Class switching and Expression of immunoglobulin genes (Part 10- Antibody Basics)
Welcome to the 10th part of the 13-part series on Antibody basics.
Previous parts: Part 1, Part 2, Part 3, Part 4, Part 5, Part 6, Part 7, Part 8, and Part 9
Class switching
As already discussed in Part 3, there are five classes of antibodies: IgM, IgA, IgG, IgE, and IgD based on the type of heavy chain they have µ, α, γ, ε and δ respectively. Each of these five different heavy chains is called an isotype. The antibody class of a B cell changes during B cell development and activation. The immature naive B cells, which have never been exposed to an antigen before, express only membrane-bound IgM antibodies. But the mature naive B cells express both membrane-bound IgM and IgD. The coexpression of IgM and IgD antibodies makes the mature B cells ready to recognize and bind to an antigen. Upon binding to an antigen, B cells get activated. The activated B cells begin to divide and differentiate into antibody-producing plasma cells.
If these activated B cells encounter specific signals, they undergo antibody class switching to produce IgG, IgA, or IgE antibodies that have defined roles in the immune system.
In this process, the heavy chain DNA can undergo a further rearrangement in which the VHDHJH rearranged unit can combine with any constant gene segment.
Therefore, antibody class switching is defined as the process that changes a B cell’s antibody production from one class to another, such as from the class IgM to class IgG. During the class switching process, the constant region of the antibody’s heavy chain is changed, but the variable region of the heavy chain stays the same.
Since the variable region does not change, therefore, class switching does not affect the antigen specificity of a B cell. Instead, the antibody retains affinity for the same antigens but can interact with different effector molecules. It has already been discussed in Part 3 that the unique amino acids present in the constant region of the heavy chains of different classes of antibodies confer structural and functional properties to the antibodies. For instance, secretory IgA is present in body secretions and prevents the attachment of pathogens to mucosal membranes. IgG antibody crosses the placenta and provides protection to the fetus. IgM antibody is most efficient in activating the complement system. IgE antibody plays a very crucial role in allergic reactions and defense against parasitic worms. And IgD antibody signals B cells to get activated. Thus, the specific functions of each class of an antibody can be attributed to the unique amino acids present in the constant region of the respective heavy chains.
Mechanism of class switching
- Class switching process requires the switch sequences present on the 5’ side of each of the CH domain gene segments. Note that there is no switch region between the Cµ and Cδ.
These switch sequences are designated as Sμ, Sγ3, Sγ2, Sγ1, Sγ4, Sε, Sα1 and Sα2. The switch sequences are 5–10 kb long and are composed of multiple copies of short repeats of GAGCT and TGGGG. The switch region Sμ needs to recombine with any of the switch regions Sγ, Sε, and Sα for the class switching.
2. Class switching is mediated by an enzyme called switch recombinase that recognizes these switch sequences and carries out recombination. Then the switch sequences are cleaved by an enzyme named Activation Induced Deaminase, abbreviated as AID, followed by repair and ligation of broken DNA ends by a non-homologous end-joining repair system.
Let’s take a scenario where the cell wants to undergo isotype switching from Cμ to Cγ2. The switch recombinase enzyme recognizes the switch regions Sμ and Sγ2 and then carries out the recombination between these switch regions. During the process of recombination, looping of all the sequences that are between Sμ and Sγ2 occurs.
Then the enzyme activation-induced deaminase AID cleaves the switch sequences Sμ and Sγ2 causing the deletion of all the sequences which are in between Sμ and Sγ2 in the form of circular excision product. Since Cµ, Cδ, and Cγ3 are excised out, therefore, the cell is now unable to produce IgM, IgD, or IgG3 antibodies. The constant region domains which come after Cγ2 continue to be present.
Then the repair and ligation of broken DNA ends occur by a non-homologous end-joining repair system. The recombination product then undergoes transcription and splicing to form γ2 primary RNA transcript, which upon polyadenylation, forms mRNA. The mRNA is then translated into γ2 heavy chains. Finally, the leader peptide present in the nascent polypeptide is cleaved, generating finished γ2 heavy chains. The γ2 heavy chains then interact with two light chains to form an IgG2 antibody.
If the cell has undergone switching to IgG2, it can further undergo class switching to IgA1. For this, again, the recombination between switch regions Sγ2 and Sα1 occurs.
During the process of recombination, looping of the sequences which are between Sγ2 and Sα1 occurs followed by the excision of the constant region and switch sequences that are between Sγ2 and Sα1. Since the constant region of IgG2 gets excised out, therefore, the cell can no longer be able to make IgG2.
The recombination product then undergoes transcription and splicing to form α1 primary RNA transcript, which upon polyadenylation, forms mRNA. The mRNA is then translated into α1 heavy chains.
The leader peptide present in the nascent polypeptide is cleaved, generating finished α1 heavy chains. The α1 heavy chains then interact with two light chains to form an IgA1 antibody. Thus, in the class switching process, the constant region of the antibodies changed, but the variable region remained the same.
Factors that govern class switching
- For class switching to take place, it is essential that CD40 and CD40 Ligand interaction takes place. CD40 receptor is constitutively expressed on B cells. To activate B cells, CD40 must interact with the CD40 ligand (CD40L) present on the surface of helper T cells. If there is no CD40-CD40 ligand interaction, class switching does not take place.
2. Certain regulatory proteins such as cytokines act as switch factors and determine the particular immunoglobulin class to be expressed during switching. For example, cytokine IL-4 induces class switching from IgM to IgG1 or IgE. In this case, the class switching will be done successively. Firstly, class switching will be from IgM to IgG1, followed by class switching to IgE.
3. Class switching also depends on the types of antigens encountered. For instance, stimulation of B cells by virus induces the generation of IgG2 antibodies, and on the other hand, helminths or allergens induce IgE antibody production, etc. It is because of the difference in the affinity of the antibodies that bind to these antigens. For example, IgG2 antibodies are induced in response to the virus because the Fc region of IgG2 can bind to natural killer cells and activate the ADCC pathway to destroy the virus-infected cells. And allergens induce class switching to IgE because IgE induces mast cell degranulation and releases active mediators to mediate allergic response.
4. Another factor governing class switching could also be the microenvironment of the plasma cells. For example, the activated plasma cells leaving Peyer’s patches in the intestine exhibit class switching to IgA because IgA is the predominant antibody present in the mucous secretions of the digestive tract. IgA can bind to the membranes of the pathogens, thus preventing the attachment of pathogens to the mucosal cells.
Therefore, we can say that class switching happens so that the immune system gets better and better at eradicating almost all types of pathogens or other substances that are foreign to our body.
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