Molecular Indices of Functional Competence in Developing T Cells

The development of mature T cells in the thymus involves changes in the regulation of numerous genes. In some cases genes are turned on or off, while in others they acquire the ability to be expressed in response to appropriate stimuli. One set of genes whose expression is initiated in the thymus encodes the T-cell receptor chains. Although surface T-cell receptor expression is a prerequisite for mature responsiveness, it is not sufficient. Mature T cells must also be able to induce the expression of various batteries of unlinked genes in response to antigenic stimulation. These inducible genes include interleukin-2 (IL-2) and interferon-γ in some cells, interleukins-4 and -5 in others, perforins in still others, and the IL-2 receptor 55 kd chain, which must explain changes in the regulation of both constitutively expressed and inducible genes.