Lymphatic Endothelial Cells Are Essential Components of the Subcapsular Sinus Macrophage Niche

Role of lymphatic endothelial cells in the tumor microenvironment-a narrative review of recent advances

Background

As lymphatic vessel is a major route for solid tumor metastasis, they are considered an essential part of tumor drainage conduits. Apart from forming the walls of lymphatic vessels, lymphatic endothelial cells (LECs) have been found to play multiple other roles in the tumor microenvironment, calling for a more in-depth review. We hope that this review may help researchers gain a detailed understanding of this fast-developing field and shed some light upon future research.

Methods

To achieve an informative review of recent advance, we carefully searched the Medline database for English literature that are openly published from the January 1995 to December 2020 and covered the topic of LEC or lymphangiogenesis in tumor progression and therapies. Two different authors independently examined the literature abstracts to exclude possible unqualified ones, and 310 papers with full texts were finally retrieved.

Results

In this paper, we discussed the structural and molecular basis of tumor-associated LECs, together with their roles in tumor metastasis and drug therapy. We then focused on their impacts on tumor cells, tumor stroma, and anti-tumor immunity, and the molecular and cellular mechanisms involved. Special emphasis on lung cancer and possible therapeutic targets based on LECs were also discussed.

Conclusions

LECs can play a much more complex role than simply forming conduits for tumor cell dissemination. Therapies targeting tumor-associated lymphatics for lung cancer and other tumors are promising, but more research is needed to clarify the mechanisms involved.

CNS-Draining Meningeal Lymphatic Vasculature: Roles, Conundrums and Future Challenges

A genuine and functional lymphatic vascular system is found in the meninges that sheath the central nervous system (CNS). This unexpected (re)discovery led to a reevaluation of CNS fluid and solute drainage mechanisms, neuroimmune interactions and the involvement of meningeal lymphatics in the initiation and progression of neurological disorders. In this manuscript, we provide an overview of the development, morphology and unique functional features of meningeal lymphatics. An outline of the different factors that affect meningeal lymphatic function, such as growth factor signaling and aging, and their impact on the continuous drainage of brain-derived molecules and meningeal immune cells into the cervical lymph nodes is also provided. We also highlight the most recent discoveries about the roles of the CNS-draining lymphatic vasculature in different pathologies that have a strong neuroinflammatory component, including brain trauma, tumors, and aging-associated neurodegenerative diseases like Alzheimer's and Parkinson's. Lastly, we provide a critical appraisal of the conundrums, challenges and exciting questions involving the meningeal lymphatic system that ought to be investigated in years to come.

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.

CD169+ lymph node macrophages have protective functions in mouse breast cancer metastasis

Although the contribution of macrophages to metastasis is widely studied in primary tumors, the involvement of macrophages in tumor-draining lymph nodes (LNs) in this process is less clear. We find CD169⁺ macrophages as the predominant macrophage subtype in naive LNs, which undergo proliferative expansion in response to tumor stimuli. CD169⁺ LN macrophage depletion, using an anti-CSF-1R antibody or clodronate-loaded liposomes, leads to increased metastatic burden in two mouse breast cancer models. The expansion of CD169⁺ macrophages is tightly connected to B cell expansion in tumor-draining LNs, and B cell depletion abrogates the effect of CD169⁺ macrophage absence on metastasis, indicating that the CD169⁺ macrophage anti-metastatic effects require B cell presence. These results reveal a protective role of CD169⁺ LN macrophages in breast cancer metastasis and raise caution for the use of drugs aiming at the depletion of tumor-associated macrophages, which might simultaneously deplete macrophages in tumor-draining LNs.

Targeting macrophages in cancer immunotherapy

Immunotherapy is regarded as the most promising treatment for cancers. Various cancer immunotherapies, including adoptive cellular immunotherapy, tumor vaccines, antibodies, immune checkpoint inhibitors, and small-molecule inhibitors, have achieved certain successes. In this review, we summarize the role of macrophages in current immunotherapies and the advantages of targeting macrophages. To better understand and make better use of this type of cell, their development and differentiation characteristics, categories, typical markers, and functions were collated at the beginning of the review. Therapeutic strategies based on or combined with macrophages have the potential to improve the treatment efficacy of cancer therapies.

Recruited macrophages that colonize the post-inflammatory peritoneal niche convert into functionally divergent resident cells

Inflammation generally leads to recruitment of monocyte-derived macrophages. What regulates the fate of these cells and to what extent they can assume the identity and function of resident macrophages is unclear. Here, we show that macrophages elicited into the peritoneal cavity during mild inflammation persist long-term but are retained in an immature transitory state of differentiation due to the presence of enduring resident macrophages. By contrast, severe inflammation results in ablation of resident macrophages and a protracted phase wherein the cavity is incapable of sustaining a resident phenotype, yet ultimately elicited cells acquire a mature resident identity. These macrophages also have transcriptionally and functionally divergent features that result from inflammation-driven alterations to the peritoneal cavity micro-environment and, to a lesser extent, effects of origin and time-of-residency. Hence, rather than being predetermined, the fate of inflammation-elicited peritoneal macrophages seems to be regulated by the environment.

Development and Immunological Function of Lymph Node Stromal Cells

Stromal cells have for a long time been viewed as structural cells that support distinct compartments within lymphoid tissues and little more. Instead, an active cross-talk between endothelial and fibroblastic stromal cells drives the maturation of lymphoid niches, a relationship that is recapitulated during lymph node organogenesis, steady-state conditions, and following inflammation. In this review, we go over recent advances in genetic models and high-resolution transcriptomic analyses that have propelled the finer resolution of the stromal cell infrastructure of lymph nodes, revealing that the distinct subsets are strategically positioned to deliver a catered mixture of niche factors to interacting immune cell populations. Moreover, we discuss how changes in the activation state of poised stromal cell-underpinned niches rather than on-demand differentiation of new stromal cell subsets govern the efficient interaction of Ag, APC, and cognate B and T lymphocytes during adaptive immune responses.

A Destruction Model of the Vascular and Lymphatic Systems in the Emergence of Psychiatric Symptoms

The lymphatic system is important for antigen presentation and immune surveillance. The lymphatic system in the brain was originally introduced by Giovanni Mascagni in 1787, while the rediscovery of it by Jonathan Kipnis and Kari Kustaa Alitalo now opens the door for a new interpretation of neurological diseases and therapeutic applications. The glymphatic system for the exchanges of cerebrospinal fluid (CSF) and interstitial fluid (ISF) is associated with the blood-brain barrier (BBB), which is involved in the maintenance of immune privilege and homeostasis in the brain. Recent notions from studies of postmortem brains and clinical studies of neurodegenerative diseases, infection, and cerebral hemorrhage, implied that the breakdown of those barrier systems and infiltration of activated immune cells disrupt the function of both neurons and glia in the parenchyma (e.g., modulation of neurophysiological properties and maturation of myelination), which causes the abnormality in the functional connectivity of the entire brain network. Due to the vulnerability, such dysfunction may occur in developing brains as well as in senile or neurodegenerative diseases and may raise the risk of emergence of psychosis symptoms. Here, we introduce this hypothesis with a series of studies and cellular mechanisms.

Origins and diversity of macrophages in health and disease

Macrophages are the first immune cells in the developing embryo and have a central role in organ development, homeostasis, immunity and repair. Over the last century, our understanding of these cells has evolved from being thought of as simple phagocytic cells to master regulators involved in governing a myriad of cellular processes. A better appreciation of macrophage biology has been matched with a clearer understanding of their diverse origins and the flexibility of their metabolic and transcriptional machinery. The understanding of the classical mononuclear phagocyte system in its original form has now been expanded to include the embryonic origin of tissue-resident macrophages. A better knowledge of the intrinsic similarities and differences between macrophages of embryonic or monocyte origin has highlighted the importance of ontogeny in macrophage dysfunction in disease. In this review, we provide an update on origin and classification of tissue macrophages, the mechanisms of macrophage specialisation and their role in health and disease. The importance of the macrophage niche in providing trophic factors and a specialised environment for macrophage differentiation and specialisation is also discussed.

Reprogramming of synovial macrophage metabolism by synovial fibroblasts under inflammatory conditions

Background

Macrophages adapt to microenvironments, and change metabolic status and functions to regulate inflammation and/or maintain homeostasis. In joint cavities, synovial macrophages (SM) and synovial fibroblasts (SF) maintain homeostasis. However, under inflammatory conditions such as rheumatoid arthritis (RA), crosstalk between SM and SF remains largely unclear.

Methods

Immunofluorescent staining was performed to identify localization of SM and SF in synovium of collagen antibody induced arthritis (CAIA) model mice and normal mice. Murine arthritis tissue-derived SM (ADSM), arthritis tissue-derived SF (ADSF) and normal tissue-derived SF (NDSF) were isolated and the purity of isolated cells was examined by RT-qPCR and flow cytometry analysis. RNA-seq was conducted to reveal gene expression profile in ADSM, NDSF and ADSF. Cellular metabolic status and expression levels of metabolic genes and inflammatory genes were analyzed in ADSM treated with ADSM-conditioned medium (ADSM-CM), NDSF-CM and ADSF-CM.

Results

SM and SF were dispersed in murine hyperplastic synovium. Isolations of ADSM, NDSF and ADSF to analyze the crosstalk were successful with high purity. From gene expression profiles by RNA-seq, we focused on secretory factors in ADSF-CM, which can affect metabolism and inflammatory activity of ADSM. ADSM exposed to ADSF-CM showed significantly upregulated glycolysis and mitochondrial respiration as well as glucose and glutamine uptake relative to ADSM exposed to ADSM-CM and NDSF-CM. Furthermore, mRNA expression levels of metabolic genes, such as Slc2a1, Slc1a5, CD36, Pfkfb1, Pfkfb3 and Irg1, were significantly upregulated in ADSM treated with ADSF-CM. Inflammation marker genes, including Nos2, Tnf, Il-1b and CD86, and the anti-inflammatory marker gene, Il-10, were also substantially upregulated by ADSF-CM. On the other hand, NDSF-CM did not affect metabolism and gene expression in ADSM.

Conclusions

These findings suggest that crosstalk between SM and SF under inflammatory conditions can induce metabolic reprogramming and extend SM viability that together can contribute to chronic inflammation in RA.

Tissue-specific features of microglial innate immune responses

As tissue-resident macrophages of the brain, microglia are increasingly considered as cellular targets for therapeutical intervention. Innate immune responses in particular have been implicated in central nervous system (CNS) infections, neuro-oncology, neuroinflammatory and neurodegenerative diseases. We here review the impact of 'nature and nurture' on microglial innate immune responses and summarize documented tissue-specific adaptations. Overall, such adaptations are associated with regulatory processes rather than with overt differences in the expressed repertoire of activating receptors of different tissue-resident macrophages. Microglial responses are characterized by slower kinetics, by a more persistent nature and by a differential usage of downstream enzymes and accessory receptors. We further consider factors like aging, previous exposure to inflammatory stimuli, and differences in the microenvironment that can modulate innate immune responses. The long-life span of microglia in the metabolically active CNS renders them susceptible to the phenomenon of 'inflammaging', and major challenges lie in the unraveling of the factors that underlie age-related alterations in microglial behavior.

Hematogenous Dissemination of Breast Cancer Cells From Lymph Nodes Is Mediated by Tumor MicroEnvironment of Metastasis Doorways

In primary breast tumors, cancer cells hematogenously disseminate through doorways in the vasculature composed of three-cell complexes (known as Tumor MicroEnvironment of Metastasis) comprising a perivascular macrophage, a tumor cell overexpressing the actin-regulatory protein Mammalian Enabled (Mena), and an endothelial cell, all in direct physical contact. It has been previously shown that once tumor cells establish lymph node metastases in patients, TMEM doorways form in the metastatic tumor cell nests. However, it has not been established if such lymph node-TMEM doorways actively transit tumor cells into the peripheral circulation and on to tertiary sites. To address this question in this short report, we used a mouse model of lymph node metastasis to demonstrate that TMEM doorways: (1) exist in tumor-positive lymph nodes of mice, (2) are restricted to the blood vascular endothelium, (3) serve as a mechanism for further dissemination to peripheral sites such as to the lungs, and (4) their activity can be abrogated by a pharmaceutical intervention. Our data suggest that cancer cell dissemination via TMEM doorways is a common mechanism of breast cancer cell dissemination to distant sites and thus the pharmacological targeting of TMEM may be necessary, even after resection of the primary tumor, to suppress cancer cell dissemination.

Type I interferon signaling in fibroblastic reticular cells prevents exhaustive activation of antiviral CD8+ T cells

Fibroblastic reticular cells (FRCs) are stromal cells that actively promote the induction of immune responses by coordinating the interaction of innate and adaptive immune cells. However, whether and to which extent immune cell activation is determined by lymph node FRC reprogramming during acute viral infection has remained unexplored. Here, we genetically ablated expression of the type I interferon-α receptor (Ifnar) in Ccl19-Cre⁺ cells and found that sensing of type I interferon imprints an antiviral state in FRCs and thereby preserves myeloid cell composition in lymph nodes of naive mice. During localized lymphocytic choriomeningitis virus infection, IFNAR signaling precipitated profound phenotypic adaptation of all FRC subsets enhancing antigen presentation, chemokine-driven immune cell recruitment, and immune regulation. The IFNAR-dependent shift of all FRC subsets toward an immunostimulatory state reduced exhaustive CD8⁺ T cell activation. In sum, these results unveil intricate circuits underlying type I IFN sensing in lymph node FRCs that enable protective antiviral immunity.

Biological functions of lymphatic vessels

The general functions of lymphatic vessels in fluid transport and immunosurveillance are well recognized. However, accumulating evidence indicates that lymphatic vessels play active and versatile roles in a tissue- and organ-specific manner during homeostasis and in multiple disease processes. This Review discusses recent advances to understand previously unidentified functions of adult mammalian lymphatic vessels, including immunosurveillance and immunomodulation upon pathogen invasion, transport of dietary fat, drainage of cerebrospinal fluid and aqueous humor, possible contributions toward neurodegenerative and neuroinflammatory diseases, and response to anticancer therapies.

Reticular Fibroblasts Expressing the Transcription Factor WT1 Define a Stromal Niche that Maintains and Replenishes Splenic Red Pulp Macrophages

Located within red pulp cords, splenic red pulp macrophages (RPMs) are constantly exposed to the blood flow, clearing senescent red blood cells (RBCs) and recycling iron from hemoglobin. Here, we studied the mechanisms underlying RPM homeostasis, focusing on the involvement of stromal cells as these cells perform anchoring and nurturing macrophage niche functions in lymph nodes and liver. Microscopy revealed that RPMs are embedded in a reticular meshwork of red pulp fibroblasts characterized by the expression of the transcription factor Wilms' Tumor 1 (WT1) and colony stimulating factor 1 (CSF1). Conditional deletion of Csf1 in WT1+ red pulp fibroblasts, but not white pulp fibroblasts, drastically altered the RPM network without altering circulating CSF1 levels. Upon RPM depletion, red pulp fibroblasts transiently produced the monocyte chemoattractants CCL2 and CCL7, thereby contributing to the replenishment of the RPM network. Thus, red pulp fibroblasts anchor and nurture RPM, a function likely conserved in humans.

A Single-Cell Transcriptional Roadmap of the Mouse and Human Lymph Node Lymphatic Vasculature

Single-cell transcriptomics promise to revolutionize our understanding of the vasculature. Emerging computational methods applied to high-dimensional single-cell data allow integration of results between samples and species and illuminate the diversity and underlying developmental and architectural organization of cell populations. Here, we illustrate these methods in the analysis of mouse lymph node (LN) lymphatic endothelial cells (LEC) at single-cell resolution. Clustering identifies five well-delineated subsets, including two medullary sinus subsets not previously recognized as distinct. Nearest neighbor alignments in trajectory space position the major subsets in a sequence that recapitulates the known features and suggests novel features of LN lymphatic organization, providing a transcriptional map of the lymphatic endothelial niches and of the transitions between them. Differences in gene expression reveal specialized programs for (1) subcapsular ceiling endothelial interactions with the capsule connective tissue and cells; (2) subcapsular floor regulation of lymph borne cell entry into the LN parenchyma and antigen presentation; and (3) pathogen interactions and (4) LN remodeling in distinct medullary subsets. LEC of the subcapsular sinus floor and medulla, which represent major sites of cell entry and exit from the LN parenchyma respectively, respond robustly to oxazolone inflammation challenge with enriched signaling pathways that converge on both innate and adaptive immune responses. Integration of mouse and human single-cell profiles reveals a conserved cross-species pattern of lymphatic vascular niches and gene expression, as well as specialized human subsets and genes unique to each species. The examples provided demonstrate the power of single-cell analysis in elucidating endothelial cell heterogeneity, vascular organization, and endothelial cell responses. We discuss the findings from the perspective of LEC functions in relation to niche formations in the unique stromal and highly immunological environment of the LN.

Stromal cell control of conventional and ectopic germinal centre reactions

The germinal centre (GC) is a specialized cellular structure that forms in response to antigenic stimulation. It generates long-term humoral immunity through the production of memory B cells and long-lived antibody-secreting plasma cells. Conventional GCs form within secondary lymphoid organs, where networks of specialised stromal cells that form during embryogenesis act as the stage upon which the various GC immune cell players are brought together, nurtured and co-ordinated to generate a productive response. In non-lymphoid organs, ectopic GCs can form in response to persistent antigenic and inflammatory stimuli. Unlike secondary lymphoid tissues, non-lymphoid organs do not have a developmentally programmed stromal cell network capable of supporting the germinal centre reaction; therefore, the local tissue stroma must be remodelled by inflammatory stimuli in order to host a GC reaction. These ectopic GCs produce memory B cells and plasma cells that form a critical component of the humoral immune response.

Stromal cells and immune cells involved in formation of lymph nodes and their niches

Secondary lymphoid organs are critical for efficient interaction between innate antigen presenting cells and adaptive lymphocytes in order to start adaptive immune responses. The efficiency by which these cellular subsets meet is highly increased by the orchestrating role of stromal cells within the secondary lymphoid organs. These cells provide cytokines, chemokines and cell surface receptors necessary for survival and guided migration. This increases the likelihood that antigen specific adaptive immune responses occur. Already from initial formation of secondary lymphoid organs, the interaction of immune cells with stromal cells is crucial and this interaction continues during immune activation. With the recent discovery of many stromal cell subsets new immune micro-niches with specific functions that are orchestrated by stromal cells will be discovered. Here, we will discuss how the development of lymph nodes as well as their specific niches is guided by the interaction of immune cells and stromal cells.

Regulation and function of macrophage colony-stimulating factor (CSF1) in the chicken immune system

Macrophage colony-stimulating factor (CSF1) is an essential growth factor to control the proliferation, differentiation and survival of cells of the macrophage lineage in vertebrates. We have previously produced a recombinant chicken CSF1-Fc fusion protein and administrated it to birds which produced a substantial expansion of tissue macrophage populations. To further study the biology of CSF1 in the chicken, here we generated anti-chicken CSF1 antibodies (ROS-AV181 and 183) using CSF1-Fc as an immunogen. The specific binding of each monoclonal antibody was confirmed by ELISA, Western blotting and immunohistochemistry on tissue sections. Using the anti-CSF1 antibodies, we show that chicken bone marrow derived macrophages (BMDM) express CSF1 on their surface, and that the level appears to be regulated further by exogenous CSF1. By capture ELISA circulating CSF1 levels increased transiently in both layer and broiler embryos around the day of hatch. The levels of CSF1 in broilers was higher than in layers during the first week after hatch. Antibody ROS-AV183 was able to block CSF1 biological activity in vitro and treatment of hatchlings using this neutralising antibody in vivo impacted on some tissue macrophage populations, but not blood monocytes. After anti-CSF1 treatment, CSF1R-transgene reporter expressing cells were reduced in the bursa of Fabricius and cecal tonsil and TIM4⁺ Kupffer cells in the liver were almost completely ablated. Anti-CSF1 treatment also produced a reduction in overall bone density, trabecular volume and TRAP⁺ osteoclasts. Our novel neutralising antibody provides a new tool to study the roles of CSF1 in birds.

Lymph node stromal cells: cartographers of the immune system

Lymph nodes (LNs) are strategically positioned at dedicated sites throughout the body to facilitate rapid and efficient immunity. Central to the structural integrity and framework of LNs, and the recruitment and positioning of leukocytes therein, are mesenchymal and endothelial lymph node stromal cells (LNSCs). Advances in the last decade have expanded our understanding and appreciation of LNSC heterogeneity, and the role they play in coordinating immunity has grown rapidly. In this review, we will highlight the functional contributions of distinct stromal cell populations during LN development in maintaining immune homeostasis and tolerance and in the activation and control of immune responses.

The lymph node at a glance - how spatial organization optimizes the immune response

A hallmark of the mammalian immune system is its ability to respond efficiently to foreign antigens without eliciting an inappropriate response to self-antigens. Furthermore, a robust immune response requires the coordination of a diverse range of cells present at low frequencies within the host. This problem is solved, in part, by concentrating antigens, antigen-presenting cells and antigen-responsive cells in lymph nodes (LNs). Beyond housing these cell types in one location, LNs are highly organized structures consisting of pre-positioned cells within well-defined microanatomical niches. In this Cell Science at a Glance article and accompanying poster, we outline the key cellular populations and components of the LN microenvironment that are present at steady state and chronicle the dynamic changes in these elements following an immune response. This review highlights the LN as a staging ground for both innate and adaptive immune responses, while providing an elegant example of how structure informs function.

Lymphatic endothelial cells of the lymph node

The influx and efflux of cells and antigens to and from the draining lymph nodes largely take place through the subcapsular, cortical and medullary sinus systems. Recent analyses in mice and humans have revealed unexpected diversity in the lymphatic endothelial cells, which form the distinct regions of the sinuses. As a semipermeable barrier, the lymphatic endothelial cells regulate the sorting of lymph-borne antigens to the lymph node parenchyma and can themselves serve as antigen-presenting cells. The leukocytes entering the lymph node via the sinus system and the lymphocytes egressing from the parenchyma migrate through the lymphatic endothelial cell layer. The sinus lymphatic endothelial cells also orchestrate the organogenesis of lymph nodes, and they undergo bidirectional signalling with other sinus-resident cells, such as subcapsular sinus macrophages, to generate a unique lymphatic niche. In this Review, we consider the structural and functional basis of how the lymph node sinus system coordinates immune responses under physiological conditions, and in inflammation and cancer.

Cavity Macrophages Get to the Heart of the Issue

In this issue of Immunity, Deniset et al. (2019) reveal a reparative function for GATA6⁺ pericardial cavity macrophages following cardiac injury. Their findings call for reconsideration of surgical procedures that involve the removal of the pericardium.

The Stromal Niche Guarding the Gatekeepers

In this issue of Immunity, Mondor et al. (2019) and Camara et al. (2019) show that lymphatic endothelial cells are essential components of the niche that forms and maintains the subcapsular sinusoidal macrophage network in homeostasis and throughout an immune challenge.

Developmental Stage-Specific Distribution of Macrophages in Mouse Mammary Gland

Mammary gland development begins in the embryo and continues throughout the reproductive life of female mammals. Tissue macrophages (Mϕs), dependent on signals from the Mϕ colony stimulating factor 1 receptor (CSF1R), have been shown to regulate the generation, regression and regeneration of this organ, which is central for mammalian offspring survival. However, the distribution of Mϕs in the pre- and post-natal mammary gland, as it undergoes distinct phases of development and regression, is unknown or has been inferred from immunostaining of thin tissue sections. Here, we used optical tissue clearing and 3-dimensional imaging of mammary tissue obtained from Csf1r-EGFP mice. Whilst tissue Mϕs were observed at all developmental phases, their abundance, morphology, localization and association with luminal and basal epithelial cells exhibited stage-specific differences. Furthermore, sexual dimorphism was observed at E14.5, when the male mammary bud is severed from the overlying epidermis. These findings provide new insights into the localization and possible functions of heterogeneous tissue Mϕ populations in mammogenesis.

Single-Cell Survey of Human Lymphatics Unveils Marked Endothelial Cell Heterogeneity and Mechanisms of Homing for Neutrophils

Lymphatic vessels form a critical component in the regulation of human health and disease. While their functional significance is increasingly being recognized, the comprehensive heterogeneity of lymphatics remains uncharacterized. Here, we report the profiling of 33,000 lymphatic endothelial cells (LECs) in human lymph nodes (LNs) by single-cell RNA sequencing. Unbiased clustering revealed six major types of human LECs. LECs lining the subcapsular sinus (SCS) of LNs abundantly expressed neutrophil chemoattractants, whereas LECs lining the medullary sinus (MS) expressed a C-type lectin CD209. Binding of a carbohydrate Lewis X (CD15) to CD209 mediated neutrophil binding to the MS. The neutrophil-selective homing by MS LECs may retain neutrophils in the LN medulla and allow lymph-borne pathogens to clear, preventing their spread through LNs in humans. Our study provides a comprehensive characterization of LEC heterogeneity and unveils a previously undefined role for medullary LECs in human immunity.

A Stromal Niche Defined by Expression of the Transcription Factor WT1 Mediates Programming and Homeostasis of Cavity-Resident Macrophages

Tissue-resident macrophages require specific milieus for the maintenance of defining gene-expression programs. Expression of the transcription factor GATA6 is required for the homeostasis, function and localization of peritoneal cavity-resident macrophages. Gata6 expression is maintained in a non-cell autonomous manner and is elicited by the vitamin A metabolite, retinoic acid. Here, we found that the GATA6 transcriptional program is a common feature of macrophages residing in all visceral body cavities. Retinoic acid-dependent and -independent hallmark genes of GATA6⁺ macrophages were induced by mesothelial and fibroblastic stromal cells that express the transcription factor Wilms' Tumor 1 (WT1), which drives the expression of two rate-limiting enzymes in retinol metabolism. Depletion of Wt1⁺ stromal cells reduced the frequency of GATA6⁺ macrophages in the peritoneal, pleural and pericardial cavities. Thus, Wt1⁺ mesothelial and fibroblastic stromal cells constitute essential niche components supporting the tissue-specifying transcriptional landscape and homeostasis of cavity-resident macrophages.