A breakthrough in understanding the molecular mechanism of somatic hyper-mutation came with the identification of an enzyme that is required for the process. It is called activation-induced deaminase (AID) because it is expressed specifically in activated B cells and deaminates cytosine (C) to uracil (U) during transcription of V-region coding DNA. The deamination produces U:G mismatches in the DNA double helix, and the repair of these mismatches produces various types of mutations, depending on the repair pathway used. Somatic hypermutation affects only actively transcribed DNA, because AID works only on single-stranded DNA (which is transiently exposed during transcription) and because proteins involved in the transcription of V-region coding sequences are required to recruit the AID enzyme. AID is also required for activated B cells to switch from IgM and IgD production to the production of the other classes of Ig, as we now discuss.
B cells can Switch the class of Ig They Make
After a developing B cell leaves the bone marrow, before it interacts with antigen, it expresses both IgM and IgD BCRs on its surface, both with the same antigen-binding sites (see Figure 24–24). Stimulation by antigen and helper T cells activates many of these mature naïve B cells to become IgM-secreting effector cells, so that IgM antibodies dominate the primary antibody response. Later in the immune response, however, when activated B cells are undergoing somatic hypermutation, the combination of antigen and helper-T-cell-derived cytokines (discussed later) stimulates many of the B cells to switch from making membrane-bound IgM and IgD to making IgG, IgE, or IgA, in the process of class switching. Some of these cells become memory cells that express the corresponding class of Ig as BCRs on their surface, while others become effector cells that secrete the Ig molecules as antibodies. The IgG, IgE, and IgA molecules retain their original antigen-binding site and are collectively referred to as secondary classes of Igs, because they are produced only after antigen stimulation, dominate secondary antibody responses, and make up the secondary Ig repertoire.
As discussed earlier, the constant region of an Ig heavy chain determines the class of the Ig. Thus, the ability of B cells to switch the class of antibody they make without changing the antigen-binding site implies that the same assembled VH-region coding sequence (which specifies the antigen-binding part of the heavy chain) can sequentially associate with different CH-coding sequences. This has important functional implications. It means that, in an individual animal, a particular antigen-binding site that has been selected by environmental antigens can be distributed among the various classes of antibodies, thereby acquiring the different biological properties of each class.
Cµ C° TRANSCRIPTION, RNA SPLICING,5ˆ
When a B cell switches from making IgM and IgD to one of the secondary classes of Ig, an irreversible change occurs in the DNA—a process called class switch recombination. It entails the deletion of all the CH-coding sequences between the assembled VDJ-coding sequence and the particular CH-coding sequence that the cell is destined to express. Class switch recombination differs from V(D)J recombination in several ways. (1) It happens after antigen stimulation, mainly in germinal centers, and depends on helper T cells. (2) It uses different recombination signal sequences, called switch sequences, which flank the different CH-coding segments. (3) It involves cutting and joining the switch sequences, which are noncoding sequences, and leaves the assembled VH-region coding sequence unchanged (Figure 24–30). (4) Most importantly, the molecular mechanism is different. It depends on AID, which is also involved in somatic hypermutation, rather than on the V(D)J recombinase. The cytokines that activate class switching induce the production of transcription regulators that activate transcription from the relevant switch sequences, allowing the recruitment of AID to these sites.