Abstract
Natural killer (NK) cell therapies are being increasingly used as an adoptive cell therapy for cancer because they can recognize tumor cells in an antigen-independent manner. While promising, the understanding of NK cell persistence, particularly within a harsh tumor microenvironment, is limited. Fluorine-19 (19 F) MRI is a noninvasive imaging modality that has shown promise in longitudinally tracking cell populations in vivo; however, it has not been studied on murine NK cells. In this study, the impact of 19 F labeling on murine NK cell viability and function was assessed in vitro and then used to quantify NK cell persistence in vivo. While there was no noticeable impact on viability, labeling NK cells with 19 F did attenuate cytotoxicity against lymphoma cells in vitro. Fluorescent microscopy verified 19 F labeling in both the cytoplasm and nucleus of NK cells. Lymphoma-bearing mice were given intratumoral injections of 19 F-labeled NK cells in which signal was detectable across the 6 day observation period via 19 F MRI. Quantification from the composite images detected 78-94% of the initially injected NK cells across 6 days, with a significant decrease between Days 3 and 6. Postmortem flow cytometry demonstrated retention of 19 F intracellularly within adoptively transferred NK cells with less than 1% of 19 F-containing cells identified as tumor-associated macrophages that presumably ingested nonviable NK cells. This work demonstrates that 19 F MRI offers a specific imaging platform to track and quantify murine NK cells within tumors noninvasively.