Ex vivo MRI cell tracking of autologous mesenchymal stromal cells in an ovine osteochondral defect model

Abstract

Background
Osteochondral injuries represent a significant clinical problem requiring novel cell-based therapies to
restore function of the damaged joint with the use of mesenchymal stromal cells (MSCs) leading research efforts.
Pre-clinical studies are fundamental in translating such therapies; however, technologies to minimally invasively
assess in vivo cell fate are currently limited. We investigate the potential of a MRI- (magnetic resonance imaging)
and superparamagnetic iron oxide nanoparticle (SPION)-based technique to monitor cellular bio-distribution in an
ovine osteochondral model of acute and chronic injuries.

Methods
MSCs were isolated, expanded and labelled with Nanomag, a 250-nm SPION, and using a novel cellpenetrating technique, glycosaminoglycan-binding enhanced transduction (GET). MRI visibility thresholds, cellular
toxicity and differentiation potential post-labelling were assessed in vitro. A single osteochondral defect was created
in the medial femoral condyle in the left knee joint of each sheep with the contralateral joint serving as the control.
Cells, either GET-Nanomag labelled or unlabelled, were delivered 1 week or 4.5 weeks later. Sheep were sacrificed 7
days post implantation and immediately MR imaged using a 0.2-T MRI scanner and validated on a 3-T MRI scanner
prior to histological evaluation.

Results
MRI data demonstrated a significant increase in MRI contrast as a result of GET-Nanomag labelling whilst
cell viability, proliferation and differentiation capabilities were not affected. MRI results revealed evidence of
implanted cells within the synovial joint of the injured leg of the chronic model only with no signs of cell
localisation to the defect site in either model. This was validated histologically determining the location of
implanted cells in the synovium. Evidence of engulfment of Nanomag-labelled cells by leukocytes is observed in
the injured legs of the chronic model only. Finally, serum c-reactive protein (CRP) levels were measured by ELISA
with no obvious increase in CRP levels observed as a result of P21-8R:Nanomag delivery.

Conclusion
This study has the potential to be a powerful translational tool with great implications in the clinical
translation of stem cell-based therapies. Further, we have demonstrated the ability to obtain information linked to
key biological events occurring post implantation, essential in designing therapies and selecting pre-clinical models.