Histamine receptor H1 signaling on dendritic cells plays a key role in the IFN-γ/IL-17 balance in T cell-mediated skin inflammation.

BACKGROUND: The diverse effects of histamine on immune regulation are a result of the differential expression and regulation of 4 histamine receptors. Many of the immediate allergic and inflammatory actions of histamine are mediated via the type 1 receptor (H1R). OBJECTIVES: We hypothesized that H1R was involved in the fine-tuning of the initiation of T cell-mediated skin pathology-that is, dermatitis. METHODS: The impact of the H1R invalidation on the development of skin inflammation was analyzed in a mouse model of atopic dermatitis. RESULTS: We show that H1r(-)/(-) mice developed reduced allergen-specific skin lesions. Lack of H1R expression on dendritic cells (DCs) led to diminished IL-12, upregulated IL-23, and IL-6 production upon allergen stimulation. H1R engagement on dendritic cells was necessary for DC activation and subsequent priming of effector IFN-γ(+)CD8(+) T cells. We demonstrate here that H1R blockade on DCs promotes generation of noneffector IL-17(+)CD8(+) T cells that are unable to initiate the skin inflammation. CONCLUSION: Our data identify that histamine signaling through the H1R on DCs is an important early event conditioning the quality of the skin effector immune response.

Inducible costimulator (ICOS) is a marker for highly suppressive antigen-specific T cells sharing features of TH17/TH1 and regulatory T cells.

BACKGROUND: CD4(+)CD25(+) regulatory T (Treg) cells are involved in the downmodulation of numerous immune responses to pathogens, tumors, or allergens. OBJECTIVE: In this study, we further characterized the nature of Treg cells that control skin inflammatory reactions to haptens. METHODS: In a model of contact hypersensitivity to 2,4-dinitro-fluorobenzene, we have investigated the phenotype, the specificity, and the origin of Treg cells that modulate the priming of effector CD8(+) T cells responsible for the development of the pathology. RESULTS: 2,4-Dinitrofluorobenzene immunization induced a population of CD4(+)CD25(+) Treg cells that controlled CD8(+) T-cell effector responses in a hapten-specific manner in vivo. High levels of inducible costimulator (ICOS) expression defined a population of CD4(+)CD25(+)FoxP3(+) (forkhead box protein 3) Treg cells that presented superior suppressive activity. Importantly, ICOS(+) Treg cells were distinguishable from all other FoxP3(+) Treg cells by the expression of IL-10, IL-17, and IFN-gamma. Hapten-specific Treg cells proliferating in response to their cognate antigen in vivo predominantly displayed a CD25(+)FoxP3(+)ICOS(+) phenotype. By using reporter mice, we showed that ICOS(+) Treg cells derived from the expansion of natural CD4(+)FoxP3(+) Treg cells rather than generation of adaptive Treg cells. Furthermore, the generation of ICOS(+) Treg cells depended on innate cells rather than the effector CD8(+) T-cell population. CONCLUSION: Taken together, our data show that a population of CD4(+)CD25(+)FoxP3(+) T cells upregulates ICOS on in vivo sensitization and specifically suppresses hapten-reactive CD8(+) T cells both in vivo and in vitro.

Skin exposure to weak and moderate contact allergens induces IFNgamma production by lymph node cells of CD4+ T-cell-depleted mice.

Allergic contact dermatitis (ACD) is mediated by hapten-specific CD8+ T cells and downregulated by CD4+ T cells. We have recently shown in a model of ACD to weak haptens that priming of IFNgamma-producing CD8+ T cells and the development of skin inflammation could be obtained in mice deficient in CD4+ T cells. Here we show that IFNgamma production by lymph node (LN) cells draining the site of skin sensitization of CD4+ T-cell-deficient mice is a marker of the sensitizing properties of weak haptens. LN cells from mice sensitized as in the classical local lymph node assay (LLNA) were recovered at day 5, then cultured for 20 hours in the presence of submitogenic doses of phytohemagglutinin, and finally tested for the production of IFNgamma. Results show that: (i) production of INFgamma by LN cells was induced by weak and moderate allergens in a dose-dependent fashion; (ii) the magnitude of IFNgamma production paralleled the sensitizing properties of allergens allowing to classify them as moderate or weak haptens; (iii) chemicals without sensitizing properties were unable to stimulate IFNgamma production by LN cells. Therefore, the IFNgamma LLNA appears as a sensitive, specific, and robust assay to detect weak contact allergens.

Skin-infiltrating CD8+ T cells initiate atopic dermatitis lesions.

Skin lesions in the allergic form of atopic dermatitis (AD) are induced by allergen-specific T cells that infiltrate the skin at the site of allergen exposure. Although Th2-type CD4+ T cells appear to be crucial in AD pathophysiology, little is known about the contribution of CD8+ T cells in the development of the allergic skin inflammation. In the present study, we have analyzed the respective role of CD8+ and CD4+ T cells in the development of AD skin lesions in a mouse model of allergen-induced AD. In sensitized mice, CD8+ T cells are rapidly and transiently recruited to the allergen-exposed site and initiate the inflammatory process leading to skin infiltration with eosinophils and Th1/Th2-producing cells. CD8+ T cell-depleted mice show no inflammation, demonstrating that these cells are mandatory for the development of AD. In contrast, CD4+ T cell-depleted mice develop a severe form of eczema. Furthermore, adoptive transfer of CD8+ T cells from sensitized mice into naive recipient mice leads to skin inflammation soon after allergen exposure. These data indicate that allergen-primed CD8+ T cells are required for the development of AD-like lesions in mice.

Skin contact irritation conditions the development and severity of allergic contact dermatitis.

Irritant contact dermatitis (ICD) is a frequent inflammatory skin disease induced by skin contact with low molecular weight chemicals such as haptens endowed with proinflammatory properties. Allergic contact dermatitis (ACD) is a frequent complication of ICD and is mediated by hapten-specific T cells primed in lymph nodes by skin emigrating dendritic cells. The aim of this study was to analyze the relationship between ICD and ACD to 2,4-dinitrofluorobenezene (DNFB) in C57BL/6 and BALB/C mice, which develop a severe and a moderate skin inflammation, respectively. Upon a single skin painting with DNFB, C57BL/6 developed within hours a more severe dose-dependent ICD response as compared to BALB/C mice, which was associated with enhanced upregulation of IL-1beta, IL-6, and IL-10. Skin exposure to a low dose of DNFB resulted, in both strains, in a low ICD that resolved in a few hours. Alternatively, skin painting with either an intermediate or a high DNFB concentration induced an ICD that subsequently gave rise to an ACD reaction whose intensity was proportional to the magnitude of the ICD response and was more severe in C57BL/6 mice than in BALB/C mice. In conclusion, the hapten-induced skin contact irritation conditions the development and the severity of ACD.

CD8+ T cells are effector cells of contact dermatitis to common skin allergens in mice.

Allergic contact dermatitis (ACD) to strong experimental haptens is mediated by specific CD8+ T cells. Here, we show that similar mechanisms occur for weak haptens, which comprise the vast majority of chemicals responsible for human ACD. We used a model of ACD, that is, the contact hypersensitivity reaction, to test for the allergenicity of three weak haptens involved in fragrance allergy. ACD to weak haptens could not be induced in normal mice. In contrast, mice acutely depleted in CD4+ T cells developed a typical ACD reaction to the three weak fragrance allergens that peaked 24 hours after challenge. Priming of CD8+ T cells was observed in draining lymph nodes 5 days after sensitization and development of ACD was associated with the infiltration of activated CD8+ T cells in the challenged skin. CD8+ T cells were effectors of the ACD reaction as in vivo treatment with depleting anti-CD8 mAbs abrogated the ACD responses and as purified CD8+ T cells could adoptively transfer ACD to naive recipients. In conclusion, our data demonstrate a dominant role of CD8+ T cells as initiators of ACD to weak haptens, and suggest that CD8+ T cells may represent potential targets for preventing or treating ACD.

Immunosuppressive antimetabolites inhibit induction of contact hypersensitivity while lymphoablative drugs also prevent its expression.

Contact hypersensitivity is one of the most common skin diseases and its pharmacological control is an important clinical issue. We investigated the control of contact hypersensitivity by immunosuppressive drugs administered during sensitization or challenge. Mycophenolate mofetil, methotrexate and 5-fluorouracil completely inhibited contact hypersensitivity when administered during sensitization whereas they did not decrease inflammatory reaction when administered during challenge. Conversely, mitoxantrone, and cyclophosphamide, given as a single injection at the time of sensitization or challenge, completely inhibited the reaction, a property associated with T and B cell depletion. The data indicate that antimetabolites which are cell cycle dependent inhibit clonal expansion and subsequent differentiation of cytotoxic CD8+ T cells. Their lack of effect at the time of challenge indicates that T cell proliferation is not required for the expression of effector or regulatory T cell activation. Conversely lymphoablative drugs can inactivate or destroy differentiated cytotoxic T cells with rapid kinetics.