CLINICAL AND SEROLOGICAL CORRELATES OF SERUM C1Q AND ANTI-C1Q ANTIBODIES IN SOUTH AFRICANS WITH SYSTEMIC LUPUS ERYTHEMATOSUS

OBJECTIVE: To investigate the prevalence and clinical correlates of serum C1q and anti-C1q antibody titres in black South Africans with systemic lupus erythematosus (SLE). METHODS: Cross-sectional study of 96 black South African SLE patients, 49 with lupus nephritis (LN). Anti-C1q antibodies were tested using an enzyme linked immunoassay. Serum C1q was measured as a percentage of normal by immunoelectrophoresis, using an in-house rabbit-anti-C1q antiserum. Disease activity was assessed using SELENA-SLE disease activity index. RESULTS: Most patients were female (90.7%), mean (SD) age and follow-up period diagnosis of 38.1(13.0) and 4.2 (4.4) years, respectively. Low serum C1q and positive anti-C1q antibody test were detected in 17 (17.7%) and 12 (12.5%) patients, respectively, overall. There was inverse correlation between serum C1q and anti-C1q antibodies titres (r=-0.22, p=0.03) and a direct correlation of anti-C1q antibodies titres with SELENA-SLEDAI scores (r=0.27, p=0.008). Patients with an active urine sediment (n=21) had higher anti-C1q antibodies titres (p=0.007) and low serum C1q (OR=4.51, p=0.01), compared to the remainder of patients. Anti-C1q antibody titres were higher in patients with C3/C4 hypocomplementaemia (n=14) than those with normal C3/C4 (p=0.02). A positive Coombs test (without evidence of red cell haemolysis) (n=17) was associated with a positive anti-C1q antibody test (OR=4.29, p=0.02), low serum C1q (OR=3.37, p=0.04), and C3/C4 hypocomplementaemia (OR=4.84, p=0.02). CONCLUSION: Our findings broadly confirm the clinical utility of anti-C1q antibody test in SLE, particularly as a biomarker of active LN, as evidenced by an active urine sediment. The association and clinical relevance of a Direct Coombs test with elevated anti-C1q antibodies and low C1q merits further study and confirmation.