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 6.1 A 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 Vl, 4 Jl & 4Cl ; In human, 30 Vl, 4 Jl & 4Cl) (In mouse, 85 Vk, 5 Jk & 1 Ck ; In human, 40 Vk, 5 Jk & 1 Ck) (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) coded for by two separate gene segments 1) V (variable) gene --- codes for N-term 96 residues 2) J (joining) gene --- codes for C-term  13 res. To generate Ig L chain, 1 V gene & 1 J gene brought together & joined with C-region gene creating a gene unit coding for Ig L chain. Occur in the absence of antigen.

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

- Occur only in B cells. - Antigenic specificity of lymphocyte becomes fixed. Figure 6.3 Rearrangement of DNA coding for a kappa light chain.

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 6.4 The 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 6.5 Mechanism 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 promoter because 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 produced. 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 junctions Somatic hypermutaion occurs in germinal centers, 104 higher than normal mutation rate, largely random, substitutions rather than deletion or insertion, resulting in affinity maturation Somatic gene conversion most notably in birds and rabbits

Figure 6.6 Somatic 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.