CD8 cis-targeted IL-2 drives potent antiviral activity against hepatitis B virus

CD8+ T cells are key antiviral effectors against hepatitis B virus (HBV), yet their number and function can be compromised in chronic infections. Preclinical HBV models displaying CD8+ T cell dysfunction showed that interleukin-2 (IL-2)-based treatment, unlike programmed cell death ligand 1 (PD-L1) checkpoint blockade, could reverse this defect, suggesting its therapeutic potential against HBV. However, IL-2's effectiveness is hindered by its pleiotropic nature, because its receptor is found on various immune cells, including regulatory T (Treg) cells and natural killer (NK) cells, which can counteract antiviral responses or contribute to toxicity, respectively. To address this, we developed a cis-targeted CD8-IL2 fusion protein, aiming to selectively stimulate dysfunctional CD8+ T cells in chronic HBV. In a mouse model, CD8-IL2 boosted the number of HBV-reactive CD8+ T cells in the liver without substantially altering Treg or NK cell counts. These expanded CD8+ T cells exhibited increased interferon-γ and granzyme B production, demonstrating enhanced functionality. CD8-IL2 treatment resulted in substantial antiviral effects, evidenced by marked reductions in viremia and antigenemia and HBV core antigen-positive hepatocytes. In contrast, an untargeted CTRL-IL2 led to predominant NK cell expansion, minimal CD8+ T cell expansion, negligible changes in effector molecules, and minimal antiviral activity. Human CD8-IL2 trials in cynomolgus monkeys mirrored these results, achieving a roughly 20-fold increase in peripheral blood CD8+ T cells without affecting NK or Treg cell numbers. These data support the development of CD8-IL2 as a therapy for chronic HBV infection.

Abstract 3518: AB248 is a CD8+ T cell selective IL-2 designed for superior safety and anti-tumor efficacy

High dose IL-2 was the first modern immunotherapy to show complete responses in a subset of cancer patients. The liabilities of IL-2 prompted development of second-generation molecules, which signal through the IL-2Rβγ (“not-α” IL-2 and IL-15 variants). Although these molecules avoid vascular leak syndrome, their clinical efficacy appears to be suboptimal compared to high dose IL-2 [1]. Furthermore, not-α IL-2s show biased expansion of NK cells over CD8+ T cells in patients due to their high expression of IL-2Rβ on NK cells, which can act as a sink and contribute to toxicity, and these molecules do not eliminate IL-2Rβγ-mediated activation of Tregs [2, 3]. To maximize the therapeutic potential of IL-2, we developed AB248, a cis-targeted IL-2 fusion protein that selectively signals on CD8+ T-cells with limited activity on the highly IL-2/IL-15-sensitive NK cells and immunosuppressive Tregs. We have previously shown that selective expansion of CD8+ T cells over NK cells and Tregs is achieved in vivo using muAB248, AB248’s murine surrogate. Furthermore, a single dose of muAB248 elicits strong efficacy in multiple tumor models without body weight loss. Here, we detail the properties of AB248, a fusion of a human CD8-targeting antibody that selectively binds to CD8+ T cells over NK cells, and an IL-2 mutein with reduced affinity to IL-2Rα and IL-2Rβ. In STAT5 assays, AB248 showed approximately 1000-fold preference for the activation of CD8+ T cells over NK cells and Tregs. AB248 mediated a dose-dependent selective expansion of CD8 T cells over NK and Tregs in non-human primates. Furthermore, AB248 reproduced the expected in vitro gene signatures of an IL2Rβγ agonist, demonstrating that AB248 recapitulates native IL-2Rβγ signaling selectively on CD8+ T cells.We previously demonstrated in mice that while the efficacy of not-α IL-2 was mediated by CD8+ T cells and not NK cells, toxicity as measured by body weight loss was dependent upon NK cells but not CD8+ T cells. Here, we show that human NK cells may also drive IL-2βγ agonist-induced toxicities. Both IL-2 and not-α IL-2 induced IFN-γ secretion from human PBMCs, whereas AB248 did not. Strikingly, depletion of CD56+ NK cells eliminated IL-2-induced cytokine secretion, demonstrating that human NK cells are capable of spontaneously secreting IFN-γ in response to IL-2 signaling. In the context of TCR stimulation, AB248 induced robust secretion of effector cytokines from CD8+ T cells, but no cytokine secretion was seen in absence of TCR activation. Our data suggest that while AB248 can induce antigen-independent expansion of CD8+ T cells, it only induces effector cytokine secretion in the context of an additional activating signal via the TCR. Collectively, these data show that AB248 demonstrates an improved safety and efficacy profile as compared to not-α IL-2 and is a promising immuno-oncology therapeutic. Ref: 1. Overwijk, Ann. Rev. Med., 2020; 2. Janku, AACR, 2021; 3. Italiano, JCO, 2021

Citation Format: Kelly D. Moynihan, Danielle Pappas, Terrence Park, Wei Chen, Irene Ni, Paul Bessette, Henry Nguyen, David Liu, Mike Chin, Ruth Lan, Aaron Arvey, Ton N. Schumacher, Yik Andy Yeung, Ivana Djuretic. AB248 is a CD8+ T cell selective IL-2 designed for superior safety and anti-tumor efficacy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 3518.

Abstract 561: CAR-targeted IL-2 drives selective CAR-T cell expansion and improves anti-tumor efficacy

Chimeric antigen receptor T cell (CAR-T) therapies have transformed the treatment of some hematological malignancies and are showing promising preliminary results in solid tumors. Recent studies have shown that in vivo expansion and persistence of CAR-Ts are correlated with improved therapeutic outcomes in patients. Administration of IL-2 enhances CAR-T engraftment, persistence, and functionality in preclinical models. However, the clinical potential of IL-2 stimulation with cell therapies is limited using current molecules due to severe toxicity of high-dose IL-2 and the inadequate selectivity of existing engineered IL-2 variants which expand multiple endogenous cells in addition to CAR-T cells. To address this challenge, we have applied our cis-targeting technology to develop CAR-T selective IL-2 fusion molecules that specifically activate CAR-Ts, while exhibiting minimal activity on CAR-negative cells. Cis-targeted IL-2 fusions are comprised of a fusion between a targeting antibody and an IL-2 mutein having attenuated binding to IL2R⍺ and IL2Rβ. The attenuated cytokine selectively binds and activates IL-2 receptors on CAR-Ts via the avidity provided by the targeting arm. We engineered two cis-targeted CAR-T selective IL-2 fusions, CAR-IL2 or EGFRt-IL2. CAR-IL2 targets the FMC63 CAR directly without blocking CD19 antigen recognition, enabling targeting of approved anti-CD19 CAR-T products, and EGFRt-IL2 targets the EGFRt tag co-expressed with the CAR. Molecules were characterized in vitro using primary human CAR-transduced T cells. The in vivo activity was examined using tumor bearing NSG mice infused with human CAR-Ts. The specificity of the CAR-IL2 or EGFRt-IL2 molecules was demonstrated by their ability to selectively induce pSTAT5 signaling resulting in >1000-fold preferential STAT5 activity in CAR-expressing cells compared to CAR-negative cells. The ability of the fusion proteins to selectively expand CAR-Ts in vivo was shown through a substantial and specific expansion of the infused CAR-Ts. The CAR-T fraction increased from approximately 50% of infused T cells, to over 93% of T cells, demonstrating a nearly 40-fold expansion in vivo. Re-expansion of CAR-Ts was demonstrated after allowing the CAR-Ts to rest in vivo >100 days; re-dosing resulted in significant and selective re-expansion of CAR-T cells. Furthermore, treatment with the CAR-IL2 molecule enhanced tumor regression of CD19+ lymphoma-bearing mice infused with suboptimal doses of CAR-Ts. Together, these data demonstrate that our cis-targeted IL-2 molecules selectively activate CAR-Ts in vitro and enhance CAR-T expansion and anti-tumor efficacy in vivo. Cis-targeted IL-2 fusion molecules directed by anti-CAR or anti-tag antibodies represent a promising approach to confer enhanced CAR-T activity in a specific and temporally controlled manner.

Citation Format: Nathan D. Mathewson, Kelly D. Moynihan, Sara Sleiman, Wei Chen, Paul Bessette, Christopher Kimberlin, Eric Wigton, Danielle Pappas, Terrence Park, Ton N. Schumacher, Saar I. Gill, Yik Andy Yeung, Ivana Djuretic. CAR-targeted IL-2 drives selective CAR-T cell expansion and improves anti-tumor efficacy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 561.

Abstract 2882: Selective activation of CD8+ T cells by a CD8-targeted IL-21 results in enhanced anti-tumor efficacy and safety

Interleukin-21 (IL-21) is a pleiotropic cytokine that activates multiple lymphoid and myeloid immune cell subsets. IL-21 can have beneficial effects on anti-tumor immune responses due to its ability to activate STAT3 in CD8+ T cells and promote cytotoxicity, memory cell differentiation and survival. However, IL-21 can also induce immunosuppressive effects, such as suppression of antigen presentation, which may directly oppose its anti-tumor effects on CD8+ T cells. Although recombinant IL-21 did show monotherapy activity in the clinic, overall response rates were modest. In addition to the pharmacological sink created by the broad expression of IL-21R and potential immunosuppressive effects, clinical utility of IL-21 was further limited by its low bioavailability and dose-limiting toxicities. We hypothesized that maximizing IL-21’s activity on CD8+ T cells while avoiding non-CD8 cells would enhance efficacy and improve tolerability. We developed cis-targeted IL-21 (CD8-IL21) fusion proteins that exhibit improved bioavailability and selectively activate CD8+ T cells while exhibiting minimal activity on non-CD8 cells. In vitro phospho-STAT3 assays with CD8-IL21 in mouse splenocytes and human peripheral blood cells showed >1000-fold selective activation of CD8+ T cells over non-CD8 cells. A mouse CD8-IL21 surrogate demonstrated profound single agent efficacy in multiple syngeneic tumor models at considerably lower doses than an untargeted IL-21 control, despite similar in vitro potency. CD8-IL21 mediated anti-tumor efficacy without inducing body weight loss. Whereas prior data showed that repeated high doses of tumor-targeted IL-21 fusion protein (>3 mg/kg) had single agent efficacy in the MC38 tumor model overexpressing a tumor antigen [1], here we show that a single dose of CD8-targeted IL-21 as low as 0.1 mg/kg induced curative responses in mice bearing established MC38 tumors. We demonstrated using the lymphocyte migration inhibitor FTY720 that this enhanced potency of low dose CD8-IL21 is predominately driven by the activation of peripheral CD8 T cells. Furthermore, curative effects in the MC38 model were elicited in the absence of substantial CD8+ T cell expansion (<1.5-fold) suggesting that CD8-IL21 mediated anti-tumor efficacy through modulation of T cell phenotype. A CD8-IL21 selective for human and cynomolgus CD8+ T cells with improved bioavailability also exhibited improved tolerability over untargeted IL-21 in cynomolgus monkeys, showing that there is a tolerability benefit to CD8-selective activation. In summary, CD8-IL21 showed improved efficacy and safety over untargeted IL-21, demonstrating clear promise as a novel therapeutic agent which by activating STAT3 signaling in CD8+ T cells may offer complementary benefits to activation of STAT5 signaling by cytokines such as IL-2 and IL-15.Ref: 1. Deng et al., 2020

Citation Format: Renee Greer, David Liu, Henry Nguyen, Ruth Lan, Wei Wei Prior, Meghana Sukthankar, Paul Mesko, irene Ni, Mike Chin, Kelly Moynihan, Ton Schumacher, Ivana Djuretic, Andy Yeung. Selective activation of CD8+ T cells by a CD8-targeted IL-21 results in enhanced anti-tumor efficacy and safety [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2882.

717 Selective activation of CD8+ T cells by a CD8-targeted IL-2 results in enhanced anti-tumor efficacy and safety

Background

High-dose IL-2 induces complete responses in a subset of cancer patients, but severe toxicity, including vascular leak syndrome (VLS), limits its clinical potential. Insights into IL2Rα's role in the development of VLS sparked a wave of second-generation IL-2 molecules referred to as “not-α” IL-2s. Emerging clinical data suggests that although not-α IL-2s avoid VLS, they induce suboptimal monotherapy activity in patients. Given the observation that CD8+ T cells are the dominant effector cells with IL-2-based therapies,1 2 we hypothesized that maximizing the activity of IL-2 on CD8+ T-cells and limiting its activity on immunosuppressive Tregs and highly IL-2-sensitive innate populations would improve IL-2's efficacy and tolerability. We developed cis-targeted IL-2 (CD8-IL2) fusion proteins that selectively activate CD8+ T cells and have minimal activity on CD8-negative cells.

Methods

In vitro selectivity of CD8-IL2 molecules was tested on primary immune cells including mouse splenocytes and human PBMCs. In vivo activity was evaluated in syngeneic tumor models and non-human primates.

Results

Due to the 10–20x higher expression of IL2Rβ on NK cells over other lymphocytes, not-α IL-2 induced preferential NK cell expansion in mice. Toxicity-induced body weight loss with not-α IL-2 treatment was dependent on cells expressing NK1.1 but not CD8. To avoid overt activation of IL2Rβhigh NK cells, IL-2Rα-associated toxicity, and Treg activation, we generated cis-targeted fusion proteins consisting of anti-CD8 antibodies and IL-2 muteins with attenuated binding to IL2Rα and IL2Rβ. We demonstrated that CD8-IL2 fusions preferentially activated CD8+ T cells within mouse, human, and cynomolgus immune populations, with 100–1000 fold selectivity over NK cells and Tregs for all three species. Selective expansion of CD8+ T cells over NK cells and Tregs was demonstrated in tumor and peripheral blood compartments in mice. Selective CD8+ T cell expansion was also demonstrated in cynomolgus monkeys. Furthermore, a single dose of CD8-IL2 in mice elicited strong monotherapy efficacy in MC38 tumors, with a majority of mice demonstrating complete responses without detectable body weight loss at doses that were well tolerated in cynomolgus monkeys. In contrast, not alpha IL-2 induced >10% body weight loss prior to reaching efficacious doses in mice and did not drive any complete anti-tumor responses.

Conclusions

CD8-targeted IL-2 has superior efficacy and lower toxicity compared to second-generation not-α IL-2. Development of AB248, a novel CD8-targeted IL-2 molecule is underway.

References

Rakhmilevich A, North R. Elimination of CD4+ T cells in mice bearing an advanced sarcoma augments the antitumor action of interleukin-2. Cancer Immunol Immunother 1994;38(2):107–12.

Sun Z, Ren Z. A next-generation tumor-targeting IL-2 preferentially promotes tumor-infiltrating CD8+ T-cell response and effective tumor control. Nat Commun 2019;3874:1–12.

578 CD8-targeted IL-2 drives potent anti-tumor efficacy and promotes action of tumor specific vaccines

Background

IL-2 and currently available engineered variants are of interest for solid tumor treatment, but their efficacy and toxicity profiles remain suboptimal. These results reflect the pleiotropic signaling via IL-2 receptors on different cell types that may simultaneously drive desired and undesired responses. We hypothesized that restricting IL-2’s activity to CD8+ T cells would improve efficacy while also lowering its toxicity profile.

Methods

We developed a cis-targeted IL-2 that selectively acts on CD8+ T cells (CD8-IL2) and assessed its activity using the T3 progressor MCA sarcoma model, which was selected because (a) it is sensitive to anti-PD-1 therapy when tumors are small but develops insensitivity as tumor size increase, (b) rejection requires both CD4+ and CD8+ T cells and (c) rejection is dependent on tumor expression of two neoantigens: mItgb1 (MHC-II) and mLama4 (MHC-I).

Results

Whereas mice bearing 8-day T3 tumors had become insensitive to anti-PD-1 mediated tumor rejection, 90% of mice treated with single dose CD8-IL2 monotherapy rejected their tumors, while high dose IL-2 produced minimal efficacy. Efficacy occurred without body weight loss. These results suggest that CD8-IL2 can induce therapeutic effects at a time when tumors became insensitive to anti-PD-1. To assess this possibility in a more controlled manner, we used a tumor neoantigen vaccine model that depends on CD4+ T cell help for development of functional CD8+ T cells at both the priming stage in the lymph node as well as the effector stage at the tumor site. Mice bearing T3 tumors were vaccinated with a synthetic long peptide (SLP) containing the mLama4 neoepitope and either a high or low dose of an SLP containing the mItgb1 neoepitope. Whereas 85% of tumor bearing mice that received the vaccine containing mLama4 plus low dose mItgb1 SLP rejected their tumors, surprisingly none of the mice receiving high dose mItgb1 underwent tumor rejection. This high dose inhibition was reversed when CD8-IL2 was administered after high dose vaccination and at concentrations that had only modest activity in tumor bearing, non-vaccinated mice. With CD8-IL2 treatment, antigen specific T cells were expanded and displayed increased expression of activation-associated markers and reduced expression of exhaustion-associated markers.

Conclusions

CD8-IL2 outperformed other forms of engineered IL-2 in anti-tumor efficacy, showed a significantly improved toxicity profile, and rescued deficient CD8 T cell responses resulting from poor CD4 help. In sum, we demonstrate high level antitumor efficacy and tolerability with a new form of targeted IL-2.

Ethics Approval

Mice used in this study were between 8 and 12 weeks of age and were maintained in accordance with procedures approved by the Association for Assessment and Accreditation of Laboratory Animal Care and Accredited Animal Studies Committee of Washington University in St. Louis

An Engineered IL15 Cytokine Mutein Fused to an Anti-PD-1 Improves Intratumoral T-Cell Function and Antitumor Immunity

The use of cytokines for immunotherapy shows clinical efficacy but is frequently accompanied by severe adverse events caused by excessive and systemic immune activation. Here, we set out to address these challenges by engineering a fusion protein of a single, potency-reduced, IL15 mutein and a PD-1-specific antibody (anti-PD1-IL15m). This immunocytokine was designed to deliver PD-1-mediated, avidity-driven IL2/15 receptor stimulation to PD-1+ tumor-infiltrating lymphocytes (TILs) while minimally affecting circulating peripheral natural killer (NK) cells and T cells. Treatment of tumor-bearing mice with a mouse cross-reactive fusion, anti-mPD1-IL15m demonstrated potent antitumor efficacy without exacerbating body weight loss in B16 and MC38 syngeneic tumor models. Moreover, anti-mPD1-IL15m was more efficacious than an IL15 superagonist, an anti-mPD-1, or the combination thereof in the B16 melanoma model. Mechanistically, anti-PD1-IL15m preferentially targeted CD8+ TILs and scRNA-seq analyses revealed that anti-mPD1-IL15m treatment induced the expansion of an exhausted CD8+ TILs cluster with high proliferative capacity and effector-like signatures. Antitumor efficacy of anti-mPD1-IL15m was dependent on CD8+ T cells, as depletion of CD8+ cells resulted in the loss of antitumor activity, whereas depletion of NK cells had little impact on efficacy. The impact of anti-hPD1-IL15m on primary human TILs from cancer patients was also evaluated. Anti-hPD1-IL15m robustly enhanced the proliferation, activation, and cytotoxicity of CD8+ and CD4+ TILs from human primary cancers in vitro, whereas tumor-derived regulatory T cells were largely unaffected. Taken together, we showed that anti-PD1-IL15m exhibits a high translational promise with improved efficacy and safety of IL15 for cancer immunotherapy via targeting PD-1+ TILs.

Abstract 4557: BCMA-CD3 bispecific antibody PF-06863135: Preclinical rationale for therapeutic combinations

BCMA-CD3 bispecific antibody PF-06863135 is currently under evaluation in an ongoing Phase 1 clinical trial in relapsed/refractory (RR) multiple myeloma (MM) patients (NCT03269136). We showed previously that BCMA-CD3 bispecific has potent single agent activity in RR MM human bone marrow aspirates in vitro and in preclinical models of human myeloma (Panowski et al, MCT 2019). Herein we used in vitro assays, ex vivo MM patient 3D cultures, and in vivo efficacy models to study therapeutic combinations of PF-06863135 with immune checkpoint inhibitors, immunomodulatory drugs (IMiD), and gamma secretase inhibitors (GSI). Upregulation of immune checkpoints, such as programmed cell death protein-1 (PD-1) or programmed death ligand 1 (PD-L1) has been demonstrated downstream of response to CD3 bispecifics. While PF-06863135 is effective as a single agent in ex vivo 3D primary MM cultures (EC50 0.2 nM), PD-L1 was upregulated on myeloma cells. We combined PF-06863135 with anti-PD-1 in subcutaneous and orthotopic myeloma models MM.1S and MM.1S-PD-L1 in NSG mice engrafted with human T cells. PF-06863135 demonstrated single agent anti-tumor activity in the MM.1S models; however, in the MM.1S-PD-L1 model, PF-06863135 activity was blunted. When given in combination with an anti-PD-1 blocking antibody, the full thrust of the single agent anti-tumor activity of PF-06863135 was restored. Dysfunctional, senescent like T cells may emerge downstream of T cell immunotherapies. IMiDs, such as lenalidomide, are a standard of care therapy in MM and result in immunomodulatory effects on T cells leading us to hypothesize PF-06863135 mediated tumor growth inhibition could be enhanced by combining with lenalidomide. When PF-06863135 and lenalidomide were given in combination in the human T cell engrafted established orthotopic MM.1S and MOLP8 models, an even greater anti-tumor activity was observed as compared to either agent alone. These studies suggest that PF-06863135 activity can be augmented by combinations with immunomodulatory agents such as anti-PD-1 or lenalidomide. BCMA is shed from the surface of myeloma cells by gamma secretases, not only reducing cell surface target density but contributing to a soluble sink. When treated with GSI in vitro, myeloma cell surface BCMA expression increased concurrent with reduction in soluble/shed BCMA across a panel of myeloma and lymphoma cell lines expressing a range of BCMA. In cytotoxic T lymphocyte co-culture assays, the activity of PF-06863135 was potentiated by combining with GSI. Studies of the combination of PF-06863135 and GSI are ongoing in preclinical in vivo models of myeloma. Taken together, our preclinical studies provide insights into mechanisms of action and resistance for PF-06863135, which has potential for profound single agent activity that can be enhanced with therapeutic combinations.

Citation Format: Katarzyna Karwacz, Andrea T. Hooper, Chao-Pei Betty Chang, Heike Krupka, Jeffrey Chou, Viola Lam, Ivana Djuretic, Javier Chaparro-Riggers, Puja Sapra. BCMA-CD3 bispecific antibody PF-06863135: Preclinical rationale for therapeutic combinations [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4557.

Allogeneic FLT3 CAR T Cells with an Off-Switch Exhibit Potent Activity against AML and Can Be Depleted to Expedite Bone Marrow Recovery

Patients with relapsed or refractory acute myeloid leukemia (AML) have a dismal prognosis and limited treatment options. Chimeric antigen receptor (CAR) T cells have achieved unprecedented clinical responses in patients with B cell leukemias and lymphomas and could prove highly efficacious in AML. However, a significant number of patients with AML may not receive treatment with an autologous product due to manufacturing failures associated with low lymphocyte counts or rapid disease progression while the therapeutic is being produced. We report the preclinical evaluation of an off-the-shelf CAR T cell therapy targeting Fms-related tyrosine kinase 3 (FLT3) for the treatment of AML. Single-chain variable fragments (scFvs) targeting various epitopes in the extracellular region of FLT3 were inserted into CAR constructs and tested for their ability to redirect T cell specificity and effector function to FLT3⁺ AML cells. A lead CAR, exhibiting minimal tonic signaling and robust activity in vitro and in vivo, was selected and then modified to incorporate a rituximab-responsive off-switch in cis. We found that allogeneic FLT3 CAR T cells, generated from healthy-donor T cells, eliminate primary AML blasts but are also active against mouse and human hematopoietic stem and progenitor cells, indicating risk of myelotoxicity. By employing a surrogate CAR with affinity to murine FLT3, we show that rituximab-mediated depletion of FLT3 CAR T cells after AML eradication enables bone marrow recovery without compromising leukemia remission. These results support clinical investigation of allogeneic FLT3 CAR T cells in AML and other FLT3⁺ hematologic malignancies.

An Optimized Full-Length FLT3/CD3 Bispecific Antibody Demonstrates Potent Anti-leukemia Activity and Reversible Hematological Toxicity

FLT3 (FMS-like tyrosine kinase 3), expressed on the surface of acute myeloid leukemia (AML) blasts, is a promising AML target, given its role in the development and progression of leukemia, and its limited expression in tissues outside the hematopoietic system. Small molecule FLT3 kinase inhibitors have been developed, but despite having clinical efficacy, they are effective only on a subset of patients and associated with high risk of relapse. A durable therapy that can target a wider population of AML patients is needed. Here, we developed an anti-FLT3-CD3 immunoglobulin G (IgG)-based bispecific antibody (7370) with a high affinity for FLT3 and a long half-life, to target FLT3-expressing AML blasts, irrespective of FLT3 mutational status. We demonstrated that 7370 has picomolar potency against AML cell lines in vitro and in vivo. 7370 was also capable of activating T cells from AML patients, redirecting their cytotoxic activity against autologous blasts at low effector-to-target (E:T) ratio. Additionally, under our dosing regimen, 7370 was well tolerated and exhibited potent efficacy in cynomolgus monkeys by inducing complete but reversible depletion of peripheral FLT3⁺ dendritic cells (DCs) and bone marrow FLT3⁺ stem cells and progenitors. Overall, our results support further clinical development of 7370 to broadly target AML patients.

Bispecific Antibodies in the Treatment of Hematologic Malignancies

Monoclonal antibody therapies are an important approach for the treatment of hematologic malignancies, but typically show low single‐agent activity. Bispecific antibodies, however, redirect immune cells to the tumor for subsequent lysis, and preclinical and accruing clinical data support single‐agent efficacy of these agents in hematologic malignancies, presaging an exciting era in the development of novel bispecific formats. This review discusses recent developments in this area, highlighting the challenges in delivering effective immunotherapies for patients.

Human Placenta-Derived Mesenchymal Stromal-Like Cells Enhance Angiogenesis via T Cell-Dependent Reprogramming of Macrophage Differentiation

Peripheral arterial disease (PAD) is a leading cause of limb loss and mortality worldwide with limited treatment options. Mesenchymal stromal cell (MSC) therapy has demonstrated positive effects on angiogenesis in preclinical models and promising therapeutic efficacy signals in early stage clinical studies; however, the mechanisms underlying MSC-mediated angiogenesis remain largely undefined. Here, we investigated the mechanism of action of human placenta-derived MSC-like cells (PDA-002) in inducing angiogenesis using mice hind limb ischemia model. We showed that PDA-002 improved blood flow and promoted collateral vessel formation in the injured limb. Histological analysis demonstrated that PDA-002 increased M2-like macrophages in ischemic tissue. Analysis of the changes in functional T cell phenotype in the draining lymph nodes revealed that PDA-002 treatment was associated with the induction of cytokine and gene expression signatures of Th2 response. Angiogenic effect of PDA-002 was markedly reduced in Balb/c nude mice compared with wild type. This reduction in efficacy was reversed by T cell reconstitution, suggesting T cells are essential for PDA-002-mediated angiogenesis. Furthermore, effect of PDA-002 on macrophage differentiation was also T cell-dependent as a PDA-002-mediated M2-like macrophage skewing was only observed in wild type and T cell reconstituted nude mice, but not in nude mice. Finally, we showed that PDA-002-treated animals had enhanced angiogenic recovery in response to the second injury when PDA-002 no longer persisted in vivo. These results suggest that PDA-002 enhances angiogenesis through an immunomodulatory mechanism involving T cell-dependent reprogramming of macrophage differentiation toward M2-like phenotype. Stem Cells 2017;35:1603-1613.

Modulation of Cell Attachment, Proliferation, and Angiogenesis by Decellularized, Dehydrated Human Amniotic Membrane in In Vitro Models

BACKGROUND

Decellularized, dehydrated human amniotic membrane (DDHAM) is an extracellular matrix devoid of cells, cell debris, and growth factors. This study examines the effect of cell attachment to the DDHAM and the induced cellular responses.

MATERIALS AND METHODS

The cell types employed in this study were human dermal fibroblasts (HDF), human epithelial keratinocytes (HEK), and human dermal microvascular endothelial cells (HDMEC), all of which play critical roles in the wound healing process. Further, the DDHAM was compared to a dehydrated human amnion/chorion membrane (dHACM), which contains and releases biological entities including growth factors and cytokines. The HDF and HEK were cultured on the DDHAM and the dHACM, and cell imaging and proliferation assays were performed to evaluate cell attachment to and the ability to proliferate on the DDHAM relative to the dHACM. In addition, the effect of soluble factors released by the DDHAM and the dHACM on cell survival, attachment, and proliferation were examined. The authors also evaluated the effect of soluble factors produced by culturing cells on the DDHAM in in vitro functional assays, including cell survival and endothelial cell migration in a wound closure angiogenesis assay.

RESULTS

The HDF and HEK cells readily attached to and proliferated on the DDHAM, while the dHACM did not support cell attachment and proliferation when cultured under the same conditions. Soluble factors secreted when HDF were cultured on the DDHAM enhanced both endothelial cell and keratinocyte survival and endothelial cell migration in a wound closure assay.

CONCLUSIONS

Although DDHAM is only an extracellular matrix and serves primarily as a scaffold, it has sufficient cues to allow for cell attachment and proliferation. Further, the biological entities released as a consequence of cell attachment promote cell survival and migration.

Multi-drug resistant Th17 cells: new players in autoimmune and steroid-resistant inflammation (HUM8P.351)

Th17 cells are generally regarded as key effectors of autoimmune disease. However, not all Th17 cells are pro-inflammatory. Furthermore, the defining features of "pathogenic" Th17 cells in humans are unknown. We have found that pro-inflammatory human Th17 cells are restricted to a subset of CCR6+ CXCR3(hi) CCR4(lo) CCR10- CD161+ T cells that expresses the drug transporter multi-drug resistance-1. Unlike MDR1- human memory T cell subsets, MDR1+ Th17 cells produce both Th17 and Th1 cytokines upon stimulation; do not express IL-10 or other anti-inflammatory molecules; express high levels of IL23R as well as other genes selectively expressed by pathogenic mouse Th17 cells; and display heightened responsiveness to IL-23 stimulation. In vivo, MDR1+ Th17 cells accumulate in the gut of patients with active Crohn's Disease, where they display a marked pro-inflammatory transcriptional signature relative to local MDR1- effector T cells. In addition, MDR1+ Th17 cells are refractory to the immunosuppressive effects of several steroids used to treat autoimmune disease in the clinic. Thus, MDR1+ Th17 cells may be important mediators of autoimmune inflammation, particularly in clinical settings of steroid-resistant disease.

Transcriptional partners in regulatory T cells: Foxp3, Runx and NFAT

A general theme in gene regulation is that transcription factors never function alone. Recent studies have emphasized this concept for regulatory T cells, a unique lineage of CD4+ T cells that exert active immune suppression and are essential to maintaining self-tolerance.

Regulation of Th2 differentiation and Il4 locus accessibility

Helper T cells coordinate immune responses through the production of cytokines. Th2 cells express the closely linked Il4, Il13, and Il5 cytokine genes, whereas these same genes are silenced in the Th1 lineage. The Th1/Th2 lineage choice has become a textbook example for the regulation of cell differentiation, and recent discoveries have further refined and expanded our understanding of how Th2 differentiation is initiated and reinforced by signals from antigen-presenting cells and cytokine-driven feedback loops. Epigenetic changes that stabilize the active or silent state of the Il4 locus in differentiating helper T cells have been a major focus of recent research. Overall, the field is progressing toward an integrated model of the signaling and transcription factor networks, cis-regulatory elements, epigenetic modifications, and RNA interference mechanisms that converge to determine the lineage fate and gene expression patterns of differentiating helper T cells.