A Mass General Brigham-led study in Science Immunology used multiomic analysis of malignant ascites to identify a rare immune cell population that may help metastatic tumors evade immunity. The work reframes ascites—often discarded after drainage—as a high-information clinical tissue for discovering mechanisms of treatment resistance. The study links the biology uncovered in ascites to metastatic immune suppression, supporting a model where the peritoneal environment helps sustain disease progression. For translational teams, the immediate signal is biomarker discovery potential and new immune-cell targets that could improve immunotherapy stratification in advanced solid tumors with recurrent ascites.