By
From the Departments of Medicine and Molecular Biology & Genetics, The Johns Hopkins University School of Medicine, Baltimore Maryland 21205
The process of V(D)J recombination is crucial for regulating the development of B cells and for determining their eventual antigen specificity. Here we assess the developmental regulation of the V(D)J recombinase directly, by monitoring the double-stranded DNA breaks produced in the process of V(D)J recombination. This analysis provides a measure of recombinase activity at immunoglobulin heavy and light chain loci across defined developmental stages spanning the process of B cell development. We find that expression of a complete immunoglobulin heavy chain protein is accompanied by a drastic change in the targeting of V(D)J recombinase activity, from being predominantly active at the heavy chain locus in pro-B cells to being exclusively restricted to the light chain loci in pre-B cells. This switch in locus-specific recombinase activity results in allelic exclusion at the immunoglobulin heavy chain locus. Allelic exclusion is maintained by a different mechanism at the light chain locus. We find that immature, but not mature, B cells that already express a functional light chain protein can undergo continued light chain gene rearrangement, by replacement of the original rearrangement on the same allele. Finally, we find that the developmentally regulated targeting of V(D)J recombination is unaffected by enforced rapid transit through the cell cycle induced by an Eµ-myc transgene.
Bcell antigen receptor genes are assembled from germline-encoded segments, VH, DH, and JH at the immunoglobulin heavy chain (IgH) locus and V V(D)J recombination is dependent on the recombinase
activating genes, RAG-1 and RAG-2, whose expression is
strictly limited to immature lymphoid cells (6, 7). Both
gene products are necessary for recombination, and their
coexpression in non-lymphoid cells is sufficient to confer
the ability to recombine plasmid substrates. However, additional levels of regulation must determine which loci are
targeted for recombination in developing lymphocytes, because the process shows both lineage specificity (Ig genes
are fully rearranged only in B cells and T cell receptor genes
only in T cells) and temporal regulation. In B cells, studies
of completed rearrangements in cell lines (8) or ex vivo B
lineage cells (9) representing various stages of development
have shown that DH-to-JH rearrangement occurs first on both
chromosomes before VH-to-DJH rearrangement at either allele. Successful, in-frame, VH-to-DJH rearrangement completes the assembly of a heavy chain gene of the µ isotype
(Igµ), whose expressed product then forms a signaling complex with the surrogate light chain (SLC) proteins VpreB and
The simplest hypothesis for explaining these data is that
the observed sequence of completed rearrangements reflects
the developmental regulation of V(D)J recombinase activity
by Ig gene products. According to this Ig-regulated model,
the signal mediated by the product of a functional heavy
chain gene inhibits rearrangement at the allelic heavy chain
locus, accounting for the phenomenon of heavy chain allelic
exclusion, and also activates rearrangement at the IgL loci
(8, 18). However, the complexity of B cell development
and the multiple selection steps that demonstrably involve
Ig gene products make it difficult to determine whether completed rearrangements reflect regulation of the V(D)J
recombinase or the results of selection at the cellular level
after recombination. Two alternative models have been proposed for allelic exclusion. The stochastic model proposes
that gene rearrangement is intrinsically inefficient and has a
very low probability of producing functional heavy or light
chain gene rearrangements at both alleles before all recombination is stopped (19). The cellular selection model postulates that expression of Igµ from both alleles is toxic and leads to the elimination of the double producing cells (20, 21). Also, with regard to the effect of Igµ signaling on light chain gene rearrangement, it has been proposed that it is
limited to providing the proliferative signal that expands the
precursor population undergoing light chain gene rearrangement as a whole, without any effect on the rate or efficiency of the recombination reaction itself (22).
Previous studies of V(D)J recombination have relied on
analysis of completed rearrangements (9, 23). This approach
has the disadvantages of being cumulative, and thus not
able to distinguish rearrangements completed at earlier stages
from recently generated ones. Analysis of completed rearrangements is also biased by selection events occurring at
the cellular level after rearrangement. The definition of intermediates in V(D)J recombination provides a way to follow this process directly and to distinguish between different models of B cell development. Gene segments used in V(D)J recombination are identified by a recombination signal sequence (RSS) (24). Recent work on the biochemistry
of V(D)J recombination has shown that the initial step in
the reaction is precise cleavage between a coding gene segment and its flanking RSS, resulting in two species of broken-ended DNA intermediates, a pair of signal ends and a
pair of coding ends (25, 26). These broken-ended DNA
intermediates can be detected by means of a ligation-mediated PCR (LMPCR) assay and used as a measure of V(D)J
recombinase activity at a given locus. This assay is applicable to both signal and coding ends at any of the rearranging
loci by using different primers for amplification (M.S. Schlissel,
manuscript in preparation and references 27).
We undertook to study the developmental regulation of
V(D)J recombination directly, by assaying double stranded
DNA breaks produced in the process of V(D)J recombination at the immunoglobulin heavy and light chain gene loci.
This approach allowed us to measure rearrangement activity at individual Ig loci across stages of B cell development
defined by the expression of antigenic markers and isolated
by cell sorting. Furthermore, because V(D)J recombination is known to be sensitive to cell cycle status (27) and different stages of B cell development differ greatly in proliferative behavior, we addressed the question whether cell cycle
regulation contributes to the developmental control of V(D)J
recombination by comparing B cell development in normal
and Eµ-myc transgenic mice. In these mutant mice, overexpression of a c-myc transgene targeted to the B cell lineage by the IgH (Eµ) enhancer results in vigorous proliferation of developing B cells (30). Finally, we examined the
issue of V(D)J recombinase inactivation. The simplest interpretation of the Ig-regulated model would hold that
V(D)J recombination is shut off as soon as a functional light
chain gene rearrangement is generated, resulting in expression of the complete BCR on the cell surface at the immature B cell stage (Table 1). However, it has been observed
that cells expressing IgM on their surface (sIgM+ cells) are
capable of further light chain gene rearrangement under
special circumstances, either in vitro after IL-7 withdrawal (31), or in vivo in mice expressing a transgenic BCR with
anti-self specificity (32, 33). It has not been clear, however,
whether such secondary light chain gene rearrangements
are a significant factor in normal B cell development. Our
approach allowed us to determine directly whether light
chain gene rearrangement continues to occur in sIgM+
cells in normal development, and at what stage of development V(D)J recombination finally ceases.
Table 1.
Sequential Expression of Antigens in B Cell
Development
and J
or V
and J
at the light chain (IgL) loci, by a series of site-specific
recombination events collectively termed V(D)J recombination (1). In addition to determining the antigen specificity
of mature B cells, Ig gene products play a crucial role in
guiding B cell development through a series of checkpoints
based on the successful assembly of Ig genes. Thus, B cells
from mice that lack the capacity to rearrange their Ig genes
are arrested at an early (pro-B cell) stage in development (2,
3), but the introduction of a rearranged IgH transgene allows the cells to progress to an intermediate (pre-B cell)
stage, and if both a heavy and a light chain transgene are
provided the cells can reach the mature B cell stage (4, 5).
5 (10). Signaling through this pre-B cell receptor (preBCR) complex mediates the checkpoint function of Igµ by
effecting progression to the pre-B cell stage (4, 5), proliferative expansion of the population (13), and heavy chain allelic exclusion (see below). Light chain gene rearrangements
(V
-to-J
) can occur before VH-to-DJH rearrangement (1315), but the frequency of V
J
rearrangements is much greater
at later stages of development, where cells already have a
complete heavy chain gene (9, 16, 17).
B cell stage
B220
cIgµ
CD2
sIgM
sIgD
%
B200+
% in
S-G2/M
Pro-B cells
+
15
18
Early pre-B cells
+
+
9
28
Late pre-B cells
+
+
+
48
9
Immature B cells
+
+
+
+
22
6
Mature B cells
+
+
+
+
+
8
6
*
Percentage of all B220+ cells in adult BALB/c bone marrow represented by each subpopulation. Results of one typical experiment are
shown.
Percentage of cells in each subpopulation having a DNA content
greater than G1 as shown by DNA staining with 7-AAD. Results of
one typical experiment are shown.
Mice. Female 4-6-wk-old Balb/c mice were purchased from NCI (Frederick, MD). The mice used in these experiments were between 6 and 8 wk old. Eµ-myc transgenic mice (34) were bred in our animal facility from mice originally obtained from Dr. C. Sidman (University of Cincinnati).
Antibodies. The PE-conjugated mAb RA3-6B2 (anti-CD45R, B220), RM2-5 (anti-CD2, LFA-2), and 11-26c.2a (anti-IgD) were purchased from PharMingen (San Diego, CA). Biotinylated and FITC-conjugated goat anti-mouse Igµ antisera were purchased from Southern Biotechnology Associates. The RA3-6B2 (antiCD45R, B220) antibody was also purified in our own lab and conjugated to biotin. Streptavidin-Quantum Red conjugate was purchased from Sigma Chem. Co. (St. Louis, MO). All antibodies were titered for flow cytometric staining.
Cell Staining and Sorting by Flow Cytometry. Antigens on the surface of cells were stained by standard methods (35). For staining of cytoplasmic Igµ and DNA, we used the method described by Schmid et al. (36). In brief, cells were stained for surface antigens in the usual manner, fixed in wash media containing 0.25% paraformaldehyde for 1 h on ice, and then permeabilized by incubation in PBS containing 0.2% Tween 20 for 15 min at 37°C. The extent of permeabilization was monitored with a microscope by Trypan blue exclusion, and if necessary the permeabilization step was repeated. For cytoplasmic Igµ staining, the cells were then incubated with a carefully titrated amount of FITC-conjugated goat anti-mouse Igµ serum on ice for 20 min and then washed twice with PBS containing 0.2% Tween 20. For DNA staining, 7-AAD was added to the permeabilized cells in suspension to a final concentration of 15 µg/ml and the cells were incubated on ice for 30 min before analysis. Flow cytometric analysis was performed on a Becton-Dickinson FACScan® instrument using the CellQuest software package. Sorting was performed on a flow cytometer (Coulter Epics Elite, Coulter Corp., Hialeah, FL) using single laser excitation at 488 nm.
RT-PCR. RNA from 5 × 105 sorted cells was isolated by lysis in a guanidinum thiocyanate buffer in the presence of 10 µg carrier tRNA and centrifugation through a cushion of cesium chloride (37). The RNA was reverse-transcribed as described elsewhere (38). 2 µl (one-tenth) of each reverse transcription reaction was used for each PCR amplification. PCR conditions were 94°C for 1 min followed by 66°C for 2 min. 30 s, for 24 cycles (H2 transcript) or 27 cycles (cyclin D1 transcript). Primer sequences are listed below. All amplifications were completed by a long extension step of 10 min at 72°C. One-half of the final product was analyzed by electrophoresis on a 1% agarose, 1% NuSieve (FMC) gel, and blotted under alkaline conditions to a nylon membrane (Zetabind, Cuno). Blots were hybridized to the appropriate PCR fragment labeled with 32P by random priming (Life Technologies kit and manufacturer's instructions) and analyzed with a PhosphorImager using ImageQuant software (Molecular Dynamics).
Purification of DNA for LMPCR. DNA purification, T4 polymerase polishing, and linker ligation were all carried out in agarose plugs, as described elsewhere (Schlissel, M.S., manuscript in preparation). In brief, 5 × 105 sorted cells from each stage were resuspended in 40 µl PBS, mixed with an equal volume of molten 1% agarose, and immediately poured into plug molds (BioRad). After the plugs solidified, they were incubated overnight at 55°C (in 100 mM Tris, pH 8.0, 25 mM EDTA, 1% sarkosyl, and 400 µg/ml proteinase K), washed in TE containing 0.5 mM PMSF for 30 min, then washed three more times in TE over 24 h. The plugs were next treated with T4 DNA polymerase by adding in a 40 µl reaction mixture containing polymerase buffer, three units T4 DNA polymerase (Life Technologies), and 100 µM dNTPs, for 1 h at 37°C. Plugs were then washed two more times in excess TE over 12 h and used for linker ligation.
LMPCR Assay for Detecting Signal and Coding DNA Broken Ends. The LMPCR assays for detecting signal (27) and coding end (Schlissel, M.S., manuscript in preparation) breaks have been described elsewhere. In brief, plugs containing purified DNA from sorted cells were subjected to linker ligation in 40 µl ligation mixture containing ligation buffer (Boehringer), 40 pmol BW linker, and 2 u T4 DNA ligase (Boehringer) and incubated overnight at 16°C. The reaction was then mixed with 100 µl PCR-L buffer (10 mM Tris, pH 8.8, 50 mM KCl, 0.25% Tween-20, 0.25% NP-40) and heated to 95°C for 10 min. For PCR, the linker-ligated DNA was brought to 65°C, and 4 µl of each sample were added to 21 µl amplification mix containing the appropriate primers and Taq DNA polymerase (Life Technologies). The primary amplification consisted of 12 cycles of 94°C for 1 min and by 66°C for 2 min 30 s each. 1 µl of that reaction was used for a second amplification with a nested locus-specific primer (see below) consisting of 26 cycles (signal ends) or 29 cycles (coding ends). One half of the final product was analyzed by electrophoresis on a 1% agarose, 1% NuSieve (FMC) gel, and blotted under alkaline conditions to a nylon membrane (Zetabind, Cuno). Blots were hybridized to 32P end-labeled locus-specific internal oligonucleotides and analyzed with a PhosphorImager using ImageQuant software (Molecular Dynamics). Due to multiple sequential reactions and the large numbers of amplification cycles required to detect signal and coding ends, these assays are not strictly quantitative. Template dilution controls, however, confirmed that the differences in intensity of the hybridization signals represent differences in the initial frequency of the corresponding broken DNA ends (data not shown).
Primers. The primers used are listed below.
Despite the wealth
of known regulated surface antigens on developing B cells
(9, 39), the only markers of unambiguous functional significance with regard to V(D)J recombination are the products of the Ig genes themselves. Accordingly, we have divided the B cell lineage into pro-B cells, which do not
express functional Igµ, pre-B cells, which express Igµ in
their cytoplasm (cIgµ) but not on their surface, immature
B cells, which express Igµ and IgL (IgM complex) on their
surface, but little or no isotype heavy chains (IgD), and
mature B cells, which express both IgM and high levels of
IgD on their surface. The pre-B cell population was further
subdivided based on the expression of the CD2 surface antigen into early CD2
and late CD2+ pre-B cells. This
marker was chosen because it is developmentally regulated
to appear at a point intermediate between cIgµ and sIgM (5) (Fig. 1 A), and its acquisition correlates well with the cell cycle behavior of pre-B cells (see below). These populations were identified by staining bone marrow cells from
adult Balb/c mice with various combinations of antibodies
to the B220, cIgµ, CD2, sIgM, and sIgD antigens and analyzing them by 3-color flow cytometry (Fig. 1 A and Table 1).
To assess the proliferative status of cells at each developmental stage, the DNA content of these cells was determined by flow cytometry. This was done using the DNAbinding dye 7-aminoactinomycin D (7-AAD) in conjunction
with staining for two antigenic markers to identify the subpopulation of interest (B220+cIgµ for pro-B cells, cIgµ+
CD2
for early pre-B cells, CD2+sIgM
for late pre-B cells,
sIgM+sIgD
for immature B cells, and sIgM+sIgD+ for
mature B cells). This analysis revealed that early pre-B cells
had the highest fraction of cells with DNA content greater than G1 (Fig. 2 A). This population presumably comprises
cells undergoing a proliferative burst in response to preBCR signaling. In contrast, late pre-B cells, as well as immature and mature B cells, appeared to be almost entirely
quiescent, with very few cells in the S or G2/M phases of
the cell cycle (Fig. 2 A).
This transition to quiescence at the late pre-B cell stage
was confirmed by RT-PCR analysis of cyclin gene transcripts from B220+CD2 pro/early pre-B cells, B220+
CD2+sIgM
late pre-B cells, and sIgM+ immature/mature
B cells isolated by cell sorting from Balb/c bone marrow.
As shown in figure 2B, cyclin D1 transcripts were abundant
at the pro/early pre-B cell stage, but were rare or undetectable at the subsequent late pre-B or sIgM+ B cell stages.
RT-PCR analysis of the expression of the cyclin D3, cyclin
A, and E2F1 genes showed similar results (data not shown).
As shown in Fig. 3 A, cleavage by V(D)J recombinase at an RSS results in two broken-ended species, a signal end and a coding end. Signal broken ends (SBE) are the more abundant, as they remain in the blunt-ended conformation until they are joined. This broken-ended species can persist in a resting cell for an extended period of time. However, cell cycling places a limit on the persistence of these SBE, because they are joined before the onset of DNA synthesis (27, 29). Thus, in a population of cycling cells, SBE can be a reliable measure of locus-specific V(D)J recombinase activity. This is in contrast to analysis of completed joints, which can be affected by pre-existing rearrangements or by selection at the cellular level occurring after rearrangement. Coding broken ends (CBE) are a more specific indicator of V(D)J recombinase activity, because they are rapidly processed to coding joints regardless of the cycling status of the cell (29). However, they are initially sealed in a hairpin structure and become available to linker ligation only in the interval between hairpin opening and coding joint formation (Schlissel, M.S., manuscript in preparation and 26, 29). The short half-life and the intermediate processing steps of coding ends make them much less abundant than the corresponding signal ends and more difficult to detect reliably.
To characterize the targeting of V(D)J recombinase activity at different stages of development, populations of
cells within the lineage were isolated from the bone marrow of adult Balb/c mice by cell sorting based on their expression of antigenic markers. The populations isolated
were pro-B cells (B220+cIgµCD2
), early pre-B cells
(cIgµ+CD2
sIgM
), late pre-B cells (cIgµ+CD2+sIgM
),
immature B cells (B220+sIgM+sIgD
/low), and mature B
cells (B220+sIgM+sIgDhi). Analysis of the sorted fractions
showed them to be 85-95% pure (Fig. 1 B).
DNA from equal numbers of cells was purified, treated
with T4 DNA polymerase to blunt any overhanging CBE,
and subjected to ligation with a double stranded linker capable of ligating in only one orientation. Linker-ligated
DNA was then used for amplification by PCR using a
linker-specific primer and a pair of nested locus-specific primers (Fig. 3 A). Control amplification of a non-rearranging locus showed that all samples contained similar
amounts of amplifiable DNA. Because heavy chain gene
rearrangements occur almost exclusively by deletion, using
the RSS 5 of DH genes for V-to-DJH joining (38), SBE
upstream of DH genes represent intermediates in the VHto-DJH rearrangement step. We focused on SBE upstream
of DFL16.1 because it is the most frequently used of the DH
segments (40). At the
locus we measured SBE upstream
of the most frequently used J
segments, J
1 and J
2, as indicative of V
-to-J
rearrangement (Fig. 3 B). To distinguish between ongoing rearrangement and the persistence
of unrepaired SBE in non-cycling cells at later stages of development, we also assayed for CBE at the J
1, J
2, and J
5
segments, as well as at V
gene segments (Fig. 4 A). In each
instance, the data shown is representative of that obtained
from at least three independent cell sorting experiments.
The identities of the various SBE and CBE fragments have
been confirmed by DNA sequence analysis (Schlissel, M.S., manuscript in preparation and 27).
Retargeting of V(D)J Recombinase.
The immediately striking observation that emerges from these studies is that the pro-B to pre-B cell transition is accompanied by a drastic change in the targeting of V(D)J recombinase activity, from being predominantly active at the heavy chain locus in pro-B cells to being exclusively restricted to the light chain loci in pre-B cells (Fig. 3 B, lanes 1-3). The course of DH SBE, present at high levels at the pro-B cell stage and undetectable (at least 10-fold less frequent, data not shown) at all subsequent stages, reflects the phenomenon of allelic exclusion, where a productively rearranged IgH gene (obligatory at all stages past pro-B cell) prevents subsequent rearrangement at the allelic locus, even though light chain genes are actively rearranged at later stages. This result argues that locus-specific modulation of V(D)J recombinase activity alone might be sufficient to account for the phenomenon of heavy chain allelic exclusion.
SBE upstream of J1 and J
2 show a different pattern (Fig.
3 B). We consistently detected J
SBE in pro-B cells (Fig.
3 B, lane 1, and data not shown) representing some level of
active rearrangement at the Ig
locus. We interpret this
signal to represent a low "constitutive" level of V
-to-J
rearrangement independent of signaling through the preBCR. This interpretation is consistent with evidence from
mutant mice incapable of assembling a pre-BCR, showing
that V
-to-J
rearrangements can occur at a detectable level
regardless of the state of the IgH loci (13). Assembly
and expression of IgH resulted in a dramatic induction of J
SBE levels (~10-fold as assessed by template dilution, data
not shown), but interestingly, this occurred only at the late
pre-B cell stage, after the proliferative burst occurring at
the early pre-B cell stage (Fig. 3 B, lanes 2 and 3). This suggests that signaling through the pre-BCR does induce IgL
rearrangement, but as a delayed effect, not an immediate one.
Because CD2+sIgM late pre-B cells are quiescent and
might harbor Ig
SBE for a longer time before forming a
signal joint, we also assayed CBE at several J
segments as a
more reliable indicator of ongoing recombinase activity
(Fig. 4 A). The results confirmed that maximal induction of
light chain gene rearrangement occurs at the late pre-B cell
stage (Fig. 4 A, lanes 1-5). No CBE were detectable in
pro-B cells (Fig. 4 A, lane 1) possibly due to the lower sensitivity of the CBE assay.
Taken together, these results show that the known progression of rearrangement products in B cell development
(8, 9, 16, 41) can be explained by the developmentally regulated targeting of the recombinase alone, and that the developmental stages at which IgH and IgL rearrangements
predominantly occur are sharply demarcated and separated
by a highly proliferative stage during which little rearrangement occurs at either locus. These data also show that IgH
rearrangement is suppressed at the same time that pre-B cells
are actively rearranging Ig, arguing against the stochastic
and the cellular selection models of allelic exclusion (19).
The observation that Igµ
expression maximally induced IgL gene rearrangement
only after the cells had become quiescent (Figs. 3 B and 4 A,
compare lanes 2 and 3) led us to consider a causal relationship between proliferation and the delayed induction of light chain gene rearrangement. To study the effect of proliferation on the regulation of V(D)J rearrangement, we examined this process in the B lymphocyte lineage cells from
Eµ-myc transgenic mice (34). B cell development in these
animals is grossly normal up to the immature B cell stage,
with the exception that the forced overexpression of the
c-myc transgene in the B cell lineage results in vigorous
proliferation at all stages of development (Fig. 2 C; unpublished data; and 30). If induction of IgL gene rearrangement requires that pre-B cells stop proliferating, then it should be abrogated in Eµ-myc mice, because B cells at all stages of
development are cycling rapidly. However, in sorted cells
from Eµ-myc mice, SBE upstream of DFL16.1, J1 and J
2
showed the same pattern of developmental regulation of
V(D)J rearrangement as wild-type cells, even though the
absolute level of the signal was decreased (Fig. 3 B). One
point of difference between wild-type and Eµ-myc mice is
the absence of J
SBE in pro/early pre-B cells of the latter
genotype (Fig. 3 B, lanes 1 and 2 vs. lane 7). Levels of several SBE are reduced in Eµ-myc mice, possibly as a consequence of reduced RAG-2 levels in cycling cells (42). It is possible that a signal that was weak in wild-type pro-B and early pre-B cells dropped below the limits of detection
by this assay in Eµ-myc mice because of the general decrease in SBE levels associated with this genotype. Another
point of difference concerns the high level of J
SBE, especially at J
2, found in cells from Eµ-myc spleen (Fig. 3 B,
lane 6 vs. 10). This is probably due to the fact that B cells
from wild-type spleen are almost exclusively of the mature
sIgM+IgDhi phenotype and were isolated by that criterion,
whereas B lineage cells from Eµ-myc spleens are largely
pre-B and immature sIgM+IgD
B cells and were isolated
on the basis of sIgM expression alone. A previous report
(45) demonstrated that sIgM+ cells from N-myc transgenic
mice fail to inactivate RAG expression, a factor which might
also contribute to a relatively higher level of J
2 SBEs.
These data indicate that quiescence is not a requirement
for induction of Ig rearrangement, and that other differences between early and late pre-B cells must determine
the level of recombinase activity at the Ig
locus.
B cells at later
stages of development are largely quiescent, making SBE
levels a potentially misleading indicator of ongoing V(D)J
recombination due to the long half-life of signal ends in non-cycling cells (29). Thus, it is possible that the SBE detected in immature B cells may have been generated at the
pre-B cell stage. In order to assess the inactivation of the
V(D)J recombinase, we focused our analysis on the shortlived coding ends associated with rearrangement at the J1,
J
2, and J
5 gene segments, and at V
gene segments recognized by a set of degenerate primers (46). As shown in Fig.
4 A, J
1 coding ends are abundant in late pre-B cells, but
become undetectable in either immature (sIgM+D
/low) or
mature (sIgM+Dhi) B cells (top panel, lanes 3-5), even
though ~40% of Ig
alleles remain in germline configuration (9). However, rearrangements to the downstream J
2
and J
5 gene segments do occur in immature, but not mature, B cells, as shown by our ability to detect J
2, J
5, and
V
CBE (Fig. 4 A, bottom three panels). Negative samples
contained at least 5-10-fold fewer CBE than any positive
sample in these assays (determined by template dilution,
data not shown). This demonstrates ongoing rearrangement at the Ig
locus in immature B cells, but not in mature B cells from bone marrow or spleen and implies that
the signal mediating allelic exclusion at the IgL loci is more
complex than simple expression of a functional gene product in the sIgM complex. Also, the absence of CBE at J
1 is
of significance in determining the mechanism by which
these rearrangements occur and remain allelically excluded
(see Discussion).
Several labs have reported strategies for fractionating bone marrow B cells according to developmental
stage (9, 44). Each of these schemes relies on the differential expression of surface markers whose expression is correlated with various regulated events. Because our goal was
to assess the effects of immunoglobulins on targeting of the
V(D)J recombinase, we chose to rely on Ig heavy-chain
rather than CD25 (44) or CD43 (9) as a definitive marker
for pre-B cells. We incorporated the membrane protein
CD2 into this scheme because it was shown previously to
correlate with pre-B cell cycle activity (5). Other workers
have used cell size (as assessed by forward light scatter) to
identify rapidly cycling pre-B cells. Because cell cycle stage
correlates with cell size, this approach tends to overestimate
the fraction of pre-B cells with greater than 2n DNA content (60-70%), while ignoring cells which are actively cycling but are in the G1 phase of the cycle (44). Using our
approach, which does not involve cell size as a criteria, we
found that 28% of cIgµ+CD2 had a greater than 2n DNA
content. We believe our fractionation strategy more completely identifies the population of early, cycling pre-B
cells.
The dramatic change in the loci undergoing rearrangement
across the pro-B to pre-B cell transition supports the hypothesis that V(D)J recombinase targeting is a major mechanism by which Igµ controls B cell development. Allelic
exclusion has been previously shown to depend on the
membrane-embedded form of Igµ and on the surrogate
light chain 5 (13, 14) leading to the conclusion that it is a
consequence of pre-BCR signaling. More recently, it has
also been shown that heavy chain allelic exclusion is already established at the early pre-B cell stage in wild-type mice
(23, 47). However, studies of completed heavy chain gene
rearrangements in developing B cells cannot exclude the
possibility of bias due to selection at the cellular level, because both positive and negative selection based on Igµ
expression can occur in early development (22, 48, 49). Interestingly, the few mature B cells that accumulate in
5
/
mice are allelically excluded (13), whereas their newly generated early pre-B cell progenitors are not (47), implying
that selective processes after rearrangement do contribute
to allelic exclusion under certain conditions. The complete
absence of DNA breaks associated with heavy chain gene
rearrangement after the pro-B cell stage seen in this study
demonstrates that in normal development allelic exclusion
at the IgH locus is mediated at the level of V(D)J recombination.
The segregation of heavy and light chain gene rearrangements to distinct stages of development separated by a
highly proliferative stage is consistent with the hypothesis
that the proliferative signal transduced by the pre-BCR initially stops all recombination, possibly through the cell cycle-dependent degradation of RAG-2 protein (42, 43).
This hypothesis is supported by the absence of RAG-2
protein noted previously in highly proliferative early pre-B
cells (44). A separate mechanism would then be responsible for maintaining heavy chain allelic exclusion once RAG
protein levels build up again, by limiting recombination to
light chain loci. The differential accessibility of heavy and
light chain loci in intact nuclei to in vitro cleavage by
V(D)J recombinase has recently been shown to be mediated at the level of chromatin conformation (50). It is
therefore tempting to speculate that one or more rounds of
DNA replication after signaling through the pre-BCR are
necessary in order to reprogram the chromatin conformation at Ig loci, making it inaccessible at the heavy chain locus and more accessible at the light chain loci. The increase in SBE and CBE levels at multiple gene segments in the
Ig locus across the pro-B to pre-B cell transition shows
that the role of pre-BCR signaling is not limited to expanding the population undergoing light chain gene rearrangement (22), but actually increases the activity of V(D)J
recombination at the Ig
locus.
The cell cycle
regulation of V(D)J recombination is evidenced by the restriction of recombination-associated, double stranded DNA
breaks to the G0 and/or G1 phases of the cell cycle (27). A
satisfying, but unproven explanation for this phenomenon
is the cell cycle-dependent degradation of the RAG-2 protein (42, 43). The concordance of induction of light chain
gene rearrangement and quiescence suggests the possibility
that an actively cycling state is not permissive for light
chain gene rearrangement, with the effect that differentiation is linked to cell cycle control. There is a parallel in the
103 pre-B cell line, which is transformed with a temperature-sensitive allele of the v-abl gene. When shifted to the
nonpermissive temperature, this cell line simultaneously arrests in G0, increases expression of the RAG genes, and induces rearrangement at the Ig locus (51). However, the
correct developmental regulation of V(D)J rearrangement at
both heavy and light chain loci in Eµ-myc mice (Fig. 3 B,
lanes 7-10) indicates that quiescence is not a requirement
for the induction of light chain gene rearrangement. The
decreased levels of SBE at several loci seen in Eµ-myc mice compared to wild-type can be explained either by rapid
joining linked to cell cycle progression or by lower overall
efficiency of the recombinase due to a shorter average G1
phase and the consequent lower levels of RAG-2 protein.
We demonstrate in this study that DNA breaks indicative of light chain gene rearrangement continue to occur in immature, but not mature B cells (Fig. 4 A, lanes 4 and 5). Because a functional light chain gene is necessary for cell surface expression of the IgM complex, these breaks must reflect secondary rearrangements in cells that have already generated one functional light chain gene. The generation of secondary rearrangements represents a potential violation of the principle of allelic exclusion, which ensures that only one immune specificity is generated per cell.
Two mechanisms can be envisioned for these secondary
light chain gene rearrangements in immature B cells, depending on the allele of the Ig locus on which they occur.
At the locus containing the initial rearrangement, an upstream V
segment can be joined to a downstream J
segment,
deleting the initial rearrangement (replacement mechanism,
Fig. 4 B). It should be noted that the J
1 segment cannot
participate in this kind of secondary rearrangement, because its RSS is obligatorily deleted by the initial rearrangement. If the Ig
locus allelic to the initial rearrangement is
in germline configuration, any V
can be joined to any J
,
leaving the original rearrangement intact (direct mechanism,
Fig. 4 B). This mechanism predicts prominent involvement
of J
1 in secondary rearrangements, matching its over-representation in completed rearrangements at all stages of development, the abundance of J
1 CBE in pre-B cells, and
the availability of J
1 segments in germline configuration (9, 52, 53; Fig. 4 A; and data not shown). The observed exclusion of the J
1 segment from secondary rearrangements indicates that these must occur by replacement of the original
rearrangement on the same chromosome. Replacement rearrangement probably accounts for the relatively high level
of J
2 SBEs in myc-transgenic B cells as well (Fig. 3 B,
lanes 8 and 9). This preference for restricting secondary rearrangements to the same "active" allele could be due to
the original rearrangement deleting the large segment of DNA between the V
and J
loci, bringing upstream V
segments into proximity to the actively transcribed J-C
locus. These V
segments might then be favored in secondary rearrangements because of their proximity to J
segments, or because they are transcriptionally activated by the
nearby
enhancers (46). It should also be noted that this
mechanism of replacement rearrangement limited to one
active allele maintains light chain gene allelic exclusion in
the setting of continued expression of RAG proteins and
rearrangement capability.
Secondary light chain gene rearrangements have been hypothesized to allow rescue of cells that generate a self-reactive BCR (32, 33). In our studies, the specificity of cells undergoing secondary rearrangements during normal development could not be determined; it is possible that they were autoreactive. However, it is equally plausible to envision a model where replacement rearrangement occurs at an intrinsic rate independent of the original specificity, and is in competition with the processes of negative selection and recruitment to the mature IgDhi population, which are determined by the nature or strength of the BCR signal.
Inactivation of V(D)J Recombinase.The mechanism by which
V(D)J recombination is inactivated in B cells has remained
unclear. Comparison with T cell development may be relevant, because T cells also continue to undergo V(D)J rearrangement after the expression of the antigen receptor on the cell surface, at the CD4+CD8+TCRlow stage (54). In
T cells, it has been shown that high-level surface expression
of the TCR and cessation of RAG-1 and RAG-2 gene expression and of TCR gene rearrangement are correlated
with a positive selection step dependent on TCR-MHC
interaction (54, 55). Furthermore, the changes in RAG gene
expression could be mimicked in vitro by cross-linking of
the TCR on the cell surface (56).
Direct evidence of a positive selection step for B cells has
been more elusive, and support for this notion has come
mainly from the disparity in the representation of different
VH gene families between immature and mature B cells
(57, 58). More direct evidence of the involvement of BCR
signaling in V(D)J recombinase inactivation in B cells is provided by sIgM+ B cell lines derived from tumors induced
by an Eµ-N-myc transgene. These cell lines constitutively
express RAG-1 and RAG-2 transcripts, but their expression is shut off by cross-linking of IgM on the cell surface
(45). Our observation that CBE are generated at the Ig
locus in immature but not mature cells (Fig. 4 A, lanes 4 and 5) is consistent with the observation that RAG-2 protein is present in immature but not mature cells (44) and
indicates that V(D)J recombination is inactivated before cells
attain the mature phenotype characterized by high expression of IgD. In view of the preceding observations, this
suggests that V(D)J recombinase inactivation and recruitment into the mature IgDhi B cell pool are correlated and
may be the result of a positive selection signal mediated by
the BCR.
Address correspondence to Mark S. Schlissel, Department of Medicine, Department of Molecular Biology and Genetics, Ross Building Rm 1068, 720 Rutland Ave., Baltimore, MD 21205.
Received for publication 8 October 1996
A. Constantinescu was supported by grant no. GM07309 from the National Scientist Training Program. M.S. Schlissel is a Culpeper Scholar and a Leukemia Society Scholar. This work was supported by National Institutes of Health grant No. RO1 HL48702.We would like to thank Dr. F.W. Alt for the 63-12 RAG-2 deficient pro-B cell line, Drs. S. Cory and C. Sidman for the Eµ-myc transgenic mice, K. Chadwick for expert assistance with flow cytometric cell sorting, and members of the Schlissel lab for careful reading of the manuscript and helpful comments.
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