Introduction Ascites associated with high grade serious ovarian cancer (HGSOC) is immuno-suppressive which may contribute to the lack of efficacy immune checkpoint inhibitors. The underlying mechanism of this immunosuppressive effect is poorly understood and there is an urgent need to describe the interaction between ascites and the HGSOC tumour microenvironment.

Methods We analysed the function and metabolism of cytotoxic lymphocytes (NK, T, and innate T cells) from chemo-naïve primary tumours and matched metastatic sites (n=6) from HGSOC patients (n=22) using flow cytometry, metabolomics, lipidomics, SCENITH and SEAHORSE metabolic flux analysis.

Results NK cell dysfunction (defined by activation marker expression, cytokine production and cytotoxicity assays) is present in primary HGSOC and across all metastatic sites of disease and in is exacerbated by malignant ascitic fluid. Many of the dysfunctional and metabolic features observed in tumour and ascites-infiltrating NK cells were also observed in other lymphocyte subsets (CD4⁺ and CD8⁺ T cells, innate T cells). LC-MS based metabolomics and lipidomics demonstrated that lipid metabolism (characterised increased uptake of polar lipids and defects in lipid handing capacity) is the dominant pathway affected in NK cells exposed to ascites. Treatment with polar lipids in vitro recapitulated the phenotype and treatment with lipid depleted ascites of lipids or inhibition of lipid uptake through FATP2 protected NK cell and T cell anti-tumours functions.

Conclusion/Implications We show for the first time that polar lipids derived from ascites are a key driver of NK cell dysfunction in HGSOC. These findings have important implications for the design of future immunotherapies for HGSOC.