The infection biology of piscine myocarditis virus in Atlantic salmon: Can a local immune response aid eliminating the virus from the heart

Cardiomyopathy syndrome, caused by piscine myocarditis virus (PMCV), continues to pose a significant threat to the Norwegian Atlantic salmon farming industry. In this study, we assessed expression of various innate and adaptive immune marker genes in atrium, ventricle, spleen, and head kidney (HK) over a period of 16 weeks post PMCV infection (WPI), with samples collected weekly or biweekly. The results showed early detection of virus in HK at 1 WPI, while other tissues tested positive from 2 WPI. A trend of virus clearance was noted in ventricle, with 3 out of 6 fish testing negative for virus at 16 WPI, while a low but persistent virus load was observed in other tissues. An overall positive correlation between virus load and cell-mediated immune markers was identified. When comparing the two heart compartments, atrium exhibited higher expression of proinflammatory, anti-inflammatory, and cytotoxic T cell marker genes than ventricle. However, similar difference was not evident between the HK and spleen. Furthermore, these marker genes exhibited higher expression in heart compartments compared to lymphoid tissues. An antiviral response mediated by Mx was evident across all four tissues, while the type I IFN response did not display a clear pattern of expression during the infection process. Overall, the results indicate that more severe CMS disease progression in atrium compared to ventricle could be attributed to higher inflammatory and cytotoxic T cell responses related to higher viral load/replication. Additionally, our findings highlight early primary virus replication and long-term persistence in lymphoid tissues, with no signs of clearance.