Lymph flow pattern in the intact thoracic duct in sheep

Restoration of cervical lymphatic vessel function in aging rescues cerebrospinal fluid drainage

Cervical lymphatic vessels (cLVs) have been shown to drain solutes and cerebrospinal fluid (CSF) from the brain. However, their hydrodynamical properties have never been evaluated in vivo. Here, we developed two-photon optical imaging with particle tracking in vivo of CSF tracers (2P-OPTIC) in superficial and deep cLVs of mice, characterizing their flow and showing that the major driver is intrinsic pumping by contraction of the lymphatic vessel wall. Moreover, contraction frequency and flow velocity were reduced in aged mice, which coincided with a reduction in smooth muscle actin expression. Slowed flow in aged mice was rescued using topical application of prostaglandin F2α, a prostanoid that increases smooth muscle contractility, which restored lymphatic function in aged mice and enhanced central nervous system clearance. We show that cLVs are important regulators of CSF drainage and that restoring their function is an effective therapy for improving clearance in aging.

The interstitial compartment as a therapeutic target in heart failure

Congestion is the single most important contributor to heart failure (HF) decompensation. Most of the excess volume in patients with HF resides in the interstitial compartment. Inadequate decongestion implies persistent interstitial congestion and is associated with worse outcomes. Therefore, effective interstitial decongestion represents an unmet need to improve quality of life and reduce clinical events. The key processes that underlie incomplete interstitial decongestion are often ignored. In this review, we provide a summary of the pathophysiology of the interstitial compartment in HF and the factors governing the movement of fluids between the interstitial and vascular compartments. Disruption of the extracellular matrix compaction occurs with edema, such that the interstitium becomes highly compliant, and large changes in volume marginally increase interstitial pressure and allow progressive capillary filtration into the interstitium. Augmentation of lymph flow is required to prevent interstitial edema, and the lymphatic system can increase fluid removal by at least 10-fold. In HF, lymphatic remodeling can become insufficient or maladaptive such that the capacity of the lymphatic system to remove fluid from the interstitium is exceeded. Increased central venous pressure at the site of the thoracic duct outlet also impairs lymphatic drainage. Owing to the kinetics of extracellular fluid, microvascular absorption tends to be transient (as determined by the revised Starling equation). Therefore, effective interstitial decongestion with adequate transcapillary plasma refill requires a substantial reduction in plasma volume and capillary pressure that are prolonged and sustained, which is not always achieved in clinical practice. The critical importance of the interstitium in the congestive state underscores the need to directly decongest the interstitial compartment without relying on the lowering of intracapillary pressure with diuretics. This unmet need may be addressed by novel device therapies in the near future.

Lymphoedema conditions disrupt endothelial barrier function in vitro

Lymphatic vessel contractions generate net antegrade pulsatile lymph flow. By contrast, impaired lymphatic vessels are often associated with lymphoedema and altered lymph flow. The effect of lymphoedema on the lymph flow field and endothelium is not completely known. Here, we characterized the lymphatic flow field of a platelet-specific receptor C-type lectin-like receptor 2 (CLEC2) deficient lymphoedema mouse model. In regions of lymphoedema, collecting vessels were significantly distended, vessel contractility was greatly diminished and pulsatile lymph flow was replaced by quasi-steady flow. In vitro exposure of human dermal lymphatic endothelial cells (LECs) to lymphoedema-like quasi-steady flow conditions increased intercellular gap formation and permeability in comparison to normal pulsatile lymph flow. In the absence of flow, LECs exposed to steady pressure (SP) increased intercellular gap formation in contrast with pulsatile pressure (PP). The absence of pulsatility in steady fluid flow and SP conditions without flow-induced upregulation of myosin light chain (MLCs) regulatory subunits 9 and 12B mRNA expression and phosphorylation of MLCs, in contrast with pulsatile flow and PP without flow. These studies reveal that the loss of pulsatility, which can occur with lymphoedema, causes LEC contraction and an increase in intercellular gap formation mediated by MLC phosphorylation.

Lymph Flow Before and After Lymphaticovenous Anastomosis Measured Using Transonic Transit-Time Ultrasound Microvascular Flowprobe

Background: Assessment of lymph flow has proven challenging. Transit-time ultrasound technique (TTUT) is the first technique that provides real-time quantitative lymphatic flow values. In cardiothoracic surgery and neurosurgery, this technique has tremendous clinical value in assessing surgery quality and predicting outcomes. The objective of this study was to measure lymph flow before and after lymphaticovenous anastomosis (LVA), using TTUT. Methods and Results: Consecutive patients with peripheral lymphedema undergoing LVA were included. Preoperative workup was performed using indocyanine green (ICG) lymphangiography. Perioperatively, the Transonic^® Microvascular Flowprobe was used to measure lymph flow before and after anastomosis. Twenty-five patients with International Society of Lymphology stage IIA (68%) and stage IIB (32%) peripheral lymphedema were included. Lymph flow velocities ranged from 0.02 to 0.80 mL/min (mean 0.25 ± 0.19) before anastomosis and from 0.02 to 0.86 mL/min (mean 0.27 ± 0.22) after anastomosis (p = 0.340). Mean flow values were significantly higher in the upper extremities compared with the lower extremities. Furthermore, there was a decrease in flow in patients with ICG stage IV in comparison with ICG stage III. Clinical outcomes could not be directly correlated with flow values in these individual cases. Conclusion: TTUT micro-flowprobe is a suitable instrument to measure real-time quantitative lymphatic flow in both lymphatics and LVA. It can confirm patency of lymphatic collectors and LVA peroperatively. Significantly higher lymph flow velocities were found in upper extremities in comparison with lower extremities, both before and after LVA. Further studies should be performed to evaluate lymph flow values and clinical correlation.

Morphometry and Lymph Dynamics of Swine Thoracic Duct

Background: The goal of this study was to characterize the thoracic duct (TD) both morphologically and hemodynamically. Methods and Results: The lymphatic flow and pressure gradient from the cisterna chyli (CC) to the lymphovenous junction were measured in anesthetized swine (n = 9). After the animals were euthanized, the TD were harvested for histomorphometric analyses in which three samples were perfused with 9% gelatin to obtain the morphometry of the TD valve in both the open and closed configuration. Spectral analyses were performed. An afferent lymphatic vessel of the CC was accessed and cannulated after the animal was euthanized for casting (n = 3) to obtain morphometric data. The in vivo flow rate was 0.7 ± 0.49 mL/minute. Spectral analysis (Fast Fourier Transformation) showed correlation coefficients of 0.858 ± 0.063 and 0.586 ± 0.112 (p < 0.05) for the TD and JVPs, respectively. The average pressure gradient was 8.1 mmHg along the TD. The length of the TD was 35.6 ± 2.2 cm. The maximal width of the CC ranged from 11.4 to 15 mm. The diameter of the TD varied irregularly from 2 to 4.3 mm. The geometry of the TD leaflets was determined to have an area of 1.99 ± 0.53 mm², a leaflet length of 3.26 ± 0.86 mm, a packet depth of 0.66 ± 0.19 mm, and a wall length of 5.46 ± 2.16 mm. The TD media thickness was ∼7 ± 3 μm. The number of valves ranged from 9 to 13 in the full length of the TD. Conclusions: A relatively constant pressure gradient in the swine TD drives lymph flow from the CC to the jugular vein. The TD is a thin-walled vessel with valves that prevent reflux of lymph flow. This study of morphometric and lymphatic dynamics is important for interventionalists to understand the anatomy and physiology of the TD to design new diagnostic, interventional procedures, and devices.

Proteolysis and Oxidation of Therapeutic Proteins After Intradermal or Subcutaneous Administration

The intradermal (ID) and subcutaneous (SC) routes are commonly used for therapeutic proteins (TPs) and vaccines; however, the bioavailability of TPs is typically less than small molecule drugs given via the same routes. Proteolytic enzymes in the dermal, SC, and lymphatic tissues may be responsible for the loss of TPs. In addition, the TPs may be exposed to reactive oxygen species generated in the SC tissue and the lymphatic system in response to injection-related trauma and impurities within the formulation. The reactive oxygen species can oxidize TPs to alter their efficacy and immunogenicity potential. Mechanistic understandings of the dominant proteolysis and oxidative routes are useful in the drug discovery process, formulation development, and to assess the potential for immunogenicity and altered pharmacokinetics (PK). Furthermore, in vitro tools representing the ID or SC and lymphatic system can be used to evaluate the extent of proteolysis of the TPs after the injection and before systemic entry. The in vitro clearance data may be included in physiologically based pharmacokinetic models for improved PK predictions. In this review, we have summarized various physiological factors responsible for proteolysis and oxidation of TPs after ID and SC administration.

Microparticle image velocimetry approach to flow measurements in isolated contracting lymphatic vessels

We describe the development of an optical flow visualization method for resolving the flow velocity vector field in lymphatic vessels in vitro. The aim is to develop an experimental protocol for accurately estimating flow parameters, such as flow rate and shear stresses, with high spatial and temporal resolution. Previous studies in situ have relied on lymphocytes as tracers, but their low density resulted in a reduced spatial resolution whereas the assumption that the flow was fully developed in order to determine the flow parameters of interest may not be valid, especially in the vicinity of the valves, where the flow is undoubtedly more complex. To overcome these issues, we have applied the time-resolved microparticle image velocimetry (μ -PIV) technique, a well-established method that can provide increased spatial and temporal resolution that this transient flow demands. To that end, we have developed a custom light source, utilizing high-power light-emitting diodes, and associated control and image processing software. This paper reports the performance of the system and the results of a series of preliminary experiments performed on vessels isolated from rat mesenteries, demonstrating, for the first time, the successful application of the μ -PIV technique in these vessels.

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.

Strategies to address low drug solubility in discovery and development

Drugs with low water solubility are predisposed to low and variable oral bioavailability and, therefore, to variability in clinical response. Despite significant efforts to "design in" acceptable developability properties (including aqueous solubility) during lead optimization, approximately 40% of currently marketed compounds and most current drug development candidates remain poorly water-soluble. The fact that so many drug candidates of this type are advanced into development and clinical assessment is testament to an increasingly sophisticated understanding of the approaches that can be taken to promote apparent solubility in the gastrointestinal tract and to support drug exposure after oral administration. Here we provide a detailed commentary on the major challenges to the progression of a poorly water-soluble lead or development candidate and review the approaches and strategies that can be taken to facilitate compound progression. In particular, we address the fundamental principles that underpin the use of strategies, including pH adjustment and salt-form selection, polymorphs, cocrystals, cosolvents, surfactants, cyclodextrins, particle size reduction, amorphous solid dispersions, and lipid-based formulations. In each case, the theoretical basis for utility is described along with a detailed review of recent advances in the field. The article provides an integrated and contemporary discussion of current approaches to solubility and dissolution enhancement but has been deliberately structured as a series of stand-alone sections to allow also directed access to a specific technology (e.g., solid dispersions, lipid-based formulations, or salt forms) where required.

Identification of interstitial Cajal-like cells in the human thoracic duct

Interstitial Cajal-like cells (ICLCs) are speculated to be pacemakers in smooth muscle tissues. While the human thoracic duct (TD) is spontaneously active, the origin of this activity is unknown. We hypothesized that ICLCs could be present in the TD and using histological techniques, immunohistochemistry and immunofluorescence we have investigated the presence of ICLCs, protein markers for ICLCs and the cellular morphology of the human TD. Transmission electron microscopy was employed to investigate ultrastructure. Methylene blue staining, calcium-dependent fluorophores and confocal microscopy were used to identify ICLCs in live tissue. Methylene blue stained cells with morphology suggestive of ICLCs in the TD. Immunoreactivity localized the ICLC protein markers c-kit, CD34 and vimentin to many cells and processes associated with smooth muscle cells (SMCs): coexpression of c-kit with vimentin or CD34 was observed in some cells. Electron microscopy analysis confirmed ICLCs as a major cell type of the human TD. Lymphatic ICLCs possess caveolae, dense bands, a patchy basal lamina, intermediate filaments and specific junctions to SMCs. ICLCs were ultrastructurally differentiable from other interstitial cells observed: fibroblasts, mast cells, macrophages and pericytes. Lymphatic ICLCs were localized to the subendothelial region of the wall as well as in intimate association with smooth muscle bundles throughout the media. ICLCs were morphologically distinct with multiple processes and also spindle shapes. Confocal imaging with calcium-dependent fluorophores corroborated cell morphology and localization observed in fixed tissues. Lymphatic ICLCs thus constitute a significant cell type of the human TD and physically interact with lymphatic SMCs.

Intestinal lymphatic transport for drug delivery

Intestinal lymphatic transport has been shown to be an absorptive pathway following oral administration of lipids and an increasing number of lipophilic drugs, which once absorbed, diffuse across the intestinal enterocyte and while in transit associate with secretable enterocyte lipoproteins. The chylomicron-associated drug is then secreted from the enterocyte into the lymphatic circulation, rather than the portal circulation, thus avoiding the metabolically-active liver, but still ultimately returning to the systemic circulation. Because of this parallel and potentially alternative absorptive pathway, first-pass metabolism can be reduced while increasing lymphatic drug exposure, which opens the potential for novel therapeutic modalities and allows the implementation of lipid-based drug delivery systems. This review discusses the physiological features of the lymphatics, enterocyte uptake and metabolism, links between drug transport and lipid digestion/re-acylation, experimental model (in vivo, in vitro, and in silico) of lymphatic transport, and the design of lipid- or prodrug-based drug delivery systems for enhancing lymphatic drug transport.

Is there enhanced lymphatic function in upper body trained females?

BACKGROUND

Chronic physical activity results in adaptations in many aspects of human physiology, while specific training can directly influence structural changes. It remains unknown if habitual exercise influences upper extremity lymphatic function in females; thus, the purpose of this cross-sectional study was to compare different exercise stresses on lymphatic function in ten upper body trained females with ten untrained females.

METHODS AND RESULTS

Participants underwent a maximal upper body aerobic test on an arm crank ergometer before undergoing three randomly assigned lymphatic stress tests. Lymphoscintigraphy was used to quantify lymphatic function. (99m)Tc-antimony colloid was injected into the third web space of each hand, followed by 1 min spot views taken with a gamma-radiation camera. The maximal stress test required individuals to repeat their initial maximal exercise test. The subjects were then imaged every 10 min until 60 min were reached. The submaximal stress test involved arm cranking for 2.5 min at 0.6 W x kg(-1), followed by 2.5 min of rest, repeated for 60 min. The final stress test was a 60 min seated resting session. The clearance rate (CR) and axillary uptake (AX) were determined. Only AX post maximal exercise was significantly different between trained and untrained, p=0.009. All other measures of lymphatic function between groups were similar.

CONCLUSION

This study demonstrates no significant difference in lymphatic function between upper body trained and untrained females.

Lipid-based delivery systems and intestinal lymphatic drug transport: a mechanistic update

After oral administration, the majority of drug molecules are absorbed across the small intestine and enter the systemic circulation via the portal vein and the liver. For some highly lipophilic drugs (typically log P > 5, lipid solubility > 50 mg/g), however, association with lymph lipoproteins in the enterocyte leads to transport to the systemic circulation via the intestinal lymph. The attendant delivery benefits associated with lymphatic drug transport include a reduction in first-pass metabolism and lymphatic exposure to drug concentrations orders of magnitude higher than that attained in systemic blood. In the current review we briefly describe the mechanisms by which drug molecules access the lymph and the formulation strategies that may be utilised to enhance lymphatic drug transport. Specific focus is directed toward recent advances in understanding regarding the impact of lipid source (both endogenous and exogenous) and intracellular lipid trafficking pathways on lymphatic drug transport and enterocyte-based first-pass metabolism.

Abdominal lymph flow in an endotoxin sepsis model: influence of spontaneous breathing and mechanical ventilation

OBJECTIVE

Lymph flow from the abdomen was investigated in a sepsis model. We also compared the effect on thoracic duct lymph flow of mechanical ventilation with different levels of positive end-expiratory pressure (PEEP) and spontaneous breathing with continuous positive airway pressure (CPAP).

DESIGN

Experimental study.

SETTING

Research laboratory in a university hospital.

SUBJECTS

Thirty-two pigs.

INTERVENTIONS

Animals were anesthetized. In study 1 (n = 18), an ultrasonic flow probe was put around the intact thoracic duct just caudal to the diaphragm, and animals were randomized to receive mechanical ventilation with a PEEP of 5 cm H2O or 15 cm H2O or breathed spontaneously in CPAP with a PEEP of 5 cm H2O. In study 2 (n = 6), the thoracic duct was cannulated and the cannula externalized through the abdominal wall for lymph collection; animals were then ventilated as in study 1. In all animals, endotoxin was infused at 15 microg/kg/hr for 2.5 hrs and then continued at 5 microg/kg/hr. In study 3, healthy (n = 4) and endotoxin-exposed (n = 4) pigs had intra-abdominal pressure increased to 27 cm H2O for 2 hrs by pneumoperitoneum. Lymph flow was measured as in study 1.

MEASUREMENTS AND MAIN RESULTS

Lymph flow (mean +/- SE) was 2.5 +/- 0.4 mL/min at baseline and increased to 3.9 +/- 0.8 mL/min after 90 mins and 6.3 +/- 1.6 mL/min after 150 mins (p < .005) of endotoxin exposure. PEEP 15 cm H2O decreased lymph flow in pigs with intact thoracic duct (flow probe recording) and in pigs with cannulated lymph duct when drained against the central venous pressure. However, when drained against atmospheric pressure, PEEP increased flow. Spontaneous breathing increased flow both in intact and in cannulated animals.

CONCLUSIONS

Endotoxin increases lymph flow from the abdomen. Mechanical ventilation with high PEEP impedes lymph drainage and could increase lymph production. Spontaneous breathing increases flow and improves drainage of abdominal edema.

New method for evaluation of lung lymph flow rate with intact lymphatics in anaesthetized sheep

AIM

Lung lymph has commonly been studied using a lymph fistula created by tube cannulation into the efferent duct of the caudal mediastinal node in sheep. In this method, the tail region of the caudal mediastinal node is resected and the diaphragm is cauterized to exclude systemic lymph contamination, and cannulation is performed into one of the multiple efferent ducts originating from the caudal mediastinal node. Moreover, the pumping activity of lymphatics might be diminished by cannulation. Therefore, the purpose of the study was to evaluate the flow rate of lung lymph with maintenance of intact lymphatic networks around the caudal mediastinal node to the thoracic duct in sheep.

METHODS

An ultrasound transit-time flow meter was used to measure lung lymph flow. The thoracic duct was clamped just above the diaphragm and the flow probe was attached to the thoracic duct just after the last junction with an efferent duct from the caudal mediastinal node. The lung lymph flow rate was measured at baseline and under conditions of lung-oedema formation.

RESULTS

The baseline lung lymph flow rate in our model was three- to sixfold greater than values obtained with the cannulation method. With oedema-formation, the lung lymph flow rate was the same as that measured using cannulation.

CONCLUSION

The lung lymph flow was unexpectedly large under the conditions of the study, and our data suggest that the drainage effect of lymphatics is significant as a safety factor against pulmonary oedema formation.

Arnold Heller and the lymph pump

This article reviews studies on lymph propulsion in the lymph vessels by active contraction of the vessels, first described by Arnold Heller in 1869 in German language, and here translated into English. His observations were first confirmed by Beatrice Carrier (1926) and Howard Flory et al. (1927), and several groups were active up to World war II. Few publications appeared in the period 1940--1960, followed by increasing activity and development of new experimental techniques for use both in various experimental animals and in humans. Recently it has been shown that passive lymph flow may add to active propulsion. Both mechanisms depend on lymph formation, i.e. the uptake of interstitial fluid by the initial lymph vessels which is still not well understood.

Popliteal and mesenteric lymph node injection with methylene blue for coloration of the thoracic duct in dogs

OBJECTIVE

To describe and compare the time of onset and intensity of thoracic duct coloration after injection of methylene blue into a mesenteric or popliteal lymph node.

STUDY DESIGN

Experimental study.

ANIMALS

Twenty adult dogs.

METHODS

A right tenth intercostal thoracotomy, a right paracostal laparotomy, and an approach to the right popliteal lymph node were performed on each dog. Methylene blue (0.5 mg/kg of a 1% solution, maximum 10 mg) was injected into either a mesenteric (group M, 10 dogs) or popliteal (group P, 10 dogs) lymph node. Thoracic duct color was graded (0 to 3) every 5 minutes for 60 minutes. Statistical analysis was performed on mean thoracic duct color grade data, on number of successful outcomes between groups M and P, and between weight groups.

RESULTS

Coloration of the thoracic duct occurred in all group M dogs and 6 group P dogs. Coloration was first recorded 0 to 10 minutes after injection in all dogs and persisted for 60 minutes in 15 dogs. Mean thoracic duct color grade was significantly increased postinjection compared with preinjection at all times in group M. More successful outcomes occurred in group M (P =.03).

CONCLUSIONS

Methylene blue injected into mesenteric or popliteal lymph nodes was successful in coloring the thoracic duct, but both mean grade and number of successful outcomes were significantly higher after mesenteric injection.

CLINICAL RELEVANCE

Thoracic duct coloration after lymph node injection occurred within 10 minutes and persisted for 60 minutes. This information is useful in planning thoracic duct ligation in cases of chylothorax when observation of the duct is desired. Injection of both lymph node sites was successful, but mesenteric node injection was a more reliable technique.