Effect of oral booster vaccination of rainbow trout against Yersinia ruckeri depends on type of primary immunization

Novel development of cationic surfactant-based mucoadhesive nanovaccine for direct immersion vaccination against Francisella noatunensis subsp. orientalis in red tilapia (Oreochromis sp.)

Francisella noatunensis subsp. orientalis (Fno) is one of the infectious diseases that causes economic losses associated with tilapia mortality. Even though direct immersion administration of vaccines is more practicable for small fish and fry compared with oral and injection vaccination in the fields, the efficacy is still insufficient due to lower potency of antigen uptake. Herein, we accomplished the development of a mucoadhesive nanovaccine platform using cetyltrimethylammonium bromide (CTAB), a cationic surfactant, to improve the efficiency of immersion vaccination against Fno in tilapia. Cationic Fno nanovaccine (CAT-Fno-NV) was prepared though emulsification using an ultrasonic method. In our investigation, the CAT-Fno-NV increased the opportunity of Fno vaccine uptake by extending the contact time between vaccine and mucosal surface of fish gills and enhancing the protective efficacy against Fno infection. Fish were vaccinated with the CAT-Fno-NV by a direct immersion protocol. The challenge trial by Fno injection revealed that CAT-Fno-NV at the concentration 1:100 ratio (approximately 1 × 10⁶ cfu/mL) had the highest efficacy to protect fish from Fno infection at day 30 after post challenge period according to the total number of Fno detected in head kidney, spleen and liver. A significant upregulation of IgM gene was observed in gills, skin, head kidney, serum and peripheral blood lymphocytes (PBLs) and spleen tissues treated with WC and CAT-Fno-NV (1:100) vaccines, while IgT gene was highly expressed in only gills and skin tissues for treated WC and CAT-Fno-NV (1:100) groups. We anticipate that the cationic surfactant-based nanovaccine developed in this study could become an efficient alternative for direct immersion vaccination to induce humoral immune responses against Fno in vaccinated tilapia.

Primary immunization using low antigen dosages and immunological tolerance in rainbow trout

Enteric redmouth disease (ERM), caused by the Gram negative enterobacterium Yersinia ruckeri, affects farming of salmonids, but vaccination against ERM confers a certain degree of protection dependent on the administration route. Recent studies on oral vaccination of rainbow trout suggest that immunological tolerance may be induced by primary immunization using a low antigen dosage. We have examined if low dosages of Y. ruckeri antigens, applied in feed or bath exposure over a prolonged period of time, leave rainbow trout more susceptible to infection. Groups of rainbow trout were immunized, either by immersion or feeding using different vaccine dosages, and subsequently challenged by live Y. ruckeri. Survival was recorded and immune reactions in surviving fish were evaluated (ELISA and qPCR). Trout, bath-vaccinated in a highly diluted vaccine or fed the same amount of bacterin in feed over 10 days, were not protected against Y. ruckeri challenge infection and in some cases these sub-optimally immunized fish experienced lower survival compared to non-primed controls. Genes encoding FoxP3 and immune-suppressive cytokines were down-regulated in fish vaccinated with a high antigen dosage when compared to groups exposed to low antigen dosages, suggesting a higher regulatory T cell activity in the latter fish groups. The study suggests that repeated exposure to low antigen concentrations induces some degree of immune tolerance in rainbow trout and we recommend application of high antigen dosages for primary immunization of trout.