EBV-induced gene 3 (EBI-3) codes for a soluble type I receptor homologous to the p40 subunit of IL-12 that is expressed by APCs following activation. In this study, we assessed the role of EBI-3 in a model of lung melanoma metastasis. Intravenous injection of the B16-F10 cell line resulted in a significant reduction of lung tumor metastasis in EBI-3-/- recipient mice compared with wild-type mice. The immunological finding accompanying this effect was the expansion of a newly described cell subset called IFN-γproducing killer dendritic cells associated with CD8+ T cell responses in the lung of EBI-3-/- mice including IFN-γ release and TNF-α-induced programmed tumor cell death. Depletion of CD8+ T cells as well as targeting T-bet abrogated the protective effects of EBI-3 deficiency on lung melanoma metastases. Finally, adoptive transfer of EBI-3-/- CD8+ T cells into tumor bearing wild-type mice inhibited lung metastasis in recipient mice. Taken together, these data demonstrate that targeting EBI-3 leads to a T-bet-mediated antitumor CD8+ T cell responses in the lung.