In vivo magnetic resonance imaging of iron oxide-labeled, intravenous-injected mesenchymal stem cells in kidneys of rabbits with acute ischemic kidney injury: detection and monitoring at 1.5 T

Background: Acute kidney injury (AKI) is one of the most important causes of mortality and morbidity worldwide. Mesenchymal stem cells (MSCs) can be used for stem cell-based therapy containing AKI. Magnetic resonance imaging (MRI) is an ideal mean for stem cells tracking by labeling with superparamagnetic iron oxide (SPIO). Therefore, using the iron oxide-labeled mesenchymal stem cells (MSC) to treat the AKI and evaluating migration, distribution, and homing of cells by MRI is an ideal method for cell therapy and cell tracking in vivo. Methods: In vitro, the MSCs were labeled with 25 μg/mL for 24 h, and test the labeled efficiency and cells viability. In vitro experiments, magnetic resonance imaging (MRI) measurement of non-labeled and SPIO-labeled MSCs (SPIO-MSCs) was performed in correlation to detectable cells concentrations and detectable time windows. In vivo experiments, MRI evaluation was performed before and after ischemic/reperfusion AKI (N = 14) and intravenous injection of 5 × 105 SPIO-MSCs (N = 10), PBS (N = 6) up to 8 days using a clinical 1.5 T scanner. Signal intensity of kidneys were measured and tested for statistical significance (unpaired Student’s t-test, p < 0.05) in comparison histology (hematoxylin and eosin [H&E], Prussian blue). Results: In vitro, MSCs can be labeled with the SPIO without affecting the viability and labeling efficiency. SPIO-MSCs showed a reduction of signal intensity at T2WI and T2*WI, 5 × 104 cells/mL, SPIO-MSCs were the minimum imaging cells concentration using a 1.5 T MR in vitro. In vivo, SPIO-MSCs administration resulted in a T2*WI signal intensity decrease in renal medulla caused by SPIO-MSCs accumulation in contrast to control groups (p < 0.05) up to day 3 after transplantation, but T2*WI low signal intensity region of the renal medulla revealed an decrease at day 5, and no significant differences between SPIO-MSCs and control animals at day 8. Conclusion: Our data demonstrate that in vitro and in vivo, cell-tracking and monitoring of kidney distribution of intravenous injected SPIO-MSCs after AKI is feasible in MRI at 1.5 T.

背景：急性肾损伤（Acute Kidney Injury, AKI）是全球范围内引发死亡与发病的关键病因之一。间充质干细胞（Mesenchymal Stem Cells, MSCs）可应用于针对急性肾损伤的干细胞治疗。超顺磁性氧化铁（Superparamagnetic Iron Oxide, SPIO）标记联合磁共振成像（Magnetic Resonance Imaging, MRI）是干细胞追踪的理想手段。因此，采用氧化铁标记的间充质干细胞治疗急性肾损伤，并通过磁共振成像评估细胞的迁移、分布及归巢行为，是体内细胞治疗与细胞追踪的理想方案。
方法：体外实验阶段，以25μg/mL的浓度对间充质干细胞进行标记24小时，检测标记效率与细胞活力。体外实验还开展了未标记与超顺磁性氧化铁标记的间充质干细胞（SPIO-MSCs）的磁共振成像检测，以关联可检测的细胞浓度与可检测时间窗。体内实验阶段，使用1.5T临床扫描仪，对14只缺血再灌注型急性肾损伤模型小鼠，在静脉注射5×10⁵个SPIO-MSCs（n=10）、磷酸盐缓冲液（Phosphate Buffered Saline, PBS，n=6）前后进行磁共振成像评估，扫描周期最长至造模后8天。对肾脏的信号强度进行测量，并采用独立样本t检验（unpaired Student’s t-test）进行统计学分析（p<0.05），同时结合组织学检测（苏木精-伊红[H&E]染色、普鲁士蓝染色）进行对照验证。
结果：体外实验证实，超顺磁性氧化铁可对间充质干细胞实现有效标记，且不会影响细胞活力与标记效率。SPIO-MSCs在T2加权成像（T2WI）与T2*加权成像（T2*WI）中呈现信号强度降低；在1.5T磁共振体外成像条件下，5×10⁴个/mL的SPIO-MSCs为最低可检测细胞浓度。体内实验显示，与对照组相比，SPIO-MSCs注射后可导致肾髓质的T2*WI信号强度降低，该现象由SPIO-MSCs在肾髓质的聚集所介导，差异具有统计学意义（p<0.05），且该效应可持续至移植后第3天；至第5天时，肾髓质的T2*WI低信号区域有所减少，至第8天时，SPIO-MSCs组与对照组动物之间已无显著统计学差异。
结论：本研究数据表明，在1.5T磁共振成像条件下，对急性肾损伤模型小鼠静脉注射SPIO-MSCs后，实现其体内细胞追踪以及肾脏分布的监测是切实可行的。