Using a murine metastasis model, we have previously shown that antigen-presenting cells (APC) loaded with unfractionated peptides derived from poorly immunogenic, highly metastatic tumor cells represent a potent form of tumor vaccine. The antimetastatic effect of peptide pulsed APC could be further enhanced by pretreating the cells with antisense oligonucleotides directed against the TAP-2 gene to increase the density of specific peptide-major histocompatibility complex (MHC) class I complexes and thereby improve the APC function of the treated cells (Nair SK et al., J Immunol 1996;156:1772). In this study, we investigated whether similar strategies can be used to enhance the potency of human dendritic cells (DC) to present antigen. We show that human DC pulsed with peptides encoding known cytotoxic T-lymphocyte (CTL) epitopes stimulate both memory and primary CTL responses in vitro after two cycles of stimulation with the peptide-pulsed DC. Two approaches were used to increase the density of specific peptide-MHC complexes on the surface of DC. One approach was to inhibit transporter associated with antigen presentation (TAP) function using TAP antisense oligonucleotides. The second approach was to inhibit the endogenous generation of the peptide epitopes by pretreating the DC with a proteasome inhibitor. Treatment of DC with either TAP antisense oligonucleotides or with a proteasome inhibitor resulted in a dramatic enhancement of primary CTL induction.