CpG-containing oligodeoxynucleotides (CpG ODN) have broad-ranging immunostimulatory effects, including the generation of antitumor immune responses. Analysis of different CpG ODN have identified two classes: CpG-A ODN, which stimulate high levels of IFN-α production from plasmacytoid dendritic cells and weakly activate B cells, and CpG-B ODN, which strongly activate B cells but stimulate low production of IFN-α from plasmacytoid dendritic cells. Previously, we observed that CpG-B ODN (2006) induces TRAIL/Apo-2 ligand (Apo-2L)-mediated killing of tumor cells by CD14+ PBMC. In this study, we extend our investigation of CpG ODN-induced TRAIL/Apo-2L expression and activity in PBMC to include CpG-A ODN. Of the two classes, IFN-α production and TRAIL/Apo-2L-mediated killing of tumor cells was greatest with CpG-A ODN. Surprisingly, CD3+, CD14+, CD19+, and CD56+ PBMC expressed high levels of TRAIL/Apo-2L following CpG-A ODN stimulation. When isolated, the CD19 + PBMC (B cells) were able to kill tumor cells in a TRAIL/Apo-2L-dependent manner. As with CD14+ PBMC, CD19+ sorted B cells were capable of up-regulating TRAIL/Apo-2L expression when stimulated with IFN-α alone. Interestingly, agonist anti-CD40 mAb further enhanced the IFN-α-induced TRAIL/Apo-2L expression on CD19+ B cells. These results are the first to demonstrate human B cell-mediated killing of tumor cells in a TRAIL/Apo-2L-dependent fashion.