CTLA4 and Autoimmunity: Whilst Tregs possess multiple functions that mediate immune suppression, CTLA-4 function is widely appreciated as a fundamental characteristic of Treg suppressive abilities [1] Accordingly, global loss of CTLA-4 expression or conditional deletion of Treg CTLA-4 in mice results in fatal lymphoproliferative disease and systemic T cell-mediated autoimmunity [2–4].