CD8⁺Foxp3⁺ tumor infiltrating lymphocytes accumulate in the context of an effective anti-tumor response

The composition of tumor infiltrating lymphocytes (TIL) is heterogeneous. In addition, the ratio of various subpopulations in the tumor microenvironment is highly dependent on the nature of the host's immune response. Here, we characterize Foxp3-expressing CD8⁺ T cells in the tumor that demonstrate effector function and accumulate in the context of an effective anti-tumor response. CD8⁺Foxp3⁺ T cells are induced in TIL in regressing tumors of FVB/N mice treated with a GM-CSF secreting HER-2/neu targeted whole cell vaccine. Foxp3 expression in tumor antigen-specific CD8 T cells is restricted to the tumor microenvironment and influenced by cues in the tumor. Interestingly, Foxp3⁺ and Foxp3^- CD8⁺ T cells have similar IFN-γ production and antigen-specific degranulation after stimulation with RNEU_420-429, the immunodominant HER-2/neu (neu) epitope in this model. Adoptive transfer studies, using RNEU _(420-429)-specific effector T cells into neu-N mice (a model that results in immune tolerance to neu), confirm that CD8⁺Foxp3 ⁺ T cells are present in tumors only if there is an existing pool of tumor-rejecting effector T cells. CD8⁺Foxp3⁺ TILs mark the presence of tumor-rejecting antigen-specific T cells and their accumulation serves as a marker for an effective T cell response.