Intestinal epithelial cell endoplasmic reticulum stress promotes MULT1 up-regulation and NKG2D-mediated inflammation

Endoplasmic reticulum (ER) stress is commonly observed in intestinal epithelial cells (IECs) and can, if excessive, cause spontaneous intestinal inflammation as shown by mice with IEC-specific deletion of X-box-binding protein 1 (Xbp1), an unfolded protein response-related transcription factor. In this study, Xbp1 deletion in the epithelium (Xbp1.IEC) is shown to cause increased expression of natural killer group 2 member D (NKG2D) ligand (NKG2DL) mouse UL16-binding protein (ULBP)-like transcript 1 and its human orthologue cytomegalovirus ULBP via ER stress-related transcription factor C/EBP homology protein. Increased NKG2DL expression on mouse IECs is associated with increased numbers of intraepithelial NKG2D-expressing group 1 innate lymphoid cells (ILCs; NK cells or ILC1). Blockade of NKG2D suppresses cytolysis against ER-stressed epithelial cells in vitro and spontaneous enteritis in vivo. Pharmacological depletion of NK1.1(+) cells also significantly improved enteritis, whereas enteritis was not ameliorated in Recombinase activating gene 1(-/-); Xbp1(Delta IEC) mice. These experiments reveal innate immune sensing of ER stress in IECs as an important mechanism of intestinal inflammation.