Eosinophil Granule Major Basic Protein in Basophils

The large specific granule of the human eosinophil contains a number of low molecular weight, highly basic proteins that have been purified to homogeneity, including the major basic protein (MBP) (1-3), 1 the eosinophil cationic protein (4, 5), and the eosinophil-derived neurotoxin (6, 7). The comparative physicochemical and immunochemical characteristics of these proteins have been examined in detail, and they clearly represent distinctive cationic moieties of the granule. ~ MBP, the most abundant of the three proteins, is a small polypeptide of 9,300 D (3) that has been localized by subcellular fractionation and immunoelectron microscopy to the electron-dense crystalloid core of the guinea pig (8) and human (S. J. Ackerman, D. A. Loegering, and G. J. Gleich, unpublished observations) eosinophil granule. MBP has been regarded as a unique eosinophil protein and its release both in vitro and in vivo has been used as a specific marker for eosinophil localization, degranulation, and function (9-18). Using a sensitive double-antibody immunofluorescent method (19), MBP has been localized extracellularly in a number of tissues and organs whose dysfunction in disease is generally associated with eosinophil infiltration (11, 20, 21).3 In these experiments, a small number of cells stained for MBP by immunofluorescence that subsequently could not be clearly identified as eosinophils. Because we recently demonstrated that the Charcot-Leyden crystal (CLC) protein (lysophospholipase), another eosinophil protein, was also present in and crystallized from human basophils (22), we postulated that MBP might also be a constituent