CTLA-4-Fc inhibits PHA-induced IL-2 production and CD4⁺ T-cell activation by an IDO-dependent mechanism. CD69 expression on gated CD4⁺ T cells from PBMCs in 4 different conditions: (A) no pretreatment and no activation; (B) no pretreatment and subsequent PHA activation; (C) CTLA-4-Fc pretreatment and subsequent PHA activation; and (D) CTLA-4-Fc + 1-MT pretreatment and subsequent PHA activation. A single representative experiment is shown. (E) IL-2 production using the 4 different conditions mentioned in panels A-D. Mean values obtained from experiments using cells from 3 separate donors are presented. Mean values plus standard errors are shown.