Hyperactivation of 4.1R^−/− T cells. Purified 4.1R^+/+ or 4.1R^−/− CD4⁺ T cells were stimulated with control IgG or anti-CD3ϵ (1 μg/mL) for 16 hours, and the surface expression of CD69 was assessed by flow cytometry. The representative profiles of CD69 expression of 4.1R^+/+ and 4.1R^−/− CD4⁺ T cells were shown (A), and the quantitative analysis was shown (B). Note a significantly higher percentage of 4.1R^−/− CD4⁺ T cells express CD69. Error bars indicate SD.