Chronic Rejection of Cardiac Allografts Is Associated With Increased Lymphatic Flow and Cellular Trafficking

Disease-Specific Alteration of Cardiac Lymphatics: A Review from Animal Disease Models to Clinics

For many years, the significance of cardiac lymphatic vessels was largely overlooked in clinical practice, with little consideration given to their role in the pathophysiology or treatment of cardiac diseases. However, recent research has brought renewed attention to these vessels, progressively illuminating their function and importance within the realm of cardiovascular science. Experimental studies, particularly those utilizing animal models of cardiac disease, have demonstrated a clear relationship between cardiac lymphatic vessels and both the pathogenesis and progression of these conditions. These findings have prompted a growing interest in potential therapeutic applications that specifically target the cardiac lymphatic system. Conversely, while clinical investigations into cardiac lymphatics remain limited, recent studies have begun to explore their identification through specific surface markers, as well as the expression dynamics of lymphangiogenic factors. These studies have increasingly highlighted associations of lymphatic dysfunction with inflammation and fibrosis, both of which negatively impact cardiac function and remodeling across various pathological states. Despite these advances, comprehensive reviews of the current knowledge regarding the cardiac lymphatic vasculature, particularly within specific disease contexts, remain scarce. This review aims to address this gap by providing a detailed synthesis of existing reports, encompassing both animal model research and studies on human clinical specimens, with a special focus on the role of cardiac lymphatic vessels in different disease states.

Role of the Lymphatics in Cardiac Disease

Cardiovascular diseases remain the largest cause of death worldwide with recent evidence increasingly attributing the development and progression of these diseases to an exacerbated inflammatory response. As a result, significant research is now focused on modifying the immune environment to prevent the disease progression. This in turn has highlighted the lymphatic system in the pathophysiology of cardiovascular diseases owing, in part, to its established function in immune cell surveillance and trafficking. In this review, we highlight the role of the cardiac lymphatic system and its potential as an immunomodulatory therapeutic target in selected cardiovascular diseases.

Alkali injury-induced pathological lymphangiogenesis in the iris facilitates the infiltration of T cells and ocular inflammation

Inflammatory lymphangiogenesis is intimately linked to immune regulation and tissue homeostasis. However, current evidence has suggested that classic lymphatic vessels are physiologically absent in intraocular structures. Here, we show that neolymphatic vessels were induced in the iris after corneal alkali injury (CAI) in a VEGFR3-dependent manner. Cre-loxP-based lineage tracing revealed that these lymphatic endothelial cells (LECs) originate from existing Prox1+ lymphatic vessels. Notably, the ablation of iridial lymphangiogenesis via conditional deletion of VEGFR3 alleviated the ocular inflammatory response and pathological T cell infiltration. Our findings demonstrate that iridial neolymphatics actively participate in pathological immune responses following injury and suggest intraocular lymphangiogenesis as a valuable therapeutic target for the treatment of ocular inflammation.

Fibroblasts facilitate lymphatic vessel formation in transplanted heart

Rationale: Lymphangiogenesis plays a critical role in the transplanted heart. The remodeling of lymphatics in the transplanted heart and the source of newly formed lymphatic vessels are still controversial, especially the mechanism of lymphangiogenesis remains limited. Methods: Heart transplantation was performed among BALB/c, C57BL/6J, Cag-Cre, Lyve1-CreERT2;Rosa26-tdTomato and Postn(2A-CreERT2-wpre-pA)1;Rosa26-DTA mice. scRNA-seq, Elisa assay, Western blotting, Q-PCR and immunohistochemical staining were used to identify the cells and cell-cell communications of allograft heart. Cell depletion was applied to in vivo and in vitro experiments. Whole-mount staining and three-dimensional reconstruction depicted the cell distribution within transparent transplanted heart. Results: Genetic lineage tracing mice and scRNA-seq analysis have revealed that these newly formed lymphatic vessels mainly originate from recipient LYVE1+ cells. It was found that LECs primarily interact with activated fibroblasts. Inhibition of lymphatic vessel formation using a VEGFR3 inhibitor resulted in a decreased survival time of transplanted hearts. Furthermore, when activated fibroblasts were ablated in transplanted hearts, there was a significant suppression of lymphatic vessel generation, leading to earlier graft failure. Additional investigations have shown that activated fibroblasts promote tube formation of LECs primarily through the activation of various signaling pathways, including VEGFD/VEGFR3, MDK/NCL, and SEMA3C/NRP2. Interestingly, knockdown of VEGFD and MDK in activated fibroblasts impaired cardiac lymphangiogenesis after heart transplantation. Conclusions: Our study indicates that cardiac lymphangiogenesis primarily originates from recipient cells, and activated fibroblasts play a crucial role in facilitating the generation of lymphatic vessels after heart transplantation. These findings provide valuable insights into potential therapeutic targets for enhancing graft survival.

Lymphatic vessel: origin, heterogeneity, biological functions, and therapeutic targets

Lymphatic vessels, comprising the secondary circulatory system in human body, play a multifaceted role in maintaining homeostasis among various tissues and organs. They are tasked with a serious of responsibilities, including the regulation of lymph absorption and transport, the orchestration of immune surveillance and responses. Lymphatic vessel development undergoes a series of sophisticated regulatory signaling pathways governing heterogeneous-origin cell populations stepwise to assemble into the highly specialized lymphatic vessel networks. Lymphangiogenesis, as defined by new lymphatic vessels sprouting from preexisting lymphatic vessels/embryonic veins, is the main developmental mechanism underlying the formation and expansion of lymphatic vessel networks in an embryo. However, abnormal lymphangiogenesis could be observed in many pathological conditions and has a close relationship with the development and progression of various diseases. Mechanistic studies have revealed a set of lymphangiogenic factors and cascades that may serve as the potential targets for regulating abnormal lymphangiogenesis, to further modulate the progression of diseases. Actually, an increasing number of clinical trials have demonstrated the promising interventions and showed the feasibility of currently available treatments for future clinical translation. Targeting lymphangiogenic promoters or inhibitors not only directly regulates abnormal lymphangiogenesis, but improves the efficacy of diverse treatments. In conclusion, we present a comprehensive overview of lymphatic vessel development and physiological functions, and describe the critical involvement of abnormal lymphangiogenesis in multiple diseases. Moreover, we summarize the targeting therapeutic values of abnormal lymphangiogenesis, providing novel perspectives for treatment strategy of multiple human diseases.

Dysregulation of Lymphatic Endothelial VEGFR3 Signaling in Disease

Vascular endothelial growth factor (VEGF) receptor 3 (VEGFR3), a receptor tyrosine kinase encoded by the FLT4 gene, plays a significant role in the morphogenesis and maintenance of lymphatic vessels. Under both normal and pathologic conditions, VEGF-C and VEGF-D bind VEGFR3 on the surface of lymphatic endothelial cells (LECs) and induce lymphatic proliferation, migration, and survival by activating intracellular PI3K-Akt and MAPK-ERK signaling pathways. Impaired lymphatic function and VEGFR3 signaling has been linked with a myriad of commonly encountered clinical conditions. This review provides a brief overview of intracellular VEGFR3 signaling in LECs and explores examples of dysregulated VEGFR3 signaling in various disease states, including (1) lymphedema, (2) tumor growth and metastasis, (3) obesity and metabolic syndrome, (4) organ transplant rejection, and (5) autoimmune disorders. A more complete understanding of the molecular mechanisms underlying the lymphatic pathology of each disease will allow for the development of novel strategies to treat these chronic and often debilitating illnesses.

Role of Lymphangiogenesis in Cardiac Repair and Regeneration

This article highlights the importance of the structure and function of cardiac lymphatics in cardiovascular diseases and the therapeutic potential of cardiac lymphangiogenesis. Specifically, we explore the innate lymphangiogenic response to damaged cardiac tissue or cardiac injury, derive key findings from regenerative models demonstrating how robust lymphangiogenic responses can be supported to improve cardiac function, and introduce an approach to imaging the structure and function of cardiac lymphatics.

Immune surveillance and humoral immune responses in kidney transplantation - A look back at T follicular helper cells

T follicular helper cells comprise a specialized, heterogeneous subset of immune-competent T helper cells capable of influencing B cell responses in lymphoid tissues. In physiology, for example in response to microbial challenges or vaccination, this interaction chiefly results in the production of protecting antibodies and humoral memory. In the context of kidney transplantation, however, immune surveillance provided by T follicular helper cells can take a life of its own despite matching of human leukocyte antigens and employing the latest immunosuppressive regiments. This puts kidney transplant recipients at risk of subclinical and clinical rejection episodes with a potential risk for allograft loss. In this review, the current understanding of immune surveillance provided by T follicular helper cells is briefly described in physiological responses to contrast those pathological responses observed after kidney transplantation. Sensitization of T follicular helper cells with the subsequent emergence of detectable donor-specific human leukocyte antigen antibodies, non-human leukocyte antigen antibodies their implication for kidney transplantation and lessons learnt from other transplantation "settings" with special attention to antibody-mediated rejection will be addressed.

The Lymphatic System in Kidney Disease

The high-capacity vessels of the lymphatic system drain extravasated fluid and macromolecules from nearly every part of the body. However, far from merely a passive conduit for fluid removal, the lymphatic system also plays a critical and active role in immune surveillance and immune response modulation through the presentation of fluid, macromolecules, and trafficking immune cells to surveillance cells in regional draining lymph nodes before their return to the systemic circulation. The potential effect of this system in numerous disease states both within and outside of the kidney is increasingly being explored for their therapeutic potential. In the kidneys, the lymphatics play a critical role in both fluid and macromolecule removal to maintain oncotic and hydrostatic pressure gradients for normal kidney function, as well as in shaping kidney immunity, and potentially in balancing physiological pathways that promote healthy organ maintenance and responses to injury. In many states of kidney disease, including AKI, the demand on the preexisting lymphatic network increases for clearance of injury-related tissue edema and inflammatory infiltrates. Lymphangiogenesis, stimulated by macrophages, injured resident cells, and other drivers in kidney tissue, is highly prevalent in settings of AKI, CKD, and transplantation. Accumulating evidence points toward lymphangiogenesis being possibly harmful in AKI and kidney allograft rejection, which would potentially position lymphatics as another target for novel therapies to improve outcomes. However, the extent to which lymphangiogenesis is protective rather than maladaptive in the kidney in various settings remains poorly understood and thus an area of active research.

The Lymphatic Vasculature in Cardiac Development and Ischemic Heart Disease

In recent years, the lymphatic system has received increasing attention due to the fast-growing number of findings about its diverse novel functional roles in health and disease. It is well documented that the lymphatic vasculature plays major roles in the maintenance of tissue-fluid balance, the immune response, and in lipid absorption. However, recent studies have identified an additional growing number of novel and sometimes unexpected functional roles of the lymphatic vasculature in normal and pathological conditions in different organs. Among those, cardiac lymphatics have been shown to play important roles in heart development, ischemic cardiac disease, and cardiac disorders. In this review, we will discuss some of those novel functional roles of cardiac lymphatics, as well as the therapeutic potential of targeting lymphatics for the treatment of cardiovascular diseases.

The ebb and flow of cardiac lymphatics: a tidal wave of new discoveries

The heart is imbued with a vast lymphatic network that is responsible for fluid homeostasis and immune cell trafficking. Disturbances in the forces that regulate microvascular fluid movement can result in myocardial edema, which has profibrotic and proinflammatory consequences and contributes to cardiovascular dysfunction. This review explores the complex relationship between cardiac lymphatics, myocardial edema, and cardiac disease. It covers the revised paradigm of microvascular forces and fluid movement around the capillary as well as the arsenal of preclinical tools and animal models used to model myocardial edema and cardiac disease. Clinical studies of myocardial edema and their prognostic significance are examined in parallel to the recent elegant animal studies discerning the pathophysiological role and therapeutic potential of cardiac lymphatics in different cardiovascular disease models. This review highlights the outstanding questions of interest to both basic scientists and clinicians regarding the roles of cardiac lymphatics in health and disease.

Transcript Engineered Extracellular Vesicles Alleviate Alloreactive Dynamics in Renal Transplantation

Direct contact of membrane molecules and cytokine interactions orchestrate immune homeostasis. However, overcoming the threshold of distance and velocity barriers, and achieving adhesion mediated immune interaction remain difficult. Here, inspired by the natural chemotaxis of regulatory T cells, multifunctionalized FOXP3 genetic engineered extracellular vesicles, termed Foe-TEVs, are designed, which display with adhesive molecules, regulatory cytokines, and coinhibitory contact molecules involving CTLA-4 and PD-1, by limited exogenous gene transduction. Foe-TEVs effectively adhere to the tubular, endothelial, and glomerular regions of allogeneic injury in the renal allograft, mitigating cell death in situ and chronic fibrosis transition. Remarkably, transcript engineering reverses the tracking velocity of vesicles to a retained phenotype and enhanced arrest coefficient by a factor of 2.16, directly interacting and attenuating excessive allosensitization kinetics in adaptive lymphoid organs. In murine allogeneic transplantation, immune adhesive Foe-TEVs alleviate pathological responses, restore renal function with well ordered ultrastructure and improved glomerular filtration rate, and prolong the survival period of the recipient from 30.16 to 92.81 days, demonstrating that the delivery of extracellular vesicles, genetically engineered for immune adhesive, is a promising strategy for the treatment of graft rejection.

Low-dose IL-2 prevents murine chronic cardiac allograft rejection: Role for IL-2-induced T regulatory cells and exosomes with PD-L1 and CD73

To determine the effects and immunological mechanisms of low-dose interleukin-2 (IL-2) in a murine model of chronic cardiac allograft rejection (BALB/c to C57BL/6) after costimulatory blockade consisting of MR1 (250 μg/ip day 0) and CTLA4-Ig (200 μg/ip day 2), we administered low-dose IL-2 (2000 IU/day) starting on posttransplant day 14 for 3 weeks. T regulatory (Treg) cell infiltration of the grafts was determined by immunohistochemistry; circulating exosomes by western blot and aldehyde bead flow cytometry; antibodies to donor MHC by immunofluorescent staining of donor cells; and antibodies to cardiac self-antigens (myosin, vimentin) by ELISA. We demonstrated that costimulation blockade after allogeneic heart transplantation induced circulating exosomes containing cardiac self-antigens and antibodies to both donor MHC and self-antigens, leading to chronic rejection by day 45. Treatment with low-dose IL-2 prolonged allograft survival (>100 days), prevented chronic rejection, and induced splenic and graft-infiltrating CD4+ CD25+ Foxp3 Treg cells by day 45 and circulating exosomes (Foxp3+) with PD-L1 and CD73. MicroRNA 142, associated with the TGFβ pathway, was significantly downregulated in exosomes from IL-2-treated mice. In conclusion, low-dose IL-2 delays rejection in a murine model of chronic cardiac allograft rejection and also induces graft-infiltrating Tregs and circulating exosomes with immunoregulatory molecules.

Donor Macrophages Modulate Rejection After Heart Transplantation

BACKGROUND

Cellular rejection after heart transplantation imparts significant morbidity and mortality. Current immunosuppressive strategies are imperfect, target recipient T cells, and have adverse effects. The innate immune response plays an essential role in the recruitment and activation of T cells. Targeting the donor innate immune response would represent the earliest interventional opportunity within the immune response cascade. There is limited knowledge about donor immune cell types and functions in the setting of cardiac transplantation, and no current therapeutics exist for targeting these cell populations.

METHODS

Using genetic lineage tracing, cell ablation, and conditional gene deletion, we examined donor mononuclear phagocyte diversity and macrophage function during acute cellular rejection of transplanted hearts in mice. We performed single-cell RNA sequencing on donor and recipient macrophages and monocytes at multiple time points after transplantation. On the basis of our imaging and single-cell RNA sequencing data, we evaluated the functional relevance of donor CCR2+ (C-C chemokine receptor 2) and CCR2- macrophages using selective cell ablation strategies in donor grafts before transplant. Last, we performed functional validation that donor macrophages signal through MYD88 (myeloid differentiation primary response protein 88) to facilitate cellular rejection.

RESULTS

Donor macrophages persisted in the rejecting transplanted heart and coexisted with recipient monocyte-derived macrophages. Single-cell RNA sequencing identified donor CCR2+ and CCR2- macrophage populations and revealed remarkable diversity among recipient monocytes, macrophages, and dendritic cells. Temporal analysis demonstrated that donor CCR2+ and CCR2- macrophages were transcriptionally distinct, underwent significant morphologic changes, and displayed unique activation signatures after transplantation. Although selective depletion of donor CCR2- macrophages reduced allograft survival, depletion of donor CCR2+ macrophages prolonged allograft survival. Pathway analysis revealed that donor CCR2+ macrophages are activated through MYD88/nuclear factor kappa light chain enhancer of activated B cells signaling. Deletion of MYD88 in donor macrophages resulted in reduced antigen-presenting cell recruitment, reduced ability of antigen-presenting cells to present antigen to T cells, decreased emergence of allograft-reactive T cells, and extended allograft survival.

CONCLUSIONS

Distinct populations of donor and recipient macrophages coexist within the transplanted heart. Donor CCR2+ macrophages are key mediators of allograft rejection, and deletion of MYD88 signaling in donor macrophages is sufficient to suppress rejection and extend allograft survival. This highlights the therapeutic potential of donor heart-based interventions.

Lymphatic Reconstruction in Kidney Allograft Aggravates Chronic Rejection by Promoting Alloantigen Presentation

Chronic rejection of the renal allograft remains a major cause of graft loss. Here, we demonstrated that the remodeling of lymphatic vessels (LVs) after their broken during transplantation contributes to the antigen presenting and lymph nodes activating. Our studies observed a rebuilt of interrupted lymph draining one week after mouse kidney transplantation, involving preexisting lymphatic endothelial cells (LECs) from both the donor and recipient. These expanding LVs also release C-C chemokine ligand 21 (CCL21) and recruit CCR7⁺ cells, mainly dendritic cells (DCs), toward lymph nodes and spleen, evoking the adaptive response. This rejection could be relieved by LYVE-1 specific LVs knockout or CCR7 migration inhibition in mouse model. Moreover, in retrospective analysis, posttransplant patients exhibiting higher area density of LVs presented with lower eGFR, severe serum creatinine and proteinuria, and greater interstitial fibrosis. These results reveal a rebuilt pathway for alloantigen trafficking and lymphocytes activation, providing strategies to alleviate chronic transplantation rejection.

The role of lymphangiogenesis in cardiovascular diseases and heart transplantation

Cardiac lymphangiogenesis plays an important physiological role in the regulation of interstitial fluid homeostasis, inflammatory, and immune responses. Impaired or excessive cardiac lymphatic remodeling and insufficient lymph drainage have been implicated in several cardiovascular diseases including atherosclerosis and myocardial infarction (MI). Although the molecular mechanisms underlying the regulation of functional lymphatics are not fully understood, the interplay between lymphangiogenesis and immune regulation has recently been explored in relation to the initiation and development of these diseases. In this field, experimental therapeutic strategies targeting lymphangiogenesis have shown promise by reducing myocardial inflammation, edema and fibrosis, and improving cardiac function. On the other hand, however, whether lymphangiogenesis is beneficial or detrimental to cardiac transplant survival remains controversial. In the light of recent evidence, cardiac lymphangiogenesis, a thriving and challenging field has been summarized and discussed, which may improve our knowledge in the pathogenesis of cardiovascular diseases and transplant biology.

miR-21 antagonism reprograms macrophage metabolism and abrogates chronic allograft vasculopathy

Despite much progress in improving graft outcome during cardiac transplantation, chronic allograft vasculopathy (CAV) remains an impediment to long-term graft survival. MicroRNAs (miRNAs) emerged as regulators of the immune response. Here, we aimed to examine the miRNA network involved in CAV. miRNA profiling of heart samples obtained from a murine model of CAV and from cardiac-transplanted patients with CAV demonstrated that miR-21 was most significantly expressed and was primarily localized to macrophages. Interestingly, macrophage depletion with clodronate did not significantly prolong allograft survival in mice, while conditional deletion of miR-21 in macrophages or the use of a specific miR-21 antagomir resulted in indefinite cardiac allograft survival and abrogated CAV. The immunophenotype, secretome, ability to phagocytose, migration, and antigen presentation of macrophages were unaffected by miR-21 targeting, while macrophage metabolism was reprogrammed, with a shift toward oxidative phosphorylation in naïve macrophages and with an inhibition of glycolysis in pro-inflammatory macrophages. The aforementioned effects resulted in an increase in M2-like macrophages, which could be reverted by the addition of L-arginine. RNA-seq analysis confirmed alterations in arginase-associated pathways associated with miR-21 antagonism. In conclusion, miR-21 is overexpressed in murine and human CAV, and its targeting delays CAV onset by reprogramming macrophages metabolism.

Cardiac macrophage subsets differentially regulate lymphatic network remodeling during pressure overload

The lymphatic network of mammalian heart is an important regulator of interstitial fluid compartment and immune cell trafficking. We observed a remodeling of the cardiac lymphatic vessels and a reduced lymphatic efficiency during heart hypertrophy and failure induced by transverse aortic constriction. The lymphatic endothelial cell number of the failing hearts was positively correlated with cardiac function and with a subset of cardiac macrophages. This macrophage population distinguished by LYVE-1 (Lymphatic vessel endothelial hyaluronic acid receptor-1) and by resident macrophage gene expression signature, appeared not replenished by CCR2 mediated monocyte infiltration during pressure overload. Isolation of macrophage subpopulations showed that the LYVE-1 positive subset sustained in vitro and in vivo lymphangiogenesis through the expression of pro-lymphangiogenic factors. In contrast, the LYVE-1 negative macrophage subset strongly expressed MMP12 and decreased the endothelial LYVE-1 receptors in lymphatic endothelial cells, a feature of cardiac lymphatic remodeling in failing hearts. The treatment of mice with a CCR2 antagonist during pressure overload modified the proportion of macrophage subsets within the pathological heart and preserved lymphatic network from remodeling. This study reports unknown and differential functions of macrophage subpopulations in the regulation of cardiac lymphatic during pathological hypertrophy and may constitute a key mechanism underlying the progression of heart failure.

Subconjunctival injections of dimethyl fumarate inhibit lymphangiogenesis and allograft rejection in the rat cornea

Corneal lymphangiogenesis induced by macrophages played a critical role in corneal allograft rejection (CGR). However, there are few Food and Drug Administration (FDA)-approved drugs that target lymphangiogenesis. The aim of our study is to evaluate the effects of dimethyl fumarate (DMF) on corneal allograft survival in rats. Penetrating corneal transplantation was performed in rats. Subconjunctival injections of dimethyl fumarate (20 µg) were administered at the end of the operation and postoperative day 3 to day 11. The clinical signs of corneal allografts were evaluated. Immunohistochemistry, quantitative real-time PCR (qPCR), flow cytometry and western blot were performed respectively. The effects and mechanism of DMF on RAW264.7 cells were determined by qPCR, enzyme-linked immunosorbent assay (ELISA), and western blot in vitro. The results showed that subconjunctival injections of DMF could significantly inhibit corneal lymphangiogenesis and CGR with decreased corneal macrophage infiltration compared with the vehicle group. Moreover, DMF could reduce the mRNA expression of monocyte chemoattractant protein 1 (MCP-1), tumor necrosis factor-α (TNF-α), interleukin-6 (IL-6), interleukin-1β (IL-1β), and vascular endothelial growth factor-C (VEGF-C) in the corneal grafts and RAW264.7 macrophages by inhibiting NF-κB activation. Furthermore, compared with the vehicle group, the number of dendritic cells in the ipsilateral cervical lymph nodes of the DMF-treated group was decreased significantly. Collectively, our findings showed that DMF could suppress CGR by inhibiting the macrophage-induced corneal lymphoangiogenesis.

Lymphatic Vasculature: An Emerging Therapeutic Target and Drug Delivery Route

The lymphatic system has received increasing scientific and clinical attention because a wide variety of diseases are linked to lymphatic pathologies and because the lymphatic system serves as an ideal conduit for drug delivery. Lymphatic vessels exert heterogeneous roles in different organs and vascular beds, and consequently, their dysfunction leads to distinct organ-specific outcomes. Although studies in animal model systems have led to the identification of crucial lymphatic genes with potential therapeutic benefit, effective lymphatic-targeted therapeutics are currently lacking for human lymphatic pathological conditions. Here, we focus on the therapeutic roles of lymphatic vessels in diseases and summarize the promising therapeutic targets for modulating lymphangiogenesis or lymphatic function in preclinical or clinical settings. We also discuss considerations for drug delivery or targeting of lymphatic vessels for treatment of lymphatic-related diseases. The lymphatic vasculature is rapidly emerging as a critical system for targeted modulation of its function and as a vehicle for innovative drug delivery.

Lymphatic vessels in solid organ transplantation and immunobiology

With the recent advances in our understanding of the function and biology of the lymphatic vascular system, it is clear that the lymphatic system plays an integral role in physiology, and in pathological settings, may contribute to either enhance or repress inflammation and disease progression. Inflammation is central to both acute and chronic rejection in the context of solid organ transplantation, and emerging evidence suggests the lymphatic system plays a key role in shaping outcomes. The goals of this review are to highlight and contextualize the roles of lymphatic vessels and lymphangiogenesis in immunobiology, the impact immunosuppressive therapies have on the lymphatic system and emerging evidence of organ-specific heterogeneity of lymphatic vessels in the context of solid organ transplantation.

Assessing functional status of cardiac lymphatics: From macroscopic imaging to molecular profiling

Here we describe various techniques for visualization of the lymphatic vasculature, particularly in the heart. Addressing macro-, microscopic, and molecular levels of lymphatic organization, we give examples of how to explore the roles of specific antigens/markers expressed in lymphatic vessels and their extracellular matrix as structural and functional elements involved in various biological functions of lymphatics. Some obstacles and technical challenges related to lymphatic visualization are also discussed.

Triptolide Attenuates Transplant Vasculopathy Through Multiple Pathways

Transplant vasculopathy (TV), a hallmark of chronic allograft rejection, is the primary cause of allograft loss after organ transplantation. Because multiple mechanisms are involved in TV pathogenesis, effective therapy for it remains elusive. Here, we identify the role of triptolide, which has a wide spectrum of immuno-suppressive activities, in inhibiting TV development. Murine aortic transplants models were constructed and divided into triptolide-treated and untreated groups. We found that triptolide significantly alleviated intima thickening of allografts by inhibiting multiple pathways. Triptolide significantly reduced infiltration of T lymphocytes and macrophages and inhibited the levels of pro-inflammatory (TNF-α, IL-2, and IL-6) and pro-fibrotic factors (TGF-β, α-SMA, and MMP-9) in the graft. Additionally, triptolide significantly decreased the numbers of IFN-γ-producing T lymphocytes, as well as the expression of IFN-γ and IFN-γ-inducing factor (CXCL9 and CXCL10) in recipient. Moreover, triptolide decreased the numbers of B lymphocytes and plasma cells, as well as the levels of donor specific antibodies (DSAs) in recipient. Furthermore, triptolide not only inhibited vascular smooth muscle cell (VSMC) viability and promoted VSMC apoptosis but also significantly inhibited VSMC migration in vitro. These results emphasize the efficacy of triptolide in inhibiting TV development and provide a basis for developing new treatments to prevent TV-related complications and improve the long-term survival of transplant recipients.

Lymphatic Vessels Enhancing Adaptive Immunity Deteriorates Renal Inflammation and Renal Fibrosis

Background

Lymphatic vessels transport lymph away from microvascular beds into the cardiovascular system. The basic function of the lymphatic system include absorption of water and macromolecules in the interstitial fluid, which plays an important role in maintaining osmotic balance of the body. Recent studies have shown that lymphangiogenesis is associated with tumor metabolism, injury repair, and chronic inflammation, and deteriorates disease progression via immune cell trafficking.

Summary

Renal interstitial lymph-angiogenesis is found in patients with chronic kidney disease and a series of animal models of renal fibrosis. Lymphatic vessels transfer antigen and antigen-presenting cells from peripheral tissues to lymph nodes, which initiates adaptive immunity and in turn deteriorates renal inflammation and renal fibrosis, even in non-autoimmune renal diseases.

Key Messages

This review summarizes the latest findings on how lymphatics participate in the progression of chronic kidney disease. This discussion will serve to highlight the role of adaptive immunity in non-infectious and non-autoimmune nephropathy, in order to provide new ideas and methods for prevention and treatment of kidney diseases.

Role of the lymphatic vasculature in cardiovascular medicine

The lymphatic vasculature has traditionally been considered important for removal of excessive fluid from the interstitial space, absorption of fat from the intestine and the immune system. Advances in molecular medicine and imaging have provided us with new tools to study the lymphatics. This has revealed that the vessels are actively involved in regulation of immune cell trafficking and inflammation. We now know much about how new lymphatic vessels are created (lymphangiogenesis) and that this is important in, for example, wound healing and tissue repair. The best characterised pathway for lymphangiogenesis is the vascular endothelial growth factor C (VEGF-C)/VEGFR3 pathway. Over recent years, there has been an increasing interest in the role of the lymphatics in cardiovascular medicine. Preclinical studies have shown that lymphangiogenesis and immune cell trafficking play a role in cardiovascular conditions such as atherosclerosis, recovery after myocardial infarction and rejection of cardiac allografts. Targeting the VEGF-C/VEGFR3 pathway can be beneficial in these conditions. The clinical spectrum of lymphatic abnormalities and lymphoedema is wide and overlaps with congenital heart disease. Important long-term complications to the Fontan circulation involves the lymphatics. New and improved imaging modalities has improved our understanding and management of these patients. Lymphatic leaks and flow abnormalities can be successfully treated, minimally invasively, with percutaneous embolisation. Future research will prove if the preclinical findings that point to a role of the lymphatics in several cardiovascular conditions will result in new treatment options.

Acute and chronic phagocyte determinants of cardiac allograft vasculopathy

Post-transplant immunosuppression has reduced the incidence of T cell-mediated acute rejection, yet long-term cardiac graft survival rates remain a challenge. An important determinant of chronic solid organ allograft complication is accelerated vascular disease of the transplanted graft. In the case of cardiac allograft vasculopathy (CAV), the precise cellular etiology remains inadequately understood; however, histologic evidence hints at the accumulation and activation of innate phagocytes as a causal contributing factor. This includes monocytes, macrophages, and immature dendritic cell subsets. In addition to crosstalk with adaptive T and B immune cells, myeloid phagocytes secrete paracrine signals that directly activate fibroblasts and vascular smooth muscle cells, both of which contribute to fibrous intimal thickening. Though maladaptive phagocyte functions may promote CAV, directed modulation of myeloid cell function, at the molecular level, holds promise for tolerance and prolonged cardiac graft function.

Immune cell trafficking, lymphatics and hypertension

Activated immune cell infiltration into organs contributes to the development and maintenance of hypertension. Studies targeting specific immune cell populations or reducing their inflammatory signalling have demonstrated a reduction in BP. Lymphatic vessels play a key role in immune cell trafficking and in resolving inflammation, but little is known about their role in hypertension. Studies from our laboratory and others suggest that inflammation-associated or induction of lymphangiogenesis is organ protective and anti-hypertensive. This review provides the basis for hypertension as a disease of chronic inflammation in various tissues and highlights how renal lymphangiogenesis is a novel regulator of kidney health and BP. LINKED ARTICLES: This article is part of a themed section on Immune Targets in Hypertension. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.12/issuetoc.