As specialized postcapillary venules, HEVs are primarily located in other secondary lymphoid tissues except the spleen (11). High endothelial cells (HECs) are unique microstructures in HEVs, characterized by highly columnar and approximately cuboidal endothelial cells, with a width of 7–10 μm and a height of 5–7 μm (12–14), thus distinguishing them from other common endothelial cells. HECs occupy most of the luminal space, and hence the lumen of HEVs is narrow or even closed and covered with a thick basement membrane and a perivascular sheath (5, 15, 16). HECs have a large round nucleus with one or two nucleoli and abundant organelles such as mitochondria, rough endoplasmic reticulum, ribosomes, and Golgi complex. They show the characteristics of secretory cells with highly fast metabolism (15, 17–19). Ultrastructural studies revealed that HECs have a thick carbohydrate-rich glycocalyx coating on their luminal surface (20–22). In addition, sulfated carbohydrates and glycoproteins are important recognition determinants of L-selectin in lymphocytes (23). Moreover, the glycocalyx in HECs may also facilitate the retention of secreted molecules on the luminal surface of HECs (14, 20).

As HECs are specialized capillary endothelial cells, we focused on their mode of transformation. The pathway represented by lymphotoxin and tumor necrosis factor α (TNF-α), which are mainly secreted by activated lymphocytes and natural killer cells (NK cells), is the most critical signaling pathway during HEVs formation (4). Lymphotoxin α3(LTα3) or TNF-α binds to tumor necrosis factor receptor 1 or 2 (TNFR1/2), whereas LTα1β2 or tumor necrosis factor superfamily 14 (LIGHT) signals bind to LTβR. TNF-α or LTα3 drives spontaneous HEVs formation, whereas LTα1β2 and LIGHT are the major inducers of HEVs. After HEVs formation, their special morphology also needs to be maintained (24, 25). LTβR signaling is important in maintaining the HEVs phenotype because LTαβ/LTβR signaling can maintain the cube-like morphology of HECs (26, 27). Sphingosine-1-phosphate (S1P)–S1PR1 axis also plays a role in regulating HEVs maintenance (28–30). The complex relationship between LTβR signaling and TA-HEVs plays an important role in the differentiation and growth of TA-HEVs (27, 31). As such, LTβR agonists are potent inducers of TA-HEVs in tumors (32).

The special structure of HECs contributes to the special role of HEVs in lymphocyte homing and recycling (33–35). Lymphocytes may enter lymph nodes via two transendothelial migration pathways: the paracellular route (through intercellular spaces) and the transcellular route (through endothelial cells). However, the paracellular route is predominant (36–39). First, the ligands of L-selectin synthesized by HECs are processed by the Golgi complex, stored in secretory granules, and released on the luminal surface of HECs for recognition by homing receptors on the surface of passing lymphocytes to initiate the homing of lymphocytes (40). Next, the circulating lymphocytes in the blood bind to the 6-sulfo sialyl Lewis X motif modifying HEVs via L-selectin, and then tether and roll on the HEVs wall, allowing them to immobilize with heparan sulfate and interact with chemokines on the luminal surface of HEVs (41, 42). These chemokines can mediate the activation of integrins essential for lymphocyte arrest in HEVs (43–47). Integrin lymphocyte function–associated antigen 1 (LFA1) is the major integrin responsible for T and B cell arrest in HEVs in peripheral lymph nodes. The combination of the shear stress of blood flow and the G protein-coupled chemokine receptor signaling induces a conformational change in the LFA1 molecule, leading to the firm adhesion of lymphocytes to intercellular adhesion molecules 1 and 2 (ICAM1 and ICAM2) expressed on HECs (44, 46, 48). After stable arrest, the lymphocytes crawl along the luminal surface of the HEVs in search of suitable transport sites. Thereafter, they migrate through transcellular or paracellular pathways (49, 50). During migration through the paracellular pathway, the high columnar HECs rapidly close the opened intercellular space on the side of the lumen after lymphocyte passage, thus minimizing the leakage of intravascular fluid (36, 51).

The antitumor immune response primarily depends on the activity of tumor-specific lymphocytes that can recognize and eliminate tumor cells. TA-HEVs are a major gateway for lymphocyte infiltration into human tumors. They are usually found in areas with B cell–rich tertiary lymphoid structures (TLS) as well as in areas with high densities of T cells and mature dendritic cells (DCs) (52, 53). TA-HEVs can be induced in different types of tumors (54, 55). However, MECA-79⁺ TA-HEVs are sometimes formed spontaneously in the TME even without any treatment (55–57). MECA-79⁺ TA-HEVs facilitate the recruitment of naive lymphocytes to tumors (4, 58). Further studies suggested that a high density of MECA-79⁺ TA-HEVs was associated with increased infiltration of naive and central memory T cells into TME (4, 7, 52). Therefore, an increase in the number of T cells in different morphological stages within the tumor is proposed to accelerate and foster antitumor response (59, 60). The induction of MECA-79⁺ TA-HEVs is also observed in a number of mouse tumor models. This is associated with the infiltration of T cells, especially CD8⁺ T cells, and the inhibition of tumor growth (61, 62). Moreover, increasing the differentiation and maturation of HEVs in tumors can reduce CD8⁺ T cell depletion and lead to an increase in the proportion of stem-like CD8⁺ T cells (52) ( Figure 1C ).

Theoretically, the efficacy of immunotherapy can be potentiated by the increased generation of HEVs, which in turn promotes lymphocyte entry into TME. In fact, many studies have suggested that the therapeutic induction of TA-HEVs in tumors might enhance the trafficking of endogenous lymphocytes, as well as adoptive transferred lymphocytes, and improve the efficacy of various cancer therapies, including immunotherapy with immune checkpoint inhibitors (ICIs), adoptive T cell therapy (ACT), vaccines, and potential targeted and conventional cancer therapies (radiotherapy and chemotherapy) (2, 63) ( Figure 2 ). Additionally, the immune cells, especially lymphocytes, may also have an effect on TA-HEVs. CD8⁺ T cells may be the major inducers of TA-HEVs in tumors (64, 65). CD8⁺ T cells can produce LTα3, LTα1β2, TNF-α, IFN-γ, and other signals that induce the activation of lymphotoxin beta receptor (LTβR), thus contributing to the production of TA-HEVs (66). NK cells express LTα1β2 and IFN-γ. Follicular helper T cells express LIGHT. Macrophages express TNF-α, and LTα3 plays a role in contributing to the production of TA-HEVs. In other tumor models, the generation of TA-HEVs also requires the participation of B cells and DCs (65, 67–69). Some DCs can express LTβR and directly promote the differentiation of DC-dependent HEVs (31). CD11c integrin is important in DC physiology; Moussion and Girard demonstrated in mice that CD11c cells were essential for HEVs formation (6, 70). Thus, a larger number of CD11c cells could trigger increased HEVs formation (70). The additional regulatory role of DCs is to promote the growth of TA-HEVs in a vascular endothelial growth factor (VEGF)-dependent manner (71, 72). Regulatory T cells (Tregs) appear to limit the development of TA-HEVs in tumors, but their mechanism of action remains unclear (69, 73). The appropriate ways to treat LTβR should be explored, and the number of CD8⁺ T cells, DCs, and Tregs should be reasonably regulated ( Figure 3 ). Tumor progression may also affect the presence of TA-HEVs, which needs further investigation. Since the last decades, ICB therapy, including programmed death 1 and programmed death-ligand 1(PD-1 and PD-L1) blockade treatments, has exhibited positive effects in antitumor immunotherapy for several solid tumors (2). Therefore, we focused on the association between ICB therapy and TA-HEVs. Previous studies suggested that anti-PD-1 therapy might have the potential to promote tumor immunity by stimulating the formation of TA-HEVs (74). At the same time, the formation of TA-HEVs can cooperate with anti-PD-1 therapy to promote the infiltration of lymphocytes, so that they can play a potent immune role against tumors (5, 75). Some other ICIs, such as anti-CTLA-4, can increase the abundance of TA-HECs and tumor-infiltrating CD4⁺ and CD8⁺ T cells (52, 74). A combination of antiangiogenic therapy and ICB also achieved similar efficacy, thus improving the infiltration of CD8⁺ T cells from the periphery (52). Some mouse experiments revealed a significant increase in the homing efficiency of TA-HEV-mediated lymphocytes after ICB treatment, possibly contributing to an increase in the tumor-specific T cell repertoire (76, 77).

Tumor-associated lymphatic vessels carry interstitial fluid and leukocytes unidirectionally from peripheral tissues to lymph nodes and are mainly involved in the generation, maintenance, and regression of adaptive immunity (78). Tumor-associated lymphatic vessels and their transport function are important conditions for controlling the exit of lymphocytes from tumor tissues. Tumor-associated lymphatic vessels can exert immunosuppressive effects, cross-present antigens, and limit CD8⁺ T cell-dependent tumor control in an interferon-γ (IFN-γ)-dependent manner, directly leading to tumor immune escape (79–83). However, the lymphatic system also contributes to regulating the diversity and functional status of CD8⁺ T cells within the tumor (84).Therefore, the inhibition of CD8⁺ T cell shedding by the lymphatic system is an important control point to enhance the response to immunotherapy.

The exit of lymphocytes from tumors through tumor-associated lymphatic vessels is mainly regulated by various chemokines. For example, chemokine12 (CXCL12) and its receptor CXCR4 mediated the exit of CD8⁺ T cells from the tumor through tumor-associated lymphatic vessels. In this process, CXCL12, a ligand for CXCR4, has been shown to promote the egress of DCs and B-lymphocytes (85, 86). CXCL12 can also promote the exit of CXCR4 CD8⁺ T cells from the tumor, and lymphangiogenic CXCL12 is sufficient to affect the accumulation and location of T cells in the tumor (10). Antigen contacts regulate surface CXCR4, and therefore CD8⁺ T cells are sensitive to CXCL12. The peritumoral tumor-associated lymphatic vessels direct the location and retention of CD8⁺ T cells in the TME by expressing CXCL12, which recruits and ultimately egresses a wide range of functional tumor-specific CXCR4 CD8⁺ T cells (87). Blocking CD8⁺ T cell shedding through this pathway can improve local tumor control and ICB response (88, 89). Atypical chemokine receptor 3 (ACKR3) is a decoy receptor for CXCL12. CD8⁺ T cells can modulate their CXCL12 sensitivity in vivo by downregulating the expression of surface CXCR4 and upregulating the expression of ACKR3 [ (10, 90). Therefore, the retention of memory and effector CD8⁺ T cells in tumor tissue can be promoted by downregulating the CXCR4 expression or reducing its sensitivity to CXCL12, thus allowing it to play an immune role against the tumor for a longer time (87).

Not all the T cells in the tumor are exported under the action of CXCL12–CXCR4, but selectively. Besides the role of CXCR4, T cell differentiation is also coordinated to participate in exported and retained lymphocyte screening. Exported effector CD8⁺ T cells have various transcriptional modules with retained effector CD8⁺ T cells (91). Tumor-retained effector CD8⁺ T cells are enriched for transcriptional modules related to hypoxia, leukocyte activation, and extracellular matrix organization, whereas exported CD8⁺ T cells are enriched for transcriptional modules related to leukocyte migration, glutamate receptor signaling, and branched-chain amino acid catabolism. The retained intratumoral CD8⁺ T cells are enriched for a gene set that is upregulated upon exhaustion (e.g., PD-1, nuclear receptor subfamily 4,group a(Nr4a2), cytotoxic T-lymphocyte-associated protein 4(Ctla4), and T cell immune receptor with Ig and ITIM domains(Tigit)), whereas exported CD8^+.T cells are enriched for a gene set that is downregulated upon exhaustion (e.g., transcription factor 7(Tcf7), selectin L(Sell), Lympho-enhancing factor 1(Lef1), chemokine receptors 7(Ccr7), and Sphingosine-1-phosphate receptor 1(S1pr1)) (92). In addition, CD8⁺ T cells within the tumor predominantly express TCF1. However, a subset of exported CD8⁺ T cells expresses both TCF1 and PD-1, with PD-1 exported cells expressing lower levels of PD-1 than retained cells (93, 94). Finally, a fraction of the exported CD8⁺ T cells could produce effector cytokines after restimulation in vitro, whereas the retained CD8⁺ T cells could not produce effector cytokines (95).