Yersinia ruckeri lipopolysaccharide is necessary and sufficient for eliciting a protective immune response in rainbow trout (Oncorhynchus mykiss, Walbaum)

Authors: Welch, Timothy - Tim; LAPATRA, SCOTT - Clear Springs Foods, Inc

Submitted to: Fish and Shellfish Immunology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/22/2015
Publication Date: 1/4/2016
Citation: Welch, T.J., Lapatra, S. 2016. Yersinia ruckeri lipopolysaccharide is necessary and sufficient for eliciting a protective immune response in rainbow trout (Oncorhynchus mykiss, Walbaum). Fish and Shellfish Immunology. 49:420-426. doi:10.1016/j.fsi.2015.12.037

Interpretive Summary: Aquaculture is a significant commercial activity worldwide and infectious diseases are a major impediment to production efficiency and animal welfare. A highly effective vaccine was developed in the 1970's to prevent infection caused by the bacterial pathogen Yersinia ruckeri. The unusual success of this vaccine has led to the use of Y. ruckeri vaccination as a model system for better understanding immersion vaccination. While much has been learned regarding host response to Y. ruckeri vaccination the bacterial components necessary for eliciting this protective response remain unclear. We demonstrate that highly purified Y. ruckeri lipopolysaccharide (LPS) alone is a highly potent immunogen and sufficient for eliciting a strong protective response. Bacterial LPS is the major surface component present in most gram-negative bacteria. We also created a defined Y. ruckeri mutant lacking LPS and use this mutant to demonstrate that LPS is an essential component of the whole cell vaccine. Together these results suggest that LPS is the only cellular component contributing to the protective response elicited by the Y. ruckeri bacterin vaccine. We propose that the exceptionally high potency of Y. ruckeri LPS accounts for the unusual success of this vaccine when delivered by immersion. This work contributes to a better understanding of Y. ruckeri vaccination by identifying the bacterial factors necessary for eliciting a protective response.

Technical Abstract: Enteric redmouth disease (ERM), caused by Yersinia ruckeri, has been controlled successfully using immersion-applied bacterin vaccines for several decades. While the host response to vaccination and the mechanism of protection of this vaccine have been elucidated, the bacterial components eliciting protection have remained unclear. Here we show that highly purified serotype 01 Y. ruckeri lipopolysaccharide (LPS) is sufficient to induce a protective response to experimental challenge. Dose response experiments demonstrated that Y. ruckeri LPS at doses of 1 ng/fish and above resulted in essentially complete protection and doses as low as 0.01 ng/fish (1.38 ng/kg) resulted in significant protection, thus demonstrating the extremely high potency of this immunogen. Analysis of the Y. ruckeri genome identified a cluster of putative 0-antigen biosynthetic genes specific to serotype 01 strains. This cluster primarily consisted of genes encoding proteins predicted to function in the biosynthesis of legionamic acid, a nonulosonic acid know to be part of the 0-polysaccharide repeat of 01 Y. ruckeri. Mutation of one of the identified nonulosonic acid biosynthesis genes (nab2) resulted in production of severely truncated forms of LPS. Vaccination with bacterin vaccines derived from the nab2 mutant and its wild type parent strain demonstrated that LPS is a required component of the whole-cell bacterin vaccine and suggests that LPS is the only cellular component contributing to the protective response elicited by this vaccine. We speculate that the exceptionally high potency of Y. ruckeri LPS accounts for the unusual success of this vaccine when delivered by immersion.