Objective. Smoking is a well-established risk factor in rheumatoid arthritis (RA), and citrullination of selfantigens plays a pathogenic role in the majority of patients. Increased numbers of peptidylarginine deiminase 2 (PAD2)-containing macrophages have been demonstrated in bronchoalveolar lavage (BAL) fluid from smokers, but intracellularly located PAD cannot be responsible for citrullination of extracellular self-antigens. We aimed to establish a link between smoking and extracellular PAD2 in the lungs. Methods. BAL fluid samples were obtained from 13 smokers and 11 nonsmoking controls. Total protein content and C-reactive protein (CRP) concentration were determined after separating cells from the samples. PAD2 content in cell-free BAL fluids was measured by means of a PAD2-specific sandwich ELISA.

Results. Significantly increased levels of soluble PAD2 were detected in cellfree BAL fluids from smokers as compared to non-smokers (p= 0.018). The PAD2 content correlated with the overall CRP levels (p= 0.009) and cell count (p= 0.016).

Conclusion. This first demonstration of increased levels of extracellular PAD2 in the lungs of smokers supports the hypothesis that smoking promotes extracellular citrullination of proteins. This may represent a pathological event upstream for the production of anti-citrullinated protein antibodies (ACPAs) among RA patients carrying HLA-molecules capable of binding citrullinated self-peptides.