TLR9 triggering/STAT3 inhibition to reprogram leukemic cells into antigen-presenting cells and trigger T-cell responses.

Abstract

118 Background: Clinical studies demonstrated potential of cancer immunotherapies while underscoring the need to block immune checkpoints shielding tumors from immune attack. We recently generated a strategy to target STAT3 transcription factor, critical immune checkpoint regulator, using TLR9-targeted delivery of STAT3 decoy inhibitor (CpG-STAT3dODN) targeting TLR9+ immune cells and all subsets of leukemic cells, including leukemic progenitor/stem cells in vitro and in vivo experiments in murine and human AML cells. Methods: Studies were conducted using mouse Cbfb/MYH11/Mpl and C1498 models of AML and acombination of flow cytometry, electron microscopy, and sequencing techniques were used to define changes in the AML cells after CpG-STAT3dODN treatment. Results: Intravenous injections of CpG-STAT3dODN into engrafted mice, led to significant changes in CD11b/CD11c expression, cell morphology, mitochondrial shape and number within the first 5 days of treatment. RNA-sequencing studies indicated increased expression of genes regulating differentiation and antigen presentation such as IRF8, GADD45A, CIITA, IL-12A, IFNγ. These effects associated with increased number of activated T cells in the leukemic microenvironment, thereby resulting in regression of AML in majority of treated mice. Two week CpG-STAT3dODN treatment was sufficient to generate protective antitumor immunity as demonstrated in rechallenge experiments. Conclusions: Our studies suggested that concurrent TLR9-stimulation and STAT3 inhibition trigger potent immunogenicity of AML cells sufficient to drive effector and memory T cell responses. We hypothesize that STAT3-inhibition can unleash immune-stimulation downstream of TLR9 signaling transforming leukemic cells into antigen-presenting cells Our further studies will dissect role of STAT3 and TLR9-induced signaling mediators (IRF, NF-κB) in regulation of the process.This information should support the design of new therapeutic regimens for AML immunotherapy using CpG-STAT3dODN alone or in combination with T cell-based therapies.