The National Heart, Lung, and Blood Institute (NHLBI) and the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), of the National Institutes of Health (NIH), hosted a research workshop on September 19-20, 2022 to better understand how the lymphatic system develops and functions in sickness and in health with the aim of improving how we manage and treat lymphatic disorders.
The workshop brought together international experts—including researchers, clinicians, community and federal partners—who presented the current state of research, explored knowledge gaps, and identified novel opportunities in lymphatic disease research. Community representatives with lymphedema, lipedema, and other lymphatic diseases shared their personal experiences during the event. The workshop aimed to provide a platform for collaborative partnerships among scientists, clinicians, patients, and patient advocates. Featured areas of active research included lymphedema and the role of lymphatics in health and disease states with particular emphasis in the digestive system and obesity.
Directors of the NIDDK Division of Digestive Diseases and Nutrition and NHLBI Division of Cardiovascular Sciences gave opening remarks. Keynote speakers highlighted the mechanisms of lymphatic contractile and valve dysfunction in metabolic syndrome and discussed the development of novel therapies for lymphedema. The NHLBI presented the efforts of the NIH to facilitate lymphatic research, touching on the current knowledge in lymphatic mapping. NHLBI and NIDDK staff, the Organizing Committee, and moderators took part in the discussions. The workshop was open to the public and over 700 people registered to attend.
The workshop highlighted the following topics:
- State of Research in Lymphatics
- Lymphatic Development and Homeostatic Function
- Lymphatics in Inflammation and Diseases of the Gastrointestinal System
- Lymphatic Dysfunction Diseases
- The Community’s Perspective: Living with Lymphatic Disease
- New Technologies in Lymphatic Imaging and Mapping
Watch the Videocast
The lymphatic system (LS) is a complex and intricate network of capillaries and collecting vessels that permeate nearly every organ of the body. Lymphatic vessels are dynamic and play critical roles in the maintenance of fluid homeostasis, trafficking of immune cells and antigens, absorption of key nutrients, transport of some hormones, regulation of autoimmunity, and modulation of inflammatory responses. Disturbances in lymphatic function such as leaky lymphatics, impaired collecting lymphatic pumping, and alterations in lymphatic valvular competence are noted in many disease states and thought to contribute to pathological changes. As a result, there is growing interest in studying the lymphatic system in development and disease in diverse areas, including lymphatic malformations, lymphedema, cancer biology, metabolism, obesity, and degenerative diseases of the central nervous system. A better understanding of the lymphatic system in health and disease could lead to the development of novel therapies for many of these conditions.
Lymphatic Development and Homeostatic Function
In contrast to the extensive molecular and functional characterization of blood vascular cells, little is known about the mechanisms that regulate the formation and lineage specific differentiation of lymphatic vascular cells. Research on the developmental origins of the lymphatic vasculature highlighted the homeobox gene PROX1 as a master regulator of lymphatic fate and function. Recent lineage tracing experiments of lymphatic endothelial cells (LECs) have revealed novel signaling mechanisms that regulate lymphatic differentiation during development. During development, molecular markers dictate organ laterality and are essential for organ morphogenesis and function. Other studies have shown that the LECs are heterogenous and specify a detailed fate map of the lymphatic vasculature. The heterogeneity and specialization of LECs depending on their origin within the body suggesting that this diversity contributes organ-specific programs. Intestinal lymphatics (lacteals) play a key role as a signaling hub to regulate intestinal stem cell development and activity.
Novel animal models for studying lymphatic valve formation and the signaling axis responsible for lymphatic button formation have improved the understanding of the development and function of the lymphatic system. Molecular mechanisms regulating lymphatic cell fate and function, organ morphogenesis, and lymphangiogenesis, including lymphatic valve and button junction development, are essential to understanding development of the lymphatic system, as well as targeting novel therapeutics for diseases such as non-alcoholic fatty liver disease, obesity, metabolic syndrome, lymphedema, inflammatory bowel disease (IBD), and ischemic heart disease.
Lymphatics in Inflammation, Gastrointestinal System Diseases and Lymphatic Dysfunction Diseases
About 70% of the total lymph formed in the body originates in the small intestine and the liver. Defects in gastrointestinal (GI) lymphatic development and lymphatic pump function, as well as lymphatic obstruction, result in various forms of GI pathology. Lymphatic dysfunction has been implicated in many disease states such as obesity, metabolic syndrome, IBD, liver fibrosis, cirrhosis, atherosclerosis, myocardial infarction, hypertension, kidney disease, and cardiovascular disorders. The lymphatic vasculature plays a key role in maintaining intestinal function. Button and zipper junctions of the lacteals are important for interstitial fluid drainage repair, immune cell migration, dietary lipid transport, and oral tolerance. Impairment to lymphatic vessel integrity and lymphatic valve stiffening may contribute to obesity and risk of type 2 diabetes. Inflammatory lymphangiogenesis is observed in various liver diseases, including liver fibrosis. Additionally, gut-draining lymph nodes across the gastrointestinal tract are phenotypically and functionally distinct and are essential for mounting immune response to various infections, including intestinal helminths.
Similarly, rheumatoid arthritis, kidney disease, and lymphatic malformations, like Kaposiform lymphangiomatosis and Gorham-Stout disease, also display evidence of impaired lymphatic function. Genetic mutations, altered signaling pathways, and inflammation are associated with these disease conditions. Identification of biomarkers and development of better imaging modalities are essential for improving the diagnosis and development of targeted therapeutics.
The pathophysiology of lymphedema is complex and is characterized by inflammation and fibrosis. Additional complications such as recurrent skin and soft tissue infections arise due to impaired immune function and possibly from alterations in skin barrier function. Anti-inflammatory and anti-fibrotic approaches may be helpful for treating patients with lymphedema. Ongoing clinical trials have shown modest changes in limb volume, suggesting that additional study is needed. Nevertheless, early results in clinical trials and preclinical studies are encouraging.
Prevention of lymphedema using surgical approaches to bypass damaged lymphatics and improved understanding of lymphatic anatomy in the upper extremity is another area of active study. The early results of these studies are encouraging. Future studies will determine how anatomic differences may contribute to lower extremity lymphedema. Avoiding lymphatic injury or repairing injured lymphatics at the time of surgery may therefore decrease the disease burden for patients.
It is clear that developing methods to quantify lymphatic function is important both for diagnosis and following lymphatic disease progression. Using advanced imaging with sodium MRI and intranodal injections is promising and translating this to practice and disseminating these techniques will be crucial to improve patient care. However, additional studies are needed to develop non-invasive (or minimally invasive techniques) that enable repeated measurements of lymphatic function in a quantitative manner.
The Community’s Perspective: Living with Lymphatic Disease
People living with lymphatic diseases shared their experiences, and experts in the field moderated the discussion. The objective was to provide personal account of living with lymphatic diseases for the scientific community. The main focus of this roundtable discussion was to better understand the individual and shared challenges, highlight what is not understood about lymphatic diseases, and identify what problems or needs researchers and clinicians should address in the future.
The panelists shared that lymphatic diseases impact an individual’s quality of life and well-being and the mental health component often is not considered. The lymphatic community will benefit from improved mental health treatment, support groups, patient education about concomitant medications (especially during pregnancy), education about the effects of treatments on fertility, and physician education. Lack of proper health care coverage was identified as an impediment to access care and treatments. Establishing mechanisms to facilitate the transition from pediatric to adult care was also identified as a priority.
In addition, many researchers do not have opportunities to engage with patients. Interactions between all stakeholders are critical to imparting change and fostering new opportunities for research.
In moderated discussions, participants highlighted current challenges and roadblocks to lymphatics research:
- Unique aspects of mouse lymphatic physiology may not apply to humans. Therefore, large animal models may be helpful.
- Very few functional studies of human vessels have been performed, highlighting the importance of developing techniques that enable non-invasive or minimally invasive methods to measure lymphatic function.
- The extent to which lymphatic contractile and/or valve dysfunction results from, or causes, other pathologies is not well understood.
Knowledge Gaps and Research Opportunities
The workshop provided the NHLBI and NIDDK with valuable information regarding research gaps and opportunities for advancing lymphatic research. The importance of the lymphatic system in health and disease was discussed, including the body-wide reach of the lymphatic system, specialization, importance for normal physiology, and implications in a wide range of diseases were presented. Despite the enormous challenges, reasons to be hopeful for the future of lymphatic research were also offered. The significant investment of the NIH in lymphatic research including increased lymphatic research funding over the years, establishment of lymphatic related Research Condition Disease Categories, and a multidisciplinary approach supported by the NIH, including the creation of the Human BioMolecular Atlas Program (HuBMAP). HuBMAP is developing the tools to create an open, global atlas of the human body. Within this effort, normal lymphatic anatomy will be discovered and catalogued to better understand the clinical level features and the variants of the lymphatic anatomy in normal individuals.
Basic, translational, and clinical experts presented the latest advances in the field and led interdisciplinary discussions on the lymphatic system. A list of knowledge gaps and opportunities in lymphatics research was identified. Overall, five main areas were identified for urgent need of improvement to facilitate lymphatic research:
- Early diagnosis,
- Quantification of lymphatic function and evaluation of lymphatic anatomy and heterogeneity of lymphatic cells in humans,
- Lymphatic-specific pharmacotherapies,
- Development of robust animal models for the study of lymphatic biology in health and disease states, and
- Epidemiological studies to determine the prevalence of lymphedema and other lymphatic diseases, along with increased physician education and psychosocial counseling for patients with lymphatic disorders.
Lack of non-invasive tests or blood biomarkers for lymphatic diseases and drugs that improve lymphatic function without significant off target effects were highlighted as the major challenge for clinicians and patients. Improved access and decreased costs of genetic tools and screening can improve our understanding of the genetic basis for lymphatic diseases and lead to better treatments and earlier diagnosis of lymphatic malformations, lymphedema, lipedema, lymphangiomatosis, and other lymphatic disorders.
Animal models of various diseases, including myocardial infarction, lymphedema, kidney disease, liver disease and others have shown that improving lymphatic function with lymphangiogenic growth factors such as VEGF-C can improve disease outcomes. However, this approach has significant limitations due to the lack of feasible clinical translation due to difficulties in delivering high doses of VEGF-C—particularly in chronic conditions—as well as off-target effects of this treatment. There is therefore an urgent need for development of pharmacotherapies that can be used safely and with a high degree of specificity to improve lymphatic function. There is also an important need for lymphatic muscle cell specific Cre- drivers to reveal why and how lymphatic muscle cells are altered in disease and development. These models would be particularly helpful in diseases such as lymphedema and obesity, where collecting lymphatic abnormalities are known to occur. Technical difficulties in studying lymphatic smooth muscle have resulted in an abundance of work on LECs and a relative paucity on the diversity or function of lymphatic muscle cells. The development of lymphatic muscle specific Cre-mice will therefore be helpful to bridge this gap in our knowledge.
Other studies are needed to understand the contribution of lymphatic valvular abnormalities. These studies are difficult due in part to the complexity of accurately imaging lymphatic valves in vivo. As a result, there is a significant lack of functional studies on healthy human vessels studying lymphatic pumping and valvular function. Thus, although in vitro and animal models have increased our understanding of collecting lymphatic function, these studies are limited due to our inability to study the effects of gravitational loads and other physiologic conditions.
Limitations and obstacles for basic research were also identified. The workshop highlighted the consensus that LECs have high level of heterogeneity and specialization throughout the body and there is a need for spatial biomolecular mapping. The greatest challenges are developing tools and technologies to accurately profile LECs heterogeneity at single cell resolution, identifying gene and protein biomarkers to phenotype these populations, and developing transgenic animals to examine these populations in vivo.
Although the discussions highlighted multiple gaps and deficiencies in our understanding of lymphatic disorders, there are reasons for optimism. Interest in lymphatic disorders is at an all-time high and an increasing number of outstanding investigators, surgeon-scientists, and clinicians are now involved in lymphatic research. Enhancing our understanding of lymphatic anatomy and individual variations that may increase the risk of developing lymphedema have been important advances. Similarly, development of effective surgical techniques in combination with medical therapies to improve lymphatic function will help treat patients with lymphatic disorders. This increased understanding will undoubtedly lead to targeted treatments. Identifying the gaps in our knowledge is the first step towards this goal that may lead to collaborations and breakthroughs.
Patient advocates and physician presenters/panelists also emphasized the need for the creation of more Lymphatic Centers of Excellence; specialized training for medical staff in the treatment and imaging of lymphatic disorders; increased awareness of the effects and side effects of the treatments, such as fertility and mental health issues; creative approaches to treat lymphatic diseases, such as state-of-the-art surgeries; and vascular anomalies centers for children and adult patients. The workshop discussions noted exciting reports about the role of baseline differences in lymphatic anatomy and the effect of these differences on the risk of lymphedema development.
Future research should determine how anatomic differences may contribute to lower extremity lymphedema. There is a need for large scale prospective trials analyzing outcomes of lymphedema surgery with a goal of defining optimal surgical algorithms and analyzing long term outcomes. These studies are needed for proper insurance coverage and will decrease the financial burden on patients. Finally, there is a major need for epidemiological studies to determine the prevalence of lymphedema and other lymphatic diseases, along with increased physician education and psychosocial counseling for patients with lymphatic disorders, as these diseases have historically been neglected.
Despite the various challenges identified by workshop participants, patients, and advocates, there is ample reason for optimism. Advances in the field include a novel model for studying lymphatic valve formation, the first animal model for Gorham–Stout Disease, and the ability to dissect the signaling axis responsible for lymphatic button formation. Additionally, new technologies have been developed and applied, including spatial transcriptomics and advanced macro-imaging. The progress by the Human Cell Atlas consortium to map all cells of the human body using single-cell -omics and imaging methods has been significant. The speakers showcased exciting approaches and tools to study multiscale data leveraging HuBMAP tools and resources.
Future directions should focus on broad, cross-disciplinary collaborations to solve major problems that researchers and clinicians in this field continuously encounter. Efforts should also focus on targeting the knowledge gaps that hinder clinical translation of lymphatic research. The construction of an atlas of the vasculature, including endothelial cells of the lymphatic system, is a great need. In addition, a commitment to open science and knowledge—including the open sharing of data, protocols, and transgenic animals and research findings—is needed to advance the field. Dissemination is key to improving access and developing treatments for lymphatic disorders. The NIH, NHLBI, and NIDDK are empowered by hearing patient stories and inspired by researchers’ passion, clarity of purpose, and a commitment to improve care broadly. Workshop organizers envision that the event will build collaborative partnerships among scientists, clinicians, patients, and patient advocates.
Babak J. Mehrara, M.D., Memorial Sloan Kettering Cancer Center
Andrea J. Radtke, Ph.D., National Institute of Allergy and Infectious Diseases (NIAID)/NIH
Workshop Organizing Committee
Babak J. Mehrara, M.D., Memorial Sloan Kettering Cancer Center
Andrea J. Radtke, Ph.D., NIAID/NIH
Gwendalyn J. Randolph, Ph.D., Washington University in St. Louis
Selen M. Catania, Ph.D., NHLBI/NIH
Patricia Greenwel, Ph.D., NIDDK/NIH
Ilsa I. Rovira, M.S., NHLBI/NIH
Nada Abumrad, Ph.D. - Washington University School of Medicine in St. Louis
Cameron (Cam) Ayala – Patient/advocate, Lymphatic Education & Research Network
Helene Benveniste, M.D., Ph.D. - Yale School of Medicine
Jorge A. Castorena-Gonzalez, Ph.D. - Tulane University School of Medicine
Selen Catania, Ph.D. - National Heart, Lung, and Blood Institute
Rachelle L. Crescenzi, Ph.D. - Vanderbilt University
Rafael Czepielewski, Ph.D. - Washington University School of Medicine in St Louis
Michael J. Davis, Ph.D. - University of Missouri School of Medicine
Michael Dellinger, Ph.D. - University of Texas Southwestern Medical Center
Yoav Dori, M.D., Ph.D. - Children’s Hospital of Philadelphia
Daria Esterhazy, Ph.D. - The University of Chicago
Zorina Galis, Ph.D. - National Heart, Lung, and Blood Institute
Nils Gehlenborg, Ph.D. - Harvard Medical School
David C. Goff Jr., M.D., Ph.D., FACP, FAHA - National Heart, Lung, and Blood Institute
Patricia Greenwel, Ph.D. - National Institute of Diabetes and Digestive and Kidney Diseases
Kasi Grosvenor – Patient/advocate, Lipedema Foundation
Cynthia Hudson – Patient/advocate
Maxim Itkin, M.D. - University of Pennsylvania
Yasuko Iwakiri, Ph.D. - Yale School of Medicine
Stephen P. James, M.D. - National Institute of Diabetes and Digestive and Kidney Diseases
Raghu Kataru, Ph.D. - Memorial Sloan Kettering Cancer Center
Natasza Kurpios, Ph.D. - Cornell University
Timothy Le Cras, Ph.D. - Cincinnati Children’s Hospital Medical Center
Taylor Lewis – patient/advocate, Lymphangiomatosis & Gorham’s Disease Alliance
Xiaolei Liu, Ph.D. - Temple University School of Medicine
Babak J. Mehrara, M.D. - Memorial Sloan Kettering Cancer Center
Mark Nicolls, M.D. - Stanford University School of Medicine
Rachel Niec, M.D., Ph.D. - The Rockefeller University
Timothy Padera, Ph.D. - Massachusetts General Research Institute
Susan E. Quaggin, M.D. - Northwestern University
Andrea J. Radtke, Ph.D. - National Institute of Allergy and Infectious Diseases
Reema Railkar, Ph.D. - I-Corps™ at NIH, National Cancer Institute
Gwendalyn J. Randolph, Ph.D. - Washington University School of Medicine in St. Louis
William Repicci, M.A. - Lymphatic Education & Research Network
Stanley Rockson, M.D., FACP, FACC - Stanford University School of Medicine
Ilsa I. Rovira, M.S. - National Heart, Lung, and Blood Institute
Joshua Scallan, Ph.D. -University of South Florida
Mark V. Schaverien, M.D. - The University of Texas MD Anderson Cancer Center
Edward M. Schwarz, Ph.D. - University of Rochester Medical Center
Dhruv Singhal, M.D. - Beth Israel Deaconess Medical Center
Oliver Stone, Ph.D. - University of Oxford, UK
Sarah Teichmann, Ph.D. - Wellcome Sanger Institute, UK
Ying Yang, Ph.D. - University of South Florida
- NIH HuBMAP NIH Common Fund https://commonfund.nih.gov/hubmap
- HuBMAP information: https://commonfund.nih.gov/HuBMAP/fundedresearch
- HuBMAP Consortium https://hubmapconsortium.org/
- HuBMAP Consortium. The human body at cellular resolution: the NIH Human Biomolecular Atlas Program. Nature 574, 187–192 (2019) https://www.nature.com/articles/s41586-019-1629-x
- For additional information on NCI’s Small Business Innovation Research: https://sbir.cancer.gov/
- HuBMAP Consortium. The human body at cellular resolution: the NIH Human Biomolecular Atlas Program. Nature 574, 187–192 (2019). https://doi.org/10.1038/s41586-019-1629-x
- Radtke, A.J., Lukacs, J.M., Praskievicz, N.E. et al. The centuries long pursuit to map the human lymphatic system. Nat Med 28, 1518–1520 (2022). https://doi.org/10.1038/s41591-022-01865-5