The epigenetic regulation of V(D)J recombination

The adaptive immune response utilizes a diverse repertoire of receptors, expressed on the cell surface of lymphocytes, to bind to the infinite collection of foreign, pathogenic antigens. The immune system generates antigen receptors through carefully orchestrated site-specific DNA rearrangement events between vast arrays of gene segments in a process known as V(D)J recombination. These arrays consist of numerous variable (V), diversity (D), and joining (J) gene segments distributed across six large, structurally unique antigen receptor loci. All antigen receptor gene segments are immediately flanked by recombination signal sequences (RSS), which are recognized, bound, and subsequently cleaved by the lymphocyte-specific V(D)J recombinase complex. Ubiquitously expressed components of the non-homologous end-joining pathway process the DNA double strand breaks and imprecisely join the gene segments. The combinatorial diversity inherent within the component gene segment arrays and the junctional diversity created during the imprecise joining step both contribute to the tremendous binding potential of antigen receptors. V(D)J rearrangement events are regulated by a combination of recombinase expression and the accessibility of antigen receptor loci and individual gene segments within a receptor locus to the recombinase machinery. Recombination occurs only in lymphoid cells and within the lymphocyte lineage, Immunoglobulin (Ig) loci are only rearranged in B cells, while T cell receptor (TCR) genes are only completely assembled in T cells. Furthermore, heavy chain (H) receptor loci rearrange prior to light chain (L) loci and within a heavy chain locus, D-to-J joining precedes the fusion of a V gene segment to the preassembled DJ element. In recent years it has become increasingly clear that the chromatin structure of a particular antigen receptor locus governs the accessibility of that locus to the recombinase machinery. In an effort to better understand the chromatin architecture associated with antigen receptor loci, we utilized chromatin immunoprecipitation to map the distribution of covalent histone modifications and remodeling enzymes across Ig and TCR loci. In recombinase-deficient pro-B and pro-T cells poised to undergo D-to-J rearrangement, we observed an association of acetylated histone H3, di-methylated H3-K4, tri-methylated H3-K4, and di-methylated H3-K79 with D and J gene segments. BRG1 enrichment directly correlated with acetylation at D and J gene segments. In contrast, recombinationally-poised gene segments were devoid of di-methylated H3-K9, a covalent modification known to mark heterochromatic regions. However, all TCR gene segments in pro-B cells and all Ig gene segments in pro-T cells were associated with H3-K9 dimethylation. The results presented here begin to define the domains created by the chromatin architecture associated with antigen receptor loci in developing lymphocytes. These observations are reminiscent of the chromatin domains seen within other complex genetic loci, such as the yeast mating-type locus and the chicken β-globin locus. In light of the chromatin structure associated with V, D, and J gene segments as well as the domains defined by that structure, we wanted to search for the presence of chromatin insulator elements within antigen receptor loci. To accomplish this, we searched the DNA sequence of antigen receptor loci for CTCF binding sites. CTCF is a ubiquitously expressed nuclear protein involved in transcription, chromatin insulation, and higher-order chromosomal dynamics. An array of evolutionarily conserved CTCF DNA binding sites was discovered at intergenic and RSS-associated positions throughout the VH region of IgH loci. These IgH binding sites possess potent enhancer blocking activity and are bound in vivo by CTCF in cell lines and B cell populations isolated from the bone marrow of mice. Il-7 receptor signaling, a B cell survival signal shown to be involved in regulating VH gene segment accessibility and rearrangement, can regulate the binding of CTCF to these IgH sites. These results suggest a role for CTCF in the regulation of accessibility and recombination at IgH loci.