Phenotypic modulation of smooth muscle cells in lymphoedema

Lymphatic Patterns in the Superficial Circumflex Iliac Artery Perforator Flap

BACKGROUND

 Lymphedema is a chronic condition, characterized by fluid buildup and tissue swelling and is caused by impairment of the lymphatic system. The lymph interpositional flap transfer technique, in which lymph flow is restored with a flap that includes subdermal lymphatic channels, is an option for surgical reconstruction. The superficial circumflex iliac artery perforator (SCIP) flap can be used for this purpose. This study aimed to describe and characterize the lymphatic patterns within the vascular territory of the SCIP flap.

METHODS

 This cross-sectional multicenter study involved 19 healthy volunteers aged ≥18 years of both sexes assessing the bilateral SCIP flap zone. Superficial lymphatic patterns were evaluated at 4-, 14-, and 24 minutes after indocyanine green (ICG) lymphography injection. Standardized procedures were implemented for all participants in both hospitals.

RESULTS

 The linear pattern was predominant bilaterally. The median number of lymphatic vessels and their length increased over time. Most lymphatic vessels in the SCIP flap were oriented toward the inguinal lymph node (ILN). However, the left SCIP zone lymphatic vessels were directed opposite to the ILN.

CONCLUSION

 The two sides SCIP zones were not significantly different. The primary direction of the bilateral lymphatic vessels was toward the ILN, although only single-side lymphatic vessels were in the opposite direction. These findings emphasize the importance of assessing lymphatic axiality and coherent lymphatic patterns prior to undertaking the SCIP as an interposition flap, to ensure effective restoration of lymphatic flow.

Overview of Lymphatic Muscle Cells in Development, Physiology, and Disease

Lymphatic muscle cells (LMCs) are indispensable for proper functioning of the lymphatic system, as they provide the driving force for lymph transport. Recent studies have advanced our understanding of the molecular mechanisms that regulate LMCs, which control rhythmic contraction and vessel tone of lymphatic vessels-traits also found in cardiac and vascular smooth muscle. In this review, we discuss the molecular pathways that orchestrate LMC-mediated contractility and summarize current knowledge about their developmental origin, which may shed light on the distinct contractile characteristics of LMCs. Further, we highlight the growing evidence implicating LMC dysregulation in the pathogenesis of lymphedema and other diseases related to lymphatic vessel dysfunction. Given the limited number and efficacy of existing therapies to treat lymphedema, LMCs present a promising focus for identifying novel therapeutic targets aimed at improving lymphatic vessel contractility. Here, we discuss LMCs in health and disease, as well as therapeutic strategies aimed at targeting them to improve lymphatic vessel function.

Molecular pathophysiology of secondary lymphedema

Lymphedema occurs as a result of lymphatic vessel damage or obstruction, leading to the lymphatic fluid stasis, which triggers inflammation, tissue fibrosis, and adipose tissue deposition with adipocyte hypertrophy. The treatment of lymphedema is divided into conservative and surgical approaches. Among surgical treatments, methods like lymphaticovenular anastomosis and vascularized lymph node transfer are gaining attention as they focus on restoring lymphatic flow, constituting a physiologic treatment approach. Lymphatic endothelial cells form the structure of lymphatic vessels. These cells possess button-like junctions that facilitate the influx of fluid and leukocytes. Approximately 10% of interstitial fluid is connected to venous return through lymphatic capillaries. Damage to lymphatic vessels leads to lymphatic fluid stasis, resulting in the clinical condition of lymphedema through three mechanisms: Inflammation involving CD4+ T cells as the principal contributing factor, along with the effects of immune cells on the VEGF-C/VEGFR axis, consequently resulting in abnormal lymphangiogenesis; adipocyte hypertrophy and adipose tissue deposition regulated by the interaction of CCAAT/enhancer-binding protein α and peroxisome proliferator-activated receptor-γ; and tissue fibrosis initiated by the overactivity of Th2 cells, leading to the secretion of profibrotic cytokines such as IL-4, IL-13, and the growth factor TGF-β1. Surgical treatments aimed at reconstructing the lymphatic system help facilitate lymphatic fluid drainage, but their effectiveness in treating already damaged lymphatic vessels is limited. Therefore, reviewing the pathophysiology and molecular mechanisms of lymphedema is crucial to complement surgical treatments and explore novel therapeutic approaches.

Lymphatic drainage dysfunction via narrowing of the lumen of cisterna chyli and thoracic duct after luminal dilation

BACKGROUND

The chronological pattern of extrahepatic lymphatic vessel progression in the course of chronic liver disease has not been clarified. This study aimed to clarify the chronological changes in lymphatic vessels with liver disease progression.

METHODS

This was a prospective cross-sectional study that enrolled a total of 199 patients. The maximum diameter of the cisterna chyli (CC) or terminal thoracic duct (tTD) was measured using computed tomography or ultrasonography, respectively. Changes in the maximum diameters of the CC and tTD were evaluated with patients with chronic liver disease as the pilot set (n = 138). Subsequently, we examined whether CC/tTD could be used to re-allocate unclassified patients by the Baveno-VII criteria to appropriately diagnose clinically significant portal hypertension (CSPH) in the pilot and validation sets.

RESULTS

In the pilot set, a scatter-plot showed that both CC and tTD were narrowed as terminal features in chronic liver disease after dilation. Because there was a significant correlation between the CC diameter and hepatic venous pressure gradient (r = 0.724) in unclassified patients, the diagnostic value of CC and tTD for CSPH was good (AUC: 0.961 and 0.913, respectively). After re-allocation, 68 and 27 unclassified patients were reduced to 4 and 5 in the pilot and validation sets, respectively.

CONCLUSION

Both the CC and tTD narrow in the course of liver disease after dilation. Moreover, the maximum diameter of the CC and tTD can be used to re-allocate patients who are unclassified according to the Baveno-VII criteria.

CLINICAL TRIAL NUMBER

UMIN trial no. 000044857.

Tryptase-Positive Mast Cells Promote Adipose Fibrosis in Secondary Lymphedema through PDGF

Lymphedema is a chronic and progressive condition that causes physical disfigurement and psychological trauma due to the accumulation of lymphatic fluid in the interstitial space. Once it develops, lymphedema is difficult to treat because it leads to the fibrosis of adipose tissue. However, the mechanism behind this remains unclear. The purpose of this study was to investigate the involvement of mast cells (MCs) in the adipose tissues of patients with lymphedema. We found that fibrosis spread through blood vessels in the adipose tissues of lymphedema patients, and the expression of the collagen I and III genes was significantly increased compared to that of those in normal adipose tissue. Immunostaining of vimentin and α-smooth muscle actin showed that fibroblasts were the main cellular components in severely fibrotic regions. Toluidine blue staining confirmed a significant increase in the number of MCs in the adipose tissues of lymphedema patients, and immunostaining of serial sections of adipose tissue showed a significant increase in the number of tryptase-positive cells in lymphedema tissues compared with those in normal adipose tissues. Linear regression analyses revealed significant positive correlations between tryptase and the expressions of the TNF-α, platelet-derived growth factor (PDGF)-A, and PDGFR-α genes. PDGF-A-positive staining was observed in both fibroblasts and granules of tryptase-positive MCs. These results suggest that MC-derived tryptase plays a role in the fibrosis of adipose tissue due to lymphedema directly or in cooperation with other mediators.

Edema and lymphatic clearance: molecular mechanisms and ongoing challenges

Resolution of edema remains a significant clinical challenge. Conditions such as traumatic shock, sepsis, or diabetes often involve microvascular hyperpermeability, which leads to tissue and organ dysfunction. Lymphatic insufficiency due to genetic causes, surgical removal of lymph nodes, or infections, leads to varying degrees of tissue swelling that impair mobility and immune defenses. Treatment options are limited to management of edema as there are no specific therapeutics that have demonstrated significant success for ameliorating microvascular leakage or impaired lymphatic function. This review examines current knowledge about the physiological, cellular, and molecular mechanisms that control microvascular permeability and lymphatic clearance, the respective processes for interstitial fluid formation and removal. Clinical conditions featuring edema, along with potential future directions are discussed.

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.

TGF-β1 mediates pathologic changes of secondary lymphedema by promoting fibrosis and inflammation

Background

Secondary lymphedema is a common complication of cancer treatment, and previous studies have shown that the expression of transforming growth factor‐beta 1 (TGF‐β1), a pro‐fibrotic and anti‐lymphangiogenic growth factor, is increased in this disease. Inhibition of TGF‐β1 decreases the severity of the disease in mouse models; however, the mechanisms that regulate this improvement remain unknown.

Methods

Expression of TGF‐β1 and extracellular matrix molecules (ECM) was assessed in biopsy specimens from patients with unilateral breast cancer‐related lymphedema (BCRL). The effects of TGF‐β1 inhibition using neutralizing antibodies or a topical formulation of pirfenidone (PFD) were analyzed in mouse models of lymphedema. We also assessed the direct effects of TGF‐β1 on lymphatic endothelial cells (LECs) using transgenic mice that expressed a dominant‐negative TGF‐β receptor selectively on LECs (LEC^DN‐RII).

Results

The expression of TGF‐β1 and ECM molecules is significantly increased in BCRL skin biopsies. Inhibition of TGF‐β1 in mouse models of lymphedema using neutralizing antibodies or with topical PFD decreased ECM deposition, increased the formation of collateral lymphatics, and inhibited infiltration of T cells. In vitro studies showed that TGF‐β1 in lymphedematous tissues increases fibroblast, lymphatic endothelial cell (LEC), and lymphatic smooth muscle cell stiffness. Knockdown of TGF‐β1 responsiveness in LEC^DN‐RII resulted in increased lymphangiogenesis and collateral lymphatic formation; however, ECM deposition and fibrosis persisted, and the severity of lymphedema was indistinguishable from controls.

Conclusions

Our results show that TGF‐β1 is an essential regulator of ECM deposition in secondary lymphedema and that inhibition of this response is a promising means of treating lymphedema.

Lymphatic Collecting Vessels in Health and Disease: A Review of Histopathological Modifications in Lymphedema

Secondary lymphedema of the extremities affects millions of people in the world as a common side effect of oncological treatments with heavy impact on every day life of patients and on the health care system. One of the surgical techniques for lymphedema treatment is the creation of a local connection between lymphatic vessels and veins, facilitating drainage of lymphatic fluid into the circulatory system. Successful results, however, rely on using a functional vessel for the anastomosis, and vessel function, in turn, depends on its structure. The structure of lymphatic collecting vessels changes with the progression of lymphedema. They appear initially dilated by excess interstitial fluid entered at capillary level. The number of lymphatic smooth muscle cells in their media then increases in the attempt to overcome the impaired drainage. When lymphatic muscle cells hyperplasia occurs at the expenses of the lumen, vessel patency decreases hampering lymph flow. Finally, collagen fiber accumulation leads to complete occlusion of the lumen rendering the vessel unfit to conduct lymph. Different types of vessels may coexist in the same patient but usually the distal part of the limb contains less affected vessels that are more likely to perform efficient lymphatic–venular anastomosis. Here we review the structure of the lymphatic collecting vessels in health and in lymphedema, focusing on the histopathological changes of the lymphatic vessel wall based on the observations on segments of the vessels used for lymphatic–venular anastomoses.

A Novel Approach to Subcutaneous Collecting Lymph Ducts Using a Small Diameter Wire in Animal Experiments and Clinical Trials

Background: While performing microsurgery, including lymphaticovenous anastomosis (LVA) for chronic limb lymphedema, it is a common procedure to identify the subcutaneous collecting lymph ducts with near-infrared fluorescence lymphangiography (NIR) using indocyanine green. However, due to limitations such as minimum observable depth, only a few lymphatic ducts can be identified with this procedure. Hence, we developed a new smaller-diameter "lymphatic wire" (LW) that could be inserted directly into lymphatic collecting ducts of the limbs, enabling accurate identification and localization. Methods and Results: First, used the LW on the hind limbs of 6 swine, and 36 porcine lymphatic collecting ducts were identified, the outer diameter of which varied from 0.3-0.7 mm (mean 0.41 ± 0.11 mm). We could insert the LW after creating a side opening in 30 of these ducts. We encountered no difficulties during the procedure. In the pathological examination, adverse events such as valve dysfunction and perforation were not identified. Based on the results, a clinical evaluation of the LW was performed in two patients with lower extremity lymphedema, and the LW helped us identify lymphatic ducts in the subcutaneous layer, even at the sites where the NIR had proved ineffective. Conclusion: Based on our results, we suggest that the procedure for identifying lymphatic vessels using the newly developed LW is a useful technique that can be utilized before performing a LVA for lymphedema. However, further clinical study is required to develop this device and technique, for wider clinical application in the future.

New Insights into the Pathophysiology of Primary and Secondary Lymphedema: Histopathological Studies on Human Lymphatic Collecting Vessels

Background: Lymphedema is characterized by an accumulation of interstitial fluids due to inefficient lymphatic drainage. Primary lymphedema is a rare condition, including congenital and idiopathic forms. Secondary lymphedema is a common complication of lymph node ablation in cancer treatment. Previous studies on secondary lymphedema lymphatic vessels have shown that after an initial phase of ectasia, worsening of the disease is associated with wall thickening accompanied by a progressive loss of the endothelial marker podoplanin. Methods and Results: We enrolled 17 patients with primary and 29 patients with secondary lymphedema who underwent lymphaticovenous anastomoses surgery. Histological sections were stained with Masson's trichrome, and immunohistochemistry was performed with antibodies to podoplanin, smooth muscle α-actin (α-SMA), and myosin heavy chain 11 (MyH11). In secondary lymphedema, we found ectasis, contraction, and sclerosis vessel types. In primary lymphedema, the majority of vessels were of the sclerosis type, with no contraction vessels. In both primary and secondary lymphedema, not all α-SMA-positive cells were also positive for MyH11, suggesting transformation into myofibroblasts. The endothelial marker podoplanin had a variable expression unrelatedly with the morphological vessel type. Conclusions: Secondary lymphedema collecting vessels included all the three types described in literature, that is, ectasis, contraction, and sclerosis, whereas in primary lymphedema, we found the ectasis and the sclerosis but not the contraction type. Some cells in the media stained positively for α-SMA but not for MyH11. These cells, possibly myofibroblasts, may contribute to collagen deposition.

Mechanical Dilation Using Nylon Monofilament Aids Multisite Lymphaticovenous Anastomosis Through Improving the Quality of Anastomosis

Lymphaticovenous anastomosis (LVA) is now a common treatment for lymphedema. It is important to create as many bypasses as possible to maximize the efficacy of LVA. We have developed a method whereby nylon monofilaments are placed inside the vessel lumen to act as dilators. We refer to this technique as mechanical dilation (MD) to distinguish it from intravascular stenting. In this study, we investigated the efficacy of the conventional supermicrosurgery technique performed with and without MD as a treatment for lower limb lymphedema. The LVA was performed using conventional supermicrosurgery alone in 10 patients (group without MD) and in combination with MD in another 10 patients (group with MD). The mean number of successful LVAs performed per hour was significantly higher in the group with MD than in the group without MD (1.42 ± 0.16 vs 1.14 ± 0.15; P < 0.05). The mean amount of improvement in the lower extremity lymphedema index was significantly greater in the group with MD than in the group without MD (7.34 ± 1.57 vs 4.41 ± 1.53; P = 0.003 < 0.05). A statistically significant correlation was found between the number of successful LVAs and amount of improvement in lymphedema (r = 0.449, P = 0.047 < 0.05). Our findings suggest that use of MD does not shorten the operating time or increase the number of LVAs that can be performed but may make it possible to increase the number of successful LVAs that can be performed between vessels with a diameter of less than 0.3 mm. Use of MD could increase the improvement rate of lymphedema to a greater extent than that achieved by conventional microsurgery alone.

The Unresolved Pathophysiology of Lymphedema

Lymphedema is the clinical manifestation of impaired lymphatic transport. It remains an under-recognized and under-documented clinical condition that still lacks a cure. Despite the substantial advances in the understanding of lymphatic vessel biology and function in the past two decades, there are still unsolved questions regarding the pathophysiology of lymphedema, especially in humans. As a consequence of impaired lymphatic drainage, proteins and lipids accumulate in the interstitial space, causing the regional tissue to undergo extensive and progressive architectural changes, including adipose tissue deposition and fibrosis. These changes are also associated with inflammation. However, the temporal sequence of these events, the relationship between these events, and their interplay during the progression are not clearly understood. Here, we review our current knowledge on the pathophysiology of lymphedema derived from human and animal studies. We also discuss the possible cellular and molecular mechanisms involved in adipose tissue and collagen accumulation during lymphedema. We suggest that more studies should be dedicated to enhancing our understanding of the human pathophysiology of lymphedema to pave the way for new diagnostic and therapeutic avenues for this condition.

Preliminary Report: The Relevance of Tumor Necrosis Factor-α in Acquired Primary Lymphedema-A Histopathological Investigation

Background: Lymphedema includes primary lymphedema (P-LE) and secondary lymphedema (S-LE), which is a chronic progressive disease. The former group is further classified as congenital and acquired P-LE (AP-LE); its etiology is unclear, and only a few studies on its pathophysiology exist. We hypothesized that an autoimmune disease or self-inflammatory mechanism occurs in lymphatic vessels, leading to obstruction. Methods and Results: We enrolled 46 patients with lymphedema who underwent lymphaticovenous anastomosis (LVA) from January to October 2015. Collecting lymph ducts were obtained during LVA. We performed hematoxylin/eosin staining and immunostaining for LYVE-1, IL-1β, IL-6, and TNF-α. There were no substantial histological differences between the two types of lymphedema, whereas some differences in expression of inflammatory cytokines, including interleukin (IL)-1β and tumor necrosis factor (TNF)-α, were observed. Only a few inflammatory cells could be seen around the vessels. Although no significant differences in expression of IL-1β were found between AP-LE and S-LE, TNF-α was more highly expressed in the smooth muscle layer in AP-LE patients than in S-LE patients. There were no significant morphological differences in the collecting ducts of lymphatic vessels between S-LE and P-LE. Nevertheless, higher levels of TNF-α accumulation were found in the thick smooth muscle layer of P-LE patients than in that of S-LE patients. Conclusion: TNF-α-related inflammation in collecting ducts of lymphatic vessels is an important characteristic of the pathology of P-LE. TNF-α inhibitors might improve symptoms of AP-LE.

A novel mouse tail lymphedema model for observing lymphatic pump failure during lymphedema development

It has been suggested that many forms of secondary lymphedema in humans are driven by a progressive loss of lymphatic pump function after an initial risk-inducing event. However, the link between pump failure and disease progression has remained elusive due to experimental challenges in the clinical setting and a lack of adequate animal models. Using a novel surgical model of lymphatic injury, we track the adaptation and functional decline of the lymphatic network in response to surgery. This model mimics the histological hallmarks of the typical mouse tail lymphedema model while leaving an intact collecting vessel for analysis of functional changes during disease progression. Lymphatic function in the intact collecting vessel negatively correlated with swelling, while a loss of pumping pressure generation remained even after resolution of swelling. By using this model to study the role of obesity in lymphedema development, we show that obesity exacerbates acquired lymphatic pump failure following lymphatic injury, suggesting one mechanism through which obesity may worsen lymphedema. This lymphatic injury model will allow for future studies investigating the molecular mechanisms leading to lymphedema development.

In vitro model reveals a role for mechanical stretch in the remodeling response of lymphatic muscle cells

OBJECTIVE

Using primary LMCs in vitro, we sought to characterize the impact of LMC remodeling on their functional and molecular response to mechanical loading and culture conditions.

METHODS

Primary "wounded leg" LMCs were derived from the hindlimb of three sheep who underwent lymphatic injury 6 weeks prior, while "control leg" LMCs were derived from the contralateral, unwounded, limb. Function of the LMCs was characterized in response to media of variable levels of serum (10% vs 0.2%) and glucose (4.5 vs 1 g/L). Functional and proteomic data were evaluated in LMCs exposed to cyclic stretch (0.1 Hz, 7.5% elongation) for 1 week.

RESULTS

LMCs were sensitive to changes in serum levels, significantly reducing overall activity and collagen synthesis under low serum conditions. LMCs from the remodeled vessel had higher baseline levels of metabolic activity but not collagen synthesis. Cyclic loading induced cellular alignment perpendicular to the axis of stretch and alterations in signaling pathways associated with metabolism. Remodeled LMCs had consistently higher levels of metabolic activity and were more resistant to strain-induced apoptosis.

CONCLUSIONS

LMCs exist on a functional spectrum, becoming more active in response to stretching and maintaining phenotypic remodeling in response to local lymphatic/tissue damage.

Lymphatic Vessel Network Structure and Physiology

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

Liposuction Gives Complete Reduction of Arm Lymphedema following Breast Cancer Treatment-A 5-year Prospective Study in 105 Patients without Recurrence

BACKGROUND

Arm lymphedema is a well-recognized complication after breast cancer surgery that negatively impacts patients' quality of life, both physiologically and psychologically. Lymph stasis and inflammation result in excess formation of adipose tissue, which makes removal of the deposited subcutaneous fat necessary to eliminate the excess volume. Liposuction, combined with postoperative controlled compression therapy (CCT), is the only treatment that gives complete reduction of the excess volume. The aim of this study was to evaluate the 5-year results after liposuction in combination with CCT.

METHODS

Patients consecutively operated on between 1993 and 2012 were identified from the lymphedema registry, comprising all patients with nonpitting lymphedema treated with liposuction and CCT in our department. Standardized forms were used to collect pre-, peri-, and postoperative data.

RESULTS

One hundred five women with nonpitting edema were treated. The mean interval between the breast cancer operation and lymphedema start was 2.9 ± 5.0 years, the mean duration of lymphedema was 10 ± 7.4 years, and the preoperative mean excess volume was 1,573 ± 645 ml. The mean volume aspirated was 1,831 ± 599 ml. Postoperative mean reduction 5 years postoperatively was 117% ± 26% as compared with the healthy arm.

CONCLUSION

Liposuction is an effective method for the treatment of chronic, nonpitting, arm lymphedema resistant to conservative treatment. The volume reduction remains complete after 5 years.

Smooth muscle cell recruitment to lymphatic vessels requires PDGFB and impacts vessel size but not identity

Tissue fluid drains through blind-ended lymphatic capillaries, via smooth muscle cell (SMC)-covered collecting vessels into venous circulation. Both defective SMC recruitment to collecting vessels and ectopic recruitment to lymphatic capillaries are thought to contribute to vessel failure, leading to lymphedema. However, mechanisms controlling lymphatic SMC recruitment and its role in vessel maturation are unknown. Here, we demonstrate that platelet-derived growth factor B (PDGFB) regulates lymphatic SMC recruitment in multiple vascular beds. PDGFB is selectively expressed by lymphatic endothelial cells (LECs) of collecting vessels. LEC-specific deletion of Pdgfb prevented SMC recruitment causing dilation and failure of pulsatile contraction of collecting vessels. However, vessel remodelling and identity were unaffected. Unexpectedly, Pdgfb overexpression in LECs did not induce SMC recruitment to capillaries. This was explained by the demonstrated requirement of PDGFB extracellular matrix (ECM) retention for lymphatic SMC recruitment, and the low presence of PDGFB-binding ECM components around lymphatic capillaries. These results demonstrate the requirement of LEC-autonomous PDGFB expression and retention for SMC recruitment to lymphatic vessels, and suggest an ECM-controlled checkpoint that prevents SMC investment of capillaries, which is a common feature in lymphedematous skin.

Summary:Pdgfb mutant mice provide insight into the recruitment and function of smooth muscle cells in the lymphatic vasculature, and shed new light on mechanisms of lymph vessel-associated diseases.

Pathological changes of adipose tissue in secondary lymphoedema

BACKGROUND

The pathophysiology of lymphoedema is poorly understood. Current treatment options include compression therapy, resection, liposuction and lymphatic microsurgery, but determining the optimal treatment approach for each patient remains challenging.

OBJECTIVES

We characterized skin and adipose tissue alterations in the setting of secondary lymphoedema.

METHODS

Morphological and histopathological evaluations were conducted for 70 specimens collected from 26 female patients with lower-extremity secondary lymphoedema following surgical intervention for gynaecological cancers. Indocyanine green lymphography was performed for each patient to assess lymphoedema severity.

RESULTS

Macroscopic and ultrasound findings revealed that lymphoedema adipose tissue had larger lobules of adipose tissue, with these lobules surrounded by thick collagen fibres and interstitial lymphatic fluid. In lymphoedema specimens, adipocytes displayed hypertrophic changes and more collagen fibre deposits when examined using electron microscopy, whole-mount staining and immunohistochemistry. The number of capillary lymphatic channels was also found to be increased in the dermis of lymphoedema limbs. Crown-like structures (dead adipocytes surrounded by M1 macrophages) were less frequently seen in lymphoedema samples. Flow cytometry revealed that, among the cellular components of adipose tissue, adipose-derived stem/stromal cells and M2 macrophages were decreased in number in lymphoedema adipose tissue compared with normal controls.

CONCLUSIONS

These findings suggest that long-term lymphatic volume overload can induce chronic tissue inflammation, progressive fibrosis, impaired homeostasis, altered remodelling of adipose tissue, impaired regenerative capacity and immunological dysfunction. Further elucidation of the pathophysiological mechanisms underlying lymphoedema will lead to more reliable therapeutic strategies.

A lumped parameter model of mechanically mediated acute and long-term adaptations of contractility and geometry in lymphatics for characterization of lymphedema

A primary purpose of the lymphatic system is to transport fluid from peripheral tissues to the central venous system in order to maintain tissue-fluid balance. Failure to perform this task results in lymphedema marked by swelling of the affected limb as well as geometric remodeling and reduced contractility of the affected lymphatic vessels. The mechanical environment has been implicated in the regulation of lymphatic contractility, but it is unknown how changes in the mechanical environment are related to loss of contractile function and remodeling of the tissue. The purpose of this paper was to introduce a new theoretical framework for acute and long-term adaptations of lymphatic vessels to changes in mechanical loading. This theoretical framework combines a simplified version of a published lumped parameter model for lymphangion function and lymph transport, a published microstructurally motivated constitutive model for the active and passive mechanical behavior of isolated rat thoracic ducts, and novel models for acute mechanically mediated vasoreactive adaptations and long-term volumetric growth to simulate changes in muscle contractility and geometry of a single isolated rat thoracic duct in response to a sustained elevation in afterload. The illustrative examples highlight the potential role of the mechanical environment in the acute maintenance of contractility and long-term geometric remodeling, presumably aimed at meeting fluid flow demands while also maintaining mechanical homeostasis. Results demonstrate that contractility may adapt in response to shear stress to meet fluid flow demands and show that pressure-induced long-term geometric remodeling may attenuate these adaptations and reduce fluid flow. The modeling framework and illustrative simulations help suggest relevant experiments that are necessary to accurately quantify and predict the acute and long-term adaptations of lymphangions to altered mechanical loading.

Intralymphatic histiocytosis comprises M2 macrophages in superficial dermal lymphatics with or without smooth muscles

Intralymphatic histiocytosis represents a rare reactive disorder, which is characterized by the accumulation of macrophages within lymphatic vessels and observed predominantly in upper extremities. The infiltration and preferential M2 differentiation of macrophage are observed in chronic lymphedema, and lymphedema is considered a causative factor of intralymphatic histiocytosis. However, what causes accumulation of histiocytes in the lymphatic vessels remains unclear, and investigation regarding the characteristics of the macrophages has not been evaluated. We present a case of intralymphatic histiocytosis, in which immunohistochemical staining for both macrophages and lymphatic vessels was performed to evaluate the nature of macrophages within lymphatic vessels and to determine the causative factor. Aggregated macrophages were shown to be M2 macrophages positive for CD68, CD163 and CD206 but negative for inducible nitric oxide synthase. Thick lymphatic vessels positive for D2-40 and α-SMA in the superficial dermis were observed. We speculate that chronic lymphedema leads to hypertrophy of lymphatic vessels with smooth muscle in the superficial dermis, which may be a kind of malformation, and these lymphatic vessels produce some chemokines that induce intralymphatic aggregation of macrophages.

Novel Mechanisms of Compromised Lymphatic Endothelial Cell Homeostasis in Obesity: The Role of Leptin in Lymphatic Endothelial Cell Tube Formation and Proliferation

Leptin is a hormone produced by adipose tissue that regulates various physiological processes. Recent studies have shown that the level of circulating leptin is elevated in obese patients and have suggested a relationship between obesity and postoperative lymphedema. However, the mechanisms by which postoperative lymphedema develops in obese patients and the mechanisms by which leptin regulates lymphatic endothelial cell homeostasis such as tube formation and cell proliferation remain unknown. Here we report that leptin regulates tube formation and cell proliferation in human dermal lymphatic endothelial cells (HDLECs) by activation of the signal transducer and activator of transcription 3 pathway, which is downstream signaling of the leptin receptor. Additionally, we found that upregulation of suppressor of cytokine signaling 3 underlies the mechanisms by which a high dose of leptin inhibits cell proliferation and tube formation. Leptin also enhanced expression of the proinflammatory cytokine IL-6 in HDLECs. Interestingly, IL-6 rescues the compromised cell proliferation and tube formation caused by treatment with a high dose of leptin in an autocrine or paracrine manner. Taken together, our findings reveal a novel mechanism by which compromised HDLECs maintain their homeostasis during inflammation mediated by leptin and IL-6. Thus, regulating the level of leptin or IL-6 may be a viable strategy to reduce the incidence of postoperative lymphedema.