Identification of a diversity segment of human T-cell receptor β-chain, and comparison with the analogous murine element

The humoral immune system antigen-binding proteins (immunoglobulins) are disulphide-linked heterodimers of light and heavy chains. The gene for the variable region which determines antigen specificity is assembled when one member from each of the dispersed clusters of variable (V) gene segments, diversity (D) elements (for the heavy chains only) and joining (J) segments rearrange and fuse during B-cell development (reviewed in ref. 1). Short recognition sequences adjacent to these elements appear to be involved in the recombination process. The cellular immune system antigen recognition proteins are receptors on the surface of T cells, which are composed of disulphide-linked α-chains and β-chains, each of which has a variable and constant region^2-4. Recently, cDNA clones of the β-chain⁵ mRNA have been isolated^6,7; the genomic arrangement is very similar to immunoglobulin genes with multiple V_β genes⁸, and two clusters of J_β segments, each of which is upstream from a constant-region gene segment⁹. The V_β and J _β segments have adjacent recombinational recognition sequences like the immunoglobulin elements. However, ∼10 nucleotides of the cDNA clones between the V_β and J_β regions were not present in the corresponding genomic elements and may have been due to intervening D_β segments^8,10. Here we describe a diversity element (D_β1.1) in a region of high human-mouse homology about 650 bases 5′ to the first J_β cluster. Two transcripts which include sequences upstream of D_β1.1 are found in the human thymus. This region may have some other function besides providing the β-chain with a diversity segment.