Mass, charge, and subcellular localization of a unique secretory product identified by the basophil-specific antibody BB1

Background: BB1 is a basophil-specific mAb (Lab Invest 1999;79:27-38). The Identity of the corresponding antigen has not been determined, but it gives a granular appearance on staining and is secreted on activation of basophils. Objective: We sought to further characterize the basophil-specific antigen identified by BB1. Methods: Intracellular localization was determined by flow cytometry and by immunogold labeling and electron microscopy. Physical chemical properties were investigated by gel filtration chromatography and preparative isoelectric focusing. Results: In flow cytometry, permeabilization of cells increased immunofluorescence 100-fold, confirming the predominantly intracellular localization of the antigen. It was further localized to the secretory granules by immunoelectron microscopy. Double labeling with a CD63-specific antibody demonstrated selective binding of BB1 to the granule matrix. Gel filtration chromatography indicated that the antigen is secreted as a complex of approximately 5 x 10(6)d, which was well resolved from the 210-kd supramolecular complex containing tryptase. The antigen was degraded by pronase. Isoelectric focusing indicated a highly basic protein with an isoelectric point of 9.6. Conclusion: With its granule localization, release on cell activation, and unique properties, the antigen identified by BB1 could be a novel mediator of allergic disease. We propose the name basogranulin for this novel basophil-specific protein.