SIRPα-αCD123 fusion antibodies targeting CD123 in conjunction with CD47 blockade enhance the clearance of AML-initiating cells.

BACKGROUND: Acute myeloid leukaemia (AML) stem cells (LSCs) cause disease relapse. The CD47 "don't eat me signal" is upregulated on LSCs and contributes to immune evasion by inhibiting phagocytosis through interacting with myeloid-specific signal regulatory protein alpha (SIRPα). Activation of macrophages by blocking CD47 has been successful, but the ubiquitous expression of CD47 on healthy cells poses potential limitations for such therapies. In contrast, CD123 is a well-known LSC-specific surface marker utilized as a therapeutic target. Here, we report the development of SIRPα-αCD123 fusion antibodies that localize the disruption of CD47/SIRPα signalling to AML while specifically enhancing LSC clearance. METHODS: SIRPα-αCD123 antibodies were generated by fusing the extracellular domain of SIRPα to an αCD123 antibody. The binding properties of the antibodies were analysed by flow cytometry and surface plasmon resonance. The functional characteristics of the fusion antibodies were determined by antibody-dependent cellular phagocytosis and antibody-dependent cellular cytotoxicity assays using primary AML patient cells. Finally, an in vivo engraftment assay was utilized to assess LSC targeting. RESULTS: SIRPα-αCD123 fusion antibodies exhibited increased binding and preferential targeting of CD123+ CD47+ AML cells even in the presence of CD47+ healthy cells. Furthermore, SIRPα-αCD123 fusion antibodies confined disruption of the CD47-SIRPα axis locally to AML cells. In vitro experiments demonstrated that SIRPα-αCD123 antibodies greatly enhanced AML cell phagocytosis mediated by allogeneic and autologous macrophages. Moreover, SIRPα-αCD123 fusion antibodies efficiently targeted LSCs with in vivo engraftment potential. CONCLUSIONS: SIRPα-αCD123 antibodies combine local CD47 blockade with specific LSC targeting in a single molecule, minimize the risk of targeting healthy cells and efficiently eliminate AML LSCs. These results validate SIRPα-αCD123 antibodies as promising therapeutic interventions for AML.

A modular and controllable T cell therapy platform for acute myeloid leukemia.

Targeted T cell therapy is highly effective in disease settings where tumor antigens are uniformly expressed on malignant cells and where off-tumor on-target-associated toxicity is manageable. Although acute myeloid leukemia (AML) has in principle been shown to be a T cell-sensitive disease by the graft-versus-leukemia activity of allogeneic stem cell transplantation, T cell therapy has so far failed in this setting. This is largely due to the lack of target structures both sufficiently selective and uniformly expressed on AML, causing unacceptable myeloid cell toxicity. To address this, we developed a modular and controllable MHC-unrestricted adoptive T cell therapy platform tailored to AML. This platform combines synthetic agonistic receptor (SAR) -transduced T cells with AML-targeting tandem single chain variable fragment (scFv) constructs. Construct exchange allows SAR T cells to be redirected toward alternative targets, a process enabled by the short half-life and controllability of these antibody fragments. Combining SAR-transduced T cells with the scFv constructs resulted in selective killing of CD33+ and CD123+ AML cell lines, as well as of patient-derived AML blasts. Durable responses and persistence of SAR-transduced T cells could also be demonstrated in AML xenograft models. Together these results warrant further translation of this novel platform for AML treatment.

Fusion of Bacterial Flagellin to a Dendritic Cell-Targeting αCD40 Antibody Construct Coupled With Viral or Leukemia-Specific Antigens Enhances Dendritic Cell Maturation and Activates Peptide-Responsive T Cells.

Conventional dendritic cell (DC) vaccine strategies, in which DCs are loaded with antigens ex vivo, suffer biological issues such as impaired DC migration capacity and laborious GMP production procedures. In a promising alternative, antigens are targeted to DC-associated endocytic receptors in vivo with antibody-antigen conjugates co-administered with toll-like receptor (TLR) agonists as adjuvants. To combine the potential advantages of in vivo targeting of DCs with those of conjugated TLR agonists, we generated a multifunctional antibody construct integrating the DC-specific delivery of viral- or tumor-associated antigens and DC activation by TLR ligation in one molecule. We validated its functionality in vitro and determined if TLR ligation might improve the efficacy of such a molecule. In proof-of-principle studies, an αCD40 antibody containing a CMV pp65-derived peptide as an antigen domain (αCD40CMV) was genetically fused to the TLR5-binding D0/D1 domain of bacterial flagellin (αCD40.FlgCMV). The analysis of surface maturation markers on immature DCs revealed that fusion of flagellin to αCD40CMV highly increased DC maturation (3.4-fold elevation of CD80 expression compared to αCD40CMV alone) by specifically interacting with TLR5. Immature DCs loaded with αCD40.FlgCMV induced significantly higher CMVNLV-specific T cell activation and proliferation compared to αCD40CMV in co-culture experiments with allogeneic and autologous T cells (1.8-fold increase in % IFN-γ/TNF-α+ CD8+ T cells and 3.9-fold increase in % CMVNLV-specific dextramer+ CD8+ T cells). More importantly, we confirmed the beneficial effects of flagellin-dependent DC stimulation using a tumor-specific neoantigen as the antigen domain. Specifically, the acute myeloid leukemia (AML)-specific mutated NPM1 (mNPM1)-derived neoantigen CLAVEEVSL was delivered to DCs in the form of αCD40mNPM1 and αCD40.FlgmNPM1 antibody constructs, making this study the first to investigate mNPM1 in a DC vaccination context. Again, αCD40.FlgmNPM1-loaded DCs more potently activated allogeneic mNPM1CLA-specific T cells compared to αCD40mNPM1. These in vitro results confirmed the functionality of our multifunctional antibody construct and demonstrated that TLR5 ligation improved the efficacy of the molecule. Future mouse studies are required to examine the T cell-activating potential of αCD40.FlgmNPM1 after targeting of dendritic cells in vivo using AML xenograft models.

Bifunctional PD-1 × αCD3 × αCD33 fusion protein reverses adaptive immune escape in acute myeloid leukemia.

The CD33-targeting bispecific T-cell engager (BiTE) AMG 330 proved to be highly efficient in mediating cytolysis of acute myeloid leukemia (AML) cells in vitro and in mouse models. Yet, T-cell activation is correlated with upregulation of programmed cell death-ligand 1 (PD-L1) and other inhibitory checkpoints on AML cells that confer adaptive immune resistance. PD-1 and PD-L1 blocking agents may counteract T-cell dysfunction, however, at the expense of broadly distributed immune-related adverse events (irAEs). We developed a bifunctional checkpoint inhibitory T cell-engaging (CiTE) antibody that combines T-cell redirection to CD33 on AML cells with locally restricted immune checkpoint blockade. This is accomplished by fusing the extracellular domain of PD-1 (PD-1ex), which naturally holds a low affinity to PD-L1, to an αCD3.αCD33 BiTE-like scaffold. By a synergistic effect of checkpoint blockade and avidity-dependent binding, the PD-1ex attachment increases T-cell activation (3.3-fold elevation of interferon-γ) and leads to efficient and highly selective cytotoxicity against CD33+PD-L1+ cell lines (50% effective concentration = 2.3-26.9 pM) as well as patient-derived AML cells (n = 8). In a murine xenograft model, the CiTE induces complete AML eradication without initial signs of irAEs as measured by body weight loss. We conclude that our molecule preferentially targets AML cells, whereas high-affinity blockers, such as clinically approved anticancer agents, also address PD-L1+ non-AML cells. By combining the high efficacy of T-cell engagers with immune checkpoint blockade in a single molecule, we expect to minimize irAEs associated with the systemic application of immune checkpoint inhibitors and suggest high therapeutic potential, particularly for patients with relapsed/ refractory AML.

Dual-targeting triplebody 33-16-123 (SPM-2) mediates effective redirected lysis of primary blasts from patients with a broad range of AML subtypes in combination with natural killer cells.

A number of agents designed for immunotherapy of Acute Myeloid Leukemia (AML) are in preclinical and early clinical development. Most of them target a single antigen on the surface of AML cells. Here we describe the development and key biological properties of a tri-specific agent, the dual-targeting triplebody SPM-2, with binding sites for target antigens CD33 and CD123, and for CD16 to engage NK cells as cytolytic effectors. Primary blasts of nearly all AML patients carry at least one of these target antigens and the pair is particularly promising for the elimination of blasts and leukemia stem cells (LSCs) from a majority of AML patients by dual-targeting agents. The cytolytic activity of NK cells mediated by SPM-2 was analyzed in vitro for primary leukemic cells from 29 patients with a broad range of AML-subtypes. Blasts from all 29 patients, including patients with genomic alterations associated with an unfavorable genetic subtype, were lysed at nanomolar concentrations of SPM-2. Maximum susceptibility was observed for cells with a combined density of CD33 and CD123 above 10,000 copies/cell. Cell populations enriched for AML-LSCs (CD34pos and CD34pos CD38neg cells) from 2 AML patients carried an increased combined antigen density and were lysed at correspondingly lower concentrations of SPM-2 than unsorted blasts. These initial findings raise the expectation that SPM-2 may also be capable of eliminating AML-LSCs and thus of prolonging survival. In the future, patients with a broad range of AML subtypes may benefit from treatment with SPM-2.

SIRPα-antibody fusion proteins stimulate phagocytosis and promote elimination of acute myeloid leukemia cells.

CD47, expressed on a variety of tumor cells, confers immune resistance by delivering an inhibitory "don't eat me" signal to phagocytic cells via its myeloid-specific receptor SIRPα. Recent studies have shown that blocking the CD47-SIRPα axis with CD47-directed antibodies or antibody-derivatives enhances phagocytosis and increases antitumor immune effects. However, CD47 expression on healthy cells creates an antigen sink and potential sites of toxicity, limiting the efficacy of CD47-directed therapies. In this study, we first characterized CD47 expression in Acute Myeloid Leukemia (AML) patients (n = 213) and found that CD47 is highly expressed on both AML bulk and stem cells irrespective of the disease state. Furthermore, to inhibit the CD47-SIRPα signaling pathway at the tumor site, we developed a so-called local inhibitory checkpoint monoclonal antibody (licMAB) by grafting the endogenous SIRPα domain to the N-terminus of the light chain of an antibody targeting CD33, a surface antigen expressed in AML. LicMABs selectively bind CD33-expressing cells even in the presence of a large CD33-negative CD47-positive antigen sink, stimulate phagocytosis of AML cells and eliminate AML cell lines and primary, patient-derived AML cells. Our findings qualify licMABs as a promising therapeutic approach to confine the benefit of disrupting the CD47-SIRPα axis to tumor antigen-expressing cells.

CD19-specific triplebody SPM-1 engages NK and γδ T cells for rapid and efficient lysis of malignant B-lymphoid cells.

Triplebodies are antibody-derived recombinant proteins carrying 3 antigen-binding domains in a single polypeptide chain. Triplebody SPM-1 was designed for lysis of CD19-bearing malignant B-lymphoid cells through the engagement of CD16-expressing cytolytic effectors, including NK and γδ T cells.SPM-1 is an optimized version of triplebody ds(19-16-19) and includes humanization, disulfide stabilization and the removal of potentially immunogenic sequences. A three-step chromatographic procedure yielded 1.7 - 5.5 mg of purified, monomeric protein per liter of culture medium. In cytolysis assays with NK cell effectors, SPM-1 mediated potent lysis of cancer-derived B cell lines and primary cells from patients with various B-lymphoid malignancies, which surpassed the ADCC activity of the therapeutic antibody Rituximab. EC50-values ranged from 3 to 86 pM. Finally, in an impedance-based assay, SPM-1 mediated a particularly rapid lysis of CD19-bearing target cells by engaging and activating both primary and expanded human γδ T cells from healthy donors as effectors.These data establish SPM-1 as a useful tool for a kinetic analysis of the cytolytic reactions mediated by γδ T and NK cells and as an agent deserving further development towards clinical use for the treatment of B-lymphoid malignancies.

Chip-based platform for dynamic analysis of NK cell cytolysis mediated by a triplebody.

Cancer therapy via redirected lysis mediated by antibodies and antibody-derived agents relies on the availability of substantial numbers of sufficiently active immune effector cells. To monitor antitumor responses before and during therapy, sensitive methods are needed, capable of quantitating specific lysis of target cells. Here we present a chip-based single-cell cytometric assay, which uses adherent human target cells arrayed in structured micro-fields. Using a fluorescent indicator of cell death and time-lapse microscopy in an automated high-throughput mode, we measured specific target cell lysis by activated human NK cells, mediated by the therapeutic single chain triplebody SPM-2 (33-16-123). This antibody-derived tri-specific fusion protein carries binding sites for the myeloid antigens CD33 and CD123 and recruits NK cells via a binding site for the Fc-receptor CD16. Specific lysis increased with increasing triplebody concentration, and the single-cell assay was validated by direct comparison with a standard calcein-release assay. The chip-based approach allowed measurement of lysis events over 16 hours (compared to 4 hours for the calcein assay) and required far smaller numbers of primary cells. In addition, dynamic properties inaccessible to conventional methods provide new details about the activation of cytolytic effector cells by antibody-derived agents. Thus, the killing rate exhibited a dose-dependent maximum during the reaction interval. In clinical applications ex vivo monitoring of NK activity of patient's endogenous cells will likely help to choose appropriate therapy, to detect impaired or recovered NK function, and possibly to identify rare subsets of cancer cells with particular sensitivity to effector-cell mediated lysis.

Dual-targeting triplebody 33-3-19 mediates selective lysis of biphenotypic CD19+ CD33+ leukemia cells.

Simultaneous targeting of multiple tumor-associated antigens (TAAs) in cancer immunotherapy is presumed to enhance tumor cell selectivity and to reduce immune escape.The combination of B lymphoid marker CD19 and myeloid marker CD33 is exclusively present on biphenotypic B/myeloid leukemia cells. Triplebody 33-3-19 binds specifically to both of these TAAs and activates T cells as immune effectors. Thereby it induces specific lysis of established myeloid (MOLM13, THP-1) and B-lymphoid cell lines (BV173, SEM, Raji, ARH77) as well as of primary patient cells. EC50 values range from 3 pM to 2.4 nM. In accordance with our hypothesis, 33-3-19 is able to induce preferential lysis of double- rather than single-positive leukemia cells in a target cell mixture: CD19/CD33 double-positive BV173 cells were eliminated to a significantly greater extent than CD19 single-positive SEM cells (36.6% vs. 20.9% in 3 hours, p = 0.0048) in the presence of both cell lines. In contrast, equivalent elimination efficiencies were observed for both cell lines, when control triplebody 19-3-19 or a mixture of the bispecific single chain variable fragments 19-3 and 33-3 were used. This result highlights the potential of dual-targeting agents for efficient and selective immune-intervention in leukemia patients.