ICOS blockade blocks expansion of alloantigen-specific BM graft-rejecting cells. 2C and TEa LN cells (10 × 10⁶ each) were adoptively transferred into B6 Rag^-/- mice on day -2, irradiated on day -1 (2.0 Gy), infused with BALB/c BM (40 × 10⁶) on day 0, and administered rIgG or anti-ICOS mAb from day -1 to day +9. Control of irradiated mice receiving adoptive transfer of 2C and TEa LN, but not BALB/c BM, is also shown (designated as AT Only). On day 10, spleens were harvested and the number of Tg T cells was determined. Shown is average absolute number of CD8⁺ (left) and CD4⁺ Tg T cells (right) per spleen ± 1 SEM (n = 10/group, pool of 2 experiments; ^* indicates statistical significance. P = .0197 for 2C Tg CD8⁺ cells, P = .0006 for TEa Tg CD4⁺ cells; rIgG vs anti-ICOS).