**2.2 Diversity of B-cell receptors: primary/preimmune repertoire of Igs: V(D)J/ somatic recombination**

#### *2.2.1 Ordered rearrangement model*

The ordered model was described by Alt and Baltimore in 1984 [5]. It states that during the V(D)J recombination, rearrangements at the H chain locus occur before those of the L chains.

#### *2.2.2 Sequential recombination*

The sequential recombination refers to the order of the stages of rearrangements from DH to JH occurring before rearrangements from VH to DJH [5]. In contrast, the genes of the TCR δ locus, in the precursors of T-cells, can recombine in any order. In the case of the IGH locus in B-cell precursors (and TCR β locus in T-cell precursors), one of the multiple DH segments (genes) recombines first with one of

#### **Figure 1.**

*The organization of the IGH chain C-region genes in humans. Eμ: intronic enhancer, S: switch region, IGH: immunoglobulin heavy chain.*

**25**

form a single exon [3].

**Figure 2.**

*diversity by genetic substitution*

*Immunogenetic Aspect of B-Cell Antigen Receptor Diversity Generation*

the multiple JH segments, by deletion of the DNA separating them. Then, one of the VH genes is recombined/juxtaposed with the rearranged DJ site, again by deletion of the intermediate DNA (**Figure 2**). After recombination, V, D, and J genes

*rearrangements lead first to the recombination of one of the DH segments with one of the JH segments, and then a VH segment is recombined with the DJH recombination. Although domain D is absent, the principle of IG light chain loci recombination is similar. Ig H chain: immunoglobulin heavy chain, VH: heavy chain variable genes, DH: heavy chain diversity genes, JH: heavy chain joining genes, CH: heavy chain constant genes*

*Ordered rearrangement of gene segments (adapted from [3]). In the ordered rearrangement model, rearrangements of gene segments at the H chain locus precede those at the L chain locus. Sequential* 

Like the TCR, B-cell receptor (BCR) diversity results from (i) the choice of segments to recombine, (ii) the "P" junctional variability of the nucleotides at the V-D-J or V-J junction sites between gene segments during rearrangements, (iii) the "N" diversity by insertion or deletion of a nucleotide during recombination under the action of terminal deoxynucleotidyl transferase (TdT), but also from (iv) the recombination between VH genes by substitution of a or part of a second VH gene to an already recombined VH-D-JH segment. The diversity of the B-cell repertoire is also increased by the process of somatic hypermutation (SHM) of IG variable genes. The V(D)J recombination process generates a much greater diversity at the level of the H chain loci compared to those of the L chains, where simply a V region is joined to a J region. Thus, if the human IGH locus contained only about 40 functional VH segments, 27 DH segments, and 6 JH segments, V(D)J recombination would generate about 6480 (40 × 27 × 6) H chains, whereas, human IGK, which contains about 40 Vκ genes and 5 Jκ genes, would give rise to approximately 200 κ chains, following 200 (40 × 5) different combinations. On the other hand, the variability of nucleotides/point mutations can occur at the junction sites, and bases can be lost (by deletion) or added (by insertion), giving additional diversity of the hypervariable CDR3 region of the H and L chains, which is coded by an additional sequence of DNA created by the junction of V, D, and J segments, for the H chain, and V and J segments, for the L chain; such a phenomenon is responsible for the junctional

*2.2.3 Combinatorial diversity, "P" junctional variability, "N" diversity, and* 

*DOI: http://dx.doi.org/10.5772/intechopen.90637*

*Immunogenetic Aspect of B-Cell Antigen Receptor Diversity Generation DOI: http://dx.doi.org/10.5772/intechopen.90637*

#### **Figure 2.**

*Normal and Malignant B-cell*

(for review see [1]).

**Key Point 1|Germline.**

the offspring.

**somatic recombination**

**Key Point 2|Somatic DNA.**

*2.2.2 Sequential recombination*

those of the L chains.

*2.2.1 Ordered rearrangement model*

extracted from both tumor (or matched normal) tissue and plasma.

Similar to T-cell receptors (TCRs), functional genes of immunoglobulins (Igs) are the result of somatic recombination of DNA containing the relatively limited germinal genetic information, using the so-called V(D)J recombination process that occurs between individual genes (also referred to as gene segments) of the variable domains of the H and L chains (or α, β, γ, and δ chains of TCRs). Each of these genes is present in multiple copies in most antigen receptor loci. The locus of IGH genes (like TCR β and TCR δ loci) contains variable (V), diversity (D), and joining (J) genes, whereas the IGL and IGK loci (like TCR α and TCR γ loci) contain only V and J genes [3]. So, individual V, D, and J genes at the IGH locus, and V and J genes at either the IGL or IGK loci rearrange somatically at the DNA level to generate V-D-J and V-J regions that, after transcription and translation, encode the variable domains of the antibody [4]

**2.2 Diversity of B-cell receptors: primary/preimmune repertoire of Igs: V(D)J/**

Somatic DNA is found in all cells of the body (tissues, skin, organs, and blood), except germ cells and embryonic cells, which are the source of gametes. Therefore, a mutation in the somatic DNA is not transmitted to the offspring, but it can lead to the genesis of diseases, especially tumors. So, somatic DNA can be

• Germline DNA is a constitutional DNA because it is related to the DNA of tissues derived from reproductive cells, i.e., egg or sperm that is derived from stem cells, and incorporated into the DNA of each cell of the body of the offspring. Therefore, the mutated parental germline DNA can be passed to

• Germline DNA can be extracted from bone marrow or peripheral blood nuclear cells.

The ordered model was described by Alt and Baltimore in 1984 [5]. It states that during the V(D)J recombination, rearrangements at the H chain locus occur before

The sequential recombination refers to the order of the stages of rearrangements from DH to JH occurring before rearrangements from VH to DJH [5]. In contrast, the genes of the TCR δ locus, in the precursors of T-cells, can recombine in any order. In the case of the IGH locus in B-cell precursors (and TCR β locus in T-cell precursors), one of the multiple DH segments (genes) recombines first with one of

*The organization of the IGH chain C-region genes in humans. Eμ: intronic enhancer, S: switch region, IGH:* 

**24**

**Figure 1.**

*immunoglobulin heavy chain.*

*Ordered rearrangement of gene segments (adapted from [3]). In the ordered rearrangement model, rearrangements of gene segments at the H chain locus precede those at the L chain locus. Sequential rearrangements lead first to the recombination of one of the DH segments with one of the JH segments, and then a VH segment is recombined with the DJH recombination. Although domain D is absent, the principle of IG light chain loci recombination is similar. Ig H chain: immunoglobulin heavy chain, VH: heavy chain variable genes, DH: heavy chain diversity genes, JH: heavy chain joining genes, CH: heavy chain constant genes*

the multiple JH segments, by deletion of the DNA separating them. Then, one of the VH genes is recombined/juxtaposed with the rearranged DJ site, again by deletion of the intermediate DNA (**Figure 2**). After recombination, V, D, and J genes form a single exon [3].

#### *2.2.3 Combinatorial diversity, "P" junctional variability, "N" diversity, and diversity by genetic substitution*

Like the TCR, B-cell receptor (BCR) diversity results from (i) the choice of segments to recombine, (ii) the "P" junctional variability of the nucleotides at the V-D-J or V-J junction sites between gene segments during rearrangements, (iii) the "N" diversity by insertion or deletion of a nucleotide during recombination under the action of terminal deoxynucleotidyl transferase (TdT), but also from (iv) the recombination between VH genes by substitution of a or part of a second VH gene to an already recombined VH-D-JH segment. The diversity of the B-cell repertoire is also increased by the process of somatic hypermutation (SHM) of IG variable genes. The V(D)J recombination process generates a much greater diversity at the level of the H chain loci compared to those of the L chains, where simply a V region is joined to a J region. Thus, if the human IGH locus contained only about 40 functional VH segments, 27 DH segments, and 6 JH segments, V(D)J recombination would generate about 6480 (40 × 27 × 6) H chains, whereas, human IGK, which contains about 40 Vκ genes and 5 Jκ genes, would give rise to approximately 200 κ chains, following 200 (40 × 5) different combinations. On the other hand, the variability of nucleotides/point mutations can occur at the junction sites, and bases can be lost (by deletion) or added (by insertion), giving additional diversity of the hypervariable CDR3 region of the H and L chains, which is coded by an additional sequence of DNA created by the junction of V, D, and J segments, for the H chain, and V and J segments, for the L chain; such a phenomenon is responsible for the junctional

#### *Normal and Malignant B-cell*

diversity. In total, molecular mechanisms of genetic recombination could result in a potential repertoire of at least 107 antigen-specific recognition sites/receptors. Each clone of such a cell repertoire contains only a few cells that are capable of recognizing only one antigen; exceptionally, a T-cell clone can express two different receptors and can therefore recognize up to two antigens.
