Elucidation of the mechanisms involved in the recognition and engulfment of apoptotic human eosinophils by airway epithelial cells
Elucidation of the inflammatory mechanisms of asthma has highlighted the role of the eosinophil as a major effector cell in asthma pathogenesis due to the arsenal of toxic granule proteins and other biological mediators secreted by this cell. Apoptosis induction in eosinophils and their consequent phagocytic removal from the inflamed lung would prevent the release of their detrimental cell constituents and would facilitate inflammation resolution. This thesis first established that airway epithelial cells (AEC) could specifically recognise and ingest apoptotic and not freshly isolated peripheral blood eosinophils. AEC also differentiated between apoptotic neutrophils and apoptotic eosinophils with the former not being recognised or ingested. Colon, mammary and retinal-pigmented epithelium exhibited the same selective phagocytosis suggesting that this was a ubiquitous epithelial phenomenon. Exposure of the AEC to the pro-inflammatory cytokines IL-1 and TNFa, or the glucorcorticoid dexamethasone enhanced the phagocytic capacity of these cells in a time and concentration dependent manner. Enhancement was observed both in terms of the number of cells within a monolayer phagocytosing apoptotic eosinophils and also with respect to the number of cells phagocytosed by these phagocytic AEC. CD44 ligation on the surface of A549 was also revealed to enhance phagocytic ability. The receptors utilised for apoptotic eosinophil recognition and phagocytosis were avb3, avb5, b5, CD36, PSr and unidentified lectins, which recognise the amino sugars, glucosamine, n-acetyl-glucosamine and n-acetyl-galactosamine but not their parent sugars. Neither cytokine nor glucorcorticoid of phagocytosis was associated with an increase in the expression of these receptors. Preliminary data was also found suggesting that cell-cell interactions between freshly isolated eosinophils and small airway epithelial cells (SAECs) that were co-cultured for 24h resulted in eosinophil apoptosis induction that was above that of constitutive apoptosis and only partially abrogated by IL-5 addition to the co-culture medium.