Lymphatic vessels in the age of cancer immunotherapy

Abstract

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.

Fig. 1 |. Antigen and leukocyte transport via the lymphatic vasculature.

a, Peripheral lymphatic vessels facilitate the uptake of proteins, lipids, metabolites, extracellular vesicles (EVs), leukocytes and fluid from peripheral tissues and tumours for transport to draining lymph nodes (LNs) where they are compartmentalized for appropriate activation. Lymphatic capillaries express adhesion molecules (for example, intercellular adhesion molecule 1 (ICAM1) and lymphatic vessel endothelial hyaluronic acid receptor 1 (LYVE-1)) and chemoattractants that direct the entry of migrating leukocytes. Tumour-associated lymphangiogenesis is regulated by the balance of vascular endothelial growth factors (VEGFs) and inflammatory cytokines (for example, interferon-γ (IFNγ), interleukin 4 (IL-4), IL-13, IL-1β and tumour necrosis factor (TNF)). Lymphatic vessels can limit local inflammation by scavenging inflammatory chemokines with decoy receptors (for example, atypical chemokine receptor 2 (ACKR2)). b, After entry into peripheral lymphatic capillaries, leukocytes move directionally to the LN first by crawling and then flowing through the collector lymphatic vessels. c, Macromolecules, antigens and leukocytes are sorted at the subcapsular sinus (SCS), which is lined by an outer (ceiling) and inner (floor) layer of lymphatic endothelial cells (LECs). Specialized structures in floor LECs (fLECs) filter molecules based on size into the conduit system and restrict large particulates to the SCS. The conduits, which consist of a core of organized collagen fibrils lined by fibroblastic reticular cells (FRCs), form channels that run towards high endothelial venules (HEVs), the sites of naive lymphocyte infiltration. These conduits are also lined by dendritic cells (DCs) capable of sampling and presenting antigens to naive T cells. Large particulates may also be transcytosed by LECs or sampled by SCS macrophages for transfer to follicular dendritic cells (FDCs) and B cells. Migratory DCs from the periphery follow CC-motif chemokine ligand 21 (CCL21) gradients formed by SCS LECs, which shape a directional gradient by scavenging CCL21 with the decoy receptor, ACKR4. Once in the LN paracortex, migratory DCs travel along the conduit system to engage cognate naive lymphocytes. Ag, antigen; CCR7, CC-motif chemokine receptor 7; cLECs, ceiling LECs; CXCL12, CXC-chemokine ligand 12; CXCR4, CXC-chemokine receptor 4; S1P, sphingosine 1-phosphate; S1PR1, S1P receptor 1.

Fig. 2 |. Lymphatic egress contributes to the anatomical compartmentalization of T cell responses.

a, CD8⁺ T lymphocytes continuously circulate between lymph nodes (LNs), blood and tumours. Effector and memory T cells enter tumours through the inflamed blood vasculature and are retained by high-affinity antigen (Ag) encounters with dendritic cells (DCs) or tumour cells. These same antigen signals, particularly when chronic, drive terminal differentiation leading to T cell dysfunction. By contrast, functional, low-to-moderate affinity T cells are protected from local dysfunction and appear to recirculate or egress from the tumour microenvironment (TME) via lymphatic vessels. At least, a subset of these egressing T cells are T cell-specific transcription factor 1 (TCF1)⁺ and programmed cell death protein 1 (PD1) intermediate, consistent with stem-like T (T_SC) cells that are critical for maintaining persistent T cell responses in chronic infection and tumours. LN retention of T_SC cells depends in part on transforming growth factor β (TGFβ) signalling and together with protective tissue-resident memory T (T_RM) cells represents an important depot of antigen-specific immunity in mouse and human tumour-draining LNs. Therefore, the mechanisms that regulate rates of T cell entry, retention and exit across various T cell states within the TME determine the anatomical segregation of T cell repertoires and may present novel therapeutic targets to tune the duration of T cell transit time and improve response to immunotherapy. b, As T cells transit between LNs, blood and tumours, and then back over time they encounter sequential signals that simultaneously tune their migratory potential, effector function and repertoire diversity, and thereby potential to contribute to reinvigoration in response to immune checkpoint blockade (ICB). Protecting a diverse T cell repertoire from the dysfunctional TME by maintenance in the LN may be essential for long-term tumour control both locally and systemically. CCL21, CC-motif chemokine ligand 21; CXCL12, CXC-chemokine ligand 12; CXCR, CXC-chemokine receptor; S1P, sphingosine 1-phosphate; S1PR, S1P receptor; T_dys cell, dysfunctional T cell.

Fig. 3 |. Reconciling the paradox of tumour-associated lymphangiogenesis and response to immunotherapy.

Lymphangiogenesis in tumours is associated with both lymph node (LN) metastasis and response to immune checkpoint blockade (ICB) in preclinical models. This apparent paradox in which lymphatic vessels and their transport are described as both ‘good’ and ‘bad’ limits efforts to consider translational approaches to leverage the lymphatic system for therapy. However, a working model must integrate time and the adaptive mechanisms that progressively tune the contribution of lymphatic vessels. Existing evidence indicates that (1) tumour-associated lymphatic vessels are necessary for de novo adaptive immunity and can facilitate antigen spreading; (2) driving intratumoural lymphatic growth with VEGFC can boost antigen presentation but the benefit is counterbalanced by exacerbated local inflammation, progressive immune dysfunction and metastasis; (3) lymphatic vessels facilitate the recirculation of lymphocytes away from the tumour microenvironment (TME) and promote immune resolution and (4) lymphatic transport and eventually metastasis itself conditions the LN microenvironment to be more suppressive, which impairs systemic immune surveillance. On the basis of these lines of evidence, we move away from a simple binary explanation for the role of tumour-associated lymphangiogenesis and propose a time-dependent framework whereby the progressive activation and growth of the lymphatic vasculature can shift the balance within the TME towards chronic inflammation, lymphatic efflux that drives immune resolution, and metastasis. These time-dependent changes thereby limit systemic immune surveillance and response to immunotherapy. T_eff cell, effector T cell; T_reg cell, regulatory T cell; T_dys cell, dysfunctional T cell.

Fig. 4 |. Lymphatic transport and its relationship with the systemic tumour macroenvironment.

a, The lymphatic vasculature acts to regulate both local and systemic interactions between the tumour and its macroenvironment with implications for immunotherapy. Notably, data implicate lymphatic transport in the formation of tertiary lymphoid structures (TLSs), immune surveillance in the central nervous system (CNS), the gut microbiome, ageing and obesity, complex and overlapping processes that converge to determine systemic immune surveillance. Homeostatic lymphatic function is both affected by and directly shapes each of these areas, indicating that lymphatic health may systemically shape host potential for tumour progression. b, The lymphatic vasculature sustains homeostatic health by mediating the flow of fluid, cells and lipids from peripheral tissues to lymph nodes (LNs) and back to the blood via the thoracic duct. This schematic of the host macroenvironment in which red lines indicate vascular connections and grey lines indicate lymphatic connections demonstrates the potential for lymphatic-dependent long-range signalling to regulate host homeostasis and response to malignancy. The interconnectedness of transport between (for example, gut, pancreas and liver) and possibly within (for example, TLS and tumour) tissues, and the requirement for lymphatic function for efficient efflux of diverse species from cells to lipids and even microorganisms to the systemic vasculature, demands a more holistic and systems-level interrogation of lymphatic function in the setting of tumour progression and response to immune checkpoint blockade (ICB). DC, dendritic cell; HEV, high endothelial venule.