Excessive amounts of mu heavy chain block B-cell development

Lingqiao Zhu; Cheong-Hee Chang; Wesley Dunnick

doi:10.1093/intimm/dxr049

. 2011 Jul 4;23(9):545–551. doi: 10.1093/intimm/dxr049

Excessive amounts of mu heavy chain block B-cell development

Lingqiao Zhu ¹, Cheong-Hee Chang ¹, Wesley Dunnick ^1,^✉

PMCID: PMC3157007 PMID: 21727177

Abstract

Antigen-independent B-cell development occurs in several stages that depend on the expression of Ig heavy and light chain. We identified a line of mice that lacked mature B cells in the spleen. This mouse line carried approximately 11 copies of a transgene of the murine heavy chain constant region locus, and B-lineage cells expressed excessive amounts of the intracellular μ heavy chain. B-cell development failed in the bone marrow at the pro/pre B-cell transition, and examination of other lines with various copy numbers of the same transgene suggested that deficiencies in B-cell development increased with increased transgene copy number. Expression of a transgenic (Tg) light chain along with the Tg μ heavy chain led to minimal rescue of B-cell development in the bone marrow and B cells in the spleen. There are several potential mechanisms for the death of pro/pre B cells as a consequence of excess heavy chain expression.

Keywords: antigen-independent development, transgenic mice

Introduction

Antigen-independent B-cell development occurs in several stages that depend on the expression of Ig heavy and light chain (1, 2). Stem cells become committed to the B-cell lineage, express B-lineage-specific surface markers such as B220 or CD19 and are termed pro B cells. Upon VDJ rearrangement, the cells become pre B cells and express intracellular μ heavy chains. Later, pre B cells begin to rearrange light chain genes. If the expressed light chain protein can combine with the heavy chain, the resulting IgM comes to the cell surface and becomes an immature B cell. These cells exit the bone marrow and become mature B cells, expressing cell surface IgD and increased amounts of B220 (1, 2).

We have studied a series of mice with a transgene of the entire heavy chain constant region locus. B-cell development in mice with one or two copies of the transgene is similar to that in non-transgenic (Tg) mice (3). However, we identified a line of mice, with 11 copies of the heavy chain constant region locus transgene, in which B-cell development failed. Pro B cells in this Tg line express excessive amounts of heavy chain protein and then disappear from the bone marrow before light chains or surface IgM can be expressed. We found that co-expression of a light chain transgene could rescue a small amount of B-cell development. We discuss potential mechanisms for the death of pro B cells in this line of Tg mice.

Methods

Tg mice

The ARS/Igh transgene is 230 kb long and includes a VDJH2 exon recombined into a bacterial artificial chromosome (BAC) at its normal location, all eight murine H chain C region genes and 42 kb 3′ of the Cα gene, including all known 3′ regulatory elements. The production, gene analysis and functional characterization of Tg mice used in this study have been described (3). Tg copy number was determined by amplification of the JH region as described in the Supplementary Materials of ref. (3) and digestion with DdeI and BamHI.

Flow cytometry

Splenocytes or bone marrow cells were stained with the following antibodies (all from BD Biosciences, San Diego, CA, USA): anti-B220 (PE), anti-CD19 (APC-Cy7), anti-CD3 (biotin), anti-CD43 (biotin), anti-CD24 (APC), anti-BP-1 (PE), anti-CD25 (APC), anti-c-kit (FITC), anti-κ (FITC), anti-μ (biotin) or anti-γ3 (biotin). Biotinylated antibodies were detected with streptavidin-PerCP or streptavidin-FITC. Data were collected using a FACS Canto and analyzed with WinMidi software.

B-cell development in OP9 culture

Bone marrow-derived stromal cell line OP9-GFP, provided by Juan Carlos Zúñiga-Pflücker (University of Toronto, Toronto, Canada), was maintained as described (4). The bone marrow OP9 co-culture was performed according to the protocol (5) with small modifications. Total bone marrow cells were harvested from the femurs and tibias of C57BL/6, 234 Hi and 234 mice. After red cell lysis, cells were incubated with Biotin-labeled B220, CD11b, Gr-1, CD3ϵ and TER-119 and then anti-Biotin microbeads (Miltenyi Biotec, Bergisch Gladbach, Germany) and applied to a MACS column to deplete the lineage-positive cells. Cells were then stained with FITC-conjugated anti-CD117 (2B8) and APC anti-Sca-1 (D7; all from BD Biosciences). CD117⁺ Sca-1^hi progenitor cells were sorted by FACSAria (BD Biosciences). The purity of the sorted cells was ≥98%. The purified progenitor cell (5 × 10⁴ cells per well) were seeded onto the monolayer of OP9-GFP cells in six-well plate in DMEM with 10% FBS, 10 mM HEPES, 1 mM Sodium pyruvate, 2 mM l-glutamine, 55 μM β-mercaptoethanol, penicillin, streptomycin and 50 μg ml⁻¹ gentamicin. Murine IL-7 (1 ng ml⁻¹; Peprotech, Rocky Hill, NJ, USA) and 5 ng ml⁻¹ human Flt3L (R&D Systems, Minneapolis, MN, USA) were also added. Every 4 or 5 days, the cells were diluted and transferred to fresh OP9-GFP monolayers. On the days indicated, cells were collected by vigorous pipetting, counted and analyzed by flow cytometry. RNA was prepared (6) from the non-adherent cells on day 12 of OP9-Tg bone marrow cultures. After reverse transcription with Superscript (Invitrogen, Carlsbad, CA, USA), HPRT transcripts were amplified as described (7). Xbp1-l and Xbp1-s transcripts were amplified with annealing at 60°C for 33 cycles, using primers 407S (5′-ACACGCTTGGGAATGGACAC-3′) and 551A (5′-CC ATGGGAAGATGTTCTGGG-3′) (8). RNA from ARS/Igh Tg splenic B cells (line 518, one copy, to be published elsewhere) cultured in CD40L + IFN-γ for 3 days, with 0.2 μg ml⁻¹tunicamycin during the last 36 h, was used as a positive control for the unfolded protein response. PCR products were visualized by PhosphorImager analysis after incorporation of ³²P-dATP during the amplification reaction.

Results

We have studied mice with a BAC-based transgene of the entire heavy chain constant region locus, called ‘ARS/Igh’ for the anti-arsonate (AR) activity encoded by its VDJ exon and IgH for the constant region locus. In the normal course of monitoring offspring of one ARS/Igh founder designated ‘234’, we tested the transgene copy number by amplification of the JH region and comparison of the intensity of the Tg band with that of the endogenous JH region (two copies). We found that two of three offspring of this founder had ∼11 copies of the ARS/Igh transgene and so we designated the resulting Tg line ‘234 Hi’ (Fig. 1, lane 11). However, the third offspring had only two copies of the transgene. We bred one of the high copy number offspring; six offspring in this N2 generation retained 11 copies of the transgene, but two N2 offspring had two copies of the transgene. Breeding of an N2 generation mouse with 11 transgene copies produced 10 offspring, all retaining 11 copies. We also established a Tg line from one of the two-copy N2 mice and called it 234 (Fig. 1, lane 4).

Fig. 1. — Transgene copy number. Transgenic and endogenous JH regions, from the indicated lines of Tg mice, were amplified as described (3). Digestion of the resulting PCR product with *Bam*H1 (absent in the transgene) and *Dde*1 yields a 103 bp fragment (transgene), 67 and 36 bp fragments (endogenous gene, the 36 bp fragment is run off this gel) and 128, 99 and 27 bp fragments from both genes. Transgene copy number was deduced after calculating the ratio of the 103 bp fragment density to the 67 bp fragment density. The ratio of density of the 103 bp fragment to the 67 bp fragment is 1.55 for line 820, which has a single copy of all gene segments in the transgene (3). Copy number of other Tg mice was calculated by defining a ratio of 1.55 as one copy.

Mice in the 234 line (two transgene copies) had normal numbers of splenic B cells (Fig. 2, panel 2), expressed their ARS/Igh transgene normally and undergo class switch recombination to all Tg H chain genes (3). However, we found that offspring from line 234 Hi (11 copies) had essentially no splenic B cells (Fig. 2A, panel 3). We investigated if the deficiency in B cells in line 234 Hi could be traced to B-cell development in the bone marrow. Line 234 Hi mice produce ∼50% the number of pro B cells found in C57BL/6 mice (Fig. 2B, panel 7, gate ‘P’, B220 intermediate and CD43 intermediate), but these pro B cells fail to progress to immature B cells (gate ‘I’, B220 intermediate and CD43 low) or mature B cells (gate ‘M’, B220 high and CD43 low). Like non-Tg C57BL/6, line 234 mice produce pro B cells, immature B cells and mature B cells (Fig. 2B, panels 5 and 6). There are modest changes in the portion of cells at various stages of B-cell development in line 234 compared with C57BL/6 mice, which are similar to changes imposed by the expression of other heavy chain transgenes (9). Consistent with termination of B-cell differentiation in line 234 Hi near the pro/pre B-cell transition (1, 2), the majority of gate P cells in line 234 Hi bone marrow are BP-1 negative or low (Fig. 2C, panel 12). Almost all the gate P cells in line 234 Hi also express an intermediate level of CD24 (Fig. 2C, panel 18). This is in contrast to the majority of cells in gate P in C57BL/6 or line 234 mice, which demonstrate the characteristic low CD24 expression (10) of a later stage of pro B-cell development (called ‘fraction C’). A small portion of the gate P cells in C57BL/6 and in both Tg lines express the high CD24 of the last stage of pro B-cell development (called ‘fraction C′’) (Fig. 2C, panel 18). Rag-deficient or SCID B-lineage cells stop development at fraction C, while expression of a μ transgene in Rag-deficient cells allows development through fraction C′ to pre B cells (9). The intermediate level of CD24 is consistent with the interpretation that B-cell development in line 234 Hi mice is arrested during the transition from pro B cell fraction C to C′. However, the expression of significant amounts of CD25 by line 234 Hi B220+ cells (Fig. 2B, panel 11) is more characteristic of fraction C′ pro B cells or pre B cells (1); perhaps, B-lineage cells in line 234 Hi go to an unusual stage of differentiation not found in B-cell development in wild-type mice.

During normal B-cell differentiation, pre B cells begin to produce intracellular μ after they express B220 (or CD19) (refs. 1, 2, and Fig. 2B, panel 13). We were surprised to find that lines 234 and 234 Hi bone marrow cells express intracellular μ before they express B220 (panels 14 and 15). In line 234 bone marrow, the amount of intracellular μ is about the same as non-Tg C57BL/6 mice (Fig. 2B, panels 13 and 14). However, some line 234 Hi bone marrow cells (panel 15) express four times more intracellular μ (by mean fluorescence intensity) than C57BL/6 B220+ cells. The intracellular μ+/B220+ cells have progressed beyond the earliest B lineage stages as they are negative for c-kit on the cell surface (Fig. 2B, panel 17 and not shown).

Thus, there is a relationship among transgene copy number, amount of intracellular μ expression by bone marrow cells and ability to develop mature B cells that populate the spleen. Several years ago, we had identified another ARS/Igh Tg line (called ‘688’) that had only 2.8% splenic B cells. (Analysis of two other mice in line 688 yielded profiles consistent with few or no splenic B cells, but we lost the quantitative information from these analyses.) We lost line 688 before we had the capability to complete the B-cell differentiation analysis. However, analysis of transgene content from DNA revealed that line 688 had 10 copies of the ARS/Igh transgene (Fig. 1, lane 12). We also confirmed the copy number of several other Tg lines used in this study (Fig. 1).

To further investigate how increased heavy chain transgene copy number might alter antigen-independent B-cell development, we bred two lines with four copies of the transgene to have copies of the transgene on both chromosomes (‘Tg/Tg’—Fig. 1, lanes 8 and 9), so that B cells could potentially express eight copies of the transgene. The resulting Tg mice recapitulate some aspects of bone marrow development of line 234 Hi. Both lines 556 and 347 with homozygous transgenes have gate P pro B cells but reduced numbers of gate ‘M’ mature B cells in the bone marrow (gate shown in Fig. 3A, upper row of panels). Both lines with increased transgene copy number express large amounts of internal μ compared with line 820 with a single copy of the same transgene (Fig. 3A, lower row of panels). In addition, like 234 Hi bone marrow cells, the internal μ is expressed before the cells become B220 positive. Similar to line 234 Hi bone marrow cells, the Tg/Tg pro B cells express intermediate levels of CD24 (Fig. 3B). Unlike line 234 Hi, the small numbers of mature B cells produced in the bone marrow are sufficient to populate the spleen (for example, line 556 Tg/Tg Fig. 3C). Thus, most of the effects on antigen-independent development are not dependent on the insertion site; multiple Tg lines show the same effects. We investigated several Tg lines, with copy numbers of 2–11 (Fig. 1), and found an inverse correlation between transgene copy number and percent mature B cells (B220 high/CD43 low) in the bone marrow (Fig. 3D).

Fig. 3. — B-cell development in mice with eight transgenic copies. (A) Development and internal μ expression of bone marrow cells. The mature B-cell gate, with percentages of cells in the lymphocyte gate, is shown. (B) CD24 expression of B220+/CD43 intermediate (gate P) bone marrow cells. (C) Splenic B lymphocytes from line 556 Tg/Tg mice. (D) Relationship between transgene copy number and mature B cells in the bone marrow. Data from lines 336, 231 and 995 (copy number shown in Fig. 1) contribute to this graph.

Bone marrow stromal cell lines support the differentiation of stem cells to almost exclusively pro B cells (11). Many of the resulting pro B cells divide continuously in the mixed lymphoid:stromal cell culture. We used growth on the bone marrow stromal cell line OP9 to determine if the effect on development in line 234 Hi was B cell intrinsic. Beginning with C57BL/6 stem cells, the vast majority of stem cells become B220 positive, but only a small number progress to the pre B cell stage and express intracellular μ (Fig. 4A, top two panels). Line 234 cells, with two copies of the ARS/Igh transgene, all express intracellular μ, some even before they become B220 positive (Fig. 4A, middle two panels). Similar to development in bone marrow, line 234 Hi cells express intracellular μ, in greater amounts than line 234. Furthermore, the percentage of B220+ cells is much lower than either C57BL/6 or line 234 (Fig. 4A, bottom two panels). Consistent with a block in B-cell development, the increase in numbers of line 234 Hi bone marrow cells slows compared with C57BL/6 or line 234 after 10 days in culture (Fig. 4B). Similar to bone marrow development, B220+ 234 Hi cells seem to be at a different stage of development as these cells express a slightly higher level of CD24 than do C57BL/6 or 234 cells (Fig. 4C). Therefore, antigen-independent development of line 234 Hi in vitro is much like B-cell differentiation in vivo. This suggests that the defect in development is B-cell intrinsic.

Fig. 4. — B-cell development in tissue culture with OP9 cells. (A) Lymphoid cells from OP9 cultures (day 15) were analyzed for expression of B220, CD43 and internal μ. The bars in the left panels indicate the cut-off used for B220+ and B220− cells, with percentages of each above and below the bar. To summarize all experiments, the mean (±SD) of percent B220+ cells: C57BL/6, 74.5 ± 16 (n = 2); line 234, 68 ± 7 (n = 3) and line 234 Hi, 32 ± 17 (n =3). (B) Growth of bone marrow cells in OP9 cultures. C57BL/6, diamonds; line 234, squares and dotted line and line 234 Hi, triangles. (C) B220+ lymphoid cells were tested for CD24 expression. (D) The Xbp1-l and Xbp1-s versions of the Xbp transcripts (the latter is induced during the unfolded protein response) were amplified from cDNA from OP9 cultures (234 and 234 Hi) or from mature B cells cultured in CD40L + IFN-γ + tunicamycin (518 + Tun). The same cDNA samples were tested at two concentrations for HPRT transcripts.

We considered the possibility that the excessive amount of intracellular μ heavy chains in line 234 Hi led to an unfolded protein response in the endoplasmic reticulum (ER) and then cell death of the developing B-lineage cells. One of the first events in the mammalian unfolded protein response is the splicing of a 26-bp intron, converting Xbp1-l to Xbp1-s, which in turn allows Xbp1-s to act as a transcriptional activator of unfolded protein response genes (8, 12). We reasoned that if the unfolded protein response were active in line 234 Hi cells, then we should observe a large portion of Xbp1-s in the RNA from the 234 Hi cells after 12 days of culture with OP9 cells. We found that only 4 or 5% of the Xbp1 transcripts were spliced to the ‘short’ version in both line 234 and 234 Hi cells, even though 65% of the Xbp1 transcripts were spliced to the short version in control (518 + Tun) cells in which the unfolded protein response had been induced by tunicamycin (Fig. 4D).

During B-cell development of line 234 Hi cells, if a co-expressed light chain could combine with the μ heavy chain, it might then form an IgM molecule that could be transported to the cell surface or secreted in small amounts. This might relieve a putative unfolded protein response in the ER enough so that some B cells would complete development to the mature B cell stage and populate the spleen. We tested if light chain expression would rescue the defect in B-cell development by breeding mice with both the 234 Hi heavy chain transgene and an anti-AR light chain (Vκ10) transgene (13, 14). Line 234 (two copies of the transgene) demonstrates abundant B cells both in the bone marrow and in the spleen. Almost all splenic B cells express the Tg allotype only since allelic exclusion by the ARS/Igh transgene is strong (Fig. 5A and B, top panels). B-cell development in the bone marrow in 234 Hi/Vκ10 doubly Tg mice is similar to that in 234 Hi Tg mice, with a deficiency of B220 positive, CD43 low immature and mature B cells (Fig. 5A, left set of panels). The light chain can be detected in a subset of bone marrow cells (Fig. 5A, right set of panels, compare 234 Hi/Vκ10 to 234 Hi). However, the μκ combination does not promote efficient B-cell development, as <1% of splenocytes express IgM with the Tg allotype (Fig. 5B). We repeated this experiment four times, with poor, but statistically significant, B-cell development in every case (Fig. 5C).

Fig. 5. — Effect on B-cell development by expression of a Tg L chain. Transgene composition is shown to the left of the panels. (A) Expression of B220, CD43, surface μ and intracellular κ light chain by bone marrow cells. Percentages of gated bone marrow cells (as defined in panel 8 of Fig. 2B) that are surface μ positive are shown in the upper right corner of the panels. (B) Splenic B cells. Percentages of gated splenocytes (as defined in panel 4 of Fig. 2A) that are IgM^a (transgenic) positive are shown in the upper right corner of the panels. (C) Summary of four experiments testing rescue of splenic B cells by co-expression of a κ light chain. Data are expressed as the percentage of splenocytes that are B220 and IgM positive, but any cells expressing the endogenous (IgM^b) in Tg samples were excluded. Diamonds, 234 Hi splenocytes; triangles, 234 Hi/Vκ10 splenocytes; circles, C57BL/6 splenocytes and squares, 234 splenocytes. The mean of the 234 Hi/Vκ10 samples, 0.59% ± 0.21 (SD, n = 4) is different than the mean of the 234 Hi samples, 0.18% ± 0.15, P = 0.011, paired T-test.

Discussion

We found that mice from a line with a high copy number of a heavy chain transgene failed to produce splenic B cells (Fig. 2A). This failure to produce B cells had four informative features. First, B-cell development failed at the stage where heavy chains begin to be expressed in normal B-cell development (Fig. 2B). Second, the failure in B-cell development was B-cell intrinsic (Fig. 4). Third, similar problems in B-cell development were found in several lines with four or more copies of the same heavy chain transgene (Fig. 3). B-cell development was most severely impaired in mice with 10 or 11 copies of the heavy chain transgene. Fourth, impairment of B-cell development was associated with expression of excessive amounts of intracellular μ chain (Figs. 2–4).

Larger transgene copy numbers led to greater intracellular μ expression and greater impairment of development in the bone marrow (Figs. 2 and 3). However, the ability to populate the spleen with B cells seemed to have a more abrupt transition; only mice with ≥10 Tg copies had significant deficits in splenic B cells, while those with eight copies had reasonable numbers of splenic B cells, although perhaps somewhat reduced in some mice. This difference may arise from two non-exclusive explanations. One explanation is that transgene copy numbers of 4–8 may allow production (albeit inefficient) of mature B cells. The phenotype of normal or reduced numbers of splenic B cells, but inefficient B-cell development in the bone marrow has been observed in deficiencies of surrogate light chain or cytokine receptors (15, 16). Within mice with four to eight copies of the heavy chain transgene, perhaps, it is those pro/pre B cells with the lowest amount of intracellular μ expression that can complete B-cell maturation. All of the pro B cells in mice with 10 or 11 copies of the heavy chain transgene express so much intracellular μ chain that none of them can complete development. This explanation suggests that there is an abrupt threshold level for intracellular μ that prevents B -cell development. The relative amounts of intracellular μ expressed by various Tg mice (Figs. 2–5) are consistent with this explanation. Another explanation is that the transgene insertion site in line 234 Hi disrupts some function important in B-cell development. Several Tg lines, each with an independent insertion site, show defects in B-cell development, at the same stage in the bone marrow. These observations are not consistent with this explanation. Furthermore, that the development is impaired at point where cells in the B-cell lineage express the highest amounts of heavy chains supports the first explanation more than the second. However, it is difficult to rule out a model wherein amount of intracellular heavy chain expression is important but that the insertion site in line 234 Hi also contributes to the lack of B cells in the spleen. The insertion site contribution may be to cause earlier or greater μ expression.

The association of pro B cell death with excessive expression of heavy chains in line 234 Hi has some parallels with heavy chain expression in cells at the other end of B-cell differentiation. Plasma cells, one product of antigen-driven B-cell differentiation, express large amounts of Ig, as much as 100 times more per cell than a mature B cell (17, 18). To express such large amounts of secreted protein, plasma cells have developed an extensive ER. Murine plasmacytoma (also called myeloma), the transformed counterpart of plasma cells, also secretes large amounts of Ig. While plasmacytoma cells expressing only light chains can be isolated, it has not been possible to isolate plasmacytomas expressing only full-length heavy chains. Presumably, heavy chains not paired with light chains cannot be secreted and thus lead to cell death via an unfolded protein response (19).

The inverse correlation between the amount of intracellular μ protein and the number of mature B cells suggests that excess μ protein is the cause of cell death and therefore the halt in B-cell development. One mechanism consistent with all observations is that pro B cells in line 234 Hi die of an unfolded protein response, due to excess heavy chain expression that cannot pair with a light chain and move to the cell surface or be secreted. However, two lines of experimentation failed to produce evidence in favor of this mechanism. The development of splenic B cells in mice expressing both the 234 Hi heavy chain and the Vκ10 light chain was very inefficient (Fig. 5B and C), not consistent with a rescue of an unfolded protein response by μκ pairing. The light chain rescue could fail due to technical problems that have little to do with the mechanism of the block in B-cell development in line 234 Hi. For example, the light chain transgene could be expressed too late or in insufficient quantities to rescue B-cell development. Alternatively, the light chain:heavy chain combination may result in an anti-self specificity. The same anti-ARS VH gene (albeit with a few somatic mutations) paired with the Vκ10 light chain is subject to the induction of anti-self tolerance mechanisms (13, 14), and increased VH gene expression leads to increased tolerance induction (20). In a second line of experimentation, Xbp1-s, a hallmark of the unfolded protein response, was not induced in line 234 Hi cells as they underwent B-cell development in an OP9 culture and expressed large amounts of intracellular μ protein (Figs. 4A and D). It is possible that only a small portion of the line 234 Hi cells in OP9 cultures are actually undergoing an unfolded protein response. However, there is not even a hint of induction of Xbp1-s compared with line 234 cells. In the unfolded protein response, the induction of Xbp1-s is robust (8, 12), and we believe that we would have observed this induction if only 10% of the cells in culture participated in the unfolded protein response. Although we cannot rule out the unfolded protein response in the termination of B-cell differentiation in line 234 Hi, direct experiments do not support this idea. Alternative mechanisms for B lineage death in line 234 Hi include (i) a failure to induce the unfolded protein response in the pro B cells, leading to death due to large amounts of unfolded heavy chain protein in the ER, (ii) anti-self reactivity exacerbated by the large amounts of heavy chain expressed, (iii) some other sort of aberrant signaling (perhaps even intracellular) induced by the excess heavy chain and (iv) disruption of a critical gene in B-cell development by insertion of the 234 Hi transgene. Each of these putative mechanisms could be tested by direct (and technically challenging) experiments but are already inconsistent with some data presented herein.

Funding

National Institute of Allergy and Infectious Diseases of the National Institutes of Health (AI076057). Core support for the production of Tg mice was provided by a grant from the National Institutes of Health to the University of Michigan Cancer Center (CA46592).

Acknowledgments

We thank Richard Hardy (Fox-Chase Cancer Institute, Philadelphia) for helpful comments on B-cell development, Tim Manser (Thomas Jefferson University, Philadelphia) for the gift of Vκ10 Tg mice and insight on their biology, Laurie Glimcher and Ann-Hwee Lee (Harvard School of Public Health) and Malini Raghavan and Peter Arvan (University of Michigan) for advice on the unfolded protein response. We thank Jian Shi for expert technical assistance, and we acknowledge Wanda Filipiak, Galina Gavrilina and Maggie Van Keuren for preparation of Tg mice and the Transgenic Animal Model Core of the University of Michigan’s Biomedical Research Core Facilities.

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PERMALINK

Excessive amounts of mu heavy chain block B-cell development

Lingqiao Zhu

Cheong-Hee Chang

Wesley Dunnick

Abstract

Introduction