Fluorescence tracking CD8 T cells from tumor-draining lymph nodes to irradiated tumors and their activation in the tumor environment.

Radiation is an effective site-specific cytotoxic therapy for cancer cells. In preclinical models, CD8 T cells are essential for optimum tumor control by radiation therapy, yet CD8 T cells are radiosensitive and depleted within the treatment field. Irradiated tumors can be refilled with T cells that were outside the field at the time of treatment. Using the Kaede photoconvertible mouse model we tracked CD8 T cells that originated in the tumor-draining lymph node and entered the tumor following radiation therapy. We show that radiation induces CD69 expression in newly infiltrating CD8 T cells within the tumor; however, this activation is not antigen-specific activation, but rather ois mediated by type I interferon released following radiation therapy. This effect is radiation dose dependent and requires cGAS expression in the cancer cells. Mathematical modeling demonstrates that radiation results in increased retention of surviving CD8 T cells in the tumor environment following treatment. By tracking the movement of tumor antigen-specific CD8 T cells from the tumor-draining lymph node to the tumor, we can observe that these cells rapidly express exhaustion markers in the tumor environment. These events occur in advance of radiation-mediated antigen-specific T cell expansion in the tumor-draining lymph node. These data formally demonstrate movement of CD8 T cells from the tumor-draining lymph node to the irradiated tumor and identify a mechanism of innate inflammation-mediated T cell activation that may impact T cell retention and accelerates refilling of the irradiated tumor.