Complement fixation by small, DNase-resistant DNA-anti-DNA immune complexes.

125I-ds DNA-anti-DNA immune complexes (IC) formed at antibody excess and containing DNA of 300-350 base pairs (bp) fixed complement, incorporated C3b and bound to the C3b receptors (CR1) on human red blood cells (RBC). When the IC were treated with DNase to generate small, DNase-resistant IC, some of the IC incorporated C3b, but did not bind to RBC. In order to examine C3b incorporation and RBC binding by IC of specific sizes, the DNase treated IC were fractionated by sucrose density gradient (SDG) ultracentrifugation. Small IC containing one, two, three or four IgG molecules per fragment of 125I-ds DNA were identified by autoradiography after electrophoresis of the SDG fractions on 3-12% linear polyacrylamide gradient gels. The SDG fractions were tested for C3b incorporation and RBC binding ability. There was neither C3b incorporation nor RBC binding activity in fractions which corresponded to 9-11S (containing IC with one IgG/DNA). Fractions which corresponded to 12-22S (containing IC with up to four IgG/DNA fragment) demonstrated increased C3b incorporation with increased size, but did not show significant RBC binding activity. Fractions with IC containing four or more IgGs (22-24S) incorporated C3b and bound to RBC at approximately the same level. It is concluded that DNase digested IC which contain three-four IgG/DNA fragment are large enough to activate complement and incorporate C3b, but are too small to bind to RBC CR1. These IC could therefore escape rapid clearance from the circulation via the erythrocyte CR1 clearance mechanism. Such IC could persist in the circulation and potentially elicit pathogenic effects in patients with systemic lupus erythematosus.