Mast cells express IL17A, IL17F and RORC in lesional psoriatic skin, are activated before therapy and persist in high numbers in a resting state with IL-17A positivity after treatment.

Background: The proinflammatory cytokine IL-17 is known to play an important role in psoriasis pathogenesis, but little is known about its regulation in psoriasis or after treatment.

Objective: Aiming to investigate the role of IL-17 regulation in the resolution of psoriasis, we analyzed biopsy samples from patients with plaque psoriasis, including non-lesional and lesional skin at baseline and after anti-IL-17 and -IL-23 antibody, topical dithranol or UVB treatment as well as skin from healthy donors.

Methods: Skin biopsy samples were analyzed using immunostaining, RNA sequencing and in situ mRNA detection. In addition, we investigated IL-17 concentration of primary human skin mast cell supernatants after stimulation with the pro-inflammatory cytokines TNFα, IL-22, IL-23 and IFNγ.

Results: A high number of IL-17A+ mast cells persisted in resolved lesions after treatment and correlated inversely with the time span in remission. IL-17A+ mast cells were found in T cell-rich areas and near resident memory T cells in active and resolved lesions. CIBERSORTx deconvolution of RNA-seq data of active and dithranol-treated psoriasis lesions showed that activated mast cells were increased in psoriatic skin but returned to normal levels after treatment. Primary skin mast cells responded with an increased release of IL-17A after stimulation with the pro-inflammatory cytokines and in situ mRNA detection revealed positive signals for IL17A, IL17F and RORC in mast cells.

Conclusion: Thus, together with T cells, mast cells seem to be important players of the IL-23/IL17 axis underlying psoriasis pathogenesis and relevant for early disease recurrence.