Summary It is thought that monocytes rapidly differentiate to macrophages or dendritic cells (DCs) upon leaving blood. Here we have shown that Ly-6C+ monocytes constitutively trafficked into skin, lung, and lymph nodes (LNs). Entry was unaffected in gnotobiotic mice. Monocytes in resting lung and LN had similar gene expression profiles to blood monocytes, but elevated transcripts of a limited number of genes including cyclo-oxygenase-2 (COX-2) and major histocompatibility complex class II (MHC II), induced by monocyte interaction with endothelium. Parabiosis, bromodoxyuridine (BrdU) pulse-chase analysis, and intranasal instillation of tracers indicated that instead of contributing to resident macrophages in the lung, recruited endogenous monocytes acquired antigen for carriage to draining LNs, a function redundant with DCs though differentiation to DCs did not occur. Thus, monocytes can enter steady state non-lymphoid organs and recirculate to LNs without differentiation to macrophages or DCs, revising a long-held view that monocytes become tissue-resident macrophages by default.

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.

Rationale: The pulmonary mononuclear phagocyte system is a critical host defense mechanism composed of macrophages, monocytes, monocyte-derived cells, and dendritic cells. However, our current characterization of these cells is limited because it is derived largely from animal studies and analysis of human mononuclear phagocytes from blood and small tissue resections around tumors.Objectives: Phenotypic and morphologic characterization of mononuclear phagocytes that potentially access inhaled antigens in human lungs.Methods: We acquired and analyzed pulmonary mononuclear phagocytes from fully intact nondiseased human lungs (including the major blood vessels and draining lymph nodes) obtained en bloc from 72 individual donors. Differential labeling of hematopoietic cells via intrabronchial and intravenous administration of antibodies within the same lobe was used to identify extravascular tissue-resident mononuclear phagocytes and exclude cells within the vascular lumen. Multiparameter flow cytometry was used to identify mononuclear phagocyte populations among cells labeled by each route of antibody delivery. Measurements and Main Results:We performed a phenotypic analysis of pulmonary mononuclear phagocytes isolated from whole nondiseased human lungs and lung-draining lymph nodes. Five pulmonary mononuclear phagocytes were observed, including macrophages, monocyte-derived cells, and dendritic cells that were phenotypically distinct from cell populations found in blood.Conclusions: Different mononuclear phagocytes, particularly dendritic cells, were labeled by intravascular and intrabronchial antibody delivery, countering the notion that tissue and blood mononuclear phagocytes are equivalent systems. Phenotypic descriptions of the mononuclear phagocytes in nondiseased lungs provide a precedent for comparative studies in diseased lungs and potential targets for therapeutics. Correspondence and requests for reprints should be addressed to Claudia V. Jakubzick, Ph.D., Department of Pediatrics and Immunology, National Jewish Health, 1400 Jackson Street, Denver, CO 80206. E-mail: jakubzickc@njhealth.org This article has an online supplement, which is accessible from this issue's table of contents at www.atsjournals.org The human respiratory tract has a branching structure that terminates in millions of alveoli, whose luminal surface covers an approximate area of 50 to 100 m 2 . In comparison to other barrier surfaces, such as the skin (2 m 2 ) and the gut (10 m 2 ), this surface area is massive, and therefore, comprises the body's largest interface with the ambient environment. Because of normal respiratory function, the average human exchanges 7,000 to 9,000 L of gas each day and inhales billions of particles, allergens, and microbes. Accordingly, the human lung constitutes a major site for the innate and adaptive immune responses. In this context, cells in the mononuclear phagocyte system (MPS), which consists of macrophages, monocytes, monocytederived cells, and dendritic cells (DCs), play critical roles. T...

• 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...

Idiopathic pulmonary fibrosis is a progressive lung disease with complex pathophysiology and fatal prognosis. Macrophages (MΦ) contribute to the development of lung fibrosis; however, the underlying mechanisms and specific MΦ subsets involved remain unclear. During lung injury, two subsets of lung MΦ coexist: Siglec-F resident alveolar MΦ and a mixed population of CD11b MΦ that primarily mature from immigrating monocytes. Using a novel inducible transgenic system driven by a fragment of the human CD68 promoter, we targeted deletion of the antiapoptotic protein cellular FADD-like IL-1β-converting enzyme-inhibitory protein (c-FLIP) to CD11b MΦ. Upon loss of c-FLIP, CD11b MΦ became susceptible to cell death. Using this system, we were able to show that eliminating CD11b MΦ present 7-14 days after bleomycin injury was sufficient to protect mice from fibrosis. RNA-seq analysis of lung MΦ present during this time showed that CD11b MΦ, but not Siglec-F MΦ, expressed high levels of profibrotic chemokines and growth factors. Human MΦ from patients with idiopathic pulmonary fibrosis expressed many of the same profibrotic chemokines identified in murine CD11b MΦ. Elimination of monocyte-derived MΦ may help in the treatment of fibrosis. We identify c-FLIP and the associated extrinsic cell death program as a potential pathway through which these profibrotic MΦ may be pharmacologically targeted.