Modifying Adenoviral Vectors for Use as Gene-Based Cancer Vaccines

The past decade has produced significant advances in our understanding of antigen-presenting cells, tumor antigens, and other components of the immune response to cancer. Gene-based vaccination is emerging as one of the more promising approaches for loading dendritic cells (DC) with tumor-associated antigens. In this respect, it is proposed that adenoviral (AdV) vectors can deliver high antigen concentrations, promote effective processing and MHC expression, and stimulate potent cell-mediated immunity. While AdV vectors have performed well in pre-clinical vaccine models, their application to patient care has limitations. The in vivo administration of AdV vectors is associated with both innate and adaptive host responses that result in tissue inflammation and injury, viral neutralization, and premature clearance of AdV-transduced cells. A variety of strategies have been developed to address these limitations. The ideal vaccine would avoid vector-related immune responses, have relative specificity for transducing DC, and induce high levels of transgene expression. This review describes the range of host responses to AdV vaccines, identifies strategies to reduce viral recognition and enhance transgene antigen expression, and suggests future approaches to vector development and administration. There is every reason to believe that safer and more effective forms of AdV-based vaccines can be developed and applied to patient therapy.