In summary, our study indicates that active SLE has seasonal distribution and its mechanism remains to be further explored. Ceacam1 Separates Graft-versus-Host-Disease from Graft-versus-Tumor Activity after Experimental Allogeneic Bone Marrow Transplantation BACKGROUND: Allogeneic bone marrow transplantation (allo-BMT) is a potentially curative therapy for a variety of hematologic diseases, but benefits, including graft-versus-tumor (GVT) activity are limited by graft-versus-host-disease (GVHD). Carcinoembryonic antigen related cell adhesion molecule 1 (Ceacam1) is a transmembrane glycoprotein found on epithelium, T cells, and many tumors. It regulates a variety of physiologic and pathological processes such as tumor biology, leukocyte activation, and energy homeostasis. Previous studies suggest that Ceacam1 negatively regulates inflammation in inflammatory bowel disease models. METHODS: We studied Ceacam1 as a regulator of GVHD and GVT after allogeneic bone marrow transplantation (allo-BMT) in mouse models. In vivo, Ceacam1(−/−) T cells caused increased GVHD mortality and GVHD of the colon, and greater numbers of donor T cells were positive for activation markers (CD25(hi), CD62L(lo)). Additionally, Ceacam1(−/−) CD8 T cells had greater expression of the gut-trafficking integrin α(4)β(7), though both CD4 and CD8 T cells were found increased numbers in the gut post-transplant. Ceacam1(−/−) recipients also experienced increased GVHD mortality and GVHD of the colon, and alloreactive T cells displayed increased activation. Additionally, Ceacam1(−/−) mice had increased mortality and decreased numbers of regenerating small intestinal crypts upon radiation exposure. Conversely, Ceacam1-overexpressing T cells caused attenuated target-organ and systemic GVHD, which correlated with decreased donor T cell numbers in target tissues, and mortality. Finally, graft-versus-tumor survival in a Ceacam1(+) lymphoma model was improved in animals receiving Ceacam1(−/−) vs. control T cells. CONCLUSIONS: We conclude that Ceacam1 regulates T cell activation, GVHD target organ damage, and numbers of donor T cells in lymphoid organs and GVHD target tissues. In recipients of allo-BMT, Ceacam1 may also regulate tissue radiosensitivity. Because of its expression on both the donor graft and host tissues, this suggests that targeting Ceacam1 may represent a potent strategy for the regulation of GVHD and GVT after allogeneic transplantation. Ceacam1 is a member of a large family of carcinoembryonic antigen proteins [2] . It is primarily a type I transmembrane protein with multiple splice variants [9] , though soluble forms also exist. Ceacam1 is widely expressed on a variety of tissues including endothelium [10] , epithelium [11] , hematopoietic cells [12] and both hematologic and solid tumors, and interacts in a homophilic and heterophilic fashion with physiological and pathogenassociated ligands, including carcinoembryonic antigen and the Neisseria spp. proteins [13] .