IMMU-38. LEVERAGING INNATE IMMUNE SENSORS TO GENERATE DURABLE ANTI-GLIOMA ADAPTIVE IMMUNE RESPONSES

Abstract Glioblastoma harbors a tumor microenvironment (TME) that is mostly devoid of T cell effector infiltration but is dominated by immune suppressive microglia, macrophages, and myeloid-derived suppresser cells. The innate immune cells in the TME have impaired phagocytosis and antigen presentation and are inadequate for triggering the subsequent immune activating signals needed for anti-tumor effector responses. Phagocytic activity can also be blocked by the upregulation of CD47 on the tumor cells. In the immune checkpoint resistant Qki-/- Pten-/- P53-/- (QPP) glioma model, intratumoral treatment with the synthetic cyclic di-nucleotide STING agonist IACS-8803 (8803) induces global immunological reprogramming and long-term survival resistant to tumor rechallenge. More specifically, in the infiltrating myeloid stroma 8803 increases costimulatory protein CD86 while reducing the expression of suppressive markers CD163 and CD206. Conventional type 1 dendritic cells are increased in both the TME (p<0.05) and the draining cervical lymph node (p<0.01). Intratumoral infiltration of both CD8 T (p<0.05) and NK (p<0.001) cells increases. However, only T cells, but not NK cells, are necessary for 8803 therapeutic activity when assessed with in vivo NK cell depletion studies and treatment in the RAG-/- background. STING activation increases microglial phagocytic capacity, but also induces a compensatory increase in SIRPα expression (p<0.001) – 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 (QPP8v; vehicle control MS=39d, anti-CD47 MS=40d, 8803 MS=67d 20% long-term survival, 8803 + anti-CD47 MS=107.5d 40% long-term survival). 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.