The role of inflammatory monocytes in human metastatic colorectal cancer.

624 Background: Colorectal cancer (CRC) is the most common gastrointestinal malignancy. 60% of CRC patients are diagnosed with metastatic (m) CRC and the 5-year survival is <20%. Myeloid cells, particularly inflammatory monocytes (IM), are recruited from the bone marrow to the tumor microenvironment where they become tumor associated macrophages and play a crucial role in tumor progression, metastasis, and chemoresistance. While the importance of IM have been shown in other malignancies, little is known about their role in human CRC. Methods: Human tissue was collected under an IRB approved protocol at Washington University. Flow cytometry was performed on PBMCs and single cell suspensions of normal tissue and tumor samples. Qualitative RT-PCR and confocal microscopy were performed for CCL2. T-cell suppression assays were performed using CD14+ IM isolated from patient peripheral blood and tumor samples. Results: Analysis of pre-operative blood revealed that monocyte levels correlate with the extent of disease burden. Monocytes were elevated in CRC patients compared with controls (p<0.0001), additionally patients with liver metastasis had further elevation in monocytes compared with patient’s with primary disease (p=0.01). In metastatic patients, monocyte levels also correlate with survival following resection of hepatic metastasis (p=0.0002). FACS analysis confirmed these findings and demonstrated that the circulating CD11b+/CD14+/CCR2+ subset of IM was responsible for the increase. Both primary CRC and liver mCRC had increased expression of CCL2 compared to uninvolved tissue (p=0.008 and p=0.03, respectively). Production of CCL2 was localized to CRC cells. FACS analysis showed CCR2+ tumor infiltrating macrophages were elevated in CRC liver metastasis compared to adjacent normal liver and a paucity of effector T-cells. CD14+ TAMs isolated from mCRC inhibited T-cell proliferation, illustrating the immune suppressive phenotype of these cells. Conclusions: Inflammatory monocytes correlate to prognosis in mCRC. Therefore, targeting CCR2+ myeloid cells may improve anti-tumor immunity and patient survival in metastatic disease.