Adjuvant and immunostimulatory effects of LPS and β-glucan on immune response in Japanese flounder, Paralichthys olivaceus

In order to investigate the adjuvant and immunostimulatory effects of LPS and β-glucan, a highly effective vaccine candidate for Edwardsiella tarda known as glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was recombinantly expressed in Escherichia coli, which was intramuscularly injected into Japanese flounder alone or containing LPS or β-glucan. ELISA analysis of sera showed that the fish immunized with rGAPDH containing LPS or β-glucan had significant higher antibody titers than rGAPDH immunization group at day 14 and 30 (P < 0.05). Flow cytometric analysis showed that the surface immunoglobulin positive (sIg+) lymphocytes percentages in head kidney of the immunized fish exhibited significant higher than the fish of control group at day 14 and 30, and immunization with rGAPDH containing LPS or β-glucan could induce significant higher sIg+ percentages than immunization with rGAPDH alone. Simultaneously, the expressions of genes coding for immunoregulatory molecules involved in innate and acquired responses, including IFN-γ, IL-1β, IL-6, TNF-α, TCRα, IgM, C-type lysozyme and MHC-IIα were analyzed by quantitative PCR. The transcription levels of all selected genes except TNF-α displayed up-regulation after immunization, and immunization with rGAPDH containing LPS or β-glucan could induce significant higher levels than that induced only by rGAPDH. Moreover, to investigate the protection against infection, challenge with E. tarda was performed at day 30 after immunization. Fish immunized with rGAPDH containing β-glucan exhibited the highest relative percentage survival (RPS) of 63% compared to the other groups. These results indicated that LPS and β-glucan employed as co-immunostimulatory molecules could significantly trigger the innate and acquired immune responses in Japanese flounder.