Class-switch recombination (CSR) is a programmed rearrangement of DNA in activated B cells that changes the class, or isotype, of antibody they express. It replaces the immunoglobulin heavy-chain constant region while retaining the previously assembled variable-region sequence. A cell expressing IgM can consequently produce IgG, IgA, or IgE without rebuilding its antigen-binding region. CSR enables adaptive immunity to combine an established recognition specificity with different antibody effector functions. It involves targeted DNA breaks followed by the joining of distant sequences within the immunoglobulin heavy-chain locus. (pubmed.ncbi.nlm.nih.gov)
Genetic organization and consequences
An antibody heavy chain contains a variable region and a constant region. The variable-region coding sequence is assembled during B-cell development through V(D)J recombination, which brings together variable, diversity, and joining segments. Downstream lies a series of constant-region genes. CSR changes which downstream constant region is expressed alongside the existing VDJ sequence; it is therefore distinct from the earlier process that creates the receptor’s variable region. (pubmed.ncbi.nlm.nih.gov)
The principal recombination substrates are repetitive DNA sequences called switch regions, conventionally designated Sμ, Sγ, Sε, and Sα. These lie upstream of the corresponding constant-region genes. During a typical switch from IgM to another class, breaks in the donor Sμ region and an acceptor switch region are joined. The intervening DNA loops out and is excised as a circular molecule, leaving the VDJ sequence associated with the selected downstream constant-region gene. Isolation of these switch circles provided direct experimental evidence for deletional recombination. (pubmed.ncbi.nlm.nih.gov)
Deletion makes conventional switching directionally constrained: a cell cannot restore a constant-region gene removed from its expressed heavy-chain locus simply by switching again. Further switching can nevertheless use constant-region genes that remain downstream. Sequential switching from IgM through IgG1 to IgE has been demonstrated in mice by examining the sequences of excised switch circles. (pubmed.ncbi.nlm.nih.gov)
Activation and transcriptional targeting
CSR depends on both B-cell activation and selective accessibility of the target switch regions. Signals involving CD40 and cytokines can induce switching and influence the destination isotype. In experiments with human B cells, interleukin-4 induces transcription associated with the ε constant region, while additional CD40 stimulation enables deletional recombination to IgE. Thus, transcriptional activation of a prospective target is not itself equivalent to completed switching. (pubmed.ncbi.nlm.nih.gov)
Before recombination, transcription passes through the selected switch region and associated constant-region sequences, producing so-called germline or sterile transcripts. Here, “germline” describes transcription from the unrecombined locus, not inheritance of the eventual switch event. These transcripts precede DNA rearrangement and help identify which regions are available to the switching machinery. Experiments also show that cells lacking the initiating enzyme can produce germline transcripts yet fail to recombine. (pubmed.ncbi.nlm.nih.gov)
Transcription can generate R-loops, structures in which RNA pairs with one DNA strand while the other is displaced. R-loops extending over substantial portions of switch regions have been detected in activated mouse B cells. Their exposed single-stranded DNA provides a substrate relevant to the chemistry that initiates antibody diversification. (pubmed.ncbi.nlm.nih.gov)
Initiation of DNA damage
The essential initiating enzyme is activation-induced cytidine deaminase (AID), encoded by AICDA. Mouse knockout experiments established that AID is required for both CSR and somatic hypermutation. Subsequent biochemical and genetic experiments showed that AID acts on DNA, rather than requiring an RNA-editing mechanism: it converts cytosine to uracil in single-stranded DNA, generating uracil–guanine mismatches. (pubmed.ncbi.nlm.nih.gov)
The outcome depends on how these lesions are processed. During CSR, pathways associated with base excision repair and DNA mismatch repair help convert the initial lesions into strand interruptions and double-strand breaks. Uracil removal and subsequent processing of the DNA backbone are therefore intermediate steps between AID’s base modification and recombination. CSR illustrates how ordinary genome-maintenance activities can be redirected toward programmed genetic change rather than restoration of the original sequence. (citeseerx.ist.psu.edu)
Joining distant switch regions
Break formation must be followed by DNA repair that connects the appropriate donor and acceptor regions. Classical non-homologous end joining is a major pathway for this reaction. Its components include Ku proteins, which recognize DNA ends, and an XRCC4–DNA ligase IV complex that supports ligation. Switch junctions can contain either no shared sequence or short stretches of matching sequence called microhomology. (pubmed.ncbi.nlm.nih.gov)
Classical end joining is not the only possible route. Genetically altered mouse B cells lacking key components can still undergo substantial switching through alternative end-joining pathways. These junctions are often more strongly biased toward microhomology. However, impaired classical end joining also increases persistent chromosome breaks and inappropriate joining to other chromosomes, demonstrating that successful antibody diversification depends on controlling both break formation and repair. (pubmed.ncbi.nlm.nih.gov)
Timing and distinction from affinity maturation
CSR and affinity maturation change different aspects of an antibody response. CSR replaces the constant region; somatic hypermutation introduces mutations into variable-region sequences, providing variation on which affinity-based selection can act. Both require AID, but neither process should be treated as a necessary consequence of the other. (pubmed.ncbi.nlm.nih.gov)
Switching is not restricted to the germinal center. Studies of mouse immune responses found that much CSR occurred during early B-cell activation, before established germinal-center reactions, and declined as somatic hypermutation developed. Earlier studies of human tonsils detected substantial switching within germinal centers. The anatomical distribution and timing must therefore be described with attention to species, tissue, and experimental setting. (pmc.ncbi.nlm.nih.gov)
Human genetic evidence
Inherited AID deficiency provides direct evidence of CSR’s importance in humans. Disease-associated AICDA mutations were identified in an autosomal recessive form of hyper-IgM syndrome. Studied patients showed absent class-switch recombination, deficient somatic hypermutation, and enlarged germinal centers. These findings connect the molecular switching mechanism to a defined form of immunodeficiency, while showing that strong B-cell activation alone cannot substitute for functional AID. (pubmed.ncbi.nlm.nih.gov)