Stem-like cells traffic from heart ex vivo, expand in vitro, and can be transplanted in vivo

Background: Cells with stem cell surface markers have been identified in heart tissue. Early indications suggest that these are cardiac progenitor cells that could contribute to cardiac repair/regeneration. Clinically relevant therapeutic strategies based on these cells will require improved methods for their isolation and characterization of determinants of their mobilization, proliferation and differentiation. Methods: An ex vivo culture system was developed that promotes trafficking of progenitor-like cells from mouse ventricles to a culture surface. Cells that "trafficked" from cardiac tissue were phenotyped by flow cytometry and immunohistochemistry. Results: Morphologically distinct cells spontaneously trafficked from mouse ventricular tissue, adhered in culture, and proliferated for up to 4 weeks in Dulbecco's minimal essential media supplemented with fetal calf serum. After 4 weeks in culture, cell number declined. Co-culture with unfractionated bone marrow restored the proliferation of these trafficked cells. A significant population of the trafficked cells expressed a phenotype consistent with that of a myogenic progenitor such as: c-kit⁺, Sca-1⁺, CD45^-, CD34^-, CD90.2^-, MyoD1^-, desmin^-, muscle-specific actin^-, and, infrequently, myogenin⁺. An expanded population of trafficked cells from ventricles of mice expressing green fluorescent protein (GFP⁺) and containing cardiac-derived progenitor cells were injected into the pericardial space of GFP^- mice. GFP⁺ cells trafficked throughout the heart but retained a primitive undifferentiated morphology. However, when injected into the pericardial space of Apo-E-deficient mice with coronary vasculopathy, progenitor-like cells trafficked into myocardium, and GFP⁺ cells differentiated into vessel-lining endothelial cells and, rarely, smooth muscle and cardiomyocytes. Conclusions: Progenitor-like cells in the heart can be mobilized by tissue injury to spontaneously traffic from cardiac tissue and can expand in culture by co-culture with bone marrow. When re-infused by pericardiocentesis, these primitive cells traffic into heart, retain immature morphology, but are capable of undergoing injury-induced differentiation. The novel method described herein permits further characterization of cardiac-derived progenitor cells, which are a candidate for cardiac regeneration strategies.