Measurement of Serum DNA-Binding Activity in Systemic Lupus Erythematosus

Antibodies to DNA were demonstrated in the serums of patients with systemic lupus erythematosus (SLE) by ammonium sulfate precipitation. DNA is soluble in 50 per cent saturated ammonium sulfate, whereas immunoglobulins and immunoglobulin-bound DNA are insoluble. When ammonium sulfate is added to a mixture of radioactive DNA and serum, the precipitate contains radioactivity if DNA is bound to immunoglobulins. Abnormal binding was found in serum of 75 per cent of unselected patients with SLE, 25 per cent with Sjögren's syndrome, 5 per cent with related disease and 2 per cent of normal subjects. All of 52 selected SLE serums with positive, as well as 21 of 32 SLE serums with negative, complement-fixation tests for anti-DNA antibodies had abnormal binding. Binding activity was associated with immunoglobulin G of serum. High binding values were seen chiefly in patients with active SLE renal disease; marked reductions accompanied clinical improvement. The test overcomes problems inherent in other methods to detect anti-DNA antibodies, is easy to perform and is useful in the diagnosis and assessment of the course of SLE.