Rostic Gorbatov

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.

Local proliferation dominates lesional macrophage accumulation in atherosclerosis

During the inflammatory response that drives atherogenesis, macrophages accumulate progressively in the expanding arterial wall. The observation that circulating monocytes give rise to lesional macrophages has reinforced the concept that monocyte infiltration dictates macrophage buildup. Recent work has indicated, however, that macrophage accumulation does not depend on monocyte recruitment in some inflammatory contexts. We therefore revisited the mechanism underlying macrophage accumulation in atherosclerosis. In murine atherosclerotic lesions, we found that macrophages turn over rapidly, after 4 weeks. Replenishment of macrophages in these experimental atheromata depends predominantly on local macrophage proliferation rather than monocyte influx. The microenvironment orchestrates macrophage proliferation through the involvement of scavenger receptor A (SR-A). Our study reveals macrophage proliferation as a key event in atherosclerosis and identifies macrophage self-renewal as a therapeutic target for cardiovascular disease.

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.

Origins of tumor-associated macrophages and neutrophils

Tumor-associated macrophages (TAMs) and tumor-associated neutrophils (TANs) can control cancer growth and exist in almost all solid neoplasms. The cells are known to descend from immature monocytic and granulocytic cells, respectively, which are produced in the bone marrow. However, the spleen is also a recently identified reservoir of monocytes, which can play a significant role in the inflammatory response that follows acute injury. Here, we evaluated the role of the splenic reservoir in a genetic mouse model of lung adenocarcinoma driven by activation of oncogenic Kras and inactivation of p53. We found that high numbers of TAM and TAN precursors physically relocated from the spleen to the tumor stroma, and that recruitment of tumor-promoting spleen-derived TAMs required signaling of the chemokine receptor CCR2. Also, removal of the spleen, either before or after tumor initiation, reduced TAM and TAN responses significantly and delayed tumor growth. The mechanism by which the spleen was able to maintain its reservoir capacity throughout tumor progression involved, in part, local accumulation in the splenic red pulp of typically rare extramedullary hematopoietic stem and progenitor cells, notably granulocyte and macrophage progenitors, which produced CD11b+ Ly-6Chi monocytic and CD11b+ Ly-6Ghi granulocytic cells locally. Splenic granulocyte and macrophage progenitors and their descendants were likewise identified in clinical specimens. The present study sheds light on the origins of TAMs and TANs, and positions the spleen as an important extramedullary site, which can continuously supply growing tumors with these cells.

Modified mRNA directs the fate of heart progenitor cells and induces vascular regeneration after myocardial infarction

In a cell-free approach to regenerative therapeutics, transient application of paracrine factors in vivo could be used to alter the behavior and fate of progenitor cells to achieve sustained clinical benefits. Here we show that intramyocardial injection of synthetic modified RNA (modRNA) encoding human vascular endothelial growth factor-A (VEGF-A) results in the expansion and directed differentiation of endogenous heart progenitors in a mouse myocardial infarction model. VEGF-A modRNA markedly improved heart function and enhanced long-term survival of recipients. This improvement was in part due to mobilization of epicardial progenitor cells and redirection of their differentiation toward cardiovascular cell types. Direct in vivo comparison with DNA vectors and temporal control with VEGF inhibitors revealed the greatly increased efficacy of pulse-like delivery of VEGF-A. Our results suggest that modRNA is a versatile approach for expressing paracrine factors as cell fate switches to control progenitor cell fate and thereby enhance long-term organ repair.

Extramedullary Hematopoiesis Generates Ly-6C(high) Monocytes That Infiltrate Atherosclerotic Lesions

Background— Atherosclerotic lesions are believed to grow via the recruitment of bone marrow–derived monocytes. Among the known murine monocyte subsets, Ly-6Chigh monocytes are inflammatory, accumulate in lesions preferentially, and differentiate. Here, we hypothesized that the bone marrow outsources the production of Ly-6Chigh monocytes during atherosclerosis. Methods and Results— Using murine models of atherosclerosis and fate-mapping approaches, we show that hematopoietic stem and progenitor cells progressively relocate from the bone marrow to the splenic red pulp, where they encounter granulocyte macrophage colony-stimulating factor and interleukin-3, clonally expand, and differentiate to Ly-6Chigh monocytes. Monocytes born in such extramedullary niches intravasate, circulate, and accumulate abundantly in atheromata. On lesional infiltration, Ly-6Chigh monocytes secrete inflammatory cytokines, reactive oxygen species, and proteases. Eventually, they ingest lipids and become foam cells. Conclusions— Our findings indicate that extramedullary sites supplement the hematopoietic function of the bone marrow by producing circulating inflammatory cells that infiltrate atherosclerotic lesions.

Innate Response Activator B Cells Protect Against Microbial Sepsis

Immune Sentinels A classic paradigm in immunology holds that the immune response occurs in two waves: Rapidly responding cells of the innate immune system help to contain the invading pathogen and alert lymphocytes. These cells of the adaptive immune system then help to clear the infection and go on to form long-lasting memory. However, some specialized populations of lymphocytes can also respond quickly to an infection and carry out functions that overlap with the innate immune system. Now, Rauch et al. (p. 597, published online 12 January) describe one such cell type—innate response activator (IRA) B cells. IRA B cells recognize bacterial liposaccharide through Toll-like receptor 4 and, in response, produce the cytokine GM-CSF, which activates other innate immune cells. Deletion of IRA B cells in mice impaired their ability to clear a bacterial infection and promoted septic shock. A specialized population of B lymphocytes is important for controlling bacterial infections and preventing sepsis. Recognition and clearance of a bacterial infection are a fundamental properties of innate immunity. Here, we describe an effector B cell population that protects against microbial sepsis. Innate response activator (IRA) B cells are phenotypically and functionally distinct, develop and diverge from B1a B cells, depend on pattern-recognition receptors, and produce granulocyte-macrophage colony-stimulating factor. Specific deletion of IRA B cell activity impairs bacterial clearance, elicits a cytokine storm, and precipitates septic shock. These observations enrich our understanding of innate immunity, position IRA B cells as gatekeepers of bacterial infection, and identify new treatment avenues for infectious diseases.

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.