Intracellular pools of FcalphaR (CD89) in human neutrophils are localized in tertiary granules and secretory vesicles, and two FcalphaR isoforms are found in tertiary granules.

The human FcalphaRIota (CD89) is expressed on cells of myeloid lineage and plays an important role in host defense. Neutrophils make up the majority of FcalphaRIota-positive cells. Previous reports suggested that FcalphaR was stored in neutrophil intracellular pools, and it could be mobilized quickly once neutrophils were activated. However, the subcellular localization of FcalphaR in neutrophils has not been defined yet. In this study, we identified that FcalphaR was stored in secretory vesicles and tertiary granules of neutrophils by flow cytometry analysis, ELISA, confocal microscopy, and Western blotting. The molecular mass of FcalphaR in secretory vesicles was different from that in tertiary granules. FcalphaR stored in tertiary granules had a molecular mass of 50-70 kDa, whereas FcalphaR in secretory vesicles and membranes had a molecular mass of 55-75 kDa. After treatment by peptide-N-glycosidase F, an enzyme that removes N-glycosylation, FcalphaR from secretory vesicles and tertiary granules revealed a core protein of 32 kDa, which was the same as the backbone of full length of FcalphaR. A smaller FcalphaR variant with a core protein of 29-30 kDa was found in tertiary granules but not in secretory vesicles. The nature of the small variant is not clear at present and remains to be investigated further.