OX40 costimulation synergizes with GM-CSF whole-cell vaccination to overcome established CD8⁺ T cell tolerance to an endogenous tumor antigen

T cell costimulation via OX40 is known increase CD4⁺ T cell expansion and effector function and enhances the development of T cell memory. OX40 costimulation can also prevent, and even reverse, CD4⁺ T cell anergy. However, the role of OX40 in CD8⁺ T cell function is less well defined, particularly in the setting of immune tolerance. To determine the effects of OX40 costimulation on the induction of the host CO8⁺ T cell repertoire to an endogenous tumor Ag, we examined the fate of CD8 ⁺ T cells specific for the immunodominant rat HER-2/neu epitope, RNEU_420-429, in FVB MMTV-neu (neu-N) mice, which express rat HER-2/neu protein in a predominantly mammary-restricted fashion. We show that the RNEU_420-429-specific T cell repertoire in neu-N mice expands transiently after vaccination with a neu-targeted GM-CSF-secreting whole-cell vaccine, but quickly declines to an undetectable level. However, OX40 costimulation, when combined with GM-CSF-secreting tumor-targeted vaccination, can break established CD8⁺ T cell tolerance in vivo by enhancing the expansion, and prolonging the survival and effector function of CO8⁺ T cells specific for RNEU_420-429. Moreover, we demonstrate thai OX40 expression is up-regulated on both CD4⁺ and CD8⁺ T cells shortly after administration of a GM-CSF expressing vaccine. These studies highlight the increased efficacy of OX40 costimulation when combined with a GM-CSF-secreting vaccine, and define a new role for OX40 costimulation of CD8⁺ T cells in overcoming tolerance and boosting antitumor immunity.