In health, the non-recirculating nature and long-term persistence of tissue-resident memory T cells (TRMs) in tissues protects against invading pathogens. In disease, pathogenic TRMs contribute to the recurring traits of many skin diseases. We aimed to conduct a systematic literature review on the current understanding of the role of TRMs in skin diseases and identify gaps as well as future research paths. EMBASE, PubMed, SCOPUS, Web of Science, Clinicaltrials.gov and WHO Trials Registry were searched systematically for relevant studies from their inception to October 2020. Included studies were reviewed independently by two authors. This study was conducted in accordance with the PRISMA-S guidelines. This protocol was registered with the PROSPERO database (ref: CRD42020206416). We identified 96 studies meeting the inclusion criteria. TRMs have mostly been investigated in murine skin and in relation to infectious skin diseases. Pathogenic TRMs have been characterized in various skin diseases including psoriasis, vitiligo and cutaneous T-cell lymphoma. Studies are needed to discover biomarkers that may delineate TRMs poised for pathogenic activity in skin diseases and establish to which extent TRMs are contingent on the local skin microenvironment. Additionally, future studies may investigate the effects of current treatments on the persistence of pathogenic TRMs in human skin.

Objective: Psoriatic arthritis mutilans (PAM) is the most severe phenotype of psoriatic arthritis due to excessive bone erosion causing joint destruction and decreased functional capacity. The aim of this study was to investigate the prevalence of comorbidities among patients with PAM and the association between comorbidities and joint involvement.Methods: A total of 66 patients aged ≥18 years from the Nordic countries with past or present psoriasis along with at least one mutilated joint were included in the present study.Results: The median number of comorbid conditions per patient was 1 [interquartile range (IQR) 0–2] and 16.7% reported three or more comorbidities. The most frequent comorbidity was hypertension (36.4%). The median number of mutilated joints per patient was 3 (IQR 1–8.3; range 1–38).Conclusion: Two thirds of the patients with PAM reported comorbid conditions and the most frequent was hypertension which affected more than a third of the patients. However, this study was unable to detect any association between comorbidities and the severity of PAM.