The importance of ATP in the immune system of molluscs

Molluscs rely on an innate immune system for defence against infection. Invading organisms are phagocytosed by circulating hemocytes and neutralised by a combination of hydrolytic enzymes and the production of reactive oxygen species. Phagocytosis, the central feature of the molluscan innate immune system, is an energy-demanding process as a consequence of actin polymerisation which requires a constant supply of ATP. Adenylate kinase, shown to couple local ATP supply and actomyosin assembly in macrophage cell lines, may play a similar role in molluscan hemocytes. Hemocyte conformational changes and chemotaxis are regulated via protein kinase C which is phosphorylated in response to bacterial lipopolysaccharide. ATP utilisation in protein kinase cascades is thought to be quite significant. The vast majority of the ATP required to support cellular functions in animal cells is generated via the mitochondrial electron transport chain. Disruption of this process with antimycin A resulted in a 50 % decrease in ATP levels in Haliotis midae hemocytes which also exhibited reduced phagocytic activity. Phosphoarginine and arginine kinase are thought to play a role in invertebrates that is similar to phosphocreatine and creatine kinase in vertebrates, by supplying additional ATP during periods of high energy demand. The energetically-expensive processes associated with the molluscan innate immune system, such as chemotaxis, phagocytosis, formation and acidification of the phagolysosome, are severely compromised by abiotic stresses such as elevated water temperature and pollutants, reflecting the fine balance that must be maintained to ensure sufficient ATP is available for both general metabolism and the immune response.