Morula cells and non-self recognition in the compound ascidian Botryllus schlosseri

In the ascidian Botryllus schlosseri, we studied the effects of hemocyte incubation with foreign cells, such as bacteria, bacterial spores and yeast. In the presence of yeast and bacterial spores, morula cells, a common cell type in botryllid ascidians, changed their morphology, release phenoloxidase in the medium, thus causing an increase in cytotoxicity, and express molecules recognised by anti-IL-1-a- and anti-TNF-a-antibodies. These effects were not observed when hemocytes were incubated with both gram-positive (Staphylococcus aureus) and gram-negative (Escherichia coli) bacteria. Considering that morula cells are the main source of molecules recognised by anti-cytokine-antibodies we suggest an immunosurveillance role of these cells, which may influence immune responses such as phagocytosis.