Class switch recombination in the absence of the IgH 3′ regulatory region

Abstract

Abstract The ~28 kb 3′-regulatory region (3′RR) which is located at the most distal 3′ region of the immunoglobulin (Ig) heavy (H) chain locus has multiple regulatory functions that control IgH expression, class switch recombination (CSR) and somatic hypermutation. Here, 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 the 3′RR is not absolutely required for CSR, and thus is not essential for targeting activation-induced cytidine deaminase to switch regions as has been suggested. Moreover, replacing the 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 wild type cells, suggesting that the four HS sites contain most of the CSR-promoting functions of the 3′RR. Stimulated cells express abundant germline transcripts, with the presence or absence of the 3′RR, providing evidence that the 3′RR has a separate role in promoting CSR, that is unique from enhancing S region transcription.