Binding of monoclonal anti-native DNA autoantibodies to DNA of varying size and conformation.

A microchemical assay for phosphorus was applied to the measurement of DNA in immune complexes formed with monoclonal or serum anti-DNA autoantibodies and DNA of varying size and conformation. Two monoclonal antibodies were produced by hybridomas derived from spleen cells of autoimmune MRL-lpr/lpr mice and were purified from culture fluid by affinity chromatography on columns of goat anti-mouse Ig-Sepharose. Double-helical DNA fragments were prepared by brief digestion of calf thymus DNA with micrococcal and S1 nucleases and fractionation on Sepharose 4B; their double-stranded structures was confirmed by measurement of thermal denaturation. Immune complexes were formed with monoclonal or serum antibodies and native DNA or DNA fragments or denatured DNA; the complexes were precipitated with goat anti-mouse IgG and washed, and DNA phosphorus content of the precipitates was measured. With one monoclonal autoantibody (H241), there were discontinuous increases in the amount of DNA that could be bound (and decreases in the antigen concn required for half-maximal binding) as the DNA size increased. There were especially marked increases in binding efficiency as fragment size increased from an average of 100 (range 85-105) to an average of 150 (range 105-170) base pairs, and again between 450 (range 360-620) and 600 (range 425-825) base pairs. A second monoclonal antibody (H143) did not show significant variation in binding with DNA fragments larger than 300 base pairs. With smaller fragments, the amount of DNA bound by H143 was reduced, but the DNA concn required for half-maximal binding was not. Affinities of these monoclonal antibodies were within the spectrum of human systemic lupus erythematosus serum IgG anti-DNA autoantibodies. The dependence of binding on mol. wt is important in the evaluation of these monoclonal antibodies as biochemical reagents and as potential participants in formation of immune complexes in vivo.