Title: Normal Tissue Irradiation Promotes Tumor and Immune Cell Infiltration in a Breast Cancer Model
Despite aggressive surgical, radiological, and chemotherapeutic intervention, triple-negative breast cancer patients experience high rates of recurrence. Recent studies suggest that recurrence may be facilitated by circulating tumor cell re-seeding of primary tumors. The role of the tumor microenvironment in recurrence, however, is not well understood. Radiation has been shown to promote tumor cell migration in a preclinical breast cancer model. We hypothesize that the irradiated tumor microenvironment influences tumor and immune cell behavior. Irradiation of the tumor bed occurs following tumor resection and chemotherapy in breast cancer treatment. 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. The effect of radiation on tumor cell migration to normal tissues was studied both in vitro and in vivo. Supernatant collected from irradiated mouse embryonic fibroblasts enhanced tumor cell migration in an in vitro transwell assay. Luciferase-labeled cells were used for in vivo and ex vivo bioluminescence imaging (BLI). Mice were inoculated with breast cancer cells orthotopically in the mammary fat pad (MFP), and the contralateral normal MFP was irradiated. Cell migration was monitored with BLI following irradiation, and it was found that irradiation promoted tumor cell migration to normal tissues. Immunohistochemistry was performed on irradiated and control tissues to determine the extent of immune cell infiltration. Flow cytometry was also performed on dissociated irradiated and control tissues to characterize immune cell populations. Tumor cell migration to normal tissues was stimulated by radiation, and tumor cell recruitment was shown to be coupled with immune cell infiltration. This study establishes that normal tissue radiation response modulates tumor and immune cell migration after radiation. 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 vitrotumor microenvironment models to probe the complex physical, chemical, and biological cues that influence cancer recurrence and metastasis.