Evidence for the role of extrusomes in evading attack by the host immune system in a scuticociliate parasite

Like other ciliates, the scuticociliate parasite of turbot, Philasterides dicentrarchi, produces only a feeding or growing stage called a trophont during its life cycle. Exposure of the trophonts to immune serum extracted from the host and containing specific antibodies that induce agglutination / immobilization leads to the production of a mucoid capsule from which the trophonts later emerge. We investigated how these capsules are generated, observing that the mechanism was associated with the process of exocytosis involved in the release of a matrix material from the extrusomes. The extruded material contains mucin-like glycoproteins that are deposited on the surface of the cell and whose expression increases with time of exposure to the turbot antibodies, at both protein expression and gene expression levels. Stimulation of the trophonts with host immune serum also causes an increase in discharge of the intracellular storage compartments of calcium necessary for the exocytosis processes in the extrusomes. The results obtained suggest that P. dicentrarchi uses the extrusion mechanism to generate a physical barrier protecting the ciliate from attack by soluble factors of the host immune system. Data on the proteins involved and the potential development of molecules that interfere with this exocytic process could contribute to the development of glycosylated recombinant vaccines and drugs to improve the prevention and control of scuticociliatosis in turbot.