Recurrence of Intracranial Tumors following Adoptive T Cell Therapy Can Be Prevented by Direct and Indirect Killing Aided by High Levels of Tumor Antigen Cross-Presented on Stromal Cells

Abstract

Elimination of peripheral tumors by adoptively transferred tumor-specific T cells may require killing of cancer cells and tumor stromal cells. Tumor Ags are cross-presented on stromal cells, resulting in direct cytotoxic T cell (CTL) killing of both Ag-expressing cancer cells and stromal cells. Indirect killing of Ag loss variant cells also occurs. We show here that similar processes occur in a brain tumor stromal environment. We used murine cancer cell lines that express high or low levels of a peptide Ag, SIYRYYGL (SIY), recognized by transgenic 2C CD8+ T cells. The two cell lines are killed with equivalent efficiency by 2C T cells in vitro. Following adoptive transfer of 2C T cells into mice with established SIY-Hi or SIY-Lo brain tumors, tumors of both types regressed, but low-Ag-expressing tumors recurred. High-Ag-expressing tumors contained CD11b+ cells cross-presenting SIY peptide and were completely eliminated by 2C T cells. To further test the role of cross-presentation, RAG1−/− H-2b mice were infused with H-2k tumor cells expressing high levels of SIY peptide. Adoptively transferred 2C T cells are able to kill cross-presenting H-2b stromal cells but not H-2k tumor cells. In peripheral models, this paradigm led to a small static tumor. In the brain, activated 2C T cells were able to kill cross-presenting CD11b+ cells and completely eliminate the H-2k tumors in most mice. Targeting brain tumor stroma or increasing Ag shedding from tumor cells to enhance cross-presentation may improve the clinical success of T cell adoptive therapies.