Regulatory T cell-resistant CD8+ T cells induced by glucocorticoid-induced tumor necrosis factor receptor signaling

We previously found that a Salmonella typhimurium vector engineered to secrete soluble tumor antigen induces CD4(+) T cells resistant to CD4(+)CD25(+) regulatory T cells (Treg) and that glucocorticoid-induced tumor necrosis factor receptor family-related gene (GITR) signal is involved in the development of this resistance. In this study, we address the potential of incorporating GITR ligand (GITRL) as a way to augment the immunogenicity of cancer vaccines. BALB/c mice were immunized by gene gun with plasmids encoding the mutated extracellular signal-regulated kinase 2 (mERK) with or without plasmids encoding mouse GITRL. Coadministration with GITRL during primary and secondary immunization enhanced the induction of mERK-specific CD8(+) T cells. Antibody depletion and minigene analysis suggested that GITRL directly activated CTL epitope-specific CD8(+) T cells independently of CD4(+) T cells. Immunization with plasmids encoding a CTL epitope and GITRL resulted in strong tumor inhibition in a CD8(+) T cell-dependent manner. Furthermore, CTL epitope-specific CD8(+) T cells induced by immunization with plasmids encoding CTL epitope coadministered with GITRL were refractory to suppression by CD4(+)CD25(+) Tregs compared with CD8(+) T cells induced without GITR signaling. We propose that coadministration of GITR signaling agents with tumor antigens constitutes a promising novel strategy for cancer vaccine development.