The endothelial protein PLVAP in lymphatics controls the entry of lymphocytes and antigens into lymph nodes

Innate and adaptive immune responses that control lymph-borne viruses in the draining lymph node

The interstitial fluids in tissues are constantly drained into the lymph nodes (LNs) as lymph through afferent lymphatic vessels and from LNs into the blood through efferent lymphatics. LNs are strategically positioned and have the appropriate cellular composition to serve as sites of adaptive immune initiation against invading pathogens. However, for lymph-borne viruses, which disseminate from the entry site to other tissues through the lymphatic system, immune cells in the draining LN (dLN) also play critical roles in curbing systemic viral dissemination during primary and secondary infections. Lymph-borne viruses in tissues can be transported to dLNs as free virions in the lymph or within infected cells. Regardless of the entry mechanism, infected myeloid antigen-presenting cells, including various subtypes of dendritic cells, inflammatory monocytes, and macrophages, play a critical role in initiating the innate immune response within the dLN. This innate immune response involves cellular crosstalk between infected and bystander innate immune cells that ultimately produce type I interferons (IFN-Is) and other cytokines and recruit inflammatory monocytes and natural killer (NK) cells. IFN-I and NK cell cytotoxicity can restrict systemic viral spread during primary infections and prevent serious disease. Additionally, the memory CD8+ T-cells that reside or rapidly migrate to the dLN can contribute to disease prevention during secondary viral infections. This review explores the intricate innate immune responses orchestrated within dLNs that contain primary viral infections and the role of memory CD8+ T-cells following secondary infection or CD8+ T-cell vaccination.

Distal Immunization and Systemic Cytokines Establish a Transient Immune Alert State in the Intestine

Conventionally, immune responses are studied in the context of inflamed tissues and their corresponding draining lymph nodes (LNs). However, little is known about the effects of systemic inflammatory signals generated during local inflammation on distal tissues and nondraining LNs. Using a mouse model of cutaneous immunization, we found that systemic inflammatory stimuli triggered a rapid and selective distal response in the small intestine and the mesenteric LN (mesLN). This consisted of increased permeability of intestinal blood vessels and lymphatic drainage of bloodborne solutes into the mesLN, enhanced activation and migration of intestinal dendritic cells, as well as amplified T cell responses in the mesLNs to systemic but not orally derived Ags. Mechanistically, we found that the small intestine endothelial cells preferentially expressed molecules involved in TNF-α signaling and that TNF-α blockade markedly diminished distal intestinal responses to cutaneous immunization. Together, these findings reveal that the intestinal immune system is rapidly and selectively activated in response to inflammatory cues regardless of their origin, thus identifying an additional layer of defense and enhanced surveillance of a key barrier organ at constant risk of pathogen encounter.

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 Male Reproductive Toxicity Caused by 2-Naphthylamine Was Related to Testicular Immunity Disorders

2-naphthylamine (NAP) was classified as a group I carcinogen associated with bladder cancer. The daily exposure is mostly from cigarette and E-cigarette smoke. NAP can lead to testicular atrophy and interstitial tissue hyperplasia; however, the outcomes of NAP treatment on spermatogenesis and the associated mechanisms have not been reported. The study aimed to investigate the effect of NAP on spermatogenesis and sperm physiologic functions after being persistently exposed to NAP at 5, 20, and 40 mg/kg for 35 days. We found that sperm motility, progressive motility, sperm average path velocity, and straight-line velocity declined remarkably in the NAP (40 mg/kg) treated group, and the sperm deformation rate rose upon NAP administration. The testis immunity- and lipid metabolism-associated processes were enriched from RNA-sequence profiling. Plvap, Ccr7, Foxn1, Trim29, Sirpb1c, Cfd, and Lpar4 involved in testis immunity and Pnliprp1 that inhibit triglyceride and cholesterol absorption were confirmed to rise dramatically in the NAP-exposed group. The increased total cholesterol and CD68 levels were observed in the testis from the NAP-exposed group. Gpx5, serving as an antioxidant in sperm plasma, and Semg1, which contributes to sperm progressive motility, were both down-regulated. We concluded that the short-term exposure to NAP caused reproductive toxicity, primarily due to the inflammatory abnormality in the testis.

Modulating Antigen Availability in Lymphoid Organs to Shape the Humoral Immune Response to Vaccines

Primary immune responses following vaccination are initiated in draining lymph nodes, where naive T and B cells encounter Ag and undergo coordinated steps of activation. For humoral immunity, the amount of Ag present over time, its localization to follicles and follicular dendritic cells, and the Ag's structural state all play important roles in determining the subsequent immune response. Recent studies have shown that multiple elements of vaccine design can impact Ag availability in lymphoid tissues, including the choice of adjuvant, physical form of the immunogen, and dosing kinetics. These vaccine design elements affect the transport of Ag to lymph nodes, Ag's localization in the tissue, the duration of Ag availability, and the structural integrity of the Ag. In this review, we discuss these findings and their implications for engineering more effective vaccines, particularly for difficult to neutralize pathogens.

Deciphering decidual leukocyte traffic with serial intravascular staining

The decidual immunome is dynamic, dramatically changing its composition across gestation. Early pregnancy is dominated by decidual NK cells, with a shift towards T cells later in pregnancy. However, the degree, timing, and subset-specific nature of leukocyte traffic between the decidua and systemic circulation during gestation remains poorly understood. Herein, we employed intravascular staining in pregnant C57BL/6J mice and cynomolgus macaques (Macaca fascicularis) to examine leukocyte traffic into the decidual basalis during pregnancy. Timed-mated or virgin mice were tail-vein injected with labelled αCD45 antibodies 24 hours and 5 minutes before sacrifice. Pregnant cynomolgus macaques (GD155) were infused with labelled αCD45 at 2 hours or 5 mins before necropsy. Decidual cells were isolated and resulting suspensions analyzed by flow cytometry. We found that the proportion of intravascular (IVAs)-negative leukocytes (cells labeled by the 24h infusion of αCD45 or unlabeled) decreased across murine gestation while recent immigrants (24h label only) increased in mid- to late-gestation. In the cynomolgus model our data confirmed differential labeling of decidual leukocytes by the infused antibody, with the 5 min infused animal having a higher proportion of IVAs+ cells compared to the 2hr infused animal. Decidual tissue sections from both macaques showed the presence of intravascularly labeled cells, either in proximity to blood vessels (5min infused animal) or deeper into decidual stroma (2hr infused animal). These results demonstrate the value of serial intravascular staining as a sensitive tool for defining decidual leukocyte traffic during pregnancy.

Collinsella aerofaciens as a predictive marker of response to probiotic treatment in non-constipated irritable bowel syndrome

Probiotics are exploited for adjuvant treatment in IBS, but reliable guidance for selecting the appropriate probiotic to adopt for different forms of IBS is lacking. We aimed to identify markers for recognizing non-constipated (NC) IBS patients that may show significant clinical improvements upon treatment with the probiotic strain Lacticaseibacillus paracasei DG (LDG). To this purpose, we performed a post-hoc analysis of samples collected during a multicenter, double-blind, parallel-group, placebo-controlled trial in which NC-IBS patients were randomized to receive at least 24 billion CFU LDG or placebo capsules b.i.d. for 12 weeks. The primary clinical endpoint was the composite response based on improved abdominal pain and fecal type. The fecal microbiome and serum markers of intestinal (PV1 and zonulin), liver, and kidney functions were investigated. We found that responders (R) in the probiotic arm (25%) differed from non-responders (NR) based on the abundance of 18 bacterial taxa, including the families Coriobacteriaceae, Dorea spp. and Collinsella aerofaciens, which were overrepresented in R patients. These taxa also distinguished R (but not NR) patients from healthy controls. Probiotic intervention significantly reduced the abundance of these bacteria in R, but not in NR. Analogous results emerged for C. aerofaciens from the analysis of data from a previous trial on IBS with the same probiotic. Finally, C. aerofaciens was positively correlated with the plasmalemmal vesicle associated protein-1 (PV-1) and the markers of liver function. In conclusion, LDG is effective on NC-IBS patients with NC-IBS with a greater abundance of potential pathobionts. Among these, C. aerofaciens has emerged as a potential predictor of probiotic efficacy.

Expression analysis of plvap in mouse heart development, homeostasis and injury

Plasmalemma vesicle associated protein (PLVAP) is commonly considered to be specifically expressed in endothelial cells in which it localized to diaphragms of caveolae, fenestrae, and transendothelial channels. PLVAP is reported to be an important regulator of heart development and a novel target to promote cardiac repair in the ischemic heart. However, the dynamics of plvap expression in heart development, homeostasis and pathology have not been comprehensively described. In this study, we analyzed the temporal and spatial expression of plvap in mouse heart under different conditions. We found that, during embryonic and neonatal stages, PLVAP was detected in endocardial endothelial cells, epicardial mesothelial cells, and a small amount of coronary vascular endothelial cells. In adult heart, PLVAP was also identified in endocardial cells and a few coronary vascular endothelial cells. However, epicardial expression of PLVAP was lost during postnatal heart development and cannot be detected in mouse heart by immunostaining since 3-week-old. We also analyzed the expression of plvap in a model of cardiac hypertrophy and failure induced by transverse aortic constriction surgery, and identified expression of PLVAP in endocardial cells and coronary vascular endothelial cells in the injured heart. This study provides new evidence to better understand the role of plvap in mouse heart development and injury.

The senescent secretome drives PLVAP expression in cultured human hepatic endothelial cells to promote monocyte transmigration

Liver sinusoidal endothelial cells (LSEC) undergo significant phenotypic change in chronic liver disease (CLD), and yet the factors that drive this process and the impact on their function as a vascular barrier and gatekeeper for immune cell recruitment are poorly understood. Plasmalemma-vesicle-associated protein (PLVAP) has been characterized as a marker of LSEC in CLD; notably we found that PLVAP upregulation strongly correlated with markers of tissue senescence. Furthermore, exposure of human LSEC to the senescence-associated secretory phenotype (SASP) led to a significant upregulation of PLVAP. Flow-based assays demonstrated that SASP-driven leukocyte recruitment was characterized by paracellular transmigration of monocytes while the majority of lymphocytes migrated transcellularly. Knockdown studies confirmed that PLVAP selectively supported monocyte transmigration mediated through PLVAP's impact on LSEC permeability by regulating phospho-VE-cadherin expression and endothelial gap formation. PLVAP may therefore represent an endothelial target that selectively shapes the senescence-mediated immune microenvironment in liver disease.

Biomechanical control of lymphatic vessel physiology and functions

The ever-growing research on lymphatic biology has clearly identified lymphatic vessels as key players that maintain human health through their functional roles in tissue fluid homeostasis, immunosurveillance, lipid metabolism and inflammation. It is therefore not surprising that the list of human diseases associated with lymphatic malfunctions has grown larger, including issues beyond lymphedema, a pathology traditionally associated with lymphatic drainage insufficiency. Thus, the discovery of factors and pathways that can promote optimal lymphatic functions may offer new therapeutic options. Accumulating evidence indicates that aside from biochemical factors, biomechanical signals also regulate lymphatic vessel expansion and functions postnatally. Here, we review how mechanical forces induced by fluid shear stress affect the behavior and functions of lymphatic vessels and the mechanisms lymphatic vessels employ to sense and transduce these mechanical cues into biological signals.

Understanding tumour endothelial cell heterogeneity and function from single-cell omics

Anti-angiogenic therapies (AATs) are used to treat different types of cancers. However, their success is limited owing to insufficient efficacy and resistance. Recently, single-cell omics studies of tumour endothelial cells (TECs) have provided new mechanistic insight. Here, we overview the heterogeneity of human TECs of all tumour types studied to date, at the single-cell level. Notably, most human tumour types contain varying numbers but only a small population of angiogenic TECs, the presumed targets of AATs, possibly contributing to the limited efficacy of and resistance to AATs. In general, TECs are heterogeneous within and across all tumour types, but comparing TEC phenotypes across tumours is currently challenging, owing to the lack of a uniform nomenclature for endothelial cells and consistent single-cell analysis protocols, urgently raising the need for a more consistent approach. Nonetheless, across most tumour types, universal TEC markers (ACKR1, PLVAP and IGFBP3) can be identified. Besides angiogenesis, biological processes such as immunomodulation and extracellular matrix organization are among the most commonly predicted enriched signatures of TECs across different tumour types. Although angiogenesis and extracellular matrix targets have been considered for AAT (without the hoped success), the immunomodulatory properties of TECs have not been fully considered as a novel anticancer therapeutic approach. Therefore, we also discuss progress, limitations, solutions and novel targets for AAT development.

Tumour associated endothelial cells: origin, characteristics and role in metastasis and anti-angiogenic resistance

Tumour progression and metastasis remain the leading causes of cancer-related death worldwide. Tumour angiogenesis is essential for tumour progression. The vasculature surrounding tumours is not only a transport channel for nutrients, oxygen, and metabolites, but also a pathway for metastasis. There is a close interaction between tumour cells and endothelial cells in the tumour microenvironment. Recent studies have shown that tumour-associated endothelial cells have different characteristics from normal vascular endothelial cells, play an important role in tumour progression and metastasis, and are expected to be a key target for cancer therapy. This article reviews the tissue and cellular origin of tumour-associated endothelial cells and analyses the characteristics of tumour-associated endothelial cells. Finally, it summarises the role of tumour-associated endothelial cells in tumour progression and metastasis and the prospects for their use in clinical anti-angiogenic therapy.

Nanotechnologies for Physiology-Informed Drug Delivery to the Lymphatic System

Accompanying the increasing translational impact of immunotherapeutic strategies to treat and prevent disease has been a broadening interest across both bioscience and bioengineering in the lymphatic system. Herein, the lymphatic system physiology, ranging from its tissue structures to immune functions and effects, is described. Design principles and engineering approaches to analyze and manipulate this tissue system in nanoparticle-based drug delivery applications are also elaborated.

The role of PLVAP in endothelial cells

Endothelial cells play a major part in the regulation of vascular permeability and angiogenesis. According to their duty to fit the needs of the underlying tissue, endothelial cells developed different subtypes with specific endothelial microdomains as caveolae, fenestrae and transendothelial channels which regulate nutrient exchange, leukocyte migration, and permeability. These microdomains can exhibit diaphragms that are formed by the endothelial cell-specific protein plasmalemma vesicle-associated protein (PLVAP), the only known protein component of these diaphragms. Several studies displayed an involvement of PLVAP in diseases as cancer, traumatic spinal cord injury, acute ischemic brain disease, transplant glomerulopathy, Norrie disease and diabetic retinopathy. Besides an upregulation of PLVAP expression within these diseases, pro-angiogenic or pro-inflammatory responses were observed. On the other hand, loss of PLVAP in knockout mice leads to premature mortality due to disrupted homeostasis. Generally, PLVAP is considered as a major factor influencing the permeability of endothelial cells and, finally, to be involved in the regulation of vascular permeability. Following these observations, PLVAP is debated as a novel therapeutic target with respect to the different vascular beds and tissues. In this review, we highlight the structure and functions of PLVAP in different endothelial types in health and disease.

Lessons of Vascular Specialization From Secondary Lymphoid Organ Lymphatic Endothelial Cells

Secondary lymphoid organs, such as lymph nodes, harbor highly specialized and compartmentalized niches. These niches are optimized to facilitate the encounter of naive lymphocytes with antigens and antigen-presenting cells, enabling optimal generation of adaptive immune responses. Lymphatic vessels of lymphoid organs are uniquely specialized to perform a staggering variety of tasks. These include antigen presentation, directing the trafficking of immune cells but also modulating immune cell activation and providing factors for their survival. Recent studies have provided insights into the molecular basis of such specialization, opening avenues for better understanding the mechanisms of immune-vascular interactions and their applications. Such knowledge is essential for designing better treatments for human diseases given the central role of the immune system in infection, aging, tissue regeneration and repair. In addition, principles established in studies of lymphoid organ lymphatic vessel functions and organization may be applied to guide our understanding of specialization of vascular beds in other organs.

Structural insights into plasmalemma vesicle-associated protein (PLVAP): Implications for vascular endothelial diaphragms and fenestrae

In many organs, small openings across capillary endothelial cells (ECs) allow the diffusion of low-molecular weight compounds and small proteins between the blood and tissue spaces. These openings contain a diaphragm composed of radially arranged fibers, and current evidence suggests that a single-span type II transmembrane protein, plasmalemma vesicle-associated protein-1 (PLVAP), constitutes these fibers. Here, we present the three-dimensional crystal structure of an 89-amino acid segment of the PLVAP extracellular domain (ECD) and show that it adopts a parallel dimeric alpha-helical coiled-coil configuration with five interchain disulfide bonds. The structure was solved using single-wavelength anomalous diffraction from sulfur-containing residues (sulfur SAD) to generate phase information. Biochemical and circular dichroism (CD) experiments show that a second PLVAP ECD segment also has a parallel dimeric alpha-helical configuration-presumably a coiled coil-held together with interchain disulfide bonds. Overall, ~2/3 of the ~390 amino acids within the PLVAP ECD adopt a helical configuration, as determined by CD. We also determined the sequence and epitope of MECA-32, an anti-PLVAP antibody. Taken together, these data lend strong support to the model of capillary diaphragms formulated by Tse and Stan in which approximately ten PLVAP dimers are arranged within each 60- to 80-nm-diameter opening like the spokes of a bicycle wheel. Passage of molecules through the wedge-shaped pores is presumably determined both by the length of PLVAP-i.e., the long dimension of the pore-and by the chemical properties of amino acid side chains and N-linked glycans on the solvent-accessible faces of PLVAP.

Host variation in type I interferon signaling genes (MX1), C-C chemokine receptor type 5 gene, and major histocompatibility complex class I alleles in treated HIV+ noncontrollers predict viral reservoir size

OBJECTIVE

Prior genomewide association studies have identified variation in major histocompatibility complex (MHC) class I alleles and C-C chemokine receptor type 5 gene (CCR5Δ32) as genetic predictors of viral control, especially in 'elite' controllers, individuals who remain virally suppressed in the absence of therapy.

DESIGN

Cross-sectional genomewide association study.

METHODS

We analyzed custom whole exome sequencing and direct human leukocyte antigen (HLA) typing from 202 antiretroviral therapy (ART)-suppressed HIV+ noncontrollers in relation to four measures of the peripheral CD4+ T-cell reservoir: HIV intact DNA, total (t)DNA, unspliced (us)RNA, and RNA/DNA. Linear mixed models were adjusted for potential covariates including age, sex, nadir CD4+ T-cell count, pre-ART HIV RNA, timing of ART initiation, and duration of ART suppression.

RESULTS

Previously reported 'protective' host genetic mutations related to viral setpoint (e.g. among elite controllers) were found to predict smaller HIV reservoir size. The HLA 'protective' B∗57:01 was associated with significantly lower HIV usRNA (q = 3.3 × 10-3), and among the largest subgroup, European ancestry individuals, the CCR5Δ32 deletion was associated with smaller HIV tDNA (P = 4.3 × 10-3) and usRNA (P = 8.7 × 10-3). In addition, genomewide analysis identified several single nucleotide polymorphisms in MX1 (an interferon stimulated gene) that were significantly associated with HIV tDNA (q = 0.02), and the direction of these associations paralleled MX1 gene eQTL expression.

CONCLUSIONS

We observed a significant association between previously reported 'protective' MHC class I alleles and CCR5Δ32 with the HIV reservoir size in noncontrollers. We also found a novel association between MX1 and HIV total DNA (in addition to other interferon signaling relevant genes, PPP1CB, DDX3X). These findings warrant further investigation in future validation studies.

Beyond IBD: the genetics of other early-onset diarrhoeal disorders

Diarrhoeal disorders in childhood extend beyond the inflammatory bowel diseases. Persistent and severe forms of diarrhoea can occur from birth and are associated with significant morbidity and mortality. These disorders can affect not only the gastrointestinal tract but frequently have extraintestinal manifestations, immunodeficiencies and endocrinopathies. Genomic analysis has advanced our understanding of these conditions and has revealed precision-based treatment options such as potentially curative haematopoietic stem cell transplant. Although many new mutations have been discovered, there is frequently no clear genotype-phenotype correlation. The functional effects of gene mutations can be studied in model systems such as patient-derived organoids. This allows us to further characterise these disorders and advance our understanding of the pathophysiology of the intestinal mucosa. In this review, we will provide an up to date overview of genes involved in diarrhoeal disorders of early onset, particularly focussing on the more recently described gene defects associated with protein loosing enteropathy.

Lymph node lymphatic endothelial cells as multifaceted gatekeepers in the immune system

Single-cell technologies have recently allowed the identification of multiple lymphatic endothelial cell (LEC) subsets in subcapsular, paracortical, medullary, and other lymph node (LN) sinus systems in mice and humans. New analyses show that LECs serve key immunological functions in the LN stroma during immune responses. We discuss the roles of different LEC types in guiding leukocyte and cancer cell trafficking to and from the LN parenchyma, in capturing microbes, and in transporting, presenting, and storing lymph-borne antigens in distinct types of lymphatic sinuses. We underscore specific adaptations of human LECs and raise unanswered questions concerning LEC functions in human disease. Despite our limited understanding of human lymphatics - hampering clinical translation in inflammation and metastasis - we support the potential of LN LECs as putative targets for boosting/inhibiting immunoreactivity.

The Vasculature in Pulmonary Fibrosis

Purpose of Review

The current paradigm of idiopathic pulmonary fibrosis (IPF) pathogenesis involves recurrent injury to a sensitive alveolar epithelium followed by impaired repair responses marked by fibroblast activation and deposition of extracellular matrix. Multiple cell types are involved in this response with potential roles suggested by advances in single-cell RNA sequencing and lung developmental biology. Notably, recent work has better characterized the cell types present in the pulmonary endothelium and identified vascular changes in patients with IPF.

Recent Findings

Lung tissue from patients with IPF has been examined at single-cell resolution, revealing reductions in lung capillary cells and expansion of a population of vascular cells expressing markers associated with bronchial endothelium. In addition, pre-clinical models have demonstrated a fundamental role for aging and vascular permeability in the development of pulmonary fibrosis.

Summary

Mounting evidence suggests that the endothelium undergoes changes in the context of fibrosis, and these changes may contribute to the development and/or progression of pulmonary fibrosis. Additional studies will be needed to further define the functional role of these vascular changes.

Generation of Plvap-CreER and Car4-CreER for genetic targeting of distinct lung capillary populations

It has been reported recently that there are two distinct subpopulations of capillary endothelial cells in the mammalian lungs: gCap (general capillary) and aCap (aerocyte). They are identified by two unique markers, respectively: plasmalemmal vesicle-associated protein (PLVAP) and carbonic anhydrase IV (CAR4). Here, we report two novel knock-in mouse lines Plvap-CreER and Car4-CreER, which genetically target gCap and aCap, respectively. Induced by tamoxifen, the Plvap-CreER and Car4-CreER alleles mediate specific and efficient Cre-loxP recombinations in PLVAP⁺ gCap and CAR4⁺ aCap, respectively, in the lungs. These two mouse lines are useful genetic tools to investigate cell fates and functions of PLVAP⁺ and CAR4⁺ cells in lung homeostasis, injury and repair.

Oncofetal reprogramming in tumour development and progression

Embryonic development is characterized by rapidly dividing cells, cellular plasticity and a highly vascular microenvironment. These features are similar to those of tumour tissue, in that malignant cells are characterized by their ability to proliferate and exhibit cellular plasticity. The tumour microenvironment also often includes immunosuppressive features. Reciprocal communication between various cellular subpopulations enables fetal and tumour tissues to proliferate, migrate and escape immune responses. Fetal-like reprogramming has been demonstrated in the tumour microenvironment, indicating extraordinary cellular plasticity and bringing an additional layer of cellular heterogeneity. More importantly, some of these features are also present during inflammation. This Perspective discusses the similarity between embryogenesis, inflammation and tumorigenesis, and describes the mechanisms of oncofetal reprogramming that enable tumour cells to escape from immune responses, promoting tumour growth and metastasis.

PLVAP is associated with glioma-associated malignant processes and immunosuppressive cell infiltration as a promising marker for prognosis

Previous reports have confirmed the significance of plasmalemma vesicle-associated protein (PLVAP) in the progression of multiple tumors; however, there are few studies examining its immune properties in the context of gliomas. Here, we methodically investigated the pathophysiological characteristics and clinical manifestations of gliomas. A total of 699 patients diagnosed with gliomas in the cancer genome atlas along with 325 glioma patients in the Chinese glioma genome atlas were collected for the training and validation sets. We analyzed and visualized the total statistics using RStudio. PLVAP was markedly upregulated among high grade gliomas, O⁶-methylguanine-DNA methyltransferase promoter unmethylated subforms, isocitrate dehydrogenase wild forms, 1p19q non-codeletion subforms, and mesenchyme type gliomas. The receiver operating characteristics analysis illustrated the favorable applicability of PLVAP in regard to estimating mesenchyme subform gliomas. Subsequent Kaplan–Meier curves together with multivariable Cox analyses upon survival identified high-expression PLVAP as a distinct prognostic variable for patients with gliomas. Gene ontology analysis of PLVAP among gliomas has documented the predominant role of this protein in glioma-associated immunobiological processes and also in inflammatory responses. We consequently examined the associations of PLVAP with immune-related meta-genes, and PLVAP was positively correlated with hematopoietic cell kinase, lymphocyte-specific protein tyrosine kinase, major histocompatibility complex (MHC) I, MHC II, signal transducer and activator of transcription 1, and interferon and was negatively correlated with immunoglobulin G. Moreover, association analyses between PLVAP and glioma-infiltrating immunocytes indicated that the infiltrating degrees of most immune cells exhibited positive correlations with PLVAP expression, particularly immunosuppressive subsets such as tumor-related macrophages, myeloid-derived suppressor cells, and regulatory T lymphocytes. In summary, we originally demonstrated that PLVAP is markedly associated with immunosuppressive immune cell infiltration degrees, unfavorable survival, and adverse pathology types among gliomas, thus identifying PLVAP as a practicable marker and a promising target for glioma-based precise diagnosis and therapeutic strategies.

Lymphatic vessels in cancer

The lymphatic system, composed of initial and collecting lymphatic vessels as well as lymph nodes that are present in almost every tissue of the human body, acts as an essential transport system for fluids, biomolecules and cells between peripheral tissues and the central circulation. Consequently, it is required for normal body physiology but is also involved in the pathogenesis of various diseases, most notably cancer. The important role of tumor-associated lymphatic vessels and lymphangiogenesis in the formation of lymph node metastasis has been elucidated during the last two decades, whereas the underlying mechanisms and the relation between lymphatic and peripheral organ dissemination of cancer cells are incompletely understood. Lymphatic vessels are also important for tumor-host communication, relaying molecular information from a primary or metastatic tumor to regional lymph nodes and the circulatory system. Beyond antigen transport, lymphatic endothelial cells, particularly those residing in lymph node sinuses, have recently been recognized as direct regulators of tumor immunity and immunotherapy responsiveness, presenting tumor antigens and expressing several immune-modulatory signals including PD-L1. In this review, we summarize recent discoveries in this rapidly evolving field and highlight strategies and challenges of therapeutic targeting of lymphatic vessels or specific lymphatic functions in cancer patients.

Single-Cell RNA-Seq Reveals the Heterogeneity of Cell Communications between Schwann Cells and Fibroblasts within the Microenvironment in Vestibular Schwannoma

Vestibular schwannomas (VSs), which develop from Schwann cells (SCs) of the vestibular nerve, are the most prevalent benign tumors of the cerebellopontine angle and internal auditory canal. Despite advances in treatment, the cellular components and mechanisms of VS tumor progression remain unclear. Here, we performed single-cell RNA-sequencing on clinically surgically isolated VS samples and clarified their cellular composition in a sophisticated manner, including the heterogeneous SC subtypes. Advanced bioinformatics analysis also revealed the associated biological functions, pseudotime trajectory, and transcriptional network of the SC subgroups. We also found that there was tight intercellular communication between SCs and tumor-associated fibroblasts via integrin and growth factor signaling and that the gene expression differences in SCs and fibroblasts determined the heterogeneity of cellular communication in different individuals. Our findings suggest a microenvironmental mechanism underlying the development of VS.

Imaging viral infection in vivo to gain unique perspectives on cellular antiviral immunity

The past decade has seen near continual global public health crises caused by emerging viral infections. Extraordinary increases in our knowledge of the mechanisms underlying successful antiviral immune responses in animal models and during human infection have accompanied these viral outbreaks. Keeping pace with the rapidly advancing field of viral immunology, innovations in microscopy have afforded a previously unseen view of viral infection occurring in real-time in living animals. Here, we review the contribution of intravital imaging to our understanding of cell-mediated immune responses to viral infections, with a particular focus on studies that visualize the antiviral effector cells responding to infection as well as virus-infected cells. We discuss methods to visualize viral infection in vivo using intravital microscopy (IVM) and significant findings arising through the application of IVM to viral infection. Collectively, these works underscore the importance of developing a comprehensive spatial understanding of the relationships between immune effectors and virus-infected cells and how this has enabled unique discoveries about virus/host interactions and antiviral effector cell biology.

Information flow in the spatiotemporal organization of immune responses

Immune responses must be rapid, tightly orchestrated, and tailored to the encountered stimulus. Lymphatic vessels facilitate this process by continuously collecting immunological information (ie, antigens, immune cells, and soluble mediators) about the current state of peripheral tissues, and transporting these via the lymph across the lymphatic system. Lymph nodes (LNs), which are critical meeting points for innate and adaptive immune cells, are strategically located along the lymphatic network to intercept this information. Within LNs, immune cells are spatially organized, allowing them to efficiently respond to information delivered by the lymph, and to either promote immune homeostasis or mount protective immune responses. These responses involve the activation and functional cooperation of multiple distinct cell types and are tailored to the specific inflammatory conditions. The natural patterns of lymph flow can also generate spatial gradients of antigens and agonists within draining LNs, which can in turn further regulate innate cell function and localization, as well as the downstream generation of adaptive immunity. In this review, we explore how information transmitted by the lymph shapes the spatiotemporal organization of innate and adaptive immune responses in LNs, with particular focus on steady state and Type-I vs. Type-II inflammation.

Bacterial Lymphatic Metastasis in Infection and Immunity

Lymphatic vessels permeate tissues around the body, returning fluid from interstitial spaces back to the blood after passage through the lymph nodes, which are important sites for adaptive responses to all types of pathogens. Involvement of the lymphatics in the pathogenesis of bacterial infections is not well studied. Despite offering an obvious conduit for pathogen spread, the lymphatic system has long been regarded to bar the onward progression of most bacteria. There is little direct data on live virulent bacteria, instead understanding is largely inferred from studies investigating immune responses to viruses or antigens in lymph nodes. Recently, we have demonstrated that extracellular bacterial lymphatic metastasis of virulent strains of Streptococcus pyogenes drives systemic infection. Accordingly, it is timely to reconsider the role of lymph nodes as absolute barriers to bacterial dissemination in the lymphatics. Here, we summarise the routes and mechanisms by which an increasing variety of bacteria are acknowledged to transit through the lymphatic system, including those that do not necessarily require internalisation by host cells. We discuss the anatomy of the lymphatics and other factors that influence bacterial dissemination, as well as the consequences of underappreciated bacterial lymphatic metastasis on disease and immunity.

Visualization and functional characterization of lymphoid organ fibroblasts

Fibroblastic reticular cells (FRCs) are specialized stromal cells of lymphoid organs that generate the structural foundation of the tissue and actively interact with immune cells. Distinct FRC subsets position lymphocytes and myeloid cells in specialized niches where they present processed or native antigen and provide essential growth factors and cytokines for immune cell activation and differentiation. Niche‐specific functions of FRC subpopulations have been defined using genetic targeting, high‐dimensional transcriptomic analyses, and advanced imaging methods. Here, we review recent findings on FRC‐immune cell interaction and the elaboration of FRC development and differentiation. We discuss how imaging approaches have not only shaped our understanding of FRC biology, but have critically advanced the niche concept of immune cell maintenance and control of immune reactivity.

Vaccine delivery systems toward lymph nodes

Strategies of improving vaccine targeting ability toward lymph nodes have been attracting considerable interest in recent years, though there are remaining delivery barriers based on the inherent properties of lymphatic systems and limited administration routes of vaccination. Recently, emerging vaccine delivery systems using various materials as carriers are widely developed to achieve efficient lymph node targeting and improve vaccine-triggered adaptive immune response. In this review, to further optimize the vaccine targeting ability for future research, the design principles of lymph node targeting vaccine delivery based on the anatomy of lymph nodes and vaccine administration routes are first summarized. Then different designs of lymph node targeting vaccine delivery systems, including vaccine delivery systems in clinical applications, are carefully surveyed. Also, the challenges and opportunities of current delivery systems for vaccines are concluded in the end.

Immunological perspectives on spatial and temporal vaccine delivery

The so-called rational design of vaccines has been a very attractive concept and also an important direction for vaccine research and development. However, the underlying rationales, especially on the immunological aspect, remain less systemically and deeply understood. Given the critical role of lymph nodes (LNs) in the induction of B and T cell responses upon vaccination, LN targeting has been a popular strategy in vaccine design. The LN is a highly organized structure; induction of adaptive immune response is highly orchestrated by various types of LN stromal cells and hematopoietic immune cells both spatially and temporally. Thus, not only LN targeting, but also cellular targeting and even subcellular compartment targeting should be considered for specifically enhanced vaccine efficacy. Moreover, temporal control of vaccine antigen and adjuvant delivery may also optimize the immune response.

Interplay of immune and kidney resident cells in the formation of tertiary lymphoid structures in lupus nephritis

Kidney involvement confers significant morbidity and mortality in patients with systemic lupus erythematosus (SLE). The pathogenesis of lupus nephritis (LN) involves diverse mechanisms instigated by elements of the autoimmune response which alter the biology of kidney resident cells. Processes in the glomeruli and in the interstitium may proceed independently albeit crosstalk between the two is inevitable. Podocytes, mesangial cells, tubular epithelial cells, kidney resident macrophages and stromal cells with input from cytokines and autoantibodies present in the circulation alter the expression of enzymes, produce cytokines and chemokines which lead to their injury and damage of the kidney. Several of these molecules can be targeted independently to prevent and reverse kidney failure. Tertiary lymphoid structures with true germinal centers are present in the kidneys of patients with lupus nephritis and have been increasingly recognized to associate with poorer renal outcomes. Stromal cells, tubular epithelial cells, high endothelial vessel and lymphatic venule cells produce chemokines which enable the formation of structures composed of a T-cell-rich zone with mature dendritic cells next to a B-cell follicle with the characteristics of a germinal center surrounded by plasma cells. Following an overview on the interaction of the immune cells with kidney resident cells, we discuss the cellular and molecular events which lead to the formation of tertiary lymphoid structures in the interstitium of the kidneys of mice and patients with lupus nephritis. In parallel, molecules and processes that can be targeted therapeutically are presented.

The lymphatics in kidney health and disease

The mammalian vascular system consists of two networks: the blood vascular system and the lymphatic vascular system. Throughout the body, the lymphatic system contributes to homeostatic mechanisms by draining extravasated interstitial fluid and facilitating the trafficking and activation of immune cells. In the kidney, lymphatic vessels exist mainly in the kidney cortex. In the medulla, the ascending vasa recta represent a hybrid lymphatic-like vessel that performs lymphatic-like roles in interstitial fluid reabsorption. Although the lymphatic network is mainly derived from the venous system, evidence supports the existence of lymphatic beds that are of non-venous origin. Following their development and maturation, lymphatic vessel density remains relatively stable; however, these vessels undergo dynamic functional changes to meet tissue demands. Additionally, new lymphatic growth, or lymphangiogenesis, can be induced by pathological conditions such as tissue injury, interstitial fluid overload, hyperglycaemia and inflammation. Lymphangiogenesis is also associated with conditions such as polycystic kidney disease, hypertension, ultrafiltration failure and transplant rejection. Although lymphangiogenesis has protective functions in clearing accumulated fluid and immune cells, the kidney lymphatics may also propagate an inflammatory feedback loop, exacerbating inflammation and fibrosis. Greater understanding of lymphatic biology, including the developmental origin and function of the lymphatics and their response to pathogenic stimuli, may aid the development of new therapeutic agents that target the lymphatic system.

The lymphatic vasculature: An active and dynamic player in cancer progression

The lymphatic vasculature has been widely described and explored for its key functions in fluid homeostasis and in the organization and modulation of the immune response. Besides transporting immune cells, lymphatic vessels play relevant roles in tumor growth and tumor cell dissemination. Cancer cells that have invaded into afferent lymphatics are propagated to tumor‐draining lymph nodes (LNs), which represent an important hub for metastatic cell arrest and growth, immune modulation, and secondary dissemination to distant sites. In recent years many studies have reported new mechanisms by which the lymphatic vasculature affects cancer progression, ranging from induction of lymphangiogenesis to metastatic niche preconditioning or immune modulation. In this review, we provide an up‐to‐date description of lymphatic organization and function in peripheral tissues and in LNs and the changes induced to this system by tumor growth and progression. We will specifically focus on the reported interactions that occur between tumor cells and lymphatic endothelial cells (LECs), as well as on interactions between immune cells and LECs, both in the tumor microenvironment and in tumor‐draining LNs. Moreover, the most recent prognostic and therapeutic implications of lymphatics in cancer will be reported and discussed in light of the new immune‐modulatory roles that have been ascribed to LECs.

Communication, construction, and fluid control: lymphoid organ fibroblastic reticular cell and conduit networks

Fibroblastic reticular cells (FRCs) are a crucial part of the stromal cell infrastructure of secondary lymphoid organs (SLOs). Lymphoid organ fibroblasts form specialized niches for immune cell interactions and thereby govern lymphocyte activation and differentiation. Moreover, FRCs produce and ensheath a network of extracellular matrix (ECM) microfibers called the conduit system. FRC-generated conduits contribute to fluid and immune cell control by funneling fluids containing antigens and inflammatory mediators through the SLOs. We review recent progress in FRC biology that has advanced our understanding of immune cell functions and interactions. We discuss the intricate relationships between the cellular FRC and the fibrillar conduit networks, which together form the basis for efficient communication between immune cells and the tissues they survey.

Dichotomous effects on lymphatic transport with loss of caveolae in mice

AIM

Fluid and macromolecule transport from the interstitium into and through lymphatic vessels is necessary for tissue homeostasis. While lymphatic capillary structure suggests that passive, paracellular transport would be the predominant route of macromolecule entry, active caveolae-mediated transcellular transport has been identified in lymphatic endothelial cells (LECs) in vitro. Caveolae also mediate a wide array of endothelial cell processes, including nitric oxide regulation. Thus, how does the lack of caveolae impact "lymphatic function"?

METHODS

Various aspects of lymphatic transport were measured in mice constitutively lacking caveolin-1 ("CavKO"), the protein required for caveolae formation in endothelial cells, and in mice with a LEC-specific Cav1 gene deletion (Lyve1-Cre x Cav1^flox/flox ; "LyCav") and ex vivo in their vessels and cells.

RESULTS

In each model, lymphatic architecture was largely unchanged. The lymphatic conductance, or initial tissue uptake, was significantly higher in both CavKO mice and LyCav mice by quantitative microlymphangiography and the permeability to 70 kDa dextran was significantly increased in monolayers of LECs isolated from CavKO mice. Conversely, transport within the lymphatic system to the sentinel node was significantly reduced in anaesthetized CavKO and LyCav mice. Isolated, cannulated collecting vessel studies identified significantly reduced phasic contractility when lymphatic endothelium lacks caveolae. Inhibition of nitric oxide synthase was able to partially restore ex vivo vessel contractility.

CONCLUSION

Macromolecule transport across lymphatics is increased with loss of caveolae, yet phasic contractility reduced, resulting in reduced overall lymphatic transport function. These studies identify lymphatic caveolar biology as a key regulator of active lymphatic transport functions.

Homeostatic maintenance of the lymphatic vasculature

The lymphatic vasculature is emerging as a multifaceted regulator of tissue homeostasis and regeneration. Lymphatic vessels drain fluid, macromolecules, and immune cells from peripheral tissues to lymph nodes (LNs) and the systemic circulation. Their recently uncovered functions extend beyond drainage and include direct modulation of adaptive immunity and paracrine regulation of organ growth. The developmental mechanisms controlling lymphatic vessel growth have been described with increasing precision. It is less clear how the essential functional features of lymphatic vessels are established and maintained. We discuss the mechanisms that maintain lymphatic vessel integrity in adult tissues and control vessel repair and regeneration. This knowledge is crucial for understanding the pathological vessel changes that contribute to disease, and provides an opportunity for therapy development.

Single-Cell Proteomics Reveals the Defined Heterogeneity of Resident Macrophages in White Adipose Tissue

Adipose tissue macrophages (ATMs) regulate homeostasis and contribute to the metabolically harmful chronic inflammation in obese individuals. While evident heterogeneity of resident ATMs has been described previously, their phenotype, developmental origin, and functionality remain inconsistent. We analyzed white adipose tissue (WAT) during homeostasis and diet interventions using comprehensive and unbiased single-cell mass cytometry and genetic lineage tracking models. We now provide a uniform definition of individual subsets of resident ATMs. We show that in lean mice, WAT co-harbors eight kinetically evolving CD206⁺ macrophage subpopulations (defined by TIM4, CD163, and MHC II) and two CD206^- macrophage subpopulations. TIM4^-CD163⁺, TIM4^-CD163^- and CD206^- macrophage populations are largely bone marrow-derived, while the proliferating TIM4⁺CD163⁺ subpopulation is of embryonic origin. All macrophage subtypes are active in phagocytosis, endocytosis, and antigen processing in vitro, whereas TIM4⁺CD163⁺ cells are superior in scavenging in vivo. A high-fat diet induces massive infiltration of CD206^- macrophages and selective down-regulation of MHC II on TIM4⁺ macrophages. These changes are reversed by dietary intervention. Thus, the developmental origin and environment jointly regulate the functional malleability of resident ATMs.

Single-Cell Transcriptional Heterogeneity of Lymphatic Endothelial Cells in Normal and Inflamed Murine Lymph Nodes

The lymphatic system plays a crucial role in immunity and lymph nodes (LNs) undergo drastic remodeling during inflammation. Here, we used single-cell RNA sequencing to investigate transcriptional changes in lymphatic endothelial cells (LECs) in LNs draining naïve and inflamed skin. We found that subsets of LECs lining the different LN sinuses responded individually to skin inflammation, suggesting that they exert distinct functions under pathological conditions. Among the genes dysregulated during inflammation, we confirmed an up-regulation of CD200 in the LECs lining the subcapsular sinus floor with a possible function in immune regulation. Furthermore, by in silico analysis, we predicted numerous possible interactions of LECs with diverse immune cells in the LNs and found similarities in the transcriptional changes of LN LECs in different skin inflammation settings. In summary, we provide an in-depth analysis of the transcriptional landscape of LN LECs in the naïve state and in skin inflammation.

Plasmalemma vesicle-associated protein promotes angiogenesis in cholangiocarcinoma via the DKK1/CKAP4/PI3K signaling pathway

Cholangiocarcinoma (CCA) is aggressive and has poor clinical outcomes because of typically delayed diagnosis and a lack of effective non-surgical therapeutic options. Recent studies have shown that plasmalemma vesicle-associated protein (PLVAP) is related to angiogenesis in various tumors, and in vivo PLVAP targeting therapy has been proven effective against hepatocellular carcinoma and pancreatic cancer. The goal of this study was to determine the potential therapeutic utility of targeting PLVAP and thus angiogenesis in CCA and explore the underlying molecular mechanisms. We found that the PLVAP expression levels were significantly higher in CCA tissues when compared with matched adjacent non-tumor tissues obtained from a total of 90 CCA patients; higher expression levels of PLVAP were associated with shorter overall survival of patients. In addition, overexpression of PLVAP was associated with higher micro-vessel density in CCA tissues. In a PLVAP overexpressing CCA patient-derived xenograft model, a novel humanized anti-PLVAP antibody in combination with Gemcitabine plus Cisplatin was significantly inhibited tumor growth. Molecular analysis of CCA cells co-cultured with human umbilical vascular endothelial cells or human hepatic sinusoidal endothelial cells showed that Dickkopf-related protein 1 (DKK1) secreted by CCA cells activated the PI3K/Akt pathway after binding to its receptor, cytoskeleton-associated protein 4 (CKAP4), resulting in the upregulation of PLVAP. Thus, CCA cells increased the angiogenic potency of endothelial cells in a paracrine fashion. Consistently, patients bearing CKAP4 and PLVAP overexpressing tumors had a poor prognosis. In conclusion, the DKK1/CKAP4/PI3K/PLVAP pathway increases angiogenesis in CCA and is therefore a potential anti-angiogenic target.

Stromal Cells Underlining the Paths From Autoimmunity, Inflammation to Cancer With Roles Beyond Structural and Nutritional Support

Stromal cells provide structural support and nutrients in secondary lymphoid organs and non-lymphoid tissues. However, accumulating evidence suggests that a complex relationship exists between stromal cells and immune cells. Interactions between immune cells and stromal cells have been shown to influence the pathology of both autoimmunity and cancer. This review examines the heterogeneity of stromal cells within the lymph node and non-lymphoid tissues during both homeostatic and inflammatory conditions, in particular autoimmunity and cancer, with the goal of better understanding the complex and apparently paradoxical relationship between these two classes of diseases. The review surveys potential novel mechanisms involving the interactions between stromal cells and immune cells which may contribute to the development, pathology and underlying connection between autoimmunity and cancer, including potential pathways from autoimmune inflammation to either “hot” or “cold” tumors. These interactions may provide some insights to explain the rising incidence of both autoimmunity and cancer in young women in industrialized countries and have the potential to be exploited in the development of new interventions for preventions and treatments of both autoimmune diseases and cancer.

Structure and Immune Function of Afferent Lymphatics and Their Mechanistic Contribution to Dendritic Cell and T Cell Trafficking

Afferent lymphatic vessels (LVs) mediate the transport of antigen and leukocytes to draining lymph nodes (dLNs), thereby serving as immunologic communication highways between peripheral tissues and LNs. The main cell types migrating via this route are antigen-presenting dendritic cells (DCs) and antigen-experienced T cells. While DC migration is important for maintenance of tolerance and for induction of protective immunity, T cell migration through afferent LVs contributes to immune surveillance. In recent years, great progress has been made in elucidating the mechanisms of lymphatic migration. Specifically, time-lapse imaging has revealed that, upon entry into capillaries, both DCs and T cells are not simply flushed away with the lymph flow, but actively crawl and patrol and even interact with each other in this compartment. Detachment and passive transport to the dLN only takes place once the cells have reached the downstream, contracting collecting vessel segments. In this review, we describe how the anatomy of the lymphatic network supports leukocyte trafficking and provide updated knowledge regarding the cellular and molecular mechanisms responsible for lymphatic migration of DCs and T cells. In addition, we discuss the relevance of DC and T cell migration through afferent LVs and its presumed implications on immunity.

Role of lymphatic endothelial cells in the tumor microenvironment-a narrative review of recent advances

Background

As lymphatic vessel is a major route for solid tumor metastasis, they are considered an essential part of tumor drainage conduits. Apart from forming the walls of lymphatic vessels, lymphatic endothelial cells (LECs) have been found to play multiple other roles in the tumor microenvironment, calling for a more in-depth review. We hope that this review may help researchers gain a detailed understanding of this fast-developing field and shed some light upon future research.

Methods

To achieve an informative review of recent advance, we carefully searched the Medline database for English literature that are openly published from the January 1995 to December 2020 and covered the topic of LEC or lymphangiogenesis in tumor progression and therapies. Two different authors independently examined the literature abstracts to exclude possible unqualified ones, and 310 papers with full texts were finally retrieved.

Results

In this paper, we discussed the structural and molecular basis of tumor-associated LECs, together with their roles in tumor metastasis and drug therapy. We then focused on their impacts on tumor cells, tumor stroma, and anti-tumor immunity, and the molecular and cellular mechanisms involved. Special emphasis on lung cancer and possible therapeutic targets based on LECs were also discussed.

Conclusions

LECs can play a much more complex role than simply forming conduits for tumor cell dissemination. Therapies targeting tumor-associated lymphatics for lung cancer and other tumors are promising, but more research is needed to clarify the mechanisms involved.

In Sickness and in Health: The Immunological Roles of the Lymphatic System

The lymphatic system plays crucial roles in immunity far beyond those of simply providing conduits for leukocytes and antigens in lymph fluid. Endothelial cells within this vasculature are distinct and highly specialized to perform roles based upon their location. Afferent lymphatic capillaries have unique intercellular junctions for efficient uptake of fluid and macromolecules, while expressing chemotactic and adhesion molecules that permit selective trafficking of specific immune cell subsets. Moreover, in response to events within peripheral tissue such as inflammation or infection, soluble factors from lymphatic endothelial cells exert “remote control” to modulate leukocyte migration across high endothelial venules from the blood to lymph nodes draining the tissue. These immune hubs are highly organized and perfectly arrayed to survey antigens from peripheral tissue while optimizing encounters between antigen-presenting cells and cognate lymphocytes. Furthermore, subsets of lymphatic endothelial cells exhibit differences in gene expression relating to specific functions and locality within the lymph node, facilitating both innate and acquired immune responses through antigen presentation, lymph node remodeling and regulation of leukocyte entry and exit. This review details the immune cell subsets in afferent and efferent lymph, and explores the mechanisms by which endothelial cells of the lymphatic system regulate such trafficking, for immune surveillance and tolerance during steady-state conditions, and in response to infection, acute and chronic inflammation, and subsequent resolution.

Topological Structure and Robustness of the Lymph Node Conduit System

Fibroblastic reticular cells (FRCs) form a road-like cellular network in lymph nodes (LNs) that provides essential chemotactic, survival, and regulatory signals for immune cells. While the topological characteristics of the FRC network have been elaborated, the network properties of the micro-tubular conduit system generated by FRCs, which drains lymph fluid through a pipeline-like system to distribute small molecules and antigens, has remained unexplored. Here, we quantify the crucial 3D morphometric parameters and determine the topological properties governing the structural organization of the intertwined networks. We find that the conduit system exhibits lesser small-worldness and lower resilience to perturbation compared to the FRC network, while the robust topological organization of both networks is maintained in a lymphotoxin-β-receptor-independent manner. Overall, the high-resolution topological analysis of the "roads-and-pipes" networks highlights essential parameters underlying the functional organization of LN micro-environments and will, hence, advance the development of multi-scale LN models.

Lymph node stromal cell subsets-Emerging specialists for tailored tissue-specific immune responses

The effective priming of adaptive immune responses depends on the precise dispatching of lymphocytes and antigens into and within lymph nodes (LNs), which are strategically dispersed throughout the body. Over the past decade, a growing body of evidence has advanced our understanding of lymph node stromal cells (LNSCs) from viewing them as mere accessory cells to seeing them as critical cellular players for the modulation of adaptive immune responses. In this review, we summarize current advances on the pivotal roles that LNSCs play in orchestrating adaptive immune responses during homeostasis and infection, and highlight the imprinting of location-specific information by micro-environmental cues into LNSCs, thereby tailoring tissue-specific immune responses.

Contributions of Embryonic HSC-Independent Hematopoiesis to Organogenesis and the Adult Hematopoietic System

During ontogeny, the establishment of the hematopoietic system takes place in several phases, separated both in time and location. The process is initiated extra-embryonically in the yolk sac (YS) and concludes in the main arteries of the embryo with the formation of hematopoietic stem cells (HSC). Initially, it was thought that HSC-independent hematopoietic YS cells were transient, and only required to bridge the gap to HSC activity. However, in recent years it has become clear that these cells also contribute to embryonic organogenesis, including the emergence of HSCs. Furthermore, some of these early HSC-independent YS cells persist into adulthood as distinct hematopoietic populations. These previously unrecognized abilities of embryonic HSC-independent hematopoietic cells constitute a new field of interest. Here, we aim to provide a succinct overview of the current knowledge regarding the contribution of YS-derived hematopoietic cells to the development of the embryo and the adult hematopoietic system.

Development and Immunological Function of Lymph Node Stromal Cells

Stromal cells have for a long time been viewed as structural cells that support distinct compartments within lymphoid tissues and little more. Instead, an active cross-talk between endothelial and fibroblastic stromal cells drives the maturation of lymphoid niches, a relationship that is recapitulated during lymph node organogenesis, steady-state conditions, and following inflammation. In this review, we go over recent advances in genetic models and high-resolution transcriptomic analyses that have propelled the finer resolution of the stromal cell infrastructure of lymph nodes, revealing that the distinct subsets are strategically positioned to deliver a catered mixture of niche factors to interacting immune cell populations. Moreover, we discuss how changes in the activation state of poised stromal cell-underpinned niches rather than on-demand differentiation of new stromal cell subsets govern the efficient interaction of Ag, APC, and cognate B and T lymphocytes during adaptive immune responses.

Dawn of lipid nanoparticles in lymph node targeting: Potential in cancer immunotherapy

It is generally known that the lymph nodes (LNs) are important tissues in cancer immunotherapy. Therefore, delivering immune functional compounds to LNs is a useful strategy for enhancing cancer immunotherapy. Lipid-based nanocarriers have been widely used as delivery systems that target LNs, but lipid nanoparticle (LNP) technology has recently attracted increased interest. High levels of nucleic acids can be efficiently loaded in LNPs, they can be used to actively deliver nucleic acids into the cytoplasm, and they can be produced on an industrial scale. The use of microfluidic devices has been particularly valuable for producing small-sized LNPs, thus paving the way for successful LN targeting. In the review, we focus on the potential of LNP technology for targeting LNs.

Technological approaches to streamline vaccination schedules, progressing towards single-dose vaccines

Vaccines represent the most successful medical intervention in history, with billions of lives saved. Although multiple doses of the same vaccine are typically required to reach an adequate level of protection, it would be advantageous to develop vaccines that induce protective immunity with fewer doses, ideally just one. Single-dose vaccines would be ideal to maximize vaccination coverage, help stakeholders to greatly reduce the costs associated with vaccination, and improve patient convenience. Here we describe past attempts to develop potent single dose vaccines and explore the reasons they failed. Then, we review key immunological mechanisms of the vaccine-specific immune responses, and how innovative technologies and approaches are guiding the preclinical and clinical development of potent single-dose vaccines. By modulating the spatio-temporal delivery of the vaccine components, by providing the appropriate stimuli to the innate immunity, and by designing better antigens, the new technologies and approaches leverage our current knowledge of the immune system and may synergize to enable the rational design of next-generation vaccination strategies. This review provides a rational perspective on the possible development of future single-dose vaccines.