The genetic basis of antibody structure

Overview of B cell development

• Immune response system extremely diverse (106 - 107 B & T cells)

• Genes coding for Ig & TCR use unique strategy to attain diversity; mechanisms unique to B & T cells

– Variable & constant region genes coded for by different genes, & different V genes can be linked to single C gene (instead of having 1 gene coding/Ab molecule)

– Ab genes can move & rearrange in genome of differentiating cell; brings together genes for V & C regions for transcription-translation to complete H & L chains

– Generation of diversity of antigen specific receptors on B & T cells have many common features

Figure 6.1A prototypical gene coding for a membrane protein.

Experimental demonstration of kappa gene rearrangement

Organization of germline Ig gene segments in the mouse

(In mouse, 2 V, 4 J & 4C ; In human, 30 V, 4 J & 4C)

(In mouse, 85 V, 5 J & 1 C ; In human, 40 V, 5 J & 1 C)

(In mouse, 134 VH, 13 DH & 4 JH ; In human, 51 VH, 27 DH & 6 JH )

Organization & rearrangement of light chain genes

Variable region (N terminal) codedfor by two separate gene segments1) V (variable) gene --- codes for N-term 96 residues2) J (joining) gene --- codes for C-term 13 res.

To generate Ig L chain, 1 V gene & 1 J gene brought together & joinedwith C-region gene creating a gene unit coding for Ig L chain.

Occur in the absence of antigen.

Figure 6.2The genetic events leading to the synthesis of a kappa light

chain.

Figure 6.3

Rearrangement of DNA coding for a kappa light chain.

- Occur only in B cells.- Antigenic specificity of lymphocyte becomes fixed.

Organization of H chain genes is different from L chain genes.

- Involves 3 gene segments (V, J & D); J & D code for 3rd hypervariable region (CDR3) of H chain.

- Multiple genes code for C region in germ line; C region determines class, biological function of Ig

Figure 6.4The genetic events leading to the synthesis of a human heavy chain.

A single B cell produces an Ig of only one antigenic specificity --- allelic exclusion

Switching

• One B cell forms specific Ab determined by nature of VJ & VDJ.

• Cell can switch to make different class Ig (e.g., IgG or IgE) while retaining the same antigenic specificity = class or isotype switch

• VJ & VDJ rearrangements occur prior to Ag exposure in development of B cells; switching occurs in mature B cells depending on Ag stimulation & factors released by T cells (cytokines)

Figure 6.5Mechanism of class switching in immunoglobulin synthesis. S ;eq switch region, upstream of each heavy-chain constant region.

Regulation of Ig-gene transcription

Enhancer is unable to turn on promoterbecause of long distance.

Effective only after VDJ rearrangement

Differential RNA processing of heavy-chain primary transcripts

Secreted & membrane forms of the heavy chain

Generation of Antibody Diversity

• Multiple V genes in the germ line– constitutes baseline & minimum number of different Ab that could be pro

duced.

• VJ & VDJ combinatorial association– any V any J, any V any D any J

• Random assortment of H and L chains– any H any L

• Junctional & insertional diversity– imprecise joining and insertion of small sets of nucleotides at the junction

s

• Somatic hypermutaion– occurs in germinal centers, 104 higher than normal mutation rate, largely r

andom, substitutions rather than deletion or insertion, resulting in affinity maturation

• Somatic gene conversion– most notably in birds and rabbits

Figure 6.6Somatic gene conversion generates diversity in Ig genes of several species. The Figure illustrates the phenomenon in the chicken Ig heavy-chain locus: short sequences of DNA from one or more pseudogenes (3 and 8 in the Figure) are copied into the rearranged B-cell VDJ unit.