While the benefits of transplantation to society are substantial, the ever-growing population of immunosuppressed recipients poses a unique challenge in development of immunization and containment strategies to protect the population from communicable pathogens and weaponized infectious agents. The immunosuppressive regimens that have allowed the emergence of successful transplant therapy not only inhibit T cell-dependent rejection but also cause systemic immunosuppression, which attenuates the response to vaccines in general and precludes the use of live attenuated vaccines. To date, there has been relatively little detailed systematic study of the immune alterations that accompany either the short- or long-term immunosuppressive regimens used in clinical organ transplantation. Despite the recent development of increasingly effective, but also increasingly complex, regimens using drugs with very distinct molecular targets, current policies on vaccination of transplant recipients are generic and remain based on old concepts rather than on any new understanding of the cellular and molecular effects of these therapies on the human immune system. This proposal seeks to improve our understanding of the biological mechanisms that underlie the distinct immunosuppressive regimens in practice today (calcineurin-inhibitor, or CNI, and sirolimus-based regimens) and in emerging regimens that employ agents with novel mechanisms of action, such as the CD28 costimulation blockers, and/or JAK3 kinase inhibitors. Such knowledge will be critical to strategies for enhancing desirable immune responses while not precipitating rejection.