Systemic lupus erythematosus (SLE) is a chronic autoimmune disease characterized by the production of multiple autoantibodies and accumulation of immune complexes resulting in systemic inflammatory response and tissue damage. Dysfunction of proteins initially known to initiate the classical pathway for complement activation (C1q and C1s), and their functional interference with the multifunctional protein HMGB1 (High-Mobility Group Box 1), appears to be associated with SLE. On the other hand, C1s, HMGB1 and C1q can be targeted by anti-C1s, anti-HMGB1 and anti-C1q autoantibodies from lupus patients, whose functional impact remains to be explored, in particular for non-canonical functions, independent of the complement activation cascade. Studies are needed to investigate the pathogenic role of these autoantibodies in SLE, including possible interference with the inactivation of HMGB1. This project plans to investigate the role of anti-C1s, anti-HMGB1 and anti-C1q autoantibodies in the pathogenesis of Systemic Lupus Erythematosus. This pilot study will be performed for 30 patients with active SLE on serum, realized for routine patient care. The investigators will identify the molecular targets recognized by anti-C1s, anti-HMGB1 and anti-C1q autoantibodies purified from the SLE patients' serum. The investigators will also explore the functional role of these purified autoantibodies.