Summary Cardiac macrophages are crucial for tissue repair after cardiac injury but have not been well characterized. Here we identify four populations of cardiac macrophages. At steady state, resident macrophages were primarily maintained through local proliferation. However, after macrophage depletion or during cardiac inflammation, Ly6chi monocytes contributed to all four macrophage populations, whereas resident macrophages also expanded numerically through proliferation. Genetic fate mapping revealed that yolk-sac and fetal monocyte progenitors gave rise to the majority of cardiac macrophages, and the heart was among a minority of organs in which substantial numbers of yolk-sac macrophages persisted in adulthood. CCR2 expression and dependence distinguished cardiac macrophages of adult monocyte versus embryonic origin. Transcriptional and functional data revealed that monocyte-derived macrophages coordinate cardiac inflammation, while playing redundant but lesser roles in antigen sampling and efferocytosis. These data highlight the presence of multiple cardiac macrophage subsets, with different functions, origins and strategies to regulate compartment.

Perivascular, meningeal and choroid plexus macrophages are non-parenchymal macrophages that mediate immune responses at brain boundaries. Although the origin of parenchymal microglia has recently been elucidated, much less is known about the precursors, the underlying transcriptional program and the dynamics of the other macrophages in the central nervous system (CNS). It has been assumed that they have a high turnover with blood-borne monocytes. However, large scale single-cell RNA-sequencing reveals a striking molecular overlap between perivascular macrophages and microglia but not monocytes. Using several fate mapping approaches and parabiosis we demonstrate that CNS macrophages arise from yolk sac precursors during embryonic development and remain a stable population. Notably, the generation of CNS macrophages relies on the transcription factor Pu.1 whereas myb, Batf3 and Nr4a1 are not required. Upon autoimmune inflammation, macrophages undergo extensive self-renewal by local proliferation. Our data provide challenging new insights into brains innate immune system.

Macrophages are distributed in tissues throughout the body and contribute to both homeostasis and disease. Recently, it has become evident that most adult tissue macrophages originate during embryonic development and not from circulating monocytes. Each tissue has its own composition of embryonically derived and adult-derived macrophages, but it is unclear whether macrophages of distinct origins are functionally interchangeable or have unique roles at steady state. This new understanding also prompts reconsideration of the function of circulating monocytes. Classical Ly6chi monocytes patrol the extravascular space in resting organs, and Ly6clo nonclassical monocytes patrol the vasculature. Inflammation triggers monocytes to differentiate into macrophages, but whether resident and newly recruited macrophages possess similar functions during inflammation is unclear. Here, we define the tools used for identifying the complex origin of tissue macrophages and discuss the relative contributions of tissue niche versus ontological origin to the regulation of macrophage functions during steady state and inflammation.

Significance This study addresses a fundamentally important and widely debated issue in the field of inflammation, which is why inflammation can be simultaneously deleterious after injury and yet is essential for tissue repair. Recently, an important new paradigm has emerged in the macrophage field: Organs are replete with resident macrophages of embryonic origin, distinct from monocyte-derived macrophages. In this article, we use a new model of cardiac injury and show that distinct macrophage populations derived from embryonic and adult lineages are important determinants of tissue repair and inflammation, respectively. Our data suggest that therapeutics, which inhibit monocyte-derived macrophages and/or selectively harness the function of embryonic-derived macrophages, may serve as novel treatments for heart failure.

Macrophages promote both injury and repair following myocardial infarction, but discriminating functions within mixed populations remains challenging. Here we used fate mapping and single-cell transcriptomics to demonstrate that at steady state, TIMD4 + LYVE1 + MHC-II lo CCR2 − resident cardiac macrophages self-renew with negligible blood monocyte input. Monocytes partially replaced resident TIMD4 − LYVE1 − MHC-II hi CCR2 − macrophages and fully replaced TIMD4 − LYVE1 − MHC-II hi CCR2 + macrophages, revealing a hierarchy of monocyte contribution to functionally distinct macrophage subsets. Ischemic injury reduced TIMD4 + and TIMD4 − resident macrophage abundance within infarcted tissue while recruited, CCR2 + monocyte-derived macrophages adopted multiple cell fates, including those nearly indistinguishable from resident macrophages. Despite this similarity, inducible depletion of resident macrophages using a Cx3cr1 -based system led to impaired cardiac function and promoted adverse remodeling primarily within the peri-infarct zone, highlighting a non-redundant, cardioprotective role of resident cardiac macrophages. Lastly, we demonstrate the ability of TIMD4 to be used as a durable lineage marker of a subset of resident cardiac macrophages.