Assays of Thymic Selection Fetal Thymus Organ Culture and In Vitro Thymocyte Dulling Assay

Many of the important properties of T-cells are not imprinted by the genetic program of the cell. Clonotypic expression of receptors, major histocompat- ibility complex (MHC) restriction, and self tolerance are all properties that are determined somatically during the development of an individual T-cell precursor in the thymus. Three selection checkpoints operate during development to imprint these properties: β selection, positive selection and negative selection. β selection occurs during an early stage of thymic development (1 ). This checkpoint ensures clonotypic expression of the T-cell receptor (TCR) β-chain, by signaling the termination of further rearrangement at that locus, in cells that have successfully produced a β-chain to pair with the invariant pre-Tα-chain. Positive and negative selection act at a later stage of development (2 ). Here, intact α/β-TCRs are checked for their ability to interact with MHC-self ligands. The prevailing affinity model suggests that receptors with a low affinity for MHC-self ligands trigger survival, subsequent maturation, and lineage commitment (3 ). This positive selection step assures that the T-cell repertoire of an individual is MHC restricted to the alleles that individual expresses. Receptors having a high affinity for MHC-self ligands trigger activation-induced apoptosis. It is thought that the threshold for activation of peripheral T-cells is at least as high as that of apoptosis in the thymus. Thus, negative selection contributes to a self-tolerant T-cell repertoire.