Allergic contact hypersensitivity (CHS) is a T cell–mediated inflammatory skin disease. Interleukin (IL)-12 is considered to be important in the generation of the allergen-specific T cell response. Loss of IL-12 function in IL-12Rβ2–deficient mice, however, did not ameliorate the allergic immune response, suggesting alternate IL-12–independent pathways in the induction of CHS. Because exposure to contact allergens always takes place in the presence of microbial skin flora, we investigated the potential role of Toll-like receptors (TLRs) in the induction of CHS. Using mice deficient in TLR4, the receptor for bacterial lipopolysaccharide (LPS), IL-12 receptor (R) β2, or both, we show that the concomitant absence of TLR4 and IL-12Rβ2, but not the absence of TLR4 or IL-12Rβ2 alone, prevented DC-mediated sensitization, generation of effector T cells, and the subsequent CHS response to 2,4,6-trinitro-1-chlorobenzene (TNCB), oxazolone, and fluorescein isothiocyanate. Introduction of the TLR4 transgene into the TLR4/IL-12Rβ2 mutant restored the CHS inducibility, showing a requirement for TLR4 in IL-12–independent CHS induction. Furthermore, the concomitant absence of TLR2 and TLR4 prevented the induction of CHS to TNCB in IL-12–competent mice. Finally, CHS was inducible in germ-free wild-type and IL-12Rβ2–deficient mice, but not in germ-free TLR4/IL-12Rβ2 double deficient mice, suggesting that the necessary TLR activation may proceed via endogenous ligands.

Tissue-selective homing is established during naive T cell activation by the tissue microenvironment and tissue-specific dendritic cells (DC). The factors driving induction and maintenance of T cell homing patterns are still largely unknown. Here we show that soluble factors produced during the interaction of T cells with CD11c + DC isolated from skin-or small intestine-associated tissues differentially modulate expression of the corresponding tissue-selective homing receptors (E-selectin ligands and a4b7 integrin/ CCR9, respectively) on murine CD8 + T cells. Injection of tissue-specific DC via different routes induces T cells with homing receptors characteristic of the corresponding local tissue microenvironment, independent of the origin of the DC. These data indicate an important role for signals delivered in trans. Moreover, DC can reprogram the homing receptor expression on T cells previously polarized in vitro for homing to skin or small intestine. Importantly, skin-homing memory T cells stimulated directly ex vivo can also be reprogrammed by intestinal DC to a gut-homing phenotype. Our results show that tissue-selective homing receptor expression on effector and memory T cells is governed by inductive as well as suppressive signals from both DC and tissue microenvironments.

Tolerance against self-and harmless environmental antigens involves several levels of immune regulation. Whereas deletional mechanisms attempt to purge the B and T cell repertoire of self-reactive specifi cities, antigen-specifi c suppression of autoimmunity by CD4 + Foxp3 + regulatory (T reg) T cells also plays a critical role in maintaining self-tolerance. The importance of T reg cells is highlighted by the severe autoimmunity in humans and mice rendered T reg cell -defi cient because of lesions in the FOXP3 gene, which in mice can be prevented by adoptive transfer of purifi ed T reg cells ( 1 -3 ). In addition, neonatal or adult ablation of T reg cells, or adoptive transfer of T reg cell -depleted CD4 + T cells to immunodefi cient mice rapidly triggers severe autoimmune or infl ammatory disease ( 4 -6 ). Collectively, these data demonstrate that T reg cells are required for both the establishment and maintenance of self-tolerance in vivo.T reg cells can be subdivided based on expression of homing receptors thought to direct their migration to lymphoid versus nonlymphoid tissues ( 7 ). However, the relative contributions of the various T reg cell subsets to the maintenance of self-tolerance are not well defi ned. Several reports have demonstrated that T reg cells function within nonlymphoid sites to restrict T cell responses to foreign antigens during experimentally induced acute infl ammation or infection ( 8 -11 ). In addition, we have shown that mice whose T reg cells lack the chemokine receptor CCR4 develop spontaneous infl ammatory disease in skin and lungs ( 12 ). However, although CCR4 helps direct T cell migration to the skin and lung airways, it may also facilitate interactions between T reg cells and antigenpresenting cells within secondary lymphoid tissues. Thus, the infl ammatory disease in these animals is likely the result of impaired T reg cell function in both lymphoid and nonlymphoid organs, and the importance of nonlymphoid T reg cells in maintaining tolerance in the absence of a strong infl ammatory stimulus has not been adequately addressed.Surface expression of functional E-and Pselectin ligands (E-/P-lig) is required for optimal migration of eff ector T cells to infl amed skin ( 13 ). Interestingly, a high percentage of circulating human and mouse T reg cells express Eand/or P-lig, and Foxp3 + T reg cells make up a large fraction of the CD4 + T cells in normal skin from both humans and mice ( 12,14 ). This Cutaneous immune responses must be tightly controlled to prevent unwanted infl ammation in response to innocuous antigens, while maintaining the ability to combat skin-tropic pathogens. Foxp3 + regulatory T (T reg) cells are potent immune regulators and are found at high frequency in both human and mouse skin. Although T reg cells migrate to the skin and can dampen immune responses during experimentally induced infl ammation or infection, the importance of cutaneous T reg cells for maintaining normal immune homeostasis in the skin has not been addressed. To selectively block T reg cell...

One of the unusual properties of chemically reactive haptens is their capacity to simultaneously generate immunogenic determinants for hapten-specific CD8+ and CD4+ T cells. Surprisingly, however, a clear dominance of CD8+ effector T cells is observed in murine contact hypersensitivity to various haptens and upon T cell priming with hapten-modified APCs in vitro. In this study we show that trinitrophenyl-specific CD8+ T cells actively prevent CD4+ T cell priming in vitro. This process requires cell-cell contact and is dependent on the expression of Fas on the CD4+ T cells. Our results reveal an important Fas-dependent mechanism for the regulation of hapten-specific CD4+ T cell responses by CD8+ T cells, which causes the dominance of CD8+ effector T cells and the active suppression of a CD4+ T cell response. Moreover, our demonstration of reduced contact hypersensitivity to trinitrophenyl in the absence of Fas, but not of perforin and/or granzymes A and B, underlines the important role of Fas as a pathogenetic factor for contact hypersensitivity.

Dendritic cells (DC) presenting tumor antigens are crucial to induce potent T cell-mediated anti-tumor immune responses. Therefore DC-based cancer vaccines have been established for therapy, however clinical outcomes are often poor and need improvement. Using a mouse model of B16 melanoma, we found that the route of preventive DC vaccination critically determined tumor control. While repeated DC vaccination did not show an impact of the route of DC application on the prevention of tumor growth, a single DC vaccination revealed that both the imprinting of skin homing receptors and an enhanced proliferation state of effector T cells was seen only upon intracutaneous but not intravenous or intraperitoneal immunization. Tumor growth was prevented only by intracutaneous DC vaccination. Our results indicate that under suboptimal conditions the route of DC vaccination crucially determines the efficiency of tumor defense. DC-based strategies for immunotherapy of cancer should take into account the immunization route in order to optimize tissue targeting of tumor antigen specific T cells.