Transcription of germ line Vα segments correlates with ongoing T-cell receptor α-chain rearrangement

M14T is a virally transformed immature T-cell line which continues to rearrange its T-cell antigen receptor (TCR) α-chain genes in vitro and thus represents a dynamic system for studying TCR assembly. In an effort to investigate whether the TCRα locus is accessible for V(D)J rearrangement events, we examined M14T cells for the presence of germ line TCRα transcripts. Several unrearranged Vα segments were found to be transcriptionally active in M14T cells. By comparison, germ line Vα transcripts are absent in nonlymphoid and pro-T-cell lines and barely detectable in mature T-cell lines, suggesting that this phenomenon is likely stage and tissue specific. We demonstrate a perfect correlation between transcriptionally active Vα segments and their involvement in ongoing Vα- to-Jα rearrangements. In addition, data suggesting that the unrearranged Jα locus is also transcriptionally active in the M14T line are presented. Furthermore, the recombination-activating genes RAG-1 and RAG-2 are differentially expressed, with RAG-2 detectable only by polymerase chain reaction, implying that very low levels of one of these gene products are sufficient to complement the other to facilitate VJ rearrangements. These findings provide the first direct evidence for an accessibility model of antigen receptor rearrangement in T cells.