Elucidation of IgH intronic enhancer functions via germ-fine deletion

Studies of chimeric mice demonstrated that the core Ig heavy chain (IgH) intronic enhancer (iE mu) functions in V(D)J and class switch recombination at the IgH locus. To more fully evaluate the role of this element in these and other processes, we generated mice homozygous for germ-line mutations in which the core sequences of iE mu (cE mu) were either deleted (cE mu(Delta/Delta),11,1 mice) or replaced with a pgk-Neo(R) cassette (cE mu(N/N) mice). The cEI mu(Delta/Delta) mice had reduced B cell numbers, in association with impaired D to J(H) and V-H to DJ(H) rearrangement, whereas cE mu(N/N) mice had a complete block in IgH V(D)JH recombination, confirming that additional cis elements cooperate with iE mu to enforce D to JH recombination. In addition developing cE mu(Delta/Delta) and cE mu(N/N) B lineage cells had correspondingly decreased levels of germ-line transcripts from the J(H) region of the IgH locus (mu 0 and I mu transcripts); although both had normal levels of germ-line VH transcripts, suggesting that cE mu may influence IgH locus V(D)J recombination by influencing accessibility Of JH proximal regions of the locus. Consistent with chimera studies, peripheral cE mu(Delta/Delta) B cells had normal surface Ig and relatively normal class switch recombination. However, cE mu(Delta/Delta) B cells also had relatively normal somatic hypermutation of their IgH variable region genes, showing unexpectedly that the cE mu is not required for this process. The availability of mice with the iE mu mutation in their germ line will facilitate future studies to elucidate the roles of iE mu in V-H(D)J(H) recombination in the context of IgH chromatin structure and germ-line transcription.