Dissecting myocardial and systemic drivers of cardiac dysfunction in the murine CVB3 myocarditis model using microRNA-guided viral detargeting

In murine coxsackievirus B3 myocarditis, the most widely used experimental model of viral myocarditis, functional alterations such as reduced cardiac output are frequently interpreted as surrogates of myocardial injury and inflammation, despite concurrent systemic illness. The relative contribution of systemic disease processes to these functional changes remains poorly defined. We therefore aimed to disentangle myocardial and systemic drivers of cardiac dysfunction using a microRNA-guided viral detargeting approach. We engineered a cardiomyocyte-detargeted virus by inserting target sequences for the muscle-enriched microRNA-1 into the viral genome, selectively suppressing viral replication in cardiac muscle while largely preserving systemic infection. This enabled direct comparison between conventional myocarditis and cardiac-attenuated infection in mice, using serial echocardiography as the primary non-invasive functional readout of cardiac performance. Day 3 post infection was analyzed as the acute phase and day 7 as a later stage associated with peak myocardial inflammation. Cardiac viral titers were profoundly reduced by detargeting (approximately 3-log at day 3 and 6-7-log at day 7), accompanied by absence of troponin T release, inflammatory cell infiltration, and histological myocarditis. Despite this, cardiac output was reduced at day 3 and remained impaired at day 7 in both groups. This reduction was associated with decreased left ventricular end-diastolic volume, while ejection fraction remained preserved. At day 3, additional reductions in cardiac output and global longitudinal strain were observed in control-virus-infected mice. At day 7, cardiac output remained comparably reduced despite marked differences in myocardial injury. These findings demonstrate that systemic infection substantially contributes to impaired cardiac function and that echocardiographic readouts reflect a composite phenotype. Functional alterations should therefore be interpreted in the context of systemic effects and integrated with cross-organ analyses to enable accurate interpretation in preclinical myocarditis models.