IL-21R is up-regulated on donor T cells early after BMT but is not required for efficient alloactivation. (A) WT B6 T cells were cultured in vitro for 24 hours in the presence or absence of PMA and ionomycin, then stained for FACS analysis of IL-21R levels. (B) WT B6 BM-TCD and T cells (1 × 10⁶) were transplanted into lethally irradiated BALB/c recipients, which were killed on day 4 or day 7 after BMT. Splenocytes were harvested and stained for FACS analysis of donor T-cell IL-21R levels. Data are combined from 2 independent transplantations per time point, with 8-10 total recipients per group. **P < .01 and ***P < .001.