Caveolae-mediated mesenchymal stem cell labelling by PSS-coated PLGA PFOB nano-contrast agent for MRI.

Rationale: Non-invasive tracking of transplanted cells is critical in evaluating delivery, migration and prognosis of cell therapies. Methods: We formulated a nano-contrast agent consisting of a perfluorooctylbromide (PFOB) core within a shell of poly (lactic-co-glycolic acid) (PLGA) followed by a coat of polystyrene sulfonate (PSS) for F-19 MRI. The nano-contrast agent (PSS-NP) was characterised by DLS and the uptake efficiency of the nano-contrast agent (PSS-NP) was tested using flow cytometry, in vitro MRI and confocal microscopy. In vitro and in vivo assays of labelled cells were tested for their ability to provide an MRI signal while retaining their osteoblastic differentiation capabilities. Results: PSS-NPs were internalised via caveolae-mediated endocytosis in mesenchymal stromal/stem cells without affecting cell proliferation and differentiation in osteoblasts, both in vitro and in vivo. Furthermore, labelled cells were monitored by F-19 MRI for up to 2 months after transplantation in mice. In particular, PSS-NP-labelled cells can be used to monitor the enhanced immune rejection of grafted human cells in normal BALB/c mice compared to immune-compromised NOD/SCID mice. One week after transplantation, 40% of the F-19 MRI signal was lost in normal mice, whereas only 10% was lost in immune-compromised mice. Conclusion: Overall, these results show that PSS-NPs can label MSCs effectively, and be employed in vivo as a novel nano-contrast agent for non-invasive cell tracking using clinically relevant F-19 MRI techniques.