Human AML activates the aryl hydrocarbon receptor pathway to impair NK cell development and function

The future of cellular immunotherapy for childhood leukemia

PURPOSE OF REVIEW

Exciting translational discoveries in recent years have brought realized promise of immunotherapy for children with high-risk leukemias. This review summarizes the current immunotherapeutic landscape with a focus on key clinical trials for patients with acute lymphoblastic leukemia or acute myeloid leukemia.

RECENT FINDINGS

Chemotherapy resistance remains a major barrier to cure in children with high-risk leukemias. Immunotherapy approaches have potential to overcome this resistance given alternative mechanisms of action. Based upon preclinical activity and/or success in adult patients, recent clinical trials have demonstrated safety and efficacy of various mAb, antibody-drug conjugate, bispecific T-cell-engaging antibody, natural killer cell, and chimeric antigen receptor-redirected T-cell immunotherapies for children with acute lymphoblastic leukemia or acute myeloid leukemia. Food and Drug Administration approval of several of these immunotherapies has increased the pediatric leukemia therapeutic portfolio and improved clinical outcomes for previously incurable patients.

SUMMARY

Several antibody-based or cellular immunotherapy modalities have demonstrated appreciable efficacy in children with relapsed or chemotherapy-refractory leukemia via early-phase clinical trials. Some studies have also identified critical biomarkers of treatment response and resistance that merit further investigation. Continued preclinical and clinical evaluation of novel immunotherapies is imperative to improve cure rates for children with high-risk leukemias.

Regulation of Aryl Hydrocarbon Receptor Signaling Pathway and Dioxin Toxicity by Novel Agonists and Antagonists

Dioxins, mostly through activation of aryl hydrocarbon receptor (AhR), are potent toxic substances widely distributed in the environment, while moderated suppression of AhR also exhibits anti-tumor effects. Therefore, the proper modulation of AhR activity may counteract AhR-mediated toxicities and certain diseases. In this investigation, we identified several novel AhR moderate agonists and antagonists using chemical biology approaches. The mechanisms and mode of interactions with AhR by these hits were also revealed using both experimental and computational studies. The newly identified AhR moderate agonists and antagonists were predicted to bind to AhR and modulate AhR signaling. The structure-activity relationships of moderate agonists and antagonists and their unique binding features with AhR have created a solid framework for further optimization of the next generation of AhR modulators.

The therapeutic potential of targeting tryptophan catabolism in cancer

Based on its effects on both tumour cell intrinsic malignant properties as well as anti-tumour immune responses, tryptophan catabolism has emerged as an important metabolic regulator of cancer progression. Three enzymes, indoleamine-2,3-dioxygenase 1 and 2 (IDO1/2) and tryptophan-2,3-dioxygenase (TDO2), catalyse the first step of the degradation of the essential amino acid tryptophan (Trp) to kynurenine (Kyn). The notion of inhibiting IDO1 using small-molecule inhibitors elicited high hopes of a positive impact in the field of immuno-oncology, by restoring anti-tumour immune responses and synergising with other immunotherapies such as immune checkpoint inhibition. However, clinical trials with IDO1 inhibitors have yielded disappointing results, hence raising many questions. This review will discuss strategies to target Trp-degrading enzymes and possible down-stream consequences of their inhibition. We aim to provide comprehensive background information on Trp catabolic enzymes as targets in immuno-oncology and their current state of development. Details of the clinical trials with IDO1 inhibitors, including patient stratification, possible effects of the inhibitors themselves, effects of pre-treatments and the therapies the inhibitors were combined with, are discussed and mechanisms proposed that might have compensated for IDO1 inhibition. Finally, alternative approaches are suggested to circumvent these problems.

The role of AhR in transcriptional regulation of immune cell development and function

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcriptional factor (TF) that is a member of the Per-Arnt-Sim family of proteins. AhR regulates diverse processes, including malignant transformation, hematopoietic cell development, and fate determination of immune cell lineages. Moreover, AhR forms a crucial link between innate and adaptive arms of the immune system. Malignant cells frequently evolve multiple mechanisms for suppressing tumor-specific responses, including the induction of suppressive pathways involving AhR and its metabolic byproducts in the tumor microenvironment that promote immune evasion and tumor progression. Thus, interest is high in further defining the role of AhR in carcinogenesis and immune development and regulation, particularly regarding the therapeutic interventions that unleash immune responses to cancer cells. Here, we provide an overview of the role of AhR in the regulation of innate and adaptive immune response and discuss the implications of targeting this pathway to augment the immune response in cancer patients.

Natural Killer Cells in Myeloid Malignancies: Immune Surveillance, NK Cell Dysfunction, and Pharmacological Opportunities to Bolster the Endogenous NK Cells

Natural killer (NK) cells are large granular lymphocytes involved in our defense against certain virus-infected and malignant cells. In contrast to T cells, NK cells elicit rapid anti-tumor responses based on signals from activating and inhibitory cell surface receptors. They also lyse target cells via antibody-dependent cellular cytotoxicity, a critical mode of action of several therapeutic antibodies used to treat cancer. A body of evidence shows that NK cells can exhibit potent anti-tumor activity against chronic myeloid leukemia (CML), acute myeloid leukemia (AML), and myelodysplastic syndromes (MDS). However, disease-associated mechanisms often restrain the proper functions of endogenous NK cells, leading to inadequate tumor control and risk for disease progression. Although allogeneic NK cells can prevent leukemia relapse in certain settings of stem cell transplantation, not all patients are eligible for this type of therapy. Moreover, remissions induced by adoptively infused NK cells are only transient and require subsequent therapy to maintain durable responses. Hence, new strategies are needed to trigger full and durable anti-leukemia responses by NK cells in patients with myeloid malignancies. To achieve this, we need to better understand the interplay between the malignant cells, their microenvironment, and the NK cells. This review focuses on mechanisms that are involved in suppressing NK cells in patients with myeloid leukemia and MDS, and means to restore their full anti-tumor potential. It also discusses novel molecular targets and approaches, such as bi- and tri-specific antibodies and immune checkpoint inhibitors, to redirect and/or unleash the NK cells against the leukemic cells.

Diminished AHR Signaling Drives Human Acute Myeloid Leukemia Stem Cell Maintenance

Eliminating leukemic stem cells (LSC) is a sought after therapeutic paradigm for the treatment of acute myeloid leukemia (AML). While repression of aryl hydrocarbon receptor (AHR) signaling has been shown to promote short-term maintenance of primitive AML cells in culture, no work to date has examined whether altered AHR signaling plays a pathologic role in human AML or whether it contributes at all to endogenous LSC function. Here, we show AHR signaling is repressed in human AML blasts and preferentially downregulated in LSC-enriched populations within leukemias. A core set of AHR targets are uniquely repressed in LSCs across diverse genetic AML subtypes. In vitro and in vivo administration of the specific AHR agonist FICZ significantly impaired leukemic growth, promoted differentiation, and repressed self-renewal. Furthermore, LSCs suppressed a set of FICZ-responsive AHR target genes that function as tumor suppressors and promoters of differentiation. FICZ stimulation did not impair normal hematopoietic stem and progenitor (HSPC) function, and failed to upregulate a prominent LSC-specific AHR target in HSPCs, suggesting that differential mechanisms govern FICZ-induced AHR signaling manifestations in HSCs versus LSCs. Altogether, this work highlights AHR signaling suppression as a key LSC-regulating control mechanism and provides proof of concept in a preclinical model that FICZ-mediated AHR pathway activation enacts unique transcriptional programs in AML that identify it as a novel chemotherapeutic approach to selectively target human LSCs. SIGNIFICANCE: The AHR pathway is suppressed in leukemic stem cells (LSC), therefore activating AHR signaling is a potential therapeutic option to target LSCs and to treat acute myeloid leukemia.

NK Cell Dysfunction and Checkpoint Immunotherapy

NK cells play important roles in the innate immune responses against tumors. The effector function of NK cells relies on the integration of activating and inhibitory signals. Emerging checkpoint receptors and molecules are being revealed to mediate NK cell dysfunction in the tumor microenvironment. Inhibition of some NK cell surface checkpoint receptors has displayed the potential to reverse NK cell dysfunction in tumors, and to boost anti-tumor immunity, both in clinical trials (anti-KIR and anti-NKG2A), and in preclinical studies (e.g., anti-TIGIT, and anti-CD96). To fully exploit the potential of NK-based checkpoint immunotherapy, more understanding of the regional features of NK cells in the tumor microenvironment is required. This will provide valuable information regarding the dynamic nature of NK cell immune response against tumors, as well as novel checkpoints or pathways to be targeted. In this Review, we discuss recent advances in the understanding of NK cell dysfunction in tumors, as well as emerging strategies of NK-based checkpoint immunotherapy for tumors.

ILCs in hematologic malignancies: Tumor cell killers and tissue healers

Innate lymphoid cells (ILCs) have attracted considerable attention in the past years. As modulators of epithelial barrier immunology and homeostasis they play important roles in (auto)immunity and inflammation. Here we review the role of ILCs in hematologic malignancies, where ILCs act as efficient killer cells and as tissue healers, in the context of chemotherapy, radiotherapy and after allogeneic hematopoietic stem cell transplantation (HSCT).

Role of microRNAs, circRNAs and long noncoding RNAs in acute myeloid leukemia

Acute myeloid leukemia (AML) is a malignant tumor of the immature myeloid hematopoietic cells in the bone marrow (BM). It is a highly heterogeneous disease, with rising morbidity and mortality in older patients. Although researches over the past decades have improved our understanding of AML, its pathogenesis has not yet been fully elucidated. Long noncoding RNAs (lncRNAs), microRNAs (miRNAs), and circular RNAs (circRNAs) are three noncoding RNA (ncRNA) molecules that regulate DNA transcription and translation. With the development of RNA-Seq technology, more and more ncRNAs that are closely related to AML leukemogenesis have been discovered. Numerous studies have found that these ncRNAs play an important role in leukemia cell proliferation, differentiation, and apoptosis. Some may potentially be used as prognostic biomarkers. In this systematic review, we briefly described the characteristics and molecular functions of three groups of ncRNAs, including lncRNAs, miRNAs, and circRNAs, and discussed their relationships with AML in detail.

Targeting immune cells for cancer therapy

Recent years have seen a renaissance in the research linking inflammation and cancer with immune cells playing a central role in smouldering inflammation in the tumor microenvironment. Diverse immune cell types infiltrate the tumor microenvironment, and the dynamic tumor-immune cell interplay gives rise to a rich milieu of cytokines and growth factors. Fundamentally, this intricate cross-talk creates the conducive condition for tumor cell proliferation, survival and metastasis. Interestingly, the prominent impact of immune cells is expounded in their contrary pro-tumoral role, as well as their potential anti-cancer cellular weaponry. The latter is known as immunotherapy, a concept born out of evidence that tumors are susceptible to immune defence and that by manipulating the immune system, tumor growth can be successfully restrained. Naturally, a deeper understanding of the multifaceted roles of various immune cell types thus contributes toward developing innovative anti-cancer strategies. Therefore, in this review we first outline the roles played by the major immune cell types, such as macrophages, neutrophils, natural killer cells, T cells and B cells. We then explain the recently-explored strategies of immunomodulation and discuss some important approaches via an immunology perspective.

[Aryl-hydrocarbon receptor as a potential target for anticancer therapy]

Aryl-hydrocarbon receptor (Aryl Hydrocarbon Receptor, AHR) is a ligand-dependent transcription factor, whose functions are related to xenobiotic detoxification, response to inflammation, and maintenance of tissue homeostasis. Recent investigations suggest that AHR also plays an important role in the processes of carcinogenesis. Increased expression of AHR is observed in several types of tumors and tumor cell lines. In addition, it turned out that the composition of pharmaceutical drugs used in oncotherapy includes some ligands AHR. These facts allow us to consider an aryl-hydrocarbon receptor as a potential target for anticancer therapy, especially for the treatment of severe cancers whose treatment options are very limited or do not exist at all. In this review the examples of AHR ligands' effect on tumor cell cultures and on model mice lines with AHR-dependent response are discussed.