Dissociation studies of DNA/anti-DNA complexes in relation to anti-DNA avidity

Antibodies to dsDNA differ in their avidity towards the antigen. The electrostatic interaction between DNA and anti-DNA is sensitive to increases in pH and/or ionic strength and therefore, elution studies employing either of these permit discrimination between anti-dsDNA populations that differ in avidity. Another way to determine anti-dsDNA avidity is the calculation of Farr/PEG ratios. These are obtained by division of the amount of anti-DNA measured with the Farr assay (which does not detect low avidity anti-dsDNA) by the amount measured with the PEG assay (which does detect low avidity anti-dsDNA). With these separate approaches, we compared the sera of 17 SLE patients with nephritis with the sera of 17 patients with central nervous system (CNS) involvement. Farr/PEG ratios and sensitivity to high pH elution of anti-dsDNA in the sera of these patients both permitted discrimination between the two groups of patients. The anti-dsDNA of patients with nephritis was found to have a significantly higher avidity towards DNA than anti-dsDNA of patients with cerebral disease. We also observed a significant correlation between Farr/PEG ratios and the salt lability of anti-dsDNA.