Binding characteristics of aggregated IgGa to rat basophilic leukaemia (RBL) cells and rat mast cells.

The binding properties of h. a. IgGa to the Fc-gamma receptor on rat basophilic leukaemia cells and rat mast cells were analysed. When h. a. IgGa was coupled to SRBC, rosette formation was observed with normal mast cells as well as with cells obtained from Nippostrongylus brasiliensis-infected rats. The binding of aggregates was detected by fluorescent analysis. With monomeric IgGa, no uptake was observed. Binding analysis was also performed with aggregates of different sedimentation coefficients varying from 10S to 130S. It was demonstrated that h. a. 125I-IgGa binds to rat mast cells as well as to RBL cells. The rate of binding is dependent on the number of Fc portions present within the aggregate. Pre-incubation of aggregates with protein A partially inhibited the subsequent binding. The binding of aggregates is more potent at 37 degrees, increases by five-fold at pH 6.4 as compared to pH 7.4 and is reduced by 75% at pH 8.4, while the binding of 125I-IgE did not differ at the various pH values. In the presence of Ca2+ ions, a 30-40% increase in the uptake of aggregates was noted. Our results show the presence of Fc-gamma-receptors on RBL-cells and rat mast cells by direct binding studies and suggest molecular differences of the Fc-gamma and Fc-epsilon receptors.