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Department of Microbiology and Immunology, Temple University School of Medicine, Philadelphia, PA 19140
| Abstract |
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| Introduction |
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In the autoimmune disease systemic lupus erythematosus, B cells produce Abs reactive to self Ags such as DNA or chromatin (1, 2, 3, 4, 5). The B cells producing these Abs are deleted, anergized, or edited in normal individuals (6, 7, 8, 9). Although somatic mutation and V gene usage are partly accountable for the specificity of these autoreactive Abs, unconventional Ig gene rearrangements at the heavy chain locus such as D-D fusions may also be responsible (10, 11, 12). D-D fusions result from the joining of two heavy chain D genes and create a drastic change in the amino acid sequence of the heavy chain complementarity-determining region 3 (CDR3).3 The presence of positively charged amino acids such as arginine increase the affinity for Ab binding to DNA or DNA complexed to nuclear proteins such as histones (4, 10, 13). Although D-D fusions are uncommon in the normal Ab repertoire, these unusual rearrangements have frequently been observed in Abs with antinuclear specificities (12, 14).
Previous results in our laboratory demonstrated that these unusual rearrangements (D-D fusions and D inversions) occur more frequently in the Ab repertoire of newborn autoimmune-prone MRL/MpJ-+/+ (MRL) mice when compared with C3H/HeJ (C3H) normal controls (15). In addition, the autoimmune strain used more frequently upstream D genes and the most D-distal JH genes. In comparison, the nonautoimmune C3H mice tended to use the most 3' D gene, DQ52, and the most D-proximal JH gene, JH 1. This suggests that MRL mice may have undergone secondary gene rearrangements that delete evidence of a primary rearrangement. Thus, the MRL strain may be prone to generate secondary gene rearrangements at the heavy chain locus that are more likely to include atypical junctions (15).
In this study, we wanted to determine whether similar rearrangement patterns and atypical junctions are also present in the MRL adult pre-B cell repertoire. Thus, we analyzed the heavy chain gene rearrangements in B220+IgM- cells in both MRL and C3H mice. Again, the MRL strain demonstrated an increased frequency of unconventional Ig heavy chain rearrangements when compared with C3H mice. However, the pattern of D and JH use was different in adult pre-B cells compared with newborns. Therefore, we propose a model of secondary gene rearrangements at the heavy chain locus in MRL mice, which explains the differences in gene usage between the newborn and adult libraries and the frequent occurrence of atypical rearrangements in MRL mice.
| Materials and Methods |
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Male and female animals from the lupus-prone MRL/MpJ++ (MRL) and the nonautoimmune C3H/HeJ (C3H) strains were obtained from The Jackson Laboratory (Bar Harbor, ME; Ref. 16). C3H was chosen as a control for MRL because the MRL strain is partly derived from C3H and both strains share the same Igh j allotype. The animals were maintained in our facility and sacrificed at 3 mo of age.
Flow cytometry
Bone marrow cells were obtained as previously described (17, 18). Briefly, a single-cell suspension was obtained by flushing femurs from five mice with ice-cold staining media (deficient RPMI 1640 medium without L-glutamine or phenol red (Cellgro, Herndon, VA) containing 10 mM HEPES, 3% FBS, and 0.1% NaN3). The cells were then mixed with a 1 ml syringe and treated with 0.165 M NH4Cl to eliminate erythrocytes. After washing with staining medium, the bone marrow cells were incubated with FITC-RA3.6B2 anti-B220 mAb (Southern Biotechnology Associates, Birmingham, AL) and PE-goat anti-mouse IgM (Jackson ImmunoResearch, West Grove, PA) for 15 min at 4°C, followed by 3 washes in staining media. Flow cytometric analysis and sorting of the B220+ IgM- subpopulation was conducted at the Temple University FACS core facility using an Epics Elite equipped with an autocloning attachment. The sorted fractions were reanalyzed to verify that their purity exceeds 95%. We further verified that there was virtually no (<0.34%) light chain expression in the sorted pre-B cell populations (data not shown; Ref. 19).
Primers
DNA was extracted from the B220+ IgM- bone marrow cells by proteinase K digestion and was then subjected to PCR amplification. The PCR primers and protocols were adapted from previously described methods (20, 21, 22). Amplifications were conducted using nested PCR as follows. In the first PCR, DNA was amplified with a mixed set of antisense JH primers (15) and either a sense J558 primer (GGGCAAGGCCACATTGACTGTAG) or a sense 7183 primer (GGGCCGATTCACCATCTCCAGAG). In the second PCR, 10 µl of the first PCR product was reamplified with primers that were internal to those used in the first PCR. The internal JH primers were identical with those previously used (15), while the internal VH primers were as follows: J558 sense (GTAGACAAATCCTCCAGCACAGC) and 7183 sense (GAGACAATGCCAAGAACACCCTG). All PCR were conducted in a volume of 50 µl containing 1.5 mM MgCl2, dNTPs (200 µM each), primers (0.4 µM each), and 2 units Taq DNA polymerase. Cycling conditions were as follows: an initial 5-min denaturation at 94°C; 35 cycles (1 min at 94°C, 2 min at 50°C, 1 min at 72°C); and a final 5-min extension at 72°C. After the second (internal) PCR, amplification products were separated on a 1% agarose gel and bands of the appropriate size were isolated with the QIAEX II agarose gel extraction kit (Qiagen, Chatsworth, CA). The purified products were directly ligated into the pGEM-T vector (Promega, Madison, WI) and transformed into JM109 cells. After blue/white selection, positive colonies were grown in 5 ml Luria-Bertani/Amp for plasmid purification. The inserts were sequenced using the fmol DNA sequencing system (Promega) with a 32P-labeled primer complementarity to the T7 promoter (GTAATACGACTCACTATAGGGC). Virtually all plasmids contain a VH-D-JH rearrangement and their sequences were analyzed using the GCG program by comparison with known D and JH germline sequences (23, 24, 25). A minimum of four contiguous identical nucleotides was required for assignment to a germline D sequence.
Statistical analyses
All analyses were conducted with the Prism software (Release 2.01; GraphPad, San Diego, CA).
| Results |
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B220+IgM- pre-B cells were sorted from MRL and C3H bone marrow. Ig gene rearrangements were amplified using a nested PCR with primers specific for the 7183 and J558 VH gene families and for each of the four JH genes. A total of 180 MRL and 153 C3H VH-D-JH clones were sequenced and analyzed for this study. There was no significant difference in the frequency of productive vs nonproductive rearrangements between both strains (73% productive rearrangements in MRL mice and 67% in C3H mice). These sequences are available from GenBank under accession numbers AF265709 to AF266041.
Atypical rearrangements
Conventional heavy chain Ab gene rearrangements result from the combining of single VH, D, and JH genes in the appropriate orientation, whereas we define atypical rearrangements as D-D fusions and D inversions. D-D fusions are possible due to an cryptic heptamer embedded within most D genes that simulates a 23-bp recombination signal sequence (26). Further, D inversions result from the fact that D genes are flanked by symmetric recombination signal sequences with a 12-bp spacer that allows recombination in either orientation.
As previously observed in a newborn
VH-D-JH rearrangement
library (15), MRL mice displayed more atypical
rearrangements than their Igh allotype-matched C3H controls
(48/180 for MRL vs 25/153 for C3H) (p = 0.016
using the Fishers exact test; Table I
).
This difference was mostly due to D-D fusions in that MRL mice
exhibited 44 D-D fusions, whereas C3H mice had only 15 D-D fusions. The
increased frequency of atypical junctions in MRL mice was observed for
both productive and nonproductive rearrangements (Table I
).
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The D genes at the heavy chain locus can be divided into four gene families. The DFL16 family is comprised of two D genes, DFL16.1 and DFL16.2, which are VH proximal. The next D family, DSP2, is the largest and is composed of 10 known members, including DSP 2.x, which we have observed to be frequently used in MRL and C3H mice (15). The last two families are composed of single genes, DST4 and DQ52. The DQ52 gene is the most JH-proximal D gene, mapping only 700 bp upstream of JH 1.
In a previous study of newborn MRL and C3H mice, we observed a clear
difference in the pattern of D gene usage (15). Although
C3H mice tended to use DQ52 in the majority of their heavy chain
rearrangements, MRL mice used most often members of the more upstream D
gene family, DSP2 (15). In the present study, there was
also a significant difference in the overall distribution of D genes
used between the MRL and C3H pre B cells (Table II
), although the difference was less
pronounced than in the newborn repertoire. Most notably, MRL mice used
the JH-distal DFL16 gene family more often than
their C3H counterparts. The pattern of D gene usage was similar for
productive and nonproductive rearrangements in MRL and C3H mice
(Table II
).
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Productively rearranged heavy chain D genes can be read in three
different reading frames (RF; Ref. 32, 33). The majority
of rearrangements use RF1 which encodes a glycine-tyrosine-rich neutral
amino acid sequence. Often, RF2 usage results in the expression of a
truncated Dµ protein that is selected against, whereas RF3 frequently
contains a premature stop codon (34). As in the newborn
library, both MRL and C3H mice favored RF1 and there was no significant
difference between the two strains (Table III
).
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The junctional diversity of Ab genes is enhanced through the
addition of N and P nucleotides. N nucleotides are added by TdT, which
is absent in newborn mice and up-regulated in the adult (20, 37). As expected, almost all of the adult pre-B cell sequences
contained N nucleotide additions (Table V
), whereas newborn MRL and C3H mice
showed a limited number of N nucleotides (15). Further,
there was no significant difference in the amount of N nucleotide
additions between the two strains. MRL mice possessed an average of
5.71 (± 3.54) N nucleotides per CDR3, and C3H mice had an average
of 5.34 (±3.31) N nucleotides per CDR3. P nucleotides are
presumably created by uneven cutting of DNA that resolves hairpins
created by intermediate coding joints of the Ab genes
(38). As with N nucleotide additions, there was no
significant difference between MRL and C3H adult pre-B cells in the
frequency of P nucleotide additions (Table V
).
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The deduced amino acid sequences for the productive MRL and C3H
VH-D-JH rearrangements are
listed in Fig. 1
. The heavy chain CDR3 is
critical for interaction with Ag. In particular, specific residues such
as arginine have been linked to mediating CDR3 binding of Abs to DNA or
DNA/histone complexes (4, 10, 29). Overall, the CDR3
sequences deduced from productive pre-B cell rearrangements displayed
similar properties for both strains (Table V
, Fig. 1
). For instance,
there was no significant difference in length between the MRL and C3H
CDR3 sequences (12.14 ± 2.562 amino acids for MRL and 11.29
± 2.768 for C3H) and their average arginine contents were similar
(0.37 ± 0.62 arginine per CDR3 for MRL and 0.31 ± 0.51
arginine per CDR3 for C3H). Taken together, these data suggest that at
least some of the events leading to the presence of antichromatin CDR3
sequences in MRL mice occur later than the pre-B cell stage.
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| Discussion |
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In this study, we wanted to evaluate the VH-D-JH rearrangements amplified from adult MRL and C3H B220+IgM- pre-B cells which have not yet undergone Ag selection. Our current results confirm our earlier study (15) and suggest that MRL mice indeed exhibit an intrinsic defect that leads to the development of B cell precursors with atypical Ig heavy chain gene rearrangements. It is interesting that most of the difference between the strains is due to D-D fusions in that MRL mice had 44 D-D fusions compared with 15 in C3H. This observation is revealing in that while D inversions are uncommon, they are "conventional" in that they follow established recombination rules. However, D-D fusions may be more predictive of an inherent recombination defect because of the unconventional nature of this particular rearrangement which breaks the 12/23 rule.
When comparing the current results to our previous report (15), we observed that the bias in upstream D and downstream JH gene usage previously seen in newborn MRL rearrangements was not present in the adult libraries. A significant difference is that the neonatal libraries were unselected and contained a large proportion of mature B cells, whereas the adult libraries were restricted to pre-B cells. Therefore, the difference in D and JH usage pattern between the newborn and adult libraries in MRL mice may be related to the stage of B cell maturation. Our results with the newborn library led us to propose that MRL mice may undergo more secondary D-JH rearrangements because they typically recombine more VH-proximal D genes to more distal JH genes when compared with C3H controls. Because of this biased gene usage and the presence of atypical rearrangements in MRL mice, we hypothesized that the mechanisms of atypical heavy chain gene rearrangement and secondary gene rearrangements were related. In a conventional secondary rearrangement, an upstream D gene recombines to a downstream JH gene, thus eliminating the intervening DNA sequence and any evidence of a primary rearrangement. In a D-D fusion, an upstream D gene combines with a preformed D-JH rearrangement forming a D-DJH complex. However, although our MRL pre-B cells contained a significant number of D-D fusions, we did not observe the strong bias in D and JH gene usage seen in the newborn library (15). This suggests that during autoimmunity D-D fusions may also arise from mechanisms other than incomplete secondary rearrangements. For example, VH genes could directly recombine with downstream D genes in accordance with the 12/23 rule. This VH-D product may then rearrange to a downstream D-JH complex resulting in a heavy chain with a D-D fusion (26). It has also been suggested that TdT may play a role in both the production of anti-DNA Abs and Ig gene usage. TdT-mediated N nucleotide additions increase the length of the CDR3 thus increasing the potential to generate arginine residues in the Ag binding site (39). C57BL/6-Fas(lpr) mice deficient in TdT had significantly fewer arginines in their CDR3 and a lower frequency of anti-DNA Abs (39). Further, Tuaillon and Capra (40) recently demonstrated that the presence or absence of TdT can modulate the choice of VH, D, and JH use independent of B cell Ag stimulation. Nevertheless, the mechanism by which TdT could affect Ig gene choice during rearrangement remains unknown and our data do not indicate a quantitative difference in TdT activity between MRL and C3H mice.
The analysis of the VH-D-JH
libraries from newborn (15) and adult pre-B cells (this
study) leads us to propose a model of D-JH
revision in the periphery of autoimmune mice (Fig. 2
). After D to JH
rearrangements on both alleles at the pro-B cell stage, the first
allele undergoes a VH to
DJH rearrangement (Fig. 2
, A and
B). If the rearrangement on the first heavy chain allele is
productive, the B cell will proceed through normal maturation stages.
However, subsequent D to JH rearrangements may
continue on the other allele, resulting in a biased usage of upstream D
and downstream JH sequences or in D-D fusions
(Fig. 2
B; Ref. 41). Later in B cell life, the
productive rearrangement on the first allele is inactivated (for
instance by a nonproductive VH replacement), but
the B cell may be rescued by a productive VH to
DJH (or VH to
D-DJH) rearrangement on the second allele (Fig. 2
D; Ref. 42). The B cell now possesses a new
productive rearrangement on the second allele that will be biased in
its D and JH usage and may also contain D-D
fusions that will predispose the B cell to react with DNA or
DNA-protein complexes (Fig. 2
E).
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Another critical element of our model is the rescue of a doomed B cell by a V to D-JH rearrangement on the second heavy chain allele. In our model, the B cell should be doomed because its initial VH-D-JH rearrangement has become inactivated. A likely reason for this event could be a stop codon resulting from the somatic hypermutation process, but this may also result from an additional rearrangement event within the VH-D-JH region. Most VH genes contain embedded heptamers that may serve to mediate V(D)J recombination-type events (46). When the heptamer located in the 3' region of the VH segment is used, the rearrangement may be functional, leading to a so-called VH replacement, but rearrangements at other heptamers will be nonfunctional and remain undetected because they result in the death of the B cell (47, 48). Studies with transgenic mice as well as recent human work suggest that such recombination within pre-existing VH-D-JH rearrangements are more common than previously thought (49, 50, 51, 52). RAG1 and RAG2 are likely to be required for these types of VH replacements. This is supported by studies showing that these enzymes can be expressed in the periphery although it is unclear whether this is due to re-activation or continuous expression in B cells that have left the bone marrow only recently (53, 54, 55, 56). Irrespective of the mechanism, the involvement of RAG1 and RAG2 in the recombination on the first allele would indicate that they are also available to mediate the recombination on the second allele.
Another critical factor is the existence of apoptosis defects during lupus. Although no single consistent defect has been identified, there is evidence that apoptosis is impaired in lupus individuals (57, 58, 59, 60). This decrease in apoptosis efficiency may be critical in this situation because it would allow enough survival time for the productive rearrangement to take place on the second allele, whereas a normal B cell would otherwise die before being rescued. An additional important element for autoimmunity is the timing of this secondary rearrangement. As mentioned above, the rearrangements on the second allele will be biased toward the use of certain D and JH genes. This is of particular relevance during systemic autoimmunity where self-reactive Abs often express the downstream JH 4 gene, an observation consistent with our hypothesis that successive rearrangements may have taken place on the second allele (61, 62, 63, 64). Further, these secondary rearrangements may include D-D fusions that favor reactivity with chromatin Ags. Because these "rescue-type" rearrangements will take place in the periphery, they will not be subject to the rigorous mechanisms of central tolerance and they may even occur within a context of B cell activation, resulting in the expansion of potentially self-reactive clones.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Marc Monestier, Department of Microbiology and Immunology, Temple University School of Medicine, 3400 North Broad Street, Philadelphia, PA 19140. ![]()
3 Abbreviations used in this paper: CDR3, complementarity-determining region 3; MRL, MRL/MpJ-+/+ mice; C3H, C3H/HeJ mice; RF, reading frames. ![]()
Received for publication May 12, 2000. Accepted for publication July 28, 2000.
| References |
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and heavy chain gene usage in early untreated systemic lupus erythematosus suggests intensive B cell stimulation. J. Immunol 163:1027.
-chain receptor editing in systemic lupus erythematosus. J. Clin. Invest. 102:688.[Medline]
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