Immunogenicity of the Truncated NDV F Protein Surface-Displayed on Lactobacillus Casei

To evaluate immune efficacy of the recombinant Lactobacillus casei, we constructed pLA-Newcastle disease virus (NDV)-F/L. casei and obtained the expression products. PCR amplified the NDV F gene carrying part of the major epitopes. The target gene was inserted to the shuttle plasmid pLA, and then transformed into Escherichia coli BL21 (DE3) in order to screen positive recombinant plasmid. The positive recombinant plasmid was transformed into L. casei by electroporation to construct pLA-NDV-F/L. casei. The positive strains were identified by PCR. The reactivity of the recombinant bacteria was identified by Western blotting and the protein expression was detected by indirect immunofluorescence, flow cytometry and laser confocal microscopy. The 14-day-old chickens in each group were vaccinated by oral plus nose drops. The pLA-NDV-F/L. casei twice immunization group and three times immunization group, the commercial vaccine group, the pLA/L. casei group, the unchallenge PBS and the challenge PBS group were established. IgG in serum and sIgA in the lavage fluid of intestinal, nasal and lung were detected by ELISA. The protection rate of chickens was evaluated. The results showed that 94.10% of the recombinant bacteria expressed the F protein. The recombinant protein was highly expressed on the surface of L. casei with a protein size of 62 kDa, which specifically bound to anti-NDV serum. The levels of anti-F IgG and sIgA antibodies in each test group were significantly higher than those in the control groups. The duration of antibody in the pLA-NDV-F/L. casei three-time immunization group lasted 28 days longer than that in the twice immunized group, and there was no significant difference between antibody peak values. The attack protection rates in each group of immunized pLA-NDV-F/L. casei three times, twice, attenuated vaccine, pLA/L. casei and PBS were 80%, 80%, 90%, 0% and 0%, respectively. Therefore, the antigenic protein of NDV F was successfully expressed by L. casei expression system, which has of reactogenicity and immunogenicity, and could induce protective immune responses in chickens.