Immune presensitization and local intrauterine defences as determinants of success or failure of murine interspecies pregnancies
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Putatively immuno-incompetent Mus musculus females exhibited failure to support pregnancy of Mus caroli embryos. These results for M. musculus females (i.e. treated by cyclosporine A, of the nu/nu genotype, and as an interspecies chimaera) can be explained in immunological terms. Mus musculus females possessed pre-sensitized cytotoxic T cells against Mus caroli antigen. Nu/nu mice possessed activated NK cells and macrophages, and selectively discriminated against Mus caroli embryos early in pregnancy unlike normal +/+ females; the requirement for T cells to activate non-specific cytotoxic effector mechanisms was bypassed in nu/nu mice. Mus caroli are not inbred, and interspecies chimaeras which are tolerant of the antigens on the Mus musculus donor strain were not tolerant of cells from unrelated Mus caroli. Interspecies chimaeras also behaved as if they were pre-sensitized to Mus caroli. Our results show that Mus caroli embryos recruit fewer active suppressor cells even when gestating in Mus caroli decidua as compared to Mus musculus embryos in Mus musculus decidua and that the ability of Mus caroli placental cells to directly inhibit cytotoxic effector cell killing was inherently less than the inhibitory activity of placental cells from Mus musculus. Mus caroli embryos therefore appear to be less well defended against maternal immune attack even when gestating in a uterus possessing compatible Mus caroli decidual tissue.