Compared with control T cells, extrathymic T lymphocytes show a higher apoptosis rate and produce more IFN-γ. T lymphocytes of thymic and extrathymic origin were stimulated with anti-CD3 with or without anti-CD28 as in Figure 1. The proportion of cells stained with annexin V (mean ± SD) was expressed for (A) whole CD4 (○; •) and CD8 (▵; ▴) populations of thymic (○; ▵) and extrathymic (•; ▴) origin, and (B) as a function of the CFSE content (after 72 hours) for thymic (□) and extrathymic (▪) T cells. Three mice were in each group. For panel A, statistically significant differences between thymic and extrathymic T cells are indicated for CD4 (^*) and CD8 (‡) T cells. (C) Production of cytokines by thymic (shaded histograms) and extrathymic (solid boldface lines) T cells was assessed by intracellular staining with IL-2- and IFN-γ-specific antibodies 24 and 72 hours after stimulation, respectively. Thin line histograms represent isotype controls. Percentages represent the proportion of cytokine producing thymic (upper numbers) and extrathymic (boldface lower numbers) T cells. This is one representative experiment of 3.