Clinical trials in stem cell therapy: pitfalls and lessons for the future

Phenotypic modulation and turnover of bone marrow-derived cells after myocardial infarction in rats

BACKGROUND

Bone marrow-derived cells (BMCs) are critically involved in inflammation and regeneration after myocardial infarction (MI). However, the participation of BMCs in the reconstruction of infarcted myocardium remains unclear. In this study, we investigated phenotypic modulation of BMCs and their turnover in the heart following MI.

METHODS AND RESULTS

MI was produced in rats with intra-bone marrow-bone marrow transplantation from the syngenic rats expressing green fluorescence protein (GFP). The number of GFP-positive BMCs recruited to the infarcted myocardium peaked at 3 days after MI, and the majority of BMCs recruited to the heart after MI underwent turnover within 2 weeks. This turnover rate was unchanged for up to 16 weeks after MI, although the number of BMCs recruited to the infarcted myocardium rapidly decreased between 2 and 8 weeks after MI. A small number of BMCs recruited to the heart were positive for CD31 and α-smooth muscle actin, and the majority of these were positive for vimentin at 3 days and 4 weeks after MI. None of BMCs expressed α-actinin or von Willebrand factor 4 weeks after MI.

CONCLUSIONS

These results suggest that BMCs recruited to the heart underwent phenotypic modulation to a fibroblastic cell type and turnover within 2 weeks after MI without differentiating into cardiomyocytes or endothelial cells, and that although the number of BMCs in the infarcted myocardium decreased over time, the rate of turnover remained relatively constant during the chronic phase of MI.

Disruption of a Quorum Sensing mechanism triggers tumorigenesis: a simple discrete model corroborated by experiments in mammary cancer stem cells

Background

The balance between self-renewal and differentiation of stem cells is expected to be tightly controlled in order to maintain tissue homeostasis throughout life, also in the face of environmental hazards. Theory, predicting that homeostasis is maintained by a negative feedback on stem cell proliferation, implies a Quorum Sensing mechanism in higher vertebrates.

Results

Application of this theory to a cellular automata model of stem cell development in disrupted environments shows a sharply dichotomous growth dynamics: maturation within 50-400 cell cycles, or immortalization. This dichotomy is mainly driven by intercellular communication, low intensity of which causes perpetual proliferation. Another driving force is the cells' kinetic parameters. Reduced tissue life span of differentiated cells results in uncontrolled proliferation. Model's analysis, showing that under the Quorum Sensing control, stem cell fraction within a steady state population is fixed, is corroborated by experiments in breast carcinoma cells. Experimental results show that the plating densities of CD44+ cells and of CD44+/24lo/ESA+ cells do not affect stem cell fraction near confluence.

Conclusions

This study suggests that stem cell immortalization may be triggered by reduced intercellular communication, rather than exclusively result from somatic evolution, and implies that stem cell proliferation can be attenuated by signal manipulation, or enhanced by cytotoxics targeted to differentiated cells. In vivo verification and identification of the Quorum Sensing mediating molecules will pave the way to a higher level control of stem cell proliferation in cancer and in tissue engineering.

Reviewers

This article was reviewed by Glenn Webb and Marek Kimmel.

Cell therapy in congestive heart failure

Congestive heart failure (CHF) has emerged as a major worldwide epidemic and its main causes seem to be the aging of the population and the survival of patients with post-myocardial infarction. Cardiomyocyte dropout (necrosis and apoptosis) plays a critical role in the progress of CHF; thus treatment of CHF by exogenous cell implantation will be a promising medical approach. In the acute phase of cardiac damage cardiac stem cells (CSCs) within the heart divide symmetrically and/or asymmetrically in response to the change of heart homeostasis, and at the same time homing of bone marrow stem cells (BMCs) to injured area is thought to occur, which not only reconstitutes CSC population to normal levels but also repairs the heart by differentiation into cardiac tissue. So far, basic studies by using potential sources such as BMCs and CSCs to treat animal CHF have shown improved ventricular remodelling and heart function. Recently, however, a few of randomized, double-blind, placebo-controlled clinical trials demonstrated mixed results in heart failure with BMC therapy during acute myocardial infarction.

G-CSF-Based Stem Cell Therapy for the Heart?Unresolved Issues Part A: Paracrine Actions, Mobilization, and Delivery

The results of large-scale clinical trials involving granulocyte colony-stimulating factor (G-CSF)-based mobilization of bone marrow stem cells to improve cardiac remodeling and function after acute myocardial infarction have been disappointing. These trials came about as the result of an explosion of animal studies reporting dramatic successes with this therapeutic approach and small-scale nonrandomized, nonblinded clinical trials suggesting beneficial effects in humans as well. It would be rash to conclude, however, that G-CSF-based stem cell therapies for repairing the injured or failing heart are not worth pursuing. Recent advances in basic science not only help explain the failure of the larger clinical trials but have revitalized interest into using G-CSF-based or G-CSF-related therapies for the injured heart. This article will provide an overview of recent advances that have been made in the direct protective actions of G-CSF on cardiac cells, the mobilization of stem cells from the bone marrow, and the delivery of these cells to the heart. Such knowledge could be readily exploited to make G-CSF-based therapy a reality for the clinician.