Mechanism of binding of multivalent immune complexes to Fc receptors. 2. Kinetics of binding.

The kinetics of association and dissociation of affinity cross-linked IgG oligomers with Fc receptors on P388D1 cells have been studied at 0 degrees C. For dimers prepared with two different affinity cross-linking reagents (DIBADL and BDPE) and also trimers and a heavy oligomer fraction prepared with BDPE, monomeric IgG accelerates the rate of dissociation of bound oligomer from the cells. This observation is consistent with a binding mechanism in which monovalently, and multivalently bound species are in rapid equilibrium with one another on the cell surface and in which the rate of formation of the divalently bound species is faster than the dissociation of the monovalent species from the cells. As predicted, the rates of dissociation of BDPE cross-linked oligomers decrease with size, and dimers (cross-linked with DIBADL) which can self-aggregate on the cell surface dissociate more slowly than nonaggregating (BDPE) dimers. The association and dissociation of oligomers with P388D1 cells proceed by parallel fast and slow processes, even with cells that bind dimers with a single affinity. The origin of this heterogeneity is not known but could arise from a fraction of receptors in environments with limited accessibility to bulk solution. The fast component of the association reaction is second order, and the rate-limiting step of this process is the formation of the monovalently bound intermediate from solution-phase oligomer.