You Can’t Judge a Book by Its Cover

Abstract

The current issue of Circulation reports 2 apparently conflicting studies1,2 pertaining to the highly controversial issue of transdifferentiation of bone marrow–derived cells into cardiomyocytes. The study by Iwasaki et al1 entailed intramyocardial injections of human CD34+ progenitors 20 minutes after the creation of a myocardial infarction in sex-mismatched, immunodeficient athymic mice. The results, assessed 28 days after transplantation, demonstrate a dose-dependent improvement in regional and global left ventricular function, a limitation of remodeling, and an increase in capillary density. These functional benefits are associated with a cardiac differentiation of the engrafted CD34+ cells demonstrated by a double immunofluorescent staining for human- and cardiac-specific markers. Expression of cardiac genes unraveled by real-time polymerase chain reaction is also taken as additional evidence for the cardiomyocytic conversion of the CD34+ progenitor cells, although this conclusion is compounded by the concomitant observation of fusion events between human and mouse cells demonstrated by fluorescent hybridization in situ, using probes specific for these 2 species. Differentiation of the CD34+ progenitors into smooth muscle and endothelial cells is also reported on the basis of similar double-staining immunofluorescent patterns. In contrast, the second article, by Gruh et al,2 does not demonstrate any conversion of human endothelial progenitor cells cocultured with neonatal rat cardiomyocytes. This study has carefully deciphered the potential causes of artifacts that may arise from the use of flow cytometry and conventional 2-dimensional immunofluorescence microscopy and lead to misinterpretations of phenotypic changes incurred by transplanted cells. Indeed, even 3-dimensional confocal microscopy failed to provide a conclusive interpretation for approximately 12% of the double-stained cells (ie, those expressing markers of both endothelial and cardiac lineages), thereby highlighting the caution with which these putative transdifferentiation events should be analyzed. Failure of additional real-time polymerase chain reaction to detect cardiac transcription factors …