Today, the dairy industry is comprised of cows that can produce around 20,000 pounds of milk in a year, yet producers often see only 30% to 50% of animals produce a live calf after each breeding. While many factors may be at play, there is evidence that the immune system plays a large role in regulating pregnancy and in preventing immune mediated abortions. Maternal immune system changes during pregnancy have been shown to occur both locally in the uterus, which is in direct contact with the placenta, and systemically in the blood. Research in humans and mice has shown that the immune cell populations of CD4+CD25+ γ/δ TCR+ play substantial roles as T regulatory cells and they support the development of the conceptus through trophoblast cell growth. However, in cattle these cells have not been identified. We proposed a co-culture experiment to identify which T cell populations in the cow support placental growth. We isolated and cultured the following T cell populations: CD4+, CD4+CD25+, CD4+CD25-, CD8+, and γ/δ TCR+ from pregnant and non-pregnant cows to obtain T cell conditioned medium and treat placental trophoblast cells. T cells secrete cytokines and growth factors that might alter trophoblast cell growth and development. We analyzed trophoblast cell proliferation, apoptosis, and gene expression to identify which T cell populations support placental development. Our results show that trophoblast cell gene expression is modulated by T cell supernatant obtained from pregnant cows differently than T cell supernatant from non-pregnant cows. These results will contribute to the development of strategies that could reduce rates of immune-mediated pregnancy loss. A cost-effective approach to reduce pregnancy loss in cattle could reduce monetary loss to farmers by allowing production animals to be more reproductively efficient and stay in the herd for longer. Advisors/Committee Members: Heloisa M. Rutigliano, S. Clay Isom, Aaron Thomas, ;.