Epithelial cell-specific MyD88 signaling mediates ischemia/reperfusion-induced intestinal injury independent of microbial status.

The intestinal epithelium constitutes a crucial physical barrier separating the host from the luminal compartment containing various noxious agents such as bacteria, bacterial products, and food-derived particles.1–3 In addition to its barrier function, the intestinal epithelium responds to numerous bacterial stimuli through the action of various conserved innate sensors such as Toll-like receptors (TLRs).4 In the case of commensal bacteria, this dialogue between the microbiota and intestinal epithelial cells (IECs) is important for the maintenance of intestinal homeostasis through the induction of antimicrobial peptides, toning of the innate and adaptive immune responses, and the fortification of the barrier.5 A compromised intestinal barrier could have deleterious consequences for the host as luminal contents may gain access to the mucosal immune system and enter systemic circulation, triggering a damaging immune cell response. The epithelium is constantly facing injury caused by various luminal irritants, but these episodes rarely lead to life-threatening situations because the host has developed an efficient adaptive wound-healing response to maintain homeostasis.6 However, extreme challenges such as hypoxic, ischemic, or radiation events often overpower the wound-healing response, thereby exposing the host to damaging luminal products. For example, intestinal ischemia can lead to sepsis through bacterial translocation.7 Intestinal ischemia–reperfusion (I/R) injury can occur under several clinical conditions like sepsis, hemorrhage, mesenteric ischemia due to blood clots, neonatal necrotizing enterocolitis, and even small bowel transplantation.8 Reperfusion after ischemic events causes epithelial cell death and disruption of barrier function leading to invasion of bacteria.9 Antibiotic use is the mainstay of treatment to prevent sepsis from bacterial translocation after intestinal ischemic events and has even been shown to be protective before the inciting event.10 Interestingly, the microbiota seems to play a protective role in various experimental injury models including dextran sodium sulfate and radiation.11–13 In contrast, global deletion of myeloid differentiation primary response gene (Myd88), an adaptor protein in innate immune signaling, protects mice from I/R-induced injury.14 Moreover, in a model of necrotizing enterocolitis in newborn mice, TLR/MyD88-mediated intestinal injury seems to be controlled by nucleotide-binding oligomerization domain-2 signaling.15 However, the role of microbes in injury response to I/R is still unclear because most studies linking bacteria to I/R response have used reduction strategies (antibiotic treatment) and/or innate sensor-deficient mice. Numerous cells, including IEC and mucosal immune cells, express MyD88, and the cell type specific function of this molecule in I/R-mediated injury response is unknown. In this study, we investigated the impact of the microbiota using germ-free and conventionalized mice and defined the role of IEC-specific MyD88 signaling in I/R-induced intestinal injury. We report that IEC-derived MyD88 signaling promotes I/R-induced injury. This deleterious effect is likely mediated by 2 main events: (1) IEC-dependent MyD88 signaling leading to increased amount of luminal IgA and subsequent complement activation during the ischemic phase and (2) IEC-MyD88–dependent expression of neutrophil chemoattractants in IECs, further damaging intestinal tissue.