Chimerism as a mechanism of self-repair.

To the Editor: We read with interest the editorial by Taylor et al1 concerning the ongoing debate about cardiac chimerism. We were surprised that the authors cited the article of Glaser et al2 as proof that chimerism would not exist but failed to mention our study showing that cardiomyocytes can be regenerated by non-cardiac cells in human transplanted hearts. Importantly, in our article, which appeared in the same issue of Circulation,3 we found chimerism to involve up to 1.6% of cardiomyocytes. We agree with the authors of the editorial that there are differences among the studies published to date on cardiac chimerism.3–5 However, there are no qualitative differences, but simply variability in the extent of this phenomenon. There are also distinct methodological differences among the studies; the first used fluorescence microscopy,3 the second light microscopy,4 and the third confocal microscopy.5 The Y-chromosome probes and the time between transplantation and sampling varied largely among studies. The claim of Taylor et al1 that potential acute rejection could be a cause for misinterpretation is not plausible. In our study,3 we also performed immunostaining for inflammatory cells and found them only in a small number. Furthermore, we were able to exclude cell fusion between donor and recipient cells using combined Yand X-chromosome fluorescence in situ hybridization. The fluorescence picture by which Taylor et al1 try to prove that nuclei of overlapping lymphocytes mimic cardiomyocyte nuclei is more disappointing than convincing. The lymphocyte shown cannot be confused with a myocyte nucleus by any examiner with experience in cardiovascular pathology. In summary, the existence of cardiac chimerism was proven sufficiently by 3 independent groups of investigators.3–5 The reasons for the different extent of cardiac chimerism need to be examined in future studies comparing different methodologies of detection, as well as different time points of material acquisition.