Hydrodynamic regulation of lymphatic transport and the impact of aging

The Lymphatic Vascular System: Does Nonuniform Lymphangion Length Limit Flow-Rate?

A previously developed model of a lymphatic vessel as a chain of lymphangions was investigated to determine whether lymphangions of unequal length reduce pumping relative to a similar chain of equal-length ones. The model incorporates passive elastic and active contractile properties taken from ex vivo measurements, and intravascular lymphatic valves as transvalvular pressure-dependent resistances to flow with hysteresis and transmural pressure-dependent bias to the open state as observed experimentally. Coordination of lymphangion contractions is managed by marrying an autonomous transmural pressure-dependent pacemaker for each lymphangion with bidirectional transmission of activation signals between lymphangions, qualitatively matching empirical observations. With eight lymphangions as used here and many nonlinear constraints, the model is capable of complex outcomes. The expected flow-rate advantage conferred by longer lymphangions everywhere was confirmed. However, the anticipated advantage of uniform lymphangions over those of unequal length, compared in chains of equal overall length, was not found. A wide variety of dynamical outcomes was observed, with the most powerful determinant being the adverse pressure difference, rather than the arrangement of long and short lymphangions. This work suggests that the wide variation in lymphangion length which is commonly observed in collecting lymphatic vessels does not confer disadvantage in pumping lymph.

Lymphatic Capillarization in Different Fiber Types of Rat Skeletal Muscles With Growth and Age

OBJECTIVE

To clarify the effect of growth and advancing age on lymphatic capillarization in rat skeletal muscles, we examined the histological and biochemical changes of lymphatic capillaries in different fiber types of skeletal muscles across juvenile, young, and middle-aged generations.

METHODS

We collected the tibialis anterior (TA), extensor digitorum longus (EDL), and soleus (SOL) muscles. Immunohistochemical staining using LYVE-1 and CD31 markers was used for lymphatic and blood capillaries, respectively. Real-time PCR was used to analyze mRNA expression of lymphangiogenic factors.

RESULTS

The density of LYVE-1-positive lymphatic capillaries in the muscles peaked during the juvenile period and subsequently decreased with increasing age. In contrast to blood capillaries, fast-twitch dominant muscles (i.e., TA and EDL) exhibited an age-related decrease in lymphatic capillaries. Similar to blood capillaries, lymphatic capillaries were abundant in SOL, a slow-twitch dominant muscle, which showed less susceptibility to age-related lymphatic decline. The mRNA expression of lymphangiogenic factors was significantly upregulated in SOL and decreased in all muscles of middle-aged rats.

CONCLUSIONS

The age-related decrease of lymphatic capillaries in fast-twitch muscles might be associated with age-related muscle atrophy.

Signaling mechanisms underlying lymphatic vessel dysfunction in skin aging and possible anti-aging strategies

Aging-related skin diseases are gradually increasing due to the imbalance of cutaneous homeostasis in the aging population. Skin aging-induced inflammation promotes systemic inflammation and may lead to whole-body aging. Lymphatic vessels play an important role in maintaining fluid and homeostasis balance. In intrinsically aged skin, the number of lymphatic vessels decrease and their functions decline, which is related to the reduced adhesion junctions between lymphatic endothelial cells, particularly VE-cadherin. VEGFC/VEGFR-3 signal pathway plays an important role in remodeling and expansion of lymphatic vessels; the downregulation of this pathway contributes to the dysfunction of lymphatic vessels. Meanwhile, we proposed some additional mechanisms. Decline of the pumping activity of lymphatic vessels might be related to age-related changes in extracellular matrix, ROS increase, and eNOS/iNOS disturbances. In extrinsically aged skin, the hyperpermeability of lymphatic vessels results from a decrease in endothelial-specific tight junction molecules, upregulation of VEGF-A, and downregulation of the VEGFC/VEGFR-3 signaling pathway. Furthermore, some of the Phyto therapeutics could attenuate skin aging by modulating the lymphatic vessels. This review summarized the lymphatic vessel dysfunction in skin aging and anti-aging strategies based on lymphatic vessel modulation.

Artificial intelligence velocimetry reveals in vivo flow rates, pressure gradients, and shear stresses in murine perivascular flows

Quantifying the flow of cerebrospinal fluid (CSF) is crucial for understanding brain waste clearance and nutrient delivery, as well as edema in pathological conditions such as stroke. However, existing in vivo techniques are limited to sparse velocity measurements in pial perivascular spaces (PVSs) or low-resolution measurements from brain-wide imaging. Additionally, volume flow rate, pressure, and shear stress variation in PVSs are essentially impossible to measure in vivo. Here, we show that artificial intelligence velocimetry (AIV) can integrate sparse velocity measurements with physics-informed neural networks to quantify CSF flow in PVSs. With AIV, we infer three-dimensional (3D), high-resolution velocity, pressure, and shear stress. Validation comes from training with 70% of PTV measurements and demonstrating close agreement with the remaining 30%. A sensitivity analysis on the AIV inputs shows that the uncertainty in AIV inferred quantities due to uncertainties in the PVS boundary locations inherent to in vivo imaging is less than 30%, and the uncertainty from the neural net initialization is less than 1%. In PVSs of N = 4 wild-type mice we find mean flow speed 16.33 ± 11.09 µm/s, volume flow rate 2.22 ± 1.983 × 103 µm3/s, axial pressure gradient ( - 2.75 ± 2.01)×10-4 Pa/µm (-2.07 ± 1.51 mmHg/m), and wall shear stress (3.00 ± 1.45)×10-3 Pa (all mean ± SE). Pressure gradients, flow rates, and resistances agree with prior predictions. AIV infers in vivo PVS flows in remarkable detail, which will improve fluid dynamic models and potentially clarify how CSF flow changes with aging, Alzheimer's disease, and small vessel disease.

Mapping the lymphatic system across body scales and expertise domains: A report from the 2021 National Heart, Lung, and Blood Institute workshop at the Boston Lymphatic Symposium

Enhancing our understanding of lymphatic anatomy from the microscopic to the anatomical scale is essential to discern how the structure and function of the lymphatic system interacts with different tissues and organs within the body and contributes to health and disease. The knowledge of molecular aspects of the lymphatic network is fundamental to understand the mechanisms of disease progression and prevention. Recent advances in mapping components of the lymphatic system using state of the art single cell technologies, the identification of novel biomarkers, new clinical imaging efforts, and computational tools which attempt to identify connections between these diverse technologies hold the potential to catalyze new strategies to address lymphatic diseases such as lymphedema and lipedema. This manuscript summarizes current knowledge of the lymphatic system and identifies prevailing challenges and opportunities to advance the field of lymphatic research as discussed by the experts in the workshop.

Cellular Senescence as a Brake or Accelerator for Oncogenic Transformation and Role in Lymphatic Metastasis

Cellular senescence-the irreversible cell cycle arrest driven by a variety of mechanisms and, more specifically, the senescence-associated secretory phenotype (SASP)-is an important area of research in the context of different age-related diseases, such as cardiovascular disease and cancer. SASP factors play both beneficial and detrimental roles in age-related disease progression depending on the source of the SASPs, the target cells, and the microenvironment. The impact of senescence and the SASP on different cell types, the immune system, and the vascular system has been widely discussed. However, the impact of replicative or stress-induced senescence on lymphatic biology and pathological lymphangiogenesis remains underexplored. The lymphatic system plays a crucial role in the maintenance of body fluid homeostasis and immune surveillance. The perturbation of lymphatic function can hamper normal physiological function. Natural aging or stress-induced premature aging influences the lymphatic vessel structure and function, which significantly affect the role of lymphatics in tumor dissemination and metastasis. In this review, we focus on the role of senescence on lymphatic pathobiology, its impact on cancer, and potential therapeutic interventions to manipulate the aged or senescent lymphatic system for disease management.

Perivascular mechanical environment: A narrative review of the role of externally applied mechanical force in the pathogenesis of atherosclerosis

Atherosclerosis is promoted by systemic factors, such as dyslipidemia, hypertension, diabetes, and smoking, which cause atherosclerosis in blood vessels throughout the body. However, atherosclerotic lesions are characterized by their frequent occurrence in specific vessels and sites. Blood vessels are exposed to various mechanical forces related to blood pressure and flow. Although shear stress promotes the initiation and progression of atherosclerotic lesions, the pathogenesis of site specificity of atherosclerosis is not sufficiently explained by shear stress. We propose the concept of a perivascular mechanical environment (PVME). Compelling evidence suggests that site specificity in atherosclerotic lesions depends on a distinct local PVME. Atheroprone arteries, such as the coronary artery, are markedly affected by externally applied mechanical force (EMF), whereas atheroprotective arteries, such as the internal thoracic artery, are less affected. Recent studies have shown that the coronary artery is affected by cardiac muscle contraction, the carotid artery by the hyoid bone and the thyroid cartilage, and the abdominal aorta and lower extremity arteries by musculoskeletal motion. We speculate that the thoracic cage protects the internal thoracic artery from EMF owing to a favorable PVME. Furthermore, evidence suggests that plaque eccentricity is provided by EMF; plaques are frequently observed on an external force-applied side. In each vascular tree, site-specific characteristics of the PVME differ substantially, inducing individual atherogenicity. From the perspective of the mechanical environment, hemodynamic stress occurs in an inside-out manner, whereas EMF occurs in an outside-in manner. These inward and outward forces apply mechanical load individually, but interact synergistically. The concept of a PVME is a novel pathogenesis of atherosclerosis and also might be a pathogenesis of other arterial diseases.

Vmeasur: A software package for experimental and clinical measurement of mesenteric lymphatic contractile function over an extended vessel length

OBJECTIVE

Conventionally, in vivo mesenteric lymphatic contractile function is measured using a high magnification transmission microscope (field of view 0.3-1.5 mm), which precludes visualization of extended lengths of vessels embedded in mesenteric fat. Existing software is not optimized for imaging at a low magnification using a contrast agent. We aimed to develop a simple and clinically transferable method for in situ visualization, image analysis, and quantitative assessment of mesenteric lymphatic contractile function over an extended area.

METHODS

Subserosal injection of various blue dyes was taken up by mesenteric lymphatics and visualized under a stereomicroscope (25×), allowing for video recording of 1.4 × 1.1 cm of intact mesentery. A new R package ("vmeasur") that combines multiple high-performance image analyses into a single workflow was developed. The edges of each vessel were determined by applying an automated threshold to each frame (with real-time manual verification). The vessel width at every point in each frame was plotted to provide contractile parameters over time and along the lymphatic vessel length.

RESULTS

Contractile parameters and their differences along the length of the vessel were accurately calculated in a rodent model. In a human mesenteric lymphatic, the algorithm was also able to measure changes in diameter over length.

CONCLUSION

This software offers a low cost, rapid, and accessible method to measure lymphatic contractile function over a wide area, showing differences in contractility along the length of a vessel. Because the presence of mesenteric fat has less of an impact on imaging, due to the use of an exogenous contrast agent, there is potential for clinical application.

The application of artificial gravity in medicine and space

Gravity plays a crucial role in physiology. The lack of gravity, like in long duration spaceflight missions, cause pathologies in e.g., the musculoskeletal system, cardiovascular deconditioning, immune system deprivation or brain abnormalities, to just mention a few. The application of artificial gravity through short-arm human centrifugation (SAHC) has been studied as a possible countermeasure to treat spaceflight deconditioning. However, hypergravity protocols applied by using SAHC have also been used to treat different, ground-based pathologies. Such gravitational therapies have been applied in Uruguay for more than four decades now. The aim of this overview is to summarize the most important findings about the effects of gravitational therapy in different, mainly vascular based pathologies according to the experience in the Gravitational Therapy Center and to discuss the current research in the field of hypergravity applications in medicine but also as multisystem countermeasure for near weightlessness pathologies. New insight is needed on the use of hypergravity in medicine and space research and application.

Infectious eccrine hidradenitis: sweat glands as the portal of entry for cellulitis

Neutrophilic eccrine hidradenitis (NEH) is a rare neutrophilic dermatosis involving the eccrine glands. It is commonly associated with haematological malignancy and administration of chemotherapy. An infective aetiology for NEH is termed infectious eccrine hidradenitis (IEH). Pathogens that have been associated with IEH include Nocardia, Serratia, Enterobacter sp., Staphylococcus aureus and Mycobacterium chelonae We describe a case of IEH in a patient with prolonged use of a compression sleeve for their upper limb lymphoedema. The histopathological findings of NEH and IEH are almost identical. Skin tissue culture and rapid clinical improvement with antibiotic therapy are keys in delineating the two subtypes.

Structural and Functional Changes in Aged Skin Lymphatic Vessels

Lymphatic structure and function play a critical role in fluid transport, antigen delivery, and immune homeostasis. A dysfunctional lymphatic system is associated with chronic low-grade inflammation of peripheral tissues, poor immune responses, and recurrent infections, which are also hallmarks of aging pathology. Previous studies have shown that aging impairs lymphatic structure and function in a variety of organ systems, including the intestines and central nervous system. However, previous studies are mostly limited to qualitative analysis of lymphatic structural changes and quantification of intestinal collecting vessel contractile function. It is not clear whether decreased lymphatic function contributes to pathological conditions related to aging, nor how it affects the skin immune microenvironment. Further, the effects of aging on skin initial and collecting lymphatic vessels, dendritic cell (DC) migration, cutaneous lymphatic pumping, and VEGFR-3 signaling in lymphatic endothelial cells (LECs) have not been quantitatively analyzed. Here, using fluorescent immunohistochemistry and flow cytometry, we confirm that aging decreases skin initial and collecting lymphatic vessel density. Indocyanine green (ICG) lymphangiography and DC migration assays confirm that aging decreases both fluid pumping and cell migration via lymphatic vessels. At the cellular level, aging causes decreased VEGFR-3 signaling, leading to increased LEC apoptosis and senescence. Finally, we determined that aging causes decreased lymphatic production of chemokines and alters LEC expression of junctional and adhesion molecules. This in turn leads to increased peri-lymphatic inflammation and nitrosative stress that might contribute to aging pathology in a feed-forward manner. Taken together, our study, in addition to quantitatively corroborating previous findings, suggests diverse mechanisms that contribute to lymphatic dysfunction in aging that in turn exacerbate the pathology of aging in a feed-forward manner.

Intracellular calcium dynamics of lymphatic endothelial and muscle cells co-cultured in a Lymphangion-Chip under pulsatile flow

A Lymphangion-Chip consisting an endothelial lumen co-cultured with muscle cells was exposed to step or pulsatile flow. The real-time analyses of intracellular calcium dynamics reveal the coupling of signaling between these cells under complex flows.

Adipose Tissue and Biological Factors. Possible Link between Lymphatic System Dysfunction and Obesity

The World Health Organization (WHO) has recognised obesity as one of the top ten threats to human health. Obesity is not only a state of abnormally increased adipose tissue in the body, but also of an increased release of biologically active metabolites. Moreover, obesity predisposes the development of metabolic syndrome and increases the incidence of type 2 diabetes (T2DM), increases the risk of developing insulin resistance, atherosclerosis, ischemic heart disease, polycystic ovary syndrome, hypertension and cancer. The lymphatic system is a one-directional network of thin-walled capillaries and larger vessels covered by a continuous layer of endothelial cells that provides a unidirectional conduit to return filtered arterial and tissue metabolites towards the venous circulation. Recent studies have shown that obesity can markedly impair lymphatic function. Conversely, dysfunction in the lymphatic system may also be involved in the pathogenesis of obesity. This review highlights the important findings regarding obesity related to lymphatic system dysfunction, including clinical implications and experimental studies. Moreover, we present the role of biological factors in the pathophysiology of the lymphatic system and we propose the possibility of a therapy supporting the function of the lymphatic system in the course of obesity.

Lymphatic Contractile Function: A Comprehensive Review of Drug Effects and Potential Clinical Application

BACKGROUND

The lymphatic system and the cardiovascular system work together to maintain body fluid homeostasis. Despite that, the lymphatic system has been relatively neglected as a potential drug target and a source of adverse effects from cardiovascular drugs. Like the heart, the lymphatic vessels undergo phasic contractions to promote lymph flow against a pressure gradient. Dysfunction or failure of the lymphatic pump results in fluid imbalance and tissue oedema. While this can due to drug effects, it is also a feature of breast cancer-associated lymphoedema, chronic venous insufficiency, congestive heart failure and acute systemic inflammation. There are currently no specific drug treatments for lymphatic pump dysfunction in clinical use despite the wealth of data from pre-clinical studies.

AIM

To identify (1) drugs with direct effects on lymphatic tonic and phasic contractions with potential for clinical application, and (2) drugs in current clinical use that have a positive or negative side effect on lymphatic function.

METHODS

We comprehensively reviewed all studies that tested the direct effect of a drug on the contractile function of lymphatic vessels.

RESULTS

Of the 208 drugs identified from 193 studies, about a quarter had only stimulatory effects on lymphatic tone, contraction frequency and/or contraction amplitude. Of FDA-approved drugs, there were 14 that increased lymphatic phasic contractile function. The most frequently used class of drug with inhibitory effects on lymphatic pump function were the calcium channels blockers.

CONCLUSION

This review highlights the opportunity for specific drug treatments of lymphatic dysfunction in various disease states and for avoiding adverse drug effects on lymphatic contractile function.

Go with the flow: modeling unique biological flows in engineered in vitro platforms

Interest in recapitulating in vivo phenomena in vitro using organ-on-a-chip technology has grown rapidly and with it, attention to the types of fluid flow experienced in the body has followed suit. These platforms offer distinct advantages over in vivo models with regards to human relevance, cost, and control of inputs (e.g., controlled manipulation of biomechanical cues from fluid perfusion). Given the critical role biophysical forces play in several tissues and organs, it is therefore imperative that engineered in vitro platforms capture the complex, unique flow profiles experienced in the body that are intimately tied with organ function. In this review, we outline the complex and unique flow regimes experienced by three different organ systems: blood vasculature, lymphatic vasculature, and the intestinal system. We highlight current state-of-the-art platforms that strive to replicate physiological flows within engineered tissues while introducing potential limitations in current approaches.

Lymphatic Senescence: Current Updates and Perspectives

Simple Summary

The lymphatic system is involved in tissue homeostasis, immune processes as well as transport of lipids, proteins and pathogens. Aging affects all physiological systems. However, it is not well studied how aging affects the lymphatic vasculature. Therefore, this review aims at investigating how senescence could lead to changes in the structure and function of the lymphatic vessels. We report that lymphatic senescence is associated with alterations in lymphatic muscles and nerve fibers, lymphatic endothelial cells membrane dysfunction, as well as changes in lymphatic pump, acute inflammation responses and immune function.

Abstract

Lymphatic flow is necessary for maintenance of vital physiological functions in humans and animals. To carry out optimal lymphatic flow, adequate contractile activity of the lymphatic collectors is necessary. Like in all body systems, aging has also an effect on the lymphatic system. However, limited knowledge is available on how aging directly affects the lymphatic system anatomy, physiology and function. We investigated how senescence leads to alterations in morphology and function of the lymphatic vessels. We used the strategy of a review to summarize the scientific literature of studies that have been published in the area of lymphatic senescence. Searches were carried out on PubMed and Web of Science using predefined search queries. We obtained an initial set of 1060 publications. They were filtered to 114 publications based on strict inclusion and exclusion criteria. Finally, the most appropriate 57 studies that specifically addressed lymphatic senescence have been selected for the preparation of this review. Analysis of the literature showed that lymphatic senescence is associated with alterations in lymphatic muscles and nerve fibers, lymphatic glycocalyx function of lymphatic endothelial cells, effects of chronic ultraviolet light exposure and oxidative stress as well as changes in lymphatic pump, acute inflammation responses and immune function. The current review underscores the relevance of the understudied area of lymphatic senescence. Continued research on the impact of aging on the structure and function of the lymphatic vasculature is needed to provide further insights to develop innovative clinical diagnostic—and treatment—modalities as well as to reduce the morbidity associated with diseases related to the lymphatic system.

Characterization of rat tail lymphatic contractility and biomechanics: incorporating nitric oxide-mediated vasoregulation

The lymphatic system transports lymph from the interstitial space back to the great veins via a series of orchestrated contractions of chains of lymphangions. Biomechanical models of lymph transport, validated with ex vivo or in vivo experimental results, have proved useful in revealing novel insight into lymphatic pumping; however, a need remains to characterize the contributions of vasoregulatory compounds in these modelling tools. Nitric oxide (NO) is a key mediator of lymphatic pumping. We quantified the active contractile and passive biaxial biomechanical response of rat tail collecting lymphatics and changes in the contractile response to the exogenous NO administration and integrated these findings into a biomechanical model. The passive mechanical response was characterized with a three-fibre family model. Nonlinear regression and non-parametric bootstrapping were used to identify best-fit material parameters to passive cylindrical biaxial mechanical data, assessing uniqueness and parameter confidence intervals; this model yielded a good fit (R² = 0.90). Exogenous delivery of NO via sodium nitroprusside (SNP) elicited a dose-dependent suppression of contractions; the amplitude of contractions decreased by 30% and the contraction frequency decreased by 70%. Contractile function was characterized with a modified Rachev-Hayashi model, introducing a parameter that is related to SNP concentration; the model provided a good fit (R² = 0.89) to changes in contractile responses to varying concentrations of SNP. These results demonstrated the significant role of NO in lymphatic pumping and provide a predictive biomechanical model to integrate the combined effect of mechanical loading and NO on lymphatic contractility and mechanical response.

Lymphatic remodelling in response to lymphatic injury in the hind limbs of sheep

Contractile activity in the lymphatic vasculature is essential for maintaining fluid balance within organs and tissues. However, the mechanisms by which collecting lymphatics adapt to changes in fluid load and how these adaptations influence lymphatic contractile activity are unknown. Here we report a model of lymphatic injury based on the ligation of one of two parallel lymphatic vessels in the hind limb of sheep and the evaluation of structural and functional changes in the intact, remodelling lymphatic vessel over a 42-day period. We show that the remodelled lymphatic vessel displayed increasing intrinsic contractile frequency, force generation and vessel compliance, as well as decreasing flow-mediated contractile inhibition via the enzyme endothelial nitric oxide synthase. A computational model of a chain of lymphatic contractile segments incorporating these adaptations predicted increases in the flow-generation capacity of the remodelled vessel at the expense of normal mitochondrial function and elevated oxidative stress within the lymphatic muscle. Our findings may inform interventions for mitigating lymphatic muscle fatigue in patients with dysfunctional lymphatics.

The Biophysics of Lymphatic Transport: Engineering Tools and Immunological Consequences

Lymphatic vessels mediate fluid flows that affect antigen distribution and delivery, lymph node stromal remodeling, and cell-cell interactions, to thus regulate immune activation. Here we review the functional role of lymphatic transport and lymph node biomechanics in immunity. We present experimental tools that enable quantitative analysis of lymphatic transport and lymph node dynamics in vitro and in vivo. Finally, we discuss the current understanding for how changes in lymphatic transport and lymph node biomechanics contribute to pathogenesis of conditions including cancer, aging, neurodegeneration, and infection.

Lymphatic Cannulation for Lymph Sampling and Molecular Delivery

Unlike the blood, the interstitial fluid and the deriving lymph are directly bathing the cellular layer of each organ. As such, composition analysis of the lymphatic fluid can provide more precise biochemical and cellular information on an organ's health and be a valuable resource for biomarker discovery. In this study, we describe a protocol for cannulation of mouse and rat lymphatic collectors that is suitable for the following: the "omic" sampling of pre- and postnodal lymph, collected from different anatomical districts; the phenotyping of immune cells circulating between parenchymal organs and draining lymph nodes; injection of known amounts of molecules for quantitative immunological studies of nodal trafficking and/or clearance; and monitoring an organ's biochemical omic changes in pathological conditions. Our data indicate that probing the lymphatic fluid can provide an accurate snapshot of an organ's physiology/pathology, making it an ideal target for liquid biopsy.

Pathophysiology of aged lymphatic vessels

Lymphatic vessels maintain body homeostasis by recirculation of fluid and cells. Cell senescence induces lymphatic dysfunction. Impaired contractile function is caused by low muscle cell investiture and decrease of nitric oxide in aged lymphatic collectors, leading to poor drainage of lymph. Aging-induced loss of endothelial glycocalyx and production of inflammatory cytokines increases permeability of lymphatic vessels. In addition, aging-associated basal activation of mast cells delays immune response. In this review, we summarize the structural and pathological changes of aged lymphatic vessels, and discuss the underlying molecular mechanisms.

Arterial Lymphatics in Atherosclerosis: Old Questions, New Insights, and Remaining Challenges

The lymphatic network is well known for its role in the maintenance of tissue fluid homeostasis, absorption of dietary lipids, trafficking of immune cells, and adaptive immunity. Aberrant lymphatic function has been linked to lymphedema and immune disorders for a long time. Discovery of lymphatic cell markers, novel insights into developmental and postnatal lymphangiogenesis, development of genetic mouse models, and the introduction of new imaging techniques have improved our understanding of lymphatic function in both health and disease, especially in the last decade. Previous studies linked the lymphatic vasculature to atherosclerosis through regulation of immune responses, reverse cholesterol transport, and inflammation. Despite extensive research, many aspects of the lymphatic circulation in atherosclerosis are still unknown and future studies are required to confirm that arterial lymphangiogenesis truly represents a therapeutic target in patients with cardiovascular disease. In this review article, we provide an overview of factors and mechanisms that regulate lymphangiogenesis, summarize recent findings on the role of lymphatics in macrophage reverse cholesterol transport, immune cell trafficking and pathogenesis of atherosclerosis, and present an overview of pharmacological and genetic strategies to modulate lymphatic vessel density in cardiovascular tissue.

Entrainment of Lymphatic Contraction to Oscillatory Flow

Lymphedema, a disfiguring condition characterized by an asymmetrical swelling of the limbs, is suspected to be caused by dysfunctions in the lymphatic system. A possible source of lymphatic dysfunction is the reduced mechanosensitivity of lymphangions, the spontaneously contracting units of the lymphatic system. In this study, the entrainment of lymphangions to an oscillatory wall shear stress (OWSS) is characterized in rat thoracic ducts in relation to their shear sensitivity. The critical shear stress above which the thoracic ducts show a substantial inhibition of contraction was found to be significantly negatively correlated to the diameter of the lymphangion. The entrainment of the lymphangion to an applied OWSS was found to be significantly dependent on the difference between the applied frequency and the intrinsic frequency of contraction of the lymphangion. The strength of the entrainment was also positively correlated to the applied shear stress when the applied shear was less than the critical shear stress of the vessel. The ejection fraction and fractional pump flow were also affected by the difference between the frequency of the applied OWSS and the vessel's intrinsic contraction frequency. The results suggest an adaptation of the lymphangion contractility to the existing oscillatory shear stress as a function of its intrinsic contractility and shear sensitivity. These adaptations might be crucial to ensure synchronized contraction of lymphangions through mechanosensitive means and might help explain the lymphatic dysfunctions that result from impaired mechanosensitivity.

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.

Temporal Dynamics of the Rat Thoracic Duct Contractility in the Presence of Imposed Flow

BACKGROUND

The initial periods of increased flow inside lymphatic vessels demonstrate specific temporary patterns of self-tuning of lymphatic vessel contractility that are heterogeneous across regional lymphatic networks. The current literature primarily refers to the immediate and fast reactions of the lymphangions to increases in basal flow. Until now, there were no available data on how the lymphatic vessels react to comparatively longer periods of imposed flow.

METHODS AND RESULTS

In this study, we measured and analyzed the contractility of the rat thoracic duct segments, isolated, cannulated, and pressurized at 3 cm H₂O at no imposed flow conditions and during 4 hours of imposed flow (constant transaxial pressure gradient of 2 cm H₂O). We found the development of a progressing lymphatic tonic relaxation and inhibition of the lymphatic contraction frequency over 4 hours of imposed flow. After a short initial decrease, lymphatic phasic contraction amplitude rose significantly during the first hour of imposed flow, and it demonstrated a trend to return toward control levels after 3 hours of imposed flow. As a result, the fractional pump flow (active lymph pumping per minute) of isolated thoracic duct segments reached and maintained a statistically significant decrease (from control no-flow conditions) at the end of the third hour of imposed flow.

CONCLUSIONS

Our new findings provide a better understanding of how lymphatic contractility changes during the development of prolonged periods of steady lymph flow. The latter may occur during the initial phases of development of an inflammatory-related tissue edema.

Quantitative Profiling of the Lymph Node Clearance Capacity

Transport of tissue-derived lymphatic fluid and clearance by draining lymph nodes are pivotal for maintenance of fluid homeostasis in the body and for immune-surveillance of the self- and non-self-proteomes. Yet a quantitative analysis of nodal filtration of the tissue-derived proteome present in lymphatic fluid has not been reported. Here we quantified the efficiency of nodal clearance of the composite proteomic load using label-free and isotope-labeling proteomic analysis of pre-nodal and post-nodal samples collected by direct cannulation. These results were extended by quantitation of the filtration efficiency of fluorophore-labeled proteins, bacteria, and beads infused at physiological flow rates into pre-nodal lymphatic collectors and collected by post-nodal cannulation. We developed a linear model of nodal filtration efficiency dependent on pre-nodal protein concentrations and molecular weight, and uncovered criteria for disposing the proteome incoming from defined anatomical districts under physiological conditions. These findings are pivotal to understanding the maximal antigenic load sustainable by a draining node, and promote understanding of pathogen spreading and nodal filtration of tumor metastasis, potentially helping to improve design of vaccination protocols, immunization strategies and drug delivery.

Magnetic resonance thoracic ductography assessment of serial changes in the thoracic duct after the intake of a fatty meal

The thoracic duct, a terminal lymph vessel, is thought to dilate after the intake of a fatty meal. However, this physiological change has not been well explored in vivo. Therefore, the present study aimed to assess serial changes in the thoracic duct after the intake of a fatty meal using magnetic resonance thoracic ductography (MRTD). Eight healthy volunteers were subjected to one MRTD scan before a fatty meal and eight serial MRTD scans every hour thereafter. The cross-sectional areas of the thoracic duct were estimated using MRTD measurements of the diameters of the thoracic duct at the upper edge of the aortic arch, the tracheal bifurcation, the mid-point between the tracheal bifurcation and the left part of the diaphragm and the left part of the diaphragm. The change-rates in these areas were calculated before and after the fatty meal intake, and the maximal change-rate and timing of its achievement were determined for each subject. The summed change-rates in the four portions of the thoracic duct ranged from -40.1 to 81.3%, with maximal change-rates for each subject ranging from 22.8 to 81.3% (mean, 50.4%). Although individual variations were observed, most subjects (88.9%) exhibited a maximal change-rate at 4-6 h after meal intake, with subsequent decreases at 7-8 h. In conclusion, MRTD revealed a tendency toward thoracic duct enlargement at 4-6 h after the intake of a fatty meal, followed by contraction.

Opportunities for Studying the Hydrodynamic Context for Breast Cancer Cell Spread Through Lymph Flow

The lymphatic system serves as the primary route for the metastatic spread of breast cancer cells (BCCs). A scarcity of information exists with regard to the advection of BCCs in lymph flow and a fundamental understanding of the response of BCCs to the forces in the lymphatics needs to be established. This review summarizes the flow environment metastatic BCCs are exposed to in the lymphatics. Special attention is paid to the behavior of cells/particles in microflows in an attempt to elucidate the behavior of BCCs under lymph flow conditions (Reynolds number <1).

Histamine as an Endothelium-Derived Relaxing Factor in Aged Mesenteric Lymphatic Vessels

BACKGROUND

Knowledge of the mechanisms by which aging affects contracting lymphatic vessels remains incomplete; therefore, the functional role of histamine in the reaction of aged lymphatic vessels to increases in flow remains unknown.

METHODS AND RESULTS

We measured and analyzed parameters of lymphatic contractility in isolated and pressurized rat mesenteric lymphatic vessels (MLVs) obtained from 9- and 24-month Fischer-344 rats under control conditions and after pharmacological blockade of nitric oxide (NO) by Nω-Nitro-L-arginine methyl ester hydrochloride (L-NAME, 100 μM) or/and blockade of histamine production by α-methyl-DL-histidine dihydrochloride (α-MHD, 10 μM). We also quantitatively compared results of immunohistochemical labeling of the histamine-producing enzyme, histidine decarboxylase (HDC) in adult and aged MLVs. Our data provide the first demonstration of an increased functional role of histamine as an endothelial-derived relaxing factor in aged MLVs, which appears in parallel with the abolished role of NO in the reactions of these lymph vessels to increases in flow. In addition, we found an increased expression of HDC in endothelium of aged MLVs.

CONCLUSIONS

Our findings provide the basis for better understanding of the processes of aging in lymphatic vessels and for setting new important directions for investigations of the aging-associated disturbances in lymph flow and the immune response.

Lower Extremity Lymphedema Management after Gynecologic Cancer Surgery: A Review of Current Management Strategies

Lymphedema can be described as an accumulation of protein-rich fluid in interstitial spaces. It affects patients in multiple aspects. Gynecologic cancer survivors might experience lower extremity lymphedema after cancer surgery or treatment. In literature, most of the studies have been performed on upper extremity lymphedema. As gynecologic cancer malignancies have increased in the recent years, treatment options and related complications have been gaining attention in studies. In this manner, this review focused on the management of lower extremity lymphedema after gynecologic surgery. Studies indicated that the incidence of lower extremity lymphedema ranges between 2.4% and 41% after pelvic lymph node dissection in patients with gynecologic malignancies. Thus, management of lower extremity lymphedema in patients after gynecologic cancer surgery is an important issue. Complex decongestive therapy method is still the gold standard of lymphedema management. Controlling, evaluating, and preventing the risk factors are also substantial points; hence, it is very important to provide accurate knowledge in the management of lower extremity lymphedema.

Aged Lymphatic Vessels and Mast Cells in Perilymphatic Tissues

This review provides a comprehensive summary of research on aging-associated alterations in lymphatic vessels and mast cells in perilymphatic tissues. Aging alters structure (by increasing the size of zones with low muscle cell investiture), ultrastructure (through loss of the glycocalyx), and proteome composition with a concomitant increase in permeability of aged lymphatic vessels. The contractile function of aged lymphatic vessels is depleted with the abolished role of nitric oxide and an increased role of lymphatic-born histamine in flow-dependent regulation of lymphatic phasic contractions and tone. In addition, aging induces oxidative stress in lymphatic vessels and facilitates the spread of pathogens from these vessels into perilymphatic tissues. Aging causes the basal activation of perilymphatic mast cells, which, in turn, restricts recruitment/activation of immune cells in perilymphatic tissues. This aging-associated basal activation of mast cells limits proper functioning of the mast cell/histamine/NF-κB axis that is essential for the regulation of lymphatic vessel transport and barrier functions as well as for both the interaction and trafficking of immune cells near and within lymphatic collecting vessels. Cumulatively, these changes play important roles in the pathogenesis of alterations in inflammation and immunity associated with aging.

Lipopolysaccharide modulates neutrophil recruitment and macrophage polarization on lymphatic vessels and impairs lymphatic function in rat mesentery

Impairment of the lymphatic system is apparent in multiple inflammatory pathologies connected to elevated endotoxins such as LPS. However, the direct mechanisms by which LPS influences the lymphatic contractility are not well understood. We hypothesized that a dynamic modulation of innate immune cell populations in mesentery under inflammatory conditions perturbs tissue cytokine/chemokine homeostasis and subsequently influences lymphatic function. We used rats that were intraperitoneally injected with LPS (10 mg/kg) to determine the changes in the profiles of innate immune cells in the mesentery and in the stretch-mediated contractile responses of isolated lymphatic preparations. Results demonstrated a reduction in the phasic contractile activity of mesenteric lymphatic vessels from LPS-injected rats and a severe impairment of lymphatic pump function and flow. There was a significant reduction in the number of neutrophils and an increase in monocytes/macrophages present on the lymphatic vessels and in the clear mesentery of the LPS group. This population of monocytes and macrophages established a robust M2 phenotype, with the majority showing high expression of CD163 and CD206. Several cytokines and chemoattractants for neutrophils and macrophages were significantly changed in the mesentery of LPS-injected rats. Treatment of lymphatic muscle cells (LMCs) with LPS showed significant changes in the expression of adhesion molecules, VCAM1, ICAM1, CXCR2, and galectin-9. LPS-TLR4-mediated regulation of pAKT, pERK pI-κB, and pMLC20 in LMCs promoted both contractile and inflammatory pathways. Thus, our data provide the first evidence connecting the dynamic changes in innate immune cells on or near the lymphatics and complex cytokine milieu during inflammation with lymphatic dysfunction.

Radiation-induced impairment in lung lymphatic vasculature

BACKGROUND

The lymphatic vasculature has been shown to play important roles in lung injury and repair, particularly in lung fibrosis. The effects of ionizing radiation on lung lymphatic vasculature have not been previously reported.

METHODS AND RESULTS

C57Bl/6 mice were immobilized in a lead shield exposing only the thoracic cavity, and were irradiated with a single dose of 14 Gy. Animals were sacrificed and lungs collected at different time points (1, 4, 8, and 16 weeks) following radiation. To identify lymphatic vessels in lung tissue sections, we used antibodies that are specific for lymphatic vessel endothelial receptor 1 (LYVE-1), a marker of lymphatic endothelial cells (LEC). To evaluate LEC cell death and oxidative damage, lung tissue sections were stained for LYVE-1 and with TUNEL staining, or 8-oxo-dG respectively. Images were imported into ImageJ v1.36b and analyzed. Compared to a non-irradiated control group, we observed a durable and progressive decrease in the density, perimeter, and area of lymphatic vessels over the study period. The decline in the density of lymphatic vessels was observed in both subpleural and interstitial lymphatics. Histopathologically discernible pulmonary fibrosis was not apparent until 16 weeks after irradiation. Furthermore, there was significantly increased LEC apoptosis and oxidative damage at one week post-irradiation that persisted at 16 weeks.

CONCLUSIONS

There is impairment of lymphatic vasculature after a single dose of ionizing radiation that precedes architectural distortion and fibrosis, suggesting important roles for the lymphatic circulation in the pathogenesis of the radiation-induced lung injury.

Involvement of histamine in endothelium-dependent relaxation of mesenteric lymphatic vessels

OBJECTIVES

The knowledge of the basic principles of lymphatic function, still remains, to a large degree, rudimentary and will require significant research efforts. Recent studies of the physiology of the MLVs suggested the presence of an EDRF other than NO. In this study, we tested the hypothesis that lymphatic endothelium-derived histamine relaxes MLVs.

METHODS

We measured and analyzed parameters of lymphatic contractility in isolated and pressurized rat MLVs under control conditions and after pharmacological blockade of NO by L-NAME (100 μM) or/and histamine production by α-MHD (10 μM). Effectiveness of α-MHD was confirmed immunohistochemically. We also used immunohistochemical labeling and Western blot analysis of the histamine-producing enzyme, HDC. In addition, we blocked HDC protein expression in MLVs by transient transfection with vivo-morpholino oligos.

RESULTS

We found that only combined pharmacological blockade of NO and histamine production completely eliminates flow-dependent relaxation of lymphatic vessels, thus confirming a role for histamine as an EDRF in MLVs. We also confirmed the presence of HDC and histamine inside lymphatic endothelial cells.

CONCLUSIONS

This study supports a role for histamine as an EDRF in MLVs.

Lymph transport in rat mesenteric lymphatics experiencing edemagenic stress

OBJECTIVE

To assess lymphatic flow adaptations to edema, we evaluated lymph transport function in rat mesenteric lymphatics under normal and increased fluid volume (edemagenic) conditions in situ.

METHODS

Twelve rats were infused with saline (intravenous infusion, 0.2 mL/min/100 g body weight) to induce edema. We intravitally measured mesenteric lymphatic diameter and contraction frequency, as well as lymphocyte velocity and density before, during, and after infusion.

RESULTS

A 10-fold increase in lymphocyte velocity (0.1-1 mm/s) and a sixfold increase in flow rate (0.1-0.6 μL/min), were observed post infusion, respectively. There were also increases in contraction frequency and fractional pump flow one minute post infusion. Time-averaged wall shear stress increased 10 fold post infusion to nearly 1.5 dynes/cm(2) . Similarly, maximum shear stress rose from 5 to 40 dynes/cm(2) .

CONCLUSIONS

Lymphatic vessels adapted to edemagenic stress by increasing lymph transport. Specifically, the increases in lymphatic contraction frequency, lymphocyte velocity, and shear stress were significant. Lymph pumping increased post infusion, though changes in lymphatic diameter were not statistically significant. These results indicate that edemagenic conditions stimulate lymph transport via increases in lymphatic contraction frequency, lymphocyte velocity, and flow. These changes, consequently, resulted in large increases in wall shear stress, which could then activate NO pathways and modulate lymphatic transport function.

Mechanobiology of lymphatic contractions

The lymphatic system is responsible for controlling tissue fluid pressure by facilitating flow of lymph (i.e. the plasma and cells that enter the lymphatic system). Because lymph contains cells of the immune system, its transport is not only important for fluid homeostasis, but also immune function. Lymph drainage can occur via passive flow or active pumping, and much research has identified the key biochemical and mechanical factors that affect output. Although many studies and reviews have addressed how tissue properties and fluid mechanics (i.e. pressure gradients) affect lymph transport [1-3] there is less known about lymphatic mechanobiology. As opposed to passive mechanical properties, mechanobiology describes the active coupling of mechanical signals and biochemical pathways. Lymphatic vasomotion is the result of a fascinating system affected by mechanical forces exerted by the flowing lymph, including pressure-induced vessel stretch and flow-induced shear stresses. These forces can trigger or modulate biochemical pathways important for controlling the lymphatic contractions. Here, I review the current understanding of lymphatic vessel function, focusing on vessel mechanobiology, and summarize the prospects for a comprehensive understanding that integrates the mechanical and biomechanical control mechanisms in the lymphatic system.

Emerging trends in the pathophysiology of lymphatic contractile function

Lymphatic contractile dysfunction is central to a number of pathologies that affect millions of people worldwide. Due to its critical role in the process of inflammation, a dysfunctional lymphatic system also compromises the immune response, further exacerbating a number of inflammation related diseases. Despite the critical physiological functions accomplished by the transport of lymph, a complete understanding of the contractile machinery of the lymphatic system lags far behind that of the blood vasculature. However, there has been a surge of recent research focusing on different mechanisms that underlie both physiological and pathophysiological aspects of lymphatic contractile function. This review summarizes those emerging paradigms that shed some novel insights into the contractile physiology of the lymphatics in normal as well as different disease states. In addition, this review emphasizes the recent progress made in our understanding of various contractile parameters and regulatory elements that contribute to the normal functioning of the lymphatics.

Photoacoustic lymphatic imaging with high spatial-temporal resolution

Despite its critical function in coordinating the egress of inflammatory and immune cells out of tissues and maintaining fluid balance, the causative role of lymphatic network dysfunction in pathological settings is still understudied. Engineered-animal models and better noninvasive high spatial-temporal resolution imaging techniques in both preclinical and clinical studies will help to improve our understanding of different lymphatic-related pathologic disorders. Our aim was to take advantage of our newly optimized noninvasive wide-field fast-scanning photoacoustic (PA) microcopy system to coordinately image the lymphatic vasculature and its flow dynamics, while maintaining high resolution and detection sensitivity. Here, by combining the optical-resolution PA microscopy with a fast-scanning water-immersible microelectromechanical system scanning mirror, we have imaged the lymph dynamics over a large field-of-view, with high spatial resolution and advanced detection sensitivity. Depending on the application, lymphatic vessels (LV) were spectrally or temporally differentiated from blood vessels. Validation experiments were performed on phantoms and in vivo to identify the LV. Lymphatic flow dynamics in nonpathological and pathological conditions were also visualized. These results indicate that our newly developed PA microscopy is a promising tool for lymphatic-related biological research.

Lymphaticovenular anastomosis to prevent cellulitis associated with lymphoedema

BACKGROUND

One of the complications of lymphoedema is recurrent cellulitis. The aim was to determine whether lymphaticovenous anastomosis (LVA) was effective at reducing cellulitis in patients with lymphoedema.

METHODS

This was a retrospective review of patients with arm/leg lymphoedema who underwent LVA. The frequency of cellulitis was compared before and after surgery. The diagnostic criteria for cellulitis were a fever of 38·5°C or higher, and warmth/redness in the affected limb(s).

RESULTS

A total of 95 patients were included. The mean number of episodes of cellulitis in the year preceding surgery was 1·46, compared with 0·18 in the year after surgery (P < 0·001).

CONCLUSION

LVA reduced the rate of cellulitis in these patients with lymphoedema.

Method for the quantitative measurement of collecting lymphatic vessel contraction in mice

Collecting lymphatic vessels are critical for the transport of lymph and its cellular contents to lymph nodes for both immune surveillance and the maintenance of tissue-fluid balance. Collecting lymphatic vessels drive lymph flow by autonomous contraction of smooth muscle cells that cover these vessels. Here we describe methods using intravital microscopy to image and quantify collecting lymphatic vessel contraction in mice. Our methods allow for the measurement of the strength of lymphatic contraction of an individual lymphangion in a mouse, which has not yet been demonstrated using other published methods. The ability to study murine collecting lymphatic vessel contraction-using the methods described here or other recently published techniques-allows the field to dissect the molecular mechanisms controlling lymphatic pumping under normal and pathological conditions using the wide variety of molecular tools and genetic models available in the mouse. We have used our methods to study lymphatic contraction in physiological and inflammatory conditions. The methods described here will facilitate the further study of lymphatic function in other pathological conditions that feature lymphatic complications.

Minimally invasive method for determining the effective lymphatic pumping pressure in rats using near-infrared imaging

The ability to quantify collecting vessel function in a minimally invasive fashion is crucial to the study of lymphatic physiology and the role of lymphatic pump function in disease progression. Therefore, we developed a highly sensitive, minimally invasive research platform for quantifying the pumping capacity of collecting lymphatic vessels in the rodent tail and forelimb. To achieve this, we have integrated a near-infrared lymphatic imaging system with a feedback-controlled pressure cuff to modulate lymph flow. After occluding lymphatic flow by inflating a pressure cuff on the limb or tail, we gradually deflate the cuff while imaging flow restoration proximal to the cuff. Using prescribed pressure applications and automated image processing of fluorescence intensity levels in the vessels, we were able to noninvasively quantify the effective pumping pressure (P(eff), pressure at which flow is restored after occlusion) and vessel emptying rate (rate of fluorescence clearance during flow occlusion) of lymphatics in the rat. To demonstrate the sensitivity of this system to changes in lymphatic function, a nitric oxide (NO) donor cream, glyceryl trinitrate ointment (GTNO), was applied to the tails. GTNO decreased P(eff) of the vessels by nearly 50% and the average emptying rate by more than 60%. We also demonstrate the suitability of this approach for acquiring measurements on the rat forelimb. Thus, this novel research platform provides the first minimally invasive measurements of P(eff) and emptying rate in rodents. This experimental platform holds strong potential for future in vivo studies that seek to evaluate changes in lymphatic health and disease.

Lymphatic filariasis: perspectives on lymphatic remodeling and contractile dysfunction in filarial disease pathogenesis

Lymphatic filariasis, one of the most debilitating diseases associated with the lymphatic system, affects over a hundred million people worldwide and manifests itself in a variety of severe clinical pathologies. The filarial parasites specifically target the lymphatics and impair lymph flow, which is critical for the normal functions of the lymphatic system in maintenance of body fluid balance and physiological interstitial fluid transport. The resultant contractile dysfunction of the lymphatics causes fluid accumulation and lymphedema, one of the major pathologies associated with filarial infection. In this review, we take a closer look at the contractile mechanisms of the lymphatics, its altered functions, and remodeling during an inflammatory state and how it relates to the severe pathogenesis underlying a filarial infection. We further elaborate on the complex host-parasite interactions, and molecular mechanisms contributing to the disease pathogenesis. The overall emphasis is on elucidating some of the emerging concepts and new directions that aim to harness the process of lymphangiogenesis or enhance contractility in a dysfunctional lymphatics, thereby restoring the fluid imbalance and mitigating the pathological conditions of lymphatic filariasis.

Exercise-induced interstitial pulmonary edema at sea-level in young and old healthy humans

We asked whether aged adults are more susceptible to exercise-induced pulmonary edema relative to younger individuals. Lung diffusing capacity for carbon monoxide (DLCO), alveolar-capillary membrane conductance (Dm) and pulmonary-capillary blood volume (Vc) were measured before and after exhaustive discontinuous incremental exercise in 10 young (YNG; 27±3 years) and 10 old (OLD; 69±5 years) males. In YNG subjects, Dm increased (11±7%, P=0.031), Vc decreased (-10±9%, P=0.01) and DLCO was unchanged (30.5±4.1 vs. 29.7±2.9mL/min/mmHg, P=0.44) pre- to post-exercise. In OLD subjects, DLCO and Dm increased (11±14%, P=0.042; 16±14%, P=0.025) but Vc was unchanged (58±23 vs. 56±23mL, P=0.570) pre- to post-exercise. Group-mean Dm/Vc was greater after vs. before exercise in the YNG and OLD subjects. However, Dm/Vc was lower post-exercise in 2 of the 10 YNG (-7±4%) and 2 of the 10 OLD subjects (-10±5%). These data suggest that exercise decreases interstitial lung fluid in most YNG and OLD subjects, with a small number exhibiting evidence for exercise-induced pulmonary edema.

Minimally invasive method for determining the effective lymphatic pumping pressure in rats using near-infrared imaging

The ability to quantify collecting vessel function in a minimally invasive fashion is crucial to the study of lymphatic physiology and the role of lymphatic pump function in disease progression. Therefore, we developed a highly sensitive, minimally invasive research platform for quantifying the pumping capacity of collecting lymphatic vessels in the rodent tail and forelimb. To achieve this, we have integrated a near-infrared lymphatic imaging system with a feedback-controlled pressure cuff to modulate lymph flow. After occluding lymphatic flow by inflating a pressure cuff on the limb or tail, we gradually deflate the cuff while imaging flow restoration proximal to the cuff. Using prescribed pressure applications and automated image processing of fluorescence intensity levels in the vessels, we were able to noninvasively quantify the effective pumping pressure (P(eff), pressure at which flow is restored after occlusion) and vessel emptying rate (rate of fluorescence clearance during flow occlusion) of lymphatics in the rat. To demonstrate the sensitivity of this system to changes in lymphatic function, a nitric oxide (NO) donor cream, glyceryl trinitrate ointment (GTNO), was applied to the tails. GTNO decreased P(eff) of the vessels by nearly 50% and the average emptying rate by more than 60%. We also demonstrate the suitability of this approach for acquiring measurements on the rat forelimb. Thus, this novel research platform provides the first minimally invasive measurements of P(eff) and emptying rate in rodents. This experimental platform holds strong potential for future in vivo studies that seek to evaluate changes in lymphatic health and disease.

Mechanosensing in developing lymphatic vessels

The lymphatic vasculature is responsible for fluid homeostasis, transport of immune cells, inflammatory molecules, and dietary lipids. It is composed of a network of lymphatic capillaries that drain into collecting lymphatic vessels and ultimately bring fluid back to the blood circulation. Lymphatic endothelial cells (LECs) that line lymphatic capillaries present loose overlapping intercellular junctions and anchoring filaments that support fluid drainage. When interstitial fluid accumulates within tissues, the extracellular matrix (ECM) swells and pulls the anchoring filaments. This results in opening of the LEC junctions and permits interstitial fluid uptake. The absorbed fluid is then transported within collecting lymphatic vessels, which exhibit intraluminal valves that prevent lymph backflow and smooth muscle cells that sequentially contract to propel lymph.Mechanotransduction involves translation of mechanical stimuli into biological responses. LECs have been shown to sense and respond to changes in ECM stiffness, fluid pressure-induced cell stretch, and fluid flow-induced shear stress. How these signals influence LEC function and lymphatic vessel growth can be investigated by using different mechanotransduction assays in vitro and to some extent in vivo.In this chapter, we will focus on the mechanical forces that regulate lymphatic vessel expansion during embryonic development and possibly secondary lymphedema. In mouse embryos, it has been recently shown that the amount of interstitial fluid determines the extent of lymphatic vessel expansion via a mechanosensory complex formed by β1 integrin and vascular endothelial growth factor receptor-3 (VEGFR3). This model might as well apply to secondary lymphedema.

The lymph self-antigen repertoire

The lymphatic fluid originates from the interstitial fluid which bathes every parenchymal organ and reflects the “omic” composition of the tissue from which it originates in its physiological or pathological signature. Several recent proteomic analyses have mapped the proteome-degradome and peptidome of this immunologically relevant fluid pointing to the lymph as an important source of tissue-derived self-antigens. A vast array of lymph-circulating peptides have been mapped deriving from a variety of processing pathways including caspases, cathepsins, MMPs, ADAMs, kallikreins, calpains, and granzymes, among others. These self peptides can be directly loaded on circulatory dendritic cells and expand the self-antigenic repertoire available for central and peripheral tolerance.

Mechanical forces in lymphatic vascular development and disease

The lymphatic vasculature is essential for fluid homeostasis and transport of immune cells, inflammatory molecules, and dietary lipids. It is composed of a hierarchical network of blind-ended lymphatic capillaries and collecting lymphatic vessels, both lined by lymphatic endothelial cells (LECs). The low hydrostatic pressure in lymphatic capillaries, their loose intercellular junctions, and attachment to the surrounding extracellular matrix (ECM) permit passage of extravasated blood plasma from the interstitium into the lumen of the lymphatic capillaries. It is generally thought that interstitial fluid accumulation leads to a swelling of the ECM, to which the LECs of lymphatic capillaries adhere, for example via anchoring filaments. As a result, LECs are pulled away from the vascular lumen, the junctions open, and fluid enters the lymphatic vasculature. The collecting lymphatic vessels then gather the plasma fluid from the capillaries and carry it through the lymph nodes to the blood circulation. The collecting vessels contain intraluminal bicuspid valves that prevent fluid backflow, and are embraced by smooth muscle cells that contribute to fluid transport. Although the lymphatic vessels are regular subject to mechanical strain, our knowledge of its influence on lymphatic development and pathologies is scarce. Here, we discuss the mechanical forces and molecular mechanisms regulating lymphatic vascular growth and maturation in the developing mouse embryo. We also consider how the lymphatic vasculature might be affected by similar mechanomechanisms in two pathological processes, namely cancer cell dissemination and secondary lymphedema.