Development and plasticity of meningeal lymphatic vessels

CSF transport at the brain-meningeal border: effects on neurological health and disease

The existence of specialised structures that allow a continuous exchange of CSF between different anatomical compartments at the brain-meningeal border is challenging conventional notions around molecular transport within the brain. Experimental findings highlight the conduits and cellular structures controlling the transport of CSF and immune cells between the brain parenchyma (via the glymphatic system), the subarachnoid space (enclosed by the meningeal pia and arachnoid layers), and the outmost meningeal dural layer and calvaria (via the so-called arachnoid cuff exit points). Studies in both rodent models and humans show new mechanisms of brain glymphatic molecular transport, meningeal lymphatic vascular drainage, and immune surveillance at the brain-draining skull bone marrow and cervical lymph nodes. Pathological alterations at the brain-meningeal border have been implicated in disorders of diverse causes, from traumatic brain injury to Alzheimer's disease.

The Impact of VEGF-C-Induced Dural Lymphatic Vessel Growth on Ischemic Stroke Pathology

Timely relief of edema and clearance of waste products, as well as promotion of anti-inflammatory immune responses, reduce ischemic stroke pathology, and attenuate harmful long-term effects post-stroke. The discovery of an extensive and functional lymphatic vessel system in the outermost meningeal layer, dura mater, has opened up new possibilities to facilitate post-stroke recovery by inducing dural lymphatic vessel (dLV) growth via a single injection of a vector encoding vascular endothelial growth factor C (VEGF-C). In the present study, we aimed to improve post-stroke outcomes by inducing dLV growth in mice. We injected mice with a single intracerebroventricular dose of adeno-associated viral particles encoding VEGF-C before subjecting them to transient middle cerebral artery occlusion (tMCAo). Behavioral testing, Gadolinium (Gd) contrast agent-enhanced magnetic resonance imaging (MRI), and immunohistochemical analysis were performed to define the impact of VEGF-C on the post-stroke outcome. VEGF-C improved stroke-induced behavioral deficits, such as gait disturbances and neurological deficits, ameliorated post-stroke inflammation, and enhanced an alternative glial immune response. Importantly, VEGF-C treatment increased the drainage of brain interstitial fluid (ISF) and cerebrospinal fluid (CSF), as shown by Gd-enhanced MRI. These outcomes were closely associated with an increase in the growth of dLVs around the region where we observed increased vefgc mRNA expression within the brain, including the olfactory bulb, cortex, and cerebellum. Strikingly, VEGF-C-treated ischemic mice exhibited a faster and stronger Gd-signal accumulation in ischemic core area and an enhanced fluid outflow via the cribriform plate. In conclusion, the VEGF-C-induced dLV growth improved the overall outcome post-stroke, indicating that VEGF-C has potential to be included in the treatment strategies of post-ischemic stroke. However, to maximize the therapeutic potential of VEGF-C treatment, further studies on the impact of an enhanced dural lymphatic system at clinically relevant time points are essential.

Spatiotemporal dynamic changes of meningeal microenvironment influence meningeal lymphatic function following subarachnoid hemorrhage: from inflammatory response to tissue remodeling

BACKGROUND

Meningeal lymphatic vessels (mLVs) play a critical role in clearing erythrocytes from the subarachnoid space and immune cells from the brain parenchyma following subarachnoid hemorrhage (SAH). However, the drainage function of mLVs is impaired during the acute stage after SAH and gradually recovers in the subacute phase. We aimed to investigate the meningeal transcriptional response post-SAH and elucidate the dynamic influence of meningeal microenvironment on meningeal lymphatic function.

METHODS

We employed bioinformatics analysis of single-cell RNA sequencing and spatial transcriptomics to characterize the spatiotemporal dynamic changes in the early meningeal microenvironment post-SAH. In a mouse model of SAH, the early dynamic changes of the meningeal immune cells and the potential growth factor that promoted the early repair of the mLVs were further investigated and validated.

RESULTS

During the acute phase, myeloid cells early infiltrated the meninges and triggered inflammatory responses. In the subacute phase, the fibroblast population expanded significantly, contributing to tissue remodeling. The interplay between immune cells and fibroblasts regulated cell migration and phenotypic transition, potentially affecting the function of mLVs. Notably, placental growth factor (PGF) emerged as the most prominent ligand within the VEGF signaling pathway received by meningeal lymphatic endothelial cells (mLECs) post-SAH. This signaling event was associated with the early recovery of mLVs after acute immune responses.

CONCLUSIONS

Our study revealed a spatiotemporal transformation of the meningeal microenvironment from an "inflammatory response" phase to a "tissue remodeling" phase following SAH. Monocyte-derived macrophages and self-recruiting neutrophils contributed to impairment of mLVs in the acute stage, while PGF might serve as a key factor promoting early meningeal lymphatic function repair following the inflammatory response. These findings provided novel insights into the cellular dynamics underlying mLVs dysfunction and recovery post-SAH.

Meningeal lymphatics-microglia axis regulates synaptic physiology

Meningeal lymphatics serve as an outlet for cerebrospinal fluid, and their dysfunction is associated with various neurodegenerative conditions. Previous studies have demonstrated that dysfunctional meningeal lymphatics evoke behavioral changes, but the neural mechanisms underlying these changes have remained elusive. Here, we show that prolonged impairment of meningeal lymphatics alters the balance of cortical excitatory and inhibitory synaptic inputs, accompanied by deficits in memory tasks. These synaptic and behavioral alterations induced by lymphatic dysfunction are mediated by microglia, leading to increased expression of the interleukin 6 gene (Il6). IL-6 drives inhibitory synapse phenotypes via a combination of trans- and classical IL-6 signaling. Restoring meningeal lymphatic function in aged mice reverses age-associated synaptic and behavioral alterations. Our findings suggest that dysfunctional meningeal lymphatics adversely impact cortical circuitry through an IL-6-dependent mechanism and identify a potential target for treating aging-associated cognitive decline.

Border-associated macrophages as gatekeepers of brain homeostasis and immunity

The brain's border tissues serve as essential hubs for neuroimmune regulation and the trafficking of biomaterials to and from the brain. These complex tissues-including the meninges, perivascular spaces, choroid plexus, and circumventricular organs-balance the brain's need for immune privilege with immune surveillance and blood-brain communication. Macrophages are integral components of these tissues, taking up key strategic positions within the brain's circulatory system. These border-associated macrophages, or "BAMs," are therefore emerging as pivotal for brain homeostasis and disease. BAMs perform trophic functions that help to support border homeostasis but also act as immune sentinels essential for border defense. In this review, we integrate recent findings on BAM origins, cell states, and functions, aiming to provide global insights and perspectives on the complex relationship between these macrophages and their border niche.

Niche-specific therapeutic targeting of myeloid cells in the central nervous system

The central nervous system (CNS) can be subdivided into distinct anatomical and functional compartments, including the parenchyma, perivascular space, leptomeninges, and dura mater, etc. Each compartment hosts distinct immune cell populations, such as monocytes and diverse macrophages, which play critical roles in local tissue homeostasis and regional disease pathogenesis. Advances in single-cell technologies have revealed complex immune cell compositions and functions in these anatomical regions. This review summarizes the latest approaches for modulating myeloid cell subsets in a compartment-specific manner, including cellular strategies such as stem cell therapy, ex vivo gene treatment, bone marrow transplantation, as well as non-cellular strategies like antibodies, small molecules, and viral gene delivery to augment CNS immune responses and improve disease outcomes. We also discuss the challenges and requirements of translating targeting strategies from mice to humans.

Meningeal lymphatic drainage: novel insights into central nervous system disease

In recent years, increasing evidence has suggested that meningeal lymphatic drainage plays a significant role in central nervous system (CNS) diseases. Studies have indicated that CNS diseases and conditions associated with meningeal lymphatic drainage dysfunction include neurodegenerative diseases, stroke, infections, traumatic brain injury, tumors, functional cranial disorders, and hydrocephalus. However, the understanding of the regulatory and damage mechanisms of meningeal lymphatics under physiological and pathological conditions is currently limited. Given the importance of a profound understanding of the interplay between meningeal lymphatic drainage and CNS diseases, this review covers seven key aspects: the development and structure of meningeal lymphatic vessels, methods for observing meningeal lymphatics, the function of meningeal lymphatics, the molecular mechanisms of meningeal lymphatic injury, the relationships between meningeal lymphatic vessels and CNS diseases, potential regulatory mechanisms of meningeal lymphatics, and conclusions and outstanding questions. We will explore the relationship between the development, structure, and function of meningeal lymphatics, review current methods for observing meningeal lymphatic vessels in both animal models and humans, and identify unresolved key points in meningeal lymphatic research. The aim of this review is to provide new directions for future research and therapeutic strategies targeting meningeal lymphatics by critically analyzing recent advancements in the field, identifying gaps in current knowledge, and proposing innovative approaches to address these gaps.

Redefining CNS immune privilege

The central nervous system (CNS) has a unique relationship with the immune system, referred to as immune privilege. For many years it was thought that immune privilege was due to isolation of the CNS from the immune system, but recent findings have shown that this theory is flawed and that there is substantial neuroimmune communication, particularly at border sites that encase the CNS. These border sites include perivascular and subarachnoid spaces, the choroid plexus, the meninges and the vasculature, including the recently discovered meningeal lymphatic vessels. CNS border tissues have extensive interaction with the cerebrospinal fluid, which acts as an immune mediator, allowing the immune system at the CNS borders to respond to challenges within the CNS parenchyma. Together, CNS border tissues enable immune surveillance and protection against infections while preventing inflammatory damage to the parenchyma. A better understanding of the mechanisms of immune privilege as an accord, as opposed to isolation, between the two systems would help us obtain effective immunotherapies for neurological diseases.

MR Imaging of Neurofluids in the Developing Brain

The different fluid compartments in the developing brain work together to facilitate the delivery of nutrients, neurotransmitters, and neuromodulators. The cerebrospinal fluid and interstitial fluid are essential for clearing macromolecules from the brain, a process that involves the recently discovered meningeal lymphatics. Disruptions in these interactions can hinder normal brain development. Additionally, alterations in systemic fluid dynamics may contribute to neurologic complications, highlighting the need for a more holistic approach to understanding and treating neurologic diseases. MR imaging techniques show potential for detecting these pathologic processes in pediatric neurologic disorders.

Emerging Roles of Meningeal Lymphatic Vessels in Ischemic Stroke

This review highlights the emerging relevance of meningeal lymphatic vessels (MLVs) in the context of ischemic stroke, challenging the conventional view of a privileged immunological central nervous system. MLVs facilitate immunological surveillance by modulating the entry of peripheral immune cells into the meningeal compartment, a process not impeded by the blood-brain barrier. In ischemic stroke, these vessels play a crucial role in the neuroinflammatory cascade, contributing to immune responses by draining antigens and signals to cervical lymph nodes. Their involvement extends to potential contributions to resolving ischemia-induced cerebral edema, impacting fluid homeostasis. The dynamic interaction among MLVs, neuroinflammation, and fluid dynamics suggests promising therapeutic approaches. Targeting these vessels for immunomodulation, fluid drainage, and preserving blood-brain barrier integrity emerges as an innovative approach to improve ischemic stroke outcomes. However, successful clinical translation awaits further exploration of the therapeutic potential of these vessels. The multifaceted contributions of MLVs provide a compelling rationale for ongoing research, aiming to fully harness their therapeutic impact in ischemic stroke management.

Neural-activity-regulated and glia-mediated control of brain lymphatic development

The nervous system regulates peripheral immune responses under physiological and pathological conditions, but the brain's impact on immune system development remains unknown. Meningeal mural lymphatic endothelial cells (muLECs), embedded in the leptomeninges, form an immune niche surrounding the brain that contributes to brain immunosurveillance. Here, we report that the brain controls the development of muLECs via a specialized glial subpopulation, slc6a11b+ radial astrocytes (RAs), a process modulated by neural activity in zebrafish. slc6a11b+ RAs, with processes extending to the meninges, govern muLEC formation by expressing vascular endothelial growth factor C (vegfc). Moreover, neural activity regulates muLEC development, and this regulation requires Vegfc in slc6a11b+ RAs. Intriguingly, slc6a11b+ RAs cooperate with calcium-binding EGF domain 1 (ccbe1)+ fibroblasts to restrict muLEC growth on the brain surface via controlling mature Vegfc distribution. Thus, our study uncovers a glia-mediated and neural-activity-regulated control of brain lymphatic development and highlights the importance of inter-tissue cellular cooperation in development.

Age-related meningeal extracellular matrix remodeling compromises CNS lymphatic function

Efficient clearance of central nervous system (CNS) waste proteins and appropriate immune surveillance is essential for brain health. These processes are facilitated by lymphatic networks present in the meninges that drain cerebrospinal fluid (CSF). Age-related impairments to meningeal lymphatic drainage contribute to CNS waste accumulation and immune dysfunction, yet the underlying mechanisms remain poorly understood. Here, we identify extracellular matrix (ECM) remodeling in the aged dura as a key driver of CSF clearance deficits, demonstrating that peri-lymphatic collagen accumulation disrupts lymphatic function. Exploring immune-derived factors contributing to this ECM remodeling, we identify transforming growth factor beta 1 (TGFβ1) as a major regulator using primary human dural fibroblasts. Using a novel mouse model with constitutively active TGFβ receptor 1 (TGFβR1) signaling in dural fibroblasts, we show that excessive peri-lymphatic collagen deposition impairs meningeal lymphatic drainage and alters meningeal immunity. Mechanistically, we reveal that ECM-associated matrix stiffness disrupts lymphatic junction integrity and impairs lymphangiogenesis in human lymphatic endothelial cells. These findings establish dural immune cell and fibroblast-mediated ECM remodeling as a critical regulator of CSF clearance and highlight it as a potential therapeutic target for restoring brain waste clearance in aging.

Microglia Promote Lymphangiogenesis Around the Spinal Cord Through VEGF-C/VEGFR3-Dependent Autophagy and Polarization After Acute Spinal Cord Injury

Reducing secondary injury is a key focus in the field of spinal cord injury (SCI). Recent studies have revealed the role of lymphangiogenesis in reducing secondary damage to central nerve. However, the mechanism of lymphangiogenesis is not yet clear. Macrophages have been shown to play an important role in peripheral tissue lymphangiogenesis. Microglia is believed to play a role similar to macrophages in the central nervous system (CNS); we hypothesized that there was a close relationship between microglia and central nerve system lymphangiogenesis. Herein, we used an in vivo model of SCI to explored the relationship between microglia and spinal cord lymphangiogenesis and further investigated the polarization of microglia and its role in promoting spinal cord lymphangiogenesis by a series of in vitro experiments. The current study elucidated for the first time the relationship between microglia and lymphangiogenesis around the spinal cord after SCI. Classical activated (M1) microglia can promote lymphangiogenesis by secreting VEGF-C which further increases polarization and secretion of lymphatic growth factor by activating VEGFR3. The VEGF-C/VEGFR3 pathway activation downregulates microglia autophagy, thereby regulating the microglia phenotype. These results indicate that M1 microglia promote lymphangiogenesis after SCI, and activated VEGF-C/VEGFR3 signaling promotes M1 microglia polarization by inhibiting autophagy, thereby facilitates lymphangiogenesis.

Meningeal lymphatic vessel crosstalk with central nervous system immune cells in aging and neurodegenerative diseases

Meningeal lymphatic vessels form a relationship between the nervous system and periphery, which is relevant in both health and disease. Meningeal lymphatic vessels not only play a key role in the drainage of brain metabolites but also contribute to antigen delivery and immune cell activation. The advent of novel genomic technologies has enabled rapid progress in the characterization of myeloid and lymphoid cells and their interactions with meningeal lymphatic vessels within the central nervous system. In this review, we provide an overview of the multifaceted roles of meningeal lymphatic vessels within the context of the central nervous system immune network, highlighting recent discoveries on the immunological niche provided by meningeal lymphatic vessels. Furthermore, we delve into the mechanisms of crosstalk between meningeal lymphatic vessels and immune cells in the central nervous system under both homeostatic conditions and neurodegenerative diseases, discussing how these interactions shape the pathological outcomes. Regulation of meningeal lymphatic vessel function and structure can influence lymphatic drainage, cerebrospinal fluid-borne immune modulators, and immune cell populations in aging and neurodegenerative disorders, thereby playing a key role in shaping meningeal and brain parenchyma immunity.

Regulation of VEGFR3 signaling in lymphatic endothelial cells

The receptor tyrosine kinase vascular endothelial growth factor (VEGF) receptor 3 (VEGFR3) is the principal transmembrane receptor responsible for sensing and coordinating cellular responses to environmental lymphangiogenic stimuli in lymphatic endothelial cells (LECs). VEGFC and D (VEGFC/D) function as the cognate ligands to VEGFR3 by stimulating autophosphorylation of intracellular VEGFR3 tyrosine kinase domains that activate signal cascades involved in lymphatic growth and survival. VEGFR3 primarily promotes downstream signaling through the phosphoinositide 3-kinase (PI3K) and Ras signaling cascades that promote functions including cell proliferation and migration. The importance of VEGFR3 cascades in lymphatic physiology is underscored by identification of dysfunctional VEGFR3 signaling across several lymphatic-related diseases. Recently, our group has shown that intracellular modification of VEGFR3 signaling is a potent means of inducing lymphangiogenesis independent of VEGFC. This is important because long-term treatment with recombinant VEGFC may have deleterious consequences due to off-target effects. A more complete understanding of VEGFR3 signaling pathways may lead to novel drug development strategies. The purpose of this review is to 1) characterize molecular mediators of VEGFC/VEGFR3 downstream signaling activation and their functional roles in LEC physiology and 2) explore molecular regulation of overall VEGFR3 expression and activity within LECs.

Cranial bone maneuver ameliorates Alzheimer's disease pathology via enhancing meningeal lymphatic drainage function

INTRODUCTION

Alzheimer's disease (AD) is a progressive neurodegenerative disease and the leading cause of dementia. Recent research highlights meningeal lymphatics as key regulators in neurological diseases, suggesting that enhancing their drainage function could be a potential therapeutic strategy for AD. Our proof-of-concept study demonstrated that cranial bone transport can improve meningeal lymphatic drainage function and promote ischemic stroke recovery.

METHODS

This study defined cranial bone maneuver (CBM) technique. After osteotomy, a small circular bone flap was made and attached to an external fixator for subsequent maneuver in a controlled fashion for a defined period using 5xFAD mice.

RESULTS

CBM treatment improved memory functions, reduced amyloid deposits, and promoted meningeal lymphatic drainage function. CBM induced cascades of inflammatory and lymphangiogenic processes in skull and meninges. Meningeal lymphatics are indispensable elements for the therapeutic effects of CBM.

DISCUSSION

CBM might be a promising innovative therapy for AD management, warranting further clinical investigation.

HIGHLIGHTS

Cranial bone maneuver (CBM) alleviated memory deficits and amyloid depositions. CBM promoted meningeal lymphangiogenesis and lymphatic drainage function. The beneficial effects of CBM lasted for a long time following the CBM procedures. CBM induced cascades of inflammatory and lymphangiogenic processes in the meninges. Meningeal lymphatic vessels are indispensable elements for CBM therapeutic effects.

Immune control of brain physiology

The peripheral immune system communicates with the brain through complex anatomical routes involving the skull, the brain borders, circumventricular organs and peripheral nerves. These immune-brain communication pathways were classically considered to be dormant under physiological conditions and active only in cases of infection or damage. Yet, peripheral immune cells and signals are key in brain development, function and maintenance. In this Perspective, we propose an alternative framework for understanding the mechanisms of immune-brain communication. During brain development and in homeostasis, these anatomical structures allow selected elements of the peripheral immune system to affect the brain directly or indirectly, within physiological limits. By contrast, in ageing and pathological settings, detrimental peripheral immune signals hijack the existing communication routes or alter their structure. We discuss why a diversity of communication channels is needed and how they work in relation to one another to maintain homeostasis of the brain.

Lymphatic-derived oxysterols promote anti-tumor immunity and response to immunotherapy in melanoma

In melanoma, lymphangiogenesis correlates with metastasis and poor prognosis and promotes immunosuppression. However, it also potentiates immunotherapy by supporting immune cell trafficking. We show in a lymphangiogenic murine melanoma that lymphatic endothelial cells (LECs) upregulate the enzyme Ch25h, which catalyzes the formation of 25-hydroxycholesterol (25-HC) from cholesterol and plays important roles in lipid metabolism, gene regulation, and immune activation. We identify a role for LECs as a source of extracellular 25-HC in tumors inhibiting PPAR-γ in intra-tumoral macrophages and monocytes, preventing their immunosuppressive function and instead promoting their conversion into proinflammatory myeloid cells that support effector T cell functions. In human melanoma, LECs also upregulate Ch25h, and its expression correlates with the lymphatic vessel signature, infiltration of pro-inflammatory macrophages, better patient survival, and better response to immunotherapy. We identify here in mechanistic detail an important LEC function that supports anti-tumor immunity, which can be therapeutically exploited in combination with immunotherapy.

Emerging role of adaptive immunity in diabetes-induced cognitive impairment: from the periphery to the brain

Diabetic cognitive impairment (DCI) is a central nervous system complication induced by peripheral metabolic dysfunction of diabetes mellitus. Cumulative studies have shown that neuro-immune crosstalk is involved in the pathological progression of DCI. However, current studies mostly focus on the interaction between innate immunity cells and neurons, while ignoring the role of adaptive immunity cells in DCI. Notably, recent studies have revealed adaptive immune cells are involved in cognitive development and the progression of neurodegenerative diseases. Equally important, accumulated past studies have also shown that diabetic patients experience imbalanced peripheral adaptive immune homeostasis and disrupted transmission of adaptive immune cells to the central system. Therefore, this review first updated the cognitive mechanism of adaptive immune regulation, and then summarized the contribution of adaptive immunity to DCI from the aspects of peripheral adaptive immune homeostasis, transmission pathways, and brain tissue infiltration. Furthermore, we also summarized the potential of anti-diabetic drugs to regulate adaptive immunity, and looked forward to the potential value of regulatory adaptive immunity in the prevention and treatment of DCI, to provide a new strategy for the prevention and treatment of DCI.

Modulation of glymphatic system by visual circuit activation alleviates memory impairment and apathy in a mouse model of Alzheimer's disease

Alzheimer's disease is characterized by progressive amyloid deposition and cognitive decline, yet the pathological mechanisms and treatments remain elusive. Here we report the therapeutic potential of low-intensity 40 hertz blue light exposure in a 5xFAD mouse model of Alzheimer's disease. Our findings reveal that light treatment prevents memory decline in 4-month-old 5xFAD mice and motivation loss in 14-month-old 5xFAD mice, accompanied by restoration of glial water channel aquaporin-4 polarity, improved brain drainage efficiency, and a reduction in hippocampal lipid accumulation. We further demonstrate the beneficial effects of 40 hertz blue light are mediated through the activation of the vLGN/IGL-Re visual circuit. Notably, concomitant use of anti-Aβ antibody with 40 hertz blue light demonstrates improved soluble Aβ clearance and cognitive performance in 5xFAD mice. These findings offer functional evidence on the therapeutic effects of 40 hertz blue light in Aβ-related pathologies and suggest its potential as a supplementary strategy to augment the efficacy of antibody-based therapy.

Remodeling of the brain angioarchitecture in experimental chronic neurodegeneration

Chronic neurodegenerative diseases are characterized by substantial inflammation with putative impairment of the brain vasculature also commonly observed. To address effects of chronic neurodegeneration on the regional vasculature under experimentally controlled circumstances, the glutamate receptor agonist ibotenic acid was injected into striatum of adult rats, which causes excitotoxicity in the substantia nigra pars reticulata (SNpr) due to imbalance between inhibitory inputs from the striatum and excitatory signals from the subthalamic nucleus. Brains were examined at 28 days (short-term neurodegeneration) and 91 days (long-term neurodegeneration) and analyzed for vascular remodeling taking both 2D and 3D approaches, the latter involving confocal microscopy of optically cleared samples combined with machine learning-based image analysis. Crysectioned and microdissected samples were analyzed for protein and gene expression respectively. The resulting neurodegeneration was accompanied by regional tissue loss and inflammation. The 3D analysis of the degenerating SNpr revealed substantial changes of the vasculature with higher density, increased diameter, and number of tortuous vessels already after 28 days, evidently continuing at 91 days. Interestingly, the vascular remodeling changes occurred without changes in the expression of endothelial tight junction proteins, vascular basement membrane proteins, or markers of angiogenesis. We propose that remodeling of the vasculature in neurodegeneration occurs due to regional tissue atrophy, which leaves the vasculature operating but prone to additional pathologies.

Lumped parameter simulations of cervical lymphatic vessels: dynamics of murine cerebrospinal fluid efflux from the skull

BACKGROUND

Growing evidence suggests that for rodents, a substantial fraction of cerebrospinal fluid (CSF) drains by crossing the cribriform plate into the nasopharyngeal lymphatics, eventually reaching the cervical lymphatic vessels (CLVs). Disruption of this drainage pathway is associated with various neurological disorders.

METHODS

We employ a lumped parameter method to numerically model CSF drainage across the cribriform plate to CLVs. Our model uses intracranial pressure as an inlet pressure and central venous blood pressure as an outlet pressure. The model incorporates initial lymphatic vessels (modeling those in the nasal region) that absorb the CSF and collecting lymphatic vessels (modeling CLVs) to transport the CSF against an adverse pressure gradient. To determine unknown parameters such as wall stiffness and valve properties, we utilize a Monte Carlo approach and validate our simulation against recent in vivo experimental measurements.

RESULTS

Our parameter analysis reveals the physical characteristics of CLVs. Our results suggest that the stiffness of the vessel wall and the closing state of the valve are crucial for maintaining the vessel size and volume flow rate observed in vivo. We find that a decreased contraction amplitude and frequency leads to a reduction in volume flow rate, and we test the effects of varying the different pressures acting on the CLVs. Finally, we provide evidence that branching of initial lymphatic vessels may deviate from Murray's law to reduce sensitivity to elevated intracranial pressure.

CONCLUSIONS

This is the first numerical study of CSF drainage through CLVs. Our comprehensive parameter analysis offers guidance for future numerical modeling of CLVs. This study also provides a foundation for understanding physiology of CSF drainage, helping guide future experimental studies aimed at identifying causal mechanisms of reduction in CLV transport and potential therapeutic approaches to enhance flow.

Advances in Research on Meningeal Lymphatic Vessels in Central Nervous System Diseases

Meningeal lymphatic vessels (mLVs), located around the dural sinuses, are considered significant participants in cerebrospinal fluid (CSF) circulation. Meningeal lymphatic vessels not only drain fluids and metabolic waste from the brain into deep cervical lymph nodes (dCLNs) but also transport immune cells from the brain to dCLNs, thus regulating the interaction between the central and peripheral immune systems. These vessels play a crucial role in maintaining normal physiological functions of the central nervous system (CNS). Meningeal lymphatic vessels are involved in the pathophysiological processes of various CNS diseases, including neurodegenerative diseases, cerebrovascular diseases, and brain tumors. In aging and various CNS diseases, damage and dysfunction of mLVs have been observed, leading to the abnormal accumulation of toxic substances and exacerbating neural damage. By transporting antigen-presenting cells that have taken up antigens within the brain to dCLNs, mLVs modulate the activation of peripheral immune cells and their migration and infiltration into brain lesions. Certain drug interventions or physical therapies can modulate the drainage function of mLVs, effectively improving the prognosis of CNS diseases. This review provides a detailed introduction to the anatomic structure, physiological roles, and research advances of mLVs in CNS diseases. In addition, we propose new strategies for targeting mLVs in the treatment of CNS diseases.

Panax Notoginseng Saponins promotes the meningeal lymphatic system-mediated hematoma absorption in intracerebral hemorrhage

BACKGROUND

Hematoma clearance is crucial for treating intracerebral hemorrhage (ICH). Currently, there is a lack of pharmacological therapy aimed at promoting hematoma absorption. Meningeal lymphatic system, as a drain of brain, is a potential therapeutic approach in ICH. Panax Notoginseng Saponins (PNS), proven to promote lymphangiogenesis in periphery, effectively reduces hematoma in ICH patients. However, the potential pharmacological effect of PNS on meningeal lymphatic vessels (MLVs) remains unknown.

PURPOSE

In this study, we aimed to investigate the impact of PNS on the meningeal lymphatic system and ICH.

METHODS

The collagenase-ICH model was conducted to investigate the effect of PNS. Behavioral tests, including modified neurological severity score (mNSS) and foot-fault test, and hematoma volume were used to estimate the neurological function and curative effect. The structure and drainage function of MLVs was detected by immunohistochemical staining. Visudyne intracisternal magna injection combined with red laser photoconversion was performed to ablate MLVs. RNA-sequencing was used to obtain mRNA profiles for mechanistic investigation.

RESULTS

The meningeal lymphatic drainage function was enhanced after ICH on day 14 without obvious lymphangiogenesis. Additionally, PNS further facilitated the process of drain with simultaneously inducing lymphangiogenesis. Moreover, ablation of MLVs by photoconverting of visudyne significantly blocked the benefits of neurological deficits improvement and hematoma absorption conducted by PNS. Furthermore, RNA-sequencing revealed that PNS regulated axonogenesis and inflammation, relying on the intact MLVs. In which, solute carrier family 17 member 7 (Slc17a7) and tumor necrosis factor (Tnf) were identified as bottleneck and hub nodes of the protein-protein interaction network of target genes, respectively.

CONCLUSION

PNS might be effective for ICH treatment by enhancing lymphangiogenesis and the meningeal lymphatic drainage function, thereby attenuating inflammation and promoting neurological recovery. The role of PNS in regulation of MLVs was investigated for the first time. This study provides a novel insight for PNS in the medical therapy of ICH.

Reimagining the meninges from a neuroimmune perspective: a boundary, but not peripheral

Recent advances in neuroscience have transformed our understanding of the meninges, the layers surrounding the central nervous system (CNS). Two key findings have advanced our understanding: researchers identified cranial bone marrow as a reservoir for meningeal immune cells, and rediscovered a brain lymphatic system. Once viewed merely as a protective barrier, the meninges are now recognized as a dynamic interface crucial for neuroimmune interactions. This shift in perspective highlights their unique role in maintaining CNS balance, shaping brain development, and regulating responses to injury and disease. This review synthesizes the latest insights into meningeal anatomy and function, with a focus on newly identified structures such as dural-associated lymphoid tissues (DALT) and arachnoid cuff exit (ACE) points. We also examine the diverse immune cell populations within the meninges and their interactions with the CNS, underscoring the emerging view of the meninges as active participants in brain immunity. Finally, we outline critical unanswered questions about meningeal immunity, proposing directions for future research. By addressing these knowledge gaps, we aim to deepen our understanding of the meninges' role in brain health and disease, potentially paving the way for novel therapeutic approaches.

A neuro-lymphatic communication guides lymphatic development by CXCL12 and CXCR4 signaling

Lymphatic vessels grow through active sprouting and mature into a vascular complex that includes lymphatic capillaries and collecting vessels that ensure fluid transport. However, the signaling cues that direct lymphatic sprouting and patterning remain unclear. In this study, we demonstrate that chemokine signaling, specifically through CXCL12 and CXCR4, plays crucial roles in regulating lymphatic development. We show that LEC-specific Cxcr4-deficient mouse embryos and CXCL12 mutant embryos exhibit severe defects in lymphatic sprouting, migration and lymphatic valve formation. We also discovered that CXCL12, originating from peripheral nerves, directs the migration of dermal lymphatic vessels to align with nerves in developing skin. Deletion of Cxcr4 or blockage of CXCL12 and CXCR4 activity results in reduced VEGFR3 levels on the LEC surface. This, in turn, impairs VEGFC-mediated VEGFR3 signaling and downstream PI3K and AKT activities. Taken together, these data identify previously unknown chemokine signaling originating from peripheral nerves that guides dermal lymphatic sprouting and patterning. Our work identifies for the first time a neuro-lymphatics communication during mouse development and reveals a previously unreported mechanism by which CXCR4 modulates VEGFC, VEGFR3 and AKT signaling.

Cerebral small vessel disease and glymphatic system dysfunction in multiple sclerosis: A narrative review

As the multiple sclerosis (MS) population ages, the prevalence of vascular comorbidities increases, potentially accelerating disease progression and brain atrophy. Recent studies highlight the prevalence of cerebral small vessel disease (CSVD) in MS, suggesting a potential link between vascular comorbidities and accelerated disability. CSVD affects the brain's small vessels, often leading to identifiable markers on MRI such as enlarged perivascular spaces (EPVS). EPVS are increasingly recognized also in MS and have been associated with vascular comorbidities, lower percentage of MS-specific perivenular lesions, brain atrophy and aging. The exact sequence of event leading to MRI visible EPVS is yet to be determined, but an impaired perivascular brain fluid drainage appears a possible physiopathological explanation for EPVS in both CSVD and MS. In this context, a dysfunction of the brain fluid clearance system - also known as "glymphatic system" - appears associated in MS to aging, neuroinflammation, and vascular dysfunction. Advanced imaging techniques show an impaired glymphatic function in both MS and CSVD. Additionally, lifestyle factors such as physical exercise, diet, and sleep quality appear to influence glymphatic function, potentially revealing novel therapeutic strategies to mitigate microangiopathy and neuroinflammation in MS. This review underscores the potential role of glymphatic dysfunction in the complex and not-yet elucidated interplay between neuroinflammation and CSVD in MS.

Impairment of spinal CSF flow precedes immune cell infiltration in an active EAE model

Accumulation of immune cells and proteins in the subarachnoid space (SAS) is found during multiple sclerosis and in the animal model experimental autoimmune encephalomyelitis (EAE). Whether the flow of cerebrospinal fluid (CSF) along the SAS of the spinal cord is impacted is yet unknown. Combining intravital near-infrared (NIR) imaging with histopathological analyses, we observed a significantly impaired bulk flow of CSF tracers within the SAS of the spinal cord prior to EAE onset, which persisted until peak stage and was only partially recovered during chronic disease. The impairment of spinal CSF flow coincided with the appearance of fibrin aggregates in the SAS, however, it preceded immune cell infiltration and breakdown of the glia limitans superficialis. Conversely, cranial CSF efflux to cervical lymph nodes was not altered during the disease course. Our study highlights an early and persistent impairment of spinal CSF flow and suggests it as a sensitive imaging biomarker for pathological changes within the leptomeninges.

Meningeal lymphatic function promotes oligodendrocyte survival and brain myelination

The precise neurophysiological changes prompted by meningeal lymphatic dysfunction remain unclear. Here, we showed that inducing meningeal lymphatic vessel ablation in adult mice led to gene expression changes in glial cells, followed by reductions in mature oligodendrocyte numbers and specific lipid species in the brain. These phenomena were accompanied by altered meningeal adaptive immunity and brain myeloid cell activation. During brain remyelination, meningeal lymphatic dysfunction provoked a state of immunosuppression that contributed to delayed spontaneous oligodendrocyte replenishment and axonal loss. The deficiencies in mature oligodendrocytes and neuroinflammation due to impaired meningeal lymphatic function were solely recapitulated in immunocompetent mice. Patients diagnosed with multiple sclerosis presented reduced vascular endothelial growth factor C in the cerebrospinal fluid, particularly shortly after clinical relapses, possibly indicative of poor meningeal lymphatic function. These data demonstrate that meningeal lymphatics regulate oligodendrocyte function and brain myelination, which might have implications for human demyelinating diseases.

Parasagittal dural volume correlates with cerebrospinal fluid volume and developmental delay in children with autism spectrum disorder

BACKGROUND

The parasagittal dura, a tissue that lines the walls of the superior sagittal sinus, acts as an active site for immune-surveillance, promotes the reabsorption of cerebrospinal fluid, and facilitates the removal of metabolic waste products from the brain. Cerebrospinal fluid is important for the distribution of growth factors that signal immature neurons to proliferate and migrate. Autism spectrum disorder is characterized by altered cerebrospinal fluid dynamics.

METHODS

In this retrospective study, we investigated potential correlations between parasagittal dura volume, brain structure volumes, and clinical severity scales in young children with autism spectrum disorder. We employed a semi-supervised two step pipeline to extract parasagittal dura volume from 3D-T2 Fluid Attenuated Inversion Recovery sequences, based on U-Net followed by manual refinement of the extracted parasagittal dura masks.

RESULTS

Here we show that the parasagittal dura volume does not change with age but is significantly correlated with cerebrospinal fluid (p-value = 0.002), extra-axial cerebrospinal fluid volume (p-value = 0.0003) and severity of developmental delay (p-value = 0.024).

CONCLUSIONS

These findings suggest that autism spectrum disorder children with severe developmental delay may have a maldeveloped parasagittal dura that potentially perturbs cerebrospinal fluid dynamics.

The emerging importance of lymphangiogenesis in aging and aging-associated diseases

Lymphatic aging represented by cellular and functional changes, is involved in increased geriatric disorders, but the intersection between aging and lymphatic modulation is less clear. Lymphatic vessels play an essential role in maintaining tissue fluid homeostasis, regulating immune function, and promoting macromolecular transport. Lymphangiogenesis and lymphatic remodeling following cellular senescence and organ deterioration are crosslinked with the progression of some lymphatic-associated diseases, e.g., atherosclerosis, inflammation, lymphoedema, and cancer. Age-related detrimental tissue changes may occur in lymphatic vessels with diverse etiologies, and gradually shift towards chronic low-grade inflammation, so-called inflammaging, and lead to decreased immune response. The investigation of the relationship between advanced age and organ deterioration is becoming an area of rapidly increasing significance in lymphatic biology and medicine. Here we highlight the emerging importance of lymphangiogenesis and lymphatic remodeling in the regulation of aging-related pathological processes, which will help to find new avenues for effective intervention to promote healthy aging.

Simvastatin alleviates glymphatic system damage via the VEGF-C/VEGFR3/PI3K-Akt pathway after experimental intracerebral hemorrhage

Current clinical practice primarily relies on surgical intervention to remove hematomas in patients with intracerebral hemorrhage (ICH), given the lack of effective drug therapies. Previous research indicates that simvastatin (SIM) may enhance hematoma absorption and resolution in the acute phase of ICH, though the precise mechanisms remain unclear. Recent findings have highlighted the glymphatic system (GS) as a crucial component in intracranial cerebrospinal fluid circulation, playing a significant role in hematoma clearance post-ICH. This study investigates the link between SIM efficacy in hematoma resolution and the GS. Our experimental results show that SIM alleviates GS damage in ICH-induced rats, resulting in improved outcomes such as reduced brain edema, neuronal apoptosis, and degeneration. Further analysis reveals that SIM's effects are mediated through the VEGF-C/VEGFR3/PI3K-Akt pathway. This study advances our understanding of SIM's mechanism in promoting intracranial hematoma clearance and underscores the potential of targeting the GS for ICH treatment.

Subarachnoid hemorrhage distinctively disrupts the glymphatic and meningeal lymphatic systems in beagles

Subarachnoid hemorrhage (SAH) induced acute impairment of the glymphatic system, but few have investigated the dysfunction of the meningeal lymphatic system and their contribution to the pathophysiology of SAH. In addition, most studies were conducted in rodent animals. We aimed to investigate the impact of SAH on glymphatic and meningeal lymphatic function in a large animal model using beagles and to evaluate the effects of intermittent cistern magna CSF drainage on these systems. Methods: The SAH model was created in beagles via endovascular perforation using a digital subtraction angiography machine. Intermittent cistern magna CSF drain was performed daily from 1 d to 3 d after SAH. We examined CSF pressure, neuronal death, enlargement of perivascular space (PVS), hydrocephalus, and neurological and cognitive deficits before and after SAH. The dynamics of glymphatic and meningeal lymphatic functions were analyzed by quantifying the signal intensity of dimeglumine gadopentetate (Gd-DTPA) using T1-weighted magnetic resonance imaging (MRI). Measurements were taken before SAH and at 1 h, 1 week, and 2 weeks post-SAH. Results: SAH in beagles caused significant blood clots, neuronal death, increased CSF pressure, hydrocephalus, and neurological and cognitive deficits. MRI revealed dilated ventricles and enlarged PVS post-SAH. The glymphatic system's function, assessed by Gd-DTPA distribution, showed reduced CSF influx and glymphatic impairment after SAH, particularly in the ipsilateral hemisphere, persisting for a week with partial recovery at 2 weeks. For lymphatic clearance, Gd-DTPA rapidly filled the olfactory bulbs, optic nerves, facial and vestibulocochlear nerves, and spinal nerves under normal conditions. SAH caused delayed and reduced Gd-DTPA efflux outflow in these areas, disrupting lymphatic clearance. Despite initial dysfunction, increased hemoglobin levels in cervical lymph nodes indicated active blood clearance post-SAH, with recovery by 2 weeks. Treatment with intermittent cistern magna CSF drain significantly ameliorated the glymphatic and meningeal lymphatic dysfunction after SAH. Conclusion: SAH impaired both glymphatic and meningeal lymphatic functions in beagles, with better restoration of lymphatic function post-SAH, which may contribute to functional recovery after SAH. External CSF drain is an effective therapeutic approach to facilitate the recovery of glymphatic and meningeal lymphatic function following SAH.

Therapeutic approaches to CNS diseases via the meningeal lymphatic and glymphatic system: prospects and challenges

The brain has traditionally been considered an "immune-privileged" organ lacking a lymphatic system. However, recent studies have challenged this view by identifying the presence of the glymphatic system and meningeal lymphatic vessels (MLVs). These discoveries offer new opportunities for waste clearance and treatment of central nervous system (CNS) diseases. Various strategies have been developed based on these pathways, including modulation of glymphatic system function, enhancement of meningeal lymphatic drainage, and utilization of these routes for drug delivery. Consequently, this review explores the developmental features and physiological roles of the cerebral lymphatic system as well as its significance in various CNS disorders. Notably, strategies for ameliorating CNS diseases have been discussed with a focus on enhancing glymphatic system and MLVs functionality through modulation of physiological factors along with implementing pharmacological and physical treatments. Additionally, emphasis is placed on the potential use of the CNS lymphatic system in drug delivery while envisioning future directions in terms of mechanisms, applications, and translational research.

Acupuncture promotes neurological recovery and regulates lymphatic function after acute inflammatory nerve root injury

Aims

To investigate the therapeutic effect of acupuncture on acute inflammatory nerve root injury by regulating lymphatic function.

Main methods

A mouse model of L5 nerve root compression was used to simulate acute nerve root injury. After modeling, acupuncture treatment was given each day for one week. Pain thresholds were assessed before and after modeling and treatment. Immunofluorescence staining was performed to observe the distribution astrocytes and neurons in the lumbar spinal cord, the innervation rate of neuromuscular junctions (NMJs), lymphatic endothelial cells (LECs) of lumbar aortic lymph nodes, and the percentage of M1 macrophages. The number of each type of immune cells in the lumbar aortic lymph nodes (LALNs) was measured by flow cytometry.

Key findings

The model group showed a significant decrease in pain threshold in the affected lower limb, while acupuncture treatment was able to significantly increase it. Acupuncture significantly repaired astrocytes and neurons in the lumbar spinal cord of the compressed segment, increased the innervation rate of nerve endings at NMJs, reduced LECs in the LALNs, reduced the proportion of M1 macrophages in the LALNs, and significantly reduced mononuclear neutrophils and monocytic neutrophils.

Significance

Acupuncture can reduce pain, promote nerve repair in mice with acute nerve root injury, and suppress immune responses in lumbar aortic lymph nodes.

Meningeal Lymphatics in Central Nervous System Diseases

Since its recent discovery, the meningeal lymphatic system has reshaped our understanding of central nervous system (CNS) fluid exchange, waste clearance, immune cell trafficking, and immune privilege. Meningeal lymphatics have also been demonstrated to functionally modify the outcome of neurological disorders and their responses to treatment, including brain tumors, inflammatory diseases such as multiple sclerosis, CNS injuries, and neurodegenerative disorders such as Alzheimer's and Parkinson's diseases. In this review, we discuss recent evidence of the contribution of meningeal lymphatics to neurological diseases, as well as the available experimental methods for manipulating meningeal lymphatics in these conditions. Finally, we also provide a discussion of the pressing questions and challenges in utilizing meningeal lymphatics as a prime target for CNS therapeutic intervention and possibly drug delivery for brain disorders.

Neuroimmune modulation in liver pathophysiology

The liver, the largest organ in the human body, plays a multifaceted role in digestion, coagulation, synthesis, metabolism, detoxification, and immune defense. Changes in liver function often coincide with disruptions in both the central and peripheral nervous systems. The intricate interplay between the nervous and immune systems is vital for maintaining tissue balance and combating diseases. Signaling molecules and pathways, including cytokines, inflammatory mediators, neuropeptides, neurotransmitters, chemoreceptors, and neural pathways, facilitate this complex communication. They establish feedback loops among diverse immune cell populations and the central, peripheral, sympathetic, parasympathetic, and enteric nervous systems within the liver. In this concise review, we provide an overview of the structural and compositional aspects of the hepatic neural and immune systems. We further explore the molecular mechanisms and pathways that govern neuroimmune communication, highlighting their significance in liver pathology. Finally, we summarize the current clinical implications of therapeutic approaches targeting neuroimmune interactions and present prospects for future research in this area.

Dynamics of Endothelial Cell Diversity and Plasticity in Health and Disease

Endothelial cells (ECs) are vital structural units of the cardiovascular system possessing two principal distinctive properties: heterogeneity and plasticity. Endothelial heterogeneity is defined by differences in tissue-specific endothelial phenotypes and their high predisposition to modification along the length of the vascular bed. This aspect of heterogeneity is closely associated with plasticity, the ability of ECs to adapt to environmental cues through the mobilization of genetic, molecular, and structural alterations. The specific endothelial cytoarchitectonics facilitate a quick structural cell reorganization and, furthermore, easy adaptation to the extrinsic and intrinsic environmental stimuli, known as the epigenetic landscape. ECs, as universally distributed and ubiquitous cells of the human body, play a role that extends far beyond their structural function in the cardiovascular system. They play a crucial role in terms of barrier function, cell-to-cell communication, and a myriad of physiological and pathologic processes. These include development, ontogenesis, disease initiation, and progression, as well as growth, regeneration, and repair. Despite substantial progress in the understanding of endothelial cell biology, the role of ECs in healthy conditions and pathologies remains a fascinating area of exploration. This review aims to summarize knowledge and concepts in endothelial biology. It focuses on the development and functional characteristics of endothelial cells in health and pathological conditions, with a particular emphasis on endothelial phenotypic and functional heterogeneity.

Neuroimmunology of Cardiovascular Disease

PURPOSE OF REVIEW

Cardiovascular disease (CVD) is a leading cause of death and chronic disability worldwide. Yet, despite extensive intervention strategies the number of persons affected by CVD continues to rise. Thus, there is great interest in unveiling novel mechanisms that may lead to new treatments. Considering this dilemma, recent focus has turned to the neuroimmune mechanisms involved in CVD pathology leading to a deeper understanding of the brain's involvement in disease pathology. This review provides an overview of new and salient findings regarding the neuroimmune mechanisms that contribute to CVD.

RECENT FINDINGS

The brain contains neuroimmune niches comprised of glia in the parenchyma and immune cells at the brain's borders, and there is strong evidence that these neuroimmune niches are important in both health and disease. Mechanistic studies suggest that the activation of glia and immune cells in these niches modulates CVD progression in hypertension and heart failure and contributes to the inevitable end-organ damage to the brain. This review provides evidence supporting the role of neuroimmune niches in CVD progression. However, additional research is needed to understand the effects of prolonged neuroimmune activation on brain function.

Impaired Meningeal Lymphatics and Glymphatic Pathway in Patients with White Matter Hyperintensity

White matter hyperintensity (WMH) represents a critical global medical concern linked to cognitive decline and dementia, yet its underlying mechanisms remain poorly understood. Here, humans are directly demonstrated that high WMH burden correlates with delayed drainage of meningeal lymphatic vessels (mLVs) and glymphatic pathway. Additionally, a longitudinal cohort study reveals that glymphatic dysfunction predicts WMH progression. Next, in a rat model of WMH, the presence of impaired lymphangiogenesis and glymphatic drainage is confirmed, followed by elevated microglial activation and white matter demyelination. Notably, enhancing meningeal lymphangiogenesis through adeno-associated virus delivery of vascular endothelial growth factor-C (VEGF-C) mitigates microglial gliosis and white matter demyelination. Conversely, blocking the growth of mLVs with a VEGF-C trap strategy exacerbates these changes. The findings highlight the role of mLVs and glymphatic pathway dysfunction in aggravating brain white matter injury, providing a potential novel strategy for WMH prevention and treatment.

Lymphatic Vessels in the Inner Ear of Patients With Meniere Disease: A Novel Pathological Finding

Background

Meniere disease, characterized by intermittent episodes of vertigo, fluctuating sensorineural hearing loss, tinnitus, and aural pressure, is a common cause of vertigo in humans. The pathogenesis of Meniere disease remains unknown. The current study aimed to describe a novel pathological change discovered in the inner ears of patients with Meniere disease who underwent labyrinthectomy.

Methods

This retrospective case-control study was conducted with 21 patients with MD who underwent labyrinthectomy. A total of 15 patients diagnosed with acoustic neuroma or glomus jugular tumor were review over the same period of time as control. The clinical information of the patients and the pathological features of the membrane are described.

Results

The new pathological tissue was a morbid membrane structure sealing the round window, characterized by the formation of lymphatic capillaries. Histochemical and immunofluorescent staining was positive for D2-40, LYVE-1, podoplanin, and PROX1, which are the classical markers of the lymphatic vessels. Transmission electron microscopy revealed that the lymph capillaries lacked a typical basement membrane and that their ends were blind, composed of a single layer of endothelial cells with valval connection structures between adjacent capillary epithelial cells.

Conclusion

This is the first report of lymphatic vessels in the human inner ear, and this pathological structure is a completely new discovery. The lymphatic vessels may develop due to inflammation or decompensation of pressure in the inner ear, suggesting that the inner ear can reactively form lymphatic vessels in some inflammation and fluid flow-dependent pathological conditions. The current findings help in improving our understanding of the pathogenesis of Meniere disease.

Spinal Lymphatic Dysfunction Aggravates the Recovery Process After Spinal Cord Injury

We aimed to evaluate the role of the spinal lymphatic system in spinal cord injury and whether it has an impact on recovery after spinal cord injury. Flow cytometry was used to evaluate the changes in the number of microvesicles after spinal cord injury. Evans blue extravasation was used to evaluate the function of the lymphatic system. Evans blue extravasation and immunofluorescence were used to evaluate the permeability of blood spinal cord barrier. The spinal cord edema was evaluated by dry and wet weight.Terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL) assay was used to evaluate apoptosis after spinal cord injury. Nuclear factor-kappa B pathway was detected by Western blot. Behavioral tests were used to evaluate limb function. Microvesicles released after spinal cord injury can enter the thoracic duct and then enter the blood through the lymph around the spine. After ligation of the thoracic duct, it can aggravate the neuropathological manifestations and limb function after spinal cord injury. The potential mechanism may involve nuclear factor-kappa B pathway.

Age-related changes in meningeal lymphatic function are closely associated with vascular endothelial growth factor-C expression

Meningeal lymphatic vessels (MLVs) have crucial roles in removing metabolic waste and toxic proteins from the brain and transporting them to the periphery. Aged mice show impaired meningeal lymphatic function. Nevertheless, as the disease progresses, and significant pathological changes manifest in the brain, treating the condition becomes increasingly challenging. Therefore, investigating the alterations in the structure and function of MLVs in the early stages of aging is critical for preventing age-related central nervous system degenerative diseases. We detected the structure and function of MLVs in young, middle-aged, and aged mice. Middle-aged mice, compared with young and aged mice, showed enhanced meningeal lymphatic function along with MLV expansion and performed better in the Y maze test. Moreover, age-related changes in meningeal lymphatic function were closely associated with vascular endothelial growth factor-C (VEGF-C) expression in the brain cortex. Our data suggested that the cerebral cortex may serve as a target for VEGF-C supplementation to ameliorate meningeal lymphatic dysfunction, thus providing a new strategy for preventing age-related central nervous system diseases.

Lymphatic vessels in the age of cancer immunotherapy

Lymphatic transport maintains homeostatic health and is necessary for immune surveillance, and yet lymphatic growth is often associated with solid tumour development and dissemination. Although tumour-associated lymphatic remodelling and growth were initially presumed to simply expand a passive route for regional metastasis, emerging research puts lymphatic vessels and their active transport at the interface of metastasis, tumour-associated inflammation and systemic immune surveillance. Here, we discuss active mechanisms through which lymphatic vessels shape their transport function to influence peripheral tissue immunity and the current understanding of how tumour-associated lymphatic vessels may both augment and disrupt antitumour immune surveillance. We end by looking forward to emerging areas of interest in the field of cancer immunotherapy in which lymphatic vessels and their transport function are likely key players: the formation of tertiary lymphoid structures, immune surveillance in the central nervous system, the microbiome, obesity and ageing. The lessons learnt support a working framework that defines the lymphatic system as a key determinant of both local and systemic inflammatory networks and thereby a crucial player in the response to cancer immunotherapy.

The contribution of the meningeal immune interface to neuroinflammation in traumatic brain injury

Traumatic brain injury (TBI) is a major cause of disability and mortality worldwide, particularly among the elderly, yet our mechanistic understanding of what renders the post-traumatic brain vulnerable to poor outcomes, and susceptible to neurological disease, is incomplete. It is well established that dysregulated and sustained immune responses elicit negative consequences after TBI; however, our understanding of the neuroimmune interface that facilitates crosstalk between central and peripheral immune reservoirs is in its infancy. The meninges serve as the interface between the brain and the immune system, facilitating important bi-directional roles in both healthy and disease settings. It has been previously shown that disruption of this system exacerbates neuroinflammation in age-related neurodegenerative disorders such as Alzheimer's disease; however, we have an incomplete understanding of how the meningeal compartment influences immune responses after TBI. In this manuscript, we will offer a detailed overview of the holistic nature of neuroinflammatory responses in TBI, including hallmark features observed across clinical and animal models. We will highlight the structure and function of the meningeal lymphatic system, including its role in immuno-surveillance and immune responses within the meninges and the brain. We will provide a comprehensive update on our current knowledge of meningeal-derived responses across the spectrum of TBI, and identify new avenues for neuroimmune modulation within the neurotrauma field.

Water channels in the brain and spinal cord-overview of the role of aquaporins in traumatic brain injury and traumatic spinal cord injury

Knowledge about the mechanisms underlying the fluid flow in the brain and spinal cord is essential for discovering the mechanisms implicated in the pathophysiology of central nervous system diseases. During recent years, research has highlighted the complexity of the fluid flow movement in the brain through a glymphatic system and a lymphatic network. Less is known about these pathways in the spinal cord. An important aspect of fluid flow movement through the glymphatic pathway is the role of water channels, especially aquaporin 1 and 4. This review provides an overview of the role of these aquaporins in brain and spinal cord, and give a short introduction to the fluid flow in brain and spinal cord during in the healthy brain and spinal cord as well as during traumatic brain and spinal cord injury. Finally, this review gives an overview of the current knowledge about the role of aquaporins in traumatic brain and spinal cord injury, highlighting some of the complexities and knowledge gaps in the field.

Piezo1 regulates meningeal lymphatic vessel drainage and alleviates excessive CSF accumulation

Piezo1 regulates multiple aspects of the vascular system by converting mechanical signals generated by fluid flow into biological processes. Here, we find that Piezo1 is necessary for the proper development and function of meningeal lymphatic vessels and that activating Piezo1 through transgenic overexpression or treatment with the chemical agonist Yoda1 is sufficient to increase cerebrospinal fluid (CSF) outflow by improving lymphatic absorption and transport. The abnormal accumulation of CSF, which often leads to hydrocephalus and ventriculomegaly, currently lacks effective treatments. We discovered that meningeal lymphatics in mouse models of Down syndrome were incompletely developed and abnormally formed. Selective overexpression of Piezo1 in lymphatics or systemic administration of Yoda1 in mice with hydrocephalus or Down syndrome resulted in a notable decrease in pathological CSF accumulation, ventricular enlargement and other associated disease symptoms. Together, our study highlights the importance of Piezo1-mediated lymphatic mechanotransduction in maintaining brain fluid drainage and identifies Piezo1 as a promising therapeutic target for treating excessive CSF accumulation and ventricular enlargement.

Glymphatic Imaging in Pediatrics

The glymphatic system, which facilitates cerebrospinal fluid (CSF) flow through the brain parenchyma, is important for brain development and waste clearance. Advances in imaging techniques, particularly magnetic resonance imaging, have make it possible to evaluate glymphatic structures and functions in vivo. Recently, several studies have focused on the development and alterations of the glymphatic system in pediatric disorders. This review discusses the development of the glymphatic system, advances of imaging techniques and their applications in pediatric disorders. First, the results of the reviewed studies indicate that the development of the glymphatic system is a long-lasting process that continues into adulthood. Second, there is a need for improved glymphatic imaging techniques that are non-invasive and fast to improve suitability for pediatric applications, as some of existing methods use contrast injection and are susceptible to motion artifacts from long scanning times. Several novel techniques are potentially feasible for pediatric patients and may be used in the future. Third, the glymphatic dysfunction is associated with a large number of pediatric disorders, although only a few have recently been investigated. In conclusion, research on the pediatric glymphatic system remains an emerging field. The preliminary applications of glymphatic imaging techniques have provided unique insight into the pathological mechanism of pediatric diseases, but mainly limited in visualization of enlarged perivascular spaces and morphological measurements on CSF volumes. More in-depth studies on glymphatic functions are required to improve our understanding of the mechanisms underlying brain development and pediatric diseases. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY: Stage 3.

Advancing insights into in vivo meningeal lymphatic vessels with stereoscopic wide-field photoacoustic microscopy

Meningeal lymphatic vessels (mLVs) play a pivotal role in regulating metabolic waste from cerebrospinal fluid (CSF). However, the current limitations in field of view and resolution of existing imaging techniques impede understanding the stereoscopic morphology and dynamic behavior of mLVs in vivo. Here, we utilized dual-contrast functional photoacoustic microscopy to achieve wide-field intravital imaging of the lymphatic system, including mLVs and glymphatic pathways. The stereoscopic photoacoustic microscopy based on opto-acoustic confocal features has a depth imaging capability of 3.75 mm, facilitating differentiation between mLVs on the meninges and glymphatic pathways within the brain parenchyma. Subsequently, using this imaging technique, we were able to visualize the dynamic drainage of mLVs and identify a peak drainage period occurring around 20-40 min after injection, along with determining the flow direction from CSF to lymph nodes. Inspiringly, in the Alzheimer's disease (AD) mouse model, we observed that AD mice exhibit a ~ 70% reduction in drainage volume of mLVs compared to wild-type mice. With the development of AD, there is be continued decline in mLVs drainage volume. This finding clearly demonstrates that the AD mouse model has impaired CSF drainage. Our study opens up a horizon for understanding the brain's drainage mechanism and dissecting mLVs-associated neurological disorders.