Spatially Confined T-Cell Mediated Anti-AML Tumor Immunity By Dual Recognition Antibodies (CD123/CD200)

Definitive cure of acute myeloid leukemia (AML) remains an unmet need. While recombinant antibody-based and CAR-T cell mediated immunotherapies bear great potential in targeting chemotherapy-resistant leukemic stem cells, these approaches are largely limited by the lack of specific targets uniquely expressed by leukemic cells.

Here, we propose a combinatorial strategy using a two-target approach which may have improved on-target and reduced off-target effects. The well-known stem cell marker interleukin-3 receptor α (CD123) is expressed on most AML cells. CD200 recently emerged as a cell surface marker mediating immuno-evasion on high-risk AML cells. We engineered the "Prodrug-activating chain Exchange"(PACE) TriFAB system (Dickopf S et al. Biological Chemistry, 2022) to yield a combinatorial anti-AML directed T-cell response.

PACE-TriFAB educts are complementary Knob-into-hole IgG derived prodrugs in which one half is exchanged by a dummy containing an interface destabilizing mutation in the CH3 domain. The CH2 domain of the main body is replaced by a CD3 FAB light or heavy chain, respectively. With the hinge region being replaced by a flexible linker and therefore lacking disulfide bonds, a chain exchange between two educts is enabled when in close proximity on the cell surface. This leads to an activated anti CD3 domain and a dummy-dummy by-product.

To demonstrate the T-cell activating potential, we co-cultured fresh PBMCs from healthy donors together with the CD200⁺/CD123⁺ AML cell line Kasumi-1 and checked T-cell activation by flow cytometry (E:T ratio 5:1, prodrug conc. 20 nM, 24 h stimulation). We observed a marked increase in CD69, CD25 and CD137 expression on CD8⁺ cells in the case when the CD123 and CD200 prodrugs were applicated simultaneously together with target cells, but not when single prodrugs alone or without target cells were analyzed (Median fluorescence intensity for CD25, CD69 and CD137: No construct: 450, 8.900 & 2.100; prodrug CD200: 460, 8.700 & 2.100; prodrug CD123: 460, 9.600 & 2.000; both prodrugs: 6.500, 344.300 & 43.100; results confirmed in repeat experiments)

For the evaluation of the specific cytotoxic effect of the construct we transduced Kasumi-1 cells with an mCherry lentiviral vector and imaged the cells co-cultured with PBMCs every 2 h over a time span of 60 h (E:T ratio 5:1, prodrug conc. 10 nM). Proliferation of the Kasumi-1-mCherry cells was observed when no prodrug or only a single prodrug was added. However, application of both prodrugs together led to a decrease in mCherry positive cell count (log2 fold changes to starting point: No PBMCs 1,2; no construct 0,9; prodrug CD200 0,9; prodrug CD123 0,4; both prodrugs -2,1; n=3). Not only could we show that this elimination of tumor cells was dose dependent but also it was constrained by the E:T ratio (pivot point from killing to proliferation at 0,1 nM and E:T of 1:2 n=3).

Similar results were obtained using patient derived xenograft (PDX) AML cells. In contrast to Kasumi-1 AML cells, specific killing of patient-derived PDX cells could be documented with lower E:T ratios and lower prodrug concentrations (pivot point from killing to proliferation at 0,05 nM and E:T of 1:5)

We also started to analyze the anti-leukemic effect in vivo. Luciferase-expressing CD200⁺/CD123⁺ AML PDX cells were injected into NSG mice. On day +15, PBMC from a healthy individual were transplanted, followed by additional applications of the antibodies on days +16, +18, +21, +23, +25 & +29. Tumor burden was analyzed by bioluminescence imaging (log10 fold change [P/sec] 2 weeks after PBMC injection: Control 1,9; treated -0,4; group size = 3).

In conclusion the combination of anti CD200 and anti CD123 as prodrugs of an on-target activating trispecific T-cell engager kills AML cells in vitro and in vivo and has the potential to eliminate AML LSC while minimizing the risk for severe on-target off-leukemia toxicity.

Dickopf:Roche: Ended employment in the past 24 months. Brinkmann:Roche: Current Employment, Current equity holder in publicly-traded company, Patents & Royalties.