Dendritic cells of mucosa and skin: "recruited for vaccination"

Mucosae and skin are exposed to environmental antigens and are natural entry routes for most infectious agents. To maintain immunological tolerance and ensure protective immunity against pathogens, epithelial surfaces are surveyed permanently by antigen-presenting dendritic cells (DCs). Many DC subsets have been described in epithelial tissues, depending on the inflammatory state and the type of epithelium. Identification of the DC subset able to induce cytotoxic CD8+ T cells against antigens delivered via mucosae or skin, is a major issue for the development of efficient anti-infectious and anti-tumoral vaccines. Until recently, it was commonly accepted that Langerhans cells (LC), the prototype of immature DCs residing in skin and certain mucosae, can capture and process antigens and, in response to danger signals, undergo a maturation program allowing their migration to the draining lymph nodes for priming of na?ve T cells. This concept likely needs to be revisited. Recent evidence from animal models revealed that resident epithelial tissue DCs, including LCs, do not play a direct role in T cell priming, but may contribute to maintenance of peripheral tolerance. Alternatively, DCs newly recruited into muco-cutaneous tissues exposed to pro-inflammatory stimuli are responsible for efficient priming and differentiation of CD8+ T cells into cytolytic effectors. These DC originate from blood monocytes and can cross-present protein antigens to CD8+ T cells, which subsequently give rise to specific CTL effectors. Remarkably, components derived from bacteria, virus and chemicals capable to enhance CCL20 production in epithelia, promote CCR6-dependent DC recruitment and behave as adjuvants allowing for cross-primed CD8+ CTL. These advances in the dynamic and function of epithelial tissue DC provide a rationale for the screening of novel CD8+ T cell adjuvants and the design of novel mucosal and skin vaccines.