Paradigm shifting studies in the mouse have identified tissue macrophage heterogeneity as a critical determinant of immune responses. In contrast, surprisingly little is known regarding macrophage heterogeneity in humans. Macrophages within the mouse heart are partitioned into CCR2- and CCR2+ subsets with divergent origins, repopulation mechanisms, and functions. Here we demonstrate that the human myocardium also contains distinct subsets of CCR2- and CCR2+ macrophages. Analysis of sex mismatched heart transplant recipients revealed that CCR2- macrophages are a tissue-resident population exclusively replenished through local proliferation, whereas CCR2+ macrophages are maintained through monocyte recruitment and proliferation. Moreover, CCR2- and CCR2+ macrophages have distinct functional properties, analogous to reparative CCR2- and inflammatory CCR2+ macrophages in the mouse heart. Clinically, CCR2+ macrophage abundance is associated with LV remodeling and systolic function in heart failure patients. Collectively, these observations provide initial evidence for the functional importance of macrophage heterogeneity in the human heart.

Rationale-Recent advancements have brought to light the origins, complexity, and functions of tissue-resident macrophages. However, in the context of tissue injury or disease, large numbers of monocytes infiltrate the heart and are thought to contribute to adverse remodeling and heart failure pathogenesis. Little is understood about the diversity of monocytes and monocyte-derived macrophages recruited to the heart after myocardial injury, including the mechanisms that regulate monocyte recruitment and fate specification.Objective-We sought to test the hypothesis that distinct subsets of tissue-resident CCR2− (C-C chemokine receptor 2) and CCR2+ macrophages orchestrate monocyte recruitment and fate specification after myocardial injury.Methods and Results-We reveal that in numerous mouse models of cardiomyocyte cell death (permanent myocardial infarction, reperfused myocardial infarction, and diphtheria toxin cardiomyocyte ablation), there is a shift in macrophage ontogeny whereby tissue-resident macrophages are predominately replaced by infiltrating monocytes and monocyte-derived macrophages. Using syngeneic cardiac transplantation to model ischemia-reperfusion injury and distinguish tissue-resident from recruited cell populations in combination with intravital 2-photon microscopy, we demonstrate that monocyte recruitment is differentially orchestrated by distinct subsets of tissue-resident cardiac macrophages. Tissue-resident CCR2+ macrophages promote monocyte recruitment through an MYD88 (myeloid differentiation primary response 88)dependent mechanism that results in release of MCPs (monocyte chemoattractant proteins) and monocyte mobilization. In contrast, tissue-resident CCR2− macrophages inhibit monocyte recruitment. Using CD (cluster of differentiation) 169-DTR (diphtheria toxin receptor) and CCR2-DTR mice, we further show that selective depletion of either tissue-resident CCR2− or CCR2+ macrophages before myocardial infarction results in divergent effects on left ventricular function, Bajpai et al.

Heart failure represents a major cause of morbidity and mortality worldwide. Single-cell transcriptomics have revolutionized our understanding of cell composition and associated gene expression. Through integrated analysis of single-cell and single-nucleus RNA-sequencing data generated from 27 healthy donors and 18 individuals with dilated cardiomyopathy, here we define the cell composition of the healthy and failing human heart. We identify cell-specific transcriptional signatures associated with age and heart failure and reveal the emergence of disease-associated cell states. Notably, cardiomyocytes converge toward common disease-associated cell states, whereas fibroblasts and myeloid cells undergo dramatic diversification. Endothelial cells and pericytes display global transcriptional shifts without changes in cell complexity. Collectively, our findings provide a comprehensive analysis of the cellular and transcriptomic landscape of human heart failure, identify cell type-specific transcriptional programs and disease-associated cell states and establish a valuable resource for the investigation of human heart failure.

Despite the long-standing recognition that the immune response to acute myocardial injury contributes to adverse left ventricular (LV) remodeling, it has not been possible to effectively target this clinically. Using 2 different in vivo models of acute myocardial injury, we show that pirfenidone confers beneficial effects in the murine heart through an unexpected mechanism that depends on cardiac B lymphocytes. Naive hearts contained a large population of CD19+CD11b-CD23-CD21-IgD+IgMlo lymphocytes, and 2 smaller populations of CD19+CD11b+ B1a and B1b cells. In response to tissue injury, there was an increase in neutrophils, monocytes, macrophages, as well as an increase in CD19+ CD11b- B lymphocytes. Treatment with pirfenidone had no effect on the number of neutrophils, monocytes, or macrophages, but decreased CD19+CD11b- lymphocytes. B cell depletion abrogated the beneficial effects of pirfenidone. In vitro studies demonstrated that stimulation with lipopolysaccharide and extracts from necrotic cells activated CD19+ lymphocytes through a TIRAP-dependent pathway. Treatment with pirfenidone attenuated this activation of B cells. These findings reveal a previously unappreciated complexity of myocardial B lymphocytes within the inflammatory infiltrate triggered by cardiac injury and suggest that pirfenidone exerts beneficial effects in the heart through a unique mechanism that involves modulation of cardiac B lymphocytes.

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