The immunoglobulin heavy chain locus hs3b and hs4 3’ enhancers are dispensable for VDJ assembly and somatic hypermutation. Short title: Hypermutation in the absence of hs3b/hs4 enhancers

The more distal enhancers of the immunoglobulin heavy-chain 3’ regulatory region, hs3b and hs4, were recently demonstrated as master control elements of germline transcription and class switch recombination to most immunoglobulin constant genes. In addition, they were shown to enhance the accumulation of somatic mutations on linked transgenes. Since somatic hypermutation and class switch recombination are tightly linked processes, their common dependency on the endogenous locus 3’ enhancers could be an attractive hypothesis. VDJ structure and somatic hypermutation were analyzed in B cells from mice carrying either a heterozygous or a homozygous deletion of these enhancers. We find that hs3b and hs4 are dispensable both for VDJ assembly and for the occurrence of mutations at a physiological frequency in the endogenous locus. In addition, we show that cells functionally expressing the IgM-class BCR encoded by an hs3b/hs4 deficient locus were fully able to enter germinal centers, undergo affinity maturation and yield specific antibody responses in homozygous mutant mice, where IgG1 antibodies compensated for the defect in other IgG isotypes. By contrast, analysis of Peyer’s patches from heterozygous animals showed that PNA B cells functionally expressing the hs3b/hs4 deficient allele were dramatically outclassed by B cells expressing the wild-type locus and normally switching to IgA. This study thus also highlights the role of germinal centers in the competition between B cells for affinity maturation and suggests that membrane IgA may promote recruitment in an activated B cell compartment, or proliferation of activated B cells, more efficiently than IgM in Peyer’s patches. Corresponding author: Pr M. Cogné, CNRS UMR 6101, Laboratoire d’Immunologie, 2, rue du Dr. Marcland, 87025 Limoges Cedex, France, Phone # 33 555 435 848, [email protected] For personal use only. on October 3, 2017. by guest www.bloodjournal.org From