CD8+ T cell responses differentially recognized Nef epitopes in the 2 T/F viruses. The epitopes Nef68-76 (A) and Nef19-27 (C) varied between the major and minor T/F viruses. T cell responses were measured by ex vivo IFN-γ ELISpots with responses against the major T/F shown by blue squares and responses against the minor T/F shown as red diamonds. T cell responses to subsequently emerging escape variants are shown as triangles. To the right of these graphs, the changes in each epitope are shown over time. The EC50 of the Nef68-76 T cell response targeting the major variant was lower than that of the response targeting the minor variant (B). dps = days post-screening (Fiebig I/II).