AhR as a Common Denominator in Immunity and Inflammation in Chronic Lung Diseases: Molecular and Clinical Insights

The respiratory system is directly exposed to various environmental factors, and specifically allergens and environmental pollutants, which are ligands/agonists of the aryl hydrocarbon receptor (AhR) and promote chronic lung diseases in humans. AhR, a ligand-activated transcription factor, is involved in the metabolism of xenobiotics, assigning their carcinogenic and toxic effects, and is also involved in normal homeostasis, organogenesis, and immune system function. Exogenous and endogenous AhR ligands are both high-molecular-weight compounds with a planar structure and low-molecular-weight compounds of diverse chemical structures. After entering the cell, the ligands bind to AhR and induce the activation of signaling cascades. The lung immune system responds to pathogens and environmental toxins first with a pro-inflammatory innate immune response, and then with an anti-inflammatory adaptive immune response. An imbalance between these immune systems may have an effect on the course of the disease. Activation of AhR by exogenous or endogenous ligands can affect this balance and lead to dysregulation of the immune response, leading to inflammatory complications in the lungs. Individual features of AhR expression or components of the AhR-dependent signaling pathway may also play a role in the superposition of the functions of these two links of immunity. This review summarizes advances in the comprehension of AhR's role in immunomodulation and inflammatory responses in the lungs following data in experimental rodent models, in vitro studies utilizing lung structural cells and isolated immune cell lines, and humans. The molecular mechanisms of AhR's regulation of immunity and inflammation and the potential of AhR as a therapeutic target for inflammatory lung disease are also considered.