Immunosuppression required for lifesaving solid organ transplantation induces significant morbidity including increased recurrence of latent viral infection in transplant recipients. Approximately 60-70% of adults in the United States are latently infected with Cytomegalovirus (CMV). Control of latent CMV is thought to depend on a constantly active immune response, specifically by memory T lymphocytes. In those with a normal immune system, control of latent CMV is a constant “battle” and results in so called memory inflation. In aged individuals, CMV responsive cells may constitute upward of 50% of the total CD8 T cell population. Despite active monitoring and anti-viral prophylaxis, immunosuppressed transplant recipients have a 10-15% incidence of recurrent CMV manifesting either as isolated viremia or may include end-organ damage. To better understand the immune response to CMV in solid organ transplant recipients, we have previously used multi- parameter flow cytometry to characterize CMV-responsive CD8+ T cells from pre-transplant to one year post transplant in a cohort of heart or kidney transplant recipients. Strikingly, despite a lack of clinical evidence for CMV reactivation, CMV-responsive CD8 T cells increased from approximately 4% to 12% of the CD8 repertoire during the first year post- transplant. Preliminary data indicates that the expansion maintains clonal competition and suggests intraclonal heterogeneity of function that changes with time. These data lead us to the hypothesis that immunosuppression induces oligoclonal expansion of CMV- responsive T cells resulting in accelerated inflation and premature immune senescence. To address this hypothesis, we have initiated collaborations with Mark Davis, Jorg Goronzy and Purvesh Khatri at Stanford University. Subjects will be recruited from the Portland, Nashville, Pittsburgh and Palo Alto VA transplant centers. We will use innovative approaches involving cutting edge cytometry and single cell analysis . Specifically we propose the following aims: Aim1, Determine if transplantation and immunosuppression induced expansion alters the T cell repertoire and function and Aim 2, Determine if immunosuppression in the setting of organ transplantation leads to pre-mature aging of the CMV-responsive population and decreased functional responses. The results of these studies will give us new fundamental insight into the biology of CMV-responsive T cells and can be used to develop new therapies that will extend the lives of veterans.