Modulating Costimulation to Break T Cell Tolerance: Potential Therapeutic Insight into Breaking Tolerance to Tumor Antigens

Tumor-associated antigens (TAAs) are a form of self-antigens, and thus tumor immunity can be dampened by the tolerization of TAA-specific T cells. Breaking T cell tolerance can thus augment tumor immunity and may be achieved by either inhibiting negative regulators of T cell responsiveness or positively stimulating costimulatory pathways that are otherwise not engaged during the induction of tolerance. The E3 ubiquitin ligase Cbl-b negatively regulates T cell priming by controlling the CD28 costimulatory pathway. Cbl-b -/- mice are predisposed to autoimmunity and Cbl-b-/- T cells can mediate enhanced tumor rejection. Additionally, Cbl-b-/- T cells hyper-proliferate and are resistant to anergy induction in vitro. In a model where HA-specific TCR Transgenic CD4 T cells are tolerized when they encounter HA expressed as a self-antigen, we found that while HA-specific Cbl-b-/- CD4 T cells hyper-proliferated, they were not resistant to tolerance. Nevertheless, their enhanced proliferative potential enabled tolerized Cbl-b-/- HA-specific CD4 T cells to more readily expand and regain function when self-HA was replaced with viral-HA. Thus, Cbl-b deficiency may predispose towards autoimmunity not by preventing T cell tolerance, but rather by increasing the potential of tolerized self-reactive T cells to recover function.;We next analyzed how tolerance is broken by positively enforcing costimulation. Agonistic monoclonal antibodies to CD134 plus CD137 (dual costimulation or DCo) protect HA-specific CD4 and CD8 T cells from self-HA-induced tolerance. Notably, DCo programs HA-specific CD4 T cells to expand and differentiate into cytotoxic Thl cells but can only program HA-specific CD8 T cells to expand. We found that the inability of DCo to couple CD8 T cell effector differentiation to expansion mainly results from the absence of CD4 helper T cell function. More specifically, operating via a T cellintrinsic mechanism CD4 helper T cell-elicited IL-2 and IFN-gamma cooperatively programed CD8 T cells to express IFN-gamma and granzyme B. IFN-gamma also limited IL-2-supported expression of the IL-2 receptor alpha chain via an indirect mechanism on both HAspecific effector T cells and bystanding CD4+Foxp3 + Tregs, and also limited Treg expansion during DCo. Importantly, these effects could not be explained by the ability of IFN-gamma to either limit IL-2 availability or to induce the transcription factor T-bet (that has the potential to repress IL-2). Thus, during DCo IFN-gamma cooperates with IL-2 through distinct mechanisms to establish the balance between effector and regulatory T cells by programing maximal expression of effector molecules in effector T cells while simultaneously limiting Treg expansion.