Résumé :
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Purpose: High spatial resolution and non-invasiveness feature NMR imaging, and may allow longitudinal assessment of cell therapies. However, cells must be pre-loaded with an appropriate NMR contrast agent (CA). We compared the ability of two classes of CA, iron oxide nanoparticles (SPIO) and lanthanide complexes (Gd-DTPA), to monitor the fate of labeled myoblasts transplanted in a xenogenic context. Methods: Primary human myoblasts were loaded by direct incubation with 25mM of Gd-DTPA (Magnevist) or 100µg/mL of SPIO (Endorem). 2.106 labelled cells were injected in the tibialis anterior of C57/Bl6 immunocompetent mice. NMR acquisitions were performed in a 4T Bruker Biospec NMR spectrometer. Interleaved axial T1-weighted spin echo images (in-plane resolution:120x120µm) were acquired immediately after cell injection (D0) and repeated on D1, D4, D6, D8, D11, D14, D21 and 3 months after transplantation. Size and contrast of the labeled area were measured. Comparatively, muscle cryosections were labeled with anti-human COX-2 (mitochondrial localization) and lamin A/C (nuclear localization) antibodies (immunohistofluorescence study). Prussian blue staining detected SPIO in tissue. Results: T1-weighted images showed hyper-intensity (Gd-DTPA) and hypo-intensity (SPIO) signals at injection site on D0. The size of SPIO spots halved between D1 and D11 and plateaued thereafter, while the contrast remained stable over time. The Gd-DTPA label significantly decreased and totally disappeared at D21. Immunolabelings performed on D8 and D11 confirmed the immunorejection of human cells as expected. However, Prussian blue staining revealed the presence of SPIO at D8, D11 and at 3 months, in agreement with the persistence of the NMR signal. Conclusions: Time-courses of CAs detection were different after intramuscular injection of loaded cells and the remanence of Gd-DTPA exceeded the known survival of xenografted loaded cells. Most importantly, SPIO label was still visible after 3 months, confirming its poor relevance for therapeutic cell monitoring in vivo. Supported by grants from AFM.
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