Background-Viral myocarditis is among the most common causes of heart failure in children and young adults. The RNA helicase melanoma differentiation-associated gene-5 (MDA5) is critical for host antiviral responses against members of the Picornaviridae family, such as encephalomyocarditis virus (EMCV). MDA5-knockout mice are highly susceptible to EMCV infection and develop significant myocardial injury and left ventricular dysfunction. However, the mechanisms by which MDA5 signaling within cardiac myocytes contributes to the host response against viral infection have not been defined. Methods and Results-We generated cardiac-specific MDA5 transgenic (alpha-myosin heavy chain [αMHC]-MDA5) mice. These mice showed increased baseline cardiac expression of antiviral cytokines and increased cellular infiltration but no alterations in cardiac function and structure. αMHC-MDA5 mice were less susceptible to EMCV infection and had a significantly lower cardiac viral load compared with littermate control mice. The severity of myocarditis, prevalence of cardiac myocyte apoptosis, and cleavage of caspase 3 after EMCV infection were attenuated in αMHC-MDA5 mice. Furthermore, αMHC-MDA5 mice were protected against EMCV-induced myocardial dysfunction. Conclusions-Our data suggest that myocardial MDA5 may be a key molecule in protecting the heart from direct viral injury and myocardial dysfunction. (Circ Heart Fail. 2013;6:326-334.)

TIR-domain-containing adaptor-inducing interferon-β (TRIF) plays a major role in Toll-like receptor 3 (TLR3) mediated signaling. Mice deficient in TLR3 and TRIF have been shown to be highly susceptible to enterovirus-induced myocardial injury. These mice have decreased production of antiviral cytokines and increased viral replication in the heart. Therefore, we hypothesized that conditional overexpression of TRIF would change cardiac myocyte susceptibility to virus infection by augmenting the antiviral response. We generated double-transgenic MHC-tTA/MHC(tetO)-TRIF mice (DT), with conditional cardiac-specific overexpression of TRIF. Naive DT mice had increased cardiac expression of antiviral cytokines and increased cellular infiltration but no alterations in cardiac function. DT mice were less susceptible to encephalomyocarditis virus (EMCV) infection and had a significantly lower viral load in the heart when compared to littermate (LM) and MHC(tetO)-TRIF (ST) mice. Histopathological examination showed that the severity of myocarditis was also attenuated in DT mice. Furthermore, the decreased virus titers in the DT mouse hearts led to less cardiac damage and better cardiac function when compared to LM and ST mice. Administration of doxycycline to DT mice suppressed the protective effects of TRIF overexpression in the heart. The findings of the present study establish the importance of cardiac-specific TRIF-mediated signaling in the heart in acute viral myocarditis and identify potentially important targets for diagnostic and therapeutic strategies.