Ambient urban dust particulate matter reduces pathologic T cells in the CNS and severity of EAE.

Background: Autoimmune diseases have increased in incidence and prevalence worldwide. While genetic predispositions play a role, environmental factors are a major contributor. Atmospheric particulate matter (PM) is a complex mixture composed of metals, nitrates, sulfates and diverse adsorbed organic compounds like polycyclic aromatic hydrocarbons (PAHs) and dioxins. Exposure to atmospheric PM aggravates autoimmune diseases such as type 1 diabetes, rheumatoid arthritis, multiple sclerosis, and systemic lupus erythematosus, among others. PAHs and dioxins are known aryl hydrocarbon receptor (AHR) ligands. The AHR modulates T cell differentiation and directs the balance between effector and regulatory T cells in vitro and in experimental autoimmune encephalomyelitis (EAE), a murine model of autoimmune disease. This study aims to identify pathways that contribute to autoimmune disease and their potential use as therapeutic targets to alleviate symptoms and the need for global immunosuppression. This study tests the hypothesis that atmospheric PM enhances effector T cell differentiation and aggravates autoimmune disease.

Results: An atmospheric ambient urban dust PM sample, standard reference material (SRM)1649b, was tested for its effects on autoimmunity. SRM1649b PM enhanced Th17 differentiation in an AHR-dependent manner in vitro, however intranasal treatment of SRM1649b PM delayed onset of EAE and reduced cumulative and peak clinical scores. Chronic and acute intranasal exposure of SRM1649b PM delayed onset of EAE. Chronic intranasal exposure did not reduce severity of EAE while acute intranasal exposure significantly reduced severity of disease. Acute intranasal treatment of low dose SRM1649b PM had no effect on clinical score or day of onset in EAE. Delayed onset of EAE by intranasal SRM1649b PM was AHR-dependent in vivo. Oral gavage of SRM1649b PM, in the absence of AHR ligands in the diet, had no effect on day of disease onset or severity of EAE. Day 10 analysis of T cells in the CNS after intranasal treatment of SRM1649b PM showed a reduction of pathologic T cell subsets in vivo. Moreover, MOG-specific splenocytes require AHR to generate or maintain IL-10 producing cells and reduce IFNγ producing cells in vitro.

Conclusions: These results identify the AHR pathway as a potential target for driving targeted immunosuppression in the CNS in the context of atmospheric PM-mediated autoimmune disease. The effects of SRM1649b PM on EAE are dependent on route of exposure, with intranasal treatment reducing severity of EAE and delaying disease onset while oral gavage has no effect. Intranasal SRM1649b PM reduces pathologic T cells in the CNS, specifically Th1 cells and Th1Th17 double positive cells, leading to reduced severity of EAE and AHR-dependent delayed disease onset. Additionally, SRM1649b PM treatment of antigen-specific T cells leads to AHR-dependent increase in percent IL-10 positive cells in vitro. These findings may shed light on the known increase of infection after exposure to atmospheric PM and serve as the first step in identifying components of the AHR pathway responsible for Th1-mediated immunosuppression in response to atmospheric PM exposure.