The role of negative charge in the complement-dependent dissociation of IgG-mediated aggregation

The IgG-mediated aggregation of yeast particles was used as a model to study the dissociating effect of human serum complement on immune aggregates. A 92% decrease in non-aggregated particles was obtained by coating particles with IgG. After incubation of the aggregates in normal human serum (NHS) at 37 degrees C the number of free particles increased almost 10 times. The effect could be obtained with ethyleneglycol-bis-(beta-aminoethylether)-N,N'-tetraacetic acid-containing serum but not ethylenediaminetetraacetic acid-containing serum, indicating the importance of the alternate pathway of complement activation. Treatment with NHS did not release anti-yeast IgG from the particles, indicating that disruption of antigen-antibody bonds did not explain the phenomenon. The negative charge of the different particles was studied by measuring the interaction to diethylaminoethyl-Sephacel beads. Compared with non-coated and IgG-coated particles, the NHS-treated particles were not released until the ionic strength increased to 200 mM NaCl or the pH decreased to 4.4, indicating a strong negative charge of NHS-treated particles. We propose that complement increases the negative charge of immune complexes, thereby inducing repulsive forces that counteract Fc-Fc interactions and increase the solubility of the complexes.