Pathophysiology of Skin Resident Memory T Cells

Tissue resident memory T (T(RM)) cells reside in peripheral, non-lymphoid tissues such as the skin, where they act as alarm-sensor cells or cytotoxic cells. Physiologically, skin T(RM) cells persist for a long term and can be reactivated upon reinfection with the same antigen, thus serving as peripheral sentinels in the immune surveillance network. CD8(+)CD69(+)CD103(+) T(RM) cells are the well-characterized subtype that develops in the epidermis. The local mediators such as interleukin (IL)-15 and transforming growth factor (TGF)-β are required for the formation of long-lived T(RM) cell population in skin. Skin T(RM) cells engage virus-infected cells, proliferate in situ in response to local antigens and do not migrate out of the epidermis. Secondary T(RM) cell populations are derived from pre-existing T(RM) cells and newly recruited T(RM) precursors from the circulation. In addition to microbial pathogens, topical application of chemical allergen to skin causes delayed-type hypersensitivity and amplifies the number of antigen-specific CD8(+) T(RM) cells at challenged site. Skin T(RM) cells are also involved in the pathological conditions, including vitiligo, psoriasis, fixed drug eruption and cutaneous T-cell lymphoma (CTCL). The functions of these T(RM) cells seem to be different, depending on each pathology. Psoriasis plaques are seen in a recurrent manner especially at the originally affected sites. Upon stimulation of the skin of psoriasis patients, the CD8(+)CD103(+)CD49a(-) T(RM) cells in the epidermis seem to be reactivated and initiate IL-17A production. Meanwhile, autoreactive CD8(+)CD103(+)CD49a(+) T(RM) cells secreting interferon-γ are present in lesional vitiligo skin. Fixed drug eruption is another disease where skin T(RM) cells evoke its characteristic clinical appearance upon administration of a causative drug. Intraepidermal CD8(+) T(RM) cells with an effector-memory phenotype resident in the skin lesions of fixed drug eruption play a major contributing role in the development of localized tissue damage. CTCL develops primarily in the skin by a clonal expansion of a transformed T(RM) cells. CD8(+) CTCL with the pagetoid epidermotropic histology is considered to originate from epidermal CD8(+) T(RM) cells. This review will discuss the current understanding of skin T(RM) biology and their contribution to skin homeostasis and diseases.