Sequences of human immunoglobulin switch regions: implications for recombination and transcription

Role of the mechanisms for antibody repertoire diversification in monoclonal light chain deposition disorders: when a friend becomes foe

The adaptive immune system of jawed vertebrates generates a highly diverse repertoire of antibodies to meet the antigenic challenges of a constantly evolving biological ecosystem. Most of the diversity is generated by two mechanisms: V(D)J gene recombination and somatic hypermutation (SHM). SHM introduces changes in the variable domain of antibodies, mostly in the regions that form the paratope, yielding antibodies with higher antigen binding affinity. However, antigen recognition is only possible if the antibody folds into a stable functional conformation. Therefore, a key force determining the survival of B cell clones undergoing somatic hypermutation is the ability of the mutated heavy and light chains to efficiently fold and assemble into a functional antibody. The antibody is the structural context where the selection of the somatic mutations occurs, and where both the heavy and light chains benefit from protective mechanisms that counteract the potentially deleterious impact of the changes. However, in patients with monoclonal gammopathies, the proliferating plasma cell clone may overproduce the light chain, which is then secreted into the bloodstream. This places the light chain out of the protective context provided by the quaternary structure of the antibody, increasing the risk of misfolding and aggregation due to destabilizing somatic mutations. Light chain-derived (AL) amyloidosis, light chain deposition disease (LCDD), Fanconi syndrome, and myeloma (cast) nephropathy are a diverse group of diseases derived from the pathologic aggregation of light chains, in which somatic mutations are recognized to play a role. In this review, we address the mechanisms by which somatic mutations promote the misfolding and pathological aggregation of the light chains, with an emphasis on AL amyloidosis. We also analyze the contribution of the variable domain (V_L) gene segments and somatic mutations on light chain cytotoxicity, organ tropism, and structure of the AL fibrils. Finally, we analyze the most recent advances in the development of computational algorithms to predict the role of somatic mutations in the cardiotoxicity of amyloidogenic light chains and discuss the challenges and perspectives that this approach faces.

Identification of a Transcriptionally Forward α Gene and Two υ Genes within the Pigeon (Columba livia) IgH Gene Locus

Compared with mammals, the bird Ig genetic system relies on gene conversion to create an Ab repertoire, with inversion of the IgA-encoding gene and very few cases of Ig subclass diversification. Although gene conversion has been studied intensively, class-switch recombination, a mechanism by which the IgH C region is exchanged, has rarely been investigated in birds. In this study, based on the published genome of pigeon (Columba livia) and high-throughput transcriptome sequencing of immune-related tissues, we identified a transcriptionally forward α gene and found that the pigeon IgH gene locus is arranged as μ-α-υ1-υ2. In this article, we show that both DNA deletion and inversion may result from IgA and IgY class switching, and similar junction patterns were observed for both types of class-switch recombination. We also identified two subclasses of υ genes in pigeon, which share low sequence identity. Phylogenetic analysis suggests that divergence of the two pigeon υ genes occurred during the early stage of bird evolution. The data obtained in this study provide new insight into class-switch recombination and Ig gene evolution in birds.

DNA/RNA hybrid substrates modulate the catalytic activity of purified AID

Activation-induced cytidine deaminase (AID) converts cytidine to uridine at Immunoglobulin (Ig) loci, initiating somatic hypermutation and class switching of antibodies. In vitro, AID acts on single stranded DNA (ssDNA), but neither double-stranded DNA (dsDNA) oligonucleotides nor RNA, and it is believed that transcription is the in vivo generator of ssDNA targeted by AID. It is also known that the Ig loci, particularly the switch (S) regions targeted by AID are rich in transcription-generated DNA/RNA hybrids. Here, we examined the binding and catalytic behavior of purified AID on DNA/RNA hybrid substrates bearing either random sequences or GC-rich sequences simulating Ig S regions. If substrates were made up of a random sequence, AID preferred substrates composed entirely of DNA over DNA/RNA hybrids. In contrast, if substrates were composed of S region sequences, AID preferred to mutate DNA/RNA hybrids over substrates composed entirely of DNA. Accordingly, AID exhibited a significantly higher affinity for binding DNA/RNA hybrid substrates composed specifically of S region sequences, than any other substrates composed of DNA. Thus, in the absence of any other cellular processes or factors, AID itself favors binding and mutating DNA/RNA hybrids composed of S region sequences. AID:DNA/RNA complex formation and supporting mutational analyses suggest that recognition of DNA/RNA hybrids is an inherent structural property of AID.

Binding of estrogen receptors to switch sites and regulatory elements in the immunoglobulin heavy chain locus of activated B cells suggests a direct influence of estrogen on antibody expression

Females and males differ in antibody isotype expression patterns and in immune responses to foreign- and self-antigens. For example, systemic lupus erythematosus is a condition that associates with the production of isotype-skewed anti-self antibodies, and exhibits a 9:1 female:male disease ratio. To explain differences between B cell responses in males and females, we sought to identify direct interactions of the estrogen receptor (ER) with the immunoglobulin heavy chain locus. This effort was encouraged by our previous identification of estrogen response elements (ERE) in heavy chain switch (S) regions. We conducted a full-genome chromatin immunoprecipitation analysis (ChIP-seq) using DNA from LPS-activated B cells and an ERα-specific antibody. Results revealed ER binding to a wide region of DNA, spanning sequences from the JH cluster to Cδ, with peaks in Eμ and Sμ sites. Additional peaks of ERα binding were coincident with hs1,2 and hs4 sites in the 3' regulatory region (3'RR) of the heavy chain locus. This first demonstration of direct binding of ER to key regulatory elements in the immunoglobulin locus supports our hypothesis that estrogen and other nuclear hormone receptors and ligands may directly influence antibody expression and class switch recombination (CSR). Our hypothesis encourages the conduct of new experiments to evaluate the consequences of ER binding. A better understanding of ER:DNA interactions in the immunoglobulin heavy chain locus, and respective mechanisms, may ultimately translate to better control of antibody expression, better protection against pathogens, and prevention of pathologies caused by auto-immune disease.

Hotspots for Vitamin-Steroid-Thyroid Hormone Response Elements Within Switch Regions of Immunoglobulin Heavy Chain Loci Predict a Direct Influence of Vitamins and Hormones on B Cell Class Switch Recombination

Vitamin A deficiencies are common throughout the world and have a significant negative influence on immune protection against viral infections. Mouse models demonstrate that the production of IgA, a first line of defense against viruses at mucosal sites, is inhibited in the context of vitamin A deficiency. In vitro, the addition of vitamin A to activated B cells can enhance IgA expression, but downregulate IgE. Previous reports have demonstrated that vitamin A modifies cytokine patterns, and in so doing may influence antibody isotype expression by an indirect mechanism. However, we have now discovered hundreds of potential response elements among Sμ, Sɛ, and Sα switch sites within immunoglobulin heavy chain loci. These hotspots appear in both mouse and human loci and include targets for vitamin receptors and related proteins (e.g., estrogen receptors) in the nuclear receptor superfamily. Full response elements with direct repeats are relatively infrequent or absent in Sγ regions although half-sites are present. Based on these results, we pose a hypothesis that nuclear receptors have a direct effect on the immunoglobulin heavy chain class switch recombination event. We propose that vitamin A may alter S site accessibility to activation-induced deaminase and nonhomologous end-joining machinery, thereby influencing the isotype switch, antibody production, and protection against viral infections at mucosal sites.

The Significance of Interferon-γ in HIV-1 Pathogenesis, Therapy, and Prophylaxis

Interferon-γ (IFNγ) plays various roles in the pathogenesis of HIV/AIDS. In an HIV-1 infected individual, the production of IFNγ is detected as early as the acute phase and continually detected throughout the course of infection. Initially produced to clear the primary infection, IFNγ together with other inflammatory cytokines are involved in establishing a chronic immune activation that exacerbates clinical diseases associated with AIDS. Unlike Type 1 IFNs, IFNγ has no direct antiviral activity against HIV-1 in primary cultures, as supported by the in vivo findings of IFNγ therapy in infected subjects. Results from both in vitro and ex vivo studies show that IFNγ can instead enhance HIV-1 replication and its associated diseases, and therapies aimed at decreasing its production are under consideration. On the other hand, IFNγ has been shown to enhance cytotoxic T lymphocytes and NK cell activities against HIV-1 infected cells. These activities are important in controlling HIV-1 replication in an individual and will most likely play a role in the prophylaxis of an effective vaccine against HIV-1. Additionally, IFNγ has been used in combination with HIV-1 vaccine to augment antiviral immunity. Technological advancements have focused on using IFNγ as a biological marker to analyze the type(s) of immunity generated by candidate HIV vaccines and the levels of immunity restored by anti-retroviral drug therapies or novel immunotherapies. Hence, in addition to its valuable ancillary role as a biological marker for the development of effective HIV-1 prophylactic and therapeutic strategies, IFNγ has a vital role in promoting the pathogenesis of HIV.

Cernunnos influences human immunoglobulin class switch recombination and may be associated with B cell lymphomagenesis

B cells from Cernunnos-deficient patients contain aberrant class switch recombination junctions, and a dominant-negative Cernunnos mutation was detected in a diffuse large B cell lymphoma sample.

Cernunnos is involved in the nonhomologous end-joining (NHEJ) process during DNA double-strand break (DSB) repair. Here, we studied immunoglobulin (Ig) class switch recombination (CSR), a physiological process which relies on proper repair of the DSBs, in B cells from Cernunnos-deficient patients. The pattern of in vivo generated CSR junctions is altered in these cells, with unusually long microhomologies and a lack of direct end-joining. The CSR junctions from Cernunnos-deficient patients largely resemble those from patients lacking DNA ligase IV, Artemis, or ATM, suggesting that these factors are involved in the same end-joining pathway during CSR. By screening 269 mature B cell lymphoma biopsies, we also identified a somatic missense Cernunnos mutation in a diffuse large B cell lymphoma sample. This mutation has a dominant-negative effect on joining of a subset of DNA ends in an in vitro NHEJ assay. Translocations involving both Ig heavy chain loci and clonal-like, dynamic IgA switching activities were observed in this tumor. Collectively, our results suggest a link between defects in the Cernunnos-dependent NHEJ pathway and aberrant CSR or switch translocations during the development of B cell malignancies.

Rapid high-resolution mapping of balanced chromosomal rearrangements on tiling CGH arrays

The diagnosis and classification of many cancers depends in part on the identification of large-scale genomic aberrations such as chromosomal deletions, duplications, and balanced translocations. Array-based comparative genomic hybridization (array CGH) can detect chromosomal imbalances on a genome-wide scale but cannot reliably identify balanced chromosomal rearrangements. We describe a simple modification of array CGH that enables simultaneous identification of recurrent balanced rearrangements and genomic imbalances on the same microarray. Using custom tiling oligonucleotide arrays and gene-specific linear amplification primers, translocation CGH (tCGH) maps balanced rearrangements to ∼100-base resolution and facilitates the rapid cloning and sequencing of novel rearrangement breakpoints. As proof of principle, we used tCGH to characterize nine of the most common gene fusions in mature B-cell neoplasms and myeloid leukemias. Because tCGH can be performed in any CGH-capable laboratory and can screen for multiple recurrent translocations and genome-wide imbalances, it should be of broad utility in the diagnosis and classification of various types of lymphomas, leukemias, and solid tumors.

Overlapping activation-induced cytidine deaminase hotspot motifs in Ig class-switch recombination

Ig class-switch recombination (CSR) is directed by the long and repetitive switch regions and requires activation-induced cytidine deaminase (AID). One of the conserved switch-region sequence motifs (AGCT) is a preferred site for AID-mediated DNA-cytosine deamination. By using somatic gene targeting and recombinase-mediated cassette exchange, we established a cell line-based CSR assay that allows manipulation of switch sequences at the endogenous locus. We show that AGCT is only one of a family of four WGCW motifs in the switch region that can facilitate CSR. We go on to show that it is the overlap of AID hotspots at WGCW sites on the top and bottom strands that is critical. This finding leads to a much clearer model for the difference between CSR and somatic hypermutation.

On the genomics of immunoglobulins in the gray, short-tailed opossum Monodelphis domestica

Annotated maps of the IGH, IGK, and IGL loci in the gray, short-tailed opossum Monodelphis domestica were generated from analyses of the available whole genome sequence for this species. Analyses of their content and organization confirmed a number of previous conclusions based on characterization of complementary DNAs encoding opossum immunoglobulin heavy and light chains and limited genomic analysis, including (a) the predominance of a single immunoglobulin heavy chain variable region (IGHV) subgroup and clan, (b) the presence of a single immunoglobulin (Ig)G subclass, (c) the apparent absence of an IgD, and (d) the general organization and V gene complexity of the IGK and IGL light chain loci. In addition, several unexpected discoveries were made including the presence of a partial V to D, germline-joined IGHV segment, the first germline-joined Ig V gene to be found in a mammal. In addition was the presence of a larger number of IGKV subgroups than had been previously identified. With this report, annotated maps of the major histocompatibility complex, T-cell receptor, and immunoglobulin loci have been completed for M. domestica, the only non-eutherian mammalian species for which this has been accomplished, strengthening the utility of this species as a model organism.

Non-homologous end joining in class switch recombination: the beginning of the end

Immunoglobulin class switch recombination (CSR) is initiated by a B-cell-specific factor, activation-induced deaminase, probably through deamination of deoxycytidine residues within the switch (S) regions. The initial lesions in the S regions are subsequently processed, resulting in the production of DNA double-strand breaks (DSBs). These breaks will then be recognized, edited and repaired, finally leading to the recombination of the two S regions. Two major repair pathways have been implicated in CSR, the predominant non-homologous end joining (NHEJ) and the alternative end-joining (A-EJ) pathways. The former requires not only components of the ‘classical’ NHEJ machinery, i.e. Ku70/Ku80, DNA-dependent protein kinase catalytic subunit, DNA ligase IV and XRCC4, but also a number of DNA-damage sensors or adaptors, such as ataxia–telangiectasia mutated, γH2AX, 53BP1, MDC1, the Mre11–Rad50–NBS1 complex and the ataxia telangiectasia and Rad3-related protein (ATR). The latter pathway is not well characterized yet and probably requires microhomologies. In this review, we will focus on the current knowledge of the predominant NHEJ pathway in CSR and will also give a perspective on the A-EJ pathway.

Analysis of clonotypic switch junctions reveals multiple myeloma originates from a single class switch event with ongoing mutation in the isotype-switched progeny

Multiple myeloma (MM) is a cancer of plasma cells (PCs) expressing immunoglobulin heavy chain (IgH) postswitch isotypes. The discovery of earlier stage cells related to postswitch PCs, called preswitch clonotypic IgM (cIgM) cells led to the hypothesis that cIgM cells may be MM progenitors, replenishing the tumor throughout malignancy. cIgM cells may do this by undergoing class switch recombination (CSR), a process detectable in postswitch PCs as multiple IgH switch junctions associated with a single clonotypic IgH V/D/J. We addressed this with a specific clonotypic-switch polymerase chain reaction (PCR), informative for 32 of 41 cases. Here we made 2 significant discoveries: (1) in all cases, we detected only a single clonotypic switch fragment that persists over time (1-7.6 years), and (2) we detected ongoing mutation upstream of the switch junction in 5 of 6 patients, often targeting the intronic enhancer, a key control region in IgH expression. The presence of a single, unchanging clonotypic switch junction suggests that cIgM cells are not MM-PC progenitors; rather, postswitch PCs arise from a single cIgM cell, and MM-PC progenitors reside in the postswitch population. Furthermore, mutations revealed here provide a new marker to identify MM-PC progenitors and aggressive clones that evolve throughout malignancy.

Class switch recombination: a comparison between mouse and human

Humans and mice separated more than 60 million years ago. Since then, evolution has led to a multitude of changes in their genomic sequences. The divergence of genes has resulted in differences both in the innate and adaptive immune systems. In this chapter, we focus on species difference with regard to immunoglobulin class switch recombination (CSR). We have compared the immunoglobulin constant region gene loci from human and mouse, with an emphasis on the switch regions, germ line transcription promoters, and 3' enhancers. We have also compared pathways/factors that are involved in CSR. Although there are remarkable similarities in the cellular machinery involved in CSR, there are also a number of unique features in each species.

Regulation of IgE homeostasis, and the identification of potential targets for therapeutic intervention

Atopic allergies have increased during the past 20-30 years in frequency quite dramatically and in many countries have reached almost epidemic proportions. Allergies have thereby become one of the major medical issues of the western world. Immunoglobulin E (IgE) is here a central player. IgE is the Ig class that is present in the lowest concentration in human plasma. IgG is, for example, 10 000 to 1 million times more abundant than IgE. However, despite of its low plasma levels IgE is a very important inducer of inflammation, due to its interaction with high-affinity receptors on mast cell and basophils. IgE has been conserved as a single active gene in all placental mammals studied, and the expression of this gene is under a very stringent control, most likely due to its very potent inflammatory characteristics. IgE expression is being regulated at many levels: by cytokines, switch region length, positive and negatively acting transcription factors and suppressors of cytokine signaling (SOCS). In addition, the plasma half-life differs markedly for IgG and IgE, with 21 and 2.5 days, respectively. This review summarizes the rapid progress in our understanding of the complex network of regulatory mechanisms acting on IgE and also how this new information may help us in our efforts to control IgE-mediated inflammatory conditions.

Association of Ig/BCL6 translocations with germinal center B lymphocytes in human lymphoid tissues: implications for malignant transformation

Chromosomal translocations (CTs) between immunoglobulin (Ig) genes and the BCL6 proto-oncogene are frequently associated with diffuse large B-cell lymphomas (DLBCLs) and follicular lymphomas (FLs) and are implicated in the development of these lymphomas. However, whether Ig/BCL6 translocation per se is sufficient to drive malignant transformation is not clear. To understand the biology of Ig/BCL6-translocated cells prior to their malignant transformation, we developed a system capable of detecting 1 to 3 Igmu/BCL6 CT cells in 1 million mixed cells through the detection of chimeric Imu-BCL6E2 and BCL6E1-Cmu1 transcripts that reflect reciprocal Igmu/BCL6 translocations. The chimeric transcripts that existed in the vast majority of normal lymphoid tissues are due to Igmu/BCL6 CT and were not generated from trans-splicing. Both Imu-BCL6E2 and BCL6E1-Cmu1 transcripts were coexpressed in the same cell populations. The Ig/BCL6 recombination junctions themselves were isolated from B-cell subpopulations expressing the Imu-BCL6 transcripts. The appearance of Igmu/BCL6 CT was associated with cells expressing germinal center but not naive B-cell markers. This study shows that Ig/BCL6 translocations occur in germinal center-stage B cells in healthy humans, and that Ig/BCL6 CTs per se are not likely sufficient to cause the malignant transformation in the context of human B cells.

The involvement of immunoglobulin E isotype switch in scleroderma skin tissue

BACKGROUND

The involvement of mast cell, which is activated by immunoglobulin E (IgE), has been reported in the formation of systemic sclerosis (SSc) abnormality. IgE is generated with isotype switch. During isotype switch, switch circles resulting from direct mu to epsilon, or from sequential mu to gamma via epsilon switching will be created.

OBJECTIVE

We studied whether switching occurs in SSc.

METHODS

We used nested polymerase chain reaction to analyze the S fragments from switch circles. Fifty-two patients with SSc, and 62 healthy women were studied.

RESULTS

Neither of 62 normal skin tissues showed direct switch, nor sequential switch. Neither of seven normal whole blood cells showed direct switch, nor sequential switch. In 52SSc skin tissues, three (5.8%) showed direct switch, and two (3.8%) showed sequential switch. As a result, five (9.6%) of SSc skin tissue showed immunogobulin E class switch. These results were confirmed by DNA sequencing.

CONCLUSION

These results demonstrated that isotype switch to the epsilon locus achieved by direct and/or sequential switch are involved in SSc skin.

Ongoing immunoglobulin class switch DNA recombination in lupus B cells: analysis of switch regulatory regions

Inflammation and tissue damage in systemic lupus erythematosus (SLE) are mediated by class-switched autoantibodies reactive with nucleic acids, nucleic acid-binding proteins, phospholipids and other self-antigens. While some healthy individuals produce IgM antibodies with specificities similar to those of lupus patients, immunoglobulin class switching to mature downstream isotypes appears to be required for the generation of pathogenic autoantibodies. To characterize the cellular and molecular basis of pathogenic autoantibody production in SLE, we studied the capacity of peripheral blood B cells of naive phenotype from patients with SLE, rheumatoid arthritis (RA) or healthy control subjects to spontaneously switch to IgG and IgA. In addition, we determined the DNA sequences of the upstream evolutionary conserved sequence (ECS)-Igamma promoter regulatory regions that control germline IH-CH transcription and class switch DNA recombination (CSR) to IgG1, IgG2 and IgG4. IgM+IgD+ B cells from patients with SLE, but not those from RA or healthy control subjects, underwent spontaneous CSR, as assessed by expression of germline Igamma1-Cgamma1, Igamma2-Cgamma2, Igamma3-Cgamma3, Igamma4-Cgamma4 and Ialpha1-Calpha1 transcripts, mature (switched) VHDJH-Cgamma1, VHDJH-Cgamma2, VHDJH-Cgamma3 and VHDJH-Calpha1 transcripts and secreted IgG and IgA. Although polymorphic DNA sequences were identified in the ECS-Igamma1, ECS-Igamma2 and ECS-Igamma4 promoter regions, the transcription factor-binding sites that mediate germline Igamma-Cgamma transcription were conserved in patients and controls. However, distinct patterns of nuclear protein binding to an ECS-Igamma promoter sequence that contains both positive and negative regulatory elements were observed in SLE patients and controls. These results support a role for exogenous signals, such as through CD40 ligation, rather than altered genomic sequence, in the increased production of class switched autoantibodies in SLE.

DNA acrobats of the Ig class switch

Small resting B lymphocytes all start out producing IgM Abs. Upon encountering Ag, the cells become activated and make a switch from IgM to other Ig classes. This class switch serves to distribute a particular V region to different Ig C regions. Each C region mediates a specialized effector function, and so, through switching, an organism can guide its Abs to various sites. Creating the new H chain requires loop-out and deletion of DNA between switch regions. These DNA acrobatics require transcription of the switch regions, presumably so that necessary factors can gain access to the DNA. These requisite switching factors include activation-induced cytidine deaminase and components of general DNA repair, including base excision repair, mismatch repair, and double-strand break repair. Despite much recent progress, not all important factors have been discovered, especially those that may guide recombination to a particular subclass.

Selective inhibition of class switching to IgG and IgE by recruitment of the HoxC4 and Oct-1 homeodomain proteins and Ku70/Ku86 to newly identified ATTT cis-elements

Immunoglobulin (Ig) class switching is central to the maturation of the antibody response as IgG, IgA, and IgE are endowed with more diverse biological effector functions than IgM. It is induced upon engagement of CD40 on B lymphocytes by CD40L expressed by activated CD4+ T cells and exposure of B cells to T cell-secreted cytokines including interleukin-4 and transforming growth factor-beta. It begins with germ line IH-CH transcription and unfolds through class switch DNA recombination (CSR). We show here that the HoxC4 and Oct-1 homeodomain proteins together with the Ku70/Ku86 heterodimer bind as a complex to newly identified switch (S) regulatory ATTT elements (SREs) in the Igamma and Iepsilon promoters and downstream regions to dampen basal germ line Igamma-Cgamma and Iepsilon-Cepsilon transcriptions and repress CSR to Cgamma and Cepsilon. This mechanism is inactive in the Calpha1/Calpha2 loci because of the lack of SREs in the Ialpha1/Ialpha2 promoters. Accordingly, in resting human IgM+IgD+ B cells, HoxC4, Oct-1, and Ku70/Ku86 can be readily identified as bound to the Igamma and Iepsilon promoters but not the Ialpha1/Ialpha2 promoters. CD40 signaling dissociates the HoxC4.Oct-1. Ku complex from the Igamma and Iepsilon promoter SREs, thereby relieving the IH-CH transcriptional repression and allowing CSR to unfold. Dissociation of HoxC4.Oct-1. Ku from DNA is hampered by CD153 engagement, a CD40-signaling inhibitor. Thus, these findings outline a HoxC4.Oct-1. Ku-dependent mechanism of selective regulation of class switching to IgG and IgE and further suggest distinct co-evolution and shared CSR activation pathways in the Cgamma and Cepsilon as opposed to the Calpha1/Calpha2 loci.

Clinical and genetic characteristics of Japanese Burkitt lymphomas with or without leukemic presentation

To clarify the clinical and genetic features of Burkitt lymphoma with or without leukemic presentation, we have conducted clinical, cytogenetic, and genetic studies. Of 40 Japanese patients with Burkitt lymphoma examined by cytogenetic and/or fluorescence in situ hybridization analysis or Southern blot analysis using MYC probes, 35 patients had t(8;14) translocations, and 5 had t(8;22). Breakpoints were located far upstream of MYC in 4 (12%) of 33 tumors with t(8;14), and Epstein-Barr virus infection was found in 3 (8%) of 40 tumors. These findings are similar to those reported for non-Japanese patients with the sporadic form of Burkitt lymphoma. Clinical and genetic characteristic were compared for 30 patients presenting with lymphoma and 10 presenting with leukemia. The overall survival was shorter in aggressively treated leukemia patients than in aggressively treated lymphoma patients (P = .003); however, the incidence rates of TP53 mutation, p16INK4a deletion, and p15INK4b deletion that were found in 6 (15%) of 40,3 (9%) of 35, and 2 (6%) of 35 tumors, respectively, were similar between the 2 subtypes. Thus, the present study has shown the different prognoses for the 2 subtypes of Burkitt lymphoma but has failed to clarify the genetic backgrounds that may explain the different outcomes.

Efficiency of Iε promoter-directed switch recombination in GFP expression-based switch constructs works synergistically with other promoter and/or enhancer elements but is not tightly linked to the strength of transcription

One key unresolved issue in immunoglobulin class switch recombination (CSR) is how the accessibility of the switch region for CSR is controlled. To better understand the nature of accessibility control for human Ig CSR, we developed a novel inducible switch recombination assay based on expression of green fluorescence protein (GFP) from switch constructs undergoing substrate switch recombination (SSR). Efficient SSR depends on the cytokine-inducible Iepsilon promoter and co-stimulation with IL-4+anti-CD40. Characterization of SSR reveals that both S-S deletional recombination and S-S inversion occur. We show that the IL-4-inducible Iepsilon promoter (pIepsilon) selectively determines the efficiency of the accessibility for SSR. However, the pIepsilon-induced transcription, by itself,is not sufficient to direct efficient SSR. For efficient SSR, both pIepsilon-driven transcriptional activity and an additional promoter/enhancer-derived activity are required. The efficiency of SSR is not tightly correlated with the strength of the combined transcriptional activity. Our results suggest that the mechanism(s) underlying the transcriptional activity, e.g. DNA modification is important for controlling the accessibility for efficient switch recombination.

A study of Gm allotypes and immunoglobulin heavy gamma IGHG genes in Berbers, Arabs and sub-Saharan Africans from Jerba Island, Tunisia

The Gm polymorphism of human IgG immunoglobulins was investigated in three different ethnic groups--Arabs, Berbers and 'dark-skinned people'--on Jerba Island, Tunisia. The genetic relationships among these groups and several populations from North Africa, sub-Saharan Africa, west Asia and Europe were analysed by principal coordinate analysis, Fst significance testing, and analysis of molecular variance based on haplotype frequencies. The results revealed a non-significant genetic differentiation between Arabs and Berbers from Jerba. However, the Jerbian population of sub-Saharan African origin was close to Ethiopians. Gene flow among the three Jerbian populations, as well as an East African origin of the dark-skinned individuals, is proposed to account for the observed genetic pattern. However, the genetic diversity observed among the different Tunisian populations did not show any significant correlation with either geographic or linguistic differentiation. A preliminary analysis of the restriction fragment length polymorphism of the IGHG genes in Arabs and Berbers from Jerba confirmed the close genetic relationship between the two populations. However, it also indicated a lower level of genetic diversity in the Berbers, which may be explained by more rapid genetic drift due to longer isolation on the island.

Rapid recombination among transfected plasmids, chimeric episome formation and trans gene expression in Plasmodium falciparum

Although recombination is known to be important to generating diversity in the human malaria parasite P. falciparum, the low efficiencies of transfection and the fact that integration of transfected DNA into chromosomes is observed only after long periods (typically 12 weeks or more) have made it difficult to genetically manipulate the blood stages of this major human pathogen. Here we show that co-transfection of a P. falciparum line with two plasmids, one expressing a green fluorescent protein (gfp) reporter and the other expressing a drug resistance marker (Tgdhfr-ts M23), allowed selection of a population in which about approximately 30% of the parasites produce GFP. In these GFP-producing parasites, the transfected plasmids had recombined into chimeric episomes as large as 20 kb and could be maintained under drug pressure for at least 16 weeks. Our data suggest that chimera formation occurs early (detected by 7--14 days) and that it involves homologous recombination favored by presence of the same P. falciparum 5'hrp3 UTR promoting transcription from each plasmid. This indicates the presence of high levels of homologous recombination activity in blood stage parasites that can be used to drive rapid recombination of newly introduced DNA, study mechanisms of recombination, and introduce genes for trans expression in P. falciparum.

Insertional events as well as translocations may arise during aberrant immunoglobulin switch recombination in a patient with multiple myeloma

The majority of patients with multiple myeloma have translocations involving the immunoglobulin heavy chain switch regions on chromosome 14q32 and a promiscuous range of partner chromosomes. We describe a patient with an insertion of 132 bp of chromosome 22q12 sequence into the 5' region flanking S(mu) on chromosome 14q32. The 132 bp region from chromosome 22q12 contains the whole of exon 3 from a novel gene of unknown function in man. The significance of such insertional events remains unclear. The description of insertional events occurring as a result of abnormal switch recombination suggests that, in myeloma, dysregulation of oncogenes may occur by a mechanism other than chromosomal translocation.

Class switch recombination of the chicken IgH chain genes: implications for the primordial switch region repeats

In mammals and the amphibian, Xenopus, isotypes of antibodies have been shown to be changed through class switch recombination within the IgH chain gene locus. Here, we identified switch (S) repetitive sequences in the 5' introns of the Ig C(mu) and C(gamma) genes of the chicken. The S(mu) region is composed of two homologous regions, S(mu)1 and S(mu)2. The S(mu)1 region is an upstream 3.7 kb sequence composed of 37 repeats of a consensus sequence containing tandem repeats of the decamer ACCAGTATGG. The S(mu)2 region is a downstream 1.4 kb sequence consisting of simple tandem repeats of a decamer CCCAGTACAG. The S(gamma) region contains repeats of the decamer TATGGGGCAG. Analysis of chicken IgG-producing hybridomas revealed that the C(mu) gene was deleted from the chromosome by the recombination occurring between the S(mu) and S(gamma) regions. Recombination breakpoints at the C(mu) gene of splenocytes from an immunized chicken were scattered around the S(mu) region and two such breakpoints, the precise position of which were determined, were located within possible hairpin loop structures at the palindromic sequence of S(mu)1. A primordial palindromic sequence from which the prevalent switch repeat motifs of mammals, chickens and amphibians may have diverged is presented.

Stable RNA/DNA hybrids in the mammalian genome: inducible intermediates in immunoglobulin class switch recombination

Although it is well established that mammalian class switch recombination is responsible for altering the class of immunoglobulins, the mechanistic details of the process have remained unclear. Here, we show that stable RNA/DNA hybrids form at class switch sequences in the mouse genome upon cytokine-specific stimulation of class switch in primary splenic B cells. The RNA hybridized to the switch DNA is transcribed in the physiological orientation. Mice that constitutively express an Escherichia coli ribonuclease H transgene show a marked reduction in RNA/DNA hybrid formation, an impaired ability to generate serum immunoglobulin G antibodies, and significant inhibition of class switch recombination in their splenic B cells. These data provide evidence that stable RNA/DNA hybrids exist in the mammalian nuclear genome, can serve as intermediates for physiologic processes, and are mechanistically important for efficient class switching in vivo.

Transcription-dependent R-loop formation at mammalian class switch sequences

Immunoglobulin class switching is mediated by recombination between switch sequences located immediately upstream of the immunoglobulin constant heavy chain genes. Targeting of recombination to particular switch sequences is associated temporally with transcription through these regions. We recently have provided evidence for inducible and stable RNA-DNA hybrid formation at switch sequences in the mouse genome that are mechanistically important for class switching in vivo. Here, we define in vitro the precise configuration of the DNA and RNA strands within this hybrid structure at the Smicro, Sgamma3 and Sgamma2b mouse switch sequences. We find that the G-rich (non-template) DNA strand of each switch sequence is hypersensitive to probes throughout much of its length, while the C-rich (template) DNA strand is essentially resistant. These results demonstrate formation of an R-loop, whereby the G-rich RNA strand forms a stable heteroduplex with its C-rich DNA strand counterpart, and the G-rich DNA strand exists primarily in a single-stranded state. We propose that the organized structure of the R-loop is essential for targeting the class switch recombination machinery to these sequences.

Cell-free recombination of immunoglobulin switch-region DNA with nuclear extracts

We have developed an in vitro recombination system employing cell-free nuclear extracts from human B lymphocytes capable of detecting the recombination between human mu switch (Smu) and Sepsilon sequences in a model plasmid. Nuclear extracts from CD40-stimulated B lymphocytes gave a higher frequency of recombination in the assay than the unstimulated B cells. Recombination between Smu and Sepsilon was mediated by the nuclear extracts as the recombinational products could be amplified prior to bacterial transformation. Characterization of the recombination products demonstrated that the recombination process had the characteristics of immunoglobulin (Ig) isotype switching, as it was (i) switch-region-sequence specific, (ii) nonhomologous recombination, and (iii) enhanced by CD40 stimulation. Transcription through the S region DNA was not required for recombination in the system. These results demonstrate that Ig switch-region DNA recombination can be accomplished in vitro by cell-free nuclear extracts. This in vitro system for Ig switch-region DNA recombination using cell-free nuclear extracts will permit the dissection of the events involved in IgE class switch recombination, a critical event in the development of allergic diseases.

CAGGG repeats and the pericentromeric duplication of the hominoid genome

Gene duplication is one of the primary forces of evolutionary change. We present data from three different pericentromeric regions of human chromosomes, which indicate that such regions of the genome have been sites of recent genomic duplication. This form of duplication has involved the evolutionary movement of segments of genomic material, including both intronic and exonic sequence, from diverse regions of the genome toward the pericentromeric regions. Sequence analyses of the target sites of duplication have identified a novel class of interspersed GC-rich repeats located precisely at the boundaries of duplication. Estimates of the evolutionary age of these duplications indicate that they have occurred between 10 and 25 mya. In contrast, comparative analyses confirm that the GC-rich pericentromeric repeats have existed within the pericentromeric regions of primate chromosomes before the divergence of the cercopithecoid and hominoid lineages ( approximately 30 mya). These data provide molecular evidence for considerable interchromosomal duplication of genic segments during the evolution of the hominoid genome and strongly implicate GC-rich repeat elements as playing a direct role in the pericentromeric localization of these events

Clonality and specificity of cryoglobulins associated with HCV: pathophysiological implications

BACKGROUND/AIMS

Hepatitis C virus (HCV) infection plays a central role in the pathogenesis of mixed cryoglobulinemia through molecular mechanisms which remain to be elucidated. The aim of this study was to investigate the role of antibody responses to HCV in the pathogenesis of cryoglobulinemia through characterization of the anti-HCV specificity and immunochemical characteristics of the immunoglobulins involved in cryoprecipitation.

METHODS

Sera from 50 consecutive patients with chronic HCV infection (RNA positive) were screened for the presence of cryoglobulins. The two major components of cryoprecipitates, IgM rheumatoid factors and IgG, were separated by high performance liquid chromatography and analyzed for immunochemical composition by immunoblotting and antibody specificity by ELISA and immunoblotting using recombinant HCV proteins and synthetic peptides as antigens.

RESULTS

Cryoprecipitates were observed in 27 patients and characterized by immunofixation: 13 (48%) were classified as type II and 14 (52%) as type III. Monoclonal immunoglobulins were detected by immunoblotting in 20 cryoprecipitates: IgM in 14 samples and IgG in 14, with a clear preponderance of IgG3 (12/14). Specificity studies on sera and purified IgM and IgG fractions from cryoprecipitates revealed enrichment in cryoglobulins, predominantly polyclonal IgG1, reactive with the HCV structural proteins, whereas specificities for nonstructural viral proteins were relatively less represented compared to whole serum. No restricted pattern of fine specificity was observed. IgG3 subclass was apparently not involved in HCV nucleoprotein binding.

CONCLUSIONS

Our findings do not support a direct link between monoclonal cryoglobulins and immune response to HCV According to the proposed pathogenetic model, HCV infection can induce the formation of cryoprecipitable rheumatoid factors, sustain their production, and eventually lead to monoclonal B-cell expansion through several cooperative mechanisms.

The mouse Ms6-hm hypervariable microsatellite forms a hairpin and two unusual tetraplexes

The mouse Ms6-hm microsatellite consists of a tandem array of the pentamer d(CAGGG)n. This microsatellite is extremely hypervariable, showing a germ line mutation rate of 2.5%/gamete. The mechanism responsible for this instability is not known. The ability to form intrastrand structures is a conserved feature of many hypervariable sequences, and it has been suggested that the formation of such structures might account for instability by affecting DNA replication, repair, or recombination. Here we show that this microsatellite is able to form intrastrand structures as well. Under physiological conditions, the Ms6-hm microsatellite forms a hairpin as well as two different unusual intrastrand tetraplexes. The hairpin forms in the absence of monovalent cation and contains G.A, G.C, and G.G base pairs in a 1:1:1 ratio. In the presence of K+, a tetraplex is formed in which the adenines are unpaired and extrahelical, and the cytosines are involved in C.C pairs. In Na+, a tetraplex forms that contains C.C+ pairs, with the adenines being intrahelical and hydrogen-bonded to guanines. Tetraplex formation in the presence of Na+ requires both cytosines and adenines and might reflect the altered internal dimensions of this tetraplex, perhaps resulting from the ability of the C.C+ pairs to become intercalated in this sequence context. Our demonstration of the stabilization of tetraplexes by hydrogen bonding between adenines and guanines expands the hydrogen-bonding possibilities for tetraplexes and suggests that the category of sequences with tetraplex-forming potential may be larger than previously appreciated.

Aberrant rearrangements of the immunoglobulin heavy chain switch region in chronic B-cell leukemia

Analysis of the organisation of the Cmu-switch region of the immunoglobulin heavy chain locus in B-lymphocytes from 80 patients with chronic B-cell leukemia revealed 25 patients with abnormal rearrangements that could not be explained by the normal recombination events that take place in B-lymphocytes. Detailed analysis with probes spanning the Cmu -switch region and various restriction digests localised the rearrangements in two thirds of the patients to a 1300 bp region at the 5' end of the switch region while in the remaining patients the rearrangements occurred in the switch region. The consequences of these aberrant rearrangements remain to be determined, but their clustering to a defined region of the switch region suggests a "hot spot" that may be involved in the aetiology of the disease.

The region 3' to Calpha1 gene of human IG heavy chain displays a polymorphic duplicated sequence and encodes an RNA associated with polysomes

A highly spread polymorphism flanking the 3. Calpha1 human IG heavy chain gene was identified. This polymorphism allowed the detection of an internal duplication within the 3' flanking region of both Calpha1 and Calpha2. This region has a regulatory function with four enhancer structures also present at the 3' end of the human Calpha2 as well as in that of mouse and rat single Calpha genes. The 5682-bp sequence of clone lambdapl8 described here starts 3' of Calpha1 and presents three open reading frames; one of them contains part of the tandem repeats with the 20-bp consensus described previously that is expressed in a poly(A)+ RNA and found in three dbEST clones of the human tonsillar cDNA library. Here, we demonstrate that in the CLF1 B lymphoblastoid cell line, this transcript is associated with polysomes. We also discuss the possibility of the presence of a new regulatory gene that does not encode an immunoglobulin and maps in the human IG heavy chain gene cluster.

Combined nasal challenge with diesel exhaust particles and allergen induces In vivo IgE isotype switching

In this study we undertook to provide evidence for local in vivo isotype switching to IgE following nasal challenges. Detection of deleted switch circular DNA (switch circles) by a novel nested polymerase chain reaction-based approach was employed as definitive molecular evidence of Ig isotype switching. Nasal challenge in humans with diesel exhaust particles (DEP) plus ragweed antigen has been shown to enhance local IgE production, stimulate local cytokine production, and markedly increase mucosal IgE antibody to ragweed. Four days after combined intranasal DEP plus ragweed challenge, we detected and characterized clones of deleted switch circular DNA (Sepsilon /Smu) representing switching from mu to epsilon from nasal lavage cells. No switch circular DNA was detected in nasal lavage cells following challenge with DEP alone nor with ragweed allergen alone. These results indicate that the combination of mucosal stimulation with DEP and ragweed allergen is capable of driving in vivo isotype switching to IgE in humans with ragweed allergy. These results are the first direct demonstration of in vivo IgE isotype switching in humans.

Somatic mutations in the Ig variable region genes and expression of novel Cmu-germline transcripts in a B-lymphoma cell line ("Farage") not producing Ig polypeptide chains

Non-Hodgkin's B-lymphomas (B-NHL) are a very heterogeneous group of B-cell neoplasias originating from the germinal centers of lymphatic follicles. Thus, they represent a suitable experimental model to study the molecular basis of certain key events which take place in the lymphatic follicles, including somatic hypermutation and heavy chain isotypic switch. An unusual B-NHL cell line ("Farage") not producing Ig polypeptide chains was previously shown to rearrange its IgH and Igkappa genes and transcribe seemingly normal size mu and kappa mRNAs. In an attempt to characterize the phenotype of Farage cells better and to elucidate the molecular basis of the failure of Farage cells to synthesize Ig chains, we sequenced its VH and Vkappa rearranged gene segments by PCR and RT-PCR. It was found that both V genes are somatically, heavily mutated compared to their germline counterparts. In addition, this rearranged VDJ gene of the heavy chain is not transcribed. Instead, the Farage cells express a low level of a new family of germline transcripts starting with a VH like sequence, continuing with a small segment of the 3'VH germline flanking region, and ending within the Cmu region. These transcripts lack D and J segments and do not contain the open reading frame of the full-length Cmu protein. Thus, Farage cells fail to produce mu heavy chains due to silencing of the expression of the conventional VDJCmu transcript and expression of unusual Cmu-germline transcripts. In contrast to the IgH genes, the rearranged VJ gene of Farage is transcribed and gives rise to a full-size kappa-mRNA. This transcript, however, is not translated to a full-length kappa-chain, as it contains a stop codon in its coding region. All the above show that Farage cells are unable to produce Ig polypeptide chains, due to somatic mutations altering the kappa-chain gene, and mutations and/or regulatory events that shutoff the transcription of the IgH gene. The heavily mutated Vkappa and Vkappa genes found, support the conclusion that the Farage cell line originated either from germinal center cells or from the mantle zone of the lymphoid follicle.

A highly sensitive and specific assay using a novel human growth hormone cDNA reporter gene regulated by the human interleukin-4 inducible germline epsilon transcript promoter

We have successfully developed a highly sensitive and specific assay system for human interleukin-4 (IL-4) regulated gene expression. It is based on a human Jijoye cell line with the germline epsilon transcript promoter joined to the human growth hormone (hGH) cDNA. The germline epsilon transcript promoter is responsive to IL-4 and involved in immunoglobulin heavy chain class switching. We cloned hGH complementary DNA (cDNA) as the reporter gene instead of using conventional hGH genomic DNA which failed to generate any IL-4 inducible clone in human Jijoye cells. The two IL-4 inducible cell lines with the hGH cDNA reporter show high signal/noise ratio for IL-4-mediated induction (60-90 fold). The response to IL-4 is dose-dependent with ED50 of 10 pM. As expected, there is no response to other human cytokines and growth factors, as well as mouse IL-4. The mutant hIL-4 antagonist hIL-4.Y124D inhibits the induction mediated by native hIL-4. These IL-4 inducible cell lines provide a sensitive, specific assay system to study IL-4-regulated gene expression, and in particular the regulation of the germline epsilon promoter.

CD40 Ligand Exerts Differential Effects on the Expression of Iγ Transcripts in Subclones of an IgM+ Human B Cell Lymphoma Line

The CD40:CD40 ligand (CD40L) interaction plays a critical role in T cell-dependent isotype switching. To elucidate the role of CD40 signaling in the activation of γ germline transcription and as an extension, in targeting Cγ regions for isotype switching, an IgM+ Burkitt lymphoma cell line (Ramos 2G6) was assayed for the up-regulation of germline γ transcripts after CD40L stimulation. Independent Ramos 2G6 subclones that either expressed (Iγ+) or did not express (Iγ−) basal levels of Iγ transcripts were assessed for their transcriptional response to CD40L signaling by contact with either a Jurkat T cell line (D1.1) or a transfected CD40L-expressing epithelial cell line (293/CD40L) in the presence or absence of IL-4. Both Iγ− and Iγ+ Ramos 2G6 subclones cultured with IL-4 and CD40L markedly up-regulated germline transcription predominantly from the γ1, γ2, and γ3 subclasses over levels obtained with IL-4 alone. In addition, these two signals were required to obtain de novo switch recombination. However, incubation with CD40L alone resulted in a substantial increase in germline transcription only in the Iγ+ and not the Iγ− subclones. Observed basal transcription at the γ1 locus also correlated with the ability of not only the γ1 locus, but also the γ2 and γ3 loci, to up-regulate germline transcripts in response to CD40 signaling. These data are consistent with CD40:CD40L contact up-regulating germline transcription only after the B cell has received a signal that alters the transcriptional state of the heavy chain locus.

Structural analysis of human gamma 3 intervening regions and switch regions: implication for the low frequency of switching in IgG3-deficient patients

High and low serum concentrations of IgG3 are associated with the human G3m(b) and G3m(g) allotypes, respectively. In the present study, we analyzed the structure of the S gamma 3 and I gamma 3, the switch frequency, switch breakpoints and the levels and initiation sites of I gamma 3 transcripts both in normal blood donors expressing (b) or (g) allotypes as well as IgG3-deficient (D) patients. A low switch frequency to gamma 3 was found in the (g) allotype IgG3D patients which may be caused in part by the allotype-associated mutations in the S gamma 3 region and in part by additional individual mutations observed in the A (SNAP) and B (SNIP/ NF-kappa B) sites in the S gamma 3 repeat region. A higher I gamma 3 germ-line (GL) transcriptional rate was seen in cells from the IgG3D patient, suggesting that low levels of GL I gamma 3 transcripts are not a major contributing factor to the defect. However, individual mutations in the I gamma 3 region and differential splicing of GL I gamma 3 transcripts were found which may affect the switching process.

Microsites for immunoglobulin switch recombination breakpoints from Xenopus to mammals

Immunoglobulin (Ig) heavy chain class switch recombination has been studied at the DNA level in a non-mammalian vertebrate, the amphibian Xenopus. A switch (S) region of about 5 kb has been identified in the JH-C mu intron of the Ig heavy chain locus in Xenopus. S mu contains 23 repeats approximately 150 bp long. Each repeat consists of internal shorter repeats and palindromic sequences, such as AGCT, which they share with mammalian switch regions. A deletion of the mu gene and the joining of the S regions of mu and chi occurs in B cells expressing IgX, one of the two non-mu isotypes in Xenopus. S chi shows no sequence homology with S mu and is characterized by 16 and 121 bp repeats and a high frequency of CATG, AGCA and TGCA palindromes. Both IgM and IgX S regions are AT rich and not GC rich like mammalian S regions. Recombination occurs, most of the time, at positions (microsites) where a single-stranded DNA folding program predicts the transition from a stem to a loop structure. This feature is conserved in most mammalian switch junctions which points to the general existence and involvement of microsites at one step of the determination of the recombination break-point. The recombinogenic nature of the switch regions is therefore linked to its structure rather than to its base composition, the repetitive occurrence of palindromes being essential at creating many microsites.

Sp1 binds to the precise locus of end processing within the terminal repeats of Epstein-Barr virus DNA

Interconversion between the linear genome of Epstein-Barr virus (EBV) present in virions and intracellular circular EBV DNA is a novel DNA recombination process. A previously characterized DNA binding activity called terminal repeat or tandem repeat binding protein (TRBP) was found to recognize several G-rich recombinogenic sequences in the EBV genome and in cellular DNA. TRBP was also found to be an autoantigen recognized by sera from certain patients with undifferentiated connective-tissue disorders. Here the transcription factor Sp1 has been identified as a component of TRBP and has been shown to be an autoantigen. Sp1 bound to recombination junctions of EBV DNA, such as those in the terminal repeats and in the large internal repeats, as well as to recombinogenic regions of cellular DNA, such as variable-number tandem repeats and switch regions of the immunoglobulin genes. We defined the ends of the linear EBV genome present in virions and showed that Sp1 binds to the sequence (GGGGTGGGGCATGGG) within EBV terminal repeats at the precise locus of interconversion of linear and circular viral DNA. Sp1 may be involved in DNA recombination.

Vasoactive intestinal peptide induces S(alpha)/S(mu) switch circular DNA in human B cells

Vasoactive intestinal peptide (VIP), a major neurotransmitter of peripheral nerves, has been suggested to function in host defense by regulating local human immune function. Indirect evidence has been marshaled that VIP can function as a switch factor for IgA in human Ig isotype recombination. In this study we directly tested the ability of VIP to function as a factor driving human B cells into IgA producing cells by assessing its ability to induce switch circular DNA representing direct mu to alpha switching. In addition we determined the generation of alpha germ-line transcripts and measured the level of IgA protein produced. Stimulation with VIP and CD40 mAb induced IgA production by human IgD+ B cells while VIP or CD40 alone failed to do so. Stimulation of purified IgD+ B cells with VIP plus CD40 mAb induced generation of switch circular DNA representing in vitro driven isotype switching from mu to alpha. CD40 mAb alone induced alpha germ-line transcripts but not IgA switch circles. Thus VIP, a neurogenic factor, can induce alpha-specific switching in CD40-activated human B cells and may thereby play an important role in directing the humoral immune response at mucosal surfaces.

The human IgE germline promoter is regulated by interleukin-4, interleukin-13, interferon-alpha and interferon-gamma via an interferon-gamma-activated site and its flanking regions

Class switching to IgE is preceded by the appearance of epsilon germline transcripts, which are induced by interleukin-4 (IL-4) and by IL-13. A 51-bp fragment of the human epsilon germline promoter conferred in reporter gene assays with the erythroleukemic cell line TF-1 upregulation of transcription by IL-4 or IL-13, and repression by interferon-alpha (IFN-alpha) and IFN-gamma. A central IFN-gamma activated sequence within the 51-bp fragment was sufficient for transcriptional regulation by the cytokines in the absence of its normal flanking regions. In contrast, deletion of either upstream or downstream sequences abolished repression by IFN-alpha or INF-gamma, but not upregulation by IL-4 or IL-13. IL-4 stimulated reporter gene transcription required more than ten times higher concentrations than cell proliferation or tyrosine phosphorylation of the IL-4 receptor.

A CD30 responsive element in the germline epsilon promoter that is distinct from and inhibitory to the CD40 response element

Germline epsilon (I epsilon) transcription is requisite for IgE switch recombination. I epsilon transcription is markedly increased by ligation of CD40 and/or by IL-4 stimulation. By contrast, we found previously that stimulation through CD30 inhibits I epsilon transcription in EBV-transformed B cell lines. To characterize the molecular mechanisms involved in these contradictory events, the promoter elements that are responsible for I epsilon transcriptional regulation were determined using stable CAT reporter gene constructs. The results define a 95 bp CD30 responsive element (CD30RE) located 5' of the previously defined CD40 responsive element (CD40RE) that resides within the same 95 bp fragment as the IL-4RE and ablates CD40L induced I epsilon promoter activity. However, IL-4 overrides the inhibitory effect of CD30L. These results define a CD30RE and provide further evidence for the complex regulation of I epsilon transcription by various members of the CD40L/TNF alpha family of molecules.

Composite exon structure of an unusual Ig lambda-like gene located at human 22q11 position

The surrogate light chain, composed of the VpreB and the lambda-like proteins, plays a critical role in controlling the early stages of B lymphocyte development. The lambda-like locus, located on the q11. 2-q11.3 region of human Chromosome (Chr) 22, contains three genes (14.1 Flambda-1, and 16.1) among which only the 14.1 is functional. This gene contains three exons, whereas the others lack exon 1. We have isolated in fetal liver a transcript of the Flambda-1 gene that contains the exon 3 sequence and a long non-Ig related sequence upstream. We show that this sequence resulted from the splicing of three new exons located telomeric to the Flambda-1 gene, highly homologous to beta-glucuronidase exon 11 (Chr 7), to the ABR exon 8 (Chr 17), and to an Expressed Sequence Tag (EST), respectively. We also show that this chimeric transcript is expressed in cells or tissues from various origins. This composite gene structure appears to be a new example of human genome flexibility, which can be explained by mechanisms such as exon shuffling and which results in the emergence of new transcription units inserted in regions involved in translocations.

Interleukin-10 induces immunoglobulin G isotype switch recombination in human CD40-activated naive B lymphocytes

Upon activation, B lymphocytes can change the isotype of the antibody they express by immunoglobulin (Ig) isotype switch recombination. In previous studies on the regulation of human IgG expression, we demonstrated that interleukin 10 (IL-10) could stimulate IgG1 and IgG3 secretion by human CD40-activated naive (sIgD+) tonsillar B cells. To assess whether IL-10 actually promotes the DNA recombination underlying switching to these isotypes, we examined the effect of IL-10 on the generation of reciprocal products that form DNA circles as by-products of switch recombination. The content of reciprocal products characteristic of mu-gamma recombination was elevated after culture of CD40-activated tonsillar sIgD+ B cells with either IL-4 or IL-10, although high levels of IgG secretion were observed only with IL-10. Unlike IL-4, IL-10 did not induce reciprocal products of mu-epsilon and gamma-epsilon switch recombination. These results demonstrate that IL-10 promotes both switching to gamma and IgG secretion.