Targeting mechanisms of immune evasion in chemotherapy-induced senescent cells

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Project Summary/Abstract Breast tumors are rarely eradicated by chemotherapy. Residual disease exists for 2 reasons: the tumor cells 1) avoid death from apoptosis, mitotic catastrophe, and nutrient deprivation; 2) avoid immune clearance. Our previous research has established that by activating p53-mediated programs of arrest and senescence, tumor cells avoid death by mitotic catastrophe, apoptosis, and nutrient deprivation. We showed chemotherapy treated breast cancer patients with p53 wild type tumors have dismal survival compared to patients with p53 mutant tumors. Senescent cells promote relapse by their production of cytokines and chemokines (termed the senescence associated secretory phenotype, or SASP). The weakness of prior research has been a failure to identify how the treated, senescent tumor cells avoid immune clearance. In this proposal, we present compelling preliminary data addressing this weakness, and plans to fully characterize and investigate potential therapeutic avenues that target the evasion of cell death, thus facilitating immune clearance and improving survival. In preliminary data, we identify a remarkable upregulation of PD-L1 in the persisting residual disease likely mediated through interactions with stromal cells, and expression is maintained in the relapse, suggesting a strong selective pressure. The goal of this project is to identify mechanisms and develop strategies to target immune evasion specifically in tumor cells that have survived chemotherapy by entering senescence. Hypothesis: Stromal interferon gamma (IFNγ) induces PD-L1 in a specific subset of cells in a chemotherapy- treated, senescent tumor. These persisting, senescent tumor cells can be targeted using a novel strategy that combines senolytic drugs to induce immunogenic cell death followed by immune checkpoint inhibitors. In Aim 1 we will use mouse mammary tumor models to determine if PD-L1 expressing tumor cells have greater relapse potential than cells within the same treated tumor that do not express PD-L1, and how each responds to retreatment. We will characterize gene expression differences in cells that express PD-L1 vs. those that do not. In Aim 2 we will identify the factors in vivo that are required for PD-L1 upregulation on senescent tumor cells. We will transplant mammary tumors into syngeneic mice that lack specific immune components such as T cells, treat with chemotherapy, and determine changes in PD-L1 expression and immune contexture. In Aim 3, preliminary data show only one third of senescent mouse mammary tumors responded to anti-PD-L1 therapy following chemotherapy. We will test if inducing immunogenic cell death with BH3 mimetic drugs we have shown to have senolytic activity can improve the response to immune checkpoint inhibitors. In Aim 4, we will use multiplex immunohistochemistry staining of human breast cancer specimens pre and post chemotherapy to determine if PD-L1 expression is induced in senescent tumors.