Immune escape is a prerequisite for tumor development. To avoid the immune system, tumors develop different mechanisms, including T cell exhaustion, which is characterized by expression of immune inhibitory receptors, such as PD-1, CTLA-4, Tim-3, and a progressive loss of function. The recent development of therapies targeting PD-1 and CTLA-4 have raised great interest since they induced long-lasting objective responses in patients suffering from advanced metastatic tumors. However, the regulation of PD-1 expression, and thereby of exhaustion, is unclear. VEGF-A, a proangiogenic molecule produced by the tumors, plays a key role in the development of an immunosuppressive microenvironment. We report in the present work that VEGF-A produced in the tumor microenvironment enhances expression of PD-1 and other inhibitory checkpoints involved in CD8⁺ T cell exhaustion, which could be reverted by anti-angiogenic agents targeting VEGF-A–VEGFR. In view of these results, association of anti-angiogenic molecules with immunomodulators of inhibitory checkpoints may be of particular interest in VEGF-A-producing tumors.

T cells can inhibit tumor growth, but their function in the tumor microenvironment is often suppressed. Many solid tumors exhibit abundant macrophage infiltration and low oxygen tension, yet how hypoxic conditions may affect innate immune cells and their role in tumor progression is poorly understood. Targeted deletion of the hypoxia-responsive transcription factor hypoxia-inducible factor-1α (HIF-1α) in macrophages in a progressive murine model of breast cancer resulted in reduced tumor growth, although vascular endothelial growth factor-A levels and vascularization were unchanged. Tumor-associated macrophages can suppress tumor-infiltrating T cells by several mechanisms, and we found that hypoxia powerfully augmented macrophage-mediated T-cell suppression in vitro in a manner dependent on macrophage expression of HIF-1α. Our findings link the innate immune hypoxic response to tumor progression through induction of T-cell suppression in the tumor microenvironment. Cancer Res; 70(19); 7465–75. ©2010 AACR.

Hypoxic response and inflammation both involve the action of the hypoxia-inducible transcription factors HIF-1α and HIF-2α. Previous studies have revealed that both HIF-α proteins are in a number of aspects similarly regulated post-translationally. However, the functional interrelationship of these two isoforms remains largely unclear. The polarization of macrophages controls functionally divergent processes; one of these is nitric oxide (NO) production, which in turn is controlled in part by HIF factors. We show here that the HIF-α isoforms can be differentially activated: HIF-1α is induced by Th1 cytokines in M1 macrophage polarization, whereas HIF-2α is induced by Th2 cytokines during an M2 response. This differential response was most evident in polarized macrophages through HIF-α isoform-specific regulation of the inducible NO synthase gene by HIF-1α, and the arginase1 gene by HIF-2α. In silico modeling predicted that regulation of overall NO availability is due to differential regulation of HIF-1α versus HIF-2α, acting to, respectively, either increase or suppress NO synthesis. An in vivo model of endotoxin challenge confirmed this; thus, these studies reveal that the two homologous transcription factors, HIF-1α and HIF-2α, can have physiologically antagonistic functions, but that their antiphase regulation allows them to coordinately regulate NO production in a cytokine-induced and transcription-dependent fashion.

VEGF (vascular endothelial growth factor) is an important angiogenic factor that has been implicated in tumorigenesis. Two papers now show that the function of VEGF is far more complex, as VEFG can negatively regulate angiogenesis and limit tumorigenesis. In one study, Greenberg et al. found that VEGF can inhibit angiogenesis, by impeding the function of the PDGF (platelet-derived growth factor) receptor on pericytes, leading to a loss of pericyte coverage of blood vessels. This involves the formation of heterodimers between the receptors for VEGF and PDGF. In another paper, Stockmann et al. deleted VEGF production in myeloid cells, but not other cell types. Unexpectedly, they found more rapid tumour development in these mice, at the same time as attenuated tumour vascularization and the formation of morphologically and functionally normalized blood vessels. In contrast, tumours lacking VEGF altogether grew more slowly. VEGF is an important angiogenic factor that has been implicated in tumourigenesis. Two papers now show that the function of VEGF is far more complex, as VEGF can negatively regulate angiogenesis and limit tumourigenesis. In the second paper, VEGF production was deleted in myeloid cells, but not other cell types. Unexpectedly, more rapid tumour development in these mice was found at the same time as attenuated tumour vascularization and the formation of morphologically and functionally normalized blood vessels. In contrast, tumours lacking VEGF altogether grew more slowly. Angiogenesis and the development of a vascular network are required for tumour progression, and they involve the release of angiogenic factors, including vascular endothelial growth factor (VEGF-A), from both malignant and stromal cell types1. Infiltration by cells of the myeloid lineage is a hallmark of many tumours, and in many cases the macrophages in these infiltrates express VEGF-A2. Here we show that the deletion of inflammatory-cell-derived VEGF-A attenuates the formation of a typical high-density vessel network, thus blocking the angiogenic switch in solid tumours in mice. Vasculature in tumours lacking myeloid-cell-derived VEGF-A was less tortuous, with increased pericyte coverage and decreased vessel length, indicating vascular normalization. In addition, loss of myeloid-derived VEGF-A decreases the phosphorylation of VEGF receptor 2 (VEGFR2) in tumours, even though overall VEGF-A levels in the tumours are unaffected. However, deletion of myeloid-cell VEGF-A resulted in an accelerated tumour progression in multiple subcutaneous isograft models and an autochthonous transgenic model of mammary tumorigenesis, with less overall tumour cell death and decreased tumour hypoxia. Furthermore, loss of myeloid-cell VEGF-A increased the susceptibility of tumours to chemotherapeutic cytotoxicity. This shows that myeloid-derived VEGF-A is essential for the tumorigenic alteration of vasculature and signalling to VEGFR2, and that these changes act to retard, not promote, tumour progression.