Background/Purpose: Antibodies targeting citrullinated proteins (ACPA) are implicated in the pathogenesis of interstitial lung disease (ILD) in rheumatoid arthritis (RA).  Citrullinated proteins and the enzymes that catalyze citrullination, the peptidylarginine deiminases (PADs), are detected in RA-ILD lung tissue and in the lungs of heavy smokers.  A subset of RA patients demonstrates cross-reactive antibodies against PADs 3 and 4 that have been shown to lower the calcium threshold required for PAD activation, an effect potentially relevant to the pathogenesis of RA-ILD.  We sought to explore the association between anti-PADs and radiographic RA-ILD.

Methods: RA patients participating in a study of subclinical cardiovascular disease underwent multi-detector computed tomography (MDCT) of the chest with interpretation by a pulmonary radiologist for ILD features.  A semi-quantitative ILD Score (ILDS; range 0-32) was calculated.  Concurrent serum samples were assessed for antibodies against PAD by immunoprecipitation of S³⁵-labeled PAD3 and 4.

Results: Among the 176 RA patients studied [60% female, 86% Caucasian, mean age 59±9 years, 11% current smokers, median RA duration=8 years, median DAS28=3.7], any CT-ILD was observed in 58 (33%).  Anti-PAD3/4 cross-reactive antibodies were detected in 19 (11%) and 37 (21%) had antibodies directed against PAD4 alone.  In univariate analysis, the frequency of any CT-ILD among those with anti-PAD3/4 was 68%, vs. 27 and 29% among those with anti-PAD4 only and neither reactivity, respectively (crude OR=5.39; p=0.001 for the comparison of anti-PAD3/4 vs. no anti-PAD).  The association was stronger after adjustment for relevant demographic and RA disease/treatment confounders (Fig 1A: adjusted OR=7.22; p=0.001).  Anti-PAD3/4 antibodies were significantly associated with all CT-ILD predominant patterns (i.e. ground glass, honeycombing, etc…) and quantitative ILD scores were significantly higher for the anti-PAD3/4 vs. the no anti-PAD group (median ILD score 2 vs. 0 units, respectively; p=0.020).  The association of anti-PAD3/4 antibodies with CT-ILD was stronger in ever smokers than never smokers.  Among never smokers, the adjusted frequency of any ILD was 39 vs. 17%, respectively, for those with vs. without anti-PAD3/4 (Fig 1B, never smokers: OR=3.01; p=0.19) compared with 93 vs. 30%, respectively, for ever smokers with vs. without anti-PAD3/4 (Fig 1B, ever smokers: OR=29.5; p=0.004, p-value for interaction<0.05)

Conclusion: The prevalence of CT-ILD was markedly higher among RA patients with anti-PAD3/4 antibodies, even after accounting for relevant confounders, particularly among ever smokers.  Further mechanistic studies are needed to determine whether there is a biologic interaction between smoking and pulmonary hyper-citrullination facilitated by anti-PAD3/4 antibodies that contributes to ILD pathogenesis.