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-Igβ, whereas the mechanism for counterselection against Dµ has not been determined. We have examined the role of the Ig
-Igβ signal transducers in counterselection against Dµ using mice that lack Igβ. We found that Dµ expression is not selected against in developing B cells in Igβ mutant mice. Thus, the molecular mechanism for counterselection against Dµ in pre-B cells resembles positive selection in that it requires interaction between mDµ and Ig
-Igβ.
The object of B lymphocyte development is to produce cells with a diverse group of clonally restricted antigen receptors that are not self reactive (1). Antigen receptor diversification is achieved through regulated genomic rearrangements that result in the random assembly of Ig gene segments into productive transcription units (2, 3). These gene rearrangements are in large part regulated by the preB cell receptor (BCR)1.
B cells undergoing Ig heavy chain gene rearrangements (pre-B) can express at least two types of BCRs. One form of the receptor is composed of membrane immunoglobulin heavy chain (mIgµ),
Fluorescence Analysis and Cell Sorting.
DNA and PCR.
Isolation and Sequencing of VH–DJH and DH–JH Joints.
To determine whether mIgµ could induce the pre-B cell transition in the absence of Igβ, we introduced a productively rearranged immunoglobulin gene (20) into the Igβ–/– background (TG.mµ Igβ–/–). We then measured B cell development by staining bone marrow cells with antiCD43 and anti-B220 monoclonal antibodies (30). We found that expression of a pre-rearranged Ig transgene was not sufficient to activate the pre-B cell transition in the absence of Igβ (Fig. 1). TG.mµ Igβ–/– B cells did not develop past the CD43+B220+ pre-B cell stage (Fig. 1). In control experiments, the same mIgµ transgene did induce the appearance of more mature CD43–B220+ pre-B cells in a RAG–/– mutant background where B cell development was similarly arrested at the CD43+B220+ stage (20, 25, 26; data not shown). We conclude that in the absence of Igβ, a productively rearranged mIgµ is unable to activate the pre-B cell transition.
5, V–pre-B, and Ig
-Igβ, and is referred to as the pre-BCR (4–6). A second form of the preB cell receptor, known as the Dµ pre-BCR (7), is found only in pre-B1 cells (8) and contains truncated mIgµ chains lacking a VH domain (mDµ). mDµ is produced by Ig genes that have rearranged DJH gene segments in reading frame (RF) 2 producing an in-frame start codon and a truncated transcription unit (7). Like authentic mIgµ, mDµ is a membrane protein that forms a complex with
5, V–pre-B, and Ig
-Igβ, and in tissue culture cell lines the Dµ pre-BCR can activate cellular signaling responses (9–14). But despite its ability to activate nonreceptor tyrosine kinases, Dµ preBCR producing pre-B cells are selected against by a process that is mediated through the transmembrane domain of the mDµ protein (15). In contrast, pre-B cells that express intact mIgµ containing pre-BCRs are positively selected. Counterselection is reflected in the relative lack of mature B cells that express mIgµ in RF2 (15–17). The mechanism by which mDµ activates counterselection has not been defined, but is known to require expression of syk (18). Here we report on experiments showing that Igβ is essential for counterselection against mDµ in vivo.
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Materials and Methods
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Abstract
Materials and Methods
Results
Discussion
References
Mice.
Igβ–/–, mIgµ, and Bcl-2 transgenic strains have been previously described and were maintained by backcrossing with BALB/c mice under specific pathogen-free conditions (19–21). All experiments were performed with 4–8-wk-old female mice.
Single cell suspensions prepared from bone marrow or spleen were stained with PE-labeled anti-B220 and FITC-labeled anti-CD43 (PharMingen, San Diego, CA) or FITC-labeled anti-IgM, and analyzed on a FACScan®. For cell sorting, bone marrow cells from four to six mice were stained with the same reagents and separated on a FACSvantage®. CD43+B220– and CD43+B220+ cells were collected based on gating with RAG-1–/– controls.
Total bone marrow DNA was prepared for PCR as previously described (22). DNA from sorted cells was prepared for PCR in agarose plugs (23). Primers for VH–DJH and DH–JH rearrangement were as in reference 22; these primers are mouse specific and do not detect the human Igµ transgene. All experiments were performed a minimum of three times with two independently derived DNA samples. Nonrearranging Ig gene intervening sequences were amplified in parallel with other reactions and used as a loading control (22). Amplified DNA was visualized after transfer to nylon membranes by hybridization with a 6-kb EcoR1 fragment that spans the mouse JH region.
A JH4 primer was combined with either a DH primer or a VHJ558L primer to amplify DJH and VDJH rearrangements, respectively. The primers were: (a) JH4, ACGGATCCGGTGACTGAGGTTCCT; (b) DH, ACAAGCTTCAAAGCACAATGCCTGGCT; and (c) VHJ558L, GCGAAGCTTA(A,G)GCCTGGG(A,G)CTTCAGTGAAG. PCR amplification for DJH joints was for 35 cycles of 0.5 min at 94°C, and 2 min at 72°C; for VDJH joints, it was for 0.5 min at 94°C, 1 min at 68°C, and 1.5 min at 72°C. PCR products were purified by agarose gel electrophoresis, subcloned into pBluescript, sequenced using an Applied Biosystems (Foster City, CA) DNA sequencing kit, and analyzed on a genetic analyzer (ABI-310; Applied Biosystems).
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Results
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Abstract
Materials and Methods
Results
Discussion
References
mIgM Cannot Induce the Pre-B Cell Transition or Allelic Exclusion in the Absence of Igβ.
Expression of Igβ is required for B cells to efficiently complete Ig VH to DJH gene rearrangements (19). B cells in Igβ–/– mice fail to express normal levels of mIgµ, and B cell development is arrested at the CD43+B220+ pre-B1 stage (19). A similar celltype specific developmental arrest is also found in mice that carry a mutation in the transmembrane domain of mIgµ (24), and mice that fail to complete Ig V(D)J recombination (25–29). In view of the abnormally low levels of mIgµ in Igβ–/– mice, failed pre-B cell development might simply be due to lack of Ig expression.
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and Igβ are sufficient to activate allelic exclusion (20, 35). The finding that mIgµ is unable to induce allelic exclusion in the absence of Igβ suggests that Igβ is essential for allelic exclusion.
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and Igβ signaling proteins to activate B cell responses in vitro and in vivo.
The earliest developmental checkpoint regulated by Ig
Igβ appears to involve either activation of cellular competence to complete VH to DJH rearrangements, or positive selection for cells that express mIgµ (19). In the next phase of the B cell pathway, the same transducers are necessary (Fig. 2) and sufficient to produce the signals that activate allelic exclusion and the pre-B cell transition (19, 20, 35, 41). In the present report, we show that in addition to these functions, Ig
-Igβ transducers are also necessary for negative selection against Dµ.
Two models have been proposed to explain counterselection against mDµ. The first model states that mDµ is toxic, and that cells expressing this protein are deleted by a mechanism that involves inhibition of proliferation (31). A second theory postulates that Dµ proteins produce the signal for heavy chain allelic exclusion and block the completion of productive heavy chain gene rearrangements (15). According to this second model, cells expressing mDµ are then unable to continue along the B cell pathway. Support for the active signaling model comes from three sets of observations: (a) that there is no counterselection in the absence of a Igµ transmembrane exon (15); (b) that there is no RF counterselection in the absence of syk (18); and (c) that there is no counterselection in early CD43+B220+ B cell precursors in the absence of
5 (33). These experiments partially define the receptor structure for counterselection as composed of mDµ associated with
5. Our observation that negative selection against Dµ does not occur in the absence of Igβ supports the signaling model, and identifies Ig
-Igβ as the transducers that activate counterselection possibly by linking mDµ to nonreceptor tyrosine kinases.
Why does the expression of the Dµ pre-BCR lead to arrested development, whereas mature mIgµ in the same complex activates positive selection in early B cells? Both signals are produced in CD43+B220+ pre-B cells, both require
5 (33, 39, 42), and the Ig
-Igβ coreceptors (19, 41), and both are transmitted through a cascade that induces syk (18, 43). One way to explain the difference between the cellular response to mDµ pre-BCR and mIgµ pre-BCR expression might be an inability of Dµ to pair with conventional
or
Ig light chains (14). According to this model, cells expressing mDµ should be trapped in the CD43–B220+ preB cell compartment since B cell development can progress to the CD43–B220+ stage in the absence of conventional light chains (44, 45). However, elegant single cell sorting experiments have shown that mDµ-producing cells are selected against before this stage in CD43+B220+ pre-B cells (33, 42). Thus, the idea that abnormal pairing of mDµ with light chains is responsible for counterselection fails to take into account the observation that counterselection normally occurs independently of light chain gene rearrangements.
Two alternative explanations for the disparate cellular responses to the Dµ pre-BCR and the mIgµ pre-BCR are: (a) that there are qualitative differences between signals generated by a mDµ and a mIgµ receptor complex, and (b) pre-B-I cells that contain DJH rearrangements are in a different stage of differentiation than pre-B-II cells that have completed VDJH and express mIgµ (8). An example of two qualitatively distinct signals resulting in alternative biologic responses has been found in the highly homologous TCR receptor (46, 47). TCR interaction with ligand can produce either anergy or activation, depending on the affinity of the TCR for the peptide-MHC complex (48). High affinity ligands that produce T cell responses fully activate CD3 tyrosine phosphorylation, whereas peptides that induce anergy bind with low affinity and induce a reduced level of CD3 phosphorylation. The low level CD3 phosphorylation induced by the anergizing peptides is associated with less than optimal ZAP-70 kinase activation (46, 47).
Less is known about the physiologic responses activated by Ig
-Igβ in developing B cells, but experiments in transgenic mice have shown that early B cell development requires tyrosine phosphorylation of Igβ (20), and by inference, receptor cross-linking. Although the cytoplasmic domains Ig
and Igβ appear to have redundant functions in allelic exclusion and the pre-B cell transition (20, 35), neither Ig
, (41) nor Igβ (Papavasiliou, N., and M.C. Nussenzweig, manuscript in preparation) alone are able to fully restore B cell development in the bone marrow, suggesting that there are specific functions for Ig
and Igβ, or the Ig
Igβ heterodimer. Biochemical support for the idea that individual coreceptors could have unique biologic functions also comes from transfection experiments in B cell lines (49–51) and from the observation that the cytoplasmic domains of Ig
and Igβ bind to different sets of nonreceptor tyrosine kinases (52).
We would like to propose that positive and negative selection in developing B cells, like activation and anergy in T cells, may be mediated by differential phosphorylation of Ig
and Igβ in the pre-BCR. Given the requirement for cross-linking in pre-BCR activation, the mechanism that produces the proposed differential phosphorylation of the mDµ and mIgµ pre-BCRs may be a function of their affinities for the cross-linker.
| Acknowledgments |
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Submitted: 13 September 1996
Revised: 15 October 1996
1Abbreviations used in this paper: BCR, B cell receptor; mIgµ, membrane immunoglobulin heavy chain; RF, reading frame.
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