Allogeneic hematopoietic cell transplantation (HCT) is a well-established therapeutic modality effective for a variety of hematological malignancies but unfortunately is associated with significant morbidity and mortality related to cancer relapse as well as to transplant related complications, including graft-versus-host-disease (GvHD). Natural killer (NK) cells are the first donor-derived lymphocyte subset to recover after HCT and their crucial role in protection against cancer relapse and infections is well established. Conversely, the role played by NK cells in GvHD is still controversial. Early studies suggested a participation of NK cells in GvHD induction or exacerbation. Subsequently, experimental evidence obtained in mice as well observational studies performed in humans led to a model in which NK cells play a regulatory role in GvHD by repressing alloreactive T cell responses. This widely accepted model has been recently challenged by clinical evidence indicating that NK cells can in some cases promote GvHD. In this review we summarize available knowledge about the role of NK cells in GVHD pathogenesis. We review studies uncovering cellular mechanisms through which NK cells interact with other immune cell subsets during GvHD leading to a model in which NK cells naturally suppress GvHD through their cytotoxic ability to inhibit T cell activation unless exogenous hyper-activation lead them to produce proinflammatory cytokines that can conversely sustain T cell-mediated GvHD induction.

Recent reports give insights into the role of the T-box transcription factors T-bet and Eomesodermin (Eomes) in NK cell biology. In this mini-review we recapitulate the initial reports that delineate T-bet and Eomes as master regulators of NK cell development, maturation and function. We discuss how T-bet and Eomes expression is regulated during NK cell development and peripheral maturation. Furthermore, we summarize the current literature on the role of T-bet and Eomes in the transcriptional regulation of NK cell function and review possible effects of T-box transcription factor anomalies during aging, infection, cancer and after hematopoietic stem cell transplantation. We discuss how the current data argue in favor of a model of T-bet and Eomes synergy in transcriptional regulation of NK cell function and identify T-box transcription factors as potential targets for therapeutic interventions.

Regulatory T cells (Treg) are commonly identified by CD25 (IL‐2Rα) surface expression and/or intracellular expression of the FOXP3 transcription factor. In addition, Treg are also characterized by low CD127 (IL‐7Rα) expression when compared to conventional T cells and their biology in the periphery is considered essentially independent of IL‐7. We further investigated CD127 expression on Treg and we demonstrated differential CD127 expression depending on Treg subsets considered. Notably, we observed high CD127 expression on inducible costimulatory molecule (ICOS)‐ and CD103‐expressing Treg subsets. Since these two markers reflect activation status, we addressed whether Treg activation modulated CD127 expression. We demonstrated that in contrast to conventional T cells, Treg significantly upregulated CD127 expression during in vitro and in vivo activation using adoptive transfer and contact dermatitis models. High CD127 expression on Treg was also predominantly detected ex vivo in some specific sites, notably bone marrow and skin. Importantly, higher CD127 expression on Treg correlated with higher phosphorylation of STAT5 upon IL‐7 exposure. High CD127 expression on Treg also provided survival advantage upon in vitro incubation with IL‐7. We thus demonstrated that low CD127 expression is not an intrinsic characteristic of Treg and we identified activated Treg as a potential target of endogenous or therapeutic IL‐7.

Although transcription factor CCAAT-enhancer binding protein α (C/EBPα) is critical for normal and leukemic differentiation, its role in cell and metabolic homeostasis is largely unknown in cancer. Here, multiomics analyses uncovered a coordinated activation of C/EBPα and Fms-like tyrosine kinase 3 (FLT3) that increased lipid anabolism in vivo and in patients with FLT3-mutant acute myeloid leukemia (AML). Mechanistically, C/EBPα regulated the fatty acid synthase (FASN)–stearoyl-CoA desaturase (SCD) axis to promote fatty acid (FA) biosynthesis and desaturation. We further demonstrated that FLT3 or C/EBPα inactivation decreased monounsaturated FA incorporation to membrane phospholipids through SCD downregulation. Consequently, SCD inhibition enhanced susceptibility to lipid redox stress that was exploited by combining FLT3 and glutathione peroxidase 4 inhibition to trigger lipid oxidative stress, enhancing ferroptotic death of FLT3-mutant AML cells. Altogether, our study reveals a C/EBPα function in lipid homeostasis and adaptation to redox stress, and a previously unreported vulnerability of FLT3-mutant AML to ferroptosis with promising therapeutic application. Significance: FLT3 mutations are found in 30% of AML cases and are actionable by tyrosine kinase inhibitors. Here, we discovered that C/EBPα regulates FA biosynthesis and protection from lipid redox stress downstream mutant-FLT3 signaling, which confers a vulnerability to ferroptosis upon FLT3 inhibition with therapeutic potential in AML. This article is highlighted in the In This Issue feature, p. 1501

NK cell exhaustion (NCE) due to sustained proliferation results in impaired NK cell function with loss of cytokine production and lytic activity. Using murine models of chronic NK cell stimulation, we have identified a phenotypic signature of NCE, characterized by upregulation of the terminal differentiation marker KLRG1 and by downregulation of eomesodermin and the activating receptor NKG2D. Chronic stimulation of mice lacking NKG2D resulted in minimal NCE compared with control mice, thus identifying NKG2D as a crucial mediator of NCE. NKG2D internalization and downregulation on NK cells have been previously observed in the presence of tumor cells with high expression of NKG2D ligands (NKG2DL) due to the activation of the DNA damage repair pathways. Interestingly, our study revealed that during NK cell activation, there is an increase of MULT1, an NKG2DL, that correlates with an induction of DNA damage. Treatment with the ATM DNA damage repair pathway inhibitor KU55933 (KU) during activation reduced NCE by improving expression of activation markers and genes involved in cell survival, through sustaining NKG2D expression and preserving cell functionality. Importantly, NK cells expanded ex vivo in the presence of KU displayed increased antitumor efficacy in both NKG2D-dependent and -independent mouse models. Collectively, these data demonstrate that NCE is caused by DNA damage and is regulated, at least in part, by NKG2D. Further, the prevention of NCE is a promising strategy to improve NK cell–based immunotherapy.

Immune checkpoint inhibitor (ICI) therapy has shown a significant benefit in the treatment of a variety of cancer entities. However, immune-related adverse events (irAEs) occur frequently and can lead to ICI treatment termination. MicroRNA-146a (miR-146a) has regulatory functions in immune cells. We observed that mice lacking miR-146a developed markedly more severe irAEs compared with WT mice in several irAE target organs in 2 different murine models. miR-146a ^–/– mice exhibited increased T cell activation and effector function upon ICI treatment. Moreover, neutrophil numbers in the spleen and the inflamed intestine were highly increased in ICI-treated miR-146a ^–/– mice. Therapeutic administration of a miR-146a mimic reduced irAE severity. To validate our preclinical findings in patients, we analyzed the effect of a SNP in the MIR146A gene on irAE severity in 167 patients treated with ICIs. We found that the SNP rs2910164 leading to reduced miR-146a expression was associated with an increased risk of developing severe irAEs, reduced progression-free survival, and increased neutrophil counts both at baseline and during ICI therapy. In conclusion, we characterized miR-146a as a molecular target for preventing ICI-mediated autoimmune dysregulation. Furthermore, we identified the MIR146A SNP rs2910164 as a biomarker to predict severe irAE development in ICI-treated patients.