Viral resistance and immune responses of the shrimp Litopenaeus vannamei vaccinated by two WSSV structural proteins

Abstract

Although adaptive immunity is believed to exist only in higher vertebrates, recent studies showed the ability to vaccinate shrimp and other crayfish against white spot syndrome virus (WSSV). This study attempted to establish parameters of vaccination coordinated with subsequent viral challenge to gain insights into the mechanisms of the protective response of penaeid shrimp. Two WSSV envelope proteins, VP28 and VP36B, were used as subunit vaccines expressed in Escherichia coli followed by histidine-tag affinity chromatographic purification. Shrimp vaccinated with the recombinant WSSV proteins and challenged with diluted WSSV inocula were intramuscularly injected in order to give a precise load. Results of the viral challenge trials showed complete survival in the rVP28 group in contrast to the rVP36B and PBS groups which exhibited 100% mortality. But this effective protection was exclusively induced from a combination of a higher dosage of rVP28 and a lower viral challenge pressure. The innate immune parameters analyzed among the three groups revealed that rVP28-treated shrimp showed the highest activity level (p<0.05) of phenoloxidase and superoxide dismutase during the entire period of 7 days post-vaccination. But there were no significant differences (p>0.05) in mRNA abundances of the Down syndrome cell adhesion molecule (Dscam) among all groups. In addition, total hemocyte counts significantly decreased in shrimp treated with the recombinant viral proteins compared to the PBS group. These results indicated that the existence of structure- and dose-dependent protective responses and the elevated innate immunity in shrimp following a protein-based vaccination might be responsible for conferring resistance against WSSV.