This hypothesis is sustained by the identification and functional characterization of CD8+ Treg cells in T1DM patients, revealing a new CD8+ Treg cell population, which is defective—i.e., less immunosuppressive—due to lower expression of PD-1, pointing to the immunosuppressive role of PD-1 in Tregs [44]. This evidence concerns the gene PDCD1 and type 1 diabetes mellitus.