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Epigenetic modifications of the VH region after DJH recombination in Pro‐B cells

The variable region of murine immunoglobulin heavy chain (Igh) is assembled by sequential DH–JH and VH–DJH recombination. The accessibility of the Igh locus determines the order of rearrangement. Because of the large number of VH genes and the lack of a suitable model, the epigenetic modifications of VH genes after DJH recombination have not previously been characterized. Here, we employed two v‐Abl pro‐B cell lines, in which the Igh locus is in germline and DJH‐recombined configurations, respectively. The DJH junction displays the characteristics of a recombination centre, such as high levels of activation‐associated histone modifications and recombination‐activating gene protein (RAG) binding in DJH‐rearranged pro‐B cells, which extend the recombination centre model proposed for the germline Igh locus. The different domains of the VH region have distinct epigenetic characteristics after DJH recombination. Distal VH genes have higher levels of active histone modifications, germline transcription and Pax5 binding, and good quality recombination signal sequences. Proximal VH genes are relatively close to the DJH recombination centre, which partially compensates for the low levels of the above active epigenetic modifications. DJH recombination centre might serve as a cis‐acting element to regulate the accessibility of the VH region. Furthermore, we demonstrate that RAG weakly binds to functional VH genes, which is the first detailed assessment of RAG dynamic binding to VH genes. We provide a way for VH–DJH recombination in which the VH gene is brought into close proximity with the DJH recombination centre for RAG binding by a Pax5‐dependent chromosomal compaction event, and held in this position for subsequent cleavage and VH–DJH joining.