Activation of the Sting Pathway Enhances Immunity and Improves Survival in a Murine Myeloid Leukemia Model

Type I interferon (IFN) production by innate immune cells is critical to prime spontaneous T cell responses against solid tumors. Emerging data suggests that tumor-derived DNA induces IFN-β production through activation of a cytosolic DNA sensing receptor called STING (Stimulator of Interferon Genes). While this leads to functional priming of antigen-specific T cells and tumor rejection in solid tumor models, the disseminated growth pattern of hematological malignancies may result in insufficient quantities of DNA being released upon apoptosis to activate the STING pathway and may contribute to T cell tolerance. Thus, we hypothesized that activating the STING pathway would promote a type I IFN response sufficient to facilitate priming of leukemia-specific T cells and prolong survival of mice with acute myeloid leukemia (AML). Murine C1498 AML cells engineered to express a model peptide antigen called SIY (C1498.SIY) were inoculated intravenously into syngeneic C57BL/6 mice and then treated with a selective murine STING agonist, DMXAA (5,6-dimethylxanthenone-4-acetic acid), or vehicle control. Preliminary experiments confirmed that DMXAA administration resulted in robust IFN-β production by C57BL/6 spleen cells (90-fold induction over vehicle control). STING pathway activation via DMXAA treatment of C1498.SIY-bearing animals resulted in a massive expansion (10-fold over vehicle control-treated animals) of endogenous SIY antigen-specific T cells as analyzed by SIY/K^b pentamer analysis. Intracellular cytokine staining of SIY-peptide restimulated spleen cells from these animals revealed that the expanded SIY-specific CD8⁺ T cells were functionally active, and produced high-levels of IFN-gamma. This enhanced immune response also translated to a benefit in survival. After C1498.SIY AML cell-challenge, the majority (80%) of mice treated with DMXAA rejected the leukemia and survived long-term, while control-treated mice succumbed to AML within approximately 3 weeks, as expected. DMXAA-treated mice surviving a primary C1498.SIY cell challenge also rejected a secondary challenge with parental C1498 cells (SIY-negative), suggesting DMXAA treatment led to potent immunological memory against native C1498-expressed antigens. DMXAA treatment of mice following a systemic challenge with parental C1498 cells also led to enhanced survival compared to control-treated animals. This effect was T and or B cell-dependent, as DMXAA treatment of RAG^-/- mice had no effect on survival. IFNAR^-/- mice unable to sense type I IFNs succumbed more rapidly following AML induction compared to C57BL/6 mice, but demonstrated prolonged survival following DMXAA treatment compared to vehicle-control-treated IFNAR^-/- hosts, suggesting that STING activation may stimulate other pathways important for generating anti-leukemia immunity in addition to type I IFN or may directly affect the viability of AML cells directly as was observed following in vitro cultures of AML cells with DMXAA. Collectively, these data suggest that DMXAA-mediated activation of the STING pathway potently enhances adaptive immunity to AML, culminating in prolonged survival and even disease cure. If successfully manipulated in other murine leukemia models, the STING pathway may be a promising therapeutic target to enhance endogenous immunity in AML patients.