Macrophage depletion decreases tumorigenesis and alters gut microbiota in the AOM/DSS mouse model of colon cancer

Abstract

Abstract Macrophages are recognized as major players in the connection between inflammation and cancer; they represent up to 50% of the tumor mass and produce a wide array of inflammatory mediators with pro-tumoral functions. Emerging evidence links gut microbiota to the development of colorectal cancer and host-derived immune and inflammatory responses are important driving forces that can shape microbial community composition. We used a macrophage depletion technique to examine the role of macrophages on colon tumorigenesis and gut microbiota. To induce colorectal cancer, male C57BL/6 mice (n=32) received a single injection of AOM followed by three cycles of DSS supplemented water at concentrations of 2%, 1%, 1% at weeks 1, 4, and 7, respectively. Prior to the final DSS cycle (week 7) and twice weekly until sacrifice, mice (n=16/group) received 200ul i.p. of clodronate (CL2MDP) or PBS encapsulated liposomes to deplete macrophages. Mice were sacrificed at 12 weeks following the initial AOM injection and colon tissue was analyzed for tumor burden, macrophage markers, and inflammatory mediators. Stool samples were collected and DNA was isolated and subsequently sequenced for 16S rRNA. Macrophage depletion decreased tumor number by ~35% and specifically large (≥1mm) tumors by ~35% (p<0.05). This was consistent with a decrease in gene expression of EMR1 (overall macrophage marker) as well as expression of markers associated with M2 macrophages (IL-13, IL-10, TGFβ & CCL17) (p<0.05) but not M1 markers (NOS2, IL-12a & IL-23). Macrophage depletion also increased relative abundance of the Firmicutes phylum (p<0.05) in stool. We demonstrate that macrophages are important mediators of tumor growth and changes in gut microbiota in colorectal cancer.