Development of immunoglobulin diversity at the heavy chain locus of cattle

[摘要] Immunoglobulins (Ig) are formed by combinatorial recombination of a set of germline genes and undergo mutation in B-lymphocytes as they develop and mature. Combinatorial assortment can result in enormous diversification of the primary Ig repertoire in species where the number and variability within these segments is high (e.g. humans and mice). Many other species are unable to diversify the primary repertoire through rearrangement. In these animals, somatic hypermutation and gene conversion play a major role in driving pre-immune diversity. The bovine humoral immune system is not capable of generating a significant level of heavy chain Ig diversity through combinatorial assortment due to the small size of the IgH gene family and the low diversity apparent within the CDRs of the VH segments of this family. To learn more of the molecular processes responsible for diversification, this study hypothesized that somatic hypermutation would introduce nucleotide substitutions throughout the Ig reading frame including the JH-Cintron whereas if gene conversion were the dominant process behind diversification, modifications would be confined to the Ig reading frame. To distinguish these possibilities, Ig heavy chain sequences were recovered from the rearranged locus using lymphoid tissues of cattle of different ages for sequence analysis. This also allowed determination of the timing of Ig diversification and comparison of the extent of diversification. Analysis revealed that single base substitutions predominated, with purines targeted more frequently than pyrimidines and transitions favoured over transversions. Seventeen deletions spanning 1 to 26 nucleotides and 5 insertions in the range of 1 to 3 bases were also observed. As would be expected, mutational hotspots were encountered in CDR1 (complementary determining region) and CDR2 and the sequences downstream of FR4. The modified region extended into unutilized parts of the JH locus and downstream intron and with mutations occurring as frequently as in CDR2. The frequency of mutation decreased over the 579 bases lying 3' to the rearranged VDJ gene. Therefore, the data were consistent with the predicted pattern of somatic hypermutation.(Abstract shortened by ProQuest.).

[发布机构] University:University of Glasgow

[关键词] Animal sciences, Immunology