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Immunoglobulin heavy chain (IgH) alleles have ambivalent relationships: they feature both allelic exclusion, ensuring monoallelic expression of a single immunoglobulin (Ig) allele, and frequent inter-allelic class-switch recombination (CSR) reassembling genes from both alleles. The IgH locus 3' regulatory region (3'RR) includes several transcriptional cis-enhancers promoting activation-induced cytidine deaminase (AID)-dependent somatic hypermutation (SHM) and CSR, and altogether behaves as a strong super-enhancer. It can also promote deregulated expression of translocated oncogenes during lymphomagenesis. Besides these rare, illegitimate and pathogenic interactions, we now show that under physiological conditions, the 3'RR super-enhancer supports not only legitimate cis- , but also trans-recruitment of AID, contributing to IgH inter-allelic proximity and enabling the super-enhancer on one allele to stimulate biallelic SHM and CSR. Such inter-allelic activating interactions define transvection, a phenomenon well-known in drosophila but rarely observed in mammalian cells, now appearing as a unique feature of the IgH 3'RR super-enhancer.

The role of the transcription factor Yin Yang 1 (YY1) in development is largely unknown. Here we show that specific ablation of YY1 in mouse B cells caused a defect in somatic rearrangement in the immunoglobulin heavy-chain (IgH) locus and a block in the progenitor-B-to-precursor-B-cell transition, which was partially rescued by a prerearranged IgH transgene. Three-dimensional DNA fluorescence in situ hybridization analysis revealed an important function for YY1 in IgH locus contraction, a process indispensable for distal V(H) to D(H)J(H) recombination. We provide evidence that YY1 binds the intronic Ei mu enhancer within the IgH locus, consistent with a direct role for YY1 in V(H)D(H)J(H) recombination. These findings identified YY1 as a critical regulator of early B-cell development.

The subnuclear location and chromatin state of the immunoglobulin heavy-chain (IgH) locus have been implicated in the control of VDJ recombination. VH-to-DJH rearrangement of distal, but not proximal V(H) genes, furthermore, depends on the B-lineage commitment factor Pax5 (BSAP). He e we demonstrate that ectopic Pax5 expression from the Ikaros promote induces proximal rather than distal VH-DJH rearrangements in Ik(Pax5/+) thymocytes, thus recapitulating the loss-of-function phenotype of Pax5-/- pro-B cells. The phenotypic similarities of both cell types include (1) chromatin accessibility of distal VH genes in the absence of VH-DJH rearrangements, (2) expression of the B-cell-specific regulator EBF, (3) central location of IgH alleles within the nucleus, and (4) physical separation of distal VH genes from proximal segments in an extended IgH locus. Reconstitution of Pax5 expression in Pax5-/- pro-B cells induced large-scale contraction and distal VH-DJH rearrangements of the IgH locus. Hence, VH-DJH recombination is regulated in two steps during early B-lymphopoiesis. The IgH locus is first repositioned from its default location at the nuclear periphery toward the center of the nucleus, which facilitates proximal VH-DJH recombination. Pax5 subsequently activates locus contraction and distal VH-DJH rearrangements in collaboration with an unknown factor that is present in pro-B cells, but absent in thymocytes.

Individual B lymphocytes normally express immunoglobulin (Ig) proteins derived from single Ig heavy chain (H) and light chain (L) alleles. Allelic exclusion ensures monoallelic expression of Ig genes by each B cell to maintain single receptor specificity. Here we provide evidence that at later stages of B cell development, additional mechanisms may contribute to prioritizing expression of single IgH and IgL alleles. Fluorescent in situ hybridization analysis of primary splenic B cells isolated from normal and genetically manipulated mice showed that endogenous IgH, kappa and lambda alleles localized to different subnuclear environments after activation and had differential expression patterns. However, this differential recruitment and expression of Ig alleles was not typically seen among transformed B cell lines. These data raise the possibility that epigenetic factors help maintain the monoallelic expression of Ig.