Modeling environmental inhalant exposure in rheumatoid arthritis

The mucosal origins hypothesis posits that environmental inhalant exposures, including cigarette smoke (CS) and crystalline silica (c-silica), trigger immune responses in the lung mucosa, an extra-articular site, which precede initiating events of rheumatoid arthritis (RA) pathogenesis in distant joints. Epidemiological data strongly associates these exposures with RA risk, especially in genetically susceptible individuals carrying HLA-DRB1 alleles, and with the production of autoantibodies such as anti-citrullinated peptide antibodies (ACPA) and rheumatoid factor (RF). However, establishing causality remains challenging due to unsynchronized exposure and disease onset and the lack of suitable animal models to study early disease events. This review synthesizes evidence linking inhalant exposures to RA, focusing on CS and c-silica, and evaluates experimental animal models used to investigate disease initiation and progression in the context of inhalant exposures. While models like collagen-induced arthritis (CIA) replicate joint pathology, they often fail to capture the lung-joint axis and gene-environment interactions critical for RA onset. We highlight the need for refined models with genetic susceptibility to subclinical autoimmunity to better mimic human RA, emphasizing the importance of standardized exposure protocols to address variability in outcomes. These advancements are crucial for elucidating mechanisms of inhalant exposure-induced RA and developing preventive strategies.