Triggering the succinate receptor GPR91 on dendritic cells enhances immunity.

Succinate acts as an extracellular mediator signaling through the G protein-coupled receptor GPR91. Here we show that dendritic cells had high expression of GPR91. In these cells, succinate triggered intracellular calcium mobilization, induced migratory responses and acted in synergy with Toll-like receptor ligands for the production of proinflammatory cytokines. Succinate also enhanced antigen-specific activation of human and mouse helper T cells. GPR91-deficient mice had less migration of Langerhans cells to draining lymph nodes and impaired tetanus toxoid-specific recall T cell responses. Furthermore, GPR91-deficient allografts elicited weaker transplant rejection than did the corresponding grafts from wild-type mice. Our results suggest that the succinate receptor GPR91 is involved in sensing immunological danger, which establishes a link between immunity and a metabolite of cellular respiration.

IL-12 instructs skin homing of human Th2 cells.

Distinct pattern of homing receptors determines the tissue preference for T cells to exert their effector functions. This homing competence is mostly determined early during T cell activation of naive T cells. In contrast, mechanisms governing the acquisition of particular homing receptors by T cells of the memory phenotype remain enigmatic. Th2 cell-mediated allergic diseases tend to flare during infections despite that these infections prime APCs to produce the prototypic Th1 cell-differentiating cytokine IL-12. In this study, we investigate the effect of IL-12 on the regulation of cutaneous lymphocyte Ag (CLA) on differentiated Th2 cells and consequences of this expression for allergic inflammation. Upon activation with IL-12, CLA- Th2 cells rapidly up-regulated IL-12Rbeta2 chain, alpha(1-3)-fucosyltransferase VII, and CLA molecules. IL-12-mediated CLA expression on Th2 cells was functional because it mediated rolling of these Th2 cells on E-selectin in vitro and migration into human skin grafts in SCID mice. CLA induction occurred immediately after exposure to IL-12 and was independent of IFN-gamma expression. In accordance, the transcription factor mediating IFN-gamma expression, T-bet, does not directly affect CLA expression. However, CLA expression was further enhanced after IL-12 treatment of T-bet+ -transfected Th2 cells in agreement with an increased IL-12 responsiveness of these cells caused by T-bet. The finding that IL-12 conferred skin-homing potential to already differentiated Th2 cells before inducing a switch in their cytokine production profile may explain the observed exacerbation of allergic skin diseases following bacterial infections.

CCL18 is expressed in atopic dermatitis and mediates skin homing of human memory T cells.

CCL18 is a human chemokine secreted by monocytes and dendritic cells. The receptor for CCL18 is not yet known and the functions of this chemokine on immune cells are not fully elucidated. In this study, we describe that CCL18 is present in skin biopsies of atopic dermatitis (AD) patients but not in normal or psoriatic skin. CCL18 was specifically expressed by APCs in the dermis and by Langerhans and inflammatory dendritic epidermal cells in the epidermis. In addition, the serum levels of CCL18 and the percentages of CCL18-producing monocyte/macrophages and dendritic cells were significantly increased in AD patients compared with healthy controls. Furthermore, we demonstrate that CCL18 binds to CLA(+) T cells in peripheral blood of AD patients and healthy individuals and induces migration of AD-derived memory T cells in vitro and in human skin-transplanted SCID mice. These findings highlight a unique role of CCL18 in AD and reveal a novel function of this chemokine mediating skin homing of a subpopulation of human memory T cells.

Sustained T-bet expression confers polarized human TH2 cells with TH1-like cytokine production and migratory capacities.

BACKGROUND: The transcription factor T-bet mediates IFN-gamma production by T(H)1 cells and suppresses T(H)2 cytokine production when ectopically expressed in polarized murine T(H)2 cells. Thus T-bet-mediated inhibition of T(H)2 cytokine production might be beneficial for the treatment of allergic diseases like asthma or atopic dermatitis. OBJECTIVE: We sought to investigate the effects of ectopic T-bet expression in highly polarized human T(H)2 cells obtained from skin biopsy specimens of patients with atopic dermatitis. METHODS: The cytokine production of T(H)2 cells retrovirally transfected with a vector expressing human T-bet was determined by means of intracellular FACS staining and ELISA. The effects of T-bet transfection were analyzed at the mRNA level by means of real-time PCR and DNA microarrays and confirmed by using functional chemokine response assays. RESULTS: Transfection of T-bet into T(H)2 cells induced high levels of IFN-gamma and suppressed IL-5, but IL-2 and IL-4 production remained unchanged. T-bet transfection also induced IL-12Rbeta2 and CXCR3 expression on human T(H)2 cells, whereas the IL-18 receptor was only induced as a consequence of T-bet-mediated increased responsiveness to IL-12. Furthermore, sustained T-bet expression in human T(H)2 cells induced IL-2 production and decreased the secretion of IL-4. In addition, the chemokine receptor repertoire of these cells was changed toward a T(H)1-like profile. CONCLUSION: The combined switch in cytokine pattern and migratory potential of highly polarized human T(H)2 cells mediated by T-bet might provide an additional advantage for the treatment of allergic diseases.

Targeting CLA/E-selectin interactions prevents CCR4-mediated recruitment of human Th2 memory cells to human skin in vivo.

Naive Th cells, bearing receptors for cutaneous antigens, become activated in skin-draining lymph nodes and express cutaneous lymphocyte antigen (CLA), which confers to these cells the capacity to migrate into the skin to exert their normal effector functions. In the case of atopic dermatitis (AD), allergen-specific Th2 cells generate exacerbated responses and induce skin inflammation. In such a situation, interfering with the specific mechanism of skin homing would provide a therapeutic benefit. Here we report that CLA+ Th2 memory cells, derived from skin lesions of AD patients, selectively migrate to human skin grafts transplanted onto SCID mice in response to CCR4 but not CCR3, CCR8 or CXCR3 ligands. Skin homing of human CCR4+ Th2 memory cells was Pertussis toxin sensitive and restricted to the CLA+ subset. Furthermore, treatment of these mice with anti-E-selectin monoclonal antibody was sufficient to prevent CCL22-mediated Th2 cell migration to human skin, which both, validates the model and highlights the importance of CLA/E-selectin interactions in the homing process of Th2 cells to the skin. Using this mechanistic model we demonstrate that skin homing of human Th2 memory cells can be efficiently suppressed using a low molecular weight E-selectin antagonist, which is of clinical relevance for the treatment of inflammatory skin diseases, including AD.