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* Arthritis and Immunology Research Program and
Molecular Immunogenetics Research Program, Oklahoma Medical Research Foundation, Oklahoma City, OK 73104
| Abstract |
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T2
mature B cell developmental pathway and are hyporesponsive to stimulation through the BCR. Significantly reduced numbers of T3 B cells in young lupus-prone mice further suggest that the specificity of this subset holds clues to understanding autoimmunity. | Introduction |
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2 x 107 surface (s) IgM+ cells produced in murine BM each day, only 10% enter the spleen and, furthermore, only 13% reach maturity (1). Transitional populations of B cells have been described previously (2, 3) and can be divided into three populations designated as T1 (CD93+sIgMhighCD23), T2 (CD93+ IgMhighCD23+), and T3 (CD93+sIgMlowCD23+) (4). Continuous in vivo BrdU labeling further supports a developmental sequence in which T1 B cells give rise to T2 B cells, and T2 cells give rise to T3 B cells (4). DNA content analysis revealed that the transitional subsets do not undergo significant proliferation, suggesting that peripheral development is not associated with a proliferative burst (4). Adoptive transfer studies have shown that T2 cells can give rise to mature follicular B cells (MB) in wild-type recipient mice and marginal zone (MZ) B cells in lymphopenic recipients (5). CD21int/highCD24high cells, which likely encompass T2 cells, T3 cells, and MZ precursors, undergo maturation in vitro when given soluble B cell-activating factor (BAFF) in the presence of anti-IgM BCR stimulation (6). However, details concerning the role of the T3 subset in B cell development and tolerance are still unclear.
The T1 stage has been shown to be an important deletion checkpoint, as supported by studies showing that BCR cross-linking induces exaggerated apoptosis at this stage (2, 7, 8). T2 cells are also susceptible to anti-IgM-mediated apoptosis, consistent with susceptibility of T2 cells to deletional tolerance (4). Mice overexpressing or underexpressing BAFF, an important B cell survival factor (6), have also revealed the importance of transitional B cell tolerance checkpoints. BAFF-deficient mice have a developmental block at the T1 stage. In contrast, BAFFtransgenic mice have enhanced transitional and MZ B cell populations and develop a lupus-like disorder (9, 10). It has been demonstrated that excess soluble BAFF can rescue the lower affinity hen egg lysozyme (HEL)-reactive CD21int/highCD23high population, which may include T2 and T3 B cells in HEL-transgenic mice. These self-reactive B cells were allowed to enter forbidden splenic microenvironments from which they are normally excluded (10). BAFF did not rescue higher affinity HEL-reactive B cells deleted in the BM; thus, excess BAFF specifically perturbs transitional B cell tolerance (10).
Finally, a significant increase in the number of autoreactive B cells in the peripheral naive mature compartment compared with newly emigrated peripheral B cells in human systemic lupus erythematosus (SLE) further suggests that transitional B cell tolerance is faulty in human SLE (11). Together, these studies highlight the importance of transitional B cell tolerance in regulating autoimmunity. The lack of knowledge concerning the relative role that T3 B cells play in B cell development and tolerance led us to further characterize this population and to determine whether it can give rise to MB cells.
| Materials and Methods |
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Four- to 5-wk-old C57BL/6 (B6) female mice were used as a source for splenic B cell populations for maturation, Ca2+ flux, and H chain usage studies. For quantitation of B cell subsets, 8- to 10-wk-old lupus-prone (BxSB/MpJ, NZM2410, NZBWF1, and MRL/MpJ, MRL/lpr/lpr) and control strains (BALB/c, B6, AKR/J, and 129/SvJ) were used. B6.SJL-PtprcaPepcb/BoyJ (B6.Ly5SJL) were used at the age of 45 wk. All mice were purchased from The Jackson Laboratory). All studies were approved by the Oklahoma Medical Research Foundation Institutional Animal Care and Use Committee.
Abs and flow cytometric analyses
Abs used were as follows: anti-mouse IgM F(ab')2-FITC or -Cy5 (Caltag Laboratories), anti-mouse CD23-PE (B3B4), anti-mouse B220 (RA3-6B2)-PerCP or PE-Cy5, and anti-mouse CD93 (493)-biotin. Biotinylated Abs were detected with secondary streptavidin (SA)-allophycocyanin, SA-allophycocyanin-Cy7, or SA-allophycocyanin-Cy5.5 (Molecular Probes). All Abs were purchased from BD Pharmingen, with the exceptions noted above. Anti-CD16/32 was used to block FcRs before staining 2 x 106 cells, unless otherwise noted. Events were collected on FACSCalibur or LSRII instruments (BD Biosciences).
Cell sorting
Splenic B cell subsets were isolated on a MoFlo cell sorter (DakoCytomation) or FACSAria (BD Biosciences). Purity of FACSAria-sorted cells ranged from 8998%, and purity of MoFlo-sorted cells ranged between 50 and 70%. Gating strategy is shown in Fig. 1. An anti-IgM F(ab')2 was used during purification to avoid BCR cross-linking.
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Maturation assays were performed as previously described (6). Sorted cells were incubated at 3 x 106 cells/ml in 96-well microtiter plates in DMEM supplemented with 10% FCS, 100 U/ml penicillin, 100 µg/ml streptomycin, 2 mM glutamine, 2 mM sodium pyruvate, 0.1 mM Nonessential amino acids solution, and 50 µM 2-ME. Cells were stimulated for 72 h with 1.5 µg/ml recombinant soluble human myc-tagged BAFF, provided by Dr. S. Kalled (Biogen Idec, Cambridge, MA) (12). This treatment was shown to eliminate any contamination of MB cells by the 50-h time point (6). F(ab')2 rabbit anti-mouse Ig (Zymed Laboratories) was added to cultures at a final concentration of 10 µg/ml for the last 24 h of culture. At 72 h, cells were stained with mAbs to IgM, B220, and CD93 and analyzed by flow cytometry.
In vivo maturation assay
Sorted cells from B6 (CD45.2) donors were resuspended at 250,000 cells/250 µl of PBS containing 0.5% FCS per recipient. Cells were adoptively transferred into B6.Ly5SJL (CD45.1) hosts by way of retro-orbital plexus. Ninety-six hours after transfer, B6.Ly5SJL hosts were euthanized and the phenotype of CD45.1CD45.2+ or CD45.1 donor B cells was assessed using mAbs to IgM, B220, and CD93.
Ca2+ mobilization assay
Sorted cells (1 x 106) from each population (T1, T2, T3, and MB) were resuspended in 1 ml of HBSS with Ca2+ (Sigma-Aldrich) and loaded with 5 µM Indo-1 acetoxymethyl dye in the presence of 2% pluronic F-127 (Molecular Probes). Cells were washed twice, resuspended, and stimulated with 50 µg/ml F(ab')2 rabbit anti-mouse Ig (Zymed Laboratories). Ratiometric measurements of free intracellular Ca2+ concentrations were obtained by a Photon Technology International QuantaMaster spectrofluorometer equipped with an excitation monochromator set at 350 nm and two emission monochromators set at 405 and 485 nm.
Single-cell RT-PCR
Presorted cells were resorted into 96-well plates, subjected to RT-PCR, and sequenced using recently described primers and procedures (13).
| Results and Discussion |
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In vitro studies have shown that immature B cells give rise to mature B cell populations; however, reports using similar strategies to determine the developmental fate of T3 B cells are not available. It was recently reported that BAFF delivered in combination with BCR stimulation enabled the maturation of CD21int/highCD24high immature B cells into a MB phenotype (6). We used a similar approach to determine whether the T2 and T3 populations can give rise to MB cells. As expected, BCR stimulation in the presence of BAFF led to maturation of a subset of the immature T2 population (Fig. 2). However, no maturation of the immature T3 population was observed as assessed by cell surface retention of the immature marker CD93 under similar conditions (Fig. 2). Although the purity (see Materials and Methods) and overall viability of isolated T2 and T3 cells varied with the sorter used, the degree of purity of the two subsets within a single experiment was always similar. Moreover, similar results were obtained in each experiment performed, as shown in Fig. 2B, which depicts results from one experiment using lower purity/higher viability MoFlo-sorted cells and from two experiments using higher purity/lower viability FACSAria-sorted cells. Even though BAFF receptor levels were marginally lower in T3 cells compared with T2 B cell (data not shown), there was no significant difference in the number of live-gated cells recovered between T2 and T3 cultures under conditions of BAFF alone vs BAFF plus anti-sIgM (p = 0.127 for three independent experiments, Kruskal-Wallis test). This indicated that increases in the fraction of mature cells in T2 cultures reflected phenotypic changes and not differential cell death.
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Studies have shown that T1 and T2 immature B cells mobilize Ca2+ to a greater extent than mature B cells following BCR cross-linking (14), whereas anergic B cells have a reduced mobilization of Ca2+ (15, 16). To examine the relative capacity of T3 B cells to respond to BCR cross-linking, we sorted the immature transitional cells into T1, T2, T3, and MB phenotypes, then measured Ca2+ mobilization following loading of cells with Indo-1. In response to BCR cross-linking, we found a reproducibly reduced Ca2+ flux to anti-IgM BCR stimulation in the T3 population when compared to T1, T2, and MB B cell populations (Fig. 4A). T1 and T2 cells both mobilized Ca2+ more efficiently than mature B cells as predicted. All populations fluxed Ca2+ to similar extents with the addition of ionomycin, demonstrating efficient Indo-1 loading in all populations (Fig. 4A). Similar results were observed in Fluo-4-labeled cells by flow cytometry (data not shown). Our results are consistent with the contention that the T3 population exhibits an anergic phenotype in response to BCR cross-linking (17).
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T2
MB pathway, we performed single-cell RT-PCR on the T1, T2, T3, and MB populations to analyze their Ig VH/JH gene usage. Interestingly, we observed a significant enhancement of JH3 segment usage in the T3 population (46 ± 13%), compared with T2 (17 ± 10%; p = 0.022) and mature cells having a follicular phenotype (17 ± 5%; p = 0.027; Fig. 4B). A predominance of JH6 usage has been observed within an edited autoreactive B cell subset in humans (18); however, no association between particular JH segments and potentially autoreactive B cell subsets has been described in the mouse. To determine whether JH3 might demonstrate an association with autoantibodies in the mouse, the Immunogenetics database was exhaustively searched for murine Abs directed to self-Ags or nonrodent Ags. Significantly more autoantibodies (38 of 110; 35%) than Abs to nonrodent Ags (64 of 262, 24%) used the JH3 segment (p = 0.014; Fishers exact test), suggesting an association of JH3 with autoreactivity (Supplemental Tables Ia and Ib). 4 Thus, biased JH3 usage within the T3 subset might be explained by enrichment of this subset with autoreactive clones. These data are consistent with T3 B cells being anergic and autoreactive. Lupus-prone mice have reduced numbers of T3 B cells
Given the impaired BCR responsiveness of the T3 population, reports of reversal of B cell anergy (19) led us to hypothesize that a tolerance defect at this selection point would most likely manifest itself as a significant reduction in the T3 population in autoimmune strains of mice. To explore this possibility, we enumerated T1, T2, T3, and MB cells in lupus-prone (BxSB/MpJ, NZBWF1 NZM2410, MRL/lprlpr, and MRL/MpJ) and normal inbred (BALB/c, B6, AKR/J, and 129/SvJ) strains of mice. Groups (n = 10) of mice were examined at 810 wk of age to reduce the influence of chronic inflammation on B cell numbers and to enhance the likelihood of detecting primary defects in B cell development, selection, or homeostasis. Examination of total spleen cellularity among the strains tested revealed two statistically separable groups (Fig. 5A) that were consistent with strain-to-strain variation in spleen size. We analyzed the two groups separately and compared the lupus prone strains to the non-autoimmune control strain harboring the closest number of T3 B cells. In the large spleen group, the T3 population in MRL/MpJ and MRL/lprlpr mice was significantly reduced compared to the T3 population in the AKR/J strain (closest normal control) (Fig. 5B). In the small spleen group, significantly reduced numbers of T3 B cells were also observed in both NZM2410 and NZBWF1 strains compared with 129/SvJ mice (Fig. 5B). No reduction in T3 B cell numbers was observed in BXSB males.
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Cumulatively, these data strongly suggest that the T3 transitional B cell population is not a normal stage of B cell development and is unlikely to give rise to mature B cells under normal conditions. The BAFF in vitro maturation assays and in vivo adoptive transfer assays demonstrate that T3 B cells, contrary to the T2 population, remain phenotypically immature when given survival factors plus BCR stimulation or when placed into a normal peripheral environment in the mouse. Furthermore, defective mobilization of Ca2+ in response to BCR stimulation and a biased JH gene usage provide evidence that the T3 population harbors cells of low responsiveness that have been selected away from the T2
MB pathway. The observations that JH3 usage is associated with autoreactivity and T3 B cells are selectively reduced in two genetically related lupus-prone models, along with recent data highlighting defective peripheral tolerance checkpoints in human SLE patients, suggest that T3 B cells are likely to be important in the regulation of autoimmunity.
| Acknowledgments |
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| Disclosures |
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| Footnotes |
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1 This work was supported by National Institutes of Health Grants P50 AR 48940, KO2 AI51647, and R01 AI48097. ![]()
2 Address correspondence and reprint requests to Dr. A. Darise Farris, Oklahoma Medical Research Foundation, 825 North East 13th Street, Oklahoma City, OK 73104. E-mail address: farrisd{at}omrf.ouhsc.edu ![]()
3 Abbreviations used in this paper: BM, bone marrow; MB, mature follicular B cell; MZ, marginal zone; BAFF, B cell-activating factor; SLE, systemic lupus erythematosus; s, soluble; SA, streptavidin; int, intermediate; HEL, hen egg lysozyme. ![]()
4 The online version of this article contains supplemental material. ![]()
Received for publication October 16, 2006. Accepted for publication April 24, 2007.
| References |
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allelic inclusion is a consequence of receptor editing. J. Exp. Med. 204: 153-160. This article has been cited by other articles:
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A. Getahun, S. K. O'Neill, and J. C. Cambier Establishing Anergy as a Bona Fide In Vivo Mechanism of B Cell Tolerance J. Immunol., November 1, 2009; 183(9): 5439 - 5441. [Full Text] [PDF] |
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J. A. Duty, P. Szodoray, N.-Y. Zheng, K. A. Koelsch, Q. Zhang, M. Swiatkowski, M. Mathias, L. Garman, C. Helms, B. Nakken, et al. Functional anergy in a subpopulation of naive B cells from healthy humans that express autoreactive immunoglobulin receptors J. Exp. Med., January 16, 2009; 206(1): 139 - 151. [Abstract] [Full Text] [PDF] |
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J. B. Carey, C. S. Moffatt-Blue, L. C. Watson, A. L. Gavin, and A. J. Feeney Repertoire-based selection into the marginal zone compartment during B cell development J. Exp. Med., September 1, 2008; 205(9): 2043 - 2052. [Abstract] [Full Text] [PDF] |
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K. Kiefer, P. B. Nakajima, J. Oshinsky, S. H. Seeholzer, M. Radic, G. C. Bosma, and M. J. Bosma Antigen Receptor Editing in Anti-DNA Transitional B Cells Deficient for Surface IgM J. Immunol., May 1, 2008; 180(9): 6094 - 6106. [Abstract] [Full Text] [PDF] |
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