Ancient Use of Ig Variable Domains Contributes Significantly to the TCRδ Repertoire

The loci encoding B and T cell Ag receptors are generally distinct in commonly studied mammals, with each receptor's gene segments limited to intralocus, cis chromosomal rearrangements. The nurse shark (Ginglymostoma cirratum) represents the oldest vertebrate class, the cartilaginous fish, with adaptive immunity provided via Ig and TCR lineages, and is one species among a growing number of taxa employing Ig-TCR delta rearrangements that blend these distinct lineages. Analysis of the nurse shark Ig-TCR delta repertoire found that these rearrangements possess CDR3 characteristics highly similar to canonical TCR rearrangements. Furthermore, the Ig-TCR delta rearrangements are expressed with TCR gamma, canonically found in the TCR delta heterodimer. We also quantified BCR and TCR transcripts in the thymus for BCR (IgHV-IgHC), chimeric (IgHV-TCR delta C), and canonical (TCR delta V-TCR delta C) transcripts, finding equivalent expression levels in both thymus and spleen. We also characterized the nurse shark TCR alpha delta locus with a targeted bacterial artifical chromosome sequencing approach and found that the TCR delta locus houses a complex of V segments from multiple lineages. An IgH-like V segment, nestled within the nurse shark TCR delta translocus, grouped with IgHV-like rearrangements we found expressed with TCR delta (but not IgH) rearrangements in our phylogenetic analysis. This distinct lineage of TCR delta-associated IgH-like V segments was termed TAILVs. Our data illustrate a dynamic TCR delta repertoire employing TCR delta Vs, NARTCRVs, bona fide trans-rearrangements from shark IgH clusters, and a novel lineage in the TCR delta-associated Ig-like V segments.