Immunosuppressive myeloid-derived suppressor cells (MDSC) subvert antitumor immunity and limit the efficacy of chimeric antigen receptor T cells (CAR-T). Previously, we reported that the GM-CSF/JAK2/STAT3 axis drives liver-associated MDSC (L-MDSC) proliferation and blockade of this axis rescued antitumor immunity. We extended these findings in our murine liver metastasis (LM) model, by treating tumor-bearing mice with STAT3 inhibitors (STATTIC or BBI608) to further our understanding of how STAT3 drives L-MDSC suppressive function. STAT3 inhibition caused significant reduction of tumor burden as well as L-MDSC frequencies due to decrease in pSTAT3 levels. L-MDSC isolated from STATTIC or BBI608-treated mice had significantly reduced suppressive function. STAT3 inhibition of L-MDSC was associated with enhanced antitumor activity of CAR-T. Further investigation demonstrated activation of apoptotic signaling pathways in L-MDSC following STAT3 inhibition as evidenced by an upregulation of the pro-apoptotic proteins Bax, cleaved caspase-3, and downregulation of the anti-apoptotic protein Bcl-2. Accordingly, there was also a decrease of pro-survival markers, pErk and pAkt, and an increase in pro-death marker, Fas, with activation of downstream JNK and p38 MAPK. These findings represent a previously unrecognized link between STAT3 inhibition and Fas-induced apoptosis of MDSCs. Our findings suggest that inhibiting STAT3 has potential clinical application for enhancing the efficacy of CAR-T cells in LM through modulation of L-MDSC.
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The authors thank Sorrento Therapeutics, Inc. for generously preparing the CEA-Fc for anti-CEA CAR detection and Prometheus, Inc. for providing Proleukin (IL2). We also thank Dr. John Morgan and Roger Williams Medical Center Core Facility for providing us with the flow cytometry core facility. Support for this work was provided by the National Institutes of Health (1K08CA160662-01A1).
Conflict of interest
The authors declare that they have no conflict of interest.
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Guha, P., Gardell, J., Darpolor, J. et al. STAT3 inhibition induces Bax-dependent apoptosis in liver tumor myeloid-derived suppressor cells. Oncogene 38, 533–548 (2019) doi:10.1038/s41388-018-0449-z
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