BTK inhibition potentiates anti-PD-L1 treatment in murine melanoma: potential role for MDSC modulation in immunotherapy.

Myeloid-derived suppressor cells (MDSC) have been linked to loss of immune effector cell function through a variety of mechanisms such as the generation of reactive oxygen and nitrogen species and the production of inhibitory cytokines. Our group has shown that signaling through Bruton's tyrosine kinase (BTK) is important for MDSC function. Ibrutinib is an orally administered targeted agent that inhibits BTK activation and is currently used for the treatment of B cell malignancies. Using a syngeneic murine model of melanoma, the effect of BTK inhibition with ibrutinib on the therapeutic response to systemic PD-L1 blockade was studied. BTK was expressed by murine MDSC and their activation was inhibited by ibrutinib. Ibrutinib was not directly cytotoxic to cancer cells in vitro, but it inhibited BTK activation in MDSC and reduced expression of inducible nitric oxide synthase (NOS2) and production of nitric oxide. Ibrutinib treatments decreased the levels of circulating MDSC in vivo and increased the therapeutic efficacy of anti-PD-L1 antibody treatment. Gene expression profiling showed that ibrutinib decreased Cybb (NOX2) signaling, and increased IL-17 signaling (upregulating downstream targets Mmp9, Ptgs2, and S100a8). These results suggest that further exploration of MDSC inhibition could enhance the immunotherapy of advanced melanoma.PrécisInhibition of Bruton's tyrosine kinase, a key enzyme in myeloid cellular function, improves therapeutic response to an anti-PD-L1 antibody in an otherwise fairly resistant murine melanoma model.

Education-dependent activation of glycolysis promotes the cytolytic potency of licensed human natural killer cells.

BACKGROUND: The mechanism by which natural killer (NK) cell education results in licensed NK cells with heightened effector function against missing self-targets is not known. OBJECTIVE: We sought to identify potential mechanisms of enhanced function in licensed human NK cells. METHODS: We used expanded human NK cells from killer immunoglobulin-like receptor (KIR)/HLA-genotyped donors sorted for single-KIR+ cells to generate pure populations of licensed and unlicensed NK cells. We performed proteomic and gene expression analysis of these cells before and after receptor cross-linking and performed functional and metabolic analysis before and after interference with selected metabolic pathways. We verified key findings using freshly isolated and sorted NK cells from peripheral blood. RESULTS: We confirmed that licensed human NK cells are greater in number in peripheral blood and proliferate more in vitro than unlicensed NK cells. Using high-throughput protein analysis, we found that unstimulated licensed NK cells have increased expression of the glycolytic enzyme pyruvate kinase muscle isozyme M2 and after KIR cross-linking have increased phosphorylation of the metabolic modulators p38-α and 5' adenosine monophosphate-activated protein kinase α. After cytokine expansion and activation, unlicensed NK cells depended solely on mitochondrial respiration for cytolytic function, whereas licensed NK cells demonstrated metabolic reprogramming toward glycolysis and mitochondrial-dependent glutaminolysis, leading to accumulation of glycolytic metabolites and depletion of glutamate. As such, blocking both glycolysis and mitochondrial-dependent respiration was required to suppress the cytotoxicity of licensed NK cells. CONCLUSIONS: Collectively, our data support an arming model of education in which enhanced glycolysis in licensed NK cells supports proliferative and cytotoxic capacity.

Circulating myeloid-derived suppressor cells increase in patients undergoing neo-adjuvant chemotherapy for breast cancer.

This study sought to evaluate whether myeloid-derived suppressor cells (MDSC) could be affected by chemotherapy and correlate with pathologic complete response (pCR) in breast cancer patients receiving neo-adjuvant chemotherapy. Peripheral blood levels of granulocytic (G-MDSC) and monocytic (M-MDSC) MDSC were measured by flow cytometry prior to cycle 1 and 2 of doxorubicin and cyclophosphamide and 1st and last administration of paclitaxel or paclitaxel/anti-HER2 therapy. Of 24 patients, 11, 6 and 7 patients were triple negative, HER2+ and hormone receptor+, respectively. 45.8% had pCR. Mean M-MDSC% were <1. Mean G-MDSC% and 95% confidence intervals were 0.88 (0.23-1.54), 5.07 (2.45-7.69), 9.32 (4.02-14.61) and 1.97 (0.53-3.41) at draws 1-4. The increase in G-MDSC by draw 3 was significant (p < 0.0001) in all breast cancer types. G-MDSC levels at the last draw were numerically lower in patients with pCR (1.15; 95% CI 0.14-2.16) versus patients with no pCR (2.71; 95% CI 0-5.47). There was no significant rise in G-MDSC from draw 1 to 3 in African American patients, and at draw 3 G-MDSC levels were significantly lower in African Americans versus Caucasians (p < 0.05). It was concluded that G-MDSC% increased during doxorubicin and cyclophosphamide therapy, but did not significantly differ between patients based on pathologic complete response.

The Raf Kinase Inhibitor Sorafenib Inhibits JAK-STAT Signal Transduction in Human Immune Cells.

Sorafenib is an oral multikinase inhibitor that was originally developed as a Raf kinase inhibitor. We hypothesized that sorafenib would also have inhibitory effects on cytokine signaling pathways in immune cells. PBMCs from normal donors were treated with varying concentrations of sorafenib and stimulated with IFN-α or IL-2. Phosphorylation of STAT1 and STAT5 was measured by flow cytometry and confirmed by immunoblot analysis. Changes in IFN-α- and IL-2-stimulated gene expression were measured by quantitative PCR, and changes in cytokine production were evaluated by ELISA. Cryopreserved PBMCs were obtained from cancer patients before and after receiving 400 mg sorafenib twice daily. Patient PBMCs were thawed, stimulated with IL-2 or IFN-α, and evaluated for phosphorylation of STAT1 and STAT5. Pretreatment of PBMCs with 10 µM sorafenib decreased STAT1 and STAT5 phosphorylation after treatment with IFN-α or IL-2. This inhibitory effect was observed in PBMCs from healthy donors over a range of concentrations of sorafenib (5-20 µM), IL-2 (2-24 nM), and IFN-α (10(1)-10(6) U/ml). This effect was observed in immune cell subsets, including T cells, B cells, NK cells, regulatory T cells, and myeloid-derived suppressor cells. Pretreatment with sorafenib also inhibited PBMC expression of IFN-α- and IL-2-regulated genes and inhibited NK cell production of IFN-γ, RANTES, MIP1-α, and MIG in response to IFN-α stimulation. PBMCs from patients receiving sorafenib therapy showed decreased responsiveness to IL-2 and IFN-α treatment. Sorafenib is a Raf kinase inhibitor that could have off-target effects on cytokine-induced signal transduction in immune effector cells.

Signaling pathways involved in MDSC regulation.

The immune system has evolved mechanisms to protect the host from the deleterious effects of inflammation. The generation of immune suppressive cells like myeloid derived suppressor cells (MDSCs) that can counteract T cell responses represents one such strategy. There is an accumulation of immature myeloid cells or MDSCs in bone marrow (BM) and lymphoid organs under pathological conditions such as cancer. MDSCs represent a population of heterogeneous myeloid cells comprising of macrophages, granulocytes and dendritic cells that are at early stages of development. Although, the precise signaling pathways and molecular mechanisms that lead to MDSC generation and expansion in cancer remains to be elucidated. It is widely believed that perturbation of signaling pathways involved during normal hematopoietic and myeloid development under pathological conditions such as tumorogenesis contributes to the development of suppressive myeloid cells. In this review we discuss the role played by key signaling pathways such as PI3K, Ras, Jak/Stat and TGFb during myeloid development and how their deregulation under pathological conditions can lead to the generation of suppressive myeloid cells or MDSCs. Targeting these pathways should help in elucidating mechanisms that lead to the expansion of MDSCs in cancer and point to methods for eliminating these cells from the tumor microenvironment.

E2f1-3 switch from activators in progenitor cells to repressors in differentiating cells.

In the established model of mammalian cell cycle control, the retinoblastoma protein (Rb) functions to restrict cells from entering S phase by binding and sequestering E2f activators (E2f1, E2f2 and E2f3), which are invariably portrayed as the ultimate effectors of a transcriptional program that commit cells to enter and progress through S phase. Using a panel of tissue-specific cre-transgenic mice and conditional E2f alleles we examined the effects of E2f1, E2f2 and E2f3 triple deficiency in murine embryonic stem cells, embryos and small intestines. We show that in normal dividing progenitor cells E2f1-3 function as transcriptional activators, but contrary to the current view, are dispensable for cell division and instead are necessary for cell survival. In differentiating cells E2f1-3 function in a complex with Rb as repressors to silence E2f targets and facilitate exit from the cell cycle. The inactivation of Rb in differentiating cells resulted in a switch of E2f1-3 from repressors to activators, leading to the superactivation of E2f responsive targets and ectopic cell divisions. Loss of E2f1-3 completely suppressed these phenotypes caused by Rb deficiency. This work contextualizes the activator versus repressor functions of E2f1-3 in vivo, revealing distinct roles in dividing versus differentiating cells and in normal versus cancer-like cell cycles.

Mouse development with a single E2F activator.

The E2F family is conserved from Caenorhabditis elegans to mammals, with some family members having transcription activation functions and others having repressor functions. Whereas C. elegans and Drosophila melanogaster have a single E2F activator protein and repressor protein, mammals have at least three activator and five repressor proteins. Why such genetic complexity evolved in mammals is not known. To begin to evaluate this genetic complexity, we targeted the inactivation of the entire subset of activators, E2f1, E2f2, E2f3a and E2f3b, singly or in combination in mice. We demonstrate that E2f3a is sufficient to support mouse embryonic and postnatal development. Remarkably, expression of E2f3b or E2f1 from the E2f3a locus (E2f3a(3bki) or E2f3a(1ki), respectively) suppressed all the postnatal phenotypes associated with the inactivation of E2f3a. We conclude that there is significant functional redundancy among activators and that the specific requirement for E2f3a during postnatal development is dictated by regulatory sequences governing its selective spatiotemporal expression and not by its intrinsic protein functions. These findings provide a molecular basis for the observed specificity among E2F activators during development.

E2f1-3 are critical for myeloid development.

Hematopoietic development involves the coordinated activity of differentiation and cell cycle regulators. In current models of mammalian cell cycle control, E2f activators (E2f1, E2f2, and E2f3) are portrayed as the ultimate transcriptional effectors that commit cells to enter and progress through S phase. Using conditional gene knock-out strategies, we show that E2f1-3 are not required for the proliferation of early myeloid progenitors. Rather, these E2fs are critical for cell survival and proliferation at two distinct steps of myeloid development. First, E2f1-3 are required as transcriptional repressors for the survival of CD11b(+) myeloid progenitors, and then they are required as activators for the proliferation of CD11b(+) macrophages. In bone marrow macrophages, we show that E2f1-3 respond to CSF1-Myc mitogenic signals and serve to activate E2f target genes and promote their proliferation. Together, these findings expose dual functions for E2f1-3 at distinct stages of myeloid development in vivo, first as repressors in cell survival and then as activators in cell proliferation. In summary, this work places E2f1-3 in a specific signaling cascade that is critical for myeloid development in vivo.

Decrease post-transplant relapse using donor-derived expanded NK-cells.

In this phase I/II clinical trial, we investigated the safety and efficacy of high doses of mb-IL21 ex vivo expanded donor-derived NK cells to decrease relapse in 25 patients with myeloid malignancies receiving haploidentical stem-cell transplantation (HSCT). Three doses of donor NK cells (1 × 105-1 × 108 cells/kg/dose) were administered on days -2, +7, and +28. Results were compared with an independent contemporaneously treated case-matched cohort of 160 patients from the CIBMTR database.After a median follow-up of 24 months, the 2-year relapse rate was 4% vs. 38% (p = 0.014), and disease-free survival (DFS) was 66% vs. 44% (p = 0.1) in the cases and controls, respectively. Only one relapse occurred in the study group, in a patient with the high level of donor-specific anti-HLA antibodies (DSA) presented before transplantation. The 2-year relapse and DFS in patients without DSA was 0% vs. 40% and 72% vs. 44%, respectively with HR for DFS in controls of 2.64 (p = 0.029). NK cells in recipient blood were increased at day +30 in a dose-dependent manner compared with historical controls, and had a proliferating, mature, highly cytotoxic, NKG2C+/KIR+ phenotype.Administration of donor-derived expanded NK cells after haploidentical transplantation was safe, associated with NK cell-dominant immune reconstitution early post-transplant, preserved T-cell reconstitution, and improved relapse and DFS. TRIAL REGISTRATION: NCT01904136 ( https://clinicaltrials.gov/ct2/show/NCT01904136 ).

Defining the AHR-regulated transcriptome in NK cells reveals gene expression programs relevant to development and function.

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that regulates cellular processes in cancer and immunity, including innate immune cell development and effector function. However, the transcriptional repertoire through which AHR mediates these effects remains largely unexplored. To elucidate the transcriptional elements directly regulated by AHR in natural killer (NK) cells, we performed RNA and chromatin immunoprecipitation sequencing on NK cells exposed to AHR agonist or antagonist. We show that mature peripheral blood NK cells lack AHR, but its expression is induced by Stat3 during interleukin-21-driven activation and proliferation, coincident with increased NCAM1 (CD56) expression resulting in a CD56bright phenotype. Compared with control conditions, NK cells expanded in the presence of the AHR antagonist, StemRegenin-1, were unaffected in proliferation or cytotoxicity, had no increase in NCAM1 transcription, and maintained the CD56dim phenotype. However, it showed altered expression of 1004 genes including those strongly associated with signaling pathways. In contrast, NK cells expanded in the presence of the AHR agonist, kynurenine, showed decreased cytotoxicity and altered expression of 97 genes including those strongly associated with oxidative stress and cellular metabolism. By overlaying these differentially expressed genes with AHR chromatin binding, we identified 160 genes directly regulated by AHR, including hallmark AHR targets AHRR and CYP1B1 and known regulators of phenotype, development, metabolism, and function such as NCAM1, KIT, NQO1, and TXN. In summary, we define the AHR transcriptome in NK cells, propose a model of AHR and Stat3 coregulation, and identify potential pathways that may be targeted to overcome AHR-mediated immune suppression.

E2f1, E2f2, and E2f3 control E2F target expression and cellular proliferation via a p53-dependent negative feedback loop.

E2F-mediated control of gene expression is believed to have an essential role in the control of cellular proliferation. Using a conditional gene-targeting approach, we show that the targeted disruption of the entire E2F activator subclass composed of E2f1, E2f2, and E2f3 in mouse embryonic fibroblasts leads to the activation of p53 and the induction of p53 target genes, including p21(CIP1). Consequently, cyclin-dependent kinase activity and retinoblastoma (Rb) phosphorylation are dramatically inhibited, leading to Rb/E2F-mediated repression of E2F target gene expression and a severe block in cellular proliferation. Inactivation of p53 in E2f1-, E2f2-, and E2f3-deficient cells, either by spontaneous mutation or by conditional gene ablation, prevented the induction of p21(CIP1) and many other p53 target genes. As a result, cyclin-dependent kinase activity, Rb phosphorylation, and E2F target gene expression were restored to nearly normal levels, rendering cells responsive to normal growth signals. These findings suggest that a critical function of the E2F1, E2F2, and E2F3 activators is in the control of a p53-dependent axis that indirectly regulates E2F-mediated transcriptional repression and cellular proliferation.

CpG island methylation in a mouse model of lymphoma is driven by the genetic configuration of tumor cells.

Hypermethylation of CpG islands is a common epigenetic alteration associated with cancer. Global patterns of hypermethylation are tumor-type specific and nonrandom. The biological significance and the underlying mechanisms of tumor-specific aberrant promoter methylation remain unclear, but some evidence suggests that this specificity involves differential sequence susceptibilities, the targeting of DNA methylation activity to specific promoter sequences, or the selection of rare DNA methylation events during disease progression. Using restriction landmark genomic scanning on samples derived from tissue culture and in vivo models of T cell lymphomas, we found that MYC overexpression gave rise to a specific signature of CpG island hypermethylation. This signature reflected gene transcription profiles and was detected only in advanced stages of disease. The further inactivation of the Pten, p53, and E2f2 tumor suppressors in MYC-induced lymphomas resulted in distinct and diagnostic CpG island methylation signatures. Our data suggest that tumor-specific DNA methylation in lymphomas arises as a result of the selection of rare DNA methylation events during the course of tumor development. This selection appears to be driven by the genetic configuration of tumor cells, providing experimental evidence for a causal role of DNA hypermethylation in tumor progression and an explanation for the tremendous epigenetic heterogeneity observed in the evolution of human cancers. The ability to predict genome-wide epigenetic silencing based on relatively few genetic alterations will allow for a more complete classification of tumors and understanding of tumor cell biology.

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.

Phase I study of intraventricular infusions of autologous ex vivo expanded NK cells in children with recurrent medulloblastoma and ependymoma.

BACKGROUND: Recurrent pediatric medulloblastoma and ependymoma have a grim prognosis. We report a first-in-human, phase I study of intraventricular infusions of ex vivo expanded autologous natural killer (NK) cells in these tumors, with correlative studies. METHODS: Twelve patients were enrolled, 9 received protocol therapy up to 3 infusions weekly, in escalating doses from 3 × 106 to 3 × 108 NK cells/m2/infusion, for up to 3 cycles. Cerebrospinal fluid (CSF) was obtained for cellular profile, persistence, and phenotypic analysis of NK cells. Radiomic characterization on pretreatment MRI scans was performed in 7 patients, to develop a non-invasive imaging-based signature. RESULTS: Primary objectives of NK cell harvest, expansion, release, and safety of 112 intraventricular infusions of NK cells were achieved in all 9 patients. There were no dose-limiting toxicities. All patients showed progressive disease (PD), except 1 patient showed stable disease for one month at end of study follow-up. Another patient had transient radiographic response of the intraventricular tumor after 5 infusions of NK cell before progressing to PD. At higher dose levels, NK cells increased in the CSF during treatment with repetitive infusions (mean 11.6-fold). Frequent infusions of NK cells resulted in CSF pleocytosis. Radiomic signatures were profiled in 7 patients, evaluating ability to predict upfront radiographic changes, although they did not attain statistical significance. CONCLUSIONS: This study demonstrated feasibility of production and safety of intraventricular infusions of autologous NK cells. These findings support further investigation of locoregional NK cell infusions in children with brain malignancies.

Generation of Knock-out Primary and Expanded Human NK Cells Using Cas9 Ribonucleoproteins.

CRISPR/Cas9 technology is accelerating genome engineering in many cell types, but so far, gene delivery and stable gene modification have been challenging in primary NK cells. For example, transgene delivery using lentiviral or retroviral transduction resulted in a limited yield of genetically-engineered NK cells due to substantial procedure-associated NK cell apoptosis. We describe here a DNA-free method for genome editing of human primary and expanded NK cells using Cas9 ribonucleoprotein complexes (Cas9/RNPs). This method allowed efficient knockout of the TGFBR2 and HPRT1 genes in NK cells. RT-PCR data showed a significant decrease in gene expression level, and a cytotoxicity assay of a representative cell product suggested that the RNP-modified NK cells became less sensitive to TGFß. Genetically modified cells could be expanded post-electroporation by stimulation with irradiated mbIL21-expressing feeder cells.

Nitric Oxide Production by Myeloid-Derived Suppressor Cells Plays a Role in Impairing Fc Receptor-Mediated Natural Killer Cell Function.

Purpose: mAbs are used to treat solid and hematologic malignancies and work in part through Fc receptors (FcRs) on natural killer cells (NK). However, FcR-mediated functions of NK cells from patients with cancer are significantly impaired. Identifying the mechanisms of this dysfunction and impaired response to mAb therapy could lead to combination therapies and enhance mAb therapy.Experimental Design: Cocultures of autologous NK cells and MDSC from patients with cancer were used to study the effect of myeloid-derived suppressor cells (MDSCs) on NK-cell FcR-mediated functions including antibody-dependent cellular cytotoxicity, cytokine production, and signal transduction in vitro Mouse breast cancer models were utilized to study the effect of MDSCs on antibody therapy in vivo and test the efficacy of combination therapies including a mAb and an MDSC-targeting agent.Results: MDSCs from patients with cancer were found to significantly inhibit NK-cell FcR-mediated functions including antibody-dependent cellular cytotoxicity, cytokine production, and signal transduction in a contact-independent manner. In addition, adoptive transfer of MDSCs abolished the efficacy of mAb therapy in a mouse model of pancreatic cancer. Inhibition of iNOS restored NK-cell functions and signal transduction. Finally, nonspecific elimination of MDSCs or inhibition of iNOS in vivo significantly improved the efficacy of mAb therapy in a mouse model of breast cancer.Conclusions: MDSCs antagonize NK-cell FcR-mediated function and signal transduction leading to impaired response to mAb therapy in part through nitric oxide production. Thus, elimination of MDSCs or inhibition of nitric oxide production offers a strategy to improve mAb therapy. Clin Cancer Res; 24(8); 1891-904. ©2018 AACR.

MICA-Expressing Monocytes Enhance Natural Killer Cell Fc Receptor-Mediated Antitumor Functions.

Natural killer (NK) cells are large granular lymphocytes that promote the antitumor response via communication with other cell types in the tumor microenvironment. Previously, we have shown that NK cells secrete a profile of immune stimulatory factors (e.g., IFNγ, MIP-1α, and TNFα) in response to dual stimulation with the combination of antibody (Ab)-coated tumor cells and cytokines, such as IL12. We now demonstrate that this response is enhanced in the presence of autologous monocytes. Monocyte enhancement of NK cell activity was dependent on cell-to-cell contact as determined by a Transwell assay. It was hypothesized that NK cell effector functions against Ab-coated tumor cells were enhanced via binding of MICA on monocytes to NK cell NKG2D receptors. Strategies to block MICA-NKG2D interactions resulted in reductions in IFNγ production. Depletion of monocytes in vivo resulted in decreased IFNγ production by murine NK cells upon exposure to Ab-coated tumor cells. In mice receiving trastuzumab and IL12 therapy, monocyte depletion resulted in significantly greater tumor growth in comparison to mock-depleted controls (P < 0.05). These data suggest that NK cell-monocyte interactions enhance NK cell antitumor activity in the setting of monoclonal Ab therapy for cancer. Cancer Immunol Res; 5(9); 778-89. ©2017 AACR.

A Phase I Trial to Evaluate Antibody-Dependent Cellular Cytotoxicity of Cetuximab and Lenalidomide in Advanced Colorectal and Head and Neck Cancer.

mAbs can induce antibody-dependent cellular cytotoxicity (ADCC) via the innate immune system's ability to recognize mAb-coated cancer cells and activate immune effector cells. Lenalidomide is an immunomodulatory agent with the capacity to stimulate immune cell cytokine production and ADCC activity. This phase I trial evaluated the combination of cetuximab with lenalidomide for the treatment of advanced colorectal and head and neck squamous cell cancers (HNSCC). This trial included patients with advanced colorectal cancer or HNSCC. Treatment consisted of cetuximab 500 mg/m(2) i.v. every two weeks with lenalidomide given orally days 1-21 on a 28-day cycle. Three dose levels of lenalidomide were evaluated (15, 20, 25 mg). Correlative studies included measurement of ADCC, FcγRIIIA polymorphism genotyping, measurement of serum cytokine levels, and flow cytometric analysis of immune cell subtypes. Twenty-two patients were enrolled (19 colorectal cancer, 3 HNSCC). Fatigue was the only dose-limiting toxicity. One partial response was observed and 8 patients had stable disease at least 12 weeks. The recommended phase II dose is cetuximab 500 mg/m(2) with lenalidomide 25 mg daily, days 1-21. Correlative studies demonstrated a dose-dependent increase in natural killer cytotoxic activity with increasing doses of lenalidomide. Cetuximab and lenalidomide were well tolerated. There was a lenalidomide dose-dependent increase in ADCC with higher activity in patients enrolled in cohort 3 than those enrolled in cohorts 1/2. Although response was not a primary endpoint, there was evidence of antitumor activity for the combination therapy. Further investigation of lenalidomide as an immunomodulator in solid tumors is warranted. Mol Cancer Ther; 15(9); 2244-50. ©2016 AACR.

Targeting myeloid-derived suppressor cells using a novel adenosine monophosphate-activated protein kinase (AMPK) activator.

Myeloid-derived suppressor cells (MDSC) are a heterogeneous population of early myeloid cells that accumulate in the blood and tumors of patients with cancer. MDSC play a critical role during tumor evasion and promote immune suppression through variety of mechanisms, such as the generation of reactive oxygen and nitrogen species (ROS and RNS) and cytokines. AMPactivated protein kinase (AMPK) is an evolutionarily conserved serine/threonine kinase that regulates energy homeostasis and metabolic stress. However, the role of AMPK in the regulation of MDSC function remains largely unexplored. This study was designed to investigate whether treatment of MDSC with OSU-53, a PPAR-inactive derivative that stimulates AMPK kinase, can modulate MDSC function. Our results demonstrate that OSU-53 treatment increases the phosphorylation of AMPK, significantly reduces nitric oxide production, inhibits MDSC migration, and reduces the levels of IL-6 in murine MDSC cell line (MSC2 cells). OSU53 treatment mitigated the immune suppressive functions of murine MDSC, promoting T-cell proliferation. Although OSU-53 had a modest effect on tumor growth in mice inoculated with EMT-6 cells, importantly, administration of OSU53 significantly (p < 0.05) reduced the levels of MDSC in the spleens and tumors. Furthermore, mouse MDSC from EMT-6 tumor-bearing mice and human MDSC isolated from melanoma patients treated with OSU-53 showed a significant reduction in the expression of immune suppressive genes iNOS and arginase. In summary, these results demonstrate a novel role of AMPK in the regulation of MDSC functions and provide a rationale of combining OSU-53 with immune checkpoint inhibitors to augment their response in cancer patients.

Myeloid-Derived Suppressor Cells Express Bruton's Tyrosine Kinase and Can Be Depleted in Tumor-Bearing Hosts by Ibrutinib Treatment.

Myeloid-derived suppressor cells (MDSC) are a heterogeneous group of immature myeloid cells that expand in tumor-bearing hosts in response to soluble factors produced by tumor and stromal cells. MDSC expansion has been linked to loss of immune effector cell function and reduced efficacy of immune-based cancer therapies, highlighting the MDSC population as an attractive therapeutic target. Ibrutinib, an irreversible inhibitor of Bruton's tyrosine kinase (BTK) and IL2-inducible T-cell kinase (ITK), is in clinical use for the treatment of B-cell malignancies. Here, we report that BTK is expressed by murine and human MDSCs, and that ibrutinib is able to inhibit BTK phosphorylation in these cells. Treatment of MDSCs with ibrutinib significantly impaired nitric oxide production and cell migration. In addition, ibrutinib inhibited in vitro generation of human MDSCs and reduced mRNA expression of indolamine 2,3-dioxygenase, an immunosuppressive factor. Treatment of mice bearing EMT6 mammary tumors with ibrutinib resulted in reduced frequency of MDSCs in both the spleen and tumor. Ibrutinib treatment also resulted in a significant reduction of MDSCs in wild-type mice bearing B16F10 melanoma tumors, but not in X-linked immunodeficiency mice (XID) harboring a BTK mutation, suggesting that BTK inhibition plays an important role in the observed reduction of MDSCs in vivo Finally, ibrutinib significantly enhanced the efficacy of anti-PD-L1 (CD274) therapy in a murine breast cancer model. Together, these results demonstrate that ibrutinib modulates MDSC function and generation, revealing a potential strategy for enhancing immune-based therapies in solid malignancies. Cancer Res; 76(8); 2125-36. ©2016 AACR.