Binding properties and histamine release in variants of rat basophilic leukemia cells with changes in the IgE receptor

Variants of the rat basophilic leukemia cell line were selected that had a decreased number of high affinity IgE receptors (Fc ϵR). Cloned lines with fewer than 10,000 Fc ϵR on their surface could release histamine following IgE-mediated stimulation. These variant lines were further characterized by binding studies with 125I-labeled IgE or a series of monoclonal antibodies (mAb) that inhibit IgE binding. Three of these mAb (mAb BC4, mAb CA5 and mAb CD3) bind competitively with IgE. A fourth mAb (AA4) inhibits IgE binding but its binding is not inhibited by IgE. In all the variant cloned lines, binding by the 3 mAb was highly correlated to IgE binding. Therefore, these epitopes are closely related. In contrast, there was poor correlation between mAb AA4 and IgE binding. However, even in these lines, mAb AA4 inhibited labeled IgE binding. Therefore, there is independent variation of the mAb AA4 epitope compared to the sites to which the other mAb and IgE bind.