The Absence of CCR7 Results in Dysregulated Monocyte Migration and Immunosuppression Facilitating Chronic Cutaneous Leishmaniasis

The protozoan parasite Leishmania major causes cutaneous lesions to develop at the site of infection, which are resolved with a strong Th1 immune response in resistant hosts, such as C57BL/6 mice. In contrast, the lesions ulcerate in susceptible hosts which display a Th2 response, such as BALB/c mice. The migration of cells in the immune response to L. major is regulated by chemokines and their receptors. The chemokine receptor CCR7 is expressed on activated DCs and naïve T cells, allowing them to migrate to the correct micro-anatomical positions within secondary lymphoid organs. While there have been many studies on the function of CCR7 during homeostasis or using model antigens, there are very few studies on the role of CCR7 during infection. In this study, we show that B6.CCR7^-/- mice were unable to resolve the lesion and developed a chronic disease. The composition of the local infiltrate at the lesion was significantly skewed toward neutrophils while the proportion of CCR2⁺ monocytes was reduced. Furthermore, a greater percentage of CCR2⁺ monocytes expressed CCR7 in the footpad than in the lymph node or spleen of B6.WT mice. We also found an increased percentage of regulatory T cells in the draining lymph node of B6.CCR7^-/- mice throughout infection. Additionally, the cytokine milieu of the lymph node showed a Th2 bias, rather than the resistant Th1 phenotype. This data shows that CCR7 is required for a protective immune response to intracellular L. major infection.     

An Early Reduction in Treg Cells Correlates with Enhanced Local Inflammation in Cutaneous Leishmaniasis in CCR6-Deficient Mice

Resistance to Leishmania major infection is dependent on the development of a cell-mediated Th1 immune response in resistant C57BL/6 mice whereas Th2-prone BALB/c mice develop non-healing lesions after infection. The chemokine receptor CCR6 is shared by anti-inflammatory regulatory T cells and pro-inflammatory Th17 cells. In a recent study we showed that C57BL/6 mice deficient in CCR6 exhibited enhanced footpad swelling and impaired T helper cell migration indicated by reduced recruitment of total T helper cells into the skin after infection and a reduced delayed type hypersensitivity reaction. Based on these findings we tested whether the lack of CCR6 alters Treg or Th17 cell responses during the course of Leishmania major infection. When we analyzed T cell subsets in the lymph nodes of CCR6-deficient mice, Th17 cell numbers were not different. However, reduced numbers of Treg cells paralleled with a stronger IFNγ response. Furthermore, the early increase in IFNγ-producing cells correlated with increased local tissue inflammation at later time points. Our data indicate an important role of CCR6 for Treg cells and a redundant role for Th17 cells in a Th1 cell-driven anti-parasitic immune response against Leishmania major parasites in resistant C57BL/6 mice.     

Blocking Junctional Adhesion Molecule C Enhances Dendritic Cell Migration and Boosts the Immune Responses against Leishmania major

The recruitment of dendritic cells to sites of infections and their migration to lymph nodes is fundamental for antigen processing and presentation to T cells. In the present study, we showed that antibody blockade of junctional adhesion molecule C (JAM-C) on endothelial cells removed JAM-C away from junctions and increased vascular permeability after L. major infection. This has multiple consequences on the output of the immune response. In resistant C57BL/6 and susceptible BALB/c mice, we found higher numbers of innate immune cells migrating from blood to the site of infection. The subsequent migration of dendritic cells (DCs) from the skin to the draining lymph node was also improved, thereby boosting the induction of the adaptive immune response. In C57BL/6 mice, JAM-C blockade after L. major injection led to an enhanced IFN-γ dominated T helper 1 (Th1) response with reduced skin lesions and parasite burden. Conversely, anti JAM-C treatment increased the IL-4-driven T helper 2 (Th2) response in BALB/c mice with disease exacerbation. Overall, our results show that JAM-C blockade can finely-tune the innate cell migration and accelerate the consequent immune response to L. major without changing the type of the T helper cell response.     

Neutrophil-Derived CCL3 Is Essential for the Rapid Recruitment of Dendritic Cells to the Site of Leishmania major Inoculation in Resistant Mice

Neutrophils are rapidly and massively recruited to sites of microbial infection, where they can influence the recruitment of dendritic cells. Here, we have analyzed the role of neutrophil released chemokines in the early recruitment of dendritic cells (DCs) in an experimental model of Leishmania major infection. We show in vitro, as well as during infection, that the parasite induced the expression of CCL3 selectively in neutrophils from L. major resistant mice. Neutrophil-secreted CCL3 was critical in chemotaxis of immature DCs, an effect lost upon CCL3 neutralisation. Depletion of neutrophils prior to infection, as well as pharmacological or genetic inhibition of CCL3, resulted in a significant decrease in DC recruitment at the site of parasite inoculation. Decreased DC recruitment in CCL3^−/− mice was corrected by the transfer of wild type neutrophils at the time of infection. The early release of CCL3 by neutrophils was further shown to have a transient impact on the development of a protective immune response. Altogether, we identified a novel role for neutrophil-secreted CCL3 in the first wave of DC recruitment to the site of infection with L. major, suggesting that the selective release of neutrophil-secreted chemokines may regulate the development of immune response to pathogens.     

Leishmania mexicana Induces Limited Recruitment and Activation of Monocytes and Monocyte-Derived Dendritic Cells Early during Infection

While C57BL/6 mice infected in the ear with L. major mount a vigorous Th1 response and resolve their lesions, the Th1 response in C57BL/6 mice infected with L. mexicana is more limited, resulting in chronic, non-healing lesions. The aim of this study was to determine if the limited immune response following infection with L. mexicana is related to a deficiency in the ability of monocyte-derived dendritic cells (mo-DCs) to prime a sufficient Th1 response. To address this issue we compared the early immune response following L. mexicana infection with that seen in L. major infected mice. Our data show that fewer monocytes are recruited to the lesions of L. mexicana infected mice as compared to mice infected with L. major. Moreover, monocytes that differentiate into mo-DCs in L. mexicana lesions produced less iNOS and migrated less efficiently to the draining lymph node as compared to those from L. major infected mice. Treatment of L. mexicana infected mice with α-IL-10R antibody resulted in increased recruitment of monocytes to the lesion along with greater production of IFN-γ and iNOS. Additionally, injection of DCs into the ear at the time of infection with L. mexicana also led to a more robust Th1 response. Taken together, these data suggest that during L. mexicana infection reduced recruitment, activation and subsequent migration of monocytes and mo-DCs to the draining lymph nodes may result in the insufficient priming of a Th1 response.     

CD1d-restricted NKT cells modulate placental and uterine leukocyte populations during chlamydial infection in mice

Invariant CD1d-restricted natural killer T cells play an important immunoregulatory role and can influence a broad spectrum of immunological responses including against bacterial infections. They are present at the fetal–maternal interface and although it has been reported that experimental systemic iNKT cell activation can induce mouse abortion, their role during pregnancy remain poorly understood. In the present work, using a physiological Chlamydia muridarum infection model, we have shown that, in vaginally infected pregnant mice, C. muridarum is cleared similarly in C57BL/6 wild type (WT) and CD1d^−/− mice. We have also shown that infected- as well as uninfected-CD1d^−/− mice have the same litter size as WT counterparts. Thus, CD1d-restricted cells are required neither for the resolution of chlamydial infection of the lower-genital tract, nor for the maintenance of reproductive capacity. However, unexpected differences in T cell populations were observed in uninfected pregnant females, as CD1d^−/− placentas contained significantly higher percentages of CD4⁺ and CD8⁺ T cells than WT counterparts. However, infection triggered a significant decrease in the percentages of CD4⁺ T cells in CD1d^−/− mice. In infected WT pregnant mice, the numbers of uterine CD4⁺ and CD8⁺ T cells, monocytes and granulocytes were greatly increased, changes not observed in infected CD1d^−/− mice. An increase in the percentage of CD8⁺ T cells seems independent of CD1d-restricted cells as it occurred in both WT and CD1d^−/− mice. Thus, in the steady state, the lack of CD1d-restricted NKT cells affects leukocyte populations only in the placenta. In Chlamydia-infected pregnant mice, the immune response against Chlamydia is dampened in the uterus. Our results suggest that CD1d-restricted NKT cells play a role in the recruitment or homeostasis of leukocyte populations at the maternal–fetal interface in the presence or absence of Chlamydia infection.     

Redundant Notch1 and Notch2 Signaling Is Necessary for IFN�� Secretion by T Helper 1 Cells During Infection with Leishmania major

The protective immune response to intracellular parasites involves in most cases the differentiation of IFNγ-secreting CD4⁺ T helper (Th) 1 cells. Notch receptors regulate cell differentiation during development but their implication in the polarization of peripheral CD4⁺ T helper 1 cells is not well understood. Of the four Notch receptors, only Notch1 (N1) and Notch2 (N2) are expressed on activated CD4⁺ T cells. To investigate the role of Notch in Th1 cell differentiation following parasite infection, mice with T cell-specific gene ablation of N1, N2 or both (N1N2^ΔCD4Cre) were infected with the protozoan parasite Leishmania major. N1N2^ΔCD4Cre mice, on the C57BL/6 L. major-resistant genetic background, developed unhealing lesions and uncontrolled parasitemia. Susceptibility correlated with impaired secretion of IFNγ by draining lymph node CD4⁺ T cells and increased secretion of the IL-5 and IL-13 Th2 cytokines. Mice with single inactivation of N1 or N2 in their T cells were resistant to infection and developed a protective Th1 immune response, showing that CD4⁺ T cell expression of N1 or N2 is redundant in driving Th1 differentiation. Furthermore, we show that Notch signaling is required for the secretion of IFNγ by Th1 cells. This effect is independent of CSL/RBP-Jκ, the major effector of Notch receptors, since L. major-infected mice with a RBP-Jκ deletion in their T cells were able to develop IFNγ-secreting Th1 cells, kill parasites and heal their lesions. Collectively, we demonstrate here a crucial role for RBP-Jκ-independent Notch signaling in the differentiation of a functional Th1 immune response following L. major infection.     

Activation and IL-10 production of specific CD4+ T cells are regulated by IL-27 during chronic infection with Plasmodium chabaudi

IL-27, a regulatory cytokine, plays critical roles in the prevention of immunopathology during Plasmodium infection. We examined these roles in the immune responses against Plasmodium chabaudi infection using the Il-27ra^−/− mice. While IL-27 was expressed at high levels during the early phase of the infection, enhanced CD4⁺ T cell function and reduction in parasitemia were observed mainly during the chronic phase in the mutant mice. In mice infected with P. chabaudi and cured with drug, CD4⁺ T cells in the Il-27ra^−/− mice exhibited enhanced CD4⁺ T-cell responses, indicating the inhibitory role of IL-27 on the protective immune responses. To determine the role of IL-27 in detail, we performed CD4⁺ T-cell transfer experiments. The Il-27ra^−/− and Il27p28^−/− mice were first infected with P. chabaudi and then cured using drug treatment. Plasmodium-antigen primed CD4⁺ T cells were prepared from these mice and transferred into the recipient mice, followed by infection with the heterologous parasite P. berghei ANKA. Il-27ra^−/− CD4⁺ T cells in the infected recipient mice did not produce IL-10, indicating that IL-10 production by primed CD4⁺ T cells is IL-27 dependent. Il27p28^−/− CD4⁺ T cells that were primed in the absence of IL-27 exhibited enhanced recall responses during the challenge infection with P. berghei ANKA, implying that IL-27 receptor signaling during the primary infection affects recall responses in the long-term via the regulation of the memory CD4⁺ T cell generation. These features highlighted direct and time-transcending roles of IL-27 in the regulation of immune responses against chronic infection with Plasmodium parasites.     

Engagement of NKG2D on Bystander Memory CD8 T Cells Promotes Increased Immunopathology following Leishmania major Infection

One of the hallmarks of adaptive immunity is the development of a long-term pathogen specific memory response. While persistent memory T cells certainly impact the immune response during a secondary challenge, their role in unrelated infections is less clear. To address this issue, we utilized lymphocytic choriomeningitis virus (LCMV) and Listeria monocytogenes immune mice to investigate whether bystander memory T cells influence Leishmania major infection. Despite similar parasite burdens, LCMV and Listeria immune mice exhibited a significant increase in leishmanial lesion size compared to mice infected with L. major alone. This increased lesion size was due to a severe inflammatory response, consisting not only of monocytes and neutrophils, but also significantly more CD8 T cells. Many of the CD8 T cells were LCMV specific and expressed gzmB and NKG2D, but unexpectedly expressed very little IFN-γ. Moreover, if CD8 T cells were depleted in LCMV immune mice prior to challenge with L. major, the increase in lesion size was lost. Strikingly, treating with NKG2D blocking antibodies abrogated the increased immunopathology observed in LCMV immune mice, showing that NKG2D engagement on LCMV specific memory CD8 T cells was required for the observed phenotype. These results indicate that bystander memory CD8 T cells can participate in an unrelated immune response and induce immunopathology through an NKG2D dependent mechanism without providing increased protection.     

CD8+ T cells Are Preferentially Activated during Primary Low Dose Leishmania major Infection but Are Completely Dispensable during Secondary Anti-Leishmania Immunity

We previously showed that CD8⁺ T cells are required for optimal primary immunity to low dose Leishmania major infection. However, it is not known whether immunity induced by low dose infection is durable and whether CD8⁺ T cells contribute to secondary immunity following recovery from low dose infection. Here, we compared primary and secondary immunity to low and high dose L. major infections and assessed the influence of infectious dose on the quality and magnitude of secondary anti-Leishmania immunity. In addition, we investigated the contribution of CD8⁺ T cells in secondary anti-Leishmania immunity following recovery from low and high dose infections. We found that the early immune response to low and high dose infections were strikingly different: while low dose infection preferentially induced proliferation and effector cytokine production by CD8⁺ T cells, high dose infection predominantly induced proliferation and cytokine production by CD4⁺ T cells. This differential activation of CD4⁺ and CD8⁺ T cells by high and low dose infections respectively, was imprinted during in vitro and in vivo recall responses in healed mice. Both low and high dose-infected mice displayed strong infection-induced immunity and were protected against secondary L. major challenge. While depletion of CD4⁺ cells in mice that healed low and high dose infections abolished resistance to secondary challenge, depletion of CD8⁺ cells had no effect. Collectively, our results show that although CD8⁺ T cells are preferentially activated and may contribute to optimal primary anti-Leishmania immunity following low dose infection, they are completely dispensable during secondary immunity.     

Dok proteins are recruited to the phagosome and degraded in a GP63-dependent manner during Leishmania major infection

Three adaptor molecules of the Dok family, Dok-1, Dok-2 and Dok-3 are expressed in macrophages and are involved in the negative regulation of signaling in response to lipopolysaccharide and various cytokines and growth factors. We investigated the role and the fate of these proteins following infection with Leishmania major promastigotes in macrophages. The protozoan parasite L. major causes cutaneous leishmaniasis and is known for its capacity to alter host-cell signaling and function. Dok-1/Dok-2^−/− bone marrow-derived macrophages displayed normal uptake of L. major promastigotes. Following Leishmania infection, Dok-1 was barely detectable by confocal microscopy. By contrast, phagocytosis of latex beads or zymosan led to the recruitment of Dok-1 to phagosomes. In the absence of the Leishmania pathogenesis-associated metalloprotease GP63, Dok-1 was also, partially, recruited to phagosomes containing L. major promastigotes. Further biochemical analyses revealed that similar to Dok-1, Dok-2 and Dok-3 were targets of GP63. Moreover, we showed that upon infection with wild-type or Δgp63 L. major promastigotes, production of nitric oxide and tumor necrosis factor by interferon-γ-primed Dok-1/Dok-2^−/− macrophages was reduced compared to WT macrophages. These results suggest that Dok proteins may be important regulators of macrophage responses to Leishmania infection.     

IL-4 Attenuates Th1-Associated Chemokine Expression and Th1 Trafficking to Inflamed Tissues and Limits Pathogen Clearance

Interleukin 4 (IL-4) plays a central role in the orchestration of Type 2 immunity. During T cell activation in the lymph node, IL-4 promotes Th2 differentiation and inhibits Th1 generation. In the inflamed tissue, IL-4 signals promote innate and adaptive Type-2 immune recruitment and effector function, positively amplifying the local Th2 response. In this study, we identify an additional negative regulatory role for IL-4 in limiting the recruitment of Th1 cells to inflamed tissues. To test IL-4 effects on inflammation subsequent to Th2 differentiation, we transiently blocked IL-4 during ongoing dermal inflammation (using anti-IL-4 mAb) and analyzed changes in gene expression. Neutralization of IL-4 led to the upregulation of a number of genes linked to Th1 trafficking, including CXCR3 chemokines, CCL5 and CCR5 and an associated increase in IFNγ, Tbet and TNFα genes. These gene expression changes correlated with increased numbers of IFNγ-producing CD4+ T cells in the inflamed dermis. Moreover, using an adoptive transfer approach to directly test the role of IL-4 in T cell trafficking to the inflamed tissues, we found IL-4 neutralization led to an early increase in Th1 cell recruitment to the inflamed dermis. These data support a model whereby IL-4 dampens Th1-chemokines at the site of inflammation limiting Th1 recruitment. To determine biological significance, we infected mice with Leishmania major, as pathogen clearance is highly dependent on IFNγ-producing CD4+ T cells at the infection site. Short-term IL-4 blockade in established L. major infection led to a significant increase in the number of IFNγ-producing CD4+ T cells in the infected ear dermis, with no change in the draining LN. Increased lymphocyte influx into the infected tissue correlated with a significant decrease in parasite number. Thus, independent of IL-4''s role in the generation of immune effectors, IL-4 attenuates lymphocyte recruitment to the inflamed/infected dermis and limits pathogen clearance.     

TLR2 mediates immunity to experimental cysticercosis

Information concerning TLR-mediated antigen recognition and regulation of immune responses during helminth infections is scarce. TLR2 is a key molecule required for innate immunity and is involved in the recognition of a wide range of viruses, bacteria, fungi and parasites. Here, we evaluated the role of TLR2 in a Taenia crassiceps cysticercosis model. We compared the course of T. crassiceps infection in C57BL/6 TLR2 knockout mice (TLR2^-/-) with that in wild type C57BL/6 (TLR2^+/+) mice. In addition, we assessed serum antibody and cytokine profiles, splenic cellular responses and cytokine profiles and the recruitment of alternatively activated macrophages (AAMφs) to the site of the infection. Unlike wild type mice, TLR2^-/- mice failed to produce significant levels of inflammatory cytokines in either the serum or the spleen during the first two weeks of Taenia infection. TLR2^-/- mice developed a Th2-dominant immune response, whereas TLR2^+/+ mice developed a Th1-dominant immune response after Taenia infection. The insufficient production of inflammatory cytokines at early time points and the lack of Th1-dominant adaptive immunity in TLR2^-/- mice were associated with significantly elevated parasite burdens; in contrast, TLR2^+/+ mice were resistant to infection. Furthermore, increased recruitment of AAMφs expressing PD-L1, PD-L2, OX40L and mannose receptor was observed in TLR2^-/- mice. Collectively, these findings indicate that TLR2-dependent signaling pathways are involved in the recognition of T. crassiceps and in the subsequent activation of the innate immune system and production of inflammatory cytokines, which appear to be essential to limit infection during experimental cysticercosis.     

iNOS-Producing Inflammatory Dendritic Cells Constitute the Major Infected Cell Type during the Chronic Leishmania major Infection Phase of C57BL/6 Resistant Mice

Leishmania major parasites reside and multiply in late endosomal compartments of host phagocytic cells. Immune control of Leishmania growth absolutely requires expression of inducible Nitric Oxide Synthase (iNOS/NOS2) and subsequent production of NO. Here, we show that CD11b⁺ CD11c⁺ Ly-6C⁺ MHC-II⁺ cells are the main iNOS-producing cells in the footpad lesion and in the draining lymph node of Leishmania major-infected C57BL/6 mice. These cells are phenotypically similar to iNOS-producing inflammatory DC (iNOS-DC) observed in the mouse models of Listeria monocytogenes and Brucella melitensis infection. The use of DsRed-expressing parasites demonstrated that these iNOS-producing cells are the major infected population in the lesions and the draining lymph nodes. Analysis of various genetically deficient mouse strains revealed the requirement of CCR2 expression for the recruitment of iNOS-DC in the draining lymph nodes, whereas their activation is strongly dependent on CD40, IL-12, IFN-γ and MyD88 molecules with a partial contribution of TNF-α and TLR9. In contrast, STAT-6 deficiency enhanced iNOS-DC recruitment and activation in susceptible BALB/c mice, demonstrating a key role for IL-4 and IL-13 as negative regulators. Taken together, our results suggest that iNOS-DC represent a major class of Th1-regulated effector cell population and constitute the most frequent infected cell type during chronic Leishmania major infection phase of C57BL/6 resistant mice.     

Redundancy of interleukin-6 in the differentiation of T cell and monocyte subsets during cutaneous leishmaniasis

Leishmania (L.) major is a protozoan parasite that infects mammalian hosts and causes a spectrum of disease manifestations that is strongly associated with the genetic background of the host. Interleukin (IL)-6 is an acute phase proinflammatory cytokine, known in vitro to be involved in the inhibition of the generation of regulatory T cells. IL-6-deficient mice were infected with L. major, and T cell and monocyte subsets were analyzed with flow cytometry. Our data show that at the site of infection in the footpad and in the draining popliteal lymph node, numbers of regulatory T cells remain unchanged between WT and IL-6-deficient mice. However, the spleens of IL-6^−/− mice contained fewer regulatory T cells after infection with L. major. The development of cutaneous lesions is similar between WT and IL-6-deficient mice, while parasite burden in IL-6^−/− mice is reduced compared to WT. The development of IFN-γ or IL-10 producing T cells is similar in IL-6^−/− mice. Despite a comparable adaptive T cell response, IL-6-deficient mice develop an earlier peak of some inflammatory cytokines than WT mice. This data indicate that the role of IL-6 in the differentiation of regulatory T cells is complex in vivo, and the effect of an absence of this cytokine can be counter-intuitive.     

Decoding systems immunological model of sphingolipids with IL-6/IL-17/IL-23 axes in L. major infection

IL-6, IL-17, IL-23 and IL-1β are the crucial cytokines controlling inflammatory and immune response during L. major infection. During cutaneous leishmaniasis, an important T helper cell type CD4⁺ Th17 subset plays a deterministic role in lesion formation through channelling infected macrophages and production of IL-1β, IL-6, IL-23 and IFN-γ. Ceramide derived sphingosine precursors may assist in pro-inflammatory cytokine response. However, the role of these metabolites in inflammation with pleiotropic pro-inflammatory cytokines in L. major infection is unknown. The present study indicates IL-6/IL-17/IL-23 and SPHK1-S1P-S1PRs signaling axes with the overexpression of SATB1 aiding in disease progression. Targeting SATB1 might modulate the secretion of pro-inflammatory cytokines and abnormal immune functioning, thereby killing the intracellular parasite. Systems immunological methods assisted in a step towards identifying the key to the mystery of crucial components and serving as an approach for therapeutic intervention in L. major infection.     

Chemokine receptor 5 blockade modulates macrophage trafficking in renal ischaemic-reperfusion injury

Chemokine receptor 5 (CCR5) is a pivotal regulator of macrophage trafficking in the kidneys in response to an inflammatory cascade. We investigated the role of CCR5 in experimental ischaemic-reperfusion injury (IRI) pathogenesis. To establish IRI, we clamped the bilateral renal artery pedicle for 30 min and then reperfused the kidney. We performed adoptive transfer of lipopolysaccharide (LPS)-treated RAW 264.7 macrophages following macrophage depletion in mice. B6.CCR5^−/− mice showed less severe IRI based on tubular epithelial cell apoptosis than did wild-type mice. CXCR3 expression in CD11b⁺ cells and inducible nitric oxide synthase levels were more attenuated in B6.CCR5^−/− mice. B6.CCR5^−/− mice showed increased arginase-1 and CD206 expression. Macrophage-depleted wild-type mice showed more injury than B6.CCR5^−/− mice after M1 macrophage transfer. Adoptive transfer of LPS-treated RAW 264.7 macrophages reversed the protection against IRI in wild-type, but not B6.CCR5^−/− mice. Upon knocking out CCR5 in macrophages, migration of bone marrow-derived macrophages from wild-type mice towards primary tubular epithelial cells with recombinant CCR5 increased. Phospho-CCR5 expression in renal tissues of patients with acute tubular necrosis was increased, showing a positive correlation with tubular inflammation. In conclusion, CCR5 deficiency favours M2 macrophage activation, and blocking CCR5 might aid in treating acute kidney injury.     

T Cell-Derived IL-10 Determines Leishmaniasis Disease Outcome and Is Suppressed by a Dendritic Cell Based Vaccine

In the murine model of Leishmania major infection, resistance or susceptibility to the parasite has been associated with the development of a Th1 or Th2 type of immune response. Recently, however, the immunosuppressive effects of IL-10 have been ascribed a crucial role in the development of the different clinical correlates of Leishmania infection in humans. Since T cells and professional APC are important cellular sources of IL-10, we compared leishmaniasis disease progression in T cell-specific, macrophage/neutrophil-specific and complete IL-10-deficient C57BL/6 as well as T cell-specific and complete IL-10-deficient BALB/c mice. As early as two weeks after infection of these mice with L. major, T cell-specific and complete IL-10-deficient animals showed significantly increased lesion development accompanied by a markedly elevated secretion of IFN-γ or IFN-γ and IL-4 in the lymph nodes draining the lesions of the C57BL/6 or BALB/c mutants, respectively. In contrast, macrophage/neutrophil-specific IL-10-deficient C57BL/6 mice did not show any altered phenotype. During the further course of disease, the T cell-specific as well as the complete IL-10-deficient BALB/c mice were able to control the infection. Furthermore, a dendritic cell-based vaccination against leishmaniasis efficiently suppresses the early secretion of IL-10, thus contributing to the control of parasite spread. Taken together, IL-10 secretion by T cells has an influence on immune activation early after infection and is sufficient to render BALB/c mice susceptible to an uncontrolled Leishmania major infection.     

Monocyte Recruitment to the Dermis and Differentiation to Dendritic Cells Increases the Targets for Dengue Virus Replication

Dengue virus (DENV) causes the most prevalent arthropod-borne viral disease in humans. Although Aedes mosquitoes transmit DENV when probing for blood in the skin, no information exists on DENV infection and immune response in the dermis, where the blood vessels are found. DENV suppresses the interferon response, replicates, and causes disease in humans but not wild-type mice. Here, we used mice lacking the interferon-α/β receptor (Ifnar ^–/–), which had normal cell populations in the skin and were susceptible to intradermal DENV infection, to investigate the dynamics of early DENV infection of immune cells in the skin. CD103⁺ classical dendritic cells (cDCs), Ly6C^– CD11b⁺ cDCs, and macrophages in the steady-state dermis were initial targets of DENV infection 12-24 hours post-inoculation but then decreased in frequency. We demonstrated recruitment of adoptively-transferred Ly6C^high monocytes from wild-type and Ifnar ^–/– origin to the DENV-infected dermis and differentiation to Ly6C⁺ CD11b⁺ monocyte-derived DCs (moDCs), which became DENV-infected after 48 hours, and were then the major targets for virus replication. Ly6C^high monocytes that entered the DENV-infected dermis expressed chemokine receptor CCR2, likely mediating recruitment. Further, we show that ∼100-fold more hematopoietic cells in the dermis were DENV-infected compared to Langerhans cells in the epidermis. Overall, these results identify the dermis as the main site of early DENV replication and show that DENV infection in the skin occurs in two waves: initial infection of resident cDCs and macrophages, followed by infection of monocytes and moDCs that are recruited to the dermis. Our study reveals a novel viral strategy of exploiting monocyte recruitment to increase the number of targets for infection at the site of invasion in the skin and highlights the skin as a potential site for therapeutic action or intradermal vaccination.