Natural IgM Blockade Limits Infarct Expansion and Left Ventricular Dysfunction in a Swine Myocardial Infarct Model

Abstract

Background: Acute coronary syndrome is the leading cause of mortality worldwide. However, treatment of acute coronary occlusion inevitably results in ischemia-reperfusion injury. Circulating natural IgM has been shown to play a significant role in mouse models of ischemia-reperfusion injury. A highly conserved self-antigen, nonmuscle myosin heavy chain II, has been identified as a target of pathogenic IgM. We hypothesized that a monoclonal antibody (m21G6) directed against nonmuscle myosin heavy chain II may inhibit IgM binding and reduce injury in a preclinical model of myocardial infarction. Thus, our objective was to evaluate the efficacy of intravenous m21G6 treatment in limiting infarct expansion, troponin release, and left ventricular dysfunction in a swine myocardial infarction model.

Methods and results: Massachusetts General Hospital miniature swine underwent occlusion of the midleft anterior descending coronary artery for 60 minutes, followed by 1 hour, 5-day, or 21-day reperfusion. Specificity and localization of m21G6 to injured myocardium were confirmed using fluorescently labeled m21G6. Treatment with m21G6 before reperfusion resulted in a 49% reduction in infarct size (P<0.005) and a 61% reduction in troponin-T levels (P<0.05) in comparison with saline controls at 5-day reperfusion. Furthermore, m21G6-treated animals recovered 85.4% of their baseline left ventricular function as measured by 2-dimensional transthoracic echocardiography in contrast to 67.1% in controls at 21-day reperfusion (P<0.05).

Conclusions: Treatment with m21G6 significantly reduced infarct size and troponin-T release, and led to marked preservation of cardiac function in our study. Overall, these findings suggest that pathogenic IgM blockade represents a valid therapeutic strategy in mitigating myocardial ischemia-reperfusion injury.

Keywords: immunology; inflammation; left ventricular function; myocardial infarction; troponin T.

Figure 1. Specificity of m21G6 in the swine model of MI

Animals were subjected to 1h LAD occlusion and 1h reperfusion. Prior to reperfusion, fluorescently labeled m21G6 (2mg/kg) (arrow heads, red) was injected IV. Tissue sections were also stained with CD31 (green) to visualize vasculature and analyzed by confocal microscopy. Scale bar, 10µm.

Figure 2. Effect of m21G6 on serum cTnT and infarct size at 5d reperfusion in the swine MI model

Swine were subjected to 1h LAD occlusion and 5d reperfusion. Prior to reperfusion either saline or m21G6 (2mg/kg) was injected IV. (A) Peak cTnT levels. (B) Integrated cTnT levels over 5d reperfusion (area under the curve). (C) AAR expressed as a percentage of the LV. (D) Myocardial infarct size size expressed as a percentage of the AAR. #, p<0.05; *, p<0.01; **, p<0.001. For all panels, N=11 and N=5–6 for saline and m21G6, respectively.

Figure 3. Effect of m21G6 on cardiac function over 21d reperfusion

Swine were subjected to 1h LAD occlusion and 21d reperfusion. Prior to reperfusion either saline or m21G6 (2mg/kg) was administered IV. At the indicated reperfusion times, (A) left ventricular ejection fraction (LVEF), (B) LV wall thickness, (C) left ventricular end diastolic volume (LVEDV), and (D) left ventricular end diastolic dimension (LVEDd) were measured. #, p<0.05; *, p<0.01; **, p<0.001. For all panels, N=3/group.

Figure 4. The N2 Pathway

During myocardial infarction, hypoxia and reperfusion-induced neo-antigen expression of NMHC-II at the cell surface triggers an IgM-complement cascade leading to cell death. Murine 21G6 masks this neo-antigen thereby blocking IgM binding and salvaging myocardium and preventing loss of cardiac function.