Citrobacter rodentium Infection Induces Persistent Molecular Changes and Interferon Gamma-Dependent Major Histocompatibility Complex Class II Expression in the Colonic Epithelium.

Most studies of infections at mucosal surfaces have focused on the acute phase of the disease. Consequently, little is known about the molecular processes that underpin tissue recovery and the long-term consequences postinfection. Here, we conducted temporal deep quantitative proteomic analysis of colonic intestinal epithelial cells (cIECs) from mice infected with the natural mouse pathogen Citrobacter rodentium over time points corresponding to the late steady-state phase (10 days postinfection [DPI]), the clearance phase (13 to 20 DPI), and 4 weeks after the pathogen has been cleared (48 DPI). C. rodentium , which relies on a type III secretion system to infect, is used to model infections with enteropathogenic and enterohemorrhagic Escherichia coli . We observe a strong upregulation of inflammatory signaling and nutritional immunity responses during the clearance phase of the infection. Despite morphological tissue recovery, chromogranin B (ChgB)-positive endocrine cells remained significantly below baseline levels at 48 DPI. In contrast, we observed an increased abundance of proteins involved in antigen processing and presentation 4 weeks after pathogen clearance. In particular, long-term changes were characterized by a persistent interferon gamma (IFN-γ) response and the expression of major histocompatibility complex class II (MHCII) molecules in 60% of the EpCAM + cIECs, which were not seen in Ifn γ -/- mice. Nonetheless, both wild-type and Ifn γ -/- mice mounted similar systemic and colonic IgG responses to C. rodentium and were equally protected from rechallenge, suggesting that cIEC MHCII is not necessary for protective immunity against C. rodentium. IMPORTANCE Mucosal surfaces respond to infection by mounting an array of metabolic, inflammatory, and tissue repair responses. While these have been well studied during acute infection, less is known about tissue recovery after pathogen clearance. We employ the mouse pathogen Citrobacter rodentium, which binds colonic intestinal epithelial cells (cIECs), to investigate the long-term effects of bacterial infection on gut physiology. Using global proteomic analysis, we study cIEC temporal responses during and after the clearance phase of infection. While the overall tissue morphology recovered, cIECs showed persistent signs of infection 4 weeks after pathogen clearance. These were characterized by a strong IFN-γ signature, including the upregulation of major histocompatibility complex class II (MHCII) antigen presentation proteins, suggesting that the tissue remains on "high alert" for weeks after the acute insult is resolved. However, we demonstrate that cIEC MHCII expression, which is induced by IFN-γ, is not required for protective IgG-mediated immunity against C. rodentium; instead, it may play a role in mucosal recovery.