Abstract

Evaluation of the preclinical phases of the classic autoimmune diseases including rheumatoid arthritis has been facilitated by the availability of autoantibody and genetic markers that point firmly towards the early dysregulation of the adaptive immune responses. The association of psoriatic disease with the human leucocyte antigen—Cw0602 (HLA-Cw0602) gene has likewise led to the perception that autoimmunity has a pivotal role in early psoriatic arthritis (PsA). However, this HLA-Cw0602 genetic association does not appear to hold for PsA or associated nail, scalp and intergluteal skin involvement. Of note, these three sites of psoriasis are predictive of PsA evolution. For initiation of both skin and nail disease there is a link with Koebnerisation, or site-specific trauma. Nail disease is most common in the dominant hand thumbnail, pointing towards local tissue factors as disease initiators Likewise, for PsA, there is also good evidence for a history of previous joint trauma and histological studies showing microdamage in normal entheses which are typical locations where PsA frequently occurs. Furthermore, subclinical enthesopathy including osteitis is common in subjects with psoriasis but without arthritis. Collectively, these findings indicate that the classic model of adaptive immune dysregulation does not generally hold for the early stages of PsA. The way in which knowledge pertaining to tissue-specific factors in PsA, combined with the emerging data relating to monogenic disorders and animal models, points towards perturbation in the healing response and dysregulation of innate immune responses in early PsA is discussed. The way in which this model explains the clinical disconnect between skin and joint disease and the emerging human data that support it are demonstrated.