Use of Magnetotactic Bacteria as an MRI Contrast Agent for In Vivo Tracking of Adoptively Transferred Immune Cells

Abstract Purpose In vivo immune cell tracking using MRI is a valuable tool for studying the mechanisms underlying successful cancer therapies. Current cell labeling methods using superparamagnetic iron oxide (SPIO) lack the specificity and persistence needed to track the fate and location of transplanted cells long-term. Magnetospirillium magneticum is a commercially available, iron-producing bacterium that can be taken up by, and live harmoniously within, mammalian cells as magneto-endosymbionts (MEs). MEs have shown promise as labeling agents for in vivo stem and cancer cell tracking but have yet to be evaluated in immune cells. This pilot study examined ME labeling in myeloid-derived suppressor cells (MDSCs), cytotoxic T lymphocytes (CTLs) and dendritic cells (DCs) and its effects on cell purity, function and MRI contrast. Procedures: MDSCs, CTLs and DCs were incubated with MEs at various ME labelling ratios (MLR) and various biological metrics and iron uptake were assessed. For in vivo imaging, MDSCs were labeled overnight with either MEs or SPIO (Molday ION Rhodamine B) and injected into C3 tumor-bearing mice via tail vein injection 24 days post-implant and scanned daily with MRI for one week to assess cellular quantification. Results Following incubations MDSCs contained 0.62 and 2.22 pg Fe/ cell. CTLs achieved Fe loading of < 0.5 pg/ cell and DCs achieved Fe loading of ~ 1.4pg/cell. The suppressive functionality of MDSCs at 1000MLR was not affected by ME labeling but was affected at 2000MLR. Markers of CTL dysfunction were not markedly affected by ME labeling, nor were DC markers. In vivo data demonstrated that the MDSCs labeled with MEs generated sufficient contrast to be detectable using TurboSPI, similar to SPIO-labeled cells. Conclusions Cells can be labeled with pre-clinically relevant amounts of MEs without compromising cell viability. Care must be taken at higher concentrations of MEs, which may affect the functional activity and/or morphology of some cell types. Immune cells with minimal phagocytic behaviour have much lower iron content per cells after incubation with MEs vs SPIO; however, MEs can successfully be used as a contrast agent for phagocytic immune cells.