T cell-recruiting triplebody 19-3-19 mediates serial lysis of malignant B-lymphoid cells by a single T cell

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-1_ex), 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-1_ex 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.

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.

Antigen-specific T cell Redirectors: a nanoparticle based approach for redirecting T cells

Redirection of T cells to target and destroy tumors has become an important clinical tool and major area of research in tumor immunology. Here we present a novel, nanoparticle-based approach to selectively bind antigen-specific cytotoxic T cells (CTL) and redirect them to kill tumors, termed ATR (Antigen-specific T cell Redirectors). ATR were generated by decorating nanoparticles with both an antigen-specific T cell binding moiety, either peptide loaded MHC-Ig dimer or clonotypic anti-TCR antibody, and a model tumor cell binding moiety, anti-CD19 antibody to engage CD19+ tumor cells. ATR stably bind tumor cells and CTL in a dose dependent fashion and stimulate antigen-specific conjugate formation between those cells. ATR induced redirected lysis of tumor cells in vitro, as demonstrated by 51Cr-release killing. In vivo ATR administration led to reduced tumor growth in a SCID/beige human lymphoma treatment model. In summary, ATR represent a novel, nanoparticle based approach for redirecting antigen-specific CTL to kill tumors.

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.

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.