Optimizing Natural Killer (NK) Cell Labeling Using Magnetic Particle Imaging (MPI) Tailored Tracers

Immunotherapy is one of the most promising cancer treatments available. Despite their incredible promise, clinically approved T-cell immunotherapies present considerable challenges such as accessibility limitations due to their autologous nature, as well as the potential treatment-related toxicities. As such, natural killer (NK) cells are being studied as an alternative with unique advantages. Being the major cytotoxic effectors in the innate immune system, NK cells provide immune surveillance and can induce an immune response without the need for priming. Additionally, NK cells do not carry a risk of graft-versus-host disease, making them attractive as an allogenic source and a more feasible alternative for clinical settings.

To better understand the mechanisms of NK cell therapies, there is a need to track the localization and persistence of these cells after their delivery. Magnetic particle imaging (MPI) is a technology that allows non-invasive visualization of superparamagnetic iron oxide nanoparticles (SPIONs), allowing tracking of SPION-labeled cells. This imaging modality does not use ionizing radiation and has proven to have signal intensity proportional to SPION's mass. The objective of this research was to investigate the feasibility of labeling NK cells using SPIONs tailored for MPI.

NK-92 cells, were labeled using RL-1 PMAO-FL. This RL-1 tracer is engineered for MPI sensitivity, coated with a charged polymer (PMAO) that enhances cell uptake and tagged with fluorseceinamine (FL) to allow their microscopy visualization. NK-92 cells were incubated with tracers at different incubation times, concentrations, and media conditions. Cell viability, as well as cell uptake were compared between conditions. Fluorescence imaging was also performed, to confirm uptake and determine intracellular localization of the tracers.

It was observed that the NK-92 cells incubated with RL-1 PMAO-FL for 2 hours at a concentration of 25 μgFe/mL in low-serum media had better uptake of the tracers. Cell viability was not significantly affected by the different tracer concentrations and incubation times studied. In contrast, observations showed that media conditions seemed to pose an inversely proportional relationship between cellular uptake and cell viability, an interaction that will have to be further studied. Microscopy images suggested tracer internalization in the cells. Microscopy images suggested tracer internalization in the cells. Together this study indicates that NK-92 cells can be labeled using RL-1 PMAO FL particles and traced using both confocal imaging and MPI. Future efforts will include measuring NK cell functionality after labeling, as well as experiments validating MPI tracking of NK cells in vivo.