Engineering the Lymphatic System

Dynamic multispectral NIR/SWIR for in vivo lymphovascular architectural and functional quantification

Significance

Although the lymphatic system is the second largest circulatory system in the body, there are limited techniques available for characterizing lymphatic vessel function. We report shortwave-infrared (SWIR) imaging for minimally invasive in vivo quantification of lymphatic circulation with superior contrast and resolution compared with near-infrared first window imaging.

Aim

We aim to study the lymphatic structure and function in vivo via SWIR fluorescence imaging.

Approach

We evaluated subsurface lymphatic circulation in healthy, adult immunocompromised salt-sensitive Sprague-Dawley rats using two fluorescence imaging modalities: near-infrared first window (NIR-I, 700 to 900 nm) and SWIR (900 to 1800 nm) imaging. We also compared two fluorescent imaging probes: indocyanine green (ICG) and silver sulfide quantum dots (QDs) as SWIR lymphatic contrast agents following intradermal footpad delivery in these rats.

Results

SWIR imaging exhibits reduced scattering and autofluorescence background relative to NIR-I imaging. SWIR imaging with ICG provides 1.7 times better resolution and sensitivity than NIR-I, and SWIR imaging with QDs provides nearly two times better resolution and sensitivity with enhanced vessel distinguishability. SWIR images thus provide a more accurate estimation of in vivo vessel size than conventional NIR-I images.

Conclusions

SWIR imaging of silver sulfide QDs into the intradermal footpad injection provides superior image resolution compared with conventional imaging techniques using NIR-I imaging with ICG dye.

The Role of Inflammation in Lymphedema: A Narrative Review of Pathogenesis and Opportunities for Therapeutic Intervention

Lymphedema is a chronic and progressive disease of the lymphatic system characterized by inflammation, increased adipose deposition, and tissue fibrosis. Despite early hypotheses identifying lymphedema as a disease of mechanical lymphatic disruption alone, the progressive inflammatory nature underlying this condition is now well-established. In this review, we provide an overview of the various inflammatory mechanisms that characterize lymphedema development and progression. These mechanisms contribute to the acute and chronic phases of lymphedema, which manifest clinically as inflammation, fibrosis, and adiposity. Furthermore, we highlight the interplay between current therapeutic modalities and the underlying inflammatory microenvironment, as well as opportunities for future therapeutic development.

Real-Time Evaluation of Absolute, Cytosolic, Free Ca2+ and Corresponding Contractility in Isolated, Pressurized Lymph Vessels

The lymphatic vasculature, now often referred to as "the third circulation," is located in many vital organ systems. A principal mechanical function of the lymphatic vasculature is to return fluid from extracellular spaces back to the central venous ducts. Lymph transport is mediated by spontaneous rhythmic contractions of lymph vessels (LVs). LV contractions are largely regulated by the cyclic rise and fall of cytosolic, free calcium ([Ca2+]i). This paper presents a method to concurrently calculate changes in absolute concentrations of [Ca2+]i and vessel contractility/rhythmicity in real time in isolated, pressurized LVs. Using isolated rat mesenteric LVs, we studied changes in [Ca2+]i and contractility/rhythmicity in response to drug addition. Isolated LVs were loaded with the ratiometric Ca2+-sensing indicator Fura-2AM, and video microscopy coupled with edge-detection software was used to capture [Ca2+]i and diameter measurements continuously in real time. The Fura-2AM signal from each LV was calibrated to the minimum and maximum signal for each vessel and used to calculate absolute [Ca2+]i. Diameter measurements were used to calculate contractile parameters (amplitude, end diastolic diameter, end systolic diameter, calculated flow) and rhythmicity (frequency, contraction time, relaxation time) and correlated with absolute [Ca2+]i measurements.

Local electroneutrality breakdown for electrolytes within varying-section nanopores

We determine the local charge dynamics of a [Formula: see text] electrolyte embedded in a varying-section channel. By means of an expansion based on the length scale separation between the axial and transverse direction of the channel, we derive closed formulas for the local excess charge for both, dielectric and conducting walls, in 2D (planar geometry) as well as in 3D (cylindrical geometry). Our results show that, even at equilibrium, the local charge electroneutrality is broken whenever the section of the channel is not homogeneous for both dielectric and conducting walls as well as for 2D and 3D channels. Interestingly, even within our expansion, the local excess charge in the fluid can be comparable to the net charge on the walls. We critically discuss the onset of such local electroneutrality breakdown in particular with respect to the correction that it induces on the effective free energy profile experienced by tracer ions.

Clinical application of magnetic resonance lymphangiography in the vascularized omental lymph nodes transfer with or without lymphaticovenous anastomosis for cancer-related lower extremity lymphedema

Background

The recent increase in the number of patients with lower extremities lymphedema and the development of microsurgery techniques have led to a rise in lymphedema treatment. Vascularized omental lymph node transfer (VOLT), an emerging treatment modality for extremity lymphedema, has shown its unique advantages in reconstructing lymphatic circulation and absorbing exudated lymphatic fluid. Patients who underwent radical tumor resection with/without radiation therapy treatment often present with impairment or degeneration of the inguinal lymph nodes. For such cases, VOLT could provide adequate lymph nodes and tissue to absorb edema fluid in these areas. Therefore, we analyzed the operative outcomes of VOLT under the guidance of magnetic resonance lymphangiography (MRL) in this study, as this individualized and precise surgical procedure could benefit patients and improve their quality of life.

Methods

From November 2021 to September 2022, a total of 14 patients' 19 legs with extremity lymphedema underwent a VOLT with or without lymphaticovenous anastomosis (LVA). Outcomes, including circumference reduction rates, preoperative and postoperative MRL results, and other complications, were analyzed.

Results

The mean follow-up period was 8.86±1.41 months (range, 7-11 months). The mean circumference reduction rates {circumference reduction rate (%) = [1 - (postoperative affected limb - healthy limb)/(preoperative affected limb - healthy limb)] × 100%} of different planes (i.e., ankle, 10 cm above the knee, 10 cm below the knee, 10 cm above the ankle, and 20 cm above the knee) were 15.64%±40.08%, 11.79%±30.69%, 20.25%±24.94%, 7.73%±30.05%, -1.517%±16.75%. Notably, one patient had multi-drug-resistant gram-negative infections, which resulted in the loss of three flaps. The postoperative MRL showed improved lymphatic drainage and lower extremity volume in the remaining 13 cases.

Conclusions

The precision evaluation of inguinal lymph nodes and lower extremities lymphatic system through MRL using VOLT can provide surgeons with a comprehensive understanding and reliable evidence for the treatment of cancer-related lower extremity lymphedema.

Closed Formula for Transport across Constrictions

In the last decade, the Fick-Jacobs approximation has been exploited to capture transport across constrictions. Here, we review the derivation of the Fick-Jacobs equation with particular emphasis on its linear response regime. We show that, for fore-aft symmetric channels, the flux of noninteracting systems is fully captured by its linear response regime. For this case, we derive a very simple formula that captures the correct trends and can be exploited as a simple tool to design experiments or simulations. Lastly, we show that higher-order corrections in the flux may appear for nonsymmetric channels.

Engineering transplantable human lymphatic and blood capillary networks in a porous scaffold

Due to a relative paucity of studies on human lymphatic assembly in vitro and subsequent in vivo transplantation, capillary formation and survival of primary human lymphatic (hLEC) and blood endothelial cells (hBEC) ± primary human vascular smooth muscle cells (hvSMC) were evaluated and compared in vitro and in vivo. hLEC ± hvSMC or hBEC ± hvSMC were seeded in a 3D porous scaffold in vitro, and capillary percent vascular volume (PVV) and vascular density (VD)/mm² assessed. Scaffolds were also transplanted into a sub-cutaneous rat wound with morphology/morphometry assessment. Initially hBEC formed a larger vessel network in vitro than hLEC, with interconnected capillaries evident at 2 days. Interconnected lymphatic capillaries were slower (3 days) to assemble. hLEC capillaries demonstrated a significant overall increase in PVV (p = 0.0083) and VD (p = 0.0039) in vitro when co-cultured with hvSMC. A similar increase did not occur for hBEC + hvSMC in vitro, but hBEC + hvSMC in vivo significantly increased PVV (p = 0.0035) and VD (p = 0.0087). Morphology/morphometry established that hLEC vessels maintained distinct cell markers, and demonstrated significantly increased individual vessel and network size, and longer survival than hBEC capillaries in vivo, and established inosculation with rat lymphatics, with evidence of lymphatic function. The porous polyurethane scaffold provided advantages to capillary network formation due to its large (300-600 μm diameter) interconnected pores, and sufficient stability to ensure successful surgical transplantation in vivo. Given their successful survival and function in vivo within the porous scaffold, in vitro assembled hLEC networks using this method are potentially applicable to clinical scenarios requiring replacement of dysfunctional or absent lymphatic networks.

In Vitro, In Vivo, and In Silico Models of Lymphangiogenesis in Solid Malignancies

Lymphangiogenesis (LA) is the formation of new lymphatic vessels by lymphatic endothelial cells (LECs) sprouting from pre-existing lymphatic vessels. It is increasingly recognized as being involved in many diseases, such as in cancer and secondary lymphedema, which most often results from cancer treatments. For some cancers, excessive LA is associated with cancer progression and metastatic dissemination to the lymph nodes (LNs) through lymphatic vessels. The study of LA through in vitro, in vivo, and, more recently, in silico models is of paramount importance in providing novel insights and identifying the key molecular actors in the biological dysregulation of this process under pathological conditions. In this review, the different biological (in vitro and in vivo) models of LA, especially in a cancer context, are explained and discussed, highlighting their principal modeled features as well as their advantages and drawbacks. Imaging techniques of the lymphatics, complementary or even essential to in vivo models, are also clarified and allow the establishment of the link with computational approaches. In silico models are introduced, theoretically described, and illustrated with examples specific to the lymphatic system and the LA. Together, these models constitute a toolbox allowing the LA research to be brought to the next level.

Drug-Related Lymphedema: Mysteries, Mechanisms, and Potential Therapies

The lymphatic circulation is an important component of the circulatory system in humans, playing a critical role in the transport of lymph fluid containing proteins, white blood cells, and lipids from the interstitial space to the central venous circulation. The efficient transport of lymph fluid critically relies on the rhythmic contractions of collecting lymph vessels, which function to "pump" fluid in the distal to proximal direction through the lymphatic circulation with backflow prevented by the presence of valves. When rhythmic contractions are disrupted or valves are incompetent, the loss of lymph flow results in fluid accumulation in the interstitial space and the development of lymphedema. There is growing recognition that many pharmacological agents modify the activity of ion channels and other protein structures in lymph muscle cells to disrupt the cyclic contraction and relaxation of lymph vessels, thereby compromising lymph flow and predisposing to the development of lymphedema. The effects of different medications on lymph flow can be understood by appreciating the intricate intracellular calcium signaling that underlies the contraction and relaxation cycle of collecting lymph vessels. For example, voltage-sensitive calcium influx through long-lasting ("L-type") calcium channels mediates the rise in cytosolic calcium concentration that triggers lymph vessel contraction. Accordingly, calcium channel antagonists that are mainstay cardiovascular medications, attenuate the cyclic influx of calcium through L-type calcium channels in lymph muscle cells, thereby disrupting rhythmic contractions and compromising lymph flow. Many other classes of medications also may contribute to the formation of lymphedema by impairing lymph flow as an off-target effect. The purpose of this review is to evaluate the evidence regarding potential mechanisms of drug-related lymphedema with an emphasis on common medications administered to treat cardiovascular diseases, metabolic disorders, and cancer. Additionally, although current pharmacological approaches used to alleviate lymphedema are largely ineffective, efforts are mounting to arrive at a deeper understanding of mechanisms that regulate lymph flow as a strategy to identify novel anti-lymphedema medications. Accordingly, this review also will provide information on studies that have explored possible anti-lymphedema therapeutics.

RhoA-ROCK competes with YAP to regulate amoeboid breast cancer cell migration in response to lymphatic-like flow

Lymphatic drainage generates force that induces prostate cancer cell motility via activation of Yes-associated protein (YAP), but whether this response to fluid force is conserved across cancer types is unclear. Here, we show that shear stress corresponding to fluid flow in the initial lymphatics modifies taxis in breast cancer, whereas some cell lines use rapid amoeboid migration behavior in response to fluid flow, a separate subset decrease movement. Positive responders displayed transcriptional profiles characteristic of an amoeboid cell state, which is typical of cells advancing at the edges of neoplastic tumors. Regulation of the HIPPO tumor suppressor pathway and YAP activity also differed between breast subsets and prostate cancer. Although subcellular localization of YAP to the nucleus positively correlated with overall velocity of locomotion, YAP gain- and loss-of-function demonstrates that YAP inhibits breast cancer motility but is outcompeted by other pro-taxis mediators in the context of flow. Specifically, we show that RhoA dictates response to flow. GTPase activity of RhoA, but not Rac1 or Cdc42 Rho family GTPases, is elevated in cells that positively respond to flow and is unchanged in cells that decelerate under flow. Disruption of RhoA or the RhoA effector, Rho-associated kinase (ROCK), blocked shear stress-induced motility. Collectively, these findings identify biomechanical force as a regulator amoeboid cell migration and demonstrate stratification of breast cancer subsets by flow-sensing mechanotransduction pathways.

Dantrolene Prevents the Lymphostasis Caused by Doxorubicin in the Rat Mesenteric Circulation

Background and Purpose: Doxorubicin (DOX) is a risk factor for arm lymphedema in breast cancer patients. We reported that DOX opens ryanodine receptors (RYRs) to enact "calcium leak," which disrupts the rhythmic contractions of lymph vessels (LVs) to attenuate lymph flow. Here, we evaluated whether dantrolene, a clinically available RYR1 subtype antagonist, prevents the detrimental effects of DOX on lymphatic function. Experimental Approach: Isolated rat mesenteric LVs were cannulated, pressurized (4-5 mm Hg) and equilibrated in physiological salt solution and Fura-2AM. Video microscopy recorded changes in diameter and Fura-2AM fluorescence tracked cytosolic free calcium ([Ca2+ i]). High-speed in vivo microscopy assessed mesenteric lymph flow in anesthetized rats. Flow cytometry evaluated RYR1 expression in freshly isolated mesenteric lymphatic muscle cells (LMCs). Key Results: DOX (10 μmol/L) increased resting [Ca2+ i] by 17.5 ± 3.7% in isolated LVs (n = 11). The rise in [Ca2+ i] was prevented by dantrolene (3 μmol/L; n = 10). A single rapid infusion of DOX (10 mg/kg i.v.) reduced positive volumetric lymph flow to 29.7 ± 10.8% (n = 7) of baseline in mesenteric LVs in vivo. In contrast, flow in LVs superfused with dantrolene (10 μmol/L) only decreased to 76.3 ± 14.0% (n = 7) of baseline in response to DOX infusion. Subsequently, expression of the RYR1 subtype protein as the presumed dantrolene binding site was confirm in isolated mesenteric LMCs by flow cytometry. Conclusion and Implications: We conclude that dantrolene attenuates the acute impairment of lymph flow by DOX and suggest that its prophylactic use in patients subjected to DOX chemotherapy may lower lymphedema risk.

Investigating lymphangiogenesis in vitro and in vivo using engineered human lymphatic vessel networks

Lymphatic vessel networks are important for various biological processes; thus, incorporating them into engineered constructs can have both research and clinical implications. Engineered lymphatic vessels can improve biomimicry and functionality of in vitro tissue assays and serve as a treatment for various diseases associated with impaired lymphatic function. In this work, we created functional engineered lymphatic vessels that anastomosed to the host lymphatic system postimplantation. We investigated the effect of supporting cells, cell-secreted extracellular matrix, and mechanical forces on lymphatic vessel formation within engineered constructs. Interestingly, lymphatic vasculature responded differently to cyclic stretch compared to blood vasculature. This phenomenon opens up an avenue for investigating the variability of cellular responses to mechanical stimulation.

The lymphatic system is involved in various biological processes, including fluid transport from the interstitium into the venous circulation, lipid absorption, and immune cell trafficking. Despite its critical role in homeostasis, lymphangiogenesis (lymphatic vessel formation) is less widely studied than its counterpart, angiogenesis (blood vessel formation). Although the incorporation of lymphatic vasculature in engineered tissues or organoids would enable more precise mimicry of native tissue, few studies have focused on creating engineered tissues containing lymphatic vessels. Here, we populated thick collagen sheets with human lymphatic endothelial cells, combined with supporting cells and blood endothelial cells, and examined lymphangiogenesis within the resulting constructs. Our model required just a few days to develop a functional lymphatic vessel network, in contrast to other reported models requiring several weeks. Coculture of lymphatic endothelial cells with the appropriate supporting cells and intact PDGFR-β signaling proved essential for the lymphangiogenesis process. Additionally, subjecting the constructs to cyclic stretch enabled the creation of complex muscle tissue aligned with the lymphatic and blood vessel networks, more precisely biomimicking native tissue. Interestingly, the response of developing lymphatic vessels to tensile forces was different from that of blood vessels; while blood vessels oriented perpendicularly to the stretch direction, lymphatic vessels mostly oriented in parallel to the stretch direction. Implantation of the engineered lymphatic constructs into a mouse abdominal wall muscle resulted in anastomosis between host and implant lymphatic vasculatures, demonstrating the engineered construct's potential functionality in vivo. Overall, this model provides a potential platform for investigating lymphangiogenesis and lymphatic disease mechanisms.

Modeling Immunity In Vitro: Slices, Chips, and Engineered Tissues

Modeling immunity in vitro has the potential to be a powerful tool for investigating fundamental biological questions, informing therapeutics and vaccines, and providing new insight into disease progression. There are two major elements to immunity that are necessary to model: primary immune tissues and peripheral tissues with immune components. Here, we systematically review progress made along three strategies to modeling immunity: ex vivo cultures, which preserve native tissue structure; microfluidic devices, which constitute a versatile approach to providing physiologically relevant fluid flow and environmental control; and engineered tissues, which provide precise control of the 3D microenvironment and biophysical cues. While many models focus on disease modeling, more primary immune tissue models are necessary to advance the field. Moving forward, we anticipate that the expansion of patient-specific models may inform why immunity varies from patient to patient and allow for the rapid comprehension and treatment of emerging diseases, such as coronavirus disease 2019.

Lymphatic injury alters the contractility and mechanosensitivity of collecting lymphatics to intermittent pneumatic compression

KEY POINTS

We present the first in vivo evidence that lymphatic contraction can entrain with an external oscillatory mechanical stimulus. Lymphatic injury can alter collecting lymphatic contractility, but not much is known about how its mechanosensitivity to external pressure is affected, which is crucial given the current pressure application methods for treating lymphoedema. We show that oscillatory pressure waves (OPW), akin to intermittent pneumatic compression (IPC) therapy, optimally entrain lymphatic contractility and modulate function depending on the frequency and propagation speed of the OPW. We show that the OPW-induced entrainment and contractile function in the intact collecting lymphatics are enhanced 28 days after a contralateral lymphatic ligation surgery. The results show that IPC efficacy can be improved through proper selection of OPW parameters, and that collecting lymphatics adapt their function and mechanosensitivity after a contralateral injury, switching their behaviour to a pump-like configuration that may be more suited to the altered microenvironment.

ABSTRACT

Intermittent pneumatic compression (IPC) is commonly used to control the swelling due to lymphoedema, possibly modulating the collecting lymphatic function. Lymphoedema causes lymphatic contractile dysfunction, but the consequent alterations in the mechanosensitivity of lymphatics to IPC is not known. In the present work, the spatiotemporally varying oscillatory pressure waves (OPW) generated during IPC were simulated to study the modulation of lymphatic function by OPW under physiological and pathological conditions. OPW with three temporal frequencies and three propagation speeds were applied to rat tail collecting lymphatics. The entrainment of the lymphatics to OPW was significantly higher at a frequency of 0.05 Hz compared with 0.1 Hz and 0.2 Hz (P = 0.0054 and P = 0.014, respectively), but did not depend on the OPW propagation speed. Lymphatic function was significantly higher at a frequency of 0.05 Hz and propagation speed of 2.55 mm/s (P = 0.015). Exogenous nitric oxide was not found to alter OPW-induced entrainment. A contralateral lymphatic ligation surgery was performed to simulate partial lymphatic injury in rat tails. The intact vessels showed a significant increase in entrainment to OPW, 28 days after ligation (compared with sham) (P = 0.016), with a similar increase in lymphatic transport function (P = 0.0029). The results suggest an enhanced mechanosensitivity of the lymphatics, along with a transition to a pump-like behaviour, in response to a lymphatic injury. These results enhance our fundamental understanding of how lymphatic mechanosensitivity assists the coordination of lymphatic contractility and how this might be leveraged in IPC therapy.

KATP Channel Openers Inhibit Lymphatic Contractions and Lymph Flow as a Possible Mechanism of Peripheral Edema

Pharmacological openers of ATP-sensitive potassium (KATP) channels are effective antihypertensive agents, but off-target effects, including severe peripheral edema, limit their clinical usefulness. It is presumed that the arterial dilation induced by KATP channel openers (KCOs) increases capillary pressure to promote filtration edema. However, KATP channels also are expressed by lymphatic muscle cells (LMCs), raising the possibility that KCOs also attenuate lymph flow to increase interstitial fluid. The present study explored the effect of KCOs on lymphatic contractile function and lymph flow. In isolated rat mesenteric lymph vessels (LVs), the prototypic KATP channel opener cromakalim (0.01-3 µmol/l) progressively inhibited rhythmic contractions and calculated intraluminal flow. Minoxidil sulfate and diazoxide (0.01-100 µmol/l) had similar effects at clinically relevant plasma concentrations. High-speed in vivo imaging of the rat mesenteric lymphatic circulation revealed that superfusion of LVs with cromakalim and minoxidil sulfate (0.01-10 µmol/l) maximally decreased lymph flow in vivo by 38.4% and 27.4%, respectively. Real-time polymerase chain reaction and flow cytometry identified the abundant KATP channel subunits in LMCs as the pore-forming Kir6.1/6.2 and regulatory sulfonylurea receptor 2 subunits. Patch-clamp studies detected cromakalim-elicited unitary K+ currents in cell-attached patches of LMCs with a single-channel conductance of 46.4 pS, which is a property consistent with Kir6.1/6.2 tetrameric channels. Addition of minoxidil sulfate and diazoxide elicited unitary currents of similar amplitude. Collectively, our findings indicate that KCOs attenuate lymph flow at clinically relevant plasma concentrations as a potential contributing mechanism to peripheral edema. SIGNIFICANCE STATEMENT: ATP-sensitive potassium (KATP) channel openers (KCOs) are potent antihypertensive medications, but off-target effects, including severe peripheral edema, limit their clinical use. Here, we demonstrate that KCOs impair the rhythmic contractions of lymph vessels and attenuate lymph flow, which may promote edema formation. Our finding that the KATP channels in lymphatic muscle cells may be unique from their counterparts in arterial muscle implies that designing arterial-selective KCOs may avoid activation of lymphatic KATP channels and peripheral edema.

Doxorubicin Activates Ryanodine Receptors in Rat Lymphatic Muscle Cells to Attenuate Rhythmic Contractions and Lymph Flow

Doxorubicin is a risk factor for secondary lymphedema in cancer patients exposed to surgery or radiation. The risk is presumed to relate to its cytotoxicity. However, the present study provides initial evidence that doxorubicin directly inhibits lymph flow and this action appears distinct from its cytotoxic activity. We used real-time edge detection to track diameter changes in isolated rat mesenteric lymph vessels. Doxorubicin (0.5-20 μmol/l) progressively constricted lymph vessels and inhibited rhythmic contractions, reducing flow to 24.2% ± 7.7% of baseline. The inhibition of rhythmic contractions by doxorubicin paralleled a tonic rise in cytosolic Ca²⁺ concentration in lymphatic muscle cells, which was prevented by pharmacological antagonism of ryanodine receptors. Washout of doxorubicin partially restored lymph vessel contractions, implying a pharmacological effect. Subsequently, high-speed optical imaging was used to assess the effect of doxorubicin on rat mesenteric lymph flow in vivo. Superfusion of doxorubicin (0.05-10 μmol/l) maximally reduced volumetric lymph flow to 34% ± 11.6% of baseline. Likewise, doxorubicin (10 mg/kg) administered intravenously to establish clinically achievable plasma concentrations also maximally reduced volumetric lymph flow to 40.3% ± 6.0% of initial values. Our findings reveal that doxorubicin at plasma concentrations achieved during chemotherapy opens ryanodine receptors to induce "calcium leak" from the sarcoplasmic reticulum in lymphatic muscle cells and reduces lymph flow, an event linked to lymph vessel damage and the development of lymphedema. These results infer that pharmacological block of ryanodine receptors in lymphatic smooth muscle cells may mitigate secondary lymphedema in cancer patients subjected to doxorubicin chemotherapy. SIGNIFICANCE STATEMENT: Doxorubicin directly inhibits the rhythmic contractions of collecting lymph vessels and reduces lymph flow as a possible mechanism of secondary lymphedema, which is associated with the administration of anthracycline-based chemotherapy. The inhibitory effects of doxorubicin on rhythmic contractions and flow in isolated lymph vessels were prevented by pharmacological block of ryanodine receptors, thereby identifying the ryanodine receptor family of proteins as potential therapeutic targets for the development of new antilymphedema medications.

Lymphatic vessels of the eye - old questions - new insights

Due to its accessible position and tissue heterogeneity, the eye is ideally suited for studying the lymphatic system. As early as the 19th century, questions about the origin and function of this system were discussed. For example, whether Schlemm's canal, which is of particular importance in the pathogenesis of glaucoma, is a lymphatic vessel, or does this vascular system begin with finger-shaped protuberances? Despite the discovery of lymphatic endothelial molecules and the use of molecular imaging technologies, these questions are still discussed controversially today. Leber demonstrated in 1873 with a solution consisting of two dyes of different particle size that only the smaller particles from the anterior chamber of the eye filled the episcleral and conjunctival veins around the corneal margin. He believed to have proven - to be read in the historical review of our article - that the Canalis Schlemmii in humans is a venous circular vessel and not a lymphatic vessel. In our own investigations, we reduced the rather contradictory and complex question of whether there are lymphatic vessels in the eye to the question of whether there are drainage connections between the different sections of the eye and the lymphatic system or not. With different radioactive tracers and combined with unilateral ligation of cervical lymph vessels, we observed outflow from the subconjunctival and retrobulbar space, from the anterior chamber and the vitreous body. The rate of discharge of the radioactive tracer was determined by the radiopharmaceutical and injection site. In analogy to the lymphatic drainage of the head we found a segmental drainage of lymphatic substances on the eye. Vitreous humour and retrobulbar space were drained by lymphatic vessels, predominantly to the deep cervical lymph nodes, while anterior chamber and subconjunctival space drains predominated over the superficial cervical lymph nodes. Eyeball tattoos - as loved by some fan communities - should therefore cause a coloured staining of the superficial cervical lymph nodes. The boundary of the drained segments would be in the area of the eyeball's equator. According to the textbooks, the lymph is actively removed from finger-shaped initial segments via pre-collectors and collectors with properly functioning intraluminal valves and smooth muscle cells in the vessels' media. In patients with spontaneous conjunctival bleeding, however, we observed phenomena in the conjunctival lymph vessels, which ca not be explained with old familiar ideas. At nozzle-shaped vessel constrictions separation of blood components occurred. The erythrocytes formed partially a so-called fluidic "resting bulk layer". Parallel vessel parts caused a retrograde filling of already emptied segments. These observations led our experimental investigations. In the literature, there are different scanning electron microscopy (SEM) images of lymphatic endothelial surfaces; nevertheless they are unassigned to a particular vessel segment. In the conjunctiva, we studied the question whether there is a dependence between vessel diameter and the surface characteristics of endothelial cells (after unfolding by lymphography). A constantly applied photo-mathematical procedure for all specimens allowed determining the size of the cross sections. The specimens were randomized into seven groups with diameters of 0.1-1.0mm and above and examined by SEM. In the smallest vessels (diameter=0.11mm), the impressions of the occasionally occurring nuclei in the lumen were clearly impressive. With increasing diameter, these impressions were lost and the individual endothelial nuclei could no longer be identified. Rather, one recognized only wall-like structures. In vessels of intermediate diameter (0.3-0.4mm), structures could be seen on the surface similar to reticular fibres. With increasing diameters, their prominent character weakened. In the group with diameters above 0.5mm, wavy surface structures were shown. Finally, in vessels of diameters over 1.0mm, a uniform, flat surface was observed. Regardless of the collection site of the specimens, we found certain surface characteristics related to the vessels' calibre. In further investigations by means of interstitial dye lymphography, we were able to demonstrate in the conjunctiva that under increasing injection pressure, additional vessels stained from finger-shaped processes. At least in the conjunctiva, the existence of so-called "blind-ending initial segments" seems doubtful (despite the fact that initial segments or "initial lymphatics" would begin in periphery, not end). Rather, these are likely to be temporary filling states. SEM investigations were carried out on the internal structure of these dome-shaped vessel parts by means of a specially developed preparation technique. Despite numerous variants in the lymphographic design of the blind bags - in the form of finger, balloon, dome, piston, pyramidal, double-humped and spearhead-like endings - slot-shaped, lip-shaped and saw blade-like structures were repeatedly found, similar to a zipper. These findings suggest preformed connections to the next segment and may control lymphatic flow. To clarify the retrograde fluid movements, we examined the lymph vessels' valves or those structures that were previously interpreted as valves. The different structures found could be subdivided into three groups. The lack of common bicuspid structures provides an explanation for retrograde fluid movement. That nevertheless a directional flow is possible, is explained by the flow model developed by Gerhart Liebau. Conjunctival lymphatics show intraluminal structures by double contrast injection, which we divided into four groups due to anatomical differences: An accurate statement about the occurrence of certain intraluminal vascular structures in certain vascular calibres was possible only conditionally. However, complex and extended structures (group d) were found almost exclusively in larger vessel calibres (diameter>0.9mm). The structures are reminiscent of published findings in the "collector channel orifices of Schlemm's canal". They should play an important role in the regulation of the intraocular pressure, or the balance between production and outflow of the aqueous humour. The influence of such structures on the function of the lymphatic vessels is not yet known. As an approach models could be used, which for instance are applied in the water industry for the drainage, the degradation of introduced substances, or the detention pond. The latter serves for the retention and purification of drainage water (storage, treatment and reuse of drainage water). Dead zones, barriers, short-circuit currents and swirling are further hydraulic terms. Can intraluminal vascular structures, for example, affect the lymphatic flow and thus the mechano-sensitivity of lymphatic endothelial cells? Whatever interpretation model we use, the warning of the Swiss anatomist His from 1862 is still true today that all theories about the formation and movement of lymph should be based on precise anatomical basics. This review article therefore tries to make a contribution therefore. Despite knowing of lymphatic endothelial molecules, despite the discovery of the role of lymphangiogenic growth factors in diseases and the use of molecular imaging technologies, we still know too little about the anatomy and function of the lymphatic system.

TAZ responds to fluid shear stress to regulate the cell cycle

Physical forces associated with tumor growth and drainage alter cancer cell invasiveness and metastatic potential. We previously showed that fluid frictional force, or shear stress, typical of lymphatic flow induces YAP1/TAZ activation in prostate cancer cells to promote motility dependent upon YAP1 but not TAZ. Here, we show that shear stress elevates TAZ protein levels and promotes TAZ nuclear localization. Increased TAZ activity drives increased DNA synthesis and induces AMOTL2, ANKRD1, and CTGF gene transcription independently of YAP1. Ectopic expression of constitutively activated TAZ increases expression of these TAZ target genes and promotes cell proliferation of prostate cancer cells. Conversely, silencing of TAZ results in reduced proliferation. Together, our data show that force-induced TAZ regulates signaling that dictates cell division, and suggest that TAZ may govern cellular proliferation of cancer cells traveling through the lymphatics in response to biophysical cues.

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

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

Towards the prediction of flow-induced shear stress distributions experienced by breast cancer cells in the lymphatics

Tumour metastasis in the lymphatics is a crucial step in the progression of breast cancer. The dynamics by which breast cancer cells (BCCs) travel in the lymphatics remains poorly understood. The goal of this work is to develop a model capable of predicting the shear stresses metastasising BCCs experience using numerical and experimental techniques. This paper models the fluidic transport of large particles ([Formula: see text] where [Formula: see text] is the particle diameter and W is the channel width) subjected to lymphatic flow conditions ([Formula: see text]), in a [Formula: see text] microchannel. The feasibility of using the dynamic fluid body interaction (DFBI) method to predict particle motion was assessed, and particle tracking experiments were performed. The experiments found that particle translational velocity decreased from the undisturbed fluid velocity with increasing particle size (5-14% velocity lag for [Formula: see text]). DFBI simulations were found to better predict particle behaviour than theoretical predictions; however, mesh restrictions in the near-wall region ([Formula: see text]) result in computationally expensive models. The simulations were in good agreement with the experiments ([Formula: see text] difference) across the channel ([Formula: see text]), with differences up to 25% in the near-wall region. Particles experience a range of shear stresses (0.002-0.12 Pa) and spatial shear gradients ([Formula: see text]) depending on their size and radial position. The predicted shear gradients are far in excess of values associated with BCC apoptosis ([Formula: see text]). Increasing our understanding of the shear stress magnitudes and gradients experienced by BCCs could be leveraged to elucidate whether a particular BCC size or location exists that encourages metastasis within the lymphatics.

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.

Cell-based approach for 3D reconstruction of lymphatic capillaries in vitro reveals distinct functions of HGF and VEGF-C in lymphangiogenesis

Regeneration of lymphatic vessels is important for treatment of various disorders of lymphatic system and for restoration of lymphatic function after surgery. We have developed a method for generating a human 3D lymphatic vascular construct. In this system, human lymphatic endothelial cells, co-cultured with fibroblasts, spontaneously organized into a stable 3D lymphatic capillary network without the use of any exogenous factors. In vitro-generated lymphatic capillaries exhibited the major molecular and ultra-structural features of native, human lymphatic microvasculature: branches in the three dimensions, wide lumen, blind ends, overlapping borders, adherens and tight junctions, anchoring filaments, lack of mural cells, and poorly developed basement membrane. Furthermore, we show that fibroblast-derived VEGF-C and HGF cooperate in the formation of lymphatic vasculature by activating ERK1/2 signaling, and demonstrate distinct functions of HGF/c-Met and VEGF-C/VEGFR-3 in lymphangiogenesis. This lymphatic vascular construct is expected to facilitate studies of lymphangiogenesis in vitro and it holds promise as a strategy for regeneration of lymphatic vessels and treatment of lymphatic disorders in various conditions.

Effects of dynamic shear and transmural pressure on wall shear stress sensitivity in collecting lymphatic vessels

Given the known mechanosensitivity of the lymphatic vasculature, we sought to investigate the effects of dynamic wall shear stress (WSS) on collecting lymphatic vessels while controlling for transmural pressure. Using a previously developed ex vivo lymphatic perfusion system (ELPS) capable of independently controlling both transaxial pressure gradient and average transmural pressure on an isolated lymphatic vessel, we imposed a multitude of flow conditions on rat thoracic ducts, while controlling for transmural pressure and measuring diameter changes. By gradually increasing the imposed flow through a vessel, we determined the WSS at which the vessel first shows sign of contraction inhibition, defining this point as the shear stress sensitivity of the vessel. The shear stress threshold that triggered a contractile response was significantly greater at a transmural pressure of 5 cmH2O (0.97 dyne/cm(2)) than at 3 cmH2O (0.64 dyne/cm(2)). While contraction frequency was reduced when a steady WSS was applied, this inhibition was reversed when the applied WSS oscillated, even though the mean wall shear stresses between the conditions were not significantly different. When the applied oscillatory WSS was large enough, flow itself synchronized the lymphatic contractions to the exact frequency of the applied waveform. Both transmural pressure and the rate of change of WSS have significant impacts on the contractile response of lymphatic vessels to flow. Specifically, time-varying shear stress can alter the inhibition of phasic contraction frequency and even coordinate contractions, providing evidence that dynamic shear could play an important role in the contractile function of collecting lymphatic vessels.

A Model for Interstitial Drainage Through a Sliding Lymphatic Valve

This study investigates fluid flow and elastic deformation in tissues that are drained by the primary lymphatic system. A model is formulated based on the Rossi hypothesis that states that the primary lymphatic valves, which are formed by overlapping endothelial cells around the circumferential lining of lymphatic capillaries, open in response to swelling of the surrounding tissue. Tissue deformation and interstitial fluid flow through the tissue are treated using the Biot equations of poroelasticity and, the fluid flux (into the interstitium) across the walls of the blood capillaries, is assumed to be linearly related to the pressure difference across the walls via a constant of proportionality (the vascular permeability). The resulting model is solved in a periodic domain containing one blood capillary and one lymphatic capillary starting from a configuration in which the tissue is undeformed. On imposition of a constant pressure difference between blood and lymphatic capillaries, the solutions are found to settle to a steady state. Given that the magnitude of pressure fluctuations in the lymphatic system is much smaller than this pressure difference between blood and lymph, it is postulated that the resulting steady-state solution gives a good representation of the state of the tissue under physiological conditions. The effects of changes to the Young's modulus of the tissue, the blood-lymphatic pressure difference, vascular permeability and valve dimensions on the steady state are investigated and discussed in terms of their effects on oedema in the context of age- and pregnancy-related changes to the body.

Bridging the divide between pathogenesis and detection in lymphedema

While our understanding of the lymphatic system has improved substantially in the past few decades, the translation of this knowledge into improved healthcare solutions for patients suffering from secondary lymphedema has been severely limited. The challenge facing clinicians is two-fold. First, there is no reliable, affordable, diagnostic capable of detecting the disease before symptoms of the lymphedema develop and the efficacy of treatment options becomes limited. Second, our understanding of the disease pathogenesis, its risk factors, and the underlying physiologic mechanisms is still in its infancy. These two challenges go hand in hand as limited diagnostic options have hindered our ability to understand lymphedema progression, and the lack of known underlying mechanisms involved in the disease prohibits the development of new diagnostic targets. This review serves to discuss the recent developments in clinical and lab research settings of both lymphedema diagnostic technologies and our understanding of the mechanisms driving disease risk and progression. We will show how these two lines of research are synergistically working with the ultimate goal of improving patient outcomes for those suffering from this horrible disease, identifying key areas of further research that are warranted to move the field forward and provide clinical relief for this neglected patient population.

Mechanobiology of lymphatic contractions

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

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.

Ex vivo lymphatic perfusion system for independently controlling pressure gradient and transmural pressure in isolated vessels

In addition to external forces, collecting lymphatic vessels intrinsically contract to transport lymph from the extremities to the venous circulation. As a result, the lymphatic endothelium is routinely exposed to a wide range of dynamic mechanical forces, primarily fluid shear stress and circumferential stress, which have both been shown to affect lymphatic pumping activity. Although various ex vivo perfusion systems exist to study this innate pumping activity in response to mechanical stimuli, none are capable of independently controlling the two primary mechanical forces affecting lymphatic contractility: transaxial pressure gradient, [Formula: see text], which governs fluid shear stress; and average transmural pressure, [Formula: see text], which governs circumferential stress. Hence, the authors describe a novel ex vivo lymphatic perfusion system (ELPS) capable of independently controlling these two outputs using a linear, explicit model predictive control (MPC) algorithm. The ELPS is capable of reproducing arbitrary waveforms within the frequency range observed in the lymphatics in vivo, including a time-varying [Formula: see text] with a constant [Formula: see text], time-varying [Formula: see text] and [Formula: see text], and a constant [Formula: see text] with a time-varying [Formula: see text]. In addition, due to its implementation of syringes to actuate the working fluid, a post-hoc method of estimating both the flow rate through the vessel and fluid wall shear stress over multiple, long (5 s) time windows is also described.

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.

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.

Differential transport function of lymphatic vessels in the rat tail model and the long-term effects of Indocyanine Green as assessed with near-infrared imaging

Introduction: Near-infrared (NIR) imaging has emerged as a novel imaging modality for assessing lymphatic function in vivo. While the technique has provided quantitative data previously unavailable, questions remain in regards to the spatiotemporal capabilities of the approach. We address three of the more important issues here using the rodent tail, one of the most widely utilized in vivo model systems in the lymphatic literature. Specifically we demonstrate (1) the transient vs. steady state response of lymphatics to tracer injection, (2) the functional characteristics of multiple collecting vessels draining the same tissue space in parallel, and (3) the long-term consequences of fluorescent tracers on lymphatic function to repeated functional measurements.

Methods: Rat tails were imaged with NIR and metrics of function were calculated for both collecting vessels that drain the tail. A nitric oxide donor cream (GTNO) was applied to the tail. Additionally, two different NIR dyes, indocyanine green (ICG) and LI-COR IRDye 800CW PEG, were utilized for function imaging at the time of initial injection and at 1, 2, and 4 week follow-up time points after which both draining lymph nodes were harvested.

Results and Discussion: Significant differences were found between the two collecting vessels such that the vessel first showing fluorescence (dominant) produced enhanced functional metrics compared to the second vessel (non-dominant). GTNO significantly reduced lymphatic function in the non-dominant vessel compared to the dominant. ICG remained visible in the tail for 2 weeks after injection and was accompanied by significant losses in lymphatic function and enlarged draining lymph nodes. The Licor tracer also remained visible for 2 weeks. However, the dye produced significantly lower effects on lymphatic function than ICG, and lymph nodes were not enlarged at any time point, suggesting that this may be a more appropriate contrast agent for longitudinal lymphatic imaging.

Low-cost microcontroller platform for studying lymphatic biomechanics in vitro

The pumping innate to collecting lymphatic vessels routinely exposes the endothelium to oscillatory wall shear stress and other dynamic forces. However, studying the mechanical sensitivity of the lymphatic endothelium remains a difficult task due to limitations of commercial or custom systems to apply a variety of time-varying stresses in vitro. Current biomechanical in vitro testing devices are very expensive, limited in capability, or highly complex; rendering them largely inaccessible to the endothelial cell biology community. To address these shortcomings, the authors propose a reliable, low-cost platform for augmenting the capabilities of commercially available pumps to produce a wide variety of flow rate waveforms. In particular, the Arduino Uno, a microcontroller development board, is used to provide open-loop control of a digital peristaltic pump using precisely timed serial commands. In addition, the flexibility of this platform is further demonstrated through its support of a custom-built cell-straining device capable of producing oscillatory strains with varying amplitudes and frequencies. Hence, this microcontroller development board is shown to be an inexpensive, precise, and easy-to-use tool for supplementing in vitro assays to quantify the effects of biomechanical forces on lymphatic endothelial cells.