Immune Profiling of Refractory cSLE Exposed to CD3×CD19 BiTE

Childhood-onset systemic lupus erythematosus (cSLE) is a severe autoimmune disease characterized by early disease onset, frequent multi-organ involvement, high disease activity, and a relapsing-remitting clinical course. Despite treatment with conventional immunosuppressive agents and biologic therapies, a subset of pediatric patients continues to experience refractory or persistently active disease. CD3×CD19 bispecific T-cell engagers (BiTEs) redirect endogenous T cells to eliminate CD19-positive B cells and have demonstrated potent immunologic effects in B-cell malignancies. In cSLE, CD3×CD19 BiTE therapy has been administered in a limited number of patients under a separate, approved exploratory clinical study, where preliminary clinical observations suggested immune changes accompanied by improvement in disease activity. However, the immunologic effects and immune dynamics associated with CD3×CD19 BiTE exposure in cSLE have not been systematically characterized, and longitudinal immune profiling data in this context remain scarce. The present study is designed as a prospective, observational, single-center cohort study. It does not assign, modify, or influence any therapeutic interventions. All participants receive CD3×CD19 BiTE therapy solely as part of an independent exploratory clinical protocol, and the current study focuses exclusively on immune profiling under this predefined treatment background. Peripheral blood samples are collected longitudinally at predefined time points before and after CD3×CD19 BiTE exposure, in parallel with routine clinical follow-up. Immune profiling is primarily performed using multiparameter flow cytometry to quantify the proportions, phenotypes, and activation states of major immune cell populations, including B-cell and T-cell subsets. In a subset of samples, single-cell sequencing is conducted to further explore cellular heterogeneity, transcriptional features, and potential immune pathways associated with observed immune changes. Clinical data are collected concurrently, including disease activity indices (such as SLEDAI), serological markers (e.g., complement levels and anti-double-stranded DNA antibodies), and organ-related clinical parameters reflecting multi-organ involvement. These clinical variables are integrated with immune profiling data to explore associations between immune features and changes in disease activity. The primary aim of this study is to describe the peripheral immune landscape and immune cell dynamics in patients with cSLE exposed to CD3×CD19 BiTE therapy. Secondary objectives include exploring immune features associated with changes in disease activity and identifying candidate immunologic biomarkers that may inform future immune-monitoring strategies or mechanistic studies. This study does not evaluate the efficacy or safety of CD3×CD19 BiTE therapy. By systematically integrating longitudinal immune profiling with clinical data, this observational study seeks to provide foundational immunologic insights into cSLE under CD3×CD19 BiTE exposure and to support future hypothesis-driven research in immune monitoring and personalized immunotherapy.