Elsevier

Vascular Pharmacology

Volume 112, January 2019, Pages 31-41
Vascular Pharmacology

Review
Cellular traffic through afferent lymphatic vessels

https://doi.org/10.1016/j.vph.2018.08.001Get rights and content

Abstract

The lymphatic system has long been known to serve as a highway for migrating leukocytes from peripheral tissue to draining lymph nodes (dLNs) and back to circulation, thereby contributing to the induction of adaptive immunity and immunesurveillance. Lymphatic vessels (LVs) present in peripheral tissues upstream of a first dLN are generally referred to as afferent LVs. In contrast to migration through blood vessels (BVs), the detailed molecular and cellular requirements of cellular traffic through afferent LVs have only recently started to be unraveled. Progress in our ability to track the migration of lymph-borne cell populations, in combination with cutting-edge imaging technologies, nowadays allows the investigation and visualization of lymphatic migration of endogenous leukocytes, both at the population and at the single-cell level. These studies have revealed that leukocyte trafficking through afferent LVs generally follows a step-wise migration pattern, relying on the active interplay of numerous molecules. In this review, we will summarize and discuss current knowledge of cellular traffic through afferent LVs. We will first outline how the structure of the afferent LV network supports leukocyte migration and highlight important molecules involved in the migration of dendritic cells (DCs), T cells and neutrophils, i.e. the most prominent cell types trafficking through afferent LVs. Additionally, we will describe how tumor cells hijack the lymphatic system for their dissemination to draining LNs. Finally, we will summarize and discuss our current understanding of the functional significance as well as the therapeutic implications of cell traffic through afferent LVs.

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.

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