Serum levels of free light chains and syndecan-1 in patients with rheumatoid arthritis and systemic lupus erythematosus

Objectives: Systemic autoimmune rheumatic diseases (SARDs) are characterized by chronic inflammation. Reliable biomarkers are crucial for diagnosis, monitoring disease progression and therapeutic responses. This study explores serum Syndecan-1 (SDC-1) as a biomarker for these conditions and its relationship with free light chain (FLC) levels. Methods: A retrospective analysis was performed on sera from 60 patients with rheumatoid arthritis (RA) and from 60 with systemic lupus erythematosus (SLE), alongside 50 healthy donors (HD). Κ- and λ- FLCs were determined by turbidimetric assay, while SDC-1 levels by ELISA. Kruskal-Wallis test, Wilcoxon-Mann-Whitney U test, multivariable linear regression and Spearman's correlation were employed to compare biomarker levels across groups and to explore correlations. Results: SDC-1, κ-FLC, and λ-FLC were significantly increased in RA and SLE patients compared with HD (p< 0.001), while no significant differences in the κ/λ ratio were observed among the groups (p= 0.4). A significant difference in subject age was also identified. However, multivariate regression analysis indicated that RA and SLE are significantly associated with the levels of these markers, with minimal confounding by age. A significant correlation was observed separately in all groups between the FLC markers. Conversely, no correlation was detected between SDC-1 and FLCs, nor between these markers and age or disease activity indices. Conclusion: Elevated serum levels of FLCs and SDC-1 in RA and SLE patients compared with HD underscore their potential as biomarkers for SARDs. The findings also suggest sustained plasma cell activation, supporting the multifaceted role of SDC-1 in the pathogenesis of SARDs.