Minimal residual disease flow cytometry of peripheral blood B-cell subsets in patients with idiopathic inflammatory myopathies following rituximab therapy: a single-centre, retrospective cohort study

BACKGROUND: Rituximab treatment has shown inconclusive results in clinical trials and observational studies in patients with idiopathic inflammatory myopathies. This study aimed to assess whether complete depletion of distinct B-cell subsets was associated with clinical response in rituximab-treated patients with myositis using highly sensitive flow cytometry (HSFC). METHODS: This single-centre, retrospective cohort study included adults (aged ≥18 years) treated with rituximab who fulfilled the American College of Rheumatology (ACR) and European Alliance of Associations for Rheumatology (EULAR) classification criteria for idiopathic inflammatory myopathies, followed up at the Leeds Teaching Hospitals NHS Trust (Leeds, UK). HSFC was performed before the first rituximab infusion and at 2 weeks. The primary outcome was a clinical response (at least moderate improvement on the 2016 ACR-EULAR total improvement score) at the first follow-up 4-6 months after infusion. Associations of clinical response with complete HSFC depletion (<0·1 cells per μL) of naive B cells, memory B cells, and plasmablasts versus the conventional flow cytometry cutoff (<5 total B cells per μL) were assessed using multivariable Firth penalised logistic regression. No patients with lived experience were involved in the study. FINDINGS: Between Jan 1, 2015, and Dec 31, 2025, 44 patients with idiopathic inflammatory myopathies were treated with rituximab and had complete datasets. Of these patients, 24 (55%) had dermatomyositis, 12 (27%) had antisynthetase syndrome, five (11%) had immune-mediated necrotising myositis, and three (7%) had polymyositis. 17 (39%) of 44 patients were male, 27 (61%) were female, 35 (80%) were White, and the median age was 45 years (IQR 38-56). Rituximab response occurred in 31 (70%) patients. The total improvement score was higher in responders than non-responders (median 50·0 [IQR 45·0-55·0] vs 17·5 [10·0-30·0]; p<0·0001). At 2 weeks, memory B-cell counts and plasmablast counts were lower in responders than non-responders and complete plasmablast depletion was significantly more frequent in responders (29 [94%] of 31 vs four [31%] of 13; p<0·0001). In multivariable models, higher 2-week plasmablast counts were associated with lower odds of response (odds ratio 0·03 [95% CI 0·00-0·23]), whereas depletion of memory B cells (8·68 [1·13-82·80]) and plasmablasts (25·20 [4·61-214·54]) was associated with response. The conventional flow cytometry cutoff showed no association with response. INTERPRETATION: Complete early B-cell depletion, measured using an early HSFC-based strategy, but not conventional flow cytometry, was associated with rituximab response in patients with idiopathic inflammatory myopathies, supporting further evaluation of a peripheral blood B-cell minimal residual disease framework as a pharmacodynamic and prognostic strategy. These findings could help to select and optimise advanced B-cell depleting strategies in patients with refractory inflammatory myopathies. FUNDING: Leeds National Institute of Health Research Biomedical Research Centre.