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BRIEF DEFINITIVE REPORT |
CORRESPONDENCE Kathryn Calame: KLC1{at}columbia.edu
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Upon initial encounter with pathogens, B cells can differentiate into two types of cells that provide humoral memory as follows: (a) memory cells that differentiate into Ig-secreting plasma cells upon secondary antigenic challenge and (b) plasma cells that survive in the bone marrow, continuously secreting Ig (1). Long-lived plasma cells in the marrow are germinal centerexperienced cells (2) that survive for months to years (3) in the absence of antigen (4) or cell division (5). These cells reside in a limited number of niches, primarily in the bone marrow, that provide them with survival signals (6). The antibodies that these long-lived plasma cells secrete provide protection for future encounters with the pathogens that led to their formation. Although long-lived plasma cells are critical for humoral memory, they can also be pathogenic when they express autoantibodies in diseases such as lupus erythematosus (7) or become transformed in multiple myeloma. Treatments designed to target B lineage cells, such as radiation, prednisone, cyclophosphamide, and anti-CD20 antibodies (8), do not eliminate nondividing long-lived plasma cells. Thus, in lupus and multiple myeloma, these treatments often do not lead to resolution of disease.
Despite the physiological and pathological significance of long-lived plasma cells, little is known about their maintenance. There is increased understanding, however, of how plasma cell formation is regulated (6). B lymphocyteinduced maturation protein-1 (Blimp-1) is a transcriptional repressor that is found both necessary (9) and sufficient (10) for plasma cell differentiation. Blimp-1 is called a master regulator of plasma cell differentiation because it directly represses transcription factors that, in turn, regulate several important gene programs (11). Blimp-1 represses c-myc and other genes involved in cell cycle progression and cell division (11, 12). Blimp-1 represses Bcl-6 (11), a key germinal center factor, and blocks other germinal center activities. Finally, Blimp-1 represses Pax-5 (13), which is required for B cell identity, germinal center function, and repression of XBP-1 (14); J chain; and Ig heavy and light chain transcription. By relieving Pax-5dependent repression of these genes, Blimp-1 drives plasmacytic differentiation and Ig secretion (9, 15). Thus, Blimp-1 both induces plasmacytic differentiation and inhibits the alternate mature B cell fate.
Blimp-1 requires association with Groucho and histone deacetylases (16, 17) and the G9a histone methyltransferase (18) for its repressive activity. Nucleosomes near functional Blimp-1 binding sites have deacetylated H3 lysines in a plasmacytoma expressing endogenous Blimp-1 (13) and methylated H3 lysine 9 in cells ectopically expressing Blimp-1 (18). Although histone acetylation/deacetylation is known to be dynamic, histone methylation appears to be more stable. Blimp-1dependent chromatin modifications might be stable because terminally differentiated plasma cells do not divide. However, Blimp-1 is expressed in bone marrow plasma cells and multiple myeloma cells, suggesting its continued presence could be required (11).
Here, using a mouse where the gene encoding Blimp-1 can be inducibly deleted, we show that Blimp-1 is required not only for the formation of plasma cells, but also for their maintenance as long-lived Ig-secreting cells in the bone marrow. This sheds light on the biology of these important cells and challenges the idea that plasma cells have a stable gene expression program. Additionally, the discovery that Blimp-1 is required to maintain long-lived plasma cells suggests that interfering with Blimp-1 may provide a new rationale for designing drugs to treat autoimmune diseases or multiple myeloma.
| RESULTS AND DISCUSSION |
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Short-lived plasma cells can be generated in vitro by treating splenocytes with the polyclonal activator, LPS. We harvested splenocytes from ERCre+prdm1F/F and control mice and cultured them in LPS for 3 d. After 3 d, 98% of cells were of the B cell lineage (B220+) and 1220% were CD138HI plasma cells in both cultures (unpublished data). We treated the cells with either 4-hydroxytamoxifen (4OHT) or vehicle for an additional 4 d. At the end of the treatment, Blimp-1 was detected by immunoblotting in control cultures, but not in ERCre+prdm1F/F cultures treated with 4OHT (Fig. 1 A), indicating that deletion in vitro was efficient and Blimp-1 was no longer present. After 7 d, control cultures treated with 4OHT had a 30% reduction in the frequency of CD138HI plasma cells compared with vehicle treated cultures, whereas ERCre+prdm1F/F cultures treated with 4OHT showed 90% reduction in plasma cell frequency compared with vehicle-treated cultures (Fig. 1 B). A similar difference was observed in multiple experiments. In one experiment, B cells in LPS culture were labeled with a division-tracking dye at the time 4OHT was added, to distinguish newly forming plasma cells (B220LOCD138HICSFELO/) from nondividing, previously formed plasma cells (B220LO CD138HICSFEHI) (Fig. 1 C). After 34 d of 4OHT treatment, both total and CSFEHI plasma cells disappeared more rapidly and to a greater overall extent in cultures of ERCre+prdm1F/F B cells than in controls. Consistent with this, ELISPOT analyses showed a substantial number of IgM secreting cells in control cultures at the end of the culture period, but in the ERCre+prdm1F/F cultures treated with 4OHT, the number of IgM-secreting cells was greatly reduced (Fig. 1 D). Thus, Blimp-1 is required to maintain previously formed plasma cells in vitro.
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Blimp-1 and plasma cell maintenance
Although much is known about genes involved in the differentiation of B cells to plasma cells (6), there is less information about genes required for maintenance of plasma cells in the bone marrow. XBP-1, a transcriptional activator required for plasma cell formation (21), is required continuously for survival of myeloma cells because treatment with a proteasome inhibitor, which reduces levels of activated XBP-1, causes apoptosis due to ER stress (22). XBP-1 is probably required for maintenance of long-lived plasma cells, but this has not been tested directly. Blimp-1 is required for XBP-1 induction during plasmacytic differentiation (9, 15) and lack of XBP-1 may be an important component in the requirement for Blimp-1 shown in this study. Aiolos, an Ikaros family member, is required for long-lived plasma cells to appear in the bone marrow, but it is unclear if it affects formation, migration, and/or maintenance of these cells (23).
Mice lacking BCMA, a receptor for the TNF superfamily factors BAFF and APRIL, have reduced bone marrow plasma cells, suggesting that BAFF and/or APRIL are necessary for survival of these cells (24). However, BAFF enhances survival of human plasmablasts, but not plasma cells (25). Interestingly, APRIL binds not only to BCMA and TACI receptors, but also to the proteoglycan CD138, which is highly expressed on plasma cells (26). Thus, APRIL may be an important survival factor for bone marrow plasma cells. IL-6 supports the survival of isolated bone marrow plasma cells and is a survival factor for myeloma cells; however, IL-6/ mice have no abnormalities of plasma cell maintenance in vivo (27), suggesting other survival factors can compensate for its absence. In other settings, STAT3 (28) (which is activated by IL-6), NF-
B (unpublished data), and AP-1 (29) (when activated by toll-like receptors and possibly TNF family receptors) induce prdm1 transcription. Thus, an important function of survival signals from the bone marrow stroma to plasma cells may be activation of transcription factors required for transcription of prdm1.
Our data have interesting implications for the molecular mechanisms by which Blimp-1 represses target gene transcription. As discussed before, Blimp-1's repressive function involves both histone deacetylation and histone methylation, the latter being thought to be a relatively stable chromatin modification. The results reported here suggest that the covalent chromatin modifications associated with Blimp-1dependent repression are not stable, even in the absence of cell division, and show that the continuous presence of Blimp-1 is required to keep its target genes repressed. Such a mechanism would fit with a model where survival signals drive continued Blimp-1 expression in plasma cells that inhabit specific survival niches. Eventual loss of Blimp-1 in plasma cells that are ejected from their bone marrow niches by competitors or never reach such niches could be the physiological basis for plasma cell loss.
Our data do not distinguish whether long-lived plasma cells die or dedifferentiate upon the loss of Blimp-1. The requirement for XBP-1 in the survival of myeloma cells and the role of Blimp-1 in inducing XBP-1 suggest that plasma cells lacking Blimp-1 might die due to ER stress secondary to absence of XBP-1. However, a recent study has shown that the plasma cell phenotype of a myeloma line can be reversed by forced coexpression of the germinal center proteins MTA3 and Bcl-6 (30). In these experiments, dedifferentiation included reduction of Blimp-1 and XBP-1 mRNAs. Because Blimp-1 represses Bcl-6 (11), deletion of prdm1 could derepress Bcl-6, causing dedifferentiation with loss of Ig secretion and CD138 expression, which are the two plasma cell phenotypes we monitored.
Long-lived plasma cells can be pathogenic in autoimmune diseases such as systemic lupus erythematosus (7), where autoantibodies contribute to pathology, and in multiple myeloma, where CD138+ tumor cells in the bone marrow apparently represent the transformed counterparts of long-lived plasma cells. However, as discussed before, long-lived plasma cells are resistant to the treatments often used in these diseases. Thus, regardless of the ultimate fate of plasma cells lacking Blimp-1, our data show that targeting Blimp-1 or its regulators provides a novel approach in drug design strategies to eliminate long-lived plasma cells or their transformed counterparts in bone marrow.
| Materials and Methods |
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In vitro cultures.
Splenocytes were cultured with LPS as previously described (11) and were treated with 100 nM 4-hydroxytamoxifen in ethanol or sham treated (ethanol or no treatment) on day 3 of culture. Splenocytes treated with CFSE were prewashed with PBS and incubated with 25 nM of CFDA SE dye (GE Healthcare) following the manufacturer's instructions. Aliquots of cells were harvested daily for 4 d and stained for FACS (see Flow cytometry section), plated for ELISPOT analysis, or lysed for Western blot analysis. The methods for the latter two experiments were described previously (11).
In vivo tamoxifen treatment.
Mice were injected intraperitoneally on three consecutive days with 0.5 ml of 10 mg/ml tamoxifen in sunflower seed oil.
Immunization.
Mice were immunized i.p. and later boosted i.p. in appropriate experiments, with 400 µg NP-KLH in Ribi adjuvant (Corixa).
BrdU feeding.
Mice were fed drinking water with 1.0 mg/ml BrdU starting at the time of tamoxifen injection until bone marrow harvest 34 wk later. Water was changed every other day and protected from light.
Bone marrow transfer.
Recipient C57B/6 or B6/SJL congenic mice were irradiated at 680 rad and rested for 1 d. BM was harvested from control and ERCre+prdm1F/F mice 79 wk after NP-KLH boost and 6.57.9 x 106 total BM cells were transferred by tail vein injection to recipients. Mice were fed water with Baytril for the duration of the experiment.
Southern blot.
DNA was extracted from BM harvested from control and ERCre+prdm1F/F mice and Southern blotting was performed as described previously (9). Wild type, floxed, and deleted prdm1 were detected as 15-, 13.5-, and 10-kb bands, respectively.
Flow cytometry.
BM was harvested from control and ERCre+prdm1F/F mice 34 wk after tamoxifen treatment, red blood cells were lysed, and cells were stained with
-CD138-PE (BD Biosciences) and
-B220-allophycocyanin or
-B220-FITC (eBioscience). The buffer for all antibody incubations and washes was PBS with 1% BSA, 2% FCS, 0.03% NaN3, and 2 mM EDTA. For BrdU experiments, after B220 and CD138 staining, cells were washed, resuspended in ice-cold 0.15 M NaCl, and permeabilized with ice-cold 100% ethanol. After washing, cells were fixed with 1% paraformaldehyde, washed, treated with DNase I (Sigma-Aldrich), and washed before incubation with
-BrdU-allophycocyanin (GE Healthcare) for flow cytometry.
ELISA.
Serum was harvested from mice at various times, processed and, diluted to an appropriate range for ELISA analysis to detect NP-specific IgG1 as described previously (9).
| Acknowledgments |
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This work was supported by National Institutes of Health grant nos. RO1AI50659 and RO1AI43576 (to K. Calame).
The authors have no conflicting interests.
Submitted: 8 August 2005
Accepted: 27 October 2005
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