PTPN22 and type 1 diabetes mellitus: ,35,38 Indeed, T cells from healthy individuals73,82 as well as T1D patients74 with homozygous expression of the PTPN22 1858T allele were reported to be hyporesponsive to TCR stimulation consistent with the notion that PTPN22 R620W is a gain-of-function variant that blocks T cell activation.