Analyzing Lymphatic Vessel Patterning in Adult Tissue.

Whole-mount immunostaining allows intact tissue to be surveyed in three dimensions, avoiding the more restricted fields of view provided by visualizing thin sections. This technique is particularly useful for imaging lymphatic and blood networks by high-resolution confocal microscopy, revealing how such vessels are spatially positioned, the subcellular arrangements of individual antigens, and interactions with individual cells within the interstitium or vessel lumen. The purpose of this chapter is to provide a practical guide for obtaining images of lymphatic vessels following immunofluorescence staining, primarily in mouse skin.

Hyaluronan and Its Receptors: Key Mediators of Immune Cell Entry and Trafficking in the Lymphatic System.

Entry to the afferent lymphatics marks the first committed step for immune cell migration from tissues to draining lymph nodes both for the generation of immune responses and for timely resolution of tissue inflammation. This critical process occurs primarily at specialised discontinuous junctions in initial lymphatic capillaries, directed by chemokines released from lymphatic endothelium and orchestrated by adhesion between lymphatic receptors and their immune cell ligands. Prominent amongst the latter is the large glycosaminoglycan hyaluronan (HA) that can form a bulky glycocalyx on the surface of certain tissue-migrating leucocytes and whose engagement with its key lymphatic receptor LYVE-1 mediates docking and entry of dendritic cells to afferent lymphatics. Here we outline the latest insights into the molecular mechanisms by which the HA glycocalyx together with LYVE-1 and the related leucocyte receptor CD44 co-operate in immune cell entry, and how the process is facilitated by the unusual character of LYVE-1 • HA-binding interactions. In addition, we describe how pro-inflammatory breakdown products of HA may also contribute to lymphatic entry by transducing signals through LYVE-1 for lymphangiogenesis and increased junctional permeability. Lastly, we outline some future perspectives and highlight the LYVE-1 • HA axis as a potential target for immunotherapy.

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.

Dendritic cell entry to lymphatic capillaries is orchestrated by CD44 and the hyaluronan glycocalyx.

DCs play a vital role in immunity by conveying antigens from peripheral tissues to draining lymph nodes, through afferent lymphatic vessels. Critical to the process is initial docking to the lymphatic endothelial receptor LYVE-1 via its ligand hyaluronan on the DC surface. How this relatively weak binding polymer is configured for specific adhesion to LYVE-1, however, is unknown. Here, we show that hyaluronan is anchored and spatially organized into a 400-500 nm dense glycocalyx by the leukocyte receptor CD44. Using gene knockout and by modulating CD44-hyaluronan interactions with monoclonal antibodies in vitro and in a mouse model of oxazolone-induced skin inflammation, we demonstrate that CD44 is required for DC adhesion and transmigration across lymphatic endothelium. In addition, we present evidence that CD44 can dynamically control the density of the hyaluronan glycocalyx, regulating the efficiency of DC trafficking to lymph nodes. Our findings define a previously unrecognized role for CD44 in lymphatic trafficking and highlight the importance of the CD44:HA:LYVE-1 axis in its regulation.

Extracellular bacterial lymphatic metastasis drives Streptococcus pyogenes systemic infection.

Unassisted metastasis through the lymphatic system is a mechanism of dissemination thus far ascribed only to cancer cells. Here, we report that Streptococcus pyogenes also hijack lymphatic vessels to escape a local infection site, transiting through sequential lymph nodes and efferent lymphatic vessels to enter the bloodstream. Contrasting with previously reported mechanisms of intracellular pathogen carriage by phagocytes, we show S. pyogenes remain extracellular during transit, first in afferent and then efferent lymphatics that carry the bacteria through successive draining lymph nodes. We identify streptococcal virulence mechanisms important for bacterial lymphatic dissemination and show that metastatic streptococci within infected lymph nodes resist and subvert clearance by phagocytes, enabling replication that can seed intense bloodstream infection. The findings establish the lymphatic system as both a survival niche and conduit to the bloodstream for S. pyogenes, explaining the phenomenon of occult bacteraemia. This work provides new perspectives in streptococcal pathogenesis with implications for immunity.

Hyaluronan Receptor LYVE-1-Expressing Macrophages Maintain Arterial Tone through Hyaluronan-Mediated Regulation of Smooth Muscle Cell Collagen.

The maintenance of appropriate arterial tone is critically important for normal physiological arterial function. However, the cellular and molecular mechanisms remain poorly defined. Here, we have shown that in the mouse aorta, resident macrophages prevented arterial stiffness and collagen deposition in the steady state. Using phenotyping, transcriptional profiling, and targeted deletion of Csf1r, we have demonstrated that these macrophages-which are a feature of blood vessels invested with smooth muscle cells (SMCs) in both mouse and human tissues-expressed the hyaluronan (HA) receptor LYVE-l. Furthermore, we have shown they possessed the unique ability to modulate collagen expression in SMCs by matrix metalloproteinase MMP-9-dependent proteolysis through engagement of LYVE-1 with the HA pericellular matrix of SMCs. Our study has unveiled a hitherto unknown homeostatic contribution of arterial LYVE-1+ macrophages through the control of collagen production by SMCs and has identified a function of LYVE-1 in leukocytes.

The cardiac lymphatic system stimulates resolution of inflammation following myocardial infarction.

Myocardial infarction (MI) arising from obstruction of the coronary circulation engenders massive cardiomyocyte loss and replacement by non-contractile scar tissue, leading to pathological remodeling, dysfunction, and ultimately heart failure. This is presently a global health problem for which there is no effective cure. Following MI, the innate immune system directs the phagocytosis of dead cell debris in an effort to stimulate cell repopulation and tissue renewal. In the mammalian adult heart, however, the persistent influx of immune cells, coupled with the lack of an inherent regenerative capacity, results in cardiac fibrosis. Here, we reveal that stimulation of cardiac lymphangiogenesis with VEGF-C improves clearance of the acute inflammatory response after MI by trafficking immune cells to draining mediastinal lymph nodes (MLNs) in a process dependent on lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1). Deletion of Lyve1 in mice, preventing docking and transit of leukocytes through the lymphatic endothelium, results in exacerbation of chronic inflammation and long-term deterioration of cardiac function. Our findings support targeting of the lymphatic/immune cell axis as a therapeutic paradigm to promote immune modulation and heart repair.

Lymphatic exosomes promote dendritic cell migration along guidance cues.

Lymphatic endothelial cells (LECs) release extracellular chemokines to guide the migration of dendritic cells. In this study, we report that LECs also release basolateral exosome-rich endothelial vesicles (EEVs) that are secreted in greater numbers in the presence of inflammatory cytokines and accumulate in the perivascular stroma of small lymphatic vessels in human chronic inflammatory diseases. Proteomic analyses of EEV fractions identified >1,700 cargo proteins and revealed a dominant motility-promoting protein signature. In vitro and ex vivo EEV fractions augmented cellular protrusion formation in a CX3CL1/fractalkine-dependent fashion and enhanced the directional migratory response of human dendritic cells along guidance cues. We conclude that perilymphatic LEC exosomes enhance exploratory behavior and thus promote directional migration of CX3CR1-expressing cells in complex tissue environments.

Dendritic cells enter lymph vessels by hyaluronan-mediated docking to the endothelial receptor LYVE-1.

Trafficking of tissue dendritic cells (DCs) via lymph is critical for the generation of cellular immune responses in draining lymph nodes (LNs). In the current study we found that DCs docked to the basolateral surface of lymphatic vessels and transited to the lumen through hyaluronan-mediated interactions with the lymph-specific endothelial receptor LYVE-1, in dynamic transmigratory-cup-like structures. Furthermore, we show that targeted deletion of the gene Lyve1, antibody blockade or depletion of the DC hyaluronan coat not only delayed lymphatic trafficking of dermal DCs but also blunted their capacity to prime CD8+ T cell responses in skin-draining LNs. Our findings uncovered a previously unknown function for LYVE-1 and show that transit through the lymphatic network is initiated by the recognition of leukocyte-derived hyaluronan.

Rapid Lymphatic Dissemination of Encapsulated Group A Streptococci via Lymphatic Vessel Endothelial Receptor-1 Interaction.

The host lymphatic network represents an important conduit for pathogen dissemination. Indeed, the lethal human pathogen group A streptococcus has a predilection to induce pathology in the lymphatic system and draining lymph nodes, however the underlying basis and subsequent consequences for disease outcome are currently unknown. Here we report that the hyaluronan capsule of group A streptococci is a crucial virulence determinant for lymphatic tropism in vivo, and further, we identify the lymphatic vessel endothelial receptor-1 as the critical host receptor for capsular hyaluronan in the lymphatic system. Interference with this interaction in vivo impeded bacterial dissemination to local draining lymph nodes and, in the case of a hyper-encapsulated M18 strain, redirected streptococcal entry into the blood circulation, suggesting a pivotal role in the manifestation of streptococcal infections. Our results reveal a novel function for bacterial capsular polysaccharide in directing lymphatic tropism, with potential implications for disease pathology.

Neutrophils rapidly transit inflamed lymphatic vessel endothelium via integrin-dependent proteolysis and lipoxin-induced junctional retraction.

Neutrophils are the first leukocyte population to be recruited from the circulation following tissue injury or infection, where they play key roles in host defense. However, recent evidence indicates recruited neutrophils can also enter lymph and shape adaptive immune responses downstream in draining lymph nodes. At present, the cellular mechanisms regulating neutrophil entry to lymphatic vessels and migration to lymph nodes are largely unknown. Here, we have investigated these events in an in vivo mouse Mycobacterium bovis bacillus Calmette-Guérin vaccination model, ex vivo mouse dermal explants, and in vitro Transwell system comprising monolayers of primary human dermal lymphatic endothelial cells. We demonstrate that neutrophils are reliant on endothelial activation for adhesion, initially via E-selectin and subsequently, by integrin-mediated binding to ICAM-1 and VCAM-1, combined with CXCL8-dependent chemotaxis. Moreover, we reveal that integrin-mediated neutrophil adhesion plays a pivotal role in subsequent transmigration by focusing the action of matrix metalloproteinases and the 15-lipoxygenase-1-derived chemorepellent 12(S)-hydroxyeicosatetraenoic acid at neutrophil:endothelial contact sites to induce transient endothelial junctional retraction and rapid, selective neutrophil trafficking. These findings reveal an unexpectedly intimate collaboration between neutrophils and the lymphatic vessel endothelium, in which these phagocytic leukocytes act as pathfinders for their own transit during inflammation.

Control of dendritic cell trafficking in lymphatics by chemokines.

Dendritic cells (DCs) are crucial participants in maintaining immune surveillance of the periphery and initiating primary immune responses within the draining lymph nodes. The afferent lymphatic vessels provide a conduit for this essential trafficking and, as this review will describe, play an active role in regulating DC migration. Afferent lymphatic capillaries support constitutive trafficking of DCs from resting, non-inflamed tissue, to maintain tolerance against self-antigen and to provide immune surveillance. Following exposure to pathogens or pro-inflammatory cytokines, DCs mature from phagocytes to professional antigen-presenting cells, whilst the lymphatic endothelium adopts an activated phenotype to support the ensuing increase in leukocyte trafficking. The lymphatic endothelial-derived chemokine CCL21 plays a well-characterized role in directing migration of CCR7+ DC in both resting and acute inflammatory conditions. However, efficient trafficking of DCs from inflamed tissue also demands additional chemokine-receptor pairs. Thus, entry of DCs to activated lymphatic vessels is an intricately regulated multi-step process involving numerous chemokines and adhesion molecules.

The chemokine CX3CL1 promotes trafficking of dendritic cells through inflamed lymphatics.

Tissue inflammation is characterised by increased trafficking of antigen-loaded dendritic cells (DCs) from the periphery via afferent lymphatics to draining lymph nodes, with a resulting stimulation of ongoing immune responses. Transmigration across lymphatic endothelium constitutes the first step in this process and is known to involve the chemokine CCL21 and its receptor CCR7. However, the precise details of DC transit remain obscure and it is likely that additional chemokine-receptor pairs have roles in lymphatic vessel entry. Here, we report that the transmembrane chemokine CX3CL1 (fractalkine) is induced in inflamed lymphatic endothelium, both in vitro in TNF-α-treated human dermal lymphatic endothelial cells (HDLECs) and in vivo in a mouse model of skin hypersensitivity. However, unlike blood endothelial cells, which express predominantly transmembrane CX3CL1 as a leukocyte adhesion molecule, HDLECs shed virtually all CX3CL1 at their basolateral surface through matrix metalloproteinases. We show for the first time that both recombinant soluble CX3CL1 and endogenous secreted CX3CL1 promote basolateral-to-luminal migration of DCs across HDLEC monolayers in vitro. Furthermore, we show in vivo that neutralising antibodies against CX3CL1 dramatically reduce allergen-induced trafficking of cutaneous DCs to draining lymph nodes as assessed by FITC skin painting in mice. Finally, we show that deletion of the CX3CL1 receptor in Cx3cr1(-/-) DCs results in markedly delayed lymphatic trafficking in vivo and impaired translymphatic migration in vitro, thus establishing a previously unrecognised role for this atypical chemokine in regulating DC trafficking through the lymphatics.

Inflammation-induced secretion of CCL21 in lymphatic endothelium is a key regulator of integrin-mediated dendritic cell transmigration.

Tissue inflammation induces rapid mobilization of antigen-charged dendritic cells (DCs), which migrate to draining lymph nodes via afferent lymphatics to elicit the immune response. This increase in DC trafficking has been shown to require integrin-dependent adhesion to ICAM-1 and VCAM-1, expressed on inflamed lymphatic endothelium. In addition, both constitutive- and inflammation-induced DC migration involves the chemokine CCL21, which most likely triggers integrin activation on DC via its receptor CCR7. Recently, however, conflicting evidence has suggested that DC entry occurs independently of integrins, implying that the role of CCL21 in lymphatics is purely chemotactic. Hence, while CCL21 is reported to be inducible during inflammation, the details of this induction and the role of CCL21 during initial DC trafficking are unclear. Here, we have characterized both the production of CCL21 and the mechanism of its action in DC transmigration using primary human dermal lymphatic endothelial cells (HDLECs) and a mouse model of skin contact hypersensitivity. We showed that CCL21 is constitutively expressed intracellularly but rapidly secreted after exposure to the inflammatory cytokine tumour necrosis factor (TNF) α following de novo RNA and protein synthesis. Furthermore, using in vitro transmigration assays, we showed that endogenous HDLEC-derived CCL21 stimulates DC translymphatic migration by a predominantly chemotactic mechanism in resting HDLEC and by a ß2 integrin-mediated mechanism in TNFα-stimulated HDLEC. These results imply a direct role for CCL21 in lymphatic transmigration that involves the selective use of integrin activation in inflammation.

Blocking development of a CD8+ T cell response by targeting lymphatic recruitment of APC.

Generating a protective immune response to viral infection is known to depend upon the priming and clonal expansion of virus-specific CD8(+) T cells by Ag-loaded dendritic cells (DC) within secondary lymphoid tissue. However, the actual initiation of the response involves critical upstream events that control the recruitment of mature Ag-charged DC from the periphery via afferent lymphatics, events that are still only partly understood. Recent evidence has revealed that transmigration of lymphatic endothelium by DC is regulated by the adhesion molecules ICAM-1 and VCAM-1 both in vitro and in vivo. These findings imply that lymphatic entry may be an important rate-limiting step in primary immunity and a possible target for immune intervention. In this study, we have explored such possibilities using an F(5) TCR-transgenic mouse model to assess the contribution of lymphatic cell adhesion molecules in the CD8(+) T cell response to influenza virus nucleoprotein (NP). We show for the first time that immunization with ICAM-1- and VCAM-1-blocking mAbs can impair the T cell response in lymph node-draining sites of dermally administered nucleoprotein vaccine (MVA.HIVA.NP) by targeting lymphatic uptake of Ag-loaded DC ahead of other cell adhesion molecule-dependent events. These results reveal lymphatic entry as an important step that may be rate limiting in the development of immunity and reconfirm its potential as a target for localized immunotherapy in inflammation and tissue rejection.

Cell traffic and the lymphatic endothelium.

The principal immune function of the afferent lymphatics is to bear antigen and leukocytes from peripheral tissues to the draining lymph nodes. Recent research has shown that passage of leukocytes into the afferent lymphatic capillaries is far from an indolent process; rather it is carefully orchestrated by an array of adhesion molecules, as well as by chemokines and their receptors. Here we review the current knowledge of leukocyte trans-lymphatic endothelial migration and its role in the development of an immune response.

Renal cells activate the platelet receptor CLEC-2 through podoplanin.

We have recently shown that the C-type lectin-like receptor, CLEC-2, is expressed on platelets and that it mediates powerful platelet aggregation by the snake venom toxin rhodocytin. In addition, we have provided indirect evidence for an endogenous ligand for CLEC-2 in renal cells expressing HIV-1. This putative ligand facilitates transmission of HIV through its incorporation into the viral envelope and binding to CLEC-2 on platelets. The aim of the present study was to identify the ligand on these cells which binds to CLEC-2 on platelets. Recombinant CLEC-2 exhibits specific binding to HEK-293T (human embryonic kidney) cells in which the HIV can be grown. Furthermore, HEK-293T cells activate both platelets and CLEC-2-transfected DT-40 B-cells. The transmembrane protein podoplanin was identified on HEK-293T cells and was demonstrated to mediate both binding of HEK-293T cells to CLEC-2 and HEK-293T cell activation of CLEC-2-transfected DT-40 B-cells. Podoplanin is expressed on renal cells (podocytes). Furthermore, a direct interaction between CLEC-2 and podoplanin was confirmed using surface plasmon resonance and was shown to be independent of glycosylation of CLEC-2. The interaction has an affinity of 24.5+/-3.7 microM. The present study identifies podoplanin as a ligand for CLEC-2 on renal cells.

Inflammation-induced uptake and degradation of the lymphatic endothelial hyaluronan receptor LYVE-1.

The hyaluronan receptor LYVE-1 is selectively expressed in the endothelium of lymphatic capillaries, where it has been proposed to function in hyaluronan clearance and hyaluronan-mediated leukocyte adhesion. However, recent studies suggest that hyaluronan homeostasis is unperturbed in LYVE-1(-/-) mice and that lymphatic adhesion/transmigration may be largely mediated by ICAM-1 and VCAM-1 rather than LYVE-1. Here we have explored the possibility that LYVE-1 functions during inflammation and report that the receptor is down-regulated by pro-inflammatory cytokines. Using cultured primary lymphatic endothelial cells, we show that surface expression of LYVE-1 is rapidly and reversibly lost after exposure to tumor necrosis factor-alpha (TNFalpha) and TNFbeta via internalization and degradation of the receptor in lysosomes, coupled with a shutdown in gene expression. Curiously, internalization does not result in significant uptake of hyaluronan, a process that is largely insensitive to the novel LYVE-1 adhesion blocking monoclonal antibody 3A, and proceeds almost equally in resting and inflammation-activated lymphatic endothelial cells. Finally, we show that TNF can induce down-modulation of LYVE-1 in ex vivo murine dermal tissue explants and present evidence that the process occurs in vivo, in the context of murine allergen-induced skin inflammation. These findings suggest that LYVE-1 can function independently of hyaluronan and have implications for the use of LYVE-1 as a histological marker for lymphangiogenesis in human pathology.

An inflammation-induced mechanism for leukocyte transmigration across lymphatic vessel endothelium.

The exit of antigen-presenting cells and lymphocytes from inflamed skin to afferent lymph is vital for the initiation and maintenance of dermal immune responses. How such an exit is achieved and how cells transmigrate the distinct endothelium of lymphatic vessels are unknown. We show that inflammatory cytokines trigger activation of dermal lymphatic endothelial cells (LECs), leading to expression of the key leukocyte adhesion receptors intercellular adhesion molecule 1 (ICAM-1), vascular cell adhesion molecule 1 (VCAM-1), and E-selectin, as well as a discrete panel of chemokines and other potential regulators of leukocyte transmigration. Furthermore, we show that both ICAM-1 and VCAM-1 are induced in the dermal lymphatic vessels of mice exposed to skin contact hypersensitivity where they mediate lymph node trafficking of dendritic cells (DCs) via afferent lymphatics. Lastly, we show that tumor necrosis factor alpha stimulates both DC adhesion and transmigration of dermal LEC monolayers in vitro and that the process is efficiently inhibited by ICAM-1 and VCAM-1 adhesion-blocking monoclonal antibodies. These results reveal a CAM-mediated mechanism for recruiting leukocytes to the lymph nodes in inflammation and highlight the process of lymphatic transmigration as a potential new target for antiinflammatory therapy.