Germ‐line transcription and methylation status of the TCR‐Jα locus in its accessible configuration

We have generated two in vivo mouse models to study the regulation of DNA accessibility to the V(D)J recombinase machinery in the T cell receptor (TCR)‐Jα locus. In recombination activating gene (RAG)‐deficient mice, both injection of a TCR‐β chain transgene (RTB mice) or anti‐CD3‐ε treatment in vivo (RT3 mice) lead to the same phenotype with homogeneous thymocyte populations blocked at the CD4+CD8+ double positive (DP) stage. At this developmental stage, the TCR‐α rearrangements are about to start, and the TCR‐Jα locus is frozen in an accessible but yet unrearranged configuration in these mice. We show high level of TCR‐α germ‐line transcription in thymocytes from RTB and RT3 mice. Transcripts are skewed towards the 5′ end of the TCR‐Jα locus, and the T early alpha (TEA) sterile transcript is predominant and therefore provides a useful marker for the TCR‐Jα locus opening. Analysis of the DNA methylation status reveals a global surmethylation of the TCR‐Jα locus in the thymus in comparison with non‐lymphoid cells in these mice. We propose that hypermethylation of the locus could precede a progressive demethylation, providing a specific protective and regulatory role in the rearrangement events.

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