Imaging Long-Term Fate of Intramyocardially Implanted Mesenchymal Stem Cells in a Porcine Myocardial Infarction Model

The long-term fate of stem cells after intramyocardial delivery is unknown. We used noninvasive, repetitive PET/CT imaging with [18F]FEAU to monitor the long-term (up to 5 months) spatial-temporal dynamics of MSCs retrovirally transduced with the sr39HSV1-tk gene (sr39HSV1-tk-MSC) and implanted intramyocardially in pigs with induced acute myocardial infarction. Repetitive [18F]FEAU PET/CT revealed a biphasic pattern of sr39HSV1-tk-MSC dynamics; cell proliferation peaked at 33–35 days after injection, in periinfarct regions and the major cardiac lymphatic vessels and lymph nodes. The sr39HSV1-tk-MSC–associated [18F]FEAU signals gradually decreased thereafter. Cardiac lymphography studies using PG-Gd-NIRF813 contrast for MRI and near-infrared fluorescence imaging showed rapid clearance of the contrast from the site of intramyocardial injection through the subepicardial lymphatic network into the lymphatic vessels and periaortic lymph nodes. Immunohistochemical analysis of cardiac tissue obtained at 35 and 150 days demonstrated several types of sr39HSV1-tk expressing cells, including fibro-myoblasts, lymphovascular cells, and microvascular and arterial endothelium. In summary, this study demonstrated the feasibility and sensitivity of [18F]FEAU PET/CT imaging for long-term, in-vivo monitoring (up to 5 months) of the fate of intramyocardially injected sr39HSV1-tk-MSC cells. Intramyocardially transplanted MSCs appear to integrate into the lymphatic endothelium and may help improve myocardial lymphatic system function after MI.

心肌内注射递送后干细胞的长期归宿尚不明确。本研究采用结合[18F]FEAU示踪剂的非侵入性重复性正电子发射断层显像/X线计算机体层成像（PET/CT），对经sr39HSV1-tk基因逆转录病毒转导的间充质干细胞（mesenchymal stem cells, MSCs，缩写为sr39HSV1-tk-MSC）在诱导型急性心肌梗死猪模型中的心肌内植入情况，开展长达5个月的长期时空动态监测。重复性[18F]FEAU PET/CT成像结果显示，sr39HSV1-tk-MSC的动态变化呈现双相模式：细胞增殖于注射后33~35天在梗死周边区域、主要心脏淋巴管及淋巴结中达到峰值，此后与sr39HSV1-tk-MSC相关的[18F]FEAU信号逐渐衰减。采用PG-Gd-NIRF813造影剂开展的心脏淋巴成像研究（包含磁共振成像（MRI）与近红外荧光成像）结果显示，造影剂可通过心外膜下淋巴网络，从心肌内注射位点快速清除至淋巴管及主动脉周淋巴结。对35天和150天采集的心脏组织开展免疫组织化学分析，结果发现多种表达sr39HSV1-tk的细胞类型，包括成纤维肌细胞、淋巴管内皮细胞、微血管内皮及动脉内皮细胞。综上，本研究证实了[18F]FEAU PET/CT成像用于长达5个月的在体监测心肌内注射的sr39HSV1-tk-MSC细胞归宿的可行性与灵敏度。心肌内移植的间充质干细胞似乎可整合至淋巴管内皮中，或有助于改善心肌梗死（MI）后心肌淋巴系统的功能。