Cieslak A, Le Noir S, Trinquand A, Lhermitte L, Franchini DM, Villarese P, Gon S, Bond J, Simonin M, Vanhille L, Vanhile L, Reimann C, Verhoeyen E, Larghero J, Six E, Spicuglia S, André-Schmutz I, Langerak A, Nadel B, Macintyre E, Payet-Bornet D, Asnafi V
J. Exp. Med. 2014 Aug;211(9):1821-32
V(D)J recombination of TCR loci is regulated by chromatin accessibility to RAG1/2 proteins, rendering RAG1/2 targeting a potentially important regulator of lymphoid differentiation. We show that within the human TCR-α/δ locus, Dδ2-Dδ3 rearrangements occur at a very immature thymic, CD34(+)/CD1a(-)/CD7(+dim) stage, before Dδ2(Dδ3)-Jδ1 rearrangements. These strictly ordered rearrangements are regulated by mechanisms acting beyond chromatin accessibility. Importantly, direct Dδ2-Jδ1 rearrangements are prohibited by a B12/23 restriction and ordered human TCR-δ gene assembly requires RUNX1 protein, which binds to the Dδ2-23RSS, interacts with RAG1, and enhances RAG1 deposition at this site. This RUNX1-mediated V(D)J recombinase targeting imposes the use of two Dδ gene segments in human TCR-δ chains. Absence of this RUNX1 binding site in the homologous mouse Dδ1-23RSS provides a molecular explanation for the lack of ordered TCR-δ gene assembly in mice and may underlie differences in early lymphoid differentiation between these species.