Transmural flow modulates cell and fluid transport functions of lymphatic endothelium

Fluid shear stress activates YAP1 to promote cancer cell motility

Mechanical stress is pervasive in egress routes of malignancy, yet the intrinsic effects of force on tumour cells remain poorly understood. Here, we demonstrate that frictional force characteristic of flow in the lymphatics stimulates YAP1 to drive cancer cell migration; whereas intensities of fluid wall shear stress (WSS) typical of venous or arterial flow inhibit taxis. YAP1, but not TAZ, is strictly required for WSS-enhanced cell movement, as blockade of YAP1, TEAD1-4 or the YAP1–TEAD interaction reduces cellular velocity to levels observed without flow. Silencing of TEAD phenocopies loss of YAP1, implicating transcriptional transactivation function in mediating force-enhanced cell migration. WSS dictates expression of a network of YAP1 effectors with executive roles in invasion, chemotaxis and adhesion downstream of the ROCK–LIMK–cofilin signalling axis. Altogether, these data implicate YAP1 as a fluid mechanosensor that functions to regulate genes that promote metastasis.

Fluid frictional forces around cancer cells influence chemokine production and delivery of chemotherapeutic drugs but it is unclear if they directly impact tumour biology through biomechanical effects. Here, the authors show that wall shear stress stimulates cancer cell migration through a ROCK–LIMK–YAP axis.

Lymph Nodes and Cancer Metastasis: New Perspectives on the Role of Intranodal Lymphatic Sinuses

The lymphatic system is essential for transporting interstitial fluid, soluble antigen, and immune cells from peripheral tissues to lymph nodes (LNs). Functional integrity of LNs is dependent on intact lymphatics and effective lymph drainage. Molecular mechanisms that facilitate interactions between tumor cells and lymphatic endothelial cells (LECs) during tumor progression still remain to be identified. The cellular and molecular structures of LNs are optimized to trigger a rapid and efficient immune response, and to participate in the process of tumor metastasis by stimulating lymphangiogenesis and establishing a premetastatic niche in LNs. Several molecules, e.g., S1P, CCR7-CCL19/CCL21, CXCL12/CXCR4, IL-7, IFN-γ, TGF-β, and integrin α4β1 play an important role in controlling the activity of LN stromal cells including LECs, fibroblastic reticular cells (FRCs) and follicular dendritic cells (DCs). The functional stromal cells are critical for reconstruction and remodeling of the LN that creates a unique microenvironment of tumor cells and LECs for cancer metastasis. LN metastasis is a major determinant for the prognosis of most human cancers and clinical management. Ongoing work to elucidate the function and molecular regulation of LN lymphatic sinuses will provide insight into cancer development mechanisms and improve therapeutic approaches for human malignancy.

The lymphatic vasculature: development and role in shaping immunity

The lymphatic vasculature is an integral component of the immune system. Lymphatic vessels are a key highway via which immune cells are trafficked, serving not simply as a passive route of transport, but to actively shape and coordinate immune responses. Reciprocally, immune cells provide signals that impact the growth, development, and activity of the lymphatic vasculature. In addition to immune cell trafficking, lymphatic vessels are crucial for fluid homeostasis and lipid absorption. The field of lymphatic vascular research is rapidly expanding, fuelled by rapidly advancing technology that has enabled the manipulation and imaging of lymphatic vessels, together with an increasing recognition of the involvement of lymphatic vessels in a myriad of human pathologies. In this review we provide an overview of the genetic pathways and cellular processes important for development and maturation of the lymphatic vasculature, discuss recent work revealing important roles for the lymphatic vasculature in directing immune cell traffic and coordinating immune responses and highlight the involvement of lymphatic vessels in a range of pathological settings.

Lymphatics in Neurological Disorders: A Neuro-Lympho-Vascular Component of Multiple Sclerosis and Alzheimer's Disease?

Lymphatic vasculature drains interstitial fluids, which contain the tissue's waste products, and ensures immune surveillance of the tissues, allowing immune cell recirculation. Until recently, the CNS was considered to be devoid of a conventional lymphatic vasculature. The recent discovery in the meninges of a lymphatic network that drains the CNS calls into question classic models for the drainage of macromolecules and immune cells from the CNS. In the context of neurological disorders, the presence of a lymphatic system draining the CNS potentially offers a new player and a new avenue for therapy. In this review, we will attempt to integrate the known primary functions of the tissue lymphatic vasculature that exists in peripheral organs with the proposed function of meningeal lymphatic vessels in neurological disorders, specifically multiple sclerosis and Alzheimer's disease. We propose that these (and potentially other) neurological afflictions can be viewed as diseases with a neuro-lympho-vascular component and should be therapeutically targeted as such.

Endothelial Cell Responses to Biomechanical Forces in Lymphatic Vessels

SIGNIFICANCE

Lymphatic vessels are important components of the cardiovascular and immune systems. They contribute both to the maintenance of normal homeostasis and to many pathological conditions, such as cancer and inflammation. The lymphatic vasculature is subjected to a variety of biomechanical forces, including fluid shear stress and vessel circumferential stretch.

RECENT ADVANCES

This review will discuss recent advances in our understanding of biomechanical forces in lymphatic vessels and their role in mammalian lymphatic vascular development and function.

CRITICAL ISSUES

We will highlight the importance of fluid shear stress generated by lymph flow in organizing the lymphatic vascular network. We will also describe how mutations in mechanosensitive genes lead to lymphatic vascular dysfunction.

FUTURE DIRECTIONS

Better understanding of how biomechanical and biochemical stimuli are perceived and interpreted by lymphatic endothelial cells is important for targeting regulation of lymphatic function in health and disease. Important remaining critical issues and future directions in the field will be discussed in this review. Antioxid. Redox Signal. 25, 451-465.

Lymphatic vessels regulate immune microenvironments in human and murine melanoma

Lymphatic remodeling in tumor microenvironments correlates with progression and metastasis, and local lymphatic vessels play complex and poorly understood roles in tumor immunity. Tumor lymphangiogenesis is associated with increased immune suppression, yet lymphatic vessels are required for fluid drainage and immune cell trafficking to lymph nodes, where adaptive immune responses are mounted. Here, we examined the contribution of lymphatic drainage to tumor inflammation and immunity using a mouse model that lacks dermal lymphatic vessels (K14-VEGFR3-Ig mice). Melanomas implanted in these mice grew robustly, but exhibited drastically reduced cytokine expression and leukocyte infiltration compared with those implanted in control animals. In the absence of local immune suppression, transferred cytotoxic T cells more effectively controlled tumors in K14-VEGFR3-Ig mice than in control mice. Furthermore, gene expression analysis of human melanoma samples revealed that patient immune parameters are markedly stratified by levels of lymphatic markers. This work suggests that the establishment of tumor-associated inflammation and immunity critically depends on lymphatic vessel remodeling and drainage. Moreover, these results have implications for immunotherapies, the efficacies of which are regulated by the tumor immune microenvironment.

E-Selectin Mediates Immune Cell Trafficking in Corneal Transplantation

BACKGROUND

Immune rejection continues to threaten all tissue transplants. Here we sought to determine whether platelet (P)- and endothelial (E)-selectin mediate T cell recruitment in corneal transplantation and whether their blockade can reduce T cell graft infiltration and improve long-term corneal allograft survival.

METHODS

In a murine model of allogeneic corneal transplantation, we used PCR and immunohistochemistry to investigate expression of P- and E-selectin in rejected versus accepted allografts and lymph node flow cytometry to assess expression of selectin ligands by effector T cells. Using P- and E-selectin neutralizing antibodies, we evaluated the effect of blockade on CD4 T cell recruitment, as well as the effect of anti-E-selectin on long-term allograft survival.

RESULTS

The P- (93.3-fold, P < 0.05) and E-selectin (17.1-fold, P < 0.005) are upregulated in rejected versus accepted allogeneic transplants. Type 1 T helper cells from hosts with accepted and rejected grafts express high levels of P-selectin glycoprotein ligand 1 and glycosylated CD43. In vivo blockade of P (0.47 ± 0.03, P < 0.05) and E selectin (0.49 ± 0.1, P < 0.05) reduced the number of recruited T cells compared with IgG control (0.98 ± 0.1). Anti-E-selectin reduced the number of mature antigen-presenting cells trafficking to lymphoid tissue compared with control (6.96 ± 0.9 vs 12.67 ± 0.5, P < 0.05). Anti-E-selectin treatment delayed graft rejection and increased survival compared with control, although this difference did not reach statistical significance.

CONCLUSIONS

In a model of corneal transplantation, P- and E-selectin mediate T cell recruitment to the graft, E-selectin mediates APC trafficking to lymphoid tissue, and blockade of E-selectin has a modest effect on improving long-term graft survival.

Lymphatic transport of exosomes as a rapid route of information dissemination to the lymph node

It is well documented that cells secrete exosomes, which can transfer biomolecules that impact recipient cells’ functionality in a variety of physiologic and disease processes. The role of lymphatic drainage and transport of exosomes is as yet unknown, although the lymphatics play critical roles in immunity and exosomes are in the ideal size-range for lymphatic transport. Through in vivo near-infrared (NIR) imaging we have shown that exosomes are rapidly transported within minutes from the periphery to the lymph node by lymphatics. Using an in vitro model of lymphatic uptake, we have shown that lymphatic endothelial cells actively enhanced lymphatic uptake and transport of exosomes to the luminal side of the vessel. Furthermore, we have demonstrated a differential distribution of exosomes in the draining lymph nodes that is dependent on the lymphatic flow. Lastly, through endpoint analysis of cellular distribution of exosomes in the node, we identified macrophages and B-cells as key players in exosome uptake. Together these results suggest that exosome transfer by lymphatic flow from the periphery to the lymph node could provide a mechanism for rapid exchange of infection-specific information that precedes the arrival of migrating cells, thus priming the node for a more effective immune response.

The Lymphatic System in Disease Processes and Cancer Progression

Advances in our understanding of the structure and function of the lymphatic system have made it possible to identify its role in a variety of disease processes. Because it is involved not only in fluid homeostasis but also in immune cell trafficking, the lymphatic system can mediate and ultimately alter immune responses. Our rapidly increasing knowledge of the molecular control of the lymphatic system will inevitably lead to new and effective therapies for patients with lymphatic dysfunction. In this review, we discuss the molecular and physiological control of lymphatic vessel function and explore how the lymphatic system contributes to many disease processes, including cancer and lymphedema.

The lymphatic system and pancreatic cancer

This review summarizes current knowledge of the biology, pathology and clinical understanding of lymphatic invasion and metastasis in pancreatic cancer. We discuss the clinical and biological consequences of lymphatic invasion and metastasis, including paraneoplastic effects on immune responses and consider the possible benefit of therapies to treat tumors that are localized to lymphatics. A review of current techniques and methods to study interactions between tumors and lymphatics is presented.

Melanoma growth effects on molecular clearance from tumors and biodistribution into systemic tissues versus draining lymph nodes

Factors produced within or administered directly into the tumor interstitium, such as cytokines, chemokines, proteases, exosomes, microvesicles, or therapeutic agents, play important and multifaceted roles in the regulation of malignant disease progression. Their bioavailability to mediate signaling in distributed tissues outside of the tumor microenvironment, however, has not been well described. We therefore sought to elucidate the relative extent to which factors from within the primary tumor disseminate to systemic tissues as well as how these distribution profiles are influenced by both hydrodynamic size and the remodeling tumor vasculature. To accomplish this goal, we intratumorally co-infused into the dermal lesions of B16F10 melanoma-bearing mice at prescribed times post tumor implantation a near infrared fluorescent tracer panel ranging from 5 to 500nm in hydrodynamic diameter and compared the in vivo clearance and biodistribution profiles to that of naïve animals. Our results indicate that tumor growth reduces tumor-draining lymph node accumulation and alters the distribution of tumor-derived factors amongst systemic tissues. Despite these changes, previously developed principles of size-dependent lymph node drug targeting are conserved in melanomas, suggesting their applicability to sentinel lymph node-targeted drug delivery. Tumor progression was also found to result in a significant increase in the hydrodynamic size of factors originating from the tumor that accumulated within systemic tissues. This suggests that tumor vascular remodeling may redirect the organism-wide signaling activity of tumor-derived factors and may negatively contribute to disease progression by altering the bioavailability of molecules important to the regulation of pre-metastatic niche formation and the induction of anti-tumor immunity.

Three-dimensional biomimetic model to reconstitute sprouting lymphangiogenesis in vitro

Formation of new lymphatic vessels, termed lymphangiogenesis, is central for diverse biological processes during development, inflammation and tumor metastasis. However, reliable in vitro model is still under demand for detailed elucidation of how sprouting lymphangiogenesis is initiated and coordinated. Here, we describe a microfluidic platform optimized for close reconstitution of lymphangiogenesis, achieved by on-chip integration of salient constituents of lymphatic microenvironment found in vivo. With flexible and precise control over the factors that include biochemical cues, interstitial flow (IF), and endothelial-stromal interactions, we found that orchestrated efforts of multiple environmental factors are necessary for robust lymphatic sprouting in 3D extracellular matrix. Especially, we demonstrate that IF serves as a central regulatory cue which defines lymphangiogenic responses and phenotypes of lymphatic endothelial cells. When synergized with pro-lymphangiogenic factors, IF significantly augmented initiation and outgrowth of lymphatic sprouts toward upstream of the flow while suppressing downstream-directed sprouting. In an appropriate synergism, lymphatic sprouts exhibited structural, molecular signatures and cellular phenotypes that closely approximate sprouting lymphatic neovessels in vivo, and precisely reflected the modulatory effects of pro- and anti-lymphangiogenic stimuli. Our study not only reveals critical but unappreciated role of mechanical cue that regulates lymphangiogenic sprouting, but also provides a novel biomimetic model that may leverage further biological studies as well as phenotypic drug screening.

Lymphatic Vessels, Inflammation, and Immunity in Skin Cancer

Skin is a highly ordered immune organ that coordinates rapid responses to external insult while maintaining self-tolerance. In healthy tissue, lymphatic vessels drain fluid and coordinate local immune responses; however, environmental factors induce lymphatic vessel dysfunction, leading to lymph stasis and perturbed regional immunity. These same environmental factors drive the formation of local malignancies, which are also influenced by local inflammation. Herein, we discuss clinical and experimental evidence supporting the tenet that lymphatic vessels participate in regulation of cutaneous inflammation and immunity, and are important contributors to malignancy and potential biomarkers and targets for immunotherapy.

SIGNIFICANCE

The tumor microenvironment and tumor-associated inflammation are now appreciated not only for their role in cancer progression but also for their response to therapy. The lymphatic vasculature is a less-appreciated component of this microenvironment that coordinates local inflammation and immunity and thereby critically shapes local responses. A mechanistic understanding of the complexities of lymphatic vessel function in the unique context of skin provides a model to understand how regional immune dysfunction drives cutaneous malignancies, and as such lymphatic vessels represent a biomarker of cutaneous immunity that may provide insight into cancer prognosis and effective therapy.

A Closer Look at Schlemm's Canal Cell Physiology: Implications for Biomimetics

Among ocular pathologies, glaucoma is the second leading cause of progressive vision loss, expected to affect 80 million people worldwide by 2020. A primary cause of glaucoma appears to be damage to the conventional outflow tract. Conventional outflow tissues, a composite of the trabecular meshwork and the Schlemm's canal, regulate and maintain homeostatic responses to intraocular pressure. In glaucoma, filtration of aqueous humor into the Schlemm's canal is hindered, leading to an increase in intraocular pressure and subsequent damage to the optic nerve, with progressive vision loss. The Schlemm's canal encompasses a unique endothelium. Recent advances in culturing and manipulating Schlemm's canal cells have elucidated several aspects of their physiology, including ultrastructure, cell-specific marker expression, and biomechanical properties. This review highlights these advances and discusses implications for engineering a 3D, biomimetic, in vitro model of the Schlemm's canal endothelium to further advance glaucoma research, including drug testing and gene therapy screening.

Modes of Antigen Presentation by Lymph Node Stromal Cells and Their Immunological Implications

Antigen presentation is no longer the exclusive domain of cells of hematopoietic origin. Recent works have demonstrated that lymph node stromal cell (LNSC) populations, such as fibroblastic reticular cells, lymphatic and blood endothelial cells, not only provide a scaffold for lymphocyte interactions but also exhibit active immunomodulatory roles that are critical to mounting and resolving effective immune responses. Importantly, LNSCs possess the ability to present antigens and establish antigen-specific interactions with T cells. One example is the expression of peripheral tissue antigens, which are presented on major histocompatibility complex (MHC)-I molecules with tolerogenic consequences on T cells. Additionally, exogenous antigens, including self and tumor antigens, can be processed and presented on MHC-I complexes, which result in dysfunctional activation of antigen-specific CD8⁺ T cells. While MHC-I is widely expressed on cells of both hematopoietic and non-hematopoietic origins, antigen presentation via MHC-II is more precisely regulated. Nevertheless, LNSCs are capable of endogenously expressing, or alternatively, acquiring MHC-II molecules. Transfer of antigen between LNSC and dendritic cells in both directions has been recently suggested to promote tolerogenic roles of LNSCs on the CD4⁺ T cell compartment. Thus, antigen presentation by LNSCs is thought to be a mechanism that promotes the maintenance of peripheral tolerance as well as generates a pool of diverse antigen-experienced T cells for protective immunity. This review aims to integrate the current and emerging literature to highlight the importance of LNSCs in immune responses, and emphasize their role in antigen trafficking, retention, and presentation.

ADAM17 Promotes Motility, Invasion, and Sprouting of Lymphatic Endothelial Cells

Tumor-associated lymphatic vessels actively participate in tumor progression and dissemination. ADAM17, a sheddase for numerous growth factors, cytokines, receptors, and cell adhesion molecules, is believed to promote tumor development, facilitating both tumor cell proliferation and migration, as well as tumor angiogenesis. In this work we addressed the issue of whether ADAM17 may also promote tumor lymphangiogenesis. First, we found that ADAM17 is important for the migratory potential of immortalized human dermal lymphatic endothelial cells (LEC). When ADAM17 was stably silenced in LEC, their proliferation was not affected, but: (i) single-cell motility, (ii) cell migration through a 3D Matrigel/collagen type I matrix, and (iii) their ability to form sprouts in a 3D matrix were significantly diminished. The differences in the cell motility between ADAM17-proficient and ADAM17-silenced cells were eliminated by inhibitors of EGFR and HER2, indicating that ADAM17-mediated shedding of growth factors accounts for LEC migratory potential. Interestingly, ADAM17 depletion affected the integrin surface expression/functionality in LEC. ADAM17-silenced cells adhered to plastic, type I collagen, and fibronectin faster than their ADAM17-proficient counterparts. The difference in adhesion to fibronectin was abolished by a cyclic RGD peptide, emphasizing the involvement of integrins in the process. Using a soluble receptor array, we identified BIG-H3 among several candidate proteins involved in the phenotypic and behavioral changes of LEC upon ADAM17 silencing. In additional assays, we confirmed the increased expression of BIG-H3, as well as TGFβ2 in ADAM17-silenced LEC. The antilymphangiogenic effects of ADAM17 silencing in lymphatic endothelial cells suggest further relevance of ADAM17 as a potential target in cancer therapy.

Lymph node biophysical remodeling is associated with melanoma lymphatic drainage

Tissue remodeling is a characteristic of many solid tumor malignancies including melanoma. By virtue of tumor lymphatic transport, remodeling pathways active within the local tumor microenvironment have the potential to be operational within lymph nodes (LNs) draining the tumor interstitium. Here, we show that lymphatic drainage from murine B16 melanomas in syngeneic, immune-competent C57Bl/6 mice is associated with LN enlargement as well as nonuniform increases in bulk tissue elasticity and viscoelasticity, as measured by the response of whole LNs to compression. These remodeling responses, which quickly manifest in tumor-draining lymph nodes (TDLNs) after tumor inoculation and before apparent metastasis, were accompanied by changes in matrix composition, including up to 3-fold increases in the abundance of soluble collagen and hyaluronic acid. Intranodal pressures were also significantly increased in TDLNs (+1 cmH2O) relative to both non-tumor-draining LNs (-1 cmH2O) and LNs from naive animals (-1 to 2 cmH2O). These data suggest that the reorganization of matrix structure, composition, and fluid microenvironment within LNs associated with tumor lymphatic drainage parallels remodeling seen in primary malignancies and has the potential to regulate the adhesion, proliferation, and signaling function of LN-resident cells involved in directing melanoma disease progression.

3D functional and perfusable microvascular networks for organotypic microfluidic models

The metastatic dissemination of cancer cells from primary tumors to secondary loci is a complex and multistep process including local invasion, intravasation, survival in the blood stream and extravasation towards the metastatic site. It is well known cancer metastases follow organ-specific pathways with selected primary tumors mainly metastasizing towards a specific panel of secondary organs (Steven Paget's theory 1889). However, circulatory patterns and microarchitecture of capillary networks play a key role in the metastatic spread as well (James Ewing's theory 1929). Taking into account both these factors would be critical to develop more complex and physiologically relevant in vitro cancer models. This review presents recent advances in the generation of microvascularized systems through microfluidic approaches and discusses promising results achieved by organ-on-a-chip platforms mimicking the pathophysiology of the functional units of specific organs. The combination of physiologically-like microvascular networks and organotypic microenvironments would foster a new generation of in vitro cancer models to more effectively screen new therapeutics, design personalized medicine treatments and investigate molecular pathways involved in cancer metastases.

The (dys)functional extracellular matrix

The extracellular matrix (ECM) is a major component of the biomechanical environment with which cells interact, and it plays important roles in both normal development and disease progression. Mechanical and biochemical factors alter the biomechanical properties of tissues by driving cellular remodeling of the ECM. This review provides an overview of the structural, compositional, and mechanical properties of the ECM that instruct cell behaviors. Case studies are reviewed that highlight mechanotransduction in the context of two distinct tissues: tendons and the heart. Although these two tissues demonstrate differences in relative cell-ECM composition and mechanical environment, they share similar mechanisms underlying ECM dysfunction and cell mechanotransduction. Together, these topics provide a framework for a fundamental understanding of the ECM and how it may vary across normal and diseased tissues in response to mechanical and biochemical cues. This article is part of a Special Issue entitled: Mechanobiology.

An in vitro model of the tumor-lymphatic microenvironment with simultaneous transendothelial and luminal flows reveals mechanisms of flow enhanced invasion

The most common cancers, including breast and skin, disseminate initially through the lymphatic system, yet the mechanisms by which tumor cells home towards, enter and interact with the lymphatic endothelium remain poorly understood. Transmural and luminal flows are important biophysical cues of the lymphatic microenvironment that can affect adhesion molecules, growth factors and chemokine expression as well as matrix remodeling, among others. Although microfluidic models are suitable for in vitro reconstruction of highly complex biological systems, the difficult assembly and operation of these systems often only allows a limited throughput. Here we present and characterize a novel flow chamber which recapitulates the lymphatic capillary microenvironment by coupling a standard Boyden chamber setup with a micro-channel and a controlled fluidic environment. The inclusion of luminal and transmural flow renders the model more biologically relevant, combining standard 3D culture techniques with advanced control of mechanical forces that are naturally present within the lymphatic microenvironment. The system can be monitored in real-time, allowing continuous quantification of different parameters of interest, such as cell intravasation and detachment from the endothelium, under varied biomechanical conditions. Moreover, the easy setup permits a medium-high throughput, thereby enabling downstream quantitative analyses. Using this model, we examined the kinetics of tumor cell (MDA-MB-231) invasion and transmigration dynamics across lymphatic endothelium under varying flow conditions. We found that luminal flow indirectly upregulates tumor cell transmigration rate via its effect on lymphatic endothelial cells. Moreover, we showed that the addition of transmural flow further increases intravasation, suggesting that distinct flow-mediated mechanisms regulate tumor cell invasion.

The role of mechanical forces in tumor growth and therapy

Tumors generate physical forces during growth and progression. These physical forces are able to compress blood and lymphatic vessels, reducing perfusion rates and creating hypoxia. When exerted directly on cancer cells, they can increase cells' invasive and metastatic potential. Tumor vessels-while nourishing the tumor-are usually leaky and tortuous, which further decreases perfusion. Hypoperfusion and hypoxia contribute to immune evasion, promote malignant progression and metastasis, and reduce the efficacy of a number of therapies, including radiation. In parallel, vessel leakiness together with vessel compression causes a uniformly elevated interstitial fluid pressure that hinders delivery of blood-borne therapeutic agents, lowering the efficacy of chemo- and nanotherapies. In addition, shear stresses exerted by flowing blood and interstitial fluid modulate the behavior of cancer and a variety of host cells. Taming these physical forces can improve therapeutic outcomes in many cancers.

Immunomodulatory roles of lymphatic vessels in cancer progression

Lymphatic vessels in the tumor microenvironment are known to foster tumor metastasis in many cancers, and they can undergo activation, hyperplasia, and lymphangiogenesis in the tumor microenvironment and in the tumor-draining lymph node. The mechanism underlying this correlation was originally considered as lymphatic vessels providing a physical route for tumor cell dissemination, but recent studies have highlighted new roles of the lymphatic endothelium in regulating host immunity. These include indirectly suppressing T-cell function by secreting immunosuppressive factors and inhibiting dendritic cell (DC) maturation, as well as directly driving T-cell tolerance by antigen presentation in the presence of inhibitory ligands. Furthermore, lymphatic endothelium scavenges and regulates transendothelial transport actively, controlling the sustained delivery of lymph-borne antigens from chronically inflamed tissues to draining lymph nodes where immature DCs, in the absence of danger signals, along with lymph node stromal cells present these antigens to T cells for maintenance of peripheral tolerance to self-antigens, a mechanism that may be hijacked by some tumors. This Masters of Immunology primer aims to present an overview of research in this area and highlight emerging evidence that suggests lymphatic vessels, and lymphangiogenesis, play important immunomodulatory roles in the tumor microenvironment.

FOXO1 regulates dendritic cell activity through ICAM-1 and CCR7

The transcription factor FOXO1 regulates cell function and is expressed in dendritic cells (DCs). We investigated the role of FOXO1 in activating DCs to stimulate a lymphocyte response to bacteria. We show that bacteria induce FOXO1 nuclear localization through the MAPK pathway and demonstrate that FOXO1 is needed for DC activation of lymphocytes in vivo. This occurs through FOXO1 regulation of DC phagocytosis, chemotaxis, and DC-lymphocyte binding. FOXO1 induces DC activity by regulating ICAM-1 and CCR7. FOXO1 binds to the CCR7 and ICAM-1 promoters, stimulates CCR7 and ICAM-1 transcriptional activity, and regulates their expression. This is functionally important because transfection of DCs from FOXO1-deleted CD11c.Cre(+)FOXO1(L/L) mice with an ICAM-1-expressing plasmid rescues the negative effect of FOXO1 deletion on DC bacterial phagocytosis and chemotaxis. Rescue with both CCR7 and ICAM-1 reverses impaired DC homing to lymph nodes in vivo when FOXO1 is deleted. Moreover, Ab production following injection of bacteria is significantly reduced with lineage-specific FOXO1 ablation. Thus, FOXO1 coordinates upregulation of DC activity through key downstream target genes that are needed for DCs to stimulate T and B lymphocytes and generate an Ab defense to bacteria.

Measurement of shear stress-mediated intracellular calcium dynamics in human dermal lymphatic endothelial cells

The shear stress applied to lymphatic endothelial cells (LEC) by lymph flow changes dramatically under normal conditions as well as in response to disease conditions and immune reactions. In general, LEC are known to regulate the contraction frequency and strength of lymphatic pumping in response to shear stress. Intracellular calcium concentration ([Ca(2+)]i) is an important factor that regulates lymphatic contraction characteristics. In this study, we measured changes in the [Ca(2+)]i under different shear stress levels and determined the source of this calcium signal. Briefly, human dermal LEC were cultured in custom-made microchannels for 3 days before loading with 2 µM fura-2 AM, a ratiometric calcium dye to measure [Ca(2+)]i. Step changes in shear stress resulted in a rapid increase in [Ca(2+)]i followed by a gradual return to the basal level and sometimes below the initial baseline (45.2 ± 2.2 nM). The [Ca(2+)]i reached a peak at 126.2 ± 5.6 nM for 10 dyn/cm(2) stimulus, whereas the peak was only 71.8 ± 5.4 nM for 1 dyn/cm(2) stimulus, indicating that the calcium signal depends on the magnitude of shear stress. Removal of the extracellular calcium from the buffer or pharmocological blockade of calcium release-activated calcium (CRAC) channels significantly reduced the peak [Ca(2+)]i, demonstrating a role of extracellular calcium entry. Inhibition of endoplasmic reticulum (ER) calcium pumps showed the importance of intracellular calcium stores in the initiation of this signal. In conclusion, we demonstrated that the shear-mediated calcium signal is dependent on the magnitude of the shear and involves ER store calcium release and extracellular calcium entry.

Assessing the roles of galectins in regulating dendritic cell migration through extracellular matrix and across lymphatic endothelial cells

Leukocyte migration from the bloodstream into tissues, and from tissues to lymph nodes, depends on expression of specific adhesion and signaling molecules by vascular endothelial cells and lymphatic endothelial cells. Tissue damage and microbial infection induce vascular endothelial cells to up-regulate expression of adhesion molecules to facilitate entry of several leukocyte populations from blood into tissues. Many of these cells then leave inflamed tissue and migrate to regional lymph nodes. A critical population that emigrates from inflamed tissue is dendritic cells. Dendritic cells in tissue have to migrate through extracellular matrix and across a layer of lymphatic endothelial cells to enter the lymphatic vasculature. Little is known about the adhesion molecules expressed by lymphatic endothelial cells or the processes required for the critical step of dendritic cell exit from tissues, specifically migration through the extracellular matrix and basal-to-apical migration across the lymphatic endothelial cell layer into lymphatic vasculature.Members of the galectin family of carbohydrate binding proteins are expressed in both vascular and lymphatic endothelial cells. Dynamic changes in galectin expression during inflammation are known to regulate leukocyte tissue entry during inflammation. However, the roles of galectin family members expressed by lymphatic endothelial cells in leukocyte tissue exit remain to be explored.Here, we describe an in vitro transmigration assay that mimics dendritic cell tissue exit in the presence and absence of galectin protein. Fluorescently labeled human dendritic cell migration through extracellular matrix and across human lymphatic endothelial cells is examined in the presence and absence of recombinant human galectin protein.

Innovative microsurgical treatment of male external genitals lymphedema

Secondary lymphedema of male external genital organs, characterized by increase in genital organs volume, tissue fibrosis, erysipelas, and objective difficulties in the normal use of lower limbs and the penis, is a very common and impairing consequence of invasive surgery, radical lymphadenectomy and radiotherapy of the pelvic-inguinal area. Standard surgical approach to lymphedema are either very invasive and/or at high risk of lymphedema recurrence and do not guarantee an efficient long-term treatment. Alternatively, we developed a microsurgical technique to perform direct anastomoses between the lymphatic collectors of the spermatic funiculum afferent to the external iliac chains and the vessels tributary to the spermatic vein. This innovative approach, although surgically demanding, provided a long term successful treatment of external genitals with no clinical complications, low invasivity, rapid post-surgical recovery, minor tissue demolition and satisfactory post-surgical functional and esthetic results. In addition, lympho-venous microsurgery seems to trigger the local development of new lymphatic vessels that not only canalize along new collecting channels, but also form complex meshes in proximity to the anastomosis area, thus improving lymphedema also in adjacent tissues like lower limbs, supplied by lymphatics emptying into common developed lymphatic shunt.

Sentinel lymph node metastases in cancer: causes, detection and their role in disease progression

Malignant tumors of ectodermal or endodermal origin may metastasize to the sentinel lymph node, the first lymph node encountered by tumor cells that enter lymphatics in the organ of origin. This pathway is enabled by the anatomy of the disease and the causes of metastasis are the result of complex interactions that include mechanical forces within the tumor and host tissues, and molecular factors initiated by tumor cell proliferation, elaboration of cytokines and changes in the tumor microenvironment. Mechanical stresses may influence complex biochemical, genetic and other molecular events and enhance the likelihood of metastasis. This paper summarizes our understanding of interacting molecular, anatomical and mechanical processes which facilitate metastasis to SLNs. Our understanding of these interacting events is based on a combination of clinical and basic science research, in vitro and in vivo, including studies in lymphatic embryology, anatomy, micro-anatomy, pathology, physiology, molecular biology and mechanobiology. The presence of metastatic tumor in the SLN is now more accurately identifiable and, based upon prospective clinical trials, paradigm-changing SLN biopsy has become the standard of clinical practice in breast cancer and melanoma.

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.

Sterile inflammation after lymph node transfer improves lymphatic function and regeneration

BACKGROUND

The aim of this study was to determine whether sterile inflammatory reactions can serve as a physiologic means of augmenting lymphangiogenesis in transplanted lymph nodes using a murine model.

METHODS

The authors used their previously reported model of lymph node transfer to study the effect of sterile inflammation on lymphatic regeneration. Mice were divided into three groups: group 1 (controls) underwent lymphadenectomy followed by immediate lymph node transplantation without inflammation; group 2 (inflammation before transfer) underwent transplantation with lymph nodes harvested from donor animals in which a sterile inflammatory reaction was induced in the ipsilateral donor limb; and group 3 (inflammation after transfer) underwent transplantation with lymph nodes and then inflammation was induced in the ipsilateral limb. Lymphatic function, lymphangiogenesis, and lymph node histology were examined 28 days after transplantation and compared with those of normal lymph nodes.

RESULTS

Animals that had sterile inflammation after transplantation (group 3) had significantly improved lymphatic function (>2-fold increase) on lympho scintigraphy, increased perinodal lymphangiogenesis, and functional lymphatics compared with the groups with no inflammation and inflammation before transplantation (p<0.01). Inflammation after transplantation was associated with a more normal lymph node architecture, expansion of B-cell zones, and decreased percentage of T cells compared with the other experimental groups.

CONCLUSIONS

Sterile inflammation is a potent method of augmenting lymphatic function and lymphangiogenesis after lymph node transplantation and is associated with maintenance of lymph node architecture. Induction of inflammation after transplantation is the most effective method and promotes maintenance of normal lymph node B- and T-cell architecture.

Non-invasive mapping of interstitial fluid pressure in microscale tissues

This study describes a non-invasive method for mapping interstitial fluid pressure within hydrogel-based microscale tissues. The method is based on embedding (or forming) a tissue within a silicone (PDMS) microfluidic device, and measuring the extremely slight displacement (<1 μm) of the PDMS optically when the device is pressurized under static and flow conditions. The displacement field under uniform pressure provides a map of the local device stiffness, which can then be used to obtain the non-uniform pressure field under flow conditions. We have validated this method numerically and applied it towards determining the hydraulic properties of tumor cell aggregates, blind-ended epithelial tubes, and perfused endothelial tubes that were all cultured within micropatterned collagen gels. The method provides an accessible tool for generating high-resolution maps of interstitial fluid pressure for studies in mechanobiology.

Interstitial fluid flow in cancer: implications for disease progression and treatment

As cancer progresses, a dynamic microenvironment develops that creates and responds to cellular and biophysical cues. Increased intratumoral pressure and corresponding increases in interstitial flow from the tumor bulk to the healthy stroma is an observational hallmark of progressing cancers. Until recently, the role of interstitial flow was thought to be mostly passive in the transport and dissemination of cancer cells to metastatic sites. With research spanning the past decade, we have seen that interstitial flow has a promigratory effect on cancer cell invasion in multiple cancer types. This invasion is one mechanism by which cancers can resist therapeutics and recur, but the role of interstitial flow in cancer therapy is limited to the understanding of transport of therapeutics. Here we outline the current understanding of the role of interstitial flow in cancer and the tumor microenvironment through cancer progression and therapy. We also discuss the current role of fluid flow in the treatment of cancer, including drug transport and therapeutic strategies. By stating the current understanding of interstitial flow in cancer progression, we can begin exploring its role in therapeutic failure and treatment resistance.

Mechanical forces and lymphatic transport

This review examines the current understanding of how the lymphatic vessel network can optimize lymph flow in response to various mechanical forces. Lymphatics are organized as a vascular tree, with blind-ended initial lymphatics, precollectors, prenodal collecting lymphatics, lymph nodes, postnodal collecting lymphatics and the larger trunks (thoracic duct and right lymph duct) that connect to the subclavian veins. The formation of lymph from interstitial fluid depends heavily on oscillating pressure gradients to drive fluid into initial lymphatics. Collecting lymphatics are segmented vessels with unidirectional valves, with each segment, called a lymphangion, possessing an intrinsic pumping mechanism. The lymphangions propel lymph forward against a hydrostatic pressure gradient. Fluid is returned to the central circulation both at lymph nodes and via the larger lymphatic trunks. Several recent developments are discussed, including evidence for the active role of endothelial cells in lymph formation; recent developments on how inflow pressure, outflow pressure, and shear stress affect the pump function of the lymphangion; lymphatic valve gating mechanisms; collecting lymphatic permeability; and current interpretations of the molecular mechanisms within lymphatic endothelial cells and smooth muscle. An improved understanding of the physiological mechanisms by which lymphatic vessels sense mechanical stimuli, integrate the information, and generate the appropriate response is key for determining the pathogenesis of lymphatic insufficiency and developing treatments for lymphedema.

Tissue-engineered lymphatic graft for the treatment of lymphedema

BACKGROUND

Lymphedema is a chronic debilitating condition and curative treatment is yet to be found. Tissue engineering approach, which combines cellular components, scaffold, and molecular signals hold great potential in the treatment of secondary lymphedema with the advent of lymphatic graft to reconstruct damaged collecting lymphatic vessel. This review highlights the ideal characteristics of lymphatic graft, the limitation and challenges faced, and the approaches in developing tissue-engineered lymphatic graft.

METHODS

Literature on tissue engineering of lymphatic system and lymphatic tissue biology was reviewed.

RESULTS

The prime challenge in the design and manufacturing of this graft is producing endothelialized conduit with intraluminal valves. Suitable scaffold material is needed to ensure stability and functionality of the construct. Endothelialization of the construct can be enhanced via biofunctionalization and nanotopography, which mimics extracellular matrix. Nanocomposite polymers with improved performance over existing biomaterials are likely to benefit the development of lymphatic graft.

CONCLUSIONS

With the in-depth understanding of tissue engineering, nanotechnology, and improved knowledge on the biology of lymphatic regeneration, the aspiration to develop successful lymphatic graft is well achievable.

Dendritic cell interactions with lymphatic endothelium

Afferent lymphatic vessels fulfill essential immune functions by transporting leukocytes and lymph-borne antigen to draining lymph nodes (dLNs). An important cell type migrating through lymphatic vessels are dendritic cells (DCs). DCs reside in peripheral tissues like the skin, where they take up antigen and transport it via the lymphatic vascular network to dLNs for subsequent presentation to T cells. As such, DCs play a key role in the induction of adaptive immune responses during infection and vaccination, but also for the maintenance of tolerance. Although the migratory pattern of DCs has been known for long time, interactions between DCs and lymphatic vessels are only now starting to be unraveled at the cellular level. In particular, new tools for visualizing lymphatic vessels in combination with time-lapse microscopy have recently generated valuable insights into the process of DC migration to dLNs. In this review we summarize and discuss current approaches for visualizing DCs and lymphatic vessels in tissues for imaging applications. Furthermore, we review the current state of knowledge about DC migration towards, into and within lymphatic vessels, particularly focusing on the cellular interactions that take place between DCs and the lymphatic endothelium.

Emerging roles of lymphatic endothelium in regulating adaptive immunity

Emerging research on the roles of stromal cells in modulating adaptive immune responses has included a new focus on lymphatic endothelial cells (LECs). LECs are presumably the first cells that come into direct contact with peripheral antigens, cytokines, danger signals, and immune cells travelling from peripheral tissues to lymph nodes. LECs can modulate dendritic cell function, present antigens to T cells on MHC class I and MHC class II molecules, and express immunomodulatory cytokines and receptors, which suggests that their roles in adaptive immunity are far more extensive than previously realized. This Review summarizes the emergent evidence that LECs are important in maintaining peripheral tolerance, limiting and resolving effector T cell responses, and modulating leukocyte function.

Mechanisms of lymphatic metastasis

Malignant tumors release growth factors such as VEGF-C to induce lymphatic vessel expansion (lymphangiogenesis) in primary tumors and in draining sentinel LNs, thereby promoting LN metastasis. Surprising recent evidence suggests that lymphatic vessels do not merely represent passive channels for tumor spread, but that they may actively promote tumor cell recruitment to LNs, cancer stem cell survival, and immune modulation. New imaging approaches allow the sensitive visualization of the earliest LN metastases and the quantitative, noninvasive measurement of the function of tumor-draining lymphatic vessels, with potential applications in the development of biomarkers for prognosis and measurement of therapeutic response.

Lymphangiogenesis and lymphatic vessel remodelling in cancer

The generation of new lymphatic vessels through lymphangiogenesis and the remodelling of existing lymphatics are thought to be important steps in cancer metastasis. The past decade has been exciting in terms of research into the molecular and cellular biology of lymphatic vessels in cancer, and it has been shown that the molecular control of tumour lymphangiogenesis has similarities to that of tumour angiogenesis. Nevertheless, there are significant mechanistic differences between these biological processes. We are now developing a greater understanding of the specific roles of distinct lymphatic vessel subtypes in cancer, and this provides opportunities to improve diagnostic and therapeutic approaches that aim to restrict the progression of cancer.

Inflammation-induced lymphangiogenesis and lymphatic dysfunction

The lymphatic system is intimately linked to tissue fluid homeostasis and immune cell trafficking. These functions are paramount in the establishment and development of an inflammatory response. In the past decade, an increasing number of reports has revealed that marked changes, such as lymphangiogenesis and lymphatic contractile dysfunction occur in both vascular and nodal parts of the lymphatic system during inflammation, as well as other disease processes. This review provides a critical update on the role of the lymphatic system in disease process such as chronic inflammation and cancer and examines the changes in lymphatic functions the diseases cause and the influence these changes have on the progression of the diseases.

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.

Taking the lymphatic route: dendritic cell migration to draining lymph nodes

In contrast to leukocyte migration through blood vessels, trafficking via lymphatic vessels (LVs) is much less well characterized. An important cell type migrating via this route is antigen-presenting dendritic cells (DCs), which are key for the induction of protective immunity as well as for the maintenance of immunological tolerance. In this review, we will summarize and discuss current knowledge of the cellular and molecular events that control DC migration from the skin towards, into, and within LVs, followed by DC arrival and migration in draining lymph nodes. Finally, we will discuss potential strategies to therapeutically target this migratory step to modulate immune responses.

Interactions of immune cells and lymphatic vessels

In addition to fluid and lipid absorption, immune cell trafficking has now become recognized as one of the major functions of the lymphatic system. Recently, several critical roles of the lymphatic vessels (LVs) in modulating immune reactions during both physiological and pathological conditions have been emerging. As LVs serve as conduits for immune cells, they come to closely interact with macrophages/monocytes, dendritic cells, and T and B lymphocytes. Accumulating evidences indicate that reciprocal interactions between the LVs and immune cells exist which cause considerable influence over the process of immune cell migration, LV growth, and ultimately certain immune reactions. This chapter discusses on the interactions of macrophages/monocytes and dendritic cells with peripheral LVs and on those of sinusoidal macrophages and T and B lymphocytes with lymph node LVs.

Lymphatic Regulation of Cellular Trafficking

Lymphatic vessels play vital roles in immune surveillance and immune regulation by conveying antigen loaded dendritic cells, memory T cells, macrophages and neutrophils from the peripheral tissues to draining lymph nodes where they initiate as well as modify immune responses. Until relatively recently however, there was little understanding of how entry and migration through lymphatic vessels is organized or the specific molecular mechanisms that might be involved. Within the last decade, the situation has been transformed by an explosion of knowledge generated largely through the application of microscopic imaging, transgenic animals, specific markers and function blocking mAbs that is beginning to provide a rational conceptual framework. This article provides a critical review of the recent literature, highlighting seminal discoveries that have revealed the fascinating ultrastructure of leucocyte entry sites in lymphatic vessels, as well as generating controversies over the involvement of integrin adhesion, chemotactic and haptotactic mechanisms in DC entry under normal and inflamed conditions. It also discusses the major changes in lymphatic architecture that occur during inflammation and the different modes of leucocyte entry and trafficking within inflamed lymphatic vessels, as well as presenting a timely update on the likely role of hyaluronan and the major lymphatic endothelial hyaluronan receptor LYVE-1 in leucocyte transit.

Initial afferent lymphatic vessels controlling outbound leukocyte traffic from skin to lymph nodes

Tissue drains fluid and macromolecules through lymphatic vessels (LVs), which are lined by a specialized endothelium that expresses peculiar differentiation proteins, not found in blood vessels (i.e., LYVE-1, Podoplanin, PROX-1, and VEGFR-3). Lymphatic capillaries are characteristically devoid of a continuous basal membrane and are anchored to the ECM by elastic fibers that act as pulling ropes which open the vessel to avoid edema if tissue volume increases, as it occurs upon inflammation. LVs are also crucial for the transit of T lymphocytes and antigen presenting cells from tissue to draining lymph nodes (LN). Importantly, cell traffic control across lymphatic endothelium is differently regulated under resting and inflammatory conditions. Under steady-state non-inflammatory conditions, leukocytes enter into the lymphatic capillaries through basal membrane gaps (portals). This entrance is integrin-independent and seems to be mainly guided by CCL21 chemokine gradients acting on leukocytes expressing CCR7. In contrast, inflammatory processes in lymphatic capillaries involve a plethora of cytokines, chemokines, leukocyte integrins, and other adhesion molecules. Importantly, under inflammation a role for integrins and their ligands becomes apparent and, as a consequence, the number of leukocytes entering the lymphatic capillaries multiplies several-fold. Enhancing transmigration of dendritic cells en route to LN is conceivably useful for vaccination and cancer immunotherapy, whereas interference with such key mechanisms may ameliorate autoimmunity or excessive inflammation. Recent findings illustrate how, transient cell-to-cell interactions between lymphatic endothelial cells and leukocytes contribute to shape the subsequent behavior of leukocytes and condition the LV for subsequent trans-migratory events.

Demonstration of ATP-dependent, transcellular transport of lipid across the lymphatic endothelium using an in vitro model of the lacteal

PURPOSE

The lymphatic system plays crucial roles in tissue fluid balance, trafficking of immune cells, and the uptake of dietary lipid from the intestine. Given these roles there has been an interest in targeting lymphatics through oral lipid-based formulations or intradermal delivery of drug carrier systems. However the mechanisms regulating lipid uptake by lymphatics remain unknown. Thus we sought to modify a previously developed in vitro model to investigate the role of ATP in lipid uptake into the lymphatics.

METHODS

Lymphatic endothelial cells were cultured on a transwell membrane and the effective permeability to free fatty acid and Caco-2 cell-secreted lipid was calculated in the presence or absence of the ATP inhibitor sodium azide.

RESULTS

ATP inhibition reduced Caco-2 cell-secreted lipid transport, but not dextran transport. FFA transport was ATP-dependent primarily during early periods of ATP inhibition, while Caco-2 cell-secreted lipid transport was lowered at all time points studied. Furthermore, the transcellular component of transport was highly ATP-dependent, a mechanism not observed in fibroblasts, suggesting these mechanisms are unique to lymphatics. Total transport of Caco-2 cell-secreted lipid was dose-dependently reduced by ATP inhibition, and transcellular lipoprotein transport was completely attenuated.

CONCLUSION

The transport of lipid across the lymphatic endothelium as demonstrated with this in vitro model occurs in part by an ATP-dependent, transcellular route independent of passive permeability. It remains to be determined the extent that this mechanism exists in vivo and future work should be directed in this area.

The inflammatory response of lymphatic endothelium

Lymphatic vessels have traditionally been regarded as a rather inert drainage system, which just passively transports fluids, leukocytes and antigen. However, it is becoming increasingly clear that the lymphatic vasculature is highly dynamic and plays a much more active role in inflammatory and immune processes. Tissue inflammation induces a rapid, stimulus-specific upregulation of chemokines and adhesion molecules in lymphatic endothelial cells and a proliferative expansion of the lymphatic network in the inflamed tissue and in draining lymph nodes. Moreover, increasing evidence suggests that inflammation-induced changes in the lymphatic vasculature have a profound impact on the course of inflammatory and immune responses, by modulating fluid drainage, leukocyte migration or the removal of inflammatory mediators from tissues. In this review we will summarize and discuss current knowledge of the inflammatory response of lymphatic endothelium and of inflammation-induced lymphangiogenesis and the current perspective on the overall functional significance of these processes.

The effects of inflammatory cytokines on lymphatic endothelial barrier function

Proper lymphatic function is necessary for the transport of fluids, macromolecules, antigens and immune cells out of the interstitium. The lymphatic endothelium plays important roles in the modulation of lymphatic contractile activity and lymph transport, but it's role as a barrier between the lymph and interstitial compartments is less well understood. Alterations in lymphatic function have long been associated with edema and inflammation although the integrity of the lymphatic endothelial barrier during inflammation is not well-defined. In this paper we evaluated the integrity of the lymphatic barrier in response to inflammatory stimuli commonly associated with increased blood endothelial permeability. We utilized in vitro assays of lymphatic endothelial cell (LEC) monolayer barrier function after treatment with different inflammatory cytokines and signaling molecules including TNF-α, IL-6, IL-1β, IFN-γ and LPS. Moderate increases in an index of monolayer barrier dysfunction were noted with all treatments (20-60 % increase) except IFN-γ which caused a greater than 2.5-fold increase. Cytokine-induced barrier dysfunction was blocked or reduced by the addition of LNAME, except for IL-1β and LPS treatments, suggesting a regulatory role for nitric oxide. The decreased LEC barrier was associated with modulation of both intercellular adhesion and intracellular cytoskeletal activation. Cytokine treatments reduced the expression of VE-cadherin and increased scavenging of β-catenin in the LECs and this was partially reversed by LNAME. Likewise the phosphorylation of myosin light chain 20 at the regulatory serine 19 site, which accompanied the elevated monolayer barrier dysfunction in response to cytokine treatment, was also blunted by LNAME application. This suggests that the lymphatic barrier is regulated during inflammation and that certain inflammatory signals may induce large increases in permeability.

Advanced drug delivery to the lymphatic system: lipid-based nanoformulations

The delivery of drugs and bioactive compounds via the lymphatic system is complex and dependent on the physiological uniqueness of the system. The lymphatic route plays an important role in transporting extracellular fluid to maintain homeostasis and in transferring immune cells to injury sites, and is able to avoid first-pass metabolism, thus acting as a bypass route for compounds with lower bioavailability, ie, those undergoing more hepatic metabolism. The lymphatic route also provides an option for the delivery of therapeutic molecules, such as drugs to treat cancer and human immunodeficiency virus, which can travel through the lymphatic system. Lymphatic imaging is useful in evaluating disease states and treatment plans for progressive diseases of the lymph system. Novel lipid-based nanoformulations, such as solid lipid nanoparticles and nanostructured lipid carriers, have unique characteristics that make them promising candidates for lymphatic delivery. These formulations are superior to colloidal carrier systems because they have controlled release properties and provide better chemical stability for drug molecules. However, multiple factors regulate the lymphatic delivery of drugs. Prior to lymphatic uptake, lipid-based nanoformulations are required to undergo interstitial hindrance that modulates drug delivery. Therefore, uptake and distribution of lipid-based nanoformulations by the lymphatic system depends on factors such as particle size, surface charge, molecular weight, and hydrophobicity. Types of lipid and concentration of the emulsifier are also important factors affecting drug delivery via the lymphatic system. All of these factors can cause changes in intermolecular interactions between the lipid nanoparticle matrix and the incorporated drug, which in turn affects uptake of drug into the lymphatic system. Two lipid-based nanoformulations, ie, solid lipid nanoparticles and nanostructured lipid carriers, have been administered via multiple routes (subcutaneous, pulmonary, and intestinal) for targeting of the lymphatic system. This paper provides a detailed review of novel lipid-based nanoformulations and their lymphatic delivery via different routes, as well as the in vivo and in vitro models used to study drug transport in the lymphatic system. Physicochemical properties that influence lymphatic delivery as well as the advantages of lipid-based nanoformulations for lymphatic delivery are also discussed.