Equilibrium between immune activation and suppression may be necessary to maintain immune homeostasis, because proinflammatory effector T cells (defined as antiregulatory T cells) counteract the functions of regulatory immune cells. These self-reactive T cells recognize human leukocyte antigen (HLA)–restricted epitopes derived from proteins expressed by regulatory immune cells such as IDO, PD-L1, PD-L2, or arginase. The activation of such proinflammatory effector T cells offers a novel way to directly target the tumor microenvironment, potentially giving them considerable clinical value, especially in patients with cancer. Vaccination against genetically stable cells with regular HLA expression is an attractive way to directly target immunosuppressive cells in addition to attracting proinflammatory cells into the tumor microenvironment. Importantly, vaccination toward IDO or PD-L1 to potentiate such T cells have proven safe, with minimal toxicity in the clinical phase I trials conducted thus far.