Immune roles of a rhamnose-binding lectin in the colonial ascidian Botryllus schlosseri

The present paper describes the immune role played by a recently identified (Gasparini et al. 2008) member of the rhamnose-binding lectin (RBL) family from the colonial ascidian Botryllus schlosseri. B. schlosseri RBL (BsRBL) can activate phagocytes through: (i) induction of their directional movement towards the source of the molecule; (ii) modification of cytoskeleton, required for shape changes; (iii) stimulation of the respiratory burst, and consequent production of reactive oxygen species (ROS) with microbicidal activity, including superoxide anions and peroxides; and (iv) increase in the ability to phagocytose foreign particles. RBL also induces the synthesis and release, by cytotoxic morula cells (MCs), of cytokines recognised by anti-IL1_ and anti-TNF_ antibodies. At high concentrations, BsRBL induces degranulation of MCs and the consequent release of the cytotoxic enzyme phenoloxidase into the medium. Results are consistent with the existence of cross-talk between B. schlosseri immunocytes (phagocytes and MCs). In addition, a three-dimensional model for BsRBL is presented.