CD4 and Autoimmunity: In the light of the foregoing discussion and to further elucidate the putative role of PD-1 in T cell function in autoimmunity development, we examined PD-1 expression in activated CD4+CD25+ T cells and Treg population after CD3/CD28 stimulation of peripheral blood lymphocytes of a group of T1D patients and of a group of healthy controls.