Abstract
The use of oral vaccination in finfish has lagged behind injectable vaccines for a long time as oral vaccines fall short of injection vaccines in conferring protective immunity. Biodegradable polymeric nanoparticles (NPs) have shown potential to serve as antigen delivery systems for oral vaccines. In this study the recombinant outer membrane protein A (rOmpA) of Edwardsiella tarda was encapsulated in chitosan NPs (NP-rOmpA) and used for oral vaccination of Labeo fimbriatus. The rOmpA purity was 85%, nanodiameter <500 nm, encapsulation efficiency 60.6%, zeta potential +19.05 mV, and there was an in vitro release of 49% of encapsulated antigen within 48 h post incubation in phosphate-buffered saline. Empty NPs and a non-formulated, inactivated whole cell E. tarda (IWC-ET) vaccine were used as controls. Post-vaccination antibody levels were significantly (p = 0.0458) higher in the NP-rOmpA vaccinated fish (Mean OD450 = 2.430) than in fish vaccinated with inactivated whole cell E. tarda (IWC-ET) vaccine (Mean OD450 = 1.735), which corresponded with post-challenge survival proportions (PCSP) of 73.3% and 48.28% for the NP-rOmpA and IWC-ET groups, respectively. Serum samples from NP-rOmpA-vaccinated fish had a higher inhibition rate for E. tarda growth on tryptic soy agar (TSA) than the IWC-ET group. There was no significant difference (p = 0.989) in PCSPs between fish vaccinated with empty NPs and the unvaccinated control fish, while serum from both groups showed no detectable antibodies against E. tarda. Overall, these data show that the NP-rOmpA vaccine produced higher antibody levels and had superior protection over the IWC-ET vaccine, showing that encapsulating OmpA in chitosan NPs confer improved protection against E. tarda mortality in L. fimbriatus. There is a need to elucidate the possible adjuvant effects of chitosan NPs and the immunological mechanisms of protective immunity induced by OMPs administered orally to fish.
Highlights
Edwardsiella tarda is a member of the Enterobacteriaceae family that infects different fish species and mammals
We wanted to determine whether oral vaccination using the recombinant outer membrane protein A (rOmpA) antigen encapsulated in chitosan nanoparticles would afford higher protection than the levels obtained in our previous studies, in which the rOmpA protein was intraperitoneally injected in fish without nanoparticle encapsulation
In our previous study [19], we showed that the infectious pancreatic necrosis virus (IPNV) variable protein 2 (VP2) that had a molecular weight (MW) of 36 KDa was encapsulated in 332 nm PLGA
Summary
Edwardsiella tarda is a member of the Enterobacteriaceae family that infects different fish species and mammals. Bacterial outer membrane proteins (OMPs) are highly immunogenic and recognized as pathogen-associated molecular patterns (PAMPs) by pattern recognition receptors (PRRs) on host cells. They are conserved among different serovars [6,7] and have attracted a lot of interest in vaccine design. We wanted to determine whether oral vaccination using the rOmpA antigen encapsulated in chitosan nanoparticles would afford higher protection than the levels obtained in our previous studies, in which the rOmpA protein was intraperitoneally injected in fish without nanoparticle encapsulation. There is a need to develop highly protective vaccines with the capacity to induce long-term protection in vaccinated fish in order to increase the population of this species for food and nutritional security
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