Temporal regulation of interleukin-12p70 (IL-12p70) and IL-12-related cytokines in splenic dendritic cell subsets during Leishmania donovani infection

Commensal bacteria promote type I interferon signaling to maintain immune tolerance in mice

Type I interferons (IFNs) exert a broad range of biological effects important in coordinating immune responses, which have classically been studied in the context of pathogen clearance. Yet, whether immunomodulatory bacteria operate through IFN pathways to support intestinal immune tolerance remains elusive. Here, we reveal that the commensal bacterium, Bacteroides fragilis, utilizes canonical antiviral pathways to modulate intestinal dendritic cells (DCs) and regulatory T cell (Treg) responses. Specifically, IFN signaling is required for commensal-induced tolerance as IFNAR1-deficient DCs display blunted IL-10 and IL-27 production in response to B. fragilis. We further establish that IFN-driven IL-27 in DCs is critical in shaping the ensuing Foxp3+ Treg via IL-27Rα signaling. Consistent with these findings, single-cell RNA sequencing of gut Tregs demonstrated that colonization with B. fragilis promotes a distinct IFN gene signature in Foxp3+ Tregs during intestinal inflammation. Altogether, our findings demonstrate a critical role of commensal-mediated immune tolerance via tonic type I IFN signaling.

Leishmaniasis: Immune Cells Crosstalk in Macrophage Polarization

Leishmaniasis is a complex infectious parasitic disease caused by protozoa of the genus Leishmania, belonging to a group of neglected tropical diseases. It establishes significant global health challenges, particularly in socio-economically disadvantaged regions. Macrophages, as innate immune cells, play a crucial role in initiating the inflammatory response against the pathogens responsible for this disease. Macrophage polarization, the process of differentiating macrophages into pro-inflammatory (M1) or anti-inflammatory (M2) phenotypes, is essential for the immune response in leishmaniasis. The M1 phenotype is associated with resistance to Leishmania infection, while the M2 phenotype is predominant in susceptible environments. Notably, various immune cells, including T cells, play a significant role in modulating macrophage polarization by releasing cytokines that influence macrophage maturation and function. Furthermore, other immune cells can also impact macrophage polarization in a T-cell-independent manner. Therefore, this review comprehensively examines macrophage polarization's role in leishmaniasis and other immune cells' potential involvement in this intricate process.

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.

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.

Role of Cytokines in Experimental and Human Visceral Leishmaniasis

Visceral Leishmaniasis (VL) is the most fatal form of disease leishmaniasis. To date, there are no effective prophylactic measures and therapeutics available against VL. Recently, new immunotherapy-based approaches have been established for the management of VL. Cytokines, which are predominantly produced by helper T cells (Th) and macrophages, have received great attention that could be an effective immunotherapeutic approach for the treatment of human VL. Cytokines play a key role in forming the host immune response and in managing the formation of protective and non-protective immunities during infection. Furthermore, immune response mediated through different cytokines varies from different host or animal models. Various cytokines viz. IFN-γ, IL-2, IL-12, and TNF-α play an important role during protection, while some other cytokines viz. IL-10, IL-6, IL-17, TGF-β, and others are associated with disease progression. Therefore, comprehensive knowledge of cytokine response and their interaction with various immune cells is very crucial to determine appropriate immunotherapies for VL. Here, we have discussed the role of cytokines involved in VL disease progression or host protection in different animal models and humans that will determine the clinical outcome of VL and open the path for the development of rapid and accurate diagnostic tools as well as therapeutic interventions against VL.

IL-27 signalling regulates glycolysis in Th1 cells to limit immunopathology during infection

Inflammation is critical for controlling pathogens, but also responsible for symptoms of infectious diseases. IL-27 is an important regulator of inflammation and can limit development of IFNγ-producing Tbet⁺ CD4⁺ T (Th1) cells. IL-27 is thought to do this by stimulating IL-10 production by CD4⁺ T cells, but the underlying mechanisms of these immunoregulatory pathways are not clear. Here we studied the role of IL-27 signalling in experimental visceral leishmaniasis (VL) caused by infection of C57BL/6 mice with the human pathogen Leishmania donovani. We found IL-27 signalling was critical for the development of IL-10-producing Th1 (Tr1) cells during infection. Furthermore, in the absence of IL-27 signalling, there was improved control of parasite growth, but accelerated splenic pathology characterised by the loss of marginal zone macrophages. Critically, we discovered that IL-27 signalling limited glycolysis in Th1 cells during infection that in turn attenuated inflammation. Furthermore, the modulation of glycolysis in the absence of IL-27 signalling restricted tissue pathology without compromising anti-parasitic immunity. Together, these findings identify a novel mechanism by which IL-27 mediates immune regulation during disease by regulating cellular metabolism.

Infectious diseases like visceral leishmaniasis caused by the protozoan parasites Leishmania donovani and L. infantum are associated with an inflammatory response generated by the host. This is needed to control parasite growth, but also contributes to the symptoms of disease. Consequently, these inflammatory responses need to be tightly regulated. Although we now recognize many of the cells and molecules involved in controlling inflammation, the underlying mechanisms mediating immune regulation are unclear. CD4⁺ T cells are critical drivers of inflammatory responses during infections and as they progress from a naïve to activated state, the metabolic pathways they use have to change to meet the new energy demands required to proliferate and produce effector molecules. In this study, we discovered that the inflammatory CD4⁺ T cells needed to control L. donovani infection switch from relying on mitochondrial oxidative pathways to glycolysis. Critically, we found the cytokine IL-27 limited glycolysis in these inflammatory CD4⁺ T cells, and in the absence of IL-27 signaling pathways, these cells expanded more rapidly to better control parasite growth, but also caused increased tissue damage in the spleen. However, pharmacological dampening of glycolysis in inflammatory CD4⁺ T cells in L. donovani-infected mice lacking IL-27 signaling pathways limited tissue damage without affecting their improved anti-parasitic activity. Together, these results demonstrate that the pathogenic activity of inflammatory CD4⁺ T cells can be modulated by altering their cellular metabolism.

Interleukin-27 Functional Duality Balances Leishmania Infectivity and Pathogenesis

IL-27 is a cytokine that exerts diverse effects on the cells of innate and adaptive immune systems. Chiefly expressed in macrophages and dendritic cells during the early phase of Leishmania infection, IL-27 contributes to the protection against L. major infection but suppresses the protective Th1 response against L. donovani, L. infantum, L. amazonensis and L. braziliensis infections, suggesting its functional duality. During the late stage of Leishmania infection, IL-27 limits the immunopathogenic reactions and tissue damages. Herein, we analyze the mechanism of the functional duality of IL-27 in the resistance or susceptibility to Leishmania infection, prompting IL-27 for anti-Leishmanial therapy.

Reciprocal changes in CD11c+CD11b+ and CD11c+CD8α+ dendritic cell subsets determine protective or permissive immune response in murine experimental VL

CD11c⁺CD8α⁺ and CD11c⁺CD11b⁺ dendritic cells are two major subsets of murine splenic CD11c⁺ DCs which play a crucial role in T cell priming and shaping Th1/Th2 responses, but their role in the context of experimental visceral leishmaniasis (VL) is poorly understood. Herein, we showed that L. donovani infection in Balb/c mice preferentially decreased the population abundance of CD11c⁺CD11b⁺ DCs and increased relative abundance of splenic CD11c⁺CD8α ⁺DCs. During infection, splenic CD11c⁺CD11b⁺ DCs induced Th1 differentiation whereas CD11c⁺CD8α⁺ DCs promoted Th2 differentiation. Additionally, treatment of infected mice with miltefosine as experimental control exhibited host defense allowing the restoration of CD11c⁺CD11b⁺ population and decrease in CD11c⁺CD8α⁺ subset. Furthermore, reciprocal regulation of immune accessory surface molecules, Sema4A and OX40L critically determined Th1/Th2 response induced by these DC subsets during VL. L. donovani infection significantly induced OX40L expression and slightly downregulated SEMA 4A expression in CD11c⁺CD8α⁺ DCs whereas miltefosine treatment significantly downregulated OX40L expression along with pronounced upregulation of SEMA 4A expression in CD11c⁺CD11b⁺ DCs. SiRNA mediated knockdown of SEMA 4A markedly reduced CD11c⁺CD11b⁺ driven IFN-γ, TNF-α and IL-12 synthesis in miltefosine treated mice whereas functional blocking of OX40L decreased CD11c⁺CD8α⁺ induced IL-10, IL-4 and TGF-β synthesis in L. donovani infected group. Vaccination of Balb/c mice with antigen-pulsed + CpG-ODN-activated DC subsets revealed that only antigen-pulsed CD11c⁺CD11b⁺ DCs eliminated parasite load in visceral organ and restored protective Th1 cytokine response. Collectively, our results suggest that differential regulation of splenic CD11c⁺ subsets by L. donovani is essential for disease progression and specific subtypes may be exploited as prophylactic measures against visceral leishmaniasis.

The genetically determined production of the alarmin eosinophil-derived neurotoxin is reduced in visceral leishmaniasis

Visceral leishmaniasis (VL) is the most severe form of leishmaniasis. Recent findings indicate that dendritic cells have a key role in the defense against the Leishmania parasite and that the activity of this cell may be modified by the eosinophil secretory protein eosinophil-derived neurotoxin (EDN). We hypothesized that the interactions between dendritic cells and EDN might be of importance in the disease development. Cellular content of EDN was analyzed by ELISA. The single-nucleotide polymorphisms at positions 405, 416, and 1122 in the EDN gene were analyzed by real-time PCR with TaqMan^® reagents. The study cohorts comprised 239 Sudanese subjects (65 healthy controls and 174 with VL) and 300 healthy Swedish controls. The eosinophil content of EDN was lower in VL as compared with controls (p < 0.0001). The EDN405 (G>C) genotype distribution was similar among Swedish and Sudanese controls, whereas VL subjects had a higher prevalence of the EDN405-GG genotype (p < 0.0001). The content of EDN in the eosinophils was closely linked to the EDN405 polymorphism (p = 0.0002). Our findings suggest that the predisposition to acquire VL is related to the genetic polymorphism of the EDN gene and the reduced production by the eosinophil of this gene product.

Interleukin-27 Early Impacts Leishmania infantum Infection in Mice and Correlates with Active Visceral Disease in Humans

The complexity of Leishmania–host interactions, one of the main leishmaniasis issues, is yet to be fully understood. We detected elevated IL-27 plasma levels in European patients with active visceral disease caused by Leishmania infantum, which returned to basal levels after successful treatment, suggesting this cytokine as a probable infection mediator. We further addressed this hypothesis recurring to two classical susceptible visceral leishmaniasis mouse models. BALB/c, but not C57BL/6 mice, showed increased IL-27 systemic levels after infection, which was associated with an upregulation of IL-27p28 expression by dendritic cells and higher parasite burdens. Neutralization of IL-27 in acutely infected BALB/c led to decreased parasite burdens and a transient increase in IFN-γ⁺ splenic T cells, while administration of IL-27 to C57BL/6 promoted a local anti-inflammatory cytokine response at the site of infection and increased parasite loads. Overall, we show that, as in humans, BALB/c IL-27 systemic levels are infection dependently upregulated and may favor parasite installation by controlling inflammation.

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.

IL-27 enhances Leishmania amazonensis infection via ds-RNA dependent kinase (PKR) and IL-10 signaling

The protozoan parasite Leishmania infects and replicates in macrophages, causing a spectrum of diseases in the human host, varying from cutaneous to visceral clinical forms. It is known that cytokines modulate the immunological response against Leishmania and are relevant for infection resolution. Here, we report that Interleukin (IL)-27 increases Leishmania amazonensis replication in human and murine macrophages and that the blockage of the IL-10 receptor on the surface of infected cells abolished the IL-27-mediated enhancement of Leishmania growth. IL-27 induced the activation/phosphorylation of protein kinase R (PKR) in macrophages, and PKR blockage or PKR gene deletion abrogated the enhancement of the parasite growth driven by IL-27, as well as the L. amazonensis-induced macrophage production of IL-27. We also observed that L. amazonensis-induced expression of IL-27 depends on type I interferon signaling and the engagement of Toll-like receptor 2. Treatment of Leishmania-infected mice with IL-27 increased lesion size and parasite loads in the footpad and lymph nodes of infected animals, indicating that this cytokine exerts a local and a systemic effect on parasite growth and propagation. In conclusion, we show that IL-27 is a L. amazonensis-enhancing factor and that the PKR/IFN1 axis and IL-10 are critical mediators of this IL-27 induced effect.

Experimental models to investigate the function of dendritic cell subsets: challenges and implications

The dendritic cell (DC) lineage is remarkably heterogeneous. It has been postulated that specialized DC subsets have evolved in order to select and support the multitude of possible T cell differentiation pathways. However, defining the function of individual DC subsets has proven remarkably difficult, and DC subset control of key T cell fates such as tolerance, T helper cell commitment and regulatory T cell induction is still not well understood. While the difficulty in assigning unique functions to particular DC subsets may be due to sharing of functions, it may also reflect a lack of appropriate physiological in-vivo models for studying DC function. In this paper we review the limitations associated with many of the current DC models and highlight some of the underlying difficulties involved in studying the function of murine DC subsets.

Stromal cell induction of regulatory dendritic cells

Dendritic cells (DCs) are specialized antigen presenting cells of bone marrow origin that can exist in tissues in either an immature or mature state. DCs have a myriad of roles in immunity and tolerance induction, but are perhaps best known for their role in the activation and differentiation of naïve T cells at the onset of an acquired immune response. Over the past decade, a body of literature has developed that suggests that DCs, as well as many other myeloid cell populations, are also capable of exerting “regulatory” effects on T cell responses. However, relatively little is known regarding the mechanisms by which such regulatory myeloid cells arise in vivo. In this mini-review, we first define the characteristics of “regulatory” DCs (rDCs) and then focus on the contribution of non-hematopoietic stromal cells to their generation within specific tissue microenvironments. We also highlight areas of research that warrant future attention, arguing for a focusing of efforts toward a better understanding of the features of stromal cell populations that enable the induction of rDCs. Finally, we discuss how an understanding of stromal cell-myeloid cell interactions may lead to new therapeutic strategies for cancer, autoimmunity, and infectious disease.

IL-10-producing Th1 cells and disease progression are regulated by distinct CD11c⁺ cell populations during visceral leishmaniasis

IL-10 is a critical regulatory cytokine involved in the pathogenesis of visceral leishmaniasis caused by Leishmania donovani and clinical and experimental data indicate that disease progression is associated with expanded numbers of CD4⁺ IFNγ⁺ T cells committed to IL-10 production. Here, combining conditional cell-specific depletion with adoptive transfer, we demonstrate that only conventional CD11c(hi) DCs that produce both IL-10 and IL-27 are capable of inducing IL-10-producing Th1 cells in vivo. In contrast, CD11c(hi) as well as CD11c(int/lo) cells isolated from infected mice were capable of reversing the host protective effect of diphtheria toxin-mediated CD11c⁺ cell depletion. This was reflected by increased splenomegaly, inhibition of NO production and increased parasite burden. Thus during chronic infection, multiple CD11c⁺ cell populations can actively suppress host resistance and enhance immunopathology, through mechanisms that do not necessarily involve IL-10-producing Th1 cells.

IRF7 regulates TLR2-mediated activation of splenic CD11c(hi) dendritic cells

Members of the Interferon Regulatory Factor (IRF) family of transcription factors play an essential role in the development and function of the immune system. Here we investigated the role of IRF7 in the functional activation of conventional CD11c(hi) splenic dendritic cells (cDCs) in vitro and in vivo. Using mice deficient in IRF7, we found that this transcription factor was dispensable for the in vivo development of cDC subsets in the spleen. However, IRF7-deficient cDCs showed enhanced activation in response to microbial stimuli, characterised by exaggerated expression of CD80, CD86 and MHCII upon TLR2 ligation in vitro. The hyper-responsiveness of Irf7(-/-) cDC to TLR ligation could not be reversed with exogenous IFNα, nor by co-culture with wild-type cDCs, suggesting an intrinsic defect due to IRF7-deficiency. Irf7(-/-) cDCs also had impaired capacity to produce IL-12p70 when stimulated ex vivo, instead producing elevated levels of IL-10 that impaired their capacity to drive Th1 responses. Finally, analysis of bone marrow microchimeric mice revealed that cDCs deficient in IRF7 were also hyper-responsive to TLR2-mediated activation in vivo. Our data suggest a previously unknown function for IRF7 as a component of the regulatory network associated with cDC activation and adds to the wide variety of situations in which these transcription factors play a role.

Amniotic fluid and amniotic membrane stem cells: marker discovery

Amniotic fluid (AF) and amniotic membrane (AM) have been recently characterized as promising sources of stem or progenitor cells. Both not only contain subpopulations with stem cell characteristics resembling to adult stem cells, such as mesenchymal stem cells, but also exhibit some embryonic stem cell properties like (i) expression of pluripotency markers, (ii) high expansion in vitro, or (iii) multilineage differentiation capacity. Recent efforts have been focused on the isolation and the detailed characterization of these stem cell types. However, variations in their phenotype, their heterogeneity described by different groups, and the absence of a single marker expressed only in these cells may prevent the isolation of a pure homogeneous stem cell population from these sources and their potential use of these cells in therapeutic applications. In this paper, we aim to summarize the recent progress in marker discovery for stem cells derived from fetal sources such as AF and AM, using novel methodologies based on transcriptomics, proteomics, or secretome analyses.

Natural killer cells in experimental and human leishmaniasis

Infections with parasites of the genus Leishmania lead to a rapid, but transient activation of natural killer (NK) cells. In mice activation of NK cells requires a toll-like-receptor 9-dependent stimulation of dendritic cells (DC) which is followed by the production of IL-12. Although NK cells appear to be non-essential for the ultimate control of cutaneous and visceral leishmaniasis (VL) and can exhibit immunosuppressive functions, they form an important source of interferon (IFN)-γ, which elicits antileishmanial activity in macrophages and helps to pave a protective T helper cell response. In contrast, the cytotoxic activity of NK cells is dispensable, because Leishmania-infected myeloid cells are largely resistant to NK-mediated lysis. In human cutaneous and VL, the functional importance of NK cells is suggested by reports that demonstrate (1) a direct activation or inhibition of NK cells by Leishmania promastigotes, (2) the suppression of NK cell numbers or activity during chronic, non-healing infections, and (3) the recovery of NK cell activity following treatment. This review aims to provide an integrated view on the migration, activation, inhibition, function, and therapeutic modulation of NK cells in experimental and human leishmaniasis.

Marginal zone B cells regulate antigen-specific T cell responses during infection

Marginal zone B cells (MZB) participate in the early immune response to several pathogens. In this study, we show that in μMT mice infected with Leishmania donovani, CD8 T cells displayed a greater cytotoxic potential and generated more effector memory cells compared with infected wild type mice. The frequency of parasite-specific, IFN-γ(+) CD4 T cells was also increased in μMT mice. B cells were able to capture parasites, which was associated with upregulation of surface IgM and MyD88-dependent IL-10 production. Moreover, MZB presented parasite Ags to CD4 T cells in vitro. Depletion of MZB also enhanced T cell responses and led to a decrease in the parasite burden but did not alter the generation of effector memory T cells. Thus, MZB appear to suppress protective T cell responses during the early stages of L. donovani infection.

Spleen-resident CD4+ and CD4- CD8α- dendritic cell subsets differ in their ability to prime invariant natural killer T lymphocytes

One important function of conventional dendritic cells (cDC) is their high capacity to capture, process and present Ag to T lymphocytes. Mouse splenic cDC subtypes, including CD8α⁺ and CD8α⁻ cDC, are not identical in their Ag presenting and T cell priming functions. Surprisingly, few studies have reported functional differences between CD4⁻ and CD4⁺ CD8α⁻ cDC subsets. We show that, when loaded in vitro with OVA peptide or whole protein, and in steady-state conditions, splenic CD4⁻ and CD4⁺ cDC are equivalent in their capacity to prime and direct CD4⁺ and CD8⁺ T cell differentiation. In contrast, in response to α-galactosylceramide (α-GalCer), CD4⁻ and CD4⁺ cDC differentially activate invariant Natural Killer T (iNKT) cells, a population of lipid-reactive non-conventional T lymphocytes. Both cDC subsets equally take up α-GalCer in vitro and in vivo to stimulate the iNKT hybridoma DN32.D3, the activation of which depends solely on TCR triggering. On the other hand, and relative to their CD4⁺ counterparts, CD4⁻ cDC more efficiently stimulate primary iNKT cells, a phenomenon likely due to differential production of co-factors (including IL-12) by cDC. Our data reveal a novel functional difference between splenic CD4⁺ and CD4⁻ cDC subsets that may be important in immune responses.

Critical role of IRF-5 in the development of T helper 1 responses to Leishmania donovani infection

The transcription factor Interferon Regulatory Factor 5 (IRF-5) has been shown to be involved in the induction of proinflammatory cytokines in response to viral infections and TLR activation and to play an essential role in the innate inflammatory response. In this study, we used the experimental model of visceral leishmaniasis to investigate the role of IRF-5 in the generation of Th1 responses and in the formation of Th1-type liver granulomas in Leishmania donovani infected mice. We show that TLR7-mediated activation of IRF-5 is essential for the development of Th1 responses to L. donovani in the spleen during chronic infection. We also demonstrate that IRF-5 deficiency leads to the incapacity to control L. donovani infection in the liver and to the formation of smaller granulomas. Granulomas in Irf5^-/- mice are characterized by an increased IL-4 and IL-10 response and concomitant low iNOS expression. Collectively, these results identify IRF-5 as a critical molecular switch for the development of Th1 immune responses following L. donovani infections and reveal an indirect role of IRF-5 in the regulation of iNOS expression.

Leishmania donovani is a parasite that currently infects 12 million people around the world. In order to better understand why this parasite causes incurable disease we chose to investigate how the immune system sees L. donovani. The immune system sees infecting organisms by the recognition of molecules that are specifically expressed by pathogens. This is done by a family of receptors expressed by cells called Toll Like Receptors (TLRs). When TLRs recognize a pathogen it leads to a molecular chain reaction within the cell resulting in the release of cytokines. Interferon Regulatory Factors (IRFs) are a very important part of this signaling chain. The protein we have studied, IRF-5, has been identified as having a key role in inducing pro-inflammatory cytokines following the recognition of viruses by TLRs. However, whether it plays a role in the immune response to parasitic disease has not yet been examined. In this study we infected mice deficient of IRF-5 with L. donovani and demonstrate for the first time that IRF-5 is essential to develop a protective response against this parasite. These results are important as they help us to understand the molecular mechanisms required for an immune response to fight L. donovani.

Immune evasion by Yersinia enterocolitica: differential targeting of dendritic cell subpopulations in vivo

CD4(+) T cells are essential for the control of Yersinia enterocolitica (Ye) infection in mice. Ye can inhibit dendritic cell (DC) antigen uptake and degradation, maturation and subsequently T-cell activation in vitro. Here we investigated the effects of Ye infection on splenic DCs and T-cell proliferation in an experimental mouse infection model. We found that OVA-specific CD4(+) T cells had a reduced potential to proliferate when stimulated with OVA after infection with Ye compared to control mice. Additionally, proliferation of OVA-specific CD4(+) T cells was markedly reduced when cultured with splenic CD8α(+) DCs from Ye infected mice in the presence of OVA. In contrast, T-cell proliferation was not impaired in cultures with CD4(+) or CD4(-)CD8α(-) DCs isolated from Ye infected mice. However, OVA uptake and degradation as well as cytokine production were impaired in CD8α(+) DCs, but not in CD4(+) and CD4(-)CD8α(-) DCs after Ye infection. Pathogenicity factors (Yops) from Ye were most frequently injected into CD8α(+) DCs, resulting in less MHC class II and CD86 expression than on non-injected CD8α(+) DCs. Three days post infection with Ye the number of splenic CD8α(+) and CD4(+) DCs was reduced by 50% and 90%, respectively. The decreased number of DC subsets, which was dependent on TLR4 and TRIF signaling, was the result of a faster proliferation and suppressed de novo DC generation. Together, we show that Ye infection negatively regulates the stimulatory capacity of some but not all splenic DC subpopulations in vivo. This leads to differential antigen uptake and degradation, cytokine production, cell loss, and cell death rates in various DC subpopulations. The data suggest that these effects might be caused directly by injection of Yops into DCs and indirectly by affecting the homeostasis of CD4(+) and CD8α(+) DCs. These events may contribute to reduced T-cell proliferation and immune evasion of Ye.

Imaging of the host/parasite interplay in cutaneous leishmaniasis

An understanding of host-parasite interplay is essential for the development of therapeutics and vaccines. Immunoparasitologists have learned a great deal from 'conventional'in vitro and in vivo approaches, but recent developments in imaging technologies have provided us (immunologists and parasitologists) with the ability to ask new and exciting questions about the dynamic nature of the parasite-immune system interface. These studies are providing us with new insights into the mechanisms involved in the initiation of a Leishmania infection and the consequent induction and regulation of the immune response. Here, we review some of the recent developments and discuss how these observations can be further developed to understand the immunology of cutaneous Leishmania infection in vivo.

Immune evasive mechanisms contributing to persistent Leishmania donovani infection

The protozoan parasite Leishmania donovani, a causative agent of visceral leishmaniasis, has evolved several strategies to interfere with the immune system and establish persistent infections that are potentially lethal. In this article, we discuss two mechanisms of immune evasion adopted by the parasite: the induction of immune suppressive IL-10 responses and the generation of poor and functionally impaired CD8(+) T-cell responses.

Dendritic cells in systemic lupus erythematosus

Systemic lupus erythematosus (SLE) persists as a chronic inflammatory autoimmune disease and is characterized by the production of autoantibodies and immune complexes that affect multiple organs. The underlying mechanism that triggers and sustains disease are complex and involve certain susceptibility genes and environmental factors. There have been several immune mediators linked to SLE including cytokines and chemokines that have been reviewed elsewhere [ 1-3 ]. A number of articles have reviewed the role of B cells and T cells in SLE [ 4-10 ]. Here, we focus on the role of dendritic cells (DC) and innate immune factors that may regulate autoreactive B cells.

Dendritic cells matured by inflammation induce CD86-dependent priming of naive CD8+ T cells in the absence of their cognate peptide antigen

Dendritic cells (DC) licensed by the interaction between pathogen products and pattern recognition receptors can activate naive T cells to undergo Ag-dependent proliferation and cytokine production. In contrast, DC induced to mature by trans-acting inflammatory stimuli are believed to only be capable of supporting Ag-dependent proliferative responses. In this study, we show that uninfected DC matured as a consequence of Leishmania-induced inflammation induce CD8(+) T cells to proliferate in the absence of their cognate Ag. We separated splenic DC from Leishmania donovani-infected mice into those that contained parasites and had been activated to induce IL-12p40, from those that had undergone only partial maturation, measured by increased CD86 expression in the absence of IL-12p40 induction. We then showed that these partially matured DC could induce exogenous peptide-independent proliferation of OT-I and F5 CD8(+) TCR transgenic T cells, as well as polyclonal CD8(+) T cells. Proliferation of OT-I cells was significantly inhibited in vitro and in vivo by anti-CD86 mAb but not by anti-CD80 mAb and could also be inhibited by cyclosporine A. Proliferating OT-I cells did not produce IFN-gamma, even when re-exposed to mature DC. However, these primed OT-I cells subsequently produced effector cytokines, not just on exposure to their cognate peptide but, more importantly, to weak exogenous TCR agonists that otherwise failed to induce IFN-gamma. We further showed that OT-I cells undergoing locally driven proliferation to another pathogen, Streptococcus pneumoniae, rapidly seeded other lymphoid tissues, suggesting that CD8(+) T cells primed in this way may play a role in rapidly countering pathogen dissemination.

Plasmacytoid and conventional dendritic cells are early producers of IL-12 in Neospora caninum-infected mice

Neospora caninum is a coccidian parasite causative of clinical infections in a wide range of animal hosts. The maturation and activation of splenic conventional dendritic cells (cDCs) and plasmacytoid dendritic cells (pDCs) were studied here in BALB/c mice challenged intraperitoneal with N. caninum tachyzoites. The number of cDCs was found to decrease in the spleen of the infected mice 12 h and 2 days after the parasitic challenge, whereas at day 5 after infection it was significantly above that of mock-infected controls. In contrast, the number of splenic pDCs did not change significantly on infection. In the infected mice, both cell subtypes displayed an activated phenotype with upregulation of costimulatory and MHC class II molecules. This stimulatory effect was more marked at the earliest assessed time point after infection, 12 h, when a clear increase in the frequency of cDCs (CD8alpha(+) and CD8alpha(-)) and pDCs producing interleukin-12 (IL-12) was also observed. N. caninum tachyzoites could be observed by confocal microscopy associated with sorted DCs. Overall, these results present the first evidence that both cDCs and pDCs mediate in vivo the innate immune response to N. caninum infection through the production of IL-12, a key cytokine for host resistance to neosporosis.

Enhancement of Th1 immune response by CD8alpha(+) dendritic cells loaded with heat shock proteins enriched tumor extract in tumor-bearing mice

The discovery of dendritic cells (DCs) as professional antigen presenting cells has opened up new possibilities for their use in the development of tumor vaccines. We investigated the effect of the CD8alpha(+) DCs loaded with heat-treated tumor lysate (HTL) as a vaccine in tumor immunotherapy. The HTL loaded CD8alpha(+) DCs, TL loaded CD8alpha(+) DCs and unloaded CD8alpha(+) DCs were subcutaneously injected in the fibrosarcoma-bearing mice. The splenocyte proliferation and the shifting of Th1/Th2 response were measured. The results indicated a significant increase in the lymphocytes proliferation and the IFN-gamma production in the test group of mouse splenocytes. According to the results, HTL loaded CD8alpha(+) DCs vaccine significantly decreased tumor growth and longer survival than the other immunized animals. These findings show that anti-tumor immune response against the fibrosarcoma can be induced by HTL loaded CD8alpha(+) DCs and may provide a useful therapeutic model for development of approaches to tumor treatments.

In vivo induction of immune responses to pathogens by conventional dendritic cells

Specific defense mechanisms against pathogens are fulfilled by different subsets of nonmucosal conventional dendritic cells (DCs), including migratory Langerhans cells (LCs), dermal DCs, and resident CD8(+) and CD8(-) DCs found in lymphoid organs. Dermal DCs capture antigens in the skin and migrate to lymph nodes, where they can transfer the antigens to CD8(+) DCs and activate CD4(+) T cells. Differential antigen-processing machinery grants CD8(+) DCs a high efficiency in activating CD8(+) T cells through crosspresentation, whereas CD8(-) DCs preferentially trigger CD4(+) T cell responses. Recent findings have revealed the important role played by monocyte-derived DCs (mo-DCs), newly formed during infection, in activating CD4(+) and CD8(+) T cells, regulating immunoglobulin production, and killing pathogens. However, a number of controversial issues regarding the function of different DC subsets during viral, bacterial, and parasitic infections remain to be resolved.

VCAM-1 and VLA-4 modulate dendritic cell IL-12p40 production in experimental visceral leishmaniasis

Vascular cell adhesion molecule-1 (VCAM-1) interacts with its major ligand very late antigen-4 (VLA-4) to mediate cell adhesion and transendothelial migration of leukocytes. We report an important role for VCAM-1/VLA-4 interactions in the generation of immune responses during experimental visceral leishmaniasis caused by Leishmania donovani. Our studies demonstrate that these molecules play no direct role in the recruitment of leukocytes to the infected liver, but instead contribute to IL-12p40-production by splenic CD8⁺ dendritic cells (DC). Blockade of VCAM-1/VLA-4 interactions using whole antibody or anti-VCAM-1 Fab′ fragments reduced IL-12p40 mRNA accumulation by splenic DC 5 hours after L. donovani infection. This was associated with reduced anti-parasitic CD4⁺ T cell activation in the spleen and lowered hepatic IFNγ, TNF and nitric oxide production by 14 days post infection. Importantly, these effects were associated with enhanced parasite growth in the liver in studies with either anti-VCAM-1 or anti-VLA-4 antibodies. These data indicate a role for VCAM-1 and VLA-4 in DC activation during infectious disease.

VCAM-1 and its major ligand VLA-4 are adhesion molecules required for the recruitment and movement of leukocytes within tissue. In this study, we have investigated the role of these molecules during an experimental infection with Leishmania donovani, a protozoan parasite that causes a chronic disease called visceral leishmaniasis. Surprisingly, we showed that VCAM-1 and VLA-4 were not required for leukocyte migration into the liver, a site of acute L. donovani infection. Instead, there was a requirement for these molecules to initiate cell-mediated immune responses in the spleen within the first 5 hours of infection. When VCAM-1 was blocked during infection, early dendritic cell production of IL-12p40, a potent pro-inflammatory cytokine required for control of L. donovani, was suppressed, associated with a reduced parasite-specific T cell response in the spleen, and impaired immunity and parasite clearance in the liver. These results are important because they identify a novel role for VCAM-1 and VLA-4 in the regulation of dendritic cell activation during infectious disease.

Comparison of the immunomodulatory effects of L. donovani and L. major excreted-secreted antigens, particulate and soluble extracts and viable parasites on human dendritic cells

In an experimental model of human monocyte-derived dendritic cells (DCs), the immunophenotype of mature DCs infected with Leishmania donovani and Leishmania major showed a weak decrease in the cell surface expression of CD40, CD86, HLA-DR and DC-SIGN compared with uninfected control DCs. This immunomodulatory effect was more pronounced after stimulation with excreted-secreted antigens (ESA) of both species but absent after stimulation with particulate and soluble extracts. Infection with viable promastigotes, as well as stimulation with ESA from L. donovani and L. major, decreased IL-10 and IL-12p70 secretion. To our knowledge, this is the first direct demonstration that ESA from Leishmania promastigotes can stimulate DCs in the same manner as viable promastigotes.