Immunobiology of compound ascidians, with particular reference to Botryllus schlosseri: state of art.

The phylogenetic position of invertebrate chordates closely related to vertebrates explains the increasing interest towards tunicate immunobiology. Most of the tunicates are ascidians which, like all other invertebrates, rely only on innate immunity for their defense. Compound ascidians differ from solitary species for the presence of colony specificity, i.e. the ability for intraspecific non-self recognition. The immunobiology of compound ascidians has been particularly studied in Botryllus schlosseri, which is an emerging model organism for this kind of studies. In B. schlosseri and related species, immunocytes are represented by phagocytes and cytotoxic morula cells, the former able to ingest foreign cell and particles, the latter representing the effectors of the inflammatory reaction which follows the contact between genetically incompatible colonies. Activated phagocytes release lectins with opsonic activity and are involved in the clearance of apoptotic cells during the colonial generational change. Morula cells recognize the presence of foreign molecules as well as allogeneic soluble factors diffusing from an alien colony and as a consequence they: i) release cytokines in the medium which have chemotactic activity and activate phagocytes; ii) degranulate and release phenoloxidase which induces necrotic cell death by oxidative stress. A better knowledge of Botryllus genome will allow a deeper insight into open problems in immunobiology of compound ascidians.