Blood vessel endothelial VEGFR-2 delays lymphangiogenesis: an endogenous trapping mechanism links lymph- and angiogenesis

A potential early-atheroprotective target: Irgm1 mediates lymphangiogenesis through LEC autophagy by Tfeb translocation

Lymphatic dysfunction is a pivotal pathological mechanism underlying the development of early atherosclerotic plaques. Potential targets of lymphatic function must be identified to realize the early prevention and treatment of atherosclerosis (AS). The immunity-related GTPase Irgm1 is involved in orchestrating cellular autophagy and apoptosis. However, the effect of Irgm1 on early AS progression, particularly through alterations in lymphatic function, remains unclear. In this study, we confirmed the protective effect of lymphangiogenesis on early-AS in vivo. Subsequently, an in vivo model of early AS mice with Irgm1 knockdown shows that Irgm1 reduces early atherosclerotic plaque burden by promoting lymphangiogenesis. Given that lymphatic endothelial cell (LEC) autophagy significantly contributes to lymphangiogenesis, Irgm1 may enhance lymphatic circulation by promoting LEC autophagy. Moreover, Irgm1 orchestrates autophagy in LECs by inhibiting mTOR and facilitating nuclear translocation of Tfeb. Collectively, these processes lead to lymphangiogenesis. Thus, this study establishes a link between Irgm1 and early AS, thus revealing a novel mechanism by which Irgm1 exerts an early protective influence on AS within the context of lymphatic circulation. The insights gained from this study have the potential to revolutionize the approach and management of AS onset.

Lymphangiogenesis Guidance Mechanisms and Therapeutic Implications in Pathological States of the Cornea

Corneal lymphangiogenesis is one component of the neovascularization observed in several inflammatory pathologies of the cornea including dry eye disease and corneal graft rejection. Following injury, corneal (lymph)angiogenic privilege is impaired, allowing ingrowth of blood and lymphatic vessels into the previously avascular cornea. While the mechanisms underlying pathological corneal hemangiogenesis have been well described, knowledge of the lymphangiogenesis guidance mechanisms in the cornea is relatively scarce. Various signaling pathways are involved in lymphangiogenesis guidance in general, each influencing one or multiple stages of lymphatic vessel development. Most endogenous factors that guide corneal lymphatic vessel growth or regression act via the vascular endothelial growth factor C signaling pathway, a central regulator of lymphangiogenesis. Several exogenous factors have recently been repurposed and shown to regulate corneal lymphangiogenesis, uncovering unique signaling pathways not previously known to influence lymphatic vessel guidance. A strong understanding of the relevant lymphangiogenesis guidance mechanisms can facilitate the development of targeted anti-lymphangiogenic therapeutics for corneal pathologies. In this review, we examine the current knowledge of lymphatic guidance cues, their regulation of inflammatory states in the cornea, and recently discovered anti-lymphangiogenic therapeutic modalities.

Association between the VEGFR-2 -604T/C polymorphism (rs2071559) and type 2 diabetic retinopathy

This retrospective case-control study examined the association between the rs2071559 (-604T/C) single nucleotide polymorphism (SNP) in the vascular endothelial growth factor receptor (VEGFR)-2 gene and the risk of diabetic retinopathy (DR) in Northern Han Chinese. This study included patients diagnosed with diabetes mellitus (DM) in Shijiazhuang between 07/2014 and 07/2016. The healthy controls were unrelated individuals who received routine physical examinations. The diabetic patients were grouped as DM (diabetes but no fundus examination abnormalities), proliferative DR (PDR), and non-proliferative DR (NPDR). Finally, 438 patients were included: 114 controls and 123, 105, and 96 patients in the DM, NPDR, and PDR groups, respectively. In the multivariable analyses and all genetic models, the VEGFR-2 rs2071559 SNP was not associated with DR (among all diabetic patients) or with PDR (among the patients with DR) after adjustment for age, sex, duration of DM, blood glucose, systolic blood pressure, diastolic blood pressure, and body mass index (all P > 0.05). In conclusion, the VEGFR-2- 604T/C rs2071559 SNP is not associated with DR or PDR in the Han Chinese population of Shijiazhuang (China).

Low expression of RBP4 in the vitreous humour of patients with proliferative diabetic retinopathy who underwent Conbercept intravitreal injection

Intravitreal injection of anti-VEGF antibodies has been widely used in the treatment of proliferative diabetic retinopathy (PDR). However, anti-VEGF drugs can exacerbate fibrosis and eventually lead to retinal detachment. To explore proteins closely related to fibrosis, we conducted proteomic analysis of human vitreous humour collected from PDR patients who have or have not intravitreal Conbercept (IVC) injection. Sixteen vitreous humour samples from PDR patients with preoperative IVC and 20 samples from those without preoperative IVC were examined. An immunodepletion kit was used to remove high-abundance vitreous proteins. Conbercept-induced changes were determined using a tandem mass tag-based quantitative proteomic strategy. Enzyme-linked immunosorbent assays were performed to confirm the concentrations of selected proteins and validate the proteomic results. Based on a false discovery rate between 0.05% and -0.05% and a fold-change > 1.5, 97 proteins were altered (49 higher levels and 48 lower levels) in response to IVC. Differentially expressed proteins were found in the extracellular and intracellular regions and were found to be involved in VEGF binding and VEGF-activated receptor activity. Protein-protein interactions indicated associations with fibrosis, neovascularisation and inflammatory signalling pathways. We found the low levels of RBP4 in the vitreous humour of PDR patients with IVC injection, as revealed by ELISA and proteomic profiling. Moreover, RBP4 significantly restored the mitochondrial function of HRMECs induced by AGEs and down regulated the level of glycolysis. Our study is the first to report that RBP4 decreases in the vitreous humour of PDR patients who underwent Conbercept treatment, thereby verifying the role of RBP4 in glucose metabolism. Results provide evidence for the potential mechanism underlying Conbercept-related fibrosis.

ANGPTL2 Promotes Inflammation via Integrin α5β1 in Chondrocytes

BACKGROUND

Angiopoietin-like protein 2 (ANGPTL2) is a secreted molecule with numerous physiologic and pathologic functions, for example, in angiogenesis, hematopoiesis, and tumorigenesis. Although recent studies implicated ANGPTL2 in chronic inflammation in mouse peritoneal macrophages, human ligamentum flavum fibroblasts, and human retinal microvascular endothelial cells, the mechanism underlying ANGPTL2-associated inflammation in chondrocytes remains unclear. Therefore, it was investigated whether ANGPTL2 is expressed in or functions in chondrocytes.

METHODS

Expression of ANGPTL2 and its receptor, integrin α5β1 were examined over time in ATDC5 cells using real-time RT-PCR (reverse transcription-polymerase chain reaction) analysis. ATDC5 cells were then incubated with or without ANGPTL2 for 3 hours, and expression of the IL-1β, TNF-α, COX-2, aggrecanase (ADAMTS)-5, matrix metalloproteinase (MMP)-3, and MMP-13 genes were examined using real-time RT-PCR. Additionally, phosphorylation of ERK, JNK, p38, Akt, and NF-κB was examined by western blotting. Furthermore, it was also investigated for the effect of anti-integrin α₅β₁ antibody on the expression of inflammatory markers and intracellular signaling pathways.

RESULTS

ANGPTL2 induced the phosphorylation of all 3 MAPKs, Akt, and NF-κB and dramatically upregulated the expression of inflammation-related factor genes. Inhibiting the activation of integrin α5β1 suppressed these reactions.

CONCLUSION

ANGPTL2 may induce inflammatory factors by stimulating the integrin α5β1/MAPKs, Akt, and NF-κB signaling pathway.

Retinal VEGF-A Overexpression Is Not Sufficient to Induce Lymphangiogenesis Regardless of VEGF-C Upregulation and Lyve1+ Macrophage Infiltration

Purpose

No lymphatic vessels have been identified in the retina. This study investigated whether pathological VEGF-A-overexpressing diabetic retina causes lymphangiogenesis.

Methods

Three genetic mouse models of diabetic retinopathy (DR) (Akita [Ins2+/-], Kimba [vegfa+/+], and Akimba [Akita × Kimba] mice) were used. Retinas were examined by fundus photography, fluorescence angiography (FA), and immunostaining to detect lymphangiogenesis or angiogenesis. Lyve1-GFP (Lyve1EGFP/Cre) mice were used to examine Lyve1-expressing cells by immunostaining. Lymphatic-related factors were investigated in mouse retina and vitreous fluid from proliferative diabetic retinopathy (PDR) patients by RT-PCR and ELISA, respectively. Aged Kimba and Akimba mice were used to examine the retinal phenotype at the late phase of VEGF overexpression.

Results

FA and immunostaining showed retinal neovascularization in Kimba and Akimba mice but not wild-type and Akita mice. Immunohistochemistry showed that lymphangiogenesis was not present in the retinas of Akita, Kimba, or Akimba mice despite the significant upregulation of lymphatic-related factors (Lyve1, podoplanin, VEGF-A, VEGF-C, VEGF-D, VEGFR2, and VEGFR3) in the retinas of Kimba and Akimba mice by RT-PCR (P < 0.005). Furthermore, lymphangiogenesis was not present in aged Kimba or Akimba mice. Significantly increased numbers of Lyve1-positive cells present in the retinas of Kimba and Akimba mice, especially in the peripheral areas, were CD11b positive, indicating a macrophage population (P < 0.005). VEGF-C in PDR vitreous with vitreous hemorrhage (VH) was higher than in PDR without VH or a macular hole.

Conclusions

Retinal VEGF-A overexpression did not cause typical lymphangiogenesis despite upregulated lymphatic-related factors and significant Lyve1-positive macrophage infiltration.

Role of R-spondin 2 in arterial lymphangiogenesis and atherosclerosis

AIMS

Impaired lymphatic drainage of the arterial wall results in intimal lipid accumulation and atherosclerosis. However, the mechanisms regulating lymphangiogenesis in atherosclerotic arteries are not well understood. Our studies identified elevated levels of matrix protein R-spondin 2 (RSPO2) in atherosclerotic arteries. In this study, we investigated the role of RSPO2 in lymphangiogenesis, arterial cholesterol efflux into lesion-draining lymph nodes (LNs) and development of atherosclerosis.

METHODS AND RESULTS

The effect of RSPO2 on lymphangiogenesis was investigated using human lymphatic endothelial cells (LEC) in vitro and implanted Matrigel plugs in vivo. Cellular and molecular approaches, pharmacological agents, and siRNA silencing of RSPO2 receptor LGR4 were used to investigate RSPO2-mediated signalling in LEC. In vivo low-density lipoprotein (LDL) tracking and perivascular blockade of RSPO2-LGR4 signalling using LGR4-extracellular domain (ECD) pluronic gel in hypercholesterolemic mice were utilized to investigate the role of RSPO2 in arterial reverse cholesterol transport and atherosclerosis. Immunoblotting and imaging experiments demonstrated increased RSPO2 expression in human and mouse atherosclerotic arteries compared to non-atherosclerotic controls. RSPO2 treatment inhibited lymphangiogenesis both in vitro and in vivo. LGR4 silencing and inhibition of RSPO2-LGR4 signalling abrogated RSPO2-induced inhibition of lymphangiogenesis. Mechanistically, we found that RSPO2 suppresses PI3K-AKT-endothelial nitric oxide synthase (eNOS) signalling via LGR4 and inhibits activation of the canonical Wnt-β-catenin pathway. ApoE-/- mice treated with LGR4-ECD developed significantly less atherosclerosis compared with control treatment. Finally, increased arterial lymphatic vessel density and improved lymphatic drainage of fluorescently labelled LDL to deep cervical LNs were observed in LGR4-ECD-treated mice.

CONCLUSION

These findings demonstrate that RSPO2 inhibits lymphangiogenesis via LGR4 and downstream impairment of AKT-eNOS-nitric oxide signalling. These results may also inform new therapeutic strategies to promote lymphangiogenesis and improve cholesterol efflux from atherosclerotic arteries.

Quantification of Angiogenesis and Lymphangiogenesis in the Dual ex vivo Aortic and Thoracic Duct Assay

Abstract: Background

Lymphatic vessel formation (lymphangiogenesis) plays important roles in cancer metastasis, organ rejection, and lymphedema, but the underlying molecular events remain unclear. Furthermore, despite significant overlap in the molecular families involved in angiogenesis and lymphangiogenesis, little is known about the crosstalk between these processes. The ex vivo aortic ring assay and lymphatic ring assay have enabled detailed studies of vessel sprouting, but harvesting and imaging clear thoracic duct samples remain challenging. Here we present a modified ex vivo dual aortic ring and thoracic duct assay using tissues from dual fluorescence reporter Prox1-GFP/Flt1-DsRed (PGFD) mice, which permit simultaneous visualization of blood and lymphatic endothelial cells.

Objective

To characterize the concurrent sprouting of intrinsically fluorescent blood and lymphatic vessels from harvested aorta and thoracic duct samples.

Methods

Dual aorta and thoracic duct specimens were harvested from PGFD mice, grown in six types of endothelial cell growth media (one control, five that each lack a specific growth factor), and visualized by confocal fluorescence microscopy. Linear mixed models were used to compare the extent of vessel growth and sprouting over a 28-day period.

Results

Angiogenesis occurred prior to lymphangiogenesis in our assay. The control medium generally induced superior growth of both vessel types compared with the different modified media formulations. The greatest decrease in lymphangiogenesis was observed in vascular endothelial growth factor-C (VEGF-C)-devoid medium, suggesting the importance of VEGF-C in lymphangiogenesis.

Conclusion

The modified ex vivo dual aortic ring and thoracic duct assay represents a powerful tool for studying angiogenesis and lymphangiogenesis in concert.

The lymphangiogenic and hemangiogenic privilege of the human sclera

PURPOSE

Most organs of the human body are supplied with a dense network of blood and lymphatic vessels. However, some tissues are either hypovascular or completely devoid of vessels for proper function, such as the ocular tissues sclera and cornea, cartilage and tendons. Since many pathological conditions are affecting the human sclera, this review is focussing on the lymphangiogenic and hemangiogenic privilege in the human sclera.

METHODS

This article gives an overview of the current literature based on a PubMed search as well as observations and experience from clinical practice.

RESULTS

The healthy human sclera is the outer covering layer of the eye globe consisting mainly of collagenous extracellular matrix and fibroblasts. Physiologically, the sclera shows only a superficial network of blood vessels and a lack of lymphatic vessels. This vascular privilege is actively regulated by balancing anti- and proangiogenic factors expressed by cells within the sclera. In pathological situations, such as open globe injuries or ciliary body melanomas with extraocular extension, lymphatic vessels can secondarily invade the sclera and the inner eye. This mechanism most likely is important for tumor cell metastasis, wound healing, immunologic defense against intruding microorganism, and autoimmune reactions against intraocular antigens.

CONCLUSIONS

The human sclera is characterized by a tightly regulated vascular network that can be compromised in pathological situations, such as injuries or intraocular tumors affecting healing outcomes Therefore, the molecular and cellular mechanisms underlying wound healing following surgical interventions deserve further attention, in order to devise more effective therapeutic strategies.

Innate immune response in retinal homeostasis and inflammatory disorders

Innate immune cells such as neutrophils, monocyte-macrophages and microglial cells are pivotal for the health and disease of the retina. For the maintenance of retinal homeostasis, these cells and immunosuppressive molecules in the eye actively regulate the induction and the expression of inflammation in order to prevent excessive activation and subsequent tissue damage. In the disease context, these regulatory mechanisms are modulated genetically and/or by environmental stimuli such as damage-associated molecular patterns (DAMPs), and a chronic innate immune response regulates or contributes to the formation of diverse retinal disorders such as uveitis, retinitis pigmentosa, retinal vascular diseases and retinal fibrosis. Here we summarize the recent knowledge regarding the innate immune response in both ocular immune regulation and inflammatory retinal diseases, and we describe the potential of the innate immune response as a biomarker and therapeutic target.

Notch Signaling Pathway Is Involved in bFGF-Induced Corneal Lymphangiogenesis and Hemangiogenesis

Background

Notch/Dll4 involvement in cornea neovascularization (CRNV) and lymphangiogenesis is unclear. This study aimed to explore the role of notch signaling in basic fibroblast growth factor- (bFGF-) induced corneal lymphangiogenesis and hemangiogenesis.

Methods

Corneal stroma of C57BL/6 mice was implanted with bFGF- or phosphate-buffered saline- (PBS-) soaked pellets. Corneal lymphangiogenesis and neovascularization were evaluated by immunofluorescence. Vascular endothelial growth factor-A (VEGF-A), Delta-like ligand 4 (Dll4), and Notch1 mRNA and protein expression were examined on days 1, 3, 7, and 14 by real-time polymerase chain reaction and western blot. Corneal cells were treated with ranibizumab, dexamethasone, and γ-secretase inhibitor (GSI). Microspheres were used to evaluate corneal hemangiogenesis in vivo.

Results

Corneal hemangiogenesis reached its peak on day 7 after bFGF implantation, and corneal lymphangiogenesis was significantly higher on day 7 and 14, compared with PBS. mRNA and protein expression of VEGF-A, Dll4, and Notch1 were higher in bFGF-induced animal models compared with controls. Corneal hemangiogenesis and lymphangiogenesis decreased after 7 days of ranibizumab or dexamethasone treatment. After adding GSI for 24 h in bFGF-induced cells, the expression of Notch1 and Dll4 were downregulated compared with that in the control group whereas the expression level of VEGF-A was upregulated. Fluorescent particle number was higher in the GSI group. Ranibizumab and dexamethasone decreased the fluorescence signal.

Conclusion

The notch signaling pathway plays a role in regulating VEGF expression, affecting corneal lymphangiogenesis and hemangiogenesis in mice. The molecular imaging probe technique can visualize the changes in the VEGF-A expression level of corneal limbus hemangiogenesis.

Human organotypic lymphatic vessel model elucidates microenvironment-dependent signaling and barrier function

The lymphatic system is an active player in the pathogenesis of several human diseases, including lymphedema and cancer. Relevant models are needed to advance our understanding of lymphatic biology in disease progression to improve therapy and patient outcomes. Currently, there are few 3D in vitro lymphatic models that can recapitulate the physiological structure, function, and interactions of lymphatic vessels in normal and diseased microenvironments. Here, we developed a 3D microscale lymphatic vessel (μLYMPH) system for generating human lymphatic vessels with physiological tubular structure and function. Consistent with characteristics of lymphatic vessels in vivo, the endothelium of cultured vessels was leaky with an average permeability of 1.38 × 10^-5 ± 0.29 × 10^-5 cm/s as compared to 0.68 × 10^-5 ± 0.13 × 10^-5 cm/s for blood vessels. This leakiness also resulted in higher uptake of solute by the lymphatic vessels under interstitial flow, demonstrating recapitulation of their natural draining function. The vessels secreted appropriate growth factors and inflammatory mediators. Our system identified the follistatin/activin axis as a novel pathway in lymphatic vessel maintenance and inflammation. Moreover, the μLYMPH system provided a platform for examining crosstalk between lymphatic vessels and tumor microenvironmental components, such as breast cancer-associated fibroblasts (CAFs). In co-culture with CAFs, vessel barrier function was significantly impaired by CAF-secreted IL-6, a possible pro-metastatic mechanism of lymphatic metastasis. Targeted blocking of the IL-6/IL-6R signaling pathway with an IL-6 neutralizing antibody fully rescued the vessels, demonstrating the potential of our system for screening therapeutic targets. These results collectively demonstrate the μLYMPH system as a powerful model for advancing lymphatic biology in health and disease.

Corneal lymphangiogenesis ameliorates corneal inflammation and edema in late stage of bacterial keratitis

Lymphatic vessels play a crucial role in systemic immune response and regulation of tissue fluid homeostasis. Corneal lymphangiogenesis in bacterial keratitis has not been studied. In this study, we investigated the mechanism and the role of corneal lymphangiogenesis in a murine bacterial keratitis model using Pseudomonas aeruginosa. We first demonstrated that corneal lymphangiogenesis was enhanced mainly in the late stage of bacterial keratitis, contrary to corneal angiogenesis that started earlier. Corresponding to the delayed lymphangiogenesis, expression of the pro-lymphangiogenic factors VEGF-C and VEGFR-3 increased in the late stage of bacterial keratitis. We further found that F4/80 and CD11b positive macrophages played an essential role in corneal lymphangiogenesis. Notably, macrophages were specifically involved in corneal lymphangiogenesis in the late stage of bacterial keratitis. Finally, we demonstrated the beneficial role of corneal lymphangiogenesis in ameliorating the clinical course of bacterial keratitis. Our study showed that bacterial activity was not directly involved in the late stage of keratitis, while corneal lymphangiogenesis reduced corneal edema and clinical manifestation in the late stage of bacterial keratitis. These findings suggest that the process of lymphangiogenesis in bacterial keratitis ameliorates corneal inflammation and edema in the late stage of bacterial keratitis.

Lymphatic Vessel Network Structure and Physiology

The lymphatic system is comprised of a network of vessels interrelated with lymphoid tissue, which has the holistic function to maintain the local physiologic environment for every cell in all tissues of the body. The lymphatic system maintains extracellular fluid homeostasis favorable for optimal tissue function, removing substances that arise due to metabolism or cell death, and optimizing immunity against bacteria, viruses, parasites, and other antigens. This article provides a comprehensive review of important findings over the past century along with recent advances in the understanding of the anatomy and physiology of lymphatic vessels, including tissue/organ specificity, development, mechanisms of lymph formation and transport, lymphangiogenesis, and the roles of lymphatics in disease. © 2019 American Physiological Society. Compr Physiol 9:207-299, 2019.

Vascular endothelial growth factors C and D may promote angiogenesis in the primate ovulatory follicle

Angiogenesis in the ovary occurs rapidly as the ovarian follicle transforms into a mature corpus luteum. Granulosa cells produce vascular endothelial growth factor A (VEGFA) in response to the ovulatory gonadotropin surge. VEGFA is established as a key mediator of angiogenesis in the primate ovulatory follicle. To determine if additional VEGF family members may be involved in angiogenesis within the ovulatory follicle, cynomolgus monkeys (Macaca fascicularis) received gonadotropins to stimulate multiple follicular development, and human chorionic gonadotropin (hCG) substituted for the luteinizing hormone surge to initiate ovulatory events. Granulosa cells of monkey ovulatory follicles contained mRNA and protein for VEGFC and VEGFD before and after hCG administration. VEGFC and VEGFD were detected in monkey follicular fluid and granulosa cell-conditioned culture media, suggesting that granulosa cells of ovulatory follicles secrete both VEGFC and VEGFD. To determine if these VEGF family members can stimulate angiogenic events, monkey ovarian microvascular endothelial cells (mOMECs) were obtained from monkey ovulatory follicles and treated in vitro with VEGFC and VEGFD. Angiogenic events are mediated via three VEGF receptors; mOMECs express all three VEGF receptors in vivo and in vitro. Exposure of mOMECs to VEGFC increased phosphorylation of AKT, while VEGFD treatment increased phosphorylation of both AKT and CREB. VEGFC and VEGFD increased mOMEC migration and the formation of endothelial cell sprouts in vitro. However, only VEGFD increased mOMEC proliferation. These findings suggest that VEGFC and VEGFD may work in conjunction with VEGFA to stimulate early events in angiogenesis of the primate ovulatory follicle.

Abnormal lymphatic vessel development is associated with decreased decidual regulatory T cells in severe preeclampsia

PROBLEM

The lymphatic vasculature controls leukocytes trafficking and limits the adaptive immune response. In previous models of preeclampsia (PE), defective immune function caused by disruption of lymphangiogenesis was shown to be involved in the disease pathophysiology. Especially, the dysfunction of regulatory T cells (Treg) at the maternal-fetal interface may be one of the causes of severe PE. In particular, activation of Tregs to obtain immune tolerance requires adequate antigen presentation through the lymphatic system. We hypothesized that impaired lymphangiogenesis and imbalanced Tregs at the maternal-fetal interface are associated with the pathophysiology of severe PE. However, the current research addressing this hypothesis is limited. Therefore, to compare differences in lymphangiogenesis in severe PE and normal conditions, we aimed to examine the location of lymphatics at the maternal-fetal interface and to investigate the association between lymphangiogenesis and Tregs in severe PE.

METHOD OF STUDY

We obtained entire uterus from normal pregnant mice. Placental and fetal membranes, including decidua, were obtained from 10 pregnant women with severe PE and 10 gestational age-matched controls. Immunohistochemistry for LYVE1 was used to localize the distribution of lymphatic vessels and CD4, CD25, and FOXP3 for Treg.

RESULTS

LYVE1-positive vessels were present in the uterine wall of mice. LYVE1-positive lymphatic vessels were localized on the human decidua. Tubular lymphatics were abundant in the control decidua, but significantly reduced in severe PE. Furthermore, lymphatic vessel density correlated with the number of decidual Tregs.

CONCLUSION

Abnormal decidual lymphangiogenesis is associated with reduced numbers of decidual Tregs in severe PE.

Cathepsin B-mediated CD18 shedding regulates leukocyte recruitment from angiogenic vessels

Cathepsin B (CtsB) contributes to atherosclerosis and cancer progression by processing the extracellular matrix and promoting angiogenesis. Although CtsB was reported to promote and reduce angiogenesis, there is no mechanistic explanation that reconciles this apparent discrepancy. CtsB cleaves CD18 from the surface of immune cells, but its contribution to angiogenesis has not been studied. We developed an in vivo technique for visualization of immune cell transmigration from corneal vessels toward implanted cytokines. Wild-type (WT) leukocytes extravasated from limbal vessels, angiogenic stalks, and growing tip vessels and migrated toward the cytokines, indicating immune competence of angiogenic vessels. Compared to WT leukocytes, CtsB^-/- leukocytes accumulated in a higher number in angiogenic vessels, but extravasated less toward the implanted cytokine. The accumulated CtsB^-/- leukocytes in angiogenic vessels expressed more CD18. CD18^-/- leukocytes extravasated later than WT leukocytes. However, once extravasated, CD18^-/- leukocytes transmigrated more rapidly than their WT counterparts. These results suggest that, although CD18 facilitates efficient extravasation, outside of the vessel CD18 interaction with the extracellular matrix, it reduced transmigration velocity. Our results reveal an unexpected role for CtsB in leukocyte extravasation and transmigration, which advances our understanding of the complex contribution of CtsB to angiogenesis.-Nakao, S., Zandi, S., Sun, D., Hafezi-Moghadam, A. Cathepsin B-mediated CD18 shedding regulates leukocyte recruitment from angiogenic vessels.

Carbohydrate-binding protein CLEC14A regulates VEGFR-2- and VEGFR-3-dependent signals during angiogenesis and lymphangiogenesis

Controlled angiogenesis and lymphangiogenesis are essential for tissue development, function, and repair. However, aberrant neovascularization is an essential pathogenic mechanism in many human diseases, including diseases involving tumor growth and survival. Here, we have demonstrated that mice deficient in C-type lectin family 14 member A (CLEC14A) display enhanced angiogenic sprouting and hemorrhage as well as enlarged jugular lymph sacs and lymphatic vessels. CLEC14A formed a complex with VEGFR-3 in endothelial cells (ECs), and CLEC14A KO resulted in a marked reduction in VEGFR-3 that was concomitant with increases in VEGFR-2 expression and downstream signaling. Implanted tumor growth was profoundly reduced in CLEC14A-KO mice compared with that seen in WT littermates, but tumor-bearing CLEC14A-KO mice died sooner. Tumors in CLEC14A-KO mice had increased numbers of nonfunctional blood vessels and severe hemorrhaging. Blockade of VEGFR-2 signaling suppressed these vascular abnormalities and enhanced the survival of tumor-bearing CLEC14A-KO mice. We conclude that CLEC14A acts in vascular homeostasis by fine-tuning VEGFR-2 and VEGFR-3 signaling in ECs, suggesting its relevance in the pathogenesis of angiogenesis-related human disorders.

Molecular underpinnings of corneal angiogenesis: advances over the past decade

The cornea is maintained in an avascular state by maintaining an environment whereby anti-angiogenic factors take the upper hand over factors promoting angiogenesis. Many of the common pathologies affecting the cornea involve the disruption of such equilibrium and the shift towards new vessel formation, leading to corneal opacity and eventually-vision loss. Therefore it is of paramount importance that the molecular underpinnings of corneal neovascularization (CNV) be clearly understood, in order to develop better targeted treatments. This article is a review of the literature on the recent discoveries regarding pro-angiogenic factors of the cornea (such as vascular endothelial growth factors, fibroblast growth factor and matrix metalloproteinases) and anti-angiogenic factors of the cornea (such as endostatins and neostatins). Further, we review the molecular underpinnings of lymphangiogenesis, a process now known to be almost separate from (yet related to) hemangiogenesis.

Phenotypic transformation of intimal and adventitial lymphatics in atherosclerosis: a regulatory role for soluble VEGF receptor 2

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 of apoE(-/-) 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 of apoE(-/-) 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.

The Corneal Micropocket Assay: A Model of Angiogenesis and Lymphangiogenesis

The cornea is a transparent tissue that lacks blood and lymphatic vessels. In addition, the cornea is readily accessible, which makes it convenient for direct visualization of angiogenesis and lymphangiogenesis. The corneal micropocket assay is a commonly used quantitative technique, in which a growth factor containing pellet is micro-surgically implanted into the cornea of a rodent. Subsequently, the growth of the preexisting limbal vessels toward the growth factor is visualized by live microscopy or immunohistochemistry. Recently, there has been significant interest in the process of lymphangiogenesis and the factors that regulate it. To facilitate these studies, we introduce a novel technique for visualization of the immune response during growth factor induced angiogenesis and lymphangiogenesis in the cornea.

Understanding lymphangiogenesis in knockout models, the cornea, and ocular diseases for the development of therapeutic interventions

A major focus of cancer research for several decades has been understand the ability of tumors to induce new blood vessel formation, a process known as angiogenesis. Unfortunately, only limited success has been achieved in the clinical application of angiogenesis inhibitors. We now know that lymphangiogenesis, the growth of lymphatic vessels, likely also plays a major role in tumor progression. Thus, therapeutic strategies targeting lymphangiogenesis or both lymphangiogenesis and angiogenesis may represent promising approaches for treating cancer and other diseases. Importantly, research progress toward understanding lymphangiogenesis is significantly behind that related to angiogenesis. A PubMed search of "angiogenesis" returns nearly 80,000 articles, whereas a search of "lymphangiogenesis" returns 2,635 articles. This stark contrast can be explained by the lack of molecular markers for identifying the invisible lymphatic vasculature that persisted until less than 2 decades ago, combined with the intensity of research interest in angiogenesis during the past half century. Still, significant strides have been made in developing strategies to modulate lymphangiogenesis, largely using ocular disease models. Here we review the current knowledge of lymphangiogenesis in the context of knockout models, ocular diseases, the biology of activators and inhibitors, and the potential for therapeutic interventions targeting this process.

Soluble vascular endothelial growth factor receptor-3 suppresses allosensitization and promotes corneal allograft survival

PURPOSE

To investigate the effect of VEGF-C and VEGF-D blockade via soluble VEGFR-3 (sVEGFR-3) on T cell allosensitization, corneal neovascularization, and transplant survival.

METHODS

Corneal intrastromal suture placement and allogeneic transplantation were performed on BALB/c mice to evaluate the effect of sVEGFR-3 on corneal neovascularization. Soluble VEGFR-3 trap was injected intraperitoneally to block VEGF-C/D (every other day starting the day of surgery). Immunohistochemical staining of corneal whole mounts was performed using anti-CD31 (PECAM-1) and anti-LYVE-1 antibodies to quantify the levels of hem- and lymphangiogenesis, respectively. Mixed lymphocyte reaction (MLR) was performed to assess indirect and direct host T cell allosensitization and the frequencies of IFN-γ-producing T cells in the draining lymph nodes were assessed using flow cytometry. Graft opacity and survival was evaluated by slit-lamp biomicroscopy.

RESULTS

Treatment with sVEGFR-3 resulted in a significant blockade of lymphangiogenesis 2 weeks post-transplantation and significantly prolonged corneal allograft survival compared to the control group at 8 weeks post-transplantation (87.5 % vs. 50 %), and this was associated with significant reduction in the frequencies of allosensitized T cells and decreased frequencies of IFN-γ-producing CD4 T cells.

CONCLUSIONS

Soluble VEGFR-3 suppresses corneal lymphangiogenesis and allograft rejection and may offer a viable therapeutic modality for corneal neovascularization and corneal transplantation.

Molecular imaging reveals elevated VEGFR-2 expression in retinal capillaries in diabetes: a novel biomarker for early diagnosis

Diabetic retinopathy (DR) is a microvascular complication of diabetes and a leading cause of vision loss. Biomarkers and methods for early diagnosis of DR are urgently needed. Using a new molecular imaging approach, we show up to 94% higher accumulation of custom designed imaging probes against vascular endothelial growth factor receptor 2 (VEGFR-2) in retinal and choroidal vessels of diabetic animals (P<0.01), compared to normal controls. More than 80% of the VEGFR-2 in the diabetic retina was in the capillaries, compared to 47% in normal controls (P<0.01). Angiography in rabbit retinas revealed microvascular capillaries to be the location for VEGF-A-induced leakage, as expressed by significantly higher rate of fluorophore spreading with VEGF-A injection when compared to vehicle control (26±2 vs. 3±1 μm/s, P<0.05). Immunohistochemistry showed VEGFR-2 expression in capillaries of diabetic animals but not in normal controls. Macular vessels from diabetic patients (n=7) showed significantly more VEGFR-2 compared to nondiabetic controls (n=5) or peripheral retinal regions of the same retinas (P<0.01 in both cases). Here we introduce a new approach for early diagnosis of DR and VEGFR-2 as a molecular marker. VEGFR-2 could become a key diagnostic target, one that might help to prevent retinal vascular leakage and proliferation in diabetic patients.

Lymphatic function is impaired following irradiation of a single lymph node

INTRODUCTION

Lymph nodes are often the target of radiotherapy procedures. Unfortunately, the impact of nodal irradiation on lymphatic function is uncertain. In this study, our aim was to quantify the impact of lymph node irradiation on lymph flow.

METHODS AND RESULTS

The popliteal node or the nodal excision site of rabbits was treated with four daily 8 Gy doses of radiation. A FITC-dextran tracer was infused into a prenodal popliteal lymphatic. The area under the tracer blood recovery curve (AUC) indicated lymphatic functionality and the inflow pressure versus flow rate relationship inferred resistance through the system. Fluoroscopic and histological examination provided supporting data. Radiation of intact nodes decreased lymph transport significantly at 1 week, 1 month, and 6 months post-treatment (AUCs of 207.9 ± 79.87, 191.6 ± 62.95, and 250.44 ± 46.45) in comparison to controls (667.32 ± 104.18). Surprisingly, this functional decline was similar to that detected with a combination of node removal and irradiation of the excision site. The pressure-flow relationships in all treatment groups were significantly different from controls. This may be due in part to fibrosis and the thickening of the nodal capsules and trabeculae observed at 1 and 6 months. Fluoroscopy and Evans blue dye studies revealed vigorous new lymphatic vessel growth and occasionally, vessels anastomosed with local veins.

CONCLUSIONS

Irradiation of the popliteal lymph node impaired lymph transport and increased the pressure required to maintain flow through the system. New vessel formation and the growth of lymph-venous anastomoses indicated the development of alternative drainage pathways as a compensatory response.

Lymphangiogenic factors, mechanisms, and applications

Lymphangiogenesis, the growth of lymphatic vessels, is essential in embryonic development. In adults, it is involved in many pathological processes such as lymphedema, inflammatory diseases, and tumor metastasis. Advances during the past decade have dramatically increased the knowledge of the mechanisms of lymphangiogenesis, including the roles of transcription factors, lymphangiogenic growth factors and their receptors, and intercellular and intracellular signaling cascades. Strategies based on these mechanisms are being tested in the treatment of various human diseases such as cancer, lymphedema, and tissue allograft rejection. This Review summarizes the recent progress on lymphangiogenic mechanisms and their applications in disease treatment.

Lack of lymphatics and lymph node-mediated immunity in choroidal neovascularization

PURPOSE

Inflammation and immune cells regulate choroidal neovascularization (CNV) and could become therapeutic targets in age-related macular degeneration (AMD). Lymphangiogenesis is a key component of various inflammatory diseases. Whether lymphangiogenesis and lymph node-mediated immunity are involved in the pathogenesis of AMD is not understood.

METHODS

To investigate lymphangiogenesis in CNV, we generated CNV in animals by laser and studied surgically removed CNV membranes from uveitis and AMD patients. Immunohistochemistry was performed with lymphatic vessel endothelial hyaluronate receptor 1 (LYVE-1) and podoplanin antibodies. VEGF-C and VEGFR-3 expressions were examined with immunohistochemistry and Western blotting. To examine the role of lymph node in CNV, we lasered lymphotoxin alpha-deficient mice (LTα-/-) and measured the CNV volume.

RESULTS

Immunohistochemistry showed that LYVE-1(+) macrophages infiltrated in acutely induced CNV, although lymphatic tubes did not form. CNV membranes from patients did not show LYVE-1(+)podoplanin(+) vessels, suggesting the lack of lymphangiogenesis in AMD and uveitis. Western blots and immunostaining revealed VEGF-C and VEGFR-3 expression in CNV lesions, mainly in macrophages and angiogenic endothelial cells. Using fluorescent microsphere tracers, we show a path for cellular migration from the eye to the cervical lymph nodes (LNs) during CNV. However, CNV injury did not cause LN swelling. CNV volume did not differ between wild-type and LN-deficient mice, suggesting that LN is not a key component of early CNV formation.

CONCLUSIONS

Laser-induced CNV is not primarily dependent on acquired immunity, nor does the fundus injury affect peripheral LNs. Our results reveal a previously unknown cellular connection between the ocular fundus and the cervical LNs. This connection that in function resembles lymphatics is actively utilized in CNV.

Relationships between lymphangiogenesis and angiogenesis during inflammation in rat mesentery microvascular networks

BACKGROUND

Lymphatic and blood microvascular systems play a coordinated role in the regulation of interstitial fluid balance and immune cell trafficking during inflammation. The objective of this study was to characterize the temporal and spatial relationships between lymphatic and blood vessel growth in the adult rat mesentery following an inflammatory stimulus.

METHODS AND RESULTS

Mesenteric tissues were harvested from unstimulated adult male Wistar rats and at 3, 10, and 30 days post compound 48/80 stimulation. Tissues were immunolabeled for PECAM, LYVE-1, Prox1, podoplanin, CD11b, and class III β-tubulin. Vascular area, capillary blind end density, and vascular length density were quantified for each vessel system per time point. Blood vascular area increased compared to unstimulated tissues by day 10 and remained increased at day 30. Following the peak in blood capillary sprouting at day 3, blood vascular area and density increased at day 10. The number of blind-ended lymphatic vessels and lymphatic density did not significantly increase until day 10, and lymphatic vascular area was not increased compared to the unstimulated level until day 30. Lymphangiogenesis correlated with the upregulation of class III β-tubulin expression by endothelial cells along lymphatic blind-ended vessels and increased lymphatic/blood endothelial cell connections. In local tissue regions containing both blood and lymphatic vessels, the presence of lymphatics attenuated blood capillary sprouting.

CONCLUSIONS

Our work suggests that lymphangiogenesis lags angiogenesis during inflammation and motivates the need for future investigations aimed at understanding lymphatic/blood endothelial cell interactions. The results also indicate that lymphatic endothelial cells undergo phenotypic changes during lymphangiogenesis.

Corneal angiogenesis and lymphangiogenesis

PURPOSE OF REVIEW

The purpose of the present review is to describe new antilymphangiogenic treatment strategies and recent findings on strain-dependency of corneal lymphangiogenesis and the interdependency between blood and lymphatic vessel growth.

RECENT FINDINGS

Studies on mice have revealed that apart from haemangiogenesis, lymphangiogenesis can also differ markedly between several mouse strains under normal and inflammatory conditions. Although haemangiogenesis and lymphangiogenesis are closely interconnected in their spatial-temporal patterning, recent data suggest that they can also occur independently.

SUMMARY

Understanding the coordinated regulation of blood and lymphatic vessel growth and genetic factors determining lymphangiogenesis in more detail could improve the development of specifically targeted antihaemangiogenic or antilymphangiogenic strategies.

An in vivo method to quantify lymphangiogenesis in zebrafish

Background

Lymphangiogenesis is a highly regulated process involved in the pathogenesis of disease. Current in vivo models to assess lymphangiogenesis are largely unphysiologic. The zebrafish is a powerful model system for studying development, due to its rapid growth and transparency during early stages of life. Identification of a network of trunk lymphatic capillaries in zebrafish provides an opportunity to quantify lymphatic growth in vivo.

Methods and Results

Late-phase microangiography was used to detect trunk lymphatic capillaries in zebrafish 2- and 3-days post-fertilization. Using this approach, real-time changes in lymphatic capillary development were measured in response to modulators of lymphangiogenesis. Recombinant human vascular endothelial growth factor (VEGF)-C added directly to the zebrafish aqueous environment as well as human endothelial and mouse melanoma cell transplantation resulted in increased lymphatic capillary growth, while morpholino-based knockdown of vegfc and chemical inhibitors of lymphangiogenesis added to the aqueous environment resulted in decreased lymphatic capillary growth.

Conclusion

Lymphatic capillaries in embryonic and larval zebrafish can be quantified using late-phase microangiography. Human activators and small molecule inhibitors of lymphangiogenesis, as well as transplanted human endothelial and mouse melanoma cells, alter lymphatic capillary development in zebrafish. The ability to rapidly quantify changes in lymphatic growth under physiologic conditions will allow for broad screening of lymphangiogenesis modulators, as well as help define cellular roles and elucidate pathways of lymphatic development.

Larger therapeutic window for steroid versus VEGF-A inhibitor in inflammatory angiogenesis: surprisingly similar impact on leukocyte infiltration

PURPOSE

The current treatments against inflammatory angiogenesis are steroids and anti-VEGF-A, such as dexamethasone and bevacizumab, respectively. However, the therapeutic windows for dexamethasone and bevacizumab against inflammatory angiogenesis are unknown.

METHODS

To investigate the therapeutic windows for dexamethasone and bevacizumab, we used the corneal pocket assay. IL-1β pellets were implanted in corneas of BALB/c mice that were then treated with dexamethasone or bevacizumab at different time points. Angiogenesis (area, number of vessels, and sprouting) was quantitated at various time points after implantation. Nuclear Factor-κB (NF-κB) signaling and leukocyte accumulation in inflammatory angiogenesis were examined by Western blotting, by immunohistochemistry, and in the authors' novel leukocyte transmigration assay.

RESULTS

Dexamethasone inhibited IL-1β-induced angiogenesis when treatment started 4 days after IL-1β implantation, while bevacizumab only inhibited angiogenesis by day 2 after implantation. Both bevacizumab and dexamethasone inhibited angiogenic sprouting. Interestingly, bevacizumab did not affect NF-κB activation, which is one of the main signaling targets for steroid action. The authors' new imaging approach revealed that bevacizumab and steroid treatment blocked leukocyte infiltration into implanted corneas.

CONCLUSIONS

VEGF-A inhibition affected angiogenic sprouting, while it was not effective against matured vessels. Both dexamethasone and bevacizumab inhibited leukocyte transmigration from angiogenic vessels; however, dexamethasone had a larger therapeutic window. These insights improve the treatment strategy in angiogenic disorders.

Expansion of cortical and medullary sinuses restrains lymph node hypertrophy during prolonged inflammation

During inflammation, accumulation of immune cells in activated lymph nodes (LNs), coupled with a transient shutdown in lymphocyte exit, results in dramatic cellular expansion. Counter-regulatory measures to restrain LN expansion must exist and may include re-establishment of lymphocyte egress to steady-state levels. Indeed, we show in a murine model that egress of lymphocytes from LNs was returned to steady-state levels during prolonged inflammation following initial retention. This restoration in lymphocyte egress was supported by a preferential expansion of cortical and medullary sinuses during late inflammation. Cortical and medullary sinus remodeling during late inflammation was dependent on temporal and spatial changes in vascular endothelial growth factor-A distribution. Specifically, its expression was restricted to the subcapsular space of the LN during early inflammation, whereas its expression was concentrated in the paracortical and medullary regions of the LN at later stages. We next showed that this process was mostly driven by the synergistic cross-talk between fibroblastic reticular cells and interstitial flow. Our data shed new light on the biological significance of LN lymphangiogenesis during prolonged inflammation and further underscore the collaborative roles of stromal cells, immune cells, and interstitial flow in modulating LN plasticity and function.

Lymphatics and lymphangiogenesis in the eye

Lymphatic is a prerequisite for the maintenance of tissue fluid balance and immunity in the body. A body of evidence also shows that lymphangiogenesis plays important roles in the pathogenesis of diseases such as tumor metastasis and inflammation. The eye was thought to lack lymphatic vessels except for the conjunctiva; however, advances in the field, including the identification of lymphatic endothelial markers (e.g., LYVE-1 or podoplanin) and lymphangiogenic factors (e.g., VEGF-C), have revealed the exsitence and possible roles of lymphatics and lymphangiogenesis in the eye. Recent studies have shown that corneal limbus, ciliary body, lacrimal gland, orbital meninges, and extraocular muscles contain lymphatic vessels and that the choroid might have a lymphatic-like system. There is no known lymphatic outflow from the eye. However, several lymphatic channels including uveolymphatic pathway might serve the ocular fluid homeostasis. Furthermore, lymphangiogenesis plays important roles in pathological conditions in the eye including corneal transplant rejection and ocular tumor progression. Yet, the role of lymphangiogenesis in most eye diseases, especially inflammatory disease or edema, remains unknown. A better understanding of lymphatic and lymphangiogenesis in the eye will open new therapeutic opportunities to prevent vision loss in ocular diseases.

Forkhead box transcription factor FoxC1 preserves corneal transparency by regulating vascular growth

Normal vision requires the precise control of vascular growth to maintain corneal transparency. Here we provide evidence for a unique mechanism by which the Forkhead box transcription factor FoxC1 regulates corneal vascular development. Murine Foxc1 is essential for development of the ocular anterior segment, and in humans, mutations have been identified in Axenfeld-Rieger syndrome, a disorder characterized by anterior segment dysgenesis. We show that FOXC1 mutations also lead to corneal angiogenesis, and that mice homozygous for either a global (Foxc1(-/-)) or neural crest (NC)-specific (NC-Foxc1(-/-)) null mutation display excessive growth of corneal blood and lymphatic vessels. This is associated with disorganization of the extracellular matrix and increased expression of multiple matrix metalloproteinases. Heterozygous mutants (Foxc1(+/-) and NC-Foxc1(+/-)) exhibit milder phenotypes, such as disrupted limbal vasculature. Moreover, environmental exposure to corneal injury significantly increases growth of both blood and lymphatic vessels in both Foxc1(+/-) and NC-Foxc1(+/-) mice compared with controls. Notably, this amplification of the angiogenic response is abolished by inhibition of VEGF receptor 2. Collectively, these findings identify a role for FoxC1 in inhibiting corneal angiogenesis, thereby maintaining corneal transparency by regulating VEGF signaling.

Discontinuous LYVE-1 expression in corneal limbal lymphatics: dual function as microvalves and immunological hot spots

LYVE-1(+) corneal lymphatics contribute to drainage and immunity. LYVE-1 is widely accepted as the most reliable lymphatic marker because of its continuous expression in lymphatic endothelium. LYVE-1 expression in corneal lymphatics has not been examined. In this study, we report intact CD31(+) corneal lymphatic capillary endothelial cells that do not express LYVE-1. The number of LYVE-1(-) gaps initially increased until 8 wk of age but was significantly reduced in aged mice. C57BL/6 mice showed a notably higher number of the LYVE-1(-)/CD31(+) lymphatic regions than BALB/c mice, which suggests a genetic predisposition for this histological feature. The LYVE-1(-) lymphatic gaps expressed podoplanin and VE-cadherin but not αSMA or FOXC2. Interestingly, the number of LYVE-1(-) gaps in FGF-2, but not VEGF-A, implanted corneas was significantly lower than in untreated corneas. Over 70% of the CD45(+) leukocytes were found in the proximity of the LYVE-1(-) gaps. Using a novel in vivo imaging technique for visualization of leukocyte migration into and out of corneal stroma, we showed reentry of extravasated leukocytes from angiogenic vessels into newly grown corneal lymphatics. This process was inhibited by VE-cadherin blockade. To date, existence of lymphatic valves in cornea is unknown. Electron microscopy showed overlapping lymphatic endothelial ends, reminiscent of microvalves in corneal lymphatics. This work introduces a novel corneal endothelial lymphatic phenotype that lacks LYVE-1. LYVE-1(-) lymphatic endothelium could serve as microvalves, supporting unidirectional flow, as well as immunological hot spots that facilitate reentry of stromal macropahges.

VAP-1-mediated M2 macrophage infiltration underlies IL-1β- but not VEGF-A-induced lymph- and angiogenesis

Vascular adhesion protein-1 (VAP-1) contributes to inflammatory and angiogenic diseases, including cancer and age-related macular degeneration. It is expressed in blood vessels and contributes to inflammatory leukocyte recruitment. The cytokines IL-1β and vascular endothelial growth factor A (VEGF-A) modulate angiogenesis, lymphangiogenesis, and leukocyte infiltration. The lymphatic endothelium expresses intercellular adhesion molecule-1 and vascular adhesion molecule-1, which facilitate leukocyte transmigration into the lymphatic vessels. However, whether lymphatics express VAP-1 and whether they contribute to cytokine-dependent lymph- and angiogenesis are unknown. We investigated the role of VAP-1 in IL-1β- and VEGF-A-induced lymph- and angiogenesis using the established corneal micropocket assay. IL-1β increased VAP-1 expression in the inflamed cornea. Our in vivo molecular imaging revealed significantly higher VAP-1 expression in neovasculature than in the preexisting vessels. VAP-1 was expressed in blood but not lymphatic vessels in vivo. IL-1β-induced M2 macrophage infiltration and lymph- and angiogenesis were blocked by VAP-1 inhibition. In contrast, VEGF-A-induced lymph- and angiogenesis were unaffected by VAP-1 inhibition. Our results indicate a key role for VAP-1 in lymph- and angiogenesis-related macrophage recruitment. VAP-1 might become a new target for treatment of inflammatory lymph- and angiogenic diseases, including cancer.