Transfer of serum and cells from Yersinia ruckeri vaccinated doubled-haploid hot creek rainbow trout into outcross F1 progeny elucidates mechanisms of vaccine-induced protection

• As little as 0.5 μL of plasma from vaccinated fish protects naïve fish against ERM.
• High molecular weight IgM is the protective component of the plasma.
• Y. ruckeri BT1 vaccination induces protective against either BT1 or BT2 strains.
• Adoptively transferred PBL provide no protection.
• A new paradigm for elucidating vaccine-induced immune mechanism(s) in rainbow trout.

Yersinia ruckeri is a well-established bacterial pathogen for many salmonid species, against which a formalin-killed bacterin vaccine has been effective in reducing disease outbreaks. Previous studies have reported conflicting results about the protective value of the systemic humoral response to Y. ruckeri vaccination. Here we directly demonstrate that plasma contains the long-term protective component elicited by both immersion and intraperitoneal injection vaccination of rainbow trout. A total of 0.5 μL of plasma from vaccinated fish provided almost complete protection against experimental challenge. Conversely, the cells obtained from peripheral blood conferred little or no protection in naïve recipients. The protective component of immune sera was IgM based on size exclusion chromatography and recognition by monoclonal antibody Warr 1–14. Immune plasma generated against a Y. ruckeri biotype 1 strain protected equally against challenges with Y. ruckeri biotype 1 and 2 strains. These results illustrate the importance of the humoral IgM response against Y. ruckeri and the use of doubled haploid rainbow trout (Oncorhynchus mykiss) and transfer of plasma/serum and cells into F1 outcross progeny as a model system for dissection of the mechanism(s) of vaccine-induced protection.