CD4 and myocarditis: Mice deficient for PD-1 or PD-L1 similarly developed fatal myocarditis with massive infiltration of CD4+ and CD8+ T cells in an autoimmune-prone MRL-Faslpr/lpr background (27), (28), but PD-1-deficient mice did not develop autoimmune cardiomyopathy in a non-autoimmune-prone C57BL/6 background (29).