Cellular aspects of allorecognition in the compound ascidian Botrylloides simodensis

When colonies of the compound ascidian Botrylloides simodensis contact each other at their cut surfaces, either fusion or rejection occurs. Contact between genetically compatible colonies leads to the complete fusion of their tunics and vasculature within 24 h. Conversely, the rejection reaction between incompatible colonies is characterized by the appearance of a melanic, necrotic band along the contact border. In the case of fusion, limited crowding of cytotoxic morula cells (MCs) was observed in the ampullae near the contact border. In rejection, limited tunic fusion occurred in the contact region and MCs were selectively recruited inside the ampullae near the cut surface: most of them leaked into the tunic where they changed their morphology and contributed to the formation of the necrotic region. Granular amebocytes, like MCs, have granules well stained by eosin and were also seen inside the ampullae involved in the rejection reaction and along the contact border between incompatible colonies. Immunohistochemical analysis using antibodies raised against Botryllus schlosseri phenoloxidase (PO) and mammalian IL-1-α and TNF-α indicate that MCs were the only cells recognized by the anti-PO antibody; they resulted immunopositive also to the anti-cytokine antibodies in both fusion and rejection, whereas granular amoebocytes were recognized by the latter antibodies only during the rejection reaction.