Simple SummaryOvarian cancer is the deadliest among gynecological cancers, and there is a huge demand for new treatments for these patients. Immunotherapy holds great potential in cancer treatment, but has not yet proven successful for the majority of ovarian cancer patients. To better understand the immunological landscape of the disease, we have characterized lymphocytes in patients with high-grade serous ovarian cancer. Natural killer cells and T cells are present in both primary tumors and in the metastasizing environment of ascites, a fluid in the abdominal cavity that is developed in many patients with ovarian cancer. Our data reveal that a large fraction of these natural killer cells and T cells express tissue-resident markers and the inhibitory receptor, NKG2A, and are able to kill ovarian cancer cells. In summary, we report a functional subset of lymphocytes that may be targeted in future immunotherapeutic approaches.AbstractWomen with ovarian cancer have limited therapy options, with immunotherapy being unsatisfactory for a large group of patients. Tumor cells spread from the ovary or the fallopian tube into the abdominal cavity, which is commonly accompanied with massive ascites production. The ascites represents a unique peritoneal liquid tumor microenvironment with the presence of both tumor and immune cells, including cytotoxic lymphocytes. We characterized lymphocytes in ascites from patients with high-grade serous ovarian cancer. Our data reveal the presence of NK and CD8+T lymphocytes expressing CD103 and CD49a, which are markers of tissue residency. Moreover, these cells express high levels of the inhibitory NKG2A receptor, with the highest expression level detected on tissue-resident NK cells. Lymphocytes with these features were also present at the primary tumor site. Functional assays showed that tissue-resident NK cells in ascites are highly responsive towards ovarian tumor cells. Similar results were observed in an in vivo mouse model, in which tissue-resident NK and CD8+T cells were detected in the peritoneal fluid upon tumor growth. Together, our data reveal the presence of highly functional lymphocyte populations that may be targeted to improve immunotherapy for patients with ovarian cancer.