Functional Diversity of Human Dendritic Cells.

At the crossroad between innate and adaptive immunity are the dendritic Cells (DCs), a "novel cell type." discovered in 1973 by Ralph Steinman. Although not entirely appreciated at first, it is clear that they play a critical role as specialized antigen-presenting cells and essential mediators in shaping immune reactivity and tolerance. Dendritic cells are now recognized as a heterogeneous group of cells in terms of cell-surface markers, anatomic location, and function adapted to protect against an array of pathogens and conditions. Importantly, these subsets are also unique to each species. While significant progress has been made on the identification and function of mouse DC subsets, much less is known about human cells. Here we review the fascinating biology of human skin DCs and describe tolerogenic principles that are critical in maintaining immune homeostasis and for controlling inflammation, as well as mechanisms that are fundamental to confer immunity. We surmise that these principles could be applied to DCs across organs, and could be harnessed for the treatment of various human autoimmune, inflammatory diseases, as well as cancer. Importantly, to leverage the relevance of basic research to the clinical setting, it is first necessary to determine the functional homology between mouse and human DCs. We discuss practical steps towards this aim.