C-C chemokine receptor type 2 (CCR2) is highly expressed in human breast cancer tissue by both cancer cells and infiltrating immune cells. High expression of CCR2 is correlated with advanced tumor stage, metastasis, and early relapse. Studies in mice have suggested that CCR2 expression on infiltrating inflammatory monocytes promotes primary tumor growth and metastasis. However, the role of CCR2 expression on cancer cells has not been well studied.
To determine how CCR2 directly impacts tumor growth and metastasis, we used a murine model of luminal breast cancer, the mouse mammary tumor virus (MMTV) promoter polyoma middle T antigen (PyMT) model. We crossed MMTV-PyMT mice to Ccr2-/- mice and found that loss of CCR2 did not alter tumor onset. However, primary tumors of MMTV-PyMT;Ccr2+/+ mice grew significantly faster than tumors of MMTV-PyMT;Ccr2-/- mice. Lung metastases were also larger in MMTV-PyMT;Ccr2+/+ mice than in MMTV-PyMT;Ccr2-/- mice.
We determined by RNA in situ hybridization that CCR2 was indeed expressed by both host cells and cancer cells in MMTV-PyMT tumors. To differentiate between the contributions of CCR2 expression in the tumor and host, we transplanted primary cancer cells derived from MMTV-PyMT;Ccr2+/+ and MMTV-PyMT;Ccr2-/- mice to syngeneic Ccr2+/+ and Ccr2-/- hosts. Surprisingly, CCR2 expression in the host had no significant effect on tumor growth. However, deletion of CCR2 from cancer cells resulted in delayed tumor growth and increased survival. Furthermore, when Ccr2+/+ and Ccr2-/- cancer cells were transplanted to athymic nude mice (lacking T cells), the resulting tumors no longer showed a delayed growth. Consistent with this finding, flow cytometry of infiltrating leukocytes in tumors transplanted to syngeneic mice revealed increased levels of CD8+ cytotoxic T cells and antigen-presenting CD103+ dendritic cells into Ccr2-/- tumors compared to wild type tumors. These data suggest that CCR2 expression on cancer cells regulates an immune response to the tumors. Supporting this conclusion, expression of programmed cell death ligand 1 (PD-L1) on Ccr2-/- cancer cells was reduced to just 30% of the level on wild type cancer cells, indicating that CCR2 expression on cancer cells suppresses the anti-tumor immune response. Together, our results establish a novel role for cancer cell CCR2 expression in breast cancer progression by suppression of cytotoxic T cells through an immune checkpoint.
Citation Format: Miriam R. Fein, Ana S. Almeida, Anaïs Eberhardt, Mikala Egeblad. Cancer cell expression of CCR2 regulates the PD-L1/ PD-1 immune checkpoint in breast cancer. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A063.