Non-invasive optical imaging of the lymphatic vasculature of a mouse

Targeting lymphatic function in cardiovascular-kidney-metabolic syndrome: preclinical methods to analyze lymphatic function and therapeutic opportunities

The lymphatic vascular system spans nearly every organ in the body and serves as an important network that maintains fluid, metabolite, and immune cell homeostasis. Recently, there has been a growing interest in the role of lymphatic biology in chronic disorders outside the realm of lymphatic abnormalities, lymphedema, or oncology, such as cardiovascular-kidney-metabolic syndrome (CKM). We propose that enhancing lymphatic function pharmacologically may be a novel and effective way to improve quality of life in patients with CKM syndrome by engaging multiple pathologies at once throughout the body. Several promising therapeutic targets that enhance lymphatic function have already been reported and may have clinical benefit. However, much remains unclear of the discreet ways the lymphatic vasculature interacts with CKM pathogenesis, and translation of these therapeutic targets to clinical development is challenging. Thus, the field must improve characterization of lymphatic function in preclinical mouse models of CKM syndrome to better understand molecular mechanisms of disease and uncover effective therapies.

A Multiresolution Approach with Method-Informed Statistical Analysis for Quantifying Lymphatic Pumping Dynamics

Despite significant strides in lymphatic system imaging, the timely diagnosis of lymphatic disorders remains elusive. One main cause for this is the absence of standardized, quantitative methods for real-time analysis of lymphatic contractility. Here, we address this unmet need by combining near-infrared lymphangiography imaging with an innovative analytical workflow. We combined data acquisition, signal processing, and statistical analysis to integrate traditional peak and-valley with advanced wavelet time-frequency analyses. Decision theory was used to evaluate the primary drivers of attributable variance in lymphangiography measurements to generate a strategy for optimizing the number of repeat measurements needed per subject to increase measurement reliability. This approach not only offers detailed insights into lymphatic pumping behaviors across species, sex and age, but also significantly boosts the reliability of these measurements by incorporating multiple regions of interest and evaluating the lymphatic system under various gravitational loads. By addressing the critical need for improved imaging and quantification methods, our study offers a new standard approach for the imaging and analysis of lymphatic function that can improve our understanding, diagnosis, and treatment of lymphatic diseases. The results highlight the importance of comprehensive data acquisition strategies to fully capture the dynamic behavior of the lymphatic system.

Lymphatic contractile dysfunction in mouse models of Cantú Syndrome with KATP channel gain-of-function

Cantú Syndrome (CS) is an autosomal dominant disorder caused by gain-of-function (GoF) mutations in the Kir6.1 and SUR2 subunits of KATP channels. KATP overactivity results in a chronic reduction in arterial tone and hypotension, leading to other systemic cardiovascular complications. However, the underlying mechanism of lymphedema, developed by >50% of CS patients, is unknown. We investigated whether lymphatic contractile dysfunction occurs in mice expressing CS mutations in Kir6.1 (Kir6.1[V65M]) or SUR2 (SUR2[A478V], SUR2[R1154Q]). Pressure myograph tests of contractile function of popliteal lymphatic vessels over the physiological pressure range revealed significantly impaired contractile strength and reduced frequency of spontaneous contractions at all pressures in heterozygous Kir6.1[V65M] vessels, compared to control littermates. Contractile dysfunction of intact popliteal lymphatics in vivo was confirmed using near-infrared fluorescence microscopy. Homozygous SUR2[A478V] vessels exhibited profound contractile dysfunction ex vivo, but heterozygous SUR2[A478V] vessels showed essentially normal contractile function. However, further investigation of vessels from all three GoF mouse strains revealed significant disruption in contraction wave entrainment, decreased conduction speed and distance, multiple pacemaker sites, and reversing wave direction. Tests of 2-valve lymphatic vessels forced to pump against an adverse pressure gradient revealed that all CS-associated genotypes were essentially incapable of pumping under an imposed outflow load. Our results show that varying degrees of lymphatic contractile dysfunction occur in proportion to the degree of molecular GoF in Kir6.1 or SUR2. This is the first example of lymphatic contractile dysfunction caused by a smooth muscle ion channel mutation and potentially explains the susceptibility of CS patients to lymphedema.

Long-Duration and Non-Invasive Photoacoustic Imaging of Multiple Anatomical Structures in a Live Mouse Using a Single Contrast Agent

Long-duration in vivo simultaneous imaging of multiple anatomical structures is useful for understanding physiological aspects of diseases, informative for molecular optimization in preclinical models, and has potential applications in surgical settings to improve clinical outcomes. Previous studies involving simultaneous imaging of multiple anatomical structures, for example, blood and lymphatic vessels as well as peripheral nerves and sebaceous glands, have used genetically engineered mice, which require expensive and time-consuming methods. Here, an IgG4 isotype control antibody is labeled with a near-infrared dye and injected into a mouse ear to enable simultaneous visualization of blood and lymphatic vessels, peripheral nerves, and sebaceous glands for up to 3 h using photoacoustic microscopy. For multiple anatomical structure imaging, peripheral nerves and sebaceous glands are imaged inside the injected dye-labeled antibody mass while the lymphatic vessels are visualized outside the mass. The efficacy of the contrast agent to label and localize deep medial lymphatic vessels and lymph nodes using photoacoustic computed tomography is demonstrated. The capability of a single injectable contrast agent to image multiple structures for several hours will potentially improve preclinical therapeutic optimization, shorten discovery timelines, and enable clinical treatments.

Lymphangiogenesis and Lymphatic Barrier Dysfunction in Renal Fibrosis

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

T helper 2 differentiation is necessary for development of lymphedema

T cells infiltrating lymphedematous tissues have a mixed T helper 1 (Th1) and Th2 differentiation profile. Treatment with neutralizing antibodies targeting cytokines that promote Th2 differentiation (interleukin 4 [IL-4] and IL-13) decreases the severity of lymphedema in preclinical models, suggesting that Th2 cells play a key role in the pathology of this disease. However, these previous studies do not address the contribution of Th1 cells and it remains unknown if IL-4 and IL-3 blockade acts primarily on T cells or decreases the pathological changes of lymphedema by other mechanisms. Therefore, this study sought to analyze the effect of lymphatic injury in transgenic mice with mutations that cause defects in Th1 and Th2 cell generation (T-bet knockout or T-betKO and STAT6 knockout or STAT6KO mice, respectively). Using both the mouse tail and popliteal lymph node dissection models of lymphedema, we show that Th2-deficient (STAT6KO) mice are protected from developing lymphedema, have decreased fibrosis, increased collateral vessel formation, and preserved collecting lymphatic vessel pumping function. In contrast, mice with defective Th1 cell generation (T-betKO) develop disease with the same severity as wild-type controls. Taken together, our results suggest that Th2 differentiation is necessary for development of lymphedema following lymphatic injury and that Th1 differentiation does not significantly contribute to the pathology of the disease. Such findings are important as immunotherapy directed at Th2 cells has been found to be effective in well-studied Th2-mediated diseases such as asthma and atopic dermatitis and may therefore be similarly useful for lymphedema management.

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.

Development of a mouse model for the visual and quantitative assessment of lymphatic trafficking and function by in vivo imaging

Methods for quantitative analysis of long distance lymphatic transport of nanoparticles in live animals are yet to be established. We established a mouse model for analysis of time-dependent transport just beneath the abdominal skin to investigate lymph node-to-lymph node trafficking by in vivo imaging. For this purpose, popliteal lymph nodes (PLNs) as well as efferent and afferent lymphatic vessels, marginal veins, and feeding blood vessels were surgically resected to change the lymphatic flow from footpad injections. Using this model, we observed a novel lymphatic flow from the footpad to the proper axillary lymph node (ALN) via the inguinal lymph node (ILN). This drainage pathway was maintained over 12 weeks. Time-dependent transportation of 1,1′-dioctadecyltetramethyl indotricarbocyanine iodide-labelled liposomes from the footpad to the ILN was successfully quantified by an in vivo imaging system. Moreover, congestion and development of a new collateral lymphatic route was visualised under a lymphedema status. Histological analysis of abdominal skin tissues of this model revealed that PLN resection had no effect on the abdominal lymphatic system between the ILN and ALN. These data indicate that this model might be useful to clarify the mechanisms of lymphedema and study direct transportation of lymph or other substances between lymph nodes.

Lymphatic function measurements influenced by contrast agent volume and body position

Several imaging modalities have been used to assess lymphatic function, including fluorescence microscopy, near-infrared fluorescence (NIRF) imaging, and Doppler optical coherence tomography (DOCT). They vary in how the mouse is positioned, the invasiveness of the experimental setup, and the volume of contrast agent injected. Here, we present how each of these experimental parameters affects functional measurements of collecting lymphatic vessels. First, fluorescence microscopy showed that supine mice have a statistically lower contraction frequency compared with mice sitting upright. To assess the effect of different injection volumes on these endpoints, mice were injected with 4, 10, or 20 μl of dye. The lowest frequencies were observed after 20-μl injections. Interestingly, lymph-flow DOCT revealed that although there was lower contraction frequency in mice injected with 20 μl versus 4 μl, mice showed a higher volumetric flow with a 20-μl injection. This indicates that contraction frequency alone is not sufficient to understand lymphatic transport. Finally, NIRF revealed that removing the skin reduced contraction frequency. Therefore, this study reveals how sensitive these techniques are to mouse position, removal of skin, and dye volume. Care should be taken when comparing results obtained under different experimental conditions.

Lymph Node Transplantation Decreases Swelling and Restores Immune Responses in a Transgenic Model of Lymphedema

INTRODUCTION

Secondary lymphedema is a common complication of cancer treatment and recent studies have demonstrated that lymph node transplantation (LNT) can decrease swelling, as well as the incidence of infections. However, although these results are exciting, the mechanisms by which LNT improves these pathologic findings of lymphedema remain unknown. Using a transgenic mouse model of lymphedema, this study sought to analyze the effect of LNT on lymphatic regeneration and T cell-mediated immune responses.

METHODS

We used a mouse model in which the expression of the human diphtheria toxin receptor is driven by the FLT4 promoter to enable the local ablation of the lymphatic system through subdermal hindlimb diphtheria toxin injections. Popliteal lymph node dissection was subsequently performed after a two-week recovery period, followed by either orthotopic LNT or sham surgery after an additional two weeks. Hindlimb swelling, lymphatic vessel regeneration, immune cell trafficking, and T cell-mediated immune responses were analyzed 10 weeks later.

RESULTS

LNT resulted in a marked decrease in hindlimb swelling, fibroadipose tissue deposition, and decreased accumulation of perilymphatic inflammatory cells, as compared to controls. In addition, LNT induced a marked lymphangiogenic response in both capillary and collecting lymphatic vessels. Interestingly, the resultant regenerated lymphatics were abnormal in appearance on lymphangiography, but LNT also led to a notable increase in dendritic cell trafficking from the periphery to the inguinal lymph nodes and improved adaptive immune responses.

CONCLUSIONS

LNT decreases pathological changes of lymphedema and was shown to potently induce lymphangiogenesis. Lymphatic vessels induced by LNT were abnormal in appearance, but were functional and able to transport antigen-presenting cells. Animals treated with LNT have an increased ability to mount T cell-mediated immune responses when sensitized to antigens in the affected hindlimb.

Obesity-induced lymphatic dysfunction is reversible with weight loss

KEY POINTS

Obesity induces lymphatic leakiness, decreases initial lymphatic vessel density, impairs collecting vessel pumping and decreases transport of macromolecules. Obesity results in perilymphatic inducible nitric oxide synthase (iNOS) expression and accumulation of T cells and macrophages. Deleterious effects of obesity on the lymphatic system correlate with weight gain. Weight loss restores lymphatic function in obese animals and decreases perilymphatic iNOS and inflammatory cell accumulation.

ABSTRACT

Although clinical and experimental studies have shown that obesity results in lymphatic dysfunction, it remains unknown whether these changes are permanent or reversible with weight loss. In the current study, we used a mouse model of diet-induced obesity to identify putative cellular mechanisms of obesity-induced lymphatic dysfunction, determine whether there is a correlation between these deleterious effects and increasing weight gain, and finally examine whether lymphatic dysfunction is reversible with diet-induced weight loss. We report that obesity is negatively correlated with cutaneous lymphatic collecting vessel pumping rate (r = -0.9812, P < 0.0005) and initial lymphatic vessel density (r = -0.9449, P < 0.005). In addition, we show a significant positive correlation between weight gain and accumulation of perilymphatic inflammatory cells (r = 0.9872, P < 0.0005) and expression of inducible nitric oxide synthase (iNOS; r = 0.9986, P < 0.0001). Weight loss resulting from conversion to a normal chow diet for 8 weeks resulted in more than a 25% decrease in body weight and normalized cutaneous lymphatic collecting vessel pumping rate, lymphatic vessel density, lymphatic leakiness, and lymphatic macromolecule clearance (all P < 0.05). In addition, weight loss markedly decreased perilymphatic inflammation and iNOS expression. Taken together, our findings show that obesity is linearly correlated with lymphatic dysfunction, perilymphatic inflammation and iNOS expression, and that weight loss via dietary modification effectively reverses these deleterious effects.

Inhibition of Inflammation and iNOS Improves Lymphatic Function in Obesity

Although recent studies have shown that obesity decreases lymphatic function, the cellular mechanisms regulating this response remain unknown. In the current study, we show that obesity results in perilymphatic accumulation of inflammatory cells and that local inhibition of this response with topical tacrolimus, an inhibitor of T cell differentiation, increases lymphatic vessel density, decreases perilymphatic iNOS expression, increases lymphatic vessel pumping frequency, and restores lymphatic clearance of interstitial fluid to normal levels. Although treatment of obese mice with 1400W, a selective inhibitor of iNOS, also improved lymphatic collecting vessel contractile function, it did not completely reverse lymphatic defects. Mice deficient in CD4(+) cells fed a high fat diet also gained weight relative to controls but were protected from lymphatic dysfunction. Taken together, our findings suggest that obesity-mediated lymphatic dysfunction is regulated by perilymphatic accumulation of inflammatory cells and that T cell inflammatory responses are necessary to initiate this effect.

Nodal lymph flow quantified with afferent vessel input function allows differentiation between normal and cancer-bearing nodes

Morbidity and complexity involved in lymph node staging via surgical resection and biopsy could ideally be improved using node assay techniques that are non-invasive. While visible blue dyes are often used to locate the sentinel lymph nodes from draining lymphatic vessels near a tumor, they do not provide an in situ metric to evaluate presence of cancer. In this study, the transport kinetics of methylene blue were analyzed to determine the potential for better in situ information about metastatic involvement in the nodes. A rat model with cancer cells in the axillary lymph nodes was used, with methylene blue injection to image the fluorescence kinetics. The lymphatic flow from injection sites to nodes was imaged and the relative kinetics from feeding lymphatic ducts relative to lymph nodes was quantified. Large variability existed in raw fluorescence and transport patterns within each cohort resulting in no systematic difference between average nodal uptake in normal, sham control and cancer-bearing nodes. However, when the signal from the afferent lymph vessel fluorescence was used to normalize the signal of the lymph nodes, the high signal heterogeneity was reduced. Using a model, the lymph flow through the nodes [Formula: see text] was estimated to be 1.49 ± 0.64 ml/g/min in normal nodes, 1.53 ± 0.45 ml/g/min in sham control nodes, and reduced to 0.50 ± 0.24 ml/g/min in cancer-cell injected nodes. This summarizes the significant difference (p = 0.0002) between cancer-free and cancer-bearing nodes in normalized flow. This process of normalized flow imaging could be used as an in situ tool to detect metastatic involvement in nodes.

Method for the quantitative measurement of collecting lymphatic vessel contraction in mice

Collecting lymphatic vessels are critical for the transport of lymph and its cellular contents to lymph nodes for both immune surveillance and the maintenance of tissue-fluid balance. Collecting lymphatic vessels drive lymph flow by autonomous contraction of smooth muscle cells that cover these vessels. Here we describe methods using intravital microscopy to image and quantify collecting lymphatic vessel contraction in mice. Our methods allow for the measurement of the strength of lymphatic contraction of an individual lymphangion in a mouse, which has not yet been demonstrated using other published methods. The ability to study murine collecting lymphatic vessel contraction-using the methods described here or other recently published techniques-allows the field to dissect the molecular mechanisms controlling lymphatic pumping under normal and pathological conditions using the wide variety of molecular tools and genetic models available in the mouse. We have used our methods to study lymphatic contraction in physiological and inflammatory conditions. The methods described here will facilitate the further study of lymphatic function in other pathological conditions that feature lymphatic complications.

Imaging the lymphatic system

Visualization of the lymphatic system is clinically necessary during diagnosis or treatment of many conditions and diseases; it is used for identifying and monitoring lymphedema, for detecting metastatic lesions during cancer staging and for locating lymphatic structures so they can be spared during surgical procedures. Imaging lymphatic anatomy and function also plays an important role in experimental studies of lymphatic development and function, where spatial resolution and accessibility are better. Here, we review technologies for visualizing and imaging the lymphatic system for clinical applications. We then describe the use of lymphatic imaging in experimental systems as well as some of the emerging technologies for improving these methodologies.

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.

Lymphatic vascular response to acute inflammation

During acute inflammation, functioning lymphatics are believed to reduce edema and to provide a transiting route for immune cells, but the extent at which the dermal lymphatic remodeling impacts lymphatic transport or the factors regulating these changes remains unclear. Herein we quantify the increase in lymphatic endothelial cells (LECs) and examine the expression of pro-angiogenenic and lymphangiogenic factors during acute cutaneous hypersensitivity (CHS). We found that LECs actively proliferate during CHS but that this proliferation does not affect the lymphatic vessel density. Instead, lymphatic remodeling is accompanied by lymphatic vessel leakiness and lower ejection of lymph fluid, which is observed only in the proximal lymphatic vessel draining the inflamed area. LECs and the immune cells release growth factors and cytokines during inflammation, which impact the lymphatic microenvironment and function. We identified that FGF-2, PLGF-2, HGF, EGF, and KC/CXCL17 are differentially expressed within tissues during acute CHS, but both VEGF-C and VEGF-D levels do not significantly change. Our results indicate that VEGF-C and VEGF-D are not the only players and other factors may be responsible for the LECs proliferation and altered lymphatic function in acute CHS.