Background. Despite the significant success of recent therapies, Acute Lymphoblastic Leukemia (ALL) remains the second leading cause of childhood death. The discordance between therapeutic improvements and poor outcomes is partially caused by the difficulty of salvaging relapsed disease. While outcomes have improved in de novo treatment, dismal rates of overall survival (less than 25%) were observed in relapsed subtypes of ALL. The clinical armamentarium for treating relapsed or refractory T-ALL must be supported with new options. Strategy. While most of T-ALL cases exhibit gain-of-function mutations in Notch signaling, therapies against Notch have not fulfilled their clinical promise. To identify alternative targets for new therapies, we propose to define how cell-intrinsic oncogenic events integrate with external signals from the microenvironment. Recent studies point to the functional impact of “stromal” signals in leukemia biology. However, one significant gap in this knowledgebase is how microenvironmental factors become essential for leukemogenesis and maintenance. Preliminary results. According to our results in primary T-ALL cells, activating mutations in Notch failed to saturate Notch signaling: Notch signal strength increased when T-ALL cells encounter Notch ligands within the microenvironment – e.g. interleukin 7 (IL-7). The increased strength of Notch signaling correlated with increased surface expression of IL-7Rα by direct transcriptional activation of the IL-7Rα promoter, resulting in T-ALL hyper- responsiveness to IL-7. IL-7 also induced the cell cycle regulator SKP2, activated STAT5, and (surprisingly) STAT3. Primary T-ALL cells showed persistent STAT3 activation and our results suggest STAT3 deletion impairs T-ALL leukemogenesis. Hypothesis. These data support significant interplay between oncogenic factors and the microenvironment. According to our hypothesis, interplay between microenvironmental signals (IL-7), Notch signaling, SKP2, and STAT3 form a reciprocal positive feedback loop that is essential for T-cell leukemogenesis; this axis also compensates for the action of standard therapies in relapsed and refractory disease. Approach. To test this, we propose: 1) To determine the temporal requirement for STAT3 deletion in initiation, progression, and relapse in T-ALL by using a model of inducible genetic deletion of STAT3 in combination with a model of Notch-induced T-ALL. 2) To map how T-ALL development is affected by Notch/IL-7/STAT3/SKP2 signaling circuitry by using overexpression and gene silencing approaches to define the reciprocal regulation of Notch, STAT3, and SKP2. 3) To identify the impact of inhibiting Notch/STAT/SKP2 circuitry in relapsed T-ALL by testing both pre-clinical and clinical inhibitors of STAT signaling and SKP2 inhibitors in pre-clinical PDX models of T-ALL. Impact. Successful completion of this proposed work will: 1) define how cooperation between oncogenic signaling and the microenvironment affects ...