Resting and cytokine-stimulated human small airway epithelial cells recognize and engulf apoptotic eosinophils

Eosinophils, which are prominent cells in asthmatic inflammation, undergo apoptosis and are recognized and engulfed by phagocytic macrophages in vitro. We have examined the ability of human small airway epithelial cells (SAEC) to recognize and ingest apoptotic human eosinophils. Cultured SAEC ingested apoptotic eosinophils but not freshly isolated eosinophils or opsonized erythrocytes. The ability of SAEC to ingest apoptotic eosinophils was enhanced by interleukin-1alpha (IL-1alpha) or tumor necrosis factor alpha (TNFalpha) in a time- and concentration-dependent fashion. IL-1alpha was found to be more potent than TNFalpha and each was optimal at 10(-10) mol/L, with a significant (P <.05) effect observed at 1 hour postcytokine incubation that was maximal at 5 hours. IL-1alpha stimulation not only increased the number of SAEC engulfing apoptotic eosinophils, but also enhanced their capacity for ingestion. The amino sugars glucosamine, n-acetyl glucosamine, and galactosamine significantly inhibited uptake of apoptotic eosinophils by both resting and IL-1alpha-stimulated SAEC, in contrast to the parent sugars glucose, galactose, mannose, and fucose. Incubation of apoptotic eosinophils with the tetrapeptide RGDS, but not RGES, significantly inhibited their uptake by both resting and IL-1alpha-stimulated SAEC, as did monoclonal antibody against alphavbeta3 and CD36. Thus, SAEC recognize apoptotic eosinophils via lectin- and integrin-dependent mechanisms. These data demonstrate a novel function for human bronchial epithelial cells that might represent an important mechanism in the resolution of eosinophil-induced asthmatic inflammation.