Directed differentiation of pluripotent stem cells: from developmental biology to therapeutic applications.

The discovery of human pluripotent stem cells has laid the foundation for an emerging new field of biomedical research that holds promise to develop models of human development and disease, establish new strategies for discovering and testing drugs, and provide systems for the generation of cells and tissues for transplantation for the treatment of disease. The remarkable potential of pluripotent stem cells has sparked interest and excitement in academia, the biotechnology and pharmaceutical industries, as well as the lay public. Although the potential of human pluripotent stem cells is truly outstanding, fulfilling this potential is solely dependent on our ability to efficiently generate functional cell types from them. Some of the most successful approaches in this area to date are those that have applied the principles of developmental biology to stem cell differentiation. In this chapter, we review these concepts and highlight specific examples demonstrating that pluripotent stem cell differentiation in culture recapitulates the key aspects of early embryonic development. By continuing to translate insights from embryology to stem cell biology, progress in our ability to generate specific cell types from pluripotent stem cells will advance, yielding enriched populations of human cell types, including cardiomyocytes, hematopoietic cells, hepatocytes, pancreatic beta cells, and neural cells, for drug discovery, functional evaluation in preclinical models of human disease, and ultimately clinical applications.

A physiological ligand of positive selection is recognized as a weak agonist.

Positive selection is a process that ensures that peripheral T cells express TCR that are self-MHC restricted. This process occurs in the thymus and requires both self-MHC and self-peptides. We have recently established a TCR transgenic (TCR(trans)(+)) mouse model using the C10.4 TCR restricted to the MHC class Ib molecule, H2-M3. Having defined H2-M3 as the positively selecting MHC molecule, the severely limited number of H2-M3 binding peptides allowed us to characterize a mitochondrial NADH dehydrogenase subunit 1-derived 9-mer peptide as the physiological ligand of positive selection. Here, we demonstrate that the NADH dehydrogenase subunit 1 self-peptide is seen by mature C10.4 TCR(trans)(+) T cells as a weak agonist and induces positive selection at a defined concentration range. We also found that the full-length cognate peptide, a strong agonist for mature C10.4 TCR(trans)(+) T cells, initiated positive selection, albeit at significantly lower concentrations. At increased peptide concentrations, and thus increased epitope densities, either peptide only induced the development of partially functional T cells. We conclude that successful positive selection only proceeded at a defined, yet fairly narrow window of avidity.