(786a) The Role of the Immune System in Modulating Tumor Cell Migration Following Radiation-Induced Changes in the Tissue Microenvironment

Despite aggressive surgical, radiological, and chemotherapeutic intervention, triple-negative breast cancer patients experience high rates of recurrence. The role of the tumor microenvironment in recurrence, however, is not well understood. We hypothesize that the irradiated tumor microenvironment influences tumor and immune cell behavior. In this study, we characterize the effects of normal tissue irradiation on tumor and immune cell migration to evaluate how tumor-stromal interactions modulate recurrence after therapy. This work represents the first step toward elucidating how stromal radiation response contributes to tumor and immune cell recruitment.

The effect of radiation on tumor cell migration to normal tissues was studied using an orthotopic breast cancer model. Nude mice were inoculated with luciferase-labeled 4T1 murine or MDA-MB-231 human triple-negative breast cancer cells in the mammary fat pad (MFP), and BALB/c mice with depleted CD4+ and/or CD8+ T cells were inoculated with 4T1 MFP tumors. The contralateral normal MFP was irradiated to a dose of 20 Gy with a 250 kVp cabinet x-ray machine when tumors were palpable. Cell migration was monitored with bioluminescence imaging (BLI) 10 d after irradiation. Irradiated and control tissues were evaluated using immunohistochemistry (IHC). Tissue sections were stained with F4/80 to determine the extent of macrophage infiltration. Flow cytometry was also performed on dissociated irradiated and control tissues to characterize immune cell populations. Of particular interest were CD11b+F4/80+ macrophages and CD11b+GR1+ myeloid-derived suppressor cells (MDSCs).

Radiation enhanced tumor cell migration to normal tissues in vivo. Ex vivo BLI analysis in nude mice revealed that normal tissue irradiation attracted tumor cells to the MFP and surrounding tissues, including the peritoneum and muscle (p<0.001). Tumor cells migrated to irradiated normal tissues in BALB/c mice with depleted CD4+ and CD8+ T cells (p<0.001). Depletion of CD8+ T cells alone significantly enhanced tumor cell migration in comparison to depleting CD4+ T cells alone (p<0.01). This indicates that CD8+ T cells may negatively regulate local recurrence following radiotherapy. An increase in macrophage infiltration was also found in irradiated tissue sections in the absence of T cells. Flow cytometry confirmed increases in macrophage and MDSC infiltration in irradiated areas.

This study establishes that normal tissue radiation response modulates tumor and immune cell migration after radiation. The increase in macrophage and MDSC infiltration after irradiation in the absence of T cells indicates a crucial role of the immune system in tumor cell migration. Our work suggests that the tumor stroma may facilitate tumor cell invasion and tumor regrowth following radiotherapy. Future studies will utilize these results to engineer improved in vitro tumor microenvironment models to probe the complex physical, chemical, and biological cues that influence cancer recurrence and metastasis.