IRF1 and type 1 diabetes mellitus: Both decreased PD-1 gene expression in peripheral CD4+ T cells and low frequency of circulating PD-1+ CD4+ T cells have been reported in patients with T1DM.[11] In addition, PD-L1 is also expressed in human islet β cells with T1DM, and it is up-regulated by both type I and type II interferons via interferon regulatory factor 1, possibly to reduce the autoimmune attack.[12]