A multiscale 3D chromatin architecture that controls development of the humoral immune system is assembled by IKAROS [HiC_LpreB_Steadystate]
A generic level of chromatin organization generated by the interplay between cohesin and CTCF suffices to limit promiscuous interactions between regulatory elements, but a lineage-specific assembly of chromatin that supercedes these constraints is required to configure the genome to support the gene expression changes that guide faithful lineage progression. Using loss-of-function approaches in B cell precursors in vivo we show that IKAROS assembles interactions between sites often separated by megabase distances to configure a significant fraction of the genome in preparation for lymphoid development. Interactions emanating from IKAROS-bound enhancers override CTCF-imposed boundaries and assemble lineage-specific regulatory units built on a backbone of smaller invariant topological domains.In vitro deletion provides temporal resolution to changes in chromatin modifications, loops, and compartmental localization. Gain-of-function experiments in epithelial cells confirm IKAROS' ability to reconfigure chromatin architecture at multiple scales.  While the compaction of the Igk locus required for genome editing represents a function of IKAROS unique to the lymphoid system, the more general function of this lineage-defining DNA binding protein to preconfigure the genome to support lineage specific gene expression and suppress activation of extra-lineage genes provides a paradigm for lineage restriction. Overall design: We performed in situ Hi-C in wild-type and IkE5?/? large pre-B cells to understand IKAROS' function in the genome organization in early B cell development.