Expansion of a population of capsid-specific CD8+ T cells after vector infusion, and working model of capsid processing and presentation in hepatocyte. (A) Time course of serum transaminases and frequency of AAV2 capsid peptide–specific CD8⁺ T cells in PBMCs, in subject infused at 4 × 10¹¹ vg/kg dose. (B) Working hypothesis of CD8⁺ T-cell–mediated destruction of AAV-transduced hepatocytes. On transduction, vector enters hepatocytes, and following escape from the endosome and uncoating, vector DNA directs synthesis of F.IX. Some capsid remains behind in the cytosol and undergoes proteasomal processing. Capsid-derived peptides are then transported to the endoplasmic reticulum and loaded onto MHC I molecules, making the transduced hepatocyte a target for circulating capsid-specific CD8⁺ T cells. Note that activation of CD8⁺ T cells and presentation of capsid-derived peptides must occur in a similar time course to result in a clinically detectable event. Figure 3A reprinted from Mingozzi et al,⁶⁷  and Figure 3B reprinted from Mingozzi and High.¹⁰