Abstract P4-04-02: Tumor-initiated peripheral myeloid cell expansion is reversed by radiation therapy

Abstract

Abstract Myeloid lineage cells, particularly monocytes and macrophages, have an important role in the development and progression of cancer. Within established tumors, macrophages are critical for angiogenesis, invasion, metastasis, immunosuppression and response to therapy. In addition, both transplantable and transgenic mouse models of mammary cancer are associated with myeloid expansions detectible in the tumor, spleen and peripheral blood, and these myeloid cells are able to suppress T cell activation in vitro. Reducing the tumor burden through chemotherapy and resection has been shown to control the myeloid expansion. However, chemotherapies that reduce tumor burden also have direct effects on myeloid cells, and surgical trauma has been shown to mobilize myeloid cells. Radiation therapy is the third major option to treat cancer, but the consequence of radiation therapy to systemic myeloid populations has not been described. We applied tumor-specific radiation therapy to test the hypothesis that the frequency of myeloid lineage cells correlates with the primary tumor burden. We demonstrate in the MMTV-PyMT model of mammary cancer that myeloid expansion in the blood closely correlates with myeloid involvement in the tumor and with tumor progression. We demonstrate that radiation therapy of 4T1 mammary tumors leads to a decline in myeloid cell numbers in the blood and a decrease in spleen size. The frequency of myeloid cells does not decline to the level seen in tumor-free mice: we demonstrate that metastatic disease can prevent myeloid cell numbers from returning to baseline, and that tumor recurrence from residual disease correlates with re-expansion of myeloid lineage cells. Despite these changes in the frequency of myeloid lineage cells, T cell numbers in the blood and spleen remains constant. However, radiation therapy improves the myeloid cell:CD8+ T cell ratio and results in increased proliferation of T cells in the spleen. Finally, we demonstrate that T cell responses to foreign antigens are not altered by tumor burden or myeloid cell expansion, while T cell responses to tumor-associated antigens are increased after radiation therapy of the tumor. These data demonstrate that myeloid cell numbers are directly linked to primary tumor burden, that this population contracts following radiation therapy, and that radiation therapy may open a therapeutic window for immunotherapy of residual disease. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P4-04-02.