PTEN and BRCA1 tumor suppressor loss associated tumor immune microenvironment exhibits differential response to therapeutic STING pathway activation in a murine model of ovarian cancer

Background High grade serous ovarian carcinoma (HGSC) is the most lethal gynecologic malignancy characterized by chemoresistance and high rates of recurrence. HGSC tumors display a high prevalence of tumor suppressor gene loss. Loss of BRCA1 and PTEN function due to mutations or epigenetic influence have been widely associated with variable clinical outcomes, where tumors with BRCA1 mutations exhibit increased chemosensitivity and those with PTEN mutations have been reported to exhibit chemoresistance. Given the established type 1 interferon regulatory function of BRCA1 and PTEN genes and associated contrasting T cell infiltrated and non-infiltrated tumor immune microenvironment (TIME) states, in this study we investigated the potential of Stimulator of Interferon Genes (STING) pathway activation in improving overall survival via enhancing chemotherapy response, specifically in tumors with PTEN deficiency. Methods Expression of PTEN protein was evaluated in tissue microarrays generated using pre-treatment tumors collected from a cohort of 110 patients with HGSC. Multiplex immunofluorescence staining was performed to determine spatial profiles and density of selected lymphoid and myeloid cells. In vivo studies using the syngeneic murine HGSC cell lines, ID8- Trp53 -/- ; Pten -/- and ID8- Trp53 -/- ; Brca1 -/- , were conducted to characterize the TIME and response to carboplatin chemotherapy in combination with exogenous STING activation therapy. Results Tumors with absence of PTEN protein exhibited a significantly decreased disease specific survival and intra-epithelial CD68+ macrophage infiltration as compared to intact PTEN expression. In vivo studies demonstrated that Pten deficient ovarian cancer cells establish an immunosuppressed TIME characterized by increased proportions of M2-like macrophages, GR1+ MDSCs in the ascites, and reduced effector CD8+ cytotoxic T cell function compared to Brca1 deficient cells; further, tumors from mice injected with Pten deficient ID8 cells exhibited an aggressive behavior due to suppressive macrophage dominance in the malignant ascites. In combination with chemotherapy, exogenous STING activation resulted in longer overall survival in mice injected with Pten deficient ID8 cells, reprogrammed intraperitoneal M2-like macrophages derived from Pten deficient ascites to a M1-like phenotype and rescued CD8+ cytotoxic T cell activation. Conclusions This study reveals the importance of considering the influence of cancer cell intrinsic genetic alterations on the TIME for therapeutic selection. We establish the rationale for the optimal incorporation of interferon activating therapies as a novel combination strategy in PTEN deficient HGSC. ### Competing Interest Statement The authors have declared no competing interest.