Death and Proliferation Time Course of Stem Cells Transplanted in the Myocardium

Purpose: Noninvasive positron emission tomography (PET) imaging of reporter gene is combined with quantitative real-time polymerase reverse transcription (RT-PCR) method to study the time course of death and proliferation of stem cells transplanted in the myocardium.

Methods: Male murine embryonic stem cells (ESCs) were stably transfected with a mutant version of herpes simplex virus type 1 thymidine kinase (HSV1-sr39tk) reporter gene; 5 x 10(6) such cells were injected into the myocardium of female athymic rats. While the transplanted cells was monitored by in vivo 9-(4-[F-18]fluoro-3-hydroxymethylbutyl)guanine ([F-18]FHBG) PET imaging of the heart, their absolute number was estimated by RT-PCR from hearts harvested at 3-5 h, 24 h, days 4, 7, and 14 after transplantation.

Results: (1) Forty percent of injected cells were retained in the heart while majority of injected cells were lost within a few hours after injection. Cell death was peaked at 24 h when 18% of donor cells retained in the heart were dead. (2) The substantial cell loss was reversed by significant proliferation of ESCs. This led to the recovery of cell number to 3.4 million (70% of injected dose) at day 4 and first visual observation of in vivo [F-18] signal in the heart. (3) A robust correlation (R (2) = 0.9) between percent of injected dose per gram of tissue derived from in vivo PET signal and the number of donor cells estimated by RT-PCR was revealed.

Conclusions: The time course of transplanted stem cells surviving in the heart reveals a process of substantial cell loss within 24 h of injection and subsequent recovery of cell number through proliferation. Such proliferation can be noninvasively monitored by reporter gene imaging.