Pregnancy represents an allogeneic immune challenge for mothers and their developing embryos, yet most mothers do not reject their offspring in utero. In Science, Erlebacher and colleagues show that entry of cells of the immune system is blocked at the site of embryo implantation. Decidual stromal tissues, which form the specialized interface that separates maternal tissues from fetal tissues, lack expression of inflammatory chemokines such as CXCL9, CXCL10 and CCL5, which prevents the recruitment of activated T cells, including those specific for fetal antigen. The differentiation of endometrial stromal cells into decidual stromal cells during pregnancy is accompanied by the trimethylation of histone H3 Lys27 at the promoters of Cxcl9, Cxcl10 and Ccl5, which suggests that active repression occurs at those loci. The factors responsible for specific targeting of those loci and how they are induced during fetal implantation remain unknown. Suppressing the expression of inflammatory chemokines provides another layer of immunosuppression that protects embryos from attack by cells of the maternal immune system.

Science 336, 1317–1321 (2012)