<i>Fusobacterium nucleatum</i> drives a pro-inflammatory intestinal microenvironment through metabolite receptor-dependent modulation of IL-17 expression

Abstract

The colorectal cancer (CRC)-associated microbiota creates a pro-tumorigenic intestinal milieu and shapes immune responses within the tumor microenvironment. However, how oncomicrobes - like <i>Fusobacterium nucleatum</i>, found in the oral cavity and associated with CRC tissues<i>-</i> affect these distinct aspects of tumorigenesis is difficult to parse. Herein, we found that neonatal inoculation of <i>Apc^Min/+</i> mice with <i>F. nucleatum</i> strain Fn7-1 circumvents technical barriers preventing its intestinal colonization, drives colonic <i>Il17a</i> expression prior to tumor formation, and potentiates intestinal tumorigenesis. Using gnotobiotic mice colonized with a minimal complexity microbiota (the altered Schaedler's flora), we observed that intestinal Fn7-1 colonization increases colonic Th17 cell frequency and their IL-17A and IL-17F expression, along with a concurrent increase in colonic lamina propria <i>Il23p19</i> expression. As Fn7-1 stably colonizes the intestinal tract in our models, we posited that microbial metabolites, specifically short-chain fatty acids (SCFA) that <i>F. nucleatum</i> abundantly produces in culture and, as we demonstrate, in the intestinal tract, might mediate part of its immunomodulatory effects <i>in vivo</i>. Supporting this hypothesis, we found that Fn7-1 did not alter RORγt⁺ CD4⁺T cell frequency in the absence of the SCFA receptor FFAR2. Taken together, our work suggests that <i>F. nucleatum</i> influences intestinal immunity by shaping Th17 responses in an FFAR2-dependent manner, although further studies are necessary to clarify the precise and multifaceted roles of FFAR2. The potential to increase intestinal Th17 responses is shared by another oncomicrobe, enterotoxigenic <i>Bacteroides fragilis</i>, highlighting a conserved pathway that could potentially be targeted to slow oncomicrobe-mediated CRC.