ReviewCellular traffic through afferent lymphatic vessels
Graphical abstract
Introduction
The lymphatic vascular system has important functions in tissue fluid homeostasis, transport of macromolecules and uptake of dietary fats from the intestine [[1], [2], [3]]. Moreover, LVs transport antigen-containing lymph and leukocytes from peripheral tissues to dLNs [2, 4], and from LNs back into the blood circulation. LVs present in peripheral tissues upstream of a first dLN are generally referred to as afferent LVs. Over the past years, increasing evidence revealed a crucial role for leukocyte trafficking through afferent LVs for the induction of adaptive immunity and general immune-surveillance [5, 6].
To date, the lymphatic system has been less well studied compared to the blood vascular system. However, lymphatic research has gained strong momentum over the past two decades, thanks to the discovery of lymphatic-specific markers, such as the lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1) [7], the mucin type-1 protein Podoplanin [8], vascular endothelial growth factor receptor-3 (VEGFR-3) [9] and the lymphatic-specific transcription factor Prox-1 [10]. Despite their expression in other tissues and by other cell types, such as hepatic blood sinusoidal endothelial cells (LYVE-1) [11], kidney podocytes and lung alveolar type I cells (Podoplanin) [8] or skeletal muscles, neurons and retinal cells (Prox-1) [12], the use and combination of these makers nowadays enables the unambiguous molecular distinction between blood and lymphatic vasculature in tissues. Moreover, the generation of lymphatic-specific conditional knock-outs [[13], [14], [15]] or fluorescent reporter mice [16, 17], together with technical advances like the isolation and cultivation of lymphatic endothelial cells (LECs) and time-lapse imaging performed in tissue explants or in vivo, has greatly enhanced our current understanding of leukocyte migration through afferent LVs. It has also become clear that not only leukocytes use LVs to reach dLNs, but that this pathway is also routinely hijacked by tumor cells. In fact, in many cancer types lymphatic involvement and the occurrence of LN metastasis has been shown to correlate with poor patient prognosis [18].
Vaccination is considered one of the crucial contributions to public health in the 20th century and highlights the importance of leukocyte migration through afferent LVs. Upon antigen recognition, following vaccine injection, antigen presenting DCs mature and start to migrate through afferent LVs to dLNs, where they induce a specific immune response, by presenting the antigen to T cells. Since their discovery approximately 40 years ago [[19], [20], [21]], it has become apparent that DC migration to dLNs is not only essential for priming the adaptive immune response in the context of vaccination and infection, but also for promoting and maintaining tolerance [4, 5, 22]. Besides DCs, different T cell subsets and neutrophils are also frequently found in afferent lymph, but the mechanisms and relevance of their migration is less well studied.
In this review, we will present and discuss current understanding of cellular migration through afferent LVs. We will first introduce the unique anatomy and morphology of the afferent lymphatic network and highlight the main leukocyte types commonly found in afferent lymph. Next, we will describe the stepwise migration of leukocytes from peripheral tissues to dLNs through afferent LVs and provide detailed insight of the molecules involved in their migration, as well as in the migration of tumor cells, which in part is guided by similar mechanisms. Finally, we will conclude with an overall discussion of the functional significance of cellular traffic through afferent LVs and its therapeutic implications.
Section snippets
Anatomical and morphological characteristics of the lymphatic vascular network
The lymphatic system is composed of central and peripheral secondary lymphoid organs (SLOs) and a highly dispersed network of LVs, which penetrates nearly all vascularized organs of the body [23, 24]. The afferent lymphatic network originates in peripheral tissues in the form of lymphatic capillaries, which sequentially merge into larger collecting vessels (Figs. 1A, 2). These collectors drain into and through one or more dLNs before converging into a single vessel, the thoracic duct.
Cells present in afferent lymph
Early cannulation studies of afferent LVs conducted in sheep and healthy humans under homeostatic conditions revealed that T lymphocytes are the most common cell type in afferent lymph (80–90%) [[39], [40], [41], [42]]. The majority of these cells represent CD4+ effector memory T cells (TEMs), while CD8+ T cells are only found in small numbers. Functionally, CD4+ TEM are thought to migrate through LVs in order to recirculate from the periphery back to the blood circulation, in constant search
Leukocyte migration through afferent lymphatic vessels
Despite the general knowledge of leukocyte migration through afferent LVs, the cellular and molecular mechanisms of this process are only now starting to be fully unraveled. In the last 20 years, cell-tracking studies revealed that leukocytes rely on specific molecules to migrate from peripheral tissues to dLNs. More recently, time-lapse imaging performed in tissue explants or in vivo intravital microscopy (IVM) has allowed us to visualize and study leukocyte migration in real-time and with
Role of CCR7 and its ligands
The undoubtedly best-studied molecules involved in DC migration are the chemokine receptor CCR7, which is upregulated on maturing DCs [77], and its two ligands, chemokines CCL21 and CCL19. Genetic deletion of CCR7 [78, 79] or antibody-mediated blockade of CCL21 [80] profoundly reduces DC migration to dLNs in mice. CCL21 is constitutively expressed by LECs of afferent LVs [[81], [82], [83]], with higher expression in capillaries than in collectors [69]. CCL21 comprises a highly positively
Molecules involved in T cell migration through afferent lymphatic vessels
In agreement with cannulation studies, the majority of endogenous T cells exiting tissues in mice are CD4+ cells displaying a TEM-like phenotype [110]. Similar to DCs, the best-known molecule involved in T cell migration via afferent LVs is CCR7. In mice, transferred CCR7-deficient T cells failed to arrive in dLNs in steady-state [50] or to exit non-lymphoid tissues through afferent lymphatics in a model of immunization-induced airway inflammation [111]. Moreover, in CCR7−/− mice endogenous
Polymorphonuclear cell migration through afferent lymphatic vessels
Cannulation studies in humans and in sheep also detected low numbers of neutrophils, monocytes, basophils, eosinophils and B cells in afferent lymph [[39], [40], [41], [42], 122]. Of these cell types, the best studied one, migrating through afferent lymphatics, are neutrophils. Neutrophils become recruited to infected tissues where they establish the primary frontline of defense against invading pathogens. In addition to several other effector functions, neutrophils display high phagocytic
Molecular mechanisms involved in tumor cell migration through afferent lymphatic vessels
Tumor dissemination via lymphatics leads to regional LN metastasis and strongly correlates with disease progression [[131], [132], [133]]. Tumor cells are known to release vascular growth factors like vascular endothelial growth factor (VEGF)-C or VEGF-D to stimulate proliferation of peripheral LVs [134]. Tumor-induced lymphangiogenesis provides an increased lymphatic density in vicinity or within the tumor, thereby promoting lymphatic tumor invasion [135]. Lymphatic pumping and lymph flow were
Conclusion and outlook
Research on cellular traffic through afferent LVs has recently made great progress thanks to technical advances like the generation of lymphatic-specific knockouts, the use of photo-convertible mice to study endogenous cell trafficking or novel imaging approaches such as IVM. The latter experiments have revealed that leukocyte migration through afferent LVs represents a more complex process than previously assumed, involving the dynamic interplay of numerous molecules and distinct cellular
Acknowledgments
The authors thank Martina Vranova (ETH Zurich) for providing confocal microscopy images of lymphatic capillaries and collectors (Fig. 1B and C) and Morgan Hunter (ETH Zurich) for critically reading and discussing the review. CH gratefully acknowledges support by the ETH Zurich.
References (196)
- et al.
Lymph node homing of T cells and dendritic cells via afferent lymphatics
Trends Immunol.
(2012) - et al.
Dendritic-cell-based therapeutic cancer vaccines
Immunity
(2013) Angiosarcomas express mixed endothelial phenotypes of blood and lymphatic capillaries: podoplanin as a specific marker for lymphatic endothelium
Am. J. Pathol.
(1999)- et al.
Prox1 function is required for the development of the murine lymphatic system
Cell
(1999) Prox 1, a prospero-related homeobox gene expressed during mouse development
Mech. Dev.
(1993)Visualization of lymphatic vessels by Prox1-promoter directed GFP reporter in a bacterial artificial chromosome-based transgenic mouse
Blood
(2011)- et al.
Lymphangiogenesis: Molecular mechanisms and future promise
Cell
(2010) Integrin-α9 Is Required for Fibronectin Matrix Assembly during Lymphatic Valve Morphogenesis, in Developmental Cell
(2009)- et al.
B cells acquire particulate antigen in a macrophage-rich area at the boundary between the follicle and the subcapsular sinus of the lymph node
Immunity
(2007) Immune cell traffic from blood through the normal human skin to lymphatics
Clin. Dermatol.
(1995)
Regulatory T cells sequentially migrate from inflamed tissues to draining lymph nodes to suppress the alloimmune response
Immunity
Differential requirement for ROCK in dendritic cell migration within lymphatic capillaries in steady-state and inflammation
Blood
T cell migration from inflamed skin to draining lymph nodes requires intralymphatic crawling supported by ICAM-1/LFA-1 interactions
Cell Rep.
Intralymphatic CCL21 promotes tissue egress of dendritic cells through afferent lymphatic vessels
CellReports
CCR7 coordinates the primary immune response by establishing functional microenvironments in secondary lymphoid organs
Cell
CCR7 governs skin dendritic cell migration under inflammatory and steady-state conditions
Immunity
Tissue inflammation modulates gene expression of lymphatic endothelial cells and dendritic cell migration in a stimulus-dependent manner
Blood
Endothelial heparan sulfate controls chemokine presentation in recruitment of lymphocytes and dendritic cells to lymph nodes
Immunity
Locally Triggered Release of the Chemokine CCL21 Promotes Dendritic Cell Transmigration across Lymphatic Endothelia
CellReports
Immobilized chemokine fields and soluble chemokine gradients cooperatively shape migration patterns of dendritic cells
Immunity
Autocrine CCL19 blocks dendritic cell migration toward weak gradients of CCL21
Cytotherapy
The lymphatic vasculature in disease
Nat. Med.
Lymphatic vascular morphogenesis in development, physiology, and disease
J. Cell Biol.
The lymphatic vasculature revisited
J. Clin. Invest.
Dendritic cell subsets in health and disease
Immunol. Rev.
LYVE-1, a new homologue of the CD44 glycoprotein, is a lymph-specific receptor for hyaluronan
J. Cell Biol.
Expression of the fms-like tyrosine kinase 4 gene becomes restricted to lymphatic endothelium during development
Proc. Natl. Acad. Sci. U. S. A.
LYVE-1 is not restricted to the lymph vessels: expression in normal liver blood sinusoids and down-regulation in human liver cancer and cirrhosis
Cancer Res.
Genes regulating lymphangiogenesis control venous valve formation and maintenance in mice
J. Clin. Invest.
Vegfr3-CreER (T2) mouse, a new genetic tool for targeting the lymphatic system
Lymphatic endothelial cell sphingosine kinase activity is required for lymphocyte egress and lymphatic patterning
J. Exp. Med.
Intravital two-photon microscopy of lymphatic vessel development and function using a transgenic Prox1 promoter-directed mOrange2 reporter mouse
Biochem. Soc. Trans.
Interaction of tumor cells and lymphatic vessels in cancer progression
Identification of a novel cell type in peripheral lymphoid organs of mice. I. Morphology, quantitation, tissue distribution
J. Exp. Med.
Identification of a novel cell type in peripheral lymphoid organs of mice: II. Functional properties in vitro
J. Exp. Med.
Identification of a novel cell type in peripheral lymphoid organs of mice: III. Functional properties in vivo
J. Exp. Med.
The importance of dendritic cells in maintaining immune tolerance
J. Immunol.
The lymphatic vasculature: Recent progress and paradigms
Annu. Rev. Cell Dev. Biol.
The Lymphatic System in Health and Disease, in Lymphatic Research and Biology
HEVs, lymphatics and homeostatic immune cell trafficking in lymph nodes
Nat. Rev. Immunol.
Physiological Reviews
Preformed portals facilitate dendritic cell entry into afferent lymphatic vessels
J. Exp. Med.
Functionally specialized junctions between endothelial cells of lymphatic vessels
J. Exp. Med.
Rapid leukocyte migration by integrin-independent flowing and squeezing
Nature
Fine structure of the lymphatic capillary and the adjoining connective tissue area
Am. J. Anat.
Mechanisms causing initial lymphatics to expand and compress to promote lymph flow
Arch. Histol. Cytol.
PDZ interaction site in ephrinB2 is required for the remodeling of lymphatic vasculature
Genes Amp. Develop.
Afferent lymph-derived T cells and DCs use different chemokine receptor CCR7-dependent routes for entry into the lymph node and intranodal migration
Nat. Immunol.
Homing and cellular traffic in lymph nodes
Nat. Rev. Immunol.
Normal structure, function, and histology of lymph nodes
Toxicol. Pathol.
Cited by (37)
Mediators of Capillary-to-Venule Conversion in the Chronic Inflammatory Skin Disease Psoriasis
2022, Journal of Investigative DermatologyCitation Excerpt :These findings highlight an important transcriptional control of vascular remodeling in psoriasis by alternating promoters and their differential interactions with distal CREs. Different segments of lymphatic vessels adopt dissimilar morphology and transcriptomic signatures, which confer differences in their barrier functions and their roles in immune cell trafficking (Schineis et al., 2019). The LEC cluster was partitioned into three subpopulations, namely the lymphatic capillaries, precollecting (PreCol) vessels and collecting (Col) vessels (Figure 6a).
Imaging of fluorescent polymer dots in relation to channels and immune cells in the lymphatic system
2022, Materials Today BioCitation Excerpt :The lymphatic system reflects a complex network of lymphatic vessels (LVs), lymph nodes (LNs) and lymphatic organs [2]. The system is responsible for maintaining tissue fluid balance, and serves as a conduit for immune cell transportation and dietary fat uptake in the intestine [1,3–6]. This fascinating vasculature is now thought to be dynamically and actively involved in health and disease [7–9].
Development and aging of the lymphatic vascular system
2021, Advanced Drug Delivery ReviewsCitation Excerpt :Sprouting lymphatic capillaries harbour continuous cell-cell junctions, thus “button-like” junctions are hallmarks of quiescent and mature lymphatic capillary endothelium [31]. Capillary LECs (capLECs) produce high levels of the chemokine CCL21, which attracts CCR7+ dendritic cells (DCs) to migrate to lymphatic capillaries (reviewed by [2,32]). DCs pass through the pores of the discontinuous basement membrane and arrive into the lumen of lymphatic capillaries, from where they migrate towards draining LNs [33,34].
- 1
These authors contributed equally to this work.