The onset site of rheumatoid arthritis: the joints or the lung?

The etiopathogenesis of rheumatoid arthritis (RA) is not yet fully elucidated and the site of inflammation onset is still a matter of debate. The presence of autoantibodies as well as clinical manifestations, such as interstitial lung disease, before the onset of arthritis seems to be in favour of the hypothesis that initial pathogenic events take place in tissues other than the joint. In this review article we summarize the most recent literature on extra-synovial autoimmunity triggers eventually leading to RA, with particular focus on the role of the lung. To date, anti-cyclic citrullinated peptide antibodies (ACPAs) are considered central players in RA pathogenesis and represent the gold-standard for disease diagnosis. Lungs and mucosae are exposed to environmental stimuli such as dusts and smoke which have been shown to foster citrullination of peptides in lungs thereby triggering the production of ACPA. In addition, other mechanisms of disease pathogenesis independent of citrullination play an important role. Deeper knowledge of these processes could represent a huge step forward in the management of RA, with dramatic impact on diagnosis, prevention, prognostic stratification and treatment of the disease.