T cell receptor beta-chain genes in BW5147 and other AKR tumors. Deletion order of murine V beta gene segments and possible 5' regulatory regions.

The AKR thymoma BW5147 has rearranged both of its TCR beta-chain loci, using the same J beta region (J beta 2.5) in each, but with different V beta gene segments. Although the two rearrangements are expressed approximately equally in cytoplasmic RNA, the principle of allelic exclusion is maintained because only one rearrangement is in-frame and capable of encoding a functional protein. In hybridomas made with BW5147 as the fusion partner, this protein may combine with the alpha-chain protein derived from the normal cell to form new Ag/MHC specificities. An analysis of the sequences upstream from the BW5147 rearrangements and additional V regions suggests that two conserved sequences, 10 and nine nucleotides in length and located adjacent to each other 70 to 100 nucleotides 5' of the initiation codon, may be important in the expression of TCR beta-chain genes. Although B and T cells derive from common stem cells, no sequences are observed in T cells that are homologous to the octamer located 5' of all Ig genes. This implies that at least some of the sequences that regulate transcription are not shared in the two major types of lymphocytes. A survey of BW5147 and six other AKR thymomas using probes for 10 of the 18 known V region families indicates a distribution of V beta rearrangements in the tumors consistent with that found in thymocytes. Four of these tumors have apparent VDJ rearrangements on both chromosomes, with the deletion of other V beta gene segments. These data suggest that the primary mechanism of VDJ beta rearrangement is by looping out and excision of the intervening DNA and that most of the V regions are located 5' to the C region. These data were also used to develop a deletion order of the V beta gene segments in the TCR beta-chain locus.