In Vitro Models of Human T Cell Development: Dishing Out Progenitor T Cells

T cells develop within the unique microenvironment provided by the thymus. T cell differentiation involves a series of commitment events and developmental checkpoints including T cell receptor (TCR) gene recombination and positive/negative selection of developing thymocytes to yield functionally mature T cells. These events occur in a sequential, temporal and spatial fashion, as developing thymocytes migrate through the thymus. In vitro studies to yield insights into human T cell development have classically employed the fetal thymic organ culture (FTOC) model system. This approach relies on the seeding of human hematopoietic stem cells (HSCs) and/or their progeny into host thymic lobes or thymic fragments, typically of mouse origin. Recently, a novel in vitro approach that makes use of the OP9 bone marrow stromal cell line expressing the Notch receptor ligand Delta-like-1 (OP9-DL1) effectively supported the generation of large numbers of human progenitor T cells from HSCs. In this review, we outline several in vitro systems employed for the generation and study of human T cells. Particular emphasis is dedicated to the OP9-DL1 coculture system. Finally, given the number of progenitor T cells that can be generated in vitro, we discuss the potential implications for the treatment of immune-related diseases such as cancer, immunedeficiency, and autoimmunity.