van Berlo et al. reply

REPLYING TO C. Vicinanza et al. Nature 555 https://doi.org/10.1038/nature25771 (2018)

The accompanying Comment by Vicinanza et al.¹ does not agree with our previous study² regarding the utility of c-Kit^pos (also known as Kit^pos) cells when investigating regeneration in the heart. The authors claim that three previous publications^2,3,4 using Kit^cre knock-in lineage-tracing question their previous work that shows that tissue-specific c-Kit^pos cardiac stem/progenitor cells (CSCs) are necessary and sufficient for cardiomyocyte regeneration and/or replenishment after injury⁵. However, the regenerative data within their previous manuscript did not address the role of endogenous c-Kit^pos cells in the heart⁵. Instead, they relied on clonally derived c-Kit^pos cells that were selected in culture and then injected into the circulatory system of mice and these cells remarkably homed to the heart and generated abundant new cardiomyocytes that repaired isoproterenol-mediated injury⁵.

The authors now claim that the number of c-Kit^pos cells with cardiomyogenic-clone-producing potential is very low in the mouse heart (approximately 1% of total c-Kit^pos cells)⁶. Indeed, the authors showed that broadly isolated primary cardiac c-Kit^pos cells, that were not clonally selected, had almost no ability to generate new cardiomyocytes or repair the infarcted heart⁶. The authors’ new interpretation¹ that only a very small portion of c-Kit^low cells can be made clonal with regenerative activity contradicts their published results from 2003 in which broadly isolated c-Kit^pos cells regenerated 70% of the myocardium after infarction injury⁷, or published results from 2013 showing that the majority of endogenous c-Kit^pos cells express Nkx2.5 (a cardiomyocyte-determining transcription factor)⁵.

The authors further claim that Kit^cre knock-in lineage tracing approaches^2,3,4 fail to track these rare c-Kit^low CSCs, because of poor recombination efficiency and heterozygosity in the Kit allele. While we have previously addressed this criticism and discussed data to the contrary⁸, we have recently published additional data showing that the majority of c-Kit^posCD45^neg cells isolated from the hearts of Kit^cre mice are correctly lineage-traced⁹, suggesting that this technique for tracking cells in mice is effective^2,3,4. Indeed, a working group of established cardiac scientists recently published a consensus statement confirming that endogenous cardiac c-Kit^pos cells are an unlikely source for meaningful heart regeneration¹⁰. This statement was also based on the known biology of the adult mammalian heart, which essentially lacks acute cardiomyocyte-forming regenerative capacity. However, it seems reasonable that one could select and isolate rare clonally derived c-Kit^pos progenitor cell lines with ectopic cardiomyogenic activity, as suggested in the Comment¹. However, the Comment does not address the role of endogenous c-Kit^pos cells in the heart. The current negative results simply address whether clonally derived c-Kit-expressing cells can have cardiomyogenic capacity, which we believe lacks all in vivo relevance and in no way contests our previous observations.