Epithelial interactions in Hydra: apoptosis in interspecies grafts is induced by detachment from the extracellular matrix.

Apoptosis plays an important role in immunity and is widely used to eliminate foreign or infected cells. Cnidaria are the most basal eumetazoans and have no specialised immune cells, but some colonial cnidarians possess a genetic system to discriminate between self and non-self. By grafting epithelia of different species we have previously shown that the freshwater polyp Hydra eliminates non-self cells by phagocytosis. Here we have investigated whether apoptosis is involved in the histocompatibility reactions. We studied epithelial interactions between Hydra vulgaris and Hydra oligactis and show that a large number of apoptotic cells accumulate in the contact region of interspecies grafts. Histological analysis of the graft site revealed that displacement of the endodermal layer of Hydra vulgaris by endoderm from Hydra oligactis coincided with impaired cell-cell and cell-matrix contacts. We therefore suggest that in interspecies grafts, apoptosis is induced by the detachment of epithelial cells from the extracellular matrix (anoikis) and not by a discriminative allorecognition system.