New insights into the expression control of the TCRα chain in mature αβ T lymphocytes

A team of scientists at the Institute of Parasitology and Biomedicine López-Neyra (IPBLN) has found that the Tcra locus enhancer, Ea, is inactivated in mature ab T lymphocytes. This finding implies the existence of an independent mechanism for the transcripcional activation of the rearranged Tcra locus in mature ab T lymphocytes.

Ea is essential for the formation of a chromatin hub that drives Tcra germline transcription and Va-to-Ja recombination in CD4+CD8+ thymocytes. Once Tcra is rearranged, it is assumed that Ea remains active for transcription of the rearranged locus and expression of the TCRa chain in later developmental stages. This study demonstrates, however, that Ea is inactivated in CD4+ thymocytes, CD8+ thymocytes, and ab T lymphocytes. The findings of this study are published in the journal Proceedings of the National Academy of Sciences, USA (PNAS).

 

The researchers Beatriz del Blanco and Úrsula Angulo, led by Cristina Hernández-Munain of the IPBLN, and with the collaboration of Michael S. Krangel at Duke University, have used various strains of genetically modified mice to demonstrate the inactivation of Ea in mature ab T lymphocytes. Ea activity was measured through the interaction with a specific promoter using chromosomal conformational capture assay and the quantitative analysis of specific transcripts in both the endogenous unrearranged Tcra locus as in a locus in which Ea has been introduced ectopically. Ea activity was not reactivated in the context of TCRab/CD3-mediated stimulation or T helper differentiation indicating that Ea inactivation remains stable during ab T cell homoestasis. These results correlate with loss of activating histone marks at specific promoters and at the enhancer itself, as well as with the lack of recruitment of the transcription factors E2A and HEB to Ea. In fact, the lack of binding of E2A is a common feature between the inactive Ea enhanceosomes assembled in CD4-CD8- thymocytes and ab T lymphocytes.

These data indicate that the major function of Ea is the activation of Tcra germline transcription to trigger Va-to-Ja rearragement in CD4+CD8+ thymocytes. Once the TCRab is expressed on the surface of the thymocytes, Ea is inactivated by TCRab/CD3-mediated signaling associated to positive selection, and it will not be required for the normal expression of the rearranged Tcra at subsequent stages. Since transcription of the rearranged Tcra locus does not decay in ab T lymphocytes compared to that present in preselected CD4+CD8+ thymocytes, these data imply the existence of an Ea-independent mechanism to ensure transcription of the rearranged Tcra locus and expression of the TCRa chain in mature ab T lymphocytes.

del Blanco B, Angulo Ú, Krangel MS, Hernández-Munain C. T cell receptor α enhancer is inactivated in αβ T lymphocytes. PNAS 2015; published ahead of print March 23, 2015, doi:10.1073/pnas.1406551112.