Radiation therapy (RT) and chemotherapy (CTX) following surgery are mainstays of treatment for breast cancer (BC). While multiple studies have recently revealed the significance of immune cells as mediators of CTX response in BC, less is known regarding roles for leukocytes as mediating outcomes following RT. To address this, we utilized a syngeneic orthotopic murine model of mammary carcinogenesis to investigate if response to RT could be improved when select immune cells or immune-based pathways in the mammary microenvironment were inhibited. Treatment of mammary tumor-bearing mice with either a neutralizing monoclonal antibody (mAb) to colony-stimulating factor-1 (CSF-1) or a small molecule inhibitor of the CSF-1 receptor kinase (i.e., PLX3397), resulting in efficient macrophage depletion, significantly delayed tumor regrowth following RT. Delayed tumor growth in this setting was associated with increased presence of CD8+ T cells, and reduced presence of CD4+ T cells, the main source of the TH2 cytokine interleukin (IL)-4 in mammary tumors. Selective depletion of CD4+ T cells or neutralization of IL-4 in combination with RT, phenocopied results following macrophage depletion, whereas depletion of CD8+ T cells abrogated improved response to RT following these therapies. Analogously, therapeutic neutralization of IL-4 or IL-13, or IL-4 receptor alpha deficiency, in combination with the CTX paclitaxel resulted in slowed primary mammary tumor growth by CD8+ T cell-dependent mechanisms. These findings indicate that clinical responses to cytotoxic therapy in general can be improved by neutralizing dominant TH2-based programs driving protumorigenic and immune suppressive pathways in mammary (breast) tumors to improve outcomes.
- Received December 9, 2014.
- Revision received January 27, 2015.
- Accepted January 29, 2015.
- Copyright © 2015, American Association for Cancer Research.