Extracellular adenosine reduces the severity of GVHD at least in part via signaling through the A_2AAR on T cells. (A) Irradiated C57BL/6 mice (H-2K^b) received 5 × 10⁶ BM cells and 20 × 10⁶ spleen cells from BALB/c mice (H-2K^d) followed by IP injection of caffeine (10 mg/kg) or an equal volume of PBS 6 times in the first week after BMT and 3 to 5 times in the second week and later. The mice were monitored for survival for 9 weeks (P = .0015). The results are combined from 2 independent experiments (n = 20/group). (B) Irradiated BALB/c (CD45.2, A_2AAR^+/+) mice received intravenous injection of 5 × 10⁶ BM cells from CD45.1⁺CD45.2⁺ C57BL/6 F1 congenic mice (A_2AAR^+/+) together with 2.5 × 10⁶ each of splenocytes prepared from C57BL/6 A_2AAR^+/+ congenic mice (CD45.1) and from C57BL/6 A_2AAR^−/− or A_2AAR^+/+ congenic mice (CD45.2), as shown in supplemental Figure 1. On day 8, spleen cells were prepared from recipient mice and analyzed by flow cytometry for the expression of CD4, CD8, H-2K^d, CD45.1, and CD45.2 to determine the degree of chimerism. The bar graph represents the changes in the percentages of CD45.2⁺CD45.1⁻ cells in total donor CD4⁺ and CD8⁺ T cells (ie, changes in chimerism) after allo-HCT compared with that in the preinjection donor cells (day 0): open bar represents A_2AAR^−/− versus A_2AAR^+/+; filled bar, A_2AAR^+/+ versus A_2AAR^+/+. Data are mean ± SD (n = 5). *P < .01. #P < .05. This experiment was performed using Cd73^+/+ (left 2 bars) and Cd73^−/− (right 2 bars) BALB/c mice as recipients. The results are representative of 2 similar independent experiments (n = 5/group).