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In addition to canonical TCR and BCR, cartilaginous fish assemble noncanonical TCR that employ various B‐cell components. For example, shark T cells associate alpha (TCR‐α) or delta (TCR‐δ) constant (C) regions with Ig heavy chain (H) variable (V) segments or TCR‐associated Ig‐like V (TAILV) segments to form chimeric IgV‐TCR, and combine TCRδC with both Ig‐like and TCR‐like V segments to form the doubly rearranging NAR‐TCR. Activation‐induced (cytidine) deaminase‐catalyzed somatic hypermutation (SHM), typically used for B‐cell affinity maturation, also is used by TCR‐α during selection in the shark thymus presumably to salvage failing receptors. Here, we found that the use of SHM by nurse shark TCR varies depending on the particular V segment or C region used. First, SHM significantly alters alpha/delta V (TCRαδV) segments using TCR αC but not δC. Second, mutation to IgHV segments associated with TCR δC was reduced compared to mutation to TCR αδV associated with TCR αC. Mutation was present but limited in V segments of all other TCR chains including NAR‐TCR. Unexpectedly, we found preferential rearrangement of the noncanonical IgHV‐TCRδC over canonical TCR αδV‐TCRδC receptors. The differential use of SHM may reveal how activation‐induced (cytidine) deaminase targets V regions.

Many γδ T cells typically bind antigen in a manner similar to that of immunoglobulins, recognizing and directly binding small molecules and intact proteins without presentation by classical MHC:Ag complexes. In this way, γδ T cells combine an innate-like immune response with an adaptive recognition strategy, providing both an immediate response to pathogen invasion and an ongoing, adaptive response to inflammation. However, γδ T cells often recombine tissue-specific, restricted sets of genes that have limited junctional diversity. Thus, somatic hypermutation (SHM) presents a useful solution for diversifying these antibody-like T cell receptors (TCR) by fine-tuning ligand recognition or permitting changes within the loci that allow receptors to evolve more rapidly to changing ligand environments. In fact, recent studies in sandbar shark have confirmed definitively that the γ chain locus of γδ T cells employs SHM as a mechanism to generate more diverse γδ TCR, and follow-up studies in dromedary camel indicated that loci of both γ and δ chains use SHM to diversify γδ TCR. In nurse sharks, we found evidence that SHM acts in the thymus, altering the α chain locus of αβ T cells to broaden the primary αβ T cell repertoire. Further, early results suggest that SHM modifies both γ and δ chain loci (but not β chain locus of αβ T cells) and this alteration may occur in the thymus, indicating that this receptor fine-tuning may precede ligand recognition.

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