Morula cells release phagocytosis-stimulating factors in the colonial ascidian Botryllus schlosseri.

We studied in vitro yeast phagocytosis by haemocytes of the compound ascidian Botryllus schlosseri, with particular attention to interactions among different immunocyte types. The supernatant from haemocyte cultures matched with yeast cells contains factors able to enhance yeast ingestion by Botryllus phagocytes. The increase in phagocytosis is not the consequence of yeast opsonisation, as the phagocytic index does not significantly increase when yeast cells, previously incubated in the cultured media, are re-suspended in filtered sea water. When haemocytes were fractionated by density gradient centrifugation and each band was incubated with yeast, the ability to stimulate phagocytosis was found in the supernatants from haemocyte cultures of fractions rich in morula cells. The above-reported enhancing effect is completely absent in the presence of anti-IL-1-beta and anti-TNF-beta, but not of anti-rabbit-IgG antibodies, which indicates that molecules immunoreactive with anti-cytokine antibodies are required for modulating phagocyte activity; results also highlight the presence of "cross-talk" between morula cells and phagocytes. A new scenario is therefore sketched, in which morula cells actively ecognise non-self molecular patterns and, upon this recognition, release humoral factors recognised by phagocytes which modulate phagocytosis.