The Elastic Elasmobranch: Nurse Sharks Commission B Cell Components and Somatic Hypermutation Mechanisms to Diversify T Cells During Thymic Development

Abstract

Since the discovery of the T cell receptor (TCR) in 1983, immunologists have assigned somatic hypermutation (SHM) as a mechanism employed solely by B cells to diversify their antigen receptors. Remarkably, we found SHM acting in the thymus on the α chain locus of shark TCR for TCR repertoire generation. SHM in developing shark T cells likely is catalyzed by activation‐induced cytidine deaminase (AID) and results in both point and tandem mutations that accumulate non‐conservative amino acid replacements within complementarity‐determining regions (CDRs). Mutation frequency at TCRα was as high as that seen at B cell receptor loci (BCR) in sharks and mammals, and the mechanism of SHM shares unique characteristics first detected at shark BCR loci. Additionally, fluorescence in situ hybridization showed the strongest AID expression in thymic corticomedullary junction and medulla. We suggest that TCRα utilizes SHM to broaden diversification of the primary αβ T cell repertoire in sharks, the first reported use of this process in thymic diversification in vertebrates. In addition to canonical T and B cell receptors, cartilaginous fish assemble non‐ canonical TCR that employ various B cell components. For example, shark T cells associate alpha (TCR‐alpha) or delta (TCR‐delta) constant (C) regions with immunoglobulin (Ig) heavy chain (H) variable (V) segments or TCR‐associated Ig‐like V (TAIL V) segments to form chimeric IgHV‐T cell receptors, and combine TCR δC with both Ig‐like and TCR‐like V segments to form the doubly‐rearranging NAR‐TCR. 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 (TCR αδV) segments using TCR αC but not TCR 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 non‐canonical IgHV‐TCR δC over canonical TCR αδV‐ TCR δC receptors. The differential use of SHM may reveal how AID targets V regions.