Lymphatic function is regulated by a coordinated expression of lymphangiogenic and anti-lymphangiogenic cytokines

Interactions of immune cells and lymphatic vessels

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

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

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

The inflammatory response of lymphatic endothelium

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

Bone morphogenetic protein 2 signaling negatively modulates lymphatic development in vertebrate embryos

RATIONALE

The emergence of lymphatic endothelial cells (LECs) seems to be highly regulated during development. Although several factors that promote the differentiation of LECs in embryonic development have been identified, those that negatively regulate this process are largely unknown.

OBJECTIVE

Our aim was to delineate the role of bone morphogenetic protein (BMP) 2 signaling in lymphatic development.

METHODS AND RESULTS

BMP2 signaling negatively regulates the formation of LECs. Developing LECs lack any detectable BMP signaling activity in both zebrafish and mouse embryos, and excess BMP2 signaling in zebrafish embryos and mouse embryonic stem cell-derived embryoid bodies substantially decrease the emergence of LECs. Mechanistically, BMP2 signaling induces expression of miR-31 and miR-181a in a SMAD-dependent mechanism, which in turn results in attenuated expression of prospero homeobox protein 1 during development.

CONCLUSIONS

Our data identify BMP2 as a key negative regulator for the emergence of the lymphatic lineage during vertebrate development.

Anti-lymphangiogenesis effects of a specific anti-interleukin 7 receptor antibody in lung cancer model in vivo

Interleukin 7 (IL-7) is known to promote lymphangiogenesis. To study the relationship between IL-7 and the lymphangiogenesis in lung cancer cells xenograft tumors, we investigated how IL-7 regulates lymphangiogenesis by Quantitative real-time reverse transcriptase-polymerase chain reaction, Western blot, co-immunoprecipitation, chromatin immunoprecipitation, and immunohistochemistry methods. We found that, in lung cancer cells xenograft tumors IL-7/IL-7 receptor (IL-7R) increase the expression of VEGF-D and lymphangiogenesis, induce c-Fos and c-Jun heterodimer formation, and enhance c-Fos/c-Jun DNA binding activity to regulate VEGF-D. Taken together, our results provided evidence that IL-7/IL-7R induce VEGF-D upregulation and promote lymphangiogenesis via c-Fos/c-Jun pathway in lung cancer.

Obesity impairs lymphatic fluid transport and dendritic cell migration to lymph nodes

INTRODUCTION

Obesity is a major cause of morbidity and mortality resulting in pathologic changes in virtually every organ system. Although the cardiovascular system has been a focus of intense study, the effects of obesity on the lymphatic system remain essentially unknown. The purpose of this study was to identify the pathologic consequences of diet induced obesity (DIO) on the lymphatic system.

METHODS

Adult male wild-type or RAG C57B6-6J mice were fed a high fat (60%) or normal chow diet for 8-10 weeks followed by analysis of lymphatic transport capacity. In addition, we assessed migration of dendritic cells (DCs) to local lymph nodes, lymph node architecture, and lymph node cellular make up.

RESULTS

High fat diet resulted in obesity in both wild-type and RAG mice and significantly impaired lymphatic fluid transport and lymph node uptake; interestingly, obese wild-type but not obese RAG mice had significantly impaired migration of DCs to the peripheral lymph nodes. Obesity also resulted in significant changes in the macro and microscopic anatomy of lymph nodes as reflected by a marked decrease in size of inguinal lymph nodes (3.4-fold), decreased number of lymph node lymphatics (1.6-fold), loss of follicular pattern of B cells, and dysregulation of CCL21 expression gradients. Finally, obesity resulted in a significant decrease in the number of lymph node T cells and increased number of B cells and macrophages.

CONCLUSIONS

Obesity has significant negative effects on lymphatic transport, DC cell migration, and lymph node architecture. Loss of T and B cell inflammatory reactions does not protect from impaired lymphatic fluid transport but preserves DC migration capacity. Future studies are needed to determine how the interplay between diet, obesity, and the lymphatic system modulate systemic complications of obesity.

Prospective transcriptomic pathway analysis of human lymphatic vascular insufficiency: identification and validation of a circulating biomarker panel

BACKGROUND

In our previous transcriptional profiling of a murine model, we have identified a remarkably small number of specific pathways with altered expression in lymphedema. In this investigation, we utilized microarray-based transcriptomics of human skin for an unbiased a priori prospective candidate identification, with subsequent validation of these candidates through direct serum assay. The resulting multi-analyte biomarker panel sensitively should sensitively discriminate human lymphedema subjects from normal individuals.

METHODS AND FINDINGS

We enrolled 63 lymphedema subjects and 27 normals in our attempt to discover protein analytes that can distinguish diseased individuals from controls. To minimize technical and biologically irrelevant variation, we first identified potential candidates by performing transcriptional microarray analysis on paired diseased and normal skin specimens sampled from the same individuals. We focused our attention on genes with corresponding protein products that are secreted and took these candidates forward to a protein multiplex assay applied to diseased and normal subjects. We developed a logistic regression-based model on an eventual group of six proteins and validated our system on a separate cohort of study subjects. The area under the receiver operating characteristic curve was calculated to be 0.87 (95% CI : 0.75 to 0.97).

CONCLUSIONS

We have developed an accurate bioassay utilizing proteins representing four central pathogenetic modalities of the disease: lymphangiogenesis, inflammation, fibrosis, and lipid metabolism, suggesting that these proteins are directly related to the pathogenesis of the tissue pathology in lymphatic vascular insufficiency. Further studies are warranted to determine whether this newly-identified biomarker panel will possess utility as an instrument for in vitro diagnosis of early and latent disease; the ultimate applicability to risk stratification, quantitation of disease burden, and response to therapy can easily be envisioned.

Th2 differentiation is necessary for soft tissue fibrosis and lymphatic dysfunction resulting from lymphedema

Lymphedema is a dreaded complication of cancer treatment. However, despite the fact that >5 million Americans are affected by this disorder, the development of effective treatments is limited by the fact that the pathology of lymphedema remains unknown. The purpose of these studies was to determine the role of inflammatory responses in lymphedema pathology. Using mouse models of lymphedema, as well as clinical lymphedema specimens, we show that lymphatic stasis results in a CD4 T-cell inflammation and T-helper 2 (Th2) differentiation. Using mice deficient in T cells or CD4 cells, we show that this inflammatory response is necessary for the pathological changes of lymphedema, including fibrosis, adipose deposition, and lymphatic dysfunction. Further, we show that inhibition of Th2 differentiation using interleukin-4 (IL-4) or IL-13 blockade prevents initiation and progression of lymphedema by decreasing tissue fibrosis and significantly improving lymphatic function, independent of lymphangiogenic growth factors. We show that CD4 inflammation is a critical regulator of tissue fibrosis and lymphatic dysfunction in lymphedema and that inhibition of Th2 differentiation markedly improves lymphatic function independent of lymphangiogenic cytokine expression. Notably, preventing and/or reversing the development of pathological tissue changes that occur in lymphedema may be a viable treatment strategy for this disorder.

CD4(+) cells regulate fibrosis and lymphangiogenesis in response to lymphatic fluid stasis

Introduction

Lymphedema is a chronic disorder that occurs commonly after lymph node removal for cancer treatment and is characterized by swelling, fibrosis, inflammation, and adipose deposition. Although previous histological studies have investigated inflammatory changes that occur in lymphedema, the precise cellular make up of the inflammatory infiltrate remains unknown. It is also unclear if this inflammatory response plays a causal role in the pathology of lymphedema. The purpose of this study was therefore to characterize the inflammatory response to lymphatic stasis and determine if these responses are necessary for the pathological changes that occur in lymphedema.

Methods

We used mouse-tail lymphedema and axillary lymph node dissection (ANLD) models in order to study tissue inflammatory changes. Single cell suspensions were created and analyzed using multi-color flow cytometry to identify individual cell types. We utilized antibody depletion techniques to analyze the causal role of CD4+, CD8+, and CD25+ cells in the regulation of inflammation, fibrosis, adipose deposition, and lymphangiogenesis.

Results

Lymphedema in the mouse-tail resulted in a mixed inflammatory cell response with significant increases in T-helper, T-regulatory, neutrophils, macrophages, and dendritic cell populations. Interestingly, we found that ALND resulted in significant increases in T-helper cells suggesting that these adaptive immune responses precede changes in macrophage and dendritic cell infiltration. In support of this we found that depletion of CD4+, but not CD8 or CD25+ cells, significantly decreased tail lymphedema, inflammation, fibrosis, and adipose deposition. In addition, depletion of CD4+ cells significantly increased lymphangiogenesis both in our tail model and also in an inflammatory lymphangiogenesis model.

Conclusions

Lymphedema and lymphatic stasis result in CD4+ cell inflammation and infiltration of mature T-helper cells. Loss of CD4+ but not CD8+ or CD25+ cell inflammation markedly decreases the pathological changes associated with lymphedema. In addition, CD4+ cells regulate lymphangiogenesis during wound repair and inflammatory lymphangiogenesis.

Microvascular filtration is increased in the forearms of patients with breast cancer-related lymphedema

Breast cancer-related lymphedema (BCRL) is a frequent and debilitating complication of breast cancer treatment. The pathophysiology is complex and remains poorly understood; however, data suggest that changes in the peripheral circulation may contribute to edema formation. In 13 volunteers with unilateral BCRL, the following aspects of upper extremity peripheral circulation were examined: muscle relative microvascular volume; capillary filtration coefficient; central and local sympathetic vascular reflexes; skin blood flow; and forearm blood flow. These were studied via real-time, contrast-enhanced ultrasound; venous occlusion strain-gauge plethysmography; lower-body negative pressure; noninvasive blood pressure measurements; and skin (99m)Tc-pertechnetate clearance technique. Measurements were performed bilaterally and simultaneously in the forearms, enabling use of the nonedematous forearm as a control. Capillary filtration coefficients were additionally measured in healthy, age-matched controls. The capillary filtration coefficient was 7.98 ± 2.52 μl·100 ml(-1)·mmHg(-1)·min(-1) (mean ± SD) in edematous forearms and 6.09 ± 1.83 μl·100ml·(-1)·mmHg(-1)·min(-1) in nonedematous forearms in the patient group (P < 0.001). The capillary filtration coefficient was 3.32 ± 1.17 μl·100ml(-1)·mmHg(-1)·min(-1) in the forearms of healthy controls; significantly less than the both the edematous and nonedematous forearms of the patient group (P < 0.001). No significant differences were found in muscle relative microvascular volume, forearm blood flow, skin blood flow, or central or local sympathetic vascular reflexes. Forearm microvascular filtration is increased in patients with BCRL, and more so in the edematous arm. The vascular sympathetic control mechanisms seem to be preserved. We propose that the increased capillary permeability may be due to low-grade inflammation promoted by reduced clearance of inflammatory mediators.

Vascular endothelial growth factor-C and -D are involved in lymphangiogenesis in mouse unilateral ureteral obstruction

Lymphatic remodeling in inflammation has been found in tracheal mycoplasma infection, human kidney transplant, skin inflammation, peritonitis, and corneal inflammation. Here we investigated lymphangiogenesis in fibrotic area in unilateral ureteral obstruction, a model of progressive renal fibrosis, and evaluated the roles of vascular endothelial growth factor (VEGF)-C and -D in the obstructed kidney. Compared to sham-operated mice, the number of LYVE-1-positive lymphatic vessels, the proliferation of LYVE-1-positive lymphatic endothelial cells, along with VEGF-C and -D mRNA expression were all significantly increased following ureteral obstruction. Depletion of macrophages with clodronate decreased lymphangiogenesis in the obstructed kidney. VEGF-C expression was higher in M2- than in M1-polarized macrophages from bone marrow-derived macrophages, and also increased in Raw 264.7 or renal proximal tubule cells by stimulation with TGF-β1 or TNF-α. VEGF-D reversed the inhibitory effect of TGF-β1 on VEGF-C-induced migration, capillary-like tube formation, and proliferation of human lymphatic endothelial cells. Additionally, the blockade of VEGF-C and VEGF-D signaling decreased obstruction-induced lymphangiogenesis. Thus, VEGF-C and VEGF-D are associated with lymphangiogenesis in the fibrotic kidney in a mouse model of ureteral obstruction.

Analysis of tumor-induced lymphangiogenesis and lymphatic vessel invasion of pancreatic carcinoma in the peripheral nerve plexus

Cancer cells can metastasize throughout the body by various mechanisms, including the lymphatic system, resulting in tumor-induced lymphangiogenesis that can profoundly affect patient survival. The aim of the present study was to examine the role of lymphangiogenesis in the metastasis of pancreatic cancer to the peripheral nerve plexus. Immunohistochemistry was performed to analyze specimens obtained from 70 ductal adenocarcinoma patients. The markers used included lymphangiogenic factor vascular endothelial growth factor (VEGF)-C, the lymphatic-specific marker D2-40, and cytokeratin 19, an independent prognostic factor for pancreatic tumors. The relationship between survival rate and invasion of both the lymphatic vessels and peripancreatic nerve plexus (PNP) was evaluated, with clearly elevated lymphatic vessel density (LVD) in tissues adjacent to the cancer tissues. In fact, LVD levels were higher in adjacent tissues than in localized cancer tissues, and lymphatic vessel invasion into tissues adjacent to the tumor was significantly correlated with both PNP invasion (P = 0.005) and lymph node metastasis (P = 0.010). Correspondingly, LVD in tissues adjacent to the tumor was correlated with both invasion of lymphatic vessels surrounding the tumor (P = 0.024) and VEGF-C expression (P = 0.031); in addition, VEGF-C expression was correlated with invasion of lymphatic vessels around the tumor (P = 0.004). Survival rates were significantly lower in patients in whom there was peritumor lymphatic vessel invasion (P < 0.001), extrapancreatic nerve plexus invasion (P = 0.001), and/or lymph node metastasis (P < 0.001). Based on these results, lymphatic invasion associated with adjacent tumor growth likely contributes to the development of metastatic tumors that invade the PNP.

Regulation and implications of inflammatory lymphangiogenesis

Lymphatic vessels (LVs) are highly dynamic structures that intimately interact with their surrounding microenvironment. They have a profound influence on the immune system and therefore can manipulate inflammatory processes. Inflammation is a major cause of adulthood lymphangiogenesis and LV remodeling. In turn, LVs can reciprocally manipulate inflammatory processes. For instance, LV growth and/or activation regulate antigen presentation and inflammatory cell recruitment to lymph nodes (LNs), and therefore critically affect adaptive immunity. The vascular endothelial growth factor (VEGF)-C-VEGF receptor-3 and VEGF-A-VEGF receptor-2 signaling pathways are particularly important in inflammatory lymphangiogenesis. LVs contribute to the pathophysiology of various inflammatory conditions. Knowledge of lymphatic biology can be applied to manipulate inflammatory disorders and divert immune responses. This review summarizes basic concepts of inflammation-relevant lymphatic biology, and describes recent progress and practical implications.