In Vivo Targeting Of Acute Myeloid Leukemia Using CpG-Stat3 siRNA Results In T Cell-Dependent Tumor Eradication

Signal Transducer and Activator of Transcription-3 (STAT3) is an oncogene and immune checkpoint commonly activated in cancer cells and in tumor-associated immune cells. As a focal point of downstream signaling from numerous cytokines and growth factors present in the tumor microenvironment, STAT3 is an attractive but also elusive pharmacological target. We previously developed an immunostimulatory strategy based on targeted Stat3 gene silencing in Toll-like Receptor (TLR9)-positive hematopoietic cells using CpG-siRNA conjugates. Here, we assessed therapeutic effect of systemic STAT3 blocking/TLR9 triggering in disseminated acute myeloid leukemia (AML). We used a model of mouse Cbfb/MYH11/Mpl-induced leukemia, which mimics human inv(16) AML. Our results demonstrate that intravenously delivered CpG-Stat3 siRNA, but not control oligonucleotides, can eradicate established AML and impair leukemia-initiating potential. These antitumor effects require host’s effector T cells but not TLR9-positive antigen-presenting cells. Instead, CpG-Stat3 siRNA has direct immunogenic effect on AML cells in vivo leading to up regulation of MHC class II, co-stimulatory molecules and proinflammatory mediators, such as IL-12 and IFNγ, while down regulating expression of co-inhibitory PD-L1 molecule. Systemic injections of CpG-Stat3 siRNA generate potent tumor antigen specific immune responses, increase the ratio of tumor-infiltrating CD8⁺ T cells to T regulatory cells (Tregs) in various organs and result in CD8⁺ T cell-dependent regression of leukemia. Targeted STAT3 inhibition/TLR9 triggering also enhances immunogenicity of primary patients’ AML cells which gain ability to stimulate T cell proliferation. Our findings underscore the potential of using targeted STAT3 inhibition/TLR9 triggering to break tumor tolerance and induce potent immunity against AML and potentially other TLR9-positive blood cancers.