Background/Purpose: Generated under oxidative stress, malondialdehyde-acetaldehyde adducts (MAA), and antibody responses to MAA appear to facilitate loss of immune tolerance and generate pro-inflammatory responses in rheumatoid arthritis (RA).  Both MAA adduct formation and anti-MAA antibody levels are enriched in joint tissues of RA patients compared to those with non-inflammatory arthritis, and MAA appears to co-localize with citrullination in joint tissue. Because oxidative stress and autoimmunity are also key components of extra-articular complications of RA, including interstitial lung disease (ILD), we assessed MAA and anti-MAA antibody responses in RA-ILD.

Methods: Using banked serum from the Veterans Affairs Rheumatoid Arthritis (VARA) registry, we assessed serum anti-MAA antibody concentrations (IgA, IgM, and IgG via ELISA) in RA patients with and without diagnosis codes for ILD. Log-transformed anti-MAA antibody concentrations were compared by group using t-tests and linear regression models adjusted for age, sex, smoking status, disease activity, and anti-CCP antibody positivity. Additionally, we analyzed lung tissue sections from subjects with RA-ILD, non-RA ILD, emphysema, and controls (n=2 per group) available through the NIH’s Lung Tissue Resource Consortium. Sections were stained using a MAA-specific rabbit polyclonal antibody and a citrulline-specific mouse IgM monoclonal antibody, and imaged with a confocal laser scanning microscope.

Results: In a male predominant (90%), established RA patient cohort (mean duration 11 years) with frequent smoking history (78%), 103 subjects had a diagnosis code for ILD. Serum concentrations of IgA and IgM anti-MAA antibody were significantly higher in RA-ILD subjects, even after adjustment for anti-CCP antibody positivity, disease activity, and smoking status (Table 1). IgG anti-MAA antibody concentrations were also marginally higher in RA-ILD subjects, but were not statistically significant. Lung tissue demonstrated staining for MAA in non-RA ILD and emphysema; however, qualitatively, staining was highest in RA-ILD (Figure 1). In addition, staining demonstrated that MAA and citrulline co-localized in lung tissue from RA-ILD.

Conclusion:  Anti-MAA antibodies, IgA and IgM, are higher in RA patients with a diagnosis of RA-ILD. Along with the enhanced staining of MAA-modified proteins in lung tissue samples from RA-ILD, our findings suggest that MAA adduct formation and resulting adaptive immune responses could play an important role in the pathogenesis of RA-ILD and also serve as informative disease biomarkers.