Double-negative B cells mediate the causal association between rheumatoid arthritis and interstitial lung disease: a two-step Mendelian randomization mediation study

Background: Rheumatoid arthritis (RA) elevates interstitial lung disease (ILD) risk, yet the underlying mediating immune cells remain unknown. This study sought peripheral immune phenotypes driving the causal pathway from RA to ILD. Methods: A two-step Mendelian randomization (MR) mediation analysis was performed using European GWAS data: RA (14,361 cases/43,923), 731 immune cell traits, and ILD from FinnGen R13 (6,733 cases/493,453). Step 1 applied inverse variance weighted (IVW) MR to estimate RA effects on immune cells; Step 2 tested immune cell effects on ILD. Mediation was validated via joint significance and parametric bootstrapping (10,000 resamples) with BCa 95% confidence intervals (CIs). Results: RA exerted a causal risk-increasing effect on ILD (β = 0.166, P = 4.26 × 10⁻⁸). Fifty-three immune traits showed nominal significance in Step 1, 15 passing FDR correction. CD19⁺IgD⁻CD27⁻ DN B cells reached nominal significance (P = 0.028) but not FDR significance (q = 0.512) in Step 1, yet were the only trait significantly associated with ILD in Step 2 (β₂ = 0.060, P = 0.027). Bootstrapping verified a small but significant indirect effect (β = 0.0039, 95% BCa CI 0.0009–0.0153, P = 0.050; mediation proportion = 2.3%), consistent across sensitivity tests. Conclusions: DN B cells partially mediate the causal RA–ILD relationship. Our genetic evidence highlights B cell dysfunction as a mechanistic link and identifies DN B cells as candidate biomarkers for RA-related interstitial lung complications. (Table presented.)