Costimulation blockade leads to the deletion of alloreactive CD8⁺ T cells and a loss in TNF-α production after allo-stimulation. Splenocytes from either naive KB5 bone marrow synchimeric mice or synchimeric mice treated with costimulation blockade were stained for cell surface makers, and the DES⁺CD8⁺ KB5 T cells are shown as a (A) percentage or (B) as total spleen cell number (n = 5). Splenocytes from the indicated group were stimulated for 5 hours with C57BL/6 (H2^b) splenocytes and then stained for cell surface markers and for TNF-α. For analysis, samples were gated on CD8⁺ cells, and the values shown represent the percentage of CD8⁺DES⁺ cells producing TNF-α (C-D, n = 5). Means were compared with a t test (*P < .001). The data are representative of 2 experiments. Error bars indicate SD.