Effect of fibronectin on the Crithidia luciliae test for anti-double-stranded DNA antibodies

The various tests for anti-double-stranded DNA antibodies do not always agree. Plasma fibronectin specifically binds DNA, is a component of immune complexes, and shows variations in concentration with disease activity in systemic lupus erythematosus. It may therefore interfere with the detection of DNA autoantibodies. This possibility was examined in a series of studies using the Crithidia luciliae test. Studies were based on serum samples received during one year (250 samples). Serum samples from 50 patients which were positive or weakly positive in the C luciliae test were used. In blocking experiments fibronectin was added either to the wells or to the serum. In a second series of experiments fibronectin was depleted by affinity chromatography from six serum samples with weak anti-DNA staining. Preincubation of wells with fibronectin or addition of fibronectin to serum invariably blocked the interaction of anti-DNA antibodies with the C luciliae kinetoplast. When fibronectin was removed from serum the intensity of staining was increased. These results indicate that fibronectin influences the detection of anti-double-stranded DNA antibodies using C luciliae and may explain the disparity between the results of different tests for DNA antibodies. Furthermore, the unmasking of positive reactivity when fibronectin is removed from serum has implications for the diagnosis and treatment of systemic lupus erythematosus.