Immunomodulatory molecules in the compound ascidian Botryllus schlosseri: evidence from conditioned media

The supernatant from cultures of haemocytes of the compound ascidian Botryllus schlosseri incubated with zymosan (conditioned medium; CM) can enhance yeast phagocytosis by Botryllus blood cells. It contains molecules recognised by antibodies raised against the mammalian pro-inflammatory cytokines IL-1-α and TNF-α which appear as a single band of 60 kDa in immunoblot analysis. The effects on phagocytosis are abolished by the presence of sugars, such as galactose and rhamnose, sharing the same hydroxyl group configuration at C2 and C4. The same sugars also inhibit the haemagglutinating activity of the CM, suggesting the presence of lectins with opsonic activity. With immunoblot analysis, we confirmed the presence, in CM, of B. schlosseri rhamnose-binding lectins (BsRBLs), recently identified and characterised by our team, as a single electrophoretic band of 37 kDa. We had already demonstrated that these molecules are synthesised and secreted by activated phagocytes. Since previous studies have demonstrated that cytotoxic morula cells, and not phagocytes, are the haemocytes responsible for the release of molecules recognised by anti-cytokine antibodies, we propose a new scenario in which morula cells act as sentinels, able to sense foreign molecules and release immunomodulatory factors which induce phagocytes to secrete lectins able to enhance phagocytosis by acting as bridges between foreign particles and phagocyte surfaces.