Characteristics and sources of tissue-resident memory T cells in psoriasis relapse.

Tissue-resident memory T cells (Trm) are a sub-population of memory T cells that reside in skin tissue. Recent studies have revealed potential role of Trm in the reoccurrence of psoriasis, as these cells tend to be profusely infiltrated in the lesions observed during psoriasis relapse. Trm can be classified into CD8 Trm cells that are distributed mainly in the epidermis and CD4 Trm cells in the dermis. CD8 Trm is derived from circulating memory T cells and CD49aCD8 Trm takes a crucial role in psoriasis relapse. In contrast, CD4 Trm may originate from exTh17 cells and exTreg cells emerging from the inflammatory process. Since IL-23 can activate Trm, neutralizing antibodies against IL-23 are suggested to be more effective in clinical treatment. This review will focus on Trm cells in psoriasis relapsed lesions to reveal their mechanisms in the pathogenesis, relapse and transformation of psoriasis.