The critical role of angiogenesis in promoting tumor growth and metastasis is strongly established. However, tumors show considerable variation in angiogenic characteristics and in their sensitivity to antiangiogenic therapy. Tumor angiogenesis involves not only cancer cells but also various tumor-associated leukocytes (TALs) and stromal cells. TALs produce chemokines, cytokines, proteases, structural proteins, and microvescicles. Vascular endothelial growth factor (VEGF) and inflammatory chemokines are not only major proangiogenic factors but are also immune modulators, which increase angiogenesis and lead to immune suppression. In our review, we discuss the regulation of angiogenesis by innate immune cells in the tumor microenvironment, specific features, and roles of major players: macrophages, neutrophils, myeloid-derived suppressor and dendritic cells, mast cells, γδT cells, innate lymphoid cells, and natural killer cells. Anti-VEGF or anti-inflammatory drugs could balance an immunosuppressive microenvironment to an immune permissive one. Anti-VEGF as well as anti-inflammatory drugs could therefore represent partners for combinations with immune checkpoint inhibitors, enhancing the effects of immune therapy.

The angiogenic activity of CXC-ELR+ chemokines, including CXCL8/IL-8, CXCL1/macrophage inflammatory protein-2 (MIP-2), and CXCL1/growth-related oncogene-α in the Matrigel sponge angiogenesis assay in vivo, is strictly neutrophil dependent, as neutrophil depletion of the animals completely abrogates the angiogenic response. In this study, we demonstrate that mice deficient in the src family kinases, Hck and Fgr (hck−/−fgr−/−), are unable to develop an angiogenic response to CXCL1/MIP-2, although they respond normally to vascular endothelial growth factor-A (VEGF-A). Histological examination of the CXCL1/MIP-2-containing Matrigel implants isolated from wild-type or hck−/−fgr−/− mice showed the presence of an extensive neutrophil infiltrate, excluding a defective neutrophil recruitment into the Matrigel sponges. Accordingly, neutrophils from hck−/−fgr−/− mice normally migrated and released gelatinase B in response to CXCL1/MIP-2 in vitro, similarly to wild-type neutrophils. However, unlike wild-type neutrophils, those from hck−/−fgr−/− mice were completely unable to release VEGF-A upon stimulation with CXCL1/MIP-2. Furthermore, neutralizing anti-VEGF-A Abs abrogated the angiogenic response to CXCL1/MIP-2 in wild-type mice and CXCL1/MIP-2 induced angiogenesis in the chick embryo chorioallantoic membrane assay, indicating that neutrophil-derived VEGF-A is a major mediator of CXCL1/MIP-2-induced angiogenesis. Finally, in vitro kinase assays confirmed that CXCL1/MIP-2 activates Hck and Fgr in murine neutrophils. Taken together, these data demonstrate that CXCL1/MIP-2 leads to recruitment of neutrophils that, in turn, release biologically active VEGF-A, resulting in angiogenesis in vivo. Our observations delineate a novel mechanism by which CXCL1/MIP-2 induces neutrophil-dependent angiogenesis in vivo.

IFNs are highly pleiotropic cytokines also endowed with marked antiangiogenic activity. In this study, the mRNA expression profiles of endothelial cells (EC) exposed in vitro to IFN-α, IFN-β, or IFN-γ were determined. We found that in HUVEC as well as in other EC types 175 genes were up-regulated (>2-fold increase) by IFNs, including genes involved in the host response to RNA viruses, inflammation, and apoptosis. Interestingly, 41 genes showed a >5-fold higher induction by IFN-α in EC compared with human fibroblasts; among them, the gene encoding the angiostatic chemokine CXCL11 was selectively induced by IFN-α in EC along with other genes associated with angiogenesis regulation, including CXCL10, TRAIL, and guanylate-binding protein 1. These transcriptional changes were confirmed and extended by quantitative PCR analysis and ELISA; whereas IFN-α and IFN-β exerted virtually identical effects on transcriptome modulation, a differential gene regulation by type I and type II IFN emerged, especially as far as quantitative aspects were concerned. In vivo, IFN-α-producing tumors overexpressed murine CXCL10 and CXCL11, guanylate-binding protein 1, and TRAIL, with evidence of CXCL11 production by tumor-associated EC. Overall, these findings improve our understanding of the antiangiogenic effects of IFNs by showing that these cytokines trigger an antiangiogenic transcriptional program in EC. Moreover, we suggest that quantitative differences in the magnitude of the transcriptional activation of IFN-responsive genes could form the basis for cell-specific transcriptional signatures.