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Diversity Is Generated by Gene Rearrangements

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Diversity Is Generated by Gene Rearrangements
IV. Responding to Environmental Changes
33. The Immune System
33.3. Antibodies Bind Specific Molecules Through Their Hypervariable Loops
Figure 33.13. Antibody - Protein Interactions. The structure of a complex between an Fab fragment and lysozyme
reveals that the binding surfaces are complementary in shape over a large area. A single residue of lysozyme,
glutamine 121, penetrates more deeply into the antibody combining site.
IV. Responding to Environmental Changes
33. The Immune System
33.4. Diversity Is Generated by Gene Rearrangements
A mammal such as a mouse or a human being can synthesize large amounts of specific antibody against virtually any
foreign determinant within a matter of days of being exposed to it. We have seen that antibody specificity is determined
by the amino acid sequences of the variable regions of both light and heavy chains, which brings us to the key question:
How are different variable-region sequences generated?
The discovery of distinct variable and constant regions in the L and H chains raised the possibility that the genes that
encode immunoglobulins have an unusual architecture that facilitates the generation of a diverse set of polypeptide
products. In 1965, William Dreyer and Claude Bennett proposed that multiple V (variable) genes are separate from a
single C (constant) gene in embryonic (germ-line) DNA. According to their model, one of these V genes becomes joined
to the C gene in the course of differentiation of the antibody-producing cell. A critical test of this novel hypothesis had to
await the isolation of pure immunoglobulin mRNA and the development of techniques for analyzing mammalian
genomes. Twenty years later, Susumu Tonegawa found that V and C genes are indeed far apart in embryonic DNA but
are closely associated in the DNA of antibody-producing cells. Thus, immunoglobulin genes are rearranged in the
differentiation of lymphocytes.
33.4.1. J (Joining) Genes and D (Diversity) Genes Increase Antibody Diversity
Sequencing studies carried out by Susumu Tonegawa, Philip Leder, and Leroy Hood revealed that V genes in embryonic
cells do not encode the entire variable region of L and H chains. Consider, for example, the region that encodes the κ
light-chain family. A tandem array of 40 segments, each of which encodes approximately the first 97 residues of the
variable domain of the L chain, is present on human chromosome 2 (Figure 33.14).
However, the variable region of the L chain extends to residue 110. Where is the DNA that encodes the last 13 residues
of the V region? For L chains in undifferentiated cells, this stretch of DNA is located in an unexpected place: near the C
gene. It is called the J gene because it joins the V and C genes in a differentiated cell. In fact, a tandem array of five J
genes is located near the C gene in embryonic cells. In the differentiation of an antibody-producing cell, a V gene
becomes spliced to a J gene to form a complete gene for the variable region (Figure 33.15). RNA splicing generates an
mRNA molecule for the complete L chain by linking the coding regions for the rearranged VJ unit with that for the C
unit (Figure 33.16).
J genes are important contributors to antibody diversity because they encode part of the last hypervariable segment
(CDR3). In forming a continuous variable-region gene, any of the 40 V genes can become linked to any of five J genes.
Thus, somatic recombination of these gene segments amplifies the diversity already present in the germ line. The linkage
between V and J is not precisely controlled. Recombination between these genes can take place at one of several bases
near the codon for residue 95, generating additional diversity. A similar array of V and J genes encoding the λ light chain
is present on human chromosome 22. This region includes 30 V gene segments and four J segments. In addition, this
λ
region includes four distinct C genes, in contrast with the single C gene in the κ locus.
λ
In human beings, the genes encoding the heavy chain are present on chromosome 14. Remarkably, the variable domain
of heavy chains is assembled from three rather than two segments. In addition to VH genes that encode residues 1 to 94
and JH segments that encode residues 98 to 113, this chromosomal region includes a distinct set of segments that encode
residues 95 to 97 (Figure 33.17). These gene segments are called D for diversity. Some 27 D segments lie between 51
VH and 6 JH segments. The recombination process first joins a D segment to a JH segment; a VH segment is then joined
to DJH. A greater variety of antigen-binding patches and clefts can be formed by the H chain than by the L chain because
the H chain is encoded by three rather than two gene segments. Moreover, CDR3 of the H chain is diversified by the
action of terminal deoxyribonucleotidyl transferase, a special DNA polymerase that requires no template. This enzyme
inserts extra nucleotides between VH and D. The V(D)J recombination of both the L and the H chains is executed by
specific enzymes present in immune cells. These proteins, called RAG-1 and RAG-2, recognize specific DNA sequences
called recombination signal sequences (RSSs) adjacent to the V, D, and J segments and facilitate the cleavage and
religation of the DNA segments.
33.4.2. More Than 108 Antibodies Can Be Formed by Combinatorial Association and
Somatic Mutation
Let us recapitulate the sources of antibody diversity. The germ line contains a rather large repertoire of variable-region
genes. For κ light chains, there are about 40 V-segment genes and five J-segment genes. Hence, a total of 40 × 5 = 200
kinds of complete V genes can be formed by the combinations of V and J. A similar analysis suggests that at least 120
κ
different λ light chains can be generated. A larger number of heavy-chain genes can be formed because of the role of the
D segments. For 51 V, 27 D, and 6 J gene segments, the number of complete VH genes that can be formed is 8262. The
association of 320 kinds of L chains with 8262 kinds of H chains would yield 2.6 × 106 different antibodies. Variability
in the exact points of segment joining and other mechanisms increases this value by at least two orders of magnitude.
Even more diversity is introduced into antibody chains by somatic mutation
that is, the introduction of mutations into
the recombined genes. In fact, a 1000-fold increase in binding affinity is seen in the course of a typical humoral immune
response, arising from somatic mutation, a process called affinity maturation. The generation of an expanded repertoire
leads to the selection of antibodies that more precisely fit the antigen. Thus, nature draws on each of three sources of
diversity
a germ-line repertoire, somatic recombination, and somatic mutation
to form the rich variety of
antibodies that protect an organism from foreign incursions.
33.4.3. The Oligomerization of Antibodies Expressed on the Surface of Immature B
Cells Triggers Antibody Secretion
a key first step in the
The processes heretofore described generate a highly diverse set of antibody molecules
generation of an immune response. The next stage is the selection of a particular set of antibodies directed against a
specific invader. How does this selection occur? Each immature B cell, produced in the bone marrow, expresses a
monomeric form of IgM attached to its surface (Figure 33.18). Each cell expresses approximately 105 IgM molecules,
but all of these molecules are identical in amino acid sequence and, hence, in antigen-binding specificity. Thus, the
selection of a particular immature B cell for growth will lead to the amplification of an antibody with a unique
specificity. The selection process begins with the binding of an antigen to the membrane-bound antibody.
Associated with each membrane-linked IgM molecule are two molecules of a heterodimeric membrane protein called Igα-Ig-β (see Figure 33.18). Examination of the amino acid sequences of Ig-α and Ig-β is highly instructive. The amino
terminus of each protein lies outside the cell and corresponds to a single immunoglobulin, and the carboxyl terminus,
which lies inside the cell, includes a sequence of 18 amino acids called an immunoreceptor tyrosine-based activation
motif (ITAM) (see Figure 33.18). As its name suggests, each ITAM includes key tyrosine residues, which are subject to
phosphorylation by particular protein kinases present in immune-system cells.
A fundamental observation with regard to the mechanism by which the binding of antigen to membrane-bound antibody
triggers the subsequent steps of the immune response is that oligomerization or clustering of the antibody molecules is
required (Figure 33.19). The requirement for oligomerization is reminiscent of the dimerization of receptors triggered by
growth hormone and epidermal growth factor encountered in Sections 15.4 and 15.4.1; indeed, the associated signaling
mechanisms appear to be quite similar. The oligomerization of the membrane-bound antibodies results in the
phosphorylation of the tyrosine residues within the ITAMs by protein tyrosine kinases including Lyn, a homolog of Src
(Section 15.5). The phosphorylated ITAMs serve as docking sites for a protein kinase termed spleen tyrosine kinase
(Syk), which has two SH2 domains that interact with the pair of phosphorylated tyrosines in each ITAM. Syk, when
activated by phosphorylation, proceeds to phosphorylate other signal-transduction proteins including an inhibitory
subunit of a transcription factor called NF-κB and an isoform of phospholipase C. The signaling processes continue
downstream to activate gene expression, leading to the stimulation of cell growth and initiating further B-cell
differentiation.
Drugs that modulate the immune system have served as sources, of insight into immune-system signaling
pathways. For example, cyclosporin, a powerful suppressor of the immune system, acts by blocking a phosphatase
called calcineurin, which normally activates a transcription factor called NF-AT by dephosphorylating it.
The potent immune supression that results reveals how crucial the activity of this transcription factor is to the
development of an immune response. Without drugs such as cyclosporin, organ transplantation would be extremely
difficult because transplanted tissue expresses a wide range of foreign antigens, which causes the immune system to
reject the new tissue.
The role of oligomerization in the B-cell signaling pathway is illuminated when we consider the nature of many antigens
presented by pathogens. The surfaces of many viruses, bacteria, and parasites are characterized by arrays of identical
membrane proteins or membrane-linked carbohydrates. Thus, most pathogens present multiple binding surfaces that will
naturally cause membrane-associated antibodies to oligomerize as they bind adjacent epitopes. In addition, the
mechanism accounts for the observation that most small molecules do not induce an immune response; however,
coupling multiple copies of the small molecule to a large oligomeric protein such as keyhole limpet hemocyanin (KLH),
which has a molecular mass of close to 1 million daltons or more, promotes antibody oligomerization and, hence, the
production of antibodies against the small-molecule epitope. The large protein is called the carrier of the attached
chemical group, which is called a haptenic determinant. The small foreign molecule by itself is called a hapten.
Antibodies elicited by attached haptens will bind unattached haptens as well.
33.4.4. Different Classes of Antibodies Are Formed by the Hopping of VH Genes
The development of an effective antibody-based immune response depends on the secretion into the blood of antibodies
that have appropriate effector functions. At the beginning of this response, an alternative mRNA splicing pathway is
activated so that the production of membrane-linked IgM is supplanted by the synthesis of secreted IgM. As noted in
Section 33.1, secreted IgM is pentameric and has a relatively high avidity for multivalent antigens. Later, the antibodyproducing cell makes either IgG, IgA, IgD, or IgE of the same specificity as the intially secreted IgM. In this switch, the
light chain is unchanged, as is the variable region of the heavy chain. Only the constant region of the heavy chain
changes. This step in the differentiation of an antibody-producing cell is called class switching (Figure 33.20). In
undifferentiated cells, the genes for the constant region of each class of heavy chain, called C , C , C , C , and C ,
µ
δ
γ
ε
α
are next to each other. There are eight in all, including four genes for the constant regions of γ chains. A complete gene
for the heavy chains of IgM antibody is formed by the translocation of a VH gene segment to a DJH gene segment.
How are other heavy chains formed? Class switching is mediated by a gene-rearrangement process that moves a VDJ
gene from a site near one C gene to a site near another C gene. Importantly, the antigen-binding specificity is conserved
in class switching because the entire V DJ gene is translocated in an intact form. For example, the antigenH
H
combining specificity of IgA produced by a particular cell is the same as that of IgM synthesized at an earlier stage of its
development. The biological significance of CH switching is that a whole recognition domain (the variable domain) is
shifted from the early constant region (C ) to one of several other constant regions that mediate different effector
µ
functions.
IV. Responding to Environmental Changes
33. The Immune System
33.4. Diversity Is Generated by Gene Rearrangements
Figure 33.14. The κ Light-Chain Locus. This part of human chromosome 2 includes an array of 40 segments that
encode the variable (V) region (approximately residues 1 97) of the light chain, an array of 5 segments that encode the
joining (J) region (residues 98 110), and a single region that encodes the constant (C) region.
IV. Responding to Environmental Changes
33. The Immune System
33.4. Diversity Is Generated by Gene Rearrangements
Figure 33.15. VJ Recombination. A single V gene (in this case, V2) is linked to a J gene (here, J4) to form an intact VJ
region. The intervening DNA is released in a circular form. Because the V and J regions are selected at random and the
joint between them is not always in exactly the same place, many VJ combinations can be generated by this process.
IV. Responding to Environmental Changes
33. The Immune System
33.4. Diversity Is Generated by Gene Rearrangements
Figure 33.16. Light-Chain Expression. The light-chain protein is expressed by transcription of the rearranged gene to
produce a pre-RNA molecule with the VJ and C regions separated. RNA splicing removes the intervening sequences to
produce an mRNA molecule with the VJ and C regions linked. Translation of the mRNA and processing of the initial
protein product produces the light chain.
IV. Responding to Environmental Changes
33. The Immune System
33.4. Diversity Is Generated by Gene Rearrangements
Figure 33.17. V( D ) J Recombination. The heavy-chain locus includes an array of 51 V segments, 27 D segments, and
6 J segments. Gene rearrangement begins with D-J joining, followed by further rearrangement to link the V segment to
the DJ segment.
IV. Responding to Environmental Changes
33. The Immune System
33.4. Diversity Is Generated by Gene Rearrangements
Figure 33.18. B-Cell Receptor. This complex consists of a membrane-bound IgM molecule noncovalently bound to two
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