Expression of constitutively-active aryl hydrocarbon receptor in T-cells enhances the down-regulation of CD62L, but does not alter expression of CD25 or suppress the allogeneic CTL response

Babies, Bugs, and Barriers: Dietary Modulation of Intestinal Barrier Function in Early Life

The intestinal barrier is essential in early life to prevent infection, inflammation, and food allergies. It consists of microbiota, a mucus layer, an epithelial layer, and the immune system. Microbial metabolites, the mucus, antimicrobial peptides, and secretory immunoglobulin A (sIgA) protect the intestinal mucosa against infection. The complex interplay between these functionalities of the intestinal barrier is crucial in early life by supporting homeostasis, development of the intestinal immune system, and long-term gut health. Exclusive breastfeeding is highly recommended during the first 6 months. When breastfeeding is not possible, milk-based infant formulas are the only safe alternative. Breast milk contains many bioactive components that help to establish the intestinal microbiota and influence the development of the intestinal epithelium and the immune system. Importantly, breastfeeding lowers the risk for intestinal and respiratory tract infections. Here we review all aspects of intestinal barrier function and the nutritional components that impact its functionality in early life, such as micronutrients, bioactive milk proteins, milk lipids, and human milk oligosaccharides. These components are present in breast milk and can be added to milk-based infant formulas to support gut health and immunity. Expected final online publication date for the Annual Review of Nutrition, Volume 42 is August 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Aryl hydrocarbon receptor and intestinal immunity

Aryl hydrocarbon receptor (AhR) is a member of the basic helix-loop-helix-(bHLH) superfamily of transcription factors, which are associated with cellular responses to environmental stimuli, such as xenobiotics and oxygen levels. Unlike other members of bHLH, AhR is the only bHLH transcription factor that is known to be ligand activated. Early AhR studies focused on understanding the role of AhR in mediating the toxicity and carcinogenesis properties of the prototypic ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In recent years, however, it has become apparent that, in addition to its toxicological involvement, AhR is highly receptive to a wide array of endogenous and exogenous ligands, and that its activation leads to a myriad of key host physiological functions. In this study, we review the current understanding of the functions of AhR in the mucosal immune system with a focus on its role in intestinal barrier function and intestinal immune cells, as well as in intestinal homeostasis.

The Aryl Hydrocarbon Receptor in Immunity: Tools and Potential

The signaling pathway of the evolutionary old transcription factor AhR is inducible by a number of small molecular weight chemicals, including toxicants such as polycyclic aromatic hydrocarbons, bacterial toxic pigments, and physiological compounds such as tryptophan derivatives or dietary indoles. AhR activation is of immunological importance, but at the same time mediates toxicity of environmental pollutants, such as immunosuppression by dioxins. Measuring AhR activity and identification of ligands is thus of great interest for a variety of research fields. In this chapter, I briefly introduce the AhR signaling pathway, its role in immunology, and the tools and assays needed to analyze AhR signaling. Both are also needed when therapeutic applications are envisioned.

The aryl hydrocarbon receptor in barrier organ physiology, immunology, and toxicology

The aryl hydrocarbon receptor (AhR) is an evolutionarily old transcription factor belonging to the Per-ARNT-Sim-basic helix-loop-helix protein family. AhR translocates into the nucleus upon binding of various small molecules into the pocket of its single-ligand binding domain. AhR binding to both xenobiotic and endogenous ligands results in highly cell-specific transcriptome changes and in changes in cellular functions. We discuss here the role of AhR for immune cells of the barrier organs: skin, gut, and lung. Both adaptive and innate immune cells require AhR signaling at critical checkpoints. We also discuss the current two prevailing views-namely, 1) AhR as a promiscuous sensor for small chemicals and 2) a role for AhR as a balancing factor for cell differentiation and function, which is controlled by levels of endogenous high-affinity ligands. AhR signaling is considered a promising drug and preventive target, particularly for cancer, inflammatory, and autoimmune diseases. Therefore, understanding its biology is of great importance.

The aryl hydrocarbon receptor: multitasking in the immune system

The aryl hydrocarbon receptor (AhR), for many years almost exclusively studied by the pharmacology/toxicology field for its role in mediating the toxicity of xenobiotics such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), has more recently attracted the attention of immunologists. The evolutionary conservation of this transcription factor and its widespread expression in the immune system point to important physiological functions that are slowly being unraveled. In particular, the emphasis is now shifting from the role of AhR in the xenobiotic pathway toward its mode of action in response to physiological ligands. In this article, we review the current understanding of the molecular interactions and functions of AhR in the immune system in steady state and in the presence of infection and inflammation, with a focus on barrier organs such as the skin, the gut, and the lung.

Differential influences of the aryl hydrocarbon receptor on Th17 mediated responses in vitro and in vivo

The aryl hydrocarbon receptor (AhR) has been attributed with anti-inflammatory effects in the development of pathological immune responses leading to experimental autoimmune encephalomyelitis (EAE) via the induction of regulatory T cells. In agreement with previously published findings, we find that TCDD administration confers protection from EAE, however, this immuno-modulatory effect was not the consequence of de novo Treg generation, but the inhibition of Th17 cell differentiation. Systemic application of FICZ at the time of immunization also reduced EAE pathology albeit to a lesser degree than TCDD. In vitro Th17 differentiation in the presence of AhR agonists, including TCDD, promoted IL-17 and IL-22 expression, but did not induce Treg differentiation. AhR affinity influenced the amounts of IL-17 and IL-22 protein that was secreted by Th17 cells, but did not seem to affect susceptibility to EAE in vivo. Making use of conditional AhR-deficient mice, we show that the anti-inflammatory effect of TCDD depends on AhR activation in both T cells and dendritic cells, further emphasising the ability of TCDD to interfere with T effector cell differentiation in vivo. The dichotomy between the in vivo and in vitro effects of AhR reveals the complexity of the AhR pathway, which has the capacity of affecting different AhR-expressing cell types involved in mounting immune responses, thus participating in defining their outcome.

Programmed death-1 shapes memory phenotype CD8 T cell subsets in a cell-intrinsic manner

Memory phenotype T cells, found in unimmunized mice, display phenotypic and functional traits of memory cells and provide essential protection against infections, playing a role in both innate and adaptive immune responses. Mechanisms governing homeostasis of these memory phenotype T cells remain ill-defined. In this study, we reveal a crucial role of the negative costimulator programmed death-1 (PD-1) in regulating developmental fates of memory phenotype cells. Thus, in lymphoid organs and tissues of PD-1 knockout (KO) mice a marked accumulation of functional effector memory (T(EM)) phenotype CD8 T cells was observed. T(EM) phenotype cells from PD-1 KO mice exhibit decreased proliferation but increased survival potential. These cells could produce effector molecules constitutively, in response to phorbol esters or through bystander activation by innate stimuli. Similarly, in lymphopenia-induced proliferating CD8 T cells, whereby normally naive T cells acquire a memory phenotype, skewing toward a T(EM) phenotype was prominent in the absence of PD-1. Acquisition of the T(EM) phenotype was a CD8 T cell-intrinsic phenomenon as demonstrated by mixed bone marrow transfer experiments. Importantly, adoptively transferred PD-1 KO CD8 central memory T (T(CM)) cells converted into the T(EM) phenotype, indicating that PD-1 sets a major checkpoint in the T(CM) to T(EM) phenotype differentiation process. This was reflected by distinct patterns of gene expression of PD-1 KO T(CM) phenotype cells revealed by global transcriptional analysis. Additionally, adoptively transferred PD-1 KO T(EM) phenotype cells converted to a lesser degree to a T(CM) phenotype. Collectively, these data suggest that PD-1 shapes memory phenotype CD8 T cell subsets.

Use of natural AhR ligands as potential therapeutic modalities against inflammatory disorders

The aim of this review is to discuss research involving ligands for the aryl hydrocarbon receptor (AhR) and their role in immunomodulation. While activation of the AhR is well known for its ability to regulate the biochemical and toxic effects of environmental chemicals, more recently an exciting discovery has been made indicating that AhR ligation can also regulate T-cell differentiation, specifically through activation of Foxp3(+) regulatory T cells (Tregs) and downregulation of the proinflammatory Th17 cells. Such findings have opened new avenues of research on the possibility of targeting the AhR to treat inflammatory and autoimmune diseases. Specifically, this review will discuss the current research involving natural and dietary AhR ligands. In addition, evidence indicating the potential use of these ligands in regulating inflammation in various diseases will be highlighted. The importance of the AhR in immunological processes can be illustrated by expression of this receptor on a majority of immune cell types. In addition, AhR signaling pathways have been reported to influence a number of genes responsible for mediating inflammation and other immune responses. As interest in the AhR and its ligands increases, it seems prudent to consolidate current research on the contributions of these ligands to immune regulation during the course of inflammatory diseases.

The aryl hydrocarbon receptor: fine-tuning the immune-response

Triggers involved in the development of an autoimmune disease, and those that are part of determining its level of severity, are a major focus of current investigative efforts. However, factors that increase the risk to disease may not be similar to those that determine its severity or its pace of progression. The aryl hydrocarbon receptor (AhR) has been highlighted as having a potential regulatory role in these processes. Here we describe the recent findings of the possible involvement of AhR in the initiation and inhibition of immune responses.

External influences on the immune system via activation of the aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AhR), subject of intensive research over three decades by the pharmacology/toxicology field has recently made its entry into mainstream immunology research and is set to continue to intrigue with ever more complex modes of modulating immune responses. The discovery of high and selective AhR expression on Th17 cells and its role in induction of the cytokine IL-22 attributed new immunological functions to this transcription factor and stimulated further research into physiological functions of the AhR in the immune system. A number of recent reviews have highlighted potential new avenues of research. This review addresses recent new insight into physiological roles of AhR in the immune system.

Aryl hydrocarbon receptor activation by TCDD reduces inflammation associated with Crohn's disease

Crohn's disease results from a combination of genetic and environmental factors that trigger an inappropriate immune response to commensal gut bacteria. The aryl hydrocarbon receptor (AhR) is well known for its involvement in the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), an environmental contaminant that affects people primarily through the diet. Recently, TCDD was shown to suppress immune responses by generating regulatory T cells (Tregs). We hypothesized that AhR activation dampens inflammation associated with Crohn's disease. To test this hypothesis, we utilized the 2,4,6-trinitrobenzenesulfonic acid (TNBS) murine model of colitis. Mice were gavaged with TCDD prior to colitis induction with TNBS. Several parameters were examined including colonic inflammation via histological and flow cytometric analyses. TCDD-treated mice recovered body weight faster and experienced significantly less colonic damage. Reduced levels of interleukin (IL) 6, IL-12, interferon-gamma, and tumor necrosis factor-α demonstrated suppression of inflammation in the gut following TCDD exposure. Forkhead box P3 (Foxp3)(egfp) mice revealed that TCDD increased the Foxp3+ Treg population in gut immune tissue following TNBS exposure. Collectively, these results suggest that activation of the AhR by TCDD decreases colonic inflammation in a murine model of colitis in part by generating regulatory immune cells. Ultimately, this work may lead to the development of more effective therapeutics for the treatment of Crohn's disease.

Dioxin and immune regulation: emerging role of aryl hydrocarbon receptor in the generation of regulatory T cells

The immune toxicity of the ubiquitous environmental contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), commonly referred to as dioxin, has been studied for over 35 years but only recently has the profound immune suppression induced by TCDD exposure been linked to induction of regulatory T cells (Tregs). The effects of TCDD are mediated through its binding to the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor. The subsequent AHR-dependent effects on immune responses are determined by the cell types involved, their activation status, and the type of antigenic stimulus. Collectively, studies indicate that TCDD inhibits CD4+ T cell differentiation into T helper (Th)1, Th2, and Th17 effector cells, while inducing Foxp3-negative and/or preserving Foxp3+ Tregs. Although it is not yet clear how activation of AHR by TCDD induces Tregs, there is a potential therapeutic role for alternative AHR ligands in the treatment of immune-mediated disorders.