Characterization of mast-cell tryptase-expressing peripheral blood cells as basophils

Background: Mast-cell tryptase is a protease with proinflammatory activity, the expression of which by peripheral blood leukocytes (PBLs) has not been fully characterized. Objective: We examined tryptase expression in human PBLs to further characterize this tryptase-expressing cell population for lineage and disease association. Methods: PBLs were fixed, permeabilized, stained with antibodies to tryptase and a panel of mast cell- and basophil-specific markers, and analyzed by means of flow cytometry. Results: Tryptase expression was restricted to a population of cells that stained positive for IgE and negative for the panel of lineage markers (IgE(+), lin(-)). This IgE(+), lin(-) population did not stain for the mast-cell markers Kit or chymase but did stain for the basophil-specific granule proteins recognized by the 2D7 and BB1 mAbs. Per-cell tryptase expression demonstrated a greater than 100-fold range of expression among donors but did not correlate with disease status (asthma or mastocytosis), FEV1, or serum tryptase concentration. Tryptase was released by purified basophils after anti-IgE activation. Conclusions: The phenotype of tryptase-expressing PBLs and their lack of increase in patients with mastocytosis demonstrates that these cells are basophils. Per-cell basophil tryptase expression is highly variable between donors, with some donors expressing levels approaching those of mast cells. As such, anti-tryptase antibodies cannot be used to distinguish these 2 cell types from one another by means of flow cytometry. These results demonstrate that tryptase represents an additional mediator through which basophils may contribute to allergic inflammation.