In experiments with Ceacam1 2/2 donor T cells, we also observed a trend for splenic donor CD8 alloactivated T cells to express higher levels of a 4 b 7 . Although integrin a 4 b 7 is important for GVHD pathogenesis, and we have previously shown that b 7 2/2 T cells cause a sustained decrease in acute systemic and intestinal GVHD [31] , differential expression of integrin a 4 b 7 by Ceacam1 2/2 T cells is almost certainly only one part of how Ceacam1 regulates target organ GVHD. Indeed, donor alloactivated CD4 T cells expressed comparable levels of integrin a 4 b 7 as wildtype cells, yet were also found in increased numbers in the gut ( Figure 3A ). This suggests that other mechanisms, such as Ceacam1 regulation of donor T cell activation ( Figure 6 ) may also contribute to its regulation of GVHD target organ damage.