Duplication of a germline promoter downstream of the IgH 3′ regulatory region impairs class switch recombination

Abstract

During an adaptive immune response, B cells can change their surface immunoglobulins from IgM to IgG, IgE or IgA through a process called class switch recombination (CSR). Switching is preceded by inducible non-coding germline transcription (GLT) of the selected constant gene(s), which is largely controlled by a super-enhancer called the 3′ regulatory region (3′RR). Despite intense efforts, the precise mechanisms that regulate GLT are still elusive. In order to gain additional insights into these mechanisms, we analyzed GLT and CSR in mutant B cells carrying a duplication of the promoter of the α constant gene (Iα) downstream of 3′RR. Duplication of the Iα promoter affected differently GLT and CSR. While for most isotypes a drop in GLT was accompanied by a decrease in CSR, that was not the case for switching to IgA, which diminished despite unchanged GLT. Unexpectedly, there was no obvious effect on GLT and CSR to IgG3. Remarkably, specific stimuli that normally induce switching to IgG2b had contrasting effects in mutant B cells; Iγ2b was now preferentially responsive to the stimulus that induced Iα promoter. We propose that one mechanism underlying the induced 3′RR-mediated activation of GL promoters involves, at least in part, specific transcription factories.

Essential role of the initial activation signal in isotype selection upon deletion of a transcriptionally committed promoter

“…8). Whether this coincides with CIT-induced formation of specialized nuclear compartments such as transcription factories 40 that would facilitate Iγ2b-3′RR interactions and recruitment of AID remains to be explored.…”

Long-Range Control of Class Switch Recombination by Transcriptional Regulatory Elements

“…Of the ectopic and the endogenous Iα promoters, only the ectopic promoter was active in resting B cells. Following stimulation, the ectopic Iα was further induced, together with the endogenous Iα ( 47 , 56 ). The duplication reduced Sγ1, Sγ2a, and Sε transcription and CSR to the corresponding isotypes.…”

“…LPS stimulation reduced Sγ2b transcripts and IgG2b CSR levels. In contrast, TGF-β stimulation (which also activates Iα) led to normal Sγ2b transcripts and IgG2b CSR levels ( 56 ).…”

Two modes ofcis-activation of switch transcription by theIgHsuperenhancer

“…To explore whether the 3′RR displays a bidirectional activity, we used a mouse line in which the Iα promoter has been duplicated downstream of the 3′RR (2Iα mice) (15,19). We derived a heterozygous mouse line, here called 2Iα/Bl6 mice, carrying a WT allele and a mutant 2Iα allele, of C57BL/6J and 129Sv1 origin, respectively (SI Appendix, Fig.…”

“…S7). In addition, the 2Iα allele transcribes human β-globin transcripts that can be detected with specific primers (15,19) (SI Appendix, Fig. S10A).…”