Brefeldin A blocks the presentation of HLA-A*02–binding peptides by RNA-transfected DCs. Autologous monocyte-derived DCs were electroporated with MUC1 RNA alone (♦) or additionally incubated with brefeldin A (▴). A standard ⁵¹Cr-release assay was performed using MUC1 IVT-transfected DCs as targets and MUC1-specific CTLs recognizing the M1.1 peptide (A) or the M1.2 peptide (B) as effector cells. To exclude an unspecific or toxic effect of the agent, brefeldin A–treated MUC1 RNA-transfected DCs were additionally pulsed with the synthetic cognate peptides (▪) and used as targets. DCs electroporated with irrelevant EGFP RNA (×) were used as control.