Lastly, we assessed the role of Ceacam1 expression on radioresistant cells in allo-BMT recipients for donor T cell alloactivation. We transferred CFSE-labeled B6 T cells into irradiated WT vs. Ceacam1 2/2 BALB/c mice, and analyzed donor T cells in spleens on day 3. Here, we did not observe differences in proliferation (data not shown), but donor CD4 T cells in Ceacam1 2/2 allogeneic recipients did exhibit an increase in alloactivation as measured by CD25 ( Figure 6F ).