Ceacam1 on T cells has previously been shown to restrain CD4 T cell polarization, cytokine secretion and cytotoxicity. In our GVHD model systems however, we found similar T cell polarization and cytokine secretion when we analyzed donor alloreactive T cells ex vivo ( Table 1) . We ascribe this to the strongly proinflammatory cytokine milieu found in recipients following myeloablative radiation treatment, as well as the ubiquitous presence of alloantigen, which together promote strong Th1 differentiation regardless of Ceacam1 expression. However, in our model systems Ceacam1 regulated T cell activation, and numbers of donor alloactivated T cells in both lymphoid tissues and GVHD target tissues, in patterns that generally correlated with levels target organ damage (Figures 2 and 3) .