Department of Medicine, University of Alabama at Birmingham, Birmingham, AL, USA.
Division of Genetic Engineering and Biotechnology, National Research Center, Cairo, Egypt.
Immunol Rev. 2018 Jul;284(1):106-119. doi: 10.1111/imr.12669.
Although at first glance the diversity of the immunoglobulin repertoire appears random, there are a number of mechanisms that act to constrain diversity. For example, key mechanisms controlling the diversity of the third complementarity determining region of the immunoglobulin heavy chain (CDR-H3) include natural selection of germline diversity (D ) gene segment sequence and somatic selection upon passage through successive B-cell developmental checkpoints. To test the role of D gene segment sequence, we generated a panel of mice limited to the use of a single germline or frameshifted D gene segment. Specific individual amino acids within core D gene segment sequence heavily influenced the absolute numbers of developing and mature B-cell subsets, antibody production, epitope recognition, protection against pathogen challenge, and susceptibility to the production of autoreactive antibodies. At the tip of the antigen-binding loop (PDB position 101) in CDR-H3, both natural (germline) and somatic selection favored tyrosine while disfavoring the presence of hydrophobic amino acids. Enrichment for arginine in CDR-H3 appeared to broaden recognition of epitopes of varying hydrophobicity, but led to diminished binding intensity and an increased likelihood of generating potentially pathogenic dsDNA-binding autoreactive antibodies. The phenotype of altering the sequence of the D was recessive for T-independent antibody production, but dominant for T-cell-dependent responses. Our work suggests that the antibody repertoire is structured, with the sequence of individual D selected by evolution to preferentially generate an apparently preferred category of antigen-binding sites. The result of this structured approach appears to be a repertoire that has been adapted, or optimized, to produce protective antibodies for a wide range of pathogen epitopes while reducing the likelihood of generating autoreactive specificities.
尽管乍一看，免疫球蛋白库的多样性似乎是随机的，但有许多机制可以限制多样性。例如，控制免疫球蛋白重链第三互补决定区（CDR-H3）多样性的关键机制包括对胚系多样性（D）基因片段序列的自然选择和通过连续 B 细胞发育检查点的体细胞选择。为了测试 D 基因片段序列的作用，我们生成了一组仅限于使用单一胚系或移码 D 基因片段的小鼠。核心 D 基因片段序列中的特定单个氨基酸极大地影响了发育中和成熟 B 细胞亚群、抗体产生、表位识别、对病原体挑战的保护以及产生自身反应性抗体的易感性的绝对数量。在 CDR-H3 的抗原结合环尖端（PDB 位置 101），自然（胚系）和体细胞选择都有利于酪氨酸，而不利于存在疏水性氨基酸。CDR-H3 中精氨酸的富集似乎拓宽了对不同疏水性表位的识别，但降低了结合强度，并增加了产生潜在致病性双链 DNA 结合自身反应性抗体的可能性。改变 D 序列的表型对 T 细胞非依赖抗体产生是隐性的，但对 T 细胞依赖的反应是显性的。我们的工作表明，抗体库是有结构的，单个 D 的序列是由进化选择的，以优先产生一种明显偏好的抗原结合位点类别。这种结构化方法的结果似乎是产生了一种已经适应或优化的 repertoire，以产生针对广泛病原体表位的保护性抗体，同时降低产生自身反应性特异性的可能性。
