Functional characterization of human Cd33+ and Cd11b+ myeloid-derived suppressor cell subsets induced from peripheral blood mononuclear cells co-cultured with a diverse set of human tumor cell lines.

BACKGROUND: Tumor immune tolerance can derive from the recruitment of suppressor cell populations, including myeloid-derived suppressor cells (MDSC). In cancer patients, MDSC accumulation correlates with increased tumor burden, but the mechanisms of MDSC induction remain poorly understood. METHODS: This study examined the ability of human tumor cell lines to induce MDSC from healthy donor PBMC using in vitro co-culture methods. These human MDSC were then characterized for morphology, phenotype, gene expression, and function. RESULTS: Of over 100 tumor cell lines examined, 45 generated canonical CD33+HLA-DR(low)Lineage- MDSC, with high frequency of induction by cervical, ovarian, colorectal, renal cell, and head and neck carcinoma cell lines. CD33+ MDSC could be induced by cancer cell lines from all tumor types with the notable exception of those derived from breast cancer (0/9, regardless of hormone and HER2 status). Upon further examination, these and others with infrequent CD33+ MDSC generation were found to induce a second subset characterized as CD11b+CD33(low)HLA-DR(low)Lineage-. Gene and protein expression, antibody neutralization, and cytokine-induction studies determined that the induction of CD33+ MDSC depended upon over-expression of IL-1ß, IL-6, TNFα, VEGF, and GM-CSF, while CD11b+ MDSC induction correlated with over-expression of FLT3L and TGFß. Morphologically, both CD33+ and CD11b+ MDSC subsets appeared as immature myeloid cells and had significantly up-regulated expression of iNOS, NADPH oxidase, and arginase-1 genes. Furthermore, increased expression of transcription factors HIF1α, STAT3, and C/EBPß distinguished MDSC from normal counterparts. CONCLUSIONS: These studies demonstrate the universal nature of MDSC induction by human solid tumors and characterize two distinct MDSC subsets: CD33+HLA-DR(low)HIF1α+/STAT3+ and CD11b+HLA-DR(low)C/EBPß+, which should enable the development of novel diagnostic and therapeutic reagents for cancer immunotherapy.

ABIN-1 is a ubiquitin sensor that restricts cell death and sustains embryonic development.

Proteins that directly regulate tumour necrosis factor receptor (TNFR) signalling have critical roles in regulating cellular activation and survival. ABIN-1 (A20 binding and inhibitor of NF-kappaB) is a novel protein that is thought to inhibit NF-kappaB signalling. Here we show that mice deficient for ABIN-1 die during embryogenesis with fetal liver apoptosis, anaemia and hypoplasia. ABIN-1 deficient cells are hypersensitive to tumour necrosis factor (TNF)-induced programmed cell death, and TNF deficiency rescues ABIN-1 deficient embryos. ABIN-1 inhibits caspase 8 recruitment to FADD (Fas-associated death domain-containing protein) in TNF-induced signalling complexes, preventing caspase 8 cleavage and programmed cell death. Moreover, ABIN-1 directly binds polyubiquitin chains and this ubiquitin sensing activity is required for ABIN-1's anti-apoptotic activity. These studies provide insights into how ubiquitination and ubiquitin sensing proteins regulate cellular and organismal survival.

IL-15Ralpha chaperones IL-15 to stable dendritic cell membrane complexes that activate NK cells via trans presentation.

Natural killer (NK) cells are innate immune effectors that mediate rapid responses to viral antigens. Interleukin (IL)-15 and its high affinity IL-15 receptor, IL-15Ralpha, support NK cell homeostasis in resting animals via a novel trans presentation mechanism. To better understand how IL-15 and IL-15Ralpha support NK cell activation during immune responses, we have used sensitive assays for detecting native IL-15 and IL-15Ralpha proteins and developed an assay for detecting complexes of these proteins. We find that IL-15 and IL-15Ralpha are preassembled in complexes within the endoplasmic reticulum/Golgi of stimulated dendritic cells (DCs) before being released from cells. IL-15Ralpha is required for IL-15 production by DCs, and IL-15 that emerges onto the cell surface of matured DCs does not bind to neighboring cells expressing IL-15Ralpha. We also find that soluble IL-15-IL-15Ralpha complexes are induced during inflammation, but membrane-bound IL-15-IL-15Ralpha complexes, rather than soluble complexes, support NK cell activation in vitro and in vivo. Finally, we provide in vivo evidence that expression of IL-15Ralpha specifically on DCs is critical for trans presenting IL-15 and activating NK cells. These studies define an unprecedented cytokine-receptor biosynthetic pathway in which IL-15Ralpha serves as a chaperone for IL-15, after which membrane-bound IL-15Ralpha-IL-15 complexes activate NK cells via direct cell-cell contact.

DNA binding and nucleotide flipping by the human DNA repair protein AGT.

O(6)-alkylguanine-DNA alkyltransferase (AGT), or O(6)-methylguanine-DNA methyltransferase (MGMT), prevents mutations and apoptosis resulting from alkylation damage to guanines. AGT irreversibly transfers the alkyl lesion to an active site cysteine in a stoichiometric, direct damage reversal pathway. AGT expression therefore elicits tumor resistance to alkylating chemotherapies, and AGT inhibitors are in clinical trials. We report here structures of human AGT in complex with double-stranded DNA containing the biological substrate O(6)-methylguanine or crosslinked to the mechanistic inhibitor N(1),O(6)-ethanoxanthosine. The prototypical DNA major groove-binding helix-turn-helix (HTH) motif mediates unprecedented minor groove DNA binding. This binding architecture has advantages for DNA repair and nucleotide flipping, and provides a paradigm for HTH interactions in sequence-independent DNA-binding proteins like RecQ and BRCA2. Structural and biochemical results further support an unpredicted role for Tyr114 in nucleotide flipping through phosphate rotation and an efficient kinetic mechanism for locating alkylated bases.