Detection of anti-C1q antibodies and anti-C1q globular head domain antibodies in sera from Chinese patients with lupus nephritis.

Serum anti-C1q antibodies are associated with disease activity in patients with lupus nephritis. Recent studies showed that anti-C1q antibodies recognize both the collagen region and the globular head regions (gC1q) of individual A (ghA), B (ghB), and C (ghC) chains of C1q molecules. However, the clinical significance of anti-gC1q antibodies was not clear. This study is to investigate the frequency of antibodies against different parts of C1q molecule in sera from patients with lupus nephritis and their clinical significance. Sera from 83 patients with renal biopsy-proven lupus nephritis were collected at the day of renal biopsy. Sera from 30 patients with non-renal SLE (NR-SLE) and 100 healthy donors were used as controls, Serum anti-C1q antibodies, anti-collagen-like region (C1qCLR) antibodies, anti-globular head region (C1qGR) antibodies and anti-ghA, anti-ghB and anti-ghC antibodies were screened by ELISA using purified human C1q, C1qCLR, C1qGR and recombinant ghA, ghB and ghC as solid phase antigens. The association of these antibodies and clinical and histopathological features was further studied. The frequencies of positive anti-C1q and anti-C1qCLR antibodies in lupus nephritis group (46/83, 55.4%; 36/83, 43.4%, respectively) were significantly higher than that in patients with NR-SLE (4/30, 13.3%, P<0.001; 5/30, 16.7%, P=0.005, respectively) and healthy donors (5/100, 5.0%, P<0.001; 4/100, 4%, P<0.001, respectively). The levels of both SLE Disease Activity Index (SLEDAI) and renal biopsy Activity Index (AI) of patients were correlated with the levels of anti-C1q antibodies (r=0.520, P<0.001; r=0.321, P=0.003, respectively) and anti-C1qCLR antibodies (r=0.387, P<0.001; r=0.261, P=0.019, respectively). The levels of anti-C1q antibodies were closely correlated with that of anti-C1qCLR antibodies (r=0.588, P<0.001). However, the prevalence of anti-C1qGR, anti-ghA, anti-ghB and anti-ghC antibodies in lupus nephritis group was only 1.2% (1/83), 3.6% (3/83), 2.4% (2/83) and 8.4% (7/83), respectively, which were comparable with that of NR-SLE and normal controls. In conclusion, serum anti-C1q antibodies and anti-C1qCLR antibodies are closely associated with disease activity in patients with lupus nephritis. However, gC1q might not be the dominant epitope of C1q molecule.