Abstract 3213: Extracellular acidosis alters polarization of macrophages

Abstract Deficits in tumor perfusion and elevated glycolysis combine to reduce the pH of the tumor microenvironment. In poorly perfused volumes, oxygen and nutrient deprivation may also elicit necrotic areas that trigger infiltration of immune cells, including tumor-associated macrophages (TAMs). Inside tumors, TAMs are biased towards an M2 phenotype with anti-inflammatory and tumor-promoting characteristics, rather than an M1 phenotype known to be essential for host defense and tumor cell killing. We aimed to test whether low pH as a common stress factor in the tumor microenvironment could independently change the phenotypic characteristics and functional activity of macrophages. Accordingly, we stimulated macrophages into M1, M2 and prostate cancer-associated macrophages (Pr-TAMs) at pH 7.4 or pH 6.8 using IFN-γ+LPS, IL-4 or 30% TRAMP-C2-conditioned medium, respectively. 120 h after initiation of activation, M2 macrophages showed higher viability at pH 6.8 compared to pH 7.4, suggesting a better fitness of M2 macrophages in an acidic microenvironment. Extracellular acidosis also decreased gene expression of common pro-inflammatory M1 markers (iNOS, MCP1 and IL-6) in M1 macrophages; while it increased gene expression of M2 markers (MRC1, ARG1 and Chi313) in M2 macrophages. In Pr-TAMs, low pH decreased iNOS/ARG1 mRNA ratio indicating a shift towards the tumor promoting M2 phenotype. Using a cytokine array, release of inflammatory cytokines into media was also decreased when M1 macrophages were activated at acidic pH. Interestingly, when macrophages were incubated at neutral or low pH for 24 h then co-cultured with the prostate cancer cell lines TRAMP-C2 or -C3, the low pH- treated macrophages showed a higher mannose receptor expression compared to neutral pH-treated macrophages, suggesting a shifting towards M2 phenotype. Furthermore, TRAMP-C2 cells co-cultured with macrophages in acidic medium had a higher proliferation rate than tumor cells co-cultured at neutral pH. Notably, treating TRAMP mice with 200 mM sodium bicarbonate to raise microenvironmental pH, strongly inhibited both tumor progression and macrophage infiltration. In conclusion, extracellular acidosis induced a macrophage phenotypic switch on both gene expression and cytokine profile levels, which would lead to a promotion of tumor growth. Also, intervention of microenvironmental tumor acidity using buffer therapy in TRAMP mice decreased the number of tumor associated macrophages, coincident with retardation of tumor progression. Citation Format: Asmaa E. El-Kenawi, Arig A. Ibrahim-Hashim, Kimberly A. Luddy, Shari A. Pilon-Thomas, Robert A. Gatenby, Robert J. Gillies. Extracellular acidosis alters polarization of macrophages. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3213. doi:10.1158/1538-7445.AM2015-3213