A novel Igκ CTCF regulatory element is a checkpoint for B cell autoreactivity

The antibodies are assembled from a heavy chain, encoded by the immunoglobulin (Ig) heavy locus, and a light chain that can be derived from either the Igκ or λ loci. The ratio between the usage of the two Light Chains (LCs) diverges significantly among species, and the regulation of their differential expression is still not fully understood. The Igκ dominates the primary B cell repertoire in mice. It is generally accepted that Igλ B cell generation is favored when rearrangement at the Igκ locus is unsuccessful or when the immature Igκ-containing LC confers autoreactivity. In the latter case, the Igκ specificity is modified by receptor editing. The Recombining Sequence (RS) element plays an exclusive role in receptor editing in murine B cells. Rag-1 binds RS and mediates recombination, resulting in the deletion of the Cĸ exon and silencing of the Igĸ locus, subsequently promoting Igλ recombination in B cells. The molecular mechanisms that govern RS recombination and generation of Igλ remain unclear. For the first time, by specific deletion of CTCF Binding Elements (CBEs) in the regulatory region of the Igĸ locus in vivo and in vitro, we show a novel CBE that affects RS recombination and λ B cell formation. In addition, CBE loss results in the activation of two unidentified Igκ enhancers (located in the Igκ super-enhancer region), promoting increased Igĸ rearrangements in pre-B cells, on the one hand, and reduction in pre-B cells, on the other hand. Interestingly, CBE loss induces receptor editing in splenic B cells. Strikingly, the absence of this CBE results in higher levels of polyreactive antibodies in young mice. Our findings reveal a new epigenetic regulatory pathway governing Vκ-RS recombination and isotype selection, facilitated by chromatin loop extrusion, which safeguards against B cell autoreactivity.