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Rationale: Immediate changes in the ECM (extracellular matrix) microenvironment occur after myocardial ischemia and reperfusion (I/R) injury. Objective: Aim of this study was to unravel the role of the early hyaluronan (HA)-rich ECM after I/R. Methods and Results: Genetic deletion ofHas2 andHas1 was used in a murine model of cardiac I/R. Chemical exchange saturation transfer imaging was adapted to image cardiac ECM post-I/R. Of note, the cardiac chemical exchange saturation transfer signal was severely suppressed byHas2 deletion and pharmacological inhibition of HA synthesis 24 hours after I/R.Has2 KO (Has2 deficient) mice showed impaired hemodynamic function suggesting a protective role for endogenous HA synthesis. In contrast toHas2 deficiency,Has1 -deficient mice developed no specific phenotype compared with control post-I/R. Importantly, inHas2 KO mice, cardiac macrophages were diminished after I/R as detected by19 F MRI (magnetic resonance imaging) of perfluorcarbon-labeled immune cells, Mac-2/Galectin-3 immunostaining, and FACS (fluorescence-activated cell sorting) analysis (CD45+ CD11b+ Ly6G− CD64+ F4/80+ cells). In contrast to macrophages, cardiac Ly6Chigh and Ly6Clow monocytes were unaffected post-I/R compared with control mice. Mechanistically, inhibition of HA synthesis led to increased macrophage apoptosis in vivo and in vitro. In addition, α-SMA (α-smooth muscle actin)–positive cells were reduced in the infarcted myocardium and in the border zone. In vitro, the myofibroblast response as measured byActa2 mRNA expression was reduced by inhibition of HA synthesis and of CD44 signaling. Furthermore,Has2 KO fibroblasts were less able to contract collagen gels in vitro. The effects of HA/CD44 on fibroblasts and macrophages post-I/R might also affect intercellular cross talk because cardiac fibroblasts were activated by monocyte/macrophages and, in turn, protected macrophages from apoptosis.Conclusions: Increased HA synthesis contributes to postinfarct healing by supporting macrophage survival and by promoting the myofibroblast response. Additionally, imaging of cardiac HA by chemical exchange saturation transfer post-I/R might have translational value.

Cardiac Hyaluronan Synthesis Is Critically Involved in the Cardiac Macrophage Response and Promotes Healing After Ischemia Reperfusion Injury