Comprehensive analysis of MHC-II expression in healthy human skin.

A number of antigen-presenting cells (APCs) expressing major histocompatibility complex class II (MHC-II) have been identified in healthy human skin including the Langerhans cells of the epidermis and the three recently defined dermal APC subsets. It is well documented that in other tissues HLA-DR expression is not exclusive to APCs. Following a comprehensive analysis of the cells in human skin using flow cytometry and fluorescence immunohistochemistry, we have identified additional cell subsets that express HLA-DR. Using markers exclusive for blood and lymphatic endothelium, we demonstrated that both of these cell populations have the capacity to express HLA-DR. In addition, a small subset of dermal T lymphocytes was found to express low-level HLA-DR suggesting an activated phenotype. Dermal T lymphocytes were often in intimate contact with either CD1a(+) CD207(-) dermal APCs or CD1a(+) CD207(+) dermal Langerhans cells, possibly explaining the activated phenotype of a subset of dermal T lymphocytes.