Qa-1–dependent CD8⁺ T cells facilitate maturation of affinity for HEL in WT mice. (A) Regulatory CD8⁺ T cells are involved in the increase of the overall affinity of HEL-reactive CD4⁺ T cells during the secondary immune response to HEL in WT BALB/c mice. CD4⁺ T cells were purified from pooled lymphocytes from draining lymph nodes of different groups of mice and assayed in a T cell proliferation assay as described in Methods. Data are representative of 3 separate experiments with 2–4 mice per group. (B) CD8⁺ T cells selectively downregulate certain HEL Vβ8.2 clones in the secondary HEL-reactive repertoire in WT BALB/c mice. Mice were prepared as described in Methods. CD4⁺ T cells were isolated and purified from the draining lymph nodes on days 7–9 after the secondary HEL immunization and stimulated in vitro for a week. Analysis of distribution of the TCR CDR3 length of CD4⁺ T cells isolated from the different groups of mice was performed as previously described (4, 47). Data are representative of 5 separate experiments with 2–4 mice per group.