Protective and pathogenic roles of resident memory T cells in human skin disorders.

The human skin is populated by recirculating T cells and skin-sessile resident memory T cells (TRM). Skin TRM are constructed during immune responses against antigens that the host immune system encounters in the skin. TRM persist in the same sites for a long time and play important protective roles in skin immune responses in collaboration with other skin-composing cells such as dendritic cells and keratinocytes. These TRM with strong effector functions possibly also engender skin inflammatory disorders. Since human skin T cells, especially TRM, are phenotypically distinct from T cells in the blood circulation, T cells residing in the skin should be directly investigated, without presuming from the activities of blood T cells, in order to understand the functional characteristics of skin T cells in skin disorders. This review summarizes the features of human skin TRM and reviews the immunopathological involvement of TRM in human skin disorders such as infectious disease, inflammatory skin disease, and malignant skin tumors.