FIX activity, FIX Padua levels, liver enzymes, and PBMC-mediated immune responses to AAV8 post–BAX 335 infusion for participants in dose cohort 2. FIX activity and FIX Padua activity levels post–BAX 335 infusion by participant in relation to bleeding episodes, administration of FIX replacement therapy, and prednisone dosing (participant 8) as well as liver enzyme markers of hepatotoxicity (ALT and aspartate aminotransferase [AST]) are shown. Study week represents nominal collection time (mean values used if ≥1 result in 1 week). The number of exogenous FIX infusions or bleeding episodes within 1 week is indicated. The lower limit of detection (LOD) for FIX Padua–specific activity (<0.020 U) is shown by the gray dashed line. Participant 8 received prophylactic treatment with prednisone. The duration of prednisone treatment is shown in the green bar chart, with the slope indicating dose tapering. The results from IFN-γ ELISpot assays are shown in the lower graph panel for each participant; the reaction of the participant’s PBMCs to AAV8 capsid peptides in 3 separate pools are plotted as the number of spots per million PBMCs. Values were considered positive if they were above the media control (background) by a factor of 2 and were >60 spots per million PBMCs (black dashed line). Maximum results across all pools were presented for each time point.