Abstract 1407: Interferon Regulatory Factor-9 (irf-9) Mediates Short Term Host Protection, But Promotes Long Term Immune Injury In Evolution Of Myocarditis Leading To Dilated Cardiomyopathy

INTRODUCTION: Evolution of viral myocarditis to dilated cardiomyopathy (DCM) represents a delicate balance between host innate immunity and T-cell acquired immunity. IRF-9 is a key member of a transcription factor family that regulates type I interferon (IFN) production, critical for innate antiviral protection. However, the influence of innate immunity in general and IRF-9 in particular on acquired immunity and DCM is unknown. HYPOTHESIS: IRF-9 signaling provides immediate host protection through interferon production, but stimulates acquired immunity leading to DCM in a coxsackievirus murine myocarditis model. METHODS: Interferon-regulatory factor 9 (IRF-9) homozygous knockout mice were generated, and show impaired type I interferon production. Wild-type (WT, n=51) and IRF-9 −/− (n=124) littermates were inoculated with 10^2 p.f.u. of coxsackievirus B3 as previously described. Survival, viral titers, cardiac hypertrophy, inflammation and fibrosis were evaluated on days 0, 4, 7, 10, 14, 28 and 42. Splenocyte subpopulations were cell sorted and quantitated by FACS. RESULTS: IRF9−/− mice showed dramatically increased mortality compared to the wild-type littermates (0% WT vs 72% IRF-9 −/− on day 14, P<0.0001). On day 42, there was less cardiac hypertrophy and inflammation in IRF-9 −/− mice compared to WT controls (p<0.05). There was no difference in fibrosis. The mature T-lymphocyte population, defined as CD4 or CD8 single positive, was statistically identical in the two populations up until and including day 28 post-infection. However on day 42 there was a dramatic increase in the number of cytotoxic and helper T-Cells in the wild-type mice that was not observed in the IRF-9 −/− spleens (p<0.05). CONCLUSIONS: These data suggest a novel dual role of IRF-9 in not only regulating interferon in acute stage of viral infection in myocarditis, but also late acquired immunity activation, including CD4/8 populations, contributing to the development of chronic cardiomyopathy.