While engagements of CTLA-4 and PD-1 to their respective ligands deactivate effector T cells, they enhance Treg function, as supported by the observation that blockade of CTLA-4 and PD-1 are known to exacerbate autoimmune diseases through deactivation of Tregs (Walker, 2013; Kong and Flynn, 2014). Here, PDCD1 is linked to autoimmune disease.