STAT1 associated immune checkpoint expression and immune cell localization represent adaptive immune resistance in high-grade serous ovarian cancer

Abstract High-grade serous carcinoma (HGSC) is a deadly malignancy leading to ~70% of the 140,000 ovarian cancer deaths each year. Resistance to platinum chemotherapy followed by recurrence and an incurable disease occurs in most HGSC patients. Contemporary immune checkpoint blockade therapies have shown minimal efficacy in this cancer. Our previous investigations established that tumour interferon (IFN) activation status and CD8+ T cell density are predictors of chemotherapy response in HGSC. Furthermore, we also showed that the IFN induced chemokine CXCL10 is a key determinant of increased survival via immune cell recruitment in the tumour immune microenvironment (TIME). Given that signal transducer and activator of transcription 1 (STAT1) is central to the feed forward loops of cellular IFN responses, we investigated STAT1 associated transcriptomic alterations and spatial profiles of immune cells in 204 pre-treatment HGSC tumours. RNA-sequencing based whole transcriptomic profiling revealed that higher STAT1 expression significantly correlated with higher immunomodulatory gene expression, including immune checkpoints and activators, in both chemotherapy sensitive and resistant tumours. Findings were independently validated in a cohort of 379 HGSC tumour RNA-Seq profiles from The Cancer Genome Atlas Network ovarian cancer dataset. Multiplex immunofluorescence based spatial profiling of CD8+ T cells, FoxP3+ T regulatory T cells, CD68+M1, and CD163+ M2 macrophages and expression of PD-L1, PD-1, IDO1 immune checkpoints was performed. Findings from our study provide evidence for IFN mediated adaptive immune resistance in the HGSC TIME and will potentially inform the design of rational chemo-immunotherapy approaches.