Recently activated CD4 T cells in tuberculosis express OX40 as a target for host-directed immunotherapy.

After Mycobacterium tuberculosis (Mtb) infection, many effector T cells traffic to the lungs, but few become activated. Here we use an antigen receptor reporter mouse (Nur77-GFP) to identify recently activated CD4 T cells in the lungs. These Nur77-GFPHI cells contain expanded TCR clonotypes, have elevated expression of co-stimulatory genes such as Tnfrsf4/OX40, and are functionally more protective than Nur77-GFPLO cells. By contrast, Nur77-GFPLO cells express markers of terminal exhaustion and cytotoxicity, and the trafficking receptor S1pr5, associated with vascular localization. A short course of immunotherapy targeting OX40+ cells transiently expands CD4 T cell numbers and shifts their phenotype towards parenchymal protective cells. Moreover, OX40 agonist immunotherapy decreases the lung bacterial burden and extends host survival, offering an additive benefit to antibiotics. CD4 T cells from the cerebrospinal fluid of humans with HIV-associated tuberculous meningitis commonly express surface OX40 protein, while CD8 T cells do not. Our data thus propose OX40 as a marker of recently activated CD4 T cells at the infection site and a potential target for immunotherapy in tuberculosis.

Tumor eradicated by combination of imiquimod and OX40 agonist for in situ vaccination.

Various cancer vaccines have been developed to generate and amplify antigen-specific T cell responses against malignancy. Among them, in situ vaccination is one of the most practical types as it can trigger immune responses without previous antigen identification. Here we reported a novel in situ vaccine by intratumoral injection of imiquimod and OX40 agonist. In mice bearing hepatic carcinoma, both the injected tumor and the noninjected tumor in the distant lesion of the same mice were suppressed after vaccination. Further studies found that this in situ vaccine triggered systemic tumor-specific responses, with one-fold increase of effector memory T cells properties and stronger toxicity of lymphocytes in spleen. Besides, we found that imiquimod upregulated the expression of OX40 on CD4+ T cells and thus enhanced the effectiveness of OX40 agonist. Five immune-positive-related pathways were activated after vaccination. This in situ vaccine caused little harm to normal organs and provided long-term protection against the same syngeneic tumor rechallenge. Due to its effectiveness, feasibility and safety, this strategy could potentially be applied to various types of late-stage solid tumors and worthy of further clinical research.

Combined OX40 Agonist and PD-1 Inhibitor Immunotherapy Improves the Efficacy of Vascular Targeted Photodynamic Therapy in a Urothelial Tumor Model.

PURPOSE: Vascular targeted photodynamic therapy (VTP) is a nonsurgical tumor ablation approach used to treat early-stage prostate cancer and may also be effective for upper tract urothelial cancer (UTUC) based on preclinical data. Toward increasing response rates to VTP, we evaluated its efficacy in combination with concurrent PD-1 inhibitor/OX40 agonist immunotherapy in a urothelial tumor-bearing model. EXPERIMENTAL DESIGN: In mice allografted with MB-49 UTUC cells, we compared the effects of combined VTP with PD-1 inhibitor/OX40 agonist with those of the component treatments on tumor growth, survival, lung metastasis, and antitumor immune responses. RESULTS: The combination of VTP with both PD-1 inhibitor and OX40 agonist inhibited tumor growth and prolonged survival to a greater degree than VTP with either immunotherapeutic individually. These effects result from increased tumor infiltration and intratumoral proliferation of cytotoxic and helper T cells, depletion of Treg cells, and suppression of myeloid-derived suppressor cells. CONCLUSIONS: Our findings suggest that VTP synergizes with PD-1 blockade and OX40 agonist to promote strong antitumor immune responses, yielding therapeutic efficacy in an animal model of urothelial cancer.

TNF receptor agonists induce distinct receptor clusters to mediate differential agonistic activity.

Monoclonal antibodies (mAb) and natural ligands targeting costimulatory tumor necrosis factor receptors (TNFR) exhibit a wide range of agonistic activities and antitumor responses. The mechanisms underlying these differential agonistic activities remain poorly understood. Here, we employ a panel of experimental and clinically-relevant molecules targeting human CD40, 4-1BB and OX40 to examine this issue. Confocal and STORM microscopy reveal that strongly agonistic reagents induce clusters characterized by small area and high receptor density. Using antibody pairs differing only in isotype we show that hIgG2 confers significantly more receptor clustering than hIgG1 across all three receptors, explaining its greater agonistic activity, with receptor clustering shielding the receptor-agonist complex from further molecular access. Nevertheless, discrete receptor clustering patterns are observed with different hIgG2 mAb, with a unique rod-shaped assembly observed with the most agonistic mAb. These findings dispel the notion that larger receptor clusters elicit greater agonism, and instead point to receptor density and subsequent super-structure as key determinants.

OX40 agonists for cancer treatment: a patent review.

INTRODUCTION: OX40 is an immune checkpoint in cancer and its presence in cancer is a good prognosis, making it a highly relevant target for the development of new immunotherapies. AREAS COVERED: The patent literature reveals vital information on new trends in cancer therapies. The authors used the patent databases of the six major patent offices in the world: United States Patent and Trademark Office, European Patent Office, World Intellectual Property Organization, Japan Patent Office, State Office of Intellectual Property of China and Korean Intellectual Property Office, to generate a panorama of patents related to OX40 agonists. Specific patents have been grouped into innovative patents and adoption patents. EXPERT OPINION: An increasing trend in the development of OX40 agonists in cancer, particularly in the years 2018 and 2019. United States was the leader in generating patents, followed by China and England. Major pharmaceutical companies have at least one anti-OX40 agonist, MEDI6469 and MEDI-0562 (AstraZeneca), PF-04518600 (Pfizer), GSK3174998 (GlaxoSmithKline), BMS-986,178 (Bristol-Myers Squibb) and MOXR0916 (Roche), which represent 68% of clinical trials conducted with OX40 agonists.

Frontline Science: OX40 agonistic antibody reverses immune suppression and improves survival in sepsis.

A defining feature of protracted sepsis is development of immunosuppression that is thought to be a major driving force in the morbidity and mortality associated with the syndrome. The immunosuppression that occurs in sepsis is characterized by profound apoptosis-induced depletion of CD4 and CD8 T cells and severely impaired T cell function. OX40, a member of the TNF receptor superfamily, is a positive co-stimulatory molecule expressed on activated T cells. When engaged by OX40 ligand, OX40 stimulates T cell proliferation and shifts the cellular immune phenotype toward TH1 with increased production of cytokines that are essential for control of invading pathogens. The purpose of the present study was to determine if administration of agonistic Ab to OX40 could reverse sepsis-induced immunosuppression, restore T cell function, and improve survival in a clinically relevant animal model of sepsis. The present study demonstrates that OX40 agonistic Ab reversed sepsis-induced impairment of T cell function, increased T cell IFN-γ production, increased the number of immune effector cells, and improved survival in the mouse cecal ligation and puncture model of sepsis. Importantly, OX40 agonistic Ab was not only effective in murine sepsis but also improved T effector cell function in PBMCs from patients with sepsis. The present results provide support for the use of immune adjuvants that target T cell depletion and T cell dysfunction in the therapy of sepsis-induced immunosuppression. In addition to the checkpoint inhibitors anti-PD-1 and anti-PD-L1, OX40 agonistic Ab may be a new therapeutic approach to the treatment of this highly lethal disorder.

OX40 Agonist BMS-986178 Alone or in Combination With Nivolumab and/or Ipilimumab in Patients With Advanced Solid Tumors.

PURPOSE: This phase I/IIa study (NCT02737475) evaluated the safety and activity of BMS-986178, a fully human OX40 agonist IgG1 mAb, ± nivolumab and/or ipilimumab in patients with advanced solid tumors. PATIENTS AND METHODS: Patients (with non-small cell lung, renal cell, bladder, other advanced cancers) received BMS-986178 (20-320 mg) ± nivolumab (240-480 mg) and/or ipilimumab (1-3 mg/kg). The primary endpoint was safety. Additional endpoints included immunogenicity, pharmacodynamics, pharmacokinetics, and antitumor activity per RECIST version 1.1. RESULTS: Twenty patients received BMS-986178 monotherapy, and 145 received combination therapy in various regimens (including two patients receiving nivolumab monotherapy). With a follow-up of 1.1 to 103.6 weeks, the most common (≥5%) treatment-related adverse events (TRAEs) included fatigue, pruritus, rash, pyrexia, diarrhea, and infusion-related reactions. Overall, grade 3-4 TRAEs occurred in one of 20 patients (5%) receiving BMS-986178 monotherapy, six of 79 (8%) receiving BMS-986178 plus nivolumab, zero of two receiving nivolumab monotherapy, six of 41 (15%) receiving BMS-986178 plus ipilimumab, and three of 23 (13%) receiving BMS-986178 plus nivolumab plus ipilimumab. No deaths occurred. No dose-limiting toxicities were observed with monotherapy, and the MTD was not reached in either the monotherapy or the combination escalation cohorts. No objective responses were seen with BMS-986178 alone; objective response rates ranged from 0% to 13% across combination therapy cohorts. CONCLUSIONS: In this study, BMS-986178 ± nivolumab and/or ipilimumab appeared to have a manageable safety profile, but no clear efficacy signal was observed above that expected for nivolumab and/or ipilimumab.

Bifunctional macromolecule activating both OX40 and interferon-α signaling displays potent therapeutic effects in mouse HBV and tumor models.

Combinatory enhancement of innate and adaptive immune responses is a promising strategy in immunotherapeutic drug development. Bifunctional macromolecules that simultaneously target two mechanisms may provide additional advantages over the combination of targeting two single pathways. Interferon alpha (IFNα) has been used clinically against viral infection such as the chronic infection of hepatitis B virus (CHB) as well as some types of cancers. OX40 is a costimulatory immune checkpoint molecule involved in the activation of T lymphocytes. To test whether simultaneously activating IFNα and OX40 signaling pathway could produce a synergistic therapeutic effect on CHB and tumors, we designed a bifunctional fusion protein composed of a mouse OX40 agonistic monoclonal antibody (OX86) and a mouse IFNα4, joined by a flexible (GGGGS)3 linker. This fusion protein, termed OX86-IFN, can activate both IFNα and OX40. We demonstrated that OX86-IFN could effectively activate T lymphocytes in the peripheral blood of mice. Furthermore, we showed that OX86-IFN had superior therapeutic effect to monotherapies in HBV hydrodynamic transfection and syngeneic tumor models. Collectively, our data suggests that simultaneously targeting interferon and OX40 signaling pathways by bifunctional molecule OX86-IFN elicits potent antiviral and antitumor activities, which could provide a new strategy in developing therapeutic agents against viral infection and tumors.

A Novel Bispecific Antibody with PD-L1-assisted OX40 Activation for Cancer Treatment.

Immunotherapy using OX40 agonist antibodies shows great preclinical efficacy in mouse tumor models. But in a clinical setting, OX40 agonist antibody alone or in combination with checkpoint blockade exhibits only modest efficacy due to lack of sufficient activation. We hypothesized that the limited antitumor activity in patients may due to insufficient clustering of OX40 antibody in the tumor. To test this hypothesis, we generated a tetravalent programmed death ligand-1 (PD-L1)/OX40 BsAb by fusing two PD-L1 VHH fragments to the C-terminus of a nonblocking agonistic anti-OX40 antibody. The resulting BsAb had intact function of each parental antibody, including efficiently blocking PD1/PD-L1 interaction and inducing OX40 activation. In addition, this BsAb showed significantly enhanced potency in activation of OX40-expressing T cells when PD-L1-expressing tumor cells or dendrite cells were present, through PD-L1-mediated cross-linking of OX40. Moreover, the BsAb exhibited superior antitumor activities over the parental monospecific antibodies alone or in combination in multiple in vivo tumor models. These results demonstrated a great potential for further clinical development of the potent immunostimulatory PD-L1/OX40 bispecific antibody.

Immune checkpoint modulation enhances HIV-1 antibody induction.

Eliciting protective titers of HIV-1 broadly neutralizing antibodies (bnAbs) is a goal of HIV-1 vaccine development, but current vaccine strategies have yet to induce bnAbs in humans. Many bnAbs isolated from HIV-1-infected individuals are encoded by immunoglobulin gene rearrangments with infrequent naive B cell precursors and with unusual genetic features that may be subject to host regulatory control. Here, we administer antibodies targeting immune cell regulatory receptors CTLA-4, PD-1 or OX40 along with HIV envelope (Env) vaccines to rhesus macaques and bnAb immunoglobulin knock-in (KI) mice expressing diverse precursors of CD4 binding site HIV-1 bnAbs. CTLA-4 blockade augments HIV-1 Env antibody responses in macaques, and in a bnAb-precursor mouse model, CTLA-4 blocking or OX40 agonist antibodies increase germinal center B and T follicular helper cells and plasma neutralizing antibodies. Thus, modulation of CTLA-4 or OX40 immune checkpoints during vaccination can promote germinal center activity and enhance HIV-1 Env antibody responses.

An Integrative Approach to Inform Optimal Administration of OX40 Agonist Antibodies in Patients with Advanced Solid Tumors.

PURPOSE: The success of checkpoint blockade has led to a significant increase in the development of a broad range of immunomodulatory molecules for the treatment of cancer, including agonists against T-cell costimulatory receptors, such as OX40. Unlike checkpoint blockade, where complete and sustained receptor saturation may be required for maximal activity, the optimal dosing regimen and receptor occupancy for agonist agents is less well understood and requires further study. EXPERIMENTAL DESIGN: We integrated both preclinical and clinical biomarker data sets centered on dose, exposure, receptor occupancy, receptor engagement, and downstream pharmacodynamic changes to model the optimal dose and schedule for the OX40 agonist antibody BMS-986178 alone and in combination with checkpoint blockade. RESULTS: Administration of the ligand-blocking anti-mouse surrogate antibody OX40.23 or BMS-986178 as monotherapy or in combination with checkpoint blockade led to increased peripheral CD4+ and CD8+ T-cell activation in tumor-bearing mice and patients with solid tumors, respectively. OX40 receptor occupancy between 20% and 50% both in vitro and in vivo was associated with maximal enhancement of T-cell effector function by anti-OX40 treatment, whereas a receptor occupancy > 40% led to a profound loss in OX40 receptor expression, with clear implications for availability for repeat dosing. CONCLUSIONS: Our results highlight the value of an integrated translational approach applied during early clinical development to aggregate preclinical and clinical data in an effort to define the optimal dose and schedule for T-cell agonists in the clinic.

A mathematical model of combined CD8 T cell costimulation by 4-1BB (CD137) and OX40 (CD134) receptors.

Combined agonist stimulation of the TNFR costimulatory receptors 4-1BB (CD137) and OX40(CD134) has been shown to generate supereffector CD8 T cells that clonally expand to greater levels, survive longer, and produce a greater quantity of cytokines compared to T cells stimulated with an agonist of either costimulatory receptor individually. In order to understand the mechanisms for this effect, we have created a mathematical model for the activation of the CD8 T cell intracellular signaling network by mono- or dual-costimulation. We show that supereffector status is generated via downstream interacting pathways that are activated upon engagement of both receptors, and in silico simulations of the model are supported by published experimental results. The model can thus be used to identify critical molecular targets of T cell dual-costimulation in the context of cancer immunotherapy.

Cancer combinatorial immunotherapy using anti-OX40 agonist and anti-PD-L1 antagonist: a patent evaluation of US2018256711A1.

Introduction: OX40 is checkpoint inhibitor in cancer that coordinates the downregulation of the proliferation of antigen-specific lymphocytes. There is a great need to discover and develop new therapies focused on inhibiting the action of OX40 and consequently improving the immune response in the various types of cancer. Authors of patent US2018256711A1 propose a method to eradicate cancer that utilizes anti-OX40 agonist antibody in combination with anti-PD-L1 antagonist antibody. Areas covered: Patent US2018256711A1 describes a method of cancer combinatorial treatment consisting of the utilization of a pharmaceutical cocktail containing anti-OX40 and an anti-PD-L1 antibody. Expert opinion: The results of the clinical trials only support trials regarding the tolerability of combinatorial therapy, even when the objectives of determining the safety and pharmacokinetics of the treatment are proposed.

The CTLA-4 x OX40 bispecific antibody ATOR-1015 induces anti-tumor effects through tumor-directed immune activation.

BACKGROUND: The CTLA-4 blocking antibody ipilimumab has demonstrated substantial and durable effects in patients with melanoma. While CTLA-4 therapy, both as monotherapy and in combination with PD-1 targeting therapies, has great potential in many indications, the toxicities of the current treatment regimens may limit their use. Thus, there is a medical need for new CTLA-4 targeting therapies with improved benefit-risk profile. METHODS: ATOR-1015 is a human CTLA-4 x OX40 targeting IgG1 bispecific antibody generated by linking an optimized version of the Ig-like V-type domain of human CD86, a natural CTLA-4 ligand, to an agonistic OX40 antibody. In vitro evaluation of T-cell activation and T regulatory cell (Treg) depletion was performed using purified cells from healthy human donors or cell lines. In vivo anti-tumor responses were studied using human OX40 transgenic (knock-in) mice with established syngeneic tumors. Tumors and spleens from treated mice were analyzed for CD8+ T cell and Treg frequencies, T-cell activation markers and tumor localization using flow cytometry. RESULTS: ATOR-1015 induces T-cell activation and Treg depletion in vitro. Treatment with ATOR-1015 reduces tumor growth and improves survival in several syngeneic tumor models, including bladder, colon and pancreas cancer models. It is further demonstrated that ATOR-1015 induces tumor-specific and long-term immunological memory and enhances the response to PD-1 inhibition. Moreover, ATOR-1015 localizes to the tumor area where it reduces the frequency of Tregs and increases the number and activation of CD8+ T cells. CONCLUSIONS: By targeting CTLA-4 and OX40 simultaneously, ATOR-1015 is directed to the tumor area where it induces enhanced immune activation, and thus has the potential to be a next generation CTLA-4 targeting therapy with improved clinical efficacy and reduced toxicity. ATOR-1015 is also expected to act synergistically with anti-PD-1/PD-L1 therapy. The pre-clinical data support clinical development of ATOR-1015, and a first-in-human trial has started (NCT03782467).

Chronic retroviral infection of mice promotes tumor development, but CD137 agonist therapy restores effective tumor immune surveillance.

T cell responses are crucial for anti-tumor immunity. In chronic viral infections, anti-tumor T cell responses can be compromised due to various immunological mechanisms, including T cell exhaustion. To study mechanisms of anti-tumor immunity during a chronic viral infection, we made use of the well-established Friend virus (FV) mouse model. Chronically FV-infected mice are impaired in their ability to reject FBL-3 cells-a virus-induced tumor cell line of C57BL/6 origin. Here we aimed to explore therapeutic strategies to overcome the influence of T cell exhaustion during chronic viral infection, and reactivate effector CD8+ and CD4+ T cells to eliminate tumor cells. For T cell stimulation, agonistic antibodies against the tumor necrosis factor receptor (TNFR) superfamily members CD137 and CD134 were used, because they were reported to augment the cytotoxic program of T cells. αCD137 agonistic therapy, but not αCD134 agonistic therapy, resulted in FBL-3 tumor elimination in chronically FV-infected mice. CD137 stimulation significantly enhanced the cytotoxic activity of both CD4+ and CD8+ T cells, which were both required for efficient tumor control. Our study suggests that agonistic antibodies to CD137 can efficiently enhance anti-tumor immunity even in the setting of chronic viral infection, which might have promising therapeutic applications.

Late-Stage Tumor Regression after PD-L1 Blockade Plus a Concurrent OX40 Agonist.

The protective capability of tumor antigen-specific T cells is regulated by costimulatory and inhibitory signals. Current approaches in cancer immunotherapy seek to restore the function of unresponsive T cells by blocking inhibitory pathways. In contrast, providing exogenous costimulatory signals to T cells also enhances antitumor functionality. By combining these two clinical approaches, we demonstrate the synergy of targeting PD-L1 together with the costimulatory molecule OX40, to enhance antitumor immunity. Concurrently blocking PD-L1 and providing a costimulatory agonist to OX40 increased the presence and functionality of tumor antigen-specific CD8+ T cells with simultaneous enhancement of T-helper type 1 (Th1)-skewed CD4+ T cells. This shift was functionally supported by increased glucose metabolism of antigen-specific CD8+ T cells and the acquisition of granzyme B by regulatory T cells. Together, this mechanism promoted tumor regression of late-stage tumors beyond that achieved by either blockade as monotherapy. These findings indicate that targeting both T-cell intrinsic (OX40) and extrinsic (PD-L1) regulatory molecules increases the bioenergetic potential of T cells, thereby expanding functional and tumor antigen-specific T cells.

Combining STING-based neoantigen-targeted vaccine with checkpoint modulators enhances antitumor immunity in murine pancreatic cancer.

Tumor neoantigens arising from somatic mutations in the cancer genome are less likely to be subject to central immune tolerance and are therefore attractive targets for vaccine immunotherapy. We utilized whole-exome sequencing, RNA sequencing (RNASeq), and an in silico immunogenicity prediction algorithm, NetMHC, to generate a neoantigen-targeted vaccine, PancVAX, which was administered together with the STING adjuvant ADU-V16 to mice bearing pancreatic adenocarcinoma (Panc02) cells. PancVAX activated a neoepitope-specific T cell repertoire within the tumor and caused transient tumor regression. When given in combination with two checkpoint modulators, namely anti-PD-1 and agonist OX40 antibodies, PancVAX resulted in enhanced and more durable tumor regression and a survival benefit. The addition of OX40 to vaccine reduced the coexpression of T cell exhaustion markers, Lag3 and PD-1, and resulted in rejection of tumors upon contralateral rechallenge, suggesting the induction of T cell memory. Together, these data provide the framework for testing personalized neoantigen-based combinatorial vaccine strategies in patients with pancreatic and other nonimmunogenic cancers.

Could OX40 agonist antibody promote activation of the anti-tumor immune response in gastric cancer?

BACKGROUND AND OBJECTIVES: OX40, a membrane-bound molecule of the tumor-necrosis-factor-receptor superfamily, is a critical costimulatory receptor during the immune response, especially to T cells, but studies described their presence of OX-40 on neutrophils and monocytes, suggesting a potential role in the activation of immune response. Our aim was to characterize costimulatory receptors OX40 expression on circulating leukocytes in gastric cancer to identify novel targets for immunotherapy. METHODS: Peripheral blood mononuclear cells were isolated from 24 gastric cancer patients and 34 healthy controls and the expression of costimulatory (OX40) receptors were analyzed on T cells, neutrophil and monocyte using monoclonal antibodies by flow cytometry. RESULTS: We found that the higher levels of OX40 + T cells, monocytes/OX40+ and neutrophils/OX40+ from gastric cancer patients when compared to controls (P < 0.0001), and also higher levels of OX40+ T cells when compared to stages III and IV (P = 0.02). Percentage levels of total T cells were similar between patients and controls. CONCLUSIONS: OX40 as a therapeutic agent has been investigated in many preclinical tumor models. Our findings suggest that of levels of costimulatory in T cells in GC will direct future studies on the role that costimulatory receptors play in the failure of T cell-mediated immunity.

Potent Immune Modulation by MEDI6383, an Engineered Human OX40 Ligand IgG4P Fc Fusion Protein.

Ligation of OX40 (CD134, TNFRSF4) on activated T cells by its natural ligand (OX40L, CD252, TNFSF4) enhances cellular survival, proliferation, and effector functions such as cytokine release and cellular cytotoxicity. We engineered a recombinant human OX40L IgG4P Fc fusion protein termed MEDI6383 that assembles into a hexameric structure and exerts potent agonist activity following engagement of OX40. MEDI6383 displayed solution-phase agonist activity that was enhanced when the fusion protein was clustered by Fc gamma receptors (FcγRs) on the surface of adjacent cells. The resulting costimulation of OX40 on T cells induced NFκB promoter activity in OX40-expressing T cells and induced Th1-type cytokine production, proliferation, and resistance to regulatory T cell (Treg)-mediated suppression. MEDI6383 enhanced the cytolytic activity of tumor-reactive T cells and reduced tumor growth in the context of an alloreactive human T cell:tumor cell admix model in immunocompromised mice. Consistent with the role of OX40 costimulation in the expansion of memory T cells, MEDI6383 administered to healthy nonhuman primates elicited peripheral blood CD4 and CD8 central and effector memory T-cell proliferation as well as B-cell proliferation. Together, these results suggest that OX40 agonism has the potential to enhance antitumor immunity in human malignancies. Mol Cancer Ther; 17(5); 1024-38. ©2018 AACR.

The Immune Checkpoint Modulator OX40 and Its Ligand OX40L in NK-Cell Immunosurveillance and Acute Myeloid Leukemia.

The TNF receptor family member OX40 promotes activation and proliferation of T cells, which fuels efforts to modulate this immune checkpoint to reinforce antitumor immunity. Besides T cells, NK cells are a second cytotoxic lymphocyte subset that contributes to antitumor immunity, particularly in leukemia. Accordingly, these cells are being clinically evaluated for cancer treatment through multiple approaches, such as adoptive transfer of ex vivo expanded polyclonal NK cells (pNKC). Here, we analyzed whether and how OX40 and its ligand (OX40L) influence NK-cell function and antileukemia reactivity. We report that OX40 is expressed on leukemic blasts in a substantial percentage of patients with acute myeloid leukemia (AML) and that OX40 can, after stimulation with agonistic OX40 antibodies, mediate proliferation and release of cytokines that act as growth and survival factors for the leukemic cells. We also demonstrate that pNKC differentially express OX40L, depending on the protocol used for their generation. OX40L signaling promoted NK-cell activation, cytokine production, and cytotoxicity, and disruption of OX40-OX40L interaction impaired pNKC reactivity against primary AML cells. Together, our data implicate OX40/OX40L in disease pathophysiology of AML and in NK-cell immunosurveillance. Our findings indicate that effects of the OX40-OX40L receptor-ligand system in other immune cell subsets and also malignant cells should be taken into account when developing OX40-targeted approaches for cancer immunotherapy. Cancer Immunol Res; 6(2); 209-21. ©2018 AACR.