Sting Activation Improves T-Cell Engaging Immunotherapy of Acute Myeloid Leukemia

Background:

Despite emerging therapeutic options for Acute myeloid leukemia (AML) patients, for the majority of relapsed/refractory patients the outcome remains poor. Allogeneic stem cell transplantation has proven the power of T cells in eradicating residual leukemic cells, but alternative strategies based on T-cell recruiting bispecifics have so far failed to induce sustained responses. Resistance is mediated by the immunosuppressive tumor-microenvironment and secretion of immune dampening metabolites by AML cells. We hypothesized that combining a CD33-directed T cell engager (CD33 BiTE^® molecule or AMG 330) with a cGAS-STING agonist has the potential to augment anti-leukemic activity.

Methods:

In vitro co-culture assays of human T cells and AML cell lines for up to 72 h were performed to study the effect of AMG 330 in combination with the physiological STING-agonist 2’-3’ cGAMP (cGAMP). The cytotoxicity against the AML cell lines HL-60, MOLM-13 and OCI-AML-3 was assessed by flow cytometry. T-cell proliferation, T-cell degranulation (CD107a), as well as Granzyme B and TRAIL expression were characterized by flow cytometry. The secretion of effector cytokines (IFNy, TNF, IL-2, IFNa2a, CXCL10) was analyzed via CBA and ELISA assays. To better understand how AMG 330 and cGAMP work together, immunoblotting, transcriptomic analysis (bulk RNA-sequencing) and comprehensive CRISPRCas9 knockout-studies were carried out.

Results:

We observed a significant increase in AMG 330-mediated cytotoxicity of AML target cells upon co-application of cGAMP (Fig1A). The latter observation was enhanced upon prolongation of the culture and was also notable at low effector-to-target ratios. Monotherapy or addition of cGAMP by itself exerted only minor cytotoxicity against AML cells. The increase in AMG 330 mediated cytotoxicity through cGAMP was accompanied by an increase in T-cell proliferation, degranulation, Granzyme B secretion and TRAIL expression. Moreover, we observed a strong increase in secretion of proinflammatory cytokines (IFNy, IL-2 and IFNa2a). Bulk RNA-sequencing of AMG 330 and cGAMP stimulated co-cultures revealed type-I-IFN signature in the target (HL-60) and the effector cell (T cells) cell population. In addition, distinct phenotypes were found in each cell population: T cells demonstrated a marked increase in TNFa, IFNy and GZMB expression through the co-treatment, whereas HL-60 cells displayed an IFNy signaling signature (Fig1B). Importantly, the responsiveness of AML target cells to STING activation in terms of interferon stimulated gene (ISG)-induction was increased by the co-treatment, evidenced by further upregulation of STAT1, IRF7, WARS, CXCL11, IFI16, RSAD2 and CXCL10. An increase in type-I-IFN production and subsequent ISG-induction could also be confirmed on the protein level by measuring IFNα2a and CXCL-10.

Knockout-studies revealed that the cGAMP enhanced AMG 330-mediated cytotoxicity was fully dependent on target cell intrinsic STING and IRF3 signaling, but also dependent on functional IFNy and TNFa signaling. Notably, the increased IFNy production by T cells upon cGAMP addition was dependent on functional target cell STING and IFNy signaling. These observations imply a crosstalk between effector and target cells and an intricate interdependence between target cell intrinsic IFNγ- and STING-signaling. Of note, the deletion of the IFNy-receptor in AML target cells also reduced AMG 330-dependent cytotoxicity under steady state conditions. Finally, we demonstrated that the combination of IFNy, TNFa and cGAMP sufficed to induce an apoptotic cell death of AML cells, similar to the cGAMP containing T-cell-HL-60 cocultures.

Conclusion:

We propose a novel mechanism by which AMG 330-activated T cells prime and sensitize AML target cells in a forward feedback loop towards STING activation, leading to increased type-I-IFN production and induction of ISGs. The beneficial effect of physiological cGAMP in enhancing AMG 330-mediated cytotoxicity was accompanied by the pronounced expression of effector cytokines and an overall cytotoxic T-cell phenotype. We established a key role for interferon gamma in AMG 330-mediated cytotoxicity of AML cells and in rendering AML cells responsive to STING agonism. We are currently conducting in vivoexperiments to further validate our findings.