The aryl hydrocarbon receptor and food allergy

Aryl hydrocarbon receptor: The master regulator of immune responses in allergic diseases

The aryl hydrocarbon receptor (AhR) is a widely studied ligand-activated cytosolic transcriptional factor that has been associated with the initiation and progression of various diseases, including autoimmune diseases, cancers, metabolic syndromes, and allergies. Generally, AhR responds and binds to environmental toxins/ligands, dietary ligands, and allergens to regulate toxicological, biological, cellular responses. In a canonical signaling manner, activation of AhR is responsible for the increase in cytochrome P450 enzymes which help individuals to degrade and metabolize these environmental toxins and ligands. However, canonical signaling cannot be applied to all the effects mediated by AhR. Recent findings indicate that activation of AhR signaling also interacts with some non-canonical factors like Kruppel-like-factor-6 (KLF6) or estrogen-receptor-alpha (Erα) to affect the expression of downstream genes. Meanwhile, enormous research has been conducted to evaluate the effect of AhR signaling on innate and adaptive immunity. It has been shown that AhR exerts numerous effects on mast cells, B cells, macrophages, antigen-presenting cells (APCs), Th1/Th2 cell balance, Th17, and regulatory T cells, thus, playing a significant role in allergens-induced diseases. This review discussed how AhR mediates immune responses in allergic diseases. Meanwhile, we believe that understanding the role of AhR in immune responses will enhance our knowledge of AhR-mediated immune regulation in allergic diseases. Also, it will help researchers to understand the role of AhR in regulating immune responses in autoimmune diseases, cancers, metabolic syndromes, and infectious diseases.

Activation of aryl hydrocarbon receptor by benzo[a]pyrene increases interleukin 33 expression and eosinophil infiltration in a mouse model of allergic airway inflammation

We recently demonstrated that benzo[a]pyrene (BaP), the aryl hydrocarbon receptor (AhR) ligand, directly contributes to aggravation of cutaneous allergy in a mouse model of allergic dermatitis. The present study aimed to determine whether BaP-induced AhR activation results in development of airway inflammation. Initially, the potential for a direct relationship between BaP-induced AhR activation and airway inflammation was investigated in vivo, using a mouse model of type 2 helper T cell (Th2) hapten toluene-2,4-diisocyanate (TDI)-induced airway inflammation. Mice were orally administered BaP at 48, 24, and 4 h before the final allergen challenge. Oral administration of BaP showed a significant increase in lung inflammation and eosinophil infiltration. While expression of Th2 cytokines such as interleukin 4 (IL-4) and IL-13 was not affected by exposure to BaP, AhR activation significantly increased IL-33 expression. To confirm the in vivo results, in vitro experiments were performed using the human eosinophilic leukemia cell line (EOL-1), human bronchial epithelial cell line (BEAS-2B), and human lung adenocarcinoma epithelial cell line (A549). Results indicated that pre-treatment with BaP increased expression of IL-8 in house dust mite-activated EOL-1, BEAS-2B, and A549 cells. In addition, IL-33 levels in BEAS-2B cells were significantly increased after BaP exposure. Our findings indicated that BaP-induced AhR activation is involved in the pro-inflammatory response in respiratory allergy, and that this effect may be mediated by increased IL-33 expression and eosinophil infiltration.

Mechanisms by which gut microorganisms influence food sensitivities

Finely tuned mechanisms enable the gastrointestinal tract to break down dietary components into nutrients without mounting, in the majority of cases, a dysregulated immune or functional host response. However, adverse reactions to food have been steadily increasing, and evidence suggests that this process is environmental. Adverse food reactions can be divided according to their underlying pathophysiology into food intolerances, when, for instance, there is deficiency of a host enzyme required to digest the food component, and food sensitivities, when immune mechanisms are involved. In this Review, we discuss the clinical and experimental evidence for enteric infections and/or alterations in the gut microbiota in inciting food sensitivity. We focus on mechanisms by which microorganisms might provide direct pro-inflammatory signals to the host promoting breakdown of oral tolerance to food antigens or indirect pathways that involve the metabolism of protein antigens and other dietary components by gut microorganisms. Better understanding of these mechanisms will help in the development of preventive and therapeutic strategies for food sensitivities.

Aryl hydrocarbon receptor and its ligands for regulation of Th17/Treg cell differentiation in ulcerative colitis

Ulcerative colitis (UC) is a chronic, non-specific intestinal mucosal inflammatory disease, and its pathogenesis is related to the imbalance of Th17/Treg. The aryl hydrocarbon receptor (AhR) is a cytoplasmic transcription factor, which is involved in the regulation of drug metabolism, cell growth and differentiation, and is closely related to the occurrence of autoimmune disease and inflammatory disease. Recent studies have found that activation of AhR can regulate the differentiation of Th17/Treg in UC patients. This paper reviews the structure and function of AhR and its effect on differentiation of Th17/Treg in UC.