Complement-mediated cooperation between immunocytes in the compound ascidian Botryllus schlosseri

Two main kinds of innate immune responses are present in ascidians: phagocytosis and cytotoxicity. They are mediated by two different types of circulating immunocytes: phagocytes and cytotoxic morula cells (MCs). MCs, once activated by non-self-recognition, can stimulate phagocytosis by the release of soluble factors able to act as opsonins. BsC3, the complement C3 homologue, like mammalian C3, contains the thioester bond required to split the molecule into BsC3a and BsC3b. BsC3b likely represents the MC opsonin as it can enhances phagocytosis. The tenet is supported by the observed reduction in phagocytosing cells after exposure of hemocytes to compstatin, a drug preventing C3 activation, or after the bsc3 knockdown by iRNA injection. In addition, the transcript for BsCR1, homologous to mammalian CR1, is present in Botryllus phagocytes and the transcription is modulated during the blastogenetic cycle. MCs also release cytokines (chemokines) able to recruit immunocytes to the infection site. The activity is inhibited by antibodies raised against human TNFa. Since no genes for TNFa are present in the Botryllus genome, the observed activity is probably related to a TNF-domain containing protein, member of the Botryllus complement system. Conversely, activated phagocytes release a rhamnose-binding lectin able to interact with microbial surfaces and act as opsonin. It can also activate MCs by inducing the release of the reported cytokine and stimulate their degranulation. Overall, the results obtained so far indicate the presence of a well-defined cross-talk between the two types of immunocytes during the immune responses of B. schlosseri.