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Clearance of dying cells by efferocytosis is necessary for cardiac repair after myocardial infarction (MI). Recent reports have suggested a protective role for vascular endothelial growth factor C (VEGFC) during acute cardiac lymphangiogenesis after MI. Here, we report that defective efferocytosis by macrophages after experimental MI led to a reduction in cardiac lymphangiogenesis and Vegfc expression. Cell-intrinsic evidence for efferocytic induction of Vegfc was revealed after adding apoptotic cells to cultured primary macrophages, which subsequently triggered Vegfc transcription and VEGFC secretion. Similarly, cardiac macrophages elevated Vegfc expression levels after MI, and mice deficient for myeloid Vegfc exhibited impaired ventricular contractility, adverse tissue remodeling, and reduced lymphangiogenesis. These results were observed in mouse models of permanent coronary occlusion and clinically relevant ischemia and reperfusion. Interestingly, myeloid Vegfc deficiency also led to increases in acute infarct size, prior to the amplitude of the acute cardiac lymphangiogenesis response. RNA-Seq and cardiac flow cytometry revealed that myeloid Vegfc deficiency was also characterized by a defective inflammatory response, and macrophage-produced VEGFC was directly effective at suppressing proinflammatory macrophage activation. Taken together, our findings indicate that cardiac macrophages promote healing through the promotion of myocardial lymphangiogenesis and the suppression of inflammatory cytokines.

Efferocytosis triggers cellular reprogramming, including the induction of mRNA transcripts which encode anti-inflammatory cytokines that promote inflammation resolution. Our current understanding of this transcriptional response is largely informed from analysis of bulk phagocyte populations; however, this precludes the resolution of heterogeneity between individual macrophages and macrophage subsets. Moreover, phagocytes may contain so called “passenger” transcripts that originate from engulfed apoptotic bodies, thus obscuring the true transcriptional reprogramming of the phagocyte. To define the transcriptional diversity during efferocytosis, we utilized single-cell mRNA sequencing after co-cultivating macrophages with apoptotic cells. Importantly, transcriptomic analyses were performed after validating the disappearance of apoptotic cell-derived RNA sequences. Our findings reveal new heterogeneity of the efferocytic response at a single-cell resolution, particularly evident between F4/80 + MHCII LO and F4/80 − MHCII HI macrophage sub-populations. After exposure to apoptotic cells, the F4/80 + MHCII LO subset significantly induced pathways associated with tissue and cellular homeostasis, while the F4/80 − MHCII HI subset downregulated these putative signaling axes. Ablation of a canonical efferocytosis receptor, MerTK, blunted efferocytic signatures and led to the escalation of cell death-associated transcriptional signatures in F4/80 + MHCII LO macrophages. Taken together, our results newly elucidate the heterogenous transcriptional response of single-cell peritoneal macrophages after exposure to apoptotic cells.

Recent studies have suggested that lymphatics help to restore heart function after cardiac injury 1 – 6 . Here we report that lymphatics promote cardiac growth, repair and cardioprotection in mice. We show that a lymphoangiocrine signal produced by lymphatic endothelial cells (LECs) controls the proliferation and survival of cardiomyocytes during heart development, improves neonatal cardiac regeneration and is cardioprotective after myocardial infarction. Embryos that lack LECs develop smaller hearts as a consequence of reduced cardiomyocyte proliferation and increased cardiomyocyte apoptosis. Culturing primary mouse cardiomyocytes in LEC-conditioned medium increases cardiomyocyte proliferation and survival, which indicates that LECs produce lymphoangiocrine signals that control cardiomyocyte homeostasis. Characterization of the LEC secretome identified the extracellular protein reelin (RELN) as a key component of this process. Moreover, we report that LEC-specific Reln -null mouse embryos develop smaller hearts, that RELN is required for efficient heart repair and function after neonatal myocardial infarction, and that cardiac delivery of RELN using collagen patches improves heart function in adult mice after myocardial infarction by a cardioprotective effect. These results highlight a lymphoangiocrine role of LECs during cardiac development and injury response, and identify RELN as an important mediator of this function. Lymphatic endothelium secretes factors needed for heart growth and repair such as RELN, which helps with heart regeneration and cardioprotection after myocardial infarction.

Neutrophil (PMN) mobilization to sites of insult is critical for host defense and requires transendothelial migration (TEM). TEM involves several well-studied sequential adhesive interactions with vascular endothelial cells (ECs); however, what initiates or terminates this process is not well-understood. Here we describe what we believe to be a new mechanism where vessel associated macrophages (VAMs) through localized interactions primed EC responses to form ICAM-1 \"hot spots\", to support PMN TEM. Using real-time intravital microscopy (IVM) on lipopolysaccharide (LPS)-inflamed intestines in CX3CR1-EGFP macrophage-reporter mice, complemented by whole-mount tissue imaging and flow cytometry, we found that macrophage vessel association is critical for the initiation of PMN-EC adhesive interactions, PMN TEM and subsequent accumulation in the intestinal mucosa. Anti-colony stimulating factor 1 receptor (CSF1R) antibody-mediated macrophage depletion in the lamina propria and at the vessel wall resulted in elimination of ICAM-1 hot spots impeding PMN-EC interactions and TEM. Mechanistically, the use of human clinical specimens, TNFα knockout macrophage chimeras, TNFα/TNF receptor (TNFR) neutralization and multi-cellular macrophage-EC-PMN cocultures revealed that macrophage-derived TNFα and EC TNFR2 axis mediated this regulatory mechanism and was required for PMN TEM. As such, our findings identified clinically relevant mechanism by which macrophages regulate PMN trafficking in inflamed mucosa.

Clearance of dying cells by efferocytosis is necessary for cardiac repair after myocardial infarction (MI). Recent reports have suggested a protective role for vascular endothelial growth factor C (VEGFC) during acute cardiac lymphangiogenesis after MI. Here, we report that defective efferocytosis by macrophages after experimental MI led to a reduction in cardiac lymphangiogenesis and Vegfc expression. Cell-intrinsic evidence for efferocytic induction of Vegfc was revealed after adding apoptotic cells to cultured primary macrophages, which subsequently triggered Vegfc transcription and VEGFC secretion. Similarly, cardiac macrophages elevated Vegfc expression levels after MI, and mice deficient for myeloid Vegfc exhibited impaired ventricular contractility, adverse tissue remodeling, and reduced lymphangiogenesis. These results were observed in mouse models of permanent coronary occlusion and clinically relevant ischemia and reperfusion. Interestingly, myeloid Vegfc deficiency also led to increases in acute infarct size, prior to the amplitude of the acute cardiac lymphangiogenesis response. RNA-Seq and cardiac flow cytometry revealed that myeloid Vegfc deficiency was also characterized by a defective inflammatory response, and macrophage-produced VEGFC was directly effective at suppressing proinflammatory macrophage activation. Taken together, our findings indicate that cardiac macrophages promote healing through the promotion of myocardial lymphangiogenesis and the suppression of inflammatory cytokines.

While largely appreciated for their antimicrobial and repair functions, macrophages have emerged as indispensable for the development, homeostasis, and regeneration of tissue, including regeneration of the neonatal heart. Upon activation, mammalian neonatal macrophages express and secrete factors that coordinate angiogenesis, resolve inflammation, and ultimately induce cardiomyocyte proliferation. This is contrary to adult macrophages, which are incapable of inducing significant levels of cardiac regeneration. The underlying mechanisms by which pro-regenerative macrophages are activated and regulated remain vague.Therefore, we compared the macrophage response after cardiac injury in newborns to adults. Single-cell RNA sequencing revealed accumulation of tissue-resident C1q+ TLF+ neonatal macrophages that were selectively polarized for apoptotic cell recognition (or efferocytosis). Genetic ablation of the apoptotic cell receptor MerTK in neonatal macrophages prevented cardiac regeneration, leading to impaired cardiac function and fibrotic scarring. Loss of MerTK greatly remodeled macrophage gene expression required for cellular metabolism, arachidonic acid metabolism, and biosynthesis of the eicosanoid thromboxane, the latter which unexpectedly activated parenchymal cell proliferation. Thromboxane receptors were discovered on neighboring cardiomyocytes and were required for a cellular metabolic shift which was coupled to mitogenic signaling. These data are consistent with a fundamental age-defined macrophage response in which lipid mitogens produced during efferocytosis support organ regeneration.Given our findings that efferocytosis is a key regulator of neonatal cardiac regeneration, we further investigated how efferocytosis triggers transcriptional and metabolic reprogramming in macrophages. Our current understanding of the transcriptional remodeling by efferocytosis is largely informed from analysis of bulk phagocyte populations; however, this precludes the heterogeneity between individual macrophages during apoptotic cell uptake. Moreover, phagocytes may contain so called “passenger” transcripts that originate from engulfed apoptotic bodies, thus obscuring the true transcriptional reprogramming of the phagocyte. To define the transcriptional diversity during efferocytosis, we utilized single-cell mRNA sequencing after co-cultivating macrophages with apoptotic cells. Importantly, transcriptomic analyses were performed after validating the disappearance of apoptotic cell-derived RNA sequences. Our findings reveal new heterogeneity of macrophage efferocytosis at a single-cell resolution, particularly evident between large peritoneal macrophages and small peritoneal macrophages. Ablation of MerTK, blunted efferocytosis signatures, disrupted macrophage metabolism, and led to the escalation of cell death-associated transcriptional signatures in large peritoneal macrophages. Taken together, our results newly elucidate the heterogenous transcriptional response of macrophages after exposure to apoptotic cells.Lastly, in vitro macrophage polarization is dependent on alterations in metabolite usage, however the in vivo relevance of this is vague and important to understanding divergence in immune function. Utilizing an optimized low-input metabolomic profiling platform in parallel with ex vivo isotope tracing, we assessed the metabolic network and transcriptome of sorted CCR2hi versus CCR2lo macrophages after cardiac injury. The data revealed a striking divergence and unexpected metabolite preference between cell subsets that was distinct from paradigms reported in vitro. Inflammatory CCR2hi macrophages prioritized glycolysis and glutamine metabolism as opposed to CCR2lo macrophages, which favored fatty acid oxidation and nucleotide metabolism, supportive of cell proliferation. CCR2lo macrophages notably also increased catabolism of polyamines, in conjunction with signatures of efferocytosis and resolution of inflammation. Genetic blockade of efferocytosis through MerTK blunted both mitochondrial metabolism and polyamine accumulation, leading to heightened inflammation and impaired cardiac repair. These findings not only reveal an atlas of in vivo metabolic preferences in macrophage subsets that are distinct from in vitro metabolism, but also identify how efferocytosis regulates polyamine metabolism to directly remodel macrophage metabolism for inflammation resolution and cardiac repair.

Myocardial infarction (MI) mobilizes macrophages, the central protagonists of tissue repair in the infarcted heart. Although necessary for repair, macrophages also contribute to adverse remodeling and progression to heart failure. In this context, specific targeting of inflammatory macrophage activation may attenuate maladaptive responses and enhance cardiac repair. Allograft inflammatory factor 1 (AIF1) is a macrophage-specific protein expressed in a variety of inflammatory settings, but its function after MI is unknown. Here we identify a maladaptive role for macrophage AIF1 after MI in mice. Mechanistic studies show that AIF1 increases actin remodeling in macrophages to promote reactive oxygen species–dependent activation of hypoxia-inducible factor (HIF)-1α. This directs a switch to glycolytic metabolism to fuel macrophage-mediated inflammation, adverse ventricular remodeling and progression to heart failure. Targeted knockdown of Aif1 using antisense oligonucleotides improved cardiac repair, supporting further exploration of macrophage AIF1 as a therapeutic target after MI. DeBerge, Glinton et al. demonstrate that allograft inflammatory factor 1 promotes inflammatory glycolytic reprogramming in cardiac macrophages, leading to adverse remodeling and progression to heart failure after myocardial infarction.

Heart failure with preserved ejection fraction (HFpEF) is a rapidly growing public health concern and an emerging contributor to dementia, yet the mechanisms linking cardiometabolic dysfunction to neurodegeneration remain poorly understood. Here, we demonstrate that HFpEF drives a sustained neuroinflammatory state through microglial metabolic reprogramming. Using a clinically relevant murine model of HFpEF, we identified robust induction of HIF-1α signaling in microglia via integrated transcriptomics and metabolomics, coupled with increased glycolytic metabolism revealed by extracellular flux analysis. Conditional deletion of in microglia during HFpEF attenuated neuroinflammation, preserved white matter integrity, and rescued cognitive performance. We further identify Sema4D as a HIF-1α-dependent, microglia-derived effector linking metabolic stress to white matter injury. These findings establish a mechanistic bridge between cardiovascular disease and cognitive dysfunction and reveal microglial HIF-1α signaling as a tractable therapeutic strategy for preventing cognitive decline in cardiometabolic disease.

The immune response to stress diverges with age, with neonatal macrophages implicated in tissue regeneration versus tissue scarring and maladaptive inflammation in adults. Integral to the macrophage stress response is the recognition of hypoxia and pathogen-associated molecular patterns (PAMPs), which are often coupled. The age-specific, cell-intrinsic nature of this stress response remains vague. To uncover age-defined divergences in macrophage crosstalk potential after exposure to hypoxia and PAMPs, we interrogated the secreted proteomes of neonatal versus adult macrophages via non-biased mass spectrometry. Through this approach, we newly identified age-specific signatures in the secretomes of neonatal versus adult macrophages in response to hypoxia and the prototypical PAMP, lipopolysaccharide (LPS). Neonatal macrophages polarized to an anti-inflammatory, regenerative phenotype protective against apoptosis and oxidative stress, dependent on ( In contrast, adult macrophages adopted a pro-inflammatory, glycolytic phenotypic signature consistent with pathogen killing. Taken together, these data uncover fundamental age and dependent macrophage programs that may be targeted to calibrate the innate immune response during stress and inflammation.

The myocardium in hypoplastic left heart syndrome (HLHS) exhibits immature metabolic programming, impaired mitochondrial quality control, and heightened susceptibility to ischemic and hypoxic injury during palliative surgery. The long non-coding RNA H19 suppresses translation of PTEN-induced putative kinase 1 (PINK1) mRNA and modulates mitochondrial quality control and ischemia/reperfusion injury (IRI) in adult hearts. Whether-and how-H19 regulates mitophagy and IRI in HLHS or in immature animals remains unknown. We investigated H19 regulation and its role in mitophagy and ischemia/reperfusion or hypoxia/reoxygenation injury in myocardial tissue from HLHS patients, HLHS-specific induced pluripotent stem cell-derived cardiomyocytes (HLHS-iPSC-CMs), and immature rat hearts. Mechanistic interactions among H19, PINK1/Parkin signaling, and mitophagosome formation were assessed using loss-of-function approaches. HLHS myocardium exhibited markedly elevated H19 expression, accompanied by reduced PINK1 and Parkin protein abundance and diminished mitophagosome formation. Similar findings were observed in HLHS-iPSC-CMs exposed to hypoxia/reoxygenation and in immature rat hearts subjected to myocardial IRI. H19 knockdown in HLHS-iPSC-CMs attenuated hypoxia/reoxygenation-induced lactate dehydrogenase release and restored PINK1 and Parkin protein levels. In immature rats, myocardial H19 silencing reduced infarct size, enhanced mitochondrial PINK1 and Parkin expression, and improved post-reperfusion cardiac function for up to 28 days. Conversely, knockdown of PINK1 or Parkin reduced mitophagosome formation and exacerbated functional deterioration during IRI. H19 upregulation impairs PINK1/Parkin-dependent mitophagy and increases susceptibility to ischemic and hypoxic injury in HLHS and the immature heart. These findings identify H19 as a key regulator of mitochondrial quality control and a potential therapeutic target for mitigating IRI in early-life cardiac disease.