Ablation of Matrix Metalloproteinase-9 Increases Severity of Viral Myocarditis in Mice

Background—Coxsackievirus B3 (CVB3) causes human myocarditis, which can result in cardiac damage, maladaptive remodeling, and heart failure. Matrix metalloproteinases (MMP)-8 and -9 have been identified in virus-infected myocardium, but their particular roles and underlying mechanisms of effect are unknown. For the first time, we examine the severity of CVB3-induced myocarditis in MMP-8–and MMP-9–deficient mice. Methods and Results—CVB3-infected MMP-8 and MMP-9 knockout (KO) mice and corresponding wild-type (WT) mice were euthanized and harvested at 9 days after infection. Expression of MMP-2, -8, -12, and -13 and tissue inhibitors of MMPs was assessed by zymography or immunoblotting on harvested hearts, and in situ hybridization was performed to detect active infection. Infected MMP-9 KO mice had greater myocardial injury and foci of infection than WT mice despite similar pancreatic infection. Increased fibrosis (10.6 2.7% versus 7.1 2.6%, P 0.04), viral titer, as well as decreased cardiac output, were evident in MMP-9 KO compared with WT mice as assessed by picrosirius red staining, plaque assay, and echocardiography, respectively. Immune infiltration was also greatly increased in MMP-9 KO compared with WT mice (15.2 12.6% versus 2.0 3.0%, P 0.002). Myocardial interferon1, interferon, interleukin-6, interleukin-10, and macrophage inflammatory protein-1 expression was elevated in MMP-9 KO mice as measured by quantitative real-time polymerase chain reaction and ELISA. In contrast, MMP-8 KO mice had the same degree of cardiac injury, fibrosis, and viral infection as their WT counterparts. Conclusions—During acute CVB3 infection, MMP-9 appears necessary to halt virus propagation in the heart, promote proper immune infiltration and remodeling, and preserve cardiac output. (Circulation. 2008;117:1574-1582.)