ULTRASTRUCTURAL-LOCALIZATION OF MAJOR BASIC-PROTEIN IN THE HUMAN EOSINOPHIL LINEAGE IN-VITRO

We examined the ultrastructural localization of(a) a secondary granule matrix protein - eosinophil peroxidase (EPO) by cytochemistry, (b) a secondary granule core protein (major basic protein, MBP) by immunogol d labeling, and (c) a primary granule protein (the Charcot-Leyden crys tal protein, CLC protein) by immunogold labeling in eosinophilic myelo cytes (EMs) and mature, activated eosinophils that differentiated from umbilical cord blood progenitors cultured in the presence of recombin ant human interleukin-5 (rhIL-5). These studies provide the first subs tructural localization of MBP to condensing cores of immature secondar y granules of EMs, as well as identification of unicompartmental, MBP- rich secondary granules that are devoid of matrix compartments and EPO content and are not primary granules by virtue of their lack of CLC p rotein. These granules occur in quantity in IL-5-activated mature huma n eosinophils, which have previously been shown to actively transport EPO from the matrix compartments of their secondary granules to the ex tracellular milieu in smooth membrane-bound cytoplasmic vesicles, a se cretory process termed piecemeal degranulation, whereby eosinophils pr ogressively empty cytoplasmic granules of their contents in the absenc e of classical granule extrusion.