Abstract 3931: The irradiated tissue microenvironment and its role in breast cancer recurrence: Enhanced macrophage infiltration promotes tumor cell recruitment
Authors: Hacker Benjamin C. , Alves Steven M. , Jiang Dadi , Koong Albert C. , Giaccia Amato J. , Graves Edward E. , Rafat Marjan 
Published: August 2019

Triple negative breast cancer (TNBC) recurrence rates remain high despite aggressive therapeutic intervention, including surgery, chemotherapy, and radiotherapy (RT). Recent studies suggest that circulating tumor cell re-seeding of primary tumors may facilitate recurrence rather than persistent tumor cells in the irradiated surgical bed. However, the role of the microenvironment in recurrence is not well understood. An emerging risk factor for breast cancer patients is lymphopenia or abnormally low systemic lymphocyte counts following therapy. In this study, we examined whether lymphopenia following therapy correlates to local recurrence. To investigate the relationship between prolonged low lymphocyte count and recurrence, we evaluated radiation effects on tumor and immune cell recruitment to normal tissues using an orthotopic breast cancer model of lymphopenia. We tested the hypothesis that local recurrence in some instances is due to the attraction of circulating tumor cells to irradiated tissues. Local cytokine secretion as well as protein expression levels were analyzed to evaluate how tumor-stromal interactions modulate tumor cell recruitment to sites of damage. Recurrence free survival at 5 years for TNBC patients with sustained low ALC 1-5 months after RT (n=37) was 62.5% compared with 97% for patients with normal ALC (n=46; p&lt;0.0001). Ex vivo BLI analysis revealed that normal tissue irradiation attracted tumor cells to the mammary fat pad (MFP) and surrounding tissues in BALB/c mice with depleted CD8+ T cells. Macrophage infiltration was greatly enhanced in BALB/c mice with depleted CD8+ T cells (p&lt;0.0001) and necessary for tumor cell recruitment. Luminex analysis of MFPs in mice lacking CD8+ T cells after RT showed enhanced cytokine secretion of factors that regulate the inflammatory microenvironment and influence tumor cell proliferation and invasion. RPPA analysis of irradiated MFPs in the absence of CD8+ T cells revealed factors that promote epithelial-mesenchymal transition. Taken together, these results demonstrate that the response of the tissue microenvironment to therapy is dependent on the immune milieu and may encourage local recurrence. Our study establishes the importance of considering tumor subtype and immune function when assessing failure and outcome risks in breast cancer. We show that normal tissue radiation response may play a role in modulating tumor and immune cell migration. These results suggest that the stroma facilitates local recurrence in breast cancer through influencing immune and tumor cell behavior following RT.

Citation Format: Benjamin C. Hacker, Steven M. Alves, Dadi Jiang, Albert C. Koong, Amato J. Giaccia, Edward E. Graves, Marjan Rafat. The irradiated tissue microenvironment and its role in breast cancer recurrence: Enhanced macrophage infiltration promotes tumor cell recruitment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3931.