Induction of durable remission by dual immunotherapy in SHIV-infected ART-suppressed macaques.

The eradication of the viral reservoir represents the major obstacle to the development of a clinical cure for established HIV-1 infection. Here, we demonstrate that the administration of N-803 (brand name Anktiva) and broadly neutralizing antibodies (bNAbs) results in sustained viral control after discontinuation of antiretroviral therapy (ART) in simian-human AD8 (SHIV-AD8)-infected, ART-suppressed rhesus macaques. N-803+bNAbs treatment induced immune activation and transient viremia but only limited reductions in the SHIV reservoir. Upon ART discontinuation, viral rebound occurred in all animals, which was followed by durable control in approximately 70% of all N-803+bNAb-treated macaques. Viral control was correlated with the reprogramming of CD8+ T cells by N-803+bNAb synergy. Thus, complete eradication of the replication-competent viral reservoir is likely not a prerequisite for the induction of sustained remission after discontinuation of ART.

Immunotherapeutic approach to reduce senescent cells and alleviate senescence-associated secretory phenotype in mice.

Accumulation of senescent cells (SNCs) with a senescence-associated secretory phenotype (SASP) has been implicated as a major source of chronic sterile inflammation leading to many age-related pathologies. Herein, we provide evidence that a bifunctional immunotherapeutic, HCW9218, with capabilities of neutralizing TGF-ß and stimulating immune cells, can be safely administered systemically to reduce SNCs and alleviate SASP in mice. In the diabetic db/db mouse model, subcutaneous administration of HCW9218 reduced senescent islet ß cells and SASP resulting in improved glucose tolerance, insulin resistance, and aging index. In naturally aged mice, subcutaneous administration of HCW9218 durably reduced the level of SNCs and SASP, leading to lower expression of pro-inflammatory genes in peripheral organs. HCW9218 treatment also reverted the pattern of key regulatory circadian gene expression in aged mice to levels observed in young mice and impacted genes associated with metabolism and fibrosis in the liver. Single-nucleus RNA Sequencing analysis further revealed that HCW9218 treatment differentially changed the transcriptomic landscape of hepatocyte subtypes involving metabolic, signaling, cell-cycle, and senescence-associated pathways in naturally aged mice. Long-term survival studies also showed that HCW9218 treatment improved physical performance without compromising the health span of naturally aged mice. Thus, HCW9218 represents a novel immunotherapeutic approach and a clinically promising new class of senotherapeutic agents targeting cellular senescence-associated diseases.

Immunotherapeutic HCW9218 augments anti-tumor activity of chemotherapy via NK cell-mediated reduction of therapy-induced senescent cells.

Therapy induced senescence (TIS) in tumors and TIS cancer cells secrete proinflammatory senescence-associated secretory phenotype (SASP) factors. SASP factors promote TIS cancer cells to re-enter the growth cycle with stemness characteristics, resulting in chemo-resistance and disease relapse. Herein, we show that the immunotherapeutic HCW9218, comprising transforming growth factor-ß (TGF-ß) receptor II and interleukin (IL)-15/IL-15 receptor α domains, enhances metabolic and cytotoxic activities of immune cells and reduces TIS tumor cells in vivo to improve the efficacy of docetaxel and gemcitabine plus nab-paclitaxel against B16F10 melanoma and SW1990 pancreatic tumors, respectively. Mechanistically, HCW9218 treatment reduces the immunosuppressive tumor microenvironment and enhances immune cell infiltration and cytotoxicity in the tumors to eliminate TIS cancer cells. Immuno-depletion analysis suggests that HCW9218-activated natural killer cells play a pivotal role in TIS cancer cell removal. HCW9218 treatment following docetaxel chemotherapy further enhances efficacy of tumor antigen-specific and anti-programmed death-ligand 1 (PD-L1) antibodies in B16F10 tumor-bearing mice. We also show that HCW9218 treatment decreases TIS cells and lowers SASP factors in off-target tissues caused by chemotherapy of tumor-bearing mice. Collectively, HCW9218 has the potential to significantly enhance anti-tumor efficacy of chemotherapy, therapeutic antibodies, and checkpoint blockade by eliminating TIS cancer cells while reducing TIS-mediated proinflammatory side effects in normal tissues.

A Fusion Protein Complex that Combines IL-12, IL-15, and IL-18 Signaling to Induce Memory-Like NK Cells for Cancer Immunotherapy.

Natural killer (NK) cells are a promising cellular therapy for cancer, with challenges in the field including persistence, functional activity, and tumor recognition. Briefly, priming blood NK cells with recombinant human (rh)IL-12, rhIL-15, and rhIL-18 (12/15/18) results in memory-like NK cell differentiation and enhanced responses against cancer. However, the lack of available, scalable Good Manufacturing Process (GMP)-grade reagents required to advance this approach beyond early-phase clinical trials is limiting. To address this challenge, we developed a novel platform centered upon an inert tissue factor scaffold for production of heteromeric fusion protein complexes (HFPC). The first use of this platform combined IL-12, IL-15, and IL-18 receptor engagement (HCW9201), and the second adds CD16 engagement (HCW9207). This unique HFPC expression platform was scalable with equivalent protein quality characteristics in small- and GMP-scale production. HCW9201 and HCW9207 stimulated activation and proliferation signals in NK cells, but HCW9207 had decreased IL-18 receptor signaling. RNA sequencing and multidimensional mass cytometry revealed parallels between HCW9201 and 12/15/18. HCW9201 stimulation improved NK cell metabolic fitness and resulted in the DNA methylation remodeling characteristic of memory-like differentiation. HCW9201 and 12/15/18 primed similar increases in short-term and memory-like NK cell cytotoxicity and IFNγ production against leukemia targets, as well as equivalent control of leukemia in NSG mice. Thus, HFPCs represent a protein engineering approach that solves many problems associated with multisignal receptor engagement on immune cells, and HCW9201-primed NK cells can be advanced as an ideal approach for clinical GMP-grade memory-like NK cell production for cancer therapy.

Bifunctional TGF-ß trap/IL-15 protein complex elicits potent NK cell and CD8+ T cell immunity against solid tumors.

Advances in immunostimulatory and anti-immunosuppressive therapeutics have revolutionized cancer treatment. However, novel immunotherapeutics with these dual functions are not frequently reported. Here we describe the creation of a heterodimeric bifunctional fusion molecule, HCW9218, constructed using our soluble tissue factor (TF)-based scaffold technology. This complex comprises extracellular domains of the human transforming growth factor-ß (TGF-ß) receptor II and a human interleukin-15 (IL-15)/IL-15 receptor α complex. HCW9218 can be readily expressed in CHO cells and purified using antibody-based affinity chromatography in a large-scale manufacturing setting. HCW9218 potently activates mouse natural killer (NK) cells and CD8+ T cells in vitro and in vivo to enhance cell proliferation, metabolism, and antitumor cytotoxic activities. Similarly, human immune cells become activated with increased cytotoxicity following incubation with HCW9218. This fusion complex also exhibits TGF-ß neutralizing activity in vitro and sequesters plasma TGF-ß in vivo. In a syngeneic B16F10 melanoma model, HCW9218 displayed strong antitumor activity mediated by NK cells and CD8+ T cells and increased their infiltration into tumors. Repeat-dose subcutaneous administration of HCW9218 was well tolerated by mice, with a half-life sufficient to provide long-lasting biological activity. Thus, HCW9218 may serve as a novel therapeutic to simultaneously provide immunostimulation and lessen immunosuppression associated with tumors.

Enhancing T cell therapy through TCR-signaling-responsive nanoparticle drug delivery.

Adoptive cell therapy (ACT) with antigen-specific T cells has shown remarkable clinical success; however, approaches to safely and effectively augment T cell function, especially in solid tumors, remain of great interest. Here we describe a strategy to 'backpack' large quantities of supporting protein drugs on T cells by using protein nanogels (NGs) that selectively release these cargos in response to T cell receptor activation. We designed cell surface-conjugated NGs that responded to an increase in T cell surface reduction potential after antigen recognition and limited drug release to sites of antigen encounter, such as the tumor microenvironment. By using NGs that carried an interleukin-15 super-agonist complex, we demonstrated that, relative to systemic administration of free cytokines, NG delivery selectively expanded T cells 16-fold in tumors and allowed at least eightfold higher doses of cytokine to be administered without toxicity. The improved therapeutic window enabled substantially increased tumor clearance by mouse T cell and human chimeric antigen receptor (CAR)-T cell therapy in vivo.

ALT-803, an IL-15 superagonist, in combination with nivolumab in patients with metastatic non-small cell lung cancer: a non-randomised, open-label, phase 1b trial.

BACKGROUND: Immunotherapy with PD-1 or PD-L1 blockade fails to induce a response in about 80% of patients with unselected non-small cell lung cancer (NSCLC), and many of those who do initially respond then develop resistance to treatment. Agonists that target the shared interleukin-2 (IL-2) and IL-15Rßγ pathway have induced complete and durable responses in some cancers, but no studies have been done to assess the safety or efficacy of these agonists in combination with anti-PD-1 immunotherapy. We aimed to define the safety, tolerability, and activity of this drug combination in patients with NSCLC. METHODS: In this non-randomised, open-label, phase 1b trial, we enrolled patients (aged ≥18 years) with previously treated histologically or cytologically confirmed stage IIIB or IV NSCLC from three academic hospitals in the USA. Key eligibility criteria included measurable disease, eligibility to receive anti-PD-1 immunotherapy, and an Eastern Cooperative Oncology Group performance status of 0 or 1. Patients received the anti-PD-1 monoclonal antibody nivolumab intravenously at 3 mg/kg (then 240 mg when US Food and Drug Administration [FDA]-approved dosing changed) every 14 days (either as new treatment or continued treatment at the time of disease progression) and the IL-15 superagonist ALT-803 subcutaneously once per week on weeks 1-5 of four 6-week cycles for 6 months. ALT-803 was administered at one of four escalating dose concentrations: 6, 10, 15, or 20 µg/kg. The primary endpoint was to define safety and tolerability and to establish a recommended phase 2 dose of ALT-803 in combination with nivolumab. Analyses were per-protocol and included any patients who received at least one dose of study treatment. This trial is registered with ClinicalTrials.gov, number NCT02523469; phase 2 enrolment of patients is ongoing. FINDINGS: Between Jan 18, 2016, and June 28, 2017, 23 patients were enrolled and 21 were treated at four dose levels of ALT-803 in combination with nivolumab. Two patients did not receive treatment because of the development of inter-current illness during enrolment, one patient due to leucopenia and one patient due to pulmonary dysfunction. No dose-limiting toxicities were recorded and the maximum tolerated dose was not reached. The most common adverse events were injection-site reactions (in 19 [90%] of 21 patients) and flu-like symptoms (15 [71%]). The most common grade 3 adverse events, occurring in two patients each, were lymphocytopenia and fatigue. A grade 3 myocardial infarction occurred in one patient. No grade 4 or 5 adverse events were recorded. The recommended phase 2 dose of ALT-803 is 20 µg/kg given once per week subcutaneously in combination with 240 mg intravenous nivolumab every 2 weeks. INTERPRETATION: ALT-803 in combination with nivolumab can be safely administered in an outpatient setting. The promising clinical activity observed with the addition of ALT-803 to the regimen of patients with PD-1 monoclonal antibody relapsed and refractory disease shows evidence of anti-tumour activity for a new class of agents in NSCLC. FUNDING: Altor BioScience (a NantWorks company), National Institutes of Health, and Medical University of South Carolina Hollings Cancer Center.

Interleukin-15 Complex Treatment Protects Mice from Cerebral Malaria by Inducing Interleukin-10-Producing Natural Killer Cells.

Cerebral malaria is a deadly complication of Plasmodium infection and involves blood brain barrier (BBB) disruption following infiltration of white blood cells. During experimental cerebral malaria (ECM), mice inoculated with Plasmodium berghei ANKA-infected red blood cells develop a fatal CM-like disease caused by CD8+ T cell-mediated pathology. We found that treatment with interleukin-15 complex (IL-15C) prevented ECM, whereas IL-2C treatment had no effect. IL-15C-expanded natural killer (NK) cells were necessary and sufficient for protection against ECM. IL-15C treatment also decreased CD8+ T cell activation in the brain and prevented BBB breakdown without influencing parasite load. IL-15C induced NK cells to express IL-10, which was required for IL-15C-mediated protection against ECM. Finally, we show that ALT-803, a modified human IL-15C, mediates similar induction of IL-10 in NK cells and protection against ECM. These data identify a regulatory role for cytokine-stimulated NK cells in the prevention of a pathogenic immune response.

Enhanced Lymphodepletion Is Insufficient to Replace Exogenous IL2 or IL15 Therapy in Augmenting the Efficacy of Adoptively Transferred Effector CD8+ T Cells.

Effector CD8+ T cells conditioned with IL12 during activation mediate enhanced antitumor efficacy after adoptive transfer into lymphodepleted hosts; this is due in part to improved IL7 responsiveness. Therefore, we hypothesized that increasing the intensity or type of lymphodepletion would deplete more IL7-consuming host cells and improve the persistence and antitumor activity of IL12-conditioned CD8+ T cells. Using cyclophosphamide, fludarabine, and total body irradiation (TBI, 6 Gy) either individually or in combination, we found that combined lymphodepletion best enhanced T-cell engraftment in mice. This improvement was strongly related to the extent of leukopenia, as posttransfer levels of donor T cells inversely correlated to host cell counts after lymphodepletion. Despite the improvement in engraftment seen with combination lymphodepletion, dual-agent lymphodepletion did not augment the antitumor efficacy of donor T cells compared with TBI alone. Similarly, IL7 supplementation after TBI and transfer of tumor-reactive T cells failed to improve persistence or antitumor immunity. However, IL15 or IL2 supplementation greatly augmented the persistence and antitumor efficacy of donor tumor-reactive T cells. Our results indicate that the amount of host IL7 induced after single agent lymphodepletion is sufficient to potentiate the expansion and antitumor activity of donor T cells, and that the efficacy of future regimens may be improved by providing posttransfer support with IL2 or IL15.Significance: The relationship between lymphodepletion and cytokine support plays a critical role in determining donor T-cell engraftment and antitumor efficacy. Cancer Res; 78(11); 3067-74. ©2018 AACR.

Evaluation of the biological activities of the IL-15 superagonist complex, ALT-803, following intravenous versus subcutaneous administration in murine models.

ALT-803 is a fusion protein complex consisting of an interleukin (IL)-15 superagonist and a dimeric IL-15 receptor alpha sushi domain IgG1 Fc fusion protein. When administered to mice, ALT-803 is capable of inducing natural killer (NK) and CD8+ T cell proliferation and activation, and effectively promoting potent anti-tumor responses. Currently, ALT-803 is in clinical trials for treatment of various solid tumors and hematological malignancies. In the initial phase of these clinical studies, intravenous (iv) injection was used according to the route used in pre-clinical efficacy studies. In order to evaluate the possible advantage of subcutaneous (sc) injection versus iv injection, this study compared the biological activity of the two treatment regimens of ALT-803 in pre-clinical in vivo models. The pharmacokinetics, immune stimulation, and anti-tumor efficacy of iv and sc injection routes of ALT-803 in C57BL/6 mice were compared. The half-life of ALT-803 was 7.5 h for iv versus 7.7 h for sc with the maximal detected serum concentration of ALT-803 to be 3926 ng/ml at 0.5 h time-point following iv injection versus 495 ng/ml at 16 h post sc injection. Biodistribution studies indicated that sc ALT-803, similarly to iv ALT-803 as previously reported, has a greater tissue distribution and longer residence time in lymphoid tissues compared to recombinant IL-15. Notably, ALT-803 when administered either iv or sc induced comparable proliferation and activation of CD8+ T and NK cells and resulted in similar reductions of tumor burden. A toxicity study of mice receiving multiple injections of ALT-803 for 4 weeks by iv or sc routes revealed equivalent immune-related changes. The gradual absorbance into the blood stream and lower maximal blood levels of ALT-803 in sc-injected mice, along with similar anti-tumor efficacy support the administration of ALT-803 by sc injection in patients with various malignancies and infectious diseases.

First-in-human phase 1 clinical study of the IL-15 superagonist complex ALT-803 to treat relapse after transplantation.

New therapies for patients with hematologic malignancies who relapse after allogeneic hematopoietic cell transplantation (allo-HCT) are needed. Interleukin 15 (IL-15) is a cytokine that stimulates CD8+ T-cell and natural killer (NK) cell antitumor responses, and we hypothesized this cytokine may augment antileukemia/antilymphoma immunity in vivo. To test this, we performed a first-in-human multicenter phase 1 trial of the IL-15 superagonist complex ALT-803 in patients who relapsed >60 days after allo-HCT. ALT-803 was administered to 33 patients via the IV or subcutaneous (SQ) routes once weekly for 4 doses (dose levels of 1, 3, 6, and 10 µg/kg). ALT-803 was well tolerated, and no dose-limiting toxicities or treatment-emergent graft-versus-host disease requiring systemic therapy was observed in this clinical setting. Adverse events following IV administration included constitutional symptoms temporally related to increased serum IL-6 and interferon-γ. To mitigate these effects, the SQ route was tested. SQ delivery resulted in self-limited injection site rashes infiltrated with lymphocytes without acute constitutional symptoms. Pharmacokinetic analysis revealed prolonged (>96 hour) serum concentrations following SQ, but not IV, injection. ALT-803 stimulated the activation, proliferation, and expansion of NK cells and CD8+ T cells without increasing regulatory T cells. Responses were observed in 19% of evaluable patients, including 1 complete remission lasting 7 months. Thus, ALT-803 is a safe, well-tolerated agent that significantly increased NK and CD8+ T cell numbers and function. This immunostimulatory IL-15 superagonist warrants further investigation to augment antitumor immunity alone and combined with other immunotherapies. This trial was registered at www.clinicaltrials.gov as #NCT01885897.

The human IL-15 superagonist ALT-803 directs SIV-specific CD8+ T cells into B-cell follicles.

Sequestering of latent HIV in follicular helper T cells within B-cell follicles that largely exclude cytotoxic T cells is a major barrier to cellular immune-based approaches to eradicate HIV. Here, we show that the clinical-grade human interleukin-15 (IL-15) superagonist ALT-803 activates and redirects simian immunodeficiency virus (SIV)-specific CD8+ T cells from the peripheral blood into B-cell follicles. In agreement with the increased trafficking of SIV-specific cytotoxic T cells to sites of cryptic viral replication, lymph nodes of elite controlling macaques contained fewer cells expressing SIV RNA or harboring SIV DNA post-ALT-803 treatment. These data establish ALT-803 as an immunotherapeutic for HIV and other chronic viral pathogens that evade host immunity by persisting in B-cell follicles.

ALT-803 Transiently Reduces Simian Immunodeficiency Virus Replication in the Absence of Antiretroviral Treatment.

Developing biological interventions to control human immunodeficiency virus (HIV) replication in the absence of antiretroviral therapy (ART) could contribute to the development of a functional cure. As a potential alternative to ART, the interleukin-15 (IL-15) superagonist ALT-803 has been shown to boost the number and function of HIV-specific CD8+ T and NK cell populations in vitro Four simian immunodeficiency virus (SIV)-positive rhesus macaques, three of whom possessed major histocompatibility complex alleles associated with control of SIV and all of whom had received SIV vaccine vectors that had the potential to elicit CD8+ T cell responses, were given ALT-803 in three treatment cycles. The first and second cycles of treatment were separated by 2 weeks, while the third cycle was administered after a 29-week break. ALT-803 transiently elevated the total CD8+ effector and central memory T cell and NK cell populations in peripheral blood, while viral loads transiently decreased by ∼2 logs in all animals. Virus suppression was not sustained as T cells became less responsive to ALT-803 and waned in numbers. No effect on viral loads was observed in the second cycle of ALT-803, concurrent with downregulation of the IL-2/15 common γC and ß chain receptors on both CD8+ T cells and NK cells. Furthermore, populations of immunosuppressive T cells increased during the second cycle of ALT-803 treatment. During the third treatment cycle, responsiveness to ALT-803 was restored. CD8+ T cells and NK cells increased again 3- to 5-fold, and viral loads transiently decreased again by 1 to 2 logs.IMPORTANCE Overall, our data show that ALT-803 has the potential to be used as an immunomodulatory agent to elicit effective immune control of HIV/SIV replication. We identify mechanisms to explain why virus control is transient, so that this model can be used to define a clinically appropriate treatment regimen.

Glycolytic requirement for NK cell cytotoxicity and cytomegalovirus control.

NK cell activation has been shown to be metabolically regulated in vitro; however, the role of metabolism during in vivo NK cell responses to infection is unknown. We examined the role of glycolysis in NK cell function during murine cytomegalovirus (MCMV) infection and the ability of IL-15 to prime NK cells during CMV infection. The glucose metabolism inhibitor 2-deoxy-ᴅ-glucose (2DG) impaired both mouse and human NK cell cytotoxicity following priming in vitro. Similarly, MCMV-infected mice treated with 2DG had impaired clearance of NK-specific targets in vivo, which was associated with higher viral burden and susceptibility to infection on the C57BL/6 background. IL-15 priming is known to alter NK cell metabolism and metabolic requirements for activation. Treatment with the IL-15 superagonist ALT-803 rescued mice from otherwise lethal infection in an NK-dependent manner. Consistent with this, treatment of a patient with ALT-803 for recurrent CMV reactivation after hematopoietic cell transplant was associated with clearance of viremia. These studies demonstrate that NK cell-mediated control of viral infection requires glucose metabolism and that IL-15 treatment in vivo can reduce this requirement and may be effective as an antiviral therapy.

New interleukin-15 superagonist (IL-15SA) significantly enhances graft-versus-tumor activity.

Interleukin-15 (IL-15) is a potent cytokine that increases CD8+ T and NK cell numbers and function in experimental models. However, obstacles remain in using IL-15 therapeutically, specifically its low potency and short in vivo half-life. To help overcome this, a new IL-15 superagonist complex comprised of an IL-15N72D mutation and IL-15RαSu/Fc fusion (IL-15SA, also known as ALT-803) was developed. IL-15SA exhibits a significantly longer serum half-life and increased in vivo activity against various tumors. Herein, we evaluated the effects of IL-15SA in recipients of allogeneic hematopoietic stem cell transplantation. Weekly administration of IL-15SA to transplant recipients significantly increased the number of CD8+ T cells (specifically CD44+ memory/activated phenotype) and NK cells. Intracellular IFN-γ and TNF-α secretion by CD8+ T cells increased in the IL-15SA-treated group. IL-15SA also upregulated NKG2D expression on CD8+ T cells. Moreover, IL-15SA enhanced proliferation and cytokine secretion of adoptively transferred CFSE-labeled T cells in syngeneic and allogeneic models by specifically stimulating the slowly proliferative and nonproliferative cells into actively proliferating cells.We then evaluated IL-15SA's effects on anti-tumor activity against murine mastocytoma (P815) and murine B cell lymphoma (A20). IL-15SA enhanced graft-versus-tumor (GVT) activity in these tumors following T cell infusion. Interestingly, IL-15 SA administration provided GVT activity against A20 lymphoma cells in the murine donor leukocyte infusion (DLI) model without increasing graft versus host disease. In conclusion, IL-15SA could be a highly potent T- cell lymphoid growth factor and novel immunotherapeutic agent to complement stem cell transplantation and adoptive immunotherapy.

A Novel Fusion of ALT-803 (Interleukin (IL)-15 Superagonist) with an Antibody Demonstrates Antigen-specific Antitumor Responses.

IL-15 and its receptor α (IL-15Rα) are co-expressed on antigen-presenting cells, allowing transpresentation of IL-15 to immune cells bearing IL-2RßγC and stimulation of effector immune responses. We reported previously that the high-affinity interactions between an IL-15 superagonist (IL-15N72D) and the extracellular IL-15Rα sushi domain (IL-15RαSu) could be exploited to create a functional scaffold for the design of multivalent disease-targeted complexes. The IL-15N72D·IL-15RαSuFc complex, also known as ALT-803, is a multimeric complex constructed by fusing IL-15N72D·IL-15RαSu to the Fc domain of IgG1. ALT-803 is an IL-15 superagonist complex that has been developed as a potent antitumor immunotherapeutic agent and is in clinical trials. Here we describe the creation of a novel fusion molecule, 2B8T2M, using the ALT-803 scaffold fused to four single chains of the tumor-targeting monoclonal antibody rituximab. This molecule displays trispecific binding activity through its recognition of the CD20 molecule on tumor cells, stimulation via IL-2RßγC displayed on immune effector cells, and binding to Fcγ receptors on natural killer cells and macrophages. 2B8T2M activates natural killer cells to enhance antibody-dependent cellular cytotoxicity, mediates complement-dependent cytotoxicity, and induces apoptosis of B-lymphoma cells. Compared with rituximab, 2B8T2M exhibits significantly stronger antitumor activity in a xenograft SCID mouse model and depletes B cells in cynomolgus monkeys more efficiently. Thus, ALT-803 can be modified as a functional scaffold for creating multispecific, targeted IL-15-based immunotherapeutic agents and may serve as a novel platform to improve the antitumor activity and clinical efficacy of therapeutic antibodies.

A Subset of Latency-Reversing Agents Expose HIV-Infected Resting CD4+ T-Cells to Recognition by Cytotoxic T-Lymphocytes.

Resting CD4+ T-cells harboring inducible HIV proviruses are a critical reservoir in antiretroviral therapy (ART)-treated subjects. These cells express little to no viral protein, and thus neither die by viral cytopathic effects, nor are efficiently cleared by immune effectors. Elimination of this reservoir is theoretically possible by combining latency-reversing agents (LRAs) with immune effectors, such as CD8+ T-cells. However, the relative efficacy of different LRAs in sensitizing latently-infected cells for recognition by HIV-specific CD8+ T-cells has not been determined. To address this, we developed an assay that utilizes HIV-specific CD8+ T-cell clones as biosensors for HIV antigen expression. By testing multiple CD8+ T-cell clones against a primary cell model of HIV latency, we identified several single agents that primed latently-infected cells for CD8+ T-cell recognition, including IL-2, IL-15, two IL-15 superagonists (IL-15SA and ALT-803), prostratin, and the TLR-2 ligand Pam3CSK4. In contrast, we did not observe CD8+ T-cell recognition of target cells following treatment with histone deacetylase inhibitors or with hexamethylene bisacetamide (HMBA). In further experiments we demonstrate that a clinically achievable concentration of the IL-15 superagonist 'ALT-803', an agent presently in clinical trials for solid and hematological tumors, primes the natural ex vivo reservoir for CD8+ T-cell recognition. Thus, our results establish a novel experimental approach for comparative evaluation of LRAs, and highlight ALT-803 as an LRA with the potential to synergize with CD8+ T-cells in HIV eradication strategies.

IL15 Agonists Overcome the Immunosuppressive Effects of MEK Inhibitors.

Many signal transduction inhibitors are being developed for cancer therapy target pathways that are also important for the proper function of antitumor lymphocytes, possibly weakening their therapeutic effects. Here we show that most inhibitors targeting multiple signaling pathways have especially strong negative effects on T-cell activation at their active doses on cancer cells. In particular, we found that recently approved MEK inhibitors displayed potent suppressive effects on T cells in vitro However, these effects could be attenuated by certain cytokines that can be administered to cancer patients. Among them, clinically available IL15 superagonists, which can activate PI3K selectively in T lymphocytes, synergized with MEK inhibitors in vivo to elicit potent and durable antitumor responses, including by a vaccine-like effect that generated resistance to tumor rechallenge. Our work identifies a clinically actionable approach to overcome the T-cell-suppressive effects of MEK inhibitors and illustrates how to reconcile the deficiencies of signal transduction inhibitors, which impede desired immunologic effects in vivo Cancer Res; 76(9); 2561-72. ©2016 AACR.

IL-15 superagonist/IL-15RαSushi-Fc fusion complex (IL-15SA/IL-15RαSu-Fc; ALT-803) markedly enhances specific subpopulations of NK and memory CD8+ T cells, and mediates potent anti-tumor activity against murine breast and colon carcinomas.

Interleukin (IL)-15-N72D superagonist-complexed with IL-15RαSushi-Fc fusion protein (IL-15SA/IL-15RαSu-Fc; ALT-803) has been reported to exhibit significant anti-tumor activity in murine myeloma, rat bladder cancer, and murine glioblastoma models. In this study, we examined the immunomodulatory and anti-tumor effects of IL-15SA/IL-15RαSu-Fc in tumor-free and highly metastatic tumor-bearing mice. Here, IL-15SA/IL-15RαSu-Fc significantly expanded natural killer (NK) and CD8+ T cells. In examining NK cell subsets, the greatest significant increase was in highly cytotoxic and migrating (CD11b+, CD27hi; high effector) NK cells, leading to enhanced function on a per-cell basis. CD8+ T cell subset analysis determined that IL-15SA/IL-15RαSu-Fc significantly increased IL-15 responding memory (CD122+, CD44+) CD8+ T cells, in particular those having the innate (NKG2D+, PD1-) phenotype. In 4T1 breast tumor-bearing mice, IL-15SA/IL-15RαSu-Fc induced significant anti-tumor activity against spontaneous pulmonary metastases, depending on CD8+ T and NK cells, and resulting in prolonged survival. Similar anti-tumor activity was seen in the experimental pulmonary metastasis model of CT26 colon carcinoma cells, particularly when IL-15SA/IL-15RαSu-Fc was combined with a cocktail of checkpoint inhibitors, anti-CTLA-4 and anti-PD-L1. Altogether, these studies showed for the first time that IL-15SA/IL-15RαSu-Fc (1) promoted the development of high effector NK cells and CD8+ T cell responders of the innate phenotype, (2) enhanced function of NK cells, and (3) played a vital role in reducing tumor metastasis and ultimately survival, especially in combination with checkpoint inhibitors.

Comparison of the Superagonist Complex, ALT-803, to IL15 as Cancer Immunotherapeutics in Animal Models.

IL15, a potent stimulant of CD8(+) T cells and natural killer (NK) cells, is a promising cancer immunotherapeutic. ALT-803 is a complex of an IL15 superagonist mutant and a dimeric IL15 receptor αSu/Fc fusion protein that was found to exhibit enhanced biologic activity in vivo, with a substantially longer serum half-life than recombinant IL15. A single intravenous dose of ALT-803, but not IL15, eliminated well-established tumors and prolonged survival of mice bearing multiple myeloma. In this study, we extended these findings to demonstrate the superior antitumor activity of ALT-803 over IL15 in mice bearing subcutaneous B16F10 melanoma tumors and CT26 colon carcinoma metastases. Tissue biodistribution studies in mice also showed much greater retention of ALT-803 in the lymphoid organs compared with IL15, consistent with its highly potent immunostimulatory and antitumor activities in vivo. Weekly dosing with 1 mg/kg ALT-803 in C57BL/6 mice was well tolerated, yet capable of increasing peripheral blood lymphocyte, neutrophil, and monocyte counts by >8-fold. ALT-803 dose-dependent stimulation of immune cell infiltration into the lymphoid organs was also observed. Similarly, cynomolgus monkeys treated weekly with ALT-803 showed dose-dependent increases of peripheral blood lymphocyte counts, including NK, CD4(+), and CD8(+) memory T-cell subsets. In vitro studies demonstrated ALT-803-mediated stimulation of mouse and human immune cell proliferation and IFNγ production without inducing a broad-based release of other proinflammatory cytokines (i.e., cytokine storm). Based on these results, a weekly dosing regimen of ALT-803 has been implemented in multiple clinical studies to evaluate the dose required for effective immune cell stimulation in humans.