Persistent transcriptional changes in cardiac adaptive immune cells following myocardial infarction: New evidence from the re-analysis of publicly available single cell and nuclei RNA-sequencing data sets

INTRODUCTION

Chronic immunopathology contributes to the development of heart failure after a myocardial infarction. Both T and B cells of the adaptive immune system are present in the myocardium and have been suggested to be involved in post-MI immunopathology.

METHODS

We analyzed the B and T cell populations isolated from previously published single cell RNA-sequencing data sets (PMID: 32130914, PMID: 35948637, PMID: 32971526 and PMID: 35926050), of the mouse and human heart, using differential expression analysis, functional enrichment analysis, gene regulatory inferences, and integration with autoimmune and cardiovascular GWAS.

RESULTS

Already at baseline, mature effector B and T cells are present in the human and mouse heart, having increased activity in transcription factors maintaining tolerance (e.g. DEAF1, JDP2, SPI-B). Following MI, T cells upregulate pro-inflammatory transcript levels (e.g. Cd11, Gzmk, Prf1), while B cells upregulate activation markers (e.g. Il6, Il1rn, Ccl6) and collagen (e.g. Col5a2, Col4a1, Col1a2). Importantly, pro-inflammatory and fibrotic transcription factors (e.g. NFKB1, CREM, REL) remain active in T cells, while B cells maintain elevated activity in transcription factors related to immunoglobulin production (e.g. ERG, REL) in both mouse and human post-MI hearts. Notably, genes differentially expressed in post-MI T and B cells are associated with cardiovascular and autoimmune disease.

CONCLUSION

These findings highlight the varied and time-dependent dynamic roles of post-MI T and B cells. They appear ready-to-go and are activated immediately after MI, thus participate in the acute wound healing response. However, they subsequently remain in a state of pro-inflammatory activation contributing to persistent immunopathology.

Integrating molecular and clinical variables to predict myocardial recovery

Mechanical unloading and circulatory support with left ventricular assist devices (LVADs) mediate significant myocardial improvement in a subset of advanced heart failure (HF) patients. The clinical and biological phenomena associated with cardiac recovery are under intensive investigation. Left ventricular (LV) apical tissue, alongside clinical data, were collected from HF patients at the time of LVAD implantation (n=208). RNA was isolated and mRNA transcripts were identified through RNA sequencing and confirmed with RT-qPCR. To our knowledge this is the first study to combine transcriptomic and clinical data to derive predictors of myocardial recovery. We used a bioinformatic approach to integrate 59 clinical variables and 22,373 mRNA transcripts at the time of LVAD implantation for the prediction of post-LVAD myocardial recovery defined as LV ejection fraction (LVEF) ≥40% and LV end-diastolic diameter (LVEDD) ≤5.9cm, as well as functional and structural LV improvement independently by using LVEF and LVEDD as continuous variables, respectively. To substantiate the predicted variables, we used a multi-model approach with logistic and linear regressions. Combining RNA and clinical data resulted in a gradient boosted model with 80 features achieving an AUC of 0.731±0.15 for predicting myocardial recovery. Variables associated with myocardial recovery from a clinical standpoint included HF duration, pre-LVAD LVEF, LVEDD, and HF pharmacologic therapy, and LRRN4CL (ligand binding and programmed cell death) from a biological standpoint. Our findings could have diagnostic, prognostic, and therapeutic implications for advanced HF patients, and inform the care of the broader HF population.

Harnessing molecular mechanism for precision medicine in dilated cardiomyopathy caused by a mutation in troponin T

Familial dilated cardiomyopathy (DCM) is frequently caused by autosomal dominant point mutations in genes involved in diverse cellular processes, including sarcomeric contraction. While patient studies have defined the genetic landscape of DCM, genetics are not currently used in patient care, and patients receive similar treatments regardless of the underlying mutation. It has been suggested that a precision medicine approach based on the molecular mechanism of the underlying mutation could improve outcomes; however, realizing this approach has been challenging due to difficulties linking genotype and phenotype and then leveraging this information to identify therapeutic approaches. Here, we used multiscale experimental and computational approaches to test whether knowledge of molecular mechanism could be harnessed to connect genotype, phenotype, and drug response for a DCM mutation in troponin T, deletion of K210. Previously, we showed that at the molecular scale, the mutation reduces thin filament activation. Here, we used computational modeling of this molecular defect to predict that the mutant will reduce cellular and tissue contractility, and we validated this prediction in human cardiomyocytes and engineered heart tissues. We then used our knowledge of molecular mechanism to computationally model the effects of a small molecule that can activate the thin filament. We demonstrate experimentally that the modeling correctly predicts that the small molecule can partially rescue systolic dysfunction at the expense of diastolic function. Taken together, our results demonstrate how molecular mechanism can be harnessed to connect genotype and phenotype and inspire strategies to optimize mechanism-based therapeutics for DCM.

CD40 is an immune checkpoint regulator that potentiates myocardial inflammation through activation and expansion of CCR2 + macrophages and CD8 T-cells

Novel immune checkpoint therapeutics including CD40 agonists have tremendous promise to elicit antitumor responses in patients resistant to current therapies. Conventional immune checkpoint inhibitors (PD-1/PD-L1, CTLA-4 antagonists) are associated with serious adverse cardiac events including life-threatening myocarditis. However, little is known regarding the potential for CD40 agonists to trigger myocardial inflammation or myocarditis. Here, we leveraged genetic mouse models, single cell sequencing, and cell depletion studies to demonstrate that an anti-CD40 agonist antibody reshapes the cardiac immune landscape through activation of CCR2 + macrophages and subsequent recruitment of effector memory CD8 T-cells. We identify a positive feedback loop between CCR2 + macrophages and CD8 T-cells driven by IL12b, TNF, and IFN-γ signaling that promotes myocardial inflammation and show that prior exposure to CD40 agonists sensitizes the heart to secondary insults and accelerates LV remodeling. Collectively, these findings highlight the potential for CD40 agonists to promote myocardial inflammation and potentiate heart failure pathogenesis.

Dilated cardiomyopathy-associated skeletal muscle actin (ACTA1) mutation R256H disrupts actin structure and function and causes cardiomyocyte hypocontractility

Skeletal muscle actin (ACTA1) mutations are a prevalent cause of skeletal myopathies consistent with ACTA1's high expression in skeletal muscle. Rare de novo mutations in ACTA1 associated with combined cardiac and skeletal myopathies have been reported, but ACTA1 represents only ~20% of the total actin pool in cardiomyocytes, making its role in cardiomyopathy controversial. Here we demonstrate how a mutation in an actin isoform expressed at low levels in cardiomyocytes can cause cardiomyopathy by focusing on a unique ACTA1 mutation, R256H. We previously identified this mutation in multiple family members with dilated cardiomyopathy (DCM), who had reduced systolic function without clinical skeletal myopathy. Using a battery of multiscale biophysical tools, we show that R256H has potent functional effects on ACTA1 function at the molecular scale and in human cardiomyocytes. Importantly, we demonstrate that R256H acts in a dominant manner, where the incorporation of small amounts of mutant protein into thin filaments is sufficient to disrupt molecular contractility, and that this effect is dependent on the presence of troponin and tropomyosin. To understand the structural basis of this change in regulation, we resolved a structure of R256H filaments using Cryo-EM, and we see alterations in actin's structure that have the potential to disrupt interactions with tropomyosin. Finally, we show that ACTA1R256H/+ human induced pluripotent stem cell cardiomyocytes demonstrate reduced contractility and sarcomeric disorganization. Taken together, we demonstrate that R256H has multiple effects on ACTA1 function that are sufficient to cause reduced contractility and establish a likely causative relationship between ACTA1 R256H and clinical cardiomyopathy.

Cardiac macrophage metabolism in health and disease

Cardiac macrophages are essential mediators of cardiac development, tissue homeostasis, and response to injury. Cell-intrinsic shifts in metabolism and availability of metabolites regulate macrophage function. The human and mouse heart contain a heterogeneous compilation of cardiac macrophages that are derived from at least two distinct lineages. In this review, we detail the unique functional roles and metabolic profiles of tissue-resident and monocyte-derived cardiac macrophages during embryonic development and adult tissue homeostasis and in response to pathologic and physiologic stressors. We discuss the metabolic preferences of each macrophage lineage and how metabolism influences monocyte fate specification. Finally, we highlight the contribution of cardiac macrophages and derived metabolites on cell-cell communication, metabolic health, and disease pathogenesis.

Assessing Cardiac Contractility From Single Molecules to Whole Hearts

Fundamentally, the heart needs to generate sufficient force and power output to dynamically meet the needs of the body. Cardiomyocytes contain specialized structures referred to as sarcomeres that power and regulate contraction. Disruption of sarcomeric function or regulation impairs contractility and leads to cardiomyopathies and heart failure. Basic, translational, and clinical studies have adapted numerous methods to assess cardiac contraction in a variety of pathophysiological contexts. These tools measure aspects of cardiac contraction at different scales ranging from single molecules to whole organisms. Moreover, these studies have revealed new pathogenic mechanisms of heart disease leading to the development of novel therapies targeting contractility. In this review, the authors explore the breadth of tools available for studying cardiac contractile function across scales, discuss their strengths and limitations, highlight new insights into cardiac physiology and pathophysiology, and describe how these insights can be harnessed for therapeutic candidate development and translational.

Smoking exposure-induced bronchus-associated lymphoid tissue in donor lungs does not prevent tolerance induction after transplantation

The presence of bronchus-associated lymphoid tissue (BALT) in donor lungs has been suggested to accelerate graft rejection after lung transplantation. Although chronic smoke exposure can induce BALT formation, the impact of donor cigarette use on alloimmune responses after lung transplantation is not well understood. Here, we show that smoking-induced BALT in mouse donor lungs contains Foxp3+ T cells and undergoes dynamic restructuring after transplantation, including recruitment of recipient-derived leukocytes to areas of pre-existing lymphoid follicles and replacement of graft-resident donor cells. Our findings from mouse and human lung transplant data support the notion that a donor's smoking history does not predispose to acute cellular rejection or prevent the establishment of allograft acceptance with comparable outcomes to nonsmoking donors. Thus, our work indicates that BALT in donor lungs is plastic in nature and may have important implications for modulating proinflammatory or tolerogenic immune responses following transplantation.

Feasibility of Interleukin-6 Receptor Blockade in Cardiac Antibody-mediated Rejection

BACKGROUND

Antibody-mediated rejection (AMR) remains a significant cause of heart transplant mortality with few effective therapies.

METHODS

This study aimed to describe initial experience of using interleukin-6 receptor blockade with tocilizumab in the treatment of acute cardiac AMR at Barnes-Jewish Hospital/Washington University Transplant Center from July 2017 to May 2021 (n = 7). Clinical, echocardiographic, and serum alloantibody data were analyzed before and after treatment.

RESULTS

All participants demonstrated marked improvement in functional status. Echocardiographic data following 4-6 mo of tocilizumab revealed significant improvements in biventricular systolic function for all participants. Consistent reductions in donor-specific HLA or angiotensin type I receptor antibodies were not observed, suggesting that tocilizumab may act downstream of antibody production. No patient experienced drug-related complications that necessitated discontinuation of therapy.

CONCLUSIONS

These findings provide initial insights into the safety and efficacy of interleukin-6 receptor blockade in the treatment of cardiac AMR and support the design of larger prospective studies.

B cells mediate lung ischemia/reperfusion injury by recruiting classical monocytes via synergistic B cell receptor/TLR4 signaling

Ischemia/reperfusion injury-mediated (IRI-mediated) primary graft dysfunction (PGD) adversely affects both short- and long-term outcomes after lung transplantation, a procedure that remains the only treatment option for patients suffering from end-stage respiratory failure. While B cells are known to regulate adaptive immune responses, their role in lung IRI is not well understood. Here, we demonstrated by intravital imaging that B cells are rapidly recruited to injured lungs, where they extravasate into the parenchyma. Using hilar clamping and transplant models, we observed that lung-infiltrating B cells produce the monocyte chemokine CCL7 in a TLR4-TRIF-dependent fashion, a critical step contributing to classical monocyte (CM) recruitment and subsequent neutrophil extravasation, resulting in worse lung function. We found that synergistic BCR-TLR4 activation on B cells is required for the recruitment of CMs to the injured lung. Finally, we corroborated our findings in reperfused human lungs, in which we observed a correlation between B cell infiltration and CM recruitment after transplantation. This study describes a role for B cells as critical orchestrators of lung IRI. As B cells can be depleted with currently available agents, our study provides a rationale for clinical trials investigating B cell-targeting therapies.

Expansion of Pathogenic Cardiac Macrophages in Immune Checkpoint Inhibitor Myocarditis

BACKGROUND

Immune checkpoint inhibitors (ICIs), antibodies targeting PD-1 (programmed cell death protein 1)/PD-L1 (programmed death-ligand 1) or CTLA4 (cytotoxic T-lymphocyte-associated protein 4), have revolutionized cancer management but are associated with devastating immune-related adverse events including myocarditis. The main risk factor for ICI myocarditis is the use of combination PD-1 and CTLA4 inhibition. ICI myocarditis is often fulminant and is pathologically characterized by myocardial infiltration of T lymphocytes and macrophages. Although much has been learned about the role of T-cells in ICI myocarditis, little is understood about the identity, transcriptional diversity, and functions of infiltrating macrophages.

METHODS

We used an established murine ICI myocarditis model (Ctla4+/-Pdcd1-/- mice) to explore the cardiac immune landscape using single-cell RNA-sequencing, immunostaining, flow cytometry, in situ RNA hybridization, molecular imaging, and antibody neutralization studies.

RESULTS

We observed marked increases in CCR2 (C-C chemokine receptor type 2)+ monocyte-derived macrophages and CD8+ T-cells in this model. The macrophage compartment was heterogeneous and displayed marked enrichment in an inflammatory CCR2+ subpopulation highly expressing Cxcl9 (chemokine [C-X-C motif] ligand 9), Cxcl10 (chemokine [C-X-C motif] ligand 10), Gbp2b (interferon-induced guanylate-binding protein 2b), and Fcgr4 (Fc receptor, IgG, low affinity IV) that originated from CCR2+ monocytes. It is important that a similar macrophage population expressing CXCL9, CXCL10, and CD16α (human homologue of mouse FcgR4) was expanded in patients with ICI myocarditis. In silico prediction of cell-cell communication suggested interactions between T-cells and Cxcl9+Cxcl10+ macrophages via IFN-γ (interferon gamma) and CXCR3 (CXC chemokine receptor 3) signaling pathways. Depleting CD8+ T-cells or macrophages and blockade of IFN-γ signaling blunted the expansion of Cxcl9+Cxcl10+ macrophages in the heart and attenuated myocarditis, suggesting that this interaction was necessary for disease pathogenesis.

CONCLUSIONS

These data demonstrate that ICI myocarditis is associated with the expansion of a specific population of IFN-γ-induced inflammatory macrophages and suggest the possibility that IFN-γ blockade may be considered as a treatment option for this devastating condition.

Imaging Inflammation Past, Present, and Future: Focus on Cardioimmunology

Growing evidence implicates the immune system as a critical mediator of cardiovascular disease progression and a viable therapeutic target. Increased inflammatory cell activity is seen in the full spectrum of disorders from early-stage atherosclerosis through myocardial infarction, cardiomyopathy, and chronic heart failure. Although therapeutic strategies to modulate inflammation have shown promise in preclinical animal models, efficacy in patients has been modest owing in part to the variable severity of inflammation across individuals. The diverse leukocyte subpopulations involved in different aspects of heart disease pose a challenge to effective therapy, wherein adverse and beneficial aspects of inflammation require appropriate balance. Noninvasive molecular imaging enables tissue-level interrogation of inflammatory cells in the heart and vasculature to provide mechanistic and temporal insights into disease progression. Although clinical imaging has relied on 18F-FDG as a nonselective and crude marker of inflammatory cell activity, new imaging probes targeting cell surface markers of different leukocyte subpopulations present the opportunity to visualize and quantify distinct phases of cardiac and vessel wall inflammation. Similarly, therapies are evolving to more effectively isolate adverse from beneficial cell populations. This parallel development of immunocardiology and molecular imaging provides the opportunity to refine treatments using imaging guidance, building toward mechanism-based precision medicine. Here, we discuss progress in molecular imaging of immune cells in cardiology from use of 18F-FDG in the past to the present expansion of the radiotracer arsenal and then to a future theranostic paradigm of tracer-therapy compound pairs with shared targets. We then highlight the critical experiments required to advance the field from preclinical concept to clinical reality.

Ferroptosis in the post-transplantation inflammatory response

Transplantation is a life-saving therapy for patients with end-stage organ disease. Successful outcomes after transplantation require mitigation of the post-transplant inflammatory response, limiting alloreactivity, and prevention of organ rejection. Traditional immunosuppressive regimens aim to dampen the adaptive immune response; however, recent studies have shown the feasibility and efficacy of targeting the innate immune response. Necroinflammation initiated by donor organ cell death is implicated as a critical mediator of primary graft dysfunction, acute rejection, and chronic rejection. Ferroptosis is a form of regulated cell death that triggers post-transplantation inflammation and drives the activation of both innate and adaptive immune cells. There is a growing acceptance of the clinical relevance of ferroptosis to solid organ transplantation. Modulating ferroptosis may be a potentially promising strategy to reduce complications after organ transplantation.

Resolvin D1 prevents injurious neutrophil swarming in transplanted lungs

Neutrophils are the primary cell type involved in lung ischemia-reperfusion injury (IRI), which remains a frequent and morbid complication after organ transplantation. Endogenous lipid mediators that become activated during acute inflammation-resolution have gained increasing recognition for their protective role(s) in promoting the restoration of homeostasis, but their influence on early immune responses following transplantation remains to be uncovered. Resolvin D1, 7S,8R,17S-trihydroxy-4Z,9E,11E,13Z,15E,19Z-docosahexaenoic acid (RvD1), is a potent stereoselective mediator that exhibits proresolving and anti-inflammatory actions in the setting of tissue injury. Here, using metabololipidomics, we demonstrate that endogenous proresolving mediators including RvD1 are increased in human and murine lung grafts immediately following transplantation. In mouse grafts, we observe lipid mediator class switching early after reperfusion. We use intravital two-photon microscopy to reveal that RvD1 treatment significantly limits early neutrophil infiltration and swarming, thereby ameliorating early graft dysfunction in transplanted syngeneic lungs subjected to severe IRI. Through integrated analysis of single-cell RNA sequencing data of donor and recipient immune cells from lung grafts, we identify transcriptomic changes induced by RvD1. These results support a role for RvD1 as a potent modality for preventing early neutrophil-mediated tissue damage after lung IRI that may be therapeutic in the clinics.

Deletion of taf1 and taf5 in zebrafish capitulate cardiac and craniofacial abnormalities associated with TAFopathies through perturbations in metabolism

Intellectual disability is a neurodevelopmental disorder that affects 2-3% of the general population. Syndromic forms of intellectual disability frequently have a genetic basis and are often accompanied by additional developmental anomalies. Pathogenic variants in components of TATA-binding protein associated factors (TAFs) have recently been identified in a subset of patients with intellectual disability, craniofacial hypoplasia, and congenital heart disease. This syndrome has been termed as a TAFopathy and includes mutations in TATA binding protein (TBP), TAF1, TAF2, and TAF6. The underlying mechanism by which TAFopathies give rise to neurodevelopmental, craniofacial, and cardiac abnormalities remains to be defined. Through a forward genetic screen in zebrafish, we have recovered a recessive mutant phenotype characterized by craniofacial hypoplasia, ventricular hypoplasia, heart failure at 96 h post-fertilization and lethality, and show it is caused by a nonsense mutation in taf5. CRISPR/CAS9 mediated gene editing revealed that these defects where phenocopied by mutations in taf1 and taf5. Mechanistically, taf5-/- zebrafish displayed misregulation in metabolic gene expression and metabolism as evidenced by RNA sequencing, respiration assays, and metabolite studies. Collectively, these findings suggest that the TAF complex may contribute to neurologic, craniofacial, and cardiac development through regulation of metabolism.

Rapid recruitment and IFN-I-mediated activation of monocytes dictate focal radiotherapy efficacy

The recruitment of monocytes and their differentiation into immunosuppressive cells is associated with the low efficacy of preclinical nonconformal radiotherapy (RT) for tumors. However, nonconformal RT (non-CRT) does not mimic clinical practice, and little is known about the role of monocytes after RT modes used in patients, such as conformal RT (CRT). Here, we investigated the acute immune response induced by after CRT. Contrary to non-CRT approaches, we found that CRT induces a rapid and robust recruitment of monocytes to the tumor that minimally differentiate into tumor-associated macrophages or dendritic cells but instead up-regulate major histocompatibility complex II and costimulatory molecules. We found that these large numbers of infiltrating monocytes are responsible for activating effector polyfunctional CD8+ tumor-infiltrating lymphocytes that reduce tumor burden. Mechanistically, we show that monocyte-derived type I interferon is pivotal in promoting monocyte accumulation and immunostimulatory function in a positive feedback loop. We also demonstrate that monocyte accumulation in the tumor microenvironment is hindered when RT inadvertently affects healthy tissues, as occurs in non-CRT. Our results unravel the immunostimulatory function of monocytes during clinically relevant modes of RT and demonstrate that limiting the exposure of healthy tissues to radiation has a positive therapeutic effect on the overall antitumor immune response.

Expansion of Disease Specific Cardiac Macrophages in Immune Checkpoint Inhibitor Myocarditis

Background

Immune checkpoint inhibitors (ICIs), antibodies targeting PD-1/PD-L1 or CTLA4 have revolutionized cancer management but are associated with devastating immune-related adverse events (irAEs) including myocarditis. The main risk factor for ICI myocarditis is the use of combination PD-1 and CTLA4 inhibition. ICI-myocarditis is often fulminant and is pathologically characterized by myocardial infiltration of T lymphocytes and macrophages. While much has been learned regarding the role of T-cells in ICI-myocarditis, little is understood regarding the identity, transcriptional diversity, and functions of infiltrating macrophages.

Methods

We employed an established murine ICI myocarditis model ( Ctla4 +/- Pdcd1 -/- mice) to explore the cardiac immune landscape using single-cell RNA-sequencing, immunostaining, flow cytometry, in situ RNA hybridization and molecular imaging and antibody neutralization studies.

Results

We observed marked increases in CCR2 + monocyte-derived macrophages and CD8 + T-cells in this model. The macrophage compartment was heterogeneous and displayed marked enrichment in an inflammatory CCR2 + subpopulation highly expressing Cxcl9 , Cxcl10 , Gbp2b , and Fcgr4 that originated from CCR2 + monocytes. Importantly, a similar macrophage population expressing CXCL9 , CXCL10 , and CD16α (human homologue of mouse FcgR4) was found selectively expanded in patients with ICI myocarditis compared to other forms of heart failure and myocarditis. In silico prediction of cell-cell communication suggested interactions between T-cells and Cxcl9 + Cxcl10 + macrophages via IFN-γ and CXCR3 signaling pathways. Depleting CD8 + T-cells, macrophages, and blockade of IFN-γ signaling blunted the expansion of Cxcl9 + Cxcl10 + macrophages in the heart and attenuated myocarditis suggesting that this interaction was necessary for disease pathogenesis.

Conclusion

These data demonstrate that ICI-myocarditis is associated with the expansion of a specific population of IFN-γ induced inflammatory macrophages and suggest the possibility that IFN-γ blockade may be considered as a treatment option for this devastating condition.

Defining cardiac functional recovery in end-stage heart failure at single-cell resolution

Recovery of cardiac function is the holy grail of heart failure therapy yet is infrequently observed and remains poorly understood. In this study, we performed single-nucleus RNA sequencing from patients with heart failure who recovered left ventricular systolic function after left ventricular assist device implantation, patients who did not recover and non-diseased donors. We identified cell-specific transcriptional signatures of recovery, most prominently in macrophages and fibroblasts. Within these cell types, inflammatory signatures were negative predictors of recovery, and downregulation of RUNX1 was associated with recovery. In silico perturbation of RUNX1 in macrophages and fibroblasts recapitulated the transcriptional state of recovery. Cardiac recovery mediated by BET inhibition in mice led to decreased macrophage and fibroblast Runx1 expression and diminished chromatin accessibility within a Runx1 intronic peak and acquisition of human recovery signatures. These findings suggest that cardiac recovery is a unique biological state and identify RUNX1 as a possible therapeutic target to facilitate cardiac recovery.

Clinicopathological classification of immune checkpoint inhibitor-associated myocarditis: possible refinement by measuring macrophage abundance

BACKGROUND

Immune checkpoint inhibitor (ICI) myocarditis is associated with high morbidity and mortality. While endomyocardial biopsy (EMB) is considered a gold standard for diagnosis, the sensitivity of EMB is not well defined. Additionally, the pathological features that correlate with the clinical diagnosis of ICI-associated myocarditis remain incompletely understood.

METHODS

We retrospectively identified and reviewed the clinicopathological features of 26 patients with suspected ICI-associated myocarditis based on institutional major and minor criteria. Seventeen of these patients underwent EMB, and the histopathological features were assessed by routine hematoxylin and eosin (H&E) staining and immunohistochemical (IHC) staining for CD68, a macrophage marker.

RESULTS

Only 2/17 EMBs obtained from patients with suspected ICI myocarditis satisfied the Dallas criteria. Supplemental IHC staining and quantification of CD68⁺ macrophages identified an additional 7 patients with pathological features of myocardial inflammation (> 50 CD68⁺ cells/HPF). Macrophage abundance positively correlated with serum Troponin I (P = 0.010) and NT-proBNP (N-terminal pro-brain natriuretic peptide, P = 0.047) concentration. Inclusion of CD68 IHC could have potentially changed the certainty of the diagnosis of ICI-associated myocarditis to definite in 6/17 cases.

CONCLUSIONS

While the Dallas criteria can identify a subset of ICI-associated myocarditis patients, quantification of macrophage abundance may expand the diagnostic role of EMB. Failure to meet the traditional Dallas Criteria should not exclude the diagnosis of myocarditis.

A TRPV4-dependent neuroimmune axis in the spinal cord promotes neuropathic pain

Microglia, resident macrophages of the CNS, are essential to brain development, homeostasis, and disease. Microglial activation and proliferation are hallmarks of many CNS diseases, including neuropathic pain. However, molecular mechanisms that govern the spinal neuroimmune axis in the setting of neuropathic pain remain incompletely understood. Here, we show that genetic ablation or pharmacological blockade of transient receptor potential vanilloid type 4 (TRPV4) markedly attenuated neuropathic pain-like behaviors in a mouse model of spared nerve injury. Mechanistically, microglia-expressed TRPV4 mediated microglial activation and proliferation and promoted functional and structural plasticity of excitatory spinal neurons through release of lipocalin-2. Our results suggest that microglial TRPV4 channels reside at the center of the neuroimmune axis in the spinal cord, which transforms peripheral nerve injury into central sensitization and neuropathic pain, thereby identifying TRPV4 as a potential new target for the treatment of chronic pain.

CCL17 Protects Against Viral Myocarditis by Suppressing the Recruitment of Regulatory T Cells

Background Viral myocarditis is characterized by leukocyte infiltration of the heart and cardiomyocyte death. We recently identified C-C chemokine ligand (CCL) 17 as a proinflammatory effector of C-C chemokine receptor 2-positive macrophages and dendritic cells that are recruited to the heart and contribute to adverse left ventricular remodeling following myocardial infarction and pressure overload. Methods and Results Mouse encephalomyocarditis virus was used to investigate the function of CCL17 in a viral myocarditis model. Ccl17^Gfp reporter and knockout mice were used to identify the cell types that express CCL17 and delineate the functional importance of CCL17 in encephalomyocarditis virus clearance and myocardial inflammation. Cardiac CCL17 was expressed in C-C chemokine receptor 2-positive macrophages and dendritic cells following encephalomyocarditis virus infection. Colony-stimulating factor 2 (granulocyte-macrophage colony-stimulating factor) signaling was identified as a key regulator of CCL17 expression. Ccl17 deletion resulted in impaired encephalomyocarditis virus clearance, increased cardiomyocyte death, and higher mortality during infection early stage, and aggravated hypertrophy and fibrotic responses in infection long-term stage. An increased abundance of regulatory T cells was detected in the myocardium of injured Ccl17-deficient mice. Depletion of regulatory T cells in Ccl17-deficient mice abrogated the detrimental role of CCL17 deletion by restoring interferon signaling. Conclusions Collectively, these findings identify CCL17 as an important mediator of the host immune response during cardiac viral infection early stage and suggest that CCL17 targeted therapies should be avoided in acute viral myocarditis.

Human Cardiac Pericytes Are Susceptible to SARS-CoV-2 Infection

COVID-19 is associated with serious cardiovascular complications, with incompletely understood mechanism(s). Pericytes have key functions in supporting endothelial cells and maintaining vascular integrity. We demonstrate that human cardiac pericytes are permissive to SARS-CoV-2 infection in organotypic slice and primary cell cultures. Viral entry into pericytes is mediated by endosomal proteases, and infection leads to up-regulation of inflammatory markers, vasoactive mediators, and nuclear factor kappa-B-dependent cell death. Furthermore, we present evidence of cardiac pericyte infection in COVID-19 myocarditis patients. These data demonstrate that human cardiac pericytes are susceptible to SARS-CoV-2 infection and suggest a role for pericyte infection in COVID-19.

Targeting Immune-Fibroblast Crosstalk in Myocardial Infarction and Cardiac Fibrosis

Inflammation and tissue fibrosis co-exist and are causally linked to organ dysfunction. However, the molecular mechanisms driving immune-fibroblast crosstalk in human cardiac disease remains unexplored and there are currently no therapeutics to target fibrosis. Here, we performed multi-omic single-cell gene expression, epitope mapping, and chromatin accessibility profiling in 38 donors, acutely infarcted, and chronically failing human hearts. We identified a disease-associated fibroblast trajectory marked by cell surface expression of fibroblast activator protein (FAP), which diverged into distinct myofibroblasts and pro-fibrotic fibroblast populations, the latter resembling matrifibrocytes. Pro-fibrotic fibroblasts were transcriptionally similar to cancer associated fibroblasts and expressed high levels of collagens and periostin (POSTN), thymocyte differentiation antigen 1 (THY-1), and endothelin receptor A (EDNRA) predicted to be driven by a RUNX1 gene regulatory network. We assessed the applicability of experimental systems to model tissue fibrosis and demonstrated that 3 different in vivo mouse models of cardiac injury were superior compared to cultured human heart and dermal fibroblasts in recapitulating the human disease phenotype. Ligand-receptor analysis and spatial transcriptomics predicted that interactions between C-C chemokine receptor type 2 (CCR2) macrophages and fibroblasts mediated by interleukin 1 beta (IL-1β) signaling drove the emergence of pro-fibrotic fibroblasts within spatially defined niches. This concept was validated through in silico transcription factor perturbation and in vivo inhibition of IL-1β signaling in fibroblasts where we observed reduced pro-fibrotic fibroblasts, preferential differentiation of fibroblasts towards myofibroblasts, and reduced cardiac fibrosis. Herein, we show a subset of macrophages signal to fibroblasts via IL-1β and rewire their gene regulatory network and differentiation trajectory towards a pro-fibrotic fibroblast phenotype. These findings highlight the broader therapeutic potential of targeting inflammation to treat tissue fibrosis and restore organ function.

Imaging Targets to Visualize the Cardiac Immune Landscape in Heart Failure

Heart failure involves a complex interplay between diverse populations of immune cells that dynamically shift across the natural history of disease. Within this context, the character of the immune response is a key determinant of clinical outcomes. Recent technological advances in single-cell transcriptomic, spatial, and proteomic technologies have fueled an explosion of new and clinically relevant insights into distinct immune cell populations that reside within the diseased heart including potential targets for molecular imaging and therapy. In this review, we will discuss the immune cell types and their respective functions with respect to myocardial infarction remodeling, dilated cardiomyopathy, and heart failure with preserved ejection fraction. In addition, we give a brief overview regarding myocarditis and cardiac sarcoidosis as inflammatory heart failure etiologies. We will highlight markers and cell populations as targets for molecular imaging to visualize inflammation and tissue healing and discuss clinical implications including the development and implementation of precision medicine approaches.

The Programmed Death-1 Signaling Axis Modulates Inflammation and LV Structure/Function in a Stress-Induced Cardiomyopathy Model

The role of immune checkpoints in the setting of tissue injury remains unknown. Using an experimental model of isoproterenol (ISO)-induced stress cardiomyopathy, we show that ISO-induced myocardial injury provokes tissue-autonomous up-regulation of the programmed death-1 (PD-1):programmed death ligand (PD-L) axis in cardiac resident innate immune cells and T cells. PD-1 signaling was responsible for modulating the acute inflammatory response, as well as normalization of impaired left ventricular structure and function after ISO injection. Necrotic cardiac extracts were sufficient to increase the expression of PD-1 in macrophages and T cells in vitro. Viewed together these studies suggest that the PD-1:PD-L signaling axis regulates immune responses to cardiac tissue injury and is important for restoring myocardial homeostasis.

Biology of myocardial recovery in advanced heart failure with long-term mechanical support

Cardiac remodeling is an adaptive, compensatory biological process following an initial insult to the myocardium that gradually becomes maladaptive and causes clinical deterioration and chronic heart failure (HF). This biological process involves several pathophysiological adaptations at the genetic, molecular, cellular, and tissue levels. A growing body of clinical and translational investigations demonstrated that cardiac remodeling and chronic HF does not invariably result in a static, end-stage phenotype but can be at least partially reversed. One of the paradigms which shed some additional light on the breadth and limits of myocardial elasticity and plasticity is long term mechanical circulatory support (MCS) in advanced HF pediatric and adult patients. MCS by providing (a) ventricular mechanical unloading and (b) effective hemodynamic support to the periphery results in functional, structural, cellular and molecular changes, known as cardiac reverse remodeling. Herein, we analyze and synthesize the advances in our understanding of the biology of MCS-mediated reverse remodeling and myocardial recovery. The MCS investigational setting offers access to human tissue, providing an unparalleled opportunity in cardiovascular medicine to perform in-depth characterizations of myocardial biology and the associated molecular, cellular, and structural recovery signatures. These human tissue findings have triggered and effectively fueled a "bedside to bench and back" approach through a variety of knockout, inhibition or overexpression mechanistic investigations in vitro and in vivo using small animal models. These follow-up translational and basic science studies leveraging human tissue findings have unveiled mechanistic myocardial recovery pathways which are currently undergoing further testing for potential therapeutic drug development. Essentially, the field is advancing by extending the lessons learned from the MCS cardiac recovery investigational setting to develop therapies applicable to the greater, not end-stage, HF population. This review article focuses on the biological aspects of the MCS-mediated myocardial recovery and together with its companion review article, focused on the clinical aspects, they aim to provide a useful framework for clinicians and investigators.

Transcriptional and Immune Landscape of Cardiac Sarcoidosis

BACKGROUND

Cardiac involvement is an important determinant of mortality among sarcoidosis patients. Although granulomatous inflammation is a hallmark finding in cardiac sarcoidosis, the precise immune cell populations that comprise the granuloma remain unresolved. Furthermore, it is unclear how the cellular and transcriptomic landscape of cardiac sarcoidosis differs from other inflammatory heart diseases.

METHODS

We leveraged spatial transcriptomics (GeoMx digital spatial profiler) and single-nucleus RNA sequencing to elucidate the cellular and transcriptional landscape of cardiac sarcoidosis. Using GeoMX digital spatial profiler technology, we compared the transcriptomal profile of CD68⁺ rich immune cell infiltrates in human cardiac sarcoidosis, giant cell myocarditis, and lymphocytic myocarditis. We performed single-nucleus RNA sequencing of human cardiac sarcoidosis to identify immune cell types and examined their transcriptomic landscape and regulation. Using multichannel immunofluorescence staining, we validated immune cell populations identified by single-nucleus RNA sequencing, determined their spatial relationship, and devised an immunostaining approach to distinguish cardiac sarcoidosis from other inflammatory heart diseases.

RESULTS

Despite overlapping histological features, spatial transcriptomics identified transcriptional signatures and associated pathways that robustly differentiated cardiac sarcoidosis from giant cell myocarditis and lymphocytic myocarditis. Single-nucleus RNA sequencing revealed the presence of diverse populations of myeloid cells in cardiac sarcoidosis with distinct molecular features. We identified GPNMB (transmembrane glycoprotein NMB) as a novel marker of multinucleated giant cells and predicted that the MITF (microphthalmia-associated transcription factor) family of transcription factors regulated this cell type. We also detected additional macrophage populations in cardiac sarcoidosis including HLA-DR (human leukocyte antigen-DR)⁺ macrophages, SYTL3 (synaptotagmin-like protein 3)⁺ macrophages and CD163⁺ resident macrophages. HLA-DR⁺ macrophages were found immediately adjacent to GPMMB⁺ giant cells, a distinct feature compared with other inflammatory cardiac diseases. SYTL3⁺ macrophages were located scattered throughout the granuloma and CD163⁺ macrophages, CD1c⁺ dendritic cells, nonclassical monocytes, and T cells were located at the periphery and outside of the granuloma. Finally, we demonstrate mTOR (mammalian target of rapamycin) pathway activation is associated with proliferation and is selectively found in HLA-DR⁺ and SYLT3⁺ macrophages.

CONCLUSIONS

In this study, we identified diverse populations of immune cells with distinct molecular signatures that comprise the sarcoid granuloma. These findings provide new insights into the pathology of cardiac sarcoidosis and highlight opportunities to improve diagnostic testing.

Donor Macrophages Modulate Rejection After Heart Transplantation

BACKGROUND

Cellular rejection after heart transplantation imparts significant morbidity and mortality. Current immunosuppressive strategies are imperfect, target recipient T cells, and have adverse effects. The innate immune response plays an essential role in the recruitment and activation of T cells. Targeting the donor innate immune response would represent the earliest interventional opportunity within the immune response cascade. There is limited knowledge about donor immune cell types and functions in the setting of cardiac transplantation, and no current therapeutics exist for targeting these cell populations.

METHODS

Using genetic lineage tracing, cell ablation, and conditional gene deletion, we examined donor mononuclear phagocyte diversity and macrophage function during acute cellular rejection of transplanted hearts in mice. We performed single-cell RNA sequencing on donor and recipient macrophages and monocytes at multiple time points after transplantation. On the basis of our imaging and single-cell RNA sequencing data, we evaluated the functional relevance of donor CCR2+ (C-C chemokine receptor 2) and CCR2- macrophages using selective cell ablation strategies in donor grafts before transplant. Last, we performed functional validation that donor macrophages signal through MYD88 (myeloid differentiation primary response protein 88) to facilitate cellular rejection.

RESULTS

Donor macrophages persisted in the rejecting transplanted heart and coexisted with recipient monocyte-derived macrophages. Single-cell RNA sequencing identified donor CCR2+ and CCR2- macrophage populations and revealed remarkable diversity among recipient monocytes, macrophages, and dendritic cells. Temporal analysis demonstrated that donor CCR2+ and CCR2- macrophages were transcriptionally distinct, underwent significant morphologic changes, and displayed unique activation signatures after transplantation. Although selective depletion of donor CCR2- macrophages reduced allograft survival, depletion of donor CCR2+ macrophages prolonged allograft survival. Pathway analysis revealed that donor CCR2+ macrophages are activated through MYD88/nuclear factor kappa light chain enhancer of activated B cells signaling. Deletion of MYD88 in donor macrophages resulted in reduced antigen-presenting cell recruitment, reduced ability of antigen-presenting cells to present antigen to T cells, decreased emergence of allograft-reactive T cells, and extended allograft survival.

CONCLUSIONS

Distinct populations of donor and recipient macrophages coexist within the transplanted heart. Donor CCR2+ macrophages are key mediators of allograft rejection, and deletion of MYD88 signaling in donor macrophages is sufficient to suppress rejection and extend allograft survival. This highlights the therapeutic potential of donor heart-based interventions.

The Dynamic Role of Cardiac Macrophages in Aging and Disease

PURPOSE OF REVIEW

The cardiac immune landscape dynamically changes in response to aging, hemodynamic stress, and myocardial injury. Here, we highlight key cardiac immune cell types, their role in reshaping the cellular landscape and promoting tissue remodeling following cardiac insults, and how understanding of these processes uncovers novel disease mechanisms that contribute to cardiac pathology.

RECENT FINDINGS

Distinct subsets of cardiac macrophages reside within the heart and exhibit divergent functions in response to myocardial injury. Parsing cardiac macrophages based on developmental origin has served as a valuable approach to define functionally divergent populations of reparative (embryonic-derived, tissue resident) and inflammatory (monocyte-derived, recruited) cardiac macrophages. Single-cell transcriptomics and elucidation of the effector mechanisms that orchestrate macrophage functions has provided new and therapeutically tractable insights into the pathogenesis of numerous cardiac diseases. The immune landscape of the heart is dynamic and represents an important mediator of disease pathogenesis across an array of cardiac pathology. Elucidation of mechanisms that drive inflammatory monocyte/macrophage recruitment, activation, and effector responses may lead to the identification of new therapeutic targets.

Pediatric and adult dilated cardiomyopathy are distinguished by distinct biomarker profiles

BACKGROUND

Emerging evidence suggests that pediatric and adult dilated cardiomyopathy (DCM) represent distinct diseases. Few diagnostic tools exist for pediatric cardiologists to assess clinical status and prognosis. We hypothesized that pediatric DCM would have a unique biomarker profile compared to adult DCM and controls.

METHODS

We utilized a DNA aptamer array (SOMAScan) to compare biomarker profiles between pediatric and adult DCM. We simultaneously measured 1310 plasma proteins and peptides from 39 healthy children (mean age 3 years, interquartile range (IQR) 1-14), 39 ambulatory subjects with pediatric DCM (mean age 2.7 years, IQR 1-13), and 40 ambulatory adults with DCM (mean age 53 years, IQR 46-63).

RESULTS

Pediatric and adult DCM patients displayed distinct biomarker profiles, despite similar clinical characteristics. We identified 20 plasma peptides and proteins that were increased in pediatric DCM compared to age- and sex-matched controls. Unbiased multidimensionality reduction analysis suggested previously unrecognized heterogeneity among pediatric DCM subjects. Biomarker profile analysis identified four subgroups of pediatric DCM with distinguishing clinical characteristics.

CONCLUSIONS

These findings support the emerging concept that pediatric and adult DCM are distinct disease entities, signify the need to develop pediatric-specific biomarkers for disease prognostication, and challenge the paradigm that pediatric DCM should be viewed as a single disease.

IMPACT

Pediatric and adult DCM patients displayed distinct biomarker profiles, despite similar clinical characteristics and outcomes. Our findings suggest that pediatric DCM may be a heterogeneous disease with various sub-phenotypes, including differing biomarker profiles and clinical findings. These data provide prerequisite information for future prospective studies that validate the identified pediatric DCM biomarkers, address their diagnostic accuracy and prognostic significance, and explore the full extent of heterogeneity amongst pediatric DCM patients.

Mouse Heterotopic Cervical Cardiac Transplantation Utilizing Vascular Cuffs

Murine models of cardiac transplantation are frequently utilized to study ischemia-reperfusion injury, innate and adaptive immune responses after transplantation, and the impact of immunomodulatory therapies on graft rejection. Heterotopic cervical heart transplantation in mice was first described in 1991 using sutured anastomoses and subsequently modified to include cuff techniques. This modification allowed for improved success rates, and since then, there have been multiple reports that have proposed further technical improvements. However, translation into more widespread utilization remains limited due to the technical difficulty associated with graft anastomoses, which requires precision to achieve adequate length and caliber of the cuffs to avoid vascular anastomotic twisting or excessive tension, which can result in damage to the graft. The present protocol describes a modified technique for performing heterotopic cervical cardiac transplantation in mice which involves cuff placement on the recipient's common carotid artery and the donor's pulmonary artery in alignment with the direction of the blood flow.

Meteorin-like promotes heart repair through endothelial KIT receptor tyrosine kinase

Effective tissue repair after myocardial infarction entails a vigorous angiogenic response, guided by incompletely defined immune cell-endothelial cell interactions. We identify the monocyte- and macrophage-derived cytokine METRNL (meteorin-like) as a driver of postinfarction angiogenesis and high-affinity ligand for the stem cell factor receptor KIT (KIT receptor tyrosine kinase). METRNL mediated angiogenic effects in cultured human endothelial cells through KIT-dependent signaling pathways. In a mouse model of myocardial infarction, METRNL promoted infarct repair by selectively expanding the KIT-expressing endothelial cell population in the infarct border zone. Metrnl-deficient mice failed to mount this KIT-dependent angiogenic response and developed severe postinfarction heart failure. Our data establish METRNL as a KIT receptor ligand in the context of ischemic tissue repair.

Derivation of extra-embryonic and intra-embryonic macrophage lineages from human pluripotent stem cells

Tissue-resident macrophages are increasingly recognized as important determinants of organ homeostasis, tissue repair, remodeling and regeneration. Although the ontogeny and function of tissue-resident macrophages has been identified as distinct from postnatal hematopoiesis, the inability to specify, in vitro, similar populations that recapitulate these developmental waves has limited our ability to study their function and potential for regenerative applications. We took advantage of the concept that tissue-resident macrophages and monocyte-derived macrophages originate from distinct extra-embryonic and definitive hematopoietic lineages to devise a system to generate pure cultures of macrophages that resemble tissue-resident or monocyte-derived subsets. We demonstrate that human pluripotent stem cell-derived extra-embryonic-like and intra-embryonic-like hematopoietic progenitors differentiate into morphologically, transcriptionally and functionally distinct macrophage populations. Single-cell RNA sequencing of developing and mature cultures uncovered distinct developmental trajectories and gene expression programs of macrophages derived from extra-embryonic-like and intra-embryonic-like hematopoietic progenitors. These findings establish a resource for the generation of human tissue resident-like macrophages to study their specification and function under defined conditions and to explore their potential use in tissue engineering and regenerative medicine applications.

CCL17 Aggravates Myocardial Injury by Suppressing Recruitment of Regulatory T Cells

BACKGROUND

Recent studies have established that CCR2 (C-C chemokine receptor type 2) marks proinflammatory subsets of monocytes, macrophages, and dendritic cells that contribute to adverse left ventricle (LV) remodeling and heart failure progression. Elucidation of the effector mechanisms that mediate adverse effects of CCR2+ monocytes, macrophages, and dendritic cells will yield important insights into therapeutic strategies to suppress myocardial inflammation.

METHODS

We used mouse models of reperfused myocardial infarction, angiotensin II and phenylephrine infusion, and diphtheria toxin cardiomyocyte ablation to investigate CCL17 (C-C chemokine ligand 17). We used Ccl17 knockout mice, flow cytometry, RNA sequencing, biochemical assays, cell trafficking studies, and in vivo cell depletion to identify the cell types that generate CCL17, define signaling pathways that controlled its expression, delineate the functional importance of CCL17 in adverse LV remodeling and heart failure progression, and determine the mechanistic basis by which CCL17 exerts its effects.

RESULTS

We demonstrated that CCL17 is expressed in CCR2+ macrophages and cluster of differentiation 11b+ conventional dendritic cells after myocardial infarction, angiotensin II and phenylephrine infusion, and diphtheria toxin cardiomyocyte ablation. We clarified the transcriptional signature of CCL17+ macrophages and dendritic cells and identified granulocyte-macrophage colony-stimulating factor (GM-CSF) signaling as a key regulator of CCL17 expression through cooperative activation of STAT5 (signal transducer and activator of transcription 5) and canonical NF-κB (nuclear factor κ-light-chain-enhancer of activated B cells) signaling. Ccl17 deletion resulted in reduced LV remodeling, decreased myocardial fibrosis and cardiomyocyte hypertrophy, and improved LV systolic function after myocardial infarction and angiotensin II and phenylephrine infusion. We observed increased abundance of regulatory T cells (Tregs) in the myocardium of injured Ccl17 knockout mice. CCL17 inhibited Treg recruitment through biased activation of CCR4. CCL17 activated Gq signaling and CCL22 (C-C chemokine ligand 22) activated both Gq and ARRB (β-arrestin) signaling downstream of CCR4. CCL17 competitively inhibited CCL22 stimulated ARRB signaling and Treg migration. We provide evidence that Tregs mediated the protective effects of Ccl17 deletion on myocardial inflammation and adverse LV remodeling.

CONCLUSIONS

These findings identify CCL17 as a proinflammatory mediator of CCR2+ macrophages and dendritic cells and suggest that inhibition of CCL17 may serve as an effective strategy to promote Treg recruitment and suppress myocardial inflammation.

Cell specific peripheral immune responses predict survival in critical COVID-19 patients

SARS-CoV-2 triggers a complex systemic immune response in circulating blood mononuclear cells. The relationship between immune cell activation of the peripheral compartment and survival in critical COVID-19 remains to be established. Here we use single-cell RNA sequencing and Cellular Indexing of Transcriptomes and Epitomes by sequence mapping to elucidate cell type specific transcriptional signatures that associate with and predict survival in critical COVID-19. Patients who survive infection display activation of antibody processing, early activation response, and cell cycle regulation pathways most prominent within B-, T-, and NK-cell subsets. We further leverage cell specific differential gene expression and machine learning to predict mortality using single cell transcriptomes. We identify interferon signaling and antigen presentation pathways within cDC2 cells, CD14 monocytes, and CD16 monocytes as predictors of mortality with 90% accuracy. Finally, we validate our findings in an independent transcriptomics dataset and provide a framework to elucidate mechanisms that promote survival in critically ill COVID-19 patients. Identifying prognostic indicators among critical COVID-19 patients holds tremendous value in risk stratification and clinical management.

Selective loss of resident macrophage-derived insulin-like growth factor-1 abolishes adaptive cardiac growth to stress

Hypertension affects one-third of the world's population, leading to cardiac dysfunction that is modulated by resident and recruited immune cells. Cardiomyocyte growth and increased cardiac mass are essential to withstand hypertensive stress; however, whether immune cells are involved in this compensatory cardioprotective process is unclear. In normotensive animals, single-cell transcriptomics of fate-mapped self-renewing cardiac resident macrophages (RMs) revealed transcriptionally diverse cell states with a core repertoire of reparative gene programs, including high expression of insulin-like growth factor-1 (Igf1). Hypertension drove selective in situ proliferation and transcriptional activation of some cardiac RM states, directly correlating with increased cardiomyocyte growth. During hypertension, inducible ablation of RMs or selective deletion of RM-derived Igf1 prevented adaptive cardiomyocyte growth, and cardiac mass failed to increase, which led to cardiac dysfunction. Single-cell transcriptomics identified a conserved IGF1-expressing macrophage subpopulation in human cardiomyopathy. Here we defined the absolute requirement of RM-produced IGF-1 in cardiac adaptation to hypertension.

Abstract 119: Donor Macrophages Modulate Cellular Rejection After Heart Transplantation

Heart transplant rejection is common and is associated with significant morbidity and mortality. Current immunosuppressive therapies primarily target recipient T-cells and have a multitude of untoward effects including infections, malignancies, and end-organ damage. Recent studies implicate the roles of antigen presenting cells towards pathogenesis of allograft rejection through recruitment and activation of T-cells. The importance of antigen presenting cell origin, identity, and functional importance remains unknown. Using complimentary imaging and single cell RNA sequencing techniques, we show that donor and recipient monocytes and macrophages co-exist after heart transplantation. These myeloid populations have diverse transcriptional signatures that evolve throughout ongoing rejection. Donor macrophages can be defined ontologically and based on their expression of C-C chemokine receptor 2 (CCR2) and expression of MHC-II. Donor CCR2+ and CCR2- populations can be further defined based on their gene expression profiles, highlighting the marked heterogeneity in the donor macrophage population. Selective depletion of CCR2+ macrophages result in prolonged allograft survival. We use longitudinal single cell RNA sequencing to show that donor CCR2+ and CCR2- macrophages have distinct activation mechanisms such that donor CCR2+ macrophages signal through MyD88/NF-kB. Conditional depletion of MyD88 in donor macrophages recapitulates the donor CCR2+ depletion phenotype. Further interrogation of MyD88 conditionally depleted allografts shows reduced T-cell alloreactivity, holding promise for a potential therapeutic target pathway. Together, we show the molecular identity, diversity, and evolution of donor and recipient monocytes and macrophages as well as the functional relevance and activation pathways of donor macrophages in cardiac allografts.

Abstract P439: Harnessing Multiscale Models Of A Dilated Cardiomyopathy Mutation For Precision Medicine

Familial dilated cardiomyopathy (DCM) is a leading cause of both adult and pediatric heart failure. Currently, there is no cure for DCM, and the 5-year transplant free survival rate is <50%. There is therefore an outstanding need to develop new therapeutics. Prior studies have established a strong genetic basis for DCM and identified causative genetic mutations. These observations provide unique opportunities to apply precision medicine approaches that target and circumvent the effects of deleterious mutations. Here, we used a multiscale approach to study the consequences of a human mutation in troponin T that causes DCM, ΔK210. We found that at the molecular scale ΔK210 changes the positioning of tropomyosin along the thin filament, leading to molecular hypocontractility. Using genome edited human stem cell derived cardiomyocytes heterozygous for the mutation, we show reduced cellular contractility at the single cell and tissue levels. Importantly, we demonstrate that mutant tissues show a reduced Frank-Starling response, increased stiffness, and misaligned myocytes. Based on our molecular mechanism, we hypothesized that treatment of ΔK210 with Omecamtiv Mecarbil (OM), a thin filament activator in clinical trials for heart failure, would improve the function of mutant tissues. We found that treatment of ΔK210 molecular complexes and tissues with OM causes a dose-dependent increase in cardiac function, reversing the mutation-induced contractile defect. Taken together, our study demonstrates how mechanistic molecular studies can be harnessed to identify precision medicine therapeutics.

Abstract MP223: CCR2 + Macrophages And T-cells Infiltrate The Murine Heart Following Immune Regulator Treatment With CD40

Immune checkpoint inhibitors have revolutionized cancer treatment but have been associated with severe adverse cardiac events including myocarditis. CD40 agonist (CD40ag) antibodies are immune regulators that have emerged as promising candidates with remarkable efficacy across tumors including those thought to be resistant to established ICIs. To investigate CD40 signaling and its potential for adverse cardiac events, C57BL/6J, CCR2-GFP, and CCR2-knockout mice were injected with CD40ag or isotype antibodies for 7 days. Flow cytometry showed a specific increase in CCR2 + macrophages that was independent of monocyte recruitment. There was no change in neutrophil recruitment and only a modest increase in dendritic cells was evident. We also observed a significant expansion of CD4 and CD8 T-cells that displayed an effector memory phenotype. Bulk cardiac tissue gene expression analysis (Figure A) revealed that CD40 activation upregulates multiple inflammatory cytokines (Ifng, Tnf, Il-12β) and chemokines (Ccl3, Ccl5, Ccl7, Cxcl9, Cxcl10), which are regulated by interferon gamma and coordinate the activation of APCs and T-cells. Finally, RNA in situ hybridization (Figure B-C) demonstrated increased levels of Cxcl9 (red) expression in cells consistent with CCR2 + (white) macrophages when comparing isotype control to treatment with CD40ag. These findings reveal that CD40 activation results in a robust expansion of CCR2 + macrophages and activation of T-cells within the heart, initiating a feed forward loop of activation that is mediated by interferon gamma and generates inflammatory cytotoxic mediators that may lead to myocardial injury.

Resident cardiac macrophages mediate adaptive myocardial remodeling

Cardiac macrophages represent a heterogeneous cell population with distinct origins, dynamics, and functions. Recent studies have revealed that C-C Chemokine Receptor 2 positive (CCR2⁺) macrophages derived from infiltrating monocytes regulate myocardial inflammation and heart failure pathogenesis. Comparatively little is known about the functions of tissue resident (CCR2^-) macrophages. Herein, we identified an essential role for CCR2^- macrophages in the chronically failing heart. Depletion of CCR2^- macrophages in mice with dilated cardiomyopathy accelerated mortality and impaired ventricular remodeling and coronary angiogenesis, adaptive changes necessary to maintain cardiac output in the setting of reduced cardiac contractility. Mechanistically, CCR2^- macrophages interacted with neighboring cardiomyocytes via focal adhesion complexes and were activated in response to mechanical stretch through a transient receptor potential vanilloid 4 (TRPV4)-dependent pathway that controlled growth factor expression. These findings establish a role for tissue-resident macrophages in adaptive cardiac remodeling and implicate mechanical sensing in cardiac macrophage activation.

Basophils balance healing after myocardial infarction via IL-4/IL-13

The inflammatory response after myocardial infarction (MI) is a precisely regulated process that greatly affects subsequent remodeling. Here, we show that basophil granulocytes infiltrated infarcted murine hearts, with a peak occurring between days 3 and 7. Antibody-mediated and genetic depletion of basophils deteriorated cardiac function and resulted in enhanced scar thinning after MI. Mechanistically, we found that basophil depletion was associated with a shift from reparative Ly6Clo macrophages toward increased numbers of inflammatory Ly6Chi monocytes in the infarcted myocardium. Restoration of basophils in basophil-deficient mice by adoptive transfer reversed this proinflammatory phenotype. Cellular alterations in the absence of basophils were accompanied by lower cardiac levels of IL-4 and IL-13, two major cytokines secreted by basophils. Mice with basophil-specific IL-4/IL-13 deficiency exhibited a similarly altered myeloid response with an increased fraction of Ly6Chi monocytes and aggravated cardiac function after MI. In contrast, IL-4 induction in basophils via administration of the glycoprotein IPSE/α-1 led to improved post-MI healing. These results in mice were corroborated by the finding that initially low counts of blood basophils in patients with acute MI were associated with a worse cardiac outcome after 1 year, characterized by a larger scar size. In conclusion, we show that basophils promoted tissue repair after MI by increasing cardiac IL-4 and IL-13 levels.

SVEP1 is a human coronary artery disease locus that promotes atherosclerosis

A low-frequency variant of sushi, von Willebrand factor type A, EGF, and pentraxin domain-containing protein 1 (SVEP1), an extracellular matrix protein, is associated with risk of coronary disease in humans independent of plasma lipids. Despite a robust statistical association, if and how SVEP1 might contribute to atherosclerosis remained unclear. Here, using Mendelian randomization and complementary mouse models, we provide evidence that SVEP1 promotes atherosclerosis in humans and mice and is expressed by vascular smooth muscle cells (VSMCs) within the atherosclerotic plaque. VSMCs also interact with SVEP1, causing proliferation and dysregulation of key differentiation pathways, including integrin and Notch signaling. Fibroblast growth factor receptor transcription increases in VSMCs interacting with SVEP1 and is further increased by the coronary disease-associated SVEP1 variant p.D2702G. These effects ultimately drive inflammation and promote atherosclerosis. Together, our results suggest that VSMC-derived SVEP1 is a proatherogenic factor and support the concept that pharmacological inhibition of SVEP1 should protect against atherosclerosis in humans.

SARS-CoV-2 Infects Human Engineered Heart Tissues and Models COVID-19 Myocarditis

There is ongoing debate as to whether cardiac complications of coronavirus disease-2019 (COVID-19) result from myocardial viral infection or are secondary to systemic inflammation and/or thrombosis. We provide evidence that cardiomyocytes are infected in patients with COVID-19 myocarditis and are susceptible to severe acute respiratory syndrome coronavirus 2. We establish an engineered heart tissue model of COVID-19 myocardial pathology, define mechanisms of viral pathogenesis, and demonstrate that cardiomyocyte severe acute respiratory syndrome coronavirus 2 infection results in contractile deficits, cytokine production, sarcomere disassembly, and cell death. These findings implicate direct infection of cardiomyocytes in the pathogenesis of COVID-19 myocardial pathology and provides a model system to study this emerging disease.

Lymphatic drainage from bronchus-associated lymphoid tissue in tolerant lung allografts promotes peripheral tolerance

Tertiary lymphoid organs are aggregates of immune and stromal cells including high endothelial venules and lymphatic vessels that resemble secondary lymphoid organs and can be induced at nonlymphoid sites during inflammation. The function of lymphatic vessels within tertiary lymphoid organs remains poorly understood. During lung transplant tolerance, Foxp3+ cells accumulate in tertiary lymphoid organs that are induced within the pulmonary grafts and are critical for the local downregulation of alloimmune responses. Here, we showed that tolerant lung allografts could induce and maintain tolerance of heterotopic donor-matched hearts through pathways that were dependent on the continued presence of the transplanted lung. Using lung retransplantation, we showed that Foxp3+ cells egressed from tolerant lung allografts via lymphatics and were recruited into donor-matched heart allografts. Indeed, survival of the heart allografts was dependent on lymphatic drainage from the tolerant lung allograft to the periphery. Thus, our work indicates that cellular trafficking from tertiary lymphoid organs regulates immune responses in the periphery. We propose that these findings have important implications for a variety of disease processes that are associated with the induction of tertiary lymphoid organs.

Chemokine Receptor 2-targeted Molecular Imaging in Pulmonary Fibrosis. A Clinical Trial

Rationale: Idiopathic pulmonary fibrosis (IPF) is a progressive inflammatory lung disease without effective molecular markers of disease activity or treatment responses. Monocyte and interstitial macrophages that express the C-C motif CCR2 (chemokine receptor 2) are active in IPF and central to fibrosis.Objectives: To phenotype patients with IPF for potential targeted therapy, we developed ⁶⁴Cu-DOTA-ECL1i, a radiotracer to noninvasively track CCR2⁺ monocytes and macrophages using positron emission tomography (PET).Methods: CCR2⁺ cells were investigated in mice with bleomycin- or radiation-induced fibrosis and in human subjects with IPF. The CCR2⁺ cell populations were localized relative to fibrotic regions in lung tissue and characterized using immunolocalization, single-cell mass cytometry, and Ccr2 RNA in situ hybridization and then correlated with parallel quantitation of lung uptake by ⁶⁴Cu-DOTA-ECL1i PET.Measurements and Main Results: Mouse models established that increased ⁶⁴Cu-DOTA-ECL1i PET uptake in the lung correlates with CCR2⁺ cell infiltration associated with fibrosis (n = 72). As therapeutic models, the inhibition of fibrosis by IL-1β blockade (n = 19) or antifibrotic pirfenidone (n = 18) reduced CCR2⁺ macrophage accumulation and uptake of the radiotracer in mouse lungs. In lung tissues from patients with IPF, CCR2⁺ cells concentrated in perifibrotic regions and correlated with radiotracer localization (n = 21). Human imaging revealed little lung uptake in healthy volunteers (n = 7), whereas subjects with IPF (n = 4) exhibited intensive signals in fibrotic zones.Conclusions: These findings support a role for imaging CCR2⁺ cells within the fibrogenic niche in IPF to provide a molecular target for personalized therapy and monitoring.Clinical trial registered with www.clinicaltrials.gov (NCT03492762).

SARS-CoV-2 Infects Human Engineered Heart Tissues and Models COVID-19 Myocarditis

Epidemiological studies of the COVID-19 pandemic have revealed evidence of cardiac involvement and documented that myocardial injury and myocarditis are predictors of poor outcomes. Nonetheless, little is understood regarding SARS-CoV-2 tropism within the heart and whether cardiac complications result directly from myocardial infection. Here, we develop a human engineered heart tissue model and demonstrate that SARS-CoV-2 selectively infects cardiomyocytes. Viral infection is dependent on expression of angiotensin-I converting enzyme 2 (ACE2) and endosomal cysteine proteases, suggesting an endosomal mechanism of cell entry. After infection with SARS-CoV-2, engineered tissues display typical features of myocarditis, including cardiomyocyte cell death, impaired cardiac contractility, and innate immune cell activation. Consistent with these findings, autopsy tissue obtained from individuals with COVID-19 myocarditis demonstrated cardiomyocyte infection, cell death, and macrophage-predominate immune cell infiltrate. These findings establish human cardiomyocyte tropism for SARS-CoV-2 and provide an experimental platform for interrogating and mitigating cardiac complications of COVID-19.