TL1A, a recently described TNF-like cytokine that interacts with DR3, costimulates T cells and augments anti-CD3 plus anti-CD28 IFN-γ production. In the current study we show that TL1A or an agonistic anti-DR3 mAb synergize with IL-12/IL-18 to augment IFN-γ production in human peripheral blood T cells and NK cells. TL1A also enhanced IFN-γ production by IL-12/IL-18 stimulated CD56+ T cells. When expressed as fold change, the synergistic effect of TL1A on cytokine-induced IFN-γ production was more pronounced on CD4+ and CD8+ T cells than on CD56+ T cells or NK cells. Intracellular cytokine staining showed that TL1A significantly enhanced both the percentage and the mean fluorescence intensity of IFN-γ-producing T cells in response to IL-12/IL-18. The combination of IL-12 and IL-18 markedly up-regulated DR3 expression in NK cells, whereas it had minimal effect in T cells. Our data suggest that TL1A/DR3 pathway plays an important role in the augmentation of cytokine-induced IFN-γ production in T cells and that DR3 expression is differentially regulated by IL-12/IL-18 in T cells and NK cells.