PROJECT SUMMARY/ABSTRACT Epstein Barr virus (EBV) is a broadly disseminated gammaherpes virus that, in immunosuppressed or immunocompromised individuals, can cause serious, life-threatening B cell lymphomas. In solid organ transplant (SOT) recipients these EBV+ B cell lymphomas are the most serious manifestation of the group of heterogeneous lymphoproliferations termed post-transplant lymphoproliferative disease (PTLD). Predisposing factors for PTLD include primary EBV infection, reactivation of EBV in recipient B cells, and impaired T cell immunity due to immunosuppression. There are major gaps in our understanding of how specific viral genes contribute to lymphomagenesis in the context of EBV+ PTLD and whether there are specific alterations in the immune response to EBV in SOT that develop EBV+ PTLD compared to those that do not. Prior work from our group has focused on latent membrane protein 1 (LMP1), the major oncogene of EBV, to better understand EBV+ PTLD pathogenesis. In a recent prospective, multicenter clinical trial in SOT recipients we demonstrated that specific gain of function mutations in LMP1 significantly correlate with the development of EBV+ PTLD. We’ve also demonstrated that EBV alters the host cell microRNA profile and that this has direct effects on survival of EBV+ B lymphoma cells. Building on our previous innovative studies of the bidirectional interactions between EBV and host immunity, and using our unique Biorepository of samples from SOT recipients that developed EBV+ PTLD and matched SOT controls that did not develop EBV+ PTLD, we propose to define the impact of viral genetic diversity on protective immune responses to EBV. We hypothesize that EBV genetic diversity leads to alterations in viral gene function and immune recognition that contribute to the pathogenesis of EBV+ PTLD. To test this hypothesis we propose the following Specific Aims:1) Determine the genetic diversity of EBV in PTLD and the impact on host cell function 2) Determine the effect of EBV+ PTLD-associated genetic diversity on host immunity to EBV and 3) Determine how extracellular vesicles and microRNA contribute to the development of EBV+ PTLD. We anticipate these studies will identify novel mechanisms underlying the EBV-driven pathogenesis of B cell lymphomas in PTLD and will reveal new opportunities for therapeutic strategies to prevent and treat EBV+ B cell lymphomas in immunosuppressed and immunocompromised individuals.