Comparison of transcriptome responses in blood cells of Atlantic salmon infected by three genotypes of Piscine orthoreovirus.

Piscine orthoreovirus (PRV) infection is common in aquaculture of salmonids. The three known PRV genotypes (PRV-1-3) have host species specificity and cause different diseases, but all infect and replicate in red blood cells (RBCs) in early infection phase. PRV-1 is the causative agent of heart and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon (Salmo salar), PRV-2 causes erythrocytic inclusion body syndrome (EIBS) in coho salmon (Oncorhynchus kisutch), while PRV-3 induces HSMI-like disease in farmed rainbow trout (Oncorhynchus mykiss). PRV-3 can also infect A. salmon without causing clinical disease and has been shown to cross-protect against PRV-1 infection and HSMI, while PRV-2 or inactivated adjuvanted PRV-1 vaccine only partially reduced HSMI pathologic changes. In the present work, we studied the transcriptional responses in blood cells of A. salmon two- and five-weeks post infection with PRV-1, PRV-2, PRV-3, or post injection with inactivated PRV-1 vaccine. PRV-1 and PRV-3 replicated well in A. salmon blood cells, and both induced the typical innate antiviral responses triggered by dsRNA viruses. Two weeks post infection, PRV-3 triggered stronger antiviral responses than PRV-1, despite their similar viral RNA replication levels, but after five weeks the induced responses were close to equal. PRV-2 and the InPRV-1 vaccine did not trigger the same typical antiviral responses as the replicating PRV-1 and PRV-3 genotypes, but induced genes involved in membrane trafficking and signaling pathways that may regulate physiological functions. These findings propose that the protection mediated by PRV-3 against a secondary infection by PRV-1 occur due to a potent and early activation of the same type of innate immune responses. The difference in the timing of antiviral responses may give PRV-1 an evolutionary edge, facilitating its dissemination to A. salmon heart, a critical step for HSMI development.