BACKGROUND & AIMS
Tight junction dysregulation and epithelial damage contribute to barrier loss in patients with inflammatory bowel disease (IBD). However, the mechanisms that regulate these processes and their relative contributions to disease pathogenesis are incompletely understood. We investigated these processes using colitis models in mice.
We induced colitis by adoptive transfer of CD4+CD45RBhi cells or administration of dextran sulfate sodium (DSS) to mice, including those deficient in tumor necrosis factor receptor (TNFR) 1, TNFR2, or the long isoform of myosin light chain kinase (MLCK). Intestinal tissues and isolated epithelial cells were analyzed by immunoblot, immunofluorescence, ELISA, and real-time PCR assays.
Induction of immune-mediated colitis by CD4+CD45RBhi adoptive transfer increased intestinal permeability; epithelial expression of claudin-2, the long isoform of MLCK, and TNFR2 (but not TNFR1); and phosphorylation of the myosin II light chain (MLC). Long MLCK upregulation, MLC phosphorylation, barrier loss, and weight loss were attenuated in TNFR2−/−, but not TNFR1−/−, recipients of wildtype CD4+CD45RBhi cells. Similarly, long MLCK−/− mice had limited increases in MLC phosphorylation, claudin-2 expression, and intestinal permeability and delayed onset of cell transfer-induced colitis. However, coincident with onset of epithelial apoptosis, colitis ultimately developed. This indicates that disease progresses via apoptosis in the absence of MLCK-dependent tight junction regulation. In support of this conclusion, long MLCK−/− mice were not protected from epithelial apoptosis-mediated, damage-dependent DSS colitis.
In immune-mediated IBD models, TNFR2 signaling increases long MLCK expression, resulting in tight junction dysregulation, barrier loss and induction of colitis. At advanced stages, colitis progresses by apoptosis and mucosal damage that results in tight junction- and MLCK-independent barrier loss. Therefore, barrier loss in immune-mediated colitis occurs via two temporally and morphologically distinct mechanisms. Differential targeting of these mechanisms may lead to improved IBD therapies.
inflammatory bowel disease; epithelial barrier; intestinal permeability; disease progression
Regulation of microbially induced cytokine secretion is critical in intestinal immune homeostasis. NOD2, the Crohn's disease-associated bacterial peptidoglycan sensor, activates the NF-κB pathway. After chronic NOD2 stimulation in human macrophages, cytokine secretion is significantly attenuated, similar to the situation in the intestinal environment. We find that NF-κB1 (p105/p50) expression is upregulated with chronic NOD2 stimulation and is required for attenuation of cytokine secretion in vitro in human macrophages and in vivo in mice. Upon chronic NOD2 stimulation, regulation of both activating (H3K4Me2 and H4Ac) and inhibitory (H3K27Me3) histone modifications was observed within cytokine gene promoters; these outcomes were NF-κB1 dependent. In addition to enhanced binding to cytokine gene promoters with chronic NOD2 stimulation, NF-κB1 bound to the promoter of the transcriptional repressor, ATF3. ATF3 was then induced and bound to cytokine gene promoters; both features were impaired upon NF-κB1 knockdown. Restoring ATF3 expression under NF-κB1 knockdown conditions restored NOD2-mediated cytokine downregulation. Finally, NF-κB1 and ATF3 cooperate with other inhibitory pathways, including IRAKM and secreted mediators, to downregulate cytokine secretion after chronic NOD2 stimulation. Therefore, we identify NF-κB1 and ATF3 as critical mechanisms through which NOD2 downregulates cytokines and contributes to intestinal immune homeostasis.
The intestinal immune system defends against pathogens and entry of
excessive intestinal microbes; simultaneously, a state of immune tolerance to
resident intestinal microbes must be maintained. Perturbation of this balance is
associated with intestinal inflammation in various mouse models and is thought
to predispose humans to inflammatory bowel disease (IBD). The innate immune
system senses microbes; dendritic cells, macrophages, and epithelial cells
produce an initial, rapid response. The immune system continuously monitors
resident microbiota and utilizes constitutive antimicrobial mechanisms to
maintain immune homeostasis. associations between IBD and genes that regulate
microbial recognition and innate immune pathways, such as nucleotide
oligomerization domain 2 (Nod2), genes that
control autophagy (eg, ATG16L1, IRGM), and
genes in the interleukin-23–T helper cell 17 pathway indicate the
important roles of host-microbe interactions in regulating intestinal immune
homeostasis. There is increasing evidence that intestinal microbes influence
host immune development, immune responses, and susceptibility to human diseases
such as IBD, diabetes mellitus, and obesity. Conversely, host factors can affect
microbes, which in turn modulate disease susceptibility. We review the cell
populations and mechanisms that mediate interactions between host defense and
tolerance and how the dysregulation of host-microbe interactions leads to
intestinal inflammation and IBD.
Inflammatory Bowel Disease; Toll-Like Receptor; Pattern Recognition Receptor
The Ashkenazi Jewish population has a several-fold higher prevalence of Crohn’s disease compared to non-Jewish European ancestry populations and has a unique genetic history. Haplotype association is critical to Crohn’s disease etiology in this population, most notably at NOD2, in which three causal, uncommon, and conditionally independent NOD2 variants reside on a shared background haplotype. We present an analysis of extended haplotypes which showed significantly greater association to Crohn’s disease in the Ashkenazi Jewish population compared to a non-Jewish population (145 haplotypes and no haplotypes with P-value < 10−3, respectively). Two haplotype regions, one each on chromosomes 16 and 21, conferred increased disease risk within established Crohn’s disease loci. We performed exome sequencing of 55 Ashkenazi Jewish individuals and follow-up genotyping focused on variants in these two regions. We observed Ashkenazi Jewish-specific nominal association at R755C in TRPM2 on chromosome 21. Within the chromosome 16 region, R642S of HEATR3 and rs9922362 of BRD7 showed genome-wide significance. Expression studies of HEATR3 demonstrated a positive role in NOD2-mediated NF-κB signaling. The BRD7 signal showed conditional dependence with only the downstream rare Crohn’s disease-causal variants in NOD2, but not with the background haplotype; this elaborates NOD2 as a key illustration of synthetic association.
haplotype association; Ashkenazi Jewish; Crohn’s disease; NF-κB signaling; synthetic association
Stimulation of nucleotide-binding oligomerization domain-containing (Nod)2 and other pattern recognition receptors (PRR) in human monocyte-derived macrophages induces interleukin (IL)-1, which increases mitogen-activated protein kinase (MAPK) activation and cytokine secretion. Activation of MAPK by different PRR has varied effects on inflammatory cytokine secretion. We investigated whether different levels of autocrine IL-1 mediate these varied effects.
Macrophage responses to PRR ligands were analyzed by enzyme-linked immunosorbent assay and flow cytometry. We overexpressed or reduced MAPK levels (using small inhibitory RNA).
Nod2 and other PRR activated signaling via extracellular signal-related kinase (ERK) and p38 that inhibited inflammatory cytokine production by human monocyte-derived macrophages; autocrine IL-1 production prevented this inhibition. ERK and p38 inhibited inflammatory cytokine production by human macrophages that produce low levels of IL-1 (such as M2, endotoxin-tolerant, and intestinal macrophages); adding exogenous IL-1 caused ERK and p38 to stimulate production of inflammatory cytokines in these cells. In mouse macrophages, which do not produce IL-1 in response to PRR stimulation alone, addition of exogenous IL-1 reversed the ERK-mediated inhibition of IL-12p40. Increasing activation of c-Jun N-terminal kinase in Nod2-stimulated human monocyte-derived macrophages, in the absence of autocrine IL-1 signaling, caused ERK and p38 to stimulate inflammatory cytokines secretion. Infection of human intestinal macrophages with pathogens that induce IL-1 production reversed the inhibition of inflammatory cytokine production by ERK and p38.
In response to PRR stimulation of macrophages, the level of MAPK signaling is regulated by autocrine IL-1 and determines whether production of inflammatory cytokines is inhibited or stimulated. This mechanism could account for reported differences in MAPK regulation of inflammatory cytokines and propagate the inflammatory response to pathogens.
TLR; IBD; Salmonella
Monocyte-derived cells display highly variable cytokine secretion upon pattern-recognition receptor (PRR) stimulation across individuals; such variability likely affects inter-individual inflammatory/autoimmune disease susceptibility. To define mechanisms for this heterogeneity, we examined pattern recognition receptor (PRR)-induced monocyte-derived-cell cytokine secretion from a large cohort healthy individuals. Although cytokine secretion ranged widely among individuals, the magnitude of cytokine induction after individual Nod2 and TLR2 stimulation (a cohort of 86 individuals) or stimulation of multiple TLRs (a cohort of 77 individuals), either alone or in combination with Nod2, was consistent intra-individually across these stimuli. Nod2 and TLRs signal through interferon-regulatory-factor-5 (IRF5) and common IRF5 polymorphisms confer risk for autoimmunity. We find that cells from rs2004640 IRF5 risk-associated allele carriers secrete increased cytokines upon individual or synergistic PRR stimulation in a gene dose- and ligand dose-dependent manner in both monocyte-derived dendritic cells and macrophages. IRF5 expression knockdown in IRF5-risk-allele carrier cells significantly decreases PRR-induced cytokines. Moreover, we find that IRF5 knockdown profoundly decreases Nod2-mediated MAPK and NF-κB pathway activation, whereas the PI3K and mTOR pathways are not impaired. Finally, the IRF5 rs2004640 polymorphism is a major determinant of the variance (r2=0.53) in Nod2-induced cytokine secretion by monocyte-derived cells from different individuals. We therefore show a profound contribution of a single gene to the variance in inter-individual PRR-induced cytokines. The hyper-responsiveness of IRF5 disease-associated polymorphisms to a wide spectrum of microbial triggers has broad implications on global immunological responses, host defenses against pathogens and inflammatory/autoimmune disease susceptibility.
human; dendritic cells; pattern-recognition receptors
Crohn’s disease (CD) and ulcerative colitis (UC), the two common forms of inflammatory bowel disease (IBD), affect over 2.5 million people of European ancestry with rising prevalence in other populations1. Genome-wide association studies (GWAS) and subsequent meta-analyses of CD and UC2,3 as separate phenotypes implicated previously unsuspected mechanisms, such as autophagy4, in pathogenesis and showed that some IBD loci are shared with other inflammatory diseases5. Here we expand knowledge of relevant pathways by undertaking a meta-analysis of CD and UC genome-wide association scans, with validation of significant findings in more than 75,000 cases and controls. We identify 71 new associations, for a total of 163 IBD loci that meet genome-wide significance thresholds. Most loci contribute to both phenotypes, and both directional and balancing selection effects are evident. Many IBD loci are also implicated in other immune-mediated disorders, most notably with ankylosing spondylitis and psoriasis. We also observe striking overlap between susceptibility loci for IBD and mycobacterial infection. Gene co-expression network analysis emphasizes this relationship, with pathways shared between host responses to mycobacteria and those predisposing to IBD.
The interleukin-23 (IL-23) pathway plays a critical role in the pathogenesis of multiple chronic inflammatory disorders, however, inter-individual variability in IL-23-induced signal transduction in circulating human lymphocytes has not been well-defined. In this study, we observed marked, reproducible inter-individual differences in IL-23 responsiveness (measured by STAT3 phosphorylation) in peripheral blood CD8+CD45RO+ memory T and CD3+CD56+ NKT cells. Age, but not gender, was a significant (Pearson’s correlation coefficient, r = −0.37, p = 0.001) source of variability observed in CD8+CD45RO+ memory T cells, with IL-23 responsiveness gradually decreasing with increasing age. Relative to cells from individuals demonstrating low responsiveness to IL-23 stimulation, CD8+CD45RO+ memory T cells from individuals demonstrating high responsiveness to IL-23 stimulation showed increased gene expression for IL-23 receptor (IL-23R), RORC (RORγt) and CD161 (KLRB1), whereas RORA (RORα) and STAT3 expression were equivalent. Similar to CD4+ memory T cells, IL-23 responsiveness is confined to the CD161+ subset in CD8+CD45RO+ memory T cells, suggesting a similar CD161+ precursor as has been reported for CD4+ Th17 cells. We observed a very strong positive correlation between IL-23 responsiveness and the fraction of CD161+, CD8+CD45RO+ memory T cells (r = 0.80, p<0.001). Moreover, the fraction of CD161+, CD8+CD45RO+ memory T cells gradually decreases with aging (r = −0.34, p = 0.05). Our data define the inter-individual differences in IL-23 responsiveness in peripheral blood lymphocytes from the general population. Variable expression of CD161, IL-23R and RORC affects IL-23 responsiveness and contributes to the inter-individual susceptibility to IL-23-mediated defenses and inflammatory processes.
The chronic autoimmune diseases include multiple complex genetic disorders. Recently, genome-wide association studies (GWAS) have identified a large number of major loci, with many associations shared between various autoimmune diseases. These associations highlight key roles for lymphocyte activation and prioritize specific cytokine pathways and mechanisms of host-microbe recognition. Despite success in identifying loci, comprehensive models of disease pathogenesis are currently lacking. Future efforts comparing association patterns between autoimmune diseases may be particularly illustrative. New genomic technologies applied to classic genetic studies involving twins, early onset cases, and phenotypic extremes may provide key insights into developmental and gene-environment interactions in autoimmunity.
Genome-wide association studies (GWAS) in immune-mediated diseases have identified over 150 associated genomic loci. Many of these loci play a role in T cell responses, and regulation of T cell differentiation plays a critical role in immune-mediated diseases; however, the relationship between implicated disease loci and T cell differentiation is incompletely understood. To further address this relationship, we examined differential gene expression in naïve human CD4+ T cells, as well as in in vitro differentiated Th1, memory Th17-negative and Th17-enriched CD4+ T cells subsets using microarray and RNASeq. We observed a marked enrichment for increased expression in memory CD4+ compared to naïve CD4+ T cells of genes contained among immune–mediated disease loci. Within memory T cells, expression of disease-associated genes was typically increased in Th17-enriched compared to Th17-negative cells. Utilizing RNASeq and promoter methylation studies, we identified a differential regulation pattern for genes solely expressed in Th17 cells (IL17A and CCL20) compared to genes expressed in both Th17 and Th1 cells (IL23R and IL12RB2), where high levels of promoter methylation are correlated to near zero RNASeq levels for IL17A and CCL20. These findings have implications for human Th17 celI plasticity and for the regulation of Th17-Th1 expression signatures. Importantly, utilizing RNASeq we found an abundant isoform of IL23R terminating before the transmembrane domain that was enriched in Th17 cells. In addition to molecular resolution, we find that RNASeq provides significantly improved power to define differential gene expression and identify alternative gene variants relative to microarray analysis. The comprehensive integration of differential gene expression between cell subsets with disease-association signals, and functional pathways provides insight into disease pathogenesis.
Crohn's disease (CD) is a complex disorder resulting from the interaction of intestinal microbiota with the host immune system in genetically susceptible individuals. The largest meta-analysis of genome-wide association to date identified 71 CD–susceptibility loci in individuals of European ancestry. An important epidemiological feature of CD is that it is 2–4 times more prevalent among individuals of Ashkenazi Jewish (AJ) descent compared to non-Jewish Europeans (NJ). To explore genetic variation associated with CD in AJs, we conducted a genome-wide association study (GWAS) by combining raw genotype data across 10 AJ cohorts consisting of 907 cases and 2,345 controls in the discovery stage, followed up by a replication study in 971 cases and 2,124 controls. We confirmed genome-wide significant associations of 9 known CD loci in AJs and replicated 3 additional loci with strong signal (p<5×10−6). Novel signals detected among AJs were mapped to chromosomes 5q21.1 (rs7705924, combined p = 2×10−8; combined odds ratio OR = 1.48), 2p15 (rs6545946, p = 7×10−9; OR = 1.16), 8q21.11 (rs12677663, p = 2×10−8; OR = 1.15), 10q26.3 (rs10734105, p = 3×10−8; OR = 1.27), and 11q12.1 (rs11229030, p = 8×10−9; OR = 1.15), implicating biologically plausible candidate genes, including RPL7, CPAMD8, PRG2, and PRG3. In all, the 16 replicated and newly discovered loci, in addition to the three coding NOD2 variants, accounted for 11.2% of the total genetic variance for CD risk in the AJ population. This study demonstrates the complementary value of genetic studies in the Ashkenazim.
Crohn's disease causes inflammation of the digestive tract resulting from the interaction of normal gut bacteria with the host immune system in genetically predisposed individuals. People of Jewish heritage have an increased risk of developing Crohn's disease compared to non-Jewish Europeans. So far, 71 genetic variants that increase the risk of Crohn's disease have been identified in individuals of European ancestry. Here, we take advantage of recent technical and methodological advances to explore Crohn's diseases-related genetic variants specific to the Ashkenazi Jewish population. We examined 6,347 individuals whose Jewish ancestry was confirmed by a large number of genetic markers and detected several variants associated with the increased risk of Crohn' disease. We confirmed the involvement of 12 known Crohn's disease risk variants in Ashkenazi Jews and identified novel genetic regions not previously found in non-Jewish European populations. Further studies of these regions may help discover biological pathways affecting susceptibility to Crohn's disease and lead to the development of novel treatments. This study also demonstrates the complementary value of genetic studies in isolated populations, like the Ashkenazim.
Background and Objectives
Nod2 polymorphisms increase the risk for developing Crohn’s disease, characterized by chronic intestinal inflammation. Bacterial peptidoglycan products chronically stimulate Nod2 in the intestine. Recent studies found that chronic Nod2 stimulation in human macrophages downregulates pro-inflammatory cytokines upon Nod2 or Toll-like receptor (TLR) restimulation. Therefore, an emerging hypothesis is that Nod2-mediated cytokine downregulation is required for intestinal homeostasis, but the mechanisms mediating this downregulation are incompletely understood.
Utilizing primary human macrophages, we examined secretory mediators as a mechanism of Nod2-mediated tolerance by inhibiting their function and assessing tolerance reversal through cytokine secretion. Signaling pathways contributing to secretory mediator induction and Nod2-mediated tolerance were identified through pathway inhibition.
We find that chronic Nod2 stimulation not only cross-tolerizes to TLRs, but also to the IL-1 receptor (IL-1R). Moreover, chronic IL-1β stimulation downregulates Nod2 responses. Accordingly, IL-1β blockade partially reverses Nod2-mediated tolerance. We find that an additional essential mechanism for Nod2-mediated tolerance is the early secretion of the anti-inflammatory mediators IL-10, TGF-β and IL-1Ra. Importantly, the mTOR pathway, involved in cell growth, differentiation and activation, significantly contributes to Nod2-induced anti- as opposed to pro-inflammatory cytokines and to Nod2-mediated tolerance.
Inflammatory responses through the IL-1R are downregulated upon chronic Nod2 stimulation, secretory mediators are a critical mechanism for Nod2-mediated cytokine downregulation, and the mTOR pathway is crucial for Nod2-mediated tolerance. These results further contribute to our understanding of the mechanisms through which Nod2, a protein critical to intestinal homeostasis, downregulates cytokine responses.
human; monocytes/macrophages; cytokines
Protein tyrosine phosphatase nonreceptor type 22 (PTPN22) gene polymorphisms are associated with many autoimmune diseases. The major risk allele encodes an R620W amino acid change that alters B cell receptor (BCR) signaling involved in the regulation of central B cell tolerance. To assess whether this PTPN22 risk allele affects the removal of developing autoreactive B cells, we tested by ELISA the reactivity of recombinant antibodies isolated from single B cells from asymptomatic healthy individuals carrying one or two PTPN22 risk allele(s) encoding the PTPN22 R620W variant. We found that new emigrant/transitional and mature naive B cells from carriers of this PTPN22 risk allele contained high frequencies of autoreactive clones compared with those from non-carriers, revealing defective central and peripheral B cell tolerance checkpoints. Hence, a single PTPN22 risk allele has a dominant effect on altering autoreactive B cell counterselection before any onset of autoimmunity. In addition, gene array experiments analyzing mature naive B cells displaying PTPN22 risk allele(s) revealed that the association strength of PTPN22 for autoimmunity may be due not only to the impaired removal of autoreactive B cells but also to the upregulation of genes such as CD40, TRAF1, and IRF5, which encode proteins that promote B cell activation and have been identified as susceptibility genes associated with autoimmune diseases. These data demonstrate that early B cell tolerance defects in autoimmunity can result from specific polymorphisms and precede the onset of disease.
Retinoic acid (RA) is a small molecule capable of shunting developing T cells away from the Th17 lineage and towards the Treg phenotype, making it a potentially useful therapeutic for autoimmune and inflammatory diseases. However, therapy can be complicated by systemic toxicity and unpredictable bioavailability, making a targeted drug delivery vehicle for local therapy desirable. A promising approach is the use of nanoparticles, which have been demonstrated to increase potency and decrease toxicity of therapies in a variety of disease models including Th17 mediated diseases. Nanoparticles can also be targeted to specific cell types via surface modification, further increasing the potential specificity of this approach. We therefore constructed a nanoparticulate drug delivery platform from poly(lactic-co-glycolic acid) (PLGA) capable of encapsulating and releasing RA. Here we report the fabrication, characterization, and in vitro bioactivity of this platform. We demonstrate that RA containing PLGA nanoparticles suppress IL-17 production and ROR-γ(t) expression in T cells polarized towards the Th17 phenotype in vitro with similar potency to that of free drug. Furthermore, we show that these particles enhance TGF-β dependent Foxp3 expression and IL-10 production of T cells in vitro with similar potency to free RA. Finally, we demonstrate that T cells polarized towards the Th17 phenotype in the presence of free and nanoparticulate RA have similarly suppressed ability to induce IL-6 production by fibroblasts. Our findings demonstrate the feasibility of RA delivery via biodegradable nanoparticles and represent an exciting technology for the treatment of autoimmune and inflammatory diseases.
nanoparticles; drug delivery; autoimmunity; retinoic acid; Th17; treg
The use of biological annotation such as genes and pathways in the analysis of gene expression data has aided the identification of genes for follow-up studies and suggested functional information to uncharacterized genes. Several studies have applied similar methods to genome wide association studies and identified a number of disease related pathways. However, many questions remain on how to best approach this problem, such as whether there is a need to obtain a score to summarize association evidence at the gene level, and whether a pathway, dominated by just a few highly significant genes, is of interest.
We evaluated the performance of two pathway-based methods (Random Set, and Binomial approximation to the hypergeometric test) based on their applications to three data sets of Crohn's disease. We consider both the disease status as a phenotype as well as the residuals after conditioning on IL23R, a known Crohn's related gene, as a phenotype.
Our results show that Random Set method has the most power to identify disease related pathways. We confirm previously reported disease related pathways and provide evidence for IL-2 Receptor Beta Chain in T cell Activation and IL-9 signaling as Crohn's disease associated pathways.
Our results highlight the need to apply powerful gene score methods prior to pathway enrichment tests, and that controlling for genes that attain genome wide significance enable further biological insight.
Natural regulatory T cells (nTregs) that develop in the thymus are essential to limit immune responses and prevent autoimmunity. However, the steps necessary for their thymic development are incompletely understood. The CARMA1/Bcl10/Malt1 (CBM) complex, comprised of adaptors that link the TCR to the transcription factor NF-κB, is required for development of Tregs but not of conventional T cells. Current models propose that TCR-NF-κB is needed in a Treg-extrinsic manner for IL-2 production by conventional T cells or in already pre-committed Treg precursors for driving IL-2/STAT5 responsiveness and further maturation into Tregs and/or for promoting cell survival. Using CARMA1-KO mice, our data show instead that the CBM complex is needed in a Treg-intrinsic rather than extrinsic manner. Constitutive activity of STAT5 or protection from apoptosis by transgenic expression of Bcl2 in developing Tregs is not sufficient to rescue CARMA1-KO Treg development. Instead, our results demonstrate that the CBM complex controls an early checkpoint in Treg development by enabling generation of thymic precursors of Tregs. These data suggest a modified model of nTreg development in which TCR-CBM-dependent signals are essential to commit immature thymocytes to the nTreg lineage.
Background & Aims
Inflammatory bowel disease (IBD) is a multifactorial disease thought to be caused by alterations in epithelial function, innate and adaptive immunity, and luminal microbiota. The specific role of epithelial barrier function remains undefined, although increased activity of intestinal epithelial myosin light chain kinase (MLCK), which is the primary mechanism of tumor necrosis factor (TNF)-induced barrier dysfunction, occurs in human IBD. We aimed to determine whether in an intact epithelium, primary dysregulation of the intestinal epithelial barrier by pathophysiologically relevant mechanisms can contribute to development of colitis.
We developed transgenic (Tg) mice that express constitutively-active MLCK (CA-MLCK) specifically within intestinal epithelia. Their physiology, immune status, and susceptibility to disease were assessed and compared to non-Tg littermate controls.
CA-MLCK Tg mice demonstrated significant barrier loss, but grew and gained weight normally and did not develop spontaneous disease. CA-MLCK Tg mice did, however, develop mucosal immune activation demonstrated by increased numbers of lamina propria CD4+ lymphocytes, redistribution of CD11c+ cells, increased production of interferon (IFN)-γ and TNF, as well as increased expression of epithelial MHC class I. When challenged with CD4+CD45+ Rbhi lymphocytes, Tg mice developed an accelerated and more severe form of colitis and had shorter survival times than non-Tg littermates.
Primary pathophysiologically relevant intestinal epithelial barrier dysfunction is insufficient to cause experimental intestinal disease but can broadly activate mucosal immune responses and accelerate the onset and severity of immune-mediated colitis.
Ulcerative colitis is a chronic inflammatory disease of the colon that presents as diarrhea and gastrointestinal bleeding. We performed a genome-wide association study using DNA samples from 1,052 individuals with ulcerative colitis and pre-existing data from 2,571 controls, all of European ancestry. In an analysis that controlled for gender and population structure, ulcerative colitis loci attaining genome-wide significance and subsequent replication in two independent populations were identified on chromosomes 1p36 (rs6426833, combined P = 5.1×10−13, combined OR = 0.73) and 12q15 (rs1558744, combined P = 2.5×10−12, combined OR = 1.35). In addition, combined genome-wide significant evidence for association was found in a region spanning BTNL2 to HLA-DQB1 on chromosome 6p21 (rs2395185, combined P = 1.0×10−16, combined OR = 0.66) and at the IL23R locus on chromosome 1p31 (rs11209026, combined P = 1.3×10−8, combined OR = 0.56; rs10889677, combined P = 1.3×10−8, combined OR = 1.29).
Background & Aims:
Tumor necrosis factor (TNF) plays critical roles in intestinal disease. In intestinal epithelia, TNF causes tight junction disruption and epithelial barrier loss by upregulating myosin light chain kinase (MLCK) activity and expression. The aim of this study was to determine the signaling pathways by which TNF causes intestinal epithelial barrier loss.
Caco-2 cells that were either non-transfected or stably-transfected with human TNF receptor 1 (TNFR1) or TNFR2 and mouse colonocytes were used for physiological, morphological, and biochemical analyses.
Colitis induced in vivo by adoptive transfer of CD4+CD45RBhi T cells was associated with increased epithelial myosin light chain kinase (MLCK) expression and myosin II regulatory light chain (MLC) phosphorylation as well as morphological tight junction disruption. In vitro studies showed that TNF caused similar increases in MLCK expression and MLC phosphorylation, as well as barrier dysfunction, in Caco-2 monolayers only after IFN-γ pretreatment. This reductionist model was therefore used to determine the molecular mechanism by which IFN-γ and TNF synergize to cause intestinal epithelial barrier loss. IFN-γ priming increased TNFR1 and TNFR2 expression and blocking antibody studies showed that TNFR2, but not TNFR1, was required for TNF-induced barrier dysfunction. Transgenic TNFR2, but not TNFR1, expression allowed IFN-γ-independent TNF responses.
IFN-γ primes intestinal epithelia to respond to TNF by inducing TNFR2 upregulation, which in turn mediates the TNF-induced MLCK-dependent barrier dysfunction. The data further suggest that epithelial TNFR2 blockade may be a novel approach to restore barrier function in intestinal disease.
tight junction; interferon-γ; tumor necrosis factor; cytoskeleton; myosin; inflammatory bowel disease