Blockade of the PD-1/PD-L1 axis augments lysis of AML cells by the CD33/CD3 BiTE antibody construct AMG 330: reversing a T-cell-induced immune escape mechanism.

Bispecific T-cell engagers (BiTEs) are very effective in recruiting and activating T cells. We tested the cytotoxicity of the CD33/CD3 BiTE antibody construct AMG 330 on primary acute myeloid leukemia (AML) cells ex vivo and characterized parameters contributing to antileukemic cytolytic activity. The E:T ratio and the CD33 expression level significantly influenced lysis kinetics in long-term cultures of primary AML cells (n=38). AMG 330 induced T-cell-mediated proinflammatory conditions, favoring the upregulation of immune checkpoints on target and effector cells. Although not constitutively expressed at the time of primary diagnosis (n=123), PD-L1 was strongly upregulated on primary AML cells upon AMG 330 addition to ex vivo cultures (n=27, P<0.0001). This phenomenon was cytokine-driven as the sole addition of interferon (IFN)-γ and tumor necrosis factor-α also induced expression. Through blockade of the PD-1/PD-L1 interaction, AMG 330-mediated lysis (n=9, P=0.03), T-cell proliferation (n=9, P=0.01) and IFN-γ secretion (n=8, P=0.008) were significantly enhanced. The combinatorial approach was most beneficial in settings of protracted AML cell lysis. Taken together, we have characterized a critical resistance mechanism employed by primary AML cells under AMG 330-mediated proinflammatory conditions. Our results support the evaluation of checkpoint molecules in upcoming clinical trials with AMG 330 to enhance BiTE antibody construct-mediated cytotoxicity.

Conserved high-affinity NF-kappa B binding site in the interferon regulatory factor-1 promoter is not occupied by NF-kappa B in vivo and is transcriptionally inactive.

The promoter of the interferon regulatory factor-1 (IRF-1) gene contains at position -47 to -38 an evolutionary conserved binding sequence for the inducible transcription factor NF-kappa B. This site is highly homologous to a transcriptionally active site from the MHC class I enhancer. In this study, we show by in vitro assays using purified NF-kappa B that the kappa B motif in the IRF-1 promoter binds the factor specifically and with high affinity, comparable to various other cis-acting kappa B elements. Two copies of the IRF-1 kappa B site fused to the heterologous c-fos promoter conferred induction of a chloramphenicol acetyl transferase (CAT) reported gene in response to stimulation of L929 fibroblasts with various NF-kappa B inducers, such as tumor necrosis factor alpha (TNF alpha) or phorbol 12-myristate 13-acetate (PMA). Mutation of the binding site completely abolished transcriptional inducibility of the heterologous promoter. Surprisingly, the same IRF-1 kappa B motif in context of the homologous IRF-1 promoter was transcriptionally inactive in CAT assays. The very weak induction of the IRF-1 promoter in response to TNF treatment or infection of fibroblasts with Newcastle disease virus (NDV) was barely affected by point mutation of the kappa B site or loss of the site by truncation of the promoter. Analysis of the occupational state of the chromosomal IRF-1 kappa B site by in vivo foot-printing revealed that no footprint was induced over the kappa B motif in the IRF-1 promoter after PMA treatment of L929 fibroblast cells, despite the simultaneous induction of IRF-1 mRNA and NF-kappa B binding activity. Constitutive footprints were detected at a CCAAT and GC-rich region in the promoter. This is the first example of a high-affinity NF-kappa B binding site within a promoter which may not participate in transcriptional regulation under conditions activating NF-kappa B DNA binding and gene expression.

Tolerance to lipopolysaccharide involves mobilization of nuclear factor kappa B with predominance of p50 homodimers.

Stimulation of the human monocytic cell line Mono Mac 6 with lipopolysaccharide (LPS) leads to rapid and transient expression of cytokines like tumor necrosis factor (TNF). When such cells are precultured for 2 days with a low dose of LPS (20 ng/ml) followed by stimulation with a high dose of LPS (1 microgram/ml), expression of the TNF gene is minimal, i.e. the cells are tolerant. In nuclear run-on analysis, such tolerant cells show only a low degree of transcription, indicating that tolerance operates at or upstream of the transcription level. The CD14 LPS receptor is, however, up-regulated (not down-regulated) in tolerant cells, and LPS can, in fact, still lead to activation of tolerant cells as evidenced by mobilization of the transcription factor nuclear factor kappa B (NF-kappa B). Resolution of the NF-kappa B complex in gel shift analysis shows that the binding protein, mobilized in naive Mono Mac 6 cells, consists mainly of p50-p65 heterodimers, while in tolerant cells, the p50 homodimer is predominant. This increase in p50 homodimers coincides with an increase in p105 mRNA, suggestive of a transcriptional up-regulation of p50. Reporter gene analysis reveals that the NF-kappa B complex mobilized in tolerant cells is functionally inactive in that NF-kappa B-dependent luciferase constructs containing the human immunodeficiency virus long terminal repeat or the TNF 5'-region show only minimal transactivation after LPS stimulation. Similar to Mono Mac 6 cells, primary blood monocytes, when precultured with a low dose of LPS, also become tolerant and produce little TNF after LPS stimulation. The tolerant blood monocytes also up-regulate CD14, and they mobilize NF-kappa B with a predominance of p50 homodimers. Taken together, these results demonstrate that tolerance to LPS is determined by post-receptor mechanisms that involve an altered composition of the NF-kappa B complex.

Identification of NF-jun, a novel inducible transcription factor that regulates c-jun gene transcription.

In this study we report the identification of a novel transcription factor, termed Nuclear Factor-jun (NF-jun). This factor contributes to inducible transcription of the c-jun gene in human myeloid leukemia cells. NF-jun was, however, undetectable in nuclear proteins from human monocytes, granulocytes, resting T lymphocytes and lung fibroblasts. NF-jun shares several features with the well characterized NF-kappa B in that binding activity can be generated in cytosolic extracts by treatment with dissociating agents. In addition, binding of NF-jun to its recognition site is enhanced by treatment of cells with 12-O-tetradecanoylphorbol-13-acetate, tumor necrosis factor alpha or the protein synthesis inhibitor cycloheximide (CHX). However, as revealed by competition assays and electrophoretic mobility shift assays, purified NF-kappa B fails to bind to the c-jun fragment which contains the NF-jun site, and this fragment fails to compete with NF-kappa B for binding. UV crosslinking showed that NF-jun contains a 55 and a 125 kDa protein species. These findings demonstrate that the c-jun gene can be regulated by a transcription factor distinct from AP-1. Our findings also indicate that while NF-jun has several features in common with the NF-kappa B binding protein including its subcellular localization and its ability to translocate from the cytoplasm to the nucleus, this factor recognizes a unique DNA sequence. Moreover, the activity of this protein is differentially regulated in various cell types. NF-jun might function as a signal transducing molecule in order to mediate rapid induction of the early response gene c-jun in a cell type- and stimulus-specific manner.