Clearance kinetics and organ uptake of complement-solubilized immune complexes in mice

C3-bearing immune complexes were prepared by in vitro solubilization of BSA-anti-BSA complexes at equivalence. Sucrose density gradient analyses showed a size-heterogeneous population of solubilized complexes with a range of 7S to greater than 29S and a peak at around 19S. The presence of C3bi was demonstrated by precipitation with antibodies to C3c and to C3d and by binding to conglutinin. Immune complexes solubilized in two and three times antigen excess were selected as controls due to their size similarities with complement-solubilized complexes. Blood clearance curves were very similar for C3-bearing complexes and controls. At 1 hr, the percentage of injected material remaining in the circulation for complement-solubilized and two and three times antigen excess complexes were 29.5 +/- 1.3, 30.9 +/- 1.7 and 26.1 +/- 2.7, respectively. Uptake by liver accounted for the majority of complement- and antigen-solubilized immune complexes removed from circulation. Although the uptake by the spleen was no more than one-tenth of the liver uptake, more complement-solubilized complexes than antigen-solubilized complexes were removed by this organ. The present data indicate that soluble immune complexes bearing C3 components and soluble immune complexes without C3 components, but of comparable size, are cleared from the circulation of mice at comparable rates. The mechanisms of clearance of these two populations of complexes, however, may differ.