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.

Various steady state and inflamed tissues have been shown to contain a heterogeneous DC population consisting of developmentally distinct subsets, including cDC1s, cDC2s and monocyte-derived DCs, displaying differential functional specializations. The identification of functionally distinct tumour-associated DC (TADC) subpopulations could prove essential for the understanding of basic TADC biology and for envisaging targeted immunotherapies. We demonstrate that multiple mouse tumours as well as human tumours harbour ontogenically discrete TADC subsets. Monocyte-derived TADCs are prominent in tumour antigen uptake, but lack strong T-cell stimulatory capacity due to NO-mediated immunosuppression. Pre-cDC-derived TADCs have lymph node migratory potential, whereby cDC1s efficiently activate CD8+ T cells and cDC2s induce Th17 cells. Mice vaccinated with cDC2s displayed a reduced tumour growth accompanied by a reprogramming of pro-tumoural TAMs and a reduction of MDSCs, while cDC1 vaccination strongly induces anti-tumour CTLs. Our data might prove important for therapeutic interventions targeted at specific TADC subsets or their precursors.

Tumors contain a heterogeneous myeloid fraction comprised of discrete MHC-II hi and MHC-II lo tumor-associated macrophage (TAM) subpopulations that originate from Ly6C hi monocytes. However, the mechanisms regulating the abundance and phenotype of distinct TAM subsets remain unknown. Here, we investigated the role of macrophage colony-stimulating factor (M-CSF) in TAM differentiation and polarization in different mouse tumor models. We demonstrate that treatment of tumor-bearing mice with a blocking anti-M-CSFR monoclonal antibody resulted in a reduction of mature TAMs due to impaired recruitment, extravasation, proliferation, and maturation of their Ly6C hi monocytic precursors. M-CSFR signaling blockade shifted the MHC-II lo / MHC-II hi TAM balance in favor of the latter as observed by the preferential differentiation of Ly6C hi monocytes into MHC-II hi TAMs. In addition, the genetic and functional signatures of MHC-II lo TAMs were downregulated upon M-CSFR blockade, indicating that M-CSFR signaling shapes the MHC-II lo TAM phenotype. Conversely, granulocyte macrophage (GM)-CSFR had no effect on the mononuclear tumor infiltrate or relative abundance of TAM subsets. However, GM-CSFR signaling played an important role in fine-tuning the MHC-II hi phenotype. Overall, our data uncover the multifaceted and opposing roles of M-CSFR and GM-CSFR signaling in governing the phenotype of macrophage subsets in tumors, and provide new insight into the mechanism of action underlying M-CSFR blockade. Cancer Res; 76(1); 35-42. Ó2015 AACR.

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.

No abstract

RXR signaling is predicted to have a major impact in macrophages, but neither the biological consequence nor the genomic basis of its ligand activation is known. Comprehensive genome-wide studies were carried out to map liganded RXR-mediated transcriptional changes, active binding sites, and cistromic interactions in the context of the macrophage genome architecture. The macrophage RXR cistrome has 5200 genomic binding sites, which are not impacted by ligand. Active enhancers are characterized by PU.1 binding, an increase of enhancer RNA, and P300 recruitment. Using these features, 387 liganded RXR-bound enhancers were linked to 226 genes, which predominantly reside in CTCF/cohesin-limited functional domains. These findings were molecularly validated using chromosome conformation capture (3C) and 3C combined with sequencing (3C-seq), and we show that selected long-range enhancers communicate with promoters via stable or RXR-induced loops and that some of the enhancers interact with each other, forming an interchromosomal network. A set of angiogenic genes, including Vegfa, has liganded RXR-controlled enhancers and provides the macrophage with a novel inducible program.

African trypanosomes are the causative agents of Human African Trypanosomosis (HAT/Sleeping Sickness) and Animal African Trypanosomosis (AAT/Nagana). A common hallmark of African trypanosome infections is inflammation. In murine trypanosomosis, the onset of inflammation occurs rapidly after infection and is manifested by an influx of myeloid cells in both liver and spleen, accompanied by a burst of serum pro-inflammatory cytokines. Within 48 hours after reaching peak parasitemia, acute anemia develops and the percentage of red blood cells drops by 50%. Using a newly developed in vivo erythrophagocytosis assay, we recently demonstrated that activated cells of the myeloid phagocytic system display enhanced erythrophagocytosis causing acute anemia. Here, we aimed to elucidate the mechanism and immune pathway behind this phenomenon in a murine model for trypanosomosis. Results indicate that IFNγ plays a crucial role in the recruitment and activation of erythrophagocytic myeloid cells, as mice lacking the IFNγ receptor were partially protected against trypanosomosis-associated inflammation and acute anemia. NK and NKT cells were the earliest source of IFNγ during T. b. brucei infection. Later in infection, CD8+ and to a lesser extent CD4+ T cells become the main IFNγ producers. Cell depletion and transfer experiments indicated that during infection the absence of NK, NKT and CD8+ T cells, but not CD4+ T cells, resulted in a reduced anemic phenotype similar to trypanosome infected IFNγR-/- mice. Collectively, this study shows that NK, NKT and CD8+ T cell-derived IFNγ is a critical mediator in trypanosomosis-associated pathology, driving enhanced erythrophagocytosis by myeloid phagocytic cells and the induction of acute inflammation-associated anemia.

No abstract