Transcellular Pathways in Lymphatic Endothelial Cells Regulate Changes in Solute Transport by Fluid Stress

Lymphatic vessels in the age of cancer immunotherapy

Lymphatic transport maintains homeostatic health and is necessary for immune surveillance, and yet lymphatic growth is often associated with solid tumour development and dissemination. Although tumour-associated lymphatic remodelling and growth were initially presumed to simply expand a passive route for regional metastasis, emerging research puts lymphatic vessels and their active transport at the interface of metastasis, tumour-associated inflammation and systemic immune surveillance. Here, we discuss active mechanisms through which lymphatic vessels shape their transport function to influence peripheral tissue immunity and the current understanding of how tumour-associated lymphatic vessels may both augment and disrupt antitumour immune surveillance. We end by looking forward to emerging areas of interest in the field of cancer immunotherapy in which lymphatic vessels and their transport function are likely key players: the formation of tertiary lymphoid structures, immune surveillance in the central nervous system, the microbiome, obesity and ageing. The lessons learnt support a working framework that defines the lymphatic system as a key determinant of both local and systemic inflammatory networks and thereby a crucial player in the response to cancer immunotherapy.

A specific and portable gene expression program underlies antigen archiving by lymphatic endothelial cells

Antigens from protein subunit vaccination traffic from the tissue to the draining lymph node, either passively via the lymph or carried by dendritic cells at the local injection site. Lymph node (LN) lymphatic endothelial cells (LEC) actively acquire and archive foreign antigens, and archived antigen can be released during subsequent inflammatory stimulus to improve immune responses. Here, we answer questions about how LECs achieve durable antigen archiving and whether there are transcriptional signatures associated with LECs containing high levels of antigen. We used single cell sequencing in dissociated LN tissue to quantify antigen levels in LEC and dendritic cell populations at multiple timepoints after immunization, and used machine learning to define a unique transcriptional program within archiving LECs that can predict LEC archiving capacity in independent data sets. Finally, we validated this modeling, showing we could predict antigen archiving from a transcriptional dataset of CHIKV infected mice and demonstrated in vivo the accuracy of our prediction. Collectively, our findings establish a unique transcriptional program in LECs that promotes antigen archiving that can be translated to other systems.

Para- and Transcellular Transport Kinetics of Nanoparticles across Lymphatic Endothelial Cells

Lymphatic vessels have received significant attention as drug delivery targets, as they shuttle materials from peripheral tissues to the lymph nodes, where adaptive immunity is formed. Delivery of immune modulatory materials to the lymph nodes via lymphatic vessels has been shown to enhance their efficacy and also improve the bioavailability of drugs when delivered to intestinal lymphatic vessels. In this study, we generated a three-compartment model of a lymphatic vessel with a set of kinematic differential equations to describe the transport of nanoparticles from the surrounding tissues into lymphatic vessels. We used previously published data and collected additional experimental parameters, including the transport efficiency of nanoparticles over time, and also examined how nanoparticle formulation affected the cellular transport mechanisms using small molecule inhibitors. These experimental data were incorporated into a system of kinematic differential equations, and nonlinear, least-squares curve fitting algorithms were employed to extrapolate transport coefficients within our model. The subsequent computational framework produced some of the first parameters to describe transport kinetics across lymphatic endothelial cells and allowed for the quantitative analysis of the driving mechanisms of transport into lymphatic vessels. Our model indicates that transcellular mechanisms, such as micro- and macropinocytosis, drive transport into lymphatics. This information is crucial to further design strategies that will modulate lymphatic transport for drug delivery, particularly in diseases like lymphedema, where normal lymphatic functions are impaired.

A compartment model for subcutaneous injection of monoclonal antibodies

Despite the growing popularity of subcutaneous (SC) administration for monoclonal antibodies (mAbs), there remains a limited understanding of the significance of mAb transport rate constants within the interstitial space and the lymphatic system on their pharmacokinetics. To bridge this knowledge gap, we introduce a compartmental model for subcutaneously administered mAbs. Our model differentiates FcRn-expressing cells across various sites, and the model predictions agree with experimental data from both human and rat studies. Our findings indicate that the time to reach the maximum mAb concentration in the plasma, denoted by Tmax, displays a weak positive correlation with mAb half-life and a negligible correlation with bioavailability. In contrast, the half-life of mAbs exhibits a strong positive correlation with bioavailability. Moreover, the rate of mAb transport from lymph to plasma significantly affects the mAb half-life. Increasing the transport rates of mAbs from the injection site to the lymph or from lymph to plasma enhances bioavailability. These insights, combined with our compartmental model, contribute to a deeper understanding of the pharmacokinetics of subcutaneously administered mAbs.

Battle of the milky way: Lymphatic targeted drug delivery for pathogen eradication

Many viruses, bacteria, and parasites rely on the lymphatic system for survival, replication, and dissemination. While conventional anti-infectives can combat infection-causing agents in the bloodstream, they do not reach the lymphatic system to eradicate the pathogens harboured there. This can result in ineffective drug exposure and reduce treatment effectiveness. By developing effective lymphatic delivery strategies for antiviral, antibacterial, and antiparasitic drugs, their systemic pharmacokinetics may be improved, as would their ability to reach their target pathogens within the lymphatics, thereby improving clinical outcomes in a variety of acute and chronic infections with lymphatic involvement (e.g., acquired immunodeficiency syndrome, tuberculosis, and filariasis). Here, we discuss approaches to targeting anti-infective drugs to the intestinal and dermal lymphatics, aiming to eliminate pathogen reservoirs and interfere with their survival and reproduction inside the lymphatic system. These include optimized lipophilic prodrugs and drug delivery systems that promote lymphatic transport after oral and dermal drug intake. For intestinal lymphatic delivery via the chylomicron pathway, molecules should have logP values >5 and long-chain triglyceride solubilities >50 mg/g, and for dermal lymphatic delivery via interstitial lymphatic drainage, nanoparticle formulations with particle size between 10 and 100 nm are generally preferred. Insight from this review may promote new and improved therapeutic solutions for pathogen eradication and combating infective diseases, as lymphatic system involvement in pathogen dissemination and drug resistance has been neglected compared to other pathways leading to treatment failure.

A 3D biomimetic model of lymphatics reveals cell-cell junction tightening and lymphedema via a cytokine-induced ROCK2/JAM-A complex

Impaired lymphatic drainage and lymphedema are major morbidities whose mechanisms have remained obscure. To study lymphatic drainage and its impairment, we engineered a microfluidic culture model of lymphatic vessels draining interstitial fluid. This lymphatic drainage-on-chip revealed that inflammatory cytokines that are known to disrupt blood vessel junctions instead tightened lymphatic cell-cell junctions and impeded lymphatic drainage. This opposing response was further demonstrated when inhibition of rho-associated protein kinase (ROCK) was found to normalize fluid drainage under cytokine challenge by simultaneously loosening lymphatic junctions and tightening blood vessel junctions. Studies also revealed a previously undescribed shift in ROCK isoforms in lymphatic endothelial cells, wherein a ROCK2/junctional adhesion molecule-A (JAM-A) complex emerges that is responsible for the cytokine-induced lymphatic junction zippering. To validate these in vitro findings, we further demonstrated in a genetic mouse model that lymphatic-specific knockout of ROCK2 reversed lymphedema in vivo. These studies provide a unique platform to generate interstitial fluid pressure and measure the drainage of interstitial fluid into lymphatics and reveal a previously unappreciated ROCK2-mediated mechanism in regulating lymphatic drainage.

Lymphangiogenesis in gastric cancer: function and mechanism

Increased lymphangiogenesis and lymph node (LN) metastasis are thought to be important steps in cancer metastasis, and are associated with patient's poor prognosis. There is increasing evidence that the lymphatic system may play a crucial role in regulating tumor immune response and limiting tumor metastasis, since tumor lymphangiogenesis is more prominent in tumor metastasis and diffusion. Lymphangiogenesis takes place in embryonic development, wound healing, and a variety of pathological conditions, including tumors. Tumor cells and tumor microenvironment cells generate growth factors (such as lymphangiogenesis factor VEGF-C/D), which can promote lymphangiogenesis, thereby inducing the metastasis and diffusion of tumor cells. Nevertheless, the current research on lymphangiogenesis in gastric cancer is relatively scattered and lacks a comprehensive understanding. Therefore, in this review, we aim to provide a detailed perspective on molecules and signal transduction pathways that regulate gastric cancer lymphogenesis, which may provide new insights for the diagnosis and treatment of cancer.

Physiological functions of caveolae in endothelium

Endothelial caveolae are essential for a wide range of physiological processes and have emerged as key players in vascular biology. Our understanding of caveolar biology in endothelial cells has expanded dramatically since their discovery revealing critical roles in mechanosensation, signal transduction, eNOS regulation, lymphatic transport, and metabolic disease progression. Furthermore, caveolae are involved in the organization of membrane domains, regulation of membrane fluidity, and endocytosis which contribute to endothelial function and integrity. Additionally, recent advances highlight the impact of caveolae-mediated signaling pathways on vascular homeostasis and pathology. Together, the diverse roles of caveolae discussed here represent a breadth of cellular functions presenting caveolae as a defining feature of endothelial form and function. In light of these new insights, targeting caveolae may hold potential for the development of novel therapeutic strategies to treat a range of vascular diseases.

The emerging importance of lymphatics in health and disease: an NIH workshop report

The lymphatic system (LS) is composed of lymphoid organs and a network of vessels that transport interstitial fluid, antigens, lipids, cholesterol, immune cells, and other materials in the body. Abnormal development or malfunction of the LS has been shown to play a key role in the pathophysiology of many disease states. Thus, improved understanding of the anatomical and molecular characteristics of the LS may provide approaches for disease prevention or treatment. Recent advances harnessing single-cell technologies, clinical imaging, discovery of biomarkers, and computational tools have led to the development of strategies to study the LS. This Review summarizes the outcomes of the NIH workshop entitled "Yet to be Charted: Lymphatic System in Health and Disease," held in September 2022, with emphasis on major areas for advancement. International experts showcased the current state of knowledge regarding the LS and highlighted remaining challenges and opportunities to advance the field.

Addressing the in vivo delivery of nucleic-acid nanostructure therapeutics

DNA and RNA nanostructures are being investigated as therapeutics, vaccines, and drug delivery systems. These nanostructures can be functionalized with guests ranging from small molecules to proteins with precise spatial and stoichiometric control. This has enabled new strategies to manipulate drug activity and to engineer devices with novel therapeutic functionalities. Although existing studies have offered encouraging in vitro or pre-clinical proof-of-concepts, establishing mechanisms of in vivo delivery is the new frontier for nucleic-acid nanotechnologies. In this review, we first provide a summary of existing literature on the in vivo uses of DNA and RNA nanostructures. Based on their application areas, we discuss current models of nanoparticle delivery, and thereby highlight knowledge gaps on the in vivo interactions of nucleic-acid nanostructures. Finally, we describe techniques and strategies for investigating and engineering these interactions. Together, we propose a framework to establish in vivo design principles and advance the in vivo translation of nucleic-acid nanotechnologies.

Mechanisms of Nanoparticle Transport across Intestinal Tissue: An Oral Delivery Perspective

Oral drug administration has been a popular choice due to patient compliance and limited clinical resources. Orally delivered drugs must circumvent the harsh gastrointestinal (GI) environment to effectively enter the systemic circulation. The GI tract has a number of structural and physiological barriers that limit drug bioavailability including mucus, the tightly regulated epithelial layer, immune cells, and associated vasculature. Nanoparticles have been used to enhance oral bioavailability of drugs, as they can act as a shield to the harsh GI environment and prevent early degradation while also increasing uptake and transport of drugs across the intestinal epithelium. Evidence suggests that different nanoparticle formulations may be transported via different intracellular mechanisms to cross the intestinal epithelium. Despite the existence of a significant body of work on intestinal transport of nanoparticles, many key questions remain: What causes the poor bioavailability of the oral drugs? What factors contribute to the ability of a nanoparticle to cross different intestinal barriers? Do nanoparticle properties such as size and charge influence the type of endocytic pathways taken? In this Review, we summarize the different components of intestinal barriers and the types of nanoparticles developed for oral delivery. In particular, we focus on the various intracellular pathways used in nanoparticle internalization and nanoparticle or cargo translocation across the epithelium. Understanding the gut barrier, nanoparticle characteristics, and transport pathways may lead to the development of more therapeutically useful nanoparticles as drug carriers.

Nanotechnologies for Physiology-Informed Drug Delivery to the Lymphatic System

Accompanying the increasing translational impact of immunotherapeutic strategies to treat and prevent disease has been a broadening interest across both bioscience and bioengineering in the lymphatic system. Herein, the lymphatic system physiology, ranging from its tissue structures to immune functions and effects, is described. Design principles and engineering approaches to analyze and manipulate this tissue system in nanoparticle-based drug delivery applications are also elaborated.

In Vitro Models of Blood and Lymphatic Vessels-Connecting Tissues and Immunity

Blood and lymphatic vessels are regulators of physiological processes, including oxygenation and fluid transport. Both vessels are ubiquitous throughout the body and are critical for sustaining tissue homeostasis. The complexity of each vessel's processes has limited the understanding of exactly how the vessels maintain their functions. Both vessels have been shown to be involved in the pathogenesis of many diseases, including cancer metastasis, and it is crucial to probe further specific mechanisms involved. In vitro models are developed to better understand blood and lymphatic physiological functions and their mechanisms. In this review, blood and lymphatic in vitro model systems, including 2D and 3D designs made using Transwells, microfluidic devices, organoid cultures, and various other methods, are described. Models studying endothelial cell-extracellular matrix interactions, endothelial barrier properties, transendothelial transport and cell migration, lymph/angiogenesis, vascular inflammation, and endothelial-cancer cell interactions are particularly focused. While the field has made significant progress in modeling and understanding lymphatic and blood vasculature, more models that include coculture of multiple cell types, complex extracellular matrix, and 3D morphologies, particularly for models mimicking disease states, will help further the understanding of the role of blood and lymphatic vasculature in health and disease.

Blood Vessel Organoids for Development and Disease

Despite enormous advances, cardiovascular disorders are still a major threat to global health and are responsible for one-third of deaths worldwide. Research for new therapeutics and the investigation of their effects on vascular parameters is often limited by species-specific pathways and a lack of high-throughput methods. The complex 3-dimensional environment of blood vessels, intricate cellular crosstalks, and organ-specific architectures further complicate the quest for a faithful human in vitro model. The development of novel organoid models of various tissues such as brain, gut, and kidney signified a leap for the field of personalized medicine and disease research. By utilizing either embryonic- or patient-derived stem cells, different developmental and pathological mechanisms can be modeled and investigated in a controlled in vitro environment. We have recently developed self-organizing human capillary blood vessel organoids that recapitulate key processes of vasculogenesis, angiogenesis, and diabetic vasculopathy. Since then, this organoid system has been utilized as a model for other disease processes, refined, and adapted for organ specificity. In this review, we will discuss novel and alternative approaches to blood vessel engineering and explore the cellular identity of engineered blood vessels in comparison to in vivo vasculature. Future perspectives and the therapeutic potential of blood vessel organoids will be discussed.

Regulating Lymphatic Vasculature in Fibrosis: Understanding the Biology to Improve the Modeling

Fibrosis occurs in many chronic diseases with lymphatic vascular insufficiency (e.g., kidney disease, tumors, and lymphedema). New lymphatic capillary growth can be triggered by fibrosis-related tissue stiffening and soluble factors, but questions remain for how related biomechanical, biophysical, and biochemical cues affect lymphatic vascular growth and function. The current preclinical standard for studying lymphatics is animal modeling, but in vitro and in vivo outcomes often do not align. In vitro models can also be limited in their ability to separate vascular growth and function as individual outcomes, and fibrosis is not traditionally included in model design. Tissue engineering provides an opportunity to address in vitro limitations and mimic microenvironmental features that impact lymphatic vasculature. This review discusses fibrosis-related lymphatic vascular growth and function in disease and the current state of in vitro lymphatic vascular models while highlighting relevant knowledge gaps. Additional insights into the future of in vitro lymphatic vascular models demonstrate how prioritizing fibrosis alongside lymphatics will help capture the complexity and dynamics of lymphatics in disease. Overall, this review aims to emphasize that an advanced understanding of lymphatics within a fibrotic disease-enabled through more accurate preclinical modeling-will significantly impact therapeutic development toward restoring lymphatic vessel growth and function in patients.

Lymphatic uptake of biotherapeutics through a 3D hybrid discrete-continuum vessel network in the skin tissue

Subcutaneous administration is a common approach for the delivery of biotherapeutics, which is achieved mainly through the absorption across lymphatic vessels. In this paper, the drug transport and lymphatic uptake through a three-dimensional hybrid discrete-continuum vessel network in the skin tissue are investigated through high-fidelity numerical simulations. We find that the local lymphatic uptake through the explicit vessels significantly affects macroscopic drug absorption. The diffusion of drug solute through the explicit vessel network affects the lymphatic uptake after the injection. This effect, however, cannot be captured using previously developed continuum models. The lymphatic uptake is dominated by the convection due to lymphatic drainage driven by the pressure difference, which is rarely studied in experiments and simulations. Furthermore, the effects of injection volume and depth on the lymphatic uptake are investigated in a multi-layered domain. We find that the injection volume significantly affects the rate of lymphatic uptake through the heterogeneous vessel network, while the injection depth has little influence, which is consistent with the experimental results. At last, the binding and metabolism of drug molecules are studied to bridge the simulations to the drug clearance experients. We provide a new approach to study the diffusion and convection of drug molecules into the lymphatic system through the hybrid vessel network.

Oxidized low density lipoprotein in the liver causes decreased permeability of liver lymphatic- but not liver sinusoidal-endothelial cells via VEGFR-3 regulation of VE-Cadherin

The lymphatic vasculature of the liver is vital for liver function as it maintains fluid and protein homeostasis and is important for immune cell transport to the lymph node. Chronic liver disease is associated with increased expression of inflammatory mediators including oxidized low-density lipoprotein (oxLDL). Intrahepatic levels of oxLDL are elevated in nonalcoholic fatty liver disease (NAFLD), chronic hepatitis C infection (HCV), alcohol-associated liver disease (ALD), and cholestatic liver diseases. To determine if liver lymphatic function is impaired in chronic liver diseases, in which increased oxLDL has been documented, we measured liver lymphatic function in murine models of NAFLD, ALD and primary sclerosing cholangitis (PSC). We found that Mdr2-/- (PSC), Lieber-DeCarli ethanol fed (ALD) and high fat and high cholesterol diet fed (NAFLD) mice all had a significant impairment in the ability to traffic FITC labeled dextran from the liver parenchyma to the liver draining lymph nodes. Utilizing an in vitro permeability assay, we found that oxLDL decreased the permeability of lymphatic endothelial cells (LEC)s, but not liver sinusoidal endothelial cells (LSEC)s. Here we demonstrate that LECs and LSECs differentially regulate SRC-family kinases, MAPK kinase and VE-Cadherin in response to oxLDL. Furthermore, Vascular Endothelial Growth Factor (VEGF)C or D (VEGFR-3 ligands) appear to regulate VE-Cadherin expression as well as decrease cellular permeability of LECs in vitro and in vivo after oxLDL treatment. These findings suggest that oxLDL acts to impede protein transport through the lymphatics through tightening of the cell-cell junctions. Importantly, engagement of VEGFR-3 by its ligands prevents VE-Cadherin upregulation and improves lymphatic permeability. These studies provide a potential therapeutic target to restore liver lymphatic function and improve liver function.

Lymphatic Vascular Permeability

Blood vessels have a regulated permeability to fluid and solutes, which allows for the delivery of nutrients and signaling molecules to all cells in the body, a process essential to life. The lymphatic vasculature is the second network of vessels in the body, making up part of the immune system, yet is not typically thought of as having a permeability to fluid and solute. However, the major function of the lymphatic vasculature is to regulate tissue fluid balance to prevent edema, so lymphatic vessels must be permeable to absorb and transport fluid efficiently. Only recently were lymphatic vessels discovered to be permeable, which has had many functional implications. In this review, we will provide an overview of what is known about lymphatic vascular permeability, discuss the biophysical and signaling mechanisms regulating lymphatic permeability, and examine the disease relevance of this new property of lymphatic vessels.

Lymphangiogenesis and Lymphatic Barrier Dysfunction in Renal Fibrosis

As an integral part of the vascular system, the lymphatic vasculature is essential for tissue fluid homeostasis, nutritional lipid assimilation and immune regulation. The composition of the lymphatic vasculature includes fluid-absorbing initial lymphatic vessels (LVs), transporting collecting vessels and anti-regurgitation valves. Although, in recent decades, research has drastically enlightened our view of LVs, investigations of initial LVs, also known as lymphatic capillaries, have been stagnant due to technical limitations. In the kidney, the lymphatic vasculature mainly presents in the cortex, keeping the local balance of fluid, solutes and immune cells. The contribution of renal LVs to various forms of pathology, especially chronic kidney diseases, has been addressed in previous studies, however with diverging and inconclusive results. In this review, we discuss the most recent advances in the proliferation and permeability of lymphatic capillaries as well as their influencing factors. Novel technologies to visualize and measure LVs function are described. Then, we highlight the role of the lymphatic network in renal fibrosis and the crosstalk between kidney and other organs, such as gut and heart.

Targeting Lymphatics for Nanoparticle Drug Delivery

The lymphatics transport material from peripheral tissues to lymph nodes, where immune responses are formed, before being transported into systemic circulation. With key roles in transport and fluid homeostasis, lymphatic dysregulation is linked to diseases, including lymphedema. Fluid within the interstitium passes into initial lymphatic vessels where a valve system prevents fluid backflow. Additionally, lymphatic endothelial cells produce key chemokines, such as CCL21, that direct the migration of dendritic cells and lymphocytes. As a result, lymphatics are an attractive delivery route for transporting immune modulatory treatments to lymph nodes where immunotherapies are potentiated in addition to being an alternative method of reaching systemic circulation. In this review, we discuss the physiology of lymphatic vessels and mechanisms used in the transport of materials from peripheral tissues to lymph nodes. We then summarize nanomaterial-based strategies to take advantage of lymphatic transport functions for delivering therapeutics to lymph nodes or systemic circulation. We also describe opportunities for targeting lymphatic endothelial cells to modulate transport and immune functions.

Nanoparticles with dense poly(ethylene glycol) coatings with near neutral charge are maximally transported across lymphatics and to the lymph nodes

Lymphatic vessels have recently been shown to effectively deliver immune modulatory therapies to the lymph nodes, which enhances their therapeutic efficacy. Prior work has shown that lymphatics transport 10-250 nm nanoparticles from peripheral tissues to the lymph node. However, the surface chemistry required to maximize this transport is poorly understood. Here, we determined the effect of surface poly(ethylene glycol) (PEG) density and size on nanoparticle transport across lymphatic endothelial cells (LECs) by differentially PEGylated model polystyrene nanoparticles. Using an established in-vitro lymphatic transport model, we found PEGylation improved the transport of 100 and 40 nm nanoparticles across LECs 50-fold compared to the unmodified nanoparticles and that transport is maximized when the PEG is in a dense brush conformation or high grafting density (Rf/D = 4.9). We also determined that these trends are not size-dependent. PEGylating 40 nm nanoparticles improved transport efficiency across LECs 68-fold compared to unmodified nanoparticles. We also found that PEGylated 100 nm and 40 nm nanoparticles accumulate in lymph nodes within 4 h after intradermal injection, while unmodified nanoparticles accumulated minimally. Densely PEGylated nanoparticles traveled the furthest distance from the injection site and densely PEGylated 40 nm nanoparticles had maximum accumulation in the lymph nodes compared to low density PEGylated and unmodified nanoparticles. Finally, we determined that nanoparticles are transported via both paracellular and transcellular mechanisms, and that PEG conformation modulates the cellular transport mechanisms. Our results suggest that PEG conformation is crucial to maximize nanoparticle transport across LECs and into lymphatic vessels, making PEG density a crucial design. Optimizing PEG density on nanoparticle formulations has the potential to enhance immunotherapeutic and vaccine outcomes. STATEMENT OF SIGNIFICANCE: Lymphatic vessels are an emerging target for drug delivery both in the context of modulating immune responses and enhancing bioavailability by avoiding first pass hepatic metabolism after oral delivery. Lymphatic vessels are the natural conduits from peripheral tissues to the lymph nodes, where the adaptive immune response is shaped, and eventually to systemic circulation via the thoracic duct. Lymphatics can be targeted via nanoparticles, but the surface chemistry required to maximize nanoparticle transport by lymphatics vessels remains poorly understood. Here, we demonstrate that coating nanoparticles with hydrophilic polyethylene glycol (PEG) effectively enhances their transport across lymphatic endothelial cells in vitro and in vivo and that both paracellular and micropinocytosis mechanisms underly this transport. We found that dense PEG coatings maximize lymphatic transport of nanoparticles, thus providing new material design criteria for lymphatic targeted drug delivery.

Recent Advances in Fluorescence Recovery after Photobleaching for Decoupling Transport and Kinetics of Biomacromolecules in Cellular Physiology

Among the new molecular tools available to scientists and engineers, some of the most useful include fluorescently tagged biomolecules. Tools, such as green fluorescence protein (GFP), have been applied to perform semi-quantitative studies on biological signal transduction and cellular structural dynamics involved in the physiology of healthy and disease states. Such studies focus on drug pharmacokinetics, receptor-mediated endocytosis, nuclear mechanobiology, viral infections, and cancer metastasis. In 1976, fluorescence recovery after photobleaching (FRAP), which involves the monitoring of fluorescence emission recovery within a photobleached spot, was developed. FRAP allowed investigators to probe two-dimensional (2D) diffusion of fluorescently-labelled biomolecules. Since then, FRAP has been refined through the advancements of optics, charged-coupled-device (CCD) cameras, confocal microscopes, and molecular probes. FRAP is now a highly quantitative tool used for transport and kinetic studies in the cytosol, organelles, and membrane of a cell. In this work, the authors intend to provide a review of recent advances in FRAP. The authors include epifluorescence spot FRAP, total internal reflection (TIR)/FRAP, and confocal microscope-based FRAP. The underlying mathematical models are also described. Finally, our understanding of coupled transport and kinetics as determined by FRAP will be discussed and the potential for future advances suggested.

Lymphatic Collecting Vessel: New Perspectives on Mechanisms of Contractile Regulation and Potential Lymphatic Contractile Pathways to Target in Obesity and Metabolic Diseases

Obesity and metabolic syndrome pose a significant risk for developing cardiovascular disease and remain a critical healthcare challenge. Given the lymphatic system's role as a nexus for lipid absorption, immune cell trafficking, interstitial fluid and macromolecule homeostasis maintenance, the impact of obesity and metabolic disease on lymphatic function is a burgeoning field in lymphatic research. Work over the past decade has progressed from the association of an obese phenotype with Prox1 haploinsufficiency and the identification of obesity as a risk factor for lymphedema to consistent findings of lymphatic collecting vessel dysfunction across multiple metabolic disease models and organisms and characterization of obesity-induced lymphedema in the morbidly obese. Critically, recent findings have suggested that restoration of lymphatic function can also ameliorate obesity and insulin resistance, positing lymphatic targeted therapies as relevant pharmacological interventions. There remain, however, significant gaps in our understanding of lymphatic collecting vessel function, particularly the mechanisms that regulate the spontaneous contractile activity required for active lymph propulsion and lymph return in humans. In this article, we will review the current findings on lymphatic architecture and collecting vessel function, including recent advances in the ionic basis of lymphatic muscle contractile activity. We will then discuss lymphatic dysfunction observed with metabolic disruption and potential pathways to target with pharmacological approaches to improve lymphatic collecting vessel function.

Endothelial Transcytosis in Acute Lung Injury: Emerging Mechanisms and Therapeutic Approaches

Acute Lung Injury (ALI) is characterized by widespread inflammation which in its severe form, Acute Respiratory Distress Syndrome (ARDS), leads to compromise in respiration causing hypoxemia and death in a substantial number of affected individuals. Loss of endothelial barrier integrity, pneumocyte necrosis, and circulating leukocyte recruitment into the injured lung are recognized mechanisms that contribute to the progression of ALI/ARDS. Additionally, damage to the pulmonary microvasculature by Gram-negative and positive bacteria or viruses (e.g., Escherichia coli, SARS-Cov-2) leads to increased protein and fluid permeability and interstitial edema, further impairing lung function. While most of the vascular leakage is attributed to loss of inter-endothelial junctional integrity, studies in animal models suggest that transendothelial transport of protein through caveolar vesicles, known as transcytosis, occurs in the early phase of ALI/ARDS. Here, we discuss the role of transcytosis in healthy and injured endothelium and highlight recent studies that have contributed to our understanding of the process during ALI/ARDS. We also cover potential approaches that utilize caveolar transport to deliver therapeutics to the lungs which may prevent further injury or improve recovery.

Lung lymphatic thrombosis and dysfunction caused by cigarette smoke exposure precedes emphysema in mice

The lymphatic vasculature is critical for lung function, but defects in lymphatic function in the pathogenesis of lung disease is understudied. In mice, lymphatic dysfunction alone is sufficient to cause lung injury that resembles human emphysema. Whether lymphatic function is disrupted in cigarette smoke (CS)-induced emphysema is unknown. In this study, we investigated the effect of CS on lung lymphatic function. Analysis of human lung tissue revealed significant lung lymphatic thrombosis in patients with emphysema compared to control smokers that increased with disease severity. In a mouse model, CS exposure led to lung lymphatic thrombosis, decreased lymphatic drainage, and impaired leukocyte trafficking that all preceded the development of emphysema. Proteomic analysis demonstrated an increased abundance of coagulation factors in the lymph draining from the lungs of CS-exposed mice compared to control mice. In addition, in vitro assays demonstrated a direct effect of CS on lymphatic endothelial cell integrity. These data show that CS exposure results in lung lymphatic dysfunction and a shift in thoracic lymph towards a prothrombic state. Furthermore, our data suggest that lymphatic dysfunction is due to effects of CS on the lymphatic vasculature that precede emphysema. These studies demonstrate a novel component of CS-induced lung injury that occurs early in the pathogenesis of emphysema.

Lymphatic Tissue and Organ Engineering for In Vitro Modeling and In Vivo Regeneration

The lymphatic system has an important role in maintaining fluid homeostasis and transporting immune cells and biomolecules, such as dietary fat, metabolic products, and antigens in different organs and tissues. Therefore, impaired lymphatic vessel function and/or lymphatic vessel deficiency can lead to numerous human diseases. The discovery of lymphatic endothelial markers and prolymphangiogenic growth factors, along with a growing number of in vitro and in vivo models and technologies has expedited research in lymphatic tissue and organ engineering, advancing therapeutic strategies. In this article, we describe lymphatic tissue and organ engineering in two- and three-dimensional culture systems and recently developed microfluidics and organ-on-a-chip systems in vitro. Next, we discuss advances in lymphatic tissue and organ engineering in vivo, focusing on biomaterial and scaffold engineering and their applications for lymphatic vessels and lymphoid organ regeneration. Last, we provide expert perspective and prospects in the field of lymphatic tissue engineering.

Multilayer microfluidic platform for the study of luminal, transmural, and interstitial flow

Efficient delivery of oxygen and nutrients to tissues requires an intricate balance of blood, lymphatic, and interstitial fluid pressures (IFPs), and gradients in fluid pressure drive the flow of blood, lymph, and interstitial fluid through tissues. While specific fluid mechanical stimuli, such as wall shear stress, have been shown to modulate cellular signaling pathways along with gene and protein expression patterns, an understanding of the key signals imparted by flowing fluid and how these signals are integrated across multiple cells and cell types in native tissues is incomplete due to limitations with current assays. Here, we introduce a multi-layer microfluidic platform (MμLTI-Flow) that enables the culture of engineered blood and lymphatic microvessels and independent control of blood, lymphatic, and IFPs. Using optical microscopy methods to measure fluid velocity for applied input pressures, we demonstrate varying rates of interstitial fluid flow as a function of blood, lymphatic, and interstitial pressure, consistent with computational fluid dynamics (CFD) models. The resulting microfluidic and computational platforms will provide for analysis of key fluid mechanical parameters and cellular mechanisms that contribute to diseases in which fluid imbalances play a role in progression, including lymphedema and solid cancer.

VE-Cadherin Is Required for Cardiac Lymphatic Maintenance and Signaling

BACKGROUND

The adherens protein VE-cadherin (vascular endothelial cadherin) has diverse roles in organ-specific lymphatic vessels. However, its physiological role in cardiac lymphatics and its interaction with lymphangiogenic factors has not been fully explored. We sought to determine the spatiotemporal functions of VE-cadherin in cardiac lymphatics and mechanistically elucidate how VE-cadherin loss influences prolymphangiogenic signaling pathways, such as adrenomedullin and VEGF (vascular endothelial growth factor)-C/VEGFR3 (vascular endothelial growth factor receptor 3) signaling.

METHODS

Cdh5^flox/flox;Prox1CreER^T2 mice were used to delete VE-cadherin in lymphatic endothelial cells across life stages, including embryonic, postnatal, and adult. Lymphatic architecture and function was characterized using immunostaining and functional lymphangiography. To evaluate the impact of temporal and functional regression of cardiac lymphatics in Cdh5^flox/flox;Prox1CreER^T2 mice, left anterior descending artery ligation was performed and cardiac function and repair after myocardial infarction was evaluated by echocardiography and histology. Cellular effects of VE-cadherin deletion on lymphatic signaling pathways were assessed by knockdown of VE-cadherin in cultured lymphatic endothelial cells.

RESULTS

Embryonic deletion of VE-cadherin produced edematous embryos with dilated cardiac lymphatics with significantly altered vessel tip morphology. Postnatal deletion of VE-cadherin caused complete disassembly of cardiac lymphatics. Adult deletion caused a temporal regression of the quiescent epicardial lymphatic network which correlated with significant dermal and cardiac lymphatic dysfunction, as measured by fluorescent and quantum dot lymphangiography, respectively. Surprisingly, despite regression of cardiac lymphatics, Cdh5^flox/flox;Prox1CreER^T2 mice exhibited preserved cardiac function, both at baseline and following myocardial infarction, compared with control mice. Mechanistically, loss of VE-cadherin leads to aberrant cellular internalization of VEGFR3, precluding the ability of VEGFR3 to be either canonically activated by VEGF-C or noncanonically transactivated by adrenomedullin signaling, impairing downstream processes such as cellular proliferation.

CONCLUSIONS

VE-cadherin is an essential scaffolding protein to maintain prolymphangiogenic signaling nodes at the plasma membrane, which are required for the development and adult maintenance of cardiac lymphatics, but not for cardiac function basally or after injury.

Innovations in lymph node targeting nanocarriers

Lymph nodes are secondary lymphoid tissues in the body that facilitate the co-mingling of immune cells to enable and regulate the adaptive immune response. They are also tissues implicated in a variety of diseases, including but not limited to malignancy. The ability to access lymph nodes is thus attractive for a variety of therapeutic and diagnostic applications. As nanotechnologies are now well established for their potential in translational biomedical applications, their high relevance to applications that involve lymph nodes is highlighted. Herein, established paradigms of nanocarrier design to enable delivery to lymph nodes are discussed, considering the unique lymph node tissue structure as well as lymphatic system physiology. The influence of delivery mechanism on how nanocarrier systems distribute to different compartments and cells that reside within lymph nodes is also elaborated. Finally, current advanced nanoparticle technologies that have been developed to enable lymph node delivery are discussed.

Three-Dimensional In Vitro Lymphangiogenesis Model in Tumor Microenvironment

Lymphangiogenesis is a stage of new lymphatic vessel formation in development and pathology, such as inflammation and tumor metastasis. Physiologically relevant models of lymphatic vessels have been in demand because studies on lymphatic vessels are required for understanding the mechanism of tumor metastasis. In this study, a new three-dimensional lymphangiogenesis model in a tumor microenvironment is proposed, using a newly designed macrofluidic platform. It is verified that controllable biochemical and biomechanical cues, which contribute to lymphangiogenesis, can be applied in this platform. In particular, this model demonstrates that a reconstituted lymphatic vessel has an in vivo–like lymphatic vessel in both physical and biochemical aspects. Since biomechanical stress with a biochemical factor influences robust directional lymphatic sprouting, whether our model closely approximates in vivo, the initial lymphatics in terms of the morphological and genetic signatures is investigated. Furthermore, attempting an incorporation with a tumor spheroid, this study successfully develops a complex tumor microenvironment model for use in lymphangiogenesis and reveals the microenvironment factors that contribute to tumor metastasis. As a first attempt at a coculture model, this reconstituted model is a novel system with a fully three-dimensional structure and can be a powerful tool for pathological drug screening or disease model.

VE-Cadherin and Vesicles Differentially Regulate Lymphatic Vascular Permeability to Solutes of Various Sizes

Lymphatic vascular permeability prevents lymph leakage that is associated with lymphedema, lymphatic malformations, obesity, and inflammation. However, the molecular control of lymphatic permeability remains poorly understood. Recent studies have suggested that adherens junctions and vesicle transport may be involved in regulating lymphatic vessel permeability. To determine the contribution of each transport pathway, we utilized an ex vivo permeability assay to directly measure the solute flux of various molecular weight solutes across a range of pressures in intact murine collecting lymphatic vessels. Pharmacological and biological tools were used to probe the relative contributions of vesicles and junction proteins in the lymphatic vasculature. We show that the permeability of collecting lymphatic vessels is inversely related to the solute molecular weight. Further, our data reveal that vesicles selectively transport BSA, as an inhibitor of vesicle formation significantly decreased the permeability to BSA (∼60% decrease, n = 8, P = 0.02), but not to 3 kDa dextran (n = 7, P = 0.41), α-lactalbumin (n = 5, P = 0.26) or 70 kDa dextran (n = 8, P = 0.13). In contrast, disruption of VE-cadherin binding with a function blocking antibody significantly increased lymphatic vessel permeability to both 3 kDa dextran (5.7-fold increase, n = 5, P < 0.0001) and BSA (5.8-fold increase, n = 5, P < 0.0001). Thus, in the lymphatic vasculature, adherens junctions did not exhibit selectivity for any of the solutes tested here, whereas vesicles specifically transport BSA. Overall, the findings suggest that disease states that disrupt VE-cadherin localization or expression will cause significant leakage of solutes and fluid from the lymphatic vasculature.

A bioengineered lymphatic vessel model for studying lymphatic endothelial cell-cell junction and barrier function

OBJECTIVE

Lymphatic vessels (LVs) maintain fluid homeostasis by draining interstitial fluid. A failure in lymphatic drainage triggers lymphatic diseases such as lymphedema. Since lymphatic drainage is regulated by lymphatic barrier function, developing experimental models that assess lymphatic barrier function is critical for better understanding of lymphatic physiology and disease.

METHODS

We built a lymphatic vessel-on-chip (LV-on-chip) by fabricating a microfluidic device that includes a hollow microchannel embedded in three-dimensional (3D) hydrogel. Employing luminal flow in the microchannel, human lymphatic endothelial cells (LECs) seeded in the microchannel formed an engineered LV exhibiting 3D conduit structure.

RESULTS

Lymphatic endothelial cells formed relatively permeable junctions in 3D collagen 1. However, adding fibronectin to the collagen 1 apparently tightened LEC junctions. We tested lymphatic barrier function by introducing dextran into LV lumens. While LECs in collagen 1 showed permeable barriers, LECs in fibronectin/collagen 1 showed reduced permeability, which was reversed by integrin α5 inhibition. Mechanistically, LECs expressed inactivated integrin α5 in collagen 1. However, integrin α5 is activated in fibronectin and enhances barrier function. Integrin α5 activation itself also tightened LEC junctions in the absence of fibronectin.

CONCLUSIONS

Lymphatic vessel-on-chip reveals integrin α5 as a regulator of lymphatic barrier function and provides a platform for studying lymphatic barrier function in various conditions.

Biochemical and mechanical signals in the lymphatic vasculature

Lymphatic vasculature is an integral part of the cardiovascular system where it maintains interstitial fluid balance. Additionally, lymphatic vasculature regulates lipid assimilation and inflammatory response. Lymphatic vasculature is composed of lymphatic capillaries, collecting lymphatic vessels and valves that function in synergy to absorb and transport fluid against gravitational and pressure gradients. Defects in lymphatic vessels or valves leads to fluid accumulation in tissues (lymphedema), chylous ascites, chylothorax, metabolic disorders and inflammation. The past three decades of research has identified numerous molecules that are necessary for the stepwise development of lymphatic vasculature. However, approaches to treat lymphatic disorders are still limited to massages and compression bandages. Hence, better understanding of the mechanisms that regulate lymphatic vascular development and function is urgently needed to develop efficient therapies. Recent research has linked mechanical signals such as shear stress and matrix stiffness with biochemical pathways that regulate lymphatic vessel growth, patterning and maturation and valve formation. The goal of this review article is to highlight these innovative developments and speculate on unanswered questions.

Dichotomous effects on lymphatic transport with loss of caveolae in mice

AIM

Fluid and macromolecule transport from the interstitium into and through lymphatic vessels is necessary for tissue homeostasis. While lymphatic capillary structure suggests that passive, paracellular transport would be the predominant route of macromolecule entry, active caveolae-mediated transcellular transport has been identified in lymphatic endothelial cells (LECs) in vitro. Caveolae also mediate a wide array of endothelial cell processes, including nitric oxide regulation. Thus, how does the lack of caveolae impact "lymphatic function"?

METHODS

Various aspects of lymphatic transport were measured in mice constitutively lacking caveolin-1 ("CavKO"), the protein required for caveolae formation in endothelial cells, and in mice with a LEC-specific Cav1 gene deletion (Lyve1-Cre x Cav1^flox/flox ; "LyCav") and ex vivo in their vessels and cells.

RESULTS

In each model, lymphatic architecture was largely unchanged. The lymphatic conductance, or initial tissue uptake, was significantly higher in both CavKO mice and LyCav mice by quantitative microlymphangiography and the permeability to 70 kDa dextran was significantly increased in monolayers of LECs isolated from CavKO mice. Conversely, transport within the lymphatic system to the sentinel node was significantly reduced in anaesthetized CavKO and LyCav mice. Isolated, cannulated collecting vessel studies identified significantly reduced phasic contractility when lymphatic endothelium lacks caveolae. Inhibition of nitric oxide synthase was able to partially restore ex vivo vessel contractility.

CONCLUSION

Macromolecule transport across lymphatics is increased with loss of caveolae, yet phasic contractility reduced, resulting in reduced overall lymphatic transport function. These studies identify lymphatic caveolar biology as a key regulator of active lymphatic transport functions.

Homeostatic maintenance of the lymphatic vasculature

The lymphatic vasculature is emerging as a multifaceted regulator of tissue homeostasis and regeneration. Lymphatic vessels drain fluid, macromolecules, and immune cells from peripheral tissues to lymph nodes (LNs) and the systemic circulation. Their recently uncovered functions extend beyond drainage and include direct modulation of adaptive immunity and paracrine regulation of organ growth. The developmental mechanisms controlling lymphatic vessel growth have been described with increasing precision. It is less clear how the essential functional features of lymphatic vessels are established and maintained. We discuss the mechanisms that maintain lymphatic vessel integrity in adult tissues and control vessel repair and regeneration. This knowledge is crucial for understanding the pathological vessel changes that contribute to disease, and provides an opportunity for therapy development.

Impaired meningeal lymphatic vessels exacerbate early brain injury after experimental subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Blood that enters the subarachnoid space (SAS) and its breakdown products are neurotoxic and are the principal inducers of brain injury after subarachnoid hemorrhage (SAH). Recently, meningeal lymphatic vessels (MLVs) have been proven to play an important role in clearing erythrocytes that arise from SAH, as well as other macromolecular solutes. However, evidence demonstrating the relationship between MLVs and brain injury after SAH is still limited. Therefore, we performed this study to observe the effects of meningeal lymphatic impairment on early brain injury (EBI) after experimental SAH.

METHODS

The MLVs of C57BL/6 male adult mice were ablated by injecting Visudyne into the cisterna magna and transcranially photoconverting it with laser light. The MLVs were then examined by immunofluorescence staining for lyve-1. Next, both the MLV-ablated group and the control group (normal mice) underwent filament perforation to model SAH or sham operation. We assessed the cortical perfusion of all the mice before SAH induction, 5 min after SAH and 24 h after SAH. In addition, we evaluated neurological function deficits by Garcia scores and measured brain water content at 24 h post SAH. Then, neuroinflammation and neural apoptosis in the mouse brain were also examined.

RESULTS

Visudyne and transcranial photoconversion treatment notably ablated mouse MLVs. Five minutes after SAH induction, cortical perfusion was significantly impaired, and after 24 h, this impairment was ameliorated considerably in the control group but ameliorated only slightly or worsened in the MLV-ablated group. Additionally, the MLVablated group presented worse neurological function deficits and more severe brain edema than the control group. More notably, neuroinflammation and neural apoptosis were also observed.

CONCLUSION

Ablation of MLVs by Visudyne treatment exacerbated EBI after experimental SAH in mice. The worsening of EBI may have arisen from limited drainage of blood and other breakdown products, which are thought to cause brain edema, neuroinflammation, neuronal apoptosis and other pathological processes.

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.

Afferent Lymphatic Transport and Peripheral Tissue Immunity

Lymphatic vessels provide an anatomical framework for immune surveillance and adaptive immune responses. Although appreciated as the route for Ag and dendritic cell transport, peripheral lymphatic vessels are often not considered active players in immune surveillance. Lymphatic vessels, however, integrate contextual cues that directly regulate transport, including changes in intrinsic pumping and capillary remodeling, and express a dynamic repertoire of inflammatory chemokines and adhesion molecules that facilitates leukocyte egress out of inflamed tissue. These mechanisms together contribute to the course of peripheral tissue immunity. In this review, we focus on context-dependent mechanisms that regulate fluid and cellular transport out of peripheral nonlymphoid tissues to provide a framework for understanding the effects of afferent lymphatic transport on immune surveillance, peripheral tissue inflammation, and adaptive immunity.

Molecular tracking devices quantify antigen distribution and archiving in the murine lymph node

The detection of foreign antigens in vivo has relied on fluorescent conjugation or indirect read-outs such as antigen presentation. In our studies, we found that these widely used techniques had several technical limitations that have precluded a complete picture of antigen trafficking or retention across lymph node cell types. To address these limitations, we developed a 'molecular tracking device' to follow the distribution, acquisition, and retention of antigen in the lymph node. Utilizing an antigen conjugated to a nuclease-resistant DNA tag, acting as a combined antigen-adjuvant conjugate, and single-cell mRNA sequencing, we quantified antigen abundance in the lymph node. Variable antigen levels enabled the identification of caveolar endocytosis as a mechanism of antigen acquisition or retention in lymphatic endothelial cells. Thus, these molecular tracking devices enable new approaches to study dynamic tissue dissemination of antigen-adjuvant conjugates and identify new mechanisms of antigen acquisition and retention at cellular resolution in vivo.

Lymphangiogenesis-inducing vaccines elicit potent and long-lasting T cell immunity against melanomas

In melanoma, the induction of lymphatic growth (lymphangiogenesis) has long been correlated with metastasis and poor prognosis, but we recently showed it can synergistically enhance cancer immunotherapy and boost T cell immunity. Here, we develop a translational approach for exploiting this "lymphangiogenic potentiation" of immunotherapy in a cancer vaccine using lethally irradiated tumor cells overexpressing vascular endothelial growth factor C (VEGF-C) and topical adjuvants. Our "VEGFC vax" induced extensive local lymphangiogenesis and promoted stronger T cell activation in both the intradermal vaccine site and draining lymph nodes, resulting in higher frequencies of antigen-specific T cells present systemically than control vaccines. In mouse melanoma models, VEGFC vax elicited potent tumor-specific T cell immunity and provided effective tumor control and long-term immunological memory. Together, these data introduce the potential of lymphangiogenesis induction as a novel immunotherapeutic strategy to consider in cancer vaccine design.

Lymph-directed nitric oxide increases immune cell access to lymph-borne nanoscale solutes

Lymph nodes (LNs) are immune organs housing high concentrations of lymphocytes, making them critical targets for therapeutic immunomodulation in a wide variety of diseases. While there is great interest in targeted drug delivery to LNs, many nanoscale drug delivery carriers have limited access to parenchymal resident immune cells compared to small molecules, limiting their efficacy. Nitric oxide (NO) is a potent regulator of vascular and lymphatic transport and a promising candidate for modulating nanocarrier access to LNs, but its lymphatic accumulation is limited by its low molecular weight and high reactivity. In this work, we employ S-nitrosated nanoparticles (SNO-NP), a lymphatic-targeted delivery system for controlled NO release, to investigate the effect of NO application on molecule accumulation and distribution within the LN. We evaluated the LN accumulation, spatial distribution, and cellular distribution of a panel of fluorescent tracers after intradermal administration alongside SNO-NP or a small molecule NO donor. While SNO-NP did not alter total tracer accumulation in draining lymph nodes (dLNs) or affect active cellular transport of large molecules from the injection site, its application enhanced the penetration of nanoscale 30 nm dextrans into the LN and their subsequent uptake by LN-resident lymphocytes, while nontargeted NO delivery did not. These results further extended to a peptide-conjugated NP drug delivery system, which showed enhanced uptake by B cells and dendritic cells when administered alongside SNO-NP. Together, these results highlight the utility of LN-targeted NO application for the enhancement of nanocarrier access to therapeutically relevant LN-resident immune cells, making NO a potentially useful tool for improving LN drug delivery and immune responses.

Brain Glymphatic/Lymphatic Imaging by MRI and PET

Since glymphatic was proposed and meningeal lymphatic was discovered, MRI and even PET were introduced to investigate brain parenchymal interstitial fluid (ISF), cerebrospinal fluid (CSF), and lymphatic outflow in rodents and humans. Previous findings by ex vivo fluorescent microscopic, and in vivo two-photon imaging in rodents were reproduced using intrathecal contrast (gadobutrol and the similar)-enhanced MRI in rodents and further in humans. On dynamic MRI of meningeal lymphatics, in contrast to rodents, humans use mainly dorsal meningeal lymphatic pathways of ISF-CSF-lymphatic efflux. In mice, ISF-CSF exchange was examined thoroughly using an intra-cistern injection of fluorescent tracers during sleep, aging, and neurodegeneration yielding many details. CSF to lymphatic efflux is across arachnoid barrier cells over the dorsal dura in rodents and in humans. Meningeal lymphatic efflux to cervical lymph nodes and systemic circulation is also well-delineated especially in humans onintrathecal contrast MRI. Sleep- or anesthesia-related changes of glymphatic-lymphatic flow and the coupling of ISF-CSF-lymphatic drainage are major confounders ininterpreting brain glymphatic/lymphatic outflow in rodents. PET imaging in humans should be interpreted based on human anatomy and physiology, different in some aspects, using MRI recently. Based on the summary in this review, we propose non-invasive and longer-term intrathecal SPECT/PET or MRI studies to unravel the roles of brain glymphatic/lymphatic in diseases.

Properties of rapamycin solid lipid nanoparticles for lymphatic access through the lungs & part I: the effect of size

Background: Lymphangioleiomyomatosis (LAM) is characterized by growth of smooth muscle-like cells in the lungs that spread to other organs via lymphatic vessels. Current oral rapamycin treatment is limited by low bioavailability of approximately 15%. Aim: The effect of inhaled rapamycin solid lipid nanoparticles (Rapa-SLNs) size on its penetration through the lymphatics. Method: Three Rapa-SLN formulations (200-1000 nm) were produced and assessed for particle characteristics and further for toxicity and performance in vitro. Results: Rapa-SLNs of 200 nm inhibited proliferation in TSC2-negative mouse embryonic fibroblast cells and penetrated the respiratory epithelium and lymphatic endothelium significantly faster compared with free rapamycin and larger Rapa-SLNs. Conclusion: Rapa-SLN approximately 200 nm allows efficient entry of rapamycin into the lymphatic system and is therefore a promising treatment for LAM patients.

Properties of rapamycin solid lipid nanoparticles for lymphatic access through the lungs & part II: the effect of nanoparticle charge

Aim: Lymphangioleiomyomatosis is characterized by smooth muscle-like cells in the lungs that spread to other organs via lymphatic vessels. Oral rapamycin is restricted by low bioavailability approximately 15%. The aim of the present study is to systematically investigate the effect of inhaled rapamycin solid lipid nanoparticles (Rapa-SLN) surface charge on efficacy and penetration into the lymphatics. Materials & methods: Rapa-SLN formulations with different charge: neutral, positive and negative, were produced and assessed for their physicochemical particle characteristics and efficacy in vitro. Results: Negative Rapa-SLNs were significantly faster at entering the lymphatic endothelium and more potent at inhibiting lymphanigiogenesis compared with neutral and positive Rapa-SLNs. Conclusion: Negative Rapa-SLNs showed efficient lymphatic access and should therefore be investigated further as a treatment for targeting extrapulmonary lymphangioleiomyomatosis.

Stromal regulation of tumor-associated lymphatics

Recent advances have identified a growing array of roles played by lymphatics in the tumor microenvironment, from providing a route of metastasis to immune modulation. The tumor microenvironment represents an exceptionally complex, dynamic niche comprised of a diverse mixture of cancer cells and normal host cells termed the stroma. This review discusses our current understanding of stromal elements and how they regulate lymphatic growth and functional properties in the tumor context.

Lymphatic targeting by albumin-hitchhiking: Applications and optimisation

The lymphatic system plays an integral role in the development and progression of a range of disease conditions, which has impelled medical researchers and clinicians to design, develop and utilize advanced lymphatic drug delivery systems. Following interstitial administration, most therapeutics and molecules are cleared from tissues via the draining blood capillaries. Macromolecules and delivery systems >20 kDa in size or 10-100 nm in diameter are, however, transported from the interstitium via draining lymphatic vessels as they are too large to cross the blood capillary endothelium. Lymphatic uptake of small molecules can be promoted by two general approaches: administration in association with synthetic macromolecular constructs, or through hitchhiking on endogenous cells or macromolecular carriers that are transported from tissues via the lymphatics. In this paper we review the latter approach where molecules are targeted to lymph by hitchhiking on endogenous albumin transport pathways after subcutaneous, intramuscular or intradermal injection. We describe the properties of the lymphatic system and albumin that are relevant to lymphatic targeting, the characteristics of drugs and delivery systems designed to hitchhike on albumin trafficking pathways and how to further optimise these properties, and finally the current applications and potential future directions for albumin-hitchhiking approaches to target the lymphatics.

The Lymphatic Vasculature in the 21st Century: Novel Functional Roles in Homeostasis and Disease

Mammals have two specialized vascular circulatory systems: the blood vasculature and the lymphatic vasculature. The lymphatic vasculature is a unidirectional conduit that returns filtered interstitial arterial fluid and tissue metabolites to the blood circulation. It also plays major roles in immune cell trafficking and lipid absorption. As we discuss in this review, the molecular characterization of lymphatic vascular development and our understanding of this vasculature's role in pathophysiological conditions has greatly improved in recent years, changing conventional views about the roles of the lymphatic vasculature in health and disease. Morphological or functional defects in the lymphatic vasculature have now been uncovered in several pathological conditions. We propose that subtle asymptomatic alterations in lymphatic vascular function could underlie the variability seen in the body's response to a wide range of human diseases.