A sensitive solid phase microradioimmunoassay has been developed for measurement of antidouble stranded
DNA (dsDNA)
antibodies. In this procedure, advantage has been taken of the capacity of poly-
L-lysine (PLL) to facilitate the binding of pure dsDNA to
plastic surfaces. In the absence of PLL, binding did not occur. Diluted sera were incubated in PLL-treated dsDNA-coated microtitration trays and anti-dsDNA Ig measured using affinity purified 125I-anti-Ig of high specific activity. The synthetic
DNA,
poly dA-dT, was used as a model for dsDNA. In initial experiments, specific anti-
DNA binding could not be demonstrated because of high background binding of patient Ig to PLL-treated surfaces. This was reduced by diluting test sera and anti-Ig in
buffer containing 2% BGG and 1% BSA. Specificity of the assay for
DNA was demonstrated by absorbing the anti-
DNA activity on
DNA-coated
plastic. The binding of
systemic lupus erythematosus (SLE) patient serum Ig to
poly dA-dT coated trays did not diminish after digestion with nuclease S1, suggesting that the synthetic
polymer is an appropriate model for dsDNA. Patient and normal sera were screened for anti-dsDNA activity using
poly dA-dT as
antigen. None of the 38 normal sera, 23 of 35 active SLE sera, 1 of 25 treated SLE, 4 of 35
rheumatoid arthritis, 3 of 35 scleroderma, and 1 of 13
polymyositis sera demonstrated positive anti-dsDNA activity. The anti-dsDNA values obtained in the radioimmunoassay correlated significantly with those obtained in the Crithidia luciliae assay.