Cell death and the immune response of the sipunculid worm Themiste petricola

Abstract

We have recently studied the role of cell death in the immune system of the sipunculan worm Themiste petricola. Typical biochemical and morphological changes of apoptosis were induced in celomocytes of these marine worms after in vitro exposure of cells to hydrogen peroxide. Apoptosis was time and dose dependent, and required several hours to become apparent. Surprisingly, in unexposed samples a subtype of granulocyte was observed to undergo homotypic aggregation, extensive cytoskeletal changes, and degranulation followed by cell death. This spontaneous response ending in cell death occurred in a divalent cation-dependent manner, served to entrap foreign particles, and was blocked by EDTA-containing saline solutions. Even though the mode of granulocyte cell death shares some features with apoptosis, it appears to be a different form of programmed cell death since it occurs within minutes and does not produce single cell-derived apoptotic bodies but transforms itself into one or several syncytial masses with haemostatic and immune purposes. Since numerous granulocyte types and multicellular masses involved in cellular immunity have been described in sipunculan worms, the review also discusses the potential influence of activation of granulocytes by sea water in expanding the variety of morphological types and multicellular structures identified through morphological studies among sipunculan species.