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), FEV(1), 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.