Anti-protein arginine deiminase antibodies are distinctly associated with joint and lung involvement in rheumatoid arthritis.

Objectives: RA is a chronic inflammatory disease in which possible interstitial lung disease (ILD) is an extra-articular manifestation that carries significant morbidity and mortality. RF and ACPA are included in the RA classification criteria but prognostic and diagnostic biomarkers for disease endotyping and RA-ILD are lacking. Anti-protein arginine deiminase antibodies (anti-PAD) are a novel class of autoantibodies identified in RA. This study aimed to assess clinical features, ACPA and anti-PAD antibodies in RA patients with articular involvement and ILD.

Methods: We retrospectively collected joint erosions, space narrowing, clinical features and lung involvement of a cohort of 71 patients fulfilling the 2010 ACR/EULAR RA classification criteria. Serum samples from these patients were tested for ACPA IgG (QUANTA Flash CCP3), and anti-PAD3 and anti-PAD4 IgG, measured with novel assays based on a particle-based multi-analyte technology (PMAT).

Results: Anti-PAD4 antibodies were significantly associated with radiographic injury (P = 0.027) and erosions (P = 0.02). Similarly, ACPA levels were associated with erosive disease (P = 0.014). Anti-PAD3/4 double-positive patients displayed more joint erosions than patients with anti-PAD4 antibodies only or negative for both (P = 0.014 and P = 0.037, respectively). RA-ILD (15.5%, 11/71 patients) was associated with older age (P < 0.001), shorter disease duration (P = 0.045) and less erosive disease (P = 0.0063). ACPA were elevated in RA-ILD, while anti-PAD4 were negatively associated (P = 0.043).

Conclusion: Anti-PAD4 and anti-PAD3 antibodies identify RA patients with higher radiographic injury and bone erosions. In our cohort, ILD is associated with lower radiographic and erosive damage, as well as low levels of anti-PAD4 antibodies.