Identification of surrogates of protection against yersiniosis in immersion vaccinated Atlantic salmon

Abstract

Simple cost-effective bacterins are the earliest and most successfully used commercial vaccines in fish. In particular, those prepared from Yersinia ruckeri have proven effective at controlling Enteric Red Mouth Disease (ERM) and yersiniosis in rainbow trout and Atlantic salmon, respectively. However, the emergence of outbreaks of ERM caused by atypical biotypes of Y. ruckeri and reports of vaccine failure resulting in mass mortality of hatchery Atlantic salmon has reinvigorated interest in vaccines against fish bacterial diseases. Therefore the objective of this study was to identify surrogates of protection against yersiniosis using cDNA microarray to characterise the response of host genes in the gills of unvaccinated and vaccinated Atlantic salmon challenged with Y. ruckeri. Differentially expressed genes were identified using two-way ANOVA and restricted to those with >2.5-fold change at P<0.05. Using cDNA microarray we identified the expression of 6 genes in response to infection and 4 genes associated with the protective host response to yersiniosis. Analysis by real-time PCR confirmed that three immunologically relevant genes, namely a cathelicidin (47-fold) and a C-type lectin (19-fold) increased in response to yersiniosis. Including collagenase (17-fold increase), an important tissue remodelling and repair enzyme, these genes represent 3 of 6 non-protective and/or pathological responses to yersiniosis. Genes associated with the protective host response included an immunoglobulin gene and a selenoprotein that showed significant fold changes (15-fold increases each), highlighting the importance of antibody-mediated protection against yersiniosis. These findings provide much needed knowledge of the host-pathogen interaction in response to bacterial infection and immunisation in fish. Significantly, we identified a transcriptional biosignature consisting of predominantly immune-relevant genes (14 up and 3 down-regulated) in the gills of Atlantic salmon after immersion vaccination and before bacterial challenge. This biosignature may be used as a surrogate of protection and therefore as a predictor of vaccine success against yersiniosis.

Figure 1. Survival analysis of naive unvaccinated and immersion vaccinated Atlantic salmon after experimental immersion challenge with Y. ruckeri at 6 weeks post-vaccination.

Kaplan-Meier survival analysis of Atlantic salmon (n = 90) immersion vaccinated with trypsinised yersinivac-B showed that vaccinated salmon had statistically significantly (P<0.01) greater survival than naive unvaccinated salmon (n = 90) 21 day post-challenge. The vaccinated salmon had a relative percent survival (RPS) of 57% in relation to the 83% mortality of the control (unvaccinated) salmon.

Figure 2. Venn diagram displaying differentially regulated genes in Atlantic salmon gill tissue that define vaccine success after vaccination but before challenge with Y. ruckeri.

The highlighted central square shows the number of differentially expressed genes (ANOVA P<0.05 and >2.5-fold change) found in common between the comparison of the vaccinated unchallenged group of fish (VU) and all the other treatment/vaccination combinations at both time points post-challenge (UI8h/UI72h and VI8h/VI72h) and was considered as a specific vaccine-induced biosignature.