A key challenge in psoriasis therapy is the tendency for lesions to recur in previously affected anatomical locations after treatment discontinuation following lesion resolution. Available evidence supports the concept of a localized immunological 'memory' that persists in resolved skin after complete disappearance of visible inflammation, as well as the role of a specific subpopulation of T cells characterized by the dermotropic CCR4+ phenotype and forming a local memory. Increasing knowledge of the interleukin (IL)-23/T helper 17 (Th17) cell pathway in psoriasis immunopathology is pointing away from the historical classification of psoriasis as primarily a Th1-type disease. Research undertaken from the 1990s to the mid-2000s provided evidence for the existence of a large population of CD8+ and CD4+ tissue-resident memory T cells in resolved skin, which can initiate and perpetuate immune responses of psoriasis in the absence of T-cell recruitment from the blood. Dendritic cells (DCs) are antigen-presenting cells that contribute to psoriasis pathology via the secretion of IL-23, the upstream regulator of Th17 cells, while plasmacytoid DCs are involved via IL-36 signalling and type I interferon activation. Overall, the evidence discussed in this review indicates that IL-23-driven/IL-17-producing T cells play a critical role in psoriasis pathology and recurrence, making these cytokines logical therapeutic targets. The review also explains the clinical efficacy of IL-17 and IL-23 receptor blockers in the treatment of psoriasis.
© 2021 British Association of Dermatologists.