Steve Shenouda

Maintenance of colonic homeostasis by distinctive apical TLR9 signalling in intestinal epithelial cells

The mechanisms by which commensal bacteria suppress inflammatory signalling in the gut are still unclear. Here, we present a cellular mechanism whereby the polarity of intestinal epithelial cells (IECs) has a major role in colonic homeostasis. TLR9 activation through apical and basolateral surface domains have distinct transcriptional responses, evident by NF-κB activation and cDNA microarray analysis. Whereas basolateral TLR9 signals IκBα degradation and activation of the NF-κB pathway, apical TLR9 stimulation invokes a unique response in which ubiquitinated IκB accumulates in the cytoplasm preventing NF-κB activation. Furthermore, apical TLR9 stimulation confers intracellular tolerance to subsequent TLR challenges. IECs in TLR9-deficient mice, when compared with wild-type and TLR2-deficient mice, display a lower NF-κB activation threshold and these mice are highly susceptible to experimental colitis. Our data provide a case for organ-specific innate immunity in which TLR expression in polarized IECs has uniquely evolved to maintain colonic homeostasis and regulate tolerance and inflammation.

Constitutive intestinal NF-κB does not trigger destructive inflammation unless accompanied by MAPK activation

Constitutive NF-κB activation in IECs induces inflammatory cytokines and chemokines in the lamina propria, but does not result in overt tissue damage unless acute inflammatory insults are present, causing TNF-dependent destruction and barrier disruption.

Chronic epithelial NF-κB activation accelerates APC loss and intestinal tumor initiation through iNOS up-regulation

The role of NF-κB activation in tumor initiation has not been thoroughly investigated. We generated Ikkβ(EE)IEC transgenic mice expressing constitutively active IκB kinase β (IKKβ) in intestinal epithelial cells (IECs). Despite absence of destructive colonic inflammation, Ikkβ(EE)IEC mice developed intestinal tumors after a long latency. However, when crossed to mice with IEC-specific allelic deletion of the adenomatous polyposis coli (Apc) tumor suppressor locus, Ikkβ(EE)IEC mice exhibited more β-catenin+ early lesions and visible small intestinal and colonic tumors relative to Apc+/ΔIEC mice, and their survival was severely compromised. IEC of Ikkβ(EE)IEC mice expressed high amounts of inducible nitric oxide synthase (iNOS) and elevated DNA damage markers and contained more oxidative DNA lesions. Treatment of Ikkβ(EE)IEC/Apc+/ΔIEC mice with an iNOS inhibitor decreased DNA damage markers and reduced early β-catenin+ lesions and tumor load. The results suggest that persistent NF-κB activation in IEC may accelerate loss of heterozygocity by enhancing nitrosative DNA damage.

Conventional dendritic cells regulate the outcome of colonic inflammation independently of T cells.

We explored the physiological role of conventional dendritic cells (cDCs) in acute colitis induced by a single cycle of dextran sodium sulfate administration. Depending on their mode of activation and independently of T cells, cDCs can enhance or attenuate the severity of dextran sodium sulfate-induced colitis. The latter beneficial effect was achieved, in part, by IFN-1 induced by Toll-like receptor 9-activated cDCs. IFN-1 inhibits colonic inflammation by regulating neutrophil and monocyte trafficking to the inflamed colon and restraining the inflammatory products of tissue macrophages. These data highlight a novel role of cDCs in the regulation of other innate immune cells and position them as major players in acute colonic inflammation.

GM-CSF Produced by Nonhematopoietic Cells Is Required for Early Epithelial Cell Proliferation and Repair of Injured Colonic Mucosa

GM-CSF is a growth factor that promotes the survival and activation of macrophages and granulocytes, as well as dendritic cell differentiation and survival in vitro. The mechanism by which exogenous GM-CSF ameliorates the severity of Crohn’s disease in humans and colitis in murine models has mainly been considered to reflect its activity on myeloid cells. We used GM-CSF–deficient (GM-CSF−/−) mice to probe the functional role of endogenous host-produced GM-CSF in a colitis model induced after injury to the colon epithelium. Dextran sodium sulfate (DSS), at doses that resulted in little epithelial damage and mucosal ulceration in wild type mice, caused marked colon ulceration and delayed ulcer healing in GM-CSF−/− mice. Colon crypt epithelial cell proliferation in vivo was significantly decreased in GM-CSF−/− mice at early times after DSS injury. This was paralleled by decreased expression of crypt epithelial cell genes involved in cell cycle, proliferation, and wound healing. Decreased crypt cell proliferation and delayed ulcer healing in GM-CSF−/− mice were rescued by exogenous GM-CSF, indicating the lack of a developmental abnormality in the epithelial cell proliferative response in those mice. Nonhematopoietic cells, and not myeloid cells, produced the GM-CSF important for colon epithelial proliferation after DSS-induced injury, as revealed by bone marrow chimera and dendritic cell–depletion experiments, with colon epithelial cells being the cellular source of GM-CSF. Endogenous epithelial cell–produced GM-CSF has a novel nonredundant role in facilitating epithelial cell proliferation and ulcer healing in response to injury of the colon crypt epithelium.

Transplanted Embryonic Stem Cells Successfully Survive, Proliferate, and Migrate to Damaged Regions of the Mouse Brain

An understanding of feasibility of implanting embryonic stem cells (ESCs), their behavior of migration in response to lesions induced in brain tissues, and the mechanism of their in vivo differentiation into neighboring neural cells is essential for developing and refining ESC transplantation strategies for repairing damages in the nervous system, as well as for understanding the molecular mechanism underlying neurogenesis. We hypothesized that damaged neural tissues offer a niche to which injected ESCs can migrate and differentiate into the neural cells. We inflicted damage in the murine (C57BL/6) brain by injecting phosphate‐buffered saline into the left frontal and right caudal regions and confirmed neural damage by histochemistry. Enhanced yellow fluorescent protein‐expressing ESCs were injected into the nondamaged left caudal portion of the brain. Using immunohistochemistry and fluorescent microscopy, we observed migration of ESCs from the injection site (left caudal) to the damaged site (right caudal and left frontal). Survival of the injected ESCs was confirmed by the real‐time polymerase chain reaction analysis of stemness genes such as Oct4, Sox2, and FGF4. The portions of the damaged neural tissues containing ESCs demonstrated a fourfold increase in expression of these genes after 1 week of injection in comparison with the noninjected ESC murine brain, suggesting proliferation. An increased level of platelet‐derived growth factor receptor demonstrated that ESCs responded to damaged neural tissues, migrated to the damaged site of the brain, and proliferated. These results demonstrate that undifferentiated ESCs migrate to the damaged regions of brain tissue, engraft, and proliferate. Thus, damaged brain tissue provides a niche that attracts ESCs to migrate and proliferate.

Chimerism and tolerance post-in utero transplantation with embryonic stem cells

Background. Clinical application of in utero transplantation (IUT) in human fetuses with intact immune systems resulted in a very low level of donor chimerism. In this study, we examined whether the fetal immune system early in the second trimester of pregnancy (13.5 dpc) can initiate immune tolerance for major histocompatibility complex (MHC)-mismatched embryonic stem (ES) cells. We also examined whether immune tolerance mechanisms respond differently to ontogenetically different stem cells. Methods. MHC-mismatched ES, fetal liver (FL), and bone-marrow (BM) cells (H-2kb) at 1×109 cells/kg fetal body weight were injected intraperitoneally into 13.5 dpc BALB/c fetuses (H-2Kd). Peripheral chimerism was determined in blood by flow cytometry (sensitivity≤0.1%) at monthly intervals. Donor-specific immune responses were determined by cytotoxic lymphocyte (CTL) assay, mixed lymphocyte reaction, and T helper (Th)1 and Th2 cytokine assays. Chimeric mice at the age of 9 months received postnatal boosts (PB) with minimal conditioning of 200 cGy by intravenous injection of 1×109 of the corresponding cells/kg body weight. Results. After IUT with ES, FL, or BM cells, the level of peripheral chimerism within the first 9 months of life was 0% to 0.4%. PB with 1×109/kg of corresponding cells resulted in a decrease in the peripheral chimerism to 0% within 2 weeks of PB. CTL and cytokine assays before and after PB demonstrated a shift toward immunity. Conclusions. Immunologic tolerance was not achieved after IUT of murine fetuses at 13.5 dpc with MHC-mismatched ES cells, and only a low level chimerism was achieved.

Lack of the programmed death‐1 receptor renders host susceptible to enteric microbial infection through impairing the production of the mucosal natural killer cell effector molecules

The programmed death‐1 receptor is expressed on a wide range of immune effector cells, including T cells, natural killer T cells, dendritic cells, macrophages, and natural killer cells. In malignancies and chronic viral infections, increased expression of programmed death‐1 by T cells is generally associated with a poor prognosis. However, its role in early host microbial defense at the intestinal mucosa is not well understood. We report that programmed death‐1 expression is increased on conventional natural killer cells but not on CD4+, CD8+ or natural killer T cells, or CD11b+ or CD11c+ macrophages or dendritic cells after infection with the mouse pathogen Citrobacter rodentium. Mice genetically deficient in programmed death‐1 or treated with anti–programmed death‐1 antibody were more susceptible to acute enteric and systemic infection with Citrobacter rodentium. Wild‐type but not programmed death‐1–deficient mice infected with Citrobacter rodentium showed significantly increased expression of the conventional mucosal NK cell effector molecules granzyme B and perforin. In contrast, natural killer cells from programmed death‐1–deficient mice had impaired expression of those mediators. Consistent with programmed death‐1 being important for intracellular expression of natural killer cell effector molecules, mice depleted of natural killer cells and perforin‐deficient mice manifested increased susceptibility to acute enteric infection with Citrobacter rodentium. Our findings suggest that increased programmed death‐1 signaling pathway expression by conventional natural killer cells promotes host protection at the intestinal mucosa during acute infection with a bacterial gut pathogen by enhancing the expression and production of important effectors of natural killer cell function.