Different immune responses of flounder (Paralichthys olivaceus) towards the full-length and N-terminal or C-terminal portion of hirame novirhabdovirus glycoprotein

Abstract

Glycoprotein is an important immunogenic protein of Hirame novirhabdovirus (HIRRV). In this study, the full-length and N-/C-terminal portions of glycoprotein were recombinantly expressed (rG, rGn and rGc protein), and the induced immune responses were investigated in flounder (Paralichthys olivaceus) model. The results showed that compared to PBS control, rG, rGn and rGc proteins and inactivated HIRRV suspension (iVS) could all stimulate significant increases of flounder CD4-1+, CD4-2+ T lymphocytes and surface IgM positive (sIgM+) B lymphocytes in peripheral blood, spleen and head kidney (p < 0.05). However, no significant differences of the percentages of CD4-1+ or CD4-2+ T lymphocytes were observed among three protein vaccination groups (p > 0.05). iVS could induce the highest mean levels of CD4+ T lymphocytes in peripheral blood and spleen. For sIgM+ B lymphocytes, the average peak percentages in rG and rGc groups were higher than rGn group. Moreover, significant increases of specific serum IgM against HIRRV or rG protein were observed in iVS, rG, rGn and rGc groups, but rG group exhibited the highest mean level. Furthermore, rG protein induced the highest titer of neutralizing antibodies against HIRRV, followed by iVS. Meanwhile, the challenge test showed that the relative percent survival (RPS) of rG, rGn, rGc and iVS groups were 75.0%, 35.7%, 53.6% and 60.7%, respectively. These results revealed that the full-length G protein would be a more effective subunit vaccine candidate against HIRRV infection.