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Center for Immunology and Department of Medicine, University of Minnesota Medical School, Minneapolis, MN 55455
| Abstract |
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| Introduction |
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As many as 20% or more of all developing B cells undergo receptor editing in vivo (7); however, relatively little is known about the mechanisms that regulate this important pathway. Previous studies suggested strong developmental constraint on receptor editing. Using an in vitro model system employing anti-class I MHC 383 conventional transgenic (Tg) Ig B cells, Melamed et al. (8) provided evidence to suggest that immature B cells with low levels of IgM (IgMlow) were competent to edit in response to membrane-bound self-Ag, while IgMhigh immature cells were incompetent, even when kept alive by virtue of a bcl-2 transgene.
We have recently used the hen egg lysozyme (HEL) Ig Tg system (4) to examine L chain receptor editing in response to soluble self-Ags (9). We tested whether the same developmental constraint reported for membrane-bound self-Ag also held for soluble Ag. Our results indicate that in the HEL Tg system, and in response to soluble Ag, IgMhigh B cells are fully competent to undergo L chain editing in vitro.
| Materials and Methods |
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MD4 HEL-Ig Tg mice (4) were originally provided by C. Goodnow (Australian National University, Canberra, Australia), while recombination-activating gene 2-green fluorescence protein (Rag2-GFP) reporter mice (10) were a gift from M. Nussenzweig (Rockefeller University, New York). The animals were intercrossed to generate Rag2-GFP/HEL-Ig double Tg mice. The mice were maintained in a specific pathogen-free facility at the University of Minnesota and were 48 wk of age at the time of the experiments.
IL-7 BM cultures
Single-cell suspensions of BM cells from HEL-Ig and HEL-Ig/Rag2-GFP mice were prepared as previously described (9). Cells were cultured at a concentration of 2 x 106 cells/ml in complete medium consisting of 1/1 RPMI 1640:EHAA (Mediatech (Washington, DC) and Biofluids (Rockville, MD), respectively), 10% heat-inactivated FBS (Life Technologies, Gaithersburg, MD), L-glutamine (BioWhittaker, Walkersville, MD), and penicillin and streptomycin (Mediatech) in the presence of 16 ng/ml recombinant murine IL-7 (R&D Systems, Minneapolis, MN) for 5 days (11, 12, 13). After sorting, cells were placed back into culture in complete medium on gamma-irradiated (2000 rad) S17 stroma (12, 14, 15). The cells were cultured in medium alone, HEL Ag (Sigma-Aldrich, St. Louis, MO), or HELMUT, a four-amino acid, site-directed mutant of HEL with a Kd of
10-7 M (
100-fold less affinity than wild-type HEL) for the HEL-Ig receptor (9, 16).
Flow cytometry and cell sorting
BM cells grown in IL-7 cultures were stained in FACS buffer (1x PBS and 10% FBS) with FITC-, PE-, or CyChrome-conjugated mAbs to IgMa and IgDa (BD PharMingen, San Diego, CA) and were sorted by FACSVantage (BD Biosciences, Mountain View, CA). Sorted B cells from secondary cultures were harvested and stained with FITC-, PE-, or CyChrome-conjugated mAbs to IgMa and IgDa and B220 (BD PharMingen) and assayed by FACSCalibur (BD Biosciences). In some staining conditions, annexin V-PE and/or 7-AAD (BD PharMingen and Calbiochem (San Diego, CA)) were used to exclude dead cells. For the quantitation of cell numbers, a known number of PKH26 reference microbeads (Sigma-Aldrich) was added to the cell suspensions just before FACS analysis. Flow cytometric analyses were performed using CellQuest (BD Biosciences) and Flowjo (Treestar, San Carlos, CA) software.
PCR analyses
Genomic DNA was extracted from sorted BM cells before and after 2-day secondary cultures in either the presence or the absence of soluble HEL or HELMUT. Genomic DNA from the equivalent of 4000 cells was then subjected to semiquantitative PCR amplifications using primers specific for HEL-Ig and V-J
1 as previously described (9, 17) (http://jcsmr.anu.edu/group_pages/mgc/PCR.html). The HEL-Ig PCR was slightly modified and performed using primers IgHF1 (5'-GCGACTCCATCACCAGCGAT-3') and IgHR1 (5'-ACCACAGACCAGCAGGCAGA-3') and using the following cycling conditions: 1 min at 94°C, 1 min at 59°C, and 1.5 min at 72°C for 25 cycles. The-V-J
1 PCR was designed to detect the rearrangements of various V
gene segments to J
1 by using degenerate V
primers and a primer downstream of the J
1 gene segment. Recombining sequence (RS) recombinations were detected with the following primers: VDEG1, 5'-GCGAAGCTTCCCTGATCGCTTCACAGGCAGTGG-3'; VDEG2, 5'-GCGAAGCTTCCCWGCTCGCTTCAGTGGCAGTGG-3'; VDEG3, 5'-GCGAAGCTTCCCAKMCAGGTTCAGTGGCAGTGG-3'; and reverse primer (RS3'), 5'-CTCAAATCTGAGCTCAACTGC-3', using the following cycling conditions: 45 s at 94°C, 1 min at 64°C, and 1 min at 72°C for 27 or 30 cycles (18). This RS PCR was designed to detect rearrangements occurring from V
regions to a conserved heptamer-nonamer recombination signal sequence found downstream of C
and 3'
enhancer, resulting in the deletion of all J
and C
regions (19). PCR products were resolved on 1% agarose gels and transferred onto nitrocellulose blots. Specific bands were detected by Southern hybridization of nitrocellulose blots with radiolabeled internal oligonucleotides and autoradiography.
| Results |
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100-fold reduced affinity for the HEL-Ig BCR (16), for 2 days on S17 BM stromal cells. The sorted cells were sensitive to cell death, as determined by forward light scatter and the incorporation of 7-amino-actinomycin D (7AAD) at the end of a 48-h culture (Fig. 2A) as well as by viable cell counts (see below). As previously noted in the 383 Tg B cell model system (8), the IgMhigh immature B cell population was more susceptible to cell death than the IgMlow immature B cells even in the absence of Ag. Fig. 2B shows that both IgMlow and IgMhigh immature B cells down-regulated surface IgM levels following incubation with Ag to a similar degree, with more extensive surface IgM down-regulation observed in the presence of sHEL than sHELMUT. In the absence of Ag, IgMlow cells became IgMhigh during the 2-day culture period.
Cells were harvested at the end of the secondary Ag culture and assayed for new endogenous V-J
1 and RS rearrangements, markers for receptor editing in this system. As shown in Fig. 3A, both the IgMlow and IgMhigh immature B cell subpopulations were equally capable of receptor editing in response to sHEL, as determined by the detection of new endogenous V-J
1 and RS rearrangements. The receptor editing responses of both subpopulations were more vigorous in the presence of HEL than in the presence of HELMUT, consistent with previous observations in unfractionated cells (9) (Fig. 3B). As noted previously (9), we did not detect surface
L chain-expressing cells in these cultures as a marker for editing, possibly due to the inability of
-chains to pair well with the HEL-Ig H chain or to an inability to displace the endogenous HEL L chain (data not shown). It was also technically difficult to determine whether there was a loss of HEL reactivity in this system as a consequence of editing due to strong down-regulation of overall Ig receptor levels upon Ag encounter and the fact that the anti-HEL L chain is not replaced during editing because it is integrated into chromosomal DNA outside the normal
locus.
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40% of these were Rag2-GFP positive (Fig. 4A). After initial cell sorting, there was an average of 6.0% contaminating IgMlow cells in the IgMhigh cell populations. Since the average viability of IgMlow cells in the presence of sHEL was 80%, they would constitute 26.7% of the viable cell population in the sHEL treated IgMhigh cultures (80% x 6.0% = 4.8%; 4.8%/18% viable cells in IgMhigh cultures after incubation with Ag = 26.7%). Because only 30% of viable IgMlow cells up-regulate Rag2-GFP in the presence of sHEL, only 8% (30% x 26.7%) of the 40%, for a total of 20% of the Rag2-GFP+ cells found in the IgMhigh cultures, could be derived from contaminating IgMlow cells (Fig. 4A). Even in a worst case scenario, where all contaminating IgMlow cells were viable, and 50% were Rag2-GFP+, they could only account for less than half of the Rag2-GFP+ cells in the IgMhigh cultures. An expansion of contaminating IgMlow cells is also very unlikely, since there was no appreciable proliferation of cells in these Ag-treated cultures, as determined by [3H]thymidine incorporation or CFSE labeling (9) (data not shown). We conclude that IgMhigh cells are capable of undergoing receptor editing in vitro in response to soluble self-Ag. | Discussion |
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10-9 M) than the 3-83 receptor has for class I MHC (Kd =
10-6 M), and it is possible that higher affinity interactions are required to potently initiate the signaling required to induce editing in IgMhigh immature B cells. Arguing against this possibility is our finding that lower affinity Ag (HELMUT; Kd =
10-7 M) for the HEL-Ig receptor was also capable of inducing editing in IgMhigh cells. Other factors relating to the Tg receptors, such as differences in BCR density and differing abilities to down-regulate BCR levels in response to Ag encounter, may also play a role. A clear difference between the systems is the nature of the Ag. For the reported 3-83 experiments, the class I MHC Ag was recognized as an abundant, high density, membrane-bound Ag on Op42 stromal cells (8), while the experiments described here involve soluble self-Ag on S17 stromal cells. The avidity of the 3-83 receptor/Ag interaction together with unknown interactionsbetween surface proteins on the Op42 stromal cell line and the IgMhigh B cells may have combined to block the ability of these B cells to initiate an editing response. On the other hand, the S17 cell line in some fashion might remain permissive for receptor editing in IgMhigh late immature B cells, particularly in response to the lower levels of cross-linking observed in the HEL system in response to soluble Ag. In experiments not shown we attempted to examine the editing of IgMhigh immature B cells in response to membrane-bound HEL, by incubating sorted cells with BM stroma derived from membrane-bound HEL mice (3). However, these experiments were inconclusive due to the very small number of viable cells remaining at the end of a 2-day culture.
In summary, the current study suggests that the window of time available for immature B cells to revise their receptors following an encounter with self-Ag may extend to the late IgMhigh stage, at least for certain Ags. Given the increased sensitivity of IgMhigh cells to cell death, it is unlikely that these cells would have a lengthy period of time available for revising receptors after contact with self-Ag. However, this mechanism may allow for "11th hour" salvage of self-reactive receptors before B cell clonal deletion.
| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Timothy W. Behrens, 6-126 BSBE Building, 312 Church Street SE, University of Minnesota Medical School, Minneapolis, MN 55455. E-mail address: behre001{at}umn.edu ![]()
3 Abbreviations used in this paper: BM, bone marrow; 7AAD, 7-amino-actinomycin D; BCR, B cell receptor; Tg, transgenic; HEL, hen egg lysozyme; Rag2-GFP, recombination-activating gene 2-green fluorescence protein; sHEL, soluble HEL; RS, recombining sequence. ![]()
Received for publication December 31, 2002. Accepted for publication May 5, 2003.
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region characterized by the circular excision products in mouse splenocytes. J. Exp. Med. 173:1065.This article has been cited by other articles:
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