Objectives: We previously reported a gene-environment interaction between HLA-DR shared epitope genes and smoking in anti-cyclic-citrullinated-peptide (anti-CCP) antibody positive rheumatoid arthritis (RA). We suggested that citrullination induced by smoking might be the first step in the pathogenic chain of RA, by identification of citrullinated proteins in bronchoalveolar lavage (BAL) cells from smokers. The objective of this study was to confirm and extend these findings.
Methods: Immunohistochemistry was performed on BAL cells and bronchial mucosal biopsy sections obtained through bronchoscopy from 14 healthy smokers and 16 healthy non-smokers. We used 2 antibodies recognizing citrullinated proteins, 2 antibodies recognizing peptidylarginine deiminase (PAD)-2 enzyme and 1 recognizing PAD4 enzyme.
Results: Citrullinated proteins are up-regulated in BAL cells of healthy smokers compared to healthy non-smokers. This was associated with higher expression of the PAD2 enzyme. Citrullinated proteins were present in bronchial mucosal biopsies to same extent in healthy smokers and non-smokers, despite higher expression of PAD2 in smokers.
Conclusion: This study provides evidence that smoking enhances PAD2 expression in the bronchial mucosal and alveolar compartment with consequent generation of citrullinated proteins in the latter. We propose smoking as an environmental factor that might lead to citrulline autoimmunity in genetically susceptible individuals.