PTPN22 and psoriatic arthritis: Interestingly, while the PTPN22 R620W polymorphism does not associate with skin psoriasis, it does increase the risk of psoriatic arthritis (Bowes et al., 2015), suggesting that the two diseases have diverging pathogeneses, and PTPN22 may in some way alter the balance or phenotype of CD8 and/or Th17 cells, particularly when the known action of PTPN22 on CD4 T cell differentiation is taken into consideration (Vang et al., 2013).