IntroductionObesity 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.MethodsAdult 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.ResultsHigh 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.ConclusionsObesity 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.

Background Although lymph node transplantation has been shown to improve lymphatic function the mechanisms regulating lymphatic vessel reconnection and functional status of lymph nodes remains poorly understood. Methods We developed and used LacZ lymphatic reporter mice to examine the lineage of lymphatic vessels infiltrating transferred lymph nodes. In addition, we analyzed lymphatic function, expression of vascular endothelial growth factor (VEGF-C), maintenance of T and B cell zone, and anatomic localization of lymphatics and high endothelial venules (HEVs). Results Reporter mice were specific and highly sensitive in identifying lymphatic vessels. Lymph node transfer was associated with rapid return of lymphatic function and clearance of Tc99 secondary to a massive infiltration of recipient mouse lymphatics and putative connections to donor lymphatics. T and B cell populations in the lymph node were maintained. These changes correlated with marked increases in the expression of VEGF-C in the perinodal fat and infiltrating lymphatics. Newly formed lymphatic channels in transferred lymph nodes were in close anatomic proximity to HEVs. Conclusions Transferred lymph nodes have rapid infiltration of functional host lymphatic vessels and maintain T and B cell populations. This process correlates with increased endogenous expression of VEGF-C in the perinodal fat and infiltrating lymphatics. Anatomic proximity of newly formed lymphatics and HEVs supports the hypothesis that lymph node transfer can improve lymphedema by exchanges with the systemic circulation.

Background/Objectives High-fat diet (HFD)-induced obesity has significant negative effects on lymphatic function, but it remains unclear whether this is a direct effect of HFD or secondary to adipose tissue deposition. Methods We compared the effects of HFD on obesity-prone and obesity-resistant mice and analyzed lymphatic function in vivo and in vitro. Results Only obesity-prone mice had impaired lymphatic function, increased perilymphatic inflammation, and accumulation of lipid droplets surrounding their lymphatic endothelial cells (LECs). LECs isolated from obesity-prone mice, in contrast to obesity-resistant animals, had decreased expression of VEGFR-3 and Prox1. Exposure of LECs to a long-chain free fatty acid increased cellular apoptosis and decreased VEGFR-3 expression, while inhibition of intracellular inhibitors of VEGFR-3 signaling pathways increased cellular viability. Conclusions Collectively, our studies suggest that HFD-induced obesity decreases lymphatic function by increasing perilymphatic inflammation and altering LEC gene expression. Reversal of diminished VEGFR-3 signaling may rescue this phenotype and improve lymphatic function.

While acute tissue injury potently induces endogenous danger signal expression, the role of these molecules in chronic wound healing and lymphedema is undefined. The purpose of this study was to determine the spatial and temporal expression patterns of the endogenous danger signals high-mobility group box 1 (HMGB1) and heat shock protein (HSP)70 during wound healing and chronic lymphatic fluid stasis. In a surgical mouse tail model of tissue injury and lymphedema, HMGB1 and HSP70 expression occurred along a spatial gradient relative to the site of injury, with peak expression at the wound and greater than twofold reduced expression within 5 mm (P < 0.05). Expression primarily occurred in cells native to injured tissue. In particular, HMGB1 was highly expressed by lymphatic endothelial cells (>40% positivity; twofold increase in chronic inflammation, P < 0.001). We found similar findings using a peritoneal inflammation model. Interestingly, upregulation of HMGB1 (2.2-fold), HSP70 (1.4-fold), and nuclear factor (NF)-κβ activation persisted at least 6 wk postoperatively only in lymphedematous tissues. Similarly, we found upregulation of endogenous danger signals in soft tissue of the arm after axillary lymphadenectomy in a mouse model and in matched biopsy samples obtained from patients with secondary lymphedema comparing normal to lymphedematous arms (2.4-fold increased HMGB1, 1.9-fold increased HSP70; P < 0.01). Finally, HMGB1 blockade significantly reduced inflammatory lymphangiogenesis within inflamed draining lymph nodes (35% reduction, P < 0.01). In conclusion, HMGB1 and HSP70 are expressed along spatial gradients and upregulated in chronic lymphatic fluid stasis. Furthermore, acute expression of endogenous danger signals may play a role in inflammatory lymphangiogenesis.

Mechanisms regulating lymphedema pathogenesis remain unknown. Recently, we have shown that lymphatic fluid stasis increases endogenous danger signal expression, and these molecules influence lymphatic repair (Zampbell JC, et al. Am J Physiol Cell Physiol 300: C1107-C1121, 2011). Endogenous danger signals activate Toll-like receptors (TLR) 2, 4, and 9 and induce homeostatic or harmful responses, depending on physiological context. The purpose of this study was to determine the role of TLRs in regulating tissue responses to lymphatic fluid stasis. A surgical model of lymphedema was used in which wild-type or TLR2, 4, or 9 knockout (KO) mice underwent tail lymphatic excision. Six weeks postoperatively, TLR KOs demonstrated markedly increased tail edema compared with wild-type animals (50-200% increase; P < 0.01), and this effect was most pronounced in TLR4 KOs (P < 0.01). TLR deficiency resulted in decreased interstitial and lymphatic transport, abnormal lymphatic architecture, and fewer capillary lymphatics (40-50% decrease; P < 0.001). Lymphedematous tissues of TLR KOs demonstrated increased leukocyte infiltration (P < 0.001 for TLR4 KOs), including higher numbers of infiltrating CD3+ cells (P < 0.05, TLR4 and TLR9 KO), yet decreased infiltrating F4/80+ macrophages (P < 0.05, all groups). Furthermore, analysis of isolated macrophages revealed twofold reductions in VEGF-C (P < 0.01) and LYVE-1 (P < 0.05) mRNA from TLR2-deficient animals. Finally, TLR deficiency was associated with increased collagen type I deposition and increased transforming growth factor-β1 expression (P < 0.01, TLR4 and TLR9 KO), contributing to dermal fibrosis. In conclusion, TLR deficiency worsens tissue responses to lymphatic fluid stasis and is associated with decreased lymphangiogenesis, increased fibrosis, and reduced macrophage infiltration. These findings suggest a role for innate immune responses, including TLR signaling, in lymphatic repair and lymphedema pathogenesis.

IntroductionLymphedema is the chronic swelling of an extremity that occurs commonly after lymph node resection for cancer treatment. Recent studies have demonstrated that transfer of healthy tissues can be used as a means of bypassing damaged lymphatics and ameliorating lymphedema. The purpose of these studies was to investigate the mechanisms that regulate lymphatic regeneration after tissue transfer.MethodsNude mice (recipients) underwent 2-mm tail skin excisions that were either left open or repaired with full-thickness skin grafts harvested from donor transgenic mice that expressed green fluorescent protein in all tissues or from LYVE-1 knockout mice. Lymphatic regeneration, expression of VEGF-C, macrophage infiltration, and potential for skin grafting to bypass damaged lymphatics were assessed.ResultsSkin grafts healed rapidly and restored lymphatic flow. Lymphatic regeneration occurred beginning at the peripheral edges of the graft, primarily from ingrowth of new lymphatic vessels originating from the recipient mouse. In addition, donor lymphatic vessels appeared to spontaneously re-anastomose with recipient vessels. Patterns of VEGF-C expression and macrophage infiltration were temporally and spatially associated with lymphatic regeneration. When compared to mice treated with excision only, there was a 4-fold decrease in tail volumes, 2.5-fold increase in lymphatic transport by lymphoscintigraphy, 40% decrease in dermal thickness, and 54% decrease in scar index in skin-grafted animals, indicating that tissue transfer could bypass damaged lymphatics and promote rapid lymphatic regeneration.ConclusionsOur studies suggest that lymphatic regeneration after tissue transfer occurs by ingrowth of lymphatic vessels and spontaneous re-connection of existing lymphatics. This process is temporally and spatially associated with VEGF-C expression and macrophage infiltration. Finally, tissue transfer can be used to bypass damaged lymphatics and promote rapid lymphatic regeneration.

Background Although fat deposition is a defining clinical characteristic of lymphedema, the cellular mechanisms that regulate this response remain unknown. The goals of this two-part study were to determine the effect of lymphatic fluid stasis on adipogenesis and inflammation (part 1) and how these changes regulate the temporal and spatial expression of fat differentiation genes (part 2). Methods Adult female mice underwent tail lymphatic ablation and were sacrificed 6 weeks after surgery (n=20). Fat deposition, fibrosis, and inflammation were then analyzed in the regions of the tail exposed to lymphatic fluid stasis as compared with normal lymphatic flow. Results Lymphatic fluid stasis in the tail resulted in significant subcutaneous fat deposition with a 2-fold increase in fat thickness (p<0.01). In addition, lymphatic stasis was associated with subcutaneous fat fibrosis and collagen deposition. Adipogenesis in response to lymphatic fluid stasis was associated with a marked mononuclear cell inflammatory response (5-fold increase in CD45+ cells; p<0.001). In addition, we noted a significant increase in the number of monocytes/macrophages as identified by F4/80 immunohistochemistry (p<0.001). Conclusions The mouse-tail model has pathological findings that are similar to clinical lymphedema including fat deposition, fibrosis, and inflammation. Adipogenesis in response to lymphatic fluid stasis closely resembles this process in obesity. This model therefore provides an excellent means to study the molecular mechanisms that regulate the pathophysiology of lymphedema.

Background Although fat deposition is a defining clinical characteristic of lymphedema, the cellular mechanisms that regulate this response remain unknown. The goal of this study was to determine how lymphatic fluid stasis regulates adipogenic gene activation and fat deposition. Methods Adult female mice underwent tail lymphatic ablation and sacrifice at 1, 3, or 6 weeks post-operatively (n=8/group). Samples were analyzed by immunohistochemistry and western blot. An alternative group of mice underwent axillary dissections or sham incisions and limb tissues were harvested 3 weeks post-operatively (n=8/group). Results Lymphatic fluid stasis resulted in significant subcutaneous fat deposition and fibrosis in lymphedematous tail regions (p<0.001). Western blot analysis demonstrated that proteins regulating adipose differentiation including CCAAT/enhancer binding protein-alpha (CEBP-α) and adiponectin were markedly upregulated in response to lymphatic fluid stasis in the tail and axillary models. Expression of these markers increased in edematous tissues according to the gradient of lymphatic stasis distal to the wound. Immunohistochemical analysis further demonstrated that adiponectin and peroxisome proliferator-activated receptor gamma (PPAR-γ), another critical adipogenic transcription factor, followed similar expression gradients. Finally, adiponectin and PPAR-γ expression localized to a variety of cell types in newly formed subcutaneous fat. Conclusions The mouse-tail model of lymphedema demonstrates pathological findings similar to clinical lymphedema including fat deposition and fibrosis. We show that lymphatic fluid stasis potently upregulates the expression of fat differentiation markers both spatially and temporally. These studies elucidate mechanisms regulating abnormal fat deposition in lymphedema pathogenesis and therefore provide a basis for developing targeted treatments.