CD45RC^low CD8 T cells suppress the proliferation of alloreactive CD4 T cells. (A) Purified naive CD4 T cells from LEW rats (5 × 10⁵ cells/well) were stimulated for 5 days with irradiated, semiallogeneic, T-cell-depleted F1 splenocytes (2.5 × 10⁵) either without (▪) or with variable numbers of LEW CD45RC^high (•) or CD45RC^low (○) CD8 T cells. Proliferation was assessed with an 18-hour (³H)-thymidine pulse added after 4 days of culture. Similar results were obtained in 4 independent experiments. (B) Results were expressed as the percentage of inhibition of proliferation at a CD4/CD8CD45RC ratio of 1:1, considering that the proliferation of CD4 T cells without CD8 T cells is set as 100%. Bars represent the mean values of 4 individual experiments, and filled circles represent the results of each individual experiment. (C-E) CFSE-labeled naive CD4 T cells from LEW rats were stimulated for 5 days with irradiated semiallogeneic T-cell-depleted F1 splenocytes. Stimulation took place without (left panels) or with either LEW CD45RC^high (middle panels) or CD45RC^low (right panels) CD8 T cells added at a 1:1 ratio. Isotype control (C) or anti-TGFβ (D) mAbs (50 μg/mL) or exogenous IL-2 (E) (10 U/mL) was added on day 0. The division of CD4 T cells was assessed by flow cytometry by gating on CD4 T cells. The percentage in each plot represents dividing CD4 T cells. These data are representative of 3 independent experiments.