Qa-1–dependent CD8⁺ T cells selectively downregulate T cell clones of intermediate affinity/avidity for HEL. (A) In this representative example, the CD8⁺ T cells differentially downregulate a susceptible but not a nonsusceptible clone in a dose-dependent manner. The CD8⁺ T cells were directly mixed with CFSE-labeled testing clones as described in Methods. In this type of setting, we used non–CFSE-labeled activated clones to set up the cutoff line for undivided or less divided and more divided fractions for each clone individually. Since the autofluorescent background of each unlabeled activated clone in the FITC channel differs among clones, slightly different cutoff lines sometimes appear among different clones. Only 2 of 5 E/Ts for 2 representative clones, 13C9 (intermediate affinity/avidity) and 9E4 (high affinity/avidity), are shown. (B) Qa-1–dependent CD8⁺ T cells selectively downregulate HEL-specific T cell clones based on their affinity/avidity to HEL. In this set of tests, CD8⁺ T cells isolated from naive WT or HEL low Tg mice, which showed no effect on target T cells, were routinely used as control for the regulatory CD8⁺ T cells. (C) The downregulation of the intermediate–affinity/avidity clones by the CD8⁺ T cells is blocked by antibodies against TCR, Qa-1, and CD8, but not by the control antibody. The experiments were performed as described in Methods. Data represent experiments at E/Ts of 0.2:1 to 1:1, and final concentrations of mAbs were 12.5–25 μg/ml.