Exploration of the Differential Expression Patterns of Immunoglobulin Heavy Chain Genes in Horses and Donkeys.

The research explores differences in expression of immunoglobulin (IG) genes between different breeds of Chinese horses and donkeys, aiming to better understand the IG expression patterns in these breeds. The main findings include the IGH locus structure of donkeys, the high frequency of particular gene combinations during recombination, differences in V, D and J junctions and the driving mechanisms of IGH diversity.

IGH Locus Structure and Recombination Patterns

• The researchers examined the immunoglobulin heavy chain (IGH) locus in the genome of Chinese horses and donkeys and found that in donkeys, the IGH locus contains 117 VH genes (23 of them are functional), 44 DH genes, and 8 JH genes. These are arranged in a specific sequence – VH-DH-JH – on the chromosome 7, spanning approximately 1189 kilobase pairs.
• During the process of VDJ recombination, which is a mechanism to produce antibody diversity, both the horse and donkey species showed a high frequency of certain gene combinations – IGHV4-IGHD2-IGHJ6, IGHV4-IGHD4-IGHJ6, and IGHV4-IGHD2-IGHJ4.

Junctional Diversity and CDR3H Region Differences

• The study found significant differences in the V, D, and J junctions (which represent variable, diversity and joining gene segments, respectively) between horses and donkeys, as well as between different breeds within each species. These were mainly due to differences in the length of N-nucleotide insertions which can contribute to added diversity in the final antibody molecule.
• The researchers also found that the CDR3H region (complementarity determining region 3 of the heavy chain), which plays a crucial part in the specificity of the antibody for its specific antigen, showed greater length diversity and a higher content of Cysteine (Cys) residues in horses than in donkeys. This indicates the possibility of species-specific differences in the spatial structure of IGH.

Somatic Hypermutation and IGH Diversity

• A noteworthy finding was the distinct influence of somatic hypermutation (SHM), a process that introduces point mutations into the variable genes of B cells after activation, in shaping the diversity of IGH chains in both horses and donkeys. There was a clear preference for A>G (adenine to guanine) and G>A (guanine to adenine) mutations in both species and breeds.
• The study concluded that V(D)J recombination, junctional diversity, and SHM are key mechanisms driving IGH diversity in horses and donkeys. However, while the basic mechanisms are similar, their specific manifestations exhibit both distinct and consistent patterns. This reflects on the differences in immune system adaptations across species and breeds and acts as a theoretical ground for understanding IGH expression diversity in equids.