Piecemeal degranulation of peripheral blood eosinophils: a study of allergic subjects during and out of the pollen season

The variability of serum and plasma levels of eosinophil granule proteins in different clinical conditions, interpreted as the result of different patterns of cytokine priming, suggests a selective mobilization of granule proteins. Inasmuch as piecemeal degranulation (PM) is the mechanism proposed for the differential release of eosinophil granule proteins, we decided to investigate whether blood eosinophils from allergic subjects show characteristics of PM during natural allergen challenge. Eosinophils from three birch-sensitive subjects were studied before and during the pollen season. Electron microscopy analysis showed that during the season, eosinophils presented morphologic features of PM. By immunogold labeling, eosinophil cationic protein (ECP) was detected not only in normal specific granules but also in the cytoplasm, in the vicinity of partially lucent specific granules. These results were confirmed by subcellular fractionation, where the amount of ECP associated with compartments containing small vesicles increased 2-fold during the pollen season. A study of the distribution of ECP, eosinophil peroxidase, and hexosaminidase in eosinophils of different densities showed that the profile of each of these proteins differed depending on cell density. All of these proteins decreased in the specific granule of hypodense cells and increased in other cell compartments. We conclude that allergen exposure causes PM of the peripheral blood eosinophils of allergic subjects, and that the density of these cells reflects the degree of degranulation. Our results provide novel information for the understanding of the selective mobilization of granule proteins into the circulation.