B cells expressing IgM and IgD have not undergone class switching, which requires irreversible changes to the DNA. Instead, these B cells produce a long primary mRNA transcript that is differentially spliced to yield either of two distinct mRNA molecules (Fig. 5.21). In one transcript, the VDJ exon is spliced to the Cμ exons and undergoes polyadenylation from a nearby site (pA1), to encode a complete IgM molecule. The second RNA transcript extends well beyond this site and includes the downstream Cδ exons. In this transcript, the VDJ exon is spliced to these Cδ exons and polyadenylation occurs at a separate site downstream (pA2). This transcript encodes an IgD molecule.
It has been known since the 1980s that the processing of the long mRNA transcript is developmentally regulated, with immature B cells making mostly the μ transcript and mature B cells making mostly the δ along with some μ, although until recently there was little to no molecular explanation. A recent forward genetic screen of N-ethyl-N-nitrosourea (ENU)-induced mutagenesis in mice identified a gene involved in IgD expression that regulates the alternative splicing process. The gene encodes ZFP318, a protein structurally related to the U1 small nuclear ribonucleoprotein of the spliceosome, the RNA– protein complex that is required for mRNA splicing. ZFP318 is not expressed in immature B cells, where the IgD transcript is not produced, but becomes expressed in mature and activated B cells that coexpress IgD with IgM. ZFP318 is required for alternative splicing of the long pre-mRNA from the VDJ exon to the Cδ exons, as mice with a fully inactivated ZFP318 gene fail to express IgD and express increased levels of IgM. While the precise mechanism is unclear, it seems likely that ZFP318 may act directly on the pre-mRNA transcript during elongation, by suppressing splicing of the VDJ exon to the Cμ exons, allowing transcript elongation and promoting splicing to the Cδ exons. In short, expression of ZFP318 promotes IgD expression, although how ZFP318 expression itself is regulated in immature and mature B cells is still unknown.
5-15 Transmembrane and secreted forms of immunoglobulin are generated from alternative heavy-chain mRNA transcripts.
Each of the immunoglobulin isotypes can be produced either as a membrane-bound receptor or as secreted antibodies. B cells initially express the transmembrane form of IgM; after stimulation by antigen, some of their
In mature B cells, transcription initiated at the VH promoter extends through both Cμ and Cδ exons. This long primary transcript is then processed by cleavage and polyadenylation (AAA), and by splicing. Cleavage and polyadenylation at the μ site (pA1) and splicing between Cμ exons yields an mRNA encoding the μ heavy chain (left panel). Cleavage and polyadenylation at the δ site (pA2) and a different pattern of splicing that joins the V region exon to the Cδ exons and removes the Cμ exons yields mRNA encoding the complete δ heavy chain (right panel). For simplicity we have not shown all the individual C-region exons.