Apoptosis and phagocytosis of tissue-dwelling eosinophils in sinonasal polyps

• Objective: Sinonasal polyps contain numerous tissue-dwelling eosinophils, but the mechanisms causing their accumulation, functional activities, and resolution are largely unknown. Study Design: Nasal polyp tissue from 14 patients was evaluated for cellular expression of CD95, CD68, and Annexin-V, for the degree of apoptosis, and for phagocytosis of eosinophils. Material and Methods: Histological sections were immunostained as single stains for CD95, CD68, and Annexin-V, and as an immunostaining for CD68 combined with a modified Vital New Red staining. The latter staining is specific for eosinophils. Other sections were stained by terminal d-UTP nick end labeling (TUNEL) assay and routinely stained for H and E. Evaluation of the amount of stained cells was performed by counting the average number in 10 randomly chosen high-power fields. The TUNEL positivity was in all cases confirmed with apoptotic morphology. Results: The inflammatory infiltrate consisted of numerous eosinophils but also a considerable amount of lymphocytes, mast cells, and macrophage-like CD68+ cells. CD95 was frequently expressed on eosinophils, on numerous other inflammatory cells, and also on morphologically apoptotic cells. Annexin-V-positive eosinophils were not as frequent as CD95+ cells, but numerous Annexin-V-positive eosinophils were found. CD68+ cells approximately equalled the number of eosinophils. The number of cells phagocytosing eosinophils varied between polyps. Apoptosis of eosinophils (as evaluated by TUNEL combined with apoptotic morphology) was a common finding in six of the polyps. Conclusions: Previous in vitro and ex vivo findings of CD95 on eosinophils are now supported by demonstration of CD95 on eosinophils in this in vivo study. This investigation revealed a switch of the membrane-bound phosphatidylserine of apoptotic cells, which is a novel observation. The study has demonstrated apoptosis of tissue-dwelling eosinophils, and that CD68+ macrophage-like cells phagocytose eosinophils within the sinonasal polyps.

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