The PD-1/PD-L1 pathway has recently been identified as a negative regulator of immunity . The PD-1 immunoreceptor is also a member of the CD28/CTLA-4 family which, upon interaction with either one of its two ligands PD-L1 or PD-L2, down-modulates TCR signaling, reduces cytokine production and affects T cell survival by recruiting the SHP-2 tyrosine phosphatase, thereby dampening the PI3K and Akt pathway (Francisco et al., 2010) . In BALB/c mice, PD-1 deficiency leads to the spontaneous development of a lethal autoimmune cardiomyopathy . This disease is mediated by autoantibodies directed against the cardiac muscle autoantigen troponin I (Okazaki et al., 2003) . Hence, PD-1 controls the physiological tolerance to muscle autoantigens. Also, PD-L1 was recently shown to regulate CD8 + T cell-mediated muscle injury in a mouse model of myocarditis (Grabie et al., 2007) . Several PD-1/PD-L1 dependant mechanisms may act synergistically to induce an effective effector T cell blockade and contribute to tolerance induction. As mentioned earlier, PD-1 engagement on the surface of T cell is known to significantly inhibit TCR signaling thereby preventing T cell activation and cytokine production. In addition to this mechanism, PD-L1 was recently shown to be a potent inducer of adaptive Tregs. Indeed, PD-L1 expressed on CD8 + dendritic cell subset plays an important role on the conversion of naive lymphocytes toward FoxP3 + regulatory T cells (Wang et al., 2008) . Thus, engagement of PD-1 on T cells could induce cell-intrinsic tolerance (by blocking TCR signaling) as well as a dominant form of immunological tolerance by promoting the emergence of FoxP3 + adaptive Tregs. Of note, manipulation of the PD-1/PD-L1 axis is also used by some tumors cells to tolerize surrounding lymphocytes. Indeed, expression of PD-L1 on the surface of malignant cells has been suggested to represent a subversive strategy used by tumors to escape to immuno-surveillance . Thus, we believe that up-regulation of the expression of PD-L1 on the surface of muscle cells by the means of gene transfer should be able to induce tolerization of the immune system by preventing cytotoxicity of PD-1-expressing activated lymphocytes. However, PD-L1 gene transfer alone was not sufficient in our model to prevent the immune response directed against transduced muscle fibers (Figure 2) , suggesting that muscle expression of PD-L1, i.e., at distance from lymphoid compartments, is probably not capable of inducing sufficient adaptive FoxP3 + Tregs. Another explanation would be that PD-L1 expressed in muscle is not effective to block fully activated T cells that have received normal costimulatory signals (i.e., in the absence of CTLA-4/Ig co-administration). This interpretation would be in line with the finding that PD-1 inhibitory pathway could be overcome by CD28 costimulatory ligation in the presence of IL-2 (Freeman et al., 2000; Latchman et al., 2001; Carter et al., 2002; Ishida et al., 2002) . Hence, the use of CTLA-4/Ig to block early CD28 costimulatory signaling may create favorable conditions where escaping T cells could become sensitive to PD-L1/2. This strengthens the notion that CTLA-4 and PD-1 signaling represent two distinct pathways acting synergistically to maintain tolerance (Fife and Bluestone, 2008) . As shown herein, neither CTLA-4/Ig alone nor combination therapies could block immune response at days 40 and 80 (Figures 3 and 4) . Our results are thus compatible with the notion that the use of rAAV2/1-PD-L1 or rAAV2/1-PD-L2 in combination with CTLA-4/Ig, does not completely inhibit the immune priming against sOva that most probably occurs at distance from the site of transduction (e.g., spleen and/or lymph nodes) but instead provides an additional protection by disarming lymphocytes within the target tissue.