Background
Glioblastoma harbors a tumor microenvironment (TME) that is mostly devoid of effector T cells but is dominated by immunosuppressive microglia, macrophages, and myeloid-derived suppresser cells. Innate immune cells in the TME have impaired phagocytosis and antigen presentation and are inadequate for triggering the immune activating signals needed for anti-tumor effector responses. Glioblastoma tumors further restrict phagocytosis and subsequent tumor antigen presentation via CD47 upregulation. We show in the Qki-/- Pten-/- P53-/- (QPP) tumor model, which reflects the immunosuppressive and immune checkpoint blockade resistant phenotypes of human glioblastoma, that agonists of the Stimulator of Interferon Genes (STING) dsDNA sensing pathway induce proinflammatory conversion of the TME and synergize with CD47 blockade to generate curative responses.
Methods
Using the synthetic cyclic di-nucleotide STING agonist IACS-8803 (8803; IMGS-203) and the anti-CD47 clone MIAP-410 we treated orthotopic murine QPP tumors intratumorally (8803) and systemically (CD47). We treated huNOG-EXL mice bearing human U87 glioma with intratumoral STING agonists 8803 and ML-RR. We analyzed survival and performed high parameter flow cytometry profiling of the tumor immune microenvironment following STING agonist treatment and CD47 blockade. T and NK cell therapeutic contributions were assessed using in vivo NK depletion studies and treatment in the RAG-/- background.
Results
In the QPP glioma model, intratumoral 8803 induces global immunological reprogramming and long-term survival resistant to tumor rechallenge. In the myeloid stroma 8803 increases costimulatory CD86 while reducing the expression of suppressive markers CD163 and CD206. cDC1 dendritic cells are increased in both the TME and the draining cervical lymph node. Intratumoral infiltration of both CD8 T and NK cells increases; however, only CD8 depletion abrogates 8803 therapeutic benefit. In the humanized U87 glioma model, intratumoral STING agonist extends survival and decreases human myeloid stroma CD163 and CD206. STING activation increases microglial phagocytic capacity, but also induces a compensatory increase in SIRPα expression – the ligand for CD47. 8803 synergizes with anti-CD47 blockade to enhance microglia phagocytosis of QPP glioblastoma cells in vitro. Finally, the combination of 8803 and anti-CD47 synergizes to extend both median survival and the fraction of long-term survivors in orthotopic QPP-bearing mice.
Conclusions
These data demonstrate that 8803 induces proinflammatory conversion of the glioblastoma TME, generates T cell-dependent survival benefits in QPP-bearing mice and synergizes with anti-CD47 blockade to enhance microglia phagocytosis of glioblastoma cells and extend survival in QPP-bearing mice. Additionally, STING activation reduces immune suppression and extends survival in a human immune infiltrate versus human glioblastoma orthotopic setting.
Ethics Approval
All in vivo mouse experiments were approved in accordance with Laboratory Animal Resources Commission Standards by the Institutional Animal Care and Use Committee (IACUC) and conducted according to the approved protocols 0001378-RN01/RN02 at MD Anderson Cancer Center.