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1168 Part IX: Lymphocytes and Plasma Cells Chapter 75: Functions of B Lymphocytes and Plasma Cells in Immunoglobulin Production 1169

lymphoid tissue, particularly in Peyer patches and mesenteric lymph (4) the coming together of the heavy- and light-chain polypeptides
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nodes (Chap. 6). Also, class-switched IgA plasmablasts have a pro- to produce a complete immunoglobulin monomer capable of binding
pensity to migrate to the lamina propria of the intestine and to other antigen; and (5) somatic mutations within the rearranged DNA seg-
mucosal sites. 76 ments themselves. Somatic mutations occurs through a process called
Immunoglobulin class switch recombination (CSR) occurs in or somatic hypermutation.
near the switch region located in the intron between the rearranged Somatic hypermutation is not active in all B cells and cannot be
V(D)J sequence and the μ gene and any one of similar regions triggered merely by mitogen-induced B-cell activation. However, dur-
H
located upstream of the C genes encoding each of the other heavy- ing discrete stages of B-cell differentiation, expressed immunoglobulin
–3
chain isotypes, with the exception of the δ gene (see Fig. 77–4). The V genes may incur new mutations at rates as high as 10 base substi-
μ switch region, designated Sμ, consists of approximately 150 repeats tutions per base pair per generation over several cell divisions, partic-
of the sequence (GAGCT) (GGGGGT), where n is generally 3 but ularly during the secondary humoral immune response to antigen.
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n
can be as many as 7. The sequences of the other switch regions (Sλ, Hypermutations begin on the 5′ end of rearranged V genes downstream
Sδ, Sε) are similar in that they also contain repeats of the GAGCT and of the transcription initiation site and continue through the V gene and
GGGGGT sequences. The switch in heavy-chain classes results from into the 3′ flanking region before tapering off. As such, the mutations
DNA recombination between Sμ, and Sλ, Sδ, or Sε, accompanied by are clustered in the region spanning from 300 bp 5′ of the rearranged
the deletion of intervening DNA segments and the apposition of the variable-region exon to approximately 1 kb 3′ of the rearranged mini-
previously rearranged variable-region gene next to the new con- gene J segment. A high frequency of mutations are clustered around
stant-region gene. “hotspots” defined by the primary DNA sequence. The sequence
In contrast to V(D)J recombination, which mostly occurs in the RGYW (R = purine, A or G; Y = pyrimidine, C or T; W = A/T) and its
G and/or G stage of the cell cycle, CSR seems to require DNA repli- complement, for example, is a hotspot for mutation that is conserved
1
0
cation. Also, unlike V(D)J recombination, CSR requires expression of among species. 83
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activation-induced deaminase (AID), an enzyme expressed in activated The process of somatic hypermutation requires the activity of
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B cells that also is required for somatic hypermutation. Patients with AID through a process that has some similarly with CSR. 84,85 In addi-
inherited defects in AID have an immune deficiency (hyper-IgM syn- tion to having the hyper-IgM immunodeficiency syndrome type II,
drome type II) characterized by relatively high serum levels of IgM and patients who have inherited defects in AID have B cells that lack the
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negligible serum levels of other immunoglobulin isotypes. Specific capacity to undergo somatic hypermutation (Chap. 80). 79,86 As with
inactivation of the C-terminal AID domain, encoded by exon 5 (E5), CSR, somatic hypermutation requires active transcription of the
allows very efficient deamination of the AID target regions, but greatly genes undergoing mutation. AID most likely deaminates the cyto-
impacts the efficiency and quality of subsequent DNA repair. Specifi- sines in the region encompassing the rearranged variable-region
cally eliminating E5 not only precludes CSR, but also causes an atyp- gene, converting the dC to dU, which are converted to T after DNA
ical, enzymatic activity-dependent, dominant-negative effect on CSR. replication, giving rise to C/G to T/A transitions. Alternatively, the
This explains the autosomal dominant inheritance of AID variants with dU are removed by UNG, resulting in abasic sites that subsequently
truncated E5 in patients with hyper-IgM syndrome type II and estab- are cleaved by AP endonuclease. This process generates staggered
lishes that AID, through the E5 domain, provides a link between DNA nick cleavage of the DNA. Repair of these staggered nicks may involve
damage and repair during CSR. 80 low-fidelity DNA synthesis, giving rise to frequent mutations. DNA
AID is expressed in germinal centers of peripheral lymphoid cleaving enzymes and DNA repair enzymes (e.g., mismatch repair
organs, the site where CSR occurs in B cells activated in response to enzymes, base-excision repair enzymes, proteins involved in NHEJ)
antigen. AID most likely deaminates the closely positioned cytosines form a complex called the mutasome, which also apparently binds the
(dC) in the S-region DNA, converting the dC to uracils (dU), which, target DNA to reduce its tendency to incur complete double-stranded
in turn, are removed by uracil-DNA glycosylase (UNG). The impor- DNA breaks.
tance of UNG is underscored by patients who have inherited defects in Immunoglobulin enhancers may account in part for the preferen-
this enzyme, resulting in an autosomal recessive form of the hyper-IgM tial somatic hypermutation of immunoglobulin genes. Combinations
immunodeficiency syndrome similar to that of patients with inherited of immunoglobulin enhancers target somatic mutation to immuno-
defects in AID (see Chap. 80). The abasic sites generated by UNG are globulin genes by recruiting AID and/or by making the immunoglob-
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cleaved by AP endonuclease, resulting in closely positioned staggered ulin genes better substrates for mutation. In addition, posttranslation
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nicks in the DNA that may result in double-stranded DNA breaks. The AID ubiquitination has an important regulatory role during CSR and
end processing, repair, and joining mechanisms for these DNA breaks somatic hypermutation. 88,89 Unlike Rag-2, AID protein stability is not
apparently involve mechanisms and proteins similar to those involved associated with phases of the cell cycle, but rather with subcellular
in NHEJ used for V(D)J recombination. Because the CSR occurs in the localization. In mouse B cells nuclear AID is subjected to rapid turnover
intron between the variable-region exon and the exon encoding the upon polyubiquitination. 88
first constant-region domain, this process does not generate mutations As a consequence somatic hypermutation, mostly transitional
in the regions encoding the variable or constant regions of the newly mutations are introduced at high frequency in the expressed immu-
generated immunoglobulin heavy chain. noglobulin V genes and in other transcriptionally active genes with
“hotspots,” which can serve as a substrate for AID, UNG, and the
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MECHANISMS FOR GENERATING ANTIBODY mutasome. Subsequent selection of the B cell and its daughter cells
DIVERSITY that express mutated V genes encoding an immunoglobulin variable
region with improved fitness for binding antigen allows for “affinity
Several mechanisms contribute to the generation of diversity among maturation” of the antibodies expressed during the immune response to
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immunoglobulin polypeptide variable regions. The mechanisms are antigen, which typically is retained on follicular dendritic cells. Such
(1) the presence in the germline DNA of multiple different V, J, and selection enhances the frequency of nonconservative base substitutions
D gene segments; (2) the random joining of these DNA segments to in the DNA sequences encoding the CDR that serves as the contact site
produce a complete variable-region exon; (3) junctional diversity; for antigen binding. 84

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