The current paradigm in macrophage biology is that some tissues mainly contain macrophages from embryonic origin, such as microglia in the brain, whereas other tissues contain postnatal-derived macrophages, such as the gut. However, in the lung and in other organs, such as the skin, there are both embryonic and postnatal-derived macrophages. In this study, we demonstrate in the steady-state lung that the mononuclear phagocyte system is comprised of three newly identified interstitial macrophages (IMs), alveolar macrophages, dendritic cells, and few extravascular monocytes. We focused on similarities and differences between the three IM subtypes, specifically, their phenotype, location, transcriptional signature, phagocytic capacity, turnover, and lack of survival dependency on fractalkine receptor, CXCR1. Pulmonary IMs were located in the bronchial interstitium but not the alveolar interstitium. At the transcriptional level, all three IMs displayed a macrophage signature and phenotype. All IMs expressed MER proto-oncogene, tyrosine kinase, CD64, CD11b, and CXCR1, and were further distinguished by differences in cell surface protein expression of CD206, Lyve-1, CD11c, CCR2, and MHC class II, along with the absence of Ly6C, Ly6G, and Siglec F. Most intriguingly, in addition to the lung, similar phenotypic populations of IMs were observed in other nonlymphoid organs, perhaps highlighting conserved functions throughout the body. These findings promote future research to track four distinct pulmonary macrophages and decipher the division of labor that exists between them.

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• Of the 30 000 genes, there are ;0.1% genes whose expression is linked to the origin of the cell rather than the environment.• Marco was most conserved by embryonic origin and not altered by the environment, whereas C1qb and Plbd1 were most conserved by adult origin.Alveolar macrophages (AMs) reside on the luminal surfaces of the airways and alveoli where they maintain host defense and promote alveolar homeostasis by ingesting inhaled particulates and regulating inflammatory responses. Recent studies have demonstrated that AMs populate the lungs during embryogenesis and self-renew throughout life with minimal replacement by circulating monocytes, except under extreme conditions of depletion or radiation injury. Here we demonstrate that on a global scale, environment appears to dictate AM development and function. Indeed, transcriptome analysis of embryonic host-derived and postnatal donor-derived AMs coexisting within the same mouse demonstrated >98% correlation and overall functional analyses were similar. However, we also identified several genes whose expression was dictated by origin rather than environment. The most differentially expressed gene not altered by environment was Marco, a gene recently demonstrated to have enhancer activity in embryonic-derived but not postnatal-derived tissue macrophages. Overall, we show that under homeostatic conditions, the environment largely dictates the programming and function of AMs, whereas the expression of a small number of genes remains linked to the origin of the cell. (Blood. 2015;126(11):1357-1366 Introduction Alveolar macrophages (AMs) reside on the luminal surfaces of the airways and airspaces where they serve critical roles in host defense and alveolar homeostasis, ingesting particulates and microbes that are constantly encountered in the lungs. Importantly, under most circumstances the phagocytosis of inhaled foreign agents is silent, such that inflammatory responses are activated only under circumstances when host defenses become overwhelmed.1 Indeed, compared with macrophages from other sites, AMs are relatively ineffective at initiating immune responses.2,3 Furthermore, compared with other tissue macrophages, they display a unique repertoire of cell surface molecules and have a distinct transcriptome profile. [4][5][6] AMs are now known to derive primarily from fetal liver monocytes and self-renew throughout life with minimal replenishment from circulating monocytes. [7][8][9][10][11][12][13][14][15] This self-renewal is not only maintained under steady-state conditions, but also during acute and chronic inflammation.16 These concepts were illustrated in lung-protected bone marrow (BM) chimera studies in which lead shields were used to protect AMs during radiation. Eight weeks after BM transplantation, the lungs of these chimeras contained AMs of host origin, whereas circulating monocytes were donor-derived. 16 In these chimeric mice, we showed that during inflammation (lipopolysaccharide or influenza A infection), BM donor-derived monocytes were rapidly...

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Two populations of alveolar macrophages (AMs) coexist in the inflamed lung: resident AMs that arise during embryogenesis, and recruited AMs that originate postnatally from circulating monocytes. The objective of this study was to determine whether origin or environment dictates the transcriptional, metabolic, and functional programming of these two ontologically distinct populations over the time course of acute inflammation. RNA sequencing demonstrated marked transcriptional differences between resident and recruited AMs affecting three main areas: proliferation, inflammatory signaling, and metabolism. Functional assays and metabolomic studies confirmed these differences and demonstrated that resident AMs proliferate locally and are governed by increased tricarboxylic acid cycle and amino acid metabolism. Conversely, recruited AMs produce inflammatory cytokines in association with increased glycolytic and arginine metabolism. Collectively, the data show that even though they coexist in the same environment, inflammatory macrophage subsets have distinct immunometabolic programs and perform specialized functions during inflammation that are associated with their cellular origin.