The dermis contains langerin + dendritic cells that develop and function independently of epidermal Langerhans cells Lionel

JEM © The Rockefeller University Press $30.00 Vol. 204, No. 13, December 24, 2007 3119-3131 www.jem.org/cgi/doi/10.1084/jem.20071724 3119 Langerhans cells (LCs) constitute a subset of DCs. In their immature state, they reside in the stratifi ed squamous epidermal layer of the skin and in the mucosal epithelia lining the ocular, oral, and vaginal surfaces. LCs are thought to detect pathogens that penetrate epithelial barriers and, after undergoing a phase of maturation, convey this information via lymphatic vessels to T cells present in LNs ( 1 – 3 ). Recent data suggest that migratory LCs play an indirect role in T cell priming, possibly in carrying over antigens to those DCs that reside throughout their life cycle in LNs ( 4 ). These last DCs are denoted as lymphoid tissue – resident DCs to distinguish them from tissue-derived (migratory) DCs ( 5, 6 ). Lymphoid tissue – resident DCs are categorized into CD8 + and CD8 subsets and have an “ immature ” phenotype, which is characterized by low levels of MHC class II (MHCII) and costimulatory molecules. They collect and present antigens in the lymphoid organ itself, and they can respond to activatory signals and mature in situ. In addition to LCs, the skin contains a second type of DC, the dermal DC (DDC). Epidermal LCs and DDCs can migrate to cutaneous-draining LNs (CLNs) under both steadystate and infl ammatory conditions, and they constitute the direct precursors of the epidermal LCand DDC-derived DCs found in CLNs, respectively. These two types of skin-derived DCs express a “ mature ” phenotype, which is characterized by a CD11c inter to high , MHCII high