Vaccination strategies and IgM responses against PKD in rainbow trout

Abstract

Abstract PKD is one of the most serious diseases affecting trout aquaculture in the UK. Caused by the myxozoan parasite, Tetracapsuloides bryosalmonae, PKD is elicited by the temperature-dependent development of parasite spore sacs in colonial bryozoans. Since recovered fish are known to exhibit protective immunity to re-infection, a successful treatment, based on the reduction of kidney pathology and parasite burden, could markedly reduce fish mortalities leading to improved productivity and fish welfare. Our investigations have focused on the selection of putative T. bryosalmonae virulence factors to unravel host immune evasion mechanisms exploited by the parasite, whilst shortlisting candidates for vaccine studies. Here we report the results of three DNA-vaccination field trials using selected antigens administered individually or in combination. Some vaccine groups were found to have a partial protective effect in reducing PKD-associated kidney pathology whilst decreasing parasite load. Pathology reduction was improved in successive trials by improving the vaccination strategy. We have functionally characterized the most promising antigen from our vaccine studies, which represents a novel micro-exon gene (Tb-MEG1). MEGs, until now, were thought to be unique to helminth parasites. In schistosomes, they exhibit extensive antigenic variability that is thought to enable greater plasticity in host protein targeting as a mechanism of host immune subversion over the course of infection. Using a validated anti Tb-MEG1 MoAb, we have demonstrated the protein to be expressed in and on the surface of parasites and a subset of immune cells within the kidneys of infected fish. We have also demonstrated potent Tb-MEG1-specific IgM responses in sera from parasite-infected (farmed) rainbow trout and have successfully induced a specific IgM response after protein vaccination. DNA vaccines encoding molecules homologous to proteins involved in nutrition acquisition, cell-cell interactions or of unknown function, have shown promise towards the development of a future PKD vaccine, that may also be applicable to the generation of vaccines against other fish parasites. The discovery and characterization of T. bryosalmonae antigens has provided valuable insights into host immune evasion by myxozoan parasites, with the present Tb-MEG1 studies also having major implications towards understanding the evolution of antigenic variability in metazoan parasites. Keywords: Vaccination, myxozoan, parasite, antigen, igm responseCopyright © 2019