Lymphatic vessels: roles and potential therapeutic intervention in rheumatoid arthritis and osteoarthritis

Lymphatic vessels: roles and potential therapeutic intervention in rheumatoid arthritis and osteoarthritis

  • Post category:Rheumatology
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Introduction

(Article introduction authored by Conquest Editorial Team)

This review explores the crucial role of lymphatic vessels in rheumatoid arthritis (RA) and osteoarthritis (OA), emphasizing their involvement in maintaining joint homeostasis. The inflammatory environment in arthritic conditions affects lymphatic vessel function, contributing to disease progression.

The review covers structural and functional aspects of lymphatic vessels in RA and OA, highlighting potential regulatory mechanisms. Therapeutic strategies targeting lymphatic vessels, such as promoting lymphangiogenesis, anti-inflammatory approaches, enhancing lymphatic function, and reducing congestion, are discussed.

The structure and function of lymphatic vessels

The lymphatic system is located in close proximity to the venous network, which plays crucial roles in conducting surveillance, and facilitating lipid absorption.

In brief, lymphatic vessels originate from preexisting blood vessels, which initiate from the lymphatic capillary, also known as initial lymphatic vessels, in the peripheral regions of the body.

Subsequently, they converge through collecting lymphatics towards draining lymph nodes (DLNs), ultimately facilitating lymph drainage into the venous system via the right lymphatic trunk and thoracic duct (Fig 1)

Lymph formation occurs in lymphatic capillaries as interstitial fluid enters these vessels. Initial lymphatic vessels are slender, measuring 35-70 μm in diameter, and have a cul-de-sac structure. These vessels consist of a monolayer of lymphatic endothelial cells (LECs) with an interrupted basal lamina. Anchoring filaments connect LECs to elastic fibers, enabling the stretching of lymphatic capillaries under elevated interstitial fluid pressure. “Button-like” junctions in LECs create highly permeable vessels, allowing the passage of fluids, lipids, macromolecules, and cells. Overlapping LECs form primary lymphatic valves, preventing lymph backflow. During edema formation, increased interstitial fluid pressure opens “button-like” junctions, facilitating enhanced lymph generation and drainage in lymphatic capillaries.( Fig 2)

Lymphatic outflow occurs through collecting lymphatic vessels, characterized by a continuous basement membrane, zipper-like junctions between lymphatic endothelial cells (LECs), and distinct lymphatic muscle cells (LMCs). Secondary valves with two layers of LECs prevent lymph backflow by opening or closing in response to fluid pressure changes. The flow of lymph in collecting vessels depends on intrinsic and extrinsic pumps. Intrinsic pumping involves the coordinated contraction or relaxation of LMCs, while extrinsic pumping results from surrounding tissue forces. LMCs, in collaboration with lymphatic valves, counteract gravitational forces for smooth lymph propulsion. Modulating LMC function can regulate lymph flow by modifying pumping force and outflow resistance. After collection by lymphatic vessels, lymph reaches draining lymph nodes (DLNs), where an autoimmune response is initiated in lymphatic sinuses. Lymph exits DLNs through efferent lymphatic vessels to enter the lymph circulation.

Assessment and visualization of lymphatic vessel distribution within joints

Lymphatic vessels in joints are assessed using imaging techniques like near-infrared indocyanine green imaging, contrast-enhanced magnetic resonance imaging, and power Doppler ultrasound.

Histological observation is crucial, employing immunostaining markers for lymphatic endothelial cells (LECs) and lymphatic muscle cells (LMCs). Common markers include LYVE1, podoplanin (PDPN), PROX1, and VEGFR3 for LECs, and α-smooth muscle actin (αSMA) for LMCs. Multiple-immunostaining, including PDPN and αSMA, aids in classification. In the synovium of human joints, lymphatic vessels are predominantly found in the outer layer, using multiple markers for identification.

Advanced techniques like whole-slide digital imaging and light-sheet microscopy enhance lymphatic vessel detection, revealing their distribution in various joint tissues.

Lymphatic vessels and RA

Rheumatoid arthritis (RA) is a chronic autoimmune disorder affecting joints, causing persistent inflammation and damage. Inflamed joints exhibit increased activated immune cells, including macrophages, lymphocytes, and plasma cells. These cells release inflammatory mediators such as TNF-α, IL-1, and IL-6, contributing to synovial inflammation, vasodilation, and joint symptoms.
Lymphatic vessels are implicated in clearing inflammatory cells and mediators from the inflamed synovium, based on clinical studies and animal models.

Vascular endothelial growth factor C (VEGF-C)/vascular endothelial growth factor receptor 3 (VEGFR3) signaling pathway in lymphangiogenesis of RA

Lymphangiogenesis, the process of forming new lymphatic vessels, is regulated by factors such as the VEGF family, including VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E, VEGF-F, and PIGF.

These ligands bind to VEGFRs, with VEGFR3 playing a central role in lymphangiogenesis.

Binding of VEGF-C/VEGF-D to VEGFR3 triggers downstream signaling pathways, including MAPK/ERK, PI3k/AKT, and JNK1/2, promoting lymphatic endothelial cell proliferation, survival, migration, and vessel remodeling.

NRP2 and VEGFR3 interaction enhances lymphangiogenesis by mediating proper lymphatic vessel sprouting in response to VEGF-C stimuli. (Fig 3)

In rheumatoid arthritis (RA), studies have highlighted the significance of VEGF-C/VEGFRs in lymphangiogenesis. Elevated expression of VEGF-C and its receptors (VEGFR3 and VEGFR2) in arthritic synovial tissue, particularly in synovial endothelial cells, fibroblasts, and macrophages, indicates their role in RA inflammation.

Synovial fluid from RA patients shows increased VEGF-C levels, correlating positively with TNF-α. Molecularly, PROX1 activation by NF-κB pathway in the inflammatory microenvironment enhances VEGFR3 expression. Furthermore, the VEGF-C/VEGFR3 pathway inhibits the TLR4/NF-κB pathway.

Nitric oxide synthase (NOS) signaling pathway in LMCs and the contraction of lymphatic vessels of RA

Lymphatic vessel contraction, crucial for fluid transport, is regulated by the rhythmic contractions of lymphatic muscle cells (LMCs). The contractile force involves calcium dynamics and contractile proteins.

LMC contraction is initiated by stretch-induced contractions mediated through Ca2+ channels. Action potentials induce Ca2+ influx, leading to muscle contraction.

LMC relaxation is primarily regulated by nitric oxide (NO), mainly synthesized by lymphatic endothelial cells (LECs) through endothelial NO synthase (eNOS). Inducible NO synthase (iNOS), induced by inflammation, increases NO production, impacting lymphatic function.

In inflammatory arthritis, elevated iNOS levels in LECs and other cells lead to impaired lymphatic contraction and excessive NO production, hindering lymphatic drainage.

Changes of lymphatic vessels during RA processes

In rheumatoid arthritis (RA), clinical studies reveal a significant increase in lymphatic vessels within the arthritic synovial membrane compared to healthy controls.

Similar observations are noted in mouse models of RA. Structural alterations, such as increased capillary lymphatics and reduced collecting lymphatics, are observed, along with impaired drainage function and elevated degenerative LMCs. The distribution and structure of lymphatic vessels undergo modifications in RA, impacting their clearance function. In RA progression, two distinct phenotypes of draining lymph nodes (DLNs) emerge—expanding and collapsed—with different characteristics.
CD11b+ macrophages play a crucial role in promoting lymphangiogenesis during the expanding phase. In the collapsing phase, B-in cells obstruct lymph channels and lymphatic sinus, leading to diminished lymphatic drainage and bone loss in adjacent joints.
This dynamic process involves macrophage-induced lymphatic changes and bone loss in RA pathogenesis.

Lymphatic vessels and OA

Osteoarthritis (OA) is a prevalent joint disorder characterized by articular cartilage degeneration and inflammation of surrounding tissues.

Unlike rheumatoid arthritis (RA), OA is attributed to gradual mechanical degradation and aging-related changes in cartilage and subchondral bone. Synovial cells in OA release inflammatory mediators, triggering the synthesis of cytokines and matrix-degrading enzymes, leading to cartilage destruction. Inflammation’s role in OA pathogenesis is increasingly recognized.

Despite extensive research on lymphatic vessels in RA, their role in OA has received limited attention. The peri-articular lymphatic system is acknowledged to play a significant role in OA, similar to RA. (Fig 5)

Dynamic alterations of lymphatic vessels involves in various stages of OA progression

In osteoarthritis (OA), clinical reports reveal increased lymphatic vessels infiltrated by inflammatory cells in the synovial membrane, suggesting their involvement in OA inflammation.

Microcirculation and lymphatic drainage dysfunction is observed in OA patients. Studies show lower lymphatic vessel density in knee synovium of OA patients, correlating with synovial effusion.

Reduced lymphatic vessel density may contribute to synovial fluid retention, potentially worsening joint inflammation. Dynamic alterations in the lymphatic system’s structure and functions play a role in various stages of OA progression, emphasizing the need for further investigation into long-term mechanisms maintaining joint homeostasis.

In a mice model of meniscal-ligamentous injury (MLI)-induced osteoarthritis (OA), there’s an increase in capillary lymphatics and decrease in collecting lymphatic vessels in OA joints at 3 months post-MLI.

Further studies reveal significantly increased lymphatic vessels, especially lymphatic capillaries, in the thicker synovial membrane at 12 weeks post-MLI. Despite increased lymphangiogenesis, there’s a decline in lymphatic drainage function, suggesting that increased lymphatic capillaries may not positively correlate with drainage capacity.

Reduced lymphatic capillaries and collecting lymphatic vessels impair lymphatic pumping function, leading to diminished lymph clearance and accumulation of pro-inflammatory factors in OA knees.

This impairment is observed in both MLI-induced and aging-related OA models, supported by findings in OA patient samples.

In osteoarthritis (OA), macrophages, particularly M1 pro-inflammatory subtype, significantly contribute to joint inflammation and bone destruction.

M1 macrophages release TNF-α, IL-1, and iNOS, influencing lymphatic drainage in the synovium. The negative regulator of NF-κB, Itch, suppresses pro-inflammatory polarization and enhances lymphatic drainage.

Global knockout of Itch in mice leads to severe OA phenotypes. Reduced fibroblast growth factor receptor 3 (FGFR3) expression in monocytes exacerbates joint destruction, promoting synovitis and macrophage accumulation.

The interplay between synovial macrophages and lymphatic vessels plays a crucial role in OA progression.

Potential impact of inflammatory environment on lymphatic vessels in RA and OA

Rheumatoid arthritis (RA) and osteoarthritis (OA) display contrasting inflammatory profiles. RA exhibits chronic, high-grade systemic inflammation, while OA has mild synovial inflammation. RA synovium has higher TNF-α and IL-1β expression, with more inflammatory cells than OA.

In leukocyte-poor RA synovium, similarities with OA suggest shared inflammatory states. Leukocyte-rich RA synovium shows pronounced impairment in lymphatic vessel drainage, warranting further investigation.

Various macrophage subgroups contribute to inflammation in both diseases. In OA, M1 macrophages influence lymphatic vessel function.

RA progression involves elevated pro-inflammatory M1-like macrophages, indicating distinct inflammatory processes in RA and OA that require further exploration.

Targeting the lymphatic vessels as a potential therapeutic strategy

The statement highlights the potential for distinct regulatory mechanisms and varying treatment approaches in addressing disorders of lymphatic vessels associated with RA and OA.

Anti-TNF treatment: TNF-α is pivotal in RA; anti-TNF therapy improves lymphatic function and reduces inflammation, providing significant pain relief.

While systemic anti-TNF treatment is generally effective, about 40% of RA patients show inadequate response.

Localized delivery targeting lymphatic dysfunction may enhance efficacy. In OA, TNF inhibition trials with Adalimumab and Etanercept showed no significant pain control in hand OA patients.

Bortezomib treatment: Bortezomib, approved for multiple myeloma, shows promise in arthritis treatment. Studies in arthritis murine models, including MLI-induced OA, demonstrate improved synovial lymphatic drainage and reduced inflammation with intra-articular Bortezomib.

B cell depletion therapy: Depletion of B-in cells using anti-CD20 mAbs or Rituximab in RA shows promise in preventing knee flare and ameliorating inflammatory-erosive arthritis.

Localized delivery of B cell depletion therapy in RA joints enhances exposure to DLNs, reducing systemic side effects.

In OA, the role of B-cells is less understood, but targeting them may hold therapeutic potential, and their activation might serve as a biomarker for disease progression in OA. Further research is needed to explore these possibilities.

VEGF-C/VEGFR3 treatment: The VEGF-C/VEGFR3 signaling pathway is a promising target for arthritis treatment, enhancing lymphangiogenesis and drainage.

In RA, systemic VEGFR3 blockade worsens inflammation, while intra-articular VEGF-C administration improves lymphatic drainage.

In OA, impaired synovial drainage is observed, and VEGF-C156S injection enhances drainage, reducing tissue damage. AAV-mediated VEGF-C delivery shows short-term safety, but long-term effects and safety in chronic arthritis need further investigation.

The pathway’s modulation of chondrocytes, macrophages, and bone homeostasis also requires more research.

iNOS inhibitors: Inflammation-induced upregulation of iNOS leads to excessive NO production, impairing lymphatic contraction and drainage in joints. iNOS inhibitors show promise in preserving lymphatic function, with positive outcomes observed in animal models of RA.

Local iNOS inhibition in TNF-Tg mice restores lymphatic contractions. Plant-derived compounds also exhibit potential for regulating lymphatic function.

In OA, iNOS inhibition is associated with chondrocyte protection, but its impact on lymphatic vessels remains unclear.

While animal studies support iNOS inhibitors as potential interventions for arthritis, including OA and early RA, successful clinical trials are lacking.

The timing of iNOS-dependent effects on severe RA and irreversible damage to lymphatic muscle cells necessitate further investigation.

Discussion

Lymphatic vessels play a crucial role in joint homeostasis, and their dysfunction is closely linked to inflammatory joint diseases like RA and OA. Further research is needed to understand dynamic changes and functional importance at different disease stages, identifying optimal intervention windows. Comprehensive investigation, including single-cell sequencing and lineage tracing, is crucial for exploring key cellular and molecular characteristics underlying lymphatic vessel alterations. Specific molecular targets and tailored drug delivery modes are essential for optimizing therapeutic strategies. Anticipated advancements hold promise in improving clinical outcomes in inflammatory joint diseases.

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