The Eμ Enhancer Region Influences H Chain Expression and B Cell Fate without Impacting IgVH Repertoire and Immune Response In Vivo

Abstract

The IgH intronic enhancer region Eμ is a combination of both a 220-bp core enhancer element and two 310–350-bp flanking scaffold/matrix attachment regions named MARsEμ. In the mouse, deletion of the core-enhancer Eμ element mainly affects VDJ recombination with minor effects on class switch recombination. We carried out endogenous deletion of the full-length Eμ region (core plus MARsEμ) in the mouse genome to study VH gene repertoire and IgH expression in developing B-lineage cells. Despite a severe defect in VDJ recombination with partial blockade at the pro–B cell stage, Eμ deletion (core or full length) did not affect VH gene usage. Deletion of this regulatory region induced both a decrease of pre–B cell and newly formed B cell compartments and a strong orientation toward the marginal zone B cell subset. Because Igμ H chain expression was decreased in Eμ-deficient pre–B cells, we propose that modification of B cell homeostasis in deficient animals was caused by “weak” pre–B cell and BCR expression. Besides imbalances in B cell compartments, Ag-specific Ab responses were not impaired in animals carrying the Eμ deletion. In addition to its role in VDJ recombination, our study points out that the full-length Eμ region does not influence VH segment usage but ensures efficient Igμ-chain expression required for strong signaling through pre–B cells and newly formed BCRs and thus participates in B cell inflow and fate.

Sequential activation and distinct functions for distal and proximal modules within the IgH 3′ regulatory region

“…This window of activity clearly overlaps with that of Eμ, recently described as active on μH-chain transcription from pre-B to transitional B-cell stages (18).…”

“…S1A). Once introduced into the mouse germ line, the selection cassette was deleted, as described previously (18), to get the 3′PAL Δ model (Fig. S1B).…”

“…Devoid of enhancer activity (14,15), 3′CBE (hs5 to hs8) likely participate in IgH folding before VDJ recombination because deletion of hs5 to -7 only impacts use of proximal V H regions (16). In pre-B cells, once a functional H chain is expressed as a component of the pre-B-cell recptor (BCR), the Eμ enhancer function switches from DJ H region accessibility to Igμ chain expression, and consequently modulates pre-BCR expression and expansion of the pre-B-cell compartment (17,18). The activity of Eμ even extends to the newly formed (NF)/immature stage, where it tunes BCR expression and influences B-cell fate (18).…”

“…In pre-B cells, once a functional H chain is expressed as a component of the pre-B-cell recptor (BCR), the Eμ enhancer function switches from DJ H region accessibility to Igμ chain expression, and consequently modulates pre-BCR expression and expansion of the pre-B-cell compartment (17,18). The activity of Eμ even extends to the newly formed (NF)/immature stage, where it tunes BCR expression and influences B-cell fate (18). The 3′RR has been proven to be dispensable for locus contraction and VDJ recombination (19,20).…”

Elucidation of IgH 3′ region regulatory role during class switch recombination via germline deletion

“…1a). Among 5 0 elements, the intronic Em enhancer is reported as a master control element of V(D)J recombination 3,4 . The IgH 3 0 regulatory region (3 0 RR), that encompasses the four transcriptional enhancers hs3a, hs1,2, hs3b and hs4, controls m transcription in mature B-cells 5 , and is the master element controlling conventional class switch recombination (CSR) 6,7 and somatic hypermutation (SHM) 8 but without role on V(D)J recombination 9,10 .…”