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The Extent of Histone Acetylation Correlates with the Differential Rearrangement Frequency of Individual V_H Genes in Pro-B Cells¹

Department of Immunology, The Scripps Research Institute, La Jolla, CA 92037

	Abstract

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During B lymphocyte development, Ig heavy and L chain genes are assembled by V(D)J recombination. Individual V, D, and J genes rearrange at very different frequencies in vivo, and the natural variation in recombination signal sequence does not account for all of these differences. Because a permissive chromatin structure is necessary for the accessibility of V_H genes for V_H to DJ_H recombination, we hypothesized that gene rearrangement frequency might be influenced by the extent of histone modifications. Indeed, we found in freshly isolated pro-B cells from µMT mice a positive correlation between the level of enrichment of V_HS107 genes in the acetylated histone fractions as assayed by chromatin immunoprecipitation, and their relative rearrangement frequency in vivo. In the V_H7183 family, the very frequently rearranging V_H81X gene showed the highest association with acetylated histones, especially in the newborn. Together, our data show that the extent of histone modifications in pro-B cells should be considered as a mechanism by which accessibility and the rearrangement level of individual V_H genes is regulated.

	Introduction

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The vast diversity of the immune repertoire is generated during lymphocyte development by assembly of Ag receptor gene segments in a process termed V(D)J recombination. In B cell precursors, there is a precise order of rearrangement of the gene segments, with D_H to J_H rearrangement occurring before V_H to DJ_H rearrangement, and H chain rearranging before L chain. In addition, there is lineage specificity in V(D)J recombination. Even though the same recombination-activating gene (RAG) enzymes perform V(D)J recombination in both lineages, only T cells rearrange TCR gene segments, and only B cells rearrange Ig segments (1).

According to the accessibility model, specific molecular mechanisms should exist in developing lymphocytes that make the appropriate TCR or Ig regions accessible to the common recombinase activity in a lineage- and stage-specific manner (2, 3). Although it has been demonstrated that cis-acting elements within the TCR and the Ig loci, including enhancers and promoters, play crucial roles in recombination by helping to establish the locus and gene segment-specific accessibility (4, 5, 6), the precise molecular nature of the DNA or chromatin epigenetic modifications that are responsible for the change in accessibility status of Ig genes is still not fully understood. Chromatin exists in a highly compacted structure in the eukaryotic nucleus, and it has long been accepted that this structure functions as a barrier to gene expression. In general, transcriptionally inactive genes are methylated, show low levels of sensitivity to DNase I digestion, and are associated with hypoacetylated histones (7, 8), and this is the status of Ig and TCR genes in nonlymphoid cells.

Several studies have demonstrated that demethylation of the H and L chain genes occurs during B cell development, and is associated with onset of V(D)J recombination (9, 10, 11, 12). In addition, DNase I analysis has shown that initiation of V(D)J recombination at the Ig loci is preceded by an increase in nuclease sensitivity of V and J segments (13). More recently, other studies have demonstrated that loci active in V(D)J recombination contain nucleosomes with acetylated histone proteins (14, 15, 16, 17), and enhancement of histone acetylation levels in a pro-B cell line by treatment with inhibitors of histone deacetylase increases recombination activity (18).

It has been shown by several groups, including our own, that V_H genes rearrange at different frequencies (19). Natural variation in the composition of the recombination signal sequence (RSS)³ is part of the reason for this differential rearrangement, but it does not explain all of the nonrandom rearrangement (19). For example, our laboratory has shown that the large V_H7183 family is composed of many genes that have identical RSS, yet they rearrange at different frequencies in vivo (20). Thus, it is clear that factors others than RSS efficiency must play an important role in controlling recombination frequency in vivo.

Because induction of histone acetylation has been associated with induction of accessibility to rearrangement for V genes, we postulated that the extent of histone acetylation may be higher for the more frequently rearranging V_H genes. We therefore examined histone acetylation and methylation by chromatin immunoprecipitation (ChIP), using freshly isolated B220⁺ pro-B cells. We found a positive correlation between the extent of the association with acetylated histones and relative rearrangement frequency in the V_HS107 and V_H7183 families. Thus, in the present work, we give the first in vivo demonstration that the extent of acetylation of histone proteins is one of the mechanisms by which V(D)J rearrangement frequency of individual V_H genes may be regulated.

	Materials and Methods

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Mice

The µMT mice (21) were initially obtained from The Jackson Laboratory, and subsequently bred and maintained in specific pathogenic-free animal facilities at The Scripps Research Institute (La Jolla, CA). Experiments were approved and performed according to the regulatory standards of the Institutional Animal Care and Use Committee.

Cell isolation and B220⁺ cell enrichment

Bone marrow (BM) cells were isolated from 6-wk-old µMT mice by flushing the femur and tibia with 10% fetal serum in PBS. Newborn liver cells were obtained from <16-h-old µMT mice. For the majority of experiments, B220⁺ cells were purified using anti-mouse CD45R Ab-coated magnetic microbeads (Miltenyi Biotec). Then, the B220⁺ fraction was stained with PE-conjugated anti-B220 Ab (eBioscience). In other experiments, B220⁺ cells were pre-enriched by first staining BM or liver cells with PE-conjugated B220 Ab, followed by purification using anti-PE microbeads (Miltenyi Biotec). In both cases, viable pro-B cells (B220⁺) were sorted by flow cytometry. Thymocytes were obtained from 4- to 6-wk-old µMT mice. NIH 3T3 fibroblasts were maintained in DMEM (Invitrogen Life Technologies) supplemented with 10% FBS.

PCR, cloning, and sequencing

B220⁺ cells from adult BM and newborn liver µMT mice were isolated as described above, and genomic DNA was prepared using QIAamp DNA Mini Kit (Qiagen). Primers used to determine the rearrangement frequency of individual V_HS107 and V_H7183 genes have been described previously (20, 22). PCR products were ligated into the T/A cloning vector pCR 2.1-TOPO (Invitrogen Life Technologies), and positive clones were sequenced using the ABI PRISM 3100 DNA analyzer (Applied Biosystems).

In vivo ChIP

ChIPs were performed according to Upstate Biotechnology’s protocol, with minor modifications. Approximately 4 x 10⁶ cells were crosslinked by adding formaldehyde to a final concentration of 1% to the medium for 15 min at room temperature. The reaction was stopped by adding 0.125 M glycine and incubation for 5 min at room temperature. Subsequent steps were performed at 4°C. Fixed cells were harvested by centrifugation (1,000 rpm for 10 min), washed twice in cold PBS buffer containing 1 mM PMSF and 0.1% protease inhibitor mixture for mammalian extracts (Sigma-Aldrich), and resuspended in cell lysis buffer containing 1 mM PMSF and 0.1% protease inhibitor mixture. After 10-min incubation on ice, lysed cells were centrifuged at 3,000 rpm for 10 min to pellet the nuclei. The nuclear pellet was resuspended in SDS lysis buffer containing 1 mM PMSF and 0.1% protease inhibitor mixture and incubated 10 min on ice. The nuclear suspension was sonicated, and debris was removed by centrifugation (14,000 rpm, 10 min). The chromatin solution was diluted 10-fold in ChIP dilution buffer containing 1 mM PMSF and 0.1% protease inhibitor mixture, and precleared with salmon sperm DNA-protein A agarose beads (Upstate Biotechnology) for 1 h at 4°C. After centrifugation, 500 µl of the supernatant was saved to be used as input DNA, and the rest of the supernatant was incubated overnight at 4°C with 5 µg Ab/10⁶ cells. Upstate Biotechnology’s Abs used included anti-acetylated H3 at lysines K9 and K14 (catalog no. 06-599), anti-acetylated H4 at lysines K5, K8, K12, and K16 (catalog no. 06-866), and anti-dimethylated H3 at lysine 4 (catalog no. 07-030).

Immune complexes were collected with salmon sperm DNA-protein A agarose beads for 1 h at 4°C. Following washes and elution, Ab-bound DNA (i.e., DNA associated with the specific histone modification under study) and input DNA (DNA not incubated with any Ab) were reverse crosslinked by heating at 65°C overnight. DNA was recovered after proteinase K treatment, 2 phenol/chloroform/isoamylalcohol and one chloroform extractions, and isopropanol precipitation. Finally, DNA was dissolved in 1x Tris/EDTA buffer and stored at 4°C until use.

Real-time PCR analysis

Real-time PCR was performed using the Quantitec SYBR PCR Kit (Qiagen) and the ABI Prism 7700 Sequence Detection System (Applied Biosystems). The input DNA and the immunoprecipitated (bound) DNA were first quantified using PicoGreen dye (Invitrogen Life Technologies). The input DNA was diluted in 1x TE to match the concentration of bound DNA. The real-time PCR was conducted using 0.2 ng of DNA at 95°C for 15 min, followed by 45 cycles at 94°C for 15 s, 60°C for 30 s, and 72°C for 30 s. Data were collected at 72°C. A melting curve analysis step was built at the end of the cycling program to verify quality of the PCR products. The sequences of the primers used and their location are described in Table I. The specificity of each PCR was first confirmed by sequencing the amplified product.

Data is presented as relative to actin values for each amplified DNA sequence, which was determined as follows: 2^ [(Ct _{input sample} – Ct _{bound sample}) – (Ct _{input actin} – Ct _{bound actin})], where Ct is the cycle threshold.

Data analysis

Real-time PCRs were performed two to three times per ChIP preparation. All data used in figures are expressed as the mean values of three to six independent ChIP preparations ± SEM. The Mann-Whitney U test was used for statistical evaluation of the results.

	Results

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The V_HS107 genes rearrange at different frequencies

The µMT mice (21) have a targeted disruption of a membrane exon of the gene encoding the IgM constant region, resulting in a complete block of B cell development at the pro-B cell step. Therefore, this strain of mice is an ideal model to analyze the initial rearrangement frequency of V_H genes.

The V_HS107 family has three functional members, V1, V11, and V13. We previously described that in newborn liver B220⁺ pro-B cells from µMT mice, V1 rearranges five times more often than V11, and V11 rearranges 7.7-fold more often than V13 (22). In this study, we performed a similar analysis in adult BM B220⁺ pro-B cells from µMT mice to determine whether the rearrangement frequency of individual members changes from newborn to adult B220⁺ cells. We isolated newborn liver and adult BM B220⁺ pro-B cells from µMT mice and amplified the V_HS107 genes as described previously (22). The sequencing of additional 15 clones from newborn B220⁺ pro-B cells did not change the differential rearrangement frequency previously observed within the members of the V_HS107 family (Fig. 1). In addition, sequencing of 45 clones from adult BM B220⁺ pro-B cells showed approximately the same rearrangement frequency pattern as observed in newborn liver pro-B cells (V1>V11>V13) (Fig. 1). Thus, this unequal rearrangement pattern is stable throughout ontogeny.

View larger version (15K): [in this window] [in a new window]   FIGURE 1. In vivo relative rearrangement frequency of the three VHS107 genes. PCRs with family-specific primers were done on DNA from newborn liver and adult BM B220+ pro-B cells from µMT mice, and the PCR products were cloned and sequenced. Results are expressed as the percentage of the total number of the VHS107 family rearrangements.

Next, we wanted to test our hypothesis that the extent of histone modifications may correlate with rearrangement frequency in vivo. ChIP assays using Abs to acetylated histone H3, acetylated histone H4, or histone H3 dimethylated at lysine 4 (H3/K4) were performed on freshly isolated B220⁺ pro-B cells from adult BM and newborn liver from µMT mice. Primers unique for individual members of the V_HS107 family were used in real-time PCR. The forward primers were located in the coding region, and the reverse primers were situated 3' of their RSS. Thus, only unrearranged genes were assayed.

In adult BM B220⁺ pro-B cells, we found a positive correlation between the rearrangement frequency of the V_HS107 family members, and their level of enrichment in the DNA immunoprecipitated with Abs against acetylated histones H3 and H4, and dimethylated H3/K4 (Fig. 2, A–C). V1 has 3- and 2.4-fold higher levels of histone H3 and H4 acetylation, respectively, compared with V11 (p < 0.10 and p < 0.05, respectively; Fig. 2, A and B). Furthermore, V11 has 2.5- and 2.9-fold higher levels of histone H3 and H4 acetylation, respectively, compared with V13 (p < 0.05 and p < 0.02, respectively). For dimethylated H3/K4, V1 was 6.2-fold more enriched in the immunoprecipitated fraction than V11, and V11 was 2.7-fold more enriched than V13 (p < 0.10; Fig. 2C). In newborn B220⁺ cells, the rearrangement frequency also correlated with association with acetylated histone H3 (Fig. 2D). V1 has 1.8- and 2.2-fold higher levels of histone H3 acetylation associated with its RSS region than that of V11 and V13, respectively. Although we could not find a significant difference between V1 and V11 in our anti-acetylated H4-ChIP preparations (Fig. 2E), V11 was 2-fold more enriched in the immunoprecipitated fraction than V13 (p < 0.10).

View larger version (24K): [in this window] [in a new window]   FIGURE 2. Analysis of histone acetylation and methylation in the VHS107 family. ChIP assays were performed on adult BM pro-B cells from µMT mice using Abs to acetylated H3 (A), acetylated H4 (B), or histone H3 dimethylated at lysine 4 (H3/K4) (C). ChIP assays were also performed on newborn liver pro-B cells from µMT mice using Abs to either acetylated H3 (D) or acetylated H4 (E). Analysis was performed by real-time PCR. Data is expressed relative to the positive control actin gene (actin = 1). Results represent the mean ± SEM of 3–5 independent ChIP preparations.

Histone modification is localized to the rearranging V_H gene

We wanted to assess whether histone proteins H3 and H4 associated with V1-flanking regions were acetylated or dimethylated to the same degree as the histones surrounding the RSS and coding regions. For this analysis, three different regions (5'-promoter, 142–421 bp upstream of leader; 3'-flanking region, 504–822 bp downstream of the RSS; and intergenic region, 2.4–2.6 kb downstream of the RSS) flanking the V1 gene were analyzed in adult BM B220⁺ pro-B cells. Histone H3 and H4 acetylation, as well as H3/K4 dimethylation reached their highest value around the coding region and RSS, decreasing dramatically within 1 kb 5' and 550 bp 3' of the RSS (Fig. 3, A–C). This data shows that the extent of histone modifications associated with accessible chromatin is localized to the rearranging gene.

View larger version (22K): [in this window] [in a new window]   FIGURE 3. Acetylation and methylation patterns of histones H3 and H4 associated with flanking regions of the V1 gene in pro-B cells from adult µMT mice. ChIP assays were performed using Abs to acetylated H3 (A), acetylated H4 (B), or dimethylated H3/K4 (C). Real-time PCR was performed using primers designed to amplify four different V1 regions (5'-promoter, coding and RSS, 3'-flanking and intergenic). Data is expressed relative to the positive control actin gene (actin = 1). Results represent the mean ± SEM of five independent ChIP preparations.

It has been well documented that V_H81X, a member of V_H7183 family, is the most frequently rearranging V_H segment in fetal life (23, 24, 25, 26). We have previously shown that in newborn B220⁺ pro B-cells from µMT mice, V_H81X clearly dominates the V_H7183 repertoire, comprising 59% of the rearrangements (20). In this study, we performed a similar analysis in B220⁺ adult BM cells to determine the rearrangement frequency of V_H81X with respect to the rest of V_H7183 family members.

Rearranged V_H7183 genes from adult BM B220⁺ pro B-cells from µMT mice were amplified, cloned, and sequenced. Of 31 clones derived from adult BM B220⁺ pro-B cells, V_H81X accounted for 35% of the rearrangements in the V_H7183 family (data not shown). The remainder of the V_H7183 genes showed similar relative rearrangement frequency as in the newborn (data not shown). Although these data indicated that V_H81X rearranges more often than any other V_H7183 gene in both newborn and adult B220⁺ cells, the relative rearrangement frequency of the V_H81X gene was much higher in neonatal than in adult B220⁺ pro-B cells.

Higher association of acetylated histones with V_H81X

To test our hypothesis that the level of enrichment of acetylated histones surrounding V_H81X will be higher than that of the rest of the V_H7183 family members, ChIP assays were performed and analyzed by real-time PCR using primers that specifically amplified the V_H81X gene and primers that amplified the rest of the V_H7183 family members, excluding the V_H81X gene. The forward primers were located in the coding region, and the common reverse primer was located 37 bp downstream of the nonamer.

In adult pro-B cells from µMT mice, V_H81X has 2.4- and 2-fold higher level of histone H3 and H4 acetylation, respectively, compared with the rest of family members (p < 0.02 and p < 0.05, respectively; Fig. 4, A and B). Interestingly, in newborn pro-B cells from µMT mice, there was even more preferential association of the V_H81X gene with acetylated histone H3, 4.2-fold higher than that of the rest of family members (p < 0.10; Fig. 4C). However, we could not detect a significant difference between V_H81X and the rest of V_H7183 family members in the acetylated H4-immunoprecipitated fraction in newborn liver pro-B cells (Fig. 4D). Thus, V_H81X has a higher level of histone H3 acetylation associated with its RSS region than that of the rest of the V_H7183 genes, and this differential association is much greater in the newborn, correlating with its very high rearrangement frequency.

View larger version (26K): [in this window] [in a new window]   FIGURE 4. Analysis of histone acetylation in the VH7183 family. ChIP assays were performed on adult BM pro-B cells from µMT mice using Abs to either acetylated H3 (A), or acetylated H4 (B). ChIP assays were also performed on newborn liver pro-B cells from µMT mice using Abs to either acetylated H3 (C) or acetylated H4 (D). Analysis was performed by real-time PCR. Data is expressed relative to the positive control actin gene (actin = 1). Results represent the mean ± SEM of 4–6 independent ChIP preparations.

The large V_HJ558 family, which is the most distal family in the V_H locus, was included in our global analysis of the histone acetylation and methylation status in adult and newborn B220⁺ pro-B cells. V_HJ558 genes undergo rearrangement much less frequently in fetal and neonatal pro-B cells than in adults (27). Because the status of histone proteins associated with the V_HJ558 family has not been analyzed in freshly isolated neonatal pro-B cells, we examined this V_H family by ChIP assay using newborn liver and also adult BM pro-B cells from µMT mice. For our real-time-PCR analysis, we designed a pair of primers that would amplify most of the V_HJ558 family members, with primers located in FR1 and FR3.

The distal V_HJ558 family showed higher association with acetylated histone H3 in adult than in newborn pro-B cells (p < 0.10; Fig. 5A), which correlates with the high rearrangement frequency of V_HJ558 genes in adult pro-B cells. However, when comparing the extent of H3 and H4 acetylation surrounding different V_H genes in newborn liver pro-B cells, the V_HJ558 family was more associated with acetylated H3 than any of the proximal V_H genes other than V_H81X, which showed the highest association with acetylated histone H3 (Fig. 5A). Furthermore, there was a higher association of acetylated H4 with distal than with proximal V_H genes in both adult BM and newborn liver pro-B cells (Fig. 5B). In addition, in adult pro-B cells, the V_HJ558 family had a much higher association with dimethylated H3/K4 than the other V_H genes analyzed (Fig. 5C).

View larger version (20K): [in this window] [in a new window]   FIGURE 5. Analysis of the acetylation and methylation patterns of histones H3 and H4 associated with many VH genes. ChIP assays were performed on newborn liver and adult BM pro-B cells from µMT mice using Abs to acetylated H3 (A) or acetylated H4 (B). C, Dimethylated H3/K4 ChIP assays were performed on adult BM pro-B cells from µMT. Analysis was performed by real-time PCR. Data is expressed relative to the positive control actin gene (actin = 1). Results represent the mean ± SEM of 3–6 independent ChIP preparations. Data on VHS107 and VH7183 genes are from Figs. 2 and 4. Genes are displayed in their 5' to 3' orientation into the IgH locus.

Increased level of histone acetylation is specific for pro-B cells

To demonstrate the lineage specificity of the acetylation of histones associated with V_H genes, we performed ChIP on freshly isolated µMT thymocytes and on cultured NIH 3T3 fibroblasts. We found that 3T3 cells displayed little acetylation of histones associated with any of the V_H genes analyzed (Fig. 6, A and B). Furthermore, although thymocytes showed a slightly increased level of H3 acetylation as compared with NIH 3T3 cells, they were still hypoacetylated compared with pro-B cells. Thus, induction of hyperacetylation is specific to the B lineage. In addition, this comparative study demonstrated that it is only in pro-B cells where the level of association of individual V_H members with acetylated histones correlates with their rearrangement frequency (Fig. 6, A and B).

View larger version (18K): [in this window] [in a new window]   FIGURE 6. Analysis of histone H3 acetylation associated with many VH genes in three cell types. ChIP assays were performed on adult BM pro-B cells, total thymocytes, and 3T3 fibroblast. VHS107 (A) and VH7183 (B) family members were assayed for their enrichment in the anti-acetylated H3-immunoprecipitated fraction. Analysis was performed by real-time PCR. Data is expressed relative to the positive control actin gene (actin = 1). Results represent the mean ± SEM of 2–6 independent ChIP preparations.

	Discussion

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Another V_H gene family in which RSS differences do not account for rearrangement frequency is the large V_H7183 gene family. Although sequencing of the RSS of all 20 V_H7183 family members indicated that several have identical RSS (20), genes with identical RSS rearranged at very different frequencies in vivo. The V_H7183 gene family contains V_H81X, a gene that rearranges at an extraordinarily high frequency, especially in fetal life. V_H81X is the most 3' functional V_H gene, and shares the same consensus heptamer with other family members, although it has a unique spacer sequence. However, we previously demonstrated that the very high rearrangement frequency of V_H81X is not due to its RSS (20). Therefore, it is clear that factors other than RSS efficiency must play a role in controlling V_H gene recombination frequencies.

Increasing evidence that acetylation of histone proteins H3 and H4 and dimethylation of histone H3 at lysine 4 are associated with active chromatin led us to hypothesize that genes that rearrange more frequently within a V_H family may be more enriched in histones that have active posttranslational modifications. Consistent with this idea, we show in freshly isolated pro-B cells a positive correlation between rearrangement frequency among the V_HS107 and V_H7183 family members and their level of enrichment in acetylated histone fractions. For the V_HS107 family, we found that in adult BM pro-B cells the extent of association with acetylated histones H3 and H4, as well as dimethylated H3/K4, was higher with the highly rearranging V1 gene than with V11 gene. This result indicates that in addition to the contribution of the RSS spacer sequence in the higher rearrangement frequency of V1 over V11, association with acetylated H3 and H4, as well as dimethylated H3/K4, may also play a role in the unequal rearrangement frequency of these two V_HS107 genes. Our data also suggest that the difference in recombination frequency between V11 and V13 in adult pro-B cells could be influenced by a preferential association of V11 with acetylated H3 and H4, and dimethylated H3/K4 histones. Our data in this study uncover the first difference between V11 and V13 genes in adult pro-B cells.

In the newborn, however, the extent of histone H3 and H4 acetylation was not significantly different between V1 and V11, and the high rearrangement frequency of V11 over V13 correlated only with the association of V11 with acetylated histone H4. Thus, additional factors may be responsible for the differential rearrangement frequency within V_HS107 family members in newborn pro-B cells.

In the present work, we also showed that the high rearrangement frequency of V_H81X with respect to the rest of V_H7183 family members could be influenced by its very high association with acetylated histone H3. That association was even more pronounced in newborn than in adult pro-B cells, which concurs with the extremely high rearrangement frequency of V_H81X early in ontogeny. However, we did not observe much dimethylation of H3/K4 on any of the V_H7183 family members. Our data therefore indicates that the extent of histone acetylation could be a factor in promoting accessibility of V_H81X for rearrangement.

In our global analysis of the modification status of histone proteins associated with other V_H genes, we found that in adult pro-B cells, the acetylation of H3 and H4 is much higher in the distal V_HJ558 genes than in any of the proximal V_H genes tested. It has been previously described that the D_H-proximal and D_H-distal V_H gene segments reside in distinct regulatory environments (30, 31, 32, 33, 34, 35). Our data showing high levels of H3 acetylation associated with the distal V_HJ558 family in adult pro-B cells agree with two previous reports in which adult BM cells from RAG-deficient mice were used (16, 17). However, both groups cultured BM cells with IL-7, a cytokine which up-regulates acetylation of histones associated with the V_HJ558 family, but has no effect on proximal V_H genes. Furthermore, in RAG-deficient mice, no V_H genes were associated with acetylated histones in the absence of IL-7 (16). Thus, direct analysis of freshly isolated pro-B cells from mice, which express functional RAG proteins as we did in this study, is preferable to determine the in vivo acetylation status of histone proteins surrounding V_H genes. Chowdhury and Sen (16) also analyzed adult pro-B cells isolated from BALB/c mice and showed lower relative levels of histone acetylation associated with V_HJ558 genes than we did in this study. The difference may be due to the fact that we used µMT mice, in which B cell development is blocked at the pro-B cell stage, possibly resulting in increased distal V_H gene rearrangement.

We report in this study the first analysis of the histone acetylation status in freshly isolated newborn pro-B cells. In agreement with the lower use of the distal V_HJ558 genes early in ontogeny, we found much lower association of acetylated H3 and H4 with V_HJ558 genes in newborn as compared with adult pro-B cells. The only other study performed on neonatal cells was done with RAG^–/– fetal liver pro-B cells after culture with IL-7, but the status of V_H genes in the absence of IL-7 culture was not determined (17).

Using a novel system in which V(D)J recombination was induced in the nonlymphoid cell line BOSC after transient transfection with E2A or EBF, we have previously shown that Ig loci are not opened globally but recombination is localized to individual genes, with neighboring genes remaining inaccessible (36). We hypothesized that this was due to localized induction of histone post-translational modifications. Thus, we wanted to investigate whether the high association of modified histone proteins with the V1 gene was extended over flanking DNA or localized to the gene itself. We found a much higher association of acetylated histones and dimethylated H3/K4 surrounding the coding region and RSS of the V1 gene than the 5'- or 3'-flanking DNA or intergenic regions. This result agrees with a previous observation in which histone H4 acetylation was higher in V_H gene segments and usually did not extend into intergenic regions (17). Thus, this observation gives a molecular basis for our previously observed localized induction of recombination.

Some studies (31, 37, 38, 39) have shown that acetylation of histones is not sufficient for induction of accessibility leading to rearrangement, suggesting that other epigenetic modifications could also be important for fully inducing accessibility resulting in V(D)J recombination. For example, H4 phosphorylation at serine 1 has been shown to be associated with the V_H81X gene in pro-B cell lines (40), and thus this modification may also contribute to the high rearrangement frequency of V_H81X. In addition to the well-documented role of DNA methylation and histone modifications in promoting V(D)J recombination, antisense intergenic transcription, as well as nucleosome positioning at RSS have been proposed as additional mechanisms (41, 42, 43, 44). Furthermore, new reports have provided evidence for at least two roles of the transcription factor Pax5: facilitating rearrangement of distal V_H genes by inducing IgH locus contraction (45), and also in removing H3 methylation at lysine 9, a marker of repressed chromatin, in the V_H locus, making V_H genes accessible for V_H to DJ_H recombination (46). Thus, it is likely that accessibility to V(D)J recombination during early B cell development is controlled by several factors.

There are at least two explanations for the observations presented in this study. The accessibility of individual V_H genes could be determined by the relative number of acetylated lysines on histones associated with individual V_H genes, and/or by the proportion of pro-B cells carrying a particular V_H gene in an active chromatin environment. The later proposal has been reinforced by the observation that the activation of germline transcription occurs only in a small fraction of otherwise apparently homogeneous pre-B cells (47). Furthermore, preliminary ChIP data in our laboratory indicate that only a small fraction of VI genes are acetylated in BOSC cells after transfection with E2A, yet the frequency of induced recombination of those genes was high, suggesting that the VI genes associated with acetylated histones are the ones accessible for rearrangement (P. Goebel, M. Cherrier, and A. J. Feeney, unpublished data). Thus, we propose that it is most likely that an increased proportion of pro-B cells have acetylated histone proteins surrounding frequently (e.g., V1 and 81X) vs infrequently rearranging genes.

Taken together, our data show that the extent of acetylation of histone proteins is one of the mechanisms by which V(D)J rearrangement frequency of individual V_H genes may be regulated in vivo, opening the field for further investigations of epigenetic modifications governing the preferential accessibility of individual V_H genes within a family for V(D)J recombination during normal B cell development.

	Acknowledgments

 
We thank L. Watson, N. Morris, and P. Lao for technical assistance, and K. Mowen, D. Nemazee, and J. Kaye for critical reading of the manuscript.

	Disclosures

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The authors have no financial conflict of interest.

	Footnotes

 
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ This work was supported by a grant from the National Institutes of Health (Grant AI 52313).

² Address correspondence and reprint requests to Dr. Ann J. Feeney, Department of Immunology, The Scripps Research Institute, La Jolla, CA 92037. E-mail address: feeney{at}scripps.edu

³ Abbreviations used in this paper: RSS, recombination signal sequence; ChIP, chromatin immunoprecipitation; BM, bone marrow; Ct, cycle threshold.

Received for publication May 5, 2005. Accepted for publication September 9, 2005.

	References

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