Abstract

The abnormal tumour vasculature

The growth and progression of tumours are facilitated by the recruitment of blood vessels. Tumour blood vessels not only supply oxygen and nutrients and remove waste products, but also sustain a favourable niche for so-called cancer stem cells and serve as a conduit for both the metastatic dissemination of tumour cells and the infiltration of immune cells^9,10. One would expect a rapidly growing organ or tissue to have normally functioning vessels; however, tumour vessels are abnormal, both structurally and functionally, relative to those of nonmalignant tissues⁷. Perhaps counter-intuitively, this abnormality promotes tumour progression by impairing perfusion, which results in tumour hypoxia and a low pH intratumourally⁷. In addition, the leaky nature of tumour blood vessels, coupled with dysfunctional lymphatic drainage, leads to elevated interstitial fluid pressure in the TME⁶. Tumour cells adapt to overcome these conditions and even to gain a survival advantage over nonmalignant cells through several mechanisms⁷. The abnormal vessels and impaired perfusion can also restrict the entry of cytotoxic drugs and immune cells from the circulation into tumours, limiting their anticancer activity. Furthermore, the abnormal TME might modulate the activity and, thus, the effectiveness of these drugs and immune cells after they accrue in the tumour. For example, an adequate level of oxygen is required for many drugs and immune cells to kill cancer cells⁷. Additionally, various cytokines produced by diverse cell types in response to hypoxia and the low pH in the TME can cause both local (FIGS 1,​,2)2) and — after entering the circulation — systemic immunosuppression (FIG. 3). Finally, the abnormal blood and lymphatic vessels also facilitate tumour invasion and metastasis via multiple mechanisms⁶. For example, fluid leaking out of a tumour, owing to insufficient drainage via the defective intratumoural lymphatic system, can carry cancer cells to nearby peri-tumour lymphatic vessels, thereby promoting lymphatic metastasis¹¹. Abnormal leaky vessels might also facilitate intravasation and shedding of cancer cells into the blood; after entering the systemic circulation, some of these tumour cells can disseminate to distal organs and form metastases. Moreover, the cytokines, such as VEGF, that seep from the TME into the systemic circulation can also potentiate the extravasation of metastatic cancer cells out of the blood vessels within distant organs¹². By contrast, while CD8⁺ cytotoxic T lymphocytes (CTLs) can also home to tumours, multiple factors present in the TME often render these cells dysfunctional and/or limit their penetration into the tumour (FIGS 1,​,2).2). Other adverse consequences of abnormal tumour vasculature and the resulting conditions in the TME include inflammation, fibrosis, genomic instability, DNA hypermethylation, activation of the unfolded protein response, a switch to anaerobic metabolism, resistance to autophagy and apoptosis, induction of a cancer stem cell programme, epithelial-to-mesenchymal transition, and tumour cell invasion and metastasis⁷. In the following sections, we elaborate on the roles of angiogenic factors in mediating two of these consequences: immunosuppression and metastasis.

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Figure 1

Direct effects of angiogenic factors on various immune cells

VEGF and other angiogenic factors, such as angiopoietin 2 (ANG2), modulate the functions of innate and adaptive immune cells towards immunosuppression. VEGF can be produced by tumour cells and immune cells (red stars) as well as by endothelial cells and stromal cells. VEGF creates a pro-tumour microenvironment by increasing the number and enhancing the suppressive functions of regulatory T (T_reg) cells and tumour-associated macrophages (TAMs) and/or monocytes. These cell populations also produce VEGF (red stars) and ANG2 (green pentagons) and thereby engage in paracrine and autocrine VEGF (and/or ANG2) signalling. Among immune cells, T_reg cells have been identified as the major source of VEGF in the tumour microenvironment using in vivo models¹⁷. This finding was further confirmed in a different study⁸⁹. While VEGF directly inhibits the development and/or activation of cytotoxic T lymphocytes (CTLs), CTL-produced VEGF can induce angiogenesis and promote tumour progression⁹⁷. Furthermore, VEGF has been shown to increase the numbers of CD4⁺ memory T cells (Supplementary Table S4). VEGF can promote the expansion of myeloid-derived suppressor cells (MDSCs) and improve their immunosuppressive function in the tumour microenvironment (TME)²³. Importantly, dendritic cells (DCs), which are required for priming of CTLs, lose their ability to mature and present antigens following VEGF exposure in vitro. Consequently, the anticancer activity of CTLs in the TME might be compromised. In this context, CTLs have a decreased capacity to traffic to the tumour, proliferate, and produce cytokines that are integral to the anticancer immune response, such as IFNγ and tumour necrosis factor¹⁷². Collectively, T_reg cells, TAMs, MDSCs, and immature DCs suppress the activity of CTLs, resulting in a pro-tumour microenvironment. The roles of ANG2 (and ANG1) in modulating various immune cells are not as well understood as those of VEGF; thus, the effects of ANG2 are not illustrated in this figure.

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Figure 2

The abnormal tumour vasculature contributes to immunosuppression in the tumour microenvironment

Abnormalities in the tumour vasculature result in hypoxia and acidosis of the tumour microenvironment (TME), which in turn contribute to immunosuppression via several mechanisms. These mechanisms include increased accumulation, activation, and expansion of immunosuppressive regulatory T (T_reg) cells; recruitment of inflammatory monocytes and tumour-associated macrophages (TAMs) and reprogramming of TAMs from an anticancer M1-like phenotype towards the pro-tumour M2 phenotype; suppression of dendritic cell (DC) maturation, which results in impaired antigen presentation and activation of tumour-specific cytotoxic T lymphocytes (CTLs); and expansion of abnormal endothelial cells (ECs) with immunosuppressive phenotypes. Importantly, the programmed cell death protein 1 (PD-1)–programmed cell death 1 ligand 1 (PD-L1) pathway is often activated in the TME as a mechanism to evade anticancer immune responses, with upregulation of PD-L1 expression on TAMs, DCs, and ECs, as well as on tumour cells. In addition, tumour-infiltrating CTLs typically upregulate PD-1, marking them as dysfunctional or ‘exhausted’ and limiting their cytotoxic potential against tumour cells. Overall, the consequence of aberrant tumour angiogenesis and vascular abnormality is an immunosuppressive TME. ANG2, angiopoietin 2; CCL, C-C-motif chemokine ligand; CXCL12, C-X-C-motif chemokine ligand 12; CSF1, macrophage colony-stimulating factor 1; FASL, FAS antigen ligand; GM-CSF, granulocyte–macrophage colony-stimulating factor; TGFβ, transforming growth factor β.

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Figure 3

Tumours secrete factors that cause systemic immunosuppression

The cells of the tumour microenvironment exert systemic immunosuppressive effects by releasing factors such as VEGF, transforming growth factor β (TGFβ), and prostaglandin E₂ (PGE₂) into the circulation. Collectively, these cytokines reduce the ability of antigen-presenting cells to prime T cells and thus reduce the anticancer responses of effector T cells. In addition, tumour-derived factors increase the presence and function of myeloid-derived suppressor cells (MDSCs) and regulatory T (T_reg) cells, which suppress anticancer immunity.

Strategies to normalize tumour vessels

Multiple molecular and physical mechanisms contribute to vascular dysfunction in tumours^7,60; chief among them is the imbalance of signalling mediated by proangiogenic versus antiangiogenic molecules²⁸. In nonmalignant tissues, this balance is exquisitely maintained to ensure the normal morphology and function of blood vessels. During carcinogenesis, however, this balance typically tips towards angiogenesis, and the vessels become chronically immature and abnormal. In 2001, we proposed to restore this balance using judicious doses of antiangiogenic agents and suggested that this strategy would transiently give rise to tumour vessels with structural and functional phenotypes closer to those of the vessels of nonmalignant tissues — a state we referred to as ‘normalized’ (REF. 61), because the vessels would not be perfectly normal (BOX 1). We also suggested that the use of other therapies during this ‘window of normalization’ (REF. 61) would result in better outcomes than if the same agents were given before or after this window.

We initially tested our vascular normalization hypothesis in mice using agents that target the central regulator of angiogenesis: VEGF (BOX 2). Our work provided compelling evidence in support of this hypothesis. For example, radiation therapy given during the window of normalization, especially when hypoxia was alleviated in tumours, had outcomes superior to those achieved when radiation was delivered outside this window⁶². A number of preclinical and clinical studies from our own institution and others have also provided evidence in support of the validity of this hypothesis for a number of malignancies^16,63. Our initial efforts focused on cytotoxic therapies⁶⁴, but we subsequently showed in preclinical studies that vascular-normalizing doses of anti-VEGF agents also convert the immunosuppressive TME to an immunosupportive one and improve the outcome of vaccine-based anticancer immunotherapy²⁷. The findings of multiple other groups provide further evidence of these effects in preclinical models using different anti-VEGF agents, as well as different immunotherapies^7,13 (TABLE 1), and such combinations are currently being tested in a number of clinical trials (TABLE 2). In fact, the results of a phase II trial demonstrate the benefit of such an approach for patients with advanced-stage renal cell carcinoma (RCC)⁶⁵, and the results of a follow-up phase III trial comparing anti-PD-1 therapy plus axitinib with standard sunitinib monotherapy in the first-line treatment of metastatic RCC are forthcoming (TABLE 2). While RCC is a highly VEGF-dependent tumour, these early results point to the multiple roles of VEGF pathway inhibition in anticancer immunity.

A major challenge with anti-VEGF therapies is that only transient vascular normalization has been observed⁶²; the window of normalization might be of short duration (days to weeks) and is dependent on the tumour and dose of anti-VEGF agent used²⁷. In our attempts to extend the normalization window in experimental models of cancer, we discovered that ANG1/ANG2–TIE2 signalling (BOX 3) mediates the recruitment of pericytes to the tumour vessels⁶², and that ectopic expression of ANG2 can abrogate the benefit of anti-VEGF therapy⁶⁶. Moreover, concomitant blockade of ANG2 and VEGF extends both the window of normalization and the survival benefit compared with inhibition of either pathway alone, in part, by reprogramming the immunosuppressive TME^35,36. Interestingly, in patients with glioblastoma, circulating ANG2 levels rebound after a marked initial decrease following treatment with a VEGF pathway inhibitor⁶⁷. Perhaps most pertinent to immunotherapy, ANG2 levels have been reported to be elevated or increased in patients with melanoma who have an unfavourable response to ICB⁶⁸. Importantly, ANG2 levels were predictive of treatment response to ICB with or without anti-VEGF therapy⁶⁸. A number of lessons can be learnt from the findings of these and other preclinical and clinical studies reported to date.

The dose of anti-VEGF agents matters

Preclinical data indicate that anti-VEGF therapy results in vascular normalization via pruning of immature blood vessels, which have poor pericyte coverage, and via active pericyte recruitment^62,69. Similar vascular changes have also been observed using advanced MRI techniques in patients with glioblastoma after VEGFR tyrosine kinase inhibitor (TKI) treatment^63,67. The onset of vessel normalization conferred by anti-VEGF treatment can occur in as little as 1 day in both mouse⁶² and human tumours⁶⁷. The normalization process is transient and reversible, in a tumour-dependent and dose-dependent manner, and lasts for around 1 week in mice and possibly for a few months in humans^7,62,63,67. The window of vessel normalization ends when an excessive number of vessels are pruned, or when tumours switch to the use of alternative angiogenic pathways or alternative mechanisms of blood vessel recruitment, such as vessel co-option^56,62. Notably, high doses of antiangiogenic agents can result in a short normalization window by causing excessive amounts of vessel pruning, thus exacerbating hypoxia and acidosis in the TME⁷. High doses of anti-VEGF agents can also lead to increased deposition of extracellular matrix that, together with hypoxia, can promote the infiltration of immunosuppressive and/or pro-tumour immune cells, such as monocytic and granulocytic MDSCs^37–39. These results suggest that high-dose anti-VEGF therapy limits the benefits of chemotherapy in mouse models and in patients with metastatic colorectal cancer (CRC) or advanced-stage NSCLC^37,70,71. Similarly, blockade of VEGF signalling using high doses of the multi-target, pan-VEGFR TKI sorafenib caused increased hypoxia and the stromal cell-derived factor 1 (SDF1)-mediated recruitment of immunosuppressive cells, such as T_reg cells and M2-like macrophages, in a mouse model of advanced-stage hepatocellular carcinoma (HCC); blockade of the SDF1 receptor C-X-C-chemokine receptor 4 (CXCR4) prevented this shift towards an immunosuppressive TME, inhibited tumour growth and metastasis, and improved the survival of sorafenib-treated mice⁴⁰. In mouse models of CRC, high-dose anti-VEGF treatment can also result in the recruitment of immunosuppressive, nonclassical, lymphocyte antigen 6C (Ly6C)^low monocytes that also confer resistance to anti-VEGF therapy, thus setting up a vicious cycle^38,39.

By contrast, use of lower doses of antiangiogenic agents — for instance, as low as one-quarter of the doses that induce antivascular effects and/or vessel pruning in animal studies — has the potential to induce prolonged vessel normalization^7,27. Achieving vascular normalization, and thereby reducing tumour hypoxia and acidosis, is crucial because these stressors confer cancer cells with an invasive metastatic phenotype⁷². Moreover, a normalized vasculature provides a conduit for the efficient infiltration of immune cells and delivery of anticancer therapeutics into tumours^6,27. Importantly, restoring tumour oxygenation and a physiological pH might improve the anticancer activity of infiltrating immune cells²⁷, as well as that of radiation therapy and many drugs⁷ (FIGS 2,​,4).4). Treatment-resistant cancer stem cells are hypothesized to reside in the hypoxic niche, and excessive antiangiogenic therapy can cause hypoxia and thereby support these cells⁷³. Therefore, low doses of antiangiogenic agents might also reduce the number of these stem cells, hence avoiding or delaying therapy resistance. In fact, the results of two retrospective studies have shown that low-dose bevacizumab (<3.6 mg/kg per week) in combination with chemoradiotherapy confers a greater survival benefit than high-dose bevacizumab (5 mg/kg per week) in patients with glioblastoma^74–77. Hence, to realize the full potential of antiangiogenic therapy, the dose and schedule of anti-VEGF agents might need to be tailored to baseline levels of tumour vascularity⁷⁸ and/or VEGF. Notably, pre-treatment levels of circulating VEGF are prognostic for treatment outcome in different solid tumour settings, but are not predictive of anti-VEGF therapy outcome⁷⁹. Whether pre-treatment VEGF levels are predictive in the context of personalized bevacizumab dosing regimens is unknown. This hypothesis needs to be tested in prospective studies using low-dose antiangiogenic therapy.

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Figure 4

Vascular-normalizing therapies can reprogramme the immunosuppressive tumour microenvironment to an immunosupportive one

The structural and functional abnormalities of tumour blood vessels lead to impaired blood flow and perfusion, thus resulting in a hypoxic tumour microenvironment (TME). Hypoxic conditions in tumours limit the effectiveness of cytotoxic therapies, such as radiation therapy and certain chemotherapies. In addition, drug delivery to the tumour via these abnormal vessels is impaired, and this problem is further exacerbated by the high interstitial fluid pressure — a consequence of the leaky blood vessels and irregular lymphatic system in the tumour. Moreover, abnormal and leaky tumour blood vessels facilitate the intravasation of cancer cells into the systemic circulation, promoting metastasis. Furthermore, the abnormal TME is associated with increased infiltration of immunosuppressive regulatory T cells and polarizes tumour-associated macrophages from an anticancer M1-like phenotype towards an immunosuppressive M2 phenotype. The dysfunctional vasculature also restricts the accumulation and homogeneous intratumoural distribution of effector T cells. Consequently, anticancer immune responses are severely impaired. Vascular normalization could potentially prevent or reverse many of these adverse effects in patients, enhance the delivery and effectiveness of chemotherapy and immunotherapy, and improve the anticancer effects of radiotherapy.

Normalization improves immunotherapy

Immunotherapy has rapidly transformed the anticancer treatment and drug development landscape; however, many patients do not benefit from this approach. The success of immunotherapy relies on the recruitment, expansion, and effective anticancer activity of immune cells, especially CTLs, in the TME.

The accumulation of T cells occurs via a multistep process. First, T cells must interact with adhesion molecules, such as E-selectin, ICAM1, and VCAM1, expressed on the luminal surface of ECs lining blood vessels and must then cross the endothelial barriers. Dual anti-VEGF–ANG2 therapy with A2V has been shown to upregulate the expression of adhesion molecules during the window of vascular normalization and can thereby facilitate the accumulation of anticancer T cells within multiple types of tumours in mice¹⁶. This finding is in line with the results of earlier preclinical studies that demonstrated increased tumour infiltration by T cells in response to low doses of an anti-VEGFR2 antibody — during the window of normalization²⁷. In these models, antiangiogenic treatment several days before immunotherapy, by either vaccination or adoptive cell transfer approaches, increased the accumulation of T cells within the tumour and thus increased anticancer efficacy compared with vaccination therapy alone^27,96. Anti-VEGF therapy is also known to induce DC maturation and thus inhibit the priming of T cells. Additionally, loss of VEGF in T cells results in vessel normalization and an improved chemotherapeutic response in mice⁹⁷. Importantly, although loss of VEGF expression by T cells might potentiate anticancer responses through vascular normalization, this effect might be compromised by the decreased capacity of VEGF-deficient T cells to infiltrate tumours and thus control tumour growth⁹⁷. Indeed, the aforementioned upregulation of PD-L1 expression on ECs, which renders PD-1-expressing CTLs dysfunctional, has been highlighted as a possible mechanism of resistance to dual VEGF–ANG2 blockade¹⁶. Similarly, upregulation of PD-L1 has been demonstrated on both ECs and tumour cells following treatment with anti-VEGFR2 therapy⁹⁸. Thus, a strong rationale supports the combination of antiangiogenic agents and therapies that improve the functions of T cells in tumours. In fact, the findings of multiple preclinical studies provide evidence of the efficacy of combining vascular-normalizing agents with interventions that alleviate functional blockade of T cells, such as immune-checkpoint inhibitors (TABLE 1). For example, ICB with an anti-PD-1 antibody improved the degree of tumour control achieved with the anti-ANG2–VEGF antibody A2V in various cancer models¹⁶.

The combination of anti-VEGFR2 and anti-PD-L1 antibodies, however, did not improve survival in a glioblastoma model⁹⁸. This lack of efficacy was attributed to a low incidence of high endothelial venules (HEVs) within these tumours, contrary to findings in models of breast cancer and PNETs, in which this combination was efficacious. HEVs are specialized vascular units, usually organized as tertiary lymphoid structures, which facilitate the recruitment and extravasation of naive T cells, and then promote the differentiation of these cells into CTLs. The ECs lining HEVs are known to express ICAM1 and VCAM1, and thus facilitate homing and migration of immune cells to tumours⁹⁸. These findings are of particular importance because anti-PD-1 treatment alone conferred a survival benefit for only a small fraction of patients with glioblastoma⁹⁹ and failed to improve the outcomes of patients with this disease in a randomized phase III trial¹⁰⁰. Anti-PD-1 antibodies have also been shown to provide limited therapeutic benefit in a less immunogenic breast tumour model (MMTV-PyMT)¹⁶. Thus, treatments that can induce HEV formation and/or increase antigen load, which include radiation, chemotherapy, and cancer vaccines, might enhance the therapeutic benefits of ICB^101–104. Notably, HEV formation in the aforementioned breast cancer and PNET models was found to be mediated by lymphotoxin-β receptor (LTβR) signalling, and treatment with an agonistic anti-LTβR antibody induced HEVs in the previously recalcitrant glioblastoma model, increased CTL activity, and thereby increased the efficacy of combined anti-VEGFR2 and anti-PD-L1 therapy⁹⁸.

Interestingly, ICB of cytotoxic T lymphocyte protein 4 (CTLA-4) and PD-1 can normalize blood vessels in multiple models of transplanted breast and other tumours¹⁰⁵. The vascular normalization effect observed in this study was attributed to increased CD4⁺ type 1 T helper (T_H1) cell accumulation and anticancer activity¹⁰⁵. However, this evidence was derived using CD4⁺ T cell-deficient animals or anti-CD4 antibody treatment before breast tumour inoculation; thus, the roles of T_reg cells versus those of other populations of CD4⁺ T cells could not be separated. Hence, the effects of T_H1 cells on vascular normalization in the context of ICB need to be further validated in other preclinical models and through comparisons of patient biopsy specimens obtained before and after ICB. Importantly, although the generalizability of the reciprocal regulation of tumour vessel normalization and immunostimulatory reprogramming is supported by analyses of large transcriptomic databases in human tumours, the experimental studies were limited to breast cancer models¹⁰⁵. Whether ICB-mediated vessel normalization is induced across a range of tumour types remains to be determined. If found to be broadly applicable, this mechanism provides another rationale for the pursuit of combinatorial approaches using antiangiogenic agents and immune-checkpoint inhibitors, as these therapies target nonredundant pathways for vessel normalization and abrogation of immunosuppression in the TME.

Perspectives

Compelling preclinical and clinical evidence indicates that anti-VEGF therapy creates a transient window of vessel normalization during which the delivery of oxygen — a radiosensitizer and immunostimulator — and various therapeutic agents to tumours is improved^7,62. The extent of normalization varies with tumour type, as well as with the duration of treatment and the dosage of drug, and closure of the normalization window is marked by the development of adaptive tumour resistance and immunosuppression⁷. Tumours might escape anti-VEGF therapies mainly through alternative modes of vascularization (such as vessel co-option) and/or upregulation of alternative angiogenic pathways (such as ANG2–TIE2 signalling). Thus, targeting both the VEGF and ANG2 pathways concomitantly increases the efficacy of antiangiogenic treatments beyond that of monotherapy targeting either of these cytokines in preclinical studies^35,36. The treatment benefits observed with antiangiogenic therapy in animal models stem, in part, from restoration of anticancer immunity in the TME¹⁶, and this mechanism has formed the basis for combining antiangiogenic therapies with various immunotherapies, including tumour vaccines, chimeric antigen receptor (CAR) T cells, and immune-checkpoint inhibitors (TABLE 1). On the basis of positive preclinical data, these combinations are now being tested in multiple clinical trials, with promising outcomes reported for patients with RCC in a phase II trial⁶⁵ (TABLE 2). However, these therapies have complex biological effects, and combining them is likely to further increase this complexity; an increased risk of toxicities, particularly immune-related adverse events such as autoimmune colitis⁵, might ensue. Here, we outline some of the challenges and opportunities that lie ahead, highlighting future research priorities.

The results of both preclinical and retrospective clinical studies have demonstrated that the dosage of agents targeting the VEGF pathway is a key consideration^6,7. Historically, the clinical development of these agents has been predicated on the use of maximum tolerated doses until disease progression. High doses and/or a prolonged duration of anti-VEGF treatment are correlated with lower levels of tumour perfusion and elevated hypoxia²⁷. The addition of agents that target the ANG2–TIE2 pathway to VEGF pathway blockade seems to extend the window of vascular normalization and convert an immunosuppressive micro-environment into an immunostimulatory milieu^35,36; however, over time, tumours again become hypoxic and escape from this combined treatment approach via upregulation of PD-L1 on tumour cells and stromal cells of the TME, including ECs and immune cells⁹⁸. Thus, a critical need exists to optimize the dosage, duration, and sequence of administration of antiangiogenic agents for use in combination with ICB.

ICB can have vessel-normalizing effects in preclinical models of breast tumours¹⁰⁵, although the kinetics of vascular normalization are unknown. Obtaining this knowledge would facilitate efforts to optimally sequence the administration of immune-checkpoint inhibitors and antiangiogenic agents, in order to extend the window of normalization and, thus, the survival benefits¹⁰⁶.

Of note, ICB is known to increase the risk of oedema in patients with brain tumours, sometimes resulting in death^107,108. This observation contrasts with the vascular normalization induced by ICB in breast cancer models¹⁰⁵ and suggests that the vascular effects are dependent on the tumour location and/or type — a possibility that needs to be further investigated. Nevertheless, anti-VEGF agents can reduce glioblastoma-associated brain oedema in mice and patients^63,109, providing a strong rationale for combining low-dose anti-VEGF therapy with ICB and other forms of immunotherapy for this disease, and potentially also brain metastases.

ICBs are known to cause sometimes severe immune-related adverse events. These toxicities can often be resolved by discontinuing ICB therapy or reducing the doses of the immune-checkpoint inhibitors used⁵. Considering that vascular normalization can improve the delivery of therapeutic agents to tumours⁷, the proposed combination strategy might require lower doses of immune-checkpoint inhibitors to confer immunostimulation but also to reduce the risk of toxicity.

Neither anti-VEGF therapy nor ICB seem to be effective in highly desmoplastic tumours, such as pancreatic ductal adenocarcinoma, breast cancer, and cholangiocarcinoma^7,110 — with the notable exception of desmoplastic melanomas, which have been shown to respond to ICB¹¹¹. The reasons for this lack of responsiveness are currently unknown, although results from our laboratory have shown that the elevated levels of extracellular matrix molecules, including collagen I and hyaluronan, in such tumours can compress blood vessels, impairing their function and resulting in hypoxia⁹⁴. We have also shown that widely prescribed ARBs can normalize the stroma and decompress blood vessels, thereby improving the outcome of chemotherapy⁹⁴. Moreover, RNA sequencing analyses of tumour specimens from patients with pancreatic ductal adenocarcinoma have revealed that ARBs can activate both the innate and adaptive immune system¹¹². Thus, the testing of combinations of ARBs with antiangiogenic agents¹¹³ and immune-checkpoint inhibitors⁹⁵ is warranted.

Obesity is associated with an unfavourable prognosis across multiple types of cancer¹¹⁴. Notably, an increased severity of adverse effects has been observed in obese versus nonobese mice following immunotherapy¹¹⁵; however, the mechanisms underlying this increase are not understood. Given that obesity is an emerging public health problem worldwide, this relationship demands urgent investigation with regard to combined antiangiogenesis and immunotherapies.

We have focused on VEGF and angiopoietins as key targets for achieving vascular normalization; however, the duration and extent of normalization that can be achieved by specifically targeting these pathways might be limited. Certain molecularly targeted agents, such as the anti-HER2 antibody trastuzumab, and metronomic chemotherapy can also induce vascular normalization via upregulation of thrombospondin expression^28,116,117. Moreover, endocrine therapy for hormone-dependent tumours can also reduce VEGF levels and normalize tumour vessels¹¹⁸. Similarly, novel approved drugs, including poly [ADP-ribose] polymerase inhibitors and cyclin-dependent kinase 4 (CDK4) and CDK6 inhibitors, might potentiate the effectiveness of immunotherapies, in part, by vascular normalization¹¹⁹. Novel vascular normalization strategies targeting these additional pathways should be further explored.

In addition to ICB, other immunotherapy approaches are also showing promise for the treatment of solid tumours¹²⁰. Thus, the concepts discussed herein might also be applicable to CAR T cells and immunostimulatory or bi-functional antibodies in terms of improving their delivery and effectiveness.

A number of preclinical studies have demonstrated that cytotoxic therapies (radiation and chemotherapy) can be used to increase the neoantigen load of tumours that have a low baseline neoantigen load, which is a barrier to effective immunotherapy¹²¹. Again, by increasing the functionality of tumour vessels and fostering an immunosupportive TME (FIG. 4), vascular normalization approaches might not only improve the development of anticancer immunity, but also decrease the required doses of cytotoxic therapies and thus reduce the adverse effects of combinations incorporating immunotherapies.

The microbiota is emerging as a determinant of responses to various anticancer therapies, including immunotherapy^122–125. Virtually nothing is known, however, about the role of the microbiome in antiangiogenic therapy and vice versa. Thus, exploring the interactions among the microbiota, immunotherapy, and antiangiogenic therapy might offer exciting opportunities to improve the survival of patients with cancer.

Finally, no validated biomarkers are available to guide the use of antiangiogenic drugs^7,126. Similarly, biomarkers to evaluate responses to ICB, beyond MSI-H, dMMR, and PD-L1 positivity (which have suboptimal predictive utility), are urgently needed⁵. Optimally combining antiangiogenic therapy with ICB would require a concerted effort in this area owing to the costs and adverse effects of these treatments, which necessitate efforts to limit their use to patients who will derive clinical benefit.

Conclusions

In summary, immune-checkpoint inhibitors and antiangiogenic drugs are widely and increasingly prescribed, and are under continued developed in clinical trials, for the treatment of a range of solid tumours (Supplementary Table S1 and Supplementary Table S2). ICB can dramatically prolong the survival of patients who have durable responses, but has generally failed to improve the outcomes of the remaining majority of patients, at the cost of substantial toxicities in many recipients. Antiangiogenic drugs offer only a modest survival benefit of a few weeks to months, with rare durable responses. Herein, we have discussed the potential to rationally combine these two approaches in order to increase patient survival beyond that currently conferred by each approach individually. Despite the outstanding challenges, including the currently high costs and considerable risks of adverse effects associated with these treatment modalities, compelling evidence supports the combination of antiangiogenic and ICB therapies. The evidence indicates that the potential benefit of such combinations will be manifested though modulation of both the tumour vasculature and the tumour immune microenvironment. We await, with interest, the outcomes of ongoing clinical trials that are investigating whether the combination of antiangiogenic therapy with ICB will realize the full potential of this approach to improve the survival of patients with cancer. We also anticipate that the development of predictive biomarkers and orally available inhibitors of immune checkpoints, as well as competition among the companies manufacturing these agents, will help bring down the costs associated with these treatments in the future.

Supplementary Material

Acknowledgments

Footnotes

References