Core enhancers of the 3'RR optimize IgH nuclear position and loop conformation for oriented CSR

Class switch recombination is an essential process which enabling B cells to adapt immunoglobulin subtypes to antigens. Transcription plays a crucial role in regulating CSR in which the IgH 3Regulatory Region (3RR) was identified as a key player. The 3RR stands at the 3 end of IgH locus and is composed of four core enhancers surrounded by inverted repeated sequences, forming a quasi-palindrome. In addition to transcriptional control, nuclear organization appears to be an important level in CSR regulation. Furthermore, the chromatin loops at IgH locus facilitate an efficient CSR recombination by bringing the donor and acceptor switch regions closer together. However, the precise control mechanisms governing both of these processes remain partially understood. Here, using the reference DNA 3D-FISH technique combined with various high throughput approaches, we showed that 3RR core enhancers are necessary and sufficient to preorganize resting B cell nuclei to facilitate a deletional CSR mechanism at activated stage. We demonstrated that the 3RR core enhancers regulate IgH locus addressing in the nuclei, control IgH locus accessibility and orchestrate IgH loops formation. Our findings pinpointed an additional regulation level of mechanisms underlying B cell diversification. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=184 SRC="FIGDIR/small/548507v1_ufig1.gif" ALT="Figure 1"> View larger version (41K): org.highwire.dtl.DTLVardef@69ca5aorg.highwire.dtl.DTLVardef@1640c29org.highwire.dtl.DTLVardef@15a7949org.highwire.dtl.DTLVardef@d0af7e_HPS_FORMAT_FIGEXP M_FIG C_FIG Position of IgH loci through B cell development (from transitional to stimulated stages) is represented by the red spots. In wt and c3RR context, IgH loci get closer from each other and from nucleus center during evolution from transitional to mature resting stage and they relocates more at nuclear periphery, away one from each other, upon in vitro stimulation. In{Delta} 3RR model, this dynamic is lost and, moreover, IgH loci are more localized to pericentromeric heterochromatin (represented by green area) since the mature resting B cell stage and remain in after in vitro stimulation.