Multimodal Imaging Reveals Improvement of Blood Supply to an Artificial Cell Transplant Site Induced by Bioluminescent Mesenchymal Stem Cells

Abstract

Purpose: An artificial site for cell or pancreatic islet transplantation can be created using a polymeric scaffold, even though it suffers subcutaneously from improper vascularisation. A sufficient blood supply is crucial for graft survival and function and can be enhanced by transplantation of mesenchymal stem cells (MSCs). The purpose of this study was to assess the effect of syngeneic MSCs on neoangiogenesis and cell engraftment in an artificial site by multimodal imaging.

Procedures: MSCs expressing a gene for luciferase were injected into the artificial subcutaneous site 7 days after scaffold implantation. MRI experiments (anatomical and dynamic contrast-enhanced images) were performed on a 4.7-T scanner using gradient echo sequences. Bioluminescent images were acquired on an IVIS Lumina optical imager. Longitudinal examination was performed for 2 months, and one animal was monitored for 16 months.

Results: We confirmed the long-term presence (lasting more than 16 months) of viable donor cells inside the scaffolds using bioluminescence imaging with an optical signal peak appearing on day 3 after MSC implantation. When compared to controls, the tissue perfusion and vessel permeability in the scaffolds were significantly improved at the site with MSCs with a maximal peak on day 9 after MSC transplantation.

Conclusions: Our data suggest that the maximal signal obtained by bioluminescence and magnetic resonance imaging from an artificially created site between 3 and 9 days after MSC transplantation can predict the optimal time range for subsequent cellular or tissue transplantation, including pancreatic islets.

Keywords: Bioluminescence; DCE; Dynamic contrast-enhanced MRI; Magnetic resonance imaging; Mesenchymal stem cells; Vascularisation.

Fig. 1.

Experiment design. a A photograph showing a device consisting of a macroporous monofilament scaffold combined with a rounded Teflon rod. b A photograph showing the implantation procedure of the scaffold into the subcutaneous space in the abdominal site of the animal and injection of MSCs into the artificial site through the skin and c engrafted scaffold. d A schematic illustration of the design of the experiment.

Fig. 2.

DCE-MRI measurement. a The graph shows MR signal intensity changes before and after contrast agent (CA) injection. Representative DCE-MR images b before and c after CA injection. d Region of interest in the artificial site chosen for evaluation. EXP experimental scaffold with MSCs, CTRL control scaffold, K kidney.

Fig. 3.

Validation of stem cell characteristics. a In vitro bioluminescent images of the isolated MSCs. b The linear relationship between the optical signal and cell numbers in the in vitro experiment. c The presence of CD29- and CD44-specific molecules on the MSC surface measured by FACS.

Fig. 4.

MRI examination of the scaffolds before MSC transplantation. Representative anatomical MR images of the scaffolds a before and b after MSC transplantation (Tx) acquired before administration of the contrast agent. MR signal enhancement related to vascularisation in the scaffolds c before and d after Tx MSCs.

Fig. 5.

Bioluminescence imaging of transplanted MCSs in the scaffolds and the suggested transplantation window. a A cumulative optical signal originating from the devices at different time points after transplantation (Tx) of MSCs. b The relationship between the MRI and BLI signal at different time points after MSC transplantation. c Representative in vivo optical images of the scaffolds with bioluminescent MSCs. Arrow indicates control scaffold without any BLI signal. D day, M month, after MSC transplantation. b The suggested transplantation window between day 3 (the maximal number of viable MSCs) and day 9 (the maximal perfusion and vessel permeability) for further transplantation of pancreatic islets is expressed as a dashed rectangle.

Fig. 6.

Histology of the scaffolds. The experimental (a, b) and control (c, d) scaffolds are filled with the mesenchymal tissue. Microvascular density is higher in the specimen with b MSCs compared to the d control. H&E staining (a–d), anti-CD31 immunohistochemistry (insets). Original magnifications ×100 (a, c) and ×400 (b, d, insets).