Clearance kinetics of soluble pre-formed immune complexes containing IgM antibodies in normal and decomplemented rabbits.

The clearance kinetics of homologous soluble pre-formed 125I-BSA/IgM anti-BSA immune complexes (IgM . IC) injected into the blood was studied in the rabbit. IgM . IC close to equivalence were cleared with a greater velocity than those in x 5 or x 10 antigen excess. Clearance of BSA specifically bound to antibody, was measured by the Farr assay and found to exhibit an exponential mode of elimination suggesting a selective removal of large over small lattice complexes or free antigen. The role of C3 in the clearance of IgM . IC was examined by treating rabbits with cobra venom factor (CVF). While more variation was noted in CVF-treated rabbits, no statistically significant differences could be detected between C3-depleted and control animals. It is, therefore, concluded that soluble preformed IgM . IC are cleared from the rabbit circulation by a C3b-independent mechanism. This finding for soluble IC is clearly different from the clearance of IgM-coated red cells from the circulation, which has previously shown to be highly C3b-dependent.