Abstract 803: Microsphere engulfment by macrophages when exposed to breast and colon tumor microenvironments

The microenvironment of most tumors is filled with a large population of macrophages. In breast cancer, studies have found that macrophages can account for >50% of the tumor mass. Their presence within the tumor microenvironment, in some cases, has been proven to increase metastasis, angiogenesis, and immunosuppression. However, colorectal cancer has been seen in some cases to exhibit decreased tumor growth and proliferation when a larger number of macrophages are associated with the tumor. Previous studies have suggested that cytokines and chemokines released by the tumor cells in the tumor microenvironment influence the polarization of the macrophages present. The M1 phenotype will often lead to aggressive destruction of the tumor, while an M2 phenotype will in fact aid tumor survival. M2 macrophages have been suggested to have a reduced phagocytic function. In this study, we investigated overall phagocytosis of microspheres by U-937 derived macrophages when exposed to nutrient-depleted media from both breast and colon cancer cell lines as well as when exposed to nutrient-depleted media containing cancer cells themselves. U-9379s were stained with Efluor 670 dye for identification and then stimulated using phorbol 12-myristate 13-acetate (PMA). Macrophages were then incubated for various time periods with cancer cells seeded in 6-well plates. Engulfment was measured by adding fluorescent latex microspheres to samples and assayed using a flow cytometer. Overall engulfment was measured as the percent of macrophages in a sample that engulfed microspheres while aggressiveness was measured as the amount of beads that macrophages were seen to phagocytose. Controls indicate that 5-10% of cancer cells non-specifically bind microspheres and that approximately 20% of unstimulated U9379s engulf beads, whereas positive controls indicate that 80% of PMA stimulated U9379s engulf microspheres. Further results show that incubation in spent media from both breast and colon cell lines did not significantly alter overall phagocytosis or aggressiveness. However, when incubated with either colon or breast cancer cell lines in spent media, there is as high as a 30% reduction in overall engulfment, indicating a shift toward an M2 phenotype. When macrophages are co-cultured with non-cancerous lymphocytes, the overall engulfment stays consistent with positive controls. Also, highly aggressive macrophages engulfed significantly less aggressively when exposed to tumor microenvironments as compared to positive controls. This study suggests that the microenvironment influences the polarization of macrophages via cell to cell contact and that both overall engulfment and aggressiveness of macrophages are affected. It also suggests that the microenvironments of both breast and colon cancer are equally able to polarize macrophages toward M2. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 803. doi:10.1158/1538-7445.AM2011-803