Two bicistronic DNA vaccines against Vibrio anguillarum and the immune effects on flounder Paralichthys olivaceus

Chemokines are cytokines that can promote the activation and migration of immune cells, and increase the recognition of antigen by antigen-presenting cells (APC). Previous studies showed that a DNA vaccine can induce humoral and cellular immune responses of flounder after immunization. To explore the improvement of chemokines on the efficiency of OmpK vaccine, two bicistronic DNA candidate vaccines were constructed and the immune responses they induced in the flounder were investigated by reverse transcription polymerase chain reaction (RT-PCR), indirect immunofluorescent assay (IFA), H&E staining, flow cytometry (FCM), and quantificational real-time polymerase chain reaction (qRT-PCR). pBudCE4.1 plasmid as an expression vector, bicistronic DNA vaccines encoding OmpK gene and CC-motif ligand 4 gene (p-OmpK-CCL4), or Ompk gene and CC-motif ligand 19 gene (p-OmpK-CCL19) were successfully constructed. The results showed that two bicistronic DNA vaccines expressed Ompk protein of Vibrio anguillarum and CCL4/CCL19 proteins of flounder both in vitro and in vivo. After immunization, a large number of leucocytes in muscle were recruited at the injection site in treatment groups. The constructed vaccines induced significant increases in CD4-1⁺ and CD4-2⁺ T lymphocytes, and sIgM⁺ B lymphocytes in peripheral blood, spleen, and head kidney. The percentage of T lymphocytes peaked on the 14^th post-vaccination day whereas that of B lymphocytes peaked in the 6^th post-vaccination week. Moreover, the expression profiles of 10 immune-related genes increased in muscles around the injection site, spleen, and head kidney. After the challenge, p-OmpK-CCL4 and p-OmpK-CCL19 conferred a relative percentage survival (RPS) of 74.1% and 63.3%, respectively, higher than p-OmpK alone (40.8%). In conclusion, both CCL4 and CCL19 can improve the protection of p-OmpK via evoking local immune response and then humoral and cellular immunity. CCL4 and CCL19 will be potential molecular adjuvants for use in DNA vaccines.

Comparative immunogenicity of outer membrane protein K and whole-cell antigens of Vibrio parahaemolyticus for diagnosis

The immunogenicity of soluble outer membrane protein K (OmpK)- small ubiquitin-like modifier, OmpK inclusion bodies, formalin, and heat-killed Vibrio parahaemolyticus cells were prepared and studied in a mouse model. The results of whole-cell ELISA and Western blot (WB) revealed that the serum against soluble OmpK and OmpK inclusion bodies reacted only with homologous V. parahaemolyticus. Furthermore, recombinant OmpK proteins were not recognized by the serum against whole-cell V. parahaemolyticus antigens. Unexpectedly, the serum against formalin and heat-killed V. parahaemolyticus reacted broadly with homologous (an immunization strain) and heterologous (non-immunization strains) V. parahaemolyticus and Vibrio species. The WB results revealed that the serum against the two V. parahaemolyticus whole-cell antigens primarily reacted with proteins that were approximately 100, 70, 36, 28, and 22 kDa in the cell lysates from different Vibrio strains, rather than the recombinant OmpK. The 70 and 28 kDa proteins exhibited specificity to Vibrio species, while the 22 kDa protein was more specific to V. parahaemolyticus. This study showed the limitation of recombinant OmpK to prepare diagnostic antibodies and revealed several specific Omps of Vibrio sp. and V. parahaemolyticus that were promising in diagnosis and vaccine development.

Potential use of chitosan nanoparticles for oral delivery of DNA vaccine in black seabream Acanthopagrus schlegelii Bleeker to protect from Vibrio parahaemolyticus

To develop an effective and easy-to-administer vaccine against vibriosis of fish, the chitosan nanoparticles-loaded DNA vaccine against Vibrio parahaemolyticus was studied. A DNA vaccine was constructed using the outer membrane protein K (ompK) gene of V. parahaemolyticus strain (OS4) and pEGFP-N2 , a eukaryotic expression vector, and the construct was named pEGFP-N2 -OMPK (pDNA). The pDNA was encapsulated in chitosan particles (chitosan/pDNA). The effective diameter, mean diameter and polydispersity of the particles were 284.4 nm, 218.9 nm and 0.160, respectively. Scanning electron microscopy showed that the particles are dispersed as individual nanoparticles with spherical shape of around 200 nm and are homogeneously distributed. Encapsulation efficiency and loading percentage of nanoparticles were 91.5% and 2.08%, respectively. Reverse transcription-polymerase chain reaction (RT-PCR) showed that RNA-containing information of the ompK gene existed in mid-intestine, liver, kidney and muscle 3 weeks after oral administration in black seabream Acanthopagrus schlegelii Bleeker. Expression of the reporter gene, green fluorescent protein (GFP), was observed in the above-mentioned tissues by fluorescence microscopy. Expression of the ompK gene within 3 weeks evoked an immune response. Black seabream was protected from V. parahaemolyticus (OS4), with 72.3% relative percentage survival (RPS) 3 weeks post-vaccination with chitosan/pDNA. The direct agglutination test indicated that oral administration with chitosan/pDNA induced an antibody immune response in fish against V. parahaemolyticus (OS4). Data obtained, here and in other related studies, suggest that chitosan nanoparticles are promising carriers for an oral pDNA vaccine.