Danger signals derived from stressed and necrotic epithelial cells activate human eosinophils

Eosinophilic granulocytes are found in tissues with an interface with the external environment, such as the gastrointestinal, genitourinary and respiratory tracts. These leucocytes have been associated with tissue damage in a variety of diseases. The aim of this study was to evaluate whether necrotic epithelial cells can activate eosinophils. The danger theory postulates that cells of the innate immune system primarily recognize substances that signal danger to the host. We damaged epithelial cell lines derived from the genital (HeLa cells), respiratory (HEp-2 cells) and intestinal tracts (HT29 cells) and assessed their capacity to cause eosinophilic migration, release of putative tissue-damaging factors, such as eosinophil peroxidase (EPO) and eosinophil cationic protein (ECP), as well as secretion of tissue-healing factors, e.g. fibroblast growth factors (FGF)-1 and -2 and transforming growth factor (TGF)-beta1. We found that necrotic intestinal cells induced chemotaxis in human eosinophils. EPO release was elicited in eosinophils stimulated with necrotic cells derived from all cell lines, as well as from viable HEp-2 and HT29 cells. Release of ECP was only seen in eosinophils incubated with necrotic intestinal or genital cells, not viable ones. Both necrotic intestinal and genital cells elicited FGF-2 secretion from eosinophils. TGF-beta1 was released from eosinophils exposed to viable and necrotic HT29 cells. These findings indicate that eosinophils are able to recognize and be activated by danger signals released from damaged epithelial cells, which may be of importance in understanding the role of eosinophils in the various inflammatory conditions in which they are involved.