Background: Acute lymphoblastic leukemia (ALL) is the most common childhood cancer, of which pre B-cell ALL with CRLF2 overexpression is a high risk subtype. CRLF2 B-ALL is associated with poor outcome, increased rate of relapse, and health disparities in Hispanic children. Together with IL-7 receptor alpha (IL-7Rα), CRLF2 makes up a receptor complex that is activated by the cytokine, thymic stromal lymphopoetin (TSLP). Activation of the receptor complex leads to JAK2/STAT5 and PI3/AKT/mTOR signals that promote leukemia cell survival and proliferation. To study the role of TSLP in CRLF2 B-ALL, we developed a patient-derived xenograft (PDX) model of CRLF2 B-ALL that allows us to alter circulating levels of human TSLP (hTSLP). We generated PDX from CRLF2 B-ALL cells harvested from patient samples and compared leukemia burden in mice with varying levels of hTSLP. In PDX models with hTSLP levels at or below physiological levels in pediatric cancer patients, CRLF2 B-ALL cells grew robustly. However, in PDX with elevated levels of hTSLP, leukemia cells were essentially eliminated. Our objective is to elaborate on the mechanism of high dose hTSLP’s antileukemic effect. Methods: We performed TSLP dose response studies and used flow cytometry to evaluate the effect of TSLP on SOCS protein expression, CRLF2 signaling shutdown, and loss of TSLP receptor components. Results: CRLF2 B-ALL cells cultured with hTSLP showed a dose-dependent loss in the ability to induce STAT5 and S6 phosphorylation following hTSLP stimulation. This loss was correlated with the loss of IL-7Rα, and maintained for 24-48 hours following a pulse of high-dose, but not low-dose, hTSLP. The loss of signaling and surface IL-7Rα could be prolonged if high-dose hTSLP levels were maintained. Flow cytometry analysis showed that high-dose hTSLP upregulated SOCS1 and SOCS3 proteins in CRLF2 B-ALL cells. Whole genome microarray showed that SOCS1, SOCS2, SOCS3 and CISH mRNA were upregulated in primary CRLF2 B-ALL cells cultured with high dose hTSLP. Conclusions: These data support the hypothesis that TSLP exerts its anti-leukemia effects through shutdown of CRLF2-mediated signals and that these effects are at least partially mediated by the loss of the IL-7Rα component, and potentially through SOCS family proteins. These studies elucidate the mechanism of the human TSLP cytokine as a potential biologic therapy to treat CRLF2 B-ALL. Supported in part by 1R01CA209829 (KJP), 1R43CA224723 (KJP), ASH HONORS Award 2018-2019 (WBW), and Alpha Omega Alpha Carolyn L. Kuckein Student Research Fellowship 2018-2019 (WBW).