The Lymphatic system: a new frontier in hepatology

Lymphatic vessels maintain tissue fluid homeostasis through the re-absorption of interstitial fluid and the transport of lipids and waste materials. They also direct trafficking of immune cells and coordinate immune responses. The most recognized clinical manifestations of lymphatic vessel disorders are lymphedema and ascites, where build-up of tissue fluids occurs because of impaired lymphatic drainage. Knowledge gained in the last decade has connected dysfunction of lymphatic vessels to various health problems including cancer, obesity, cardiovascular diseases and gastrointestinal disorders such as inflammatory bowel disease.(1)

The liver is the largest lymph-producing organ and accounts for nearly half of the body’s lymphatic fluid.(2) However, the study of the lymphatic vascular system in the liver is often overlooked. Its regulation has been poorly investigated at the cellular and molecular levels. Limited knowledge about the molecular markers specific to hepatic lymphatics and a lack of established experimental models and techniques for studying the lymphatic vasculature in postnatal livers impairs our understanding of this area. Recent advances in the study of lymphatic vessels in other organs/tissues are largely attributable to identification of specific markers and advanced imaging technologies using transgenic mice.(3)

Lymph in the liver originates from plasma components of blood flow filtered through fenestrae of liver sinusoidal endothelial cells. These plasma components (fluids and proteins) enter into the space of Disse, the hepatic version of the interstitial space, and form lymph. Drainage continues to small lymphatic vessels around the portal tract area, ultimately leaving the liver for the systemic lymphatic system.(2, 4) Anatomically, lymphatic vessels are categorized into lymphatic capillaries and collecting lymphatic vessels. These vessels consist of one layer of lymphatic endothelial cells with different degrees of basement membrane and pericytes depending on their sub-types. The morphological details of the lymphatic vasculature in the liver remain to be defined.

Hydrostatic pressure is the major driving force for plasma components to move from sinusoids to the space of Disse. Therefore, hemodynamic changes in the sinusoids influence the amount of lymph production. For example, the production of lymph increases in cirrhosis with portal hypertension as a result of an elevated pressure in the sinusoids. When the production of lymph exceeds the capacity of the lymphatic system, ascites results. To what degree the lymphatic vasculature can compensate for the increased production of lymph, or whether there is decreased uptake in cirrhosis is not well understood. Future translational research in this area may yield clinical benefits.

Besides tissue fluid homeostasis, lymphatic vessels collect fats and waste materials. In recent years, lymphatic vessels in the skin have received significant attention because of the recognition that normal lymphatic vessel function is vital to keep a healthy skin and prevent age-related skin problems. Dysfunction of lymphatic vessels causes accumulation of subcutaneous fats, resulting in “sagging” skin, typical of the aging process.(5) Like the skin, hepatic fat accumulation contributes to the pathogenesis of many liver diseases. In nonalcoholic fatty liver disease, it is plausible to consider that maintenance of normal lymph production and lymphatic vessel function would have significant benefits for a healthy liver.

Another important function of lymphatic vessels is trafficking of immune cells. Immune cells at inflamed sites migrate to lymphatic vessels and further to lymph nodes for discharge. In addition, they interact with lymphatic vessels and promote lymphangiogenesis. Lymphangiogenesis is the process of new lymphatic vessel formation from pre-existing lymphatic vessels, similar to that of angiogenesis. This occurs in embryogenesis, wound healing and a wide range of pathological conditions including cancer. Lymphangiogenesis helps infiltrated immune cells to be discharged from inflamed sites and accelerates resolution of inflammation.(6, 7) For example, in inflamed skin, this phenomenon is accompanied by increased vascular endothelial growth factor (VEGF)-C/D production. Depletion of VEGF-C/D attenuates lymphangiogenesis and delays mobilization of inflammatory cells and resolution of inflammation. Conversely, overexpression of VEGF-C/D enlarges lymphatic vessels in the dermis and enhances mobilization of inflammatory cells and resolution of inflammation.(6) Given the role of inflammation in multiple liver diseases, it is tempting to speculate that controlled lymphangiogenesis may help to reduce inflammation in early stages of liver injury and prevent its further development.

Like angiogenesis, lymphangiogenesis is also related to tumor metastasis. In malignant tumors, lymphangiogenic factors such as VEGF-C/D are produced and promote lymphangiogenesis.(8) Tumor lymphangiogenesis results in lymph node metastasis and distant metastasis.(9) Blockade of VEGF-C may be a potential therapy against malignant tumors. VEGF-C neutralizing antibody (VGX-100) inhibits VEGFR-2 and VEGFR-3, thereby blocking both angiogenesis and lymphangiogenesis.(9) Clinical trials of agents that target molecules that facilitate lymphangiogenesis are in progress. For example, the above-mentioned VEGF-C neutralizing antibody is now in a Phase I clinical trial for adult patients with advanced or metastatic solid tumors (NCT01514123).(10) Lymphangiogenesis is also related to liver fibrosis. Liver specimens from patients with viral hepatitis and cirrhotic rats have shown that the number and area of lymphatic vessels are positively associated with the severity of fibrosis around portal tracts.(3) Targeting lymphangiogenesis will likely require consideration for the stage of liver disease. In early stages of liver disease when inflammation plays a pivotal role, enhancing lymphangiogenesis may facilitate clearance of immune cells through lymphatic vessels and help to ameliorate inflammation, thereby blocking disease progression. In more chronic conditions such as hepatocellular carcinoma and liver cirrhosis, blocking lymphangiogenesis could be potentially beneficial.

In summary, the function of lymphatic vessels in the liver is an area open for new investigation. The concepts discussed above represent a small snapshot of their full role. Therefore, the study of lymphatic vessels warrants further research. This will advance our understanding of liver physiology and pathophysiology significantly and provide new insights into therapeutic strategies for many liver diseases. The first step to this end may include identification of specific markers of hepatic lymphatic endothelial cells as well as establishment of appropriate experimental models for their study. The study of lymphatic vessels in the liver offers an important opportunity to embark on a new era in hepatology research.