An exceptional immunosurveillance system of the pharynx entry in a protochordate.

In the colonial ascidian Botryllus schlosseri, we have recently identified unusual free amoebocytes, completely exposed to the sea-water flow entering into the pharynx, which inhabit the tunic surface that internally covers the oral siphon (Cima et al., 2006 Dev Comp. Immunol., 30: 463-472). All histochemical data support the hypothesis that these cells named “siphonal guard cells” (SGC) belong to the hyaline amoebocyte population of the phagocytic differentiation pathway of the circulating immunocytes. After crossing the epidermis which bounds the siphonal blood sinus, they adhere to strategic points of the tunic and can recognise and phagocytise various foreign particles. After exposure of colonies to bacterial spores, the observations at both light and electronic microscope revealed that these cells are involved in a complex series of local and systemic immune events. Already after 5 min, the SGC show bacteria inside heterophagic vacuoles. After 10-15 min, as a transitory plug of colloidal material rich in heparin and histamine is formed in the siphonal lumen by exocytosis of some SGC, other ones with engulfed bacteria cross the epidermis reaching the siphonal sinus and probably transferring an alert signal; cells of the cytotoxic blood cell line (morula cells) crowd into the siphonal sinus, where most of them are labelled by anti-TNF- and anti-CD57 antibodies and degranulate. After this time, large scavenger phagocytes increase in number in blood circulation, show bacteria engulfed in large phagosomes and are continuously eliminated through the peribranchial chamber and the cloacal siphon with a mechanism which was not previously described.