Fate-mapping Models Research Articles

Earliest Lymphoid Precursors in the Fetal Liver Contribute Constructing Mucosal Immunity in a HSC-Independent Manner

Abstract We've been clarifing the role of hematopoietic stem cell (HSC) -indepenbdent progenitors for the consisting the post-natal immune-layers. Recent our repors, combining fate-mapping (FM) study and transplantation of E11.5 AGM cells, displayed nearly half of lympoid cells are maintained by HSC-independent progenitors even in old mice. Here, we characterized eariest lymoid progenitors in the E12.5 fetal liver (FL) by using IL-7RCre:flox-dTom mice and FM study. Unlike protein expression, dTom % in the E12 FL lin-Sca1-Kit+ (LSK) cells were more than 30% (34.8 ± 7.2%, n=7). Tom+ or Tom- LSK cells were further divided into HSC (CD48loEPCRhi) and MPP (CD+EPCRlo~dim) and sorted 4-populatios were transplanted into NSG mice. Recipients with Tom-HSCs showed multi-lineage reconstitution wile MPPs (Tom- or Tom+) repopulated B-1 and T-cells. Interestingly Tom+CD48loEPCRhi, phenotypically equivalent to HSCs, exibited unique properties that reconstituted only B-1 cells in PerC and IgA+ cells and type 2 innate lymphoid cells in the small intestine. These data suggest HSC-independent PerC B-1a cells and IgA+ cells share the common progenitor. As for FM study, we labeled neonatal HSC with dTom by injecting p2 Tamoxifen into Fgd5-based FM model, resulted in very lower contribution to the IgA+ B-cells in the lamina proplia. Relative Tom percentage showed 0.382 ± 0.12, indicating IgA+ cells are maintained by largely HSC-indepent mannar than previously thought.

Fetal liver macrophages contribute to the hematopoietic stem cell niche by controlling granulopoiesis.

During embryogenesis, the fetal liver becomes the main hematopoietic organ, where stem and progenitor cells as well as immature and mature immune cells form an intricate cellular network. Hematopoietic stem cells (HSCs) reside in a specialized niche, which is essential for their proliferation and differentiation. However, the cellular and molecular determinants contributing to this fetal HSC niche remain largely unknown. Macrophages are the first differentiated hematopoietic cells found in the developing liver, where they are important for fetal erythropoiesis by promoting erythrocyte maturation and phagocytosing expelled nuclei. Yet, whether macrophages play a role in fetal hematopoiesis beyond serving as a niche for maturing erythroblasts remains elusive. Here, we investigate the heterogeneity of macrophage populations in the murine fetal liver to define their specific roles during hematopoiesis. Using a single-cell omics approach combined with spatial proteomics and genetic fate-mapping models, we found that fetal liver macrophages cluster into distinct yolk sac-derived subpopulations and that long-term HSCs are interacting preferentially with one of the macrophage subpopulations. Fetal livers lacking macrophages show a delay in erythropoiesis and have an increased number of granulocytes, which can be attributed to transcriptional reprogramming and altered differentiation potential of long-term HSCs. Together, our data provide a detailed map of fetal liver macrophage subpopulations and implicate macrophages as part of the fetal HSC niche.

Fate-Mapping of Hematopoietic Stem Cell-Derived Macrophages.

Macrophages are cells of the innate immune system, which contribute to the maintenance of tissue homeostasis and form the first line of defense against pathogens. Tissue-resident macrophages that originate from erythro-myeloid-progenitors in the yolk sac colonize the organs early during development and self-maintain in most organs throughout adulthood. Under homeostatic and pathological conditions, circulating monocytes infiltrate the tissue, where they differentiate into macrophages. However, particularly upon inflammation, phenotyping of these distinct macrophage populations using surface markers or antibody stainings is insufficient as their phenotypes converge, at least transiently. A well-established method for the developmental origin of different cell types is the use of in vivo fate-mapping models, where a fluorescent reporter will be expressed under the control of a cell type-specific promoter. Here, we describe the Cxcr4CreERT2; Rosa26LSL-tdTomato mouse fate-mapping model, which labels hematopoietic stem cells and, thus, also monocytes and monocyte-derived macrophages while most tissue-resident macrophages are not targeted.

Fate-Mapping Macrophages: From Ontogeny to Functions.

Macrophages are vital to the physiological function of most tissues, but also contribute to disease through a multitude of pathological roles. They are thus highly plastic and heterogeneous. It is now well recognized that macrophages develop from several distinct progenitors from embryogenesis onwards and extending throughout life. Tissue-resident macrophages largely originate from embryonic sources and in many cases self-maintain independently without monocyte input. However, in certain tissues, monocyte-derived macrophages replace these over time or as a result of tissue injury and inflammation. This additional layer of heterogeneity has introduced many questions regarding the influence of origin on fate and function of macrophages in health and disease. To comprehensively address these questions, appropriate methods of tracing macrophage ontogeny are required. This chapter explores why ontogeny is of vital importance in macrophage biology and how to delineate macrophage populations by origin through genetic fate mapping. First, we summarize the current view of macrophage ontogeny and briefly discuss how origin may influence macrophage function in homeostasis and pathology. We go on to make the case for genetic fate mapping as the gold standard and briefly review different fate-mapping models. We then put forward our recommendations for fate-mapping strategies best suited to answer specific research questions and finally discuss the strengths and limitations of currently available models.

Dendritic cell type 3 arises from Ly6C+ monocyte-dendritic cell progenitors

Conventional dendritic cells (cDCs) are professional antigen-presenting cells that control the adaptive immune response. Their subsets and developmental origins have been intensively investigated but are still not fully understood as their phenotypes, especially in the DC2 lineage and the recently described human DC3s, overlap with monocytes. Here, using LEGENDScreen to profile DC vs. monocyte lineages, we found sustained expression of FLT3 and CD45RB through the whole DC lineage, allowing DCs and their precursors to be distinguished from monocytes. Using fate mapping models, single-cell RNA sequencing and adoptive transfer, we identified a lineage of murine CD16/32+CD172a+ DC3, distinct from DC2, arising from Ly6C+ monocyte-DC progenitors (MDPs) through Lyz2+Ly6C+CD11c- pro-DC3s, whereas DC2s develop from common DC progenitors (CDPs) through CD7+Ly6C+CD11c+ pre-DC2s. Corresponding DC subsets, developmental stages, and lineages exist in humans. These findings reveal DC3 as a DC lineage phenotypically related to but developmentally different from monocytes and DC2s.

Abstract 462: Transcriptomic Analysis Reveals That Intimal Monocytes Drive Inflammation In Early Atherogenesis

Hypercholesterolemia (HC) is a key risk factor for atherosclerosis, a chronic inflammatory disease that causes myocardial infarction and stroke. Although all regions of arteries are exposed to HC, atherosclerosis develops in discrete regions, such as the lesser curvature (LC) of the mouse aortic arch. In Ldlr -/- mice HC induces intimal myeloid cells to develop into the foam cells of nascent lesions. The purpose of this study is to characterize the transcriptomic changes occurring in the aortic intimal myeloid cells during the earliest stages of atherogenesis, and correlate them to cellular responses. Intimal cells were isolated from the LC of the ascending aortic arch of Ldlr -/- mice at 0, 5, 14, and 56 days of CRD feeding using en face enzymatic digestion and cell microisolation. Bulk RNA-seq and RT-qPCR revealed that genes associated with lipid-loaded macrophages (e.g., Abcg1 , Lgals3 ) were progressively elevated over time. Atherogenesis-associated inflammatory transcripts and pathways, such as leukocyte transendothelial migration, cell adhesion, and cytokine/chemokine signalling were significantly elevated after 14d of CRD, but not at 5d. Single cell transcriptomic analysis of the mouse aortic arch intima of Ldlr -/- mice revealed multiple sub-populations of myeloid cells. With our intima specific approach, we have uncovered a diverse landscape of intimal myeloid cells; including monocytes, macrophages, foam cells, DCs, and neutrophils. Atherogenesis-associated inflammatory transcripts were scarcely detected in macrophage and foam cell clusters, but were prominent in non-macrophage cell types. Ccr2 + intimal monocytes exhibited a hypercholesterolemia induced increase in inflammatory pathways within 5 days of CRD feeding. Furthermore, analysis of monocyte and macrophage clusters suggests that this inflammatory signature dissipates as recently recruited monocytes transition into a macrophage/foam cell fate. We are using Ccr2 + fate mapping models, to interrogate the hypercholesterolemia-induced transition of inflammatory monocytes within a growing lesion. We suggest that the atherogenesis-associated inflammation may be initiated by recently recruited monocytes, and may not originate directly from lipid loaded macrophages.

Dysregulation of fetal-derived cochlear resident tissue macrophages mediates CMV-associated sensorineural hearing loss

Abstract Cytomegalovirus (CMV) is the most common congenital viral infection in the developed world and the leading cause of non-genetic sensorineural hearing loss (SNHL) in children. Congenital CMV causes progressive SNHL long after acute infection has resolved, suggesting a lasting effect on the developing host immune response in the cochlea. Using fate-mapping models, we investigated the contribution of fetal-derived resident tissue macrophages (RTMs) to normal cochlear development and function, and their role in mediating CMV-associated SNHL. We uncovered dual contribution of fetal-derived cochlear RTMs (F4/80 hiCD11b lo) from yolk sac (YS) and fetal liver (FL) hematopoiesis at birth. YS-derived RTMs naturally recede postnatally after cochlea development, but remain in key tissues, such as the spiral ligament and stria vascularis, through adulthood. Depletion of fetal-derived RTMs by anti-CSF1R blocking Ab led to SNHL and impairments to cochlear architecture. Similarly, embryonic deletion of YS-derived RTMs by induced deletion of CSF1R leads to SNHL at high frequencies. During acute CMV infection, RTMs expand and become transcriptionally dysregulated in developmental programs, including tissue remodeling, chemorepulsion, otic vesicle morphogenesis, and FGF signaling. Importantly, analysis of CSF1R-Cre labeling suggests YS-derived RTMs retract in response to CMV infection, suggesting RTM expansion is caused by infiltration of BM-derived macrophages. Ongoing work examines spatial distribution of fetal-derived cochlear RTMs, transcriptional differences between YS, FL, and BM-derived RTMs in response to CMV, and how replacement of fetal-derived RTMs with BM-derived RTMs affects tissue function and response to CMV. Supported by grants from NIAID (T32 5T32AI138945) and University of Utah molecular medicine and immunology, inflammation and infectious disease (3i) initiative

Determining the fate of pulmonary iILC2s in helminth infection

Abstract Group 2 Innate Lymphoid Cells (ILC2s) are innate counterparts to Th2 cells and key early responders in helminth infection. ILC2s have two subpopulations: tissue-resident “natural” nILC2s, present in the lungs at homeostasis, and “inflammatory” iILC2s, which appear transiently in the lungs during the response to helminth infection. We aim to determine the fate of pulmonary iILC2s and hypothesize that pulmonary iILC2s acquire an nILC2-like phenotype and make biologically relevant long-term contributions to the tissue-resident ILC2 pool. Early studies demonstrated that iILC2s can convert to an nILC2 phenotype, but the prevailing view in the field is that the nILC2 population is maintained through self-renewal. However, fate-mapping models show an external contribution to the pulmonary ILC2 population which corresponds with an influx of iILC2s. We have further characterized the pulmonary ILC2 population during helminth infection and found an intermediate-phenotype population 8 days post infection. Preliminary flow cytometry and scRNA-seq data from congenic transfer experiments indicate that iILC2s may acquire an nILC2 phenotype in vivo. Preliminary multiple infection experiments indicate that mice lacking a pulmonary iILC2 response have smaller nILC2 compartments compared to wildtype counterparts after 3 infections. We are further testing our hypothesis using fate-mapping models and congenic transfer experiments in conjunction with single-cell sequencing approaches to more rigorously confirm these findings. Understanding the contribution of iILC2s to the long-term tissue-resident ILC2 population could lead to therapies in diseases with ILC2 involvement including helminth infection and steroid-resistant asthma. Supported by NIH grant R01 AI56901-0

Imaging of lung resident memory CD8 +T cells reveals antigen-specific early protective responses during secondary infection

Abstract Tissue-resident memory T (T RM) cells are poised at mucosal sites to confer robust protection against secondary infection. In contrast to skin and intestine, lung T RMcells are short lived with a decline in number coinciding with a decline in protective immunity against subsequent heterosubtypic influenza infections. Despite this causal relationship, the localization, dynamics and function of lung Trm during secondary responses remains poorly defined. To address this, we used two-photon intravital microscopy of lungs to assess in real time the spatiotemporal dynamics of lung T RMcells during recall infection. At a memory time point, lung T RMcells (representing both CD103 +and CD103 −subsets) exhibit significantly lower motility and less displacement compared to an CD8 +effector T cells at an earlier time point post infection. Depletion of circulating CD8 +T cells using a low dose antibody administered systemically led to enrichment of lung T RMcells that exhibited slow to moderate speed scanning patterns. A fate mapping model for CD103 +T RMcells specifically revealed limited speed (<5 um/min) and patrolling behavior compared to CD103 −T RMcells. Within 24 hr of rechallenge, CD103 +T RMcells co-localized with infected cells and by 48 hr, lung-resident CD8 +T cells exhibit significantly lower motility with recruitment and clustering in infected regions in an antigen-specific manner. Protective immune responses by lung T RMcells is further accompanied by CXCR3-dependent migration of dendritic cells to draining lymph nodes as well as recruitment of inflammatory monocytes. Overall, these data unravel the spatiotemporal dynamics of lung T RMcells and interactions with recruited immune cells for early protective immunity. Supported by grants from NIH (GM134880, AI42767, AI114543, AI151183, AI167249, the Holden Cancer Center at the University of Iowa and its National Cancer Institute Award P30CA086862

Gutsy memory T cells stand their ground against pathogens.

Elegant fate-mapping models to label intestinal tissue-resident memory CD8+ T (TRM) cells demonstrate retention and lack of expansion of CD103+ TRM cells, whereas intestinal CD103- memory cells expand, forming both new tissue-localized CD103+ and CD103- TRM cells (see related Research Articles by Fung etal. and von Hoesslin etal.).

The contribution of fetal-derived tissue resident macrophages to cytomegalovirus-associated sensorineural hearing loss

Abstract Cytomegalovirus (CMV) is the most common congenital viral infection in the developed world and the leading cause of non-genetic sensorineural hearing loss (SNHL) in children. Congenital CMV causes progressive SNHL long after acute infection has resolved, suggesting a lasting effect on the developing host immune response within the cochlea and surrounding temporal bone marrow (TB). Using fate-mapping models, we have investigated the contribution of fetal-derived tissue resident macrophages (TRMs) to normal cochlear development and function, and their role in mediating CMV-associated SNHL. Our analysis defined two distinct myeloid population within the inner ear: 1) classical tissue resident macrophages (F4/80hiCD11blo) that highly express CD64 and are found exclusively in cochlear tissue; and 2) transitional monocytes (F4/80midCD11bhi) that express both CD64 and Ly6C and are found in cochlea and TB. Fate mapping of these populations uncovered dual contribution of fetal-derived cochlear TRMs from yolk sac (YS) and fetal liver (FL) hematopoiesis. Transitional monocytes increase during acute CMV infection, acting as drivers of expanded lymphoid populations after CMV resolves. Importantly, analysis of Flt3-Cre labeling within myeloid populations suggests that TRMs are replaced by BM-derived populations at later postnatal stages following CMV infection, coinciding with SNHL. Ongoing work examines spatial distribution of distinct fetal-derived TRMs along cochlear architecture, defining transcriptional regulation differences between YS-, FL-, and adult-derived TRMs in response to CMV, and examining how replacement of fetal-derived TRMs with adult-derived TRMs affects tissue function and response to infection. Supported by grants from University of Utah Molecular Medicine and Immunology, Inflammation and Infectious Disease (3i) Initiative and NIAID (T32 5T32AI138945)

Embryonic Origin and Subclonal Evolution of Tumor-Associated Macrophages Imply Preventive Care for Cancer.

Macrophages are widely distributed in tissues and function in homeostasis. During cancer development, tumor-associated macrophages (TAMs) dominatingly support disease progression and resistance to therapy by promoting tumor proliferation, angiogenesis, metastasis, and immunosuppression, thereby making TAMs a target for tumor immunotherapy. Here, we started with evidence that TAMs are highly plastic and heterogeneous in phenotype and function in response to microenvironmental cues. We pointed out that efforts to tear off the heterogeneous “camouflage” in TAMs conduce to target de facto protumoral TAMs efficiently. In particular, several fate-mapping models suggest that most tissue-resident macrophages (TRMs) are generated from embryonic progenitors, and new paradigms uncover the ontogeny of TAMs. First, TAMs from embryonic modeling of TRMs and circulating monocytes have distinct transcriptional profiling and function, suggesting that the ontogeny of TAMs is responsible for the functional heterogeneity of TAMs, in addition to microenvironmental cues. Second, metabolic remodeling helps determine the mechanism of phenotypic and functional characteristics in TAMs, including metabolic bias from macrophages’ ontogeny in macrophages’ functional plasticity under physiological and pathological conditions. Both models aim at dissecting the ontogeny-related metabolic regulation in the phenotypic and functional heterogeneity in TAMs. We argue that gleaning from the single-cell transcriptomics on subclonal TAMs’ origins may help understand the classification of TAMs’ population in subclonal evolution and their distinct roles in tumor development. We envision that TAM-subclone-specific metabolic reprogramming may round-up with future cancer therapies.

Fetal-derived macrophages persist and sequentially maturate in ovaries after birth in mice.

Macrophages, which are highly diverse in different tissues, play a complex and vital role in tissue development, homeostasis, and inflammation. The origin and heterogeneity of tissue-resident monocytes and macrophages in ovaries remains unknown. Here we identify three tissue-resident monocyte populations and five macrophage populations in the adult ovaries using high-dimensional single cell mass cytometry. Ontogenic analyses using cell fate mapping models and cell depletion experiments revealed the infiltration of ovaries by both yolk sac and fetal liver-derived macrophages already during the embryonic development. Moreover, we found that both embryonic and bone marrow-derived macrophages contribute to the distinct ovarian macrophage subpopulations in the adults. These assays also showed that fetal-derived MHC II-negative macrophages differentiate postnatally in the maturing ovary to MHC II-positive cells. Our analyses further unraveled that the developmentally distinct macrophage types share overlapping distribution and scavenging function in the ovaries under homeostatic conditions. In conclusion, we report here the first comprehensive analyses of ovarian monocytes and macrophages. In addition, we show that the mechanisms controlling monocyte immigration, the phenotype of different pools of interstitial macrophages, and the interconversion capacity of fetal-derived macrophages in ovaries are remarkably different from those seen in other tissue niches.

Early Fate Defines Microglia and Non-parenchymal Brain Macrophage Development

Central nervous system (CNS) macrophages comprise microglia and border-associated macrophages (BAMs) residing in the meninges, the choroid plexus, and the perivascular spaces. Most CNS macrophages emerge during development, with the exception of choroid plexus and dural macrophages, which are replaced by monocytes in adulthood. Whether microglia and BAMs share a developmental program or arise from separate lineages remains unknown. Here, we identified two phenotypically, transcriptionally, and locally distinct brain macrophages throughout development, giving rise to either microglia or BAMs. Two macrophage populations were already present in the yolk sac suggesting an early segregation. Fate-mapping models revealed that BAMs mostly derived from early erythro-myeloid progenitors in the yolk sac. The development of microglia was dependent on TGF-β, whereas the genesis of BAMs occurred independently of this cytokine. Collectively, our data show that developing parenchymal and non-parenchymal brain macrophages are separate entities in terms of ontogeny, gene signature, and requirement for TGF-β.

Soluble Alpha-Klotho Alleviates Cardiac Fibrosis without Altering Cardiomyocytes Renewal

Heart disease is the leading cause of death worldwide. The major cause of heart failure is the death of the myocardium caused by myocardial infarction, detrimental cardiac remodeling, and cardiac fibrosis occurring after the injury. This study aimed at discovering the role of the anti-aging protein α-klotho (KL), which is the co-receptor of fibroblast growth factor-23 (FGF23), in cardiac regeneration, fibrosis, and repair. We found that the anti-apoptotic function of soluble KL in isoproterenol-treated H9c2 cardiomyocytes was independent of FGF23 in vitro. In vivo, isoproterenol-induced cardiac fibrosis and cardiomyocyte and endothelial cell apoptosis were reduced by KL treatment. Moreover, the number of Ki67-positive endothelial cells and microvessel density within the isoproterenol-injured myocardium were increased upon KL treatment. However, by using genetic fate-mapping models, no evident cardiomyocyte proliferation within the injured myocardium was detected with or without KL treatment. Collectively, the cardioprotective functions of KL could be predominantly attributed to its anti-apoptotic and pro-survival activities on endothelial cells and cardiomyocytes. KL could be a potential cardioprotective therapeutic agent with anti-apoptotic and pro-survival activities on cardiomyocytes and endothelial cells.

P5370Pulmonary pressure overload stimulates cardiac stem or progenitor cell-derived cardiac regeneration in the right ventricular area

Abstract Introduction and purpose The number of patients with right heart failure due to pulmonary hypertension has been increasing. Although several drugs have reportedly improved pulmonary hypertension, no treatments have been established for decompensated right heart failure. The heart has an innate ability to regenerate, and cardiac stem or progenitor cells (e.g., side population [SP] cells) have been reported to contribute to the regeneration process. However, their contribution to right ventricular pressure overload has not been clarified. Here, this regeneration process was evaluated using a genetic fate-mapping model. Methods and results We used Cre-LacZ mice, in which more than 99.9% of the cardiomyocytes in the left ventricular field were positive for 5-bromo-4-chloro-3-indolyl-β-D-galactoside (X-gal) staining immediately after tamoxifen injection. Then, we performed either a pulmonary binding (PAB) or sham operation on the main pulmonary tract. In the PAB-treated mice, the right ventricular cavity was significantly enlarged (right-to-left ventricular [RV/LV] ratio, 0.24±0.04 in the sham group and 0.68±0.04 in the PAB group). Increased peak flow velocity in the PAB group (1021±80 vs 1351±62 mm/sec) was confirmed by echocardiography. One month after the PAB, the PAB-treated mice had more X-gal-negative (newly generated) cells than the sham mice (94.8±34.2 cells/mm2 vs 23.1±10.5 cells/mm2; p<0.01). The regeneration was biased in the RV free wall (RV free wall, 225.5±198.7 cells/mm2; septal area, 88.9±56.5/mm2; LV lateral area, 46.8±22.0/mm2; p<0.05). To examine the direct effects of PAB on the cardiac progenitor cells, bromodeoxyuridine was administered to the mice daily until 1 week after the PAB operation. Then, the hearts were isolated and SP cells were harvested. The SP cell population increased from 0.65±0.23% in the sham mice to 1.87% ± 1.18% in the PAB-treated mice. Immunostaining analysis revealed a significant increase in the number of BrdU-positive SP cells, from 11.6±2.0% to 44.0±18%, therefore showing SP cell proliferation. Conclusions Pulmonary pressure overload stimulated cardiac stem or progenitor cell-derived regeneration with a RV bias, and SP cell proliferation may partially contribute to this process. Acknowledgement/Funding JSPS KAKENHI Grant Number JP 17K17636, GSK Japan Research Grant 2016

Fate Mapping via Ms4a3-Expression History Traces Monocyte-Derived Cells.

Most tissue-resident macrophage (RTM) populations are seeded by waves of embryonic hematopoiesis and are self-maintained independently of a bone marrow contribution during adulthood. A proportion of RTMs, however, is constantly replaced by blood monocytes, and their functions compared to embryonic RTMs remain unclear. The kinetics and extent of the contribution of circulating monocytes to RTM replacement during homeostasis, inflammation, and disease are highly debated. Here, we identified Ms4a3 as a specific gene expressed by granulocyte-monocyte progenitors (GMPs) and subsequently generated Ms4a3TdT reporter, Ms4a3Cre, and Ms4a3CreERT2 fate-mapping models. These models traced efficiently monocytes and granulocytes, but no lymphocytes or tissue dendritic cells. Using these models, we precisely quantified the contribution of monocytes to the RTM pool during homeostasis and inflammation. The unambiguous identification of monocyte-derived cells will permit future studies of their function under any condition.

Uncovering the Mysteries of Langerhans Cells, Inflammatory Dendritic Epidermal Cells, and Monocyte-Derived Langerhans Cell-Like Cells in the Epidermis.

The identity of Langerhans cells (LCs) has been called into question of late due to the increasing evidence that LCs originate from macrophage lineage instead of dendritic cell (DC) lineage as previously thought. For many years, LCs have been assumed to be DCs due to its migratory capabilities. However, recent studies have demonstrated that LCs are from macrophage lineage of the adult fetal liver (FL) progenitor. Bona fide LCs are now considered tissue-resident macrophages as they originate from the FL as shown by fate mapping models. In recent years, studies have shown that there are three types of antigen-presenting cells present in the epidermis, such as LCs, monocyte-derived LC-like cells, and inflammatory dendritic epidermal cells (IDECs). Of these, LC-like cells have been characterized in both human and mouse studies, while IDECs have only been described in human studies. This has shed a new light on the area of epidermal macrophages, suggesting that there might be a misidentification and misclassification of LCs. IDECs and LC-like cells have been shown to be present in both steady state and inflammatory state, but they are present in more significant amounts under inflammatory conditions such as atopic dermatitis, ultra violet injury, and psoriasis. In this review, we discuss what is already known and discuss the possible roles of LCs, LC-like cells, and IDECs during inflammation. Most intriguingly, we discuss the possibility of LCs having a dual identity as both a macrophage and a DC. This is shown as LCs are the only tissue-resident macrophage to have shown migratory property-like DCs.

Clonal Proliferation and Stochastic Pruning Orchestrate Lymph Node Vasculature Remodeling

Lymph node (LN) expansion during an immune response relies on the transient remodeling of its vasculature. Although the mechanisms driving LN endothelial cell division are beginning to be understood, a comprehensive view of LN endothelial cell dynamics at the single-cell level is lacking. Here, we used multicolored fluorescent fate-mapping models to track the behavior of blood endothelial cells during LN expansion upon inflammation and subsequent return to homeostasis. We found that expansion of the LN vasculature relied on the sequential assembly of endothelial cell proliferative units. This segmented growth was sustained by the clonal proliferation of high endothelial venule (HEV) cells, which act as local progenitors to create capillaries and HEV neo-vessels at the periphery of the LN. Return to homeostasis was accompanied by the stochastic death of pre-existing and neo-synthesized LN endothelial cells. Thus, ourfate-mapping studies unravel-at a single-cell level-the complex dynamics of vascular-tree remodeling during LN expansion and contraction.

Most Tissue-Resident Macrophages Except Microglia Are Derived from Fetal Hematopoietic Stem Cells

Macrophages are one of the most diverse cell populations in terms of their anatomical location and functional specialization during both homeostasis and disease. Although it has been shown in different fate mapping models that some macrophages present in adult tissues are already established during fetal development, their exact origins are still under debate. In the current study, we developed a fate mapping strain, based on the Kit locus, which allowed us to readdress "the origins" question. Different types of macrophages from various adult tissues were traced to their fetal or adult sources by inducing labeling in precursors at several time points either during fetal development or in adult mice. We show that all adult macrophages, resident or infiltrating, are progenies of classical hematopoietic stem cells (HSC) with the exception of microglia and, partially epidermal Langerhans cells, which are yolk sac (YS)-derived.