Dermal Dendritic Cells are Important Members of the Skin Immune System

Abstract

Dendritic cells (DCs) were originally described as a distinct cell type by Steinman et al. in mouse spleen suspensions¹. Since then much has been learned about this bone-marrow derived member of the immune system. DCs belong to a lineage of cells that constitute a defined network of antigen-presenting cells (APCs) different from macrophages. DCs are distributed in trace amounts in barrier zones of antigen entry into the body such as the skin, gut, lung and blood. They contribute to the surveillance function of the immune system and are key initiators of primary immune responses². Many insights into the physiology of DC eminate from studies with skin-derived epidermal Langerhans cells. These cells are located in the suprabasal layer of the epidermis, and are characterized by high levels of CDla on their dendritic processes, and the presence of Birbeck granules. The current concept about the initiation of immune responses in the skin is that antigen which percolates through the epidermis is captured by Langerhans cells, processed and presented on MHC class II molecules on their surface. As motile cells they reach the afferent lymph node and prime naive T cells to the antigen of interest³. Several reports have recently challenged the concept that dendritic APCs in the epidermis are the only important APCs in skin immune responses. In contact hypersensitivity reactions a functional role for dermal APCs has been demonstrated^4,5. Furthermore a role in UV-B dependent tolerance in mice has been shown⁶. Recently a dendritic cell population has been identified in human dermis by means of immunohistochemistry. These cells are bone-marrow derived, express high levels of MHC class II molecules and factor XIIIa which discriminates them from conventional Langerhans cells⁷.