Insufficient generation of Th17 cells in IL-23p19-deficient BALB/c mice protects against progressive cutaneous leishmaniasis.

Healing of leishmaniasis-a parasitic skin disease-is associated with high levels of secreted interferon (IFN)γ and IL-12 in resistant C57BL/6 mice and humans. Susceptible BALB/c mice predominantly react with a Th17/Th2/Treg-related immune response and finally succumb to infection. Previously, we showed that BALB/c IL-17A-/- mice are protected against Leishmania (L.) major infections, indicating that IL-17A-predominantly produced by Th17 cells-plays an important role for disease outcome. We now investigated DC-derived cytokines and finally identified IL-23p19 as key cytokine responsible for induction of Leishmania-specific Th17 cells that play an important role for progressive disease in susceptible BALB/c mice.

Uptake of Leishmania major by dendritic cells is mediated by Fcgamma receptors and facilitates acquisition of protective immunity.

Uptake of Leishmania major by dendritic cells (DCs) results in activation and interleukin (IL)-12 release. Infected DCs efficiently stimulate CD4- and CD8- T cells and vaccinate against leishmaniasis. In contrast, complement receptor 3-dependent phagocytosis of L. major by macrophages (MPhi) leads exclusively to MHC class II-restricted antigen presentation to primed, but not naive, T cells, and no IL-12 production. Herein, we demonstrate that uptake of L. major by DCs required parasite-reactive immunoglobulin (Ig)G and involved FcgammaRI and FcgammaRIII. In vivo, DC infiltration of L. major-infected skin lesions coincided with the appearance of antibodies in sera. Skin of infected B cell-deficient mice and Fcgamma-/- mice contained fewer parasite-infected DCs in vivo. Infected B cell-deficient mice as well as Fcgamma-/- mice (all on the C57BL/6 background) showed similarly increased disease susceptibility as assessed by lesion volumes and parasite burdens. The B cell-deficient mice displayed impaired T cell priming and dramatically reduced IFN-gamma production, and these deficits were normalized by infection with IgG-opsonized parasites. These data demonstrate that DC and MPhi use different receptors to recognize and ingest L. major with different outcomes, and indicate that B cell-derived, parasite-reactive IgG and DC FcgammaRI and FcgammaRIII are essential for optimal development of protective immunity.

Mast cells promote Th1 and Th17 responses by modulating dendritic cell maturation and function.

Mast cells (MCs) play an important role in the regulation of protective adaptive immune responses against pathogens. However, it is still unclear whether MCs promote such host defense responses via direct effects on T cells or rather by modifying the functions of antigen-presenting cells. To identify the underlying mechanisms of the immunoregulatory capacity of MCs, we investigated the impact of MCs on dendritic cell (DC) maturation and function. We found that murine peritoneal MCs underwent direct crosstalk with immature DCs that induced DC maturation as evidenced by enhanced expression of costimulatory molecules. Furthermore, the MC/DC interaction resulted in the release of the T-cell modulating cytokines IFN-γ, IL-2, IL-6 and TGF-ß into coculture supernatants and increased the IL-12p70, IFN-γ, IL-6 and TGF-ß secretion of LPS-matured DCs. Such MC-"primed" DCs subsequently induced efficient CD4+ T-cell proliferation. Surprisingly, we observed that MC-primed DCs stimulated CD4+ T cells to release high levels of IFN-γ and IL-17, demonstrating that MCs promote Th1 and Th17 responses. Confirming our in vitro findings, we found that the enhanced disease progression of MC-deficient mice in Leishmania major infection is correlated with impaired induction of both Th1 and Th17 cells.

Interleukin 1alpha promotes Th1 differentiation and inhibits disease progression in Leishmania major-susceptible BALB/c mice.

Protective immunity against pathogens such as Leishmania major is mediated by interleukin (IL)-12-dependent Th1-immunity. We have shown previously that skin-dendritic cells (DCs) from both resistant C57BL/6 and susceptible BALB/c mice release IL-12 when infected with L. major, and infected BALB/c DCs effectively vaccinate against leishmaniasis. To determine if cytokines other than IL-12 might influence disease outcome, we surveyed DCs from both strains for production of a variety of cytokines. Skin-DCs produced significantly less IL-1alpha in response to lipopolysaccharide/interferon gamma or L. major when expanded from BALB/c as compared with C57BL/6 mice. In addition, IL-1alpha mRNA accumulation in lymph nodes of L. major-infected BALB/c mice was approximately 3-fold lower than that in C57BL/6 mice. Local injections of IL-1alpha during the first 3 d after infection led to dramatic, persistent reductions in lesion sizes. In L. major-infected BALB/c mice, IL-1alpha administration resulted in increased Th1- and strikingly decreased Th2-cytokine production. IL-1alpha and IL-12 treatments were similarly effective, and IL-1alpha efficacy was strictly IL-12 dependent. These data indicate that transient local administration of IL-1alpha acts in conjunction with IL-12 to influence Th-development in cutaneous leishmaniasis and prevents disease progression in susceptible BALB/c mice, perhaps by enhancing DC-induced Th1-education. Differential production of IL-1 by C57BL/6 and BALB/c mice may provide a partial explanation for the disparate outcomes of infection in these mouse strains.

Distinct roles for IL-1 receptor type I signaling in early versus established Leishmania major infections.

IL-1alpha/beta released by infected dendritic cells (DC) plays a critical role in the development of protective immunity against Leishmania major. Previous studies demonstrated that treatment of susceptible BALB/c mice with IL-1alpha during T-cell priming (days 1-3 post-infection) induced T helper (Th)1-mediated protection. In contrast, we now demonstrate that prolonged treatment with IL-1alpha (for 3 weeks) worsened disease outcome. To characterize the receptor involved, L. major infections in IL-1 receptor type I (IL-1RI) knockout mice were studied. In C57BL/6 IL-1RI-/- mice, the IL-1alpha-mediated protective effect was abrogated. The course of high-dose infection (2 x 10(5) parasites) in IL-1RI-/- mice was not different from controls. Surprisingly, in low-dose infections (10(3) parasites), IL-1RI-/- mice developed approximately 50% smaller lesions compared to wild types, decreased parasite loads and increased IFNgamma/IL-4 ratios. Interestingly, naive Th0 and Th2, but not Th1, cells expressed IL-1RI ex vivo. We conclude that IL-1RI mediates the effect of IL-1alpha in leishmaniasis in C57BL/6 mice. In addition, IL-1 appears to play distinct roles in Th education and maintenance. In early phases of physiologically relevant, low-dose L. major infections, IL-1 facilitates Th1 development from Th0 cells, whereas later on IL-1RI signaling promotes Th2 expansion and worsens disease outcome. Effects of IL-1 on disease outcome may be related to levels of IL-1RI on Th subpopulations.

A role for leukocyte-derived IL-1RA in DC homeostasis revealed by increased susceptibility of IL-1RA-deficient mice to cutaneous leishmaniasis.

Dendritic cell (DC)-derived IL-1α/ß plays a critical role in the induction of T helper type 1 (Th1)-dependent immunity against Leishmania. DCs from susceptible BALB/c mice produce less IL-1α/ß when compared with resistant C57BL/6 mice, contributing to aberrant Th2 development and ultimate death of infected mice. We have extended our studies of the role of IL-1 in leishmaniasis using IL-1RA(-/-) BALB/c mice that are characterized by upregulated IL-1 receptor signaling. Unexpectedly, infection of IL-1RA(-/-) mice led to significantly worsened disease outcome with larger lesions, dramatically higher parasite burdens, and decreased IFN-γ production by antigen-specific T cells. We determined that IL-1RA(-/-) DCs were more mature already in the steady state, exhibited less phagocytotic capacity, and IL-12 production in response to various stimuli was impaired. Our data suggest that in addition to effects on Th education, IL-1α/ß signaling also modulates DC homeostasis with increased signaling, leading to downmodulation of IL-12 synthesis and worsened disease outcome after infection with Leishmania major. Thus, the complex regulation of various members of the IL-1 cytokine family mediated through effects on both DCs and T cells critically contributes to disease outcome against this important human pathogen.