Mechanisms of REST-mediated immunosuppression in cancer

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Project Abstract Tumor-intrinsic factors in cancer cells modulate the immune milieu to enable prolonged survival and growth of tumors. In breast cancer (BC), the 2nd leading cause of cancer-related deaths in women, molecular subtyping of these factors effectively guides targeted therapies, but does not guide immunotherapy approaches as immune infiltrates vary significantly within each subtype. The presence of tumor-infiltrating lymphocytes (TILs) is highly prognostic for survival and therapeutic benefit in BC, but the tumor-intrinsic factors governing their presence are not well defined. Using TCGA RNA analysis, I have identified loss of function in the transcriptional repressor REST as being a key correlate to reduced lymphocyte infiltration into tumors of multiple BC molecular subtypes. Loss of functional REST in BC (~15-20% of tumors) is associated with poorer prognoses, however, the mechanisms by which loss of REST function modulates the tumor immune microenvironment are not known and may provide novel targets for enhancing therapeutic responses. I recently generated a murine Rest knockout BC line (Rest-less) and my in vivo studies confirmed that Rest-less tumors contain significantly less lymphocytes while also revealing a significant increase in pro-tumor macrophages, a cell population our lab has identified as being key modulators of lymphocyte suppression in BC. Furthermore, my in vitro studies identified lymphangiogenic Vegfc and Vegfd among the most differentially upregulated genes in Rest-less BC cells. Tumor lymphangiogenesis is linked to T cell suppression in solid tumors and may provide a reason for Rest-mediated suppression of lymphocytes. Based on my preliminary data, I hypothesize that lymphocyte suppression in Rest- less tumors is orchestrated by the polarization of tumor-associated macrophages (TAMs) and lymphatic endothelium to a suppressive phenotype. I propose two specific aims to test my hypothesis. In Aim 1, I will determine the role of tumor intrinsic Rest on TAM-mediated lymphocyte suppression using co-culture assays of TAMs from Rest-less tumors together with lymphocytes in vitro and macrophage depletion through anti-CSF-1R treatment in an antigen-specific (GFP; JEDI) Rest-less model in vivo. In Aim 2, I will evaluate how Vegfr3, the receptor for Vegfc and Vegfd, regulates the REST-less tumor microenvironment and responsiveness to immunotherapy. To accomplish this task, I will test anti-Vegfr3 blockade on Rest-less tumors with or without anti- PD1 and comprehensively analyze tumor growth, lymphatic depletion, and immune responses using RNAseq, spectral cytometry, and cyclic immunofluorescence. Our proposed research will provide an understanding of a previously uncharacterized facet of REST-mediated immune suppression in BC and will use novel murine orthotopic models to test strategies that block immune suppressive mechanisms. Ultimately, I anticipate my findings will reveal targetable mechanism(s) to inhibit Rest-mediated immunosuppression in BC and provide a considerable impact on the treatment of REST-less tumors overall.