PURPOSE:Magnetic resonance imaging (MRI) has been used to track magnetically labeled human bone marrow-derived mesenchymal stem cells (hBMSCs) in vivo after COX-2 silencing and transplantation into nude rats via tail vein injection. METHODS:In the present study, we knocked down COX-2 expression in hBMSCs through lentivirus transduction. The COX-2 knockdown was confirmed by real-time PCR and Western blotting analyses. Subsequently, we labeled cells with the novel reagent SPIO-Molday ION Rhodamine-B™ (MIRB). The viability, proliferation and differentiation of these cells were assessed in vitro. Labeled lenti-shCOX2 hBMSCs, unlabeled hBMSCs and phosphate-buffered saline (PBS) were individually injected into the tail veins of nude rat models, forming three treatment groups. All nude rats underwent GRE T2*-weighted MRI at 1 h, 7 days and 14 days post-injection. After MRI examination, the animals were sacrificed, and the brain and liver were examined by fluorescence microscopy and Prussian Blue staining. RESULTS:Our results confirmed the successful down-regulation of COX-2 at the mRNA and protein levels in hBMSCs by lentivirus transduction. The viability and differentiation of hBMSCs were not affected by MIRB labeling. After 7 days, hypointense signal void areas in the rat livers were observed on MRI. After 14 days, iron particles were detected in the blood vessels, sinusoids, interlobular septum and capsule tissues of the liver. CONCLUSION:The MIRB-labeled lenti-shCOX2 hBMSCs transplanted into nude rat models via tail vein injection can be detected and monitored in vivo using 3.0 T clinical MRI for up to 14 days after cell transplantation.