Innate CD4⁺ CD25⁺ cells inhibit hapten-specific CD8⁺ expansion in vitro. CD8⁺ T cells (10⁵) were isolated from day-5 DNFB-sensitized C57BL/6 mice and stimulated with hapten-pulsed and mitomycin C–treated BM-DCs (2 × 10⁴) (▦). Graded numbers of CD4⁺CD25⁺ T cells, purified from spleen and lymph nodes of either naive (□) or orally tolerant C57BL/6 mice (▨), were added to cultures. Proliferation was determined after 3 days by [³H]thymidine uptake (A) and γ-IFN production was titrated by ELISA in 48 hours cell free supernatants (B). Results are expressed as mean values ± SD and are representative of 3 independent experiments.