NEW INSIGHTS INTO MYOCARDIAL ISCHEMIA/REPERFUSION INJURY - HOW TO DAMP-EN A RAGE-ING HEART
University of Virginia, Charlottesville, VA, USA
Background: Mechanisms underlying ischemia/reperfusion injury have been extensively investigated. However, treatments to reduce myocardial IRI have largely failed in the clinical application. Revisiting the mechanisms underlying IRI may provide new insights that may translate into more effective treatment regimens in the fight IRI.
Methods and Results: Using an in vivo mouse model of IRI, we have shown that myocardial infarction during reperfusion will not occur in mice unless the duration of ischemia is greater than 20 minutes. Mice subject to 40-min of myocardial ischemia, infarct size (IS) was 55% of risk region at 60-min after reperfusion. Inhibition of cell-free DNA (cfDNA) or HMGB1 upon reperfusion suffices to reduce IS to 30-33% of risk region. However, no further reduction in IS could be realized if both cfDNA and HMGB1 were inhibited at the same time. We found that after 40 min of ischemia, the myocardium released both HMGB1 and cfDNA into blood stream upon reperfusion. Perfusate from ischemic hearts had higher HMGB1 and cfDNA levels than blood sampled prior to reperfusion. Injection of perfusate from 40’/0’ (I/R) hearts or plasma from 40’/5’ mice to mice with 20’/60’ IRI upon reperfusion significantly increased IS. Depleting either cfDNA or HMGB1 in those same 40’/5’ plasma samples abolished the infarct exacerbation found in mice with 20’/60’ IRI. Furthermore, the infarct exacerbating effect of the plasma from 40’/5’ mice disappeared in splenectomized mice or RAGE KO mice.
Conclusions: Myocardium rendered necrotic by ischemia releases HMGB1 and cfDNA during reperfusion. The combination of these two Damage-Associated Molecular Pattern molecules are necessary to activate splenic leukocytes via RAGE to exacerbate myocardial infarction during reperfusion. This inter-organ signaling axis creates a feed-forward amplification loop that exacerbates reperfusion injury, such that little further myocardial injury will result from reperfusion, provided that there is no injury to cardiomyocytes during ischemia.