Non-adjuvanted flagellin elicits a non-specific protective immune response in rainbow trout (Oncorhynchus mykiss, Walbaum) towards bacterial infections

Enteric redmouth disease, caused by Yersinia ruckeri, may result in high mortalities in farmed salmonids. Prophylaxis has been achieved with an immersion vaccine comprised of inactivated serovar 1 biotype 1 (motile) Y. ruckeri cultures. However, there has been a growing number of enteric redmouth outbreaks in vaccinated livestock associated with serovar 1 biotype 2 (non-motile) Y. ruckeri strains which do not produce flagellin. It was the aim of this study to evaluate the protective role of flagellin in enteric redmouth vaccines. Results showed that flagellin in the inactivated whole-cell vaccine were not the main immunoprotective molecule in eliciting a protective immune response towards infection. However, use of non-adjuvanted flagellin as a sub-unit vaccine, both in the native and recombinant form, resulted in a potent non-specific protective function towards challenge with biotype 1 (flagellin-producing) and biotype 2 (flagellin-devoid) Y. ruckeri. This vaccine can also protect rainbow trout against other microbial fish pathogens, for example Aeromonas salmonicida. Thus non-adjuvanted flagellin may have potential as a non-specific vaccine for fish towards bacterial pathogens.