We investigated the possibility to block immune responses directed against the transgene product by two distinct immunomodulatory strategies, one blocking lymphocyte priming (i.e., CTLA-4/Ig) and the other inhibiting the function of lymphocytes targeting transduced muscle fibers (i.e., activation of the PD-1 pathway by muscle gene transfer of PD-1 ligands). Indeed, we have previously shown in transgenic mice that express Ova as a neo-autoantigen in muscle that tolerant transgenic anti-Ova CD8 + T cells up-regulated PD-1, suggesting that PD-1 plays a role in the induction of tolerance to skeletal muscle-expressed Ag (Calbo et al., 2008) . As expected, the anti-Ova CD8 + T cell response that followed an i.m. injection of rAAV2/1-Ova (Figures 1B,C) was accompanied by an up-regulation of PD-1 on CD8 + T cells that exhibit an activated CD44 hi phenotype (Figure 2A) . As activated T cells express PD-1 and PD-1 ligands inhibit their activation and function, we generated rAAV2/1-PD-L1 and rAAV2/1-PD-L2 vectors and first verified that they were able to efficiently transduce HEK cells in vitro ( Figure 2B) . The expected consequence of injecting these vectors i.m. is that they should not prevent T cell priming but rather inhibit the cytotoxic activity of T cells directed against muscle-expressed transgenic proteins.