Comparative study of the immunoprotective effect of two grass carp-sourced Bacillus subtilis spore-based vaccines against grass carp reovirus

Grass carp reovirus II (GCRV II) causes severe haemorrhagic disease with high mortality in grass carp, Cyenopharyngodon idellus. Vaccination is a very effective method in conferring protection against GCRV. In the current study, we constructed two grass carp-sourced Bacillus subtilis spore-based vaccines (GC5-VP4 and GC5NS38) and investigated their potential as oral candidate vaccines against GCRV II. Our results showed that both VP4 and NS38 proteins of GCRV II were efficiently displayed on the spore surface of B. subtilis GC5, and the two recombinant spores stayed in the hindguts of grass carp for antigen presentation. After oral administration, compared with the control groups, both recombinant vaccines GC5-VP4 and GC5-NS38 increased the survival rate of grass carp against GCRV II, with relative percent survival rates of 30% and 36.4%, respectively. Although they had a similar immunoprotective effect, the mechanisms were very different. GC5-VP4 mainly elicited specific antibody responses, both systemic and mucosal, whereas GC5-NS38 mainly elicited an inflammatory response and cellular immunity via expression of major histocompatibility complex molecules. Furthermore, this study demonstrated that the recombinant spores triggered high levels of mucosal immunity and would be promising oral candidate vaccines against GCRV II in grass carp. Combined with previous research, we inferred that vaccine efficacy was not only related to antigen, but also vaccination strategies.