Lymphatic transport in anti-tumor immunity and metastasis

Lymphatic vessels (LVs) are enriched in tissues frequently exposed to pathogens, such as the skin, intestine, and mucosal tissues. The interstitial fluid, which includes a variety of nutrients, antigens, and immune cells, flows into the lymphatic capillaries, travels through the collecting LVs to the LNs, and ultimately reenters the bloodstream through the lymphatic trunks and thoracic ducts. This process relies on the highly specialized and hierarchical structure of the lymphatic network (Fig. 1 and reviewed in Swartz [2001]).

Lymphatic capillaries (or initial lymphatics) are composed of a single layer of lymphatic endothelial cells (LECs) forming blind-ended vessels, with no coverage by perivascular mural cells, such as pericytes and smooth muscle cells (SMCs), but only discontinuous basement membrane in some locations (Baluk et al., 2007). As the major entry point for fluid and cells from peripheral tissues, lymphatic capillaries are highly permeable and contain unique junctional structures (Fig. 1 a). Adjacent LECs are overlaid in an “oak leaf” pattern and connected by discontinuous “button-like” intercellular junctions (Baluk et al., 2007; Dejana et al., 2009a). Each junction contains adhesion and tight junction molecules, such as zonula occludens-1, occludin, junctional adhesion molecule-A, vascular endothelial cadherin (VE-cadherin), and intercellular adhesion molecule-1 (ICAM-1), forming a “flap valve” allowing the entrance for interstitial fluid (Baluk et al., 2005; Dejana et al., 2009b; Pfeiffer et al., 2008).

Anchoring filaments, primarily made up of fibrillin, link the abluminal membrane of endothelial cells to the adjacent elastic fibers, probably via the transmembrane integrin α 3 β 1 and the focal adhesion kinase molecule (Gerli et al., 2000). Anchoring filaments connect the extracellular matrix (ECM) and the endothelial cell cytoskeleton, maintaining the luminal structure of the lymphatic capillaries during pressure variations in the surrounding tissue and enabling the flap valves to open and facilitate the entry of the tissue fluid (Saharinen et al., 2004; Schmid-Schönbein, 1990). Furthermore, the intercellular junction space between LECs in the lymphatic capillaries exhibits an elevated anion density, favoring the entry of inorganic salts and small molecules, thereby regulating the composition of the lymph and interstitial fluid (Leak, 1986). Newly sprouted lymphatic capillaries, as well as those exposed to inflammation, show transient continuous intercellular junctions. Therefore, a button-like arrangement of junction proteins can also be regarded as a characteristic of both static and functional lymphatic capillaries (Baluk et al., 2007). After being transported in the lymphatic capillaries, the interstitial fluid is drained through the pre-collecting LVs and into the lymphatic collectors due to the contraction of specialized SMCs connected to the lymphatic pre-collectors. LECs lining larger collecting LVs are connected by continuous zipper-like junctions. In addition, lymphatic collectors exhibit a bilobed valve structure within their lumen and complete SMC and basement membrane coverage, allowing the maintenance of a unidirectional lymph flow to the draining LNs (Alitalo et al., 2005; Dejana et al., 2009a; Norrmén et al., 2011). It has been demonstrated that inflammation caused by viral or bacterial infections activates the tightening of lymphatic capillary junctions (zippering) (Baluk et al., 2007; Churchill et al., 2022; Yao et al., 2012), which is linked to decreased fluid transport and helps prevent the spread of pathogens.

At steady state, LVs mostly maintain fluid balance, lipid absorption, and transportation of immune cells. Transudate from the blood circulation flows into interstitial spaces where it might enter highly permeable lymphatic capillaries due to their discontinuous intercellular junctions, therefore enabling a rapid influx of interstitial fluid, macromolecules, particles, and immune cells when the interstitial pressure is elevated. Conversely, the overlapping structure of LECs prevents the lymph content from leaking outside the lymphatic capillaries (Angeli and Lim, 2023). The specialized features of LVs at steady state, how they connect to LNs, and later to the bloodstream are illustrated in Fig. 1, b–d. Besides, specialized LVs are found in different organs and exhibit tissue-specific structures and functions (Fig. 1 e). For example, in the brain, meningeal LVs exhibit a phenotype similar to the one of lymphatic capillaries in other organs but develop valves only near the base of the skull alongside the cranial nerves (Aspelund et al., 2015). These LVs drain the cerebrospinal fluid and parenchyma interstitial fluid into the deep cervical LNs (Ma et al., 2017) and enable immune cell trafficking (Louveau et al., 2015). In the intestine, lacteals are central LVs that are present in the gut villi. They control the uptake and the transport of chylomicrons and other interstitial fluid components from the villi to the mesenteric LNs and then return to the blood circulation (Bernier-Latmani and Petrova, 2017; Cifarelli and Eichmann, 2019). Although lung LVs mature only after birth, they begin to drain the interstitial fluid during the later stages of pregnancy, which enhances lung compliance and is essential for proper lung inflation in newborns. Due to the absence of SMC coverage in lung lymphatic collectors, it is thought that the pressure generated by respiratory movements aids in the propulsion of lymph (Jakus et al., 2014; Reed et al., 2019). The heart also has a considerable amount of LVs that encompass all layers of the heart in humans. Similar to lung LVs, heart LVs lack SMCs and therefore, lymph propulsion in heart LVs relies entirely on the cardiac muscle contraction and twist forces. A functional heart lymphatic drainage system is essential to prevent edema and maintain normal cardiac functions. Recent evidence indicates that LECs can generate lymphoangiocrine signals that regulate the proliferation and survival of cardiomyocytes during heart development, enhance cardiac regeneration in neonates, and provide cardioprotective effects following a myocardial infarction (Brakenhielm and Alitalo, 2019; Liu et al., 2020). Finally, in the skin, LVs have also been found promoting the growth of the hair follicle (Peña-Jimenez et al., 2019; Yoon et al., 2019).

LNs are one of the most important secondary lymphoid organs, where cellular interactions between T cells and antigen-presenting dendritic cells (DCs) occur. They are crucial for initiating adaptive cell-mediated immunity. Although leukocytes are the dominant population in LNs, LECs and other stromal cells, including blood endothelial cells, fibroblastic reticular cells, and follicular DCs, are essential for maintaining the structure and the functions of LNs (Jalkanen and Salmi, 2020).

LNs are composed of extensive and highly conserved lymphatic structures (Sainte-Marie, 2010; Willard-Mack, 2006), mostly constituted by afferent LVs, LN sinuses, and efferent LVs. The penetrating afferent LVs connect the lymph from peripheral tissues and the subcapsular sinus layer of LNs. Two layers of LECs line the subcapsular sinus: the ceiling LECs forming the outer layer close to the capsule and the floor LECs forming the inner layer facing the LN parenchyma (Jalkanen and Salmi, 2020). Floor LECs also interact with subcapsular sinus macrophages and sinus-resident DCs that are strategically positioned to protrude and survey the subcapsular lumen (Gerner et al., 2015; Gray and Cyster, 2012; Jalkanen and Salmi, 2020; Louie and Liao, 2019). The medullary and cortical sinuses form along the subcapsular sinus to the hilum of the LN. Finally, the medullary sinuses expand to the edge of the LNs and form efferent LVs, which support the exit of lymphocytes from the LNs (Jalkanen and Salmi, 2020).

Soluble antigens and immune cells from peripheral tissues enter lymphatic capillaries and are transported toward regional LNs via afferent LVs. LVs are actively involved in regulating the trafficking and the transportation of immune cell to LNs, which orchestrate adaptive immune responses. Circulating naïve lymphocytes enter LNs through high endothelial venules (HEVs) (Girard et al., 2012), and after getting their immune imprinting, effector cells leave the LN through efferent LVs to return to the blood circulation via the thoracic duct to head to the tissues of demand. As such, effector/memory T cells represent the main population in efferent LVs (Haig et al., 1999; Hunter et al., 2016).

Under CCL21 guidance, CCR7⁺ DCs migrate from the tissues to the lymphatic capillaries (Russo et al., 2016; Tal et al., 2011; Weber et al., 2013). Interestingly, steady-state DCs autonomously migrate into the LVs mainly by flowing and squeezing (Lämmermann et al., 2008) (Fig. 2 a). The sole force of actin-network expansion promotes protrusive flowing of the leading edge, whereas the myosin II–dependent squeezing contraction of the trailing edge propels the rigid nucleus and allows the passage through narrow gaps, which is sufficient to drive DC locomotion (Lämmermann et al., 2008). This process only becomes integrin dependent under inflammatory conditions when LECs upregulate ICAM-1 and VCAM-1 (Arasa et al., 2021; Johnson et al., 2006) (Fig. 2 b). Recent findings show that in case of inflammation, the connective tissue membrane surrounding the lymphatic collectors is subjected to degradation, whereas the collectors upregulate the DC-trafficking molecule VCAM-1, facilitating DC penetration and rapid transportation to draining LNs (Arasa et al., 2021) (Fig. 2 c). After reaching the LNs, DCs first enter the LN subcapsular sinus. Ceiling LECs produce ACKR4 (or CCRL1), a decoy receptor for CCL21 and CCL19, that scavenges these chemokines in the sinus lumen and shapes chemokine gradients directed toward the floor layer. Some studies indicate that in some contexts, CCR7 may not be sufficient for proper DC positioning within the LN and that other chemokine networks may be involved in subset-specific DC migration to LNs. For example, Th2-skewing stimulus efficiently induces the migration of CD301b⁺ DCs into draining LNs despite low surface CCR7 levels. Instead, CD301b⁺ DCs upregulate CCR8, and their migration toward the LN parenchyma is facilitated by floor LECs that produce CCL1 (Qu et al., 2004; Sokol et al., 2018) (Fig. 2 d).

For lymphocytes, live imaging of naïve lymphocytes shows that these cells initially enter the LN paracortex from the blood vasculature through HEVs (Girard et al., 2012). However, intralymphatic cotransfers of activated lymphocytes and naive lymphocytes with DCs further demonstrate possible entry at the level of LN subcapsular sinuses (Steele et al., 2023). In the subcapsular sinuses, activated T cells and memory B cells randomly survey along floor LECs. Lymphocytes approach the LN parenchyma by following CCL21 gradients (Martens et al., 2020; Zhang et al., 2022) (Fig. 2 e). The macrophage scavenger receptor (MSR1) expressed on subcapsular sinus LECs (and on different macrophage subtypes) supports the binding of lymphocytes and regulates their entry from the subcapsular sinus to the LN parenchyma (Jalkanen and Salmi, 2020). The filtering function of the lymphatic sinus endothelium is further dependent on diaphragms formed by plasmalemma vesicle–associated protein fibrils in transendothelial channels that constitute physical sieve regulating the entry of lymphocytes and soluble antigens into the LN parenchyma (Rantakari et al., 2015).

The most extensive literature on lymphatic migration regards DCs, and little is known about the mechanisms governing T cell afferent lymph migration to LNs. Although confirmation in humans is needed, mouse studies have revealed that upon inflammation, the main subset of T cells emigrating via afferent lymphatics to draining LNs are regulatory T cells (Tregs) (Ikebuchi et al., 2016; Nakanishi et al., 2018; Tomura et al., 2010). It was further shown that Tregs migrate into LVs in an LTαβ–LTβR-dependent manner to reach LNs (Brinkman et al., 2016; Heim et al., 2024) (Fig. 2 f). Finally, a recent study demonstrated that LVs further transport antigen-experienced cytotoxic CD8⁺ T cell effectors from the skin to the draining LNs, allowing the generation of LN-resident memory T cells exhibiting protective functions upon viral rechallenge (Heim et al., 2024) (Fig. 2 g). For neutrophils, a unique subset expressing MHC class-II (MHC-II) is found in mouse and human LNs (Hampton and Chtanova, 2016). Human subcapsular sinus floor and medullary sinus LECs have been shown to express neutrophil chemoattractants (CXCL1 and CXCL5), which could explain the migration of CXCR6⁺ neutrophils in the LNs (Takeda et al., 2019). In mice, neutrophils were described to enter inflamed LNs through the CXCR4/CXCL12 axis (Hampton et al., 2015) (Fig. 2 h). For other leukocytes, floor LECs express sialoglycans that bind CD169, regulating the localization of LN-resident macrophages (D’Addio et al., 2021) (Fig. 2 i). LN-resident CXCR6⁺ innate-like lymphoid cells follow CCL20 gradients and migrate across the sinus through interactions between LFA molecule-1 and ICAM-1 (Zhang et al., 2016) (Fig. 2 e). The egress of immune cells from LNs is mostly regulated by the sphingosine-1 phosphate (S1P). LECs near cortical sinus and efferent lymphatics, as well as in peripheral tissues, produce S1P, attracting S1P receptor (S1PR1)–positive immune cells and leading to their egress from LNs (Matloubian et al., 2004; Pham et al., 2010) (Fig. 2 j). The expression of α9β1 integrin on cortical and medullary LECs may also support the exit of immune cells from LNs (Ito et al., 2014).

Apart from supporting cell transportation, LVs actively contribute to immunosurveillance and immunomodulation. Human and mouse LN LECs express MHC class I (MHC-I) and MHC-II. At steady state, LN LECs endogenously express peripheral tissue–restricted antigens that they can present through MHC-I (Johnson et al., 2017; Olszewski, 2005), inducing the deletion of autoreactive CD8⁺ T cells and therefore participating in peripheral T cell tolerance (Cohen et al., 2010; Fletcher et al., 2010; Tewalt et al., 2012). LECs can also cross-present exogenous antigens via MHC-I to induce antigen-specific CD8⁺ T cell apoptosis (Hirosue et al., 2014). Although LECs seem unable to present antigens through MHC-II due to the absence of the H2-M expression at steady state (Rouhani et al., 2015), they can induce antigen-specific CD4⁺ T cell tolerance by presenting peptide–MHC-II complexes acquired from DCs or by presenting peptides through IFN-γ–induced endogenous MHC-II under inflammatory condition (Dubrot et al., 2014). For example, MHC-II deficiency in LN LECs promotes defective Tregs and increased effector CD4⁺ T cells, resulting in impaired peripheral CD4⁺ T cell tolerance and enhanced autoantibody production in aging mice (Dubrot et al., 2018). Furthermore, S1P produced by LN LECs supports naive T cell survival through the stimulation of their mitochondrial function (Mendoza et al., 2017). LECs express IL-7 in both LN and peripheral tissues, suggesting that they might support T cell survival (Hara et al., 2012). Under infection and inflammation, some factors released into the surrounding tissue activate LECs to induce their proliferation, a process called lymphangiogenesis. These factors, named lymphangiogenic factors, promote LEC proliferation, sprouting, and activation, altogether increasing the abundance of LVs in inflamed tissues. LVs experience remarkable expansion, accompanied by a transient increase of lymph flow, promoting the translocation of DCs and lymphocytes to the LN parenchyma and therefore the initiation of the immune response. Extranodal lymphangiogenesis is accompanied by intranodal lymphangiogenesis in the LNs draining the inflamed tissue, which can be reverted by a T cell–dependent IFN-γ–mediated negative regulation (Kataru et al., 2011). Upon inflammatory stimuli, LN LECs further express immunosuppressive enzymes, such as indoleamine dioxygenase and inducible nitric oxide synthase (iNOS), thereby suppressing T cell proliferation and activation (Lukacs-Kornek et al., 2011), as well as inhibiting DC maturation (Christiansen et al., 2016). During viral infection and vaccination, LN LECs function as antigen-archiving cells and slowly release these antigens into the LN parenchyma to contribute to protective immune responses (Kedl et al., 2017; Tamburini et al., 2014). During bacterial infection, such as Mycobacterium tuberculosis, the bacteria can be transported and internalized by LN LECs, in which bacterial proliferation is inhibited by the production of nitric oxide by LECs when exposed to IFN-γ (Lerner et al., 2016). Tumors are a typical condition where lymphangiogenesis occurs and where some of the remodeling of LEC features described above might apply, together with specific adaptations induced by the tumor microenvironment (TME) (developed in Lymphangiogenic factors in the TME).

Tumor-associated LV expansion is a hallmark of several human cancers and is often related to metastasis and poor prognosis (Dieterich and Detmar, 2016). Upon tumor development, lymphangiogenesis can occur in the peritumoral tissue, the tumor-draining LNs, and at the center of the tumor mass (Hu and Luo, 2018). Whereas the role of peritumoral lymphatics that exhibit a large open lumen, in promoting metastasis is well-established, flattened, collapsed intratumoral lymphatics do not seem to be involved in this process (Padera et al., 2002). Several cell types in the TME, including tumor cells, macrophages, and fibroblasts, secrete lymphangiogenic cytokines (Dieterich et al., 2022). Fig. 3 recapitulates the lymphangiogenic factors produced in the TME, the cellular sources, as well as the impact of lymphangiogenesis on tumor progression and metastasis and response to immunotherapy. Vascular endothelial growth factors (VEGF)-C and VEGF-D are the two most essential pro-lymphangiogenic factors. The VEGF receptor 3 (VEGFR3), which serves as the common receptor for VEGF-C and VEGF-D, is highly expressed by LECs. The binding of VEGFR3 and subsequent signaling induces the phosphorylation of its tyrosine kinase domain, which recruits the CT10 regulator of the kinase, the SRC homology domain-containing, and the growth factor receptor-bonus protein 2 adaptor protein. Together with the phosphatidylinositol-3-kinase, VEGFR3 signaling activates the downstream protein kinase B (Akt), MAPK, ERK 1/2, and JNK pathways, altogether promoting LECs proliferation and migration (Fournier et al., 1995; Leppänen et al., 2013; Mäkinen et al., 2001; Salameh et al., 2005). At early tumor development stages, tumor cells are the main inducers of VEGF-C/D signaling in the TME. The specific upregulation of VEGF-C/D in tumor cells results in potent tumor associated lymphangiogenesis in mouse models (Stacker et al., 2001; Tacconi et al., 2019). At later stages as well, tumor cells significantly contribute to the production of VEGF-C in the TME. Indeed, the proinflammatory cytokine IL-6 and the enzyme cyclooxygenase-2 were shown to induce VEGF-C expression in tumor cells and were correlated with lymphangiogenesis and LN metastasis in multiple human tumors (Liu et al., 2010; Lyons et al., 2014; Shinriki et al., 2011). Upon tumor development, tumor-associated macrophages (TAMs) are important contributors to VEGF-A, VEGF-C, and VEGF-D production in TME. In vivo depletion of macrophages dampens tumor-associated lymphangiogenesis in several tumor models (Fischer et al., 2007; Zumsteg and Christofori, 2012). In squamous cell carcinoma, CD169⁺ macrophages recruited to the peritumoral region secrete VEGF-C, leading to an increase in lymphatic density and to the reorganization of the tumor-associated lymphatic vasculature (Moussai et al., 2011). TNF-α and IL1-β are two cytokines that induce VEGF-C expression in TAMs (Ji et al., 2014; Peppicelli et al., 2014; Ristimäki et al., 1998). By expressing matrix metalloproteinases, plasmin, and urokinase plasminogen, TAMs contribute to the remodeling of the ECM, further promoting the migration of LECs on the tips of the capillaries and new LV sprouting (Mazzone and Bergers, 2019). VEGF-A is also a significant lymphangiogenic factor. Although it is mostly known to promote angiogenesis (the expansion of blood endothelial cells), LECs also express its receptor VEGFR2. Constitutive VEGFR2 deficiency leads to hypoplastic LVs in mice (Dellinger et al., 2013) and reduces Notch signaling–dependent LEC expansion, migration, and lymphatic capillary formation in vitro (Dellinger and Brekken, 2011; Hirakawa et al., 2003; Marino et al., 2014). VEGF-A contributes to tumor and sentinel LN lymphangiogenesis and promotes tumor metastasis in the cutaneous squamous cell carcinoma model (Hirakawa et al., 2005).

Besides VEGF signaling, several additional factors of the TME contribute to tumor-associated lymphangiogenesis. TNF-α not only promotes VEGF-C production by TAMs, as mentioned above, but also directly induces LEC proliferation and migration (Hong et al., 2016). The tyrosine receptors of angiopoietin Ang, Tie1, and Tie2 are expressed by LECs, and overproduction of their ligands Ang1/2/3/4 by tumor cells induces lymphangiogenesis in experimental models (Fagiani et al., 2011; Schulz et al., 2011). Activation of the ephrin type-B receptor 4 signaling pathway by its ligand Ephrin B2, both of which are expressed by LECs, initiates lymphangiogenesis in astrocytoma and melanoma, thereby promoting tumor progression and dissemination (Héroult et al., 2010; Sawamiphak et al., 2010). Other important lymphangiogenic factors are insulin-like growth factors (IGF)-1 and 2, as LECs express their respective receptors IGF-1R and IGF-2R. Activation of the IGF-1/2R pathway induces LEC proliferation, migration, and phosphorylation of ERK, AKT, and Src signaling (Björndahl et al., 2005). In addition, IGF-1R expression positively correlates with LV density in human colorectal cancer (CRC), and IGF-1 administration in mouse models induces lymphangiogenesis in tumors (Li et al., 2013). Fibroblast growth factor-2 is also an inducer of lymphangiogenesis. Its overexpression in mouse fibrosarcoma cells increases intratumoral LV density (Cao et al., 2012), while its neutralization in mouse lung cancer leads to reduced tumor lymphangiogenesis (Cai et al., 2017). The platelet-derived growth factor (PDGF) family is known to induce lymphangiogenesis in a VEGF-C/D independent manner. In the tumor context, increased lymphangiogenesis and metastasis have been described in PDGF-BB overexpressing fibrosarcoma mouse models, these observations being abrogated by PDGF receptor blockade (Cao et al., 2004). LECs express both the calcitonin receptor-like receptor and receptor activity-modifying protein 2, these two receptors exhibiting high affinity for adrenomedullin (AM) when combined together (McLatchie et al., 1998). Interestingly, AM overexpression in murine Lewis lung carcinoma correlates with increased tumor growth and LN lymphangiogenesis, as well as enhanced distant organ metastasis (Karpinich et al., 2013). High levels of AM are found in several human tumors as well, indicating a potential relationship between calcitonin receptor-like receptor/receptor activity-modifying protein 2–AM signaling and tumor-associated lymphangiogenesis (Hay et al., 2010). Hepatocyte growth factor and S1P are also found increased in TME, contributing to both tumor angiogenesis and lymphangiogenesis (Kajiya et al., 2005; Nagahashi et al., 2012). Although it does not directly promote lymphangiogenesis, the GPI-linked protein semaphorin 7A has been shown to be highly expressed by tumoral mammary epithelial cells during involution, leading the upregulation of podoplanin⁺ macrophages that promote lymphangiogenesis (Elder et al., 2018). Finally, S100A4 produced by LECs promotes LV sprouting in growing tumors by regulating glycolysis (Li et al., 2023).

Downstream of the LV collectors are the draining LNs, which are also exposed to tumor-derived factors that trigger intranodal remodeling changes. Even at early stages of tumor development, tumor-draining LNs undergo enlargement in response to these factors, mainly as a result of LEC proliferation (Commerford et al., 2018). It was shown that tumor-secreted VEGF-C was triggering significant expansion of sinusoidal LVs within sentinel LNs, accompanied by increased LEC proliferation and upregulation of VEGFR3 (Hirakawa et al., 2007). Similarly, tumor-associated LVs exhibit distinct transcriptional profiles compared with dermal lymphatics, and LECs within tumor-draining LNs display unique transcriptional signatures distinct from healthy LN LECs (Commerford et al., 2018). These alterations promote tumor cell survival (described more in the next sections).

Lymphatic metastasis is considered the first step for spreading to distant organs in many epithelial cancers, such as melanoma, breast cancer, and colorectal carcinoma (Stacker et al., 2014). Phenotypical and functional differences clearly distinguish tumor-associated LVs from their healthy tissue counterparts. Intratumoral LVs usually exhibit a collapsed shape, likely due to the combination of proliferating cancer cells causing mechanical forces (“solid stress”) (Padera et al., 2004) and possibly to high interstitial pressure (Nia et al., 2020) present in the tumor mass. In contrast, peritumoral LVs often appear dilated and morphologically tortuous, occupied by intraluminal cells, and are considered as the site where lymphatic metastasis initiates (Leu et al., 2000; Padera et al., 2002; Stanczyk et al., 2010). Although steady-state LVs have a discontinuous basement membrane, tumor-associated lymphatics become more permeable (Tammela et al., 2007) and less able to create and retain lymph (Jones, 2020). Discontinuities in lymphatic capillary walls allow not only the transport of extracellular content both into and out of the LV but also the entry by cancer cells, providing a route for metastatic dissemination (Tammela et al., 2007; Jones, 2020). Furthermore, tumor-draining LVs were shown to exhibit an increased pumping activity in collecting ducts, with increased frequency and amplitude of contractions, in different tumor mouse models, such as glioblastoma, lung cancer, melanoma, and fibrosarcoma (Bachmann et al., 2019; Dieterich et al., 2022; Gogineni et al., 2013; He et al., 2005; Hoshida et al., 2006; Proulx et al., 2010), making them easier routes for tumor invasion and dissemination. However, there is no clear consensus on this point, as other studies have described reduced lymphatic contraction and outflow in multiple mouse models of cancer, including glioma (Ma et al., 2019), melanoma, breast cancer, and sarcoma (Liao et al., 2019). Lymphatic drainage can also be redirected from the tumor to collateral lymphatics draining different LNs when primary sentinel LN sinus obstruction happens, generating additional routes for primary tumor metastasis (Leijte et al., 2009; Nathanson and Mahan, 2011; Proulx et al., 2013).

Evidence shows that tumor-associated LVs facilitate tumor metastasis by expressing chemokines and adhesion molecules or by undergoing junction remodeling to help tumor cell invasion. Several studies have reported that tumor cells can enter LVs by “leukocyte mimicry.” Many chemokine axes used for immune cell migration, such as CCL21 (Wiley et al., 2001), CXCL1 (Kasashima et al., 2017), CXCL7 (YAMAMOTO et al., 2019), CXCL9/10/11 (Kawada et al., 2007; Kumaravel et al., 2020), and CXCL12 (Hirakawa et al., 2009; Kim et al., 2010), can be hijacked by tumor cells through the expression of their receptors, directing tumor cell migration toward LVs. For example, CCL1, which is expressed by subcapsular sinus LECs, directs melanoma cells that express CXCR8 toward LVs. Inhibition of CCR8 leads to tumor cell arrest in afferent collecting LVs at their junction with the LN subcapsular sinus (Das et al., 2013). VEGF-C not only promote LEC proliferation but also increases their production of CCL21, promoting lymphatic metastasis of CCR7⁺ tumor cells (Issa et al., 2009). Moreover, tumor cells use LEC-expressed adhesion proteins to facilitate their crawling into the LVs. VEGF-C in the TME does not only activate tumor-associated LECs but also promotes their upregulation of integrin α4β1 expression, facilitating the adherence of VCAM-1⁺ tumor cells to LVs (Garmy-Susini et al., 2013). In human head and neck squamous cell carcinoma and colorectal carcinoma, endothelial cell–selective adhesion molecule (ESAM) expression levels in lymphatics correlate with LN metastasis, although the underlined mechanisms remain unclear (Clasper et al., 2008; Wegmann et al., 2006). While specific lymphatic capillary features have already been adapted to facilitate cell entry, some additional modulations of the tumor-associated LV barrier have been described. The upregulation of integrin α4β1 and subsequent interaction with VCAM-1⁺ tumor cells is concomitant with a decreased expression of VE-cadherin on adjacent LECs (Dieterich et al., 2019). In human melanoma, age-associated loss of VE-cadherin expression in LECs correlates with increased lymphatic permeability and extravasation of tumor cells from afferent LVs into tumor-draining LNs, which in return leads to increased visceral metastasis (Ecker et al., 2019). Another mechanism facilitating tumor lymphatic invasion through the remodeling of the lymphatic barrier is called chemorepellent-induced defects, a mechanism used by neutrophils to migrate into inflamed lymphatics. Tumor cells that express ALOX15 have been observed to release 12-hydroxyeicosatetraenoic acid and induce the formation of large pores in the LEC layer in vitro. In vivo, ALOX15 expression is negatively correlated with metastasis-free survival in some ductal adenocarcinoma patients (Kerjaschki et al., 2011). In breast cancer, tumor cells similarly hijack this mechanism to facilitate their entry into lymphatics (Kerjaschki et al., 2011).

For tumor cells localized nearby or that have successfully entered LVs, tumor-associated LECs can further facilitate their arrival, setup, and survival by establishing metastatic niches. When exposed to tumor cell–derived VEGF-D, LECs in collecting LVs undergo profound transcriptomic remodeling, with notably the downregulation of 15-hydroxyprostaglandin dehydrogenase (PGDH), a key enzyme in prostaglandin catabolism (Karnezis et al., 2012). The collecting lymphatic endothelium consequently produces lower amounts of prostaglandins, leading to the dilation of the collecting LVs and promoting metastasis (Karnezis et al., 2012). Clinical observation in melanoma patients revealed a unique phenomenon called “in-transit metastasis,” which develops along the LVs between the primary tumor and the draining LNs. Tumor cells nest in afferent LVs and adopt a stem cell–like phenotype characterized by a low proliferation rate, self-renewability, and drug resistance, suggesting that the lymphatic endothelium might provide a microenvironment favorable for the long-term survival of dormant tumor cells (Karaman and Detmar, 2014).

In melanoma and hepatoma, tumor cells that express the cancer stem-like marker CD133 are drawn to CXCL12 produced by LECs, resulting in their accumulation near LVs at both primary and metastatic sites (Kim et al., 2010; Wei et al., 2019). While this accumulation can occur in both peri-tumoral LVs and tumor-draining LNs, the mechanisms underlying the formation and function of the tumor lymphatic niche remain largely unclear. Research on hepatoma suggests that IL-17 produced by LECs may facilitate tumor cell self-renewal and immune evasion within the intra-LV niche (Wei et al., 2019). Additionally, a study utilizing the MMTV-PyMT breast tumor model indicates that the maintenance of the tumor stem cell pool relies on CCL21/CCR7 signaling (Boyle et al., 2016), implying that this pathway may also play a role in the accumulation of cancer stem-like cells in lymphatic niches.

It is somehow counterintuitive to understand how cancer cells can survive and establish metastasis in tumor-draining LNs, which are abundantly populated by immune cells and where the establishment of a pre-metastatic niche also remains mechanistically elusive. Although the formation of the LN pre-metastatic mice has also been reported in preclinical breast cancer (Commerford et al., 2018) and chemically induced skin cancer (Hirakawa et al., 2005), this phenomenon has been mostly deciphered in preclinical and clinical melanoma. Melanoma-draining LNs have been found to display a degree of immunosuppression prior to the invasion of cancer cells, both in mouse models (Ogawa et al., 2014) and in human melanoma (Mansfield et al., 2011). This is likely due to the exposure to TME-derived factors that are drained by LVs. Accordingly, tumor-associated factors are enriched in lymphatic exudate in metastatic melanoma patients (Broggi et al., 2019). Two of these factors, VEGF-C and -A, mediate lymphangiogenesis in tumor-draining LNs to promote LN metastasis prior to distant organ metastasis (Hirakawa et al., 2005, 2007). In tumor mouse models, the transcriptome of tumor-draining LN-LECs has been shown to be affected as early as tumor cell inoculation, with an upregulation of genes involved in sprouting, proliferation, adhesion, and immunomodulation (Commerford et al., 2018). Still in melanoma, early induction of the pre-metastatic niches seems uncoupled from lymphangiogenesis at primary cancer sites. Furthermore, melanoma cells secrete the heparin-binding factor midkine, which acts as a systemic inducer of neo-lymphangiogenesis by activating the mTOR pathway in LECs and pre-metastatic niche formation at distal sites (Olmeda et al., 2017). Tumor-derived vesicles have also been implicated in the establishment of the pre-metastatic LN niche, with metastatic melanoma cells being enriched in the exosome-rich area in tumor-draining LNs (Hood et al., 2011). While melanoma-derived exosomes moving through the lymph appear to be initially “blocked” by macrophages in the subcapsular sinuses, this barrier is disrupted as cancer progresses, enabling the vesicles to access the LN cortex and facilitate LN metastasis (Pucci et al., 2016).

In LNs, cancer cells can shape the immune landscape to support their own growth and survival, shifting multiple immune cell subsets, including macrophages, DCs, natural killer cells, and T cells, toward a more permissive environment (reviewed in Zhou et al. [2021]). LECs in tumor-draining LNs exposed to cancer cells further contribute to the establishment of metastasis, with, for example, the upregulation of PD-L1 and indoleamine dioxygenase, or the cross-presentation of tumor-derived antigens, leading to the inhibition of effector T cell functions in tumor mouse models (Lund et al., 2012; Tokumoto et al., 2017).

While the extent to which LN metastases can contribute to distant metastases is still a topic of debate, recent studies have shown that LNs can act as a pathway for tumor cells to reach distant organs in certain cancer models and patients. In CRC patients, this “sequential progression model” applies to one-third of the patients, as demonstrated by a reconstruction of the evolutionary relationship of primary tumors, LN metastases, and distant metastases by using phylogenetic methods (Naxerova et al., 2017). A recent analysis of the subclonal structure of primary tumors, LN metastases, and liver metastases from CRC patients further showed that 36% of distant metastases originated from LNs, indicating that tumor cell dissemination within the lymphatic network is common in CRC (Zhang et al., 2020). Genomic analyses using primary tumors and metastatic lesions from breast cancer patients showed that whereas most of the mutations conferring a metastatic phenotype to cancer cells occur in the primary tumor, LN metastasis consistently exhibits a passive role in seeding and spreading to distant sites (Siegel et al., 2018; Ullah et al., 2018). The firm demonstration of a contribution to distant metastasis by cancer cells in LNs came from studies using photo-convertible Dendra2-labeled cancer cells in mouse models of breast cancer and melanoma (Pereira et al., 2018). After colonizing the LN, cancer cells exert mechanical forces that can remodel HEVs, inducing the exclusion of lymphocytes in metastatic lesions (Jones et al., 2021). In addition, by migrating along the fibroblastic reticular cell network, cancer cells reach and invade HEVs (Brown et al., 2018; Pereira et al., 2018), further modulating HEVs and therefore impairing T cell migration in LNs, altogether promoting immune escape (Bekkhus et al., 2021).

The concept of the premetastatic niche in distant organs is based on the ability of tumors to send signals through the release of soluble molecules, including cytokines, chemokines, hormones, metabolites, or their delivery via exosomes. These factors spread throughout the body to prepare them to host disseminating metastatic cells, improving their survival and proliferation (Kaplan et al., 2005). These factors induce changes in the microenvironment of secondary distant organs devoid of cancer cells, providing a nest favorable for cancer cell metastasis. The premetastatic niche in distant organs is shaped by the crosstalk between cancer cell–derived factors and resident stromal cells, resulting in the remodeling of the ECM, vascular leakiness, lymphangiogenesis, the recruitment of immune cells such as neutrophils and other myeloid-derived suppressor cells, and their skewing toward an immunosuppressive nature. In addition, some of these factors mobilize bone marrow–derived cells and gut microbiome metabolites. This dual action leads to an immunosuppressive and inflammatory environment, characterized by various immune cells and factors that facilitate the recruitment of disseminated tumor cells to specific sites called pre-metastatic niches, such as LNs, lungs, liver, omentum, bone, and brain, altogether favoring a dysfunctional permissive environment for the incoming tumor cells (Houg and Bijlsma, 2018; Kaplan et al., 2005; Patras et al., 2023; Ren et al., 2015). Studies have shown that the remodeling of the LN vasculature, including changes in LVs and HEVs in response to VEGF-C and VEGF-A, respectively, is central for the establishment of the premetastatic niche in LNs and the promotion of distant metastasis (Qian et al., 2006).

Although there is substantial evidence that tumor-associated lymphangiogenesis facilitates tumor progression and metastasis, it has also been shown to support anti-tumor processes. In primary tumors of melanoma and CRC patients, LV density is positively correlated with inflammation and immune cell infiltration (Bordry et al., 2018; Fankhauser et al., 2017; Lund et al., 2016). Although the expression of VEGF-C is usually associated with more metastasis, lymphangiogenic melanomas are more sensitive to immunotherapy in mouse models. In human metastatic melanoma patients, VEGF-C expression in tumors is also correlated with better response to immunotherapy, such as Melan-A analog vaccination and combined anti–CTLA-4 and anti–PD-1 treatment (Fankhauser et al., 2017).

As described above, it is well-established that tumor lymphangiogenesis promotes cancer cell dissemination and metastasis, worsening patient prognosis (Farnsworth et al., 2018). However, due to their essential role in transporting immune cells and antigens from the tumor to the draining LNs, LVs are also crucial for the generation of adaptive immune responses. As such, mice devoid of dermal lymphatics develop larger melanoma tumors that contain reduced immune cell infiltrate and altered anti-tumor immunity (Lund et al., 2016). High levels of lymphangiogenesis in human melanomas are positively associated with intratumoral immune cells (Bordry et al., 2018; Lund et al., 2016), indicating that tumor LVs may facilitate immune infiltration. In murine melanoma, lymphangiogenesis enhances the infiltration of naïve T cells and consequently improves the efficacy of immunotherapy (Fankhauser et al., 2017). Tumor lymphangiogenesis further appears beneficial for response to immunotherapy, with correlative evidence supporting a “lymphangiogenic potentiation” in patients undergoing immunotherapeutic treatments (Fankhauser et al., 2017). Altogether, these observations indicate that aiming at stimulating lymphangiogenesis in tumors may represent promising strategies to increase immunotherapy efficacy in poorly responsive tumors, with nevertheless the risk of promoting tumor metastasis. A relevant example is the meningeal lymphatic drainage system, which has been correlated with either a beneficial or detrimental effect on the growth of a wide range of brain tumors. In patients with intracranial tumors, dynamic contrast-enhanced magnetic resonance imaging has shown that disturbed meningeal lymphatic function is associated with malignancy and progression (Wang et al., 2023). However, it was further shown that the local administration of VEGF-C in the cerebrospinal fluid (via viral vectors or mRNA therapy) enhances anti-tumor T cell responses and response to immunotherapy in mouse glioblastoma models (Song et al., 2020). A study provided evidence of an extensive lymphatic phenotypic and genotypic remodeling of the dorsal meningeal LVs in glioma or metastatic murine melanoma (Hu et al., 2020). As expected, the disruption of the dorsal meningeal LVs resulted in impaired intratumor fluid drainage and reduced tumor cell dissemination in deep cervical-draining LNs. However, it also diminished the efficacy of the anti–PD-1/CTLA-4 combination therapy, suggesting that VEGF-C potentiates immune checkpoint blockade therapy (Hu et al., 2020). Further investigation will determine whether meningeal lymphatics play an active role in these processes or simply result from microenvironmental changes.

An approach to exploit lymphangiogenesis while avoiding the risk of boosting metastatic dissemination could be the development of lymphangiogenic vaccines administered in a distant site from the tumor and mimicking a lymphangiogenic tumor. This strategy has been previously tested and showed efficacy in murine melanoma models, using intradermal vaccination of tumor-bearing mice with irradiated tumor cells genetically modified to overexpress VEGF-C and combined to an adjuvant (Sasso et al., 2021). An alternative strategy to promote anti-tumor immunity by targeting tumor LVs is related to their ability to control T cell exit from tumors. Indeed, in murine melanoma, tumor-associated LVs increase the egress of CXCR4⁺ effector CD8⁺ T cells at the tumor periphery by producing CXCL12 (Steele et al., 2023). This phenomenon is further promoted by their encounter of antigen that tunes CXCR4 expression by CD8⁺ T cells and therefore their susceptibility to CXCL12. Accordingly, blocking CD8⁺ T cell egress through the inhibition of the CXCL12–CXCR4 pathway improves local tumor control and response to immune checkpoint blockade, suggesting that CD8⁺ T cell egress through LVs is a control point to enhance immunotherapy efficacy.

LVs play dual roles in tumor immunity. On one hand, LECs are involved in anti-tumor immune responses. While some studies have indicated that naïve T cells can be activated locally within tumors (Broz et al., 2014; Peske et al., 2015), the transport of tumor antigens and APCs to the draining LNs is essential for triggering an antigen-specific anti-tumor immune response. Tumor antigen molecules or molecular complexes that are drained from peripheral tissues to LNs via local lymphatics can be processed into antigenic peptides. These peptides are then presented on MHC-I or MHC-II molecules by LN APCs, such as DCs and resident macrophages, to activate naïve T cells or reactivate central memory CD8⁺ and CD4⁺ T cells. Tumor-infiltrated DCs that are activated locally in tissues also travel to LNs through LVs to stimulate T cells. As such, mice with lymphatic transport deficiency exhibit impaired antigen presentation (Kimura et al., 2015) and T cell priming (Roberts et al., 2016) and accelerated tumor growth (Steinskog et al., 2016). Tumor-associated LVs can be modulated following direct interaction with immune cells. We have recently shown that LECs in tumors can cross-present tumor antigens through MHC-I molecules in an IFN-γ–dependent manner (Fig. 4 a), leading to their antigen-specific T cell killing and the dismantling of the tumor-associated lymphatic vasculature (Fig. 4 b), and consequently reducing the LN metastasis (Garnier et al., 2022). Besides, LN LECs also prevent tumor progression by supporting LN-resident CD169⁺ sinusoidal macrophages. Those macrophages have been shown to inhibit primary tumor growth and metastasis by regulating B cells and activating CD8⁺ T cell response (Asano et al., 2011; Pucci et al., 2016; Tacconi et al., 2021).

On the other hand, tumor-associated LECs also interact with immune cells to promote tumor progression and metastasis. Studies in murine lymphangiogenic melanoma showed that LECs in draining LNs and primary tumor sites capture and cross-present tumor antigens through MHC-I, which, together with enhanced expression of IFN-γ–induced PD-L1, promote apoptosis in tumor-specific CD8⁺ T cells (Dieterich et al., 2017; Lane et al., 2018). This process is also true for central memory T cells that are deleted following antigen presentation by LECs in tumor-draining LNs (Cousin et al., 2021; Leary et al., 2022; Lund et al., 2012). Besides presenting tumor antigens through MHC-I, we have shown that LECs in tumors also function as MHC-II–restricted tumor APCs to enhance Treg immunosuppressive functions (Gkountidi et al., 2021) (Fig. 4, c–e). In mouse breast cancer models, tumoral LECs in the primary site showed increased iNOS expression (Dieterich et al., 2019). Although iNOS expression by LECs suppresses T cell proliferation in vitro (Lukacs-Kornek et al., 2011), it remains to be determined whether this is also the case in vivo, and in particular in tumor context. Moreover, galectin 8 expressed by LECs in breast tumors is critical for their interaction with podoplanin⁺ macrophages, promoting the activation of the pro-migratory integrin-β1, and therefore facilitating the attachment of TAMs to LECs (Bieniasz-Krzywiec et al., 2019). In return, TAMs increase lymphangiogenesis and local ECM remodeling and subsequently promote intra-vessel tumor invasion (Bieniasz-Krzywiec et al., 2019) (Fig. 4 f). In human esophageal squamous cell carcinoma, lymphangiogenesis is associated with the accumulation of CD117⁺ Treg cells expressing high level of IL-35 and correlates with CD8⁺ T cell exhaustion (Ma et al., 2024). A recent publication showed that in both mouse models and human CRC tumors, LECs serve as a peritumoral niche promoting interactions between Tregs and a particular subset of mature DCs enriched in immunoregulatory molecules (mregDCs) (You et al., 2024). These interactions enhance Treg activation and subsequent mregDC retention in perilymphatic immunosuppressive niches, inhibiting the activation of anti-tumor T cells in LNs (You et al., 2024) (Fig. 4 g). The factors locally produced by LECs in the peripheral tumor stroma favoring the accumulation and crosstalk between Tregs and mregDCs remain to be elucidated.

Lymphedema is defined as the accumulation of extracellular fluid in tissues after lymphatic system damage or dysfunction, featured by progressive swelling of the affected regions of the body and chronic tissue inflammation. Based on etiology, lymphedema can be classified as primary or secondary lymphedema. Cancer-related lymphedema, which belongs to the category of secondary lymphedema, is the most common form of lymphedema, occurring in one out of seven patients who have undergone cancer treatment (Cormier et al., 2010), which can give substantial physical and psychological morbidity.

Frequently used cancer treatments, including the surgical removal of regional LNs (such as axillary LN dissection and extensive LN removal) and radiotherapy, are the primary contributors to cancer-related lymphedema. This condition typically occurs after treatment in cases of breast, prostate, testicular, uterine, cervical, ovarian, melanoma, and head and neck cancers (Cormier et al., 2010). Cancer-associated lymphedema can be characterized by lymphatic aggregation, fibrosis, and adipohypertrophic pathological response in the related region, together with obstruction to lymphatic drainage and iatrogenic structural damage to the lymphatic vasculature and LNs. As a direct consequence of lymphedema, lymphatic fluid stasis leads to the upregulation of key adipogenic regulators, including CCAAT/enhancer-binding protein α and peroxisome proliferator-activated receptor-γ, which drive adipocyte differentiation, proliferation, and lipid accumulation (Aschen et al., 2012; Hsiao et al., 2023; Koc et al., 2021; Sung et al., 2022). The stagnant lymph contains high levels of fatty acids, insulin, and IGF-2, which not only promotes the expression of genes related to adipogenesis and the growth of adipocytes but also encourages adipose-derived stem cells to transform into fat cells. This accumulation of adipose tissue increases the secretion of adipokines, which, in turn, elevates insulin levels, creating a vicious cycle of adipohypertrophic response (Hsiao et al., 2023). The resulting adipogenesis complicates lymphedema management, as the excess adipose tissue becomes resistant to conventional treatments focused on reducing fluid retention (Lee and Kim, 2024). Although LN dissection for cancer treatment is the most common cause of lymphedema, aggressive cancers themselves can also cause an aggressive form of lymphedema, named malignant lymphedema, which happens when cancer cells metastasize to the regional LNs or directly into the lymphedematous tissue (Rockson et al., 2019).

Cancer-related lymphedema can be divided into three stages: At the initial stage (stage 1), LECs in LVs and LNs undergo a profound remodeling (Kim et al., 2014), leading to the establishment of lymphatic vascular dysfunction. At this stage, T cell responses are initiated, including T helper 1 (Th1), Th2, and Th17, and are associated with neutrophil and macrophage-mediated innate immunity (Jiang et al., 2018). In the postsurgical murine lymphedema model, CD4⁺ T cell depletion decreases the severity of lymphedema (Zampell et al., 2012), whereas inhibition of Th2 cell differentiation improves lymphatic function and reduces the pathological change of the lymphedematous tissues (Avraham et al., 2010). While inflammation extends (stage 2), the accumulation of interstitial fluid at the lymphedema site impairs the transportation of inflammatory cells and mediators (Ly et al., 2017; Rockson et al., 2019). Inflammatory gene expression profiling revealed a notable upregulation of arachidonate 5-lipoxygenase RNA expression in lymphedema skin (Tabibiazar et al., 2006), leading to the identification of the 5-lipoxygenase metabolite eicosanoid leukotriene B₄ (LTB₄) as an inflammatory mediator controlling the initiation of the pathological events in lymphedema (Tian et al., 2017). Endothelial cells and immune cells are the main source of LTB₄. In both human and mouse, LTB₄ can impair LEC function (Tian et al., 2017). In mouse models, LTB₄ suppresses Notch pathways and inhibits the expression and the activation of VEGFR3, which is crucial for the development and maintenance of LVs (Tian et al., 2017). LTB₄ further mediates the recruitment of CD4⁺ and CD8⁺ T cells into the inflamed tissues (Goodarzi et al., 2003; Ott et al., 2003; Tager et al., 2003) and promotes Th17 cell differentiation and migration (Chen et al., 2009; Lee et al., 2015). At the late stage of lymphedema (stage 3), pathological symptoms usually come with the substantial fibrotic remodeling of the tissues, together with LV dilatation, rupture, and hardening (Daróczy, 1995). At this stage, several cytokines are involved in the regulation of disease development, such as IL-13 and IL-4 produced by Th2 cells in the fibrotic tissues, and promote M2 macrophage differentiation. Administration of blockers targeting either fibrotic cytokines (anti–IL-12 and anti–IFN-γ) or Th2 cells reduces the severity of late lymphedema stages. Unfortunately, the current diagnosis of lymphedema is based on clinical presentation and changes in tissue volume relative to the unaffected side. Consequently, the diagnosis remains challenging due to the lack of simple clinical measurement, thus limiting early intervention to treat the disease.

LVs are integral components of the TME, exerting complex, multifaceted, and opposite effects on tumor progression and anti-tumor immunity. On one hand, LVs serve as crucial transporters for antigen delivery and immune cell transportation, although this route is often hijacked by tumor cells to metastasize. On the other hand, LECs actively regulate anti-tumor immunity via their ability to present tumor antigens as well as to express activatory and/or inhibitory molecules. This dual nature of LVs associated with cancers highlights the intricate balance required to develop therapeutic strategies aiming at targeting these pathways. For example, tumor-associated lymphangiogenesis, driven by factors such as VEGF-C and VEGF-D, is often correlated with poor prognosis due to its role in facilitating metastasis. However, the same process also enhances immune cell infiltration into the tumor bed and potentiates the efficacy of immunotherapies in experimental models. As such, stimulating lymphangiogenesis may promote immunotherapy efficacy in tumors with poor immune infiltration, but it may also favor metastasis. Conversely, lymphangiogenesis inhibition or metastatic-draining LN removal to prevent distant organ metastasis may impair immune cell trafficking, dampen anti-tumor responses, as well as induce secondary lymphedema, which heavily damage the life quality of the patients. Future research is needed to precisely decipher the molecular mechanisms by which LVs affect tumor cell dissemination and immune responses. A comprehensive understanding of these mechanisms will be essential for the development of targeted therapies that can effectively harness the beneficial aspects of lymphatic function while mitigating the associated risks.

Author contributions: M. Sun: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Visualization, Writing - original draft, Writing - review & editing, J. Angelillo: Investigation, S. Hugues: Conceptualization, Visualization, Writing - original draft, Writing - review & editing.