With rare exceptions, each lymphocyte is destined to express only one antigen-receptor specificity for life, but the mechanisms by which this is enforced have not been fully resolved. During B-cell development, immunoglobulin gene segments ? variable (V), diversity (D) and joining (J) ? recombine to form functional immunoglobulin genes, generating a diverse B-cell repertoire. Regulated expression of the recombination activating genes ( RAG1 and RAG2 ) during B-cell development helps to ensure that each B cell expresses a single heavy (IgH) chain and a single light-chain (Igκ or Igλ) allele, a phenomenon known as allelic exclusion. Now, reporting in Nature Immunology, Skok et al. provide the first evidence that epigenetic mechanisms could reinforce monoallelic expression of immunoglobulin genes at later stages.

Previous studies have shown that many inactive genes localize to centromeric heterochromatin in the nucleus of actively dividing lymphocytes, in association with Ikaros (a DNA binding protein that is essential for B and T cell development). This led to the proposal that the recruitment of loci to heterochromatic domains might lead to heritable silencing of genes. In the current study, the three-dimensional nuclear location of immunoglobulin alleles in mature B cells was revealed by combining immunofluorescence and fluorescence in situ hybridization (FISH) techniques.

Resting primary B cells were stimulated to proliferate, causing the recruitment of Ikaros to heterochromatic regions. In almost all the cells observed, only one IgH allele was associated with Ikaros-containing centromeric domains. Similarly, three of the four possible light chain alleles were centromeric. By contrast, neither IgH allele was found in association with centromeric domains in progenitor B-cell clones, indicating that non-equivalent nuclear localization of immunoglobulin alleles is acquired during B-cell development. Using specific messenger RNA probes, the authors confirmed that mature B cells express only one allele, which is consistently the non-centromeric allele.

In terms of transcription, centromeres are relatively inactive regions of the nucleus. The authors propose that during B-cell proliferation, the centromeric recruitment of immunoglobulin alleles excluded during development helps to keep them quiet ? crucially maintaining the monospecificity of a B-cell clone.