Lymphocyte subsets and disease activity in treated juvenile-onset systemic lupus erythematosus: An exploratory single-center cross-sectional analysis

Juvenile-onset systemic lupus erythematosus (jSLE) is a severe autoimmune disease with high disease activity and multi-organ involvement. The relationship between lymphocyte subsets and disease activity in treated pediatric patients remains unclear. This study aimed to characterize lymphocyte subset distributions in treated jSLE and to explore their cross-sectional association with concurrent disease activity. This retrospective, single-center, exploratory study included patients who met the 2019 European League Against Rheumatism/American College of Rheumatology (EULAR/ACR) criteria. Lymphocyte subsets were measured once at the last routine follow-up after treatment initiation; disease activity at the same visit was assessed using the Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI-2K). Associations were evaluated using Spearman correlation analysis with false discovery rate (FDR) adjustment. All eligible patients with complete data were included without an a priori sample size calculation. Fourteen patients (78.6% female) were analyzed. Median SLEDAI-2K decreased from 8.0 at diagnosis to 2.0 at the last visit (p < 0.001). Anti-double-stranded DNA (anti-dsDNA) decreased from 51.5 to 15.0 IU/mL (p < 0.001), erythrocyte sedimentation rate (ESR) from 42.0 to 14.5 mm/h (p < 0.001), and C-reactive protein (CRP) from 5.0 to 3.0 mg/L (p = 0.013). Absolute lymphocyte counts increased numerically from 1.7 to 2.2×109/L, but the difference was not statistically significant (p = 0.060; FDR q = 0.083). No robust FDR-adjusted correlations were found between the evaluated major lymphocyte subsets and concurrent SLEDAI-2K. CD20+ B-cell percentages were lower in patients exposed to intravenous immunoglobulin (IVIG), but the difference was not significant after FDR adjustment. In this small exploratory cohort of treated jSLE, single-time-point lymphocyte subset distributions were not robustly associated with concurrent disease activity. Immunophenotyping should be interpreted with clinical and laboratory parameters rather than as a standalone disease activity biomarker.