RECOMBINANT SUBUNIT VACCINES AGAINST PISCINE FRANCISELLOSIS

Esteban Soto*, Fernanda de Alexandre Sebastião, Matt Rogge, Alvin Camus,  John D. Hansen
 
University of California-Davis, Department of Medicine and Epidemiology, School of Veterinary Medicine, Davis, CA
sotomartinez@ucdavis.edu
 

Francisella noatunensis subsp. orientalis (Fno) is an emerging bacterial pathogen of marine and fresh water fish with worldwide distribution. Fish francisellosis is a severe sub-acute to chronic granulomatous disease with high mortalities and high infectivity rates in cultured and wild fish. To date, there is no approved vaccine for this widespread emergent disease. To better characterize immunodominant Fno antigens, proteomic analyses was performed using serum collected from laboratory challenged Nile tilapia (Oreochromis niloticus). Western blot analysis consistently detected antigens between 20-30 kDa. Further analysis identified 19 proteins including both housekeeping and virulence-related proteins. We hypothesized that some of the identified proteins could be used as recombinant subunit vaccines against piscine francisellosis.  In this study, the Fno iglC, and vgrG were cloned into an Escherichia coli expression vector, inactivated with formalin or heat, and used to immunized Nile tilapia fingerlings via intracoelomic injection. Thirty days post-immunization, fish were challenged with wild-type Fno. Naive tilapia vaccinated with r-iglC, and r-vgrG, and subsequently challenged with wild type Fno presented lower mortality when compared to non-vaccinated controls. This information demonstrates that defined recombinant subunit vaccines have the potential to be used as a safe prophylactic practice against piscine francisellosis.

Mean percent mortality of tilapia vaccinated with different vaccine treatments by intracoelomic injection, or mock vaccinated with PBS (Controls) and challenged 4 weeks later with WT Francisella noatunensis subsp. orientalis. Fish were vaccinated with: formalin inactivated Escherichia coli vector, heat inactivated E. coli vector, formalin inactivated E. coli vector expressing recombinant iglC, heat inactivated E. coli vector expressing recombinant iglC, formalin inactivated E. coli vector expressing recombinant vgrG, or heat inactivated E. coli vector expressing recombinant vgrG. Four weeks post-immunization fish were challenged with 105 CFU/ml of WT Fno. Mean percent mortality was calculated 30 days post-challenge with WT. Each bar represents the mean percent mortality ± standard error of three tanks (15 fish/tank).