The Myeloid Immune Signature of Enterotoxigenic Bacteroides Fragilis-Induced Murine Colon Tumorigenesis

Enterotoxigenic Bacteroides fragilis (ETBF), a human commensal and candidate pathogen in colorectal cancer (CRC), is a potent initiator of IL-17-dependent colon tumorigenesis in Min(Apc+/-)mice. We examined the role of IL-17 and ETBF on the differentiation of myeloid cells into myeloid-derived suppressor cells (MDSC) and tumor-associated macrophages (TAM), which are known to promote tumorigenesis. The myeloid compartment associated with ETBF-induced colon tumorigenesis in Min mice was defined using flow cytometry and gene expression profiling. Cell sorted immature myeloid cells were functionally assayed for inhibition of T cell proliferation and iNOS expression in order to delineate MDSC populations. A comparison of ETBF infection to that of other oncogenic bacteria (Fusobacterium nucleatum or pks+ E. coli) revealed a specific, ETBF-associated colonic immune infiltrate. ETBF-triggered colon tumorigenesis is associated with an IL-17-driven myeloid signature characterized by subversion of steady-state myelopoiesis in favor of the generation of protumoral monocytic (MO)-MDSCs. Combined action of the Bacteroides fragilis enterotoxin BFT and IL-17 on colonic epithelial cells promoted the differentiation of MO-MDSCs, which selectively upregulated Arg1 and Nos2, produced NO and suppressed T cell proliferation. Evidence of a pathogenic inflammatory signature in humans colonized with ETBF may allow for the identification of populations at risk for developing colon cancer.