The co-delivery of Programmed Death 1 ligands enhances and prolongs rAAV-mediated gene expression in pre-immunized mice

Immune responses against recombinant adeno-associated virus (rAAV) are one of the major obstacles in gene therapy. We investigated the potential of Programmed Death 1 ligands 1 and 2 (PD-L1/2) to protect AAV-transduced cells from immunological clearance. Ligand compatibility for co-delivery was first evaluated using two transgenes, VEGF-B186 and muSEAP, separated from PD-L1/2 by a self-cleaving P2A peptide. After proper cleavage and biological activity of the co-produced proteins were demonstrated in vitro, the effect of PD-L1/2 co-expression on muSEAP production and persistence was studied in naïve and vector pre-immunized mice. Vectors (rAAV6-muSEAP, rAAV6-muSEAP-PD-L1, or rAAV6-muSEAP-PD-L2) were injected into two sites of the gastrocnemius muscle at a total dose of 1×10¹⁰ vg. Co-delivery of PD-L1, particularly, significantly enhanced muSEAP secretion into the bloodstream up to 12 weeks despite elevated anti-AAV6 responses in pre-immunized mice. muSEAP secretion increased 33.3- and 31.4-fold with the co-delivery of PD-L1, while the increase was only 5.6- and 9.3-fold in the muSEAP control group at 5 and 12 weeks, respectively. Ligand-treated pre-immunized animals also had less T-cell infiltration into the treated muscle compared to naïve animals. In summary, co-delivery of PD-L1/2 alongside a transgene represents a promising strategy for achieving sustained gene expression in individuals pre-exposed to AAV.