Compound exocytosis of granules in human neutrophils

Abstract: Human neutrophils are of prime impor-tance for the immune defense. Recent data from eosinophils and pancreatic beta cells have indi-cated that granules, upon exocytosis, occasionally fuse with each other in the cytosol prior to their subsequent fusion with the plasma membrane. This is termed compound exocytosis. We therefore studied exocytosis of single granules from human neutrophils by the high-resolution cell-attached patch-clamp capacitance technique. We found that 1.5 % of the capacitance steps was greater than 5 fF, i.e., significantly larger than steps expected for exocytosis of single granules. The mean step size of these events was 20.5 fF, corresponding to com-pounds formed by at least five granules. The ca-pacitance input from compound steps contributed more than 20 % of the total capacitance increase. Electron microscopy captured morphological mani-festations of transient exocytic events, confirming the functional results obtained by capacitance mea-surements. Compound exocytosis may be a mech-anism for efficient targeting of release during exo-cytosis. J. Leukoc. Biol. 71: 973–980; 2002