CD80 KO donor T cell upregulation of PD-1 and Fas is not defective. Experimental protocol is described in . The number of Fas-positive (A, B) or PD-1–positive (C, D) donor CD4 (A, C) or CD8 (B, D) T cells was determined by flow cytometry at the indicated time points for F1 mice receiving 50 × 106 WT or CD80 KO splenocytes. Results are shown as group mean ± SE. *p < 0.05; **p < 0.01; ***p < 0.005.