CTCF-mediated cis-regulatory chromatin insulation enforces a central B-cell tolerance checkpoint [ATAC-seq]

The generation of a diverse and self-tolerant B cell repertoire is essential for adaptive immunity and is achieved through V(D)J recombination. In mice, Igκ is the dominant light chain, whereas Igλ rearrangement typically occurs in response to nonproductive or autoreactive Igκ recombination, a process termed receptor editing. Recombination at the RS element deletes the Igκ constant exon, silencing the locus and enabling Igλ expression. However, the epigenetic regulatory framework that orchestrates and governs receptor editing remains poorly defined. Here, we identify a CTCF-binding insulator element (CBE) within the 3′ Igκ super-enhancer (3′-SEκ) that regulates receptor editing and directs the κ-to-λ switch required for Igλ⁺ B-cell development. Mechanistically, loss of this CBE activates an insulated enhancer within the 3′-SEκ, causing aberrant Vκ rearrangements and altered chromatin interactions through disrupted loop extrusion dynamics. Notably, loss of this CBE in mice leads to increased autoantibody production by ten weeks of age, demonstrating that CBE-mediated chromatin architecture shapes B cell fate by constraining autoreactive potential. Collectively, our findings define a novel CTCF-dependent cis- regulatory insulation checkpoint that connects chromatin loop extrusion to antigen- driven receptor editing, thereby enforcing B-cell tolerance.