Aeromonas salmonicida subsp. salmonicida, the etiologic agent of furunculosis of salmonid fish, is a major pathogen of fisheries worldwide. The mechanisms of pathogenicity and the factors leading to a protective immune response with current bactrin vaccines remain poorly understood. The ADP ribosyltransferase exotoxin AexT and the type-three secretion system (T3SS) are recognized as major virulence attributes of A. salmonicida. Deletion of central genes of the T3SS block AexT secretion and translocation to rainbow trout (Oncorhynchus mykiss) gonad cells (RTG-2), lead to a total loss of cytotoxicity toward RTG-2 and to a complete loss of virulence upon infection of rainbow trout.
Evaluating the role of T3SS antigens in mounting a protective immune response against furunculosis, we vaccinated rainbow trout intraperitoneally with bacterins prepared from a wt A. salmonicida strain and an isogenic strain having a full deletion of the T3SS (ΔascV). Fish were challenged with a hyper-virulent wt strain eight weeks after vaccination. The survival rate of trout vaccinated with the ΔascV strain was significantly higher in comparison to the group vaccinated with the wt. Also, fish vaccinated with recombinant AcrV were not protected against a challenge while fish vaccinated with surface protein VapA were partially protected.
High-throughput proteomics was used to display the differences between in vitro secretome of wt A. salmonicida and T3SS-deficient mutant. Results confirmed the secretion via T3SS of effectors AopH, AexT, AopP and AopO as well as other known as effectors and needle subunits in hyper-virulent A. salmonicida. Several of these factors have immunosuppressive activity.
The presence of T3SS proteins in the vaccine preparations decreased the level of protection against A. salmonicida infection assumingly via an immunosuppressive action and that AcrV was not a protective antigen. These results challenge the hypothesis that mounting specific antibodies against T3SS proteins should bring better protection to fish.