Autoantibodies against complement C1q (anti-C1q) are an excellent marker for active nephritis in SLE patients. Here, we describe a typical protocol for the quantification of anti-C1q using immobilized C1q (important for the presentation of relevant cryptic epitopes) and a high salt buffer for the incubation steps (to prevent immune-complex binding to intact C1q). More recently, a linear epitope on the C1q A chain, that is targeted by anti-C1q, has been described (A08). The assay using this peptide seems to be more specific and more sensitive for the detection of active nephritis in SLE patients than the conventional anti-C1q assay, but further studies are required to establish the role of anti-A08 of C1q in the clinical routine.
Keywords: Anti-C1q antibodies; C1q; Complement; Lupus nephritis; SLE; Systemic lupus erythematosus.