Development of Ig1C and GroEL recombinant vaccines for francisellosis in Nile tilapia, Oreochromis niloticus

Warm-water piscine francisellosis is a granulomatous bacterial disease caused by Francisella orientalis (Fo). The disease has been detected in a wide range of fish species globally, causing mortalities as high as 90% and significant economic losses. Currently there are no commercially available vaccines and few treatment options exist. In the current study, two novel recombinant vaccines were prepared using diatom-expressed Ig1C or bacterial-expressed GroEL proteins. The vaccine antigens were emulsified with either nanoparticles or a commercially available oil-based adjuvant. Nile tilapia, Oreochromis niloticus, fingerlings were immunized intracoelomically with the recombinant Ig1C or GroEL vaccines, diatoms alone or phosphate buffer saline. Approximately 840-degree days post-vaccination, fish were challenged via immersion with 10(6) CFU/mL of wild-type Fo. Twenty-one days post challenge (dpc), the highest relative percent survival was recorded in the Ig1C-Montanide group (75%), compared to 53%, 50%, 22%, 19% and 16% in the Ig1C-nanoparticles, GmEL-Montanide, GroEL-nanoparticles, diatoms-Montanide and diatoms-nanoparticles groups, respectively. Protection correlated with significantly higher specific antibody responses in the Ig1C-Montanide group. Moreover, a significantly lower bacterial load was detected in spleen samples from the Ig1C-Montanide survivor tilapia compared to the other experimental groups. This is the first report of recombinant vaccines against piscine francisellosis in tilapia. The Fo vaccines described in our study may facilitate development of a safe, cost-effective and highly protective vaccine against francisellosis in farmed tilapia.