Native - Human Plasma
Autoantibodies directed to the C1q component of the C1 complex of complement activation were first described in the serum of patients with systemic lupus erythematosus (SLE). The presence of anti-C1q antibodies in SLE has been shown to correlate with a greater incidence of nephritis and the absence of anti-C1q is indicative that renal flares are unlikely to occur. Anti-C1q are also present in a range of other clinical conditions such as hypocomplementemic urticarial vasculitis syndrome (HUVS), autoimmune thyroid disease and mixed connective tissue disease, anti-glomerular basement membrane nephritis, hepatitis C and HIV infection. Anti-C1q antibodies also occur in the healthy population with an incidence of 2-8% that increases with age.
C1q associates with the proenzymes C1r and C1s to yield C1, the first component of the serum complement system. The collagen-like regions of C1q interact with the Ca2+-dependent C1r2C1s2 proenzyme complex, and efficient activation of C1 takes place on interaction of the globular heads of C1q with the Fc regions of IgG or IgM antibody present in immune complexes.
C1q is made up of six copies of 3 glycosylated polypeptides: the A-subunit (223 amino acids), the B-subunit (226 aminos acids) and the C-subunit (217 amino acids). Each subunit has a short cysteine-containing N-terminal region, followed by a collagen-like region of approximately 81 amino acids and then a globular domain of approximately 135 amino acids at the C-terminal. These C-terminal domains form a heterotrimeric globular head domain; A to B and C to C interchain disulphide bonds yield a six-subunit, two globular head structural unit. Three of these structural units associate to yield the complete hexameric C1q molecule that has a tulip-like structure.
Autoantibodies to C1q are believed to be directed against a neo-epitope on the collagen-like region that is only exposed upon binding of C1q to immunoassay plates. Anti-C1q binding to linear epitopes has also been reported, as well as to conformation-dependent epitopes and to cryptic epitopes that only become exposed after fragmentation of C1q. The use of purified C1q for the detection of autoantibodies in human serum has been described elsewhere.
Systemic Lupus Erythematosus
Hypocomplementemic Urticarial Vasculitis Syndrome (HUVS)
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