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

Authors: SHAHIN, KHALID - University Of California, Davis; PIREZAN, FELIPE - University Of California, Davis; ROGGE, MATT - University Of Wisconsin; Lafrentz, Benjamin; SHRESTHA, ROSHAN - University Of California, San Diego; HILDEBRAND, MARK - University Of California, San Diego; LU, FANGJIA - Purdue University; HOGENESCH, HARM - Purdue University; SOTO, ESTEBAN - University Of California, Davis

Submitted to: Fish and Shellfish Immunology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2020
Publication Date: 8/1/2020
Publication URL: https://handle.nal.usda.gov/10113/7045982
Citation: Shahin, K., Pirezan, F., Rogge, M., Lafrentz, B.R., Shrestha, R.P., Hildebrand, M., Lu, F., Hogenesch, H., Soto, E. 2020. Development of IglC and GroEL recombinant vaccines for francisellosis in Nile tilapia, Oreochromis niloticus. Fish and Shellfish Immunology. 105:341-349. https://doi.org/10.1016/j.fsi.2020.07.045.
DOI: https://doi.org/10.1016/j.fsi.2020.07.045

Interpretive Summary: Francisellosis is a bacterial disease caused by Francisella orientalis and is one of the major emergent diseases in the tilapia industry, where high mortality rates of up to 95% have been reported in farmed tilapia from around the world. Currently there are no commercially available vaccines and few treatment options exist. Previous experimental vaccines such as live attenuated vaccines have shown effectiveness in the lab, but concerns of safety, costs and complexity of production have limited their marketability and regulatory acceptance. In this study, we developed and tested an alternative vaccine in which two proteins of the bacterium were produced recombinantly in either bacterial or diatom expression systems. Nile tilapia were vaccinated with each protein combined with different adjuvants to boost the immune system, and then fish were experimentally infected with F. orientalis. The results demonstrated that one vaccine comprised of the IglC protein expressed in diatoms and Montanide adjuvant provided signification protection with 75% of the vaccinated fish surviving infection compared to controls. Following infection, the vaccinated fish that survived exhibited a strong antibody response and reduced numbers of F. orientalis in the spleen. These results may facilitate the development of a cost effective, safe, convenient, and highly protective recombinant vaccine against francisellosis in tilapia. Future studies will be performed to test and optimize the newly developed recombinant vaccine using different routes of administration (i.e. immersion and oral) and evaluation of the vaccine performance in field trials.

Technical Abstract: 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 IglC 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 (I.C.) with the recombinant IglC or GroEL vaccines, diatoms alone or phosphate buffer saline (PBS). Approximately 840-degree days post-vaccination, fish were challenged via immersion with 106 CFU/mL of wild-type Fo. Twenty-one days post challenge (dpc), the highest relative percent survival (RPS) was recorded in the IglC-Montanide group (75%), compared to 53.15%, 50%, 21.87%, 18.75% and 15.62% in the IglC-nanoparticles, GroEL-Montanide, GroEL-nanoparticles, diatoms-Montanide and diatoms-nanoparticles groups, respectively. Protection correlated with significantly higher specific antibody responses in the IglC-Montanide group. Moreover, a significantly lower bacterial load was detected in spleen samples from the IglC-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.