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Background: Systemic lupus erythematosus (SLE) is an autoimmune disease characterized by autoantibody production and diverse tissue and organ inflammatory affections. Interleukin 21 (IL-21) is implicated in B cell survival, proliferation, differentiation, class switching, and immunoglobulin production; therefore, it is considered a key cytokine in the pathogenesis of SLE. However, its association with disease activity and clinical phenotypes remains unclear. We aimed to evaluate the association of IL-21 levels with the disease activity and clinical phenotypes in patients with SLE. Also, we analyzed the IL21 polymorphisms associated with increased IL-21 levels. Methods: The IL-21 serum levels were determined using the enzyme-linked immunosorbent assay (ELISA) method. The rs2221903 and rs2055979 polymorphisms were assessed in 300 healthy controls (HCs) and 300 patients with SLE by the polymerase chain reaction–restriction fragment length polymorphism (PCR-RFLP) technique. The levels of IL-21 were monitored during follow-up visits in 59 patients with SLE. Results: The patients with SLE showed higher IL-21 levels compared to the HCs. The IL-21 levels did not correlate with Mex-SLEDAI and were not different in patients with inactive, mild–moderate, and severe disease. The IL-21 levels were increased in patients with hematological affection. The ROC curve analysis revealed that the IL-21 levels had good predictive power in discriminating among patients with SLE and HCs. In a follow-up analysis, the levels of IL-21 remained higher in the patients with SLE even when the patients were in remission. Also, the rs2221903 polymorphism was associated with increased IL-21 levels. Conclusions: This study highlights the importance of IL-21 as a key cytokine in SLE. IL-21 levels are higher in patients with SLE and remain increased regardless of disease activity. According to the ROC analysis, IL-21 is a potential biomarker of SLE. Further longitudinal studies are needed to explore the relationship between IL-21 and the clinical phenotypes of SLE. [ABSTRACT FROM AUTHOR]

Background: The B-cell activating factor (BAFF) controls the maturation and survival of B cells. An imbalance in this cytokine has been associated with systemic autoimmunity in SLE and lupus nephritis (LN). However, few investigations have evaluated the tissular expression of BAFF in LN. This study aimed to associate BAFF system expression at the tissular level with the proliferative LN classes. Methods: The analysis included eighteen kidney tissues, with sixteen LN (class III = 5, class IV = 6, class III/IV+V = 4, and class V = 1), and two controls. The tissular expression was evaluated with an immunochemistry assay. A Cytation5 imaging reader and ImageJ software were used to analyze the quantitative expression. A p-value < 0.05 was considered significant. Results: The expressions of BAFF, A proliferation-inducing ligand (APRIL), and their receptors were observed in glomerular, tubular, and interstitial zones, with BAFF being the most strongly expressed in the overall analysis. BAFF-Receptor (BR3), transmembrane activator and CALM interactor (TACI), and B-Cell maturation antigen (BCMA) displayed higher expressions in LN class IV in all zones analyzed (p < 0.05). Additionally, a positive correlation was found between APRIL, TACI, and BCMA at the glomerular level; BCMA and APRIL in the interstitial zone; and BR3, TACI, and BCMA in the tubule (p < 0.05). Conclusions: The expression of BAFF and BAFF receptors is mainly associated with LN class IV, emphasizing the participation of these receptors as an essential pathogenic factor in kidney involvement in SLE patients. [ABSTRACT FROM AUTHOR]