Two modes ofcis-activation of switch transcription by theIgHsuperenhancer

Abstract

B cell isotype switching plays an important role in modulating adaptive immune responses. It occurs in response to specific signals that often induce different isotype (I) promoters driving transcription of switch regions, located upstream of the Ig heavy chain (IgH) constant genes. The transcribed switch regions can recombine, leading to a change of the constant gene and, consequently, of antibody isotype. Switch transcription is controlled by the superenhancer 3′ regulatory region (3′RR) that establishes long-range chromatin cis-interactions with I promoters. Most stimuli induce more than one I promoter, and switch transcription can occur on both chromosomes. Therefore, it is presently unknown whether induced I promoters compete for the 3′RR on the same chromosome. Here we performed single-chromosome RT-qPCR assays to examine switch transcription monoallelically in the endogenous context. We show that there are two modes of 3′RR-mediated activation of I promoters: coactivation and competition. The nature of the inducing signal plays a pivotal role in determining the mode of activation. Furthermore, we provide evidence that, in its endogenous setting, the 3′RR has a bidirectional activity. We propose that the coactivation and competition modes mediated by the 3′RR may have evolved to cope with the different kinetics of primary immune responses.

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

“…Moroever, Ig2b promoter was often activated on alleles with pre-activated Ig3. These findings strongly suggest that 3′RR activity, RNAPII, and transcription factors and co-factors are not limiting during I promoter activation and that initial activation of one promoter does not prevent activation of the other (17,112). In particular, the Ig2b promoter, which is induced by both LPS and TGF-b, was coactivated with Ig3 in the vast majority of alleles upon LPS stimulation, but was almost never co-activated with Ia after TGF-b stimulation (112).…”

“…The single-chromosome approach also solved the longstanding issue of the polarity of the 3'RR, i.e., if the 3'RR activity was exclusively oriented toward the upstream I promoters or if it could also target a downstream promoter (17). In this regard, analysis at the single-chromosome level of activated B cells with duplicated Ia promoter downstream of the 3'RR (47) (Figure 2) revealed that the 3′RR activated both the ectopic and the endogenous Ia promoters, which points to a bidirectional activity (112).…”

“…The use of mouse models with engineered endogenous IgH locus, polymorphic allelic differences, and a single allele-specific RT-qPCR assay revealed that the type of stimulation largely determined which mode of cis -activation, competition or co-activation, prevailed ( 112 ). In the presence of IL4, the majority of alleles displayed promoter competition, but Sγ1 single expressers prevailed over Sε single expressers.…”

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

“…the likely initial committing signal. We have recently shown that in the majority of TGFβ-activated splenic B cells, the Iγ2b and Iα promoters compete for the 3′RR 39 . Although we cannot ascertain if commitment to the Iα promoter in the original CH12 line took place following co-activation of Iγ2b and Iα or single activation of Iα, the CSR pattern in deletion clones raises the possibility that commitment to the Iα promoter has rewired the CSR machinery so that, even after deletion of Iα, it targets Iγ2b and there is optimal switching in response to the initial signal (Fig.…”

Super-Enhancers in the Regulation of Gene Transcription: General Aspects and Antitumor Targets

“…Super-enhancers are much more likely to act as regulators of the key processes in normal cells and pathological processes compared to conventional enhancers [ 6 ]. The IgH 3′RR super-enhancer located in the 3′-regulatory region of the IgH locus on chromosome 14 of the human genome regulates recombination in B cells (in particular, V(D)J recombination in B1 cells [ 17 ] and isotype switching, depending on the external signal in B2 cells [ 18 ]). Another element that is important in this process is the super-enhancer of the Aicda locus .…”