Avidity of anti-native DNA antibody and glomerular immune complex localization in lupus nephritis

Ten patients with clinically active SLE and biopsy-proven renal involvement were studied. Antigen binding capacity (ABC) and avidity of anti-nDNA antibodies were measured, using 125I-nDNA, by the ammonium sulfate test of Minden and Farr. The glomerular localization of immune complexes was determined by morphological studies. Mean avidity in 7 patients with mesangial and subendothelial deposits was 0.387 +/- 0.022 (mean +/- SE), and in those with additional subepithelial deposits was 0.247 +/- 0.036 (P less than 0.01). When measured serially, the avidity was found to alter slowly either during treatment or spontaneously. Follow-up histology in 2 cases showed morphologic transitions; in one the transition from the initial mesangial proliferative type to the membranous form was associated with a shift of the avidity from a high level to a low one. There was no significant relation between the avidity and ABC (r = 0.235, P greater than 0.1), suggesting that avidity and the quantitative ABC of antibodies in immune complexes may not necessarily behave in parallel. Thus, the avidity measurement is useful in understanding the immunological events which underlie various clinicopathological features of SLE.