Radionuclide imaging is widely used in non-invasively monitoring the metabolism and function of the cells, as a molecular imaging modality. As the most sensitive facility, its lowest detection concentration of radioactors can reach 10 -11 -10-12 mol/l, which is 6-8 times higher that MRI and CT(27). However, it has a relative low spatial resolution compared to MRI and thus can't provide a detailed anatomic structure. Nuclear imaging also involves the use of radioactive substances, which may cause potential harm to human body. But this won't limit its development.
2.1 Radioactive tracer
There are several advantages over the tracking of cells labeled with radioactive tracers, it has a higher specificity over the transplanted cells, as it allows the detection and quantification of as small as 3mm in size. (28). Moreover, the signal detected by positron emission tomography (PET)/ single photon emission computed tomography(SPECT) is directly correlated to the quantity of the transplanted cells, which facilitate the quantification of the islet graft.
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The most frequently used label agent is 18F. A good representative of this kind is 18F -fluorodexyglucose ([18F] -FDG), which is available and widely used in the clinical settings. As long as it enters the cell, [18F] -FDG is phosphorylated into [18F] -FDG-6P(4). Neither can it be metabolized nor transferred to the outside of the cell by other blood regulating vectors, so it is trapped in the cell. When the cell dies, the released 18F-FDG-6P won't be uptaken by other cells, which increase the accuracy of the imaging transplanted islets.
In the study performed by Eriksson, they found the distribution of the transplanted islet cell in the liver is heterogeneous in a transplantation into a large animal model (pig) (29). And they detected early decreasing (almost 50%) of the number of transplanted cells just a few minutes after the transplantation. Similar phenomena were observed in other researches (30, 31) However, as the half-life of 18F (109.8min) is relatively low and can't maintain for a long time, it is not possible for longitudinal observation and analysis of the reason that has caused the decrease. Researchers assume it might be affected by the autoimmune reaction, radioactivity and the blood flow of the portal vein. But little is known about the detailed influence and damage.
The first clinical study was carried out in 2009(31). Eriksson et al. performed intrahepatic islet transplantation in 5 diabetic patients through the portal vein using 18F -FDG labeled islet cells. The results showed that in all patients, HbA1c values were reduced and the dose of insulin used is less than those before transplantation, indicating the viability and normal function of the graft.
As the observation time of FDG that has been report is up to 6 hours((28), relatively short, it limits the development and clinic application of FDG. Although use of other isotopes with a longer half-life such as 64Cu permits imaging 24-36 hours post labeling, they have a higher rate of cellular outflow and have yet to be applied to islets(32). Further studies of designing a radioactive labeling agent with a longer half -life and higher intracellular stability would great help unraveling the reasons for the early decrease post transplantation.
2.2 Report gene
With the development of gene technology, report genes provide a new way of transplanted islets tracing by PET. Herpes simplex virus 1thymidine kinase (HSV-1tk) is a commonly used report gene. It can be carried by an adenovirus or a lentivirus to transfect the islet cells. After engraftment, the radiotracer [124I]- or [125I]-5-iodo-1-(2-deoxy-2-ï¬‚uoro-D-arabino-furanosyl)uracil is administered for PET or SPECT imaging, respectively. The radiotracers are then phosphorylated by thymidine kinase once inside viable islet cells and the amount of tracers that is retained by islets in vivo can be measured by PET or SPECT(4).
Lu et al. used a mutant form of HSV-1 TK, HSV1-sr39Tk ,engineered with a lentivirus, combined with a PET radiotracer 9-(4-[18F]-ï¬‚uoro-3-hydromethylbutyl)guanine ([18F]-FHBG) and monitored by a microPET in their study, as HSV1-sr39Tk can phosphorylate FHBG more effectively(33). The results showed that the transfected islet grafts transplanted in the axillary cavity of NOD-scidï¼ˆå¾…åŠ ä¸Šå…¨°ï¼‰ mice could be repeatedly detected by microPET for at least 90 days. However, in cases that are using adenovirus, the observation period was much shorter (34, 35). Within a few days post transplantation, these mice became euglycemic till the end of the observation time (90 days) (33). The histological analysis of the implants showed that the healthy islets with PET reporter-expressing cells distributed throughout the islet architecture with normal morphology without inflammatory reaction. This means that the report gene won't affect the viability and function of the cell. Moreover, there is a significant relationship between the intensity of the signal and the number of the transplanted islets (almost proportional) (33) However, the magnitude of the graft decreased number of the graft decreased by approximately one-half during the first few weeks, indicating the death of the islets and it remained stable after about 30 days. The researchers assume that it may result from less supportive environment of the axillary cavity.
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A similar study was later carried out using a therapeutic gene: interleukin-10(35). A direct proportion was confirmed in transplantation into the liver by using another report gene interleukin -10[R18]. The researchers compared the early decreasing of the signal with the nonfasting blood glucose levels, intraperitoneal glucose tolerance tests, plasma insulin levels and immunohistochemistry(IHC) simultaneously and found that the signal loss was caused by the elimination of the graft, rather than the silence of the report gene.
Although the report gene has a relative longer observation time than the radiotracer, its long term influence on the engineered cells is not clear. Some researchers wonder it may be potential harm to the genes of the cell(4).