AUTHOR=Khass Mohamed , Levinson Michael , Schelonka Robert L. , Kapoor Pratibha , Burrows Peter D. , Schroeder Harry W. 

TITLE=Preimmune Control of the Variance of TCR CDR-B3: Insights Gained From Germline Replacement of a TCR Dβ Gene Segment With an Ig DH Gene Segment

JOURNAL=Frontiers in Immunology

VOLUME=Volume 11 - 2020

YEAR=2020

URL=https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2020.02079

DOI=10.3389/fimmu.2020.02079

ISSN=1664-3224

ABSTRACT=We have previously shown that the sequence of the immunoglobulin diversity gene segment (DH) helps dictate the structure and composition of immunoglobulin heavy chain complementarity determining region 3 (CDR-H3).   In order to test the role of germline D sequence on the diversity of the preimmune TCRβ repertoire of T cells, we generated a mouse with a mutant TCRβ DJC locus wherein the Dβ2-Jβ2 gene segment cluster was deleted and the remaining diversity gene segment, Dβ1, was replaced with DSP2.3, a commonly used B cell immunoglobulin DH gene segment.  Crystallographic studies have shown that the length and thus structure of TCR CDR-B3 places amino acids at the tip of CDR-B3 in a position to directly interact with peptide bound to an MHC molecule.  The length distribution of CDR-B3 has been proposed to be restricted largely by MHC-specific selection, disfavoring CDR-B3 that are too long or too short.  Here we show that the mechanism of control of CDR-B3 length depends on the Dβ sequence, which in turn dictates exonucleolytic nibbling. By contrast, the extent of N addition and the variance of created CDR3 lengths are regulated by the cell of origin, the thymocyte.  We found that the sequence of the D and control of N addition collaborate to bias the distribution of CDR-B3 lengths in the pre-immune TCR repertoire.  Natural selection thus appears to enhance the efficiency of functional TCR production by reducing the likelihood of producing CDR-B3 intervals whose lengths and structures might impede optimal MHC:peptide interactions.