Long-term effects of reovirus strain T3D on the myocardium.

Mammalian orthoreovirus (reovirus) is a well-established model for studying viral pathogenesis. Although studies of reovirus have largely been focused on central nervous system disease, reoviruses can also cause myocarditis. Reovirus strain type 1 Lang (T1L) causes mild myocarditis in neonatal mice. However, many highly myocarditic reoviruses are reassortants between T1L and the serotype 3 (T3) Dearing strain that is non-myocarditic. Although cardiac immune responses to T1L are well described, many open questions remain regarding cardiac immune responses to T3 reoviruses. To better understand the effects of T3 reoviruses on the heart, we investigated the long-term cardiac impact of reovirus strain T3 Dearing (T3D) in neonatal C57BL/6 mice. Oral infection with T3D resulted in subclinical myocarditis that was non-lethal but still produced persistent histological myocardial alterations. Echocardiographic analysis revealed a mild decrease in diastolic left ventricular anterior wall thickness in mice infected with 10⁴ PFU, though no consistent dose-dependent functional impairments were observed. Histological examination identified myocardial lesions characterized as replacement fibrosis that developed independently of the inoculating dose. Flow cytometry showed an early immune response at 8 days post-infection, with increased CD4 T cells, CD8 T cells, B cells, and innate immune cells. By 26 days post-infection, the inflammation had largely resolved, but low-level immune infiltration persisted, characterized by CD4 T cells, CD8 T cells, and B cells. These findings suggest that T3D induces subclinical myocarditis with lasting histopathological changes. The presence of fibrosis raises concerns about potential long-term cardiac effects, emphasizing the need for further research into the myocardial impact of non-myocarditic reoviruses.IMPORTANCEViral infections that cause myocarditis are a significant cause of morbidity and mortality worldwide, particularly in children and young adults. Mammalian orthoreoviruses (reoviruses) are an established model for studying viral myocarditis in mice and are also under development as a cancer therapeutic due to their capacity to kill cancer cells. Here, we describe the long-term myocardial effects of the type 3 Dearing (T3D) reovirus strain, which is classically considered neurotropic. Our findings indicate that T3D causes subclinical myocarditis that is non-lethal but produces histopathological changes indicative of fibrosis. Thus, although T3D does not cause overt acute cardiac pathology, it can have long-term effects on the myocardium. This work will inform future studies on reovirus tropism and is relevant to ongoing efforts to harness the oncolytic properties of reoviruses for therapeutic applications.