RESEARCH ARTICLE Low Dose Focused Ultrasound Induces Enhanced Tumor Accumulation of Natural Killer Cells

Natural killer (NK) cells play a vital antitumor role as part of the innate immune system. Effi-cacy of adoptive transfer of NK cells depends on their ability to recognize and target tumors. We investigated whether low dose focused ultrasound with microbubbles (ldbFUS) could facilitate the targeting and accumulation of NK cells in a mouse xenograft of human colorec-tal adenocarcinoma (carcinoembryonic antigen (CEA)-expressing LS-174T implanted in NOD.Cg-PrkdcscidIl2rgtm1Wjl/SzJ (NSG) mice) in the presence of an anti-CEA immunocyto-kine (ICK), hT84.66/M5A-IL-2 (M5A-IL-2). Human NK cells were labeled with an FDA-approved ultra-small superparamagnetic iron oxide particle, ferumoxytol. Simultaneous with the intravenous injection of microbubbles, focused ultrasound was applied to the tumor. In vivo longitudinal magnetic resonance imaging (MRI) identified enhanced accumu-lation of NK cells in the ensonified tumor, which was validated by endpoint histology. Signifi-cant accumulation of NK cells was observed up to 24 hrs at the tumor site when ensonified with 0.50 MPa peak acoustic pressure ldbFUS, whereas tumors treated with at 0.25 MPa showed no detectable NK cell accumulation. These clinically translatable results show that ldbFUS of the tumor mass can potentiate tumor homing of NK cells that can be evaluated non-invasively using MRI