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Major Basic Protein Homolog (MBP2): A Specific Human Eosinophil Marker¹

Douglas A. Plager^2,*, David A. Loegering^*, James L. Checkel^*, Junger Tang^*, Gail M. Kephart^*, Patricia L. Caffes^*, Cheryl R. Adolphson^*, Lyo E. Ohnuki and Gerald J. Gleich

^* Allergic Diseases Research Laboratory, Mayo Clinic and Foundation, Rochester, MN 55905; and Department of Dermatology, University of Utah, Salt Lake City, UT 84132

Human eosinophil granule major basic protein (MBP1) is an exceedingly basic (isoelectric point >11) 14-kDa protein, comprising the core of the secondary eosinophil granule. Recently, a less cationic homolog of MBP, termed MBPH or simply, MBP2, has been discovered. We prepared a panel of mAbs to MBP2 and used these Abs to localize and quantitate this molecule in leukocytes and biological fluids. Specific mAbs for MBP2 were selected using slot-blot analyses and used in a two-site immunoassay, Western blotting, and immunofluorescence microscopy. The sensitivity of the immunoassay was markedly improved by reduction and alkylation of MBP2. MBP1 is more abundant than MBP2 in lysates of eosinophils and their granules, as judged by immunoassay and Western blotting. By immunofluorescence, MBP1 is present in eosinophils, basophils, and a human mast cell line (HMC1), whereas MBP2 is only detected in eosinophils. Neither MBP1 nor MBP2 could be detected in any other peripheral blood leukocyte. MBP2 levels measured in plasma and serum were essentially identical. In contrast to past measurements for MBP1, MBP2 was not detected above normal levels in sera from pregnant donors. However, measurement of serum MBP2 discriminated patients with elevated eosinophils from normal subjects, and MBP2 was also detectable in other biological specimens, such as bronchoalveolar lavage, sputum, and stool. These results indicate that MBP2 is present only in eosinophils and that it may be a useful biomarker for eosinophil-associated diseases.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ This work was supported by National Institutes of Health Grants AI09728, AI34577, and AI50494 and the Mayo Foundation.

² Address correspondence and reprint requests to Dr. Douglas A. Plager, Allergic Diseases Research Laboratory, Mayo Clinic and Foundation, Rochester, MN 55905. E-mail address: plager.douglas{at}mayo.edu

³ Abbreviations used in this paper: MBP1, major basic protein; pI, isoelectric point; CM, carboxymethyl; ECP, eosinophil cationic protein; MCP, mast cell protease.