The association between systemic inflammation as measured by calprotectin and autoantibody load in rheumatoid arthritis

Background Plasma calprotectin is a marker of neutrophil activity. Its relation to the clinical phenotype and autoantibodies in rheumatoid arthritis (RA) is only partly understood. We therefore aimed to investigate the association between calprotectin and clinical parameters, CRP, ESR, anti‐CCP2 and other anti‐modified protein autoantibodies (AMPAs) in patients with RA. Methods Rheumatoid factor (RF), anti‐cyclic citrullinated peptide 2 (CCP2) isotypes and plasma calprotectin were centrally measured in a cross‐sectional cohort of 4521 Scandinavian RA patients using fluoroenzyme immunoassays. IgG to 15 citrullinated (Cit) and four carbamylated or acetylated (Carb/Acet) peptides were measured by multiplex solid‐phase array. CRP and ESR values were obtained from clinical measurements at the time of blood sampling. Results Calprotectin levels correlated with ESR (ρ=0.53), CRP (ρ=0.47), VAS pain (ρ=0.24), SJC28 (ρ=0.39), and TJC28 (ρ=0.26; p10mg/L) and calprotectin (>15 AU/ml) (14%, N=613) had significantly higher VAS pain, SJC28, and TJC28 than patients with only elevated CRP (pPatients with elevated calprotectin, independent of CRP levels, had significantly higher proportions of AMPA positivity including anti‐CCP2. When patients were divided into six groups based on increasing number of Cit/Carb/Acet‐reactivities and anti‐CCP2 IgG levels, elevated calprotectin was associated with increasing AMPA load, particularly in the presence of RF IgM. The differences between AMPA load groups remained after adjusting for sex, age, RF IgM, and disease duration. Conclusions Plasma calprotectin is positively associated with seropositivity and AMPA load by autoantibody levels and the breadth of the autoantibody response.