Florian Leuschner

Myocardial infarction accelerates atherosclerosis

During progression of atherosclerosis, myeloid cells destabilize lipid-rich plaques in the arterial wall and cause their rupture, thus triggering myocardial infarction and stroke. Survivors of acute coronary syndromes have a high risk of recurrent events for unknown reasons. Here we show that the systemic response to ischaemic injury aggravates chronic atherosclerosis. After myocardial infarction or stroke, Apoe−/− mice developed larger atherosclerotic lesions with a more advanced morphology. This disease acceleration persisted over many weeks and was associated with markedly increased monocyte recruitment. Seeking the source of surplus monocytes in plaques, we found that myocardial infarction liberated haematopoietic stem and progenitor cells from bone marrow niches via sympathetic nervous system signalling. The progenitors then seeded the spleen, yielding a sustained boost in monocyte production. These observations provide new mechanistic insight into atherogenesis and provide a novel therapeutic opportunity to mitigate disease progression.

Therapeutic siRNA silencing in inflammatory monocytes in mice

Excessive and prolonged activity of inflammatory monocytes is a hallmark of many diseases with an inflammatory component. In such conditions, precise targeting of these cells could be therapeutically beneficial while sparing many essential functions of the innate immune system, thus limiting unwanted effects. Inflammatory monocytes—but not the noninflammatory subset—depend on the chemokine receptor CCR2 for localization to injured tissue. Here we present an optimized lipid nanoparticle and a CCR2-silencing short interfering RNA that, when administered systemically in mice, show rapid blood clearance, accumulate in spleen and bone marrow, and localize to monocytes. Efficient degradation of CCR2 mRNA in monocytes prevents their accumulation in sites of inflammation. Specifically, the treatment attenuates their number in atherosclerotic plaques, reduces infarct size after coronary artery occlusion, prolongs normoglycemia in diabetic mice after pancreatic islet transplantation, and results in reduced tumor volumes and lower numbers of tumor-associated macrophages.

Rapid monocyte kinetics in acute myocardial infarction are sustained by extramedullary monocytopoiesis

IL-1b signaling augments continued splenic monocyte supply during acute inflammation.

PET/MRI of inflammation in myocardial infarction.

OBJECTIVES The aim of this study was to explore post-myocardial infarction (MI) myocardial inflammation. BACKGROUND Innate immune cells are centrally involved in infarct healing and are emerging therapeutic targets in cardiovascular disease; however, clinical tools to assess their presence in tissue are scarce. Furthermore, it is currently not known if the nonischemic remote zone recruits monocytes. METHODS Acute inflammation was followed in mice with coronary ligation by 18-fluorodeoxyglucose ((18)FDG) positron emission tomography/magnetic resonance imaging, fluorescence-activated cell sorting, polymerase chain reaction, and histology. RESULTS Gd-DTPA-enhanced infarcts showed high (18)FDG uptake on day 5 after MI. Cell depletion and isolation data confirmed that this largely reflected inflammation; CD11b(+) cells had 4-fold higher (18)FDG uptake than the infarct tissue from which they were isolated (p < 0.01). Surprisingly, there was considerable monocyte recruitment in the remote myocardium (approximately 10(4)/mg of myocardium, 5.6-fold increase; p < 0.01), a finding mirrored by macrophage infiltration in the remote myocardium of patients with acute MI. Temporal kinetics of cell recruitment were slower than in the infarct, with peak numbers on day 10 after ischemia. Quantitative polymerase chain reaction showed a robust increase of recruiting adhesion molecules and chemokines in the remote myocardium (e.g., 12-fold increase of monocyte chemoattractant protein-1), although levels were always lower than in the infarct. Finally, matrix metalloproteinase activity was significantly increased in noninfarcted myocardium, suggesting that monocyte recruitment to the remote zone may contribute to post-MI dilation. CONCLUSIONS This study shed light on the innate inflammatory response in remote myocardium after MI.

Angiotensin-Converting Enzyme Inhibition Prevents the Release of Monocytes From Their Splenic Reservoir in Mice With Myocardial Infarction

Rationale: Monocytes recruited to ischemic myocardium originate from a reservoir in the spleen, and the release from their splenic niche relies on angiotensin (Ang) II signaling. Objective: Because monocytes are centrally involved in tissue repair after ischemia, we hypothesized that early angiotensin-converting enzyme (ACE) inhibitor therapy impacts healing after myocardial infarction partly via effects on monocyte traffic. Methods and Results: In a mouse model of permanent coronary ligation, enalapril arrested the release of monocytes from the splenic reservoir and consequently reduced their recruitment into the healing infarct by 45%, as quantified by flow cytometry of digested infarcts. Time-lapse intravital microscopy revealed that enalapril reduces monocyte motility in the spleen. In vitro migration assays and Western blotting showed that this was caused by reduced signaling through the Ang II type 1 receptor. We then studied the long-term consequences of blocked splenic monocyte release in atherosclerotic apolipoprotein (apo)E−/− mice, in which infarct healing is impaired because of excessive inflammation in the cardiac wound. Enalapril improved histologic healing biomarkers and reduced inflammation in infarcts measured by FMT-CT (fluorescence molecular tomography in conjunction with x-ray computed tomography) of proteolytic activity. ACE inhibition improved MRI-derived ejection fraction by 14% on day 21, despite initially comparable infarct size. In apoE−/− mice, ischemia/reperfusion injury resulted in larger infarct size and enhanced monocyte recruitment and was reversible by enalapril treatment. Splenectomy reproduced antiinflammatory effects of enalapril. Conclusion: This study suggests that benefits of early ACE inhibition after myocardial infarction can partially be attributed to its potent antiinflammatory impact on the splenic monocyte reservoir.

Detection of Macrophages in Aortic Aneurysms by Nanoparticle Positron Emission Tomography–Computed Tomography

Objective—Current management of aortic aneurysms (AAs) relies primarily on size criteria to determine whether invasive repair is indicated to preempt rupture. We hypothesized that emerging molecular imaging tools could be used to more sensitively gauge local inflammation. Because macrophages are key effector cells that destabilize the extracellular matrix in the arterial wall, it seemed likely that they would represent suitable imaging targets. We here aimed to develop and validate macrophage-targeted nanoparticles labeled with fluorine-18 (18F) for positron emission tomography–computed tomography (PET-CT) detection of inflammation in AAs. Methods and Results—Aneurysms were induced in apolipoprotein E−/− mice via systemic administration of angiotensin II. Mice were imaged using PET-CT and a monocyte/macrophage–targeted nanoparticle. AAs were detected by contrast-enhanced micro-CT and had a mean diameter of 1.85±0.08 mm, whereas normal aortas measured 1.07±0.03 (P<0.05). The in vivo PET signal was significantly higher in aneurysms (standard uptake value, 2.46±0.48) compared with wild-type aorta (0.82±0.05, P<0.05). Validation with scintillation counting, autoradiography, fluorescence, and immunoreactive histology and flow cytometry demonstrated that nanoparticles localized predominantly to monocytes and macrophages within the aneurysmatic wall. Conclusion—PET-CT imaging with 18F-labeled nanoparticles allows quantitation of macrophage content in a mouse model of AA.

In vivo detection of Staphylococcus aureus endocarditis by targeting pathogen-specific prothrombin activation

Coagulase-positive Staphylococcus aureus (S. aureus) is the major causal pathogen of acute endocarditis, a rapidly progressing, destructive infection of the heart valves. Bacterial colonization occurs at sites of endothelial damage, where, together with fibrin and platelets, the bacteria initiate the formation of abnormal growths known as vegetations. Here we report that an engineered analog of prothrombin could be used to detect S. aureus in endocarditic vegetations via noninvasive fluorescence or positron emission tomography (PET) imaging. These prothrombin derivatives bound staphylocoagulase and intercalated into growing bacterial vegetations. We also present evidence for bacterial quorum sensing in the regulation of staphylocoagulase expression by S. aureus. Staphylocoagulase expression was limited to the growing edge of mature vegetations, where it was exposed to the host and co-localized with the imaging probe. When endocarditis was induced with an S. aureus strain with genetic deletion of coagulases, survival of mice improved, highlighting the role of staphylocoagulase as a virulence factor.

Molecular Imaging of Coronary Atherosclerosis and Myocardial Infarction Considerations for the Bench and Perspectives for the Clinic

Motivated by the promise to transform preclinical research and clinical care, cardiovascular molecular imaging has made advances toward targeting coronary atherosclerosis and heart failure. Here, we discuss recent progress in the field, highlight how molecular imaging may facilitate preventive patient care, and review specific challenges associated with coronary and heart failure imaging. Practical considerations stress the potential of fluorescence imaging for basic research and discuss hybrid protocols such as FMT-CT and PET-MRI.

Endoscopic time-lapse imaging of immune cells in infarcted mouse hearts.

Rationale: High-resolution imaging of the heart in vivo is challenging owing to the difficulty in accessing the heart and the tissue motion caused by the heartbeat. Objective: Here, we describe a suction-assisted endoscope for visualizing fluorescently labeled cells and vessels in the beating heart tissue through a small incision made in the intercostal space. Methods and Results: A suction tube with a diameter of 2 to 3 mm stabilizes the local tissue motion safely and effectively at a suction pressure of 50 mm Hg. Using a minimally invasive endoscope integrated into a confocal microscope, we performed fluorescence cellular imaging in both normal and diseased hearts in live mice for an hour per session repeatedly over a few weeks. Real-time imaging revealed the surprisingly rapid infiltration of CX3CR1+ monocytes into the injured site within several minutes after acute myocardial infarction. Conclusions: The time-lapse analysis of flowing and rolling (patrolling) monocytes in the heart and the peripheral circulation provides evidence that the massively recruited monocytes come first from the vascular reservoir and later from the spleen. The imaging method requires minimal surgical preparation and can be implemented into standard intravital microscopes. Our results demonstrate the applicability of our imaging method for a wide range of cardiovascular research.

Absence of auto-antibodies against cardiac troponin I predicts improvement of left ventricular function after acute myocardial infarction

AIMS Application of antibodies against cardiac troponin I (cTnI-Ab) can induce dilation and dysfunction of the heart in mice. Recently, we demonstrated that immunization with cTnI induces inflammation and fibrosis in myocardium of mice. Others have shown that auto-antibodies to cTnI are present in patients with acute coronary syndrome, but little is known about the clinical relevance of detected cTnI-Ab. METHODS AND RESULTS First, anti-cTnI and anti-cTnT antibody titres were measured in sera from 272 patients with dilated- (DCM) and 185 with ischaemic- (ICM) cardiomyopathy. Secondly, 108 patients with acute myocardial infarction (AMI) were included for a follow-up study. Heart characteristics were determined by magnetic resonance imaging 4 days and 6-9 months after AMI. Altogether in 7.0% of patients with DCM and in 9.2% with ICM, an anti-cTnI IgG antibody titre >/=1:160 was measured. In contrast, only in 1.7% of patients with DCM and in 0.5% with ICM, an anti-cTnT IgG antibody titre >/=1:160 was detected. Ten out of 108 patients included in the follow-up study were tested positive for cTnI-Ab with IgG Ab titres >/=1:160. TnI-Ab negative patients showed a significant increase in left ventricular ejection fraction (LVEF) and stroke volume 6-9 months after AMI. In contrast, there was no significant increase in LVEF and stroke volume in TnI-Ab positive patients. CONCLUSION We demonstrate for the first time that the prevalence of cTnI-Abs in patients with AMI has an impact on the improvement of the LVEF over a study period of 6-9 months.