T cell responses in psoriasis and psoriatic arthritis.
According to the current view the histological features of psoriasis arise as a consequence of the interplay between T cells, dendritic cells and keratinocytes giving rise to a self-perpetuating loop that amplifies and sustains inflammation in lesional skin. In particular, myeloid dendritic cell secretion of IL-23 and IL-12 activates IL-17-producing T cells, Th22 and Th1 cells, leading to the production of inflammatory cytokines such as IL-17, IFN-γ, TNF and IL-22. These cytokines mediate effects on keratinocytes thus establishing the inflammatory loop. Unlike psoriasis the immunopathogenic features of psoriatic arthritis are poorly characterized and there is a gap in the knowledge of the pathogenic link between inflammatory T cell responses arising in the skin and the development of joint inflammation. Here we review the knowledge accumulated over the years from the early evidence of autoreactive CD8 T cells that was studied mainly in the years 1990s and 2000s to the recent findings of the role of Th17, Tc17 cells and γδ T cells in psoriatic disease pathogenesis. The review will also focus on common and distinguishing features of T cell responses in psoriatic plaques and in synovial fluid of patients with psoriatic arthritis. The integration of this information could help to distinguish the role played by T cells in the initiation phase of the disease from the role of T cells as downstream effectors sustaining inflammation in psoriatic plaques and potentially leading to disease manifestation in distant joints.