Priscilla S. Redd

The expression profiles and regulation of PD-L1 in tumor-induced myeloid-derived suppressor cells

ABSTRACT Programmed death-ligand 1 (PD-L1) is an inhibitory ligand that binds to PD-1 to suppress T cell activation. PD-L1 is constitutively expressed and inducible in tumor cells, but the expression profiles and regulatory mechanism of PD-L1 in myeloid-derived suppressor cells (MDSCs) are largely unknown. We report that PD-L1 is abundantly expressed in tumor-infiltrating leukocytes in human patients with both microsatellite instable and microsatellite stable colon cancer. About 60% CD11b+CD33+HLA-DR− MDSCs from peripheral blood of human colon cancer patients are PD-L1+. PD-L1+ MDSCs are also significantly higher in tumor-bearing mice than in tumor-free mice. Interestingly, the highest PD-L1+ MDSCs were observed in the tumor microenvironment in which 56–71% tumor-infiltrating MDSCs are PD-L1+ in vivo. In contrast, PD-L1+ MDSCs are significantly less in secondary lymphoid organs and peripheral blood as compared to the tumor tissues, whereas bone marrow MDSCs are essentially PD-L1− in tumor-bearing mice. IFNγ is highly expressed in cells of the tumor tissues and IFNγ neutralization significantly decreased PD-L1+ MDSCs in the tumor microenvironment in vivo. However, IFNγ-activated pSTAT1 does not bind to the cd274 promoter in MDSCs. Instead, pSTAT1 activates expression of IRF1, IRF5, IRF7 and IRF8 in MDSCs, and only pSTAT1-activated IRF1 binds to a unique IRF-binding sequence element in vitro and chromatin in vivo in the cd274 promoter to activate PD-L1 transcription. Our data determine that PD-L1 is highly expressed in tumor-infiltrating MDSCs and in a lesser degree in lymphoid organs, and the pSTAT1-IRF1 axis regulates PD-L1 expression in MDSCs.

SETD1B Activates iNOS Expression in Myeloid-Derived Suppressor Cells

Inducible nitric oxide synthase (iNOS) generates nitric oxide (NO) in myeloid cells that acts as a defense mechanism to suppress invading microorganisms or neoplastic cells. In tumor-bearing mice, elevated iNOS expression is a hallmark of myeloid-derived suppressor cells (MDSC). MDSCs use NO to nitrate both the T-cell receptor and STAT1, thus inhibiting T-cell activation and the antitumor immune response. The molecular mechanisms underlying iNOS expression and regulation in tumor-induced MDSCs are unknown. We report here that deficiency in IRF8 results in diminished iNOS expression in both mature CD11b+Gr1- and immature CD11b+Gr1+ myeloid cells in vivo Strikingly, although IRF8 was silenced in tumor-induced MDSCs, iNOS expression was significantly elevated in tumor-induced MDSCs, suggesting that the expression of iNOS is regulated by an IRF8-independent mechanism under pathologic conditions. Furthermore, tumor-induced MDSCs exhibited diminished STAT1 and NF-κB Rel protein levels, the essential inducers of iNOS in myeloid cells. Instead, tumor-induced MDSCs showed increased SETD1B expression as compared with their cellular equivalents in tumor-free mice. Chromatin immunoprecipitation revealed that H3K4me3, the target of SETD1B, was enriched at the nos2 promoter in tumor-induced MDSCs, and inhibition or silencing of SETD1B diminished iNOS expression in tumor-induced MDSCs. Our results show how tumor cells use the SETD1B-H3K4me3 epigenetic axis to bypass a normal role for IRF8 expression in activating iNOS expression in MDSCs when they are generated under pathologic conditions. Cancer Res; 77(11); 2834-43. ©2017 AACR.

CD133 + CD24 lo defines a 5-Fluorouracil-resistant colon cancer stem cell-like phenotype

The chemotherapeutic agent 5-Fluorouracil (5-FU) is the most commonly used drug for patients with advanced colon cancer. However, development of resistance to 5-FU is inevitable in almost all patients. The mechanism by which colon cancer develops 5-FU resistance is still unclear. One recently proposed theory is that cancer stem-like cells underlie colon cancer 5-FU resistance, but the phenotypes of 5-FU-resistant colon cancer stem cells are still controversial. We report here that 5-FU treatment selectively enriches a subset of CD133+ colon cancer cells in vitro. 5-FU chemotherapy also increases CD133+ tumor cells in human colon cancer patients. However, sorted CD133+ colon cancer cells exhibit no increased resistance to 5-FU, and CD133 levels exhibit no correlation with colon cancer patient survival or cancer recurrence. Genome-wide analysis of gene expression between sorted CD133+ colon cancer cells and 5-FU-selected colon cancer cells identifies 207 differentially expressed genes. CD24 is one of the genes whose expression level is lower in the CD133+ and 5-FU-resistant colon cancer cells as compared to CD133+ and 5-FU-sensitive colon cancer cells. Consequently, CD133+CD24lo cells exhibit decreased sensitivity to 5-FU. Therefore, we determine that CD133+CD24lo phenotype defines 5-FU-resistant human colon cancer stem cell-like cells.

H3K4me3 mediates the NF-κB p50 homodimer binding to the pdcd1 promoter to activate PD-1 transcription in T cells

ABSTRACT PD-1 is a co-repressive receptor that curbs T cell activation and thereby serves as a protection mechanism against autoimmunity under physiological conditions. Under pathological conditions, tumor cells express PD-L1 as an adaptive resistant mechanism to suppress PD-1+ T cells to evade host immunosurveillance. PD-1 therefore is a key target in cancer immunotherapy. Despite the extensive studies of PD-1 expression regulation, the pdcd1 transcription machinery and regulatory mechanisms are still not fully understood. We report here that the NF-κB p50 homodimer is a transcription regulator of PD-1 in activated T cells. A putative κB sequence exists at the pdcd1 promoter. All five NF-κB Rel subunits are activated in activated T cells. However, only the p50 homodimer directly binds to the κB sequence at the pccd1 promoter in CD4+ and CD8+ T cells. Deficiency in p50 results in reduced PD-1 expression in both CD4+ and CD8+ T cells in vitro. Using an in vivo mixed bone marrow chimera mouse model, we show that p50 regulates PD-1 expression in a cell-intrinsic way and p50 deficiency leads to decreased PD-1 expression in both antigen-specific CD4+ and CD8+ T cells in vivo. The expression levels of H3K4me3-specific histone methyltransferase increased significantly, resulting in a significant increase in H3K4me3 deposition at the pdcd1 promoter in activated CD4+ and CD8+ T cells. Inhibition of H3K4me3 significantly decreased p50 binding to the pdcd1 promoter and PD-1 expression in a T cell line. Our findings determine that the p50-H3K4me3 axis regulates pdcd1 transcription activation in activated T cells.

Abstract 2327: IFNgamma regulates PD-L1 expression through activating IRF1 transcription in tumor cells

The emerging clinical success of checkpoint blockade immunotherapy in human cancer patients has highlighted the critical importance of PD-L1, a ligand for the T cell inhibitory receptor PD-1, as a molecular target in cancer immunotherapy. PD-L1 is constitutively expressed in tumor cells and is also inducible by inflammatory cytokines such as IFNgamma. It has been proposed that tumor cells may sense the elevated IFNgamma from activated host T cells as a “threat” in the tumor microenvironment and adapt it by up-regulating PD-L1. The aim of this study is to elucidate the molecular mechanism underlying IFNgamma regulation of PD-L1 expression in tumor cells. We observed that PD-L1 is constitutively expressed, albeit at low level, in multiple types of cancer cells, including pancreatic, colon, breast, and sarcoma cells. IFNgamma treatment dramatically increased PD-L1 expression level and IFNgamma up-regulates PD-L1 expression through the Jak-STAT1 signaling pathway in vitro and in vivo. Chromatin immunoprecipitation assay did not identify IFNgamma-activated pSTAT1 binding to the pd-l1 promoter. Instead, we observed that IFNgamma activates IRF1 transcription and IRF1 is required for IFNgamma-induced PD-L1 expression. Chromatin immunoprecipitation analysis shows that pSTAT1 is associated with the irf1 but not the pd-l1 promoter. Analysis of the irf1 promoter DNA sequence revealed a pSTAT1-binding consensus sequence, and electrophoretic mobility shift assay indicates that pSTAT1 directly binds to this DNA element of the irf1 promoter. Furthermore, we demonstrated that IRF1 is associated with the pd-l1 promoter chromatin near the irf1 transcription initiation site. The pd-l1 promoter region contains a putative IRF1-binding consensus sequence and electrophoretic mobility shift assay shows that IRF1 binds to this DNA element of the pd-l1 promoter region. Taken together, our data indicate that IFNgamma activates pSTAT1 that binds to the irf1 promoter to activate irf1 transcription. IFNgamma-induced IRF1 then binds to the pd-l1 promoter to activate pd-l1 transcription in tumor cells. Citation Format: Chunwan Lu, Amy V. Paschall, Priscilla S. Redd, Iryna Lebedyeva, Kebin Liu. IFNgamma regulates PD-L1 expression through activating IRF1 transcription in tumor cells. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2327.

Abstract LB-265: IRF8 is an essential transcriptional activator of iNOS although MDSCs up-regulate iNOS expression via an IRF8-independent mechanism

Nitric Oxide Synthase (NOS) is a family of enzymes that catalyzes L-arginine metabolism to generate nitric oxide (NO). There are three main isoforms of NOS, named neuronal NOS (nNOS), endothelial NOS (eNOS), and inducible NOS (iNOS or NOS2). These three distinct isoforms of NOS have different cellular localization, regulation, and catalytic properties. iNOS is the most extensively studied one among these three NOS isoforms. iNOS was originally identified in myeloid cells, and it is known that its expression is often induced after myeloid cell activation by endotoxins or cytokines to generate NO which then acts as a defense effector to suppress invading microorganisms or neoplastic cells. It is known that IFNγ signaling pathway and NF-κB synergistically regulate iNOS expression, and that IFN regulatory factor 8 (IRF8) enhances interaction between IFNγ signaling and NF-κB in regulation of iNOS expression. However, the molecular mechanism underlying iNOS expression regulation in myeloid cell lineage is still not fully understood. We report here that IRF8 is an essential transcription activator of iNOS in myeloid cells since IRF8-deficient myeloid cells lose iNOS expression. Furthermore, a key phenotype of IRF8 KO mice is the mass accumulation of CD11b+Gr1+ myeloid-derived suppressor cells (MDSCs), suggesting that IRF8 is a key suppressor of MDSC differentiation under physiological conditions. On the other hand, accumulation of MDSCs is a hallmark of cancers and iNOS expression is dramatically elevated in tumor-induced MDSCs, despite that tumor-induced MDSCs exhibit diminished IRF8 expression. Furthermore, the IFNγ signaling pathway is impaired and NF-κB subunits are undetectable in tumor-induced MDSCs. Our data therefore indicate that iNOS expression is activated by an IRF8-independent mechanism that is also IFNγ and NF-κB-independent in tumor-induced MDSCs under pathological conditions. At the functional level, we determined that iNOS-KO mice exhibit lower tumor incidence as compared to WT mice. However, the sizes of established tumors are not different between iNOS-KO mice and WT mice. Taken together, our data determine that IRF8 is an essential transcriptional activator of iNOS expression in myeloid cells under physiological conditions, although iNOS expression is activated by an IFNγ-, NF-κB-, and IRF8-independent mechanism in tumor-induced MDSCs under pathological conditions. iNOS acts as a suppressor of tumor cell colonization but not tumor growth. Citation Format: Mohammed M. Ibrahim, Amy V. Paschall, Priscilla S. Redd, Kebin Liu. IRF8 is an essential transcriptional activator of iNOS although MDSCs up-regulate iNOS expression via an IRF8-independent mechanism. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr LB-265.