Increased cytokine production by DP CD4⁺CD8⁺ T-cell subsets in neonates. Cytokine production by mitogen-stimulated or unstimulated jejunum lamina propria (A) and spleen (B) T-cell subsets in a normal, uninfected neonate at 12 days old. Cells were unstimulated or stimulated with PMA/ionomycin, and cytokine production was detected by intracellular cytokine staining. The percentage of cytokine-producing cells is indicated in the top box of each panel. Note that DP T cells had higher IL-2, TNF-α, and IFN-γ responses compared with SP CD4⁺/CD8⁺ T cells in jejunum LPLs, whereas SP CD8⁺ T cells had higher IL-2 responses in spleen lymphocytes.