Immunohistochemical Expression of Lymphatic Endothelial Markers in Blue Rubber Bleb Nevus Syndrome.

INTRODUCTION: Blue rubber bleb nevus syndrome (BRBNS) is an uncommon vascular anomaly characterized by multifocal cutaneous, visceral, and other soft tissue or solid organ venous malformations. We observed that BRBNS lesions express immunohistochemical markers of lymphatic differentiation. METHODS: BRBNS histopathologic specimens assessed at our institution during the past 27 years were reviewed. Slides from 19 BRBNS lesions were selected from 14 patients (9 cutaneous, 9 gastrointestinal, and 1 hepatic). We recorded the involved anatomical compartments and presence/absence of thrombi or vascular smooth muscle. Immunohistochemical endothelial expression of PROX1 (nuclear) and D2-40 (membranous/cytoplasmic) was evaluated semi-quantitatively. RESULTS: Endothelial PROX1 immunopositivity was noted in all specimens; the majority (89.5%) demonstrated staining in more than 10% of cells. D2-40 immunopositivity was present in one-third (33%) of cutaneous lesions and only 1 gastrointestinal lesion. CONCLUSION: Endothelial cells in BRBNS almost always express 1 or more immunohistochemical markers of lymphatic differentiation.

Role of lymphatic endothelium specific hyaluronan receptor 1 in virus infection and associated diseases.

Lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1) serves as a prominent marker for lymphatic endothelial cells (LECs) and is pivotal in the process of lymphangiogenesis, a critical factor in cancer development and metastasis. Overexpression of LYVE-1 has been observed in various cancers, where it is recognized as an adverse prognostic indicator. Targeting LYVE-1 has demonstrated inhibitory effects on tumor cell proliferation, migration, and the formation of lymph node metastases both in vitro and in vivo. While extensive research has focused on the role of LYVE-1 in cancer cells, its involvement in virus infection and associated diseases remains largely unexplored. This review consolidates recent findings regarding the expression of LYVE-1 and its functions in lymphangiogenesis during various viral infections and the development of related diseases, with a particular emphasis on Kaposi's sarcoma herpesvirus. Despite the limited available data, it is evident that further studies are essential to comprehensively understand the contribution of LYVE-1 to viral pathogenesis and oncogenesis.

A phosphodiesterase pathway essential for lymphangiogenic growth arrest and maturation.

In developing embryos, downregulation of lymphatic endothelial proliferation is needed for maturation of lymphatic vessels into a hierarchical network. In this issue of Developmental Cell, Carlantoni discover that phosphodiesterase2A controls lymphatic endothelial growth arrest and maturation via regulation of cGMP, p38 MAP kinase, and Notch pathway.

The lymphatic endothelium-derived follistatin: activin A axis regulates neutrophil motility in response to Pseudomonas aeruginosa.

The lymphatic system plays an active role during infection, however the role of lymphatic-neutrophil interactions in host-defense responses is not well understood. During infection with pathogens such as Pseudomonas aeruginosa, Staphylococcus aureus and Yersinia pestis, neutrophils traffic from sites of infection through the lymphatic vasculature, to draining lymph nodes to interact with resident lymphocytes. This process is poorly understood, in part, due to the lack of in vitro models of the lymphatic system. Here we use a 3D microscale lymphatic vessel model to examine neutrophil-lymphatic cell interactions during host defense responses to pathogens. In previous work, we have shown that follistatin is secreted at high concentrations by lymphatic endothelial cells during inflammation. Follistatin inhibits activin A, a member of the TGF-ß superfamily, and, together, these molecules form a signaling pathway that plays a role in regulating both innate and adaptive immune responses. Although follistatin and activin A are constitutively produced in the pituitary, gonads and skin, their major source in the serum and their effects on neutrophils are poorly understood. Here we report a microfluidic model that includes both blood and lymphatic endothelial vessels, and neutrophils to investigate neutrophil-lymphatic trafficking during infection with P. aeruginosa. We found that lymphatic endothelial cells produce secreted factors that increase neutrophil migration toward P. aeruginosa, and are a significant source of both follistatin and activin A during Pseudomonas infection. We determined that follistatin produced by lymphatic endothelial cells inhibits activin A, resulting in increased neutrophil migration. These data suggest that the follistatin:activin A ratio influences neutrophil trafficking during infection with higher ratios increasing neutrophil migration.

Kidney Injury Causes Accumulation of Renal Sodium That Modulates Renal Lymphatic Dynamics.

Lymphatic vessels are highly responsive to changes in the interstitial environment. Previously, we showed renal lymphatics express the Na-K-2Cl cotransporter. Since interstitial sodium retention is a hallmark of proteinuric injury, we examined whether renal sodium affects NKCC1 expression and the dynamic pumping function of renal lymphatic vessels. Puromycin aminonucleoside (PAN)-injected rats served as a model of proteinuric kidney injury. Sodium 23Na/1H-MRI was used to measure renal sodium and water content in live animals. Renal lymph, which reflects the interstitial composition, was collected, and the sodium analyzed. The contractile dynamics of isolated renal lymphatic vessels were studied in a perfusion chamber. Cultured lymphatic endothelial cells (LECs) were used to assess direct sodium effects on NKCC1. MRI showed elevation in renal sodium and water in PAN. In addition, renal lymph contained higher sodium, although the plasma sodium showed no difference between PAN and controls. High sodium decreased contractility of renal collecting lymphatic vessels. In LECs, high sodium reduced phosphorylated NKCC1 and SPAK, an upstream activating kinase of NKCC1, and eNOS, a downstream effector of lymphatic contractility. The NKCC1 inhibitor furosemide showed a weaker effect on ejection fraction in isolated renal lymphatics of PAN vs controls. High sodium within the renal interstitium following proteinuric injury is associated with impaired renal lymphatic pumping that may, in part, involve the SPAK-NKCC1-eNOS pathway, which may contribute to sodium retention and reduce lymphatic responsiveness to furosemide. We propose that this lymphatic vessel dysfunction is a novel mechanism of impaired interstitial clearance and edema in proteinuric kidney disease.

CCN2-induced lymphangiogenesis is mediated by the integrin αvß5-ERK pathway and regulated by DUSP6.

Lymphangiogenesis is essential for the development of the lymphatic system and is important for physiological processes such as homeostasis, metabolism and immunity. Cellular communication network factor 2 (CCN2, also known as CTGF), is a modular and matricellular protein and a well-known angiogenic factor in physiological and pathological angiogenesis. However, its roles in lymphangiogenesis and intracellular signaling in lymphatic endothelial cells (LECs) remain unclear. Here, we investigated the effects of CCN2 on lymphangiogenesis. In in vivo Matrigel plug assays, exogenous CCN2 increased the number of Podoplanin-positive vessels. Subsequently, we found that CCN2 induced phosphorylation of ERK in primary cultured LECs, which was almost completely inhibited by the blockade of integrin αvß5 and partially decreased by the blockade of integrin αvß3. CCN2 promoted direct binding of ERK to dual-specific phosphatase 6 (DUSP6), which regulated the activation of excess ERK by dephosphorylating ERK. In vitro, CCN2 promoted tube formation in LECs, while suppression of Dusp6 further increased tube formation. In vivo, immunohistochemistry also detected ERK phosphorylation and DUSP6 expression in Podoplanin-positive cells on CCN2-supplemented Matrigel. These results indicated that CCN2 promotes lymphangiogenesis by enhancing integrin αvß5-mediated phosphorylation of ERK and demonstrated that DUSP6 is a negative regulator of excessive lymphangiogenesis by CCN2.

Immunohistochemical and immunofluorescence expression profile of lymphatic endothelial cell markers in oral cancer.

Lymphangiogenesis makes an important contribution to the tumour microenvironment (TME), but little is known about this in oral squamous cell carcinoma (OSCC). Archival formalin-fixed paraffin-embedded specimens (28 OSCC, 10 inflamed and 6 normal oral mucosa controls) were processed using immunohistochemistry (IHC) with antibodies against lymphatic markers D2-40 (podoplanin), LYVE-1, VEGFR3 and Prox1. After the endothelial cells had been highlighted by the various markers for lymphatic endothelium, the positive stained cells and vessels were identified and counted in a systematic manner to determine microvessel density. Double-labelling immunofluorescence (DLIF) was used to investigate the specificity of D2-40 and LYVE-1 to lymphatic endothelial cells (LECs) as opposed to blood ECs. There was higher D2-40 and Prox1 lymphatic vessel density (P = .001) in the OSCC group when compared with both control groups. Some malignant keratinocytes expressed lymphatic markers, as did a much smaller number of epithelial cells in the control groups. DLIF showed that no vessels co-expressed D2-40/CD34 or LYVE/CD34. Some D2/40+ LVs were LYVE- . D2-40 was the most specific LEC marker in OSCC tissues. These results establish that the OSCC TME contains significantly more lymphatic vessels expressing D2-40 and Prox1 than the control groups, which may play a role in facilitating lymphatic invasion and metastases.

FOXC2 controls adult lymphatic endothelial specialization, function, and gut lymphatic barrier preventing multiorgan failure.

The mechanisms maintaining adult lymphatic vascular specialization throughout life and their role in coordinating inter-organ communication to sustain homeostasis remain elusive. We report that inactivation of the mechanosensitive transcription factor Foxc2 in adult lymphatic endothelium leads to a stepwise intestine-to-lung systemic failure. Foxc2 loss compromised the gut epithelial barrier, promoted dysbiosis and bacterial translocation to peripheral lymph nodes, and increased circulating levels of purine metabolites and angiopoietin-2. Commensal microbiota depletion dampened systemic pro-inflammatory cytokine levels, corrected intestinal lymphatic dysfunction, and improved survival. Foxc2 loss skewed the specialization of lymphatic endothelial subsets, leading to populations with mixed, pro-fibrotic identities and to emergence of lymph node-like endothelial cells. Our study uncovers a cross-talk between lymphatic vascular function and commensal microbiota, provides single-cell atlas of lymphatic endothelial subtypes, and reveals organ-specific and systemic effects of dysfunctional lymphatics. These effects potentially contribute to the pathogenesis of diseases, such as inflammatory bowel disease, cancer, or lymphedema.

Targeting local lymphatics to ameliorate heterotopic ossification via FGFR3-BMPR1a pathway.

Acquired heterotopic ossification (HO) is the extraskeletal bone formation after trauma. Various mesenchymal progenitors are reported to participate in ectopic bone formation. Here we induce acquired HO in mice by Achilles tenotomy and observe that conditional knockout (cKO) of fibroblast growth factor receptor 3 (FGFR3) in Col2+ cells promote acquired HO development. Lineage tracing studies reveal that Col2+ cells adopt fate of lymphatic endothelial cells (LECs) instead of chondrocytes or osteoblasts during HO development. FGFR3 cKO in Prox1+ LECs causes even more aggravated HO formation. We further demonstrate that FGFR3 deficiency in LECs leads to decreased local lymphatic formation in a BMPR1a-pSmad1/5-dependent manner, which exacerbates inflammatory levels in the repaired tendon. Local administration of FGF9 in Matrigel inhibits heterotopic bone formation, which is dependent on FGFR3 expression in LECs. Here we uncover Col2+ lineage cells as an origin of lymphatic endothelium, which regulates local inflammatory microenvironment after trauma and thus influences HO development via FGFR3-BMPR1a pathway. Activation of FGFR3 in LECs may be a therapeutic strategy to inhibit acquired HO formation via increasing local lymphangiogenesis.

Dendritic cell entry to lymphatic capillaries is orchestrated by CD44 and the hyaluronan glycocalyx.

DCs play a vital role in immunity by conveying antigens from peripheral tissues to draining lymph nodes, through afferent lymphatic vessels. Critical to the process is initial docking to the lymphatic endothelial receptor LYVE-1 via its ligand hyaluronan on the DC surface. How this relatively weak binding polymer is configured for specific adhesion to LYVE-1, however, is unknown. Here, we show that hyaluronan is anchored and spatially organized into a 400-500 nm dense glycocalyx by the leukocyte receptor CD44. Using gene knockout and by modulating CD44-hyaluronan interactions with monoclonal antibodies in vitro and in a mouse model of oxazolone-induced skin inflammation, we demonstrate that CD44 is required for DC adhesion and transmigration across lymphatic endothelium. In addition, we present evidence that CD44 can dynamically control the density of the hyaluronan glycocalyx, regulating the efficiency of DC trafficking to lymph nodes. Our findings define a previously unrecognized role for CD44 in lymphatic trafficking and highlight the importance of the CD44:HA:LYVE-1 axis in its regulation.

A Flow Cytometric Assay for Investigating Melanoma Cell Adhesion to Lymphatic Endothelial Cells.

Lymph node invasion by tumor cells is an important process in the progression of melanoma and is a poor prognostic factor for patients with this cancer. Before they are able to spread to regional lymph nodes, though, melanoma cells must first adhere to lymphatic endothelium and transmigrate into the lymphatic vasculature. In order to study melanoma cell adhesion to lymphatic endothelial cells and the factors that regulate this process, we have developed an in vitro flow cytometry-based assay to measure melanoma cell attachment to lymphatic endothelial cells. This assay will be a useful tool for investigating the interactions that take place between melanoma cells and lymphatic endothelial cells during the adhesion process.

An immunohistochemical study of lymphatic elements in the human brain.

Almost 150 papers about brain lymphatics have been published in the last 150 years. Recently, the information in these papers has been synthesized into a picture of central nervous system (CNS) "glymphatics," but the fine structure of lymphatic elements in the human brain based on imaging specific markers of lymphatic endothelium has not been described. We used LYVE1 and PDPN antibodies to visualize lymphatic marker-positive cells (LMPCs) in postmortem human brain samples, meninges, cavernous sinus (cavum trigeminale), and cranial nerves and bolstered our findings with a VEGFR3 antibody. LMPCs were present in the perivascular space, the walls of small and large arteries and veins, the media of large vessels along smooth muscle cell membranes, and the vascular adventitia. Lymphatic marker staining was detected in the pia mater, in the arachnoid, in venous sinuses, and among the layers of the dura mater. There were many LMPCs in the perineurium and endoneurium of cranial nerves. Soluble waste may move from the brain parenchyma via perivascular and paravascular routes to the closest subarachnoid space and then travel along the dura mater and/or cranial nerves. Particulate waste products travel along the laminae of the dura mater toward the jugular fossa, lamina cribrosa, and perineurium of the cranial nerves to enter the cervical lymphatics. CD3-positive T cells appear to be in close proximity to LMPCs in perivascular/perineural spaces throughout the brain. Both immunostaining and qPCR confirmed the presence of adhesion molecules in the CNS known to be involved in T cell migration.

Fsp1 cardiac embryonic expression delineates atrioventricular endocardial cushion, coronary venous and lymphatic valve development.

Fsp1 (a.k.a S100A4 or Metastatin) is an intracellular and secreted protein widely regarded as a fibroblast marker. Recent studies have nonetheless shown that Fsp1 is also expressed by other cell types, including small subsets of endothelial cells. Since no detailed and systematic description of Fsp1 spatio-temporal expression pattern in cardiac vascular cells is available in the literature, we have used a transgenic murine line (Fsp1-GFP) to study Fsp1 expression in the developing and postnatal cardiac vasculature and endocardium. Our work shows that Fsp1 is expressed in the endocardium and mesenchyme of atrioventricular valve primordia, as well as in some coronary venous and lymphatic endothelial cells. Fsp1 expression in cardiac venous and lymphatic endothelium is progressively restricted to the leaflets of cardiac venous and lymphatic valves. Our results suggest that Fsp1 could play a role in the development of atrioventricular valves and participate in the patterning and morphogenesis of cardiac venous and lymphatic vessel valves.

Transient Expression of Lymphatic Markers in Retrobulbar Intraconal Orbital Vasculature During Fetal Development.

Purpose: The aim of this study is to investigate the presence of orbital lymphatic vessels during fetal and neonatal development and in adults using a panel of lymphatic markers. Methods: This was a retrospective observational case series. For analyzing lymphatic vessels, we used formalin-fixed paraffin-embedded enucleated eyes from 25 human fetuses between 13 and 24 weeks of gestation and postnatal eyes from 15 children and 5 adults. Immunohistochemical analysis of lymphatic vessels was performed for the markers: lymphatic vessel endothelial hyaluronic acid receptor-1 (LYVE-1), podoplanin (D2-40), Prospero-related homeobox gene-1 (Prox-1), pan-endothelial marker CD31, and blood vessel endothelium specific CD34. Results: Vasculature showing endothelial expression of LYVE-1, D2-40, Prox-1, and CD31 in combination with absence or weak expression of CD34, as would be expected for lymphatic vessels, was seen in 11 of 25 fetuses in an age range from 14 weeks to 23 weeks of gestation (44%). This lymphatic vascular staining pattern was also observed in 4 of 15 liveborn children (27%), all within 1 month of age, of which two were born prematurely at 32 and 34 weeks of gestation. Interestingly, an incomplete lymphatic staining pattern was observed in another 4 fetuses and two liveborn children of 4 months and 7 years old. No expression of lymphatic markers was observed in adult orbital vasculature. Conclusions: No retrobulbar intraorbital lymphatic vessels were observed in adults, however, we did observe transient expression of lymphatic markers in retrobulbar intraconal orbital vasculature during fetal and early neonatal development. The orbit may, therefore, be proposed to possess a full range of lymphatic plasticity.

Feed­back loops integrating RELA, SOX18 and FAK mediate the break­down of the lymph­endothelial barrier that is triggered by 12(S)­HETE.

Metastatic cancer cells cross endothelial barriers and travel through the blood or lymphatic fluid to pre­metastatic niches, leading to their colonisation. 'S' stereoisomer 12S­hydroxy­5Z,8Z,10E,14Z­eicosatetraenoic acid [12(S)­HETE] is secreted by a variety of cancer cell types and has been indicated to open up these barriers. In the present study, another aspect of the endothelial unlocking mechanism was elucidated. This was achieved by investigating 12(S)­HETE­treated lymph endothelial cells (LECs) with regard to their expression and mutual interaction with v­rel avian reticuloendotheliosis viral oncogene homolog A (RELA), intercellular adhesion molecule 1, SRY­box transcription factor 18 (SOX18), prospero homeobox 1 (PROX1) and focal adhesion kinase (FAK). These key players of LEC retraction, which is a prerequisite for cancer cell transit into vasculature, were analysed using western blot analysis, reverse transcription­quantitative PCR and transfection with small interfering (si)RNA. The silencing of a combination of these signalling and executing molecules using siRNA, or pharmacological inhibition with defactinib and Bay11­7082, extended the mono­culture experiments to co­culture settings using HCT116 colon cancer cell spheroids that were placed on top of LEC monolayers to measure their retraction using the validated 'circular chemorepellent­induced defect' assay. 12(S)­HETE was indicated to induce the upregulation of the RELA/SOX18 feedback loop causing the subsequent phosphorylation of FAK, which fed back to RELA/SOX18. Therefore, 12(S)­HETE was demonstrated to be associated with circuits involving RELA, SOX18 and FAK, which transduced signals causing the retraction of LECs. The FAK­inhibitor defactinib and the NF­κB inhibitor Bay11­7082 attenuated LEC retraction additively, which was similar to the suppression of FAK and PROX1 (the target of SOX18) by the transfection of respective siRNAs. FAK is an effector molecule at the distal end of a pro­metastatic signalling cascade. Therefore, targeting the endothelial­specific activity of FAK through the pathway demonstrated herein may provide a potential therapeutic method to combat cancer dissemination via vascular routes.

Sphingosine 1-phosphate-regulated transcriptomes in heterogenous arterial and lymphatic endothelium of the aorta.

Despite the medical importance of G protein-coupled receptors (GPCRs), in vivo cellular heterogeneity of GPCR signaling and downstream transcriptional responses are not understood. We report the comprehensive characterization of transcriptomes (bulk and single-cell) and chromatin domains regulated by sphingosine 1-phosphate receptor-1 (S1PR1) in adult mouse aortic endothelial cells. First, S1PR1 regulates NFκB and nuclear glucocorticoid receptor pathways to suppress inflammation-related mRNAs. Second, S1PR1 signaling in the heterogenous endothelial cell (EC) subtypes occurs at spatially-distinct areas of the aorta. For example, a transcriptomically distinct arterial EC population at vascular branch points (aEC1) exhibits ligand-independent S1PR1/ß-arrestin coupling. In contrast, circulatory S1P-dependent S1PR1/ß-arrestin coupling was observed in non-branch point aEC2 cells that exhibit an inflammatory gene expression signature. Moreover, S1P/S1PR1 signaling regulates the expression of lymphangiogenic and inflammation-related transcripts in an adventitial lymphatic EC (LEC) population in a ligand-dependent manner. These insights add resolution to existing concepts of endothelial heterogeneity, GPCR signaling and S1P biology.

The cortical actin network regulates avidity-dependent binding of hyaluronan by the lymphatic vessel endothelial receptor LYVE-1.

Lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1) mediates the docking and entry of dendritic cells to lymphatic vessels through selective adhesion to its ligand hyaluronan in the leukocyte surface glycocalyx. To bind hyaluronan efficiently, LYVE-1 must undergo surface clustering, a process that is induced efficiently by the large cross-linked assemblages of glycosaminoglycan present within leukocyte pericellular matrices but is induced poorly by the shorter polymer alone. These properties suggested that LYVE-1 may have limited mobility in the endothelial plasma membrane, but no biophysical investigation of these parameters has been carried out to date. Here, using super-resolution fluorescence microscopy and spectroscopy combined with biochemical analyses of the receptor in primary lymphatic endothelial cells, we provide the first evidence that LYVE-1 dynamics are indeed restricted by the submembranous actin network. We show that actin disruption not only increases LYVE-1 lateral diffusion but also enhances hyaluronan-binding activity. However, unlike the related leukocyte HA receptor CD44, which uses ERM and ankyrin motifs within its cytoplasmic tail to bind actin, LYVE-1 displays little if any direct interaction with actin, as determined by co-immunoprecipitation. Instead, as shown by super-resolution stimulated emission depletion microscopy in combination with fluorescence correlation spectroscopy, LYVE-1 diffusion is restricted by transient entrapment within submembranous actin corrals. These results point to an actin-mediated constraint on LYVE-1 clustering in lymphatic endothelium that tunes the receptor for selective engagement with hyaluronan assemblages in the glycocalyx that are large enough to cross-bridge the corral-bound LYVE-1 molecules and thereby facilitate leukocyte adhesion and transmigration.

Regulatory T Cells Condition Lymphatic Endothelia for Enhanced Transendothelial Migration.

Regulatory T cells (Tregs) express high levels of cell surface lymphotoxin alpha beta (LTα1ß2) to activate the LT beta receptor (LTßR) on the lymphatic endothelial cells (LECs), modulating LEC adhesion molecules, intercellular junctions, and chemokines. We demonstrate a role for Tregs through this pathway to condition the permissiveness of lymphatic endothelia for transendothelial migration (TEM), thus gating leukocyte traffic. Human Tregs share the same property with murine Tregs. Activation of TLR2 on Tregs during inflammation specifically augments LTα1ß2-LTßR signaling, which further enhances the permissiveness of LECs to facilitate TEM. The conditioning of endothelia may promote the resolution of inflammation by directing leukocytes out of tissues to lymphatic vessels and draining lymph nodes (dLNs). Thus, Tregs interact with lymphatic endothelia under homeostasis and inflammation and dictate endothelial permissiveness and gating mechanisms for subsequent leukocyte migration through endothelial barriers.

Brain-to-cervical lymph node signaling after stroke.

After stroke, peripheral immune cells are activated and these systemic responses may amplify brain damage, but how the injured brain sends out signals to trigger systemic inflammation remains unclear. Here we show that a brain-to-cervical lymph node (CLN) pathway is involved. In rats subjected to focal cerebral ischemia, lymphatic endothelial cells proliferate and macrophages are rapidly activated in CLNs within 24 h, in part via VEGF-C/VEGFR3 signalling. Microarray analyses of isolated lymphatic endothelium from CLNs of ischemic mice confirm the activation of transmembrane tyrosine kinase pathways. Blockade of VEGFR3 reduces lymphatic endothelial activation, decreases pro-inflammatory macrophages, and reduces brain infarction. In vitro, VEGF-C/VEGFR3 signalling in lymphatic endothelial cells enhances inflammatory responses in co-cultured macrophages. Lastly, surgical removal of CLNs in mice significantly reduces infarction after focal cerebral ischemia. These findings suggest that modulating the brain-to-CLN pathway may offer therapeutic opportunities to ameliorate systemic inflammation and brain injury after stroke.

Paraxial Mesoderm Is the Major Source of Lymphatic Endothelium.

Endothelial cells (ECs), which line blood and lymphatic vessels, are generally described to come from the lateral plate mesoderm despite experimental evidence for a broader source of origin, including the paraxial mesoderm (PXM). Current dogma suggests that following specification from mesoderm, local environmental cues establish the distinct molecular and functional characteristics of ECs in different vascular beds. Here we present evidence to challenge this view, showing that lymphatic EC fate is imprinted during transition through the PXM lineage. We show that PXM-derived cells form the lymphatic endothelium of multiple organs and tissues, with a more restricted contribution to blood vessel endothelium. By deleting Prox1 specifically in PXM-derived cells, we show that this lineage is indispensable for lymphatic vessel development. Collectively, our data establish lineage history as a critical determinant of EC specialization, a finding with broad implications for our understanding of vascular development and heterogeneity.