CTLA4 and Autoimmunity: It is thought that this regulatory function of CTLA‐4 operates constitutively in order to prevent costimulation of self‐reactive T cells in the steady state, since loss of CTLA‐4 function triggers profound autoimmunity in mice and humans (Tivol et al, 1995; Kuehn et al, 2014; Schubert et al, 2014).