Peripheral Foxp1-deficient T cells respond like activated T cells and have increased apoptosis. (A) Total spleen cells from 4-week-old Foxp1^f/+Cd4^Cre and Foxp1^f/fCd4^Cre mice were stimulated with anti-CD3 plus anti-CD28 antibodies or PMA plus ionomycin for 4 hours and analyzed for IL-2 and IFN-γ production by intracellular staining. The data are gated on CD4⁺ T or CD8⁺ T cells, and are representative of various time points (weeks 4, 8, and 21) in at least 3 independent experiments. (B) Purified CD4⁺ T or CD8⁺ T cells from 4-week-old Foxp1^f/+Cd4^Cre and Foxp1^f/fCd4^Cre mice were labeled with CFSE and stimulated with 1 μg/mL anti-CD3 antibodies for 2 days. Cell proliferation was analyzed by measuring the CFSE profile. Data are representative of 2 independent experiments. (C) Thymic and lymph node cells from 8-week-old Foxp1^f/+Cd4^Cre and Foxp1^f/fCd4^Cre mice were analyzed for apoptosis by annexin V and PI staining. The bars represent the percentages of annexin V⁺ PI⁻ cells in gated CD4⁺ or CD8⁺ T cells. Data represent average ± SD of the data of 8-week-old mice, n = 4. *P < .05; **P < .01.