Anti-C1q antibodies and antiendothelial cell antibodies in systemic lupus erythematosus -: relationship with disease activity and renal involvement

The aim of this study was to investigate the relationship between the presence and titre of antibodies against Clq (anti-Clq Ab) and disease activity and renal involvement in patients with systemic lupus erythematosus (SLE). Anti-Clq Ab were measured in 79 patients with SLE (70 women and 9 men; mean age 41.7 years; mean disease duration 8.4 years): 19 patients had active disease with lupus nephritis, 8 active disease without nephritis, 26 inactive disease with nephritis and 26 inactive disease without nephritis. Anti-dsDNA antibodies (EIA and immunofluorescence), antiendothelial cell antibodies (AECA) and complement levels (C3, C4, total haemolytic complement activity) were determined in parallel. Anti-Clq Ab were positive in 49%, anti-dsDNA Ab in 61% and AECA in 19% of the patients, respectively. Significantly higher titres of anti-Clq Ab were found in patients with active disease compared with those with inactive SLE (P < 0.01). Serum levels of anti-Clq Ab showed a positive correlation with anti-dsDNA Ab and SLEDAI score (P < 0.01) and a negative correlation with C3 (P < 0.05), C4 (P < 0.01) and CH50U (P < 0.01). The presence of anti-Clq Ab was not different between patients with or without nephritis. In patients with (P < 0.05) and without nephritis (P < 0.01) the frequency of anti-Clq Ab was significantly higher in active patients compared with inactive patients. Both anti-Clq and anti-ds-DNA Ab were detectable in 74% of patients with active nephritis but only in 30% of all other patients (P = 0.001). None of the patients with active nephritis was negative for anti-Clq and anti-dsDNA Ab, whereas 37% of the patients without active nephritis were negative for both antibodies (P < 0.01). Sensitivity, specificity, positive and negative predictive values for active lupus nephritis among SLE patients were 100%, 50%, 51.9% and 100% for anti-dsDNA Ab (EIA) and 74%, 70%, 57% and 89.4% for positive findings of both anti-dsDNA and anti-Clq Ab. The presence and titre of anti-Clq-Ab in SLE are related to disease activity. Absence of anti-dsDNA Ab excludes active nephritis; positive findings of both anti-dsDNA Ab and anti-Clq Ab are of relatively high specificity for active nephritis.