Accessibility Control of Recombination at Immunoglobulin Locus

During B cell development, two somatic DNA recombination events occur at the immunoglobulin heavy chain loci: VDJ recombination and class switch recombination (CSR). VDJ recombination assembles antigen receptor genes from a pool of gene segments. CSR exchanges the μ constant region of the immunoglobulin heavy chain gene for the other isotypes (γ1, γ2a, γ2b, γ3, α or ε). In both cases, the target specificity of recombination reactions seems to be regulated by structural changes of the target chromatin. In fact, many studies support this notion, called the “accessibility model”. In recent years, covalent modifications of histones have gained prominence as epigenetic markers that alter the properties of the associated DNA and contribute to structural changes of the target chromatin. This review focuses on the control of CSR by modulation of accessibility, and the role of histone modifications and germline transcription in CSR.