T-cell egress from the thymus: Should I stay or should I go?

T-cells bearing the alpha beta TCR play a vital role in defending the host against foreign pathogens and malignant transformation of self. Importantly, T-cells are required to remain tolerant to the host's own cells and tissues in order to prevent self-reactive responses that can lead to autoimmune disease. T-cells achieve the capacity for self/nonself discrimination by undergoing a highly selective and rigorous developmental program during their maturation in the thymus. This organ is unique in its ability to support a program of T-cell development that ensures the establishment of a functionally diverse alpha beta TCR repertoire within the peripheral T-cell pool. The thymus achieves this by virtue of specialized stromal microenvironments that contain heterogeneous cell types, whose organization and function underpins their ability to educate, support, and screen different thymocyte subsets through various stages of development. These stages range from the entry of early T-cell progenitors into the thymus, through to the positive and negative selection of the alpha beta TCR repertoire. The importance of the thymus medulla as a site for T-cell tolerance and the exit of newly generated T-cells into the periphery is well established. In this review, we summarize current knowledge on the developmental pathways that take place during alpha beta T-cell development in the thymus. In addition, we focus on the mechanisms that regulate thymic egress and contribute to the seeding of peripheral tissues with newly selected self-tolerant alpha beta T-cells.