IL-15 signaling optimizes T-bet–induced gene products in iNKT cells. Peripheral iNKT cells were activated by administering α-GalCer to each group of mice. Two and half hours later, CD1d tetramer⁺ cells from the liver and spleen were analyzed for IFN-γ production via intracellular cytokine staining. iNKT cells from untreated WT mice were used as a negative control. Representative histograms showing the percentage of hepatic (A) and splenic (B) iNKT cells producing IFN-γ after α-GalCer administration from treated mice (IL-15Rα^−/−, CD11c/IL-15Rα Tg, and WT mice) and untreated WT mice. The graph shows the average percentage of IFN-γ⁺ iNKT cells from the liver (A) and spleen (B) from all treated groups. Error bars represent SEM, derived from combined data from 2 independent experiments; n = 4 or 5 mice/group. *P ≤ .005. (C) Flow cytometric plots displaying CD44 and T-bet expression in CD1d tetramer⁺ cells from the thymus and liver of IL-15Rα^−/− and WT mice. (Upper right) Percentage of T-bet⁺ iNKT cells from the thymus and liver of the 2 groups. *P < .001. (Bottom right) Average MFI of T-bet expression in CD44⁺NK1.1⁺ iNKT cells from IL-15Rα^−/− and WT mice isolated from the thymus and liver. *P < .001. (D) Representative histogram of CD122 expression in thymic CD44⁺NK1.1⁺ iNKT cells from IL-15Rα^−/− and WT mice; the average MFI of CD122 in thymic CD44⁺NK1.1⁺ iNKT cells from both groups. *P < .001. (C-D) Combined data from 2 independent experiments; n = 6 for both IL-15Rα^−/− and WT mice. Error bars represent SEM.