific sialic acids. Here we aimed to explore the utilization of sialic acids by Leishmania donovani for siglec-mediated binding, phagocytosis, modulation of innate immune response and signaling pathways for establishment of successful infection in the host.

Methodology/Principle Findings

We have found enhanced binding of high sialic acids containing virulent strains (AG83^+Sias) with siglec-1 and siglec-5 present on macrophages compared to sialidase treated AG83^+Sias (AG83^-Sias) and low sialic acids-containing avirulent strain (UR6) by flow cytometry. This specific receptor-ligand interaction between sialic acids and siglecs were further confirmed by confocal microscopy. Sialic acids-siglec-1-mediated interaction of AG83^+Sias with macrophages induced enhanced phagocytosis. Additionally, sialic acids-siglec-5 interaction demonstrated reduced ROS, NO generation and Th2 dominant cytokine response upon infection with AG83^+Sias in contrast to AG83^-Sias and UR6. Sialic acids-siglecs binding also facilitated multiplication of intracellular amastigotes. Moreover, AG83^+Sias induced sialic acids-siglec-5-mediated upregulation of host phosphatase SHP-1. Such sialic acids-siglec interaction was responsible for further downregulation of MAPKs (p38, ERK and JNK) and PI3K/Akt pathways followed by the reduced translocation of p65 subunit of NF-κβ to the nucleus from cytosol in the downstream signaling pathways. This sequence of events was reversed in AG83^-Sias and UR6-infected macrophages. Besides, siglec-knockdown macrophages also showed the reversal of AG83^+Sias infection-induced effector functions and downstream signaling events.

Conclusions/Significances

Taken together, this study demonstrated that virulent parasite (AG83^+Sias) establish a unique sialic acids-mediated binding and subsequent phagocytosis in the host cell through the selective exploitation of siglec-1. Additionally, sialic acids-siglec-5 interaction altered the downstream signaling pathways which contributed impairment of immune effector functions of macrophages. To the best of our knowledge, this is a comprehensive report describing sialic acids-siglec interactions and their role in facilitating uptake of the virulent parasite within the host.

Sialic acids are nine carbon sugars present on terminal cell surface glycoproteins and glycolipids. Siglec is a membrane receptor that belongs to an immunoglobulin super family present in almost all the haematopoetic cell lineages. There are 14 different types of siglecs present on human immune cells that take an active part in balancing the magnitude of immunological reactions. In general, these siglecs bind with sialic acids and negatively regulate the immune response. Leishmania contains sialic acids on its surface. Virulent parasites utilize this sugar to bind with macrophages through siglec-1 and siglec-5 compared to low sialic acids containing avirulent parasites. Such sialic acids-siglec-mediated interactions exhibited a suppressed host immune response which helped them to establish successful infection compared to desialylated virulent and avirulent parasites, as well as, siglec-depleted macrophages. Interestingly, interaction between sialic acids and siglec-1 induced enhanced phagocytosis, while sialic acids-siglec-5 interaction upregulated the phosphatase SHP-1. This interaction with the virulent strain exhibited deactivation of various downstream signaling pathways and ultimately controlled translocation of a functional component of transcription factor NF-κβ for regulation of cytokines and other effector molecules in infected macrophages. Thus, the interaction between the parasite and the host cells through sialic acids-siglec binding is clearly a newly identified mechanism by which parasites can establish successful infection by subverting the host’s innate immune response.

Leishmania donovani Exploits Myeloid Cell Leukemia 1 (MCL-1) Protein to Prevent Mitochondria-dependent Host Cell Apoptosis

Apoptosis is one of the mechanisms used by host cells to remove unwanted intracellular organisms, and often found to be subverted by pathogens through use of host anti-apoptotic proteins. In the present study, with the help of in vitro and in vivo approaches, we documented that the macrophage anti-apoptotic protein myeloid cell leukemia 1 (MCL-1) is exploited by the intra-macrophage parasite Leishmania donovani to protect their "home" from actinomycin D-induced mitochondria-dependent apoptosis. Among all the anti-apoptotic BCL-2 family members, infection preferentially up-regulated expression of MCL-1 at both the mRNA and protein levels and compared with infected control, MCL-1-silenced infected macrophages documented enhanced caspase activity and increased apoptosis when subjected to actinomycin D treatment. Phosphorylation kinetics and ChIP assay demonstrated that infection-induced MCL-1 expression was regulated by transcription factor CREB (cAMP-response element-binding protein) and silencing of CREB resulted in reduced expression of MCL-1 and increased apoptosis. During infection, MCL-1 was found to be localized in mitochondria and this was significantly reduced in Tom70-silenced macrophages, suggesting the active role of TOM70 in MCL-1 transport. In the mitochondria, MCL-1 interacts with the major pro-apoptotic protein BAK and prevents BAK-BAK homo-oligomer formation thereby preventing cytochrome c release-mediated mitochondrial dysfunction. Silencing of MCL-1 in the spleen of infected mice showed decreased parasite burden and increased induction of splenocyte apoptosis. Collectively our results showed that L. donovani exploited the macrophage anti-apoptotic protein MCL-1 to prevent BAK-mediated mitochondria-dependent apoptosis thereby protecting its niche, which is essential for disease progression.

Leptin induces the phagocytosis and protective immune response in Leishmania donovani infected THP-1 cell line and human PBMCs

Visceral leishmaniasis (VL) is an infectious disease responsible for several deaths in malnourished children due to impaired cell-mediated immunity, which is accompanied by low circulating leptin levels. The cytokine function of leptin is implicated for several immune regulation activities such as hematopoiesis, angiogenesis, innate and adaptive immunity. Its deficiency associated with polarization of Th2 response, which coincides with VL pathogenesis. To determine the cytokine role of leptin in case of experimental VL, we tested the leptin associated Th1/Th2 type cytokine profile at mRNA level from Leishmania donovani infected human monocytic leukemia cell line (THP-1) and peripheral blood mononuclear cells (PBMCs). We also tested the effect of leptin on macrophages activation (viz. studying the phosphorylation of signaling moieties), phagocytic activity and intracellular reactive oxygen species (ROS) production during infection. We observed that leptin induced Th1 specific response by upregulation of IL-1α, IL-1β, IL-8 and TNF-α in THP-1 and IFN-γ, IL-12 and IL-2 in PBMCs. We also observed the downregulation of Th2 type cytokine i.e. IL-10 in THP-1 and unaltered expression of cytokines i.e. TGF-β, IL-10 and IL-4 in PBMCs. In addition, leptin stimulates the macrophages by inducing phosphorylation of Erk1/2 and Akt which are usually dephosphorylated in L. donovani infection. In concordance, leptin also induces the macrophage phagocytic activity by enhancing the intracellular ROS generation which helps in phagolysosome formation and oxidative killing of the parasite. In compilation, leptin is able to maintain the defensive environment against L. donovani infection through the classical macrophage activity.

Trypanosoma cruzi parasites fight for control of the JAK-STAT pathway by disarming their host

The zoonotic Chagas' disease is caused by infections with the hemoflagellate Trypanosoma cruzi (T. cruzi) which is endemic in Latin America. Despite recent advances in our understanding of the pathogenesis of the disease, the underlying molecular processes involved in host-parasite interactions are only poorly understood. In particular, the mechanisms for parasite persistence in host cells remain largely unknown. Cytokine-driven transcription factors from the family of STAT (signal transducer and activator of transcription) proteins appear to play a central role in the fight against T. cruzi infection. However, amastigotes proliferating in the cytoplasm of infected host cells develop effective strategies to circumvent the attack executed by STAT proteins. This review highlights the interactions between T. cruzi parasites and human host cells in terms of cytokine signaling and, in particular, discusses the impact of STATs on the balance between parasite invasion and clearance.

Prostaglandin E2 negatively regulates the production of inflammatory cytokines/chemokines and IL-17 in visceral leishmaniasis

Persistence of intracellular infection depends on the exploitation of factors that negatively regulate the host immune response. In this study, we elucidated the role of macrophage PGE2, an immunoregulatory lipid, in successful survival of Leishmania donovani, causative agent of the fatal visceral leishmaniasis. PGE2 production was induced during infection and resulted in increased cAMP level in peritoneal macrophages through G protein-coupled E-series prostanoid (EP) receptors. Among four different EPs (EP1-4), infection upregulated the expression of only EP2, and individual administration of either EP2-specific agonist, butaprost, or 8-Br-cAMP, a cell-permeable cAMP analog, promoted parasite survival. Inhibition of cAMP also induced generation of reactive oxygen species, an antileishmanial effector molecule. Negative modulation of PGE2 signaling reduced infection-induced anti-inflammatory cytokine polarization and enhanced inflammatory chemokines, CCL3 and CCL5. Effect of PGE2 on cytokine and chemokine production was found to be differentially modulated by cAMP-dependent protein kinase A (PKA) and exchange protein directly activated by cAMP (EPAC). PGE2-induced decreases in TNF-α and CCL5 were mediated specifically by PKA, whereas administration of brefeldin A, an EPAC inhibitor, could reverse decreased production of CCL3. Apart from modulating inflammatory/anti-inflammatory balance, PGE2 inhibited antileishmanial IL-17 cytokine production in splenocyte culture. Augmented PGE2 production was also found in splenocytes of infected mice, and administration of EP2 antagonist in mice resulted in reduced liver and spleen parasite burden along with host-favorable T cell response. These results suggest that Leishmania facilitates an immunosuppressive environment in macrophages by PGE2-driven, EP2-mediated cAMP signaling that is differentially regulated by PKA and EPAC.

Leishmania eukaryotic initiation factor (LeIF) inhibits parasite growth in murine macrophages

The leishmaniases constitute neglected global public health problems that require adequate control measures, prophylactic clinical vaccines and effective and non-toxic drug treatments. In this study, we explored the potential of Leishmania infantum eukaryotic initiation factor (LieIF), an exosomal protein, as a novel anti-infective therapeutic molecule. More specifically, we assessed the efficacy of recombinant LieIF, in combination with recombinant IFN-γ, in eliminating intracellular L. donovani parasites in an in vitro macrophage model. J774A.1 macrophages were initially treated with LieIF/IFN-γ prior to in vitro infection with L. donovani stationary phase promastigotes (pre-infection treatment), and resistance to infection was observed 72 h after infection. J774A.1 macrophages were also treated with LieIF/IFN-γ after L. donovani infection (post-infection treatment), and resistance to infection was also observed at both time points tested (19 h and 72 h) after infection. To elucidate the LieIF/IFN-γ-induced mechanism(s) that mediate the reduction of intracellular parasite growth, we examined the generation of potent microbicidal molecules, such as nitric oxide (NO) and reactive oxygen species (ROS), within infected macrophages. Furthermore, macrophages pre-treated with LieIF/IFN-γ showed a clear up-regulation in macrophage inflammatory protein 1α (MIP-1α) as well as tumor necrosis factor alpha (TNF-α) expression. However, significant different protein levels were not detected. In addition, macrophages pre-treated with LieIF/IFN-γ combined with anti-TNF-α monoclonal antibody produced significantly lower amounts of ROS. These data suggest that during the pre-treatment state, LieIF induces intramacrophage parasite growth inhibition through the production of TNF-α, which induces microbicidal activity by stimulating NO and ROS production. The mechanisms of NO and ROS production when macrophages are treated with LieIF after infection are probably different. Overall, these results indicate that LieIF is a good candidate for use as an anti-leishmanial molecule.

Combination of liposomal CpG oligodeoxynucleotide 2006 and miltefosine induces strong cell-mediated immunity during experimental visceral leishmaniasis

Immuno-modulators in combination with antileishmanial drug miltefosine is a better therapeutic approach for treatment of Visceral Leishmaniasis (VL) as it not only reduces the dose of miltefosine but also shortens the treatment regimen. However, immunological mechanisms behind the perceived benefits of this combination therapy have not been investigated in detail. In the present study, we hypothesized that potential use of drugs that target the host in addition to the parasite might represent an alternative strategy for combination therapy. We investigated immune responses generated in Leishmania donovani infected animals (hamsters and mice) treated with combination of CpG-ODN-2006 and miltefosine at short dose regimen. Infected animals were administered CpG-ODN-2006 (0.4 mg/kg, single dose), as free and liposomal form, either alone or in combination with miltefosine for 5 consecutive days and parasite clearance was evaluated at day 4 and 7 post treatment. Animals that received liposomal CpG-ODN-2006 (lipo-CpG-ODN-2006) and sub-curative miltefosine (5 mg/kg) showed the best inhibition of parasite multiplication (∼97%) which was associated with a biased Th1 immune response in. Moreover, compared to all the other treated groups, we observed increased mRNA expression levels of pro-inflammatory cytokines (IFN-γ, TNF-α and IL-12) and significantly suppressed levels of Th2 cytokines (IL-10 and TGF-β) on day 4 post treatment in animals that underwent combination therapy with lipo-CpG-ODN-2006 and sub-curative miltefosine. Additionally, same therapy also induced heightened iNOS mRNA levels and NO generation, increased IgG2 antibody level and strong T-cell response in these hamsters compared with all the other treated groups. Collectively, our results suggest that combination of lipo-CpG-ODN-2006 and sub-curative miltefosine generates protective T-cell response in an animal model of visceral leishmaniasis which is characterized by strong Th1 biased immune response thereby underlining our hypothesis that combination therapy, at short dose regimen can be used as a novel way of treating visceral leishmaniasis.

Activity of anti-cancer protein kinase inhibitors against Leishmania spp

OBJECTIVES

There is an urgent need to develop new and effective treatments for poverty-related neglected diseases. In light of the time required to bring a new drug to market and the cost involved (10-15 years, >1 billion US$), one approach to identifying new treatments for diseases like leishmaniasis is to evaluate drugs that are already registered for the treatment of other diseases. This paper describes the anti-leishmanial activities of 10 FDA-approved protein kinase inhibitors already available for the treatment of human cancers.

METHODS

In vitro and in vivo models of Leishmania infection were used to evaluate the potency of selected protein kinase inhibitors.

RESULTS

Sunitinib, sorafenib and lapatinib were identified as active against Leishmania donovani amastigotes in cultured murine macrophages with IC(50) values of 1.1, 3.7 and 2.5 μM, respectively, a level of potency similar to that of miltefosine (IC(50) = 1.0 μM), and were not toxic to mammalian cells. In addition, some of the protein kinase inhibitors were active against L. donovani in the BALB/c mouse model of infection; dosing on days 7-11 with a 50 mg/kg oral dose of sunitinib, lapatinib or sorafenib reduced liver amastigote burdens by 41%, 36% and 30%, respectively, compared with untreated control mice. Although less efficacious, sorafenib was also active in vitro against intracellular amastigotes of the cutaneous disease-causing species Leishmania amazonensis, Leishmania major and Leishmania mexicana.

CONCLUSIONS

This study demonstrates in vivo anti-leishmanial activity of clinically used protein kinase inhibitors and provides further evidence of the potential of drug repurposing.

The curative effect of fucoidan on visceral leishmaniasis is mediated by activation of MAP kinases through specific protein kinase C isoforms

Fucoidan can cure both antimony-sensitive and antimony-resistant visceral leishmaniasis through immune activation. However, the signaling events underlying this cellular response remain uncharacterized. The present study reveals that fucoidan induces activation of p38 and ERK1/2 and NF-κB DNA binding in both normal and Leishmania donovani-infected macrophages, as revealed by western blotting and electrophoretic mobility shift assay (EMSA), respectively. Pharmacological inhibition of p38, ERK1/2 or the NF-κB pathway markedly attenuated fucoidan-induced pro-inflammatory cytokine synthesis and inducible nitric oxide synthase (iNOS) gene transcription, resulting in a reduction of parasite clearance. To decipher the underlying mechanism of fucoidan-mediated parasite suppression, the expression and functionality of various protein kinase C (PKC) isoforms were evaluated by immunoblotting and enzyme activity assay. Fucoidan elicited an increase in expression and activity of PKC-α, -βI and -βII isoforms in infected macrophages. Functional knockdown of PKC-α and -β resulted in downregulation of p38 and ERK1/2, along with a marked reduction of IL-12 and TNF-α production in fucoidan-treated infected macrophages. Collectively, these results suggest that the curative effect of fucoidan is mediated by PKC-dependent activation of the mitogen-activated protein kinase (MAPK)/NF-κB pathway, which ultimately results in the production of nitric oxide (NO) and disease-resolving pro-inflammatory cytokines.

Effects of P-MAPA immunomodulator on Toll-like receptor 2, ROS, nitric oxide, MAPKp38 and IKK in PBMC and macrophages from dogs with visceral leishmaniasis

Leishmania (L.) chagasi is the etiologic agent of visceral leishmaniasis (VL) that can be transmitted to humans and dogs. VL in Brazil represents a serious public health problem; therefore, it is important to study new alternatives to treat infected dogs. In dogs, the therapeutic arsenal against canine VL is limited. The immunomodulator protein aggregate magnesium-ammonium phospholinoleate-palmitoleate anhydride (P-MAPA) improves immunocompetence when the immune system is impaired, but its dependence on Toll-like receptors (TLRs) and the mechanisms involved in immune response remain unclear. The in vitro action of P-MAPA on the expression of TLR2 and TLR4, reactive oxygen species (ROS), nitric oxide (NO) and p38 mitogen-activated protein kinase (p38 MAPK) and IKK phosphorylation was studied in mononuclear cells from peripheral blood and macrophages from healthy and Leishmania-infected dogs. The PBMC or macrophages were isolated and cultured with different concentrations of P-MAPA (20,100 and 200 μg/ml) in a humid environment at 37°C with 5% CO(2). Observation revealed that Leishmania-infected dogs showed a decrease in TLR2 in macrophages compared with healthy dogs and in induction with P-MAPA. ROS were increased in PBMCs from Leishmania spp.-infected dogs compared with healthy dogs and P-MAPA improved ROS production. NO production was increased in culture supernatant from macrophages stimulated by P-MAPA in both healthy and Leishmania spp. infected dogs. Treatment of macrophages from healthy dogs with immunomodulatory P-MAPA induced p38 MAPK and IKK phosphorylation, suggesting signal transduction by this pathway. These findings suggest that P-MAPA has potential as a therapeutic drug in the treatment of canine visceral leishmaniasis.

Role of MKP-1 (DUSP1) in clozapine-induced effects on the ERK1/2 signaling pathway in the rat frontal cortex

RATIONALE

Clozapine affects the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway in the brain, which plays an important role in its antipsychotic action. However, previous findings are inconsistent, and related molecular mechanisms require further clarification.

OBJECTIVES

Time- and dose-dependent effects of clozapine on the ERK1/2 pathway and its regulatory mechanism were investigated in rat frontal cortex.

METHODS AND RESULTS

At 15, 30, 60, and 120 min after intraperitoneal injection of clozapine (5, 10, and 20 mg/kg), changes in ERK1/2, its upstream canonical kinases (Raf1 and mitogen-activated protein kinase kinase 1/2 [MEK1/2]), and its downstream molecule (p90 ribosomal S6 kinase [p90RSK]) were investigated in rat frontal cortex. At 15 min, p-Raf1, p-MEK1/2, p-ERK1/2, and p-p90RSK all increased dose-dependently. At 30 min, p-ERK1/2 and p-p90RSK showed no significant changes, while dose-dependent increases in p-Raf1 and p-MEK1/2 were found. At 60 and 120 min, although p-ERK1/2 and p-p90RSK decreased, increases in p-Raf1 and p-MEK1/2 were maintained. A clozapine-induced reduction in ERK1/2 phosphorylation was evident at both tyrosine and threonine residues, suggesting the involvement of dual specificity phosphatases (DUSPs; mitogen-activated protein kinase phosphatases [MKPs]). mRNA expression of seven Dusps that can dephosphorylate ERK1/2 were examined; Mkp-1 (Dusp1) mRNA increased following clozapine treatment. Moreover, MKP-1 protein and phosphatase activity increased, and binding of MKP-1 to ERK1/2 was also upregulated by clozapine administration.

CONCLUSIONS

In rat frontal cortex, clozapine regulates ERK1/2 phosphorylation via MKP-1, which induces uncoupling between Raf1-MEK1/2 and ERK1/2-p90RSK activity. These findings suggest an important role of MKP-1 in the mechanism of action of clozapine.

Oxidative stress induces inactivation of protein phosphatase 2A, promoting proinflammatory NF-κB in aged rat kidney

The molecular inflammation hypothesis of aging proposes that redox dysregulation causes an age-related activation of NF-κB and its signaling to upregulate various proinflammatory genes. In the present study, we focused on the inactive form of the protein phosphastase 2A (PP2A). More specifically, we aimed to define the correlation between PP2A inactivation and NF-κB activation by age-related oxidative stress. Experimentations were designed to determine the effect of oxidative stress-induced PP2A inactivation on NF-κB activity, utilizing prooxidants t-BHP and AAPH, the PTP inhibitor Na3VO4, and the PP2A inhibitor Calyculin A and PP2A siRNA, in HEK293T cells. We also assessed the phosphorylation of PP2A catalytic subunit (PP2Ac) and the activities of PP2A and NF-κB in aged rat kidney, utilizing aging-retarding 40% calorie restriction (CR) -60% of food intake and inflammation-triggering LPS paradigms. Results revealed that an oxidative stress-induced PTK/PTP imbalance led to phosphorylation of PP2Ac, following exposures to t-BHP, AAPH, and Na3VO4 in HEK293T cells. Subsequently, we found that Calyculin A and PP2A siRNA activates NIK/IKK and MAPKs, leading to upregulation of NF-κB and its dependent oxidative stress. Also, the contrasting relation between PP2A inactivation and NF-κB activation was confirmed by AAPH-induced oxidative status in mice, and non-induced normal status or LPS-induced inflammatory status in aged rats while the antioxidative, anti-inflammatory, anti-aging effects of CR significantly blunted these actions. Thus, we present evidence that PP2A inactivation via PTK/PTP imbalance provoked by oxidative stress causes NF-κB activation, which contributes to the accumulation of oxidative stress in aged rat kidney.

Successful therapy of visceral leishmaniasis with curdlan involves T-helper 17 cytokines

The aim of this study was to evaluate and characterize the therapeutic potential of curdlan, a naturally occurring β-glucan immunomodulator, against visceral leishmaniasis, a fatal parasitic disease. Curdlan eliminated the liver and spleen parasite burden in a 45-day BALB/c mouse model of visceral leishmaniasis at a dosage of 10 mg/kg/day as determined by Giemsa-stained organ impression smears. Curdlan was associated with production of the disease-resolving T-helper (Th) 1 and Th17-inducing cytokines interleukin (IL)-6, IL-1β, and IL-23, as well as with production of Th17 cytokines IL-17 and IL-22, as determined by enzyme-linked immunosorbent assay (ELISA) and real time polymerase chain reaction (RT-PCR). Reversal of curdlan-mediated protection by anti-IL-17 and anti-IL-23 monoclonal antibodies showed the importance of Th17 cytokines. Significantly decreased production of both IL-17 and IL-22 by mice that received anti-IL-23 antibody suggested the essential role of IL-23 in Th17 differentiation. Although administration of recombinant IL-17 or IL-23 caused significant suppression of the organ parasite burden, with marked generation of interferon γ and nitric oxide (NO), effects were much faster for IL-17. These results documented that although both IL-23 and IL-17 play major roles in the antileishmanial effect of curdlan, the effect of IL-23 may occur indirectly, through the induction of IL-17 production.

Immunopathogenesis of non-healing American cutaneous leishmaniasis and progressive visceral leishmaniasis

The outcomes of Leishmania infection are determined by host immune and nutrition status, parasite species, and co-infection with other pathogens. While subclinical infection and self-healing cutaneous leishmaniasis (CL) are common, uncontrolled parasite replication can lead to non-healing local lesions or visceral leishmaniasis (VL). It is known that infection control requires Th1-differentiation cytokines (IL-12, IL-18, and IL-27) and Th1 cell and macrophage activation. However, there is no generalized consensus for the mechanisms of host susceptibility. The recent studies on regulatory T cells and IL-17-producing cells help explain the effector T cell responses that occur independently of the known Th1/Th2 cell signaling pathways. This review focuses on the immunopathogenesis of non-healing American CL and progressive VL. We summarize recent evidence from human and animal studies that reveals the mechanisms of dysregulated, hyper-responses to Leishmania braziliensis, as well as the presence of disease-promoting or the absence of protective responses to Leishmania amazonensis and Leishmania donovani. We highlight immune-mediated parasite growth and immunopathogenesis, with an emphasis on the putative roles of IL-17 and its related cytokines as well as arginase. A better understanding of the quality and regulation of innate immunity and T cell responses triggered by Leishmania will aid in the rational control of pathology and the infection.

Leishmania donovani exploits host deubiquitinating enzyme A20, a negative regulator of TLR signaling, to subvert host immune response

TLRs, which form an interface between mammalian host and microbe, play a key role in pathogen recognition and initiation of proinflammatory response thus stimulating antimicrobial activity and host survival. However, certain intracellular pathogens such as Leishmania can successfully manipulate the TLR signaling, thus hijacking the defensive strategies of the host. Despite the presence of lipophosphoglycan, a TLR2 ligand capable of eliciting host-defensive cytokine response, on the surface of Leishmania, the strategies adopted by the parasite to silence the TLR2-mediated proinflammatory response is not understood. In this study, we showed that Leishmania donovani modulates the TLR2-mediated pathway in macrophages through inhibition of the IKK-NF-κB cascade and suppression of IL-12 and TNF-α production. This may be due to impairment of the association of TRAF6 with the TAK-TAB complex, thus inhibiting the recruitment of TRAF6 in TLR2 signaling. L. donovani infection drastically reduced Lys 63-linked ubiquitination of TRAF6, and the deubiquitinating enzyme A20 was found to be significantly upregulated in infected macrophages. Small interfering RNA-mediated silencing of A20 restored the Lys 63-linked ubiquitination of TRAF6 as well as IL-12 and TNF-α levels with a concomitant decrease in IL-10 and TGF-β synthesis in infected macrophages. Knockdown of A20 led to lower parasite survival within macrophages. Moreover, in vivo silencing of A20 by short hairpin RNA in BALB/c mice led to increased NF-κB DNA binding and host-protective proinflammatory cytokine response resulting in effective parasite clearance. These results suggest that L. donovani might exploit host A20 to inhibit the TLR2-mediated proinflammatory gene expression, thus escaping the immune responses of the host.

Impact of Leishmania metalloprotease GP63 on macrophage signaling

The intramacrophage protozoan parasites of Leishmania genus have developed sophisticated ways to subvert the innate immune response permitting their infection and propagation within the macrophages of the mammalian host. Several Leishmania virulence factors have been identified and found to be of importance for the development of leishmaniasis. However, recent findings are now further reinforcing the critical role played by the zinc-metalloprotease GP63 as a virulence factor that greatly influence host cell signaling mechanisms and related functions. GP63 has been found to be involved not only in the cleavage and degradation of various kinases and transcription factors, but also to be the major molecule modulating host negative regulatory mechanisms involving for instance protein tyrosine phosphatases (PTPs). Those latter being well recognized for their pivotal role in the regulation of a great number of signaling pathways. In this review article, we are providing a complete overview about the role of Leishmania GP63 in the mechanisms underlying the subversion of macrophage signaling and functions.

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.

Mitogen-activated protein kinase phosphatase (MKP)-1 in immunology, physiology, and disease

Mitogen-activated protein kinases (MAPKs) are key regulators of cellular physiology and immune responses, and abnormalities in MAPKs are implicated in many diseases. MAPKs are activated by MAPK kinases through phosphorylation of the threonine and tyrosine residues in the conserved Thr-Xaa-Tyr domain, where Xaa represents amino acid residues characteristic of distinct MAPK subfamilies. Since MAPKs play a crucial role in a variety of cellular processes, a delicate regulatory network has evolved to control their activities. Over the past two decades, a group of dual specificity MAPK phosphatases (MKPs) has been identified that deactivates MAPKs. Since MAPKs can enhance MKP activities, MKPs are considered as an important feedback control mechanism that limits the MAPK cascades. This review outlines the role of MKP-1, a prototypical MKP family member, in physiology and disease. We will first discuss the basic biochemistry and regulation of MKP-1. Next, we will present the current consensus on the immunological and physiological functions of MKP-1 in infectious, inflammatory, metabolic, and nervous system diseases as revealed by studies using animal models. We will also discuss the emerging evidence implicating MKP-1 in human disorders. Finally, we will conclude with a discussion of the potential for pharmacomodulation of MKP-1 expression.

Curative effect of 18β-glycyrrhetinic acid in experimental visceral leishmaniasis depends on phosphatase-dependent modulation of cellular MAP kinases

We earlier showed that 18β-glycyrrhetinic acid (GRA), a pentacyclic triterpenoid from licorice root, could completely cure visceral leishmaniasis in BALB/c mouse model. This was associated with induction of nitric oxide and proinflammatory cytokine production through the up regulation of NF-κB. In the present study we tried to decipher the underlying cellular mechanisms of the curative effect of GRA. Analysis of MAP kinase pathways revealed that GRA caused strong activation of p38 and to a lesser extent, ERK in bone marrow-derived macrophages (BMDM). Almost complete abrogation of GRA-induced cytokine production in presence of specific inhibitors of p38 and ERK1/2 confirmed the involvement of these MAP kinases in GRA-mediated responses. GRA induced mitogen- and stress-activated protein kinase (MSK1) activity in a time-dependent manner suggested that GRA-mediated NF-κB transactivation is mediated by p38, ERK and MSK1 pathway. As kinase/phosphatase balance plays an important role in modulating infection, the effect of GRA on MAPK directed phosphatases (MKP) was studied. GRA markedly reduced the expression and activities of three phosphatases, MKP1, MKP3 and protein phosphatase 2A (PP2A) along with a substantial reduction of p38 and ERK dephosphorylation in infected BMDM. Similarly in the in vivo situation, GRA treatment of L. donovani-infected BALB/c mice caused marked reduction of spleen parasite burden associated with concomitant decrease of individual phosphatase levels. However, activation of kinases also played an important role as the protective effect of GRA was significantly abrogated by pharmacological inhibition of p38 and ERK pathway. Curative effect of GRA may, therefore, be associated with restoration of proper cellular kinase/phosphatase balance, rather than modulation of either kinases or phosphatases.

Attenuation of the acute inflammatory response by dual specificity phosphatase 1 by inhibition of p38 MAP kinase

Dual specificity phosphatase 1 (DUSP1) dephosphorylates and, hence, regulates the activity of MAP kinases. The present study investigated the effect of DUSP1 on inflammatory gene expression and on the development of carrageenan-induced inflammation. It was found that DUSP1 expression was increased by LPS, and the down-regulation of DUSP1 by siRNA enhanced the phosphorylation of p38 MAPK, while JNK phosphorylation was not affected in murine macrophages. LPS-induced interleukin (IL)-6, tumor-necrosis factor (TNF) and cyclooxygenase-2 (COX2) expression were enhanced in bone marrow-derived macrophages (BMMs) from DUSP1(-/-) mice as compared to those from wild-type mice. In addition, down-regulation of DUSP1 by siRNA enhanced IL-6, TNF and COX2 expression in J774 macrophages, while p38 MAPK inhibitors SB202190 and BIRB 796 inhibited the expression of those inflammatory factors. In vivo, the intensity of the carrageenan-induced paw edema reaction was increased in DUSP1(-/-) mice as compared to the wild-type animals. In conclusion, DUSP1 is an important negative regulator of the acute inflammatory response by limiting p38 MAPK, and compounds which enhance DUSP1 expression or activity may hold a promise as anti-inflammatory drugs.

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.

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.