STING activation improves T-cell-engaging immunotherapy for acute myeloid leukemia

T-cell recruiting bispecific antibodies (BsAbs) are in clinical development for relapsed/refractory acute myeloid leukemia (AML). Despite promising results, early clinical trials have failed to demonstrate durable responses. We investigated whether activation of the innate immune system through stimulator of interferon genes (STING) can enhance target-cell killing by a BsAb targeting CD33 (CD33 BiTE® molecule, AMG 330). Indeed, we show that cytotoxicity against AML mediated by AMG 330 can be greatly enhanced when combined with the STING agonist 2',3'-cyclic GMP-AMP (cGAMP), or diABZI. We used in vitro cytotoxicity assays, immunoblotting, transcriptomic analyses, and extensive CRISPR-Cas9 knockout experiments to investigate the enhancing effect of a STING agonist on the cytotoxicity of AMG 330 against AML. Importantly, we validated our findings with primary AML cells, and in a xenograft AML model. Mechanistically, in addition to direct cytotoxic effects of STING activation on AML cells, activated T cells render AML cells more susceptible to STING activation through their effector cytokines interferon-gamma (IFNγ) and tumor necrosis factor (TNF), resulting in enhanced type I interferon production and induction of interferon-stimulated genes. This feeds back to the T cells, leading to a further increase in effector cytokines and an overall cytotoxic T-cell phenotype, contributing to the beneficial effect of cGAMP/diABZI in enhancing AMG 330-mediated lysis. We established a key role for IFNγ in AMG 330-mediated cytotoxicity against AML cells, and in rendering AML cells responsive to STING agonism. Here, we propose to improve the efficacy of CD33-targeting BsAbs by combining them with a STING agonist.