PURPOSE: In vivo immune cell tracking using MRI can be a valuable tool for studying the mechanisms underlying successful cancer therapies. Current cell labeling methods using superparamagnetic iron oxide (SPIO) lack the persistence to track the fate and location of transplanted cells long-term. Magnetospirillum 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 labeling 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 1 week to assess cellular quantification. RESULTS: Following incubations, MDSCs contained > 0.6 pg Fe/cell. CTLs achieved Fe loading of < 0.5 pg/cell, and DCs achieved Fe loading of ~ 1.4 pg/cell. The suppressive functionality of MDSCs at 1000 MLR was not affected by ME labeling but was affected at 2000 MLR. 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 sufficient numbers of MEs to be detectable with MRI without compromising cell viability. Care must be taken at higher concentrations of MEs, which may affect some cell types' functional activity and/or morphology. Immune cells with minimal phagocytic behavior have much lower iron content per cell after incubation with MEs vs SPIO; however, MEs can successfully be used as a contrast agent for phagocytic immune cells.