how does class switching occur

Full Answer

Where in the body does class switching occur?

mature B cellsAntibody class switching occurs in mature B cells in response to antigen stimulation and costimulatory signals.

What is needed for class switching?

Class switching occurs after activation of a mature B cell via its membrane-bound antibody molecule (or B cell receptor) to generate the different classes of antibody, all with the same variable domains as the original antibody generated in the immature B cell during the process of V(D)J recombination, but possessing ...

What happens during isotype switching?

Antibody isotype switching (or class switching) is a biological feature of the humoral immune response, in which a switch from IgM to other Ig follows first exposure to an antigen during the immunization.

How does IgM switch to IgA?

Antibody-producing cells undergo a process of differentiation and class switch recombination (CSR) such that the antibodies produced start as immunoglobulin M (IgM) and then switch to IgG and IgA as the concentration of antigen changes and as the cells differentiate.

Is class switching permanent?

This is done by a process called class switch recombination and is an irreversible process. Repetitive areas of DNA known as 'switch regions' are found in the introns upstream of each isotype gene, which is used to guide AID and other enzymes to the site.

Is aid involved in class switching?

Activation-induced cytidine deaminase (AID) initiates both class switch recombination (CSR) and somatic hypermutation (SHM) in antibody diversification. Mechanisms of AID targeting and catalysis remain elusive despite its critical immunological roles and off-target effects in tumorigenesis.

How is isotype switching induced?

Isotype switching is preceded by transcriptional activation of the isotype in question (Chapter 4). Two major cytokines, IL-4 and TGF-β, induce surface IgM-positive (sIgM+) B cells to switch to downstream isotypes, including IgE and IgA (Fig.

What causes class switching to IgE?

In order for a B lymphocyte to switch to IgE production it needs two signals provided by a Th2 cell in the form of the cytokines interleukin (IL-) 4/IL-13 and ligation of the CD40. In spite of a half-life of only a few days, there is evidence that the IgE response may last for years even without allergen stimulation.

Does class switching require T cells?

Lymphokines such as interleukin-2, human B cell growth factor, helper T cell factor, or interferon-gamma were also incapable of inducing IgG production. These results suggest that the cognate interaction between T cells and B cells is necessary for the immunoglobulin class switching.

What interleukin causes class switching to IgG?

Interleukin 4 (IL-4) can regulate Ig class switching in mice to IgG1 and IgE, and in humans to some subclasses of IgG and to IgE. Other cytokines can induce activation of various Ig classes and subclasses.

What is the difference between IgA IgG and IgM?

IgG is found in your blood and tissue. IgM is mostly found in your blood. IgA is found at high levels in fluid your mucus membranes make, such as saliva, tears, and nasal secretions. IgE is mostly attached to immune system cells in your blood.

How does IgM become IgG?

IgM is the primary antibody produced during an initial antigen challenge. Yet, upon subsequent antigen exposure, follicular B cells undergo isotype switching, resulting in IgG, IgG, IgE, or IgA production.

How do B cells undergo isotype switching?

Class switching occurs by a mechanism called class switch recombination (CSR) binding. Class switch recombination is a biological mechanism that allows the class of antibody produced by an activated B cell to change during a process known as isotype or class switching.

What cytokines are involved in isotype switching?

Numerous studies have demonstrated that most isotype switching is promoted by cytokines (particularly TGFβ, IFNγ, and IL-4) in the immediate microenvironment of the activated B cell, and that different cytokines favor switching to different isotypes.

How does an isotype control work?

Isotype controls are primary antibodies that lack specificity to the target, but match the class and type of the primary antibody used in the application. Isotype controls are used as negative controls to help differentiate non-specific background signal from specific antibody signal.

Where do isotype switching somatic hypermutation and affinity maturation occur?

This occurs within the dark and light zones of the germinal centre through interactions with FDCs and T follicular helper cells presenting antigen.

How does class switching occur?

Class switching occurs by a mechanism called class switch recombination (CSR) binding. Class switch recombination is a biological mechanism that allows the class of antibody produced by an activated B cell to change during a process known as isotype or class switching. During CSR, portions of the antibody heavy chain locus are removed from the chromosome, and the gene segments surrounding the deleted portion are rejoined to retain a functional antibody gene that produces antibody of a different isotype. Double-stranded breaks are generated in DNA at conserved nucleotide motifs, called switch (S) regions, which are upstream from gene segments that encode the constant regions of antibody heavy chains; these occur adjacent to all heavy chain constant region genes with the exception of the δ-chain. DNA is nicked and broken at two selected S-regions by the activity of a series of enzymes, including activation-induced (cytidine) deaminase (AID), uracil DNA glycosylase and apyrimidic/apurinic (AP)-endonucleases. The intervening DNA between the S-regions is subsequently deleted from the chromosome, removing unwanted μ or δ heavy chain constant region exons and allowing substitution of a γ, α or ε constant region gene segment. The free ends of the DNA are rejoined by a process called non-homologous end joining (NHEJ) to link the variable domain exon to the desired downstream constant domain exon of the antibody heavy chain. In the absence of non-homologous end joining, free ends of DNA may be rejoined by an alternative pathway biased toward microhomology joins. With the exception of the μ and δ genes, only one antibody class is expressed by a B cell at any point in time. While class switch recombination is mostly a deletional process, rearranging a chromosome in "cis", it can also occur (in 10 to 20% of cases, depending upon the Ig class) as an inter-chromosomal translocation mixing immunoglobulin heavy chain genes from both alleles.

Which cytokines modulate class switching in mouse and human?

T cell cytokines modulate class switching in mouse (Table 1) and human (Table 2). These cytokines may have suppressive effect on production of IgM.

What is the name of the process that changes the B cell's production of immunoglobulin from one type to?

Immunoglobulin class switching, also known as isotype switching, isotypic commutation or class-switch recombination ( CSR ), is a biological mechanism that changes a B cell 's production of immunoglobulin from one type to another, such as from the isotype IgM to the isotype IgG.

What happens to mature B cells after activation?

After activation by antigen, these B cells proliferate. If these activated B cells encounter specific signaling molecules via their CD40 and cytokine receptors (both modulated by T helper cells ), they undergo antibody class switching to produce IgG, IgA or IgE antibodies. During class switching, the constant region of the immunoglobulin heavy chain changes but the variable regions, and therefore antigenic specificity, stay the same. This allows different daughter cells from the same activated B cell to produce antibodies of different isotypes or subtypes (e.g. IgG1, IgG2 etc.).

Where are double stranded breaks generated?

Double-stranded breaks are generated in DNA at conserved nucleotide motifs, called switch (S) regions , which are upstream from gene segments that encode the constant regions of antibody heavy chains; these occur adjacent to all heavy chain constant region genes with the exception of the δ-chain. DNA is nicked and broken at two selected S-regions by ...

Does class switching affect antigen specificity?

Since the variable region does not change, class switching does not affect antigen specificity. Instead, the antibody retains affinity for the same antigens, but can interact with different effector molecules.

Is class switch recombination a deletional process?

While class switch recombination is mostly a deletional process, rearranging a chromosome in "cis", it can also occur (in 10 to 20% of cases, depending upon the Ig class) as an inter-chromosomal translocation mixing immunoglobulin heavy chain genes from both alleles.

How does IG switch occur?

Ig isotype switching occurs by an intrachromosomal deletional recombination event, diagrammed in Figure 1for the mouse H chain locus. The human H chain locus is similarly organized but not identical. Class switch recombination (CSR) occurs between switch (S) regions located upstream of each of the CHregions except Cδ and results in a change from IgM and IgD expression by naive B cells to expression of one of the downstream isotypes. IgD expression occurs by alternative transcription termination/splicing of the Cμ-Cδ genes. S regions consist of tandem repeats of short G-rich sequences (20–80 bp), which differ for each isotype, with an overall length varying from ~1 kb to 12 kb, and CSR can occur anywhere within or near the S regions (5, 6). CSR occurs by an end-joining type of recombination, rather than by homologous recombination (7, 8).

Why are S-S recombinations preassociated?

It is attractive to expect that they are preassociated in order to increase the likelihood of correct S-S recombination. This association could be directed to the correct S region by GL transcription. Another reasonable possibility is that the association occurs immediately after AID attacks Sμ. In situ studies suggest that the Ig loci form loops, apparently bringing the V and J genes in proximity prior to V(D)J recombination (127, 128).

How are DSBs produced?

DSBs produced during DNA replication or during G2 phase of the cell cycle are generally repaired by homologous recombination, as there is a chromosomal homolog that can be copied. However, S region DSBs in B cells induced during CSR are generated and resolved during G1 phase (77), and S regions lack sufficient homology to undergo homologous recombination.

What is the mechanism of somatic hypermutation?

CSR and somatic hypermutation (SHM) are initiated by activation-induced cytidine deaminase (AID), which converts cytosines in S regions and Ig variable regions to uracils by deamination (9–14). Subsequent repair of the dU residues leads to single-strand DNA breaks (SSBs) that must be converted to double-strand breaks (DSBs) within the donor Sμ region and within an acceptor Sx region, to initiate the process of intrachromosomal DNA recombination. This review focuses mainly on the overall mechanism of CSR, which is discussed in the next section. Although there are interesting similarities and differences between CSR and SHM, we do not discuss them owing to space constraints. SHM is reviewed in another article in this volume by M.D. Scharff (15). Also, we do not extensively review all the information available about AID, as this protein is extensively discussed in the Scharff article (15) and in several other reviews (16–19).

Where do DSBs occur in wild type cells?

In wild-type cells, AID-dependent blunt and staggered DSBs in the Sμ region occur preferentially at G:C bp and at AID hotspot targets, with p values for blunt DSBs ≤ 0.002 relative to random (21, 74). This indicates that SSBs and DSBs occur at the dC nucleotides that are targeted by AID, as predicted by the DNA-deamination model (Figure 2a). In aid−/−or ung−/−or APE-deficient cells, DSBs do not specifically occur at AID hotspot targets, although B cells deficient in only one of the APEs maintain some specificity for AID targets (21, 74). These data suggest that APE1 and APE2 both serve as endonucleases to incise abasic sites introduced by AID and UNG. However, whether the third AP endonuclease PALF/APLF/XIP-1 also has a role is unknown.

What determines the class of an antibody?

Antibody class, or isotype, is determined by the heavy chain constant (CH) region, which is important for determining the antibody’s effector function. The CHregion is bound by cell-surface receptors, e.g., Fc receptors on many cell types and poly immunoglobulin (Ig) receptors on mucosal epithelial cells, and also by complement. Different CHregions have different affinities for these proteins, thus greatly influencing antibody function and determining whether antibody-antigen complexes will activate cells that help to eliminate pathogens, e.g., macrophages, NK cells, or mast cells. Also, the CHregion determines whether an antibody can be transcytosed through epithelial membranes at mucosal surfaces, can diffuse into tissues, and will polymerize and thereby have a greater avidity. The CHregion also influences the stability of the antibody (reviewed in 1). The membrane-bound forms of the various iso-types differ in their intracytoplasmic carboxy termini; the different termini result in varying abilities to associate at cell membranes with intracellular signaling proteins, although the biological roles of these differences are not yet understood (2–4).

Which cells can switch to any isotype?

Naive B cells have the potential to switch to any isotype, and cytokines secreted by T cells and other cells direct the isotype switch (reviewed in 7, 31, 32). Although there is more to be discovered, the predominant mechanism for regulating isotype specificity is by regulation of transcription through S regions, and only transcriptionally active S regions undergo CSR. The regulation of isotype specificity is further discussed in the section on “Regulation of Switching.”

How does class switching occur?

Class switching occurs by a mechanism called class switch recombination (CSR) binding. Class switch recombination is a biological mechanism that allows the class of antibody produced by an activated B cell to change during a process known as isotype or class switching. During CSR, portions of the antibody-heavy chain locus are removed from the chromosome, and the gene segments surrounding the deleted portion are rejoined to retain a functional antibody gene that produces antibody of a different isotype. Double-stranded breaks are generated in DNA at conserved nucleotide motifs, called switch (S) regions, which are upstream from gene segments that encode the constant regions of antibody-heavy chains; these occur adjacent to all heavy chain constant region genes with the exception of the δ-chain. DNA is nicked and broken at two selected S-regions by the activity of a series of enzymes, including Activation-Induced (Cytidine) Deaminase (AID), uracil DNA glycosylase and apyrimidic/apurinic (AP)-endonucleases. The intervening DNA between the S-regions is subsequently deleted from the chromosome, removing unwanted μ or δ heavy chain constant region exons and allowing substitution of a γ, α or ε constant region gene segment. The free ends of the DNA are rejoined by a process called non-homologous end joining (NHEJ) to link the variable domain exon to the desired downstream constant domain exon of the antibody-heavy chain. In the absence of non-homologous end joining, free ends of DNA may be rejoined by an alternative pathway biased toward microhomology joins. With the exception of the μ and δ genes, only one antibody class is expressed by a B cell at any point in time.

What is the mechanism of antibody class switching?

Immunoglobulin class switching (or isotype switching, or isotypic commutation, or class switch recombination (CSR)) is a biological mechanism that changes a B cell’s production of antibody from one class to another ; for example, from an isotype called IgM to an isotype called IgG. During this process, the constant region portion of the antibody-heavy chain is changed, but the variable region of the heavy chain stays the same (the terms “constant” and “variable” refer to changes or lack thereof between antibodies that target different epitopes). Since the variable region does not change, class switching does not affect antigen specificity. Instead, the antibody retains affinity for the same antigens, but can interact with different effector molecules. This allows different daughter cells from the same activated B cell to produce antibodies of different isotypes or subtypes (e.g. IgG1, IgG2 etc.).

What happens when activated B cells encounter specific signaling molecules?

If activated B cells encounter specific signaling molecules via their CD40 and cytokine receptors (both modulated by T helper cells), they undergo antibody class switching to produce IgG, IgA or IgE antibodies that have defined roles in the immune system.

What is the mechanism that changes a B cell’s production of antibody from one class to another?

Isotype class switching is a biological mechanism that changes a B cell’s production of antibody from one class to another.

When do B cells begin to express IgM and IgD?

Immature B cells have never been exposed to an antigen and are known as naïve B cells. B cells begin to express both IgM and IgD when they reach maturity and renders the B cell ‘mature’ and ready to respond to antigen.

Where are double stranded breaks generated?

Double-stranded breaks are generated in DNA at conserved nucleotide motifs, called switch (S) regions, which are upstream from gene segments that encode the constant regions of antibody-heavy chains; these occur adjacent to all heavy chain constant region genes with the exception of the δ-chain. DNA is nicked and broken at two selected S-regions by ...

Does class switching affect antigen specificity?

Since the variable region does not change, class switching does not affect antigen specificity. Instead, the antibody retains affinity for the same antigens, but can interact with different effector molecules.

How does class switching occur?

Class switching occurs by a mechanism called class switch recombination (CSR)binding. This process uses conserved nucleotide motifs, called switch (S) regions, found in DNA upstream from each of the antibody heavy chain constant region genes, except the δ-chain. DNA is nicked and broken at two selected S-regions by the activity of a series of enzymes, including Activation-Induced (Cytidine) Deaminase (AID), uracil DNA glycosylase and apyrimidic/apurinic (AP)-endonucleases. The intervening DNA between the S-regions is subsequently deleted from the chromosome, removing the unwanted μ or δ heavy chain constant region genes and possibly some of the γ, α or ε constant region genes. The free ends of the DNA are rejoined by a process called non-homologous end joining (NHEJ) to link the variable domain exon to the desired downstream constant domain exon of the immunoglobulin heavy chain. Class switching can occur more than once, but it can only switch to heavy chain segments that are downstream (in the immunoglobulin locus) from the previous heavy chain exon. With the exception of the μ and δ genes, only one antibody class is expressed by a B cell at any point in time.

What is the mechanism that changes an antibody from one class to another?

Immunoglobulin class switching (or isotype switching or Isotypic Commutation) is a biological mechanism that changes an antibody from one class to another, for example, from an isotype called IgM to an isotype called IgG. During this process, the constant region portion of the antibody heavy chain is changed, but the variable region of the heavy chain stays the same. Since the variable region does not change, class switching does not affect the antigens that are bound by the antibody. Instead, the antibody retains affinity for the same antigens, but can interact with different effector molecules.

What is IG class switching?

Ig class switching involves deletions of germline DNA resulting in religation of the VDJ complex to downstream heavy chain C region genes , such as γ3, γ1 γ2b, γ2a, ε, and α.

What is the process of isotype switching?

Isotype switching involves the recombination between DNA segments (switch or S regions) that are located 2 kb 5' (upstream) of the respective C H genes, and S regions are composed of multiple repeats of short (~5 bp) sequences. Recombination occurs when upstream and downstream S regions join to form a DNA loop containing the intervening C H genes which are subsequently deleted. In addition to the IgA isotype switching induced by Tsw cells discussed above, isotype switching can also be induced by cytokines in combination with “non-cognate” activational signals. The best studied example is IL-4-induced switching to IgG1 and IgE in cultures of LPS-stimulated mouse splenic B cells ( Isakson et al., 1982; reviewed in Coffman et al., 1988 ). Cytokine-induced switching is preceded by the induction of germline transcripts corresponding to the immunoglobulin isotype to which the B cell will switch. Thus, IL-4 will induce IgG1 and IgE germline transcripts prior to the expression of either IgG1 or IgE in LPS-stimulated splenic B cells. IFNγ has also been shown to induce isotype switching in mouse splenic B cells to IgG2a ( Snapper and Paul, 1987) if LPS is used as the primary stimulus and to IgG3 as well as IgG2a ( Snapper et al., 1992) when the stimulus is anti-IgD coupled to dextran.

How does isotype switching work?

The prevailing hypothesis is that, once the signal to switch is received, the DNA is looped out such that the selected C H exon is juxtaposed next to the rearranged VDJ exon, and the intervening sequences (containing any C H exons 5′ to the selected C H exon) are deleted. Switch recombination depends on the presence of highly conserved guanine-rich switch regions (S H ), which are tandem repeats of 52 base pairs (total length 1–10 kb) located 2–3 kb upstream in the introns 5′ of each C H exon (except Cδ) ( Fig. 9-14 ). The switch region preceding the Cμ exon is denoted Sμ, that before the Cγ1 as Sγ1, etc. A double-stranded break is introduced (by an unknown DNA cleavage mechanism) somewhere in the DNA of the Sμ and another in that of the S H region immediately 5′ to the selected C H exon. According to standard switch recombination models, the S H regions are then paired so that circular products formed by the looping out of the intervening DNA are excised, resulting in the deletion of all C H exons 5′ to the selected C H exon. The actual joining within the S H regions as the DNA strands are paired does not have to be precise and appears to happen at some random point in the switch sequence.

What happens to the C H exon during isotype switching?

During isotype switching, the C H region of the centrocyte undergoes a series of DNA cutting/rejoining events that can bring any of the downstream C H exons next to the V H D H J H exon previously established by V (D)J recombination. The antigenic specificity of subsequent progeny B cells is the same (because the V exon is unchanged), but this specificity is now linked to a C region that may confer a different effector function. The actual mechanism of isotype switching, called switch recombination, is not yet fully understood but requires the same AID enzyme involved in somatic hypermutation. Switch recombination depends on the pairing of highly conserved switch regions (Sμ, Sγ3, Sγ1, Sγ2b, Sγ2a, Sε and Sα in the mouse) that lie just upstream of each C H exon (except Cδ) ( Fig. 5-10 ). Once the signal to switch is received, the DNA is likely looped out such that the selected C H exon is juxtaposed next to the rearranged V H D H J H exon, the intervening sequences (including unused C H exons) are deleted, and the DNA is repaired to restore the Ig gene. After switching, the C H exon closest to the V H D H J H exon is preferentially transcribed, spliced and translated, generating Ig proteins of the new isotype.

How do daughter cells produce different antibody isotypes?

As an antigen-activated B cell proliferates and differentiates, various daughter cells may produce different antibody isotypes, depending on the switch recombination experienced in various members of the expanding clone. In this example (described in detail in the text), Daughter 1 undergoes a switch from IgM to IgG2b production while Daughter 2 switches directly to IgA production. Progeny of Daughter 1 can subsequently switch to the production of any isotype whose C H exon remained in the Ig gene. However, the progeny of Daughter 2 can no longer switch to any other isotype because all other 5′ C H exons have already been excised from the DNA.

How many B cells switch to IgA?

These studies show that only 2–5% of B cells actually switch to IgA, making it difficult to explain the high rate of switching which normally occurs in PP germinal centers (up to 60%). This point has been addressed recently; Mclntyre and associates (1995) showed that TGFβ together with IL-4 and IL-5 induced sIgA + B cell populations of up to 15–20%. It is unfortunate that the TGFβ gene knockout mouse dies from a generalized lymphoproliferative disease at 3–4 weeks after birth, a fact which makes it difficult to use this mouse to investigate the role of TGFβ in IgA regulation in vivo. It should be remembered, however, that prior to studies of TGFβ-induced isotype switching, this cytokine was shown to have profound suppressive effects on the proliferation of both B and T cells, as well as IgA secretion ( Kehrl et al., 1986, 1987, 1991 ), and it is possible that loss of mucosal immunity in these mice contribute to their early death.

Can DCs induce IgA?

DCs can also induce surface IgA + B cells via direct stimulation of B cells with B-cell activation factor of the TNF family (BAFF) and a proliferation-inducing ligand (APRIL). 25 APRIL–transmembrane activator and CAML interactor (TACI) signaling plays a key role in CD40-independent IgA class switching in mice. 25 In humans, functional mutations in TACI can result in IgA deficiency (IgAD; Chapter 34 ). Differentiation of sIgA + B cells into IgA-producing plasma cells is dependent on IL-5 and IL-6. 26

Overview

Mechanism

Class switching occurs after activation of a mature B cell via its membrane-bound antibody molecule (or B cell receptor) to generate the different classes of antibody, all with the same variable domains as the original antibody generated in the immature B cell during the process of V(D)J recombination, but possessing distinct constant domains in their heavy chains.
Naïve mature B cells produce both IgM and IgD, which are the first two heavy chain segments in t…

Cytokines responsible for class switching

See also

External links