Environmental factors regulate Paneth cell phenotype and host susceptibility to intestinal inflammation in Irgm1-deficient mice

Abstract

Crohn's disease (CD) represents a chronic inflammatory disorder of the intestinal tract. Several susceptibility genes have been linked to CD, though their precise role in the pathogenesis of this disorder remains unclear. Immunity-related GTPase M (<i>IRGM</i>) is an established risk allele in CD. We have shown previously that conventionally raised (CV) mice lacking the <i>IRGM</i> ortholog, <i>Irgm1</i> exhibit abnormal Paneth cells (PCs) and increased susceptibility to intestinal injury. In the present study, we sought to utilize this model system to determine if environmental conditions impact these phenotypes, as is thought to be the case in human CD. To accomplish this, wild-type and <i>Irgm1^-/-</i> mice were rederived into specific pathogen-free (SPF) and germ-free (GF) conditions. We next assessed how these differential housing environments influenced intestinal injury patterns, and epithelial cell morphology and function in wild-type and <i>Irgm1^-/-</i> mice. Remarkably, in contrast to CV mice, SPF <i>Irgm1^-/-</i> mice showed only a slight increase in susceptibility to dextran sodium sulfate-induced inflammation. SPF <i>Irgm1^-/-</i> mice also displayed minimal abnormalities in PC number and morphology, and in antimicrobial peptide expression. Goblet cell numbers and epithelial proliferation were also unaffected by Irgm1 in SPF conditions. No microbial differences were observed between wild-type and <i>Irgm1^-/-</i> mice, but gut bacterial communities differed profoundly between CV and SPF mice. Specifically, <i>Helicobacter</i> sequences were significantly increased in CV mice; however, inoculating SPF <i>Irgm1^-/-</i> mice with <i>Helicobacter hepaticus</i> was not sufficient to transmit a pro-inflammatory phenotype. In summary, our findings suggest the impact of Irgm1-deficiency on susceptibility to intestinal inflammation and epithelial function is critically dependent on environmental influences. This work establishes the importance of <i>Irgm1^-/-</i> mice as a model to elucidate host-environment interactions that regulate mucosal homeostasis and intestinal inflammatory responses. Defining such interactions will be essential for developing novel preventative and therapeutic strategies for human CD.