<i>In vivo</i> 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

<i>Background</i>: 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 <i>in vivo. Methods</i>: <i>In vitro</i>, the MSCs were labeled with 25 μg/mL for 24 h, and test the labeled efficiency and cells viability. <i>In vitro</i> 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. <i>In vivo</i> experiments, MRI evaluation was performed before and after ischemic/reperfusion AKI (<i>N</i> = 14) and intravenous injection of 5 × 10⁵ SPIO-MSCs (<i>N</i> = 10), PBS (<i>N = </i>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 <i>t</i>-test, <i>p &lt; </i>0.05) in comparison histology (hematoxylin and eosin [H&amp;E], Prussian blue). <i>Results</i>: <i>In vitro</i>, 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 × 10⁴cells/mL, SPIO-MSCs were the minimum imaging cells concentration using a 1.5 T MR <i>in vitro. In vivo</i>, SPIO-MSCs administration resulted in a T2*WI signal intensity decrease in renal medulla caused by SPIO-MSCs accumulation in contrast to control groups (<i>p &lt; </i>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. <i>Conclusion</i>: Our data demonstrate that <i>in vitro</i> and <i>in vivo</i>, cell-tracking and monitoring of kidney distribution of intravenous injected SPIO-MSCs after AKI is feasible in MRI at 1.5 T.背景（Background）：急性肾损伤（Acute kidney injury, AKI）是全球范围内致死与致残的重要诱因之一。间充质干细胞（Mesenchymal stem cells, MSCs）可用于涵盖急性肾损伤在内的干细胞治疗。磁共振成像（Magnetic resonance imaging, MRI）借助超顺磁性氧化铁（superparamagnetic iron oxide, SPIO）标记技术，成为干细胞活体追踪的理想手段。因此，采用氧化铁标记间充质干细胞（MSCs）治疗急性肾损伤，并通过磁共振成像在活体内（in vivo）追踪、评估细胞的迁移、分布与归巢行为，是细胞治疗与活体细胞追踪的理想方案。 方法（Methods）： 体外（in vitro）实验：将间充质干细胞以25μg/mL的浓度进行24小时标记，检测标记效率与细胞活力。针对未标记间充质干细胞与超顺磁性氧化铁标记间充质干细胞（SPIO-MSCs），开展体外磁共振成像检测，分析其可检测细胞浓度与可检测时间窗。 体内（in vivo）实验：采用1.5T临床磁共振扫描仪，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（PBS，n=6）前后，连续8天进行磁共振成像评估。检测肾脏的信号强度，并结合组织学染色（苏木精-伊红[H&E]、普鲁士蓝染色）结果，采用非配对Student’s t检验进行统计学分析（p<0.05）以验证组间差异的显著性。 结果（Results）： 体外实验显示，超顺磁性氧化铁标记不会影响间充质干细胞的活力与标记效率。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的特征；在1.5T磁共振体外扫描条件下，5×10⁴个/mL的SPIO-MSCs为可成像的最低细胞浓度。 体内实验表明，与对照组相比，SPIO-MSCs输注后可导致肾髓质的T2*WI信号强度降低，该现象源于SPIO-MSCs在肾髓质的聚集，此差异在移植后第3天仍具有统计学意义（p<0.05）；但在第5天，肾髓质的T2*WI低信号区域开始减少，至第8天时，SPIO-MSCs组与对照组已无显著统计学差异。 结论（Conclusion）：本研究数据证实，在1.5T磁共振成像条件下，通过体外与体内实验，追踪并监测急性肾损伤模型小鼠静脉注射SPIO-MSCs后的肾脏分布与细胞迁移情况具备可行性。