Comparison of the immune responses induced by oral immunization of mice with Lactobacillus casei-expressing porcine parvovirus VP2 and VP2 fused to Escherichia coli heat-labile enterotoxin B subunit protein

The major structural protein VP2 of porcine parvovirus (PPV) was used as the model parvovirus antigen, which has been expressed in Lactobacillus casei fusing with Escherichia coli heat-labile enterotoxin B subunit (LTB) as mucosal adjuvant. The VP2-LTB DNA fragment was cloned into vector pPG611 or pPG612 to generated inducible surface-displayed and secretion expression systems based on xylose promoter, designated as rLc:pPG611-VP2-LTB (recombinant L. casei) and rLc:pPG612-VP2-LTB, respectively. Expression of the fusion protein was verified by SDS-PAGE, Western blot immunofluorescence and electron microscopy. It was observed that the level of IgG or sIgA from mice orally immunized with VP2-LTB was higher than that from mice received VP2 and negative control, which demonstrated significantly statistically different. Especially, the titer of IgG or sIgA in mice immunized with rLc:pPG612-VP2-LTB is the highest in this study. In summary, LTB as mucosal adjuvant was able to effectively facilitate induction of mucosal and systemic immunity by L. casei-expressing VP2 fusion protein.