ABSTRACT Background Despite widespread study of dendritic cell (DC)-based cancer immunotherapies, the in vivo post-injection fate of DC remains largely unknown. Due in part to a lack of quantifiable imaging modalities, this is troubling as the amount of DC migration to secondary lymphoid organs correlates with therapeutic efficacy. Preliminary studies have identified magnetic particle imaging (MPI) as a suitable modality to quantify in vivo migration of superparamagnetic iron oxide-(SPIO)-labeled DC. Herein, we describe a lymph node- (LN)-focused MPI scan to quantify DC in vivo migration accurately and consistently. Methods Both adenovirus (Ad)-transduced SPIO + (Ad SPIO + ) and SPIO + C57BL/6 bone marrow-derived DC were generated and assessed for viability and phenotype using flow cytometry. Ad SPIO + and SPIO + DC were fluorescently-labeled and injected into C57BL/6 mouse hind footpads (n=6). Two days later, in vivo DC migration was quantified using whole animal, popliteal LN- (pLN)-focused, and ex vivo pLN MPI scans. Results No significant differences in viability, phenotype and in vivo pLN migration were noted for Ad SPIO + and SPIO + DC. Day 2 pLN-focused MPI successfully quantified DC migration in all instances while whole animal MPI only quantified pLN migration in 75% of cases. Ex vivo MPI and fluorescence microscopy confirmed MPI signal was pLN-localized and due to originally-injected Ad SPIO + and SPIO + DC. Conclusions We overcame a reported limitation of MPI by using a pLN-focused MPI scan to quantify pLN-migrated Ad SPIO + and SPIO + DC in 100% of cases. With this improved method, we detected as few as 1000 DC (4.4 ng Fe) in vivo . MPI is a suitable pre-clinical imaging modality to assess DC-based cancer immunotherapeutic efficacy.

In vivotracking of adenoviral-transduced iron oxide-labeled bone marrow-derived dendritic cells using magnetic particle imaging