Responses of naive and activated CD4 T cells from CD28^−/−scurfy mice. CD4⁺CD25⁻ NK1.1⁻ populations from mice as indicated were sorted into CD62L⁺CD44⁻ and CD62L⁻CD44⁺ populations. Sorted cells (1.5 × 10⁴ cells/well in flat-bottomed 96-well plate) were activated in triplicate wells coated with 0.1 or 1 μg/mL of anti-CD3 in the presence or absence of anti-CD28 antibody (0.5 μg/mL). (A) Proliferation (mean ± SD, n = 3) and (B) cytokine production (mean ± SD, n = 2) was determined as in Figure 3.