Pax5 induces V-to-DJ rearrangements and locus contraction of the immunoglobulin heavy-chain gene

(T)he subnuclear location and chromatin state of the immunoglobulin heavy chain (IgH) locus have been implicated in the control of V(D)J recombination. V(H)-to-DJ(H) rearrangement of distal, but not proximal V(H) genes, furthermore, depends on the B-lineage commitment factor Pax5 (BSAP). Here we demonstrate that ectopic Pax5 expression from the Ikaros promoter induces proximal rather than distal V(H)-DJ(H) 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 V. genes in the absence of V(H)-DJ(H) 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 V(H) genes from proximal segments in an extended IgH locus. Reconstitution of Pax5 expression in Pax(5-/-) pro-B cells induced large-scale contraction and distal V(H)-DJ(H) rearrangements of the IgH locus. Hence, V(H)-DJ(H) 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 V(H)-DJ(H) recombination. Pax5 subsequently activates locus contraction and distal V(H)-DJ(H) rearrangements in collaboration with an unknown factor that is present in pro-B cells, but absent in thymocytes.