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 NOS2^low/TNFα^high phenotype, restoring the <i>in vivo</i> 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.