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. Although the underlying mechanisms are complex, defects in dying cells elimination are likely to contribute to autoantigen overload and development of autoimmunity. Molecules important in damaged cell clearance, such as early complement components, may thus have a protective role. According to this hypothesis, deficiencies in C1q and MBL, the recognition proteins of the classical and lectin pathways of complement; are associated with increased susceptibility to SLE. In the proposed project, the investigators will investigate the involvement of another related recognition protein, ficolin-3, which activates the complement lectin pathway and recognizes necrotic cells. The investigators have shown in a recent study a significant association between the presence of anti-ficolin-3 antibodies and active nephritis in patients with SLE. However, the possible involvement of anti-ficolin-3 antibodies in the pathogenesis of SLE and particularly in lupus nephritis (LN) remains to be elucidated. This project plans to investigate the role of ficolin-3 and ficolin-3 autoantibodies in LN. The study associates two aspects, aiming at deciphering the role of anti-ficolin-3 antibodies in dying cells recognition and investigating the role of ficolin-3 in renal tissue damage. This pilot study will be performed for 14 patients with active LN on serum and renal biopsy, realized for routine patient care. The investigators will explore the effect of anti-ficolin-3 antibodies purified from the patient serum on ficolin-3-dependent necrotic cells recognition, in relation with possible altered clearance of dead cells, which is an important hypothesis of the pathogenesis of SLE. The investigators will also investigate ficolin-3 deposition in renal biopsy, which may contribute to the local formation of immune complexes, leading to complement activation and subsequent inflammation and tissue injury.

PRIMARY OUTCOME MEASURE Exploration of the inhibition of anti-ficolin-3 antibodies purified from the serum of 14 patients with active lupus nephritis in ficolin-3-dependent necrotic cells recognition.

The criterion used is the shift of MFI (Mean Fluorescence Intensity) measured after addition of these antibodies to necrotic Jurkat cells incubated with ficolin-3.

The study has a single visit approach with serum collection, so every outcome is measured at T0, which is the only visit for the patient.

SECONDARY OUTCOME MEASURES

1. Investigation of ficolin-3 deposition in renal biopsy of the same 14 patients with active LN.

   Analysis: deposition of ficolin-3 will be evaluated by immunostaining on renal biopsy.

2. Quantification of anti-ficolin-3 antibodies. Analysis: Anti-ficolin-3 antibodies are quantified by ELISA.

3. Quantification of serum levels of ficolin-3. Analysis: Ficolin-3 is quantified by ELISA.

4. Correlation between anti-ficolin-3 antibodies and serum levels of ficolin-3. Analysis: Anti-ficolin-3 antibodies and ficolin-3 are quantified by ELISA.

5. Correlation between serum levels of anti-ficolin-3 antibodies and ficolin-3 deposition in the kidney.

6. Correlation between serum levels of ficolin-3 and ficolin-3 deposition in the kidney.

7. Exploration of the inhibition of anti-ficolin-2 antibodies purified from the serum of 14 patients with active lupus nephritis in ficolin-2-dependent necrotic cells recognition.

   The criterion used is the shift of MFI (Mean Fluorescence Intensity) measured after addition of these antibodies to necrotic Jurkat cells incubated with ficolin-2.

8. Investigation of ficolin-2 deposition in renal biopsy of the same 14 patients with active LN.

   Analysis: deposition of ficolin-2 will be evaluated by immunostaining on renal biopsy.

9. Quantification of anti-ficolin-2 antibodies. Analysis: Anti-ficolin-2 antibodies are quantified by ELISA.

10. Quantification of serum levels of ficolin-2. Analysis: Ficolin-2 is quantified by ELISA.

11. Correlation between anti-ficolin-2 antibodies and serum levels of ficolin-2. Analysis: Anti-ficolin-2 antibodies and ficolin-2 are quantified by ELISA.

12. Correlation between serum levels of anti-ficolin-2 antibodies and ficolin-2 deposition in the kidney.

13. Correlation between serum levels of ficolin-2 and ficolin-2 deposition in the kidney.

Inclusion Criteria:

• Age ≥ 18 years old
• Patients who have valid health insurance
• Non-opposition to participation obtained
• Diagnostic de lupus according to SLICC 2012, performed more than 3 months ago.
• Active lupus nephritis defined by :

elevated SLEDAI indexes (≥ 4), the presence of a significant proteinuria (≥ 0.5 g/day) and/or the presence of hematuria, aseptic leukocyturia or urinary casts, and documented by renal biopsy and classified according to the ISN/RPS classification.

Non-inclusion Criteria:

• Patient with a known progressing cancer
• Patient who had started lupus nephritis flare treatment
• Participant involved in another interventional clinical study
• Person deprived of liberty by judicial order
• Person under guardianship or curatorship
• Hemoglobin level < 7 g/dL

SLE patients with lupus Nephritis