In vivo redundant function of the 3' IgH regulatory element HS3b in the mouse.

In the mouse, the regulatory region located at the 3' end of the IgH locus includes four transcriptional enhancers: HS3a, HS1-2, HS3b, and HS4; the first three lie in a quasi-palindromic structure. Although the upstream elements HS3a and HS1-2 proved dispensable for Ig expression and class switch recombination (CSR), the joint deletion of HS3b and HS4 led to a consistent decrease in IgH expression in resting B cells and to a major CSR defect. Within this pair of distal enhancers, it was questionable whether HS3b and HS4 could be considered individually as elements critical for IgH expression and/or CSR. Studies in HS4-deficient mice recently revealed the role of HS4 as restricted to Igmicro-chain expression from the pre-B to the mature B cell stage and left HS3b as the last candidate for CSR regulation. Our present study finally invalidates the hypothesis that CSR could mostly rely on HS3b itself. B cells from HS3b-deficient animals undergo normal proliferation, germline transcription, and CSR upon in vitro stimulation with LPS; in vivo Ag-specific responses are not affected. In conclusion, our study highlights a major effect of the global ambiance of the IgH locus; enhancers demonstrated as being strongly synergistic in transgenes turn out to be redundant in their endogenous context.