Tumor hypoxia does not drive differentiation of tumor-associated macrophages but rather fine-tunes the M2-like macrophage population

Tumor-associated macrophages (TAM) are exposed to multiple microenvironmental cues in tumors, which collaborate to endow these cells with protumoral activities. Hypoxia, caused by an imbalance in oxygen supply and demand because of a poorly organized vasculature, is often a prominent feature in solid tumors. However, to what extent tumor hypoxia regulates the TAM phenotype in vivo is unknown. Here, we show that the myeloid infiltrate in mouse lung carcinoma tumors encompasses two morphologically distinct CD11b(hi)F4/80(hi)Ly6C(lo) TAM subsets, designated as MHC-II(lo) and MHC-II(hi) TAM, both of which were derived from tumor-infiltrating Ly6C(hi) monocytes. MHC-II(lo) TAM express higher levels of prototypical M2 markers and reside in more hypoxic regions. Consequently, MHC-II(lo) TAM contain higher mRNA levels for hypoxia-regulated genes than their MHC-II(hi) counterparts. To assess the in vivo role of hypoxia on these TAM features, cancer cells were inoculated in prolyl hydroxylase domain 2 (PHD2)-haplodeficient mice, resulting in better-oxygenated tumors. Interestingly, reduced tumor hypoxia did not alter the relative abundance of TAM subsets nor their M2 marker expression, but specifically lowered hypoxia-sensitive gene expression and angiogenic activity in the MHC-II(lo) TAM subset. The same observation in PHD2(+/+) → PHD2(+/-) bone marrow chimeras also suggests organization of a better-oxygenized microenvironment. Together, our results show that hypoxia is not a major driver of TAM subset differentiation, but rather specifically fine-tunes the phenotype of M2-like MHC-II(lo) TAM

Exploring mechanisms regulating the heterogeneity of Tumor-Associated Macrophages

Accumulation of macrophages in the tumor microenvironment is associated to poor prognoses in most human cancers. Tumor-associated macrophages (TAMs) represent a heterogeneous and plastic population of cells that contribute to tumor growth, metastatic dissemination, angiogenesis, and immune suppression. Both recruitment of monocytes from the blood and in situ proliferation contribute to the accumulation of TAMs. During the course of tumor progression, the phenotype of the infiltrating TAMs is changing by influences from other cells, extracellular signal molecules and availability of oxygen and nutrients. Therapeutic targeting of TAMs as a monotherapy has limited success, however, experimental studies show promising results when selectively depleting specific subsets of TAMs or changing the function of the TAM population. Yet, the mechanisms whereby macrophage phenotypes are regulated during tumor growth are still largely unknown. In this thesis, we investigated mechanisms underlying the dynamic changes in the TAM population observed during tumor growth. In our first study, overexpression of semaphorin 3A induced the proliferation of anti-tumoral macrophages and at the same time reduced the proliferation of pro-tumoral TAMs resulting in accumulation and activation of CD8+ T-cells and NK-cells and restricted tumor growth. In study II, we identified translational regulation of gene expression as an important mechanism regulating the TAM phenotype during tumor growth. By selective inhibition of translational activation, pro-tumoral macrophages were skewed towards an anti-tumoral phenotype. In the third study, we demonstrated a functional difference between macrophages of different ontogeny in a mouse model of glioblastoma. M2-polarized microglia, but not bone-marrow derived macrophages induced the expression of platelet-derived growth factor receptor B (PDGFRB) in glioma cells, enhancing their migratory capacity. In study IV, we showed that Zoledronic acid in combination with interleukin-2 induced the expression of interferon-g by monocytes leading to an up-regulation of TNF-related apoptosis-inducing ligand (TRAIL) on NKcells, inducing their cytotoxicity against tumor cells. In summary, we describe several mechanisms whereby the TAM phenotype may be regulated i.e translational control of gene expression, regulation of proliferation and ontogeny

GALDIERO is CO-FIRST AUTHOR. Phagocytes as Corrupted Policemen in Cancer-Related Inflammation

Inflammation is a key component of the tumor microenvironment. Tumor-associated macrophages (TAMs) and tumor-associated neutrophils (TANs) are prototypic inflammatory cells in cancer-related inflammation. Macrophages provide a first line of resistance against infectious agents but in the ecological niche of cancer behave as corrupted policemen. TAMs promote tumor growth and metastasis by direct interactions with cancer cells, including cancer stem cells, as well as by promoting angiogenesis and tissue remodeling and suppressing effective adaptive immunity. In addition, the efficacy of chemotherapy, radiotherapy, and checkpoint blockade inhibitors is profoundly affected by regulation of TAMs. In particular, TAMs can protect and rescue tumor cells from cytotoxic therapy by orchestrating a misguided tissue repair response. Following extensive preclinical studies, there is now proof of concept that targeting tumor-promoting macrophages by diverse strategies (e.g., Trabectedin, anti-colony-stimulating factor-1 receptor antibodies) can result in antitumor activity in human cancer and further studies are ongoing. Neutrophils have long been overlooked as a minor component of the tumor microenvironment, but there is evidence for an important role of TANs in tumor progression. Targeting phagocytes (TAMs and TANs) as corrupted policemen in cancer may pave the way to innovative therapeutic strategies complementing cytoreductive therapies and immunotherapy

Mechanisms Driving Macrophage Diversity and Specialization in Distinct Tumor Microenvironments and Parallelisms with Other Tissues

Macrophages are extremely versatile cells that adopt a distinct phenotype in response to a changing microenvironment. Consequently, macrophages are involved in diverse functions, ranging from organogenesis and tissue homeostasis to recognition and destruction of invading pathogens. In cancer, tumor-associated macrophages (TAM) often contribute to tumor progression by increasing cancer cell migration and invasiveness, stimulating angiogenesis, and suppressing anti-tumor immunity. Accumulating evidence suggests that these different functions could be exerted by specialized TAM subpopulations. Here, we discuss the potential underlying mechanisms regulating TAM specialization and elaborate on TAM heterogeneity in terms of their ontogeny, activation state, and intra-tumoral localization. In addition, parallels are drawn between TAM and macrophages in other tissues. Together, a better understanding of TAM diversity could provide a rationale for novel strategies aimed at targeting the most potent tumor-supporting macrophages

HIF-mediated innate immune responses: cell signaling and therapeutic implications

Leukocytes recruited to infected, damaged, or inflamed tissues during an immune response must adapt to oxygen levels much lower than those in the circulation. Hypoxia inducible factors (HIFs) are key mediators of cellular responses to hypoxia and, as in other cell types, HIFs are critical for the upregulation of glycolysis, which enables innate immune cells to produce adenosine triphosphate anaerobically. An increasing body of evidence demonstrates that hypoxia also regulates many other innate immunological functions, including cell migration, apoptosis, phagocytosis of pathogens, antigen presentation and production of cytokines, chemokines, and angiogenic and antimicrobial factors. Many of these functions are mediated by HIFs, which are not only stabilized posttranslationally by hypoxia, but also transcriptionally upregulated by inflammatory signals. Here, we review the role of HIFs in the responses of innate immune cells to hypoxia, both in vitro and in vivo, with a particular focus on myeloid cells, on which the majority of studies have so far been carried out

Radiation-induced immune responses: mechanisms and therapeutic perspectives.

Recent advancement in the radiotherapy technology has allowed conformal delivery of high doses of ionizing radiation precisely to the tumors while sparing large volume of the normal tissues, which have led to better clinical responses. Despite this technological advancement many advanced tumors often recur and they do so within the previously irradiated regions. How could tumors recur after receiving such high ablative doses of radiation? In this review, we outlined how radiation can elicit anti-tumor responses by introducing some of the cytokines that can be induced by ionizing radiation. We then dis-cuss how tumor hypoxia, a major limiting factor responsible for failure of radiotherapy, may also negatively impact the anti-tumor responses. In addition, we highlight how there may be other populations of immune cells including regulatory T cells (Tregs), mye-loid-derived suppressor cells (MDSCs), and tumor-associated macrophages (TAMs) that can be recruited to tumors interfering with the anti-tumor immunity. Finally, the impact of irradiation on tumor hypoxia and the immune responses according to different radio-therapy regimen is also delineated. It is indeed an exciting time to see that radiotherapy is being combined with immunotherapy in the clinic and we hope that this review can add an excitement to the field.11Yscopuskc