<i>In Vivo</i> Tracking of Adoptively Transferred Natural Killer Cells in Rhesus Macaques Using ⁸⁹Zirconium-Oxine Cell Labeling and PET Imaging.

Abstract

Trials of adoptive natural killer (NK)-cell immunotherapy for hematologic malignancies have thus far shown only marginal effects, despite the potent <i>in vitro</i> antitumor activity of these cells. Homing of infused cells to tumor microenvironments is critical for efficacy, but has not been well characterized. We established a novel method to track and quantify the distribution of adoptively transferred NK cells using rhesus macaques (RM) as a clinically relevant preclinical model. RM NK cells were expanded <i>ex vivo</i> for 14-21 days, labeled with ⁸⁹Zr-oxine complex, and assessed for phenotype, function, and survival. Trafficking of ⁸⁹Zr-labeled <i>ex vivo</i>-expanded NK cells infused into RMs was monitored and quantitated by serial positron emission tomography (PET)/CT (<i>n</i> = 3, 2.05 ± 0.72 MBq, 23.5 ± 2.0 × 10⁶ NK cells/kg) and compared with that of ⁸⁹Zr-labeled nonexpanded NK cells, apoptotic NK cells, and hematopoietic stem and progenitor cells (HSPC). NK cells retained sufficient levels of ⁸⁹Zr for accurate <i>in vivo</i> tracking for 7 days. ⁸⁹Zr labeling did not alter cellular phenotype, viability, or function. PET/CT showed NK cells initially localized in the lungs, followed by their migration to the liver, spleen, and, at low levels, bone marrow. One day following transfer, only 3.4% of infused NK cells localized to the BM versus 22.1% of HSPCs. No clinical side effects were observed, and dosimetry analysis indicated low organ radioexposures of 6.24 mSv/MBq (spleen) or lower. These data support translation of this technique to humans to track the distribution of adoptively infused cells and to develop novel techniques to improve immune cell homing to tumor microenvironments.