Cutting Edge: Histone Acetylation and
Recombination at the TCR γ Locus Follows
IL-7 Induction
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J Immunol 2001; 167:6073-6077; ;
doi: 10.4049/jimmunol.167.11.6073
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References
Jiaqiang Huang, Scott K. Durum and Kathrin Muegge
●
Cutting Edge: Histone Acetylation and
Recombination at the TCR␥ Locus Follows IL-7
Induction
Jiaqiang Huang,* Scott K. Durum,* and Kathrin Muegge1*†
V
ariable diversity joining recombination is a site-specific
recombination process that leads to the creation of a
novel immune receptor gene repertoire (1). Recombination is crucial for lymphoid development, and aberrant regulation
of this process may lead to immune deficiency or promote tumorigenesis. Recombination is initiated by the recombinase-activating
gene products RAG-1 and RAG-2. The RAG dimer recognizes and
specifically cleaves recombination signal sequences that flank immune receptor gene segments. The recombination process is tightly
regulated in a stage- and lineage-specific manner. This level of control
cannot be explained simply by the presence or absence of the RAG
recombinase. Thus it had been postulated that access of the V(D)J
recombinase to its target sequence is regulated and that chromatin
modifications similar to those required for transcriptional processes participate in this regulation (2, 3). In support of this hypothesis, it was reported that transcription of unrearranged gene
*Laboratory of Molecular Immunoregulation and †Science Applications International
Corporation, National Cancer Institute, Frederick, MD 21702
Received for publication September 11, 2001. Accepted for publication October
2, 2001.
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
Address correspondence and reprint requests to Dr. Kathrin Muegge, National Cancer Institute, Building 469, Room 243, Frederick, MD 21702-1201. E-mail address:
[email protected]
Copyright © 2001 by The American Association of Immunologists
●
segments usually precedes V(D)J recombination and the absence
of transcriptional regulatory sequences within the TCR locus results in inhibition of recombination. Thus, an open chromatin
structure may allow access of the transcriptional machinery as well
as access for the V(D)J recombinase. Recently it was reported that
targeted deletion of regulatory sequences of the TCR␣ and ␤ locus
that abrogate recombination correlates with the level of histone
acetylation (4, 5). This suggests another close relationship between
the regulation of chromatin accessibility for transcription as well
as the process of V(D)J recombination.
IL-7 is required for normal lymphoid development and specifically for V(D)J recombination at the TCR␥ locus (reviewed in
Refs. 6 – 8). Deletion of different components of the IL-7 signal
transduction pathway inhibits ␥␦ T cell development and leads to
a specific suppression of V(D)J recombination at the TCR␥ locus
(9 –12). Although IL-7 provides survival functions for pro-T cells,
the defect in recombination cannot be corrected by substituting the
IL-7 signal with a survival signal. Thus a bcl-2 transgene did not
restore V(D)J recombination at the TCR␥ locus in IL-7R␣⫺/⫺
mice (12). We have previously reported that in the absence of the
IL-7R␣ signal, T cell precursors fail to initiate cleavage at the TCR␥
locus and that this failure is due to an inaccessible state of chromatin
for RAG-mediated cleavage (13, 14). The need for the IL-7R␣ signal
can be replaced by Trichostatin A, a specific inhibitor of histone
deacetylases, suggesting a role of histone acetylation in the opening of
chromatin structure induced by IL-7. However, Trichostatin A treatment induces a global, rather than locus-specific, acetylation of histones, and thus its mechanism of mimicking the IL-7 effect on the
TCR␥ locus might have been indirect. Thus, in the present study we
address the question of how IL-7 signaling modifies chromatin to
allow access for the V(D)J recombinase. We examined whether IL-7
induces specifically histone acetylation directly at regulatory sites
within the TCR␥ locus.
Materials and Methods
Mice
Embryonic thymus were derived from day 15 gestation of C57BL/6 mice
(Animal Production, Frederick, MD). IL-7R␣⫺/⫺ mice and Rag-2⫺/⫺ mice
(The Jackson Laboratory, Bar Harbor, ME) were ⬃4 – 8 wk of age. The
generation of IL-7⫺/⫺Rag-1⫺/⫺ were kindly provided by R. Murray (EOS
Biotechnology, San Francisco, CA) (9). Animal care was provided in accordance with the procedures outlined in the “Guide for the Care and Use
of Laboratory Animals” (National Institutes of Health Publication No. 8623, 1985).
Primers and probes
The locations of primers and probes within the TCR␥ locus are shown in
Fig. 1. The primers for detection of gene rearrangement and sterile transcripts were derived from sequences as previously published (13, 14). The
0022-1767/01/$02.00
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IL-7 signaling is required for V(D)J recombination at the
TCR␥ locus. We have recently reported that IL-7 controls
chromatin accessibility for RAG-mediated cleavage. Inhibition
of histone deacetylase substituted for the IL-7 signal, indicating a role for histone acetylation in altering chromatin accessibility. We found a greatly reduced histone 3 and histone 4
acetylation level in IL-7R␣ⴚ/ⴚ thymocytes in comparison with
RAGⴚ/ⴚ thymocytes or fetal thymocytes. Sterile transcripts,
indicating an open chromatin configuration, were suppressed
in IL-7R␣ⴚ/ⴚ and IL-7ⴚ/ⴚRAGⴚ/ⴚ thymocytes. Moreover, exogenously added IL-7 induced sterile transcripts from the TCR␥
constant region in cultured thymocytes from IL-7ⴚ/ⴚRAGⴚ/ⴚ
mice. This induction correlated with increased histone acetylation
at the J-promoter and C-enhancer regulatory elements at the
TCR␥ locus. These results suggest that IL-7 regulates chromatin
accessibility for V(D)J recombination by specifically altering histone acetylation within the TCR␥ locus. The Journal of Immunology, 2001, 167: 6073– 6077.
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CUTTING EDGE: IL-7 INDUCES HISTONE ACETYLATION FOR TCR␥ GENE REARRANGEMENT
the ratio of bound:input fractions which was very similar to the ratio
formed by bound:unbound control fractions. The results shown in the figures are expressed as means ⫾ SE from three to five independent ChIP
experiments.
FIGURE 1. Map of the murine TCR␥1 locus. Squares represent coding
gene segments, circles represent regulatory sequences, and triangles indicate primer positions used for RT-PCR or ChIPs PCR analysis.
PCR and RT-PCR analysis
The PCR after ChIP was performed with Supermix (Life Technologies,
Rockville, MD) started with 94°C for 5 min, followed by 25 cycles: 1 min
at 94°C, 2 min at 51°C, and 3 min at 72°C, followed by a 10-min extension
at 72°C. Total RNA was reverse transcribed with oligo(dT) primer by using
Sensiscript Reverse Transcriptase (Qiagen, Valencia, CA) or a control reaction was performed omitting reverse transcriptase. PCR was amplified in
50 ␮l of HotStartTaq Master Mix reaction buffer (Qiagen) containing 0.2
mM of each NTP, 0.2 ␮M of each primer, and 2.5 U of HotStartTaq
polymerase. The amplification for RT-PCR was started at 94°C for 15 min,
followed by 35 cycles: 0.5 min at 94°C, 0.5 min at 56°C, and 1 min at
72°C, followed by a 10-min extension at 72°C. (The annealing temperature
was 64°C for ␤-actin primers.) PCR products were separated by agarose
gel electrophoresis and transferred to NYTRAN membranes (Schleicher &
Schuell, Keene, NH). Hybridization was performed in Rapid-hyb buffer
(Amersham Pharmacia Biotech, Piscataway, NJ) according to the manufacturer’s protocol and visualized by autoradiography.
ChIPs assay
The procedure of chromatin immunoprecipitation was modified from the
manufacturer’s protocol (acetyl-histone H3 or H4 ChIP assay kit; Upstate
Biotechnology, Lake Placid, NY) as follows. Thymocytes were crosslinked with 1% formaldehyde for 12 min at 37°C and the reaction was
stopped by adding glycine to a final concentration of 125 mM for 5 min.
The cells were washed twice in ice-cold PBS and lysed in SDS lysis buffer.
Sonication was performed on ice and the samples were cooled on dry ice
between the pulses, but care was taken not to freeze the samples. The
length of the DNA fragments averaged between ⬃200 and 500 bp. After
precleaning, 10% of each sample was saved as input fraction. Immunoprecipitation was performed using specific Abs against acetylated histone
3 (5 ␮g/ml) and histone 4 (4 ␮l/ml), or normal rabbit IgG (5 ␮g/ml) as
control. After incubation for 16 h at 4°C with agitation, the Ab/histone/
DNA complex was captured by using salmon sperm DNA/protein A-agarose slurry for 1 h rotating at 4°C. The supernatant of each sample was kept
as unbound fraction. The complex was eluted and the cross-linking was
reversed. After purification, the DNA concentration was determined and
diluted in three-fold series (20 ng, 6.7 ng, and 2.33 ng) for PCR amplification. The blot result was imaged by PhosporImager and quantitated by
ImageQuant v5.0 software (Molecular Dynamics, Sunnyvale, CA). The
relative acetylated enrichment as shown in the figures was determined from
2
Abbreviation used in this paper: ChIP, chromatin immunoprecipitation.
Hyperacetylation of histones at the TCR␥ locus in normal fetal
thymocytes
To determine whether IL-7 stimulation effects histone acetylation at
the TCR␥ locus, we first determined the status of acetylation in the
presence of IL-7 during normal development. TCR␥ locus rearrangement occurs in thymocytes between days 14 and 16 of murine gestation. During this period, sterile transcripts and recombination intermediates are detected, indicating an open chromatin configuration and
an actively occurring recombination process, respectively (14). Thus,
fetal thymocytes were used as a nuclear source for ChIP using specific
Abs against acetylated histones. The acetylation of N-terminal lysine
residues of histone 3 as well as histone 4, components of the mononucleosomes, are thought to result in an open chromatin structure (2,
3, 15). Fragmentation of chromatin after cross-linking allowed for
specific analysis of small regulatory sequences (average size 200 –500
bp). The murine TCR␥1 locus extends 40 –50 kbp comprising four
variable region gene segments and one joining and constant region
segment (Fig. 1). We first investigated the status of histone 3 and
histone 4 acetylation at regulatory sites recently identified as transcriptional response elements within the TCR␥ locus: the DNase I
hypersensitive site A (V-HsA), an enhancer-like element between the
V5 and V2 genes, the J region promoter (J-promoter), or the constantregion enhancer (C-enhancer) (12, 16, 17). In addition, the enhancer
of the TCR␦ locus (E␦) was examined as a positive control. A representative ChIP experiment (Fig. 2A) indicates specific acetylation at
histone 3 as well as histone 4 for all three regulatory sites in comparison with the control immunoprecipitation. A critical question is
whether this demonstrated acetylation level found at the TCR␥ locus
is specifically elevated in comparison with other sites in the genome.
To evaluate this question, the ratio was calculated between the specifically precipitated material and the input fraction and expressed as
fold acetylation, as shown in Fig. 2B, summarizing five experiments.
This ratio indicates how much a specific sequence is enriched in the
precipitated fraction in comparison with the whole genome (Fig.
2B). Thus the J-promoter, the C-enhancer, and the HsA sites are
22-, 10-, and 13-fold enriched in the acetylated H3 fraction and
35-, 15-, and 25-fold in the acetylated H4 fraction. This suggests
a locus-specific enhancement of histone acetylation at the TCR␥
locus. In contrast, the tissue-specific expressed genes Oct-2 (B
cell-specific) and Pgk-2 (testis-specific) are not enriched in the
precipitated fraction, indicating no significant enhancement for
acetylation in comparison with the overall genome. To determine
the borders of hyperacetylation at the TCR␥ locus, we analyzed
additional sequences that lack any known regulatory function.
Whereas the upstream site (1300 bp upstream of V␥5) still showed
a 4-fold enrichment (2-fold over Oct-2), the downstream sequence
(1700 bp downstream of the C-enhancer) was indistinguishable in
its H3 or H4 acetylation level from the control gene Oct-2. Thus
the three known regulatory sites of the TCR␥ locus are hyperacetylated in comparison with the global genome and at least two
other tissue-specific genes. Thus the acetylation is rather focused
within the locus, tapering off with increasing distance from the
regulatory sites.
Reduced histone acetylation at the TCR␥ locus in the absence of
IL-7R␣ signaling
Next we addressed the question of whether hyperacetylation as
observed at the TCR␥ locus during normal T cell development
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following oligonucleotides were used in the chromatin immunoprecipitation (ChIP)2-PCR assay: 1) upstream: S:5⬘-GGCAAGAAACAGAA
CAACTG-3⬘; AS:5⬘-GGACAATGTCTATTCTAGTC-3⬘; and probe: 5⬘GTTCTGATTGGCTACTTTCC-3⬘. 2) V␥HsA: S5⬘-GCTCTCTCTCCC
TTCTTTG-3⬘; AS5⬘-TGGTTGCAGACATGCTGAGTG-3⬘; and P5⬘-GGA
AACACAAGACGTGACG-3⬘. 3) J␥ promoter: S5⬘-CCTCTTCTCAGAAA
TATATCCCC-3⬘; AS5⬘-TAATTTCCAGGAACTCACTCGTG-3⬘; and P5⬘ACATTGGTGCTTGTGAGAAC-3⬘. 4) Constant region enhancer: S5⬘GTTGCTTCCTGGAAAATGGTTAAAG-3⬘; AS5⬘-CTAATAGCTGTGGT
CTTTCGAAAG-3⬘; and P5⬘-AGGAGCAGTTAAACCACAGC-3⬘. 5)
Downstream: S5⬘-AGGAGCTTCAGTTGAGGAAG-3⬘; AS5⬘-TAGAAC
CAAGACTAACAGCC-3⬘; and P5⬘-CATGAGATCCAGCTGCAAGG3⬘.
6) ␦ Enhancer: S5⬘-CAAAATACATGCCCAGCCA-3⬘; AS 5⬘CAG
CAAAACTGATAACCCC-3⬘; and P5⬘-AAGAGATAGCAGGG
CTTCTG-3⬘. 7) Oct-2: S5⬘-TGGAGGAGCTGGAACAGTTTG-3⬘; AS
5⬘GGACCTTGGCATCTTTGTTCC-3⬘; and P5⬘-ATCAAGCTGGGCT
TCACACAG-3⬘. 8) Pgk-2: S5⬘-TGAAGTAGAGCAAGCCTGTG-3⬘; AS
5⬘AGACAGTGATGCTTGGAAGG-3⬘; P5⬘-TGTGGAGGAAGAAGG
TAAGG3⬘.
All these primer sets were designed to give a PCR product size ranging
from 110 to 160 bp.
Results and Discussion
The Journal of Immunology
6075
FIGURE 2. Hyperacetylation of histone 3 and histone 4 during normal fetal development. A, Fetal thymocytes were immunoprecipitated using specific
Abs against acetylated histone (AcH3) or acetylated histone 4 (AcH4), or using normal rabbit IgG (control). DNA from immunoprecipitates (bound), the
supernatant (unbound), or the starting material (input) was serial diluted (1/3) for PCR analysis. Due to the limited amount of material, the PCR analysis
and hybridization for the Oct-2 gene were not optimal in this particular experiment; however, other PCR analysis experiments verified that Oct-2 was
usually detectable in the unbound material. B, Summary of five independent experiments using phosphor imaging analysis. The ratios represent DNA bound
to acetylated H3 or H4 divided by input fraction.
Induction of sterile transcripts in IL-7⫺/⫺RAG⫺/⫺ thymocytes
by IL-7
IL-7 is also an important survival signal for T cell precursors,
although Bcl-2 cannot rescue the defective TCR␥ rearrangement in
IL-7R␣ defective mice (12). To determine whether IL-7 can directly effect accessibility at the TCR␥ locus instead of promoting
accessibility indirectly through enhanced viability, we examined
the induction of sterile transcripts. Transcription of unrearranged
gene segments usually precedes V(D)J recombination and is considered an indicator of accessible chromatin. Previously we have
reported that sterile transcripts are reduced in IL-7R␣⫺/⫺ thymocytes in contrast to RAG⫺/⫺ thymocytes (13) (Fig. 4A). We also
detected a suppression of sterile transcripts for V␥2 and V␥3 and
the constant region in IL-7⫺/⫺RAG⫺/⫺ thymocytes, indicating
that at least part of the detrimental effect of IL-7R␣ deletion is due
to IL-7 itself (rather than, for example, TSLP, which also binds
IL-7R␣). Recently it had been reported that IL-7 can induce TCR␥
transcripts within 5 h in CD4⫹ T cells purified from adult thymus
(18). Similarly, we were able to induce constant region transcripts
in IL-7⫺/⫺RAG⫺/⫺ thymocytes within 5 h of stimulation with
IL-7 in culture (Fig. 4A). This rapid process suggests that IL-7
signaling is directly able to induce opening of chromatin in thymic
precursors, rather than promoting cell survival or growth, indirectly affecting chromatin. We could not observe V␥ 2 and V␥3
transcripts in this time period, although the reduced acetylation
levels at the HsA site would suggest a control by IL-7R␣ signaling
as well. However, we observed reduced viability of IL-7⫺/⫺RAG⫺/⫺
thymocytes over time in culture that was not restored by addition of
additional cytokines such as stem cell factor. It remains possible
that a different ligand of IL-7R␣, such as TSLP (19), could provide
long-term survival in culture or that TSLP may specifically participate in the control of V␥2 and V␥3 transcription. Recently it
was reported that the histone acetylation levels at the recombination recognition sites at V␥2 and V␥3 are high in fetal thymocytes,
as reported in this work, but distinctly regulated in adult thymocytes (being lower at V␥3) (20). This suggests regulatory factors
other than simply IL-7 alone being involved in the control of a
FIGURE 3. Reduced histone acetylation at the TCR␥ locus in the absence of IL-7R␣ signaling. A, ChIP assay was performed to detect acetylation of
histone 3 or histone 4 within the TCR␥ locus in thymocytes derived from RAG⫺/⫺ or IL-7R␣⫺/⫺ mice. Serial three-fold dilutions of bound, unbound, and
input DNA were analyzed by PCR. B, Results of ChIP-PCR experiments were quantified by scanning of Southern blots and calculating the ratio of
bound:input. The average values are calculated from four independent experiments.
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occurs in response to IL-7R␣ signaling. Recently we reported a
defect in V(D)J recombination and a reduced chromatin accessibility in IL-7R␣⫺/⫺ thymocytes compared with those with
RAG⫺/⫺ thymocytes. Deletion of RAG-1 or RAG-2 genes leads to
an abrogation of V(D)J recombination and an early arrest of T cell
development at a stage similar than that of day 15 fetal thymocytes. However, RAG⫺/⫺ thymocytes have detectable sterile transcripts and their chromatin shows normal accessibility to RAGmediated cleavage in vitro in contrast to IL-7R␣⫺/⫺ thymocytes
(14). Thus we compared the level of histone acetylation at the
TCR␥ locus between IL-7R␣⫺/⫺ thymocytes and RAG⫺/⫺ thymocytes. As shown in Fig. 3A, the level of histone acetylation was
greatly reduced in the absence of IL-7R␣ signaling. The enrichment for acetylation in RAG⫺/⫺ thymocytes was 20-, 26-, and
19-fold at J-promoter, C-enhancer, and V-HsA sites for H3 and
22-, 21-, and 16-fold for H4, and thus comparable with the hyperacetylation in fetal thymocytes (Fig. 3B). The slight differences
between RAG⫺/⫺ and fetal thymocytes possibly reflect distinct
developmental stages (the pro-T3 stage for RAG⫺/⫺ thymocytes in
contrast with the pro-T1/T2 stage for d14 fetal thymocytes). In
both controls all three regulatory sites were markedly hyperacetylated. In contrast, the chromatin derived from IL-7R␣⫺/⫺ thymocytes showed an overall histone acetylation level indistinguishable
from the control site within the Oct-2 gene. The reduction in acetylation was 12- to 22-fold for H3 and 6- to 10-fold for H4 comparing IL-7R␣⫺/⫺ with RAG⫺/⫺ acetylation levels. Thus the specific histone acetylation at the regulatory sites within the TCR␥
locus that is observed during normal development is dependent on
IL-7R␣ signaling.
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CUTTING EDGE: IL-7 INDUCES HISTONE ACETYLATION FOR TCR␥ GENE REARRANGEMENT
developmentally ordered rearrangement of the variable ␥ gene
segments.
Histone acetylation at the TCR␥ locus is induced in
IL-7⫺/⫺RAG⫺/⫺ thymocytes by IL-7
The induction of constant region transcripts by IL-7 within 5 h
supports the idea that IL-7 directly effects chromatin accessibility
at the TCR␥ locus. To determine whether histone acetylation contributes to the inducible opening of chromatin at the TCR␥ locus
by IL-7 and to determine which regulatory sites are involved in
this induction, ChIP experiments were performed. Fig. 4B summarizes a quantitative analysis of three independent ChIP experiments using thymocytes derived from IL-7⫺/⫺RAG⫺/⫺ mice and
cultured with IL-7 in vitro for 5 h or left untreated. The level of H3
or H4 acetylation at the J-promoter or C-enhancer in untreated
IL-7⫺/⫺RAG⫺/⫺ thymocytes was undistinguishable from the level
observed at the Oct-2 gene. This underacetylation corresponds
with the low acetylation level detected in IL-7R␣⫺/⫺ thymocytes.
Within 5 h of IL-7 stimulation histone acetylation levels changed
at the J-promoter and C-enhancer. Although the histone acetylation
after stimulation with IL-7 was much smaller than that observed in
wild-type thymocytes (see Fig. 2) the induction by IL-7 was threeto four-fold for H3 and H4 above the untreated control (Fig. 4B).
The J-promoter and the C-enhancer are thought to drive sterile
constant region transcripts. Thus the induction of histone acetylation levels correlated closely with the induction of sterile transcripts. These results suggest that the IL-7 signal transduction
pathway controls TCR␥ locus recombination by increasing acetylation of histones at specific cis-acting elements.
The precise signal transduction pathway leading from the IL-7R
to the site of specific histone acetylation within the TCR␥ locus is
not yet known, although the transcription factor Stat5 would be a
candidate. IL-7 can activate Stat5, and there is strong evidence that
Stat5 can induce histone acetylation. Moreover, Stat5 can bind to
sequences within the J-promoter (the C-enhancer and V-HsA element contain potential binding sites) (12). The most compelling
evidence that Stat5 could mediate the observed acetylation effect
Acknowledgments
We thank Rodney Wiles and Terry Stull for excellent technical assistance
and Dr. Richard Murray for the generous gift of IL-7-deficient mice.
References
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12. Ye, S. K., K. Maki, T. Kitamura, S. Sunaga, K. Akashi, J. Domen,
I. L. Weissman, T. Honjo, and K. Ikuta. 1999. Induction of germline transcription
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13. Durum, S. K., S. Candeias, H. Nakajima, W. J. Leonard, A. M. Baird, L. J. Berg,
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FIGURE 4. IL-7 alters chromatin accessibility at the TCR␥ locus in IL7⫺/⫺RAG⫺/⫺ thymocytes. A, Thymocytes derived from IL-7⫺/⫺RAG⫺/⫺
mice (⫺IL-7) were cultured for 5 h with 200 ng/ml IL-7 (⫹IL-7). Total
RNA was isolated and reverse transcribed (⫹RT or ⫺RT), and PCR analysis was performed in serial dilution (1/5) for detection of sterile transcripts. RNA from thymus derived from adult or fetal (day 15) C57BL/6
mice or RAG⫺/⫺ or IL-7R␣⫺/⫺ mice were used as controls. B, Thymocytes
isolated from IL-7⫺/⫺RAG⫺/⫺ mice were stimulated in vitro for 5 h with
IL-7 or left untreated before ChIPs was performed. The data represent a
summary of three experiments and are expressed as ratio of bound:input
material.
comes from the report that a transgene expressing Stat5 restores
TCR␥ recombination in IL-7R␣ deleted lymphoid precursor cells
(12). Stat5 has been demonstrated to associate with the interacting
protein Nmi and with the histone acetylases CBP/P300 (21). Thus
activation of Stat5 by IL-7 could lead to site-specific recruitment
of histone acetylases at the TCR␥ locus. In contrast, knockout of
Stat5a/b was not reported to be deficient in ␥␦ T cells, implicating
additional mediators.
How can site-specific histone acetylation regulate V(D)J recombination? Previous work suggested that mononucleosomes could
inhibit RAG-mediated cleavage in vitro (22, 23). This inhibition is
thought to be caused by inhibiting access of the RAG dimer to the
recombination signal sequence as well as to reducing cleavage
capacity of the RAG recombinase (23). How can histone acetylation overcome this suppression? Acetylation of histone tails may
affect the structure of the mononucleosome or participate in unfolding of higher chromatin structure. Acetylated histone tails may
also serve as flags for recruitment of other chromatin remodeling
complexes. ATP-dependent complexes such as SWI/SNF complexes may disrupt chromatin structures by sliding mononucleosomes along the DNA, thus providing better access for the RAG
recombinase. In support of this model, it was recently reported that
both acetylation and SWI/SNF-dependent chromatin remodeling
act in concert to promote RAG-mediated cleavage in vitro (23).
However, it is not yet known whether SWI/SNF-dependent remodeling contributes in vivo to accessibility for V(D)J recombination
or whether other recombination-specific chromatin remodeling activities exist. The use of IL-7 to induce specifically TCR␥ rearrangement may serve as a valuable in vivo model to study the
molecular mechanism for control of chromatin accessibility for
V(D)J recombination.
The Journal of Immunology
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