Recombinant thioredoxin 1 protein: The immune-adjuvant effect of Streptococcus iniae and its safety in rock bream, Oplegnathus fasciatus

• Recombinant thioredoxin 1 protein was verified the immune-stimulatory effect.
• The cDNA microarray was analysed to investigate the change of gene expression.
• The immune response of rock bream to Streptococcus iniae FKC with rRbTRx1 was investigated.
• Specific antibody response was analysed at different periods after immunization.
• Plasma chemistry was analysed to examine the toxic effects of rRbTRx1.

Adjuvant is an immune enhancer commonly used during vaccination to enhance the host immune response. In the present study, we analysed the recombinant protein produced from rock bream thioredoxin 1 cDNA (rRbTRx1). To verify the immune-stimulatory effect of this recombinant protein, changes in the expression level of several genes were investigated using the cDNA microarray chips in rock bream peripheral blood leukocytes stimulated with rRbTRx1. Furthermore, the immune responses of rock bream to Streptococcus iniae FKC (formalin-killed cell) vaccination alone or in combination with recombinant proteins were analysed after an experimental challenge with living S. iniae. Microarray analysis showed that 237 unique genes were upregulated more than two-fold after rRbTRx1 stimulation. Serum agglutination titres were relatively low; however, the FKC vaccine still conferred protection against S. iniae. Moreover, the adverse effects showed no statistically significant difference between fish injected with a concentration and non-injected fish. After experimental challenge to the rock bream by injection with living bacteria (S. iniae), the relative percent survival in the vaccinated groups with FKC + rRbTRx1 was significantly higher than that of the vaccinated groups with FKC alone, which were 85.9% and 68.2%, respectively. This indicated that the recombinant protein as an adjuvant showed synergism with the injected vaccine in rock bream.