Lymphangiogenesis and lymphangiodysplasia: from molecular to clinical lymphology

Cancer metastasis through the lymphatic versus blood vessels

Whether cancer cells metastasize from the primary site to the distant sites via the lymphatic vessels or the blood vessels directly into the circulation is still under intense study. In this review article, we follow the journey of cancer cells metastasizing to the sentinel lymph nodes and beyond to the distant sites. We emphasize cancer heterogeneity and microenvironment as major determinants of cancer metastasis. Multiple molecules have been found to be associated with the complicated process of metastasis. Based on the large sentinel lymph node data, it is reasonable to conclude that cancer cells may metastasize through the blood vessels in some cases but in most cases, they use the sentinel lymph nodes as the major gateway to enter the circulation to distant sites.

Piezo1 regulates meningeal lymphatic vessel drainage and alleviates excessive CSF accumulation

Piezo1 regulates multiple aspects of the vascular system by converting mechanical signals generated by fluid flow into biological processes. Here, we find that Piezo1 is necessary for the proper development and function of meningeal lymphatic vessels and that activating Piezo1 through transgenic overexpression or treatment with the chemical agonist Yoda1 is sufficient to increase cerebrospinal fluid (CSF) outflow by improving lymphatic absorption and transport. The abnormal accumulation of CSF, which often leads to hydrocephalus and ventriculomegaly, currently lacks effective treatments. We discovered that meningeal lymphatics in mouse models of Down syndrome were incompletely developed and abnormally formed. Selective overexpression of Piezo1 in lymphatics or systemic administration of Yoda1 in mice with hydrocephalus or Down syndrome resulted in a notable decrease in pathological CSF accumulation, ventricular enlargement and other associated disease symptoms. Together, our study highlights the importance of Piezo1-mediated lymphatic mechanotransduction in maintaining brain fluid drainage and identifies Piezo1 as a promising therapeutic target for treating excessive CSF accumulation and ventricular enlargement.

Vascular Endothelial Growth Factor-C Treatment Enhances Cerebrospinal Fluid Outflow during Toxoplasma gondii Brain Infection but Does Not Improve Cerebral Edema

Cerebral edema frequently develops in the setting of brain infection and can contribute to elevated intracranial pressure, a medical emergency. How excess fluid is cleared from the brain is not well understood. Previous studies have shown that interstitial fluid is transported out of the brain along perivascular channels that collect into the cerebrospinal fluid (CSF)-filled subarachnoid space. CSF is then removed from the central nervous system through venous and lymphatic routes. The current study tested the hypothesis that increasing lymphatic drainage of CSF would promote clearance of cerebral edema fluid during infection with the neurotropic parasite Toxoplasma gondii. Fluorescent microscopy and magnetic resonance imaging was used to show that C57BL/6 mice develop vasogenic edema 4 to 5 weeks after infection with T. gondii. Tracer experiments were used to evaluate how brain infection affects meningeal lymphatic function, which demonstrated a decreased rate in CSF outflow in T. gondii-infected mice. Next, mice were treated with a vascular endothelial growth factor (VEGF)-C-expressing viral vector, which induced meningeal lymphangiogenesis and improved CSF outflow in chronically infected mice. No difference in cerebral edema was observed between mice that received VEGF-C and those that rececived sham treatment. Therefore, although VEGF-C treatment can improve lymphatic outflow in mice infected with T. gondii, this effect does not lead to increased clearance of edema fluid from the brains of these mice.

Sentinel node restoration by vascularized lymph node transfer in mice

BACKGROUND

Recent reports have indicated that vascularized lymph node transfer (VLNT) may improve the impaired immunity in lymphedema but there has been no report concerning anti-cancer immunity. In the early tumor immune response, dendritic cells (DCs) participate in tumor recognition and antigen presentation in local lymphatics. Here, we investigated the impact of VLNT on DC dynamics against cancer in mouse models.

METHODS

Forty-seven 8-week-old C57BL/6 N male mice were divided into three surgical groups: a VLNT model in which a vascularized inguinal lymph node (LN) flap was transferred into the ipsilateral fossa after a popliteal LN was removed; a LN dissection (LND) model in which the popliteal LN was dissected; and a control model in which a skin incision was made at the popliteal fossa and an ipsilateral inguinal LN was removed. Postoperative lymphatic flows were observed by indocyanine green lymphography and B16-F10-luc2 mouse melanoma were implanted into the ipsilateral footpad. The proportion of DCs in the transplanted nodes was measured by CD11c immunohistochemistry using digital imaging analysis 4 days after cancer implantation. Metastases to the lungs and LNs were quantitatively evaluated by luciferase assay 4 weeks after cancer implantation.

RESULTS

After VLNT, lymphatic reconnection was observed in 59.2% of mice. The proportion of DCs was significantly higher in the VLNT group with lymphatic reconnection (8.6% ± 1.0%) than in the naïve LN (4.3% ± 0.4%) (p < .001). The tumor burden of lung metastases was significantly less in the VLNT group with lymphatic reconnection compared with the LND group (p = .049).

CONCLUSIONS

Metastasis decreased in mice with reconnected lymphatics after VLNT. A possible explanation was that lymphatic restoration may have contributed to the tumor immune response by allowing DC migration to LNs.

Clinical Potential of Adrenomedullin Signaling in the Cardiovascular System

Numerous clinical studies have revealed the utility of circulating AM (adrenomedullin) or MR-proAM (mid-regional proAM 45-92) as an effective prognostic and diagnostic biomarker for a variety of cardiovascular-related pathophysiologies. Thus, there is strong supporting evidence encouraging the exploration of the AM-CLR (calcitonin receptor-like receptor) signaling pathway as a therapeutic target. This is further bolstered because several drugs targeting the shared CGRP (calcitonin gene-related peptide)-CLR pathway are already Food and Drug Administration-approved and on the market for the treatment of migraine. In this review, we summarize the AM-CLR signaling pathway and its modulatory mechanisms and provide an overview of the current understanding of the physiological and pathological roles of AM-CLR signaling and the yet untapped potentials of AM as a biomarker or therapeutic target in cardiac and vascular diseases and provide an outlook on the recently emerged strategies that may provide further boost to the possible clinical applications of AM signaling.

Enzymatically Crosslinked Collagen as a Versatile Matrix for In Vitro and In Vivo Co-Engineering of Blood and Lymphatic Vasculature

Adequate vascularization is required for the successful translation of many in vitro engineered tissues. This study presents a novel collagen derivative that harbors multiple recognition peptides for orthogonal enzymatic crosslinking based on sortase A (SrtA) and Factor XIII (FXIII). SrtA-mediated crosslinking enables the rapid co-engineering of human blood and lymphatic microcapillaries and mesoscale capillaries in bulk hydrogels. Whereas tuning of gel stiffness determines the extent of neovascularization, the relative number of blood and lymphatic capillaries recapitulates the ratio of blood and lymphatic endothelial cells originally seeded into the hydrogel. Bioengineered capillaries readily form luminal structures and exhibit typical maturation markers both in vitro and in vivo. The secondary crosslinking enzyme Factor XIII is used for in situ tethering of the VEGF mimetic QK peptide to collagen. This approach supports the formation of blood and lymphatic capillaries in the absence of exogenous VEGF. Orthogonal enzymatic crosslinking is further used to bioengineer hydrogels with spatially defined polymer compositions with pro- and anti-angiogenic properties. Finally, macroporous scaffolds based on secondary crosslinking of microgels enable vascularization independent from supporting fibroblasts. Overall, this work demonstrates for the first time the co-engineering of mature micro- and meso-sized blood and lymphatic capillaries using a highly versatile collagen derivative.

Design, development, in-vitro and in-vivo evaluation of polylactic acid-based multifunctional nanofibrous patches for efficient healing of diabetic wounds

Impaired healing of diabetic ulcers is one of the major complications of diabetic patients due to high susceptibility to microbial infections, impaired lymphianogenesis, edema, and consequently impairing proper healing. This could even lead to much worse complications that include severe gangrene, trauma and finally limb amputation. Therefore, this study aims to develop a multilayered durable nanofibrous wound patch loaded with three promising drugs (phenytoin, sildenafil citrate and simvastatin) each in a separate layer to target a different wound healing phase. Polylactic acid was used for the preparation of the nanofibrous matrix of the wound patch, where each drug was incorporated in a separate layer during the preparation process. Drugs release profiles were studied over 3 weeks. Results showed that both phenytoin and simvastatin were released within 14 days while sildenafil continued till 21 days. Both physicochemical and mechanical characteristics of the patches were fully assessed as well as their biodegradability, swellability, breathability and porosity. Results showed that incorporation of drugs preserved the physicochemical and mechanical properties as well as porosity of the developed nanofibers. In addition, patches were evaluated for their biocompatibility and cell adhesion capability before being tested through in-vivo diabetic wound rat model induced by alloxan for three weeks. In vivo results showed that the patches were successful in inducing proper wound healing in diabetic rat model with overcoming the above-mentioned obstacles within 3 weeks. This was confirmed through assessing wound closure as well as from histopathological studies that showed complete healing with proper cell regeneration and arrangement without forming scars.

Statins block mammalian target of rapamycin pathway: a possible novel therapeutic strategy for inflammatory, malignant and neurodegenerative diseases

Inflammation plays a critical role in several diseases such as cancer, gastric, heart and nervous system diseases. Data suggest that the activation of mammalian target of rapamycin (mTOR) pathway in epithelial cells leads to inflammation. Statins, the inhibitors of the 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA), seem to be able to inhibit the mTOR. Statins are considered to have favorable effects on inflammatory diseases by reducing the complications caused by inflammation and by regulating the inflammatory process and cytokines secretion. This critical review collected data on this topic from clinical, in vivo and in vitro studies published between 1998 and June 2022 in English from databases including PubMed, Google Scholar, Scopus, and Cochrane libraries.

Promoting Lymphangiogenesis and Lymphatic Growth and Remodeling to Treat Cardiovascular and Metabolic Diseases

Lymphatic vessels are low-pressure, blind-ended tubular structures that play a crucial role in the maintenance of tissue fluid homeostasis, immune cell trafficking, and dietary lipid uptake and transport. Emerging research has indicated that the promotion of lymphatic vascular growth, remodeling, and function can reduce inflammation and diminish disease severity in several pathophysiologic conditions. In particular, recent groundbreaking studies have shown that lymphangiogenesis, which describes the formation of new lymphatic vessels from the existing lymphatic vasculature, can be beneficial for the alleviation and resolution of metabolic and cardiovascular diseases. Therefore, promoting lymphangiogenesis represents a promising therapeutic approach. This brief review summarizes the most recent findings related to the modulation of lymphatic function to treat metabolic and cardiovascular diseases such as obesity, myocardial infarction, atherosclerosis, and hypertension. We also discuss experimental and therapeutic approaches to enforce lymphatic growth and remodeling as well as efforts to define the molecular and cellular mechanisms underlying these processes.

Reactive Oxygen Species in Regulating Lymphangiogenesis and Lymphatic Function

The lymphatic system is pivotal for immunosurveillance and the maintenance of tissue homeostasis. Lymphangiogenesis, the formation of new lymphatic vessels from pre-existing vessels, has both physiological and pathological roles. Recent advances in the molecular mechanisms regulating lymphangiogenesis have opened a new area of research on reparative lymphangiogenesis for the treatment of various pathological disorders comprising neurological disorders, cardiac repair, autoimmune disease, obesity, atherosclerosis, etc. Reactive oxygen species (ROS) produced by the various cell types serve as signaling molecules in several cellular mechanisms and regulate various aspects of growth-factor-mediated responses, including lymphangiogenesis. The ROS, including superoxide anion, hydrogen peroxide, and nitric oxide, play both beneficial and detrimental roles depending upon their levels and cellular microenvironment. Low ROS levels are essential for lymphangiogenesis. On the contrary, oxidative stress due to enhanced ROS generation and/or reduced levels of antioxidants suppresses lymphangiogenesis via promoting lymphatic endothelial cell apoptosis and death. In this review article, we provide an overview of types and sources of ROS, discuss the role of ROS in governing lymphangiogenesis and lymphatic function, and summarize the role of lymphatics in various diseases.

Role of Transcriptional and Epigenetic Regulation in Lymphatic Endothelial Cell Development

The lymphatic system is critical for maintaining the homeostasis of lipids and interstitial fluid and regulating the immune cell development and functions. Developmental anomaly-induced lymphatic dysfunction is associated with various pathological conditions, including lymphedema, inflammation, and cancer. Most lymphatic endothelial cells (LECs) are derived from a subset of endothelial cells in the cardinal vein. However, recent studies have reported that the developmental origin of LECs is heterogeneous. Multiple regulatory mechanisms, including those mediated by signaling pathways, transcription factors, and epigenetic pathways, are involved in lymphatic development and functions. Recent studies have demonstrated that the epigenetic regulation of transcription is critical for embryonic LEC development and functions. In addition to the chromatin structures, epigenetic modifications may modulate transcriptional signatures during the development or differentiation of LECs. Therefore, the understanding of the epigenetic mechanisms involved in the development and function of the lymphatic system can aid in the management of various congenital or acquired lymphatic disorders. Future studies must determine the role of other epigenetic factors and changes in mammalian lymphatic development and function. Here, the recent findings on key factors involved in the development of the lymphatic system and their epigenetic regulation, LEC origins from different organs, and lymphatic diseases are reviewed.

Genetic Determinants of the Effects of Training on Muscle and Adipose Tissue Homeostasis in Obesity Associated with Lymphedema

It is widely accepted that metabolic changes associated with training are influenced by a person's genetic background. In this review, we explore the polymorphisms underlying interindividual variability in response to training of weight loss and muscle mass increase in obese individuals, with or without lymphedema, and in normal-weight subjects. We searched PubMed for articles in English published up to May 2019 using the following keywords: (((physical training[Title/Abstract] OR sport activity[Title/Abstract]) AND predisposition[Title/Abstract]) AND polymorphism [Title/Abstract]). We identified 38 single-nucleotide polymorphisms that may modulate the genetic adaptive response to training. The identification of genetic marker(s) that improve the beneficial effects of training may in perspective make it possible to assess training programs, which in combination with dietary intervention can optimize body weight reduction in obese subjects, with or without lymphedema. This is particularly important for patients with lymphedema because obesity can worsen the clinical status, and therefore, a personalized approach that could reduce obesity would be fundamental in the clinical management of lymphedema.

Primary Lymphangiosarcoma of the Urinary Bladder in a Dog

Abdominal ultrasonographical and computed tomography examinations of a 12-year-old neutered female toy poodle revealed a protruding mass, approximately 2 cm in diameter, at the apex of the bladder. The mass was firm and haemorrhagic with a homogeneously brownish-yellow cut surface. Microscopically, it was unencapsulated and located in the muscle layer with invasion of the extra-muscular layer. It was composed of spindloid to oval neoplastic cells that formed irregular clefts and diffuse sheets that dissected bundles of collagen. Immunohistochemically, the neoplastic cells were positive for vimentin and lymphatic vessel endothelial hyaluronan receptor 1 antigens, but negative for cytokeratin AE1/AE3, factor VIII-related antigen, CD31, CD34, Prox-1, S100, desmin, α-smooth muscle actin and MyoD1. Negative immunolabelling for laminin antigen supported the absence of evidence of a basal lamina on ultrastructural examination. Based on these findings, this tumour was identified as a lymphangiosarcoma. To the best of our knowledge, this case is the first report of lymphangiosarcoma arising from the bladder in a dog.

The Lymphatic Vasculature in the 21st Century: Novel Functional Roles in Homeostasis and Disease

Mammals have two specialized vascular circulatory systems: the blood vasculature and the lymphatic vasculature. The lymphatic vasculature is a unidirectional conduit that returns filtered interstitial arterial fluid and tissue metabolites to the blood circulation. It also plays major roles in immune cell trafficking and lipid absorption. As we discuss in this review, the molecular characterization of lymphatic vascular development and our understanding of this vasculature's role in pathophysiological conditions has greatly improved in recent years, changing conventional views about the roles of the lymphatic vasculature in health and disease. Morphological or functional defects in the lymphatic vasculature have now been uncovered in several pathological conditions. We propose that subtle asymptomatic alterations in lymphatic vascular function could underlie the variability seen in the body's response to a wide range of human diseases.

Non-coding RNAs as Regulators of Lymphangiogenesis in Lymphatic Development, Inflammation, and Cancer Metastasis

Non-coding RNAs (ncRNAs), which do not encode proteins, have pivotal roles in manipulating gene expression in development, physiology, and pathology. Emerging data have shown that ncRNAs can regulate lymphangiogenesis, which refers to lymphatics deriving from preexisting vessels, becomes established during embryogenesis, and has a close relationship with pathological conditions such as lymphatic developmental diseases, inflammation, and cancer. This review summarizes the molecular mechanisms of lymphangiogenesis in lymphatic development, inflammation and cancer metastasis, and discusses ncRNAs' regulatory effects on them. Therapeutic targets with regard to lymphangiogenesis are also discussed.

Two Birds, One Stone: Double Hits on Tumor Growth and Lymphangiogenesis by Targeting Vascular Endothelial Growth Factor Receptor 3

Vascular endothelial growth factor receptor 3 (VEGFR3) has been known for its involvement in tumor-associated lymphangiogenesis and lymphatic metastasis. The VEGFR3 signaling is stimulated by its main cognate ligand, vascular endothelial growth factor C (VEGF-C), which in turn promotes tumor progression. Activation of VEGF-C/VEGFR3 signaling in lymphatic endothelial cells (LECs) was shown to enhance the proliferation of LECs and the formation of lymphatic vessels, leading to increased lymphatic metastasis of tumor cells. In the past decade, the expression and pathological roles of VEGFR3 in tumor cells have been described. Moreover, the VEGF-C/VEGFR3 axis has been implicated in regulating immune tolerance and suppression. Therefore, the inhibition of the VEGF-C/VEGFR3 axis has emerged as an important therapeutic strategy for the treatment of cancer. In this review, we discuss the current findings related to VEGF-C/VEGFR3 signaling in cancer progression and recent advances in the development of therapeutic drugs targeting VEGF-C/VEGFR3.

Lymphatic Tissue Engineering and Regeneration

The lymphatic system is a major circulatory system within the body, responsible for the transport of interstitial fluid, waste products, immune cells, and proteins. Compared to other physiological systems, the molecular mechanisms and underlying disease pathology largely remain to be understood which has hindered advancements in therapeutic options for lymphatic disorders. Dysfunction of the lymphatic system is associated with a wide range of disease phenotypes and has also been speculated as a route to rescue healthy phenotypes in areas including cardiovascular disease, metabolic syndrome, and neurological conditions. This review will discuss lymphatic system functions and structure, cell sources for regenerating lymphatic vessels, current approaches for engineering lymphatic vessels, and specific therapeutic areas that would benefit from advances in lymphatic tissue engineering and regeneration.

OxLDL promotes lymphangiogenesis and lymphatic metastasis in gastric cancer by upregulating VEGF‑C expression and secretion

Gastric cancer is one of the most malignant tumor types, and its metastasis is a notable cause of mortality. Among the methods of tumor metastasis, lymphatic metastasis is the predominant one in gastric cancer. A previous study reported that the plasma oxidized low-density lipoprotein (oxLDL) is the risk factor associated with the development of tumors in patients with abnormal lipid metabolism, but the influence of plasma oxLDL in the lymphatic metastasis of gastric cancer remains unclear. In the present study, the concentration of plasma oxLDL from patients with gastric cancer was detected with an ELISA kit, and the lymphatic vessel density in gastric cancer tissues was determined by D2-40 staining. The correlation analysis of oxLDL concentration and lymphatic vessel density demonstrated that plasma oxLDL was positively correlated with lymphatic metastasis in patients with gastric cancer. Subsequently, the popliteal lymph node metastasis animal experiment with nude mice confirmed that oxLDL could promote the lymphatic metastasis of gastric cancer. Following this, the western blotting and ELISA data demonstrated that oxLDL promoted the expression and secretion of vascular endothelia growth factor (VEGF)-C in gastric cancer cell lines. Finally, blocking the lectin-like oxLDL-1 (LOX-1) receptor, a specific receptor for oxLDL, and the nuclear factor (NF)-κB signaling pathway following oxLDL (50 µg/ml) treatment in HGC-27 cells revealed that oxLDL could activate the NF-κB signaling pathway mediated by LOX-1, with subsequent upregulation of VEGF-C expression, and secretion in and from gastric cancer cells, and finally that it could promote the lymphatic metastasis of gastric cancer. These data indicate the association between the plasma oxLDL and the lymphatic metastasis of gastric cancer, and indicate that oxLDL elimination may be a potential therapeutic target for the prevention and intervention of early lymph node metastasis in gastric cancer.

Lymphangiogenesis, lymphatic systemomics, and cancer: context, advances and unanswered questions

Ever since it was discovered that endothelial cells line lymphatic vessels, investigators have been working on unraveling the mechanisms that control the growth of this distinctive endothelium and its role in normal physiology and human disease. Recent technological advances have ushered in a new era of "omics" research on the lymphatic system. Research on the genome, transcriptome, proteome, and metabolome of lymphatics has increased our understanding of the biology of the lymphatic vasculature. Here, we introduce the context-lymphatic "systemomics," then briefly review some of the latest advances in research on tumor-associated lymphatic vessels highlighting several "omic" studies that have shed light on mechanisms controlling the growth and function of tumor-associated lymphatic vessels. We conclude by returning, with unanswered questions, to the larger context of cancer and the lymphatic system as a vasculature, circulation, route of entry and transport, and control center of the immune network.

The Lymphatic Vasculature: Its Role in Adipose Metabolism and Obesity

Obesity is a key risk factor for metabolic and cardiovascular diseases, and although we understand the mechanisms regulating weight and energy balance, the causes of some forms of obesity remain enigmatic. Despite the well-established connections between lymphatics and lipids, and the fact that intestinal lacteals play key roles in dietary fat absorption, the function of the lymphatic vasculature in adipose metabolism has only recently been recognized. It is well established that angiogenesis is tightly associated with the outgrowth of adipose tissue, as expanding adipose tissue requires increased nutrient supply from blood vessels. Results supporting a crosstalk between lymphatic vessels and adipose tissue, and linking lymphatic function with metabolic diseases, obesity, and adipose tissue, also started to accumulate in the last years. Here we review our current knowledge of the mechanisms by which defective lymphatics contribute to obesity and fat accumulation in mouse models, as well as our understanding of the lymphatic-adipose tissue relationship.

The optimum marker for the detection of lymphatic vessels

Podoplanin, lymphatic vessel endothelial hyaluronic acid receptor-1, prospero-related homeobox-1 and vascular endothelial growth factor receptor 3 have been demonstrated to have crucial roles in the development of the lymphatic system and lymphangiogenesis process by combining with their corresponding receptors. Thus, the four markers have been widely used in labelling lymphatic vessels for the detection of lymphangiogenesis and lymphatic vessel invasion. Numerous authors have aimed to identify the roles of these four markers in the lymphatic system and the mechanisms have been partly clarified at the molecular level. The aim of the present review was to comprehensively clarify the characteristics and latent action modes of the four markers in order to determine which is the best one for the detection of lymphangiogenesis and lymphatic vessel invasion.

Modified Mouse Models of Chronic Secondary Lymphedema: Tail and Hind Limb Models

BACKGROUND

Postsurgical secondary lymphedema is usually a progressive and lifelong condition lacking any curative treatment. The aim of this study was to develop new, simple surgical mouse models of chronic lymphedema, better simulating chronic nature of human postsurgical lymphedema.

METHODS

Two experimental mouse models of secondary lymphedema were created surgically without radiation by modifications of the previously described methods: the tail model and the hind limb model. Lymphedema formation was clinically assessed and quantitatively evaluated by measuring circumferences and limb volumes. Postmortem specimens were assessed histologically to examine the efficacy of the models.

RESULTS

In the tail models, although a substantial frequency of tail necrosis (30.0%) was noted and the increase in circumference was maintained for only limited times postoperatively depending on the particular tail model, the overall success rate was 65.0%. In the mouse hind limb model, the overall success rate was 88.9%, and the increased circumference and limb volume were maintained over the entire study period of 8 weeks. The overall success rate of the mouse hind limb model was significantly higher than that of the mouse tail model(s).

CONCLUSIONS

We have successfully established modified mouse tail and hind limb lymphedema models via only surgical techniques without radiation, which have characteristics of chronic secondary lymphedema. The mouse hind limb model has a higher success rate than the mouse tail model and has advantages of having the healthy contralateral hind limbs as an internal control.

Shaping of Peripheral T Cell Responses by Lymphatic Endothelial Cells

Lymph node stromal cells (LNSCs) have newly been promoted to the rank of new modulators of T cell responses. The different non-hematopoietic cell subsets in lymph node (LN) were considered for years as a simple scaffold, forming routes and proper environment for antigen (Ag)-lymphocyte encountering. Deeper characterization of those cells has recently clearly shown their impact on both dendritic cell and T cell functions. In particular, lymphatic endothelial cells (LECs) control lymphocyte trafficking and homeostasis in LNs and limit adaptive immune responses. Therefore, the new role of LECs in shaping immune responses has drawn the attention of immunologists. Striking is the discovery that LECs, among other LNSCs, ectopically express a large range of peripheral tissue-restricted Ags (PTAs), and further present PTA-derived peptides through major histocompatibility class I molecules to induce self-reactive CD8⁺ T cell deletional tolerance. In addition, both steady-state and tumor-associated LECs were described to be capable of exogenous Ag cross-presentation. Whether LECs can similarly impact CD4⁺ T cell responses through major histocompatibility class II restricted Ag presentation is still a matter of debate. Here, we review and discuss our current knowledge on the contribution of Ag-presenting LECs as regulators of peripheral T cell responses in different immunological contexts, including autoimmunity and cancer.

The lymphatic vasculature: development and role in shaping immunity

The lymphatic vasculature is an integral component of the immune system. Lymphatic vessels are a key highway via which immune cells are trafficked, serving not simply as a passive route of transport, but to actively shape and coordinate immune responses. Reciprocally, immune cells provide signals that impact the growth, development, and activity of the lymphatic vasculature. In addition to immune cell trafficking, lymphatic vessels are crucial for fluid homeostasis and lipid absorption. The field of lymphatic vascular research is rapidly expanding, fuelled by rapidly advancing technology that has enabled the manipulation and imaging of lymphatic vessels, together with an increasing recognition of the involvement of lymphatic vessels in a myriad of human pathologies. In this review we provide an overview of the genetic pathways and cellular processes important for development and maturation of the lymphatic vasculature, discuss recent work revealing important roles for the lymphatic vasculature in directing immune cell traffic and coordinating immune responses and highlight the involvement of lymphatic vessels in a range of pathological settings.

The lymphatic vascular system of the mouse head

Histological studies of the lymphatic vascular system in adult mice are hampered because bones cannot be sectioned properly. Here, we decalcified the heads of 14-day-old mice, embedded them in paraffin and stained resultant serial sections with the lymphendothelial-specific antibodies Lyve-1 and Podoplanin. We show that the tissues with the highest lymphatic vascular density are the dermis and the oral mucous membranes. In contrast, the nasal mucous membrane is devoid of lymphatics, except for its most basal parts below the vomeronasal organ. The inferior nasal turbinate contains numerous lymphatics and is connected to the nasolacrimal duct (NLD), which is ensheathed by a dense network of lymphatics. The lymphatics of the eye lids and conjunctiva are connected to those of the inferior nasal turbinate. We suggest that cerebro-spinal fluid (CSF) can drain via the optic nerve and NLD lymphatics, whereas CSF drained via the Fila olfactoria into the nasal mucous membrane is used for moisturization of the respiratory air. Tongue, palatine and buccal mucous membranes possess numerous lymphatics, whereas the dental pulp has none. Lymphatics are present in the maxillary gland and close to the temporomandibular joint, suggesting the augmentation of lymph flow by chewing and yawning. Lymphatics can also be found in the dura mater and in the dural septae entering into deeper parts of the brain. Our findings are discussed with regard to CSF drainage and potential routes for ocular tumor dissemination.

Long Term Results of Innovative Procedure in Surgical Management of Chronic Lymphedema

BACKGROUND

Lymphedema is the result of impaired lymphatic drainage by the affected organ. This abnormality can be primary or secondary. Different operative approaches have been introduced to treat chronic lymphedema.

MATERIALS AND METHODS

This retrospective study included 816 patients who were diagnosed with chronic lower extremity lymphedema and did not respond to non-operative management for at least six months. Data was collected over 25 years, between March 1987 and March 2013. Doppler ultrasonography of the deep venous system was routinely undertaken in all patients to confirm patency. The patients underwent surgery and their progress was followed for at least one year postoperatively.

RESULTS

All patients were operated by the suggested technique and long term fallow-up which is a modified form of the Homan's technique. The outcome was excellent, and 89.2% of patients were free of complication and 2% had poor results. The most common complication was wound seroma and wound infection.

CONCLUSION

The long term results and considering the difficulties associated with the treatment of chronic lymphedema and the variety of surgical options, our method achieved excellent results, and may be proposed for the standard operative procedure for treating intractable forms of this disease.

Morphological and Molecular Characterization of Human Dermal Lymphatic Collectors

Millions of patients suffer from lymphedema worldwide. Supporting the contractility of lymphatic collectors is an attractive target for pharmacological therapy of lymphedema. However, lymphatics have mostly been studied in animals, while the cellular and molecular characteristics of human lymphatic collectors are largely unknown. We studied epifascial lymphatic collectors of the thigh, which were isolated for autologous transplantations. Our immunohistological studies identify additional markers for LECs (vimentin, CCBE1). We show and confirm differences between initial and collecting lymphatics concerning the markers ESAM1, D2-40 and LYVE-1. Our transmission electron microscopic studies reveal two types of smooth muscle cells (SMCs) in the media of the collectors with dark and light cytoplasm. We observed vasa vasorum in the media of the largest collectors, as well as interstitial Cajal-like cells, which are highly ramified cells with long processes, caveolae, and lacking a basal lamina. They are in close contact with SMCs, which possess multiple caveolae at the contact sites. Immunohistologically we identified such cells with antibodies against vimentin and PDGFRα, but not CD34 and cKIT. With Next Generation Sequencing we searched for highly expressed genes in the media of lymphatic collectors, and found therapeutic targets, suitable for acceleration of lymphatic contractility, such as neuropeptide Y receptors 1, and 5; tachykinin receptors 1, and 2; purinergic receptors P2RX1, and 6, P2RY12, 13, and 14; 5-hydroxytryptamine receptors HTR2B, and 3C; and adrenoceptors α2A,B,C. Our studies represent the first comprehensive characterization of human epifascial lymphatic collectors, as a prerequisite for diagnosis and therapy.

Spatio-temporal Models of Lymphangiogenesis in Wound Healing

Several studies suggest that one possible cause of impaired wound healing is failed or insufficient lymphangiogenesis, that is the formation of new lymphatic capillaries. Although many mathematical models have been developed to describe the formation of blood capillaries (angiogenesis), very few have been proposed for the regeneration of the lymphatic network. Lymphangiogenesis is a markedly different process from angiogenesis, occurring at different times and in response to different chemical stimuli. Two main hypotheses have been proposed: (1) lymphatic capillaries sprout from existing interrupted ones at the edge of the wound in analogy to the blood angiogenesis case and (2) lymphatic endothelial cells first pool in the wound region following the lymph flow and then, once sufficiently populated, start to form a network. Here, we present two PDE models describing lymphangiogenesis according to these two different hypotheses. Further, we include the effect of advection due to interstitial flow and lymph flow coming from open capillaries. The variables represent different cell densities and growth factor concentrations, and where possible the parameters are estimated from biological data. The models are then solved numerically and the results are compared with the available biological literature.

Local inhibition of elastase reduces EMILIN1 cleavage reactivating lymphatic vessel function in a mouse lymphoedema model

Lymphatic vasculature critically depends on the connections of lymphatic endothelial cells with the extracellular matrix (ECM), which are mediated by anchoring filaments (AFs). The ECM protein EMILIN1 is a component of AFs and is involved in the regulation of lymphatic vessel functions: accordingly, Emilin1^−/− mice display lymphatic vascular morphological alterations, leading to functional defects such as mild lymphoedema, lymph leakage and compromised lymph drainage. In the present study, using a mouse post-surgical tail lymphoedema model, we show that the acute phase of acquired lymphoedema correlates with EMILIN1 degradation due to neutrophil elastase (NE) released by infiltrating neutrophils. As a consequence, the intercellular junctions of lymphatic endothelial cells are weakened and drainage to regional lymph nodes is severely affected. The local administration of sivelestat, a specific NE inhibitor, prevents EMILIN1 degradation and reduces lymphoedema, restoring a normal lymphatic functionality. The finding that, in human secondary lymphoedema samples, we also detected cleaved EMILIN1 with the typical bands of an NE-dependent pattern of fragmentation establishes a rationale for a powerful strategy that targets NE inhibition. In conclusion, the attempts to block EMILIN1 degradation locally represent the basis for a novel ‘ECM’ pharmacological approach to assessing new lymphoedema treatments.

Restoration of lymphatic function rescues obesity in Prox1-haploinsufficient mice

Prox1 heterozygous mice have a defective lymphatic vasculature and develop late-onset obesity. Chyle abnormally leaks from those vessels, accumulates in the surrounding tissues, and causes an increase in adipose tissue. We characterized the lymphatics of Prox1^+/- mice to determine whether the extent of obesity correlated with the severity of lymphatic defects. The lymphatic vasculature in Prox1^+/- mice exhibited reduced tracer clearance from the ear skin, dysfunctional perfusion of the lower legs, and reduced tracer uptake into the deep lymphatic collectors during mechanostimulation prior to the onset of obesity. Ear lymphatic vessels and leg collectors in Prox1^+/- mice were disorganized and irregular, further confirming that defective lymphatic vessels are associated with obesity in Prox1^+/- mice. We now provide conclusive in vivo evidence that demonstrates that leaky lymphatics mediate obesity in Prox1^+/- mice, as restoration of lymphatic vasculature function was sufficient to rescue the obesity features in Prox1^+/- mice. Finally, depth-lipomic profiling of lymph contents showed that free fatty acids induce adipogenesis in vitro.

The Lymphatic System in Disease Processes and Cancer Progression

Advances in our understanding of the structure and function of the lymphatic system have made it possible to identify its role in a variety of disease processes. Because it is involved not only in fluid homeostasis but also in immune cell trafficking, the lymphatic system can mediate and ultimately alter immune responses. Our rapidly increasing knowledge of the molecular control of the lymphatic system will inevitably lead to new and effective therapies for patients with lymphatic dysfunction. In this review, we discuss the molecular and physiological control of lymphatic vessel function and explore how the lymphatic system contributes to many disease processes, including cancer and lymphedema.

Lymphatic Vascular Regeneration: The Next Step in Tissue Engineering

The lymphatic system plays a crucial role in interstitial fluid drainage, lipid absorption, and immunological defense. Lymphatic dysfunction results in lymphedema, fluid accumulation, and swelling of soft tissues, as well as a potentially impaired immune response. Lymphedema significantly reduces quality of life of patients on a physical, mental, social, and economic basis. Current therapeutic approaches in treatment of lymphatic disease are limited. Over the last decades, great progress has been made in the development of therapeutic strategies to enhance vascular regeneration. These solutions to treat vascular disease may also be applicable in the treatment of lymphatic diseases. Comparison of the organogenic process and biological organization of the vascular and lymphatic systems and studies in the regulatory mechanisms involved in lymphangiogenesis and angiogenesis show many common features. In this study, we address the similarities between both transport systems, and focus in depth on the biology of lymphatic development. Based on the current advances in vascular regeneration, we propose different strategies for lymphatic tissue engineering that may be used for treatment of primary and secondary lymphedema.

A mathematical model for lymphangiogenesis in normal and diabetic wounds

Several studies suggest that one possible cause of impaired wound healing is failed or insufficient lymphangiogenesis, that is the formation of new lymphatic capillaries. Although many mathematical models have been developed to describe the formation of blood capillaries (angiogenesis) very few have been proposed for the regeneration of the lymphatic network. Moreover, lymphangiogenesis is markedly distinct from angiogenesis, occurring at different times and in a different manner. Here a model of five ordinary differential equations is presented to describe the formation of lymphatic capillaries following a skin wound. The variables represent different cell densities and growth factor concentrations, and where possible the parameters are estimated from experimental and clinical data. The system is then solved numerically and the results are compared with the available biological literature. Finally, a parameter sensitivity analysis of the model is taken as a starting point for suggesting new therapeutic approaches targeting the enhancement of lymphangiogenesis in diabetic wounds. The work provides a deeper understanding of the phenomenon in question, clarifying the main factors involved. In particular, the balance between TGF-β and VEGF levels, rather than their absolute values, is identified as crucial to effective lymphangiogenesis. In addition, the results indicate lowering the macrophage-mediated activation of TGF-β and increasing the basal lymphatic endothelial cell growth rate, inter alia, as potential treatments. It is hoped the findings of this paper may be considered in the development of future experiments investigating novel lymphangiogenic therapies.

Topical fentanyl stimulates healing of ischemic wounds in diabetic rats

BACKGROUND

Topically applied opioids promote angiogenesis and healing of ischemic wounds in rats. We examined if topical fentanyl stimulates wound healing in diabetic rats by stimulating growth-promoting signaling, angiogenesis, lymphangiogenesis and nerve regeneration.

METHODS

We used Zucker diabetic fatty rats that develop obesity and diabetes on a high fat diet due to a mutation in the Leptin receptor. Fentanyl blended with hydrocream was applied topically on ischemic wounds twice daily, and wound closure was analyzed regularly. Wound histology was analyzed by hematoxylin and eosin staining. Angiogenesis, lymphangiogenesis, nerve fibers and phospho-platelet derived growth factor receptor-β (PDGFR-β) were visualized by CD31-, lymphatic vessel endothelium-1, protein gene product 9.5- and anti-phospho PDGFR-β-immunoreactivity, respectively. Nitric oxide synthase (NOS) and PDGFR-β signaling were analyzed using Western immunoblotting.

RESULTS

Fentanyl significantly promoted wound closure as compared to phosphate-buffered saline (PBS). Histology scores were significantly higher in fentanyl-treated wounds, indicative of increased granulation tissue formation, reduced edema and inflammation, and increased matrix deposition. Fentanyl treatment resulted in increased wound angiogenesis, lymphatic vasculature, nerve fibers, nitric oxide, NOS and PDGFR-β signaling as compared to PBS. Phospho-PDGFR-β co-localized with CD31 co-staining for vasculature.

CONCLUSIONS

Topically applied fentanyl promotes closure of ischemic wounds in diabetic rats. Increased angiogenesis, lymphangiogenesis, peripheral nerve regeneration, NO and PDGFR-β signaling are associated with fentanyl-induced tissue remodeling and wound healing.

Isolation and Characterization of Human Lung Lymphatic Endothelial Cells

Characterization of lymphatic endothelial cells from the respiratory system may be crucial to investigate the role of the lymphatic system in the normal and diseased lung. We describe a simple and inexpensive method to harvest, isolate, and expand lymphatic endothelial cells from the human lung (HL-LECs). Fifty-five samples of healthy lung selected from patients undergoing lobectomy were studied. A two-step purification tool, based on paramagnetic sorting with monoclonal antibodies to CD31 and Podoplanin, was employed to select a pure population of HL-LECs. The purity of HL-LECs was assessed by morphologic criteria, immunocytochemistry, flow cytometry, and functional assays. Interestingly, these cells retain in vitro several receptor tyrosine kinases (RTKs) implicated in cell survival and proliferation. HL-LECs represent a clinically relevant cellular substrate to study lymphatic biology, lymphoangiogenesis, interaction with microbial agents, wound healing, and anticancer therapy.

Adjuvant taxanes and the development of breast cancer-related arm lymphoedema

BACKGROUND

Despite affecting approximately one-quarter of all patients undergoing axillary lymph node dissection, the pathophysiology of breast cancer-related lymphoedema (BCRL) remains poorly understood. More extensive locoregional treatment and higher body mass index have long been identified as major risk factors. This study aimed to identify risk factors for BCRL with a specific focus on the potential impact of chemotherapy on the risk of BCRL.

METHODS

This was a retrospective analysis of a cohort of consecutive patients with breast cancer treated at a major London regional teaching hospital between 1 January 2010 and 31 December 2012. All patients had node-positive disease and underwent axillary lymph node dissection. Data regarding tumour-, patient- and treatment-related characteristics were collected prospectively. The diagnosis of BCRL was based on both subjective and objective criteria. Multivariable Cox proportional hazards regression was used to assess the association between treatment and risk of BCRL.

RESULTS

Some 27.1 per cent of all patients (74 of 273) developed BCRL over the study period. Administration of taxanes showed a strong association with the development of BCRL, as 52 (33.5 per cent) of 155 patients who received taxanes developed BCRL. Multivariable Cox regression analysis demonstrated that patients who received taxanes were nearly three times more likely to develop BCRL than patients who had no chemotherapy (hazard ratio 2.82, 95 per cent c.i. 1.31 to 6.06). No such increase was observed when taxanes were administered in the neoadjuvant setting.

CONCLUSION

The present findings suggest that adjuvant taxanes play a key role in the development of BCRL after surgery. This may support the use of taxanes in a neoadjuvant rather than adjuvant setting.

Topical bFGF Improves Secondary Lymphedema through Lymphangiogenesis in a Rat Tail Model

BACKGROUND

Secondary lymphedema is a common complication of cancer therapy, but options for treating lymphedema are essentially ineffective and limited. On the contrary, lymphangiogenic growth factors accelerate lymphangiogenesis and improve lymphedema.

METHODS

Rat tail models of lymphedema were assigned to groups that received either daily topical basic fibroblast growth factor (bFGF) or saline (control) groups. Tail volume was measured, and the function of the lymphatic system was evaluated as the fluorescence intensity of indocyanine green every 3 days. The mRNA levels of vascular endothelial growth factor (VEGF)-C and VEGF-D and the protein levels of VEGF-C were evaluated at postoperative days (PODs) 7, 14, and 28. The subcutaneous and deep areas and lymphatic vessel density were histologically determined at PODs 7, 14, and 28.

RESULTS

Tail volume was significantly larger in the control than in the bFGF group (P < 0.05). The intensity of indocyanine green fluorescence significantly decreased earlier in the bFGF group (P < 0.05). The mRNA and protein levels of VEGF-C were upregulated in the bFGF group at POD 14 (P < 0.01). Both subcutaneous and deep tissues gradually withered in both groups but more rapidly in the bFGF, than in the control group, reaching statistically significant differences in the subcutaneous and deeper areas at POD 14 (P < 0.05). Lymphatic vessel density was significantly higher in the bFGF than in the control group at POD 14 (P < 0.05).

CONCLUSIONS

Topical bFGF induces lymphangiogenesis and improves lymphedema in the rat tail model.

Role of angiopoietin-2 in corneal lymphangiogenesis

PURPOSE

Lymphatic research has progressed rapidly in recent years. Lymphatic dysfunction has been found in myriad disorders from cancer metastasis to transplant rejection; however, effective treatment for lymphatic disorders is still limited. This study investigates the role of angiopoietin-2 (Ang-2) in corneal inflammatory lymphangiogenesis (LG) in vivo and in lymphatic endothelial cell (LEC) functions in vitro.

METHODS

Standard suture placement model was used to study Ang-2 expression in inflamed cornea, and corneal LG and hemangiogenesis (HG) responses in Ang-2 knockout mice. Moreover, human LEC culture system was used to examine the effect of Ang-2 gene knockdown on LEC functions using small interfering RNAs (siRNAs). The effect of siRNA treatment on corneal LG was also assessed in vivo.

RESULTS

Angiopoietin-2 was expressed on lymphatic vessels and macrophages in inflamed cornea. While corneal LG response was abolished in Ang-2 knockout mice, the HG response was also significantly suppressed with disorganized patterning. Moreover, anti-Ang-2 treatment inhibited LEC proliferation and capillary tube formation in vitro and corneal LG in vivo.

CONCLUSIONS

Angiopoietin-2 is critically involved in lymphatic processes in vivo and in vitro. Further investigation of the Ang-2 pathway may provide novel insights and therapeutic strategies for lymphatic-related disorders, which occur both inside and outside the eye.

Lymphatic filariasis: perspectives on lymphatic remodeling and contractile dysfunction in filarial disease pathogenesis

Lymphatic filariasis, one of the most debilitating diseases associated with the lymphatic system, affects over a hundred million people worldwide and manifests itself in a variety of severe clinical pathologies. The filarial parasites specifically target the lymphatics and impair lymph flow, which is critical for the normal functions of the lymphatic system in maintenance of body fluid balance and physiological interstitial fluid transport. The resultant contractile dysfunction of the lymphatics causes fluid accumulation and lymphedema, one of the major pathologies associated with filarial infection. In this review, we take a closer look at the contractile mechanisms of the lymphatics, its altered functions, and remodeling during an inflammatory state and how it relates to the severe pathogenesis underlying a filarial infection. We further elaborate on the complex host-parasite interactions, and molecular mechanisms contributing to the disease pathogenesis. The overall emphasis is on elucidating some of the emerging concepts and new directions that aim to harness the process of lymphangiogenesis or enhance contractility in a dysfunctional lymphatics, thereby restoring the fluid imbalance and mitigating the pathological conditions of lymphatic filariasis.

Mechanical forces in lymphatic vascular development and disease

The lymphatic vasculature is essential for fluid homeostasis and transport of immune cells, inflammatory molecules, and dietary lipids. It is composed of a hierarchical network of blind-ended lymphatic capillaries and collecting lymphatic vessels, both lined by lymphatic endothelial cells (LECs). The low hydrostatic pressure in lymphatic capillaries, their loose intercellular junctions, and attachment to the surrounding extracellular matrix (ECM) permit passage of extravasated blood plasma from the interstitium into the lumen of the lymphatic capillaries. It is generally thought that interstitial fluid accumulation leads to a swelling of the ECM, to which the LECs of lymphatic capillaries adhere, for example via anchoring filaments. As a result, LECs are pulled away from the vascular lumen, the junctions open, and fluid enters the lymphatic vasculature. The collecting lymphatic vessels then gather the plasma fluid from the capillaries and carry it through the lymph nodes to the blood circulation. The collecting vessels contain intraluminal bicuspid valves that prevent fluid backflow, and are embraced by smooth muscle cells that contribute to fluid transport. Although the lymphatic vessels are regular subject to mechanical strain, our knowledge of its influence on lymphatic development and pathologies is scarce. Here, we discuss the mechanical forces and molecular mechanisms regulating lymphatic vascular growth and maturation in the developing mouse embryo. We also consider how the lymphatic vasculature might be affected by similar mechanomechanisms in two pathological processes, namely cancer cell dissemination and secondary lymphedema.

Lymphedema and therapeutic lymphangiogenesis

Lymphedema is a disorder of the lymphatic vascular system characterized by impaired lymphatic return and swelling of the extremities. Lymphedema is divided into primary and secondary forms based on the underlying etiology. Despite substantial advances in both surgical and conservative techniques, therapeutic options for the management of lymphedema are limited. Although rarely lethal, lymphedema is a disfiguring and disabling condition with an associated decrease in the quality of life. The recent impressive expansion of knowledge on the molecular mechanisms governing lymphangiogenesis provides new possibilities for the treatment of lymphedema. This review highlights the lymphatic biology, the pathophysiology of lymphedema, and the therapeutic lymphangiogenesis using hepatocyte growth factor.

Blood and lymphatic vasculature in the ovary: development, function and disease

BACKGROUND

The remodelling of the blood vasculature has been the subject of much research while rapid progress in the understanding of the factors controlling lymphangiogenesis in the ovary has only been reported more recently. The ovary undergoes cyclic remodelling throughout each menstrual/estrous cycle. This process requires significant vascular remodelling to supply each new cohort of growing follicles.

METHODS

Literature searches were performed to review studies on the ovarian lymphatic vasculature that described spatial, temporal and functional data in human or animal species. The role of ovarian blood and lymphatic vasculature in the pathogenesis of ovarian disease and dysfunction was also explored.

RESULTS

Research in a number of species including zebrafish, rodents and primates has described the lymphatic vasculature within the remodelling ovary, while recent research in mouse has confirmed hormonal regulation of lymphangiogenic growth factors, their receptors and also a role for the protease, ADAMTS1 in the development of the lymphatic vasculature. With a critical role in the maintenence of fluid homeostasis, the ovarian lymphatic vasculature is important for normal ovarian function and has been linked to syndromes involving ovarian fluid imbalance, including ovarian hyperstimulation syndrome and massive ovarian edema. The lymphatic vasculature has also been heavily implicated in the metastatic cancer process.

CONCLUSION

The spatial and temporal regulation of the ovarian lymphatic vasculature has now been reported in a number of species and the data also implicate the ovarian lymphatic vasculature in ovarian pathologies, including cancer and those linked with use of artificial reproduction technologies.

Vascular endothelial growth factor receptor-2 promotes the development of the lymphatic vasculature

Vascular endothelial growth factor receptor 2 (VEGFR2) is highly expressed by lymphatic endothelial cells and has been shown to stimulate lymphangiogenesis in adult mice. However, the role VEGFR2 serves in the development of the lymphatic vascular system has not been defined. Here we use the Cre-lox system to show that the proper development of the lymphatic vasculature requires VEGFR2 expression by lymphatic endothelium. We show that Lyve-1^wt/Cre;Vegfr2^flox/flox mice possess significantly fewer dermal lymphatic vessels than Vegfr2^flox/flox mice. Although Lyve-1^wt/Cre;Vegfr2^flox/flox mice exhibit lymphatic hypoplasia, the lymphatic network is functional and contains all of the key features of a normal lymphatic network (initial lymphatic vessels and valved collecting vessels surrounded by smooth muscle cells (SMCs)). We also show that Lyve-1^Cre mice display robust Cre activity in macrophages and in blood vessels in the yolk sac, liver and lung. This activity dramatically impairs the development of blood vessels in these tissues in Lyve-1^wt/Cre;Vegfr2^flox/flox embryos, most of which die after embryonic day14.5. Lastly, we show that inactivation of Vegfr2 in the myeloid lineage does not affect the development of the lymphatic vasculature. Therefore, the abnormal lymphatic phenotype of Lyve-1^wt/Cre;Vegfr2^flox/flox mice is due to the deletion of Vegfr2 in the lymphatic vasculature not macrophages. Together, this work demonstrates that VEGFR2 directly promotes the expansion of the lymphatic network and further defines the molecular mechanisms controlling the development of the lymphatic vascular system.