Regulation of the T Cell Receptor Beta Locus by Nuclear Lamina Association

Abstract

T lymphocytes of the adaptive immune system recognize antigens using T cell receptors, with each lymphocyte bearing a receptor of unique specificity. T cell receptors are generated through V(D)J recombination, a process in which gene segments are assembled for the generation of a functional receptor protein via somatic recombination.The T cell receptor β (Tcrb) locus encodes the β chain of the T cell receptor αβ heterodimer. Among antigen receptor loci, the Tcrb locus is unique in that it frequently associates with the nuclear periphery during the developmental stage in which the locus undergoes recombination. Previous work showed that the association of the Tcrb locus with the nuclear periphery was a stochastic process and that the recombination of Tcrb alleles at the nuclear periphery was suppressed compared to Tcrb alleles in the interior of the nucleus. However, the mechanisms that instruct the frequent association of Tcrb alleles with the nuclear periphery remained unknown. We characterized the association of the Tcrb locus with the nuclear lamina (NL) at high resolution using DamID. DamID analysis revealed that the association of the Tcrb locus with the NL was heterogeneous, and that a lamina-associated domain (LAD) border within the locus segregated the Tcrb recombination center (RC) from RC-proximal chromatin. This LAD border constrains the activity of the Tcrb enhancer (Eβ) to the RC. Accordingly, deletion of the LAD border caused the Eβ-dependent activation of RC-proximal chromatin, resulting in a loss of NL association, increased chromatin looping to the RC, and increased transcription and histone acetylation of genes within the affected 300 kb region. Activated gene segments underwent recombination at higher frequencies, causing a substantial alteration of the Tcrb repertoire. Therefore, our studies identified a LAD border in the Tcrb locus that served to limit the Eβ-dependent activation of RC-proximal gene segments in order to maintain the diversity of the Tcrb repertoire.