Repolarisation of M2 Macrophages Via Cgas-Sting Activation Enhances Phagocytosis of Acute Myeloid Leukaemia

The tumour microenvironment is composed of extracellular matrix and non-mutated cells supporting tumour growth and development. Tumour-associated macrophages are among the most abundant immune cells in the microenvironment. Studies of the innate immune compartment in the bone marrow of patients with acute myeloid leukaemia (AML) reveal a shift toward a tumour-supportive M2-polarised macrophage (Weinhäuser et al., 2023). Furthermore, previous work by our group has shown that AML drives activation of the stimulator of interferon genes (STING) pathway in macrophages in the AML bone marrow microenvironment (Moore et al., 2022). We found that AML-derived mitochondrial damage-associated molecular patterns were processed by bone marrow macrophages via LC3-associated phagocytosis. This activation of STING resulted in a suppression of AML growth.

To fully characterise the role of STING pathway in AML-associated macrophages we have combined RNA sequencing, flow cytometry and in vivo characterisation to reveal a shift of M2 tumour supportive macrophages towards an M1 phenotype in response to STING activation. We used various STING agonists including DMXAA, 2’3’-cGAMP, and CpG oligonucleotides to model STING activation of bone marrow derived macrophages (BMDM) in vitro. Functionally, STING activation of BMDM induced an anti-tumour response by increasing phagocytosis of AML blasts which was also established in an in vivo model of AML in C57Bl/6 mice. Using the well-established syngeneic AML murine model (MN1), STING activation in vivo slowed tumour progression and prolonged survival. RNA sequencing of STING activation in macrophages revealed a M1 proinflammatory polarisation phenotype. Moreover, AML-induced M2 macrophage polarisation was reversed in response to STING activation.

Additionally, we show that STING activation upregulates genes involved with cell-cell adhesion, and we used blocking antibodies to assess their involvement in the increased phagocytic clearance of AML blasts. We observed that inhibiting the ICAM1/LFA1 interaction blocked STING-induced phagocytosis of AML blasts, therefore demonstrating the role this interaction plays in mediating the anti-tumour response of STING activation. Finally, we show that AML cells have higher levels of LFA1 cell surface expression than non-AML cells within the bone marrow. This is further supported by the BloodSpot database which demonstrates a higher level of LFA1 expression in human AML subtypes compared to haematopoietic stem and progenitor cells (Bagger, F.O. et al., 2016).

Our study provides insight into mechanisms by which AML-associated macrophages can be reprogrammed through STING activation towards an anti-tumour, pro-phagocytic phenotype, thus providing an alternative strategy for targeting leukaemia progression.