Immune response and gene expression in hemocytes of Portunus trituberculatus inoculated with the parasitic dinoflagellate Hematodinium

• First report of the anti-parasitic immune response of Portunus trituberculatus against the Hematodinium parasites.
• The proPO system played important roles in immune responses against the parasite.
• An immunosuppressive mechanism was likely induced against activation of the proPO system.
• The balance between serine proteinases and the proteinase inhibitors was interrupted in immune responses against the parasite.
• Hemocytes were involved in cellular immune response against the parasite.

The parasitic dinoflagellates in the genus of Hematodinium infect broad range of crustaceans around the world, causing fatal diseases in multiple species of wild and cultured crabs and subsequent economic loss. In order to explore this host–parasite interaction in the early development stage of infection, mRNA transcript levels of eight key immune-related genes, including LGBP, proPO, PPAF, serpin, α2m, and three PTcSPs were quantitatively assessed in Portunus trituberculatus artificially inoculated with the Hematodinium parasites. The fluctuation of proPO gene expression indicated that the host proPO system was disturbed overtly due to the intrusion of the parasites. And as manifested by the suppressed expression of LGBP and PPAF, an immunosuppressive mechanism was likely induced by the parasites against being entrapped or killed by the host proPO system. Furthermore, the significant variations of the transcript levels of serpin, α2m, and the three PTcSPs suggested that the parasites affected the proteinase cascade reactions associated with the immune response by destroying the balance between serine proteinases and the proteinase inhibitors. The hemocytes counts and PO activity varied accordingly over the time course of infection, showing that hemocytes were actively involved in the immune response against the parasitic invasion. This study primarily highlighted the anti-parasitic immune response of crab hosts, and presented the first report of the immune response of P. trituberculatus to the parasitic dinoflagellate Hematodinium.