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Acute myeloid leukemia (AML) is a hematological cancer with a very poor prognosis. FLT3-ITD mutations that cause constitutive FLT3 signaling are commonly seen in patients. Our lab and others have shown that AML patient samples exhibit hallmarks of immune exhaustion such as T cell dysfunction, increased MDSCs, and increased Tregs that associate with worse survival. In solid tumor models DCs have been shown to be important for anti-tumor T cell activity and we aim to further describe the role of DCs in the immune response to leukemia. FLT3 signaling is critical for DC development but the effects of FLT3-ITD on DCs and how it contributes to leukemogenesis remains unclear. In a novel FLT3-ITD driven AML mouse model we found that DCs from these mice have elevated pFLT3, indicating constitutive FLT3 signaling. Moreover, significantly expanded DC progenitors and DCs in the bone marrow and spleen are observed. Analysis of serum cytokines identified increased levels of IL-27, IL-10, and IL-17a in leukemia mice compared to healthy controls, suggesting that the expansion of DCs in these leukemia mice may be driving pathogenic expansion of T helper subsets. Furthermore, we measured higher levels of circulating Th2, Th17, and Treg phenotypes in the blood compared to healthy controls. Thus, we hypothesize that FLT3-ITD DCs are a significant contributor to T cell skewing in AML resulting in poor anti-tumor responses. We are using scRNA-seq methods to identify transcriptional changes in FLT3-ITD DCs that lead to the phenotypes we identified. Understanding how FLT3-ITD DCs contribute to AML immune suppression is critical to interpreting leukemia etiology and the development of targeted therapies such as immune checkpoint blockade or small molecule inhibitors.

Acute Myeloid Leukemia (AML) is the most common adult leukemia and has a 5-year survival of under 30%. AML is caused by uncontrolled proliferation of myeloid cells resulting from a combination of mutations that affect proliferation, differentiation and epigenetic state. For this reason, drugs targeting epigenetic modifications are being studied in AML. AML cells avoid immune recognition though inhibiting the function of multiple cell types, especially T cells and therefore immune checkpoint blockade presents a promising therapy; however, clinical trials to date have shown very modest efficacy. T cell exhaustion has been shown to be a regulated process involving transcriptional and epigenetic changes. BET proteins, which are chromatin readers, have been implicated in maintaining this exhaustion state. In these studies, we investigated the effects of the BET inhibitor (BETi) JQ1 on T cell exhaustion and checkpoint responsiveness in a murine model of AML and AML patient samples. The AML mouse model bears FLT3-ITD and deletion of TET2 restricted to myeloid lineages and is resistant to anti-PD1 therapy. This mouse model of AML expanded terminally exhausted T cells and impaired proliferative capacity after TCR stimulation. Ex vivo treatment with BETi and anti-PD1 reverses CD8+ T cell exhaustion via rescue of proliferative dysfunction and expansion of more functional precursor exhausted T cells in patient samples and AML splenocytes. Finally, we show that BETi rescues anti-PD1 resistance in vivo and reduces tumor burden in multiple organ sites and enriches CD8+ T cells in the bone marrow. In total, we demonstrate that combining BETi and anti-PD1 therapy in the treatment of AML is a rational strategy to overcome anti-PD1 resistance.