Class-Switch Recombination in the Absence of the IgH 3′ Regulatory Region

The ∼28-kb 3′ regulatory region (3′RR), which is located at the most distal 3′ region of the Ig H chain locus, has multiple regulatory functions that control IgH expression, class-switch recombination (CSR), and somatic hypermutation. In this article, we report that deletion of the entire 3′RR in a mouse B cell line that is capable of robust cytokine-dependent CSR to IgA results in reduced, but not abolished, CSR. These data suggest that 3′RR is not absolutely required for CSR and, thus, is not essential for targeting activation-induced cytidine deaminase to S regions, as was suggested. Moreover, replacing 3′RR with a DNA fragment including only its four DNase I hypersensitive sites (lacking the large spacer regions) restores CSR to a level equivalent to or even higher than in wild-type cells, suggesting that the four hypersensitive sites contain most of the CSR-promoting functions of 3′RR. Stimulated cells express abundant germline transcripts, with the presence or absence of 3′RR, providing evidence that 3′RR has a role in promoting CSR that is unique from enhancing S region transcription.