Temporal dynamics of viral open reading frames (ORFs) abundance and innate immune sensing during piscine myocarditis virus infection in Atlantic salmon

The study investigates the temporal and tissue-specific dynamics of PMCV infection in the atrium, head kidney tissue (HKT), and head kidney leukocytes (HKL) over 16 weeks. Viral RNA was detected earliest in HKT (1 WPI), followed by HKL (2 WPI) and the atrium (4 WPI). Peak viral RNA levels were highest in the atrium, coinciding with the onset of cardiac pathology, while HKT and HKL showed lower peaks. Viral RNA levels stabilized in HKT and HKL after 7 WPI but persisted at low levels in the atrium until 16 WPI, indicating chronic infection. ORF3 showed higher abundance across all examined tissues, peaking at 8 WPI in the atrium, where it aligned with maximum pathology scores and upregulation of immune-related genes. ORF1 and ORF2 exhibited lower abundance, with ORF1 consistently the least abundant. The upregulation of endosomal pattern recognition receptors ( tlr3, tlr7, and tlr22 ) and cytosolic RNA sensors ( rig-i and mda5 ) was most pronounced in the atrium, suggesting their important role in viral sensing and immune activation. Adaptor molecules mavs , ticam , and myd88 showed tissue-specific activation, with the atrium exhibiting the highest upregulation, reflecting robust immune activation. The transcription factor irf7 exhibited consistent upregulation, particularly in the atrium, aligning with viral RNA levels and indicating a strong interferon-mediated antiviral response. Overall, the study highlights the atrium as the primary site of PMCV replication and pathology, with ORF3 and PRR signaling playing central roles in tissue specific infection dynamics and immune response.