CELL COOPERATION DURING HOST DEFENSE IN THE SOLITARY TUNICATE CIONA-INTESTINALIS (L)

Phagocytosis in the solitary ascidian Ciona intestinalis was investigated using mixed and separated populations of blood cells in vitro. Only the vacuolar and granular amoebocytes were seen to ingest bacteria, and when the serine protease inhibitors, STI, or benzamidine were added to monolayers of mixed cell types, uptake was significantly reduced. Analyses carried out with isolated cells revealed that phagocytosis was enhanced by incubation of the bacteria in blood cell lysate supernatants (CLS) that had been pre-treated with LPS. By contrast, pre-incubation of the bacteria in CLS preparations that were inhibited by benzamidine produced lower levels of phagocytosis. Treatment of the bacteria with plasma also failed to promote uptake, and there was no detectable agglutination of the bacteria by CLS. As lysate supernatants made from morula cells, but not other cell types, were effective in promoting phagocytosis, we propose that opsonins are derived from the morula cells, and that phagocytosis involves cooperation between different cell populations. Moreover, as the morula cells are the principal repositories of prophenoloxidase and an associated serine protease (factors which are activated by LPS but blocked by STI or benzamidine), prophenoloxidase or the protease may be involved in the opsonic phenomenon in a similar way to that previously reported for arthropods.