ICOS^-/- T cells have intact activation, proliferation, and cytolytic activity. (A) In vivo proliferation and (B) up-regulation of activation markers. WT and ICOS^-/- B6 T cells were labeled with CFSE and transferred into sublethally irradiated (750 cGy) allogeneic C3FeB6F1 recipients. Transferred T cells were analyzed by flow cytometry 72 hours after infusion. (C) Donor T-cell expansion during GVHD. Lethally irradiated (1300 cGy split) C3FeB6F1 recipients received transplants with B6 TCD allo-HSCs (5 × 10⁶) and splenic T cells (1 × 10⁶) from WT or ICOS^-/- B6 donors. Each group contained 6 animals. Recipients were killed on days 7 and 14 after allo-HSCT. Splenocytes were isolated and analyzed by flow cytometry. Absolute numbers of cells were calculated from total cell counts and percentage of donor origin determined by flow cytometry. Data are shown as the average of absolute T-cell numbers ± SEM. Statistical analysis: no difference. Data shown are representative of 2 independent experiments (n = 5 to 10 animals). (D) Cytolytic activity of WT and ICOS^-/- T cells. The murine cell lines P815 (H-2^d) and EL4 (H-2^b) were labeled with ⁵¹Cr and incubated with in vitro–primed effector T cells from WT or ICOS ^-/- B6 mice at the ratios indicated. Cells were cocultured for 4 hours before specific lysis was determined.