A guide to thymic selection of T cells

This Review summarizes how the processes of thymic selection together ensure that the T cell repertoire is fully functional and safe. In the thymus, T cell receptor signal strength is integrated with distinct stromal cues to result in positive or negative selection of thymocytes or the generation of regulatory cells. The thymus is an evolutionarily conserved organ that supports the development of T cells. Not only does the thymic environment support the rearrangement and expression of diverse T cell receptors but also provides a unique niche for the selection of appropriate T cell clones. Thymic selection ensures that the repertoire of available T cells is both useful (being MHC-restricted) and safe (being self-tolerant). The unique antigen-presentation features of the thymus ensure that the display of self-antigens is optimal to induce tolerance to all types of self-tissue. MHC class-specific functions of CD4(+) T helper cells, CD8(+) killer T cells and CD4(+) regulatory T cells are also established in the thymus. Finally, the thymus provides signals for the development of several minor T cell subsets that promote immune and tissue homeostasis. This Review provides an introductory-level overview of our current understanding of the sophisticated thymic selection mechanisms that ensure T cells are useful and safe.