(370a) Award Submission: Tracking Dendritic Cell Vaccination Using Magnetic Particle Imaging

Dendritic cell (DC) based vaccines have proven to be a safe therapeutic approach, although with inconsistent clinical results. DCs must migrate to lymphoid organs to be effective and the magnitude of anti-tumor response is expected to be correlated to DC migration to lymph nodes (LNs). It’s reported that a small fraction of administered DCs migrate to LNs. Therefore, non-invasive methods to sensitively monitor and quantify migration of adoptive DC therapies would be tremendously helpful in evaluating immune response. Although techniques such as magnetic resonance imaging (MRI), single-photon emission computed tomography (SPECT), and positron emission tomography (PET) have previously been used to track DCs, these techniques are limited by low cell detection sensitivity in the range of 10⁴ cells or more.

Magnetic particle imaging (MPI) is a novel molecular imaging technology that can sensitively and quantitatively detect iron oxide tracers in vivo, with higher cell detection sensitivity compared to other imaging approaches. Cell detection sensitivity in MPI is determined by two factors: the properties of the tracer used to label cells and the extent of tracer uptake per cell. Other than detection sensitivity, to be able to differentiate signals from LNs and injection sites, resolution and imaging conditions are crucial factors in tracking DC migration in vivo.

In this study, cellular uptake was evaluated for MPI tailored tracers (RL-1) coated with poly(maleic anhydride-alt-1-octadecene) (PMAO). The MPI maximum signal intensity for the tracer was 101 mgFe^-1. The upper labeling limit for marrow-derived dendritic cells (BMDCs) was determined to be ~ 50 pgFe/cell by screening SPION concentration and incubation time. Cell viability, functionality, and surface biomarker expression were not affected by tracer labeling. Confocal images suggested particles are in vesicle-like structures inside cells. In vitro tracer fate studies suggested particles are not degraded or exocytosed after 3 days of culture. Cell dilution studies demonstrated detection of as little as 10³ cells with MPI using optimized labeling conditions. An in vivo BMDC tracking study was performed using high sensitivity and high-sensitivity/high-resolution scan modes and different administration routes (foot pad subcutaneous injection and hind flank intradermal injection). Administered DC numbers were quantifiable using in vivo day 0 MPI measurements. BMDC migration quantified by ex vivo lymph node MPI measurements ranged from 3000 to 5000 cells, however, signals in LNs in vivo were not detectable because the strong signal from the injection site masked the weak LN signal. An ex vivo phantom study was performed to evaluate signal differentiation and quantification accuracy as a function of separation distances, administered doses, and imaging settings. Then an in vivoBMDC tracking experiment using advanced imaging settings with partial filed of view and full field of view was performed, in vivo BMDC migration was trackable with partial field of view imaging. The correlation of in vivo LN MPI signal and ex vivo LN MPI signal was evaluated. BMDC migration efficiency was compared with and without GM-CSF. A survival study will be performed to evaluate correlation of LN MPI signal and DC vaccination therapeutic response. Together, these studies demonstrate the potential of MPI as tool for quantitative tracking of DC migration in DC vaccination studies.