“These cells do not become adult cardiac myocytes,” said study coauthor Roberto Bolli, a cardiac cell therapy researcher at the University of Louisville School of Medicine. “So the mechanism is clearly a paracrine action, where the cells release ‘something’ which makes the heart better. And the million-dollar question now is, ‘What is the something?’”
Bolli’s team investigated the fate of so-called c-kit+ cells, progenitors harvested from the heart and named for the presence of a particular kinase. These cells have been the source of a long debate about their role in building cardiac muscle, with some studies finding no evidence of them producing new cardiomyocytes in vivo and others concluding that, if the conditions are right, c-kit cells do indeed make heart muscle.
C-kit cells have also been deployed in a clinical trial on heart attack patients led by Bolli’s group and Piero Anversa’s team, then at Harvard. The Phase 1 study hinted that the treatment reduced heart damage. (Editors at The Lancet, which published the results of the trial, issued an expression of concern regarding two supplemental figures in the paper that were prepared by the Harvard-based authors.)
Studies on a variety of cardiac cell therapies have found that the vast majority of the cells don’t stick around in the heart for much longer than a few weeks, suggesting that their mode of action is likely not based on the cells themselves producing new muscle tissue directly. To test whether that’s the case with c-kit cells, Bolli’s group harvested c-kit cells from healthy male rats’ hearts and injected them into female rats who had been made to have a heart attack.
Compared to controls, the treated rats had smaller scars, more muscle in their hearts, and improvements in cardiac function. To follow what had happened to the injected c-kit cells, the researchers picked out cells with Y chromosomes, finding that they made up 4 percent to 8 percent of the nuclei in the heart. Many of them had lost c-kit positivity, and it was clear from their morphology that these cells are not heart muscle and don’t contribute to cardiac contraction. “Honestly, I do not know what they are,” said Bolli, who is the editor-in-chief of Circulation Research. “That’s what we’re trying to figure out.”
It appeared that the treated animals did have more cell proliferation, which Bolli attributes to the cell therapy. “Pretty amazingly, it lasts up to 12 months after transplantation, which is another thing I cannot explain,” he said. “How can the transplantation, done only once, stimulate a proliferative response for 12 months?”
It’s been proposed that implanted cells might release cytokines, growth factors, microRNAs, exosomes, or some combination of secretions to effect the positive results seen in animals and some human studies. “They’re just confirming a paradigm we and others established years ago,” Eduardo Marbán, who is developing cardiac cell therapies at Cedars-Sinai in Los Angeles, told The Scientist in an email.
Not all experiments have supported the paracrine hypothesis. Anversa, who is now at the Cardiocentro Ticino in Lugano, Switzerland, has reported that implanted c-kit cells can become new cardiomyocytes. (He left Harvard last year after suing the university regarding its investigation into his lab, which resulted in a retraction and a correction. He declined to give his current job title.)
Anversa said he couldn’t explain the discrepancy, but that it’s not surprising to him that the same cell works differently in different experiments. “I’m very impressed by this study. It’s extremely well done,” he told The Scientist. “And the data are consistent with their interpretation of the results.”
But Bernardo Nadal-Ginard, a researcher at King’s College London, said the paper has “several significant flaws.” For one, he noted, not all c-kit+ cells are cardiac progenitors. “That the transplanted cells have a paracrine effect has been shown multiple times,” Nadal-Ginard wrote in an email to The Scientist. “What is the effect in the ‘turnover’ ‘replication’ or ‘expansion’ of the CPCs [cardiac progenitor cells] is anyone’s guess because the authors did not look for CPCs. They just monitored c-kit+ cells.”
Regardless of the mechanism of c-kit cells—or any other cell type used in cardiac therapy—clinical trials are progressing. Bolli is part of a Phase 2 study, currently recruiting patients, to explore the efficacy of heart-derived c-kit cells, bone marrow-derived mesenchymal stem cells, or both, in combination to treat ischemic cardiomyopathy.
X.-L. Tang et al., “Long-term outcome of administration of c-kitPOS cardiac progenitor cells after acute myocardial infarction: Transplanted cells do not become cardiomyocytes, but structural and functional improvement and proliferation of endogenous cells persist for at least one year,” Circulation Research, doi:10.1161/circresaha.115.307647, 2016.