Neutrophilic activity biomarkers (plasma neutrophil extracellular traps and calprotectin) in established rheumatoid arthritis patients receiving biological or JAK inhibitors: a clinical and ultrasonographic study

Abstract Background: Research into blood biomarkers of disease activity in rheumatoid arthritis (RA) has grown significantly. Accumulating evidence suggests that neutrophils play a crucial role in the initiation and progression of RA. This study assesses the accuracy of neutrophil activation markers, including neutrophil extracellular traps (NETs) and calprotectin, as biomarkers of disease activity in patients with established RA. We also analyse the relationship between NETs and various types of therapies as well as their association with autoimmunity. Methods: We performed an observational cross-sectional study of RA patients receiving treatment with biological disease-modifying antirheumatic drugs (DMARDs) or JAK inhibitors for at least 3 months. Plasma calprotectin levels were measured using an enzyme-linked immunosorbent assay (ELISA) test kit. NETs were assessed by measuring their remnants in plasma, including neutrophil elastase–DNA (NE-DNA) and histone–DNA (H3-DNA) complexes. We also assessed clinical disease activity, joint ultrasound (US) findings and autoantibody status (rheumatoid factor (RF), anti-citrullinated peptide/protein antibodies (ACPAs) and anti-carbamylated protein (anti-CarP) antibodies). Associations between neutrophilic biomarkers and clinical or US scores were sought using correlation analysis. The discriminatory capacity of both neutrophilic biomarkers to detect US synovitis was analysed through ROC curves. Results: Plasma NET levels did not correlate with clinical disease status, regardless of the clinic index analysed or the specific biological therapy administered. Moreover, no significant correlation was observed between NET remnants and US-detected synovitis. Additionally, there was no correlation between plasma NET levels and autoantibodies (RF, ACPAs, anti-CarP). In contrast, plasma calprotectin positively correlated with clinical parameters (SJC rho=0.49, CDAI rho=0.30; p<0.001) and US parameters (rho>0.50; p<0.001). Notably, this correlation was stronger than that observed with acute phase reactants (high sensitivity C-reactive protein (hsCRP) and erythrocyte sedimentation rate (ESR)). Conclusion: While NETs formation induced by neutrophils may play a role in RA pathogenesis, our study raises questions about the utility of NET remnants in peripheral circulation as a biomarker for inflammatory activity. In contrast, this study strongly supports the usefulness of blood calprotectin as a biomarker of inflammatory activity and local synovitis in patients with RA, consistent with previous findings.