Eosinophil viability during immunoglobulin-induced degranulation

Eosinophil adhesion and degranulation appear to be associated with cell death. Eosinophils bound avidly and degranulated with secretory immunoglobulin A (sIgA)- and IgG-coupled Sepharose 4B beads but bound poorly and did not degranulate with ovalburin beads. Through the use of dye staining, we found that about 50% of the bound eosinophils were dead by 4 h, regardless of the protein coating. Colchicine and reduced calcium concentration inhibited binding to beads and eosinophil degranulation in a concentration-dependent manner but did not decrease the percentages of dead bound eosinophils. Electron microscopy showed that eosinophils bound to and spread over bead surfaces. Typical granule exocytosis with release of membrane-free granules occurred in about 20% of bound eosinophils. Eosinophil degeneration and lysis with release of membrane-coated granules occurred in about 50% of bound eosinophils; often only membrane-bound granules were present. Therefore, bound eosinophils degranulate both by classical exocytosis and by release after cytolytic degeneration. By increasing the numbers of bound cells, both IgG and sIgA increase the numbers of dying cells.