DNA rearrangement can account for in vitro switching to IgG1

H3 trimethyl K9 and H3 acetyl K9 chromatin modifications are associated with class switch recombination

Class switch recombination (CSR) involves a DNA rearrangement in the Ig heavy chain (IgH) gene that allows the same variable (V) region to be expressed with any one of the downstream constant region (C) genes to encode antibodies with many different effector functions. One hypothesis for how CSR is targeted to different C region genes is that histone modifications increase accessibility and/or recruit activation-induced cytosine deaminase (AID) and its associated processes to particular donor and recipient switch regions. In this work, we identified H3 acetyl K9 and H3 trimethyl K9 as histone modifications that correlate with the recombining pair of donor and recipient switch regions. The appearance of H3 trimethyl K9 is surprising because usually it is thought to mark silent genes and heterochromatin. Nevertheless, the time course of appearance of these histone modifications, the regions in IgH they associate with, and their appearance independent of AID damage suggest that both modifications play a role in targeting CSR.

Inhibitory receptor gp49B regulates eosinophil infiltration during allergic inflammation

gp49B, an Ig-like receptor, negatively regulates the activity of mast cells and neutrophils through cytoplasmic immunoreceptor tyrosine-based inhibition motifs. To characterize the role of gp49B further in vivo, gp49B-deficient mice were tested in two allergic models. Responses to ragweed (RW) challenge in the lung and conjunctiva were assessed in models of allergic inflammation and during an infection with parasitic larvae of the nematode Ascaris suum. Infiltration by inflammatory cells into the lung during allergic responses was under negative control of the inhibitory receptor gp49B. Furthermore, an increase in conjunctival inflammation with a predominance of eosinophils, neutrophils, and degranulated mast cells was observed in RW-sensitized, gp49B-deficient mice, which had been challenged in the eye, as compared with C57BL/6 wild-type (WT) controls. Finally, an increase in allergic inflammation in the lungs of A. suum-infected, RW-sensitized mice was observed upon RW challenge, as compared with C57BL/6 WT controls. The observed influx of eosinophils into mucus membranes is characteristic of allergic asthma and allergic conjunctivitis and may contribute to airway hyper-responsiveness, airway remodeling, and mucus production. Expression of gp49B was detected on peripheral eosinophils of control mice and on eosinophils from lungs of mice treated with RW, suggesting a role for gp49B on eosinophils in dampening allergic inflammatory responses.

53BP1 links DNA damage-response pathways to immunoglobulin heavy chain class-switch recombination

The mammalian protein 53BP1 is activated in many cell types in response to genotoxic stress, including DNA double-strand breaks (DSBs). We now examine potential functions for 53BP1 in the specific genomic alterations that occur in B lymphocytes. Although 53BP1 was dispensable for V(D)J recombination and somatic hypermutation (SHM), the processes by which immunoglobulin (Ig) variable region exons are assembled and mutated, it was required for Igh class-switch recombination (CSR), the recombination and deletion process by which Igh constant region genes are exchanged. When stimulated to undergo CSR, 53BP1-deficient cells exhibited no defect in C(H) germline transcription or AID expression, however these cells had a profound decrease in switch junctions. The current findings, in combination with the known 53BP1 functions and how it is activated, implicate the DNA damage response to DSBs in the joining phase of class-switch recombination.

CD86 and beta2-adrenergic receptor signaling pathways, respectively, increase Oct-2 and OCA-B Expression and binding to the 3'-IgH enhancer in B cells

Stimulation of CD86 (formerly known as B7-2) and/or the beta2-adrenergic receptor on a CD40 ligand/interleukin-4-activated B cell increased the rate of mature IgG1 transcription. To identify the mechanism responsible for this effect, we determined whether CD86 and/or beta2-adrenergic receptor stimulation regulated transcription factor expression and binding to the 3'-IgH enhancer in vitro and in vivo. We showed that CD86 stimulation increased the nuclear localization of NF-kappaB1 (p50) and phosphorylated RelA (p65) and increased Oct-2 expression and binding to the 3'-IgH enhancer, in a protein kinase C-dependent manner. These effects were lost when CD86-deficient or NF-kappaB1-deficient B cells were used. CD86 stimulation also increased the level of IkappaB-alpha phosphorylation but in a protein kinase C-independent manner. Beta2-adrenergic receptor stimulation increased CREB phosphorylation, OCA-B expression, and OCA-B binding to the 3'-IgH enhancer in a protein kinase A-dependent manner, an effect lost when beta2-adrenergic receptor-deficient B cells were used. Also, the beta2-adrenergic receptor-induced increase in the level of mature IgG1 transcript was lost when OCA-B-deficient B cells were used. These data are the first to show that CD86 stimulation up-regulates the expression of the transcription factor Oct-2 in a protein kinase C- and NF-kappaB1-dependent manner, and that beta2-adrenergic receptor stimulation up-regulates the expression of the coactivator OCA-B in a protein kinase A-dependent manner to cooperate with Oct-2 binding to the 3'-IgH enhancer.

IgH class switch recombination to IgG1 in DNA-PKcs-deficient B cells

To assess the role of the DNA-PKcs nonhomologous DNA end-joining (NHEJ) protein in Ig heavy chain class switch recombination (CSR), we assayed CSR ability of DNA-PKcs-deficient (DP-T) B cells generated via complementation of DP-T mice with Ig heavy chain and light chain knock-in transgenes (DP-T/HC/LC mice). DP-T/HC/LC mice were severely deficient for all serum IgH isotypes except IgM and, unexpectedly, IgG1. Upon appropriate stimulation, DP-T/HC/LC B cells showed normal proliferation, germline C(H) gene transcription, and AID induction, indicating that DNA-PKcs deficiency did not affect cellular events upstream to CSR. Yet, in vitro activated DP-T/HC/LC B cells again underwent switching only to IgG1 and, like wild-type cells, frequently underwent CSR to gamma1 on both chromosomes. We conclude that DNA-PKcs is required for CSR to most C(H) genes but that CSR to gamma1 occurs via a DNA-PKcs-independent mechanism.

Extract of Nippostrongylus brasiliensis stimulates polyclonal type-2 immunoglobulin response by inducing De novo class switch

Infection with the nematode parasite Nippostrongylus brasiliensis induces a pronounced type-2 T-cell response that is associated with marked polyclonal immunoglobulin E (IgE) and IgG1 production in mice. To examine the differential roles of the infection and products produced by nematodes, we investigated a soluble extract of N. brasiliensis for the ability to mediate this type-2 response. We found that the extract induced a marked increase in IgE and IgG1 levels, similar to that induced by the infection. The extract did not affect the level of IgG2a in serum, showing that the effect was specific to IgE and IgG1 (type-2-associated immunoglobulin) rather than inducing a nonspecific increase in all immunoglobulin isotypes. This response was also associated with increased interleukin-4 production in vitro. These results confirm that the extract, like infection, is a strong inducer of polyclonal type-2 responses and a reliable model for investigating the regulation of nematode-induced responses. The extract induced the production of IgG1 when added to in vitro cultures of lipopolysaccharide-stimulated B cells. This provides evidence for the induction of class switch. It did not induce upregulation of IgG1 in naive (unstimulated) B cells or expand B cells in in vitro cultures. Analysis of DNA from the spleens of mice treated with the extract by digestion-circularization PCR demonstrated a marked increase in the occurrence of gamma1 switch region gene recombination in the cells in vivo. These results provide strong evidence that soluble worm products are able to mediate the marked polyclonal gamma1/epsilon response and that infection is not required to mediate this response. Furthermore, these data provide evidence that the soluble nematode extract induces this effect by causing de novo class switch of B cells and not by an expansion of IgG1 B cells or an increase in antibody production by IgG1 plasma cells.

Quantifiable analysis of human immunoglobulin heavy chain class-switch recombination to all isotypes

Somatic recombinational events, including the immunoglobulin heavy chain class-switch, are a normal feature of B-cell maturation. To enable comprehensive and sensitive class-switch analysis in ex vivo human B cells, we have developed multiple digestion-circularization PCR (DC-PCR) techniques for quantifiable detection of switching to all immunoglobulin isotypes. This technology was validated by extensive sequencing of PCR products, tests with control non-lymphoid cells and B-cell lines of known isotypic specificities, and by demonstrating DC-PCR selectivity in a model system. With tonsillar B-cell DNA, switching to gamma 3, gamma 1, alpha1, gamma 2, gamma 4 and alpha2 isotypes was reproducibly detectable among different individuals. Levels of epsilon switching were relatively low and usually required higher total amounts of template DNAs for detection. Quantitation of alpha1 class switching in a panel of human tonsillar whole B cells was performed by the internal-competitor approach, and showed a pattern consistent with previous studies on IgA+ tonsillar cells. We demonstrate that these assays can rapidly show germline status or specific switch rearrangements in B lymphoid cell lines.

Discordance between IgA switching at the DNA level and IgA expression at the mRNA level in IgA-deficient patients

IgA deficiency is a common immune disorder in Caucasians and is associated with certain MHC conserved extended haplotypes, such as [HLA-B8, SC01, DR3], which presumably carry a susceptibility gene(s). We applied a competitive digestion-circularization PCR method to quantitate the number of switch (S)mu to S alpha rearrangements in peripheral B cells from IgA-deficient subjects homozygous for this haplotype and compared their number with the productive C alpha mRNA level to determine C alpha gene expression in IgA-switched B cells. Two types of defects, low expression of both secreted and membrane forms of productive C alpha mRNA in IgA-switched B cells and impaired IgA switching, were characterized in IgA-deficient subjects homozygous for [HLA-B8, SC01, DR3]. The former defect was also found in another noncarrier subject. It may directly cause low IgA secretion and reflects a blockade in post-IgA switch differentiation of B cells. These results suggest that the heterogeneity of defects in IgA deficiency is not simply ascribable to MHC susceptibility genes.

IL-4- and IL-4 Receptor-Deficient BALB/c Mice Reveal Differences in Susceptibility to Leishmania major Parasite Substrains

Using genetically pure BALB/c mice deficient in IL-4 (IL-4−/−) or IL-4 receptor α-chain (IL-4Rα−/−), we have observed different disease outcomes to Leishmania major infection depending on the parasite substrain. Infection with L. major LV39 caused progressive, nonhealing ulcers and uncontrolled parasite growth in both IL-4−/− and IL-4Rα−/− mice. In contrast, infection with L. major IR173 was partially controlled in IL-4−/− mice but efficiently controlled in IL-4Rα−/− mice. Both IL-4−/− and IL-4Rα−/− mice infected with either substrain displayed reduced Th2 responses. Surprisingly, IFN-γ secretion was not up-regulated in the mutant mice, even in the IL-4Rα−/− mice, which were resistant to L. major IR173. The lack of increased IFN-γ production suggests that cytokine cross-regulation may not be operating in this model and that the effective ratios of Th1/Th2 cytokines become more indicative of disease outcome. The partial vs complete resistance to IR173 in IL-4−/− or IL-4Rα−/− mice implies that, in addition to IL-4, IL-13 may be involved in disease progression during L. major infection. The results with LV39 infection indicate that yet another unidentified factor is capable of causing susceptibility to L. major in the absence of IL-4 or IL-4 signaling.

An extrachromosomal switch recombination substrate reveals kinetics and substrate requirements of switch recombination in primary murine B cells

Ig class switch recombination occurs in B lymphocytes upon activation, and is targeted to distinct switch (S) regions by cytokine-mediated induction of switch transcripts spanning the entire S region and the adjacent constant region gene segments. Using a novel type of switch recombination substrate, constructed according to the intron-exon structure of the IgH locus, but with heterologous elements, we here have tested the structural requirements for targeting and the kinetics of switch recombination in activated primary murine B cells. When transfected at various times after activation, up to 10% of the transfected B cells perform recombination of the substrate within 12 h. Switch recombination in primary B cells is restricted to the first 72 h after onset of activation, then rapidly decreases to background levels, as obtained in plasmacytoma cells or with substrates carrying no S region sequences. In terms of structural requirements, switch recombination is targeted to any transcription unit that contains an intronic S region and depends on processing of the primary transcript by splicing.

STAT6 Is Required for IL-4-Induced Germline Ig Gene Transcription and Switch Recombination

Transcription of the germline Cγ1 and Cε Ig genes is believed to be a necessary prerequisite for isotype switching to IgG1 and IgE, respectively. IL-4 stimulation and ligation of CD40 can each independently induce low level germline γ1 and ε transcription in murine B cells. Together these signals act synergistically to promote high level germline transcription and are normally required for T-dependent isotype switching to IgG1 and IgE. The STAT6 transcription factor has been suggested to play a critical role in IL-4-induced activation of germline Cγ1 and Cε genes. To directly assess the role of STAT6 in IL-4R- and CD40-mediated germline transcription and switching, we have analyzed these events in splenic B cells from STAT6-deficient mice. Our results demonstrate that IL-4 does not induce detectable levels of germline γ1 or ε transcripts in STAT6-deficient B cells. Germline transcript expression induced by CD40 stimulation alone is unaffected, but synergism between CD40- and IL-4R-mediated signals is completely ablated. Switch recombination to Sγ1, as measured by digestion-circularization PCR, is dramatically reduced in STAT6-deficient B cells stimulated with CD40 ligand plus IL-4. Similarly, germline γ1 transcript expression and switch recombination to Sγ1 are also impaired in STAT6-deficient B cells stimulated with IL-4, IL-5, and anti-IgD Abs conjugated to dextran, a model for T-independent type II responses. These results directly demonstrate a critical role for STAT6 in the IL-4-mediated activation of germline Ig gene transcription and switch recombination in nontransformed B cells.

Ku70 is required for late B cell development and immunoglobulin heavy chain class switching

Immunoglobulin (Ig) heavy chain (HC) class switch recombination (CSR) is a late B cell process that involves intrachromosomal DNA rearrangement. Ku70 and Ku80 form a DNA end-binding complex required for DNA double strand break repair and V(D)J recombination. Ku70(-/-) (K70T) mice, like recombination activating gene (RAG)-1- or RAG-2-deficient (R1T or R2T) mice, have impaired B and T cell development at an early progenitor stage, which is thought to result at least in part from defective V(D)J recombination (Gu, Y., K.J. Seidl, G.A. Rathbun, C. Zhu, J.P. Manis, N. van der Stoep, L. Davidson, H.L. Cheng, J.M. Sekiguchi, K. Frank, et al. 1997. Immunity. 7:653-665; Ouyang, H., A. Nussenzweig, A. Kurimasa, V.C. Soares, X. Li, C. Cordon-Cardo, W. Li, N. Cheong, M. Nussenzweig, G. Iliakis, et al. 1997. J. Exp. Med. 186:921-929). Therefore, to examine the potential role of Ku70 in CSR, we generated K70T mice that carry a germline Ig HC locus in which the JH region was replaced with a functionally rearranged VH(D)JH and Ig lambda light chain transgene (referred to as K70T/HL mice). Previously, we have shown that B cells from R1T or R2T mice carrying these rearranged Ig genes (R1T/HL or R2T/HL mice) can undergo CSR to IgG isotypes (Lansford, R., J. Manis, E. Sonoda, K. Rajewsky, and F. Alt. 1998. Int. Immunol. 10:325-332). K70T/HL mice had significant numbers of peripheral surface IgM+ B cells, which generated serum IgM levels similar to those of R2T/HL mice. However, in contrast to R2T/HL mice, K70T/HL mice had no detectable serum IgG isotypes. In vitro culture of K70T/HL B cells with agents that induce CSR in normal or R2T/HL B cells did lead to the induction of germline CH transcripts, indicating that initial signaling pathways for CSR were intact in K70T/HL cells. However, treatment with such agents did not lead to detectable CSR by K70T/HL B cells, and instead, led to cell death within 72 h. We conclude that Ku70 is required for the generation of B cells that have undergone Ig HC class switching. Potential roles for Ku70 in the CSR process are discussed.

Expressed genes in interleukin-4 treated B cells identified by cDNA representational difference analysis

Interleukin-4 (IL-4) stimulates B cell growth and differentiation, such as inducing mature B cells to switch to IgG1 and IgE production. To further characterize IL-4 effects on B cells, we used a sensitive PCR-based subtraction approach to isolate genes expressed in IL-4 treated cells. Our approach combined an adaptation of the genomic representational difference analysis (RDA) method to cDNA analysis with a physical separation method (magnetic bead depletion). This cDNA RDA technique allowed us to perform subtraction on the relatively small number of highly, characterized, purified B cells that can be conveniently prepared. In the hopes of removing genes responsible for general cell growth, we subtracted cDNA made from lipopolysaccharide (LPS)-stimulated B cells from cDNA from LPS+IL-4 stimulated B cells. Two rounds of subtraction resulted in greater than 100-fold enhancement of expected IL-4-induced Cgamma1 cDNA. At that point, we cloned this subtraction library and analysed 154 randomly picked clones for sequence similarities. From these clones, 37 individual genes were obtained. Most of these genes (30) could be functionally identified by sequence similarity. These included genes encoding Cgamma1 (1), cytoskeletal components (4) and products involved in DNA replication (3), metabolism (5), signal transduction (4), transcription (4), translation (6) and transport (3). Only 7 genes had no similarity to known sequences in the GenBank, EMBL or Swiss Prot databases. One unknown gene (designated Fig1 for IL-Four Induced Gene 1) and one gene with homology to the human transcription factor E4BP4 were confirmed by Northern blot analysis to be induced 10-20-fold by IL-4 treatment. This list of expressed genes in LPS + IL-4 treated B cells may shed further insight on the action and mechanism of IL-4 stimulation of cells.

CD3-CD4-CD8-CD56-CD19-CD14- cells and CD3+CD8 dull-positive cells produce IL-4 in AIDS patients

We used flow cytometry to identify the presence of intracellular cytokines (cytoflow) and analyse the production of IL-4 in peripheral blood from AIDS patients who have practically no CD4+ T cells. We found that IL-4 was produced by CD3-CD4-CD8-CD56-CD19-CD14- cells and CD3+CD8 dull-positive cells in AIDS patients. Moreover, CD3-CD4-CD8-CD56-CD19-CD14- cells had helper activity for immunoglobulin synthesis. These findings indicate that instead of CD4+ T helper cells, C3-CD4-CD8-CD56-CD19-CD14- cells and CD3+CD8 dull-positive cells may be an important source of IL-4 in a variety of immune responses for AIDS patients.

Fig1, an interleukin 4-induced mouse B cell gene isolated by cDNA representational difference analysis

Interleukin 4 (IL-4) is a cytokine that regulates growth and differentiation of lymphoid and nonlymphoid cells. To study the molecular basis of IL-4 function, we used a cDNA subtraction approach based on the representational difference analysis method. This subtractive amplification technique allowed us to use small amounts of RNA from lipopolysaccharide +/- IL-4-stimulated normal B cells to obtain IL-4-induced genes from these cells. We report here on one such gene, Fig1 (interleukin-four induced gene 1), the first characterized immediate-early IL-4 inducible gene from B cells. Fig1 expression is strikingly limited to the lymphoid compartment. B cells, but not T cells or mast cells, express Fig1 in response to IL-4 within 2 hr in a cycloheximide resistant manner. IL-2, IL-5, and I1-6 do not induce Fig1 in culture. Fig1 maps between Klk1 and Ldh3 on mouse chromosome 7, near two loci involved with murine lupus, Sle3 and Lbw5. The Fig1 cDNA sequence encodes a predicted 70-kDa flavoprotein with best homology to the monoamine oxidases, particularly in domains responsible for FAD binding.

The role of BSAP in immunoglobulin isotype switching and B-cell proliferation

A role for the transcription factor B cell-specific activator protein (BSAP) in switch recombination has been proposed because binding sites for this protein have been found near switch regions of several isotypes. We have attempted to assess BSAP's role by altering the expression of this protein in B cells switching in culture to IgG1. We found that a phosphorothioate oligonucleotide antisense to the BSAP translation initiation site was able, when incubated with B cells, to decrease BSAP activity in nuclear extracts, and that IgG1 expression was reduced in such cells compared to cells incubated with control oligonucleotides. However, it is not clear whether this apparent reduction in switch recombination was mediated by the known BSAP binding sites in the immunoglobulin heavy chain locus because the antisense experiments revealed an additional activity of this protein: it is a rate-limiting regulator of cell proliferation. Down-regulation of BSAP was associated with decreased proliferation, while increasing BSAP (by transfection with a BSAP expression plasmid) increased proliferation. Thus because switch recombination apparently requires cell division, the effect of BSAP down-regulation on switching might have resulted from decreased proliferation. The role of BSAP in B cell proliferation suggests that dysregulation of this protein could contribute to neoplastic transformation of B cells. Because of BSAP's many activities, experiments to elucidate the mechanisms of its effects on switching and proliferation will be challenging.

Interleukin 4 receptor: signaling mechanisms

Achsah Keegan and colleagues consider the signaling mechanisms utilized by the interleukin 4 (IL-4) receptor and review evidence suggesting that these mechanisms can account for the known responses of hematopoietic and non-hematopoietic cells to IL-4. Most of these data have been obtained from analyses of the ability of IL-4 to regulate the growth of IL-3-dependent myeloid cell lines. These results have implicated a pathway of activation homologous to that utilized by insulin and insulin-like growth factor 1 (IGF-1). However, it is possible that the regulation of growth responses through the IL-4 receptor (and other receptors), and the differentiative events elicited in lymphocytes, may not be mediated by the same post-receptor events.

Independent regulation of DNA recombination and immunoglobulin (Ig) secretion during isotype switching to IgG1 and IgE

Induction of switch recombination to the gamma 1 and epsilon immunoglobulin (Ig) heavy chain loci was examined in B cells preactivated with anti-Ig (B lymphoblasts). In B lymphoblasts cultured with interleukin 4 (IL-4), IL-5 induced the accumulation of S micro-S gamma 1 rearrangements, but not epsilon recombination. Thus, IL-5 facilitates switch recombination directed to the gamma 1 heavy chain locus by IL-4, but additional signals are required to drive rearrangements to epsilon. Lipopolysaccharide (LPS), in the presence of IL-4, induced the accumulation of both S micro-S gamma 1 and S micro-S epsilon rearrangements, and cells treated with LPS exhibited 40-50-fold more S micro-S gamma 1 rearrangements than cells cultured with IL-5. Induction of switch recombination was not always associated with secretion of the respective Ig isotype, since concentrations of IL-4 that were sufficient to direct switch recombination to gamma 1 and epsilon in blasts treated with LPS failed to elicit secretion of IgG1 and IgE. These results demonstrate differential requirements for switch recombination to the gamma 1 and epsilon loci, as well as independent regulation of Ig gene rearrangement and secretion of each isotype.