Adeno-associated virus (AAV) vectors have been developed as versatile tools for gene therapy, leading to market authorization for the treatment of monogenetic diseases. AAV vectors were initially considered to have low immunogenicity, but the understanding of immune responses elicited by AAV vectors has increased substantially with the development of gene therapy approaches.Based on this knowledge, development of AAV vector based vaccines is becoming more attractive, as engineering of the transgene cassette and the capsid are important tools for developing AAV vectors optimized for vaccine applications.In this issue of Mol. Therapy Methods & Clin. Development, Krotova et al. demonstrate that a capsid modified AAV serotype 6 (AAV6) S663V vector induces cellular and humoral antigen specific immune responses against melanoma tumor-associated antigens as well as tumor protection in B16- F10 melanoma models. In the present study, Krotova et al. first administered AAV vectors coding for single antigens and a 1:1:1:1 mixture of all four AAV vectors (multivalent vaccine) i.m. to C57BL/6 mice.Two weeks later, the mice received an i.v. injection of B16- F10 melanoma cells. Interestingly, the level of protection as measured by the numbers of tumor nodules in the lungs of the mice was antigen dependent.The AAV TRP1 vector and multivalent vaccine induced the most pronounced protective effects, with a specific T cell response induced by AAVTRP1 at 14 days post-vector application.Upon splenocyte restimulation, the authors detected specific T cell immune responses against all four individual antigens and the four antigens combined, again most pronounced for AAV TRP1. AAV TRP1 and the multivalent vaccine also induced a B cell response.Interestingly, dextramer staining of intratumoral T cells demonstrated infiltration of TRP1-specific CD8+ T cells comprising up to 30% of intratumoral T cells, which were characterized by high expression of PD-1.This finding suggests that combined treatment with AAV TRP1 and PD-1 immune checkpoint inhibitors is a potential option to further improve efficiency.7 Natural killer cell infiltration was also identified in the tumors, and the phenotype of tumor-associated macrophages shifted from M2 toward M1 (i.e., anti tumoral).These results indicate that the vaccine approach influenced the tumor microenvironment (TME), potentially increasing the efficiency of immunotherapy approaches already in clin. use (i.e., immune checkpoint inhibition).Another promising approach is the display of antigens directly on the AAV capsid to trigger the immune system in an early phase of vector encounter, followed by the expression of the resp. antigen, resembling, in part, a prime boost situation.Overall, the data provided by Krotova et al. represent an important step forward in the development of AAV based tumor vaccine strategies.They demonstrate that targeting DCs with an optimized AAV based vaccine vector works in an aggressive B16 F10 murine model, based on the use of low immunogenic melanoma associated antigens, which requires breaking self tolerance. Considering the changes in the composition of the TME identified in this study, combination approaches with immune checkpoint inhibitors already in clin. use appear very promising.
