Abstract 2871: Tumor-derived CCL2 promotes recruitment of CD11b/Gr1midmyeloid cells to sustain colorectal cancer liver metastasis

Metastatic colorectal cancer (CRC) is one of the most prominent causes of cancer-related mortality. However, therapeutic options for patients with metastatic CRC are limited and development of new and more effective therapeutics aimed at downstaging the disease and prolonging survival is urgently required. Primary tumor dissemination to secondary sites, predominantly the liver, is a dynamic process dependent on the interactions between tumor cells and the host tissue microenvironment. Immune cells in the tumor microenvironment have been shown to play pivotal roles in the metastatic process but their pro-metastatic functions in CRC liver metastasis remains incompletely understood. To simulate liver metastasis, MC38 colon carcinoma cells were injected into the spleen of C57BL/6 mice. A population of CD11b/Gr1mid cells, distinct from other metastasis-associated myeloid cells previously described, increased dramatically during establishment of liver metastases. Adoptive transfer of bone marrow cells confirmed that these CD11b/Gr1mid cells were derived from bone marrow progenitor cells. MC38 tumor cells expressed high levels of CCL2 and serum levels were elevated in tumor-bearing mice compared to controls. Inhibition of tumor-derived CCL2 using shRNA, and absence of its cognate receptor in CCR2 KO mice significantly diminished CD11b/Gr1mid cell infiltration into tumor-bearing livers. Importantly, inhibition of CD11b/Gr1mid cell recruitment led to a marked reduction in tumor burden. Moreover, when CD11b/Gr1mid cells were depleted in a transgenic CD11b-DTR mouse model, a striking decrease in tumor vessel density was observed, with a concomitant reduction in tumor cell proliferation. Serum CCL2 concentrations were similarly elevated in CRC patients, including those who present metastasis, compared to normal controls. Furthermore, CD11b/CCR2 myeloid cells, analogous to the CD11b/Gr1mid population described in our mouse model, were identified in liver metastases of several CRC patients. Together, our studies demonstrate the importance of a distinct CD11b/Gr1mid myeloid subset in sustaining CRC liver metastasis and implicate a CCL2/CCR2-dependent pathway in their recruitment to metastatic sites. Our findings reinforce how tumor cells utilize host systems for successful propagation and identify a potential target for therapeutic manipulation. Citation Format: Su Yin Lim, Lei Zhao, Alex Gordon-Weeks, Thomas Tapmeier, Ruth Muschel. Tumor-derived CCL2 promotes recruitment of CD11b/Gr1mid myeloid cells to sustain colorectal cancer liver metastasis. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2871. doi:10.1158/1538-7445.AM2013-2871