Influence of pre-B cell receptor deficiency on the immunoglobulin repertoires in peripheral blood B cells before and after immunization

During B cell development, pre-B cell receptor (pre-BCR), comprising the immunoglobulin heavy chain (HC) and surrogate light chain (SLC), plays a crucial role. The expression of pre-BCR serves as a certification of HC quality, confirming its ability to associate with the SLC and light chain (LC). In mice lacking SLC, the absence of this quality control mechanism leads to a distorted repertoire of HCs in the spleen and bone marrow. In this study, we conducted a comparative analysis of the immunoglobulin gene repertoire in peripheral blood cells of both wild-type mice and pre-BCR-deficient mice. Our findings reveal differences not only in the μ HC repertoire but also in the α HC and κ LC repertoires of the pre-BCR-deficient mice. These results suggest that the pre-BCR-mediated quality check of HC influences the selection of class-switched HC and LC repertoires. To further explore the impact of pre-BCR deficiency, we immunized these mice with thymus-dependent antigens and compared the antigen-responding repertoires. Our observations indicate that the affinity maturation pathways remain consistent between wild-type mice and pre-BCR-deficient mice, albeit with variations in the degree of maturation.

Association of IGHM polymorphisms with susceptibility to type 1 diabetes

Differentiation of B lymphocytes is accompanied by a regulated switch in the expression pattern and stability of surface and secretory immunoglobulins (Igs). Several lines of evidence show that autoimmune responses evolving in much autoimmune pathologies were associated with a high level of humoral Ig, but their pathogenic role remains elusive. The aim of this study was to test the hypothesis that variants at the immunoglobulin heavy-chain IGH locus are genetic determinants to T1D susceptibility. Here, we tested the genetic association of the variants of the immunoglobulin heavy-chain IGH locus as a genetic determinant to T1D susceptibility. A total of 255 subjects from 59 Tunisian families were genotyped for 15 SNPs mapping in 4 regions in IGH locus. We found that rs1950942, rs2180790, rs1808152, and rs1956596 of IGHM and rs2516751 variant located in the IGHA1/IGHG2 region were significantly associated with a risk for T1D p = 7E-3; p = 0.03; p = 0.02; p = 0.043; and p = 3.65E-5, respectively. The TATGG haplotype derived from LD across three SNPs from IGHM gene and two SNPs from IGHD gene was significantly over-transmitted from parents to affect offspring. Our results suggest that genetic variants at the IGH locus are associated with T1D susceptibility. These variations may predispose to IgG AutoAbs production against pancreatic antigens and AutoAbs multi-reactivity, leading to T1D development.

Screening differentially expressed genes between endometriosis and ovarian cancer to find new biomarkers for endometriosis

AIM

Endometriosis is one of the most common reproductive system diseases, but the mechanisms of disease progression are still unclear. Due to its high recurrence rate, searching for potential therapeutic biomarkers involved in the pathogenesis of endometriosis is an urgent issue.

METHODS

Due to the similarities between endometriosis and ovarian cancer, four endometriosis datasets and one ovarian cancer dataset were downloaded from Gene Expression Omnibus (GEO) database. Differentially expressed genes (DEGs) were identified, followed by gene ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway and protein-protein interaction (PPI) analyses. Then, we validated gene expression and performed survival analysis with ovarian serous cystadenocarcinoma (OV) datasets in TCGA/GTEx database, and searched for potential drugs in the Drug-Gene Interaction Database. Finally, we explored the miRNAs of key genes to find biomarkers associated with the recurrence of endometriosis.

RESULTS

In total, 104 DEGs were identified in the endometriosis datasets, and the main enriched GO functions included cell adhesion, extracellular exosome and actin binding. Fifty DEGs were identified between endometriosis and ovarian cancer datasets including 11 consistently regulated genes, and nine DEGs with significant expression in TCGA/GTEx. Only IGHM had both significant expression and an association with survival, three module DEGs and two significantly expressed DEGs had drug associations, and 10 DEGs had druggability.

CONCLUSIONS

ITGA7, ITGBL1 and SORBS1 may help us understand the invasive nature of endometriosis, and IGHM might be related to recurrence; moreover, these genes all may be potential therapeutic targets.KEY MESSAGEThis manuscript used a bioinformatics approach to find target genes for the treatment of endometriosis.This manuscript used a new approach to find target genes by drawing on common characteristics between ovarian cancer and endometriosis.We screened relevant therapeutic agents for target genes in the drug database, and performed histological validation of target genes with both expression and survival analysis difference in cancer databases.

Expansion and differentiation of IgM+ B cells in the rainbow trout peritoneal cavity in response to different antigens

To date, intraperitoneal (i.p.) injection seems to be the most effective vaccination route in aquaculture, as many i.p. administered fish vaccines are capable of conferring strong and long-lasting immune responses. Despite this, how peritoneal leukocytes are regulated upon antigen encounter has only been scarcely studied in fish. Although, in the past, myeloid cells were thought to be the main responders to peritoneal inflammation, a recent study revealed that IgM⁺ B cells are one of the main cell types in the teleost peritoneal cavity in response to pathogenic bacteria. Thus, in the current work, we have focused on establishing how IgM⁺ B cells are recruited into the peritoneum in rainbow trout (Oncorhynchus mykiss) comparing different antigens: Escherichia coli as a bacterial model, E. coli-derived lipopolysaccharide (LPS) or viral hemorrhagic septicemia virus (VHSV). In addition to studying their capacity to dominate the peritoneal cavity, we have established how these IgM⁺ B cells are regulated in response to the different antigens, determining their levels of IgM secretion, surface MHC II expression, cell size and phagocytic abilities. Our results reveal that IgM⁺ B cells are one of the main cell types amplified in the peritoneum in response to either bacterial or viral antigens and that these immunogenic stimulations provoke a differentiation of some of these cells towards plasmablasts/plasma cells whereas others seem to be implicated in antigen presentation. These findings contribute to a better understanding of the immune processes that regulate peritoneal inflammation in teleost fish.

Antigen presentation by B cells guides programing of memory CD4+ T-cell responses to a TLR4-agonist containing vaccine in mice

The contribution of B cells to immunity against many infectious diseases is unquestionably important and well characterized. Here, we sought to determine the role of B cells in the induction of T-helper 1 (T_H 1) CD4⁺ T cells upon vaccination with a tuberculosis (TB) antigen combined with a TLR4 agonist. We used B-cell deficient mice (μMT^-/- ), tetramer-positive CD4⁺ T cells, markers of memory "precursor" effector cells (MPECs), and T-cell adoptive transfers and demonstrated that the early antigen-specific cytokine-producing T_H 1 responses are unaffected in the absence of B cells, however MPEC induction is strongly impaired resulting in a deficiency of the memory T_H 1 response in μMT^-/- mice. We further show that antigen-presentation by B cells is necessary for their role in MPEC generation using B-cell adoptive transfers from wt or MHC class II knock-out mice into μMT^-/- mice. Our study challenges the view that B-cell deficiency exclusively alters the T_H 1 response at memory time-points. Collectively, our results provide new insights on the multifaceted roles of B cells that will have a high impact on vaccine development against several pathogens including those requiring T_H 1 cell-mediated immunity.

The Expression Pattern of the Pre-B Cell Receptor Components Correlates with Cellular Stage and Clinical Outcome in Acute Lymphoblastic Leukemia

Precursor-B cell receptor (pre-BCR) signaling represents a crucial checkpoint at the pre-B cell stage. Aberrant pre-BCR signaling is considered as a key factor for B-cell precursor acute lymphoblastic leukemia (BCP-ALL) development. BCP-ALL are believed to be arrested at the pre-BCR checkpoint independent of pre-BCR expression. However, the cellular stage at which BCP-ALL are arrested and whether this relates to expression of the pre-BCR components (IGHM, IGLL1 and VPREB1) is still unclear. Here, we show differential protein expression and copy number variation (CNV) patterns of the pre-BCR components in pediatric BCP-ALL. Moreover, analyzing six BCP-ALL data sets (n = 733), we demonstrate that TCF3-PBX1 ALL express high levels of IGHM, IGLL1 and VPREB1, and are arrested at the pre-B stage. By contrast, ETV6-RUNX1 ALL express low levels of IGHM or VPREB1, and are arrested at the pro-B stage. Irrespective of subtype, ALL with high levels of IGHM, IGLL1 and VPREB1 are arrested at the pre-B stage and correlate with good prognosis in high-risk pediatric BCP-ALL (n = 207). Our findings suggest that BCP-ALL are arrested at different cellular stages, which relates to the expression pattern of the pre-BCR components that could serve as prognostic markers for high-risk pediatric BCP-ALL patients.

Mechanisms of PD-L1/PD-1-mediated CD8 T-cell dysfunction in the context of aging-related immune defects in the Eµ-TCL1 CLL mouse model

T-cell defects, immune suppression, and poor antitumor immune responses are hallmarks of chronic lymphocytic leukemia (CLL), and PD-1/PD-L1 inhibitory signaling has emerged as a major immunosuppressive mechanism. However, the effect of different microenvironments and the confounding influence of aging are poorly understood. The current study uses the Eμ-TCL1 mouse model, which replicates human T-cell defects, as a preclinical platform to longitudinally examine patterns of T-cell dysfunction alongside developing CLL and in different microenvironments, with a focus on PD-1/PD-L1 interactions. The development of CLL was significantly associated with changes in T-cell phenotype across all organs and function. Although partly mirrored in aging wild-type mice, CLL-specific T-cell changes were identified. Murine CLL cells highly expressed PD-L1 and PD-L2 in all organs, with high PD-L1 expression in the spleen. CD3(+)CD8(+) T cells from leukemic and aging healthy mice highly expressed PD-1, identifying aging as a confounder, but adoptive transfer experiments demonstrated CLL-specific PD-1 induction. Direct comparisons of PD-1 expression and function between aging CLL mice and controls identified PD-1(+) T cells in CLL as a heterogeneous population with variable effector function. This is highly relevant for therapeutic targeting of CD8(+) T cells, showing the potential of reprogramming and selective subset expansion to restore antitumor immunity.

B cell signatures of BCWD-resistant and susceptible lines of rainbow trout: a shift towards more EBF-expressing progenitors and fewer mature B cells in resistant animals

Bacterial cold water disease (BCWD) is a chronic disease of rainbow trout, and is caused by the Gram-negative bacterium Flavobacterium psychrophilum (Fp), a common aquaculture pathogen. The National Center for Cool and Cold Water Aquaculture has bred two genetic lines of rainbow trout: a line of Fp-resistant trout (ARS-Fp-R or R-line trout) and a line of susceptible trout (ARS-Fp-S, or S-line). Little is known about how phenotypic selection alters immune response parameters or how such changes relate to genetic disease resistance. Herein, we quantify interindividual variation in the distribution and abundance of B cell populations (B cell signatures) and examine differences between genetic lines of naive animals. There are limited trout-specific cell surface markers currently available to resolve B cell subpopulations and thus we developed an alternative approach based on detection of differentially expressed transcription factors and intracellular cytokines. B cell signatures were compared between R-line and S-line trout by flow cytometry using antibodies against transcription factors early B cell factor-1 (EBF1) and paired domain box protein Pax5, the pro-inflammatory cytokine IL-1β, and the immunoglobulin heavy chain mu. R-line trout had higher percentages of EBF(+) B myeloid/ progenitor and pre-B cells in PBL, anterior and posterior kidney tissues compared to S-line trout. The opposite pattern was detected in more mature B cell populations: R-line trout had lower percentages of both IgM(+) mature B cells and IgM-secreting cells in anterior kidney and PBL compared to S-line trout. In vitro LPS-activation studies of PBL and spleen cell cultures revealed no significant induction differences between R-line and S-line trout. Together, our findings suggest that selective resistance to BCWD may be associated with shifts in naive animal developmental lineage commitment that result in decreased B lymphopoiesis and increased myelopoiesis in BCWD resistant trout relative to susceptible trout.

Triple immunoglobulin gene knockout transchromosomic cattle: bovine lambda cluster deletion and its effect on fully human polyclonal antibody production

Towards the goal of producing fully human polyclonal antibodies (hpAbs or hIgGs) in transchromosomic (Tc) cattle, we previously reported that Tc cattle carrying a human artificial chromosome (HAC) comprising the entire unrearranged human immunoglobulin (Ig) heavy-chain (hIGH), kappa-chain (hIGK), and lambda-chain (hIGL) germline loci produced physiological levels of hIgGs when both of the bovine immunoglobulin mu heavy-chains, bIGHM and bIGHML1, were homozygously inactivated (bIGHM^−/−, bIGHML1^−/−; double knockouts or DKO). However, because endogenous bovine immunoglobulin light chain loci are still intact, the light chains are produced both from the hIGK and hIGL genomic loci on the HAC and from the endogenous bovine kappa-chain (bIGK) and lambda-chain (bIGL) genomic loci, resulting in the production of fully hIgGs (both Ig heavy-chains and light-chains are of human origin: hIgG/hIgκ or hIgG/hIgλ) and chimeric hIgGs (Ig heavy-chains are of human origin while the Ig light-chains are of bovine origin: hIgG/bIgκ or hIgG/bIgλ). To improve fully hIgG production in Tc cattle, we here report the deletion of the entire bIGL joining (J) and constant (C) gene cluster (bIGLJ1-IGLC1 to bIGLJ5-IGLC5) by employing Cre/loxP mediated site-specific chromosome recombination and the production of triple knockout (bIGHM^−/−, bIGHML1^−/− and bIGL^−/−; TKO) Tc cattle. We further demonstrate that bIGL cluster deletion greatly improves fully hIgGs production in the sera of TKO Tc cattle, with 51.3% fully hIgGs (hIgG/hIgκ plus hIgG/hIgλ).

Physiological level production of antigen-specific human immunoglobulin in cloned transchromosomic cattle

Therapeutic human polyclonal antibodies (hpAbs) derived from pooled plasma from human donors are Food and Drug Administration approved biologics used in the treatment of a variety of human diseases. Powered by the natural diversity of immune response, hpAbs are effective in treating diseases caused by complex or quickly-evolving antigens such as viruses. We previously showed that transchromosomic (Tc) cattle carrying a human artificial chromosome (HAC) comprising the entire unrearranged human immunoglobulin heavy-chain (hIGH) and kappa-chain (hIGK) germline loci (named as κHAC) are capable of producing functional hpAbs when both of the bovine immunoglobulin mu heavy-chains, bIGHM and bIGHML1, are homozygously inactivated (double knockouts or DKO). However, B lymphocyte development in these Tc cattle is compromised, and the overall production of hpAbs is low. Here, we report the construction of an improved HAC, designated as cKSL-HACΔ, by incorporating all of the human immunoglobulin germline loci into the HAC. Furthermore, for avoiding the possible human-bovine interspecies incompatibility between the human immunoglobulin mu chain protein (hIgM) and bovine transmembrane α and β immunoglobulins (bIgα and bIgβ) in the pre-B cell receptor (pre-BCR) complex, we partially replaced (bovinized) the hIgM constant domain with the counterpart of bovine IgM (bIgM) that is involved in the interaction between bIgM and bIgα/Igβ; human IgM bovinization would also improve the functionality of hIgM in supporting B cell activation and proliferation. We also report the successful production of DKO Tc cattle carrying the cKSL-HACΔ (cKSL-HACΔ/DKO), the dramatic improvement of B cell development in these cattle and the high level production of hpAbs (as measured for the human IgG isotype) in the plasma. We further demonstrate that, upon immunization by tumor immunogens, high titer tumor immunogen-specific human IgG (hIgG) can be produced from such Tc cattle.

IL-10-producing B-cells limit CNS inflammation and infarct volume in experimental stroke

Clinical stroke induces inflammatory processes leading to cerebral injury. IL-10 expression is elevated during major CNS diseases and limits inflammation in the brain. Recent evidence demonstrated that absence of B-cells led to larger infarct volumes and increased numbers of activated T-cells, monocytes and microglial cells in the brain, thus implicating a regulatory role of B-cell subpopulations in limiting CNS damage from stroke. The aim of this study was to determine whether the IL-10-producing regulatory B-cell subset can limit CNS inflammation and reduce infarct volume following ischemic stroke in B-cell deficient (μMT(-/-)) mice. Five million IL-10-producing B-cells were obtained from IL-10-GFP reporter mice and transferred i.v. to μMT(-/-)mice. After 24 h following this transfer, recipients were subjected to 60 min of middle cerebral artery occlusion (MCAO) followed by 48 h of reperfusion. Compared to vehicle-treated controls, the IL-10(+) B-cell-replenished μMT(-/-)mice had reduced infarct volume and fewer infiltrating activated T-cells and monocytes in the affected brain hemisphere. These effects in CNS were accompanied by significant increases in regulatory T-cells and expression of the co-inhibitory receptor, PD-1, with a significant reduction in the proinflammatory milieu in the periphery. These novel observations provide the first proof of both immunoregulatory and protective functions of IL-10-secreting B-cells in MCAO that potentially could impart significant benefit for stroke patients in the clinic.

Comparative analysis of IgM sub-variants in salmonid fish and identification of a residue in μ3 which is essential for MAb4C10 reactivity

In rainbow trout (Oncorhynchus mykiss) it has been shown that high affinity IgM antibodies have a higher degree of disulfide polymerization and a longer half life time. In the present study, distinct IgM sub-variants related to ancestral tetraploidy in salmonid fish were analyzed to reveal possible characteristic differences between these. A monoclonal antibody (MAb4C10) which distinguishes between IgM-A and IgM-B in Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) was further characterized. It was shown that substitution of a proline located in the loop between the B and C beta strands of the third constant domain (μ3) of salmon μA eliminated MAb4C10 reactivity. Accordingly, the reverse substitution in salmon μB restored MAb4C10 reactivity. Molecular cloning of μ cDNA from arctic char (Salvelinus alpinus) revealed two sub-variants (μA-1 and μA-2), i.e. a similar situation as in Atlantic salmon and brown trout. However, arctic char IgM eluted in one peak by anion exchange chromatography, in contrast to salmon and brown trout IgM that are eluted in two peaks. The only characteristic residue of salmon and brown trout μB is an additional cysteine in the C-terminal part of μ4. Most likely, this cysteine is involved in inter-chain disulfide bonding and influences the elution profiles of IgM-A and IgM-B on anion exchange chromatography. Neither of the μ sub-variants in arctic char have the additional cysteine, and char IgM, as well as salmon and brown trout IgM-A, showed a lower degree of inter-chain disulfide bonding than IgM-B when subjected to denaturation and gel electrophoresis under non-reducing conditions. Hybrids of char/salmon expressed μA-1, μA-2, μA and μB, indicating that there are two paralogous Ig heavy chain gene complexes in the haploid genome of char, like in Atlantic salmon. A comparison of salmonid μ sequences is presented, including representatives of Salmoninae (trout, salmon and char), Thymallinae (grayling) and Coregoninae (whitefish).

Gammaherpesvirus latency induces antibody-associated thrombocytopenia in mice

Human herpesviruses establish lifelong latency. Viral recrudescence can lead to the development of cancers, immunoproliferative disorders, transplantation complications, and thrombocytopenia. Although platelet-specific autoantibodies have been reported in patients infected with the Epstein-Barr virus (EBV), the mechanisms by which thrombocytopenia is induced remain unclear, as do the relative contributions of lytic viral replication and latent viral gene expression. The human gammaherpesviruses are tightly restricted in their ability to infect other mammals, so they are difficult to study in live animal models. Here we show that infection of mice with murine gammaherpesvirus-68 (γHV68), a rodent-specific pathogen closely related to EBV, induces the production of platelet-binding antibodies and causes thrombocytopenia. Infection of antibody-deficient mice does not lead to thrombocytopenia, indicating the platelet decrease is mediated by antibody. Additionally, infection with a latency-null recombinant γHV68 does not induce thrombocytopenia, suggesting factors associated with viral latency drive the infection-induced antibody-mediated thrombocytopenia. These studies describe an important animal model of gammaherpesvirus-induced autoimmune thrombocytopenia and demonstrate that this pathology is mediated by antibody and dependent on viral latency. This model will allow studies of the underlying mechanisms of disease progression and the testing of therapeutic strategies for the alleviation of virus-induced thrombocytopenia.

Characterization of RAG1 and IgM (mu chain) marking development of the immune system in red-spotted grouper (Epinephelus akaara)

In vertebrates, lymphoid-specific recombinase protein encoded by recombination-activating genes (RAG1/2) plays a key role in V(D)J recombination of the T-cell receptor and B-cell receptor. In this study, both RAG1 and the immunoglobulin M (IgM) mu chain were cloned to characterize their potential role in the immune defense at developmental stages of red-spotted grouper, Epinephelus akaara. The open reading frame (ORF) of E. akaara RAG1 included 2778 nucleotide residues encoding a putative protein of 925 amino acids, while the ORF of the IgM mu chain had 1734 nucleotide residues encoding 578 amino acids including variable (VH) and constant (CH1-CH2-CH3-CH4) regions. E. akaara RAG1 was composed of a zinc-binding dimerization domain (ZDD) with a RING finger and zinc finger A (ZFA) in the non-core region and a nonamer-binding region (NBR), with a zinc finger B (ZFB), the central and C-terminal domains in the core region. Tridimensional models of the ZDD and NBR of E. akaara RAG1 were constructed for the first time in fishes, while a 3D model of the E. akaara IgM mu chain was also clarified. The RAG1 mRNA was only detected in the thymus and kidney of 4-month and 1.5-year old groupers using qPCR, and the RAG1 protein was confirmed using western blotting and immunohistochemistry. The IgM mu mRNA was examined in most tissues except the gonad. RAG1 and IgM mu gene expression were observed at 15 dph (days post-hatching) and 23 dph respectively, and increased to a higher level at 37 dph. In addition, this was the first time that the morphology of the E. akaara thymus was characterized. The oval-shaped thymus of 4-month old fish was clearly seen and there were amounts of T lymphocytes present. The results suggested that the immune action of E. akaara was likely to start to develop around 15 dph to 29 dph. The transcript level of the RAG1 gene and the number of lymphocytes in the thymus between 4-month and 1.5-year old groupers indicated that age-related thymic atrophy also occurs in fishes. The similar functional structures of RAG1 and IgM protein between fish and mammals indicated that teleost species share a similar mechanism of V(D)J recombination with higher vertebrates.

Production and characterization of monoclonal antibodies to IgM of Pacific herring (Clupea pallasii)

Pacific herring (Clupea pallasii) have a central role in the North Pacific ecosystem as a forage fish species and are natural reservoirs of several important finfish pathogens, including Viral hemorrhagic septicemia virus (VHSV). Here, we report the identification of the gene encoding the immunoglobulin mu (IgM) heavy chain, as well as the development and characterization of monoclonal antibodies (MAbs) that specifically react with Pacific herring IgM. Pacific herring immunoglobulin was purified and consisted of heavy and light chains of approximately 80 and 25 kDa. Three hybridoma clones were initially identified by ELISA as reactive with purified immunoglobulin but only one clone was able to detect an 80 kDa protein in Pacific and Atlantic herring (Clupea harengus) whole plasma by denaturing western blot. However, all three MAbs were able to precipitate an 80 kDa protein from Pacific herring and LCMS sequencing of peptide fragments derived from this protein matched the predicted amino acid sequence of the cloned, heavy chain gene. In addition, two of the MAbs stained cells within the putative lymphocyte gates for the spleen, anterior kidney and posterior kidney but were not reactive for myeloid/granulocyte gates, which is consistent with these MAbs reacting with surface IgM⁺ B-cells. To our knowledge, this is the first report of IgM-related gene sequences and anti-IgM monoclonal antibodies from any member of the family Clupeidae. The antibodies produced in this study are critical for achieving our long-term goal of conducting serological surveillance to assess pathogen exposure in natural populations of Pacific herring.

Cotranscriptional effect of a premature termination codon revealed by live-cell imaging

Aberrant mRNAs with premature translation termination codons (PTCs) are recognized and eliminated by the nonsense-mediated mRNA decay (NMD) pathway in eukaryotes. We employed a novel live-cell imaging approach to investigate the kinetics of mRNA synthesis and release at the transcription site of PTC-containing (PTC+) and PTC-free (PTC-) immunoglobulin-μ reporter genes. Fluorescence recovery after photobleaching (FRAP) and photoconversion analyses revealed that PTC+ transcripts are specifically retained at the transcription site. Remarkably, the retained PTC+ transcripts are mainly unspliced, and this RNA retention is dependent upon two important NMD factors, UPF1 and SMG6, since their depletion led to the release of the PTC+ transcripts. Finally, ChIP analysis showed a physical association of UPF1 and SMG6 with both the PTC+ and the PTC- reporter genes in vivo. Collectively, our data support a mechanism for regulation of PTC+ transcripts at the transcription site.

Excessive amounts of mu heavy chain block B-cell development

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.

[Analysis of mutation in heavy chain-micro (microHC) gene in a Chinese patient with congenital agammaglobulinemia]

OBJECTIVE

Mutation in the heavy chain micro (microHC) gene causes a rare type of autosomal recessive agammaglobulinemia. Here we report the molecular and clinical characterization of a compound heterozygous mutation in the microHC gene in a patient with autosomal recessive agammaglobulinemia firstly from China.

METHOD

A one-year and ten-month-old male patient and his parents were enrolled in this study. No mutation was found in BTK gene. The microHC gene of the patient and his parents were amplified by polymerase chain reaction (PCR) from genomic DNA. Reverse transcription polymerase chain reaction (RT-PCR) was used to amplify the microHC transcripts. Sequencing was performed directly on the PCR products bidirectionally.

RESULTS

Since 8 months of age, the patient had had recurrent fever and persistent cough. He suffered an acute right hemiplegia at 11 months of age and swelling and pain of left hip joint and right knee joint at one year and eight months of age. Cerebrospinal fluid routine examination showed that total cell count was 18 x 10(6)/L [normal range (0 - 15) x 10(6)/L], leukocyte count 7 x 10(6)/L [(0 - 15) x 10(6)/L] and biochemical examination showed protein 0.14 g/L (0.15 - 0.45 g/L), glucose 4.68 mmol/L (2.44 - 4.44 mmol/L) and chloride 116.3 mmol/L (120 - 132 mmol/L). Mycobacterium bovis was identified negative by cerebrospinal fluid smear examination. No obvious abnormity was detected on skull CT examination. Hydrothorax examination showed that total cell count was 848 x 10(6)/L, leukocyte count 785 x 10(6)/L and protein 30.8 g/L (< 30 g/L). Poliovirus isolation from stool sample of the patient was negative. The serum immunoglobulin (Ig) profile was IgG 181 mg/L (normal range, 5.09 - 10.09 g/L); IgM 28.8 mg/dl (400 - 1260 mg/dl) and IgA 22 mg/dl (310 - 670 mg/dl), IgE 4.6 U/ml (normal range < 150 U/ml). There were no B-cells but normal percentage of T-cells (67%) and NK cells (32%) were present in the peripheral blood. The patient had a compound heterozygous mutation in the microHC gene:on one allele, there was an alternative splice site mutation in exon 4 (1956 G > A), for which the patient's father was a carrier. Whereas on the other allele, an insert mutation between 170 and 175 in exon 1 with a premature stop codon (170 - 175 insert C, L11fs60X) was identified, for which the patient's mother was a carrier. The insert mutation in exon 1 of microHC gene was firstly reported. To detect the effect of the splice site mutation in exon 4, microHC cDNA of the patient was amplified by semi-nested PCR. The PCR products were purified and sequenced directly. A 136 bp of intron 4 was found in the transcripts and only the secreted isoform with a missense substitution is present in the patient, while synthesis of the membrane isoform is completely abolished.

CONCLUSION

This is the first case with autosomal recessive agammaglobulinemia with compound heterozygous mutation in the microHC gene reported from China. A novel mutation in exon 1 was found.

Equal transcription rates of productively and nonproductively rearranged immunoglobulin mu heavy chain alleles in a pro-B cell line

During B cell maturation, immunoglobulin (Ig) genes frequently acquire premature translation-termination codons (PTCs) as a result of the somatic rearrangement of V, D, and J gene segments. However, it is essential for a B lymphocyte to produce only one kind of antibody and therefore to ensure that the heavy and light chain polypeptides are expressed exclusively from the corresponding functional alleles, whereas no protein is made from the nonproductively rearranged alleles. At the post-transcriptional level, a well-studied mRNA quality control mechanism, termed nonsense-mediated mRNA decay (NMD), recognizes and degrades PTC-containing mRNAs in a translation-dependent manner. In addition, transcriptional silencing of PTC-containing Ig-mu and Ig-gamma heavy chain reporter genes was observed in HeLa cells. To investigate the silencing of nonproductively rearranged Ig genes in a more physiological context, we analyzed a monoclonal line of immortalized murine pro-B cells harboring one productively (PTC-) and one nonproductively (PTC+) rearranged Ig-mu heavy chain allele. We show that the steady-state abundance of PTC+ mRNA was approximately 40-fold lower when compared to that of the PTC- mRNA. However, both the PTC+ and PTC- allele seemed to be equally well transcribed since the abundances of PTC+ and PTC- pre-mRNA were very similar and chromatin immunoprecipitations revealed comparable occupancy of RNA polymerase II and acetylated histone H3 on both alleles. Altogether, we found no evidence for transcriptional silencing of the PTC+ allele in this pro-B cell line; hence, the efficient down-regulation of the PTC+ Ig-mu mRNA results entirely from NMD.

Removal of the BiP-retention domain in Cmicro permits surface deposition and developmental progression without L-chain

Nascent, full length, immunoglobulin (Ig) heavy (H)-chains are post-translationally associated with H-chain-binding protein (BiP or GRP78) in the endoplasmic reticulum (ER). The first constant (C) domain, CH1 of a C gene (Cmu, Cgamma, Calpha), is important for this interaction. The contact is released upon BiP replacement by conventional Ig light (L)-chain (kappa or lambda). Incomplete or mutated H-chains with removed variable (VH) and/or C(H)1 domain, as found in H-chain disease (HCD), can preclude stable BiP interaction. Progression in development after the preB cell stage is dependent on surface expression of IgM when association of a micro H-chain with a L-chain overcomes the retention by BiP. We show that IgM lacking the BiP-binding domain is displayed on the cell surface and elicits a signal that allows developmental progression even without the presence of L-chain. The results are reminiscent of single chain Ig secretion in camelids where developmental processes leading to the generation of fully functional H-chain-only antibodies are not understood. Furthermore, in the mouse the largest secondary lymphoid organ, the spleen, is not required for H-chain-only Ig expression and the CD5 survival signal may be obsolete for cells expressing truncated IgM.

[Problem of J-chain of immunoglobulins]

Molecules of secretory immunoglobulins (Ig) of classes A and M (sIgA and sIgM) play the main role in protection of mucosae from pathogenic factors. The apparatus of synthesis of these molecules represents the most powerful part of the immune system. One of the key elements of the sIgA and sIgM is J-chain. It represents an acid polypeptide of molecular mass of about 15 kDa composed of 137 amino acid residues including 8 cysteine residues and one site of N-glycosylation. The primary structure of the J-chain is unique: attempts to ascribe it to any family of known proteins so far have failed. The J-chain is inserted into the sIgA and sIgM molecules to form disulfide bonds with C-terminal sites of alpha- or mu-chains. It is necessary for formation of IgA dimers and IgM pentamers, for reception of these molecules by epithelial cells, binding of secretory component to them, and for transfer of sIgA and slgM molecules onto mucosal surfaces and into secrets of endocrine glands. The J-chain has been revealed in the cytoplasm of the early T- and B-lymphocyte precursors not producing Ig. The J-chain is detected in the human embryonic liver cells earlier than the expression of the mu-chain gene begins. Study of mice with knockout of J-chain B-lymphocytes-producents has shown their block of function of T-helpers providing formation of immunologic memory. Comparison of J-chain genes of mammals, amphibians, reptiles, and cartilaginous fishes has shown the degree of interspecies homology of these proteins to vary from 33% to 70%. The J-chain genes were revealed in representatives of all vertebrate classes except for cyclostomes and bony fishes. In 1996, data were published about the presence of the J-chain genes-homologs in invertebrates, tunicates, and cyclostomes. No papers reproducing or confirming these data have been published. On the contrary, in the literature an opinion appeared that indicate necessity to revise the notion about the presence of J-chain in invertebrates. The main unsolved issues on the J-chain involve the tertiary structure of this protein, its relation to some particular protein family, its functions in cells of the T- and B-lymphocytic differentiation lineages as well as its evolutionary age.

A new isotype of immunoglobulin heavy chain in the urodele amphibian Pleurodeles waltl predominantly expressed in larvae

Up to now, it was thought that urodele amphibians possessed only two IgH isotypes, IgM (mu) and IgY (upsilon). By screening a Pleurodeles waltl Ig cDNA mini-library, we identified three isotypes: IgM, IgY and a previously unknown class. IgM are multimeric molecules and represent the most abundant isotype throughout the life of P. waltl. IgY are likely the counterpart of mammalian IgA. The new isotype has typical Ig H-chain characteristics and is expressed as both secretory and membrane forms. Our analyses indicate that this isotype is restricted to Pleurodeles. Consequently, we named it "IgP" (pi) for Pleurodeles. This isotype is mainly expressed after hatching. Its expression decreases after metamorphosis. Our data indicate that IgP-expressing B cells present some similarities with mammalian B1-cells.

Cutting edge: a hypomorphic mutation in Igbeta (CD79b) in a patient with immunodeficiency and a leaky defect in B cell development

Although null mutations in Igalpha have been identified in patients with defects in B cell development, no mutations in Igbeta have been reported. We recently identified a patient with a homozygous amino acid substitution in Igbeta, a glycine to serine at codon 137, adjacent to the cysteine required for the disulfide bond between Igalpha and Igbeta. This patient has a small percentage of surface IgM(dim) B cells in the peripheral circulation (0.08% compared with 5-20% in healthy controls). Using expression vectors in 293T cells or Jurkat T cells, we show that the mutant Igbeta can form disulfide-linked complexes and bring the mu H chain to the cell surface as part of the BCR but is inefficient at both tasks. The results show that minor changes in the ability of the Igalpha/Igbeta complex to bring the BCR to the cell surface have profound effects on B cell development.

B lymphocyte deficiency in IgH-transgenic rabbits

We developed IgH-transgenic rabbits carrying a productive VDJ-Cmu Tg and found the rabbits were B cell-deficient, with a 50-100% reduction in serum IgM and IgG levels. The bone marrow of newborn Tg rabbits contained severely reduced levels of preB cells and almost no B cells. The few preB cells present in the bone marrow were large, cycling cells that expressed the VDJ-Cmu Tg, indicating that the block in B cell development likely occurred at or before the transition from large (early) preB to small (late) preB cells. By immunoprecipitation, the Tg mu-chain paired with VpreB and lambda5, suggesting that the B cell deficiency is not due to an inability to form a preB cell receptor. Despite the block in B cell development, a few B cells, expressing predominantly endogenous mu-chains, began the second stage of development in GALT. B cells were localized in and beneath the follicle-associated epithelium of GALT prior to B cell follicle formation, suggesting to us that B cell follicle formation is initiated near the follicle-associated epithelium, possibly through contact with intestinal microbiota. These IgH-Tg rabbits should provide a useful model for studies of B cell development both in bone marrow and in GALT.

Sequential steps and checkpoints in the early exocytic compartment during secretory IgM biogenesis

The biogenesis of secretory IgM occurs stepwise under stringent quality control, formation of micro(2)L(2) preceding polymerization. How is efficiency of IgM secretion coupled to fidelity? We show here that ERp44, a soluble protein involved in thiol-mediated retention, interacts with ERGIC-53. Binding to this hexameric lectin contributes to ERp44 localization in the ER-golgi intermediate compartment. ERp44 and ERGIC-53 increase during B-lymphocyte differentiation, concomitantly with the onset of IgM polymerization. Both preferentially bind micro(2)L(2) and higher order intermediates. Their overexpression or silencing in non-lymphoid cells promotes or decreases secretion of IgM polymers, respectively. In IgM-secreting B-lymphoma cells, micro chains interact first with BiP and later with ERp44 and ERGIC-53. Our findings suggest that ERGIC-53 provides a platform that receives micro(2)L(2) subunits from the BiP-dependent checkpoint, assisting polymerization. In this process, ERp44 couples thiol-dependent assembly and quality control.

Transcriptome analysis in primary B lymphoid precursors following induction of the pre-B cell receptor

Pre-BCR signals are part of a checkpoint where early precursor (pre-) B cells with a pairing Ig muH chain (muHC) are clonally expanded before they differentiate into IgL-rearranging, resting pre-B cells. A pre-BCR consists of two muHCs, two surrogate L chains and the signal transducer Igalpha/Igbeta. The molecular circuits by which the pre-BCR controls proliferation and differentiation of pre-B cells are poorly characterized. Therefore, we identified the differential transcriptome by genome-wide expression profiling in progenitor (pro-) B cells from a Rag2-deficient mouse, in which the expression of a transgenic muHC and thus a pre-BCR as well as pre-BCR-mediated clonal expansion can be controlled by tetracycline (muHC-inducible mouse). This analysis revealed that pre-BCR signals upregulate components of the BCR signalosome, open the IgL chain (LC) locus and induce the krüppel-like transcription factor KLF2, a key regulator of quiescence and lymphocyte migration. Hence, pre-BCR signals establish the molecular network for BCR signaling even before the production of an IgLC and induce the expression of KLF2, a candidate for controlling clonal expansion and migration of functional pre-B cells.

Sequence dependence of chromosomal R-loops at the immunoglobulin heavy-chain Smu class switch region

The mechanism by which the cytidine deaminase activation-induced deaminase (AID) acts at immunoglobulin heavy-chain class switch regions during mammalian class switch recombination (CSR) remains unclear. R-loops have been proposed as a basis for this targeting. Here, we show that the difference between various forms of the Smu locus that can or cannot undergo CSR correlates well with the locations and detectability of R-loops. The Smu R-loops can initiate hundreds of base pairs upstream of the core repeat switch regions, and the area where the R-loops initiate corresponds to the zone where the AID mutation frequency begins to rise, despite a constant density of WRC sites in this region. The frequency of R-loops is 1 in 25 alleles, regardless of the presence of the core Smu repeats, again consistent with the initiation of most R-loops upstream of the core repeats. These findings explain the surprisingly high levels of residual CSR in B cells from mice lacking the core Smu repeats but the marked reduction in CSR in mice with deletions of the region upstream of the core Smu repeats. These studies also provide the first analysis of how R-loop formation in the eukaryotic chromosome depends on the DNA sequence.

Transcriptional silencing of nonsense codon-containing immunoglobulin micro genes requires translation of its mRNA

Eukaryotes have evolved quality control mechanisms that prevent the expression of genes in which the protein coding potential is crippled by the presence of a premature translation-termination codon (PTC). In addition to nonsense-mediated mRNA decay (NMD), a well documented posttranscriptional consequence of the presence of a PTC in an mRNA, we recently reported the transcriptional silencing of PTC-containing immunoglobulin (Ig) mu and gamma minigenes when they are stably integrated into the genome of HeLa cells. Here we demonstrate that this transcriptional silencing of PTC-containing Ig-mu constructs requires active translation of the cognate mRNA, as it is not observed under conditions where translation of the PTC-containing mRNA is inhibited through an iron-responsive element in the 5'-untranslated region. Furthermore, RNA interference-mediated depletion of the essential NMD factor Upf1 not only abolishes NMD but also reduces the extent of nonsense-mediated transcriptional gene silencing (NMTGS). Collectively, our data indicate that NMTGS and NMD are linked, relying on the same mechanism for PTC recognition, and that the NMTGS pathway branches from the NMD pathway at a step after Upf1 function.

Molecular analysis of the pre-BCR complex in a large cohort of patients affected by autosomal-recessive agammaglobulinemia

Autosomal-recessive agammaglobulinemia is a rare and heterogeneous disorder, characterized by early-onset infections, profound hypogammaglobulinemia of all immunoglobulin isotypes and absence of circulating B lymphocytes. To investigate the molecular basis of the disease, 23 patients with early-onset disease and no mutations in Bruton tyrosine kinase, the gene responsible for X-linked agammaglobulinemia, were selected and analyzed by direct sequencing of candidate genes. Two novel mutations in the mu heavy chain (muHC) gene (IGHM) were identified in three patients belonging to two unrelated families. A fourth patient carries a previously described G>A nucleotide substitution at the -1 position of an alternative splice site in IGHM; here, we demonstrate that this mutation is indeed responsible for aberrant splicing. Comparison of bone marrow cytofluorimetric profiles in two patients carrying different mutations in the IGHM gene suggests a genotype-phenotype correlation with the stage at which B-cell development is blocked. Several new single nucleotide polymorphisms (SNPs) both in the muHC and in the lambda5-like/VpreB-coding genes were identified. Two unrelated patients carry compound heterozygous variations in the VpreB1 gene that may be involved in disease ethiology.

Immunoglobulin kappa enhancers are differentially regulated at the level of chromatin structure

The kappa intronic and the kappa 3' enhancers synergize to regulate recombination and transcription of the Ig kappa locus. Although these enhancers have overlapping functions, the kappa i enhancer appears to predominate during receptor editing, while the kappa 3' enhancer may be more important for initiating Ig kappa germline transcription to target locus recombination and, later in development, somatic hypermutation. Changes in chromatin structure appear to regulate both enhancers, and previous reports suggest that both enhancers are packaged into an accessible chromatin structure only in B lineage cells. Why these enhancers cannot activate the demethylated, accessible, protein-associated Ig kappa allele in pro-B cells is not known. Furthermore, how the enhancers function to reactivate the locus for receptor editing or to quantitatively promote hypermutation in B cells is vague. Quantitative analysis of Ig enhancer chromatin structure in murine pro-, pre-and splenic B cells demonstrated that the kappa i enhancer maintains a highly accessible chromatin structure under a variety of conditions. This stable chromatin structure mirrored the highly accessible structure characterizing the Ig mu intronic enhancer, despite the fact that Ig mu is activated prior to Ig kappa during B cell development. Surprisingly, parallel analysis of the kappa 3' enhancer demonstrated its accessible chromatin structure is markedly unstable, as characterized by sensitivity to changes in environmental conditions. These data unexpectedly suggest that kappa locus regulation is compartmentalized along the gene in B lineage cells. Furthermore, these findings raise the possibility that environmentally dependent regulation of kappa 3' enhancer structure underlies changes in kappa activation during B cell development.

AID-/-mus-/- mice are agammaglobulinemic and fail to maintain B220-CD138+ plasma cells

The terminal stage of B cell differentiation culminates in the formation of plasma cells (PC), which secrete large quantities of Igs. Despite recent progress in understanding the molecular aspect of PC differentiation and maintenance, the requirement for the synthesis of secretory Igs as a contributing factor has not been explored. To address this issue, we generated activation-induced cytidine deaminase (AID)/secretory mu-chain (mus) double-knockout mice, in which a normally diverse repertoire of B cell receptors is retained, yet B cells are unable to synthesize secretory Igs. These mice possess polyclonal B cells but have no serum Igs. Following immunization in vivo, PCs, identified by CD138 expression and loss of the B220 marker, were starkly reduced in number in spleen and bone marrow of AID(-/-)mus(-/-) agammaglobulinemic mice compared with wild-type mice. Upon mitogenic stimulation in vitro, AID(-/-)mus(-/-) B cells differentiated into plasmablasts to some extent, but showed reduced survival compared with wild-type B cells. We found no evidence that this reduced survival was attributable to accumulation of membrane IgM. Our results indicate that the synthesis of secretory Igs is a requirement for maintenance of B220(-)CD138(+) PCs.

A weakened transcriptional enhancer yields variegated gene expression

Identical genes in the same cellular environment are sometimes expressed differently. In some cases, including the immunoglobulin heavy chain (IgH) locus, this type of differential gene expression has been related to the absence of a transcriptional enhancer. To gain additional information on the role of the IgH enhancer, we examined expression driven by enhancers that were merely weakened, rather than fully deleted, using both mutations and insulators to impair enhancer activity. For this purpose we used a LoxP/Cre system to place a reporter gene at the same genomic site of a stable cell line. Whereas expression of the reporter gene was uniformly high in the presence of the normal, uninsulated enhancer and undetectable in its absence, weakened enhancers yielded variegated expression of the reporter gene; i.e., the average level of expression of the same gene differed in different clones, and expression varied significantly among cells within individual clones. These results indicate that the weakened enhancer allows the reporter gene to exist in at least two states. Subtle aspects of the variegation suggest that the IgH enhancer decreases the average duration (half-life) of the silent state. This analysis has also tested the conventional wisdom that enhancer activity is independent of distance and orientation. Thus, our analysis of mutant (truncated) forms of the IgH enhancer revealed that the 250 bp core enhancer was active in its normal position, ∼1.4 kb 3′ of the promoter, but inactive ∼6 kb 3′, indicating that the activity of the core enhancer was distance-dependent. A longer segment – the core enhancer plus ∼1 kb of 3′ flanking material, including the 3′ matrix attachment region – was active, and the activity of this longer segment was orientation-dependent. Our data suggest that this 3′ flank includes binding sites for at least two activators.

Structure and implied functions of truncated B-cell receptor mRNAs in early embryo and adult mesenchymal stem cells: Cdelta replaces Cmu in mu heavy chain-deficient mice

Stem cells exhibit a promiscuous gene expression pattern. We show herein that the early embryo and adult MSCs express B-cell receptor component mRNAs. To examine possible bearings of these genes on the expressing cells, we studied immunoglobulin mu chain-deficient mice. Pregnant mu chain-deficient females were found to produce a higher percentage of defective morulae compared with control females. Structure analysis indicated that the mu mRNA species found in embryos and in mesenchyme consist of the constant region of the mu heavy chain that encodes a recombinant 50-kDa protein. In situ hybridization localized the constant mu gene expression to loose mesenchymal tissues within the day-12.5 embryo proper and the yolk sac. In early embryo and in adult mesenchyme from mu-deficient mice, delta replaced mu chain, implying a possible requirement of these alternative molecules for embryo development and mesenchymal functions. Indeed, overexpression of the mesenchymal-truncated mu heavy chain in 293T cells resulted in specific subcellular localization and in G(1) growth arrest. The lack of such occurrence following overexpression of a complete, rearranged form of mu chain suggests that the mesenchymal version of this mRNA may possess unique functions.

Structure of the catfish IGH locus: analysis of the region including the single functional IGHM gene

The catfish IGH locus is large ( approximately 1 Mb) and complex, having undergone multiple internal duplications and transpositions. To define the structure of the locus that contains the single expressed IGHM gene, two overlapping bacterial-artificial-chromosome (BAC) clones spanning the most 3' end of the channel catfish immunoglobulin heavy (IGH) chain locus have been completely sequenced. The analyses created a contig of 257,153 bp containing 55 VH, 6 D, 12 JH genes and the IGH constant region genes encoding the functional secreted and membrane forms of IgM and the membrane form of IgD. This analysis revealed three major features. First, no C-region genes were found aside from the previously described IGHM1 and IGHD1, with the latter gene being the most 3' C-region gene of the catfish IGH locus. There was no evidence in the region sequenced for genes that could encode an Ig class similar to the IgZ/IgT described in zebrafish, trout and pufferfish. Second, there are a high number of VH pseudogenes, 28 out of 55 (51%). In contrast, the entire zebrafish IGH locus has 40 functional VH genes and eight pseudogenes (17%). Third, an internal duplication of a 52.4-kb block of VH genes has occurred. These observations suggest that the IGH locus of teleost fish varies significantly from species to species in the diversity of C-region genes as well as the numbers of genes encoding V regions.

EJC-independent degradation of nonsense immunoglobulin-mu mRNA depends on 3' UTR length

Inconsistent with prevailing models for nonsense-mediated mRNA decay (NMD) in mammals, the mRNA levels of immunoglobulin-mu (Ig-mu) genes with premature termination codons (PTCs) in the penultimate exon are still reduced by NMD when the intron furthest downstream is deleted. As in yeast, this exon junction complex-independent NMD of Ig-mu mRNAs depends on the distance between the termination codon and the poly(A) tail and suggests an evolutionarily conserved mode of PTC recognition.

Increased negative selection impairs neonatal B cell repertoire but does not directly lead to generation of disease-associated IgM auto-antibodies

To determine if increased negative B cell selection, due to lowered signaling threshold of responsiveness to a ligand as a result of SHP-1 deficiency, during ontogeny leads to the origin of disease-associated IgM auto-antibodies (AAbs), 47 V(H)J558+ VDJCmu rearrangements from SHP-1-deficient viable motheaten (me(v)/me(v)) and 24 J558+ VDJCmu rearrangements from normal me(v)/+ neonatal (<24 h post-birth) B cells were examined for their structural properties. None of the J558+ VDJCmu rearrangements from autoimmune-prone me(v)/me(v) had the characteristic CDR3H size restriction or arginine residues noted in disease-associated IgM AAbs. However, the MVAR2/10 genes are expressed at a high frequency in me(v)/me(v) (31.9%) as compared with me(v)/+ (16.7%), and pM11 gene expression is exclusively (14.9%) noted in me(v)/me(v) B cells. Clearly, there is a trend toward higher expression of pM11 genes (P-value < or = 0.09) in autoimmune-prone me(v)/me(v) strain. The CDR2H region of J558+ VDJCmu recombinations from me(v)/me(v) has increased hotspot triplets predisposing to mutations as compared with me(v)/+ (P-value < or = 0.01) mice. A higher DFL D-gene expression is noted in J558+ VDJCmu rearrangements from me(v)/me(v) (P-value < or = 0.1) in contrast to me(v)/+. The sophisticated logistic regression and odds ratio analysis of V-, D- and J-gene expressions in neonatal B cells from me(v)/me(v) and me(v)/+ mice demonstrates differential composition of the germ line IgM repertoire as a result of SHP-1 deficiency. These observations suggest that increased negative B cell selection during ontogeny impairs the developing IgM antibody repertoire but does not directly lead to generation of disease-associated IgM AAbs.

Biased dA/dT somatic hypermutation as regulated by the heavy chain intronic iEmu enhancer and 3'Ealpha enhancers in human lymphoblastoid B cells

Somatic hypermutation (SHM) in immunoglobulin gene (Ig) variable (V) regions is critical for the maturation of the antibody response. It is dependent on the expression of activation-induced cytidine deaminase (AID) and translesion DNA polymerases in germinal center B cells as well as Ig V transcription, as regulated by the Ig heavy chain (H) intronic enhancer (iEmu) and the 3' enhancer (3'Ealpha) region. We analyzed the role of these cis elements in SHM by stably transfecting Ramos human lymphoblastoid B cells with a rearranged human IgH chain VD (diversity) J (joining) DNA construct containing a V(H) promoter at the 5' end and C(H)1 and C(H)2 exons of Cgamma1 at the 3' end. In this construct, mutations preferentially targeted dA/dT basepairs in the RGYW/WRCY hotspot. Most of the dA/dT mutations and accompanying dC/dG mutations were transitions. Deletion of iEmu resulted in decreased SHM which could be partially restored by insertion of the IgH hs1,2 enhancer. Other two 3'Ealpha enhancers, hs3-hs4, did not significantly increase the mutation frequency, but further strengthened the dA/dT bias. The frequency and spectrum of the mutations were independent of the genomic integration of the transgene or V gene transcription level. Thus, we have established a novel in vitro system to analyze SHM and identify the role of multiple cis-regulatory elements in regulating dA/dT biased SHM. This model system will be useful to further address the role of other cis-regulating elements and recruited trans-acting factors in expressing the modalities of SHM.

The extent of histone acetylation correlates with the differential rearrangement frequency of individual VH genes in pro-B cells

During B lymphocyte development, Ig heavy and L chain genes are assembled by V(D)J recombination. Individual V, D, and J genes rearrange at very different frequencies in vivo, and the natural variation in recombination signal sequence does not account for all of these differences. Because a permissive chromatin structure is necessary for the accessibility of VH genes for VH to DJH recombination, we hypothesized that gene rearrangement frequency might be influenced by the extent of histone modifications. Indeed, we found in freshly isolated pro-B cells from muMT mice a positive correlation between the level of enrichment of VHS107 genes in the acetylated histone fractions as assayed by chromatin immunoprecipitation, and their relative rearrangement frequency in vivo. In the VH7183 family, the very frequently rearranging VH81X gene showed the highest association with acetylated histones, especially in the newborn. Together, our data show that the extent of histone modifications in pro-B cells should be considered as a mechanism by which accessibility and the rearrangement level of individual VH genes is regulated.

Disparate roles of ATR and ATM in immunoglobulin class switch recombination and somatic hypermutation

Class switch recombination (CSR) and somatic hypermutation (SHM) are mechanistically related processes initiated by activation-induced cytidine deaminase. Here, we have studied the role of ataxia telangiectasia and Rad3-related protein (ATR) in CSR by analyzing the recombinational junctions, resulting from in vivo switching, in cells from patients with mutations in the ATR gene. The proportion of cells that have switched to immunoglobulin (Ig)A and IgG in the peripheral blood seems to be normal in ATR-deficient (ATRD) patients and the recombined S regions show a normal “blunt end-joining,” but impaired end joining with partially complementary (1–3 bp) DNA ends. There was also an increased usage of microhomology at the μ-α switch junctions, but only up to 9 bp, suggesting that the end-joining pathway requiring longer microhomologies (≥10 bp) may be ATR dependent. The SHM pattern in the Ig variable heavy chain genes is altered, with fewer mutations occurring at A and more mutations at T residues and thus a loss of strand bias in targeting A/T pairs within certain hotspots. These data suggest that the role of ATR is partially overlapping with that of ataxia telangiectasia–mutated protein, but that the former is also endowed with unique functional properties in the repair processes during CSR and SHM.

Only VpreB1, but not VpreB2, is expressed at levels which allow normal development of B cells

The surrogate light chain (SLC) consists of the polypeptides lambda5 and, in the mouse, either VpreB1 or VpreB2. SLC associates with BILL-Cadherin and other glycoproteins to form the pro-B cell receptor (pro-BCR) at the pre-BI cell stage, and with the immunoglobulin mu heavy chain to form the pre-BCR at the pre-BII cell stage. The function of the pro-BCR, if any, is unknown, whereas the pre-BCR is crucial for proliferative expansion of pre-BII cells. To shed light on the functional properties of VpreB1 and VpreB2 in vivo, mice with either one or two VpreB1, or one or two VpreB2, alleles have been investigated. We show that B cell development in mice with two VpreB1 alleles is indistinguishable from that of normal mice. In contrast, mice with two VpreB2 alleles show an approximately 1.6-fold increase in pre-BI and a 35% decrease in pre-BII cell numbers, while mice with only one VpreB2 allele show a reduction in B cell development manifested in a 2-fold enrichment in pre-BI cells and a 75% reduction in pre-BII cells. However, such a gene dosage effect is not observed for VpreB1. Our results suggest that the difference between VpreB1- and VpreB2-deficient mice is due to lower VpreB2 protein expression, thus limiting the formation of pre-BCRs and thereby the number of large, cycling pre-BII cells.

Human IgM monoclonal antibodies reactive with HIV-1-infected cells generated using a trans-chromosome mouse

The trans-chromosome (TC) mouse that we used harbors human chromosomes 2, 14 and/or 22, and has undergone knock-out of its endogeneous genes coding for mu-and kappa-chains of immunoglobulin. One of these TC mice was immunized with HIV-1-infected U937 cells, and spleen cells from the immunized animal were fused with the mouse myeloma cell line to generate hybridoma cells. We selected hybridomas that produce human IgM antibodies (Abs) reactive with HIV-1-infected MOLT4 cells but not with uninfected MOLT4 cells. Two hybridoma cell lines were established termed 9F11 and 2G9. Although 0.4 mug/ml of 9F11 was able to induce complement-mediated cytolysis of the infected cells in the presence of fresh human serum, 2G9 could not. There was no difference between the two monoclonal Abs in the base sequences of cDNAs coding for the constant regions of mu-and kappa-chains. Therefore, we speculate that the ability to activate complement on homologous cell membranes might reflect the structural presentation of antigenic molecules, which could facilitate the binding of an IgM Ab to multiple binding sites resulting in escape from restriction by species-specific inhibitors of complement such as DAF (CD55) and CD59. On the other hand, 2G9 induced apoptosis of HIV-1-infected cells, including latently infected OM10.1 cells, although the Ag for 2G9 remains to be identified. Since both of the Abs had reduced reactivity toward HIV-1-infected MOLT4 cells following cultivation in the presence of tunicamycin, the responsible antigens would involve a sugar moiety.

Dmu expression causes enrichment of MZ B cells, but is non permissive for B cell maturation in Rag2-/- mice even if combined with Bcl-2

Rearrangements in reading frame 2 promote the expression of a truncated heavy chain, the Dmu protein. Dmu can assemble into a pre-B cell receptor like complex that appears to induce a subset of signals elicited by full length mu, but cannot promote the pro-B to pre-B cell transition of Rag-/- B cells. In order to determine if this could stem from an impaired survival signal not properly induced by the Dmu protein, we introduced Bcl-2 into Dmu-transgenic, Rag2-/- mice. Despite the fact that the Bcl-2 transgene expression promoted some increase in the fraction of CD43- B cells, an identical increase was also observed in Rag2-/- mice. Moreover, whereas in mu-transgenic Rag2-/-Bcl-2+ mice, CD2 and CD25 expression were up regulated and c-Kit was down regulated, these markers were unaltered in Dmu-transgenic Rag2-/- Bcl-2+ mice compared to Rag2-/- Bcl-2+ mice, indicating that Dmu cannot support pre-B cell maturation despite extended survival of B cell precursors by Bcl-2. In addition, we observed that in Dmu-transgenic recombination competent mice, the Dmu induced partial block is permissive for marginal zone B cell development whereas the formation of follicular B cells is severely reduced. While the Dmu protein is expressed in peripheral B cells escaping the block, only a minor fraction of Dmu is exposed to the outer cell surface.

XBP-1 specifically promotes IgM synthesis and secretion, but is dispensable for degradation of glycoproteins in primary B cells

Differentiation of B cells into plasma cells requires X-box binding protein–1 (XBP-1). In the absence of XBP-1, B cells develop normally, but very little immunoglobulin is secreted. XBP-1 controls the expression of a large set of genes whose products participate in expansion of the endoplasmic reticulum (ER) and in protein trafficking. We define a new role for XBP-1 in exerting selective translational control over high and sustained levels of immunoglobulin M (IgM) synthesis. XBP-1^−/− and XBP-1^+/+ primary B cells synthesize IgM at comparable levels at the onset of stimulation with lipopolysaccharide or CpG. However, later there is a profound depression in synthesis of IgM in XBP-1^−/− B cells, notwithstanding similar levels of μmRNA. In marked contrast, lack of XBP-1 does not affect synthesis and trafficking of other glycoproteins, or of immunoglobulin light chains. Contrary to expectation, degradation of proteins from the ER, using TCRα or US11-mediated degradation of class I major histocompatibility complex molecules as substrates, is normal in XBP-1^−/− B cells. Furthermore, degradation of membrane μ was unaffected by enforced expression of XBP-1. We conclude that in primary B cells, the XBP-1 pathway promotes synthesis and secretion of IgM, but does not seem to be involved in the degradation of ER proteins, including that of μ chains themselves.

Inducible DNA breaks in Ig S regions are dependent on AID and UNG

Class switch recombination (CSR) occurs by an intrachromosomal deletion whereby the IgM constant region gene (Cμ) is replaced by a downstream constant region gene. This unique recombination event involves formation of double-strand breaks (DSBs) in immunoglobulin switch (S) regions, and requires activation-induced cytidine deaminase (AID), which converts cytosines to uracils. Repair of the uracils is proposed to lead to DNA breaks required for recombination. Uracil DNA glycosylase (UNG) is required for most CSR activity although its role is disputed. Here we use ligation-mediated PCR to detect DSBs in S regions in splenic B cells undergoing CSR. We find that the kinetics of DSB induction corresponds with AID expression, and that DSBs are AID- and UNG-dependent and occur preferentially at G:C basepairs in WRC/GYW AID hotspots. Our results indicate that AID attacks cytosines on both DNA strands, and staggered breaks are processed to blunt DSBs at the initiating ss break sites. We propose a model to explain the types of end-processing events observed.

A system for precise analysis of transcription-regulating elements of immunoglobulin genes

Background

Precise analysis of expression-regulating elements, such as enhancers and insulators, requires that they be tested under reproducible, isogenic conditions. The commonly used methods of transfecting DNA into cell lines and selecting for drug resistance lack the requisite precision, as they yield cell lines in which varying numbers of gene copies have inserted at varying and undefined sites. By contrast, recombination-mediated cassette exchange (RMCE), by which a site-specific recombinase is used to place a single copy of a transgene at a constant chromosomal site of a cell line, offers the necessary precision. Although RMCE is generally applicable, many regulatory elements of interest are tissue-specific in their function and so require cell lines in the appropriate ontogenetic state.

Results

As reported here, we have used RMCE in a mouse B hybridoma cell line to establish a system with several additional advantages. To avoid the non-physiological features of prokaryotic DNA, this system uses the immunoglobulin μ heavy chain (IgH) gene from the hybridoma as the reporter. Expression can be measured simply by bulk culture assays (ELISA, Northern blot) and single cell assays (flow cytometry). Expression of the IgH reporter gene varies only 1.5 fold among independent transfectants, and expression is greatly (> 50 fold) increased by inclusion of the IgH intronic enhancer.

Conclusion

This system is suitable for precise analysis of the regulatory elements of the immunoglobulin loci.

Biased immunoglobulin G (IgG) subclass production in a case of hyper-IgM syndrome

Hyper-immunoglobulin M (IgM) syndrome (HIGM) is a rare heterogeneous primary immune deficiency. We describe a patient with HIGM characterized by skewed production of serum IgG subclasses and normal somatic hypermutation. This case may represent a subgroup of HIGM type 4 that is characterized by a biased switching to the V-region proximal constant regions.

The 3' end of the heavy chain constant region locus enhances germline transcription and switch recombination of the four gamma genes

The switch in immunoglobulin (Ig) heavy chain class is preceded by germline transcription and then mediated by a DNA recombination event. To study germline transcription and class switch recombination we used transgenic mice with a 230-kilobase bacterial artificial chromosome that included a rearranged VDJ gene and the entire heavy chain constant region locus. In addition to several lines with intact transgenes, we identified two lines in which the heavy chain locus transgene lacked Cα and everything 3′ of it, including the regulatory elements HS3a, HS1-2, HS3b, and HS4. B cells from both lines with the truncated transgenes make abundant transgenic (Tg) VDJCμ transcripts and IgM protein. Deletion of the 3′ end of the locus results in dramatically reduced expression of both germline transcripts and switched VDJCH transcripts of the γ3, γ2b, γ2a, and ɛ genes. In addition, the transgenes lacking the 3′ end of the locus express reduced amounts of γ1 germline transcripts and 2–3% of the amount of Tg IgG1 in tissue culture compared with intact transgenes. Finally, switch recombination to γ1 is undetectable in the transgenes lacking the 3′ elements, as measured by digestion circularization–polymerase chain reaction or by the expression of VDJCγ1 transcripts.

The Ig kappa 3' enhancer is activated by gradients of chromatin accessibility and protein association

The Igkappa locus is recombined following initiation of a signaling cascade during the early pre-B stage of B cell development. The Ig kappa3' enhancer plays an important role in normal B cell development by regulating kappa locus activation. Quantitative analyses of kappa3' enhancer chromatin structure by restriction endonuclease accessibility and protein association by chromatin immunoprecipitation in a developmental series of primary murine B cells and murine B cell lines demonstrate that the enhancer is activated progressively through multiple steps as cells mature. Moderate kappa3' chromatin accessibility and low levels of protein association in pro-B cells are increased substantially as the cells progress from pro- to pre-B, then eventually mature B cell stages. Chromatin immunoprecipitation assays suggest transcriptional regulators of the kappa3' enhancer, specifically PU.1 and IFN regulatory factor-4, exploit enhanced accessibility by increasing association as cells mature. Characterization of histone acetylation patterns at the kappa3' enhancer and experimental inhibition of histone deacetylation suggest changes therein may determine changes in enzyme and transcription factor accessibility. This analysis demonstrates kappa activation is a multistep process initiated in early B cell precursors before Igmu recombination and finalized only after the pre-B cell stage.

Alternative splicing induced by nonsense mutations in the immunoglobulin mu VDJ exon is independent of truncation of the open reading frame

In addition to triggering nonsense-mediated mRNA decay (NMD), premature translation-termination codons (PTCs) frequently induce alternative splicing, an observation referred to as nonsense-associated alternative splicing (NAS). In many cases, NAS is induced because the nonsense mutation alters a splicing signal, such as inactivating an exonic splicing enhancer. However, for a few genes, NAS was reported to be PTC specific, implying that a translation signal could influence splicing. Here, we investigated whether production of a previously undetected alternatively spliced transcript from immunoglobulin mu (Ig-mu) depends on premature termination of the open reading frame. We show that PTCs at different positions in the VDJ exon of an Ig-mu minigene activate usage of an alternative 3' splice site, generating an alternative transcript that lacks the initial PTC and a previously identified NMD-promoting element (NPE), but contains new PTCs because of a frame shift. Corroborating the importance of the NPE for maximal NMD response, the alternative transcript is only moderately down-regulated by NMD. We further demonstrate that NAS of Ig-mu minigene transcripts is not PTC specific. This finding, together with our results that contradict the previously reported frame dependence of TCR-beta NAS, challenges the idea that cells might possess mechanisms that would allow regulation of splice site selection in response to premature termination of the ORF.