Lung diseases, including asthma, COPD, and other autoimmune lung pathologies are aggravated by exposure to particulate matter (PM) found in air pollution. IL-17 has been shown to exacerbate airway disease in animal models. As PM is known to contain aryl hydrocarbon receptor (AHR) ligands and the AHR has recently been shown to play a role in differentiation of Th17 T cells, the aim of this study was to determine whether exposure to PM could impact Th17 polarization in an AHR-dependent manner. This study used both cell culture techniques and in vivo exposure in mice to examine the response of T cells to PM. Initially experiments were conducted with urban dust particles from a standard reference material, and ultimately repeated with freshly collected samples of diesel exhaust and cigarette smoke. The readout for the assays was increased T cell differentiation as indicated by increased generation of IL-17A in culture, and increased populations of IL-17 producing cells by intracellular flow cytometry. The data illustrate that Th17 polarization was significantly enhanced by addition of urban dust in a dose dependent fashion in cultures of wild-type but not AHR-/- mice. The data further suggest that polycyclic aromatic hydrocarbons played a primary role in this enhancement. There was both an increase of Th17 cell differentiation, and also an increase in the amount of IL-17 secreted by the cells. In summary, this paper identifies a novel mechanism whereby PM can directly act on the AHR in T cells, leading to enhanced Th17 differentiation. Further understanding of the molecular mechanisms responsible for pathologic Th17 differentiation and autoimmunity seen after exposure to pollution will allow direct targeting of proteins involved in AHR activation and function for treatment of PM exposures.

Summary The aryl hydrocarbon receptor, which has been central to studies in toxicology for years as the receptor for the toxicant dioxin, is rapidly gaining interest in immunology based on its ability to influence T cell differentiation. Multiple studies have documented that binding of this receptor with certain ligands favors T cell differentiation towards regulatory T cells, and paradoxically binding of this same receptor with different ligands enhances Th17 effector cell differentiation. This finding has been confirmed in both in vitro and in vivo models, where different ligands are able to either ameliorate or conversely aggravate autoimmunity in experimental autoimmune encephalomyelitis. The aryl hydrocarbon receptor has both an endogenous role that is important in development and normal physiology, and also has an exogenous role as a receptor for man-made toxicants, with their binding leading to transcription of cytochrome P450 enzymes that metabolize these same ligands. Based on recent reports that will be summarized in this overview, we will consider the role that the aryl hydrocarbon receptor might play as a sensor to the outside environment, leading to alteration of the acquired immune system that might have relevance in transplantation or other medical conditions. In addition to describing the data in normal physiology and T cell differentiation, we will present examples of the importance of this receptor in preclinical models of disease, and highlight specific ligands that target the aryl hydrocarbon receptor and will have efficacy in treating transplant rejection and in tolerance protocols.