Lymphangiogenic gene therapy with minimal blood vascular side effects

Review of treatment strategies after lymphadenectomy: From molecular therapeutics to immediate microsurgical lymphatic reconstruction

OBJECTIVE

Lymphedema is a common complication of cancer treatment, such as lymphadenectomy and radiation therapy. It is a debilitating condition with pathologic tissue changes that hinder effective curative treatment and jeopardize patients' quality of life. Various attempts to prevent the development of lymphedema have been made, with improvements in the incidence of the pathology. However, it is still prevalent among survivors of cancer. In this paper, we review both molecular therapeutics and immediate surgical lymphatic reconstruction as treatment strategies after lymphadenectomy. Specifically, we discuss pro-lymphangiogenic molecules that have proved efficient in animal models of lymphedema and clinical trials, and review currently available microsurgical techniques of immediate lymphatic reconstruction.

METHODS

A literature search was conducted in PubMed, Embase, Cochrane Library, and Google Scholar through May 2022. Searches were done separately for molecular therapeutics and microsurgical techniques for immediate lymphatic reconstruction. Search terms used for (1) non-surgical methods include 'lymphangiogenesis,' 'lymphedema,' 'growth factor,' and 'gene therapy.' Search terms used for (2) surgical methods include 'lymphedema,' 'lymph node excision,' 'lymphatic vessels,' 'primary prevention,' and 'microsurgery.'

RESULTS

Various pro-lymphangiogenic factors with therapeutic potential include VEGF-C, VEGF-D, HGF, bFGF, PDGF, IGF, Retinoic acid, Ang-1, S1P, TLR4, and IL-8. Microsurgical lymphatic reconstruction for prevention of secondary lymphedema includes lymphovenous anastomosis, vascularized lymph node flap transfer, and lymph-interpositional flap transfer, with promising clinical outcomes.

CONCLUSIONS

With growing knowledge of the lymphangiogenic pathway and lymphedema pathology and advances in microsurgical techniques to restore lymphatic channels, molecular and surgical approaches may represent a promising method for primary prevention of lymphedema.

The Future of Lymphedema: Potential Therapeutic Targets for Treatment

Purpose of Review

This review aims to summarize the current knowledge regarding the pharmacological interventions studied in both experimental and clinical trials for secondary lymphedema.

Recent Findings

Lymphedema is a progressive disease that results in tissue swelling, pain, and functional disability. The most common cause of secondary lymphedema in developed countries is an iatrogenic injury to the lymphatic system during cancer treatment. Despite its high incidence and severe sequelae, lymphedema is usually treated with palliative options such as compression and physical therapy. However, recent studies on the pathophysiology of lymphedema have explored pharmacological treatments in preclinical and early phase clinical trials.

Summary

Many potential treatment options for lymphedema have been explored throughout the past two decades including systemic agents and topical approaches to decrease the potential toxicity of systemic treatment. Treatment strategies including lymphangiogenic factors, anti-inflammatory agents, and anti-fibrotic therapies may be used independently or in conjunction with surgical approaches.

Utilization of Vascular Endothelial Growth Factor-C156S in Therapeutic Lymphangiogenesis: A Systematic Review

Background: Vascular endothelial growth factor (VEGF) C156S is an engineering variant of VEGF-C that has the potential to promote lymphangiogenesis, activating on VEGF receptor (VEGFR) 3, without promoting angiogenesis (i.e., not acting on VEGFR-2). We conducted a systematic review of publications assessing the use of this growth factor in lymphedema treatment. We hypothesized that VEGF-C156S specificity for VEGFR-3 was an important differential for the lymphangiogenesis promoted by it. Methods and Results: We conducted a comprehensive systematic review of the published literature on PubMed/Medline, Embase, and Cochrane Clinical Answers. Eligibility criteria included articles reporting data on the use of VEGF-C156S in lymphedema treatment. We excluded articles that investigated physiology action of VEGF-C156S and articles that focused on other therapies. From 304 potential articles found in the literature, four studies fulfilled the study eligibility criteria. To date, all studies about this growth factor have been experimental. The effect of VEGF-C156 on lymph node transfer was investigated in half of the experiments. Interestingly, delivery of VEGF-C156S was mostly performed through viral gene transfer, but injection (subcutaneously or intravenously) of it as a protein (liposomal or nonliposomal) was also investigated by one author to assess drug bioavailability. Conclusions: Although authors reported promotion of lymphangiogenesis, VEGF-C156S was correlated with lymphatic hyperplasia or nonstatistically significant lymphangiogenesis compared with controls. Scientific evidence about the use of VEGF-C156S in lymphedema treatment is still limited. However, authors have shown that its lymphangiogenic effect is inferior to VEGF-C.

Doxycycline for the treatment of breast cancer-related lymphedema

Purpose: Secondary lymphedema is a common complication of cancer treatment for which no effective drug treatments yet exist. Level I clinical data suggests that doxycycline is effective for treating filariasis-induced lymphedema, in which it decreases tissue edema and skin abnormalities; however, this treatment has not been tested for cancer-related lymphedema. Over the past year, we used doxycycline in an off-label manner in patients with breast cancer-related secondary lymphedema. The purpose of this report was to retrospectively analyze the efficacy of this treatment. Methods: Patients who presented to our lymphedema clinic between January 2021 and January 2022 were evaluated, and barring allergies or contraindications to doxycycline treatment, were counseled on the off-label use of this treatment. Patients who wished to proceed were treated with doxycycline (200 mg given orally once daily) for 6 weeks. After IRB approval of this study, lymphedema outcomes were retrospectively reviewed. Results: Seventeen patients with a mean follow-up of 17.0 ± 13.2 weeks were identified in our retrospective review. Although doxycycline treatment had no significant effect on relative limb volume change or L-Dex scores, we found a significant improvement in patient-reported quality of life. Analysis of patient responses to the Lymphedema Life Impact Scale showed a significant improvement in the total impairment score due to improvements in the physical and psychological well-being subscales (p = 0.03, p = 0.03, p = 0.04, respectively). Conclusion: This small, retrospective study did not show significant improvements in limb volume or L-Dex scores in patients with breast cancer-related lymphedema treated with doxycycline. However, our patients reported improvements in quality-of-life measures using a validated lymphedema patient-reported outcome instrument. Our results suggest that doxycycline may be of use in patients with breast cancer-related lymphedema; however, larger and more rigorous studies are needed.

Role of synovial lymphatic function in osteoarthritis

BACKGROUND

Osteoarthritis (OA) affects the entire joint, initially with a low degree of inflammation. Synovitis is correlated with the severity of OA clinical symptoms and cartilage degradation. The synovial lymphatic system (SLS) plays a prominent role in clearing macromolecules within the joint, including the pro-inflammatory cytokines in arthritic status. Scattered evidence shows that impaired SLS drainage function leads to the accumulation of inflammatory factors in the joint and aggravates the progression of OA, and the role of SLS function in OA is less studied.

DESIGN

This review summarizes the current understanding of synovial lymphatic function in OA progression and potential regulatory pathways and aims to provide a framework of knowledge for the development of OA treatments targeting lymphatic structure and functions.

RESULTS

SLS locates in the subintima layer of the synovium and consists of lymphatic capillaries and lymphatic collecting vessels. Vascular endothelial growth factor C (VEGF-C) is the most critical regulating factor of lymphatic endothelial cells (LECs) and SLS. Nitric oxide production-induced impairment of lymphatic muscle cells (LMCs) and contractile function may attribute to drainage dysfunction. Preclinical evidence suggests that promoting lymphatic drainage may help restore intra-articular homeostasis to attenuate the progression of OA.

CONCLUSION

SLS is actively involved in the homeostatic maintenance of the joint. Understanding the drainage function of the SLS at different stages of OA development is essential for further design of therapies targeting the function of these vessels.

Current Understanding of Pathological Mechanisms of Lymphedema

Significance: Lymphedema is a common disease that affects hundreds of millions of people worldwide with significant financial and social burdens. Despite increasing prevalence and associated morbidities, the mainstay treatment of lymphedema is largely palliative without an effective cure due to incomplete understanding of the disease. Recent Advances: Recent studies have described key histological and pathological processes that contribute to the progression of lymphedema, including lymphatic stasis, inflammation, adipose tissue deposition, and fibrosis. This review aims to highlight cellular and molecular mechanisms involved in each of these pathological processes. Critical Issues: Despite recent advances in the understanding of the pathophysiology of lymphedema, cellular and molecular mechanisms underlying the disease remains elusive due to its complex nature. Future Directions: Additional research is needed to gain a better insight into the cellular and molecular mechanisms underlying the pathophysiology of lymphedema, which will guide the development of therapeutic strategies that target specific pathology of the disease.

Lymphatic vessels in cancer

The lymphatic system, composed of initial and collecting lymphatic vessels as well as lymph nodes that are present in almost every tissue of the human body, acts as an essential transport system for fluids, biomolecules and cells between peripheral tissues and the central circulation. Consequently, it is required for normal body physiology but is also involved in the pathogenesis of various diseases, most notably cancer. The important role of tumor-associated lymphatic vessels and lymphangiogenesis in the formation of lymph node metastasis has been elucidated during the last two decades, whereas the underlying mechanisms and the relation between lymphatic and peripheral organ dissemination of cancer cells are incompletely understood. Lymphatic vessels are also important for tumor-host communication, relaying molecular information from a primary or metastatic tumor to regional lymph nodes and the circulatory system. Beyond antigen transport, lymphatic endothelial cells, particularly those residing in lymph node sinuses, have recently been recognized as direct regulators of tumor immunity and immunotherapy responsiveness, presenting tumor antigens and expressing several immune-modulatory signals including PD-L1. In this review, we summarize recent discoveries in this rapidly evolving field and highlight strategies and challenges of therapeutic targeting of lymphatic vessels or specific lymphatic functions in cancer patients.

Piper retrofractum extract and its component piperine promote lymphangiogenesis via an AKT- and ERK-dependent mechanism

Administration of Piper retrofractum extract (PRE) has been reported to alleviate edema, but the mechanism underlying this effect is unknown. Promotion of lymphangiogenesis is known to improve lymphedema, but the effect of PRE on lymphangiogenesis remains unclear. In the present study, we investigated whether PRE and specifically, piperine, the main component of PRE, can induce lymphangiogenesis. Treatments with PRE and piperine significantly promoted the proliferation, migration, and tube formation in human dermal lymphatic microvascular endothelial cells (HDLECs) but had no effect on the expression of lymphangiogenic factors. Furthermore, PRE and piperine significantly promoted the phosphorylation of the AKT and ERK proteins in HDLECs, and pretreatment with AKT and ERK inhibitors significantly attenuated the PRE- and piperine-induced lymphangiogenesis. These results indicate that PRE and piperine promote lymphangiogenesis via an AKT- and ERK-dependent mechanism. PRACTICAL APPLICATIONS: The lymphatic system plays various roles such as maintaining tissue fluid homeostasis, immune defense, and metabolism. Disruption of the lymphatic system results in insufficient fluid drainage, which causes edema. Currently, there are no effective treatments for lymphedema; therefore, the development of novel treatment strategies is desirable. In this study, we showed that PRE and its main component piperine promote lymphangiogenesis in lymphatic endothelial cells. Therefore, PRE has the potential to be used as a novel functional food for relieving lymphedema.

Signaling cascades in the failing heart and emerging therapeutic strategies

Chronic heart failure is the end stage of cardiac diseases. With a high prevalence and a high mortality rate worldwide, chronic heart failure is one of the heaviest health-related burdens. In addition to the standard neurohormonal blockade therapy, several medications have been developed for chronic heart failure treatment, but the population-wide improvement in chronic heart failure prognosis over time has been modest, and novel therapies are still needed. Mechanistic discovery and technical innovation are powerful driving forces for therapeutic development. On the one hand, the past decades have witnessed great progress in understanding the mechanism of chronic heart failure. It is now known that chronic heart failure is not only a matter involving cardiomyocytes. Instead, chronic heart failure involves numerous signaling pathways in noncardiomyocytes, including fibroblasts, immune cells, vascular cells, and lymphatic endothelial cells, and crosstalk among these cells. The complex regulatory network includes protein-protein, protein-RNA, and RNA-RNA interactions. These achievements in mechanistic studies provide novel insights for future therapeutic targets. On the other hand, with the development of modern biological techniques, targeting a protein pharmacologically is no longer the sole option for treating chronic heart failure. Gene therapy can directly manipulate the expression level of genes; gene editing techniques provide hope for curing hereditary cardiomyopathy; cell therapy aims to replace dysfunctional cardiomyocytes; and xenotransplantation may solve the problem of donor heart shortages. In this paper, we reviewed these two aspects in the field of failing heart signaling cascades and emerging therapeutic strategies based on modern biological techniques.

Pharmacological Treatment of Secondary Lymphedema

Lymphedema is a chronic disease that results in swelling and decreased function due to abnormal lymphatic fluid clearance and chronic inflammation. In Western countries, lymphedema most commonly develops following an iatrogenic injury to the lymphatic system during cancer treatment. It is estimated that as many as 10 million patients suffer from lymphedema in the United States alone. Current treatments for lymphedema are palliative in nature, relying on compression garments and physical therapy to decrease interstitial fluid accumulation in the affected extremity. However, recent discoveries have increased the hopes of therapeutic interventions that may promote lymphatic regeneration and function. The purpose of this review is to summarize current experimental pharmacological strategies in the treatment of lymphedema.

Cell therapy as a treatment of secondary lymphedema: a systematic review and meta-analysis

Background

Lymphedema, the accumulation of interstitial fluid caused by poor lymphatic drainage, is a progressive and permanent disease with no curative treatment. Several studies have evaluated cell-based therapies in secondary lymphedema, but no meta-analysis has been performed to assess their efficacy.

Methods

We conducted a systematic review and meta-analysis of all available preclinical and clinical studies, with assessment of their quality and risk of bias.

Results

A total of 20 articles using diverse cell types were selected for analysis, including six clinical trials and 14 pre-clinical studies in three species. The meta-analysis showed a positive effect of cell-based therapies on relevant disease outcomes (quantification of edema, density of lymphatic capillaries, evaluation of the lymphatic flow, and tissue fibrosis). No significant publication bias was observed.

Conclusion

Cell-based therapies have the potential to improve secondary lymphedema. The underlying mechanisms remain unclear. Due to relevant heterogeneity between studies, further randomized controlled and blinded studies are required to substantiate the use of these novel therapies in clinical practice.

Development and physiological functions of the lymphatic system: insights from human genetic studies of primary lymphedema

Primary lymphedema is a long-term (chronic) condition characterized by tissue lymph retention and swelling that can affect any part of the body, although it usually develops in the arms or legs. Due to the relevant contribution of the lymphatic system to human physiology, while this review mainly focuses on the clinical and physiological aspects related to the regulation of fluid homeostasis and edema, clinicians need to know that the impact of lymphatic dysfunction with a genetic origin can be wide ranging. Lymphatic dysfunction can affect immune function so leading to infection; it can influence cancer development and spread, and it can determine fat transport so impacting on nutrition and obesity. Genetic studies and the development of imaging techniques for the assessment of lymphatic function have enabled the recognition of primary lymphedema as a heterogenic condition in terms of genetic causes and disease mechanisms. In this review, the known biological functions of several genes crucial to the development and function of the lymphatic system are used as a basis for understanding normal lymphatic biology. The disease conditions originating from mutations in these genes are discussed together with a detailed clinical description of the phenotype and the up-to-date knowledge in terms of disease mechanisms acquired from in vitro and in vivo research models.

Gene therapy for ischaemic heart disease and heart failure

Gene therapy has been expected to become a novel treatment method since the structure of DNA was discovered in 1953. The morbidity from cardiovascular diseases remains remarkable despite the improvement of percutaneous interventions and pharmacological treatment, underlining the need for novel therapeutics. Gene therapy-mediated therapeutic angiogenesis could help those who have not gained sufficient symptom relief with traditional treatment methods. Especially patients with severe coronary artery disease and heart failure could benefit from gene therapy. Some clinical trials have reported improved myocardial perfusion and symptom relief in CAD patients, but few trials have come up with disappointing negative results. Translating preclinical success into clinical applications has encountered difficulties in successful transduction, study design, endpoint selection, and patient selection and recruitment. However, promising new methods for transducing the cells, such as retrograde delivery and cardiac-specific AAV vectors, hold great promise for myocardial gene therapy. This review introduces gene therapy for ischaemic heart disease and heart failure and discusses the current status and future developments in this field.

Impaired Meningeal Lymphatic Flow in NMOSD Patients With Acute Attack

The meningeal lymphatic vessels (mLVs) in central nervous system (CNS) have been validated by rodent and human studies. The mLVs play a vital role in draining soluble molecules and trafficking lymphocytes, antigens and antibodies from CNS into cervical lymph nodes (CLNs). This indicates that mLVs may serve as a link between the CNS and peripheral immune system, perhaps involving in the neuroinflammatory disease. However, the morphology and drainage function of mLVs in patients with neuroinflammatory disease, such as neuromyelitis optica spectrum disorders (NMOSD), remains unexplored. Using the dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI), we found that slower flow through mLVs along superior sagittal sinus in NMOSD patients with acute attack instead of NMOSD patients in chronic phase. The reduced flow in mLVs correlated with the disease severity evaluated by expanded disability status scale (EDSS). The receiver operating characteristic curve (ROC) indicated DCE-MRI might provide objective evidence to predict the acute relapse of NMOSD through evaluating the function of mLVs. Promoting or restoring the function of mLVs might be a new target for the treatment of NMOSD relapse.

Nucleoside-modified VEGFC mRNA induces organ-specific lymphatic growth and reverses experimental lymphedema

Lack or dysfunction of the lymphatics leads to secondary lymphedema formation that seriously reduces the function of the affected organs and results in degradation of quality of life. Currently, there is no definitive treatment option for lymphedema. Here, we utilized nucleoside-modified mRNA encapsulated in lipid nanoparticles (LNPs) encoding murine Vascular Endothelial Growth Factor C (VEGFC) to stimulate lymphatic growth and function and reduce experimental lymphedema in mouse models. We demonstrated that administration of a single low-dose of VEGFC mRNA-LNPs induced durable, organ-specific lymphatic growth and formation of a functional lymphatic network. Importantly, VEGFC mRNA-LNP treatment reversed experimental lymphedema by restoring lymphatic function without inducing any obvious adverse events. Collectively, we present a novel application of the nucleoside-modified mRNA-LNP platform, describe a model for identifying the organ-specific physiological and pathophysiological roles of the lymphatics, and propose an efficient and safe treatment option that may serve as a novel therapeutic tool to reduce lymphedema.

Mechanosensation and Mechanotransduction by Lymphatic Endothelial Cells Act as Important Regulators of Lymphatic Development and Function

Our understanding of the function and development of the lymphatic system is expanding rapidly due to the identification of specific molecular markers and the availability of novel genetic approaches. In connection, it has been demonstrated that mechanical forces contribute to the endothelial cell fate commitment and play a critical role in influencing lymphatic endothelial cell shape and alignment by promoting sprouting, development, maturation of the lymphatic network, and coordinating lymphatic valve morphogenesis and the stabilization of lymphatic valves. However, the mechanosignaling and mechanotransduction pathways involved in these processes are poorly understood. Here, we provide an overview of the impact of mechanical forces on lymphatics and summarize the current understanding of the molecular mechanisms involved in the mechanosensation and mechanotransduction by lymphatic endothelial cells. We also discuss how these mechanosensitive pathways affect endothelial cell fate and regulate lymphatic development and function. A better understanding of these mechanisms may provide a deeper insight into the pathophysiology of various diseases associated with impaired lymphatic function, such as lymphedema and may eventually lead to the discovery of novel therapeutic targets for these conditions.

Engineering the Lymphatic Network: A Solution to Lymphedema

Secondary lymphedema is a life-long disorder characterized by chronic tissue swelling and inflammation that obstruct interstitial fluid circulation and immune cell trafficking. Regenerating lymphatic vasculatures using various strategies represents a promising treatment for lymphedema. Growth factor injection and gene delivery have been developed to stimulate lymphangiogenesis and augment interstitial fluid resorption. Using bioengineered materials as growth factor delivery vehicles allows for a more precisely targeted lymphangiogenic activation within the injured site. The implantation of prevascularized lymphatic tissue also promotes in situ lymphatic capillary network formation. The engineering of larger scale lymphatic tissues, including lymphatic collecting vessels and lymph nodes constructed by bioengineered scaffolds or decellularized animal tissues, offers alternatives to reconnecting damaged lymphatic vessels and restoring lymph circulation. These approaches provide lymphatic vascular grafting materials to reimpose lymphatic continuity across the site of injury, without creating secondary injuries at donor sites. The present work reviews molecular mechanisms mediating lymphatic system development, approaches to promoting lymphatic network regeneration, and strategies for engineering lymphatic tissues, including lymphatic capillaries, collecting vessels, and nodes. Challenges of advanced translational applications are also discussed.

VEGF-C/VEGFR-3 axis protects against pressure-overload induced cardiac dysfunction through regulation of lymphangiogenesis

Prolonged pressure overload triggers cardiac hypertrophy and frequently leads to heart failure (HF). Vascular endothelial growth factor-C (VEGF-C) and its receptor VEGFR-3 are components of the central pathway for lymphatic vessel growth (also known as lymphangiogenesis), which has crucial functions in the maintenance of tissue fluid balance and myocardial function after ischemic injury. However, the roles of this pathway in the development of cardiac hypertrophy and dysfunction during pressure overload remain largely unknown. Eight- to 10-week-old male wild-type (WT) mice, VEGFR-3 knockdown (VEGFR-3f/- ) mice, and their WT littermates (VEGFR-3f/f ) were subjected to pressure overload induced by transverse aortic constriction (TAC) for 1-6 weeks. We found that cardiac lymphangiogenesis and the protein expression of VEGF-C and VEGFR-3 were upregulated in the early stage of cardiac hypertrophy but were markedly reduced in failing hearts. Moreover, TAC for 6 weeks significantly reduced cardiac lymphangiogenesis by inhibiting activation of VEGFR-3-mediated signals (AKT/ERK1/2, calcineurin A/NFATc1/FOXc2, and CX43), leading to increased cardiac edema, hypertrophy, fibrosis, apoptosis, inflammation, and dysfunction. These effects were further aggravated in VEGFR-3f/- mice and were dose-dependently attenuated by delivery of recombinant VEGF-C156S in WT mice. VEGF-C156s administration also reversed pre-established cardiac dysfunction induced by sustained pressure overload. Thus, these results demonstrate, for the first time, that activation of the VEGF-C-VEGFR-3 axis exerts a protective effect during the transition from cardiac hypertrophy to HF and highlight selective stimulation of cardiac lymphangiogenesis as a potential new therapeutic approach for hypertrophic heart diseases.

Lymphatic drainage from bronchus-associated lymphoid tissue in tolerant lung allografts promotes peripheral tolerance

Tertiary lymphoid organs are aggregates of immune and stromal cells including high endothelial venules and lymphatic vessels that resemble secondary lymphoid organs and can be induced at nonlymphoid sites during inflammation. The function of lymphatic vessels within tertiary lymphoid organs remains poorly understood. During lung transplant tolerance, Foxp3+ cells accumulate in tertiary lymphoid organs that are induced within the pulmonary grafts and are critical for the local downregulation of alloimmune responses. Here, we showed that tolerant lung allografts could induce and maintain tolerance of heterotopic donor-matched hearts through pathways that were dependent on the continued presence of the transplanted lung. Using lung retransplantation, we showed that Foxp3+ cells egressed from tolerant lung allografts via lymphatics and were recruited into donor-matched heart allografts. Indeed, survival of the heart allografts was dependent on lymphatic drainage from the tolerant lung allograft to the periphery. Thus, our work indicates that cellular trafficking from tertiary lymphoid organs regulates immune responses in the periphery. We propose that these findings have important implications for a variety of disease processes that are associated with the induction of tertiary lymphoid organs.

Human cathelicidin antimicrobial peptide LL-37 promotes lymphangiogenesis in lymphatic endothelial cells through the ERK and Akt signaling pathways

LL-37, the only member of the cathelicidin family of cationic antimicrobial peptides in humans has been shown to exhibit a wide variety of biological actions in addition to its antimicrobial activity. However, the lymphangiogenic effect of LL-37 has not been elucidated yet. In this study, we examined the effects of LL-37 on lymphangiogenesis and evaluated the underlying molecular mechanisms. LL-37 treatment significantly increased the migration and tube-like formation of human dermal lymphatic microvascular endothelial cells (HDLECs) and promoted the expression of lymphangiogenic factor in HDLECs. Treatment with LL-37 increased phosphorylation of ERK and Akt proteins in HDLECs, and pretreatment with ERK and Akt inhibitors significantly blocked the LL-37-induced HDLEC migration and tube-like formation. Furthermore, to investigate the involvement of formyl peptide receptor-like 1 (FPRL1) signaling in LL-37-induced lymphangiogenesis, HDLECs were treated with an FPRL1 antagonist. Pretreatment with the FPRL1 antagonist inhibited LL-37-induced phosphorylation of ERK and Akt proteins and attenuated LL-37-induced HDLEC migration and tube-like formation. These data indicated that LL-37 induces lymphangiogenesis in lymphatic endothelial cells via FPRL1, and the activation of the ERK and Akt-dependent signaling pathways.

Cardiac lymphatics in health and disease

The lymphatic vasculature, which accompanies the blood vasculature in most organs, is indispensable in the maintenance of tissue fluid homeostasis, immune cell trafficking, and nutritional lipid uptake and transport, as well as in reverse cholesterol transport. In this Review, we discuss the physiological role of the lymphatic system in the heart in the maintenance of cardiac health and describe alterations in lymphatic structure and function that occur in cardiovascular pathology, including atherosclerosis and myocardial infarction. We also briefly discuss the role that immune cells might have in the regulation of lymphatic growth (lymphangiogenesis) and function. Finally, we provide examples of how the cardiac lymphatics can be targeted therapeutically to restore lymphatic drainage in the heart to limit myocardial oedema and chronic inflammation.

Use of Gene Transfer Vectors in Lymphedema Treatment: A Systematic Review

Different delivery mechanisms have been proposed in the literature for targeted therapies in the treatment of lymphedema. They vary from simple and direct injection to sophisticated induction of gene expression in a targeted tissue. We conducted a systematic review of publications assessing the use of viral vectors for gene transfer in lymphedema treatment. We hypothesized that viral vectors are an effective way to deliver targeted therapy in lymphedema treatment. We conducted a comprehensive systematic review of the published literature on targeted therapies associated with lymphedema surgery using the PubMed database. Eligibility criteria excluded papers that reported use of viral vectors for other medical conditions. Abstracts, presentations, reviews, meta-analyses, and non-English language articles were also excluded. From 21 potential articles found in the literature, fourteen fulfilled study eligibility criteria. Positive outcomes in terms of lymphangiogenesis were seen. The viral vectors used included adenovirus and recombinant adeno-associated virus. Most of the genes expressed were growth factors, but expression of dominant-negative transforming growth factor-β1 receptor-II or Prox1 was also proposed. Five studies targeted genetic expression on lymphedema tissue, five on transplanted lymph nodes, two on skeletal muscle, and one on adipose-derived stem cells. Publications assessing use of viral vectors for gene transfer in lymphedema treatment demonstrated that it is an effective mechanism of delivering targeted therapies. However, to date, all studies were experimental and further studies must be performed before translating these therapies into clinical practice.

Local pharmacological induction of angiogenesis: Drugs for cells and cells as drugs

The past decades have seen significant advances in pro-angiogenic strategies based on delivery of molecules and cells for conditions such as coronary artery disease, critical limb ischemia and stroke. Currently, three major strategies are evolving. Firstly, various pharmacological agents (growth factors, interleukins, small molecules, DNA/RNA) are locally applied at the ischemic region. Secondly, preparations of living cells with considerable bandwidth of tissue origin, differentiation state and preconditioning are delivered locally, rarely systemically. Thirdly, based on the notion, that cellular effects can be attributed mostly to factors secreted in situ, the cellular secretome (conditioned media, exosomes) has come into the spotlight. We review these three strategies to achieve (neo)angiogenesis in ischemic tissue with focus on the angiogenic mechanisms they tackle, such as transcription cascades, specific signalling steps and cellular gases. We also include cancer-therapy relevant lymphangiogenesis, and shall seek to explain why there are often conflicting data between in vitro and in vivo. The lion's share of data encompassing all three approaches comes from experimental animal work and we shall highlight common technical obstacles in the delivery of therapeutic molecules, cells, and secretome. This plethora of preclinical data contrasts with a dearth of clinical studies. A lack of adequate delivery vehicles and standardised assessment of clinical outcomes might play a role here, as well as regulatory, IP, and manufacturing constraints of candidate compounds; in addition, completed clinical trials have yet to reveal a successful and efficacious strategy. As the biology of angiogenesis is understood well enough for clinical purposes, it will be a matter of time to achieve success for well-stratified patients, and most probably with a combination of compounds.

Prioritization of genes involved in endothelial cell apoptosis by their implication in lymphedema using an analysis of associative gene networks with ANDSystem

BACKGROUND

Currently, more than 150 million people worldwide suffer from lymphedema. It is a chronic progressive disease characterized by high-protein edema of various parts of the body due to defects in lymphatic drainage. Molecular-genetic mechanisms of the disease are still poorly understood. Beginning of a clinical manifestation of primary lymphedema in middle age and the development of secondary lymphedema after treatment of breast cancer can be genetically determined. Disruption of endothelial cell apoptosis can be considered as one of the factors contributing to the development of lymphedema. However, a study of the relationship between genes associated with lymphedema and genes involved in endothelial apoptosis, in the associative gene network was not previously conducted.

METHODS

In the current work, we used well-known methods (ToppGene and Endeavour), as well as methods previously developed by us, to prioritize genes involved in endothelial apoptosis and to find potential participants of molecular-genetic mechanisms of lymphedema among them. Original methods of prioritization took into account the overrepresented Gene Ontology biological processes, the centrality of vertices in the associative gene network, describing the interactions of endothelial apoptosis genes with genes associated with lymphedema, and the association of the analyzed genes with diseases that are comorbid to lymphedema.

RESULTS

An assessment of the quality of prioritization was performed using criteria, which involved an analysis of the enrichment of the top-most priority genes by genes, which are known to have simultaneous interactions with lymphedema and endothelial cell apoptosis, as well as by genes differentially expressed in murine model of lymphedema. In particular, among genes involved in endothelial apoptosis, KDR, TNF, TEK, BMPR2, SERPINE1, IL10, CD40LG, CCL2, FASLG and ABL1 had the highest priority. The identified priority genes can be considered as candidates for genotyping in the studies involving the search for associations with lymphedema.

CONCLUSIONS

Analysis of interactions of these genes in the associative gene network of lymphedema can improve understanding of mechanisms of interaction between endothelial apoptosis and lymphangiogenesis, and shed light on the role of disturbance of these processes in the development of edema, chronic inflammation and connective tissue transformation during the progression of the disease.

The Meningeal Lymphatic System: A New Player in Neurophysiology

The nature of fluid dynamics within the brain parenchyma is a focus of intensive research. Of particular relevance is its participation in diseases associated with protein accumulation and aggregation in the brain, such as Alzheimer's disease (AD). The meningeal lymphatic vessels have recently been recognized as an important player in the complex circulation and exchange of soluble contents between the cerebrospinal fluid (CSF) and the interstitial fluid (ISF). In aging mammals, for example, impaired functioning of the meningeal lymphatic vessels can lead to accelerated accumulation of toxic amyloid beta protein in the brain parenchyma, thus aggravating AD-related pathology. Given that meningeal lymphatic vessels are functionally linked to paravascular influx/efflux of the CSF/ISF, and in light of recent findings that certain cytokines, classically perceived as immune molecules, exert neuromodulatory effects, it is reasonable to suggest that the activity of meningeal lymphatics could alter the accessibility of CSF-borne immune neuromodulators to the brain parenchyma, thereby altering their effects on the brain. Accordingly, in this Perspective we propose that the meningeal lymphatic system can be viewed as a novel player in neurophysiology.

Functional aspects of meningeal lymphatics in ageing and Alzheimer's disease

Ageing is a major risk factor for many neurological pathologies, but its mechanisms remain unclear. Unlike other tissues, the parenchyma of the central nervous system (CNS) lacks lymphatic vasculature and waste products are removed partly through a paravascular route. (Re)discovery and characterization of meningeal lymphatic vessels has prompted an assessment of their role in waste clearance from the CNS. Here we show that meningeal lymphatic vessels drain macromolecules from the CNS (cerebrospinal and interstitial fluids) into the cervical lymph nodes in mice. Impairment of meningeal lymphatic function slows paravascular influx of macromolecules into the brain and efflux of macromolecules from the interstitial fluid, and induces cognitive impairment in mice. Treatment of aged mice with vascular endothelial growth factor C enhances meningeal lymphatic drainage of macromolecules from the cerebrospinal fluid, improving brain perfusion and learning and memory performance. Disruption of meningeal lymphatic vessels in transgenic mouse models of Alzheimer's disease promotes amyloid-β deposition in the meninges, which resembles human meningeal pathology, and aggravates parenchymal amyloid-β accumulation. Meningeal lymphatic dysfunction may be an aggravating factor in Alzheimer's disease pathology and in age-associated cognitive decline. Thus, augmentation of meningeal lymphatic function might be a promising therapeutic target for preventing or delaying age-associated neurological diseases.

Physiological Perspective on Therapies of Lymphatic Vessels

Significance: Growth of distinctive blood vessels of granulation tissue is a central step in the post-developmental tissue remodeling. Even though lymphangiogenesis is a part of the regeneration process, the significance of the controlled restoration of lymphatic vessels has only recently been recognized.

Recent Advances: Identification of lymphatic markers and growth factors paved the way for the exploration of the roles of lymphatic vessels in health and disease. Emerging pro-lymphangiogenic therapies use vascular endothelial growth factor (VEGF)-C to combat fluid retention disorders such as lymphedema and to enhance the local healing process.

Critical Issues: The relevance of recently identified lymphatic functions awaits verification by their association with pathologic conditions. Further, despite a century of research, the complete etiology of secondary lymphedema, a fluid retention disorder directly linked to the lymphatic function, is not understood. Finally, the specificity of pro-lymphangiogenic therapy depends on VEGF-C transfection efficiency, dose exposure, and the age of the subject, factors that are difficult to standardize in a heterogeneous human population.

Future Directions: Further research should reveal the role of lymphatic circulation in internal organs and connect its impairment with human diseases. Pro-lymphangiogenic therapies that aim at the acceleration of tissue healing should focus on the controlled administration of VEGF-C to increase their capillary specificity, whereas regeneration of collecting vessels might benefit from balanced maturation and differentiation of pre-existing lymphatics. Unique features of pre-nodal lymphatics, fault tolerance and functional hyperplasia of capillaries, may find applications outreaching traditional pro-lymphangiogenic therapies, such as immunomodulation or enhancement of subcutaneous grafting.

Biology of Vascular Endothelial Growth Factor C in the Morphogenesis of Lymphatic Vessels

Because virtually all tissues contain blood vessels, the importance of hemevascularization has been long recognized in regenerative medicine and tissue engineering. However, the lymphatic vasculature has only recently become a subject of interest. Central to the task of growing a lymphatic network are lymphatic endothelial cells (LECs), which constitute the innermost layer of all lymphatic vessels. The central molecule that directs proliferation and migration of LECs during embryogenesis is vascular endothelial growth factor C (VEGF-C). VEGF-C is therefore an important ingredient for LEC culture and attempts to (re)generate lymphatic vessels and networks. During its biosynthesis VEGF-C undergoes a stepwise proteolytic processing, during which its properties and affinities for its interaction partners change. Many of these fundamental aspects of VEGF-C biosynthesis have only recently been uncovered. So far, most—if not all—applications of VEGF-C do not discriminate between different forms of VEGF-C. However, for lymphatic regeneration and engineering purposes, it appears mandatory to understand these differences, since they relate, e.g., to important aspects such as biodistribution and receptor activation potential. In this review, we discuss the molecular biology of VEGF-C as it relates to the growth of LECs and lymphatic vessels. However, the properties of VEGF-C are similarly relevant for the cardiovascular system, since both old and recent data show that VEGF-C can have a profound effect on the blood vasculature.

Lymphatic Vessels: The Next Frontier in Lung Transplant

Lymphatic vessels are essential for the uptake of fluid, immune cells, macromolecules, and lipids from the interstitial space. During lung transplant surgery, the pulmonary lymphatic vessel continuum is completely disrupted, and, as a result, lymphatic drainage function is severely compromised. After transplantation, the regeneration of an effective lymphatic drainage system plays a crucial role in maintaining interstitial fluid balance in the lung allograft. In the meantime, these newly formed lymphatic vessels are commonly held responsible for the development of immune responses leading to graft rejection, because they are potentially capable of transporting antigen-presenting cells loaded with allogeneic antigens to the draining lymph nodes. However, despite remarkable progress in the understanding of lymphatic biology, there is still a paucity of consistent evidence that demonstrates the exact impacts of lymphatic vessels on lung graft function. In this review, we examine the current literature related to roles of lymphatic vessels in the pathogenesis of lung transplant rejection.

Caveolin-1 and VEGF-C promote Lymph Node Metastasis in the Absence of Intratumoral Lymphangiogenesis in Non-small Cell Lung Cancer

Aims and background

Caveolin-1 is a key component of membrane caveolae which plays an important role in cell transformation, cell migration, metastasis and angiogenesis. The mechanism of caveolin-1 and VEGF-C in lymphatic metastasis of non-small cell lung cancer (NSCLC) is still unclear. This study aimed to define the caveolin-1 and VEGF-C expression and lymph vessel density in NSCLC and look for correlations with clinicopathological features in NSCLC.

Methods

Caveolin-1, VEGF-C, and D2–40 protein expression were assessed by immunohistochemistry in a tissue microarray constructed from 70 NSCLCs and 12 normal lungs.

Results

Caveolin-1 expression was detected in 31 of 70 (44.3%) NSCLCs, which was significantly lower than its expression in normal lungs (9 of 12, 75%; P = 0.049). Expression of VEGF-C was detected in 49 of 70 (70%) NSCLCs and 4 of 12 (33.3%) normal lungs (P = 0.022). Both caveolin-1 and VEGF-C expression were correlated with lymph node metastasis of NSCLC (P = 0.001; P = 0.028). Moreover, caveolin-1 expression was correlated with tumor stage, histological type, and differentiation grade (P = 0.012; P = 0.038; P = 0.002). VEGF-C expression was correlated only with histological type (P = 0.020). There was no correlation between intratumoral lymph vessel density and any clinicopathological parameters including lymph node status. Furthermore, there was no correlation between caveolin-1 expression, VEGF-C expression, and lymph vessel density.

Conclusions

These findings indicated a reduction of caveolin-1 expression in NSCLC and suggested that caveolin-1 as well as VEGF-C might be involved in lymph node metastasis of NSCLC. The role of caveolin-1 in lymphatic metastasis and intratumoral lymphangiogenesis in NSCLC needs further study. Free full text available at www.tumorionline.it

Targeting lymphatic function as a novel therapeutic intervention for rheumatoid arthritis

Although clinical outcomes for patients with rheumatoid arthritis (RA) have greatly improved with the use of biologic and conventional DMARDs, approximately 40% of patients do not achieve primary clinical outcomes in randomized trials, and only a small proportion achieve lasting remission. Over the past decade, studies in murine models point to the critical role of the lymphatic system in the pathogenesis and therapy of inflammatory-erosive arthritis, presumably by the removal of catabolic factors, cytokines and inflammatory cells from the inflamed synovium. Murine studies demonstrate that lymphatic drainage increases at the onset of inflammatory-erosive arthritis but, as inflammation progresses to a more chronic phase, lymphatic clearance declines and both structural and cellular changes are observed in the draining lymph node. Specifically, chronic damage to the lymphatic vessel from persistent inflammation results in loss of lymphatic vessel contraction followed by lymph node collapse, reduced lymphatic drainage, and ultimately severe synovitis and joint erosion. Notably, clinical pilot studies in patients with RA report lymph node changes following treatment, and thus draining lymphatic vessels and nodes could represent a potential biomarker of arthritis activity and response to therapy. Most importantly, targeting lymphatics represents an innovative strategy for therapeutic intervention for RA.

Modulation of VEGF-induced migration and network formation by lymphatic endothelial cells: Roles of platelets and podoplanin

Lymphatic endothelial cells (LEC) express the transmembrane receptor podoplanin whose only known endogenous ligand CLEC-2 is found on platelets. Both podoplanin and CLEC-2 are required for normal lymphangiogenesis as mice lacking either protein develop a blood-lymphatic mixing phenotype. We investigated the roles of podoplanin and its interaction with platelets in migration and tube formation by LEC. Addition of platelets or antibody-mediated crosslinking of podoplanin inhibited LEC migration induced by vascular endothelial growth factors (VEGF-A or VEGF-C), but did not modify basal migration or the response to basic fibroblast growth factor or epidermal growth factor. In addition, platelets and podoplanin crosslinking disrupted networks of LEC formed in co-culture with fibroblasts. Depletion of podoplanin in LEC using siRNA negated the pro-migratory effect of VEGF-A and VEGF-C. Inhibition of RhoA or Rho-kinase reduced LEC migration induced by VEGF-C, but had no further effect after crosslinking of podoplanin, suggesting that podoplanin is required for signaling downstream of VEGF-receptors but upstream of RhoA. Together, these data reveal for the first time that podoplanin is an intrinsic specific regulator of VEGF-mediated migration and network formation in LEC and identify crosslinking of podoplanin by platelets or antibodies as mechanisms to modulate this pathway.

An Important Role of VEGF-C in Promoting Lymphedema Development

Secondary lymphedema is a common complication after cancer treatment, but the pathomechanisms underlying the disease remain unclear. Using a mouse tail lymphedema model, we found an increase in local and systemic levels of the lymphangiogenic factor vascular endothelial growth factor (VEGF)-C and identified CD68⁺ macrophages as a cellular source. Surprisingly, overexpression of VEGF-C in a transgenic mouse model led to aggravation of lymphedema with increased immune cell infiltration and vascular leakage compared with wild-type littermates. Conversely, blockage of VEGF-C by overexpression of soluble VEGF receptor-3 reduced edema development, diminishing inflammation and blood vascular leakage. Similar findings were obtained in a hind limb lymph node excision lymphedema model. Flow cytometry analyses and immunofluorescence stainings in lymphedematic tissue showed that VEGF receptor-3 expression was restricted to lymphatic endothelial cells. Our data suggest that endogenous VEGF-C causes blood vascular leakage and fluid influx into the tissue, thus actively contributing to edema formation. These data may provide the basis for future clinical therapeutic approaches.

Shaping of Peripheral T Cell Responses by Lymphatic Endothelial Cells

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

Differential Receptor Binding and Regulatory Mechanisms for the Lymphangiogenic Growth Factors Vascular Endothelial Growth Factor (VEGF)-C and -D

VEGF-C and VEGF-D are secreted glycoproteins that induce angiogenesis and lymphangiogenesis in cancer, thereby promoting tumor growth and spread. They exhibit structural homology and activate VEGFR-2 and VEGFR-3, receptors on endothelial cells that signal for growth of blood vessels and lymphatics. VEGF-C and VEGF-D were thought to exhibit similar bioactivities, yet recent studies indicated distinct signaling mechanisms (e.g. tumor-derived VEGF-C promoted expression of the prostaglandin biosynthetic enzyme COX-2 in lymphatics, a response thought to facilitate metastasis via the lymphatic vasculature, whereas VEGF-D did not). Here we explore the basis of the distinct bioactivities of VEGF-D using a neutralizing antibody, peptide mapping, and mutagenesis to demonstrate that the N-terminal α-helix of mature VEGF-D (Phe⁹³–Arg¹⁰⁸) is critical for binding VEGFR-2 and VEGFR-3. Importantly, the N-terminal part of this α-helix, from Phe⁹³ to Thr⁹⁸, is required for binding VEGFR-3 but not VEGFR-2. Surprisingly, the corresponding part of the α-helix in mature VEGF-C did not influence binding to either VEGFR-2 or VEGFR-3, indicating distinct determinants of receptor binding by these growth factors. A variant of mature VEGF-D harboring a mutation in the N-terminal α-helix, D103A, exhibited enhanced potency for activating VEGFR-3, was able to promote increased COX-2 mRNA levels in lymphatic endothelial cells, and had enhanced capacity to induce lymphatic sprouting in vivo. This mutant may be useful for developing protein-based therapeutics to drive lymphangiogenesis in clinical settings, such as lymphedema. Our studies shed light on the VEGF-D structure/function relationship and provide a basis for understanding functional differences compared with VEGF-C.

Characterization of internodal collecting lymphatic vessel function after surgical removal of an axillary lymph node in mice

Secondary lymphedema is an acquired lymphatic disorder, which occurs because of damage to the lymphatic system from surgery and/or radiation therapy for cancer treatment. However, it remains unknown how post-nodal collecting lymphatic vessels (CLVs) draining to the surgical wound area change in response to lymphadenectomy. We investigated functional and architectural changes of inguinal-to-axillary internodal CLVs (ICLVs) in mice after a single axillary LN (ALN) dissection using near-infrared fluorescence imaging. Our data showed no lymph flow in the ICLVs draining from the inguinal LN (ILN) at 2 days post-surgery. External compression enabled visualization of a small segment of contractile fluorescent ICLVs, but not all the way to the axillary region. At day 6, abnormal lymphatic drainage patterns, including lateral and retrograde lymph flow via vessels branching off the ICLVs were observed, which started to disappear beginning 9 days after surgery. The administration of vascular endothelial growth factor (VEGF)-C into the wound increased resolution of altered lymphatic drainage. Lymphatic drainage from the base of the tail to the ILN did not significantly change over time. These results demonstrate that lymph flow in the CLVs is dramatically affected by a LN dissection and long-term interruption of lymph flow might cause CLV dysfunction and thus contribute to chronic lymphatic disorders.

Therapeutic lymphangiogenesis ameliorates established acute lung allograft rejection

Lung transplantation is the only viable option for patients suffering from otherwise incurable end-stage pulmonary diseases such as chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis. Despite aggressive immunosuppression, acute rejection of the lung allograft occurs in over half of transplant recipients, and the factors that promote lung acceptance are poorly understood. The contribution of lymphatic vessels to transplant pathophysiology remains controversial, and data that directly address the exact roles of lymphatic vessels in lung allograft function and survival are limited. Here, we have shown that there is a marked decline in the density of lymphatic vessels, accompanied by accumulation of low-MW hyaluronan (HA) in mouse orthotopic allografts undergoing rejection. We found that stimulation of lymphangiogenesis with VEGF-C156S, a mutant form of VEGF-C with selective VEGFR-3 binding, alleviates an established rejection response and improves clearance of HA from the lung allograft. Longitudinal analysis of transbronchial biopsies from human lung transplant recipients demonstrated an association between resolution of acute lung rejection and decreased HA in the graft tissue. Taken together, these results indicate that lymphatic vessel formation after lung transplantation mediates HA drainage and suggest that treatments to stimulate lymphangiogenesis have promise for improving graft outcomes.

Cell-based therapy for therapeutic lymphangiogenesis

Lymphedema is a medically irreversible condition for which currently conservative and surgical therapies are either ineffective or impractical. The potential use of progenitor and stem cell-based therapies has offered a paradigm that may provide alternative treatment options for lymphatic disorders. Moreover, basic research, preclinical studies, as well as clinical trials have evaluated the therapeutic potential of various cell therapies in the field of lymphatic regeneration medicine. Among the available cell approaches, mesenchymal stem cells (MSCs) seem to be the most promising candidate mainly due to their abundant sources and easy availability as well as evitable ethical and immunological issues confronted with embryonic stem cells and induced pluripotent stem cells. In this context, the purpose of this review is to summarize various cell-based therapies for lymphedema, along with strengths and weaknesses of these therapies in the clinical application for lymphedema treatment. Particularly, we will highlight the use of MSCs for lymphatic regeneration medicine. In addition, the future perspectives of MSCs in the field of lymphatic regeneration will be discussed.

Emerging Role of Sphingosine-1-phosphate in Inflammation, Cancer, and Lymphangiogenesis

The main function of the lymphatic system is to control and maintain fluid homeostasis, lipid transport, and immune cell trafficking. In recent years, the pathological roles of lymphangiogenesis, the generation of new lymphatic vessels from preexisting ones, in inflammatory diseases and cancer progression are beginning to be elucidated. Sphingosine-1-phosphate (S1P), a bioactive lipid, mediates multiple cellular events, such as cell proliferation, differentiation, and trafficking, and is now known as an important mediator of inflammation and cancer. In this review, we will discuss recent findings showing the emerging role of S1P in lymphangiogenesis, in inflammation, and in cancer.

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.

Cell-cell interactions influence vascular reprogramming by Prox1 during embryonic development

Lymphangiogenesis is a highly regulated process that involves the reprogramming of venous endothelial cells into early lymphatic endothelial cells. This reprogramming not only displays a polarized expression pattern from the cardinal vein, but also demonstrates vascular specificity; early lymphatics only develop from the cardinal vein and not the related dorsal aorta. In our transgenic model of lymphangiogenesis, we demonstrate that Prox1 overexpression has the ability to reprogram venous endothelium but not early arterial endothelial cells in vivo, in spite of the fact that Prox1 expression is forced onto both vascular beds. Our observations suggest that this specificity during embryogenesis may be due to cell-cell interactions between the developing arterial endothelial cells and smooth muscle cells. These conclusions have far reaching implications on how we understand the vascular specificity of lymphangiogenesis.

Biomarkers of Lymphatic Function and Disease

Substantial advances have accrued over the last decade in the identification of the processes that contribute to lymphatic vascular development in health and disease. Identification of distinct regulatory milestones, from a variety of genetic models, has led to a stepwise chronology of lymphatic development. Several molecular species have been identified as important tissue biomarkers of lymphatic development and function. At present, vascular endothelial growth-factor receptor (VEGFR)-3/VEGF-C/VEGF-D signaling has proven useful in the identification of clinical lymphatic metastatic potential and the assessment of cancer prognosis. Similar biomarkers, to be utilized as surrogates for the assessment of inherited and acquired diseases of the lymphatic circulation, are actively sought, and will represent a signal advance in biomedical investigation.