Anti-C1s autoantibodies as complementary serologic biomarker in lupus nephritis

Systemic lupus erythematosus (SLE) is an autoimmune disease characterised by the presence of circulating autoantibodies. Autoantibodies targeting C1 complex proteins, particularly C1q, have already been described in lupus nephritis (LN). However, autoantibodies targeting the C1s protease remain poorly studied. We determined the prevalence of anti-C1s autoantibodies in serum of SLE patients, and evaluated their presence in relation to clinical conditions. For this purpose, sera from 187 SLE patients with different disease activity were selected and anti-C1s autoantibodies were measured by ELISA. We observed that patients with LN had significantly higher levels of anti-C1s autoantibodies than SLE patients with other flare types. Anti-C1s autoantibodies recognised mainly the C1s N-terminal part. Interestingly, the combination of anti-C1s, anti-DNA and anti-C1q autoantibodies showed high specificity (94.6 %) and a significant positive predictive value of 80 %. These results suggest the potential interest of anti-C1s autoantibodies as a complementary serological biomarker in the early screening for LN.