Tcrd Rearrangement Redirects a Processive Tcra Recombination Program to Expand the Tcra Repertoire
2017
Zachary M. Carico | Kingshuk Roy Choudhury | Baojun Zhang | Yuan Zhuang | Michael S. Krangel
Adaptive immunity depends on diverse T cell receptor repertoires generated by variable, diversity, and joining (V[D]J) recombination. Here, we define the principles by which combinatorial diversity is generated in the murine Tcra repertoire. Tcra and Tcrd gene segments share the Tcra-Tcrd locus, with interspersed Vα and Vδ segments undergoing Vδ-Dδ-Jδ rearrangement in CD4−CD8− thymocytes and then multiple rounds of Vα-Jα rearrangement in CD4+CD8+ thymocytes. We document stepwise, highly coordinated proximal-to-distal progressions of Vα and Jα use on individual Tcra alleles, limiting combinatorial diversity. This behavior is supported by an extended chromatin conformation in CD4+CD8+ thymocytes, with only nearby Vα and Jα segments contacting each other. Tcrd rearrangements can use distal Vδ segments due to a contracted Tcra-Tcrd conformation in CD4−CD8− thymocytes. These rearrangements expand the Tcra repertoire by truncating the Vα array to permit otherwise disfavored Vα-Jα combinations. Therefore, recombination events at two developmental stages with distinct chromatin conformations synergize to promote Tcra repertoire diversity.
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