PTPN22 and colitis: In this regard, naïve Ptpn22-/- CD4+ T cells more readily adopt a Treg phenotype under conditions of weak TCR stimulation in vitro, compared to control T cells,54 whilst Ptpn22-/-, but not Ptpn22+/+, Tregs are able to limit Ptpn22-/- effector T cell-mediated inflammation in an adoptive T cell transfer model of colitis.24