Effect of CTLA-4-Fc on IDO mRNA expression and tryptophan catabolism in different cell types. (A) IDO mRNA expression was analyzed in isolated CD4⁺, CD8⁺, and CD19⁺ cells, and unseparated human PBMCs after 12 hours of culture in the absence or presence of CTLA-4-Fc. (B) Unseparated PBMCs were cultured in the absence or presence of CTLA-4-Fc for 12 hours. These PBMCs were then separated into CD4⁺, CD8⁺, and CD19⁺ cells and were analyzed for IDO mRNA expression. For panels A and B, □ indicates control; ▪, CTLA-4-Fc. *P values less than .05 are based on 5 independent experiments. (C) Kyn/Trp ratio was analyzed by HPLC on supernatants collected from unseparated PBMCs cultured in the presence or absence of CTLA-4-Fc or rhIFN-γ; CD4⁺ T cells were isolated from untreated and CTLA-4-Fc–treated PBMCs and cultured for 24 hours in the same conditions; kyn/Trp ratio was then measured on collected supernatants. Mean values obtained from experiments using cells from 2 separate donors are presented. Mean values plus standard errors are shown.