Salmonid rickettsial septicemia (SRS), caused by the intracellular bacterium Piscirickettsia salmonis, is a significant infectious disease affecting farmed salmonids, resulting in substantial economic losses for the Chilean aquaculture industry. Vaccination is widely used as a preventive strategy to mitigate the impact of SRS. In this study, we evaluated the effect of a vaccine against P. salmonis on fish survival, bacterial load, and its relationship with the expression of iron metabolism-related genes under field conditions.
Fish were intraperitoneally vaccinated with either a pentavalent inactivated vaccine combined with a monovalent live-attenuated P. salmonis vaccine (SIA) or a tetravalent attenuated vaccine lacking P. salmonis antigens (SS, sham control). While survival rates did not differ between groups up to 28 weeks post-seawater transfer, SIA-vaccinated fish exhibited significantly higher survival than SS-vaccinated fish by week 41 (85% vs. 52%, respectively). Quantitative PCR analysis confirmed a lower bacterial load in the SIA group compared to the SS group during infection. Additionally, gene expression analysis revealed an upregulation of ferritin and hepcidin in freshwater. In contrast, transferrin and its receptor were upregulated in seawater in the SIA group compared to the SS group, suggesting a role for iron metabolism in vaccine efficacy. In vitro infection assays showed that TF-KO cells exhibited reduced cytopathic effects, lower formation of P. salmonis-containing vacuoles (PCVs), and increased viability compared to wild-type (TF-WT) cells. a global transcriptomic analysis comparing them to TF-WT cells, identified 311 differentially expressed genes with functional enrichment in metal-binding and zinc-dependent processes. These findings suggest that transferrin deficiency confers an infection-tolerant phenotype through transcriptional adaptations related to metal homeostasis and other cellular processes, providing new insights into host-pathogen interactions and potential strategies for SRS control.