Histone-containing immune complexes are to a large extent responsible for anti-dsDNA reactivity in the Farr assay of active SLE patients

Increased titres of anti-dsDNA antibodies, especially if of high avidity, a re associated with renal exacerbations in patients with systemic lupus eryt hematosus (SLE). One of the most reliable assays to measure anti-dsDNA anti bodies, the Farr assay, is believed to detect preferentially high avidity a ntibodies. Purified non-complexed monoclonal antibodies (mAbs) against nucl eosomes, obtained from mice with SLE, are not reactive in the Farr assay, b ut can become so once complexed to nucleosomes, These Farr-positive, nucleo some containing, immune complexes were also able to bind in vivo to the glo merular basement membrane (GBM), predominantly via heparan sulphate (HS). T o evaluate whether in SLE patients the same kind of immune complexes are re sponsible for Parr reactivity, IgG from serum or plasma was isolated under dissociating and physiological conditions. We observed that after purificat ion under dissociating conditions, Farr reactivity was significantly decrea sed (P<0.0001) in contrast to reactivity with histones and two 'control' an tigens: Epstein Barr Virus (EBV) and Ro/SS-A. Reactivity with nucleosomes a lso decreased after purification, although to a lesser extent. Plasma purif ied under physiological conditions showed no decrease in Farr reactivity. T he importance of histones for the generation of immune complexes is support ed by the two following observations. Firstly, the presence of histones cou ld be demonstrated in serum and plasma of SLE patients but not in serum of healthy controls or in IgG preparations purified under dissociating conditi ons. Secondly, Farr reactivity of purified IgG preparations could be restor ed by addition of purified histones. From these studies we conclude that hi stones containing immune complexes are responsible for a large part of the Farr reactivity in active SLE, and are therefore indirectly implicated in t he pathogenesis of lupus nephritis. (C) 2000 Academic Press.