Abstract B009: A SOX9-B7x axis safeguards dedifferentiated tumor cells from immunosurveillance to enable DCIS progression

Abstract Accumulating evidence has demonstrated a central role of dedifferentiation in carcinogenesis. In basal-like breast cancer mouse models, upregulation of SOX9 drives dedifferentiation of luminal progenitors to cells with luminal-basal hybrid features resembling embryonic multipotent mammary stem cells. Additionally, genetic inactivation of SOX9 inhibits dedifferentiation and blocks the progression of in situ tumors to malignant carcinoma. Whether the host immune system can recognize these dedifferentiated cells and how they eventually evade immunosurveillance are unclear. Here, we showed that SOX9 was necessary for limiting infiltrating T lymphocytes to protect dedifferentiated cells from immune surveillance in premalignant lesions of basal-like breast cancer. SOX9 deletion in spontaneous BLBC tumor models caused massive T lymphocyte infiltration and elimination of dedifferentiated cells. Antibody-mediated T cell depletion rescued the accumulation of dedifferentiated tumor cells in SOX9-deficient DCIS and its progression to invasive tumors. Mechanistically, through both stimulation of the STAT3 pathway and direct transcriptional activation, SOX9 upregulated the expression of the immune checkpoint B7x, which mediated the action of SOX9 in immunosuppression. In advanced tumors, B7x blockade reactivated antitumor immunity and effectively inhibited tumor growth. Furthermore, B7x depletion overcame the resistance to anti-PD-L1 immunotherapy. Collectively, these findings uncovered a dedifferentiation-associated immune evasion mechanism and demonstrated the therapeutic potential of targeting the SOX9-B7x pathway to eradicate dedifferentiated tumor cells for intercepting breast cancer progression. Citation Format: Yu Liu, Peter John, John R Christin, Maja Oktay, XingXing Zang, Wenjun Guo. A SOX9-B7x axis safeguards dedifferentiated tumor cells from immunosurveillance to enable DCIS progression [abstract]. In: Proceedings of the AACR Special Conference on Rethinking DCIS: An Opportunity for Prevention?; 2022 Sep 8-11; Philadelphia, PA. Philadelphia (PA): AACR; Can Prev Res 2022;15(12 Suppl_1): Abstract nr B009.