Anti-C1q autoantibodies in lupus nephritis: prevalence and clinical significance.

Recently, anti-C1q autoantibodies have been proposed as a useful marker in systemic lupus erythematosus (SLE) since their occurrence correlates with renal involvement and, possibly, with nephritic activity. We aimed to evaluate the prevalence of anti-C1q antibodies in patients with SLE, with and without renal involvement, and to correlate these markers' presence and levels with the activity of the disease and nephropathy. We studied 61 patients with SLE, 40 of whom had biopsy-proven lupus nephritis; 35 patients with other connective tissue diseases; and 54 healthy controls. In addition, 18 lupus nephritis patients were followed up during the disease time course. Anti-C1q antibodies were measured using "homemade" ELISA with high salt concentration (1 M sodium chloride). High anti-C1q antibody titers (> 55 AU) were present in 27 of 61 (44%) SLE patients and in 4% and 0% of normal blood donors and pathologic controls, respectively. Anti-C1q antibodies were found in 60% of patients with lupus nephritis compared with only 14% of SLE patients without nephropathy (P < 0.05). Moreover, patients who were positive for anti-C1q antibodies had a higher European Consensus Lupus Activity Measurement (ECLAM) score (4.35 vs. 2.2); 89% of patients with active lupus nephritis showed high titers of anti-C1q antibodies compared with 0% of patients with inactive nephritis. Anti-C1q and anti-dsDNA antibodies agreed in 79% of cases. Our results confirm that anti-C1q antibodies are present in a significant percentage of SLE patients, and that their presence and levels correlate with disease activity-in particular, during renal flare-ups.