Deciphering the importance of the palindromic architecture of the immunoglobulin heavy chain 3' regulatory region.

The IgH 3' regulatory region (3'RR) controls class switch recombination (CSR) and somatic hypermutation (SHM) in B cells. The mouse 3'RR contains four enhancer elements with hs1,2 flanked by inverted repeated sequences and the center of a 25-kb palindrome bounded by two hs3 enhancer inverted copies (hs3a and hs3b). hs4 lies downstream of the palindrome. Evolution maintained in mammals this unique palindromic arrangement suggesting that it is functionally significant. We report that deconstructing the palindromic IgH 3'RR strongly impacts its function even when enhancers are preserved. CSR and IgH transcription appear poorly dependent from the 3'RR architecture and are more or less preserved provided 3'RR enhancers are present. By contrast, an “architectural effect” significantly lowers VH germline transcription, AID recruitment and SHM. In conclusion, this work indicates that the IgH 3'RR does not simply pile up enhancer units but also optimally expose them into a functional architecture of crucial importance. Overall design: RNAseq analysis of B-cell splenocytes with (S=stimulated) or without (R=resting) LPS activation from wt, delta2leftPAL, and deltaIRIS mice.

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Saintamand A, Vincent-Fabert C, Garot A, Rouaud P, Oruc Z, Magnone V, Cogné M, Denizot Y