Antigen-induced IgE-mediated degranulation of human basophils

The events associated with antigen-induced, IgE-mediated degranulation of human basophils from allergic donors were studied ultrastructurally. Partially purified cells were examined prior to addition of antigen and after incubation with antigen or control buffer for 15 seconds to 60 minutes. Reactions were stopped instantaneously by adding fixative directly to the cell suspension. After fixation the cells were exposed to cationized ferritin as a sensitive probe for demonstrating possible continuities between the cytoplasmic granules and the cell surface. Ficoll-Hypaque-isolated basophils were of three types: Type I, cells containing basophil granules with a full complement of particles; type II, cells containing some full granules but also variable numbers of cytoplasmic vacuolar structures having the size and shape of basophilic granules but having reduced or no particle content (partially fixed or empty granules); and type III, basophils containing only empty granules. Following exposure to specific antigen, basophils of all three types underwent degranulation characterized by the fusion of the membranes bounding single basophilic granules with the plasma membrane and leading to extrusion of content. Cells in the process of degranulation (type IV and V basophils) were characterized by communications between individual granules and the cell exterior. Identification of such communications was facilitated by cationized ferritin which entered granules having open communications with the cell surface. Without this marker, the number of such communications would have been seriously underestimated, either because they were extremely narrow, tortuous, or outside the plane of section. The majority of individual basophilic granules fused singly and separately with the plasma membrane, in contrast to guinea pig basophil and rat mast cell degranulation where intercommunicating clusters of granules fused with the plasma membrane at a single point. The particle and membrane contents of extruded granules frequently remained adherent to the surface of type IV and V basophils and were not immediately solubilized. Morphologic evidence of degranulation progressed with time of exposure to antigen, exhibiting kinetics that paralleled histamine release. Cells in control incubations, and rare basophils that had been exposed to antigen E, failed to degranulate. Fully degranulated (type VI) basophils were viable cells that had a markedly irregular surface and were devoid of basophilic granules but retained the minor population of small perinuclear granules.