Relative Roles of Direct Regeneration Versus Paracrine Effects of Human Cardiosphere-Derived Cells Transplanted Into Infarcted Mice

TLDR

Cardiac cell therapy efficacy derives from multiple mechanisms, chiefly direct regeneration of myocardium and vasculature versus paracrine‑mediated tissue preservation and recruitment of endogenous repair, with human cardiac progenitor cells cultured as cardiospheres or CDCs shown to regenerate heart tissue in vivo. The study aimed to characterize the paracrine effects of CDC transplantation and assess their relative importance compared to direct differentiation of surviving transplanted cells. In vitro, CDC‑conditioned media contained numerous growth factors that protected neonatal rat ventricular myocytes from apoptosis and stimulated angiogenesis in human umbilical vein endothelial cells, while in vivo the CDCs were transplanted into the peri‑infarct zone of SCID mice. In vivo, CDCs secreted VEGF, HGF, and IGF‑1, increased Akt signaling, reduced apoptosis, enhanced capillary density, and improved myocardial viability, with direct differentiation accounting for 20–50 % of the effect and the role‑model paracrine activity rivaling or exceeding direct regeneration.

Abstract

Multiple biological mechanisms contribute to the efficacy of cardiac cell therapy. Most prominent among these are direct heart muscle and blood vessel regeneration from transplanted cells, as opposed to paracrine enhancement of tissue preservation and/or recruitment of endogenous repair.Human cardiac progenitor cells, cultured as cardiospheres (CSps) or as CSp-derived cells (CDCs), have been shown to be capable of direct cardiac regeneration in vivo. Here we characterized paracrine effects in CDC transplantation and investigated their relative importance versus direct differentiation of surviving transplanted cells.In vitro, many growth factors were found in media conditioned by human adult CSps and CDCs; CDC-conditioned media exerted antiapoptotic effects on neonatal rat ventricular myocytes, and proangiogenic effects on human umbilical vein endothelial cells. In vivo, human CDCs secreted vascular endothelial growth factor, hepatocyte growth factor, and insulin-like growth factor 1 when transplanted into the same SCID mouse model of acute myocardial infarction where they were previously shown to improve function and to produce tissue regeneration. Injection of CDCs in the peri-infarct zone increased the expression of Akt, decreased apoptotic rate and caspase 3 level, and increased capillary density, indicating overall higher tissue resilience. Based on the number of human-specific cells relative to overall increases in capillary density and myocardial viability, direct differentiation quantitatively accounted for 20% to 50% of the observed effects.Together with their spontaneous commitment to cardiac and angiogenic differentiation, transplanted CDCs serve as "role models," recruiting endogenous regeneration and improving tissue resistance to ischemic stress. The contribution of the role model effect rivals or exceeds that of direct regeneration.