Diversity of mammalian immunoglobulins: genes, structure and evolution

Diversity of mammalian immunoglobulins: genes, structure and evolution

Immunoglobulins (Ig) are key molecules in the animal immune system. They play a vital role in humoral immunity, helping the body to identify and eliminate foreign antigens. The diversity of immunoglobulins is the basis for their ability to cope with complex and variable antigens, and the mechanism of this diversity has always been a hot topic in immunology research. This article will explore the structure and expression mechanism of immunoglobulin genes in mammals such as pigs, cattle, sheep and rabbits, and reveal the uniqueness of the diversity of immunoglobulins in different species.

Basic structure and diversity mechanism of immunoglobulins

The basic structure of immunoglobulins consists of two identical heavy chains (H chains) and two identical light chains (L chains), forming a Y-shaped structure. The variable regions (V regions) of the heavy and light chains are responsible for specific antigen recognition, while the constant regions (C regions) determine the type and function of immunoglobulins. In mammals, the heavy chain genes of immunoglobulins include five types: μ, δ, γ, ε and α, and the light chain genes include two types: κ and λ.

The diversity of immunoglobulins is mainly generated through the following mechanisms:
V(D)J recombination: Diverse antibody variable regions are generated by randomly combining V, D and J gene segments.
Somatic hypermutation (SHM): During the development of B cells, V region genes undergo high-frequency mutations, further increasing the diversity of antibodies.
Gene conversion (GC): New genetic information is introduced through the exchange of gene segments.

Characteristics of immunoglobulins in different mammals

Porcine immunoglobulin genes and their expression mechanisms

The immunoglobulin gene structure of pigs is similar to that of other mammals, but there are unique differences in the CDR region. The VH gene segment, DH gene segment and JH gene segment of pigs jointly participate in VDJ recombination, in which the diversity of CDR3 is mainly achieved through the insertion of N-nucleotides. In addition, pigs have a balanced ratio of λ and κ chains at the protein level, which is different from other species. In the light chain genes of pigs, the frequency of use of λ and κ chains is comparable, and the number of functional Vλ and Vκ genes is also similar.

Bovine immunoglobulin genes and their expression mechanisms

Bovine immunoglobulin genes have a unique ultra-long CDRH3 structure, which produces a variety of antigen-binding configurations through different disulfide bonds, thereby increasing the diversity of antibodies. The frequency of use of λ chains in cattle light chain genes is much higher than that of κ chains, indicating that cattle may preferentially use λ chains to enhance the diversity of antibodies. In addition, in the IgH locus of cattle, the functional IGH gene is located on chromosome 21, while the IGH gene on chromosome 11 is a truncated form.

Sheep immunoglobulin genes and their expression mechanisms

Sheep immunoglobulin genes show significant length polymorphism in the CDR3 region of Vλ cDNA, and this diversity may be related to the flexibility of antigen binding. The diversity of sheep VH gene segments and Vλ gene segments is mainly achieved through somatic hypermutation (SHM) rather than V(D)J recombination. Goat immunoglobulin genes show a preference for specific V gene segments, and somatic hypermutation plays an important role in the generation of diversity.

Rabbit immunoglobulin genes and their expression mechanisms

The Cδ gene sequence of IgD is missing in the immunoglobulin genes of rabbits, but the number of Cα genes is relatively large, which may be related to the immune defense mechanism of rabbits. The VH gene fragments and JH gene fragments of rabbits are used more frequently, indicating that they have a unique mechanism in the production of antibody diversity. In addition, the expression level of κ chain in the light chain gene of rabbits is much higher than that of λ chain, which is different from other species.

Summary

Through the study of immunoglobulin genes and their expression mechanisms in different mammals, we found that these species adopted diverse strategies in dealing with pathogens. These findings not only enrich our understanding of immunoglobulin diversity, but also provide important theoretical support for animal disease-resistant breeding and the development of new antibody drugs. Future research will further reveal the molecular basis of these unique mechanisms and bring new breakthroughs to the fields of immunology and medicine.