Host-derived circulating cells generate hybrid cardiomyocytes by cell fusion in heterotopic heart xenotransplantations

The possibility to regenerate dead myocardium by cell therapy using either extra-cardiac or cardiac stem cells is the object of an intense investigation. Whereas much work has been done to assess the potential of injected cells to form new myocardium, little is known about the spontaneous recruitment of stem cells through the circulation. Host-derived cardiomyocytes were found in human sex-mismatched heart transplantations, however the relative proportion of this phenomenon was estimated very high by some authors and negligible by others. In heterotopic heart allo-transplantation, we previously demonstrated that circulating cells engraft the heart transplants but do not significantly contribute to cardiac repair. Possible fusion events, as opposed to transdifferentiation, had been hypothesized but not proved. Here, we took advantage by the use of xeno-transplantations, in which markers of both donor and recipient cells are available, to address this issue. Xeno-transplantations were performed using GFP transgenic rats as hosts, and either Syrian hamsters (n8) or transgenic mice expressing the lacZ reporter gene under the control of the cardiac troponin I promoter (n6), as heart donors. All transplants, which were retrieved 15 days after surgery, contained a large quantity of GFP inflammatory cells. In 7 of 8 hamster-to-rat heart transplants GFP mature cardiomycytes were found, with percentages ranging from 0.0001% to 0.034%. No more than 15 GFP cardiomyocytes were detected in all mouse-to-rat transplants. We also found rare small GFP cells expressing markers of cardiac progenitor cells, such as GATA-4 and MEF2C, in all xenografts. All GFP cardiomyocytes identified in our study co-expressed GFP (the host’s marker) and either hamster-specific antigens or the LacZ marker of mouse origin (the donor’s markers). Thus, using both an immunological and a genetic approach, we conclusively demonstrate that in our experimental model circulating cells do not significantly contribute to form new myocardium, rather they generate hybrid cardiomyocytes by cell fusion.