Irgm1-deficiency leads to myeloid dysfunction in colon lamina propria and susceptibility to the intestinal pathogenCitrobacter rodentium
Authors: Taylor Gregory A. , Huang Hsin-I , Fee Brian E. , Yousssef Nourhan , Cantillana Viviana , Schoenborn Alexi A. , Rogala Allison R. , Buckley Anne F. , Vallance Bruce A. , Gulati Ajay S. , Hammer Gianna E. 
Published: March 2019

IRGMand its mouse orthologueIrgm1are dynamin-like proteins that regulate vesicular remodeling, intracellular microbial killing, and pathogen immunity.IRGMdysfunction is linked to inflammatory bowel disease (IBD), and while it is thought that defective intracellular killing of microbes underscores IBD susceptibility, studies have yet to address how IRGM/Irgm1 regulates immunity to microbes relevant to intestinal inflammation. Here we find that loss of Irgm1 confers marked susceptibility toCitrobacter rodentium, a noninvasive intestinal pathogen that models inflammatory responses to intestinal bacteria. Irgm1-deficient mice fail to controlC. rodentiumoutgrowth in the intestine, leading to systemic pathogen spread and host mortality. Surprisingly, susceptibility due to loss of Irgm1 function was not linked to defective intracellular killing ofC. rodentiumor exaggerated inflammation, but was instead linked to failure to remodel specific colon lamina propria (C-LP) myeloid cells that expand in response toC. rodentiuminfection and are essential forC. rodentiumimmunity. Defective immune remodeling was most striking in C-LP monocytes, which were successfully recruited to the infected C-LP, but subsequently underwent apoptosis. Apoptotic susceptibility was induced byC. rodentiuminfection and was specific to this setting of pathogen infection, and was not apparent in other settings of intestinal inflammation. These studies reveal a novel role for Irgm1 in host defense and suggest that deficiencies in survival and remodeling of C-LP myeloid cells that control inflammatory intestinal bacteria may underpin IBD pathogenesis linked to IRGM dysfunction.

Author Summary

Intestinal macrophages are seeded by peripheral monocytes that enter the intestine and mature into an essential component of immune defense. While this process is shaped by intestinal bacteria, the mechanisms that regulate the process, and their roles in host defense to enteric pathogens are poorly defined. We find thatIrgm1– the orthologue of the human Crohn’s disease resistance gene,IRGM– is required for monocyte/macrophage remodeling in response to the gram-negative enteric pathogenCitrobacter rodentium, with absence of Irgm1 leading to systemic spread of the bacterium and host mortality. Failure to remodel colonic monocytes/macrophages was due to a key requirement for Irgm1 in colon-infiltrating monocytes, which underwent dramatically high rates of cell death, specifically during the setting of enteric bacterial infection. While Irgm1/IRGM proteins are well appreciated for their role in regulating intracellular bacterial killing, our results expand the paradigm for Irgm1/IRGM-mediated host defense by demonstrating an essential function for Irgm1/IRGM to control the survival and population remodeling of monocytes and macrophages that expand during enteric infection and defend against pathogenic bacteria. These findings have significant implications for the genesis of Crohn’s disease in individuals that carryIRGMvariants.