We have previously shown that the sequence of the immunoglobulin diversity gene segment (D_H) helps dictate the structure and composition of complementarity determining region 3 of the immunoglobulin heavy chain (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 (IMGT:TRDB1), was replaced with DSP2.3 (IMGT:IGHD2-02), a commonly used B cell immunoglobulin D_H 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 complementarity determining region 3 of the T cell receptor beta chain (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 and to focus the diversity provided by N addition and the sequence of the D on that portion of CDR-B3 that is most likely to interact with the peptide that is bound to the presenting MHC.

我们之前已经表明免疫球蛋白多样性基因片段（D_H) 有助于决定免疫球蛋白重链 (CDR-H3) 互补决定区 3 的结构和组成。为了测试生殖系 D 序列对 T 细胞免疫前 TCRβ 库多样性的作用，我们生成了一个具有突变 TCRβ DJC 基因座的小鼠，其中 Dβ2-Jβ2 基因片段簇被删除，其余的多样性基因片段 Dβ1 (IMGT:TRDB1), 替换为 DSP2.3 (IMGT:IGHD2-02)，一种常用的 B 细胞免疫球蛋白 D_H基因片段。晶体学研究表明，TCR CDR-B3 的长度和结构将 CDR-B3 尖端的氨基酸置于与 MHC 分子结合的肽直接相互作用的位置。T 细胞受体 β 链 (CDR-B3) 互补决定区 3 的长度分布已被提议主要受 MHC 特异性选择的限制，不利于过长或过短的 CDR-B3。在这里，我们表明控制 CDR-B3 长度的机制取决于 Dβ 序列，这反过来又决定了外切核酸的吞噬。相比之下，N 添加的程度和创建的 CDR3 长度的变化受起源细胞胸腺细胞的调节。