There is a need for in vivo monitoring of cell engraftment and survival after cardiac cell transplantation therapy. This study assessed the feasibility and usefulness of combined PET and MRI for monitoring cell engraftment and survival after cell transplantation. Methods: Human endothelial progenitor cells (HEPCs), derived from CD341 mononuclear cells of umbilical cord blood, were retrovirally transduced with the sodium iodide symporter (NIS) gene for reporter gene imaging by 124 I-PET and labeled with iron oxides for visualization by MRI. Imaging and histologic analysis were performed on 3 groups of nude rats on days 1, 3, and 7 after intramyocardial injection of 4 million HEPCs. Results: In vitro studies demonstrated stable expression of functional NIS protein and normal viability of HEPCs after transduction. On day 1, after intramyocardial transplantation, iron-and NIS-labeled HEPCs were visualized successfully on MRI as a regional signal void in the healthy myocardium and on PET as 124 I accumulation. The 124 I uptake decreased on day 3 and was undetectable on day 7, and the MRI signal remained unchanged throughout the follow-up period. Histologic analysis with CD31 and CD68 antibodies confirmed the presence of either labeled or nonlabeled control transplanted HEPCs at the site of injection on day 1 but not on day 7, when only iron-loaded macrophages were seen. Furthermore, deoxyuride-59-triphosphate biotin nick end labeling showed extensive apoptosis at the site of transplantation. Conclusion: The combination of MRI and PET allows imaging of localization and survival of transplanted HEPCs together with morphologic information about the heart. Although iron labeling rapidly loses specificity for cell viability because of phagocytosis of iron particles released from dead cells, reporter gene expression provided specific information on the number of surviving cells. This multimodality approach allows complementary analysis of cell localization and viability.

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It has been suggested that vascular endothelial growth factor (VEGF) and statins enhance the survival, proliferation, and function of endothelial progenitor cells (EPCs). We investigated whether reporter gene PET can be used to detect the effects of atorvastatin and VEGF on survival of EPCs after transplantation in the rat heart. Methods: Healthy nude rats received an intramyocardial injection of 4 million human EPCs retrovirally transduced with the sodium/iodide symporter gene for reporter gene imaging. Reporter gene expression was imaged at days 1 and 3 after injection on a small-animal PET scanner with 124 I, and the presence of EPCs was confirmed by immunohistochemistry with human CD31 antibodies. The control group received EPCs transduced only with the reporter gene, whereas treatment groups received oral atorvastatin (10 mg/kg/d) and EPCs cotransduced with adenoviral vectors encoding VEGF in addition to sodium/iodide symporter. Results: Immunohistochemistry showed more EPCs at the site of injection after atorvastatin treatment and in the presence of VEGF expression in EPCs than in controls. PET successfully visualized EPCs as focal 124 I accumulation at the site of injection. The quantitative amount of 124 I accumulation assessed by PET was significantly higher in the pretreatment than control group. Autoradiography confirmed 124 I accumulation in the myocardium that correlated with the number of EPCs. Conclusion: Early survival of transplanted EPCs in the rat myocardium is prolonged by pretreatment with a combination of atorvastatin and VEGF. Reporter gene PET, by successfully quantifying the effect, is an attractive tool for monitoring stem cell survival in vivo.

Objective. Treatment of cartilage defects is still challenging, primarily because of the poor self-healing capacity of articular cartilage. Gene therapy approaches have gained considerable attention, but, depending on the vector system used, they can lead to either limited or unrestrained gene expression, and therefore regulation of gene expression is necessary. This study was undertaken to construct an efficient tetracycline (Tet)-regulated, lentivirally mediated system for the expression of growth factor bone morphogenetic protein 2 (BMP-2) in primary rabbit chondrocytes that will allow for the induction and termination of growth factor gene expression once cartilage regeneration is complete.Methods. Chondrogenic ATDC5 cells and primary rabbit chondrocytes were lentivirally transduced with different tetracycline-on (Tet-On)-regulated, selfinactivating vectors for the induction of expression of enhanced green fluorescent protein (eGFP) or BMP-2, using either a 1-vector system or a 2-vector system. Conclusion. The lentivirally mediated Tet-On system is an effective strategy for efficient, repeatedly inducible expression of BMP-2 in primary rabbit chondrocytes. Therefore, use of this system in in vivo experiments may be a promising approach as a treatment strategy for cartilage defects. Results. Expression of eGFP was induced on

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