TLR9 is required for proliferation of transgenic capsid-specific CD8⁺T cells. WT or TLR9^−/− mice were injected with AAV2-SIINFEKL or unmodified AAV2 (n = 3/group). Two days later, CD8⁺ T cells were magnetically isolated from OT-1 mice, labeled with CTV, and adoptively transferred at 5 × 10⁶ cells/mouse. (A) Timeline. (B) Representative plots of proliferation of adoptively transferred OT-1 cells. (C) Quantification of the percent proliferation, percent divided (percent of original cells that divided at least once), and the division index (the average number of cells that a dividing cell became). Data points are averages ± SEM. P values: (C) percent proliferation, ***P < .0001 for WT vs TLR9^−/−, ***P =.0002 for WT vs WT(AAV2), **P =.007 for TLR9^−/− vs WT(AAV2); percent divided, *P =.02 for WT vs TLR9^−/−, **P =.007 for WT vs WT(AAV2); division index, *P =.02 for WT vs TLR9^−/−, **P =.006 for WT vs WT(AAV2). ns, not significant.