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The breakdown of the intestinal mucosal barrier is thought to underlie the progression to Crohn disease (CD), whereby numerous risk factors contribute. For example, a genetic polymorphism of the autophagy gene ATG16L1, associated with an increased risk of developing CD, contributes to the perturbation of the intestinal epithelium. We examined the role of Atg16l1 in protecting the murine small intestinal epithelium from T-cell-mediated damage using the anti-CD3 model of enteropathy. Our work showed that mice specifically deleted for Atg16l1 in intestinal epithelial cells (IECs) (Atg16l1 ΔIEC ) had exacerbated intestinal damage, characterized by crypt epithelial cell death, heightened inflammation, and decreased survival. Moreover, Atg16l1 deficiency delayed the recovery of the intestinal epithelium, and Atg16l1-deficient IECs were impaired in their proliferative response. Pathology was largely driven by interferon (IFN)-γ signaling in Atg16l1 ΔIEC mice. Mechanistically, although survival was rescued by blocking tumor necrosis factor or IFN-γ independently, only anti-IFN-γ treatment abrogated IEC death in Atg16l1 ΔIEC mice, thereby decoupling IEC death and survival. In summary, our findings suggest differential roles for IFN-γ and tumor necrosis factor in acute enteropathy and IEC death in the context of autophagy deficiency and suggest that IFN-γ-targeted therapy may be appropriate for patients with CD with variants in ATG16L1., Competing Interests: Declarations of Competing Interest The authors have no competing interests to declare., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

The aryl hydrocarbon receptor (AhR) is a sensor of products of tryptophan metabolism and a potent modulator of immunity. Here, we examined the impact of AhR in tumor-associated macrophage (TAM) function in pancreatic ductal adenocarcinoma (PDAC). TAMs exhibited high AhR activity and Ahr-deficient macrophages developed an inflammatory phenotype. Deletion of Ahr in myeloid cells or pharmacologic inhibition of AhR reduced PDAC growth, improved efficacy of immune checkpoint blockade, and increased intra-tumoral frequencies of IFNγ + CD8 + T cells. Macrophage tryptophan metabolism was not required for this effect. Rather, macrophage AhR activity was dependent on Lactobacillus metabolization of dietary tryptophan to indoles. Removal of dietary tryptophan reduced TAM AhR activity and promoted intra-tumoral accumulation of TNFα + IFNγ + CD8 + T cells; provision of dietary indoles blocked this effect. In patients with PDAC, high AHR expression associated with rapid disease progression and mortality, as well as with an immune-suppressive TAM phenotype, suggesting conservation of this regulatory axis in human disease., Competing Interests: Declaration of interests The authors have no conflicting interests to declare., (Copyright © 2022 Elsevier Inc. All rights reserved.)

Macroautophagy/autophagy is a cellular catabolic process that results in lysosome-mediated recycling of organelles and protein aggregates, as well as the destruction of intracellular pathogens. Its role in the maintenance of the intestinal epithelium is of particular interest, as several autophagy-related genes have been associated with intestinal disease. Autophagy and its regulatory mechanisms are involved in both homeostasis and repair of the intestine, supporting intestinal barrier function in response to cellular stress through tight junction regulation and protection from cell death. Furthermore, a clear role has emerged for autophagy not only in secretory cells but also in intestinal stem cells, where it affects their metabolism, as well as their proliferative and regenerative capacity. Here, we review the physiological role of autophagy in the context of intestinal epithelial maintenance and how genetic mutations affecting autophagy contribute to the development of intestinal disease. Abbreviations: AKT1S1: AKT1 substrate 1; AMBRA1: autophagy and beclin 1 regulator 1; AMPK: AMP-activated protein kinase; APC: APC regulator of WNT signaling pathway; ATF6: activating transcription factor 6; ATG: autophagy related; atg16l1[ΔIEC] mice: mice with a specific deletion of Atg16l1 in intestinal epithelial cells; ATP: adenosine triphosphate; BECN1: beclin 1; bsk/Jnk: basket; CADPR: cyclic ADP ribose; CALCOCO2: calcium binding and coiled-coil domain 2; CASP3: caspase 3; CD: Crohn disease; CDH1/E-cadherin: cadherin 1; CF: cystic fibrosis; CFTR: CF transmembrane conductance regulator; CGAS: cyclic GMP-AMP synthase; CLDN2: claudin 2; CoPEC: colibactin-producing E. coli ; CRC: colorectal cancer; CYP1A1: cytochrome P450 family 1 subfamily A member 1; DC: dendritic cell; DDIT3: DNA damage inducible transcript 3; DEPTOR: DEP domain containing MTOR interacting protein; DSS: dextran sulfate sodium; EGF: epidermal growth factor; EGFR: epidermal growth factor receptor; EIF2A: eukaryotic translation initiation factor 2A; EIF2AK3: eukaryotic translation initiation factor 2 alpha kinase 3; EIF2AK4/GCN2: eukaryotic translation initiation factor 2 alpha kinase 4; ER: endoplasmic reticulum; ERN1: endoplasmic reticulum to nucleus signaling 1; GABARAP: GABA type A receptor-associated protein; HMGB1: high mobility group box 1; HSPA5/GRP78: heat shock protein family A (Hsp70) member 5; IBD: inflammatory bowel disease; IEC: intestinal epithelial cell; IFN: interferon; IFNG/IFNγ:interferon gamma; IL: interleukin; IRGM: immunity related GTPase M; ISC: intestinal stem cell; LGR5: leucine rich repeat containing G protein-coupled receptor 5; LRRK2: leucine rich repeat kinase 2; MAP1LC3A/LC3: microtubule associated protein 1 light chain 3 alpha; MAPK/JNK: mitogen-activated protein kinase; MAPK14/p38 MAPK: mitogen-activated protein kinase 14; MAPKAP1: MAPK associated protein 1; MAVS: mitochondrial antiviral signaling protein; miRNA: microRNA; MLKL: mixed lineage kinase domain like pseudokinase; MLST8: MTOR associated protein, LST8 homolog; MNV: murine norovirus; MTOR: mechanistic target of rapamycin kinase; NBR1: NBR1 autophagy cargo receptor; NLRP: NLR family pyrin domain containing; NOD: nucleotide binding oligomerization domain containing; NRBF2: nuclear receptor binding factor 2; OPTN: optineurin; OXPHOS: oxidative phosphorylation; P: phosphorylation; Patj: PATJ crumbs cell polarity complex component; PE: phosphatidyl-ethanolamine; PI3K: phosphoinositide 3-kinase; PIK3C3/VPS34: phosphatidylinositol 3-kinase catalytic subunit type 3; PIK3R4: phosphoinositide-3-kinase regulatory subunit 4; PPARG: peroxisome proliferator activated receptor gamma; PRR5: proline rich 5; PRR5L: proline rich 5 like; PtdIns3K: phosphatidylinositol 3-kinase; PtdIns3P: phosphatidylinositol 3-phosphate; RB1CC1/FIP200: RB1 inducible coiled-coil 1; RER: rough endoplasmic reticulum; RHEB: Ras homolog, MTORC1 binding; RICTOR: RPTOR independent companion of MTOR complex 2; RIPK1: receptor interacting serine/threonine kinase 1; ROS: reactive oxygen species; RPTOR: regulatory associated protein of MTOR complex 1; RPS6KB1: ribosomal protein S6 kinase B1; SH3GLB1: SH3 domain containing GRB2 like, endophilin B1; SNP: single-nucleotide polymorphism; SQSTM1: sequestosome 1; STAT3: signal transducer and activator of transcription 3; STING1: stimulator of interferon response cGAMP interactor 1; TA: transit-amplifying; TFEB: transcription factor EB; TFE3: transcription factor binding to IGHM enhancer 3; TGM2: transglutaminase 2; TJ: tight junction; TJP1/ZO1: tight junction protein 1; TNBS: 2,4,6-trinitrobenzene sulfonic acid; TNF/TNFα: tumor necrosis factor; Tor: target of rapamycin; TRAF: TNF receptor associated factor; TRIM11: tripartite motif containing 11; TRP53: transformation related protein 53; TSC: TSC complex subunit; Ub: ubiquitin; UC: ulcerative colitis; ULK1: unc-51 like autophagy activating kinase 1; USO1/p115: USO1 vesicle transport factor; UVRAG: UV radiation resistance associated; WIPI: WD repeat domain, phosphoinositide interacting; WNT: WNT family member; XBP1: X-box binding protein 1; ZFYVE1/DFCP1: zinc finger FYVE-type containing 1.

The small intestinal epithelial barrier inputs signals from the gut microbiota in order to balance physiological inflammation and tolerance, and to promote homeostasis. Understanding the dynamic relationship between microbes and intestinal epithelial cells has been a challenge given the cellular heterogeneity associated with the epithelium and the inherent difficulty of isolating and identifying individual cell types. Here, we used single-cell RNA sequencing of small intestinal epithelial cells from germ-free and specific pathogen-free mice to study microbe-epithelium crosstalk at the single-cell resolution. The presence of microbiota did not impact overall cellular composition of the epithelium, except for an increase in Paneth cell numbers. Contrary to expectations, pattern recognition receptors and their adaptors were not induced by the microbiota but showed concentrated expression in a small proportion of epithelial cell subsets. The presence of the microbiota induced the expression of host defense- and glycosylation-associated genes in distinct epithelial cell compartments. Moreover, the microbiota altered the metabolic gene expression profile of epithelial cells, consequently inducing mTOR signaling thereby suggesting microbe-derived metabolites directly activate and regulate mTOR signaling. Altogether, these findings present a resource of the homeostatic transcriptional and cellular impact of the microbiota on the small intestinal epithelium.

Pioneering studies from the early 1980s suggested that bacterial peptidoglycan-derived muramyl peptides (MPs) could exert either stimulatory or immunosuppressive functions depending, in part, on chronicity of exposure. However, this Janus-faced property of MPs remains largely unexplored. Here, we demonstrate the immunosuppressive potential of Nod1, the bacterial sensor of diaminopimelic acid (DAP)-containing MPs. Using a model of self-limiting peritonitis, we show that systemic Nod1 activation promotes an autophagy-dependent reprogramming of macrophages toward an alternative phenotype. Moreover, Nod1 stimulation induces the expansion of myeloid-derived suppressor cells (MDSCs) and maintains their immunosuppressive potential via arginase-1 activity. Supporting the role of MDSCs and tumor-associated macrophages in cancer, we demonstrate that myeloid-intrinsic Nod1 expression sustains intra-tumoral arginase-1 levels to foster an immunosuppressive and tumor-permissive microenvironment during colorectal cancer (CRC) development. Our findings support the notion that bacterial products, via Nod1 detection, modulate the immunosuppressive activity of myeloid cells and fuel tumor progression in CRC., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2020 The Author(s). Published by Elsevier Inc. All rights reserved.)

NOD1 and NOD2 are related intracellular sensors of bacterial peptidoglycan and belong to the Nod-like receptor (NLR) family of innate immune proteins that play fundamental and pleiotropic roles in host defense against infection and in the control of inflammation. The importance of these proteins is also highlighted by the genetic association between single nucleotide polymorphisms in NOD2 and susceptibility to Crohn's disease, an inflammatory bowel disease. At the cellular level, recent efforts have delineated the signaling pathways triggered following activation of NOD1 and NOD2, and the interplay with various cellular processes, such as autophagy. In vivo studies have revealed the importance of NOD-dependent host defense in models of infection, and a crucial area of investigation focuses on understanding the role of NOD1 and NOD2 at the intestinal mucosa, as this is of prime importance for understanding the etiology of Crohn's disease., (Copyright © 2018 Elsevier Inc. All rights reserved.)

In this issue of Cell Host & Microbe, Liang et al. (2019) show that α-defensins in the gastrointestinal tract sustain defenses to enteric pathogens during starvation. mTOR-dependent sensing of nutrient loss promotes production of an α-defensin regulator, which sustains α-defensin levels, loss of which increases lethality during bacterial infection., (Copyright © 2019 Elsevier Inc. All rights reserved.)

In physiological settings, the complement protein C3 is deposited on all bacteria, including invasive pathogens. However, because experimental host-bacteria systems typically use decomplemented serum to avoid the lytic action of complement, the impact of C3 coating on epithelial cell responses to invasive bacteria remains unexplored. Here, we demonstrate that following invasion, intracellular C3-positive Listeria monocytogenes is targeted by autophagy through a direct C3/ATG16L1 interaction, resulting in autophagy-dependent bacterial growth restriction. In contrast, Shigella flexneri and Salmonella Typhimurium escape autophagy-mediated growth restriction in part through the action of bacterial outer membrane proteases that cleave bound C3. Upon oral infection with Listeria, C3-deficient mice displayed defective clearance at the intestinal mucosa. Together, these results demonstrate an intracellular role of complement in triggering antibacterial autophagy and immunity against intracellular pathogens. Since C3 indiscriminately associates with foreign surfaces, the C3-ATG16L1 interaction may provide a universal mechanism of xenophagy initiation., (Copyright © 2018 Elsevier Inc. All rights reserved.)

The mitochondrial Nod-like receptor protein NLRX1 protects against colorectal tumorigenesis through mechanisms that remain unclear. Using mice with an intestinal epithelial cells (IEC)-specific deletion of Nlrx1, we find that NLRX1 provides an IEC-intrinsic protection against colitis-associated carcinogenesis in the colon. These Nlrx1 mutant mice have increased expression of Tnf, Egf, and Tgfb1, three factors essential for wound healing, as well as increased epithelial proliferation during the epithelial regeneration phase following injury triggered by dextran sodium sulfate. In primary intestinal organoids lacking Nlrx1, stimulation with TNF resulted in exacerbated proliferation and expression of the intestinal stem cell markers Olfm4 and Myb. This hyper-proliferation response was associated with increased activation of Akt and NF-κB pathways in response to TNF stimulation. Together, these results identify NLRX1 as a suppressor of colonic tumorigenesis that acts by controlling epithelial proliferation in the intestine during the regeneration phase following mucosal injury., (Copyright © 2016 The Authors. Published by Elsevier Inc. All rights reserved.)