Apoptosis and phagocytosis of tissue-dwelling eosinophils in sinonasal polyps

Sinonasal polyps contain numerous tissue-dwelling eosinophils, but the mechanisms causing their accumulation, functional activities, and resolution are largely unknown.

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.

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&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.

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.

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.