Characteristics of eosinophils migrating around fungal hyphae in nasal discharge.

It is known that eosinophil granular proteins cause tissue damage. To explore how eosinophils degranulate, we studied the degranulation of eosinophils that had migrated around fungal hyphae. In electron microscopic observations of allergic mucin from patients with allergic fungal sinusitis, fungal hyphae were detected, surrounded by numerous eosinophils. A number of eosinophils, including many disintegrated eosinophils, adhered to the cuticular layer of the hyphal surface. Although the fungal hyphae were detected in allergic mucins in all 5 patients, fungal hyphae surrounded by eosinophils were observed in only 1 patient. In the eosinophil cytoplasm, the cell membrane was invaginated, deep, and sheetlike, and the space formed by its infolding was filled with a highly electron-dense substance. This substance appeared to be a mixture of the cuticular substance of the hyphal surface and granular proteins. Thus, the eosinophil phagocytosed the cuticular substance of the hyphae into a sheetlike invaginated space, and released granular proteins into that space. The structure invaginated in the cytoplasm retained its form even after disintegration of the eosinophil, and adhered to the cuticular layer. This structure detected in eosinophils has not been reported previously, and is considered to be an interesting finding from the viewpoint of the function of eosinophils.