Molecular and epigenetic basis of macrophage polarized activation.

Macrophages are unique cells for origin, heterogeneity and plasticity. At steady state most of macrophages are derived from fetal sources and maintained in adulthood through self-renewing. Despite sharing common progenitors, a remarkable heterogeneity characterized tissue-resident macrophages indicating that local signals educate them to express organ-specific functions. Macrophages are extremely plastic: chromatin landscape and transcriptional programs can be dynamically re-shaped in response to microenvironmental changes. Owing to their ductility, macrophages are crucial orchestrators of both initiation and resolution of immune responses and key supporters of tissue development and functions in homeostatic and pathological conditions. Herein, we describe current understanding of heterogeneity and plasticity of macrophages using the M1-M2 dichotomy as operationally useful simplification of polarized activation. We focused on the complex network of signaling cascades, metabolic pathways, transcription factors, and epigenetic changes that control macrophage activation. In particular, this network was addressed in sepsis, as a paradigm of a pathological condition determining dynamic macrophage reprogramming.

Tumor-Derived Prostaglandin E2 Promotes p50 NF-κB-Dependent Differentiation of Monocytic MDSCs.

Myeloid-derived suppressor cells (MDSC) include immature monocytic (M-MDSC) and granulocytic (PMN-MDSC) cells that share the ability to suppress adaptive immunity and to hinder the effectiveness of anticancer treatments. Of note, in response to IFNγ, M-MDSCs release the tumor-promoting and immunosuppressive molecule nitric oxide (NO), whereas macrophages largely express antitumor properties. Investigating these opposing activities, we found that tumor-derived prostaglandin E2 (PGE2) induces nuclear accumulation of p50 NF-κB in M-MDSCs, diverting their response to IFNγ toward NO-mediated immunosuppression and reducing TNFα expression. At the genome level, p50 NF-κB promoted binding of STAT1 to regulatory regions of selected IFNγ-dependent genes, including inducible nitric oxide synthase (Nos2). In agreement, ablation of p50 as well as pharmacologic inhibition of either the PGE2 receptor EP2 or NO production reprogrammed M-MDSCs toward a NOS2low/TNFαhigh phenotype, restoring the in vivo antitumor activity of IFNγ. Our results indicate that inhibition of the PGE2/p50/NO axis prevents MDSC-suppressive functions and restores the efficacy of anticancer immunotherapy. SIGNIFICANCE: Tumor-derived PGE2-mediated induction of nuclear p50 NF-κB epigenetically reprograms the response of monocytic cells to IFNγ toward an immunosuppressive phenotype, thus retrieving the anticancer properties of IFNγ. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/80/13/2874/F1.large.jpg.

Protumor Steering of Cancer Inflammation by p50 NF-κB Enhances Colorectal Cancer Progression.

Although tumor-associated macrophages (TAM) display a M2-skewed tumor-promoting phenotype in most cancers, in colorectal cancer, both TAM polarization and its impact remain controversial. We investigated the role of the M2-polarizing p50 NF-κB subunit in orchestrating TAM phenotype, tumor microenvironment composition, and colorectal cancer progression. We first demonstrated, by parallel studies in colitis-associated cancer (CAC) and in genetically driven ApcMin mouse models, that the p50-dependent inhibition of M1-polarized gut inflammation supported colorectal cancer development. In accordance with these studies, p50-/- mice displayed exacerbated CAC with fewer and smaller tumors, along with enhanced levels of M1/Th1 cytokines/chemokines, including IL12 and CXCL10, whose administration restrained CAC development in vivo The inflammatory profile supporting tumor resistance in colons from p50-/- tumor bearers correlated inversely with TAM load and positively with both recruitment of NK, NKT, CD8+ T cells and number of apoptotic tumor cells. In agreement, myeloid-specific ablation of p50 promoted tumor resistance in mice, whereas in colorectal cancer patients, a high number of p50+ TAMs at the invasive margin was associated with decreased IL12A and TBX21 expression and worse postsurgical outcome. Our findings point to p50 involvement in colorectal cancer development, through its engagement in the protumor activation of macrophages, and identify a candidate for prognostic and target therapeutic intervention. Cancer Immunol Res; 6(5); 578-93. ©2018 AACR.