Clinical Information

Anti-complement 1Q (C1Q) antibodies have been found to be prevalent in hypocomplementemic urticarial vasculitis syndrome (HUVS) (also referred to as anti-C1Q vasculitis) as well as in some patients with systemic lupus erythematosus (SLE).(1,2) These antibodies bind to the collagenous region of C1Q and activate the classic pathway of the complement system.(1,2) This is reflected in the serum by decreased circulating levels of classical pathway components C1Q and C4 as well as C3 seen in these diseases.(3,4) Therefore, testing for serum complement C3 and C4 biomarkers is important in the interpretation of anti-C1Q antibody results.(1-3)

Hypocomplementemic urticarial vasculitis (HUV) is a rare immune complex-mediated cutaneous vasculitis of small vessels characterized by recurrent episodes of wheal-like lesions and the presence of anti-C1Q antibodies.(3). In a French nationwide study, patients with HUV typically presented with low C1Q levels and normal C1 inhibitor levels, in association with anti-C1Q antibodies in 55% of cases.(5) As per the 2012 Revised International Chapel Hill Consensus Conference Nomenclature of Vasculitide, low complement levels and the presence of anti-C1Q antibodies distinguishes HUV from normocomplementemic UV (NUV), another form of UV.(6) Compared to NUV, HUV is associated with more severe disease and can indicate the presence of an underlying systemic disease such as SLE or HUVS.(7,8) In addition, HUVS is characterized by urticaria with hypocomplementemia, arthralgia/arthritis, glomerulonephritis, recurrent abdominal pain, and obstructive lung disease.

With respect to SLE, anti-C1Q antibodies together with anti-dsDNA antibodies, complement C3 and C4 may offer useful additional information to monitor lupus nephritis (LN) activity as well as patient’s overall disease activity status.(5,9,10) In a recent study, baseline levels of anti-C1Q and anti-dsDNA was reported to predict proliferative LN.(10) In this study, anti-C1Q antibodies was also a useful predictor of complete response at the time of kidney biopsy. Overall, the authors of the study concluded that tracking anti-C1Q autoantibodies over time may provide further insights into treatment response and pathogenic mechanisms in proliferative LN patients.

While the presence of anti-C1Q antibodies is considered useful in the evaluation and management of HUV and SLE, these antibodies may also be seen in several other autoimmune and infectious diseases. Therefore, all positive results must be interpreted in the context of patient’s clinical history and presentation.