Gene Rearrangement of Immunoglobulins: The Foundation of Antibody Diversity

An immunoglobulin consists of:

• Two heavy chains (H): Each chain contains a variable (V) region and several constant (C) regions.
• Two light chains (L): Each light chain has a variable (V) region and a constant (C) region.

The variable (V) region is responsible for the specific recognition of the antigen, while the constant (C) region

determines the class of the immunoglobulin (IgG, IgA, IgM, IgE, IgD) and its effector function.

Read more about structure and functions of antibodies.

The rearrangement of immunoglobulin genes, known as V(D)J recombination, is a key process in generating antibody diversity. This mechanism occurs during the maturation of B lymphocytes and involves the following steps:

Rearrangement of Heavy Chains:

The D (diversity) and J (joining) segments are first recombined.

A V (variable) segment is then added to the DJ segment, forming a complete VDJ gene.

Rearrangement of Light Chains:

The V and J segments are recombined to form a VJ gene.

This process is catalyzed by the RAG1 and RAG2 enzymes, which recognize specific sequences flanking the V, D, and J segments, allowing their cutting and rejoining.

Read More Here.

After the initial rearrangement, further antibody diversity is generated by:

• Somatic Hypermutation: Introduces point mutations into the variable region of the immunoglobulin genes, thereby refining the antibody's specificity for its target antigen.

• Class Switching: Allows a B lymphocyte to switch the class of immunoglobulin it produces (e.g., from IgM to IgG) while retaining its antigen specificity, through rearrangement of the constant regions.

1. Kappa Chain: V-J Rearrangement

The IGK (kappa) genes are located on chromosome 2 at 2p11. These genes include:

• IGKV genes for the variable (V) region (76 genes, of which 31 to 35 are functional).
• IGKJ genes for the junctional (J) region (5 genes).
• IGKC gene for the constant (C) region (1 gene).

In the genomic DNA, the V, J, and C regions are separated (germline configuration). These represent multigene families (similar to globin gene families), and duplications of the ancestral gene have occurred over time, with mutations in each gene ensuring diversity. Many of these duplicated genes remain functional.

The process of rearranging the DNA involves the following steps:

DNA Rearrangement: This step joins one V and one J gene, with intermediate sequences being deleted.

Transcription: The pre-messenger RNA (pre-mRNA) is transcribed, containing introns.

Splicing: Introns are removed from the pre-mRNA, resulting in mature messenger RNA (mRNA).

Translation: The mRNA is translated into a protein.

Note: It’s important not to confuse DNA rearrangement with RNA splicing. Only immunoglobulin genes and T-cell receptor genes undergo DNA rearrangements.

The V-J rearrangement occurs at the recombination signal sequences (RS), which include:

A heptamer sequence (7 nucleotides) and a nonamer sequence (9 nucleotides), separated by a spacer region.

Each IGKV gene is followed by a recombination signal sequence (RS) at the 3' end, which consists of a heptamer (CACAGTG), a 12 base pair spacer, and a nonamer (ACAAAAACC).

Each IGKJ gene is preceded by a recombination signal sequence (RS) at the 5' end, which includes a nonamer (GGTTTTTGT), a 23 base pair spacer, and a heptamer (CACTGTG).

2. Lambda Chain: V-J Rearrangement

The IGL (lambda) genes are located on chromosome 22 at 22q11. The rearrangement mechanism for the IGL genes is the same as for the IGK genes. The rearrangement occurs between one of the 29 to 33 functional IGLV genes and a J gene. There are also 4 to 5 functional IGLC genes, each preceded by an IGLJ gene.

3. Allelic and Isotypic Exclusion

Allelic exclusion is explained by both the timing of the rearrangements and the expression of a functional immunoglobulin on the surface. The expression of a functional immunoglobulin prevents further rearrangements and, thus, the expression of a second chain. As a result, only one chromosome 14 (heavy chain) and one chromosome 2 (or 22, for light chains) are productive in this process.

The IGH (heavy) genes are located on chromosome 14 at 14q32. There are 11 IGHC genes in total, of which 9 are functional, corresponding to the 9 isotypes of heavy chains: μ, δ, γ1, γ2, γ3, γ4, α1, α2, and ε.

1. V-D-J Rearrangement

The V-D-J rearrangement involves the DNA between one of the 38 to 46 functional IGHV (variable) genes, one of the 23 functional IGHD (diversity) genes, and one of the 6 functional IGHJ (joining) genes. Similar to light chains, recombination signal sequences (RS) are located downstream (3') of the V genes, around the D genes, and upstream (5') of the J genes. During the V-D-J rearrangement process, the following occurs:

• D and J Joining: First, a D gene joins with a J gene.
• V and D-J Joining: Then, a V gene joins the pre-formed D-J complex.

Note: There are 2 or 3 open reading frames for the D genes, allowing them to encode 2 or 3 different peptide sequences. The V-D-J junctions are characterized by nucleotide deletions (by exonuclease) and the random addition of nucleotides (by terminal deoxynucleotidyl transferase, or TdT), leading to the generation of N-regions that are not encoded in the individual's genome. These N-regions significantly increase the diversity of the V-D-J junctions in the variable domains of heavy chains in immunoglobulins.

2. Class Switching

In pre-B lymphocytes, the mu chain (IgM) is initially synthesized because the IGHM (constant) gene is located near the V-D-J rearrangement. This mu chain associates with the surrogate light chain, forming the pre-B receptor. The first complete immunoglobulin synthesized by the B lymphocyte is IgM, where the mu chain associates with a kappa or lambda light chain.

During its differentiation, the B lymphocyte can express another class or subclass of immunoglobulin. This requires the replacement of one IGHC gene with another through DNA recombination (class switching or "switch"), excising the intervening region as a deletion loop. This excision occurs at the switch sequences, which function similarly to the recombination signal sequences (RS).

The usual process then follows: transcription of the pre-messenger RNA, splicing of introns to form mature mRNA, and protein synthesis. This explains that:

A B lymphocyte can first synthesize IgM, and then, during differentiation, produce IgG (IgG1, IgG2, IgG3, or IgG4), IgA (IgA1 or IgA2), or IgE.

The B lymphocyte retains the same V-D-J rearrangement, thus maintaining the same antigen recognition site (idiotype).

3. Membrane Ig and Secreted Ig

Alternative splicing of the heavy chain pre-mRNA results in either a membrane heavy chain (membrane-bound Ig on B lymphocytes) or a secreted heavy chain (secreted Ig from plasma cells). Both forms retain the same V-D-J rearrangement (idiotype) and the same constant region (isotype).

Note: A similar mechanism (alternative splicing of pre-messenger RNA) also allows for the expression of both IgM and IgD in the same B cell (as seen in mature B cells that leave the bone marrow and travel to lymph nodes via circulation).

1. Germline Diversity ('Germline'): Multigene Families

Germline diversity is determined by the number of genes at each locus. These are gene families that provide a selection of closely related functional sequences. Possible intergenic recombinations allow the evolution of the locus over time through gene duplication or deletion. These genes undergo intragenic conversions and recombinations, ensuring genetic shuffling and diversity (polymorphism) between individuals.

The existence of multiple open reading frames (ORFs), particularly for IGHD genes, further increases the potential selection of closely related functional sequences.

2. Diversity Due to DNA Rearrangements

Combinatorial diversity – in the mathematical sense – allows for the potential synthesis of millions of immunoglobulins. Specifically:

• IGH genes enable the synthesis of approximately 6,000 heavy chains.
• IGK or IGL genes enable the synthesis of about 160 light chains.

This results in nearly one million possible combinations (6 × 10³ × 160).

Additionally, during the IGH rearrangement of heavy chains, N-regions are acquired, and the use of different reading frames from D genes at the V-D-J junctions contributes further to this diversity. For IGK or IGL rearrangements of light chains, flexibility in the V-J junctions adds more variability. These mechanisms contribute to an increase in diversity by a factor of 1,000 to 10,000, resulting in a potential synthesis of up to 10⁹ immunoglobulins.

3. Diversity through Somatic Hypermutations

Somatic hypermutations, which are numerous mutations occurring in a targeted manner in the rearranged V-J and V-D-J genes of immunoglobulins, further enhance diversity. While the exact mechanism behind these mutations is still not fully understood, AID (Activation-Induced Cytidine Deaminase) is believed to play a key role both in the mutation process and in the class-switching mechanism. These mutations occur during the differentiation of B lymphocytes in the lymph nodes and contribute an additional factor of 1,000 to the diversity of immunoglobulins. This allows for a potential diversity of 10¹² different immunoglobulins.

These diverse mechanisms allow for the generation of 10¹² unique immunoglobulins, capable of responding to the millions of known antigens.

In practice, however, the actual number of different immunoglobulins is limited by the number of B cells in a given species.