SOMATIC HYPERMUTATION OF TCRa CONTRIBUTES TO THYMIC POSITIVE SELECTION IN SHARKS
Jeannine Ott, Texas A&M
Jeannine A. Ott1, Caitlin D. Castro2, Thaddeus C. Deiss1, Yuko Ohta2, Martin F. Flajnik2 and Michael F. Criscitiello1
1. Comparative Immunogenetics Laboratory, Department of Veterinary Pathobiology College of Veterinary Medicine and Biomedical Sciences, Texas A&M University College Station, TX 77843 USA
2. Department of Microbiology and Immunology School of Medicine, University of Maryland at Baltimore
Baltimore, MD 21201 USA
In mammals and probably all vertebrates, receptor editing of TCR alpha genes enhances immature T cell positive selection over a three-day interval in the thymic cortex. Surprisingly we found extensive somatic hypermutation (SHM) operating at the TCRα locus in the nurse shark thymus, implying that SHM contributes to receptor modifications that enhance positive selection. We analyzed mutation in TCRα families of clones with the same VJ rearrangement. Additionally, in situ hybridization showed the strongest activation-induced cytidine deaminase (AID) expression in the central thymic cortex and bordering the corticomedullary junction, with weaker expression in the medulla. The frequency of mutation at TCRα was as high as that seen at B cell receptor (BCR) loci in sharks and mammals. Complementarity determining regions (CDRs) accumulated significantly more mutations than framework regions (FWs), and significantly more of CDR mutations resulted in amino acid replacement. We saw a preference for transition mutations as well as a strong bias toward G:C substitutions within AID hotspots, especially within CDR regions. We suggest that TCRα utilizes SHM to boost positive selection and perhaps to broaden diversification of the αβ T cell repertoire in sharks, the first reported use of this process in thymic diversification in vertebrates.