Saini S S, Kaushik A
Department of Pathobiology, University of Guelph, Guelph, Ontario, Canada N1G 2W1.
Scand J Immunol. 2002 Feb;55(2):140-8. doi: 10.1046/j.1365-3083.2002.01028.x.
Analysis of seven variable-diversity-joining (VDJ) gene rearrangements in B splenocytes from a 125-day-old bovine foetus revealed an extensive heavy-chain complementarity-determining region 3 (CDR3H) length variation (9-56 codons). Indeed, the global CDR3H size spectratyping of foetal VDJ rearrangements substantiated such an extensive heterogeneity and was comparable with that noted in peripheral B lymphocytes of adult cattle. These observations are in contrast to species such as humans with extensive germline combinatorial capability where shorter CDR3H length is noted early during B-cell development. Exceptionally long CDR3H (as in adult cattle) was noted in two foetal VDJ rearrangements encoded by a single germline VH gene. Further, two VH genes (gl.110.20 and BF2B5) were preferentially expressed in the foetal VDJ rearrangements. The DH gene-encoded CDR3H region of foetal VDJ rearrangements is remarkable for repetitive GGT (glycine) and TAT (tyrosine) codons that favour the recruitment of somatic hypermutations. It appears that closely related germline DH genes, preferentially used in the hydrophilic reading frame, encode varying CDR3H lengths early during B-cell ontogeny in cattle. A comparison of germline and expressed VH genes, especially in the CDR1 and CDR2, confirms that somatic hypermutations contribute to immunoglobulin (Ig)M antibody diversification in cattle. The biased nucleotide base use and high occurrence of 'hot-spot' triplet (AGPy; AG pyrimidine base) in the CDRs predisposes to somatic hypermutations. Overall, these observations suggest that extensive CDR3H length heterogeneity, including the generation of exceptionally long CDR3H (up to 56 amino acids), and somatic hypermutations contribute to IgM antibody diversification in cattle. The extensive CDR3H length heterogeneity early during the B-cell development may compensate for constraints imposed on antibody diversification owing to the limited germline sequence diversity of genetic elements in cattle.
对一头125日龄牛胎儿的B脾细胞中7种可变区-多样性区-连接区（VDJ）基因重排的分析显示，重链互补决定区3（CDR3H）长度存在广泛变异（9 - 56个密码子）。事实上，胎儿VDJ重排的整体CDR3H大小谱型分析证实了这种广泛的异质性，并且与成年牛外周B淋巴细胞中的情况相当。这些观察结果与人类等具有广泛种系组合能力的物种形成对比，在人类中，B细胞发育早期CDR3H长度较短。在由单个种系VH基因编码的两个胎儿VDJ重排中发现了异常长的CDR3H（如成年牛中所见）。此外，两个VH基因（gl.110.20和BF2B5）在胎儿VDJ重排中优先表达。胎儿VDJ重排的DH基因编码的CDR3H区域以有利于体细胞超突变募集的重复GGT（甘氨酸）和TAT（酪氨酸）密码子为显著特征。似乎密切相关的种系DH基因，优先用于亲水性阅读框，在牛B细胞个体发育早期编码不同长度的CDR3H。种系和表达的VH基因比较，特别是在CDR1和CDR2中，证实体细胞超突变有助于牛免疫球蛋白（Ig）M抗体的多样化。CDR中偏向的核苷酸碱基使用和“热点”三联体（AGPy；AG嘧啶碱基）的高出现频率易发生体细胞超突变。总体而言，这些观察结果表明，广泛的CDR3H长度异质性，包括异常长的CDR3H（长达56个氨基酸）的产生，以及体细胞超突变，有助于牛IgM抗体的多样化。B细胞发育早期广泛的CDR3H长度异质性可能弥补了由于牛遗传元件种系序列多样性有限而对抗体多样化造成的限制。
