Localization of granule proteins in human eosinophil bone marrow progenitors

Eosinophils have a characteristic content of cationic proteins, stored in core-containing specific granules and released at sites of inflammation; coreless granules (sometimes called primary) are present in eosinophil promyelocytes. In order to determine a possible relationship between the two granule subsets, immunoelectron-microscopic techniques were used to determine the presence and precise intragranular distribution of major basic protein (MBP), eosinophil cationic protein (ECP), eosinophil peroxidase (EPO), and arylsulfatase B of eosinophil granules, as well as the Charcot-Leyden crystal (CLC) protein, in eosinophil progenitors of the bone marrow. MBP, ECP, EPO, and arylsulfatase B were observed in both coreless and core-containing (specific) granules. The difference in the distribution of MBP, having a uniform distribution in coreless granules and a crystalloid distribution in core-containing (specific) granules, could indicate a maturational process of a common organelle. CLC protein was distributed in the cytosol, in the euchromatin of the nuclei, but was also present in a rare granular compartment of both immature and mature eosinophils. The present findings suggest that coreless granules develop into core-containing specific granules.