The reciprocal function and regulation of tumor vessels and immune cells offers new therapeutic opportunities in cancer

Abstract

Tumor angiogenesis and escape of immunosurveillance are two cancer hallmarks that are tightly linked and reciprocally regulated by paracrine signaling cues of cell constituents from both compartments. Formation and remodeling of new blood vessels in tumors is abnormal and facilitates immune evasion. In turn, immune cells in the tumor, specifically in context with an acidic and hypoxic environment, can promote neovascularization. Immunotherapy has emerged as a major therapeutic modality in cancer but is often hampered by the low influx of activated cytotoxic T-cells. On the other hand, anti-angiogenic therapy has been shown to transiently normalize the tumor vasculature and enhance infiltration of T lymphocytes, providing a rationale for a combination of these two therapeutic approaches to sustain and improve therapeutic efficacy in cancer. In this review, we discuss how the tumor vasculature facilitates an immunosuppressive phenotype and vice versa how innate and adaptive immune cells regulate angiogenesis during tumor progression. We further highlight recent results of antiangiogenic immunotherapies in experimental models and the clinic to evaluate the concept that targeting both the tumor vessels and immune cells increases the effectiveness in cancer patients.

Introduction

The onset of tumor neovascularization is a well-established hallmark of cancer that is initiated at a certain time during tumor progression depending on the tumor type, grade and stage. Blood vessels not only have to provide oxygen and deplete waste products to oblige the demands of a growing tumor but also form niches that enable cancer stem cell maintenance, protection and entail sites of infiltrating immune cells [[1], [2], [3], [4]]. Various mechanisms for the reinstatement of blood vessel growth have been described of which angiogenesis, the sprouting of new vessels from pre-existing capillaries and postcapillary venules is the most commonly used mode [[5], [6], [7]]. Subsequently, tip cells anastomose with tip cells from neighboring sprouts to connect the newly formed vascular structures with the help of bridging macrophages. With the initiation of blood flow, the establishment of a basement membrane, and the recruitment of vessel-stabilizing pericytes, vascular growth is then terminated [5,6]. Thus, under physiological conditions angiogenesis is tightly regulated by a homeostatic balance of a wealth of proangiogenic and inhibitory factors of which members of the vascular endothelial growth factor (VEGF), and angiopoietin tyrosine kinase receptor families as well as various members of the CXC and CC chemokines are the most prominent molecules to orchestrate this multistep process [1,8]. The balance of angiogenesis-promoting and inhibiting molecules is, however, lost in tumors. Upon initiating neovascularization, tumors continue to promote angiogenic signaling for several reasons [9]. First, tumors express oncogenes and/or lose tumor suppressor genes that result in the activation of pro-angiogenic signaling pathways [[10], [11], [12]]. Second, expanding tumors exhibit a continually and abnormally growing tumor vasculature with a typical display of irregular-shaped, hyperdilated and poorly pericyte-covered tumor vessels that cause a leaky and sluggish blood flow [13]. These typical signs of poor vessel maturation increase interstitial pressure and generate and maintain a hypoxic and acidic environment that continues to elevate proangiogenic factors thereby further aggravating a pro-angiogenic feedback loop that never ceases.

Effects of low-oxygen tension are mediated by hypoxia-inducible factor (HIF) transcription factors that coordinate a transcriptional program to ensure metabolic and vascular adaptations under hypoxic conditions by various mechanisms [[14], [15], [16]]. Hypoxia upregulates various proangiogenic growth factors and chemokines that directly engage in vessel growth like VEGF, PlGF and Ang2 [[17], [18], [19], [20], [21]]. In addition, numerous experimental models have shown that both hypoxic and acidic conditions in the tumor microenvironment change the availability of metabolites and induce the secretion of molecules in tumors (e.g. CSF1, GM-CSF, G-CSF, CX3CL1, CXCL5, CXCL12, CCL17, CCL22, IL6, Sema3) that attract innate immune cells to the tumor site where they become reprogrammed to serve as an additional source of angiogenic factors [[22], [23], [24], [25], [26], [27], [28]]. It is important to note that innate immune cells elicit a high plasticity in order to quickly adapt and serve the homeostatic repair program. Upon injury, first neutrophils and then macrophages are recruited to the wound that are immunosupportive and angiostatic due to their responsibility to phagocytose and kill bacteria and degrade necrotic tissue. In the subsequent resolution phase, myeloid cells then convert to an angiogenic and immunosuppressive phenotype to support tissue and blood vessel restoration [29]. The ability of myeloid cells to display such opposing properties to either eliminate potential infections or hinder excessive inflammation is indeed pivotal to properly regulate tissue injury and maintain tissue homeostasis [30]. In line with the concept that tumors are wounds that never heal [31], tumors take advantage of the myeloid plasticity by reprogramming these cells to exhibit both immunosuppressive and angiogenic features to enable tumors to escape immunosurveillance and facilitate vascular expansion. These two features have been defined as pivotal hallmarks in cancer propagation and progression [32].

There is increasing evidence that tumor-supporting inflammation and angiogenesis, although being distinct and separable processes, are closely related events that are in part regulated by common chemokines [32,33]. VEGF and Ang2 are proangiogenic factors that directly and indirectly convey immunosuppression in endothelial cells as well as innate and adaptive immune cells [[34], [35], [36], [37]]. VEGF induces an immunosuppressive vasculature on different levels (Fig.1). It downregulates the expression of the vascular adhesion molecules I-CAM1 and V-CAM1 that are necessary for T-cells to cross the endothelial layer and transit into the tumor. VEGF and Ang2 have been shown to enhance the expression of the negative checkpoint regulator programmed cell death protein 1 (PD-1)–programmed cell death 1 ligand 1 (PD-L1) in endothelial cells thus disabling the cytotoxic function of PD1+-T cells [[38], [39], [40], [41]]. In addition, elevated FasL levels on tumor ECs is known to trigger apoptosis of Fas-expressing CD8 + T cells [42]. Tumor-associated ECs also preferentially attract immunosuppressive Tregs by upregulating the multifunctional endothelial receptor CLEVER-1/stabilin-1 [43]. All these mechanisms consequently lead to a potent barrier that disables cytotoxic T cell infiltration into the tumor. VEGF, however, not only facilitates VEGFR-2 signaling in endothelial cells but also in VEGFR2+ immune cells. Thereby, it stimulates the expression of PD-L1 in dendritic cells and blocks the maturation and thus, functionality of dendritic cells due to their impaired ability to present tumor antigens [44]. VEGF/VEGFR2 signaling further suppresses proliferation of effector T cells but increases and promotes tumor homing of Tregs [32,44]. This effect is dependent on neuropilin-1 (nrp1) because in an experimental tumor model, nrp1 depletion resulted in reduction of Tregs with a concomitant increase in cytotoxic T cells [45]. VEGF also upregulates several negative immune checkpoint receptors besides PD-1 (e.g. CTLA-4, Lag-3, TIM-3) on T cells contributing to T-cell anergy and exhaustion [46]. Finally, elevated levels of VEGF and/or Ang-2 as well as other tumor-secreted factors promote the recruitment and expansion of innate immune cell populations including TAMs, TEMs, neutrophils, MDSC and immature dendritic cells that secrete molecules fostering an immunosuppressive and angiogenic tumor microenvironment.