Background.
Stem cells offer the promise of cardiac repair. Stem cell labeling is a prerequisite to tracking cell fate
in vivo
.
Aim.
To develop a reporter gene that permits
in vivo
stem cell labeling. We examined the sodium-iodide symporter (NIS), a protein that is not expressed in the heart, but promotes cellular uptake of
99m
Tc or
124
I, thus permitting cell tracking by SPECT or PET imaging, respectively.
Methods.
The human NIS gene (
h
NIS) was expressed in rat cardiac derived stem cells (rCDCs) using lentivirus driven by the CAG or CMV promoter. NIS function in transduced cells was confirmed by
in vitro
99m
Tc uptake. Eleven rats were injected with 1 or 2 million rCDCs intramyocardially immediately after LAD ligation; 6 with CMV-NIS and 5 with CAG-NIS cells. Dual isotope SPECT imaging was performed on a small animal SPECT/CT system, using
99m
Tc for cell detection and
201
Tl for myocardial delineation, 24 hrs after cell injection. PET was performed on a small animal PET scanner using
124
I for cell tracking and
13
NH
3
for myocardial delineation, 48hrs after cell injection. Contrast Ratio (CR) was defined as [(signal in the cells)-(signal in blood pool)]/signal in blood pool. High resolution
ex vivo
SPECT scans of explanted hearts (n=3) were obtained to confirm that
in vivo
signal was derived from the cell injection site. The presence of
h
NIS mRNA was confirmed in injected hearts after animal sacrifice (n=2), by real-time RT-PCR.
Results.
NIS expression in rCDCs did not affect cell viability/proliferation (p=0.718, ctr vs NIS). In vitro
99m
Tc uptake was 6.0±0.9% vs 0.07±0.05, without and with perchlorate (specific NIS blocker), respectively. NIS-transduced rCDCs were easily visualized as spots of
99m
Tc or
124
I uptake within a perfusion deficit in the SPECT and PET images. CR was considerably higher when cells were transduced by the CMV-NIS virus in comparison to the CAG-NIS virus (70±40% vs 28±29%, p=0.085). Ex vivo small animal SPECT imaging confirmed that
in vivo
99m
Tc signals were localized to the injection sites. PCR confirmed the presence of
h
NIS mRNA in injected hearts.
Conclusion.
NIS expression allows non invasive
in vivo
stem cell tracking in the myocardium, using both SPECT and PET. This reporter gene has great potential for translation in future clinical applications.