Genetic Modification of Human Mesenchymal Stem Cells Enables Contribution to the Cardiac Progenitor Pool and Developing Myocardium In Vivo.
Authors: Colletti Evan J , Soland Melisa , Jeor Stephen St. , Zanjani Esmail D , Porada Christopher D , Almeida-Porada Graca 
Published: October 2019

Abstract
Abstract 3730
Although a great deal of attention has been focused on developing cell based therapies for cardiac repair, only limited success has been achieved to date. Controversy still remains as to which specific type of cells should be transplanted and what role they play in the repair of damaged areas. Human autologous mesenchymal stem cells (MSC) are currently being used in clinical trials, and early results show improvement in the overall cardiac function. This improvement is mediated by inhibition of inflammatory signaling, fibroblast recruitment, and scar tissue development; however, little to none of the transplanted cells contribute to the working myocardium. It is likely that the extensive rate of cell death observed within cells efficiently delivered to the heart constitutes a key event precluding success of cell-based myocardial repair. Cytotoxic T lymphocytes (CTL), important mediators of allograft rejection, have also been implicated in immune responses against cardiac self-antigens subsequent to myocardial damage after myocardial infarction. Likewise, Natural Killer (NK) cells play an important role in targeting and destroying allogeneic and autologous cells undergoing distress. Therefore, it is possible that, in the event of myocardial damage, CTL and NK cells present at the site of injury contribute significantly to the death of the cells delivered for myocardial rescue, reducing their therapeutic effectiveness. We have shown that MSC transduced with a viral vector encoding the human cytomegalovirus unique short region 6, (hMSC-US6), are less susceptible to both NK killing and induction of CTL proliferation when compared to untransduced MSC, and to MSC transduced with a vector encoding only NPT-II (MSC-E). Therefore, in these studies we compared the ability of hMSC-US6 and hMSC-E to give rise to cardiac cells upon transplantation in a xenogeneic sheep fetal model. 5.6×104 of each cell population was transplanted into fetal sheep at 60 days of gestation (n=4). Two months after transplant, heart tissues were collected and the contribution of transplanted MSC to the fetal hearts was evaluated by confocal microscopy and NPT-II immunofluorescence. Examination of hearts from animals transplanted with MSC-US6 showed that engrafted cells contributed not only to the myocardium, as demonstrated by co-localization of NPT-II and Troponin-I (TNI), but were also able to contribute to the cardiac stem cell pool, as evidenced by co-localization of NPT-II and c-kit positivity. In the myocardium, MSC-US6 contributed to 2.6% of total TNI+ cardiomyocytes (53.9% of all cells in the heart are TNI+ at this stage of fetal gestation). Furthermore, at this stage in development, the c-kit+ cardiac progenitor pool constitutes 12.7% of the total cells in the heart, with the majority of the c-kit population localizing perivascularly. Upon examination, 4.5% of these c-kit+ cells were also NPT-II+, demonstrating the contribution of MSC-US6 to the heart stem cell pool. By contrast, the heart of animals that received MSC-E did not contain NPT-II+/TNI+ cardiomyocytes or NPT-II+/c-kit+ cardiac stem cells; the transplanted cells only contributed to the Purkinje fiber system in the heart. Although the transplantation model used is a non-injury model, MSC are still able to elicit an immune response in this non-autologous setting, activating CTL and NK cells already present in the recipient at the time of transplant. In conclusion, our results show that expression of US-6 protein allows transplanted human MSC to evade existing CTL- and NK-mediated immunity and contribute to the myocardial tissue through integration into the cardiac stem cell pool in the chimeric fetal heart. Therefore, engineering MSC to evade resident immune cells may decrease post-infusion cell death and allow these cells to contribute directly to the repair/regeneration of the injured myocardium.

Disclosures:
No relevant conflicts of interest to declare.