CD4 and Autoimmunity: In addition to its role in exhaustion, PD-1 is expressed in memory phenotype (MP), but not naïve, CD4 T cells in the steady state and plays an important role in peripheral tolerance and the prevention of autoimmunity in mouse models (Lin et al, 2007; Thangavelu et al, 2011; Pauken et al, 2015).