Understanding why T-cell receptors remain single

To ensure that T cells express a single T-cell receptor β-chain (TCR-β) — that is, allelic exclusion — TCR-β gene rearrangement is tightly regulated. Downregulation of Vβ gene-segment accessibility was thought to be crucial to the process of preventing secondary rearrangement after a functional TCR-β has been generated. However, data published in Nature Immunology indicate that additional mechanisms regulate this process.

TCR-β allelic exclusion is regulated at two stages of thymocyte development: at the double negative (DN) stage, when the TCR-β alleles must rearrange asynchronously; and at the double positive (DP) stage — which is reached only after a maturation signal from the pre-TCR indicates successful TCR-β gene rearrangement — when further TCR-β gene rearrangement must be prevented. In this study, Jackson et al. set out to investigate the proposed role of Vβ gene-segment accessibility in the inhibition of Tcr-β gene rearrangement after transition to the DP stage. They generated knock-in mice expressing a modified Tcr-β allele in which the Tcr-α enhancer (E_α) was positioned downstream of the Vβ12 gene segment. Consistent with their hypothesis that the introduced E_α (E_αi) would maintain the Vβ chromatin in an active configuration in DP thymocytes, DP thymocytes from knock-in mice showed increased Vβ germline transcription, histone modification and restriction-endonuclease accessibility compared with DP thymocytes from wild-type animals.

Vβ-to-DJβ rearrangement is downregulated in DP thymocytes from wild-type mice compared with rearrangement in DN thymocytes from the same animals. Surprisingly, despite the presence of E_αi increasing Vβ chromatin activation (and therefore accessibility) in DP thymocytes, Vβ-to-DJβ rearrangement in these cells was also downregulated compared with rearrangement in DN thymocytes.

Although inhibition of Tcr-β gene rearrangement after transition to the DP stage was not affected by the presence of E_αi, Vβ12 rearrangement was increased in DN thymocytes from E_αi-knock-in mice. This correlated with an increased proportion of peripheral T cells expressing both a TCR-β containing Vβ12 and a TCR-β containing a distinct Vβ. Both β-chains were products of chromosomal rearrangement, indicating that the presence of E_αi disturbed allelic exclusion in DN thymocytes. This defect in allelic exclusion occurred despite the fact that the presence of E_αi did not alter Vβ gene-segment accessibility in DN thymocytes.

This study shows that factors other than chromatin accessibility regulate allelic exclusion in both the DN and the DP thymocyte compartments. Future studies will focus on identifying these additional controls at the molecular level.