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Novel Control Motif Cluster in the IgH δ-γ3 Interval Exhibits B Cell-Specific Enhancer Function in Early Development1

The majority of the human Ig heavy chain (IgH) constant (C) region locus has been cloned and mapped. An exception is the region between Cδ and Cγ3, which is unstable and may be a recombination hot spot. We isolated a pBAC clone (pHuIgH3′δ-γ3) that established a 52-kb distance between Cδ and Cγ3. Sequence analysis identified a high number of repeat elements, explaining the instability of the region, and an unusually large accumulation of transcription factor-binding motifs, for both lymphocyte-specific and ubiquitous transcription activators (IKAROS, E47, Oct-1, USF, Myc/Max), and for factors that may repress transcription (ΔEF1, Gfi-1, E4BP4, C/EBPβ). Functional analysis in reporter gene assays revealed the importance of the Cδ-Cγ3 interval in lymphocyte differentiation and identified independent regions capable of either enhancement or silencing of reporter gene expression and interaction with the IgH intron enhancer Eμ. In transgenic mice, carrying a construct that links the β-globin reporter to the novel δ-γ3 intron enhancer (Eδ-γ3), transgene transcription is exclusively found in bone marrow B cells from the early stage when IgH rearrangement is initiated up to the successful completion of H and L locus recombination, resulting in Ab expression. These findings suggest that the Cδ-Cγ3 interval exerts regulatory control on Ig gene activation and expression during early lymphoid development.