Antibody Diversity and Genetics

Antibody Diversity

• The human genome contains approximately 20,000 to 25,000 genes encoding for mRNAs that produce proteins
• The immune system can produce around 10^10 to 10^11 different antibodies to protect the body
• Genes encode only a small fraction of the antibodies needed, about 1 millionth
• Antibodies have heavy and light chains
• Heavy and light chains of immunoglobulin genes consist of segments (exons) joined to form immunoglobulin genes
• Exons join via gene segment rearrangement at the DNA level, not mRNA
• Multiple copies of heavy and light chain segments within immunoglobulin genes rearrange sequentially to form heavy and light chain genes

Light Chain Genetics

• There are three gene segments with two coding for the V domain and one codes for the C domain
• The V segment contains 35 genes coding for the first 95 amino acids of the V domain
• The J segment contains 5 genes coding for the last 13 amino acids of the V domain
• There is 1 C gene that codes for the constant domain
• Each gene is preceded by a leader sequence
• Entrons separate gene exons
• V(D)J recombinase mediates random joining of a V gene with a J gene and the C gene via genetic recombination
• Unused genes are looped out and degraded
• There are four different types of constant λ genes, with 30 V genes and 4 J genes

Somatic Recombination

• Ig gene segments rearrange to form a contiguous functional gene
• Somatic recombination rearranges the gene segments, even without antigens
• V(D)J recombinase is involved in somatic recombination in lymphocytes
• RAG-1 and RAG-2 are enzymes responsible for cleavage and rejoining of DNA in rearrangement
• RAG-1 and RAG-2 are responsible for the first cleavage step in somatic recombination of Ig genes
• RAG-1 and RAG-2 are only found in lymphocytes
• Defects in the enzymes RAG-1 and RAG-2 lead to blockage of lymphocyte development

Heavy Chain Genetics

• The H chain locus contains 12 gene segments coding the V domain and 9 for constant domains of various isotypes
• There are 50 V genes, 25 diversity genes and 6 joining genes that codes for the V domain
• There are 9 constant genes that code for constant domains in the following sequence: μ, δ, γ3, γ1, α1, γ2, γ4, Ε, α2
• A switch region exists between genes for C chains, except for μ & δ which allows cells to simultaneously express IgM and IgD with the same specificity

Heavy Chains

• Mature mRNA sequences for IgM & IgD translate into IgM and IgD via alternative gene splicing
• Gene splicing produces membrane-bound (transmembrane and cytoplasmic tail exons) and secreted forms of IgM heavy chain
• Class or subclass switching from IgM to other isotypes is influenced and dependent on T cell secreted cytokines
• Class or subclass switching does not influence specificity

Gene Segments

• The Kappa light chain has 35 genes, wavelength has 30, and the heavy chain has 50
• The D for Kappa and wavelength light chain has 0 genes, and the heavy chain has 25
• The J for Kappa light chain has 5 genes, wavelength has 4, and the heavy chain has 6
• The Kappa and wavelength have 1 the heavy chain has 9

Antibody Diversity

• V, D, and J gene segments are present in multiple copies, producing germline diversity
• VJ and VDJ gene segments recombine to produce combinatorial diversity
• Junctions form between gene segments, and the joining of a V gene segment to a DJ gene segment involves DNA cleavage and nucleotide addition/subtraction
• Multiple combinations of light and heavy chains create different antibody specificities
• Somatic hypermutation can occur after antigenic stimulation and B cells respond to an antigen
• Only one chromosome is active in any given lymphocyte