Vaccination against angiogenesis-associated antigens: a novel cancer immunotherapy strategy

Therapeutic vaccines represent an attractive approach to cancer treatment. Traditionally, cancer immunotherapy targets antigens expressed by the tumor cells. Although numerous clinical trials studying different cancer vaccines have been conducted during the past twenty years, very limited clinical responses have been observed. The inefficient anti-tumor immunity is thought to be due, in major part, to the escape mechanisms exerted by the genetically unstable tumor cells, e.g., emergence of antigen-loss mutants, downregulation of MHC molecules and lack of expression of costimulatory molecules. Recently, a novel vaccine strategy has been developed to circumvent these obstacles. Taking advantage of the importance of angiogenesis in tumor growth and the genetic stability of endothelial cells, this immunotherapy strategy targets antigens (e.g., angiogenic growth factor receptors) overexpressed by the tumor neo-vasculature rather than the tumor cells per se. For example, active immunization against vascular endothelial growth factor receptor-2 (VEGFR-2) has been shown to generate strong cellular and humoral immune responses, which lead to the inhibition of angiogenesis and tumor growth and metastasis. This review provides an outline of this emerging field and discusses the advantages and potential pitfalls of such a vaccine strategy.