Immunoregulatory antigens-novel targets for cancer immunotherapy

Historically, the development of cancer vaccines has focused on the central role of tumor antigens in eliciting tumor-specific immune responses, with limited success. Recent advances with checkpoint blockade approaches have brought about a renewed appreciation of the importance of targeting immune suppression in cancer patients. Here we discuss a novel approach to cancer immunotherapy, namely to target recently described T cells that uniquely control cells with immune suppressive functions. Accumulating evidence support the existence of self-reactive T cells that are specific to antigens derived from immunoregulatory proteins ("immunoregulatory antigens"), such as indoleamine 2,3-dioxygenase (IDO) and PD-L1. Vaccination approaches to potentiate these T cells have proven safe with minimal toxicity in the clinical phase I trials conducted thus far. Given that immunoregulatory antigens can be new targets for cancer immunotherapy, we propose here that they could be considered as a new class of tumor antigens. Targeting such antigens has advantages over targeting classical tumor antigens, as there is no requirement for identification of relevant antigens that are specific for the cancer type, and the targets are genetically stable. Furthermore, targeting immunoregulatory antigen-specific T cells potentially has dual mode of actions (I) targeting immune suppression and thereby potentiating anti-tumor effector T cell responses and (II) direct killing of immunoregulatory antigen-expressing tumor cells.