Deletion of IL-4 receptor alpha on dendritic cells renders BALB/c mice hypersusceptible to Leishmania major infection

Multimodal control of dendritic cell functions by nociceptors

It is known that interactions between nociceptors and dendritic cells (DCs) can modulate immune responses in barrier tissues. However, our understanding of the underlying communication frameworks remains rudimentary. Here, we show that nociceptors control DCs in three molecularly distinct ways. First, nociceptors release the calcitonin gene-related peptide that imparts a distinct transcriptional profile on steady-state DCs characterized by expression of pro-interleukin-1β and other genes implicated in DC sentinel functions. Second, nociceptor activation induces contact-dependent calcium fluxes and membrane depolarization in DCs and enhances their production of proinflammatory cytokines when stimulated. Finally, nociceptor-derived chemokine CCL2 contributes to the orchestration of DC-dependent local inflammation and the induction of adaptive responses against skin-acquired antigens. Thus, the combined actions of nociceptor-derived chemokines, neuropeptides, and electrical activity fine-tune DC responses in barrier tissues.

Centrin-deficient Leishmania mexicana confers protection against Old World visceral leishmaniasis

Leishmaniasis is one of the top neglected tropical diseases with significant morbidity and mortality in low and middle-income countries (LMIC). However, this disease is also spreading in the developed world. Currently, there is a lack of effective strategies to control this disease. Vaccination can be an effective measure to control leishmaniasis and has the potential to achieve disease elimination. Recently, we have generated centrin gene-deleted new world L. mexicana (LmexCen−/−) parasites using CRISPR/Cas9 and showed that they protect mice against a homologous L. mexicana infection that causes cutaneous disease. In this study, we tested whether LmexCen−/− parasites can also protect against visceral leishmaniasis caused by L. donovani in a hamster model. We showed that immunization with LmexCen−/− parasites is safe and does not cause lesions. Furthermore, such immunization conferred protection against visceral leishmaniasis caused by a needle-initiated L. donovani challenge, as indicated by a significant reduction in the parasite burdens in the spleen and liver as well as reduced mortality. Similar control of parasite burden was also observed against a sand fly mediated L. donovani challenge. Importantly, immunization with LmexCen−/− down-regulated the disease promoting cytokines IL-10 and IL-4 and increased pro-inflammatory cytokine IFN-γ resulting in higher IFN-γ/IL-10 and IFN-γ/IL4 ratios compared to non-immunized animals. LmexCen−/− immunization also resulted in long-lasting protection against L. donovani infection. Taken together, our study demonstrates that immunization with LmexCen−/− parasites is safe and efficacious against the Old World visceral leishmaniasis.

Cytokines and Signaling Networks Regulating Disease Outcomes in Leishmaniasis

Cytokines play crucial roles in commencing and coordinating the organized recruitment and activation of immune cells during infection. These molecular regulators play an important part in deciding the fate of disease outcomes in leishmaniasis, a parasitic disease of tropical and subtropical countries. T helper 1 (Th1) cell-mediated inflammatory cytokines usually play a host-protective role, while T helper 2 (Th2) cell activation produces an anti-inflammatory milieu necessary for parasite survival. It is noteworthy that in such a multifaceted disease, the role played by any particular cytokine cannot be generalized as either beneficial or detrimental. For example, a "host-favorable" cytokine in one form of the disease has been found to be "pathogen friendly" in another form of leishmaniasis. On the other hand, the complex signaling network regulating the production of cytokines is further complicated by the nature of the host as well as the presence of other cytokines in the milieu. The present review focuses on the differential roles played by cytokines and the intricate signaling network responsible for the regulation of such cytokines during infection by different species of Leishmania. While many more studies are required in the future to better understand the role of these molecules in both animal models and patient samples, current studies indicate that these molecules are potential candidates to be targeted for therapy against this deadly disease.

Eosinophils, but Not Type 2 Innate Lymphoid Cells, Are the Predominant Source of Interleukin 4 during the Innate Phase of Leishmania major Infection

Interleukin (IL)-4 plays a central role in the initiation of a type 2 T helper cell (Th2) response, which leads to non-healing and progressive infections with the protozoan parasite Leishmania (L.) major. Here, we tested the hypothesis that type 2 innate lymphoid cells (ILC2), which promote the development of Th2 cells, form an important source of IL-4 early after intradermal or subcutaneous L. major infection. Lineage-marker negative CD90.2⁺CD127⁺PD1⁻ ILC2 were readily detectable in the ear or foot skin, but hardly in the draining lymph nodes of both naïve and L. major-infected self-healing C57BL/6 and non-healing BALB/c mice and made up approximately 20% to 30% of all CD45⁺SiglecF⁻ cells. Dermal ILC2 of C57BL/6 mice expressed the inducible T cell-costimulator (ICOS, CD278), whereas BALB/C ILC2 were positive for the stem cell antigen (Sca)-1. Within the first 5 days of infection, the absolute numbers of ILC2 did not significantly change in the dermis, which is in line with the unaltered expression of cytokines activating (IL-18, IL-25, IL-33, TSLP) or inhibiting ILC2 (IL-27, IFN-γ). At day 5 to 6 post infection, we observed an upregulation of IL-4, but not of IL-5, IL-10 or IL-13 mRNA. Using IL-4-reporter (4get) mice, we found that the production of IL-4 by C57BL/6 or BALB/c mice was largely restricted to CD45⁺SiglecF⁺ cells of high granularity, i.e., eosinophils. From these data, we conclude that eosinophils, but not ILC2, are a major innate source of IL-4 at the skin site of L. major infection.

Immune Responses in Leishmaniases: An Overview

Leishmaniasis is a parasitic, widespread, and neglected disease that affects more than 90 countries in the world. More than 20 Leishmania species cause different forms of leishmaniasis that range in severity from cutaneous lesions to systemic infection. The diversity of leishmaniasis forms is due to the species of parasite, vector, environmental and social factors, genetic background, nutritional status, as well as immunocompetence of the host. Here, we discuss the role of the immune system, its molecules, and responses in the establishment, development, and outcome of Leishmaniasis, focusing on innate immune cells and Leishmania major interactions.

Centrin-deficient Leishmania mexicana confers protection against New World cutaneous leishmaniasis

Leishmaniasis is a neglected protozoan disease affecting over 12 million people globally with no approved vaccines for human use. New World cutaneous leishmaniasis (CL) caused by L. mexicana is characterized by the development of chronic non-healing skin lesions. Using the CRISPR/Cas9 technique, we have generated live attenuated centrin knockout L. mexicana (LmexCen^-/-) parasites. Centrin is a cytoskeletal protein important for cellular division in eukaryotes and, in Leishmania, is required only for intracellular amastigote replication. We have investigated the safety and immunogenicity characteristics of LmexCen^-/- parasites by evaluating their survival and the cytokine production in bone-marrow-derived macrophages (BMDMs) and dendritic cells (BMDCs) in vitro. Our data shows that LmexCen^-/- amastigotes present a growth defect, which results in significantly lower parasitic burdens and increased protective cytokine production in infected BMDMs and BMDCs, compared to the wild type (WT) parasites. We have also determined the safety and efficacy of LmexCen^-/- in vivo using experimental murine models of L. mexicana. We demonstrate that LmexCen^-/- parasites are safe and do not cause lesions in susceptible mouse models. Immunization with LmexCen^-/- is also efficacious against challenge with WT L. mexicana parasites in genetically different BALB/c and C57BL/6 mouse models. Vaccinated mice did not develop cutaneous lesions, displayed protective immunity, and showed significantly lower parasitic burdens at the infection site and draining lymph nodes compared to the control group. Overall, we demonstrate that LmexCen^-/- parasites are safe and efficacious against New World cutaneous leishmaniasis in pre-clinical models.

Interleukin-4 Responsive Dendritic Cells Are Dispensable to Host Resistance Against Leishmania mexicana Infection

IL-4 and IL-13 cytokines have been associated with a non-healing phenotype in murine leishmaniasis in L. mexicana -infected BALB/c mice as demonstrated in IL-4−/−, IL-13−/− and IL-4Rα-/- global knockout mouse studies. However, it is unclear from the studies which cell-type-specific IL-4/IL-13 signaling mediates protection to L. mexicana. Previous studies have ruled out a role for IL-4-mediated protection on CD4+ T cells during L. mexicana infections. A candidate for this role may be non-lymphocyte cells, particularly DCs, as was previously shown in L. major infections, where IL-4 production drives dendritic cell-IL-12 production thereby mediating a type 1 immune response. However, it is unclear if this IL-4-instruction of type 1 immunity also occurs in CL caused by L. mexicana, since the outcome of cutaneous leishmaniasis often depends on the infecting Leishmania species. Thus, BALB/c mice with cell-specific deletion of the IL-4Rα on CD11c+ DCs (CD11ccreIL-4Rα-/lox) were infected with L. mexicana promastigotes in the footpad and the clinical phenotype, humoral and cellular immune responses were investigated, compared to the littermate control. Our results show that CL disease progression in BALB/c mice is independent of IL-4Rα signaling on DCs as CD11ccreIL-4Rα-/lox mice had similar footpad lesion progression, parasite loads, humoral responses (IgE, IgG1, IgG 2a/b), and IFN-γ cytokine secretion in comparison to littermate controls. Despite this comparable phenotype, surprisingly, IL-4 production in CD11ccreIL-4Rα-/lox mice was significantly increased with an increasing trend of IL-13 when compared to littermate controls. Moreover, the absence of IL-4Rα signaling did not significantly alter the frequency of CD4 and CD8 lymphocytes nor their activation, or memory phenotype compared to littermate controls. However, these populations were significantly increased in CD11ccreIL-4Rα-/lox mice due to greater total cell infiltration into the lymph node. A similar trend was observed for B cells whereas the recruitment of myeloid populations (macrophages, DCs, neutrophils, and Mo-DCs) into LN was comparable to littermate IL-4Rα-/lox mice. Interestingly, IL-4Rα-deficient bone marrow-derived dendritic cells (BMDCs), stimulated with LPS or L. mexicana promastigotes in presence of IL-4, showed similar levels of IL-12p70 and IL-10 to littermate controls highlighting that IL-4-mediated DC instruction was not impaired in response to L. mexicana. Similarly, IL-4 stimulation did not affect the maturation or activation of IL-4Rα-deficient BMDCs during L. mexicana infection nor their effector functions in production of nitrite and arginine-derived metabolite (urea). Together, this study suggests that IL-4 Rα signaling on DCs is not key in the regulation of immune-mediated protection in mice against L. mexicana infection.

The subunits of IL-12, originating from two distinct cells, can functionally synergize to protect against pathogen dissemination in vivo

Cytokines are typically single gene products, except for the heterodimeric interleukin (IL)-12 family. The two subunits (IL-12p40 and IL-12p35) of the prototype IL-12 are known to be simultaneously co-expressed in activated myeloid cells, which secrete the fully active heterodimer to promote interferon (IFN)γ production in innate and adaptive cells. We find that chimeric mice containing mixtures of cells that can only express either IL-12p40 or IL-12p35, but not both together, generate functional IL-12. This alternate two-cell pathway requires IL-12p40 from hematopoietic cells to extracellularly associate with IL-12p35 from radiation-resistant cells. The two-cell mechanism is sufficient to propel local T cell differentiation in sites distal to the initial infection and helps control systemic dissemination of a pathogen, although not parasite burden, at the site of infection. Broadly, this suggests that early secretion of IL-12p40 monomers by sentinel cells at the infection site may help prepare distal host tissues for potential pathogen arrival.

A Cytokine Network Balance Influences the Fate of Leishmania (Viannia) braziliensis Infection in a Cutaneous Leishmaniasis Hamster Model

The golden hamster is a suitable model for studying cutaneous leishmaniasis (CL) due to Leishmania (Viannia) braziliensis. Immunopathological mechanisms are well established in the L. (L.) major-mouse model, in which IL-4 instructs a Th2 response towards progressive infection. In the present study, we evaluated the natural history of L. braziliensis infection from its first stages up to lesion establishment, with the aim of identifying immunological parameters associated with the disease outcome and parasitism fate. To this end, hamsters infected with 10⁴, 10⁵, or 10⁶ promastigotes were monitored during the first hours (4h, 24h), early (15 days, 30 days) and late (50 days) post-infection (pi) phases. Cytokines, iNOS and arginase gene expression were quantified in the established lesions by reverse transcription-quantitative PCR. Compared to the 10⁵ or 10⁶ groups, 10⁴ animals presented lower lesions sizes, less tissue damage, and lower IgG levels. Basal gene expression in normal skin was high for TGF-β, and intermediary for TNF, IL-6, and IL-4. At 4hpi, no cytokine induction was observed in the 10⁴ group, while an upregulation of IL-6, IL-10, and IL-4 was observed in the 10⁶ group. At 15dpi, lesion appearance was accompanied by an increased expression of all assessed cytokines, markedly in the 10⁵ and 10⁶ groups. Upregulation of all investigated cytokines was observed in the late phase, although less expressive in the 10⁴ group. IFN-γ was the depending variable influencing tissue damage, while IL-6 was associated to parasite load. The network correlating gene expression and clinical and laboratorial parameters indicated inoculum-independent associations at 15 and 30dpi. A strong positive network correlation was observed in the 10⁴ group, but not in the 10⁵ or 10⁶ groups. In conclusion, IL-4, IL-6, IL-10, and TGF-β are linked o L. braziliensis progression. However, a balanced cytokine network is the key for an immune response able to reduce the ongoing infection and reduce pathological damage.

Immunological role of keratinocytes in leishmaniasis

Following inoculation of Leishmania, a protozoan parasite, into the skin of a mammal, the epidermal keratinocytes recognize the parasite and influence the local immune response that can give rise to different outcomes of leishmaniasis. The early keratinocyte-derived cytokines and keratinocytes-T cells interactions shape the anti-leishmanial immune responses that contribute to the resistance or susceptibility to leishmaniasis. The keratinocyte-derived cytokines can directly potentiate the leishmanicidal activity of monocytes and macrophages. As keratinocytes express MHC-II and enhance the expression of costimulatory molecules, these cells act as antigen-presenting cells (APCs) in cutaneous leishmaniasis (CL). Depending on the epidermal microenvironment, the keratinocytes induce various types of effector CD4⁺ T cells. Keratinocyte apoptosis and necrosis have been also implicated in ulceration in CL. Further, keratinocytes contribute to the healing of Leishmania-related cutaneous wounds. However, keratinocyte-derived IL-10 may play a key role in the development of post-kala-azar dermal leishmaniasis (PKDL). In this review, a comprehensive discussion regarding the multiple roles played by keratinocytes during leishmaniasis was provided, while highlighting novel insights concerning the immunological and pathological roles of these cells.

Unravelling the unsolved paradoxes of cytokine families in host resistance and susceptibility to Leishmania infection

Leishmaniasis is a neglected disease caused by protozoan parasites of the genus Leishmania. Successful clearance of Leishmania relies on a robust human immune response and various cytokines have been implicated in resistance and susceptibility to Leishmania infection. Accordingly, various immunotherapeutic approaches involving cytokines and cytokine receptors are being considered as novel avenues of treatment given the limited efficacy of current anti-leishmanial drugs. These approaches target canonical T helper (Th)1/Type 1 cytokines as intended mediators of host-protection to infection whilst concomitantly suppressing Th2/Type 2 cytokines and their anticipated disease-promoting roles. However, the use of cytokine and cytokine receptor gene-deficient mice over the years has challenged this simplistic view of Th1/Type 1-mediated resistance and Th2/Type 2-mediated susceptibility. Indeed, contribution to susceptibility vs resistance is only a partial consequence to cytokine action as the overall response is multi-faceted due to the pleiotropic, redundant, antagonistic and synergistic action of cytokines and interactions with immune cells in the diseased state. Notably, while the responses of certain cytokines are selectively host-protective or characteristic disease-enhancers, some ligands exert a response depending on the parasite-species initiating infection. Paradoxically, others play dual or contradictory roles in different Leishmania immunopathologies. Hence, cytokines in disease is an unsolved paradox and a comprehensive knowledge of cytokine interplay is important to guide the development of novel immunotherapeutics against leishmaniasis. In this review, we characterize various cytokine families in persistence and clearance of the Leishmania parasite and particularly elucidate unsolved cytokine puzzles in leishmaniasis based on information acquired from "gain of knowledge by loss of function" studies in cytokine and cytokine receptor gene-deficient mice.

Macrophages as host, effector and immunoregulatory cells in leishmaniasis: Impact of tissue micro-environment and metabolism

Leishmania are protozoan parasites that predominantly reside in myeloid cells within their mammalian hosts. Monocytes and macrophages play a central role in the pathogenesis of all forms of leishmaniasis, including cutaneous and visceral leishmaniasis. The present review will highlight the diverse roles of macrophages in leishmaniasis as initial replicative niche, antimicrobial effectors, immunoregulators and as safe hideaway for parasites persisting after clinical cure. These multiplex activities are either ascribed to defined subpopulations of macrophages (e.g., Ly6C^highCCR2⁺ inflammatory monocytes/monocyte-derived dendritic cells) or result from different activation statuses of tissue macrophages (e.g., macrophages carrying markers of of classical [M1] or alternative activation [M2]). The latter are shaped by immune- and stromal cell-derived cytokines (e.g., IFN-γ, IL-4, IL-10, TGF-β), micro milieu factors (e.g., hypoxia, tonicity, amino acid availability), host cell-derived enzymes, secretory products and metabolites (e.g., heme oxygenase-1, arginase 1, indoleamine 2,3-dioxygenase, NOS2/NO, NOX2/ROS, lipids) as well as by parasite products (e.g., leishmanolysin/gp63, lipophosphoglycan). Exciting avenues of current research address the transcriptional, epigenetic and translational reprogramming of macrophages in a Leishmania species- and tissue context-dependent manner.

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.

Acute IL-4 Governs Pathogenic T Cell Responses during Leishmania major Infection

Leishmania spp. infection is a global health problem affecting more than 2 million people every year with 300 million at risk worldwide. It is well established that a dominant Th1 response (IFN-γ, a hallmark Th1 cytokine) provides resistance, whereas a dominant Th2 response (IL-4, a hallmark Th2 cytokine) confers susceptibility during infection. Given the important role of IL-4 during L. major infection, we used IL-4-neutralizing Abs to investigate the cellular and molecular events regulated by IL-4 signaling. As previously published, neutralization of IL-4 in L. major-infected BALB/c mice (a Leishmania susceptible strain) provided protection when compared with control L. major-infected BALB/c mice. Despite this protection, IFN-γ production by T cells was dramatically reduced. Temporal neutralization of IL-4 revealed that acute IL-4 produced within the first days of infection is critical for not only programming IL-4-producing Th2 CD4⁺ T cells, but for promoting IFN-γ produced by CD8⁺ T cells. Mechanistically, IL-4 signaling enhances anti-CD3-induced Tbet and IFN-γ expression in both CD4⁺ and CD8⁺ T cells. Given the pathogenic role of IFN-γ-producing CD8⁺ T cells, our data suggest that IL-4 promotes cutaneous leishmaniasis pathology by not only promoting Th2 immune responses but also pathogenic CD8⁺ T cell responses. Our studies open new research grounds to investigate the unsuspected role of IL-4 in regulating both Th1 and Th2 responses.

Can We Harness Immune Responses to Improve Drug Treatment in Leishmaniasis?

Leishmaniasis is a vector-borne parasitic disease that has been neglected in priority for control and eradication of malaria, tuberculosis, and HIV/AIDS. Collectively, over one seventh of the world’s population is at risk of being infected with 0.7–1.2 million new infections reported annually. Clinical manifestations range from self-healing cutaneous lesions to fatal visceral disease. The first anti-leishmanial drugs were introduced in the 1950′s and, despite several shortcomings, remain the mainstay for treatment. Regardless of this and the steady increase in infections over the years, particularly among populations of low economic status, research on leishmaniasis remains under funded. This review looks at the drugs currently in clinical use and how they interact with the host immune response. Employing chemoimmunotherapeutic approaches may be one viable alternative to improve the efficacy of novel/existing drugs and extend their lifespan in clinical use.

Inflammatory Dendritic Cells, Regulated by IL-4 Receptor Alpha Signaling, Control Replication, and Dissemination of Leishmania major in Mice

Leishmaniasis is a vector-borne disease caused by Leishmania parasites. Macrophages are considered the primary parasite host cell, but dendritic cells (DCs) play a critical role in initiating adaptive immunity and controlling Leishmania infection. Accordingly, our previous study in CD11c^creIL-4Rα^−/lox mice, which have impaired IL-4 receptor alpha (IL-4Rα) expression on CD11c⁺ cells including DCs, confirmed a protective role for IL-4/IL-13-responsive DCs in replication and dissemination of parasites during cutaneous leishmaniasis. However, it was unclear which DC subset/s was executing this function. To investigate this, we infected CD11c^creIL-4Rα^−/lox and control mice with L. major GFP⁺ parasites and identified subsets of infected DCs by flow cytometry. Three days after infection, CD11b⁺ DCs and CD103⁺ DCs were the main infected DC subsets in the footpad and draining lymph node, respectively and by 4 weeks post-infection, Ly6C⁺ and Ly6C⁻ CD11b⁺ DCs were the main infected DC populations in both the lymph nodes and footpads. Interestingly, Ly6C⁺CD11b⁺ inflammatory monocyte-derived DCs but not Ly6C⁻CD11b⁺ DCs hosted parasites in the spleen. Importantly, intracellular parasitism was significantly higher in IL-4Rα-deficient DCs. In terms of DC effector function, we found no change in the expression of pattern-recognition receptors (TLR4 and TLR9) nor in expression of the co-stimulatory marker, CD80, but MHCII expression was lower in CD11c^creIL-4Rα^−/lox mice at later time-points compared to the controls. Interestingly, in CD11c^creIL-4Rα^−/lox mice, which have reduced Th1 responses, CD11b⁺ DCs had impaired iNOS production, suggesting that DC IL-4Rα expression and NO production is important for controlling parasite numbers and preventing dissemination. Expression of the alternative activation marker arginase was unchanged in CD11b⁺ DCs in CD11^creIL-4Rα^−/lox mice compared to littermate controls, but RELM-α was upregulated, suggesting IL-4Rα-independent alternative activation. In summary, L. major parasites may use Ly6C⁺CD11b⁺ inflammatory DCs derived from monocytes recruited to infection as “Trojan horses” to migrate to secondary lymphoid organs and peripheral sites, and DC IL-4Rα expression is important for controlling infection.

Mechanisms of Dupilumab

The Th2 cytokines interleukin 4 (IL-4) and IL-13 and the heterodimeric IL-4 receptor (IL-4R) complexes that they interact with play a key role in the pathogenesis of allergic disorders. Dupilumab is a humanized IgG4 monoclonal antibody that targets the IL-4 receptor alpha chain (IL-4Rα), common to both IL-4R complexes: type 1 (IL-4Rα/γc; IL-4 specific) and type 2 (IL-4Rα/IL-13Rα1; IL-4 and IL-13 specific). In this review, we detail the current state of knowledge of the different signalling pathways coupled to the IL-4R complexes and examine the possible mechanisms of Dupilumab action and survey its clinical efficacy in different allergic disorders. The development of Dupilumab and the widening spectrum of its clinical applications is relevant to the current emphasis on precision medicine approaches to the blockade of pathways involved in allergic diseases.

Interleukin (IL)-21 in Inflammation and Immunity During Parasitic Diseases

Parasitic diseases cause significant morbidity and mortality in the developing and underdeveloped countries. No efficacious vaccines are available against most parasitic diseases and there is a critical need for developing novel vaccine strategies for care. IL-21 is a pleiotropic cytokine whose functions in protection and immunopathology during parasitic diseases have been explored in limited ways. IL-21 and its cognate receptor, IL-21R, are highly expressed in parasitized organs of infected humans as well in murine models of the human parasitic diseases. Prior studies have indicated the ability of the IL-21/IL-21R signaling axis to regulate the effector functions (e.g., cytokine production) of T cell subsets by enhancing the expression of T-bet and STAT4 in human T cells, resulting in an augmented production of IFN-γ. Mice deficient for either IL-21 (Il21^−/−) or IL-21R (Il21r^−/−) showed significantly reduced inflammatory responses following parasitic infections as compared with their WT counterparts. Targeting the IL-21/IL-21R signaling axis may provide a novel approach for the development of new therapeutic agents for the prevention of parasite-induced immunopathology and tissue destruction.

Microglia in neuropathology caused by protozoan parasites

Involvement of the central nervous system (CNS) is the most severe consequence of some parasitic infections. Protozoal infections comprise a group of diseases that together affect billions of people worldwide and, according to the World Health Organization, are responsible for more than 500000 deaths annually. They include African and American trypanosomiasis, leishmaniasis, malaria, toxoplasmosis, and amoebiasis. Mechanisms underlying invasion of the brain parenchyma by protozoa are not well understood and may depend on parasite nature: a vascular invasion route is most common. Immunosuppression favors parasite invasion into the CNS and therefore the host immune response plays a pivotal role in the development of a neuropathology in these infectious diseases. In the brain, microglia are the resident immune cells active in defense against pathogens that target the CNS. Beside their direct role in innate immunity, they also play a principal role in coordinating the trafficking and recruitment of other immune cells from the periphery to the CNS. Despite their evident involvement in the neuropathology of protozoan infections, little attention has given to microglia-parasite interactions. This review describes the most prominent features of microglial cells and protozoan parasites and summarizes the most recent information regarding the reaction of microglial cells to parasitic infections. We highlight the involvement of the periphery-brain axis and emphasize possible scenarios for microglia-parasite interactions.

IL-4 Mediated Resistance of BALB/c Mice to Visceral Leishmaniasis Is Independent of IL-4Rα Signaling via T Cells

Previous studies infecting global IL-4Rα^−/−, IL-4^−/−, and IL-13^−/−mice on a BALB/c background with the visceralizing parasite Leishmania donovani have shown that the T helper 2 cytokines, IL-4, and IL-13, play influential but not completely overlapping roles in controlling primary infection. Subsequently, using macrophage/neutrophil-specific IL-4Rα deficient BALB/c mice, we demonstrated that macrophage/neutrophil unresponsiveness to IL-4 and IL-13 did not have a detrimental effect during L. donovani infection. Here we expand on these findings and show that CD4⁺ T cell-(Lck^cre), as well as pan T cell-(iLck^cre) specific IL-4Rα deficient mice, on a BALB/c background, unlike global IL-4Rα deficient mice, are also not adversely affected in terms of resistance to primary infection with L. donovani. Our analysis suggested only a transient and tissue specific impact on disease course due to lack of IL-4Rα on T cells, limited to a reduced hepatic parasite burden at day 30 post-infection. Consequently, the protective role(s) demonstrated for IL-4 and IL-13 during L. donovani infection are mediated by IL-4Rα-responsive cell(s) other than macrophages, neutrophils and T cells.

Activin-A in the regulation of immunity in health and disease

The TGF-β superfamily of cytokines plays pivotal roles in the regulation of immune responses protecting against or contributing to diseases, such as, allergy, autoimmunity and cancer. Activin-A, a member of the TGF-β superfamily, was initially identified as an inducer of follicle-stimulating hormone secretion. Extensive research over the past decades illuminated fundamental roles for activin-A in essential biologic processes, including embryonic development, stem cell maintenance and differentiation, haematopoiesis, cell proliferation and tissue fibrosis. Activin-A signals through two type I and two type II receptors which, upon ligand binding, activate their kinase activity, phosphorylate the SMAD2 and 3 intracellular signaling mediators that form a complex with SMAD4, translocate to the nucleus and activate or silence gene expression. Most immune cell types, including macrophages, dendritic cells (DCs), T and B lymphocytes and natural killer cells have the capacity to produce and respond to activin-A, although not in a similar manner. In innate immune cells, including macrophages, DCs and neutrophils, activin-A exerts a broad range of pro- or anti-inflammatory functions depending on the cell maturation and activation status and the spatiotemporal context. Activin-A also controls the differentiation and effector functions of Th cell subsets, including Th9 cells, T_FH cells, Tr1 Treg cells and Foxp3⁺ Treg cells. Moreover, activin-A affects B cell responses, enhancing mucosal IgA secretion and inhibiting pathogenic autoantibody production. Interestingly, an array of preclinical and clinical studies has highlighted crucial functions of activin-A in the initiation, propagation and resolution of human diseases, including autoimmune diseases, such as, systemic lupus erythematosus, rheumatoid arthritis and pulmonary alveolar proteinosis, in allergic disorders, including allergic asthma and atopic dermatitis, in cancer and in microbial infections. Here, we provide an overview of the biology of activin-A and its signaling pathways, summarize recent studies pertinent to the role of activin-A in the modulation of inflammation and immunity, and discuss the potential of targeting activin-A as a novel therapeutic approach for the control of inflammatory diseases.

Dendritic Cell-Mediated Th2 Immunity and Immune Disorders

Dendritic cells (DCs) are the professional antigen-presenting cells that recognize and present antigens to naïve T cells to induce antigen-specific adaptive immunity. Among the T-cell subsets, T helper type 2 (Th2) cells produce the humoral immune responses required for protection against helminthic disease by activating B cells. DCs induce a Th2 immune response at a certain immune environment. Basophil, eosinophil, mast cells, and type 2 innate lymphoid cells also induce Th2 immunity. However, in the case of DCs, controversy remains regarding which subsets of DCs induce Th2 immunity, which genes in DCs are directly or indirectly involved in inducing Th2 immunity, and the detailed mechanisms underlying induction, regulation, or maintenance of the DC-mediated Th2 immunity against allergic environments and parasite infection. A recent study has shown that a genetic defect in DCs causes an enhanced Th2 immunity leading to severe atopic dermatitis. We summarize the Th2 immune-inducing DC subsets, the genetic and environmental factors involved in DC-mediated Th2 immunity, and current therapeutic approaches for Th2-mediated immune disorders. This review is to provide an improved understanding of DC-mediated Th2 immunity and Th1/Th2 immune balancing, leading to control over their adverse consequences.

Strain specific maturation of Dendritic cells and production of IL-1β controls CD40-driven colitis

Intestinal integrity is maintained by balanced numbers of CD103+ Dendritic cells (DCs), which generate peripherally induced regulatory T cells (iTregs). We have developed a mouse model where DC-specific constitutive CD40 signals caused a strong reduction of CD103+ DCs in the lamina propria (LP) and intestinal lymph nodes (LN). As a consequence, also iTregs were strongly reduced and transgenic mice on the C57Bl/6-background (B6) developed fatal colitis. Here we describe that transgenic mice on a pure Balb/c-background (B/c) do not show any pathologies, while transgenic C57Bl/6 x Balb/c (F1) mice develop weak colon inflammation, without fatal colitis. This graded pathology correlated with the effects of CD40-signalling on DCs in each background, with striking loss of CD103+ DCs in B6, but reduced in F1 and diminished in B/c background. We further show direct correlation of CD103+ DC-numbers with numbers of iTregs, the frequencies of which behave correspondingly. Striking effects on B6-DCs reflected robust loss of surface MHCII, known to be crucial for iTreg induction. Furthermore, elevated levels of IL-23 together with IL-1, found only in B6 mice, support generation of intestinal IFN-γ+IL-17+ Th17 cells and IFN-γ+ Th1 cells, responsible for onset of disease. Together, this demonstrates a novel aspect of colitis-control, depending on genetic background. Moreover, strain-specific environmental sensing might alter the CD103+ DC/iTreg-axis to tip intestinal homeostatic balance to pathology.

IL-4Rα-expressing CD11c+ cells contribute to driving optimal cellular responses during Schistosoma mansoni infection in mice

Development of IL‐4 receptor alpha (IL‐4Rα)‐dependent cellular immunity regulates host protection against acute schistosomiasis. In this study, we investigated the importance of IL‐4Rα‐expressing CD11c⁺ cells in driving the development of optimal cellular responses to Schistosoma mansoni infection by using CD11c^creIL‐4Rα^−/lox BALB/c mice, which lacked IL‐4Rα expression on dendritic cells and alveolar macrophages. Abrogation of IL‐4Rα expression on CD11c⁺ cells affected activation of CD4⁺ T cells, resulting in reduced numbers of effector CD4⁺ T cells and impaired production of Th1 and Th2 cytokines by CD4⁺ T cells ex vivo. However, secretion of both type 1 and type 2 Ab isotypes was unchanged in infected CD11c‐specific IL‐4Rα‐deficient mice compared to littermate controls. Together, these data demonstrate that IL‐4Rα‐expressing CD11c⁺ cells play an important role in maintaining cellular immunity during schistosomiasis in mice.

Deletion of Interleukin-4 Receptor Alpha-Responsive Keratinocytes in BALB/c Mice Does Not Alter Susceptibility to Cutaneous Leishmaniasis

The skin microenvironment at the site of infection plays a role in the early events that determine protective T helper 1/type 1 immune responses during cutaneous leishmaniasis (CL) infection. During CL in nonhealing BALB/c mice, early interleukin-4 (IL-4) can instruct dendritic cells for protective Th1 immunity.

The skin microenvironment at the site of infection plays a role in the early events that determine protective T helper 1/type 1 immune responses during cutaneous leishmaniasis (CL) infection. During CL in nonhealing BALB/c mice, early interleukin-4 (IL-4) can instruct dendritic cells for protective Th1 immunity. Additionally, keratinocytes, which are the principal cell type in the skin epidermis, have been shown to secrete IL-4 early after Leishmania major infection. Here, we investigated whether IL-4/IL-13 signaling via the common IL-4 receptor alpha chain (IL-4Rα) on keratinocytes contributes to susceptibility during experimental CL. To address this, keratinocyte-specific IL-4Rα-deficient (KRT14^cre IL-4Rα^−/lox) mice on a BALB/c genetic background were generated by gene targeting and site-specific recombination (Cre/loxP) under the control of the keratinocyte-specific krt14 locus. Following high-dose infection with L. major IL-81 and LV39 promastigotes subcutaneously in the footpad, footpad swelling, parasite burden, IFN-γ/IL-4/IL-13 cytokine production, and type 1 and type 2 antibody responses were similar between KRT14^cre IL-4Rα^−/lox and littermate control IL-4Rα^−/lox BALB/c mice. An intradermal infection with low-dose L. major IL-81 and LV39 promastigotes in the ear showed results in infected KRT14^cre IL-4Rα^−/lox BALB/c mice similar to those of littermate control IL-4Rα^−/lox BALB/c mice, with the exception of a significant decrease observed in parasite burden only at the site of LV39 infection in the ear. Collectively, our results show that autocrine and paracrine signaling of IL-4/IL-13 through the IL-4Rα chain on keratinocytes does not influence the establishment of a nonhealing Th2 immune response in BALB/c mice during L. major infection.

House dust mite induced allergic airway disease is attenuated in CD11ccreIL-4Rα-/l°x mice

The precise mechanisms leading to development of T helper type (Th)2-driven allergic responses are unknown. We aimed to determine how IL-4 receptor alpha (IL-4Rα) signaling on CD11c⁺ cells influences allergen-induced Th2 responses in mice. CD11c^creIL-4Rα^−/l°^x mice, deficient in IL-4Rα on dendritic cells and alveolar macrophages, were compared to IL-4Rα^−/l°^x littermate controls in models of allergic airway disease induced by OVA/alum, OVA alone or house dust mite. Cytokine responses, eosinophil and neutrophil infiltration into the lungs, airway hyperreactivity and mucus hypersecretion were evaluated after allergen challenge. In the OVA/alum model, CD11c^creIL-4Rα^−/lox mice had similar airway hyperreactivity, eosinophil infiltration, Th2-type cytokine production and mucus hypersecretion to littermate controls. When alum was omitted during sensitization, CD11c^creIL-4Rα^−/lox mice had similar airway hyperreactivity and mucus secretion but reduced Th2-type cytokine production and eosinophils, suggesting alum overrides the requirement for IL-4Rα signaling on CD11c⁺ cells in enhancing Th2-type responses. In the house dust mite model, CD11c^creIL-4Rα^−/lox mice showed similar mucus secretion, but reduced Th2 responses, eosinophils, neutrophils and airway hyperreactivity, unlike previously tested LysM^creIL-4Rα^−/lox mice, which lack IL-4Rα on alveolar macrophages but not on dendritic cells. Therefore, our results indicate that IL-4Rα signaling on dendritic cells promotes allergen-induced Th2 responses and eosinophil infiltration into the lung.

All-Trans Retinoic Acid Promotes an M1- to M2-Phenotype Shift and Inhibits Macrophage-Mediated Immunity to Leishmania major

As key cells, able to host and kill Leishmania parasites, inflammatory monocytes/macrophages are potential vaccine and therapeutic targets to improve immune responses in Leishmaniasis. Macrophage phenotypes range from M1, which express NO-mediated microbial killing, to M2 macrophages that might help infection. Resistance to Leishmaniasis depends on Leishmania species, mouse strain, and both innate and adaptive immunity. C57BL/6 (B6) mice are resistant and control infection, whereas Leishmania parasites thrive in BALB/c mice, which are susceptible to develop cutaneous lesions in the course of infection with Leishmania major, but not upon infection with Leishmania braziliensis. Here, we investigated whether a deficit in early maturation of inflammatory monocytes into macrophages in BALB/c mice underlies increased susceptibility to L. major versus L. braziliensis parasites. We show that, after infection with L. braziliensis, monocytes are recruited to peritoneum, differentiate into macrophages, and develop an M1 phenotype able to produce proinflammatory cytokines in both B6 and BALB/c mice. Nonetheless, more mature macrophages from B6 mice expressed inducible NO synthase (iNOS) and higher NO production in response to L. braziliensis parasites, whereas BALB/c mice developed macrophages expressing an incomplete M1 phenotype. By contrast, monocytes recruited upon L. major infection gave rise to immature macrophages that failed to induce an M1 response in BALB/c mice. Overall, these results are consistent with the idea that resistance to Leishmania infection correlates with improved maturation of macrophages in a mouse-strain and Leishmania-species dependent manner. All-trans retinoic acid (ATRA) has been proposed as a therapy to differentiate immature myeloid cells into macrophages and help immunity to tumors. To prompt monocyte to macrophage maturation upon L. major infection, we treated B6 and BALB/c mice with ATRA. Unexpectedly, treatment with ATRA reduced proinflammatory cytokines, iNOS expression, and parasite killing by macrophages. Moreover, ATRA promoted an M1 to M2 transition in bone marrow-derived macrophages from both strains. Therefore, ATRA uncouples macrophage maturation and development of M1 phenotype and downmodulates macrophage-mediated immunity to L. major parasites. Cautions should be taken for the therapeutic use of ATRA, by considering direct effects on innate immunity to intracellular pathogens.

Interleukin-4 Receptor Alpha: From Innate to Adaptive Immunity in Murine Models of Cutaneous Leishmaniasis

The interleukin (IL)-4 receptor alpha (IL-4Rα), ubiquitously expressed on both innate and adaptive immune cells, controls the signaling of archetypal type 2 immune regulators; IL-4 and IL-13, which elicit their signaling action by the type 1 IL-4Rα/gamma common and/or the type 2 IL-4Rα/IL-13Rα complexes. Global gene-deficient mouse models targeting IL-4, IL-13, or the IL-4Rα chain, followed by the development of conditional mice and generation of important cell-type-specific IL-4Rα-deficient mouse models, were indeed critical to gaining in-depth understanding of detrimental T helper (Th) 2 mechanisms in type 1-controlled diseases. A primary example being cutaneous leishmaniasis, which is caused by the protozoan parasite Leishmania major, among others. The disease is characterized by localized self-healing cutaneous lesions and necrosis for which, currently, not a single vaccine has made it to a stage that can be considered effective. The spectrum of human leishmaniasis belongs to the top 10 infectious diseases according to the World Health Organization. As such, 350 million humans are at risk of infection and disease, with an incidence of 1.5–2 million new cases being reported annually. A major aim of our research is to identify correlates of host protection and evasion, which may aid in vaccine design and therapeutic interventions. In this review, we focus on the immune-regulatory role of the IL-4Rα chain from innate immune responses to the development of beneficial type 1 and detrimental type 2 adaptive immune responses during cutaneous Leishmania infection. We discuss the cell-specific requirements of the IL-4Rα chain on crucial innate immune cells during L. major infection, including, IL-4Rα-responsive skin keratinocytes, macrophages, and neutrophils, as well as dendritic cells (DCs). The latter, contributing to one of the paradigm shifts with respect to the role of IL-4 instructing DCs in vivo, to promote Th1 responses against L. major. Finally, we extend these innate responses and mechanisms to control of adaptive immunity and the effect of IL-4Rα-responsiveness on T and B lymphocytes orchestrating the development of CD4⁺ Th1/Th2 and B effector 1/B effector 2 B cells in response to L. major infection in the murine host.

Redundant and regulatory roles for Toll-like receptors in Leishmania infection

Toll-like receptors (TLRs) are germline-encoded, non-clonal innate immune receptors, which are often the first receptors to recognize the molecular patterns on pathogens. Therefore, the immune response initiated by TLRs has far-reaching consequences on the outcome of an infection. As soon as the cell surface TLRs and other receptors recognize a pathogen, the pathogen is phagocytosed. Inclusion of TLRs in the phagosome results in quicker phagosomal maturation and stronger adaptive immune response, as TLRs influence co-stimulatory molecule expression and determinant selection by major histocompatibility complex (MHC) class II and MHC class I for cross-presentation. The signals delivered by the TCR-peptide-MHC complex and co-stimulatory molecules are indispensable for optimal T cell activation. In addition, the cytokines induced by TLRs can skew the differentiation of activated T cells to different effector T cell subsets. However, the potential of TLRs to influence adaptive immune response into different patterns is severely restricted by multiple factors: gross specificity for the molecular patterns, lack of receptor rearrangements, sharing of limited number of adaptors that assemble signalling complexes and redundancy in ligand recognition. These features of apparent redundancy and regulation in the functioning of TLRs characterize them as important and probable contributory factors in the resistance or susceptibility to an infection.

IL-4Rα Signaling in Keratinocytes and Early IL-4 Production Are Dispensable for Generating a Curative T Helper 1 Response in Leishmania major-Infected C57BL/6 Mice

Experimental infection with the protozoan parasite Leishmania major has been extensively used to understand the mechanisms involved in T helper cell differentiation. Following infection, C57BL/6 mice develop a small self-healing cutaneous lesion and they are able to control parasite burden, a process linked to the development of T helper (Th) 1 cells. The local presence of IL-12 has been reported to be critical in driving Th1 cell differentiation. In addition, the early secretion of IL-4 was reported to potentially contribute to Th1 cell differentiation. Following infection with L. major, early keratinocyte-derived IL-4 was suggested to contribute to Th1 cell differentiation. To investigate a putative autocrine role of IL-4 signaling on keratinocytes at the site of infection, we generated C57BL/6 mice deficient for IL-4Rα expression selectively in keratinocytes. Upon infection with L. major, these mice could control their inflammatory lesion and parasite load correlating with the development of Th1 effector cells. These data demonstrate that IL-4 signaling on keratinocytes does not contribute to Th1 cell differentiation. To further investigate the source of IL-4 in the skin during the first days after L. major infection, we used C57BL/6 IL-4 reporter mice allowing the visualization of IL-4 mRNA expression and protein production. These mice were infected with L. major. During the first 3 days after infection, skin IL-4 mRNA expression was observed selectively in mast cells. However, no IL-4 protein production was detectable locally. In addition, early IL-4 blockade locally had no impact on subsequent Th1 cell differentiation and control of the disease. Taken together, the present data rule out a major role for skin IL-4 and keratinocyte IL-4Rα signaling in the development of a Th1 protective immune response following experimental infection with L. major.

IL-4-producing B cells regulate T helper cell dichotomy in type 1- and type 2-controlled diseases

Interleukin-4 (IL-4)-induced T helper (Th) 2 cells promote susceptibility to the protozoan parasite Leishmania major, while conferring immunity to the intestinal trematode Schistosoma mansoni Here, we report that abrogation of IL-4 receptor alpha (IL-4Rα) signaling on B cells in BALB/c mice (mb1^creIL-4Rα^-/lox) transformed nonhealer BALB/c to a healer phenotype with an early type 1 and dramatically reduced type 2 immune response and an absence of ulceration and necrosis during cutaneous leishmaniasis. From adoptive reconstitution and mixed bone-marrow chimera studies in B cell-deficient (µMT) mice, we reveal a central role for B cell-derived IL-4 and IL-4Rα in the optimal induction of the susceptible type 2 phenotype to L. major infection. We further demonstrate that the absence of IL-4Rα signaling on B cells exacerbated S. mansoni-induced mortality and pathology in BALB/c mice, due to a diminished type 2 immune response. In both disease models, IL-4Rα-responsive B cells displayed increased IL-4 production as early as day 1 after infection. Together, these results demonstrate that IL-4-producing and IL-4Rα-responsive B cells are critical in regulating and assisting early T helper dichotomy toward Th2 responses, which are detrimental in cutaneous leishmaniasis but beneficial in acute schistosomiasis.

Neutrophil Extracellular Traps Reprogram IL-4/GM-CSF-Induced Monocyte Differentiation to Anti-inflammatory Macrophages

Monocyte-derived dendritic cells (mo-DCs) are essential for the development of a Th1 protective immune response against Leishmania parasites. It is well known that IL-4 and GM-CSF drive differentiation of human monocytes to dendritic cells (DCs). Here, we investigate if neutrophil extracellular traps (NETs) disrupt this process. NETs-enriched supernatants, generated after human neutrophil activation by Leishmania promastigotes, were added to monocytes and differentiation monitored by expression of molecules associated with macrophage and DCs phenotypes, cytokine production, and parasite killing. We found that NETs addition to IL-4/GM-CSF-treated monocytes prevented then to fully differentiate into DCs. No effect was observed if NETs were treated with DNase or by filtering the traps. Moreover, NETs closely interact with monocytes and downregulate the expression of the IL-4 receptor, which in turn disrupts fully differentiation of monocytes into DCs. Neutrophil elastase inhibition rescues the monocytes to DCs differentiation. Monocytes cultured with IL-4/GM-CSF and NETs differentiated into macrophages, as observed by the increased expression of CD68, CD32, and CD163, and decreased expression of CD80. Moreover, NET addition to IL-4/GM-CSF-treated monocytes rendered these cells less efficient to kill Leishmania parasites. Altogether, our results show that NETs interfere with IL-4/GM-CSF driven differentiation, reprogramming the generation of mo-DCs to an anti-inflammatory macrophage.

IL-4Rα on dendritic cells in neonates and Th2 immunopathology in respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is one of the leading causes of bronchiolitis in children, and severe RSV infection early in life has been associated with asthma development. Using a neonatal mouse model, we have shown that down-regulation of IL-4 receptor α (IL-4Rα) with antisense oligonucleotides in the lung during neonatal infection protected from RSV immunopathophysiology. Significant down-regulation of IL-4Rα was observed on pulmonary CD11b⁺ myeloid dendritic cells (mDCs) suggesting a role for IL-4Rα on mDCs in the immunopathogenesis of neonatal RSV infection. Here, we demonstrated that neonatal CD11b⁺ mDCs expressed higher levels of IL-4Rα than their adult counterparts. Because CD11b⁺ mDCs mainly present antigens to CD4⁺ T cells, we hypothesized that increased expression of IL-4Rα on neonatal CD11b⁺ mDCs was responsible for Th2 - biased RSV immunopathophysiology. Indeed, when IL-4Rα was selectively deleted from CD11b⁺ mDCs, the immunopathophysiology typically observed following RSV reinfection was ablated, including Th2 inflammation, airway-mucus hyperproduction, and pulmonary dysfunction. Further, overexpression of IL-4Rα on adult CD11b⁺ DCs and their adoptive transfer into adult mice was able to recapitulate the Th2-biased RSV immunopathology typically observed only in neonates infected with RSV. IL-4Rα levels on CD11c⁺ cells were inversely correlated with maturation status of CD11b⁺ mDCs upon RSV infection. Our data demonstrate that developmentally regulated IL-4Rα expression is critical for the maturity of pulmonary CD11b⁺ mDCs and the Th2-biased immunopathogenesis of neonatal RSV infection.

Regulator Versus Effector Paradigm: Interleukin-10 as Indicator of the Switching Response

The interleukin-10 (IL-10) is generally considered as the most important cytokine with anti-inflammatory properties and one of the key cytokines preventing inflammation-mediated tissue damage. In this respect, IL-10 producing cells play a crucial role in the outcome of infections, allergy, autoimmune reactions, tumor development, and transplant tolerance. Based on recent findings with regard to the mentioned clinical conditions, this review attempts to shed some light on the IL-10 functions, considering this cytokine as inherent inducer of the switching immunity. While acute infections and vaccinations are associated by IL-10 enhanced during few weeks, chronic parasitoses, tumor diseases, allergen-specific immunotherapy, transplants, and use of immune-suppressor drugs show an increased IL-10 level along months or years. With regard to autoimmune pathologies, the IL-10 increase is prevalently observed during early stages, whereas the successive stages are characterized by reaching of immune equilibrium independently to disease's activity. Together, these findings indicate that IL-10 is mainly produced during transient immune conditions and the persistent IL-10-related effect is the indication/prediction (and maybe effectuation) of the switching immunity. Actual knowledge emphasizes that any manipulation of the IL-10 response for treatment purposes should be considered very cautiously due to its potential hazards to the immune system. Probably, the IL-10 as potential switcher of immunity response should be used in association with co-stimulatory immune effectors that are necessary to determine the appropriate deviation during treatment of respective pathologies. Hopefully, further findings would open new avenues to study the biology of this "master switch" cytokine and its therapeutic potential.

The Challenge of Stability in High-Throughput Gene Expression Analysis: Comprehensive Selection and Evaluation of Reference Genes for BALB/c Mice Spleen Samples in the Leishmania infantum Infection Model

The interaction of Leishmania with BALB/c mice induces dramatic changes in transcriptome patterns in the parasite, but also in the target organs (spleen, liver…) due to its response against infection. Real-time quantitative PCR (qPCR) is an interesting approach to analyze these changes and understand the immunological pathways that lead to protection or progression of disease. However, qPCR results need to be normalized against one or more reference genes (RG) to correct for non-specific experimental variation. The development of technical platforms for high-throughput qPCR analysis, and powerful software for analysis of qPCR data, have acknowledged the problem that some reference genes widely used due to their known or suspected “housekeeping” roles, should be avoided due to high expression variability across different tissues or experimental conditions. In this paper we evaluated the stability of 112 genes using three different algorithms: geNorm, NormFinder and RefFinder in spleen samples from BALB/c mice under different experimental conditions (control and Leishmania infantum-infected mice). Despite minor discrepancies in the stability ranking shown by the three methods, most genes show very similar performance as RG (either good or poor) across this massive data set. Our results show that some of the genes traditionally used as RG in this model (i.e. B2m, Polr2a and Tbp) are clearly outperformed by others. In particular, the combination of Il2rg + Itgb2 was identified among the best scoring candidate RG for every group of mice and every algorithm used in this experimental model. Finally, we have demonstrated that using “traditional” vs rationally-selected RG for normalization of gene expression data may lead to loss of statistical significance of gene expression changes when using large-scale platforms, and therefore misinterpretation of results. Taken together, our results highlight the need for a comprehensive, high-throughput search for the most stable reference genes in each particular experimental model.

Topical Simvastatin as Host-Directed Therapy against Severity of Cutaneous Leishmaniasis in Mice

We recently demonstrated that statins mediate protection against intracellular pathogens, Mycobacterium tuberculosis and Listeria monocytogenes in mice. Here, we investigated the immunomodulatory potential of simvastatin as a topical or systemic host-directed drug therapy in controlling inflammatory responses in an experimental mouse model of cutaneous leishmaniasis caused by Leishmania major (LV39). In an ear infection model, topical application of simvastatin directly on established lesions significantly reduced severity of the disease reflected by ear lesion size and ulceration. The host protective effect was further accompanied by decreased parasite burden in the ear and draining lymph nodes in both BALB/c and C57BL/6 mice. Pre-treatment of these mice on a low-fat cholesterol diet and systemic simvastatin also reduced footpad swelling, as well as parasite burdens and ulceration/necrosis in the more robust footpad infection model, demonstrating the prophylactic potential of simvastatin for cutaneous leishmaniasis. Mechanistically, following L. major infection, simvastatin-treated primary macrophages responded with significantly reduced cholesterol levels and increased production of hydrogen peroxide. Furthermore, simvastatin-treated macrophages displayed enhanced phagosome maturation, as revealed by increased LAMP-3 expression in fluorescent microscopy and Western blot analysis. These findings demonstrate that simvastatin treatment enhances host protection against L. major by increasing macrophage phagosome maturation and killing effector functions.

Morinda citrifolia Linn. Reduces Parasite Load and Modulates Cytokines and Extracellular Matrix Proteins in C57BL/6 Mice Infected with Leishmania (Leishmania) amazonensis

The absence of an effective vaccine and the debilitating chemotherapy for Leishmaniasis demonstrate the need for developing alternative treatments. Several studies conducted with Morinda citrifolia have shown various biological activities, including antileishmanial activity, however its mechanisms of action are unknown. This study aimed to analyze the in vivo activity of M. citrifolia fruit juice (Noni) against Leishmania (Leishmania) amazonensis in C57BL/6 mice. M. citrifolia fruit juice from the Brazilian Amazon has shown the same constitution of other juices produced around the world and liquid chromatography–mass spectrometry analysis identified five compounds: deacetylasperulosidic acid, asperulosidic acid, rutin, nonioside B and nonioside C. Daily intragastric treatment with Noni was carried out after 55 days of L. (L.) amazonensis infection in C57BL/6 mice. Parasitic loads, cytokine and extracellular protein matrix expressions of the lesion site were analyzed by qPCR. Histopathology of the lesion site, lymph nodes and liver were performed to evaluate the inflammatory processes. Cytokines and biochemical parameters of toxicity from sera were also evaluated. The Noni treatment at 500 mg.kg^-1.day^-1 for 60 days decreased the lesion size and parasitic load in the footpad infected with L. (L.) amazonensis. The site of infection also showed decreased inflammatory infiltrates and decreased cytokine expressions for IL-12, TNF-α, TGF-β and IL-10. On the other hand, Noni treatment enhanced the extracellular matrix protein expressions of collagen IV, fibronectin and laminin in the infected footpad as well collagen I and II, fibronectin and laminin in the mock-infected footpads. No toxicity was observed at the end of treatment. These data show the efficacy of Noni treatment.

Leishmaniasis is a complex of diseases caused by parasites of the Leishmania genus, which affects thousands of people around the world. The parasite lives within the cells and the disease manifests itself in different ways, one of them is wound-like lesions on the skin that do not heal. The treatment, with a medicament discovered in 1912, causes several side effects, its intramuscular administration is painful and it is given daily over a long period of time. These characteristics show the need for new alternatives for leishmaniasis treatment justifying works like this. The Morinda citrifolia is a plant native to the Polynesian islands and has a fruit commonly known as Noni. Noni has been analyzed for various targets such as anticancer, anti-inflammatory and antimicrobial effects; however, the antileishmanial has not yet been fully evaluated. This work proves that Noni treatment can promote a fast clinical cure in mice with leishmaniasis by decreasing parasite number, acting on the immune system and repairing skin components.

Arginase 1+ microglia reduce Aβ plaque deposition during IL-1β-dependent neuroinflammation

Background

Neuroinflammation has long been considered a driver of Alzheimer’s disease progression. However, experiments developed to explore the interaction between neuroinflammation and Alzheimer’s disease (AD) pathology showed a surprising reduction in amyloid beta (Aβ) plaque deposition. We sought to understand this unexpected outcome by examining microglia phenotypes during chronic neuroinflammation.

Methods

Using an adeno-associated virus vector carrying hIL-1β cDNA, inflammation was induced in one hippocampus of 8-month-old amyloid precursor protein (APP)/PS1 mice for 4 weeks, while the other hemisphere received control injections. Bone marrow chimeras and staining analysis were used to identify the origins and types of immune cells present during sustained inflammation. Arginase 1 (Arg1) and inducible nitric oxide synthase (iNOS) immunoreactivity were used as markers of alternatively activated and classically activated cells, respectively, and changes in cellular uptake of Aβ by Arg1+ or iNOS+ microglia was demonstrated by confocal microscopy. To determine if an anti-inflammatory phenotype was present during neuroinflammation, RNA was extracted on flow-sorted microglia and rt-PCR was performed. Interleukin-4 injection was used to induce alternatively activated cells, whereas a minipump and intrahippocampal cannula was used to deliver an interleukin (IL)-4Rα antibody to block the induction of Arg1+ cells in the setting of sustained IL-1β expression.

Results

We observed a robust upregulation of centrally derived Arg1+ microglia present only in the inflamed hemisphere. Furthermore, in the inflamed hemisphere, greater numbers of Arg1+ microglia contained Aβ when compared to iNOS+ microglia. RNA isolated from flow-sorted microglia from the inflamed hemisphere demonstrated elevation of mRNA species consistent with alternative activation as well as neuroprotective genes such as BDNF and IGF1. To explore if Arg1+ microglia mediated plaque reduction, we induced Arg1+ microglia with IL-4 and observed significant plaque clearance. Moreover, when we reduced Arg1+ microglia induction in the context of neuroinflammation using an anti-IL-4Rα antibody delivered via intrahippocampal cannula, we observed a clear correlation between numbers of Arg1+ microglia and plaque reduction.

Conclusions

Together, these findings suggest that Arg1+ microglia are involved in Aβ plaque reduction during sustained, IL-1β-dependent neuroinflammation, opening up possible new avenues for immunomodulatory therapy of AD.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-015-0411-8) contains supplementary material, which is available to authorized users.

Deception and manipulation: the arms of leishmania, a successful parasite

Leishmania spp. are intracellular parasitic protozoa responsible for a group of neglected tropical diseases, endemic in 98 countries around the world, called leishmaniasis. These parasites have a complex digenetic life cycle requiring a susceptible vertebrate host and a permissive insect vector, which allow their transmission. The clinical manifestations associated with leishmaniasis depend on complex interactions between the parasite and the host immune system. Consequently, leishmaniasis can be manifested as a self-healing cutaneous affliction or a visceral pathology, being the last one fatal in 85–90% of untreated cases. As a result of a long host–parasite co-evolutionary process, Leishmania spp. developed different immunomodulatory strategies that are essential for the establishment of infection. Only through deception and manipulation of the immune system, Leishmania spp. can complete its life cycle and survive. The understanding of the mechanisms associated with immune evasion and disease progression is essential for the development of novel therapies and vaccine approaches. Here, we revise how the parasite manipulates cell death and immune responses to survive and thrive in the shadow of the immune system.

Intranasal vaccination with extracellular serine proteases of Leishmania amazonensis confers protective immunity to BALB/c mice against infection

Background

Previously, we demonstrated that unlike subcutaneous or intramuscular vaccination, intranasal vaccination of BALB/c mice with whole Leishmania amazonensis antigens leads to protection against cutaneous leishmaniasis. Here, the role of parasite serine proteases in the protective immunity was investigated.

Findings

Serine Proteases were partially purified from both soluble (LaSP-Sol) and extracellular (LaSP-Ex) Leishmania amazonensis promastigote extracts by aprotinin-agarose chromatography. BALB/c mice were intranasally immunized with LaSP-Sol and LaSP-Ex prior to infection with L. amazonensis. LaSP-Ex but not LaSP-Sol vaccination led to significantly smaller lesions and parasite burdens as compared with non-vaccinated controls. Protection was accompanied by systemic Th1 polarization with increased IFN-γ and decreased IL-4 and IL-10 splenic production. Likewise, increased production of IFN-γ, IL-12 and IL-4 concomitant with decreased TGF-β and TNF-α was locally observed in the infected footpad.

Conclusion

This study indicates that extracellular serine proteases of L. amazonensis are strong candidates for a more defined intranasal vaccine against cutaneous leishmaniasis.

Electronic supplementary material

The online version of this article (doi:10.1186/1756-3305-7-448) contains supplementary material, which is available to authorized users.

Cellular factors targeting APCs to modulate adaptive T cell immunity

The fate of adaptive T cell immunity is determined by multiple cellular and molecular factors, among which the cytokine milieu plays the most important role in this process. Depending on the cytokines present during the initial T cell activation, T cells become effector cells that produce different effector molecules and execute adaptive immune functions. Studies thus far have primarily focused on defining how these factors control T cell differentiation by targeting T cells themselves. However, other non-T cells, particularly APCs, also express receptors for the factors and are capable of responding to them. In this review, we will discuss how APCs, by responding to those cytokines, influence T cell differentiation and adaptive immunity.

Innate immunity to Leishmania infection: within phagocytes

Infection by Leishmania takes place in the context of inflammation and tissue repair. Besides tissue resident macrophages, inflammatory macrophages and neutrophils are recruited to the infection site and serve both as host cells and as effectors against infection. Recent studies suggest additional important roles for monocytes and dendritic cells. This paper addresses recent experimental findings regarding the regulation of Leishmania major infection by these major phagocyte populations. In addition, the role of IL-4 on dendritic cells and monocytes is discussed.