Allergic contact dermatitis differs from most other immune reactions by its strict dose dependence during the elicitation phase. Moreover, almost all known contact allergens can also induce dose-dependent irritative dermatitis and in general only elicit allergic contact dermatitis in sensitized individuals when applied within a narrow dose range. Therefore, we hypothesized that elicitation of contact hypersensitivity (CHS) may require two signals, antigen-specific effector cell activation and a non-antigen-specific proinflammatory signal, both of which are provided by application of a sufficient dose of hapten. To dissociate these putative two signals, oxazolone-sensitized mice were ear challenged with a dose of the specific hapten which was too low to elicit CHS. At the same time, an unrelated hapten was applied in a conventional concentration to the same skin site. Whereas neither treatment alone elicited a significant CHS response, application of both compounds together resulted in a strong CHS response that was indistinguishable from that elicited by the full dose of the specific hapten. Upon coadministration of the irrelevant hapten, allergic contact dermatitis could be elicited even when the dose of the specific hapten was further reduced by a factor of 10 3 . In contrast, a dose reduction of the irrelevant hapten by a factor of two resulted in the loss of the CHS response. These data indicate that non-antigenspecific effects of epicutaneously applied haptens significantly contribute to the elicitation of CHS responses and that the capacity of the hapten to evoke this proinflammatory stimulus rather than its antigenicity is responsible for the strict concentration dependence.

Toll-like receptor (TLR) ligands lead to the induction of proinflammatory cytokines and are potent enhancers of specific immune responses. We show here that a single systemic dose of R-848, a ligand for TLR7, potently enhanced hapten sensitization during the induction of contact hypersensitivity (CHS). However, R-848 administration also resulted in a rapid and almost complete depletion of leukocytes from the blood. This effect was transient and was associated with general induction of endothelial adhesiveness. In response to R-848, endothelial cells up-regulated adhesion molecules in vitro and in vivo and leukocytes exhibited increased rolling on endothelia in R-848-treated animals. Adhesion molecule induction appeared to be a direct effect, because endothelial cells expressed TLR7 in vitro and in vivo. After R-848 treatment, the tissue residence time of leukocytes was markedly prolonged in all major peripheral organs. The resulting transiently reduced availability of peripheral-blood leukocytes (PBLs) ( IntroductionToll-like receptors (TLRs) are a family of mammalian proteins expressed on a variety of cell types of the immune system. 1 TLRs are able to recognize specific patterns conserved in microorganisms. TLR triggering leads to the induction of inflammatory responses and induces the development of specific immunity. 1 Consequently, specific TLR ligands, such as polyinosinicpolycytidylic acid (poly I:C), lipopolysaccharide (LPS), imidazoquinolines, or CpG oligodeoxynucleotides, have been employed as powerful immune adjuvants and may enhance specific antitumor and antiviral immunity. 2,3 TLR7 is predominantly expressed by plasmacytoid dendritic cells (DCs), myeloid DCs, 4 and B cells 5 and recognizes ssRNA. 6,7 R-848 and the structurally related compound imiquimod (the active substance of the drug Aldara; 3M Pharmaceuticals, St Paul, MN) are artificial ligands for TLR7. 8,9 These imidazoquinolines are immune response modifiers that possess potent antiviral 10 and antitumoral activity when applied topically to the skin. 11 This activity is in part mediated by the induction of type I interferons and inflammatory cytokines such as tumor necrosis factor-␣ (TNF-␣) and interleukin-1␣ (IL-1␣). 12 Plasmacytoid DCs have been found to be major targets of R-848, producing large amounts of interferon-␣ and IL-12p40 as well as up-regulating activation markers in response to R-848 both in vitro 13,14 and in vivo. 14 In addition, topical imiquimod has been shown to induce emigration of resident LCs from murine skin, thereby amplifying allergic contact hypersensitivity (CHS) reactions 15 as well as inducing the migration of immature human DCs into draining lymph nodes of cancer patients. 16 Imidazoquinolines also strongly enhance T helper-1 (Th1)-type immune responses: They induce IL-12 in human Langerhans cells (LCs) 17 and lead to production of high amounts of IFN-␥ and to suppression of IL-4 and IL-5 production by T cells in cultures of human peripheral-blood leukocytes (PBLs) and murine spleen cells. 18 Interestingly, pe...

Inflammatory processes are associated with the rapid migration of dendritic cells (DCs) to regional lymph nodes and depletion of these potent antigen-presenting cells (APCs) from the inflamed tissue. This study examined whether sites of cutaneous inflammation can be repopulated with DCs from a pool of immature DCs circulating in the blood. In adoptive transfer experiments with ex vivo-generated radioactively labeled primary bone marrow-derived DCs injected into mice challenged by an allergic contact dermatitis reaction, immature DCs were actively recruited from the blood to sites of cutaneous inflammation, whereas mature DCs were not. Immature, but not mature, DCs were able to adhere specifically to immobilized recombinant E-and P-selectin under static as well as under flow conditions. P-selectin-dependent adhesion of immature DCs correlates with their higher level of expression of the carbohydrate epitope cutaneous lymphocyte-associated antigen (CLA) and is blocked by a novel inhibitory antibody against mouse P-selectin glycoprotein ligand 1 (PSGL-1). Surprisingly, however, emigration of immature DCs into inflamed skin is retained in the presence of this anti-PSGL-1 antibody and is also normal when immature DCs are generated from fucosyltransferase (Fuc-T) Fuc-TVII-deficient mice. By contrast, emigration of wild-type immature DCs is reduced by adhesion-blocking anti-E-and P-selectin antibodies, and immature DCs generated ex vivo from IntroductionDendritic cells (DCs) are bone marrow-derived leukocytes that are specialized in antigen capture, processing, and presentation to T lymphocytes and are essential for the initiation and modulation of antigen-specific immune responses. 1 DC progenitors as well as more mature DCs are present in small numbers in the blood. 2 They seed nonlymphoid tissues and are primarily localized within epithelia, such as Langerhans cells in the epidermis. 3 On activation these cells undergo phenotypic changes that allow them to migrate from their site of residence to the T-cell areas of regional lymph nodes. 4 The factors that mediate trafficking from the periphery to lymphoid organs are well defined. 5-10 However, not much is known about the immigration of DC precursors to their tissue of residence and only a little information is available about immigration and turnover of DCs in inflamed tissues. Because pathogens, allergens, or contact with CD40L-expressing cells all lead to activation and emigration of resident DCs toward regional lymph nodes, 11 the site of inflammation is rapidly depleted of resident antigen-presenting cells (APCs). Thus, for the maintenance of the antigen-specific immune response, it appears necessary that nonresident APCs be actively recruited to inflamed tissue. The role of the small population of blood DCs is not clear, but it is possible that this cell type forms a "task force" of potent APCs that can rapidly relocate to sites of inflammation. 12 This would result in enhanced local antigen presentation to infiltrating effector T cells and sustained priming o...