Currently over 200 known viruses utilize the respiratory tract as an entry point into the host. Respiratory viral infections are the third leading cause of death worldwide and are a WHO priority for vaccine development. Moreover, the emergence of highly pathogenic viruses, and the potential use of naturally occurring or modified viral pathogens for bioterrorist activities have highlighted the urgency to better understand the mechanisms that govern protective immunity against pulmonary viruses. For many respiratory viruses, CD8 T cells have been shown to play a role in control of primary infection. Moreover, virus-specific memory CD8 T cells can persist in the lung tissue and airways long after infectious virus has been cleared, suggesting a role for these cells in protection from repeated infections. Central to the question of CD8 memory is defining the regulatory mechanisms that govern their effective generation and long-term persistence, as well as their reactivation during recall responses. Understanding how CD8 memory is regulated is essential for designing more effective vaccines to combat infections and in the management of adverse immune reactions. By using a murine respiratory vaccinia virus (VACV) infection model we will test the idea that generation of protective CD8 T cell responses are highly regulated by the tumor-necrosis-factor receptor (TNFR) family member, OX40 (CD134). We present preliminary data supporting this hypothesis, and we will test whether, OX40, constitutively or inducibly expressed on memory CD8 T cell subsets control antigen reactivity and protective capacity to VACV. We hypothesize that after resolution of primary infection OX40 governs the effective reactivation and effector function of memory CD8 cells when they encounter antigen in secondary responses. We also hypothesize that OX40 then dictates long-term persistence of memory CD8 cells. Lastly, we will investigate whether targeting OX40 might be useful in the future to selectively improve the protective capacity of memory CD8 T cell subsets against respiratory VACV infection. These studies will provide invaluable data on the importance of costimulatory molecules in controlling CD8 T cell responses to respiratory viral infections and therefore should help in devising strategies to augmenting immune responses to pathogenic viruses, especially members of the orthopoxvirus family. [unreadable] [unreadable] [unreadable] [unreadable]