Update June 2016

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.

Lymphangiogenesis is supported by 2 homologous VEGFR3 ligands, VEGFC and VEGFD. VEGFC is required for lymphatic development, while VEGFD is not. VEGFC and VEGFD are proteolytically cleaved after cell secretion in vitro, and recent studies have implicated the protease a disintegrin and metalloproteinase with thrombospondin motifs 3 (ADAMTS3) and the secreted factor collagen and calcium binding EGF domains 1 (CCBE1) in this process. It is not well understood how ligand proteolysis is controlled at the molecular level or how this process regulates lymphangiogenesis, because these complex molecular interactions have been difficult to follow ex vivo and test in vivo. Here, we have developed and used biochemical and cellular tools to demonstrate that an ADAMTS3-CCBE1 complex can form independently of VEGFR3 and is required to convert VEGFC, but not VEGFD, into an active ligand. Consistent with these ex vivo findings, mouse genetic studies revealed that ADAMTS3 is required for lymphatic development in a manner that is identical to the requirement of VEGFC and CCBE1 for lymphatic development. Moreover, CCBE1 was required for in vivo lymphangiogenesis stimulated by VEGFC but not VEGFD. Together, these studies reveal that lymphangiogenesis is regulated by two distinct proteolytic mechanisms of ligand activation: one in which VEGFC activation by ADAMTS3 and CCBE1 spatially and temporally patterns developing lymphatics, and one in which VEGFD activation by a distinct proteolytic mechanism may be stimulated during inflammatory lymphatic growth.

Lymphangiogenesis plays a pivotal role in diverse pathological conditions. Here, we demonstrate that a carbohydrate-binding protein, galectin-8, promotes pathological lymphangiogenesis. Galectin-8 is markedly upregulated in inflamed human and mouse corneas, and galectin-8 inhibitors reduce inflammatory lymphangiogenesis. In the mouse model of corneal allogeneic transplantation, galectin-8-induced lymphangiogenesis is associated with an increased rate of corneal graft rejection. Further, in the murine model of herpes simplex virus keratitis, corneal pathology and lymphangiogenesis are ameliorated in Lgals8(−/−) mice. Mechanistically, VEGF-C-induced lymphangiogenesis is significantly reduced in the Lgals8(−/−) and Pdpn(−/−) mice; likewise, galectin-8-induced lymphangiogenesis is reduced in Pdpn(−/−) mice. Interestingly, knockdown of VEGFR-3 does not affect galectin-8-mediated lymphatic endothelial cell (LEC) sprouting. Instead, inhibiting integrins alpha1beta1 and alpha5beta1 curtails both galectin-8- and VEGF-C-mediated LEC sprouting. Together, this study uncovers a unique molecular mechanism of lymphangiogenesis in which galectin-8-dependent crosstalk among VEGF-C, podoplanin and integrin pathways plays a key role.

In extrapulmonary tuberculosis, the most common site of infection is within the lymphatic system, and there is growing recognition that lymphatic endothelial cells (LECs) are involved in immune function. Here, we identified LECs, which line the lymphatic vessels, as a niche for Mycobacterium tuberculosis in the lymph nodes of patients with tuberculosis. In cultured primary human LECs (hLECs), we determined that M. tuberculosis replicates both in the cytosol and within autophagosomes, but the bacteria failed to replicate when the virulence locus RD1 was deleted. Activation by IFN-gamma induced a cell-autonomous response in hLECs via autophagy and NO production that restricted M. tuberculosis growth. Thus, depending on the activation status of LECs, autophagy can both promote and restrict replication. Together, these findings reveal a previously unrecognized role for hLECs and autophagy in tuberculosis pathogenesis and suggest that hLECs are a potential niche for M. tuberculosis that allows establishment of persistent infection in lymph nodes.

The possibility of formation of lymphatic vessels after introduction of autologous bone marrow-derived multipotent mesenchymal stromal cells transfected with GFP gene into thrombosed femoral vein was studied by fluorescent microscopy. Vascular thrombosis caused by ligation of the great vein with subsequent injection of thrombin solution was accompanied by blockade of regional lymph flow. The cells injected into thrombosed vein directly participate in the formation of new lymphatic vessels in the paravasal tissue surrounding the vein, its tissue region, and around regional lymph nodes. This is seen from bright specific fluorescence of individual cells in the walls of lymphatic vessels and all vascular layers and valves in UV light.

Venous valves (VVs) are critical for unidirectional blood flow from superficial and deep veins towards the heart. Congenital valve aplasia or agenesis may, in some cases, be a direct cause of vascular disease, motivating an understanding of the molecular mechanisms underlying the development and maintenance of VVs. Three gap junction proteins (Connexins), Cx37, Cx43, and Cx47, are specifically expressed at VVs in a highly polarized fashion. VVs are absent from adult mice lacking Cx37; however it is not known if Cx37 is required for the initial formation of valves. In addition, the requirement of Cx43 and Cx47 for VV development has not been studied. Here, we provide a detailed description of Cx37, Cx43, and Cx47 expression during mouse vein development and show by gene knockout that each Cx is necessary for normal valve development. The valve phenotypes in the knockout lines exhibit Cx-specific differences, however, including whether peripheral or central VVs are affected by gene inactivation. In addition, we show that a Cx47 null mutation impairs peripheral VV development but does not affect lymphatic valve formation, a finding of significance for understanding how some CX47 mutations cause inherited lymphedema in humans. Finally, we demonstrate a striking segregation of Foxc2 and NFATc1 transcription factor expression between the downstream and upstream faces, respectively, of developing VV leaflets and show that this segregation is closely associated with the highly polarized expression of Cx37, Cx43, and Cx47. The partition of Foxc2 and NFATc1 expression at VV leaflets makes it unlikely that these factors directly cooperate during the leaflet elongation stage of VV development.

Lymphedema is a debilitating progressive condition that severely restricts quality of life and is frequently observed after cancer surgery. The mechanism underlying lymphedema development remains poorly understood, and no effective pharmacological means to prevent or alleviate the ailment is currently available. Using a mouse model of lymphedema, we show here that excessive generation of immature lymphatic vessels is essential for initial edema development and that this early process is also important for later development of lymphedema pathology. We found that CD4(+) T cells interact with macrophages to promote lymphangiogenesis, and that both lymphangiogenesis and edema were greatly reduced in macrophage-depleted mice, lymphocyte-deficient Rag2(?/?) mice or CD4(+) T-cell-deficient mice. Mechanistically, T helper type 1 and T helper type 17 cells activate lesional macrophages to produce vascular endothelial growth factor-C, which promotes lymphangiogenesis, and inhibition of this mechanism suppressed not only early lymphangiogenesis, but also later development of lymphedema. Finally, we show that atorvastatin suppresses excessive lymphangiogenesis and lymphedema by inhibiting T helper type 1 and T helper type 17 cell activation. These results demonstrate that the interaction between CD4(+) T cells and macrophages is a potential therapeutic target for prevention of lymphedema after surgery.

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 it is involved in the regulation of lymphatic vessel functions: accordingly, Emilin1−/− mice display lymphatic vascular morphological alterations leading to functional defects such as mild lymphedema, lymph leakage, and compromised lymph drainage. In this study, using a mouse post-surgical tail lymphedema model, we show that the acute phase of acquired lymphedema is correlated to EMILIN1 degradation due to elastase (NE) released by infiltrating neutrophils. As a consequence the intercellular junctions of lymphatic endothelial cells are weakened and draining to regional lymph nodes is severely affected. The local administration of Sivelestat, a specific NE inhibitor, prevents EMILIN1 degradation and reduces lymphedema, restoring a normal lymphatic functionality. The finding that also in human secondary lymphedema samples we detected cleaved EMILIN1 with the typical bands of NE-dependent pattern of fragmentation establishes a rationale for a powerful strategy that targets NE inhibition. In conclusion, the attempts to locally block EMILIN1 degradation represent the basis for a novel “ECM” pharmacological approach to assess new lymphedema treatments.

The molecular mechanisms underlying lymphatic vascular development and function are not well understood. Recent studies have suggested a role for endothelial cell (EC) Mitogen activated protein kinase kinase kinase kinase 4 (Map4k4) in developmental angiogenesis and atherosclerosis. Here, we show that constitutive loss of EC Map4k4 in mice causes postnatal lethality due to chylothorax, suggesting that Map4k4 is required for normal lymphatic vascular function. Mice constitutively lacking EC Map4k4 displayed dilated lymphatic capillaries, insufficient lymphatic valves, and impaired lymphatic flow; furthermore, primary ECs derived from these animals displayed enhanced proliferation compared with controls. Yeast 2 hybrid analyses identified the Ras GTPase activating protein Rasa1, a known regulator of lymphatic development and lymphatic endothelial cell fate, as a direct interacting partner for Map4k4. Map4k4 silencing in ECs enhanced basal Ras and Erk activities, and primary ECs lacking Map4k4 displayed enhanced lymphatic EC marker expression. Taken together, these results reveal that EC Map4k4 is critical for lymphatic vascular development by regulating EC quiescence and lymphatic EC fate.

It is a common complication to develop a secondary lymphedema after surgery or radiation, for example after axillary lymph node dissection due to breast cancer and current therapies are mainly symptomatic. Since these surgical procedures result in both, loss of adipose tissue and loss of lymphatic nodes and vessels, tissue engineering could be a new promising approach, to create an adipose tissue substitute comprised with a lymphatic network. We have conducted co-culture experiments to investigate the effects of human adipose-derived stem cells (ASCs) on human lymphatic endothelial cells (LECs) in terms of gene expression profile, proliferation, migration, and tube formation in vitro. In this respect, both cell types were co-cultured either indirectly or directly with or without the recombinant growth factor VEGF-C. Indirect co-cultures were performed with the aid of a transwell chamber. In case of direct co-culture, immunomagnetic separation by CD31 magnetic beads allowed examination of the LEC population. Direct and indirect co-culture of ASCs induced mRNA expression of lymphatic marker genes, proliferation and migration by LECs without affecting tube formation. Thus, we have shown that co-culture of ASCs with LECs might be a feasible approach that could be used in cell-based tissue engineering therapies to heal or improve a secondary lymphedema. This article is protected by copyright. All rights reserved.

Adaptive immune response consists of many highly regulated, multistep cascades that protect against infection while preserving the health of autologous tissue. The proper initiation, maintenance, and resolution of such responses require the precise coordination of molecular and cellular signaling over multiple time and length scales orchestrated by lymphatic transport. In order to investigate these functions and manipulate them for therapy, a comprehensive understanding of how lymphatics influence immune physiology is needed. This review presents the current mechanistic understanding of the role of the lymphatic vasculature in regulating biomolecule and cellular transport from the interstitium, peripheral tissue immune surveillance, the lymph node stroma and microvasculature, and circulating lymphocyte homing to lymph nodes. This review also discusses the ramifications of lymphatic transport in immunity as well as tolerance and concludes with examples of how lymphatic-mediated targeting of lymph nodes has been exploited for immunotherapy applications. Expected final online publication date for the Annual Review of Biomedical Engineering Volume 18 is July 11, 2016. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.

Lymphangiogensis is involved in various pathological conditions, such as arthritis and cancer metastasis. Although many factors have been identified to stimulate lymphatic vessel growth, little is known about lymphangiogenesis inhibitors. Here we report that membrane type 1-matrix metalloproteinase (MT1-MMP) is an endogenous suppressor of lymphatic vessel growth. MT1-MMP-deficient mice exhibit spontaneous corneal lymphangiogenesis without concomitant changes in angiogenesis. Mice lacking MT1-MMP in either lymphatic endothelial cells or macrophages recapitulate corneal lymphangiogenic phenotypes observed in Mmp14(−/−) mice, suggesting that the spontaneous lymphangiogenesis is both lymphatic endothelial cells autonomous and macrophage associated. Mechanistically, MT1-MMP directly cleaves LYVE-1 on lymphatic endothelial cells to inhibit LYVE-1-mediated lymphangiogenic responses. In addition, MT1-MMP-mediated PI3Kdelta signalling restrains the production of VEGF-C from prolymphangiogenic macrophages through repressing the activation of NF-kappaB signalling. Thus, we identify MT1-MMP as an endogenous inhibitor of physiological lymphangiogenesis.

The role of lymphatics in atherosclerosis is not yet understood. Here, we investigate lymphatic growth dynamics and marker expression in atherosclerosis in apolipoprotein E-deficient (apoE−/−) mice. The prolymphangiogenic growth factor, VEGF-C, was elevated in atherosclerotic aortic walls. Despite increased VEGF-C, we found that adventitial lymphatics regress during the course of formation of atherosclerosis (P < 0.01). Similar to lymphatic regression, the number of lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1+) macrophages decreased in the aortic adventitia ofapoE−/−mice with atherosclerosis (P < 0.01). Intimal lymphatics in the atherosclerotic lesions exhibited an atypical phenotype, with the expression of podoplanin and VEGF receptor 3 (VEGFR-3) but not of LYVE-1 and prospero homeobox protein 1. In the aortas of atherosclerotic animals, we found markedly increased soluble VEGFR-2. We hypothesized that the elevated soluble VEGFR-2 that was found in the aortas ofapoE−/−mice with atherosclerosis binds to and diminishes the activity of VEGF-C. This trapping mechanism explains, despite increased VEGF-C in the atherosclerotic aortas, how adventitial lymphatics regress. Lymphatic regression impedes the drainage of lipids, growth factors, inflammatory cytokines, and immune cells. Insufficient lymphatic drainage could thus exacerbate atherosclerosis formation. Our study contributes new insights to previously unknown dynamic changes of adventitial lymphatics. Targeting soluble VEGFR-2 in atherosclerosis may provide a new strategy for the liberation of endogenous VEGF-C and the prevention of lymphatic regression.-Taher, M., Nakao, S., Zandi, S., Melhorn, M. I., Hayes, K. C., Hafezi-Moghadam, A. Phenotypic transformation of intimal and adventitial lymphatics in atherosclerosis: a regulatory role for soluble VEGF receptor 2.

We previously documented a girl with macrothrombocytopenia and developmental delay who carried a de novo mutation in CDC42, which plays pivotal roles in the cell cycle and the formation of the actin cytoskeleton. The phenotype of mice lacking Cdc42 was strikingly similar to that of the reported patient, indicating that the mutation in CDC42 causes a new syndromic form of thrombocytopenia. We, herein, report another unrelated female patient with a similar phenotype and a de novo mutation in the same CDC42. The present observation provides further evidence supporting the notion that a mutation in CDC42 causes a recognizable syndromic form of thrombocytopenia. The cardinal features of this entity include macrothrombocytopenia, developmental delay, lymphedema in the lower extremities, camptodactyly, and distinctive facial features. (c) 2015 Wiley Periodicals, Inc.