Core enhancers of the 3’RR optimizeIgHnuclear position and loop conformation for oriented CSR

Abstract

AbstractClass switch recombination is an essential process which enabling B cells to adapt immunoglobulin subtypes to antigens. Transcription plays a crucial role in regulating CSR in which theIgH 3’Regulatory Region(3’RR) was identified as a key player. The3’RRstands at the 3’ end ofIgHlocus and is composed of four core enhancers surrounded by inverted repeated sequences, forming a quasi-palindrome. In addition to transcriptional control, nuclear organization appears to be an important level in CSR regulation. Furthermore, the chromatin loops atIgHlocus facilitate an efficient CSR recombination by bringing the donor and acceptor switch regions closer together. However, the precise control mechanisms governing both of these processes remain partially understood. Here, using the reference DNA 3D-FISH technique combined with various high throughput approaches, we showed that 3’RR core enhancers are necessary and sufficient to preorganize resting B cell nuclei to facilitate a deletional CSR mechanism at activated stage. We demonstrated that the 3’RR core enhancers regulateIgHlocus addressing in the nuclei, controlIgHlocus accessibility and orchestrateIgHloops formation. Our findings pinpointed an additional regulation level of mechanisms underlying B cell diversification.Graphical abstractPosition ofIgHloci through B cell development (from transitional to stimulated stages) is represented by the red spots. Inwtandc3’RRcontext,IgHloci get closer from each other and from nucleus center during evolution from transitional to mature resting stage and they relocates more at nuclear periphery, away one from each other, uponin vitrostimulation. InΔ3’RRmodel, this dynamic is lost and, moreover,IgHloci are more localized to pericentromeric heterochromatin (represented by green area) since the mature resting B cell stage and remain in afterin vitrostimulation.