Complement-induced degranulation of human basophils

The events associated with complement-induced degranulation of human basophils were studied ultrastructurally. Partially purified cells were examined before and after the addition of complement (C5 peptide(s)) or control buffer for 15 sec to 5 min. 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. Complement-induced degranulation of human basophils was characterized primarily by extrusion of single granules through multiple cell-membrane openings. Demonstration of these openings was greatly facilitated with the use of cationized ferritin as a post-fixation probe. Extruded granules remained attached to plasma membranes and were not immediately solubilized when in continuity with the extracellular space. Cationized ferritin filled narrow membrane openings to cytoplasmic granules as well as granule-sized empty cytoplasmic vacuoles. Morphologic evidence of degranulation progressed with time of exposure to C5 peptide(s), the degranulation exhibiting kinetics that paralleled histamine release. Many basophils released only a small number of their granules, retaining many granules in the cytoplasm. A few basophils were nearly completely degranulated, and some basophils exposed to C5 peptide(s) failed to degranulate. Cells in control incubations did not degranulate. The morphology of degranulation induced by complement is qualitatively similar to that observed for IgE-mediated anaphylactic degranulation of human basophils. However, the kinetics and completeness of degranulation for both models, as assessed ultrastructurally, paralleled the kinetics of histamine release, which was different for each model.