Somatic hypermutation of T cell receptor α chain contributes to selection in nurse shark thymus

Abstract

Since the discovery of the T cell receptor (TcR), 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 α chain locus of shark TcR. 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 in vertebrates.

Nurse shark T‐cell receptors employ somatic hypermutation preferentially to alter alpha/delta variable segments associated with alpha constant region

“…We first confirmed previous results that CDRs of TCR αδV-TCR αC mutated significantly more than FRs (Fig. 3 and 4), with CDR1 accumulating the most mutation [34]. Within the TCRαδ locus, TCR αδV-TCR αC accrued more than twice as many mutations (0.0055 S/N) as TCR αδV-TCR δC (0.0025; p = 0.002; Fig.…”

“…We previously published the novel use of SHM in TCRα chain of αβ T cells within the thymus of nurse sharks [34]. The frequency of mutation at α was similar to that seen in BCR loci in sharks and mammals.…”

“…We previously confirmed that SHM occurred within TCRγ and TCR δV segments of nurse shark, but we found that SHM altered TCR αV far more than it did gamma or delta, with replacement mutation targeting AID-preferred motifs of CDR within the thymus. This suggested that TCR-α likely uses SHM to salvage failing receptors during thymic selection [34]. In both BCRs and TCRs of sharks, SHM can occur as single point mutations or tandem mutations of two or more contiguous nucleotides, indicating at least two different cellular mechanisms generate mutations [38][39][40].…”

“…Our previous data suggested that mutation to TCRα may be higher in thymus than in peripheral lymphoid tissues (spleen, spiral valve, and blood), but the limited data set constrained our ability to find a significant difference between tissue types [34]. Thus, we attempted to evaluate any difference in mutation between primary and secondary lymphoid tissues here.…”

“…Chen et al [29] presented definitive evidence that sandbar sharks utilize SHM to diversify gamma chain of γδ T cells, and camels employ SHM to diversify both TCR gamma and delta chains [29][30][31][32][33]. Additionally, nurse sharks utilize SHM for AID-catalyzed receptor salvaging to assist thymocytes through selection during thymic development [34]. Thus, gnathostome adaptive immunity displays remarkable elasticity in T-cell diversification mechanisms.…”