PDCD1 and Autoimmunity: Unlike mouse studies in which removal of FOXP3+ Tregs led to “catastrophic autoimmunity” (30), the depletion of Tregs by anti-CD25 Abs in these humanized mice did not result in fulminant autoimmune manifestations, as the animals remained asymptomatic, although some T cells acquired an activated phenotype as illustrated by the potentially transient expression of FOXP3 in some CD25– T cells that has been associated with T cell activation in humans and by a modest upregulation of programmed cell death 1 (PD-1) (Supplemental Figure 5, C and E, and refs. 31, 32).