Cutting edge: STAT1 and T-bet play distinct roles in determining outcome of visceral leishmaniasis caused by Leishmania donovani

The role of CD4+ T cells in visceral leishmaniasis; new and emerging roles for NKG7 and TGFβ

Visceral leishmaniasis is a potentially devastating neglected tropical disease caused by the protozoan parasites Leishmania donovani and L. infantum (chagasi). These parasites reside in tissue macrophages and survive by deploying a number of mechanisms aimed at subverting the host immune response. CD4+ T cells play an important role in controlling Leishmania parasites by providing help in the form of pro-inflammatory cytokines to activate microbiocidal pathways in infected macrophages. However, because these cytokines can also cause tissue damage if over-produced, regulatory immune responses develop, and the balance between pro-inflammatory and regulatory CD4+ T cells responses determines the outcomes of infection. Past studies have identified important roles for pro-inflammatory cytokines such as IFNγ and TNF, as well as regulatory co-inhibitory receptors and the potent anti-inflammatory cytokine IL-10. More recently, other immunoregulatory molecules have been identified that play important roles in CD4+ T cell responses during VL. In this review, we will discuss recent findings about two of these molecules; the NK cell granule protein Nkg7 and the anti-inflammatory cytokine TGFβ, and describe how they impact CD4+ T cell functions and immune responses during visceral leishmaniasis.

Altered IL-7 signaling in CD4+ T cells from patients with visceral leishmaniasis

BACKGROUND

CD4+ T cells play a central role in control of L. donovani infection, through IFN-γ production required for activation of macrophages and killing of intracellular parasites. Impaired control of parasites can in part be explained by hampered CD4+ T cells effector functions in visceral leishmaniasis (VL) patients. In a recent studies that defined transcriptional signatures for CD4+ T cells from active VL patients, we found that expression of the IL-7 receptor alpha chain (IL-7Rα; CD127) was downregulated, compared to CD4+ T cells from endemic controls (ECs). Since IL-7 signaling is critical for the survival and homeostatic maintenance of CD4+ T cells, we investigated this signaling pathway in VL patients, relative to ECs.

METHODS

CD4+ T cells were enriched from peripheral blood collected from VL patients and EC subjects and expression of IL7 and IL7RA mRNA was measured by real time qPCR. IL-7 signaling potential and surface expression of CD127 and CD132 on CD4+ T cell was analyzed by multicolor flow cytometry. Plasma levels of soluble IL-7 and sIL-7Rα were measured by ELISA.

RESULT

Transcriptional profiling data sets generated previously from our group showed lower IL7RA mRNA expression in VL CD4+ T cells as compared to EC. A significant reduction was, however not seen when assessing IL7RA mRNA by RT-qPCR. Yet, the levels of soluble IL-7Rα (sIL-7Rα) were reduced in plasma of VL patients compared to ECs. Furthermore, the levels of soluble IL-7 were higher in plasma from VL patients compared to ECs. Interestingly, expression of the IL-7Rα protein was higher on VL patient CD4+ T cells as compared to EC, with activated CD38+ CD4+ T cells showing higher surface expression of IL-7Rα compared to CD38- CD4+ T cells in VL patients. CD4+ T cells from VL patients had higher signaling potential baseline and after stimulation with recombinant human IL-7 (rhIL-7) compared to EC, as measured by phosphorylation of STAT5 (pSTAT5). Interestingly, it was the CD38 negative cells that had the highest level of pSTAT5 in VL patient CD4+ T cells after IL-7 stimulation. Thus, despite unaltered or potentially lowered IL7RA mRNA expression by CD4+ T cells from VL patients, the surface expression of the IL-7Rα was higher compared to EC and increased pSTAT5 was seen following exposure to rhIL-7. Accordingly, IL-7 signaling appears to be functional and even enhanced in VL CD4+ T cells and cannot explain the impaired effector function of VL CD4+ T cells. The enhanced plasma IL-7 may serve as part of homeostatic feedback mechanism regulating IL7RA expression in CD4+ T cells.

The Preclinical Validation of 405 nm Light Parasiticidal Efficacy on Leishmania donovani in Ex Vivo Platelets in a Rag2-/- Mouse Model

Violet-blue light of 405 nm in the visible spectrum at a dose of 270 J/cm2 alone has been shown to be an effective microbicidal tool for inactivating several bacteria, HIV-1, and Trypanosoma cruzi in ex vivo plasma and platelets. Unlike chemical- and ultraviolet (UV)-based pathogen inactivation methods for plasma and platelet safety, 405 nm light is shown to be less toxic to host cells at light doses that are microbicidal. In this report, we evaluated the parasiticidal activity of a 405 nm light treatment on platelets spiked with the Leishmania donovani parasite. Following the light treatment, parasite viability was observed to be near zero in both low- and high-titer-spiked platelets relative to controls. Furthermore, to test the residual infectivity after inactivation in vivo, the light-treated low-titer L. donovani-spiked platelets were evaluated in an immunodeficient Rag2-/- mouse model and monitored for 9 weeks. The parasiticidal efficacy of 405 nm light was evident from the lack of a presence of parasites in the mice spleens. Parasiticidal activity was confirmed to be mediated through 405 nm light-induced reactive oxygen species (ROS), as quantitatively measured by a 2',7'-Dichlorodihydrofluorescein diacetate (H2DCFDA)-based assay. Overall, these results confirm the complete inactivation of L. donovani spiked in ex vivo platelets by 405 nm light treatment and exemplify the utility of the Rag2-/- mouse infection model for the preclinical validation of the parasiticidal efficacy of 405 nm light and this light-based technology as a potential PRT for ex vivo platelets.

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.

Immunotherapy for visceral leishmaniasis: A trapeze of balancing counteractive forces

The protozoan parasite Leishmania donovani, residing and replicating within the cells of the monocyte-macrophage (mono-mac) lineage, causes visceral leishmaniasis (VL) in humans. While, Leishmania infantum, is the main causative agent for zoonotic VL, where dogs are the main reservoirs of the disease. The chemotherapy is a serious problem because of restricted repertoire of drugs, drug-resistant parasites, drug-toxicity and the requirement for parenteral administration, which is a problem in resource-starved countries. Moreover, immunocompromised individuals, particularly HIV-1 infected are at higher risk of VL due to impairment in T-helper cell and regulatory cell responses. Furthermore, HIV-VL co-infected patients report poor response to conventional chemotherapy. Recent efforts are therefore directed towards devising both prophylactic and therapeutic immunomodulation. As far as prophylaxis is concerned, although canine vaccines for the disease caused by Leishmania infantum or Leishmania chagasi are available, no vaccine is available for use in humans till date. Therefore, anti-leishmanial immunotherapy triggering or manipulating the host's immune response is gaining momentum during the last two decades. Immunomodulators comprised of small molecules, anti-leishmanial peptides, complex ligands for host receptors, cytokines or their agonists and antibodies have been given trials both in experimental models and in humans. However, the success of immunotherapy in humans remains a far-off target. We, therefore, propose that devising a successful immunotherapy is an act of balancing enhanced beneficial Leishmania-specific responses and deleterious immune activation/hyperinflammation just as the swings in a trapeze.

Determinants of Innate Immunity in Visceral Leishmaniasis and Their Implication in Vaccine Development

Leishmaniasis is endemic to the tropical and subtropical regions of the world and is transmitted by the bite of an infected sand fly. The multifaceted interactions between Leishmania, the host innate immune cells, and the adaptive immunity determine the severity of pathogenesis and disease development. Leishmania parasites establish a chronic infection by subversion and attenuation of the microbicidal functions of phagocytic innate immune cells such as neutrophils, macrophages and dendritic cells (DCs). Other innate cells such as inflammatory monocytes, mast cells and NK cells, also contribute to resistance and/or susceptibility to Leishmania infection. In addition to the cytokine/chemokine signals from the innate immune cells, recent studies identified the subtle shifts in the metabolic pathways of the innate cells that activate distinct immune signal cascades. The nexus between metabolic pathways, epigenetic reprogramming and the immune signaling cascades that drive the divergent innate immune responses, remains to be fully understood in Leishmania pathogenesis. Further, development of safe and efficacious vaccines against Leishmaniasis requires a broader understanding of the early interactions between the parasites and innate immune cells. In this review we focus on the current understanding of the specific role of innate immune cells, the metabolomic and epigenetic reprogramming and immune regulation that occurs during visceral leishmaniasis, and the strategies used by the parasite to evade and modulate host immunity. We highlight how such pathways could be exploited in the development of safe and efficacious Leishmania vaccines.

Monocyte dysregulation: consequences for hepatic infections

Liver disorders due to infections are a substantial health concern in underdeveloped and industrialized countries. This includes not only hepatotropic viruses (e.g., hepatitis B, hepatitis C) but also bacterial and parasitic infections such as amebiasis, leishmaniasis, schistosomiasis, or echinococcosis. Recent studies of the immune mechanisms underlying liver disease show that monocytes play an essential role in determining patient outcomes. Monocytes are derived from the mononuclear phagocyte lineage in the bone marrow and are present in nearly all tissues of the body; these cells function as part of the early innate immune response that reacts to challenge by external pathogens. Due to their special ability to develop into tissue macrophages and dendritic cells and to change from an inflammatory to an anti-inflammatory phenotype, monocytes play a pivotal role in infectious and non-infectious liver diseases: they can maintain inflammation and support resolution of inflammation. Therefore, tight regulation of monocyte recruitment and termination of monocyte-driven immune responses in the liver is prerequisite to appropriate healing of organ damage. In this review, we discuss monocyte-dependent immune mechanisms underlying hepatic infectious disorders. Better understanding of these immune mechanisms may lead to development of new interventions to treat acute liver disease and prevent progression to organ failure.

Type I Interferons Suppress Anti-parasitic Immunity and Can Be Targeted to Improve Treatment of Visceral Leishmaniasis

Type I interferons (IFNs) play critical roles in anti-viral and anti-tumor immunity. However, they also suppress protective immune responses in some infectious diseases. Here, we identify type I IFNs as major upstream regulators of CD4⁺ T cells from visceral leishmaniasis (VL) patients. Furthermore, we report that mice deficient in type I IFN signaling have significantly improved control of Leishmania donovani, a causative agent of human VL, associated with enhanced IFNγ but reduced IL-10 production by parasite-specific CD4⁺ T cells. Importantly, we identify a small-molecule inhibitor that can be used to block type I IFN signaling during established infection and acts synergistically with conventional anti-parasitic drugs to improve parasite clearance and enhance anti-parasitic CD4⁺ T cell responses in mice and humans. Thus, manipulation of type I IFN signaling is a promising strategy for improving disease outcome in VL patients.

Human T-bet Governs Innate and Innate-like Adaptive IFN-γ Immunity against Mycobacteria

Inborn errors of human interferon gamma (IFN-γ) immunity underlie mycobacterial disease. We report a patient with mycobacterial disease due to inherited deficiency of the transcription factor T-bet. The patient has extremely low counts of circulating Mycobacterium-reactive natural killer (NK), invariant NKT (iNKT), mucosal-associated invariant T (MAIT), and Vδ2+ γδ T lymphocytes, and of Mycobacterium-non reactive classic TH1 lymphocytes, with the residual populations of these cells also producing abnormally small amounts of IFN-γ. Other lymphocyte subsets develop normally but produce low levels of IFN-γ, with the exception of CD8+ αβ T and non-classic CD4+ αβ TH1∗ lymphocytes, which produce IFN-γ normally in response to mycobacterial antigens. Human T-bet deficiency thus underlies mycobacterial disease by preventing the development of innate (NK) and innate-like adaptive lymphocytes (iNKT, MAIT, and Vδ2+ γδ T cells) and IFN-γ production by them, with mycobacterium-specific, IFN-γ-producing, purely adaptive CD8+ αβ T, and CD4+ αβ TH1∗ cells unable to compensate for this deficit.

Role of host genetics and cytokines in Leishmania infection

Cytokines and chemokines are important regulators of innate and specific responses in leishmaniasis, a disease that currently affects 12 million people. We overviewed the current information about influences of genetically engineered mouse models of cytokine and chemokine on leishmaniasis. We found that genetic background of the host, parasite species and sub-strain, as well as experimental design often modify effects of genetically engineered cytokine genes. Next we analyzed genes and QTLs (quantitative trait loci) that control response to Leishmania species in mouse in order to establish relationship between genetic control of cytokine expression and organ pathology. These studies revealed a network-like complexity of the combined effects of the multiple functionally diverse QTLs and their individual specificity. Genetic control of organ pathology and systemic immune response overlap only partially. Some QTLs control both organ pathology and systemic immune response, but the effects of genes and loci with the strongest impact on disease are cytokine-independent, whereas several loci modify cytokines levels in serum without influencing organ pathology. Understanding this genetic control might be important in development of vaccines designed to stimulate certain cytokine spectrum.

Conundrums in leishmaniasis

Parasites of the genus Leishmania cause the disease leishmaniasis. As the sandfly vector transfers the promastigotes into the skin of the human host, the infection is either cured or exacerbated. In the process, there emerge several unsolved paradoxes of leishmaniasis. Chronologically, as the infections starts in skin, the role of the salivary proteins in supporting the infection or the host response to these proteins influencing the induction of immunological memory becomes a conundrum. As the parasite invokes inflammation, the infiltrating neutrophils may act as "Trojan Horse" to transfer parasites to macrophages that, along with dendritic cells, carry the parasite to lymphoid organs to start visceralization. As the visceralized infection becomes chronic, the acutely enhanced monocytopoiesis takes a downturn while neutropenia and thrombocytopenia ensue with concomitant rise in splenic colony-forming-units. These responses are accompanied by splenic and hepatic granulomas, polyclonal activation of B cells and deviation of T cell responses. The granuloma formation is both a containment process and a form of immunopathogenesis. The heterogeneity in neutrophils and macrophages contribute to both cure and progression of the disease. The differentiation of T-helper subsets presents another paradox of visceral leishmaniasis, as the counteractive T cell subsets influence the curing or non-curing outcome. Once the parasites are killed by chemotherapy, in some patients the cured visceral disease recurs as a cutaneous manifestation post-kala azar dermal leishmaniasis (PKDL). As no experimental model exists, the natural history of PKDL remains almost a black box at the end of the visceral disease.

LmjMAPK10 offers protection against Leishmania donovani infection

AIMS

This study aimed at evaluating the DNA vaccination efficacy of Leishmania major-derived MAPK10 against Leishmania donovani infection.

METHODS AND RESULTS

MAPK10 is one of the 15 mitogen-activated protein kinases (MAPKs) of Leishmania major. Herein, we expressed the gene through a mammalian vector and tested whether priming with this gene would offer protection against L donovani infection. We report that LmjMAPK10 DNA vaccination using a mammalian expression vector significantly reduces the parasite burden. The protection is accompanied by host-protective T-cell functions, T_H 1-type cytokines and elevated leishmanial antigen-specific IgG2a isotype response. T-cell response to the L donovani/challenge infection is associated with increase in IL-12 and IFN-γ, but reduced IL-10 and IL-4 production.

CONCLUSIONS

LmjMAPK10 is cross-protective against L donovani infection.

T-bet optimizes CD4 T-cell responses against influenza through CXCR3-dependent lung trafficking but not functional programming

Although clearance of many intracellular pathogens requires T-bet-dependent CD4 T cell programming, the extent to which T-bet is needed to direct protective CD4 responses against influenza is not known. Here, we characterize wild-type and T-bet-deficient CD4 cells during murine influenza infection. Surprisingly, although T-bet expression has broad impacts on cytokine production by virus-specific CD4 cells, the protective efficacy of T-bet-deficient effector cells is only marginally reduced. This reduction is due to lower CXCR3 expression, leading to suboptimal accumulation of activated T-bet-deficient cells in the infected lung. However, T-bet-deficient cells outcompete wild-type cells to form lung-resident and circulating memory populations following viral clearance, and primed T-bet-deficient mice efficiently clear supralethal heterosubtypic influenza challenges even when depleted of CD8 T cells. These results are relevant to the identification of more incisive correlates of protective T cells and for vaccines that aim to induce durable cellular immunity against influenza.

Interleukin 2 is an Upstream Regulator of CD4+ T Cells From Visceral Leishmaniasis Patients With Therapeutic Potential

Control of visceral leishmaniasis (VL) caused by Leishmania donovani requires interferon-γ production by CD4+ T cells. In VL patients, antiparasitic CD4+ T-cell responses are ineffective for unknown reasons. In this study, we measured the expression of genes associated with various immune functions in these cells from VL patients and compared them to CD4+ T cells from the same patients after drug treatment and from endemic controls. We found reduced GATA3, RORC, and FOXP3 gene expression in CD4+ T cells of VL patients, associated with reduced Th2, Th17, and FOXP3+CD4+ T regulatory cell frequencies in VL patient blood. Interleukin 2 (IL-2) was an important upstream regulator of CD4+ T cells from VL patients, and functional studies demonstrated the therapeutic potential of IL-2 for improving antiparasitic immunity. Together, these results provide new insights into the characteristics of CD4+ T cells from VL patients that can be used to improve antiparasitic immune responses.

Type I Interferon Signaling Is Required for CpG-Oligodesoxynucleotide-Induced Control of Leishmania major, but Not for Spontaneous Cure of Subcutaneous Primary or Secondary L. major Infection

We previously showed that in mice infected with Leishmania major type I interferons (IFNs) initiate the innate immune response to the parasite at day 1 and 2 of infection. Here, we investigated which type I IFN subtypes are expressed during the first 8 weeks of L. major infection and whether type I IFNs are essential for a protective immune response and clinical cure of the disease. In self-healing C57BL/6 mice infected with a high dose of L. major, IFN-α4, IFN-α5, IFN-α11, IFN-α13, and IFN-β mRNA were most prominently regulated during the course of infection. In C57BL/6 mice deficient for IFN-β or the IFN-α/β-receptor chain 1 (IFNAR1), development of skin lesions and parasite loads in skin, draining lymph node, and spleen was indistinguishable from wild-type (WT) mice. In line with the clinical findings, C57BL/6 IFN-β^−/−, IFNAR1^−/−, and WT mice exhibited similar mRNA expression levels of IFN-γ, interleukin (IL)-4, IL-12, IL-13, inducible nitric oxide synthase, and arginase 1 during the acute and late phase of the infection. Also, myeloid dendritic cells from WT and IFNAR1^−/− mice produced comparable amounts of IL-12p40/p70 protein upon exposure to L. major in vitro. In non-healing BALB/c WT mice, the mRNAs of IFN-α subtypes (α2, α4, α5, α6, and α9) were rapidly induced after high-dose L. major infection. However, genetic deletion of IFNAR1 or IFN-β did not alter the progressive course of infection seen in WT BALB/c mice. Finally, we tested whether type I IFNs and/or IL-12 are required for the prophylactic effect of CpG-oligodesoxynucleotides (ODN) in BALB/c mice. Local and systemic administration of CpG-ODN 1668 protected WT and IFN-β^−/− mice equally well from progressive leishmaniasis. By contrast, the protective effect of CpG-ODN 1668 was lost in BALB/c IFNAR1^−/− (despite a sustained suppression of IL-4) and in BALB/c IL-12p35^−/− mice. From these data, we conclude that IFN-β and IFNAR1 signaling are dispensable for a curative immune response to L. major in C57BL/6 mice and irrelevant for disease development in BALB/c mice, whereas IL-12 and IFN-α subtypes are essential for the disease prevention by CpG-ODNs in this mouse strain.

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.

Ly6Chi inflammatory monocytes promote susceptibility to Leishmania donovani infection

Ly6C^hi inflammatory monocytes (iMO) are critical for host defense against toxoplasmosis and malaria but their role in leishmaniasis is unclear. In this study, we report a detrimental role of Ly6C^hi iMOs in visceral leishmaniasis (VL) caused by Leishmania donovani. We demonstrate that Ly6C^hi iMOs are continuously recruited into the spleen and liver during L. donovani infection and they are preferential targets for the parasite. Using microarray-based gene expression profiling, we show that Ly6C^hi iMOs isolated from the infected liver and spleen have distinct phenotypic and activation profiles. Furthermore, we demonstrate that blocking the recruitment of Ly6C^hi iMOs into the liver and spleen during L. donovani infection using a CCR2 antagonist reduces the frequency of the pathogenic IFN-γ/IL10 dual producer CD4+ T cells in the spleen and leads to a significant reduction in parasite loads in the liver and spleen. Using STAT1−/− mice we show that STAT1 is critical for mediating the recruitment of Ly6C^hi iMOs into organs during L. donovani infection, and adaptive transfer of wild type Ly6C^hi iMOs into STAT1−/− recipients renders them susceptible to disease. Our findings reveal an unexpected pathogenic role for Ly6C^hi iMOs in promoting parasite survival in VL and open the possibility of targeting this population for host-directed therapy during VL.

STAT1-dependent and -independent pulmonary allergic and fibrogenic responses in mice after exposure to tangled versus rod-like multi-walled carbon nanotubes

BACKGROUND

Pulmonary toxicity of multi-walled carbon nanotubes (MWCNTs) is influenced by physicochemical characteristics and genetic susceptibility. We hypothesized that contrasting rigidities of tangled (t) versus rod-like (r) MWCNTs would result in differing immunologic or fibrogenic responses in mice and that these responses would be exaggerated in transgenic mice lacking the signal transducer and activator of transcription-1 (STAT1), a susceptible mouse model of pulmonary fibrosis.

METHODS

Male wild type (Stat1 ^+/+ ) and STAT1-deficient (Stat1 ^-/- ) mice were exposed to 4 mg/kg tMWCNTs, rMWCNTs, or vehicle alone via oropharyngeal aspiration and evaluated for inflammation at one and 21 days post-exposure via histopathology, differential cell counts, and cytokine levels in bronchoalveolar lavage fluid (BALF). Granuloma formation, mucous cell metaplasia, and airway fibrosis were evaluated by quantitative morphometry. Airway epithelial cell proliferation was assessed by bromodeoxyuridine (BrdU) incorporation. Cytokine protein levels in BALF and serum IgE levels were measured by ELISA. Lung protein Smad2/3 levels and activation were measured by Western blot. Lung mRNAs were measured by PCR.

RESULTS

There was a 7-fold difference in rigidity between tMWCNTs and rMWCNTs as determined by static bending ratio. Both MWCNT types resulted in acute inflammation (neutrophils in BALF) after one-day post-exposure, yet only rMWCNTs resulted in chronic inflammation at 21 days as indicated by neutrophil influx and larger granulomas. Both MWCNTs induced BrdU uptake in airway epithelial cells, with the greatest proliferative response observed in rMWCNT-exposed mice after one-day. Only rMWCNTs induced mucous cell metaplasia, but this index was not different between genotypes. Stat1 ^-/- mice had higher levels of baseline serum IgE than Stat1 ^+/+ mice. Greater airway fibrosis was observed with rMWCNTs compared to tMWCNTs, and exaggerated airway fibrosis was seen in the Stat1 ^-/- mouse lungs with rMWCNTs but not tMWCNTs. Increased fibrosis correlated with elevated levels of TGF-β1 protein levels in the BALF of Stat1 ^-/- mice exposed to rMWCNTs and increased lung Smad2/3 phosphorylation.

CONCLUSIONS

Rigidity plays a key role in the toxicity of MWCNTs and results in increased inflammatory, immunologic, and fibrogenic effects in the lung. STAT1 is an important protective factor in the fibroproliferative response to rMWCNTs, regulating both induced TGF-β1 production and Smad2/3 phosphorylation status. Therefore, both rigidity and genetic susceptibility should be major considerations for risk assessment of MWCNTs.

Gamma Interferon-Regulated Chemokines in Leishmania donovani Infection in the Liver

In the livers of C57BL/6 mice, gamma interferon (IFN-γ) controls intracellular Leishmania donovani infection and the efficacy of antimony (Sb) chemotherapy. Since both responses usually correlate with granulomatous inflammation, we tested six prominently expressed, IFN-γ-regulated chemokines-CXCL9, CXCL10, CXCL13, CXCL16, CCL2, and CCL5-for their roles in (i) mononuclear cell recruitment and granuloma assembly and maturation, (ii) initial control of infection and self-cure, and (iii) responsiveness to Sb treatment. Together, the results for the L. donovani-infected livers of chemokine-deficient mice (CXCR6^-/- mice were used as CXCL16-deficient surrogates) indicated that individual IFN-γ-induced chemokines have diverse affects and (i) may be entirely dispensable (CXCL13, CXCL16), (ii) may promote (CXCL10, CCL2, CCL5) or downregulate (CXCL9) initial granuloma assembly, (iii) may enhance (CCL2, CCL5) or hinder (CXCL10) early parasite control, (iv) may promote granuloma maturation (CCL2, CCL5), (v) may exert a granuloma-independent action that enables self-cure (CCL5), and (vi) may have no role in responsiveness to chemotherapy. Despite the near absence of tissue inflammation in early-stage infection, parasite replication could be controlled (in CXCL10^-/- mice) and Sb was fully active (in CXCL10^-/-, CCL2^-/-, and CCL5^-/- mice). These results characterize chemokine action in the response to L. donovani and also reemphasize that (i) recruited mononuclear cells and granulomas are not required to control infection or respond to Sb chemotherapy, (ii) granuloma assembly, control of infection, and Sb's efficacy are not invariably linked expressions of the same T cell-dependent, cytokine-mediated antileishmanial mechanism, and (iii) granulomas are not necessarily hallmarks of protective antileishmanial immunity.

Regulation of immunity during visceral Leishmania infection

Unicellular eukaryotes of the genus Leishmania are collectively responsible for a heterogeneous group of diseases known as leishmaniasis. The visceral form of leishmaniasis, caused by L. donovani or L. infantum, is a devastating condition, claiming 20,000 to 40,000 lives annually, with particular incidence in some of the poorest regions of the world. Immunity to Leishmania depends on the development of protective type I immune responses capable of activating infected phagocytes to kill intracellular amastigotes. However, despite the induction of protective responses, disease progresses due to a multitude of factors that impede an optimal response. These include the action of suppressive cytokines, exhaustion of specific T cells, loss of lymphoid tissue architecture and a defective humoral response. We will review how these responses are orchestrated during the course of infection, including both early and chronic stages, focusing on the spleen and the liver, which are the main target organs of visceral Leishmania in the host. A comprehensive understanding of the immune events that occur during visceral Leishmania infection is crucial for the implementation of immunotherapeutic approaches that complement the current anti-Leishmania chemotherapy and the development of effective vaccines to prevent disease.

Lessons from other diseases: granulomatous inflammation in leishmaniasis

The Leishmania granuloma shares some, though not all, properties with that formed following mycobacterial infection. As a simplified, noncaseating granuloma composed of relatively few and largely mononuclear cell populations, it provides a tractable model system to investigate intra-granuloma cellular dynamics, immune regulation, and antimicrobial resistance. Here, the occurrence of granulomatous pathology across the spectrum of leishmaniasis, in humans and animal reservoir hosts, is first described. However, this review focuses on the process of hepatic granuloma formation as studied in rodent models of visceral leishmaniasis, starting from the initial infection of Kupffer cells to the involution of the granuloma after pathogen clearance. It describes how the application of intravital imaging and the use of computational modeling have changed some of our thoughts on granuloma function, and illustrates how host-directed therapies have been used to manipulate granuloma form and function for therapeutic benefit. Where appropriate, lessons that may be equally applicable across the spectrum of granulomatous diseases are highlighted.

IL-17A promotes susceptibility during experimental visceral leishmaniasis caused by Leishmania donovani

Leishmania donovani is an intracellular parasite that infects professional phagocytes and causes visceral leishmaniasis (VL). The immune response during VL has been extensively studied in the context of T-helper (Th)1 and Th2 responses. Immunity against this parasite is dependent on IFN-γ production and subsequent macrophage activation, and the Th2 response promotes granuloma formation. The cytokine IL-17A is associated with neutrophilic inflammation. Depletion of neutrophils during experimental VL results in enhanced parasitic loads. Furthermore, although patients resistant to VL showed enhanced levels of IL-17A in circulation, little is known about the role of IL-17A during VL infection. Here, we used IL-17A-deficient mice and IL-17A reporter mice to address the role of IL-17A during VL. IL-17A(-/-) mice were highly resistant to VL infection, showing decreased parasites in the liver and spleen. This unexpected phenotype was associated with enhanced IFN-γ production by T cells and decreased accumulation of neutrophils and monocytes, resulting in reduced number of granulomas. We also found γδ T and Th17 cells as the main IL-17A(+) cells during VL infection. Our data reveal an unexpected role of IL-17A rendering susceptibility against L. donovani by regulating the IFN-γ response and promoting detrimental inflammation.

Signal transducer and activator of transcription 1 (STAT-1) plays a critical role in control of Trypanosoma cruzi infection

The control of Trypanosoma cruzi infection is related to interferon-γ (IFN-γ) activation leading to intracellular clearance of parasites. The transcription factor signal transducer and activator of transcription 1 (STAT-1) is a key mediator of IFN-γ intracellular signalling and knockout of this protein leads to susceptibility to several intracellular microbes. To determine the role of STAT-1 in host susceptibility to T. cruzi infection we compared the survival, parasite loads and balance of IFN-γ and interleukin-10 (IL-10) responses between wild-type and STAT-1 knockout mice. We found that the lack of STAT-1 resulted in a more robust infection, leading to higher levels of blood and tissue parasites and markedly reduced survival. In addition, infected STAT-1 knockout mice had higher systemic levels of both IFN-γ and IL-10, suggesting that the absence of STAT-1 leads to a disequilibrium of pro-inflammatory and anti-inflammatory cytokines. Analysis of spleen cells indicates that CD4, CD8 cells generate IFN-γ and natural killer cells express IL-13 in STAT-1 knockout animals. The production of IL-17 is particularly enhanced in the absence STAT-1 expression but did not reduce mortality. Overall these results indicate that STAT-1 is important for the control of T. cruzi infection in mice.

The host-protective effect of arabinosylated lipoarabinomannan against Leishmania donovani infection is associated with restoration of IFN-γ responsiveness

Visceral leishmaniasis (VL), which is endemic as a major infectious disease in the tropical and subtropical countries, is caused by a protozoan parasite Leishmania donovani. At present, restricted treatment options and lack of vaccines intensify the problem of controlling VL. Therefore, finding a novel immunoprophylactic or therapeutic principle is a pressing need. Here, we report that arabinosylated lipoarabinomannan (Ara-LAM), a TLR2-ligand isolated from Mycobacterium smegmatis, exhibits a strong immunomodulatory property that conferred protection against L. donovani infection. Although, Ara-LAM modulates TLR2 and MAPK signaling, it is not known whether Ara-LAM involves IFN-γ signaling for effective parasite clearance. Because, it is reported that IFN-γ signaling, a principle mediator of NO generation and macrophage and Tcell activation, is hampered during leishmanial pathogenesis. Ara-LAM increases IFN-γ receptor expression and potentiates IFN-γ receptor signaling through JAK-STAT pathway. Moreover, Ara-LAM reciprocally modulates IRF4 and IRF8 expression and reinstates anti-leishmanial Th1 response that eventuates in significantly reduced parasite load in spleen and liver of L. donovani-infected BALB/c mice. IFN-γRα silencing resulted in the suppression of these host-protective mechanisms affected by Ara-LAM. Thus, Ara-LAM-mediated restoration of IFN-γ responsiveness is a novel immuno-modulatory principle for protection against L. donovani susceptible host.

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.

Deficiency of p110δ isoform of the phosphoinositide 3 kinase leads to enhanced resistance to Leishmania donovani

Background

Visceral leishmaniasis is the most clinically relevant and dangerous form of human leishmaniasis. Most traditional drugs for treatment of leishmaniasis are toxic, possess many adverse reactions and drug resistance is emerging. Therefore, there is urgent need for identification of new therapeutic targets. Recently, we found that mice with an inactivating knock-in mutation in the p110δ isoform of pi3k, (p110δ^d910a) are hyper resistant to L. major, develop minimal cutaneous lesion and rapidly clear their parasite. Here, we investigated whether pi3k signaling also regulates resistance to L. donovani, one of the causative agents of visceral leishmaniasis.

Methodology/Principal Findings

WT and p110δ^D910A mice (on a BALB/c background) were infected with L. donovani. At different time points, parasite burden and granuloma formation were assessed. T and B cell responses in the liver and spleen were determined. In addition, Tregs were expanded in vivo and its impact on resistance was assessed. We found that p110δ^D910A mice had significantly reduced splenomegaly and hepatomegaly and these organs harbored significantly fewer parasites than those of WT mice. Interestingly, infected p110δ^D910A mice liver contains fewer and less organized granulomas than their infected WT counterparts. Cells from p110δ^D910A mice were significantly impaired in their ability to produce cytokines compared to WT mice. The percentage and absolute numbers of Tregs in infected p110δ^D910A mice were lower than those in WT mice throughout the course of infection. In vivo expansion of Tregs in infected p110δ^D910A mice abolished their enhanced resistance to L. donovani infection.

Conclusions/Significance

Our results indicate that the enhanced resistance of p110δ^D910A mice to L. donovani infection is due to impaired activities of Tregs. They further show that resistance to Leishmania in the absence of p110δ signaling is independent of parasite species, suggesting that targeting the PI3K signaling pathway may be useful for treatment of both visceral and cutaneous leishmaniasis.

Visceral leishmaniasis (VL) is the most dangerous form of human leishmaniasis in terms of mortality and morbidity and is spreading to several non-endemic areas because of global traveling and military conflicts. The emergence of Leishmania-HIV coinfection and increased prevalence of drug resistant strains have compounded an already bad situation. In addition, the drugs available are toxic, expensive and have several side effects. Therefore, a detailed understanding of protective immune response is extremely important in order to identify new therapeutic targets. The phosphoinositide 3 kinase (PI3K) family of enzymes mediate several important immunologic and physiologic cellular process including proliferation, differentiation, growth and host defense. We previously showed that genetic inactivation of the p110δ isoform of PI3K results in resistant to L. major (the causative agent of cutaneous leishmaniasis (CL)). Here, we investigate the role of PI3K in immunity to VL and the mechanisms underlying its protective effect. Collectively, our results demonstrate that signaling via the p110δ also regulates immunity to L. donovani, an effect that is dependent on the impact of p110δ signaling on expansion and function of regulatory T cells in vivo. Thus, our studies suggest that targeting the p110δ pathway may be a novel therapeutic strategy for controlling VL and CL.

T-bet regulates immunity to Francisella tularensis live vaccine strain infection, particularly in lungs

Upregulation of the transcription factor T-bet is correlated with the strength of protection against secondary challenge with the live vaccine strain (LVS) of Francisella tularensis. Thus, to determine if this mediator had direct consequences in immunity to LVS, we examined its role in infection. Despite substantial in vivo gamma interferon (IFN-γ) levels, T-bet-knockout (KO) mice infected intradermally (i.d.) or intranasally (i.n.) with LVS succumbed to infection with doses 2 log units less than those required for their wild-type (WT) counterparts, and exhibited significantly increased bacterial burdens in the lung and spleen. Lungs of LVS-infected T-bet-KO mice contained fewer lymphocytes and more neutrophils and interleukin-17 than WT mice. LVS-vaccinated T-bet-KO mice survived lethal LVS intraperitoneal secondary challenge but not high doses of LVS i.n. challenge, independently of the route of vaccination. Immune T lymphocytes from the spleens of i.d. LVS-vaccinated WT or KO mice controlled intracellular bacterial replication in an in vitro coculture system, but cultures with T-bet-KO splenocyte supernatants contained less IFN-γ and increased amounts of tumor necrosis factor alpha. In contrast, immune T-bet-KO lung lymphocytes were greatly impaired in controlling intramacrophage growth of LVS; this functional defect is the likely mechanism underpinning the lack of respiratory protection. Taken together, T-bet is important in host resistance to primary LVS infection and i.n. secondary challenge. Thus, T-bet represents a true, useful correlate for immunity to LVS.

STAT4 is critical for immunity but not for antileishmanial activity of antimonials in experimental visceral leishmaniasis

We and others have previously shown that IL-12 is indispensable for immunity and is required for the optimal antiparasitic activity of antimonials in experimental visceral leishmaniasis caused by Leishmania donovani. Here we investigated the role of STAT4 in immunity against L. donovani using STAT4 knockout mice and also determined the effect of STAT4 deficiency in response to antimonial therapy. Upon infection with L. donovani, stat4⁻/⁻ BALB/c and C57BL/6 mice showed enhanced susceptibility to Leishmania during late time points of infection which was associated with a marked reduction in Th1 responses and hepatic immunopathology. Interestingly, these defects in Th1 responses in stat4⁻/⁻ did not impair the antimonial chemotherapy as both stat4⁻/⁻ and WT mice showed comparable levels of parasite clearance from the liver and spleen. These findings highlight the role of STAT4 in immunity to L. donovani infection and also provide evidence that STAT4 is dispensable for antimonial-based chemotherapy.

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.

Inborn errors of human STAT1: allelic heterogeneity governs the diversity of immunological and infectious phenotypes

The genetic dissection of various human infectious diseases has led to the definition of inborn errors of human STAT1 immunity of four types, including (i) autosomal recessive (AR) complete STAT1 deficiency, (ii) AR partial STAT1 deficiency, (iii) autosomal dominant (AD) STAT1 deficiency, and (iv) AD gain of STAT1 activity. The two types of AR STAT1 defect give rise to a broad infectious phenotype with susceptibility to intramacrophagic bacteria (mostly mycobacteria) and viruses (herpes viruses at least), due principally to the impairment of IFN-γ-mediated and IFN-α/β-mediated immunity, respectively. Clinical outcome depends on the extent to which the STAT1 defect decreases responsiveness to these cytokines. AD STAT1 deficiency selectively predisposes individuals to mycobacterial disease, owing to the impairment of IFN-γ-mediated immunity, as IFN-α/β-mediated immunity is maintained. Finally, AD gain of STAT1 activity is associated with autoimmunity, probably owing to an enhancement of IFN-α/β-mediated immunity. More surprisingly, it is also associated with chronic mucocutaneous candidiasis, through as yet undetermined mechanisms involving an inhibition of the development of IL-17-producing T cells. Thus, germline mutations in human STAT1 define four distinct clinical disorders. Various combinations of viral, mycobacterial and fungal infections are therefore allelic at the human STAT1 locus. These experiments of Nature neatly highlight the clinical and immunological impact of the human genetic dissection of infectious phenotypes.

Interferon regulatory factor 7 contributes to the control of Leishmania donovani in the mouse liver

Optimal hepatic resistance to Leishmania donovani in mice requires the coordinated effort of a variety of leukocyte populations that together induce activation of local macrophages to a leishmanicidal state. Although nitric oxide and reactive oxygen intermediates are potent leishmanicidal effector molecules operating in the acquired phase of immunity, there have long been suggestions that other mechanisms of leishmanicidal activity exist. We recently discovered that Irf-7 regulates a novel innate leishmanicidal response in resident splenic macrophages that line the marginal zone. Here, we tested whether this mechanism also operates in Kupffer cells, the resident macrophage population of the liver and the major target for hepatic infection by L. donovani. Comparing the Kupffer cell responses in situ in B6 and B6.Irf-7(-/-) mice, we found no evidence that Irf-7 affected amastigote uptake or early survival. However, we did find that Irf-7-deficient mice had impaired acquired resistance to hepatic L. donovani infection. This phenotype was attributable to a reduction in the capacity of hepatic CD4(+) T cells, NK cells, and NKT cells to produce gamma interferon (IFN-γ) and also to defective induction of NOS2 in infected Kupffer cells. Our data therefore add interferon regulatory factor 7 (IRF-7) to the growing list of interferon regulatory factors that have effects on downstream events in the acquired cellular immune response to nonviral pathogens.

Cytokine and transcription factor profiles in the skin of dogs naturally infected by Leishmania (Leishmania) chagasi presenting distinct cutaneous parasite density and clinical status

The immune response in the skin of dogs infected with Leishmania chagasi and its association with distinct levels of tissue parasitism and clinical progression of canine visceral leishmaniasis (CVL) are poorly understood and limited studies are available. A detailed analysis of the profiles of cytokines (IFN-γ, IL-4, IL-5, IL-10, IL-12, IL-13, TGF-β1 and TNF-α) and transcription factors (T-bet, GATA-3 and FOXP3) in the skin of 35 naturally infected dogs was carried out using real-time PCR alongside determinations of skin parasite density and the clinical status of CVL. A mixed cytokine profile with high levels of expression of IFN-γ, TNF-α and IL-13 was determined in asymptomatic dogs. Additionally, the levels of transcription factors GATA-3 and FOXP3 were correlated with the asymptomatic disease. A mixed cytokine profile was also observed during active CVL. Moreover, high levels of IL-10 and TGF-β1, concomitant with the low expression of IL-12, may represent a key condition that allows persistence of parasite replication in the skin. The results obtained indicate that in asymptomatic disease or lower levels of skin parasite density, a mixed inflammatory, regulatory immune response profile may be of major relevance for both the maintenance of the clinical status of the dogs as well as for parasite persistence and replication at low levels.

Transcriptional regulation of the mucosal immune system mediated by T-bet

The immune system faces the arduous task of defending the mucosal surfaces from invading pathogens, but must simultaneously repress responses against commensal organisms and other inert antigens that are abundant in the external environment, as inappropriate immune activation might expose the host to increased risk of autoimmunity. The behavior of individual immune cells is governed by the expression of transcription factors that are responsible for switching immune response genes on and off. T-bet (T-box expressed in T cells) has emerged as one of the key transcription factors responsible for controlling the fate of both innate and adaptive immune cells, and its expression in different immune cells found at mucosal surfaces is capable of dictating the critical balance between permitting robust host immunity and limiting susceptibility to autoimmunity and allergy.

T-bet-independent effects of IL-12 family cytokines on regulation of Th17 responses to experimental T. cruzi infection

Tbx21 (i.e., T-bet) is an IFN-γ-inducible transcription factor that promotes Th1 differentiation. Previously, we reported that Tbx21(-/-) mice develop a robust Th17 response to the parasite Trypanosoma cruzi, including CD4(+) T cell subsets producing IL-17 and IFN-γ. Because of the known inhibitory effects of IFN-γ on Th17 cells, the purpose of this study was to determine the contribution of IFN-γ to regulation of Th17 differentiation during the course of T. cruzi infection. We observed that infection of IFN-γ(-/-) or Stat-1(-/-) mice generated increased numbers of IL-17-producing cells. In sharp contrast to infected Stat-1(-/-) or Tbx21(-/-) mice, however, IFN-γ(-/-) mice developed a lower overall Th17 response, suggesting that IFN-γ was not required for T-bet-dependent activity, including T-bet-dependent expression of CXCR3. To determine if IFN-γ could influence Th17 responses indirectly by acting on APCs, we neutralized IFN-γ in cultures containing APC and T. cruzi antigens. Although anti-IFN-γ increased IL-17 production modestly, anti-IFN-γ and anti-IL-12 led to a significant enhancement of T. cruzi-specific IL-17 (P<0.01). In contrast to the inhibitory effects of IL-12, IL-23 was able to stimulate Tbx21(-/-) T cells and cause a striking increase in T. cruzi-specific IL-17. These data show that the IL-12 family of cytokines can influence Th17 responses in a T-bet-independent manner and that the effects of IFN-γ are not necessarily related to its ability to induce T-bet expression in T cells.

Cytokines and their STATs in cutaneous and visceral leishmaniasis

Cytokines play a critical role in shaping the host immune response to Leishmania infection and directing the development of protective and non-protective immunities during infection. Cytokines exert their biological activities through the activation and translocation of transcription factors into the nucleus whether they drive the expression of specific cytokine-responsive genes. Signal transducer and activator of transcription (STATs) are transcription factors which play a critical role in mediating signaling downstream of cytokine receptors and are important for shaping the host immune response during Leishmania infection. Here we discuss the signature cytokines and their associated STATs involved in the host immune response during cutaneous and visceral leishmaniasis.

STAT1 and pathogens, not a friendly relationship

STAT1 belongs to the STAT family of transcription factors, which comprises seven factors: STAT1, STAT2, STAT3, STAT4, STAT5A, STAT5B and STAT6. STAT1 is a 91 kDa protein originally identified as the mediator of the cellular response to interferon (IFN) α, and thereafter found to be a major component of the cellular response to IFNγ. STAT1 is, in fact, involved in the response to several cytokines and to growth factors. It is activated by cytokine receptors via kinases of the JAK family. STAT1 becomes phosphorylated and forms a dimer which enters the nucleus and triggers the transcription of its targets. Although not lethal at birth, selective gene deletion of STAT1 in mice leads to rapid death from severe infections, demonstrating its major role in the response to pathogens. Similarly, in humans who do not express STAT1, there is a lack of resistance to pathogens leading to premature death. This indicates a key, non-redundant function of STAT1 in the defence against pathogens. Thus, to successfully infect organisms, bacterial, viral or parasitic pathogens must overcome the activity of STAT1, and almost all the steps of this pathway can be blocked or inhibited by proteins produced in infected cells. Interestingly, some pathogens, like the oncogenic Epstein–Barr virus, have evolved a strategy which uses STAT1 activation.

T-bet protects against exacerbation of schistosome egg-induced immunopathology by regulating Th17-mediated inflammation

C57BL/6 mice infected with Schistosoma mansoni naturally develop mild CD4(+) T-cell-mediated immunopathology characterized by small hepatic granulomas around parasite eggs. However, immunization with soluble egg Ag in CFA markedly exacerbates the lesions by inducing a potent proinflammatory environment with high levels of IFN-gamma and IL-17, which are signature cytokines of distinct Th1- versus Th17-cell lineages. To determine the relative role of these subsets in disease exacerbation, we examined mice deficient in T-bet (T-bet(-/-)), which is required for Th1 differentiation and IFN-gamma production. We now report that immunization with soluble egg Ag in CFA caused a significantly greater enhancement of egg-induced hepatic immunopathology in T-bet(-/-) mice compared with WT controls, and analysis of their granulomas disclosed a higher proportion of activated DC and CD4(+) T cells, as well as a marked influx of neutrophils. The absence of IFN-gamma in the T-bet(-/-) mice correlated with a marked increase in IL-23p19, IL-17 and TNF-alpha in granulomas and MLN. In contrast, T-bet(-/-) mice had lower levels of IL-4, IL-5 and IL-10 and a reduction in FIZZ1 and FoxP3 expression, suggesting diminished regulatory activity, respectively, by alternatively activated macrophages and Treg. These findings demonstrate that T-bet-dependent signaling negatively regulates Th17-mediated immunopathology in severe schistosomiasis.

T-bet-dependent regulation of CD8+ T-cell expansion during experimental Trypanosoma cruzi infection

The transcription factor T-bet (T-box, expressed in T cells), promotes type I immunity to pathogens through effects involving T cells and dendritic cells. In CD8(+) T cells, many of the functions of T-bet are redundant with those of eomesodermin (Eomes), a paralogue of T-bet. We therefore investigated the role of T-bet in immunity to Trypanosoma cruzi, an intracellular pathogen that causes Chagas disease, and which requires CD8(+) T cells for resistance. T-bet-deficient mice (tbx21(-/-)) were highly susceptible to T. cruzi infection, marked by severe liver pathology. CD8(+) T cells from infected tbx21(-/-) mice expressed typical markers of activation, including CD44 and CD25. In striking contrast, there was a 10-fold reduction in the number of antigen-specific CD8(+) T cells in tbx21(-/-) mice. This reduction was not a consequence of increased apoptosis or altered tissue-specific migration. Further, antigen-presenting cell (APC) functions in tbx21(-/-) mice were normal as we observed comparable levels of B7-1, B7-2 and CD40 expression as well as normal antigen-driven proliferation of wild-type CD8(+) T cells in infected tbx21(-/-) mice. However, adoptive transfer of naïve T cells from tbx21(-/-) donors into infected Rag-2-deficient mice (tbx21(+/+)) demonstrated a similar quantitative defect in CD8(+) T-cell expansion. These data demonstrate that T-bet facilitates immunity to T. cruzi by promoting the expansion of T. cruzi-specific CD8(+) T cells in a T cell-intrinsic manner. They also serve to further illustrate the multifaceted functions of T-box proteins in regulating quantitative aspects of T-cell immunity, in addition to qualitative components such as cytokine production.

Signal transducer and activator of transcription 1 in T cells plays an indispensable role in immunity to Leishmania major by mediating Th1 cell homing to the site of infection

The signal transducer and activator of transcription 1 (STAT1) signaling pathway mediates the biological functions of IFN-gamma. We have previously shown that the STAT1 pathway is indispensable for host resistance against Leishmania major infection. In this study, we examined the role of STAT1 in lymphocytes and specifically CD4(+) and CD8(+) T cells in mediating immunity against L. major by transferring T cells from wild-type (WT) and STAT1(-/-) C57BL/6 mice into Rag2(-/-) C57BL/6 mice. Rag2(-/-) mice reconstituted with unfractionated STAT1(-/-) splenocytes (B cells and T cells) failed to mount an efficient Th1 response after L. major infection, produced more IL-4, and developed large lesions full of parasites. In contrast, Rag2(-/-) mice reconstituted with WT (STAT1(+/+)) splenocytes mounted a Th1 response and developed self-resolving lesions. Studies using Rag2(-/-) recipients that received a combination of purified CD4(+) and CD8(+) T cells from WT or STAT1(-/-) mice revealed that STAT1 deficiency in CD4(+) T cells, but not in CD8(+) T cells, leads to development of chronic, nonhealing lesions and systemic dissemination of parasites into the spleen after L. major infection. Further studies using Rag2(-/-) recipients of WT Thy1.1(+) and STAT1(-/-) Thy1.2(+) T cells showed that STAT1 in CD4(+) T cells was not required for Th1 differentiation during L. major infection. However, it was critical for up-regulation of CXCR3 on CD4(+) T cells and their migration to the regional lymph node and the cutaneous site of infection. Together, these studies indicate that the STAT1 pathway in CD4(+) T cells plays a critical role in immunity against L. major by controlling the migration of Th1 cells to the site of infection rather than their generation. Further, they reveal an essential role for CD4(+) T cell STAT1 in preventing systemic dissemination of L. major infection.

Miltefosine promotes IFN-gamma-dominated anti-leishmanial immune response

Leishmania donovani, a protozoan parasite, resides and replicates as amastigotes within macrophages. The parasite inflicts the disease visceral leishmaniasis by suppressing host cell function. Neither a therapeutic vaccine nor an effective anti-leishmanial drug to reverse the immunosuppression is available. Although miltefosine (hexadecylphosphocholine or HPC) is a promising orally bioavailable anti-leishmanial drug, its efficacy is seriously compromised by contra-indications in pregnant women. Further rational redesigning of the drug requires studies on its mechanism of action, which is unknown at present. Because miltefosine is proposed to have immunomodulatory functions, we examined whether miltefosine exerts its anti-leishmanial functions by activating macrophages. We observed that miltefosine's anti-leishmanial function was significantly compromised in IFN-gamma-deficient macrophages suggesting the importance of endogenous IFN-gamma in miltefosine-induced anti-leishmanial functions of macrophages. Miltefosine induced IFN-gamma, neutralization of which reduced the anti-leishmanial functions of macrophages. IFN-gamma responsiveness is reduced in L. donovani-infected macrophages but is significantly restored by miltefosine, as it enhances IFN-gamma receptors and IFN-gamma induced STAT-1 phosphorylation but reduced activation of SHP-1, the phosphatase implicated in the down-regulation of STAT-1 phosphorylation. Miltefosine induced protein kinase C-dependent and PI3K-dependent p38MAP kinase phosphorylation and anti-leishmanial function. Miltefosine promotes p38MAP kinase-dependent anti-leishmanial functions and IL-12-dependent Th1 response. Leishmania donovani-infected macrophages induced Th2 response but miltefosine treatment reversed the response to Th1-type. Thus, our data define for the first time the mechanistic basis of host cell-dependent anti-leishmanial function of miltefosine.

T-bet inhibits the in vivo differentiation of parasite-specific CD4+ Th17 cells in a T cell-intrinsic manner

CD4(+) Th17 cells have emerged as a new T cell subset in the Th1/Th2 paradigm, and efforts have shifted toward understanding the factors that regulate their development in vivo. To analyze the role of the transcription factor T-bet in regulation of Th17 cells, we used a murine model of Trypanosoma cruzi infection, a protozoan parasite that causes Chagas disease in humans. Infection of Tbx21(-/-) mice led to normal, unimpaired development of Ag-specific CD4(+) T cells producing IFN-gamma. However, a robust Th17 response developed concomitant with Th1 responses. Despite significant IFN-gamma production, the physiological effects of Th17 responses prevailed as there was a sharp increase in Gr-1(+)Ly6G(+) neutrophils. Adoptive transfer of T cells from infected Tbx21(-/-) mice into Rag-2(-/-) mice (Tbx21(+/+)) revealed that CD4(+) T cells maintained their IL-17-producing phenotype, including those cells capable of producing both IFN-gamma and IL-17. Furthermore, and in contrast to the effects of IL-2 on Th17 development, IL-2 had no effect on IL-17 production by primed T cells. Importantly, adoptive transfer of T cells from naive Tbx21(-/-) mice into infected Rag-2(-/-) mice recapitulated the differentiation of T. cruzi-specific Th17 cells observed in infected Tbx21(-/-) mice. Conversely, transfer of wild-type T cells into infected Tbx21(-/-) mice did not reveal an increase in Th17 development. These results demonstrate that T-bet regulates the differentiation of T. cruzi-specific Th17 cells in vivo in a T cell-intrinsic manner. These data provide important insight into the role of T-bet in regulation of parasite-specific Th17 responses.

A partial form of recessive STAT1 deficiency in humans

Complete STAT1 deficiency is an autosomal recessive primary immunodeficiency caused by null mutations that abolish STAT1-dependent cellular responses to both IFN-alpha/beta and IFN-gamma. Affected children suffer from lethal intracellular bacterial and viral diseases. Here we report a recessive form of partial STAT1 deficiency, characterized by impaired but not abolished IFN-alpha/beta and IFN-gamma signaling. Two affected siblings suffered from severe but curable intracellular bacterial and viral diseases. Both were homozygous for a missense STAT1 mutation: g.C2086T (P696S). This STAT1 allele impaired the splicing of STAT1 mRNA, probably by disrupting an exonic splice enhancer. The misspliced forms were not translated into a mature protein. The allele was hypofunctional, because residual full-length mRNA production resulted in low but detectable levels of normally functional STAT1 proteins. The P696S amino acid substitution was not detrimental. The patients' cells, therefore, displayed impaired but not abolished responses to both IFN-alpha and IFN-gamma. We also show that recessive STAT1 deficiencies impaired the IL-27 and IFN-lambda1 signaling pathways, possibly contributing to the predisposition to bacterial and viral infections, respectively. Partial recessive STAT1 deficiency is what we believe to be a novel primary immunodeficiency, resulting in impairment of the response to at least 4 cytokines (IFN-alpha/beta, IFN-gamma, IFN-lambda1, and IL-27). It should be considered in patients with unexplained, severe, but curable intracellular bacterial and viral infections.

Vaccination route that induces transforming growth factor beta production fails to elicit protective immunity against Leishmania donovani infection

BALB/c mice immunized intraperitoneally (i.p.) and intravenously (i.v.) with Leishmania donovani promastigote membrane antigens (LAg), either free or encapsulated in liposomes, were protected against challenge infection with L. donovani, whereas mice immunized by the subcutaneous (s.c.) and intramuscular routes were not protected. Protected mice showed strong parasite resistance in both the liver and spleen, along with enhanced immunoglobulin G2a and delayed-type hypersensitivity responses. Again, mice vaccinated through the i.p. and i.v. routes showed high levels of NO production after challenge infection. s.c. vaccination resulted in an increased capacity of the spleen cells to produce prechallenge transforming growth factor beta (TGF-beta) levels during the in vitro antigen recall response, whereas i.p. immunization induced production of prechallenge gamma interferon, interleukin-12 (IL-12), and IL-4 levels, with a Th1 bias. Exposure to antigen-stimulated splenocyte supernatants of i.p. but not s.c. immunized mice activated macrophages for in vitro parasite killing. As an enhanced level of TGF-beta was detected in supernatants from unprotected s.c. immunized mice, neutralization by anti-TGF-beta antibody enhanced in vitro macrophage killing activity. The suppressive role of this cytokine was evaluated in vivo by vaccination with liposomal LAg and anti-TGF-beta antibody. Upon parasite challenge, these animals showed significant protection in both the liver and spleen. Moreover, the addition of recombinant TGF-beta in splenocyte supernatants of i.p. immunized mice in vitro as well as in vivo inhibited the protective ability of the macrophages by the i.p. route. Thus, the induction of high prechallenge TGF-beta limits the efficacy of vaccination by routes that are nonprotective.

Molecular cloning and expression analysis of T-bet in ginbuna crucian carp (Carassius auratus langsdorfii)

In the adaptive immune system of mammals, naive helper T (Th) cells differentiate into Th1 or Th2 cells. The T-box expressed in T cells (T-bet) is a member of a family of T-box transcription factors that regulates the expression of IFN-gamma and plays a crucial role in Th1 cell differentiation and cell-mediated immunity. We cloned and sequenced T-bet cDNA for the first time from non-mammalian species, ginbuna crucian carp. Ginbuna T-bet was composed of 608 predicted amino acids and showed 41.5% identity with human T-bet (Tbx21), and human and ginbuna T-bet share 77.3% identity in their T-box regions. Comparative genomic analysis showed conserved synteny in these regions between zebrafish, fugu, medaka and human T-bet. Phylogenetic analysis indicated that ginbuna T-bet is closely related to that of mouse and human. In unstimulated fish, ginbuna T-bet mRNA was strongly expressed in peripheral blood leukocytes (PBL), head kidney (HK) and spleen. RT-PCR analysis in kidney cells sorted by FACS revealed that T-bet was strongly expressed in surface-IgM-negative lymphocytes in comparison to IgM-positive lymphocytes. These results suggest that ginbuna T-bet is involved in the immune system, especially in T-cell function, and is an important tool to analyze teleost cell-mediated immunity.

STAT1 expression in dendritic cells, but not T cells, is required for immunity to Leishmania major

The generation of Th1 responses is important for resistance to intracellular pathogens, including the parasite, Leishmania major. Although IFN-gammaR/STAT1 signaling promotes a Th1 response via the up-regulation of T-bet, the requirement for STAT1 in Th1 cell differentiation remains controversial. Although in some cases Th1 cells develop independently of STAT1, STAT1(-/-) mice fail to develop a Th1 response during L. major infection. However, the interpretation of this result is complicated by the role STAT1 plays in Ag presentation and, more importantly, in elimination of parasites by macrophages, because both defective Ag presentation and increased parasite burden can influence Th cell development. To resolve this issue, we assessed the ability of STAT1(-/-) T cells to become Th1 cells and protect mice against L. major following adoptive transfer into STAT1-sufficient mice. We found that whereas T-bet is critical for the differentiation of protective Th1 cells during L. major infection, IFN-gammaR and STAT1 are dispensable. Given that a STAT1-independent Th1 cell response was generated by STAT1-sufficient APCs, but not by STAT1(-/-) cells, we next addressed whether dendritic cells (DCs) require STAT1 signaling to effectively present Ag. We found that STAT1(-/-) DCs had impaired up-regulation of MHC and costimulatory molecules, and, as a consequence, the absence of STAT1 resulted in reduced Th1 cell priming. Taken together, these results demonstrate that T cell expression of STAT1 is not required for the development of Th1 cells protective against L. major and instead stress the importance of STAT1 signaling in DCs for the optimal induction of Th1 responses.

Balancing immunity and pathology in visceral leishmaniasis

Experimental visceral leishmaniasis (VL) caused by infection with Leishmania donovani results in the development of organ-specific immunity in the two main target tissues of infection, the spleen and the liver. The liver is the site of an acute resolving infection associated with the development of inflammatory granulomas around infected Kupffer cells, and resistance to reinfection. Paradoxically, the spleen is an initial site for the generation of cell-mediated immune responses, but ultimately becomes a site of parasite persistence with associated immunopathological changes. These include splenomegaly and a breakdown in tissue architecture that is postulated to contribute to the immunocompromized status of the host. The progressive development of splenic pathology is largely associated with high levels of TNF and interleukin (IL)-10. Follicular dendritic cell (DC) networks are lost, whereas TNF mediates the destruction of marginal zone macrophages and gp38(+) stromal cells, and IL-10 promotes impaired DC migration into T-cell areas with consequent ineffective T-cell priming. Splenic stromal cell function is also altered, promoting the selective development of IL-10-producing DC with immunoregulatory properties. Ultimately, a fine immunological balance determines responses that effectively promote parasite clearance in the liver and those that promote pathology in the spleen, and future investigation aims to separate these responses to offer further means of parasite control in chronically infected VL patients.