Down-Regulation of Signal Transducer and Activator of Transcription 3 Enhances Acute Myeloid Leukemia-Derived Dendritic Cell Function

Abstract Abstract 3140 Immunotherapies, including stimulating patients' immune system with acute myeloid leukemia (AML)-derived dendritic cell (DC) vaccines, are being sought for the treatment of AML. However, these vaccines have proven to be less successful than anticipated. We have previously shown that STAT3 is constitutively activated in the blasts of approximately half of AML patients and to correlate with poor prognosis. STAT3 regulates a variety of cellular events and has been shown to play a role in immunosuppression by inhibiting DC differentiation, resulting in induction of T cell tolerance rather than T cell activation. We hypothesize that AML-DC with constitutively activated STAT3 are impaired to fully differentiate and to efficiently stimulate T cells. The current study investigated the correlation between STAT3 activity, DC maturation and the ability to stimulate allogeneic T cells in primary AML-derived DCs. AML blasts differentiated to DCs regardless of the level of activated STAT3, however AML-DC with high levels of activated STAT3 elicited less T cell response than AML-DC with low levels of activated STAT3. Knock-down of STAT3 protein with inhibitory shRNAmirs in 4/4 patient AML-DCs increased stimulation of allogeneic T cells compared to control DCs without affecting the immunophenotype or endocytotic activity. Treatment of AML-DC with the STAT3 inhibitors AG490, arsenic trioxide (ATO), JSI-124 and NSC-74859 during early differentiation did not significantly enhance T cell stimulation. Treatment with ATO, but not the remaining inhibitors, during the last 24 hours of maturation led to more mature phenotype and enhanced T cell stimulation in 2/2 AML cell lines and 8/9 patient samples (p=0.0001) while having minimal effect on normal cord blood-derived DC. ATO-treated AML-DCs had an increased expression of the co-stimulatory markers CD80 and CD86, the mature DC marker CD83 and increased HLA-DR expression indicating more mature DCs. Four of six ATO-treated AML-DCs secreted higher amounts of the T cell stimulating cytokine interleukin (IL)-12p40 (average increase 2.5 fold); exogenous administration of IL-12 recapitulated ATO's effect. These data demonstrate that AML-DCs have improved immunogenicity after reducing STAT3 protein levels during differentiation. The observation that treatment with ATO is superior to the more targeted JAK/STAT inhibitors suggests that DC maturation is regulated by other pathways in addition to STAT3. We propose to use ATO-treated AML-DC vaccines in future clinical trials. ATO treatment would not only lead to more immunogenic vaccines, but would also increase the number of mature AML-DCs that could be differentiated from each patient, solving two of the major obstacles for the use of AML-DC vaccines. Disclosures: No relevant conflicts of interest to declare.