High-sensitivity flow cytometry in rheumatoid arthritis shows that early B-cell subset depletion is linked to 5-year rituximab response, while failure to deplete B cells occurs early in subsequent poly-refractory disease

OBJECTIVES: To assess complete depletion of B-cell subsets using high-sensitivity flow cytometry (HSFC) across the therapeutic spectrum of rheumatoid arthritis (RA): from good initial rituximab (RTX) responders to poly-refractory RA (failed all biologic or targeted synthetic disease-modifying antirheumatic drug classes). METHODS: Single-centre real-world cohort of 232 RTX-treated RA adults undergoing peripheral blood HSFC assessment at baseline (before the first RTX infusion) and after 2 weeks (2W). Naïve, memory B cells and plasmablasts were quantified; complete depletion was defined as counts < 0.1 cells/µL. One-year response (simplified disease activity index ≤11 within 6 months, maintained for ≥ 12 months) and 5-year RTX persistence (maintained response with no therapeutic change for ≥ 5 years) were assessed. Longitudinal B-cell trajectories with multiple RTX cycles were analysed. RESULTS: 2W complete depletion rates were 91% for naïve, 92% for memory B cells, and 69% for plasmablasts. Higher 2W plasmablast counts were associated with lower odds of 1-year response (odds ratio [OR], 0.50; 95%CI, 0.32-0.75; P = .001) and 5-year RTX persistence (OR, 0.48; 95%CI, 0.30-0.79; P = .004); complete plasmablast depletion at 2W was positively associated with both outcomes. Early during disease, poly-refractory RA (n=31; 13%) showed higher 2W memory B-cell levels, and counts were associated with increasing refractoriness (OR, 1.9; 95%CI, 1.3-2.9; P = .003). Across multiple cycles, sustained suppression of memory B cells and plasmablasts were seen in patients showing RTX persistence. CONCLUSIONS: 2W plasmablast depletion on HSFC is strongly linked to 5-year RTX retention, while incomplete memory B-cell depletion characterises poly-refractory RA. Thus, responsiveness and refractoriness in RA are linked to B-cell pathology, endorsing HSFC implementation in selecting novel B-cell targeting strategies and early recognition of refractory disease.