Dual-channel in-situ optical imaging system for quantifying lipid uptake and lymphatic pump function

Intestinal Lymphatic Biology, Drug Delivery, and Therapeutics: Current Status and Future Directions

Historically, the intestinal lymphatics were considered passive conduits for fluids, immune cells, dietary lipids, lipid soluble vitamins, and lipophilic drugs. Studies of intestinal lymphatic drug delivery in the late 20th century focused primarily on the drugs' physicochemical properties, especially high lipophilicity, that resulted in intestinal lymphatic transport. More recent discoveries have changed our traditional view by demonstrating that the lymphatics are active, plastic, and tissue-specific players in a range of biological and pathological processes, including within the intestine. These findings have, in turn, inspired exploration of lymph-specific therapies for a range of diseases, as well as the development of more sophisticated strategies to actively deliver drugs or vaccines to the intestinal lymph, including a range of nanotechnologies, lipid prodrugs, and lipid-conjugated materials that "hitchhike" onto lymphatic transport pathways. With the increasing development of novel therapeutics such as biologics, there has been interest in whether these therapeutics are absorbed and transported through intestinal lymph after oral administration. Here we review the current state of understanding of the anatomy and physiology of the gastrointestinal lymphatic system in health and disease, with a focus on aspects relevant to drug delivery. We summarize the current state-of-the-art approaches to deliver drugs and quantify their uptake into the intestinal lymphatic system. Finally, and excitingly, we discuss recent examples of significant pharmacokinetic and therapeutic benefits achieved via intestinal lymphatic drug delivery. We also propose approaches to advance the development and clinical application of intestinal lymphatic delivery strategies in the future. SIGNIFICANCE STATEMENT: This comprehensive review details the understanding of the anatomy and physiology of the intestinal lymphatic system in health and disease, with a focus on aspects relevant to drug delivery. It highlights current state-of-the-art approaches to deliver drugs to the intestinal lymphatics and the shift toward the use of these strategies to achieve pharmacokinetic and therapeutic benefits for patients.

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.

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.

Pharmacokinetics of cycloheximide in rats and evaluation of its effect as a blocker of intestinal lymph formation

Cycloheximide (CHX) has been used to reduce the flow of intestinal lymph and as a non-surgical tool to study drug absorption via the intestinal lymphatics. Pharmacokinetic information on the agent, and its relationship to effect and toxicity, have not been examined. The goal of this study was to provide pharmacokinetic data and link it to lymph-blocking and toxicological effects. Jugular-vein cannulated (JVC) adult Sprague-Dawley male rats were administered 0.5 mg/kg CHX by oral, intraperitoneal (ip), and intravenous routes followed by blood draws, and CHX was assayed using LC-MS/MS. Another four JVC rats were given peanut oil (2 mL/kg) without and then with CHX to measure effects on lipid absorption as a surrogate indicator of lymph flow. One-week later plasma biochemistry measures were obtained. The results indicated that CHX had a high clearance and volume of distribution, and oral absolute bioavailability of 0.47 with 0.5 mg/kg. CHX was associated with dose- and route-dependent pharmacokinetics. The relative bioavailability after ip doses was over 3. CHX had low plasma protein binding and minor urinary excretion. Metabolism appeared to be occur by oxidation and glucuronidation. Reductions in plasma lipids (24-40 %) were seen after 2.5 mg/kg orally with signs of inflammation and increased liver enzymes persisting for a week after the dose. CHX was associated with a reduction in lipid absorption after oral doses of 2.5 mg/kg, which seems to justify its use as a non-surgical tool to evaluate the lymphatic pathway of absorption of drugs. However, it also possesses hepatotoxicity, which should be taken into consideration in its use.

Development and application of a novel cervical lymph collection method to assess lymphatic transport in rats

Background: Fluids, solutes and immune cells have been demonstrated to drain from the brain and surrounding structures to the cervical lymph vessels and nodes in the neck via meningeal lymphatics, nasal lymphatics and/or lymphatic vessels associated with cranial nerves. A method to cannulate the efferent cervical lymph duct for continuous cervical lymph fluid collection in rodents has not been described previously and would assist in evaluating the transport of molecules and immune cells from the head and brain via the lymphatics, as well as changes in lymphatic transport and lymph composition with different physiological challenges or diseases. Aim: To develop a novel method to cannulate and continuously collect lymph fluid from the cervical lymph duct in rats and to analyze the protein, lipid and immune cell composition of the collected cervical lymph fluid. Methods: Male Sprague-Dawley rats were cannulated at the carotid artery with or without cannulation or ligation at the cervical lymph duct. Samples of blood, whole lymph and isolated lipoprotein fractions of lymph were collected and analyzed for lipid and protein composition using commercial kits. Whole lymph samples were centrifuged and isolated pellets were stained and processed for flow cytometry analysis of CD3⁺, CD4⁺, CD8a⁺, CD45R⁺ (B220) and viable cell populations. Results: Flow rate, phospholipid, triglyceride, cholesterol ester, free cholesterol and protein concentrations in cervical lymph were 0.094 ± 0.014 mL/h, 0.34 ± 0.10, 0.30 ± 0.04, 0.07 ± 0.02, 0.02 ± 0.01 and 16.78 ± 2.06 mg/mL, respectively. Protein was mostly contained within the non-lipoprotein fraction but all lipoprotein types were also present. Flow cytometry analysis of cervical lymph showed that 67.1 ± 7.4% of cells were CD3⁺/CD4⁺ T lymphocytes, 5.8 ± 1.6% of cells were CD3⁺/CD8⁺ T lymphocytes, and 10.8 ± 4.6% of cells were CD3^-/CD45R⁺ B lymphocytes. The remaining 16.3 ± 4.6% cells were CD3^-/CD45^- and identified as non-lymphocytes. Conclusion: Our novel cervical lymph cannulation method enables quantitative analysis of the lymphatic transport of immune cells and molecules in the cervical lymph of rats for the first time. This valuable tool will enable more detailed quantitative analysis of changes to cervical lymph composition and transport in health and disease, and could be a valuable resource for discovery of biomarkers or therapeutic targets in future studies.

Improving obesity and lipid metabolism using conjugated linoleic acid

BACKGROUND

Conjugated linoleic acid (CLA) can prevent fatty acid accumulation induced by a high-fructose diet and improve lipid metabolism disorders in patients.

OBJECTIVES

We aimed to investigate the effect of CLA on obesity and lipid metabolism and its possible mechanism.

METHODS

Eight-month-old male BKS.Cg-Dock7m +/+ Leprdb /JNju (db/db) mice (n = 12) were fed a CLA mix composed of equivalent c9, t11-CLA and t10, c12-CLA for 1 month. The effect of CLA on body weight, water and food intake, and triglyceride (TG) and total cholesterol (TC) levels was investigated. PPARα, PPARγ and CD36 expression was determined by quantitative PCR and western blotting. Additionally, the expression of these three genes was studied in HepG2 cells treated with CLA and linoleic acid.

RESULTS

CLA treatment notably reduced the dietary and water intake of db/db mice, effectively reduced body weight, and decreased serum TG and TC levels (p < 0.05). Increased expression of PPARα (p < 0.05) and decreased expression of CD36 (p < 0.001) were observed in the liver of mice that were fed CLA. CLA increased PPARα expression (p < 0.001) and decreased PPARγ (p < 0.001) and CD36 expression (p < 0.01) in HepG2 cells.

CONCLUSIONS

Our results showed that CLA can improve lipid metabolism in obese mice through upregulation of PPARα expression and downregulation of CD36 expression.

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Fluorescent Tracers for In Vivo Imaging of Lymphatic Targets

The lymphatic system continues to gain importance in a range of conditions, and therefore, imaging of lymphatic vessels is becoming more widespread for research, diagnosis, and treatment. Fluorescent lymphatic imaging offers advantages over other methods in that it is affordable, has higher resolution, and does not require radiation exposure. However, because the lymphatic system is a one-way drainage system, the successful delivery of fluorescent tracers to lymphatic vessels represents a unique challenge. Each fluorescent tracer used for lymphatic imaging has distinct characteristics, including size, shape, charge, weight, conjugates, excitation/emission wavelength, stability, and quantum yield. These characteristics in combination with the properties of the target tissue affect the uptake of the dye into lymphatic vessels and the fluorescence quality. Here, we review the characteristics of visible wavelength and near-infrared fluorescent tracers used for in vivo lymphatic imaging and describe the various techniques used to specifically target them to lymphatic vessels for high-quality lymphatic imaging in both clinical and pre-clinical applications. We also discuss potential areas of future research to improve the lymphatic fluorescent tracer design.

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

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

Intestinal Lymph Flow, and Lipid and Drug Transport Scale Allometrically From Pre-clinical Species to Humans

The intestinal lymphatic system transports fluid, immune cells, dietary lipids, and highly lipophilic drugs from the intestine to the systemic circulation. These transport functions are important to health and when dysregulated contribute to pathology. This has generated significant interest in approaches to deliver drugs to the lymphatics. Most of the current understanding of intestinal lymph flow, and lymphatic lipid and drug transport rates, comes from in vitro studies and in vivo animal studies. In contrast, intestinal lymphatic transport studies in human subjects have been limited. Recently, three surgical patients had cannulation of the thoracic lymph duct for collection of lymph before and during a stepwise increase in enteral feed rate. We compared these data to studies where we previously enterally administered controlled quantities of lipid and the lipophilic drug halofantrine to mice, rats and dogs and collected lymph and blood (plasma). The collected lymph was analyzed to compare lymph flow rate, triglyceride (TG) and drug transport rates, and plasma was analyzed for drug concentrations, as a function of enteral lipid dose across species. Lymph flow rate, TG and drug transport increased with lipid administration in all species tested, and scaled allometrically according to the equation A = aM ^E where A is the lymph transport parameter, M is animal body mass, a is constant and E is the allometric exponent. For lymph flow rate and TG transport, the allometric exponents were 0.84-0.94 and 0.80-0.96, respectively. Accordingly, weight normalized lymph flow and TG mass transport were generally lower in larger compared to smaller species. In comparison, mass transport of drug via lymph increased in a greater than proportional manner with species body mass with an exponent of ∼1.3. The supra-proportional increase in lymphatic drug transport with species body mass appeared to be due to increased partitioning of drug into lymph rather than blood following absorption. Overall, this study proposes that intestinal lymphatic flow, and lymphatic lipid and drug transport in humans is most similar to species with higher body mass such as dogs and underestimated by studies in rodents. Notably, lymph flow and lipid transport in humans can be predicted from animal data via allometric scaling suggesting the potential for similar relationships with drug transport.

Fermentation Products of Commensal Bacteria Alter Enterocyte Lipid Metabolism

Despite the recognized capacity of the gut microbiota to regulate intestinal lipid metabolism, the role of specific commensal species remains undefined. Here, we aimed to understand the bacterial effectors and molecular mechanisms by which Lactobacillus paracasei and Escherichia coli regulate lipid metabolism in enterocytes. We show that L-lactate produced by L. paracasei inhibits chylomicron secretion from enterocytes and promotes lipid storage by a mechanism involving L-lactate absorption by enterocytes, its conversion to malonyl-CoA, and the subsequent inhibition of lipid beta-oxidation. In contrast, acetate produced by E. coli also inhibits chylomicron secretion by enterocytes but promotes lipid oxidation by a mechanism involving acetate absorption by enterocytes, its metabolism to acetyl-CoA and AMP, and the subsequent upregulation of the AMPK/PGC-1α/PPARα pathway. Our study opens perspectives for developing specific bacteria- and metabolite-based therapeutic interventions against obesity, atherosclerosis, and malnutrition by targeting lipid metabolism in enterocytes.

Emerging Role of Lymphatics in the Regulation of Intestinal Lipid Mobilization

Intestinal handling of dietary triglycerides has important implications for health and disease. Following digestion in the intestinal lumen, absorption, and re-esterification of fatty acids and monoacylglycerols in intestinal enterocytes, triglycerides are packaged into lipoprotein particles (chylomicrons) for secretion or into cytoplasmic lipid droplets for transient or more prolonged storage. Despite the recognition of prolonged retention of triglycerides in the post-absorptive phase and subsequent release from the intestine in chylomicron particles, the underlying regulatory mechanisms remain poorly understood. Chylomicron secretion involves multiple steps, including intracellular assembly and post-assembly transport through cellular organelles, the lamina propria, and the mesenteric lymphatics before being released into the circulation. Contrary to the long-held view that the intestinal lymphatic vasculature acts mainly as a passive conduit, it is increasingly recognized to play an active and regulatory role in the rate of chylomicron release into the circulation. Here, we review the latest advances in understanding the role of lymphatics in intestinal lipid handling and chylomicron secretion. We highlight emerging evidence that oral glucose and the gut hormone glucagon-like peptide-2 mobilize retained enteral lipid by differing mechanisms to promote the secretion of chylomicrons via glucose possibly by mobilizing cytoplasmic lipid droplets and via glucagon-like peptide-2 possibly by targeting post-enterocyte secretory mechanisms. We discuss other potential regulatory factors that are the focus of ongoing and future research. Regulation of lymphatic pumping and function is emerging as an area of great interest in our understanding of the integrated absorption of dietary fat and chylomicron secretion and potential implications for whole-body metabolic health.

Age-related changes in B cell metabolism

Antibody responses to vaccinations or infections decline upon aging. In this study we tested if metabolic changes in B cells may contribute to attenuation of responses to influenza vaccination in aged humans. Our data show that aging affects mitochondrial functions in B cells leading to increases in mitochondrial reactive oxygen species (MROS) and mitochondrial mass (MM) in some aged B cell subsets and decreases in expression levels of Sirtuin 1 (SIRT1), Forkhead box protein (FOX)O1 and carnitine palmitoyltransferase 1 (CPT-1). Seahorse analyses showed minor defects in glycolysis in the aged B cells after activation but a strong reduction in oxidative phosphorylation. The analyses of the transcriptome revealed further pronounced defects in one-carbon metabolism, a pathway that is essential for amino acid and nucleotide metabolism. Overall our data support the notion that the declining ability of aged B cells to increase their metabolism following activation contributes to the weakened antibody responses of the elderly.

High-speed microscopy for in vivo monitoring of lymph dynamics

The lymphatic system contributes to body homeostasis by clearing fluid, lipids, plasma proteins and immune cells from the interstitial space. Many studies have been performed to understand lymphatic function under normal conditions and during disease. Nevertheless, a further improvement in quantification of lymphatic behavior is needed. Here, we present advanced bright-field microscopy for in vivo imaging of lymph vessels (LVs) and automated quantification of lymphatic function at a temporal resolution of 2 milliseconds. Full frame videos were compressed and recorded continuously at up to 540 frames per second. A new edge detection algorithm was used to monitor vessel diameter changes across multiple cross sections, while individual cells in the LVs were tracked to estimate flow velocity. The system performance initially was verified in vitro using 6- and 10-μm microspheres as cell phantoms on slides and in 90-μm diameter tubes at flow velocities up to 4 cm/second. Using an in vivo rat model, we explored the mechanisms of lymphedema after surgical lymphadenectomy of the mesentery. The system revealed reductions of mesenteric LV contraction and flow rate. Thus, the described imaging system may be applicable to the study of lymphatic behavior during therapeutic and surgical interventions, and potentially during lymphatic system diseases.

Experimental Models Used to Assess Lymphatic Contractile Function

Recent years have seen a renewed interest in studies of the lymphatic system. This review addresses the differences between in vivo and ex vivo methods for visualization and functional studies of lymphatic networks, with an emphasis on studies of collecting lymphatic vessels. We begin with a brief summary of the historical uses of both approaches. For the purpose of detailed comparisons, we subdivide in vivo methods into those visualizing lymphatic networks through the intact skin and those using surgically opened skin. We subdivide ex vivo methods into isobaric studies (using a pressure myograph) or isometric studies (using a wire myograph). For all four categories, we compile a comprehensive list of the advantages, disadvantages, and limitations of each preparation, with the goal of informing the research community as to the appropriate kinds of experiments best suited, and ill suited, for each.

Differences in L-type Ca2+ channel activity partially underlie the regional dichotomy in pumping behavior by murine peripheral and visceral lymphatic vessels

We identified a regional dichotomy in murine lymphatic contractile function with regard to vessel location within the periphery or visceral cavity. All vessels isolated from peripheral regions [cervical, popliteal, inguinal, axillary, and internodal inguinal axillary (Ing-Ax)] developed robust contractions with maximal ejection fractions (EFs) of 50-80% in our ex vivo isobaric myograph experiments. Conversely, vessels isolated from the visceral cavity (mesenteric, thoracic duct, and iliac) demonstrated maximal EFs of ≤10%. Using pressure myography, sharp electrode membrane potential recordings, and Ca²⁺ imaging, we assessed the role of L-type Ca²⁺ channels in this contractile dichotomy. Ing-Ax membrane potential revealed a ~2-s action potential (AP) cycle (resting -35 mV, spike -5 mV, and plateau -11 mV) with a plateau phase that was significantly lengthened by the L-type Ca²⁺ channel agonist Bay K8644 (BayK; 100 nM). APs recorded from mesenteric vessels, however, displayed a slower upstroke and an elongated time over threshold. BayK (100 nM) increased the mesenteric AP upstroke velocity and plateau duration but also significantly hyperpolarized the vessel. Contractions of vessels from both regions were preceded by Ca²⁺ flashes, detected with a smooth muscle-specific endogenous Ca²⁺ reporter, that typically were coordinated over the length of the vessel. Similar to the membrane potential recordings, Ca²⁺ flashes in mesenteric vessels were weaker and had a slower rise time but were longer lasting than those in Ing-Ax vessels. BayK (100 nM) significantly increased the Ca²⁺ transient amplitude and duration in both vessels and decreased time to peak Ca²⁺ in mesenteric vessels. However, a higher concentration (1 μM) of BayK was required to produce even a modest increase in EF in visceral lymphatics, which remained at <20%. NEW & NOTEWORTHY Lymphatic collecting vessels isolated from murine peripheral tissues, but not from the visceral cavities, display robust contractile behavior similar to lymphatic vessels from other animal models and humans. These differences are partially explained by L-type Ca²⁺ channel activity as revealed by the first measurements of murine lymphatic action potentials and contraction-associated Ca²⁺ transients.

The relationship between lymphangion chain length and maximum pressure generation established through in vivo imaging and computational modeling

The intrinsic contraction of collecting lymphatic vessels serves as a pumping system to propel lymph against hydrostatic pressure gradients as it returns interstitial fluid to the venous circulation. In the present study, we proposed and validated that the maximum opposing outflow pressure along a chain of lymphangions at which flow can be achieved increases with the length of chain. Using minimally invasive near-infrared imaging to measure the effective pumping pressure at various locations in the rat tail, we demonstrated increases in pumping pressure along the length of the tail. Computational simulations based on a microstructurally motivated model of a chain of lymphangions informed from biaxial testing of isolated vessels was used to provide insights into the pumping mechanisms responsible for the pressure increases observed in vivo. These models suggest that the number of lymphangions in the chain and smooth muscle cell force generation play a significant role in determining the maximum outflow pressure, whereas the frequency of contraction has no effect. In vivo administration of nitric oxide attenuated lymphatic contraction, subsequently lowering the effective pumping pressure. Computational simulations suggest that the reduction in contractile strength of smooth muscle cells in the presence of nitric oxide can account for the reductions in outflow pressure observed along the lymphangion chain in vivo. Thus, combining modeling with multiple measurements of lymphatic pumping pressure provides a method for approximating intrinsic lymphatic muscle activity noninvasively in vivo while also providing insights into factors that determine the extent that a lymphangion chain can transport fluid against an adverse pressure gradient. NEW & NOTEWORTHY Here, we report the first minimally invasive in vivo measurements of the relationship between lymphangion chain length and lymphatic pumping pressure. We also provide the first in vivo validation of lumped parameter models of lymphangion chains previously developed through data obtained from isolated vessel testing.

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).

Lipid-associated oral delivery: Mechanisms and analysis of oral absorption enhancement

The majority of newly discovered oral drugs are poorly water soluble, and co-administration with lipids has proven effective in significantly enhancing bioavailability of some compounds with low aqueous solubility. Yet, lipid-based delivery technologies have not been widely employed in commercial oral products. Lipids can impact drug transport and fate in the gastrointestinal (GI) tract through multiple mechanisms including enhancement of solubility and dissolution kinetics, enhancement of permeation through the intestinal mucosa, and triggering drug precipitation upon lipid emulsion depletion (e.g., by digestion). The effect of lipids on drug absorption is currently not quantitatively predictable, in part due to the multiple complex dynamic processes that can be impacted by lipids. Quantitative mechanistic analysis of the processes significant to lipid system function and overall impact on drug absorption can aid in the understanding of drug-lipid interactions in the GI tract and exploitation of such interactions to achieve optimal lipid-based drug delivery. In this review, we discuss the impact of co-delivered lipids and lipid digestion on drug dissolution, partitioning, and absorption in the context of the experimental tools and associated kinetic expressions used to study and model these processes. The potential benefit of a systems-based consideration of the concurrent multiple dynamic processes occurring upon co-dosing lipids and drugs to predict the impact of lipids on drug absorption and enable rational design of lipid-based delivery systems is presented.

Postprandial lymphatic pump function after a high-fat meal: a characterization of contractility, flow, and viscosity

Dietary lipids are transported from the intestine through contractile lymphatics. Chronic lipid loads can adversely affect lymphatic function. However, the acute lymphatic pump response in the mesentery to a postprandial lipid meal has gone unexplored. In this study, we used the rat mesenteric collecting vessel as an in vivo model to quantify the effect of lipoproteins on vessel function. Lipid load was continuously monitored by using the intensity of a fluorescent fatty-acid analog, which we infused along with a fat emulsion through a duodenal cannula. The vessel contractility was simultaneously quantified. We demonstrated for the first time that collecting lymphatic vessels respond to an acute lipid load by reducing pump function. High lipid levels decreased contraction frequency and amplitude. We also showed a strong tonic response through a reduction in the end-diastolic and systolic diameters. We further characterized the changes in flow rate and viscosity and showed that both increase postprandially. In addition, shear-mediated Ca(2+) signaling in lymphatic endothelial cells differed when cultured with lipoproteins. Together these results show that the in vivo response could be both shear and lipid mediated and provide the first evidence that high postprandial lipid has an immediate negative effect on lymphatic function even in the acute setting.

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.

Dextran sulfate sodium-induced acute colitis impairs dermal lymphatic function in mice

AIM: To investigate whether dermal lymphatic function and architecture are systemically altered in dextran sulfate sodium (DSS)-induced acute colitis.

METHODS: Balb/c mice were administered 4% DSS in lieu of drinking water ad libitum for 7 d and monitored to assess disease activity including body weight, diarrhea severity, and fecal bleeding. Control mice received standard drinking water with no DSS. Changes in mesenteric lymphatics were assessed following oral administration of a fluorescently-labelled fatty acid analogue, while dermal lymphatic function and architecture was longitudinally characterized using dynamic near-infrared fluorescence (NIRF) imaging following intradermal injection of indocyanine green (ICG) at the base of the tail or to the dorsal aspect of the left paw prior to, 4, and 7 d after DSS administration. We also measured dye clearance rate after injection of Alexa680-bovine serum albumin (BSA). NIRF imaging data was analyzed to reveal lymphatic contractile activity after selecting fixed regions of interest (ROIs) of the same size in fluorescent lymphatic vessels on fluorescence images. The averaged fluorescence intensity within the ROI of each fluorescence image was plotted as a function of imaging time and the lymphatic contraction frequency was computed by assessing the number of fluorescent pulses arriving at a ROI.

RESULTS: Mice treated with DSS developed acute inflammation with clinical symptoms of loss of body weight, loose feces/watery diarrhea, and fecal blood, all of which were aggravated as disease progressed to 7 d. Histological examination of colons of DSS-treated mice confirmed acute inflammation, characterized by segmental to complete loss of colonic mucosa with an associated chronic inflammatory cell infiltrate that extended into the deeper layers of the wall of the colon, compared to control mice. In situ intravital imaging revealed that mice with acute colitis showed significantly fewer fluorescent mesenteric lymphatic vessels, indicating impaired uptake of a lipid tracer within mesenteric lymphatics. Our in vivo NIRF imaging data demonstrated dilated dermal lymphatic vessels, which were confirmed by immunohistochemical staining of lymphatic vessels, and significantly reduced lymphatic contractile function in the skin of mice with DSS-induced acute colitis. Quantification of the fluorescent intensity remaining in the depot as a function of time showed that there was significantly higher Alexa680-BSA fluorescence in mice with DSS-induced acute colitis compared to pre-treatment with DSS, indicative of impaired lymphatic drainage.

CONCLUSION: The lymphatics are locally and systemically altered in acute colitis, and functional NIRF imaging is useful for noninvasively monitoring systemic lymphatic changes during inflammation.

Intravital imaging of intestinal lacteals unveils lipid drainage through contractility

Lacteals are lymphatic vessels located at the center of each intestinal villus and provide essential transport routes for lipids and other lipophilic molecules. However, it is unclear how absorbed molecules are transported through the lacteal. Here, we used reporter mice that express GFP under the control of the lymphatic-specific promoter Prox1 and a custom-built confocal microscope and performed intravital real-time visualization of the absorption and transport dynamics of fluorescence-tagged fatty acids (FAs) and various exogenous molecules in the intestinal villi in vivo. These analyses clearly revealed transepithelial absorption of these molecules via enterocytes, diffusive distribution over the lamina propria, and subsequent transport through lacteals. Moreover, we observed active contraction of lacteals, which seemed to be directly involved in dietary lipid drainage. Our analysis revealed that the smooth muscles that surround each lacteal are responsible for contractile dynamics and that lacteal contraction is ultimately controlled by the autonomic nervous system. These results indicate that the lacteal is a unique organ-specific lymphatic system and does not merely serve as a passive conduit but as an active pump that transports lipids. Collectively, using this efficient imaging method, we uncovered drainage of absorbed molecules in small intestinal villus lacteals and the involvement of lacteal contractibility.

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.

The mesenteric lymph duct cannulated rat model: application to the assessment of intestinal lymphatic drug transport

The intestinal lymphatic system plays key roles in fluid transport, lipid absorption and immune function. Lymph flows directly from the small intestine via a series of lymphatic vessels and nodes that converge at the superior mesenteric lymph duct. Cannulation of the mesenteric lymph duct thus enables the collection of mesenteric lymph flowing from the intestine. Mesenteric lymph consists of a cellular fraction of immune cells (99% lymphocytes), aqueous fraction (fluid, peptides and proteins such as cytokines and gut hormones) and lipoprotein fraction (lipids, lipophilic molecules and apo-proteins). The mesenteric lymph duct cannulation model can therefore be used to measure the concentration and rate of transport of a range of factors from the intestine via the lymphatic system. Changes to these factors in response to different challenges (e.g., diets, antigens, drugs) and in disease (e.g., inflammatory bowel disease, HIV, diabetes) can also be determined. An area of expanding interest is the role of lymphatic transport in the absorption of orally administered lipophilic drugs and prodrugs that associate with intestinal lipid absorption pathways. Here we describe, in detail, a mesenteric lymph duct cannulated rat model which enables evaluation of the rate and extent of lipid and drug transport via the lymphatic system for several hours following intestinal delivery. The method is easily adaptable to the measurement of other parameters in lymph. We provide detailed descriptions of the difficulties that may be encountered when establishing this complex surgical method, as well as representative data from failed and successful experiments to provide instruction on how to confirm experimental success and interpret the data obtained.

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.

Emerging lymphatic imaging technologies for mouse and man

The lymphatic circulatory system has diverse functions in lipid absorption, fluid homeostasis, and immune surveillance and responds dynamically when presented with infection, inflammation, altered hemodynamics, and cancer. Visualization of these dynamic processes in human disease and animal models of disease is key to understanding the contributory role of the lymphatic circulatory system in disease and to devising effective therapeutic strategies. Longitudinal, non-destructive, and repeated imaging is necessary to expand our understanding of disease progression and regression in basic science and clinical investigations. Herein we summarize recent advances in in vivo lymphatic imaging employing magnetic resonance, computed tomography, lymphoscintigraphy, and emerging optical techniques with respect to their contributory roles in both basic science and clinical research investigations.

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.

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.