Because the expression of Ceacam1 on alloreactive T cells after adoptive transfer occurred in vivo with similar kinetics as T cell alloactivation [30] , we asked whether Ceacam1 on either donor alloreactive T cells or radio-resistant cells in allo-BMT recipients could regulate this process. We transferred CFSE-labeled purified B6 WT or Ceacam1 2/2 splenic T cells into irradiated BALB/c recipients and analyzed donor T cells in spleens on day 3.