Page 80 - Clinical Immunology_ Principles and Practice ( PDFDrive )

CHaPter 4 Antigen Receptor Genes, Gene Products, and Coreceptors 65

the system can tailor both the receptor and the effector ends of and biases on both the structure and sequence of the antibody
the antibody molecule to meet a specific need. repertoire are apparent.
The representation of individual V gene elements is nonran-
Somatic Hypermutation dom. Among Vκ and V H elements, half the potentially functional
A final mechanism of Ig diversity is engaged only after exposure V gene elements contribute minimally to the expressed reper-
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to antigen. With T-cell help, the variable domain genes of toire. Among Vλ elements, these restrictions are even greater,
germinal center lymphocytes undergo somatic hypermutation with three gene segments contributing to half the expressed
-3
(SHM) at a rate of up to 10 changes per base pair per cell cycle. repertoire.
SHM is correlated with transcription of the locus, and recent Particular patterns of amino acid composition in the sequences
studies have suggested that at least two separate mechanisms of the V domains create predictable canonical structures for
are involved. The first mechanism targets mutation hot spots several of the hypervariable regions. In κ chains, CDR2 is found
with the RGYW (purine/G/pyrimidine/A) motif, and the second in a single canonical structure, whereas four structures are possible
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mechanism incorporates an error-prone DNA synthesis that can for CDR1. In the H chain, most germline CDR1 and CDR2
lead to a nucleotide mismatch between the original template elements encode one of three or one of five distinct canonical
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and the mutated DNA strand. SHM allows affinity maturation structures, respectively. Preservation of these key amino acids
of the antibody repertoire in response to repeated immunization during affinity maturation tends to maintain the canonical
or exposure to antigen, as B cells bearing receptors that have structure of CDR1 and CDR2 even while they are undergoing
mutated to higher affinity for the cognate antigenic epitope are somatic hypermutation. 37
preferentially stimulated to proliferate, especially under conditions The enhanced sequence diversity of the CDR3 region is
of limiting antigen concentration. mirrored by its structural diversity. Few canonical structures
have been defined for the H chain CDR3, and even in κ chains,
Activation-Induced Cytidine Deaminase 30% of the L chain CDR3 can be quite variable. However, at the
Activation-induced cytidine deaminase (AID) plays a key role sequence level, there is a preference for tyrosine and glycine
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in both CSR and SHM. AID is a single-strand DNA (ssDNA) residues and a bias against the use of highly charged or hydro-
cytidine deaminase that can be expressed in activated germinal phobic amino acids in the H chain CDR3, which reflects pref-
center B cells. Both SHM and CSR require transcription. Tran- erential use of only one of the six potential D H reading frames,
scription helps target AID to the requisite chromosomal location natural selection of reading frame content, and selection during
and contributes to formation of requisite ssDNA substrates. For development. 38
example, transcription of an Ig V domain or of the switch region
upstream of the C H 1 domain opens the DNA helix to generate The TCR αδ Chain Locus
ssDNA that can then be deaminated by AID to form mismatched The α and δ loci are located on chromosome 14q11-12. This
dU/dG DNA bps. Both CSR and SHM then co-opt the activities region is unusual in that the gene segments encoding the two
of normal cellular base excision repair (BER) and mismatch different TCR chains are actually intermixed (Fig. 4.9). There
repair (MMR) to convert AID cytidine deamination lesions to are 38–40 Vα, 5 Vα/Vδ, no Dα, and 50 Jα functional gene
mutations and/or double-strand breaks. The BER protein uracil- segments, as well as one Cα gene. 39
DNA glycosylase (UNG) removes the mismatched dU base, The δ locus lies between the Vα and Jα gene segments. There
creating an abasic site. Cleavage of the DNA backbone at this are three committed Vδ, 5 Vα/Vδ, 3 Dδ, and 3 Jδ gene segments,
abasic site by an apurinic/apyrimidinic (AP) endonuclease as well as one Cδ gene. Vδ3 lies 3’ of Cδ, and thus must rearrange
generates a ssDNA nick adjacent to the abasic site. The nick is by inversion. Although V region use by α and δ chains is largely
then processed to a single-nucleotide gap. The gap is filled in independent of one another, this unusual gene organization is
by DNA polymerase β and then sealed by DNA ligase 1 or DNA accompanied by sharing of 5 V gene segments. For example,
ligase 3. The MMR proteins MSH2 and MSH6 can also bind Vδ1 can rearrange either to Dδ/Jδ or to Jα elements and thus
and process the dU:dG mismatch. Deficiencies of AID, UNG can serve as the V region for both γδ and αβ TCRs.
underlie some forms of the hyper-IgM syndrome (Chapter 34). In the large majority of αβ T cells analyzed, the α chain on
UNG and MMR double deficiency ablates CSR. It also eliminates both chromosomes was rearranged. This occurs by the rear-
both C/G transversion mutations and spreading of mutations, rangement of the 5’ RSS δRec to a pseudo-J segment, ψJα, at
leaving only C/G transition mutations. the beginning of the Jα cluster (see Fig. 4.9) as well as by the
The benefits of diversity created by AID are balanced by the subsequent rearrangement of Vα to Jα on both chromosomes.
tendency of AID to target non-Ig genes. AID can create clusters of Both types of rearrangement delete all of the Dδ, Jδ, and Cδ
mutations in a number of genes, including BCL6, CD95, CD79A, genes, thus preventing coexpression of αβ and γδ TCRs.
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CD79B, PIM1, MYC, RHOH, and paired box 5 (PAX5). The
process is termed kataegis. These mutations clusters can contribute The TCR β Chain Locus
to the development of lymphoproliferative disorders. The β locus is positioned at chromosome 7q35.44 It contains
40–48 functional Vβ genes, two Dβ, two Jβ clusters, each contain-
Diversity and Constraints on ing six or seven gene segments, and two Cβ genes (see Fig. 4.9).
Immunoglobulin Sequence There is one Vβ immediately downstream of Cβ2, which rear-
In theory, combinatorial rearrangement of V(D)J gene segments, ranges by inversion. Each Cβ is preceded by its own Dβ–Jβ
combinatorial association of H and L chains, flexibility in the cluster. There is no apparent preference for Vβ gene rearrangement
site of gene segment joining, N region addition, P junctions, to either Dβ–Jβ cluster. Dβ1 can rearrange to the Jβ1 cluster or
somatic hypermutation, and class switching can create an antibody the Dβ2–Jβ2 cluster. Dβ2 can only rearrange to Jβ2 gene segments.
repertoire whose diversity is limited only by the total number The two Cβ segments differ by only six amino acids and are
of B cells in circulation at any one time. In practice, constraints functionally indistinguishable from each other.