Immunoglobulin kappa enhancers are differentially regulated at the level of chromatin structure

The κ intronic and the κ3′ enhancers synergize to regulate recombination and transcription of the Igκ locus. Although these enhancers have overlapping functions, the κi enhancer appears to predominate during receptor editing, while the κ3′ enhancer may be more important for initiating Igκ germline transcription to target locus recombination and, later in development, somatic hypermutation. Changes in chromatin structure appear to regulate both enhancers, and previous reports suggest that both enhancers are packaged into an accessible chromatin structure only in B lineage cells. Why these enhancers cannot activate the demethylated, accessible, protein-associated Igκ allele in pro-B cells is not known. Furthermore, how the enhancers function to reactivate the locus for receptor editing or to quantitatively promote hypermutation in B cells is vague. Quantitative analysis of Ig enhancer chromatin structure in murine pro-, pre-and splenic B cells demonstrated that the κi enhancer maintains a highly accessible chromatin structure under a variety of conditions. This stable chromatin structure mirrored the highly accessible structure characterizing the Igμ intronic enhancer, despite the fact that Igμ is activated prior to Igκ during B cell development. Surprisingly, parallel analysis of the κ3′ enhancer demonstrated its accessible chromatin structure is markedly unstable, as characterized by sensitivity to changes in environmental conditions. These data unexpectedly suggest that κ locus regulation is compartmentalized along the gene in B lineage cells. Furthermore, these findings raise the possibility that environmentally dependent regulation of κ3′ enhancer structure underlies changes in κ activation during B cell development.