Sting Agonists Activate Bone Marrow Macrophages and Synergise with Anti-CD47 Blockade Improving Survival in Animal Models of Acute Myeloid Leukemia

The bone marrow (BM) microenvironment regulates acute myeloid leukemia (AML) initiation, proliferation, and chemotherapy resistance. Previous research by our group has identified activation of the stimulator of interferon genes (STING) pathway within BM macrophages supresses leukemic growth. This contrasts with solid tumours where infiltrating macrophages release immunosuppressive signals, allowing cancer progression. AML blasts also regulate the action of macrophages by activating the 'don't eat me’ signal via interactions between CD47 and SIRPα on macrophages. Recent studies have led to clinical trials investigating the potential of blocking CD47 in patients with AML. Here we hypothesise and investigate whether activation of the STING pathway in macrophages of the AML tumor microenvironment will enhance the anti-leukemic effects of anti-CD47 antibody in disease models of AML.

Initially we found that activation of STING pathway, using DMXAA or cGAMP, in bone marrow derived macrophages (BMDM) induced clearance (via phagocytosis) of MN1 and HOXA9/Meis1 cells (two murine AML models) from the BM of animals with AML. To determine if the STING agonist activated the STING pathway we performed in vitro and in vivo experiments. In vitro BMDM treated with DMXAA or cGAMP induced STING gene expression signature (upregulation of GBP, IFIT3 and IRF3). For in vivo assessment of STING activation in macrophages, C57Bl/6 animals were treated with DMXAA for 24h, followed by isolation by FACS of BM macrophages (GR1-, F4/80+/ CD115int). RNA extracted and qPCR showed upregulation of STING gene expression signature. Next using two established AML in vivo models (HOXA9/Meis1 and MN1) we investigated the effect of STING agonist on AML progression. Results show that treatment with STING agonist significantly improved survival of mice with AML. Moreover, the numbers and activity of cytotoxic T cells was not changed between treatment groups.

Next, we explored the role of anti-CD47 blockade in combination with STING agonist. In vitro CD47 blockade synergised with DMXAA to significantly increase phagocytosis of AML by macrophages. In vivo we found that mice with AML receiving the combined treatments (DMXAA and anti-CD47) demonstrated significantly prolonged survival, over animals given either treatment alone.

In summary, we report that BM macrophages activated via the STING pathway supresses AML expansion through phagocytosis and not T cell mediated cytotoxicity. A process which is enhanced by anti-CD47 blocked on AML blasts. The results reported here provide a molecular mechanistic rationale for the clinical evaluation of targeting STING and CD47 in patients with AML.

No relevant conflicts of interest to declare.