Abstract 546: Effects of high-dose radiotherapy on the bone marrow microenvironment in the 4T1 model of breast cancer

Background: Of patients with breast cancer, an estimated 40 percent relapse and develop metastatic disease with bone as a preferred site of metastases. Radiation therapy is used frequently to treat bone metastases for palliation of symptoms and to improve local control. Advances in the field of radiation oncology such as image-guided and intensity-modulated radiation therapy (IGRT/IMRT) permit the delivery of high-dose radiation to bone metastases. The tumor stroma and bone marrow-derived cells have been shown to impact the response to irradiation, potentially via radiation-induced recruitment of bone marrow-derived cells and mesenchymal stem cells. To date, little is known about changes in the bone marrow and tumor microenvironment following high-dose irradiation. Using a murine model of metastatic breast cancer, we sought to investigate the effect of local high-dose irradiation on the irradiated, non-irradiated tumor microenvironments, and on the pattern of circulating hematopoietic, endothelial progenitor and mesenchymal stem cells. Methods: The metastatic murine mammary tumor cell line, 4T1, was engineered to express GFP and luciferase for in vitro and in vivo detection of tumor cells. These cells were injected into the bilateral tibias of BALB/c mice. Five days following intra-tibial injection, animals received either high-dose irradiation or sham irradiation to the right hind limb. Growth and progression of bone and lung metastases were monitored by in vivo bioluminescence. Blood was assayed for circulating myeloid-derived suppressor cells (MDSC), endothelial progenitor cells (EPC), and mesenchymal stem cells (MSC) at several time points following irradiation. Results: Intra-tibial injection of 4T1 cells resulted in rapid tumor growth and development of pulmonary metastases. High-dose irradiation (20Gy) resulted in a significant reduction in the irradiated tumor size without a significant change in the size of contralateral non-irradiated tumor, as evident by bioluminescent imaging. Pulmonary metastases in irradiated animals were hemorrhagic as compared to non-irradiated animals. Irradiation resulted in increased macrophage recruitment to the irradiated, but not the non-irradiated tumor. As compared to control mice and irradiated non-tumor bearing mice, animals with 4T1 intra-tibial tumors have increased numbers of circulating MDSC (CD11b+/Gr1+), EPC (CD144+/VEGFR2+), and MSC (CD105+/CD29+). Conclusions: Here we introduce a model of breast cancer bone metastasis that may be utilized to study the effects of high-dose irradiation. Irradiation results in changes in immune cell composition at both the local and systemic levels. Ongoing studies will address the specific immune cell subtypes present in irradiated versus non-irradiated bone tumors and the corresponding pulmonary metastases. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 546. doi:10.1158/1538-7445.AM2011-546