Induction of germ-line immunoglobulin heavy chain transcripts by mitogens and interleukins prior to switch recombination

Attenuated strain of CVB3 with a mutation in the CAR-interacting region protects against both myocarditis and pancreatitis

Coxsackievirus B3 (CVB3), is commonly implicated in myocarditis, which can lead to dilated cardiomyopathy, in addition to causing acute pancreatitis and meningitis. Yet, no vaccines are currently available to prevent this infection. Here, we describe the derivation of a live attenuated vaccine virus, termed mutant (Mt) 10, encoding a single amino acid substitution H790A within the viral protein 1, that prevents CVB3 infection in mice and protects from both myocarditis and pancreatitis in challenge studies. We noted that animals vaccinated with Mt 10 developed virus-neutralizing antibodies, predominantly containing IgG2a and IgG2b, and to a lesser extent IgG3 and IgG1. Furthermore, by using major histocompatibility complex class II dextramers and tetramers, we demonstrated that Mt 10 induces antigen-specific T cell responses that preferentially produce interferon-γ. Finally, neither vaccine recipients nor those challenged with the wild-type virus revealed evidence of autoimmunity or cardiac injury as determined by T cell response to cardiac myosin and measurement of circulating cardiac troponin I levels, respectively. Together, our data suggest that Mt 10 is a vaccine candidate that prevents CVB3 infection through the induction of neutralizing antibodies and antigen-specific T cell responses, the two critical components needed for complete protection against virus infections in vaccine studies.

Antibody Production in Murine Polymicrobial Sepsis-Kinetics and Key Players

Although antigen-specific priming of antibody responses is impaired during sepsis, there is nevertheless a strong increase in IgM and IgG serum concentrations. Using colon ascendens stent peritonitis (CASP), a mouse model of polymicrobial abdominal sepsis, we observed substantial increases in IgM as well as IgG of all subclasses, starting at day 3 and peaking 2 weeks after sepsis induction. The dominant source of antibody-secreting cells was by far the spleen, with a minor contribution of the mesenteric lymph nodes. Remarkably, sepsis induction in splenectomized mice did not change the dynamics of the serum IgM/IgG reaction, indicating that the marginal zone B cells, which almost exclusively reside in the spleen, are dispensable in such a setting. Hence, in systemic bacterial infection, the function of the spleen as dominant niche of antibody-producing cells can be compensated by extra-splenic B cell populations as well as other lymphoid organs. Depletion of CD4+ T cells did not affect the IgM response, while it impaired IgG generation of all subclasses with the exception of IgG3. Taken together, our data demonstrate that the robust class-switched antibody response in sepsis encompasses both T cell-dependent and -independent components.

The Nod2 Agonist Muramyl Dipeptide Cooperates with the TLR4 Agonist Lipopolysaccharide to Enhance IgG2b Production in Mouse B Cells

Many studies have shown that Toll-like receptors (TLRs) and Nod-like receptors (NLRs) were expressed in B cells and their signaling affects B cell functions. Nonetheless, the roles played by these receptors in B cell antibody (Ab) production have not been completely elucidated. In the present study, we examined the effect of the Nod2 agonist muramyl dipeptide (MDP) in combination with the TLR4 agonist lipopolysaccharide (LPS), a well-known B cell mitogen, on B cell viability, proliferation, and activation, and Ab production by in vitro culture of purified mouse spleen resting B cells. MDP combined with LPS to reinforce B cell viability, proliferation, and activation. Moreover, MDP enhanced LPS-induced IgG2b production, germline γ2b transcript (GLTγ2b) expression, and surface IgG2b expression. In an experiment with Nod2- and TLR4-deficient mouse B cells, we observed that the combined effect of MDP and LPS is dependent on Nod2 and TLR4 receptors. Furthermore, the combined effect on B cell viability and IgG2b switching was not observed in Rip2-deficient mouse cells. Collectively, this study suggests that Nod2 signaling enhances TLR4-activated B cell proliferation, IgG2b switching, and IgG2b production.

The Chromatin Reader ZMYND8 Regulates Igh Enhancers to Promote Immunoglobulin Class Switch Recombination

Class switch recombination (CSR) is a DNA recombination reaction that diversifies the effector component of antibody responses. CSR is initiated by activation-induced cytidine deaminase (AID), which targets transcriptionally active immunoglobulin heavy chain (Igh) switch donor and acceptor DNA. The 3′ Igh super-enhancer, 3′ regulatory region (3′RR), is essential for acceptor region transcription, but how this function is regulated is unknown. Here, we identify the chromatin reader ZMYND8 as an essential regulator of the 3′RR. In B cells, ZMYND8 binds promoters and super-enhancers, including the Igh enhancers. ZMYND8 controls the 3′RR activity by modulating the enhancer transcriptional status. In its absence, there is increased 3′RR polymerase loading and decreased acceptor region transcription and CSR. In addition to CSR, ZMYND8 deficiency impairs somatic hypermutation (SHM) of Igh, which is also dependent on the 3′RR. Thus, ZMYND8 controls Igh diversification in mature B lymphocytes by regulating the activity of the 3′ Igh super-enhancer.

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ZMYND8 is required for GLT of acceptor S regions and Class Switch Recombination

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ZMYND8 supports efficient somatic hypermutation of the Igh variable regions

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ZMYND8 binds B cell super-enhancers, including the 3′ Igh enhancer

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ZMYND8 modulates the transcriptional status and activity of the 3′ Igh enhancer

A transcriptional serenAID: the role of noncoding RNAs in class switch recombination

During an immune response, activated B cells may undergo class switch recombination (CSR), a molecular rearrangement that allows B cells to switch from expressing IgM and IgD to a secondary antibody heavy chain isotype such as IgG, IgA or IgE. Secondary antibody isotypes provide the adaptive immune system with distinct effector functions to optimally combat various pathogens. CSR occurs between repetitive DNA elements within the immunoglobulin heavy chain (Igh) locus, termed switch (S) regions and requires the DNA-modifying enzyme activation-induced cytidine deaminase (AID). AID-mediated DNA deamination within S regions initiates the formation of DNA double-strand breaks, which serve as biochemical beacons for downstream DNA repair pathways that coordinate the ligation of DNA breaks. Myriad factors contribute to optimal AID targeting; however, many of these factors also localize to genomic regions outside of the Igh locus. Thus, a current challenge is to explain the specific targeting of AID to the Igh locus. Recent studies have implicated noncoding RNAs in CSR, suggesting a provocative mechanism that incorporates Igh-specific factors to enable precise AID targeting. Here, we chronologically recount the rich history of noncoding RNAs functioning in CSR to provide a comprehensive context for recent and future discoveries. We present a model for the RNA-guided targeting of AID that attempts to integrate historical and recent findings, and highlight potential caveats. Lastly, we discuss testable hypotheses ripe for current experimentation, and explore promising ideas for future investigations.

Toll-like Receptor 1/2 Agonist Pam3CSK4 Suppresses Lipopolysaccharide-driven IgG1 Production while Enhancing IgG2a Production by B Cells

Interaction between pathogen-associated molecular patterns and pattern recognition receptors triggers innate and adaptive immune responses. Several studies have reported that toll-like receptors (TLRs) are involved in B cell proliferation, differentiation, and Ig class switch recombination (CSR). However, roles of TLRs in B cell activation and differentiation are not completely understood. In this study, we investigated the direct effect of stimulation of TLR1/2 agonist Pam3CSK4 on mouse B cell viability, proliferation, activation, Ig production, and Ig CSR in vitro. Treatment with 0.5 µg/ml of Pam3CSK4 only barely induced IgG1 production although it enhanced B cell viability. In addition, high-dosage Pam3CSK4 diminished IgG1 production in a dose-dependent manner, whereas the production of other Igs, cell viability, and proliferation increased. Pam3CSK4 additively increased TLR4 agonist lipopolysaccharide (LPS)-induced mouse B cell growth and activation. However, interestingly, Pam3CSK4 abrogated LPS-induced IgG1 production but enhanced LPS-induced IgG2a production. Further, Pam3CSK4 decreased LPS-induced germline γ1 transcripts (GLTγ1)/GLTε expression but increased GLTγ2a expression. On the other hand, Pam3CSK4 had no effect on LPS-induced plasma cell differentiation. Taken together, these results suggest that TLR1/2 agonist Pam3CSK4 acts as a potent mouse B cell mitogen in combination with TLR4 agonist LPS, but these 2 different TLR agonists play diverse roles in regulating the Ig CSR of each isotype, particularly IgG1/IgE and IgG2a.

The TH1 phenotype of follicular helper T cells indicates an IFN-γ-associated immune dysregulation in patients with CD21low common variable immunodeficiency

BACKGROUND

A subgroup of patients with common variable immunodeficiency (CVID) experience immune dysregulation manifesting as autoimmunity, lymphoproliferation, and organ inflammation and thereby increasing morbidity and mortality. Therefore treatment of these complications demands a deeper comprehension of their cause and pathophysiology.

OBJECTIVES

On the basis of the identification of an interferon signature in patients with CVID with secondary complications and a skewed follicular helper T-cell differentiation in defined monogenic immunodeficiencies, we sought to determine the profile of CD4 memory T cells in blood and secondary lymphatic tissues of these patients.

METHODS

We quantified T_H1/T_H2/T_H17 CD4 memory T cells in blood and lymph nodes of patients with CVID using flow cytometry, analyzed their function, and correlated all findings to the burden of immune dysregulation.

RESULTS

Patients with CVID with immune dysregulation had a skewed memory CD4 T-cell differentiation toward a CXCR3⁺CCR6^- T_H1 phenotype both in blood and lymph nodes. Consistent with our phenotypic findings, we observed a higher IFN-γ production in peripheral CD4 memory T cells and lymph node-derived follicular helper T cells of patients with CVID compared with those of healthy control subjects. Increased IFN-γ production was accompanied by a poor germinal center output, an accumulation of T-box transcription factor (T-bet)⁺ B cells in lymph nodes, and an accumulation of T-bet⁺CD21^low B cells in peripheral blood of affected patients.

CONCLUSION

Identification of excessive IFN-γ production by blood and lymph node-derived T cells of patients with CVID with immune dysregulation will offer new therapeutic avenues for this subgroup. CD21^low B cells might serve as a marker of this IFN-γ-associated dysregulation.

The role of toll-like receptors in B-cell development and immunopathogenesis of common variable immunodeficiency

Common variable immunodeficiency (CVID) is the most frequent symptomatic primary immune deficiency and is characterized by hypogammaglobulinemia, defect in specific antibody response and increased susceptibility to recurrent infections, malignancy and autoimmunity. Patients with CVID often have defects in post-antigenic B-cell differentiation, with fewer memory B cells and impaired isotype switching. Toll-like receptors (TLRs) are expressed on various immune cells as key elements of innate and adaptive immunity. TLR signaling in B cells plays multiple roles in cell differentiation and activation, class-switch recombination and cytokine and antibody production. Moreover, recent studies have shown functional alteration of TLRs responses in CVID patients including poor cell proliferation, impaired upregulation of co-stimulatory molecules and failure in cytokine and immunoglobulin production. The purpose of the present review is to discuss the role of TLRs in B-cell development and function as well as their role in the immunopathogenesis of CVID.

AID targeting: old mysteries and new challenges

Activation-induced cytidine deaminase (AID) mediates cytosine deamination and underlies two central processes in antibody diversification: somatic hypermutation and class-switch recombination. AID deamination is not exclusive to immunoglobulin loci; it can instigate DNA lesions in non-immunoglobulin genes and thus stringent checks are in place to constrain and restrict its activity. Recent findings have provided new insights into the mechanisms that target AID activity to specific genomic regions, revealing an involvement for noncoding RNAs associated with polymerase pausing and with enhancer transcription as well as genomic architecture. We review these findings and integrate them into a model for multilevel regulation of AID expression and targeting in immunoglobulin and non-immunoglobulin loci. Within this framework we discuss gaps in understanding, and outline important areas of further research.

c-Myb is required for plasma cell migration to bone marrow after immunization or infection

The transcription factor c-Myb plays a role in establishing long-lived plasma cell populations in the bone marrow by affecting migration responses to chemokine gradients. The absence of c-Myb results in an absence of IgG⁺ antigen-specific plasma cells in the bone marrow following immunization or virus infection.

Plasma cell migration is crucial to immunity, but little is known about the molecular regulators of their migratory programs. Here, we detail the critical role of the transcription factor c-Myb in determining plasma cell location. In the absence of c-Myb, no IgG⁺ antigen-specific plasma cells were detected in the bone marrow after immunization or virus infection. This was correlated with a dramatic reduction of plasma cells in peripheral blood, mislocalization in spleen, and an inability of c-Myb–deficient plasma cells to migrate along a CXCL12 gradient. Therefore, c-Myb plays an essential, novel role in establishing the long-lived plasma cell population in the BM via responsiveness to chemokine migration cues.

AID targeting in antibody diversity

Antibody maturation requires class switch recombination (CSR) and somatic hypermutation (SHM), both of which are initiated by activation-induced cytidine deaminase (AID). AID deaminates cytosine residues resulting in mismatches that are differentially processed to produce double-strand breaks in Ig switch (S) regions that lead to CSR, or to point mutations in variable (V) exons resulting in SHM. Although AID was first thought to be Ig-specific, recent work indicates that it also targets a diverse group of non-Ig loci, including genes such as Bcl6 and c-myc, whose modification by AID results in lymphoma-associated mutations and translocations. Here, we review the recent literature on AID targeting and the role for transcriptional stalling in recruitment of this enzyme to Ig and non-Ig loci. We propose a model for AID recruitment based on transcriptional stalling, which reconciles several of the key features of SHM, CSR, and lymphoma-associated translocation.

iNOS potentiates mouse Ig isotype switching through AID expression

The IgA antibody plays an important role in protecting mucosal surfaces against pathogens. It has recently been shown that nitric oxide (NO) plays a critical role in mouse IgA synthesis. In the present study, we further characterized inducible-nitric oxide synthase-deficient (iNOS(-/-)) mice in the context of Ig expression. The amount of IgA in fecal pellets was substantially diminished in iNOS(-/-) mice and was paralleled by a decrease in IgA production by Peyer's patch cells. Interestingly, the amount of all IgG subisotypes, as well as IgA, was substantially diminished in sera and in cultured spleen B cells from iNOS(-/-) mice. Moreover, the synthesis of TGF-β1-inducible IgA and IgG2b in iNOS(-/-) mice was also lower than that in WT mice. However, levels of Ig germ-line transcripts, and expression of TGF-β receptor type II (TβRII) and BAFF/APRIL, were comparable between iNOS(-/-) and WT mice. Expression of activation-induced cytidine deaminase (AID) was diminished in iNOS(-/-) B cells, but restored by a NO donor, SNAP. These results indicate that iNOS regulates Ig isotype switching events at the level of AID gene expression.

The role of germline promoters and I exons in cytokine-induced gene-specific class switch recombination

Germline transcription precedes class switch recombination (CSR). The promoter regions and I exons of these germline transcripts include binding sites for activation- and cytokine-induced transcription factors, and the promoter regions/I exons are essential for CSR. Therefore, it is a strong hypothesis that the promoter/I exons regions are responsible for much of cytokine-regulated, gene-specific CSR. We tested this hypothesis by swapping the germline promoter and I exons for the murine γ1 and γ2a H chain genes in a transgene of the entire H chain C-region locus. We found that the promoter/I exon for γ1 germline transcripts can direct robust IL-4-induced recombination to the γ2a gene. In contrast, the promoter/I exon for the γ2a germline transcripts works poorly in the context of the γ1 H chain gene, resulting in expression of γ1 H chains that is <1% the wild-type level. Nevertheless, the small amount of recombination to the chimeric γ1 gene is induced by IFN-γ. These results suggest that cytokine regulation of CSR, but not the magnitude of CSR, is regulated by the promoter/I exons.

Co-encapsulation of antigen and Toll-like receptor ligand in cationic liposomes affects the quality of the immune response in mice after intradermal vaccination

Enhanced immunogenicity of subunit antigens can be achieved by antigen encapsulation in liposomes and the addition of immune potentiators. In this study we co-encapsulated ovalbumin (OVA) and a Toll-like receptor (TLR) ligand (PAM(3)CSK(4) (PAM) or CpG) in cationic liposomes and investigated the effect of the formulations on dendritic cell (DC) maturation in vitro and on the immune response in mice after intradermal immunisation. Co-encapsulation of PAM did not affect the OVA content of the liposomes, but co-encapsulation of CpG led to a decrease in OVA content by 25%. After liposomal encapsulation, both ligands retained the ability to activate TLR-transfected HEK cells, though PAM only induced activation at elevated concentrations. DC maturation induced by liposome-based adjuvant formulations was superior compared to the free adjuvants. Encapsulation of PAM and CpG in liposomes did not influence the total IgG titres compared to the antigen/adjuvant solution, but OVA/CpG liposomes shifted the IgG1/IgG2a balance more to the direction of IgG2a compared to non-encapsulated CpG. Moreover, only this formulation resulted in IFN-γ production by restimulated splenocytes from immunised mice. These data show that co-encapsulation of antigen and immune potentiator in cationic liposomes, can affect the type of immune response generated after intradermal immunisation.

Microneedle-based transcutaneous immunisation in mice with N-trimethyl chitosan adjuvanted diphtheria toxoid formulations

Purpose

The purpose of this study was to gain insight into the delivery and immunogenicity of N-trimethyl chitosan (TMC) adjuvanted diphtheria toxoid (DT) formulations applied transcutaneously with microneedles.

Methods

Mice were vaccinated with DT-loaded TMC nanoparticles, a solution of TMC and DT (TMC/DT) or DT alone. The formulations were applied onto the skin before or after microneedle treatment with two different 300-µm-long microneedle arrays and also injected intradermally (ID). As a positive control, alum-adjuvanted DT (DT-alum) was injected subcutaneously (SC). Ex vivo confocal microscopy studies were performed with rhodamine-labelled TMC.

Results

Independent of the microneedle array used and the sequence of microneedle treatment and vaccine application, transcutaneous immunisation with the TMC/DT mixture elicited 8-fold higher IgG titres compared to the TMC nanoparticles or DT solution. The toxin-neutralising antibody titres from this group were similar to those elicited by SC DT-alum. After ID immunisation, both TMC-containing formulations induced enhanced titres compared to a DT solution. Confocal microscopy studies revealed that transport of the TMC nanoparticles across the microneedle conduits was limited compared to a TMC solution.

Conclusions

In conclusion, TMC has an adjuvant function in transcutaneous immunisation with microneedles, but only if applied in a solution.

Electronic Supplementary Material

The online version of this article (doi:10.1007/s11095-010-0182-y) contains supplementary material, which is available to authorized users.

Switch region identity plays an important role in Ig class switch recombination

Ig class switch recombination (CSR) is regulated through long-range intrachromosomal interactions between germline transcript promoters and enhancers to initiate transcription and create chromatin accessible to activation-induced deaminase attack. CSR occurs between switch (S) regions that flank Cmu and downstream C(H) regions and functions via an intrachromosomal deletional event between the donor Smicro region and a downstream S region. It is unclear to what extent S region primary sequence influences differential targeting of CSR to specific isotypes. We address this issue in this study by generating mutant mice in which the endogenous Sgamma3 region was replaced with size-matched Sgamma1 sequence. B cell activation conditions are established that support robust gamma3 and gamma1 germline transcript expression and stimulate IgG1 switching but suppress IgG3 CSR. We found that the Sgamma1 replacement allele engages in micro-->gamma3 CSR, whereas the intact allele is repressed. We conclude that S region identity makes a significant contribution to CSR. We propose that the Sgamma1 region is selectively targeted for CSR following the induction of an isotype-specific factor that targets the S region and recruits CSR machinery.

Toll-like receptors--sentries in the B-cell response

Toll-like receptors (TLR) play a central role in the initiation of the innate immune response to pathogens. Upon recognition of molecular motifs specific for microbial molecules TLR mediate pro-inflammatory cytokine secretion and enhance antigen presentation; in B cells they further promote expansion, class switch recombination and immunoglobulin secretion. As a result of their adjuvant properties, TLR ligands have become an integral component of antimicrobial vaccines. In spite of this, little is known of the direct effects of TLR engagement on B-lymphocyte function. The scope of this review is to outline the differences in TLR expression and reactivity in murine and human B-cell subsets and to provide an overview of the currently available literature. We will further discuss the possible roles of TLR in regulating B-cell effector functions and shaping antibody-mediated defence against microbial pathogens in vivo.

Interleukin 5 in the link between the innate and acquired immune response

Interleukin-5 (IL-5) is an interdigitating homodimeric glycoprotein that is initially identified by its ability to support the in vitro growth and differentiation of mouse B cells and eosinophils. IL-5 transgenic mouse shows two predominant features, remarkable increase in B-1 cells resulting in enhanced serum antibody levels, predominantly IgM, IgA, and IgE classes and in expansion of eosinophil numbers in the blood and eosinophil infiltration into various tissues. Conversely, mice lacking a functional gene for IL-5 or IL-5 receptor alpha chain (IL-5Ralpha) display a number of developmental and functional impairments in B cells and eosinophils. IL-5 receptor (IL-5R) comprises alpha and betac chains. IL-5 specifically binds to IL-5Ralpha and induces the recruitment of betac to IL-5R. Although precise mechanisms on cell-lineage-specific IL-5Ralpha expression remain elusive, several transcription factors including Sp1, E12/E47, Oct-2, and c/EBPbeta have been shown to regulate its expression in B cells and eosinophils. JAK2 and JAK1 tyrosine kinase are constitutively associated with IL-5Ralpha and betac, respectively, and are activated by IL-5 stimulation. IL-5 activates at least three different signaling pathways including JAK2/STAT5 pathway, Btk pathway, and Ras/ERK pathway. IL-5 is one of key cytokines for mouse B cell differentiation in general, particularly for fate-determination of terminal B cell differentiation to antibody-secreting plasma cells. IL-5 critically regulates homeostatic proliferation and survival of and natural antibody production by B-1 cells, and enhances the AID and Blimp-1 expression in activated B-2 cells leading to induce mu to gamma1 class switch recombination and terminal differentiation to IgM- and IgG1-secreting plasma cells, respectively. In humans, major target cells of IL-5 are eosinophils. IL-5 appears to play important roles in pathogenesis of asthma, hypereosinophilic syndromes, and eosinophil-dependent inflammatory diseases. Clinical studies will provide a strong impetus for investigating the means of modulating IL-5 effects. We will discuss the role of IL-5 in the link between innate and acquired immune response, particularly emphasis of the molecular basis of IL-5-dependent B cell activation, allergen-induced chronic inflammation and hypereosinophilic syndromes on a novel target for therapy.

Different immune correlates associated with tumor progression and regression: implications for prevention and treatment of cancer

Observations show that humans and animals respond immunologically to most cancers. Why does the immune system then fail to control cancer? We argue from the literature that there is a commonality in the regulation of responses against most murine tumors, and that a major mechanism of escape may be deviation of an effective Th1, cytotoxic T lymphocyte response to a less effective response with a Th2 component. We examined this hypothesis with two well-studied murine tumors. We found, following primary tumor implantation, that resistance correlates with Th1 responses and IgG2a antibody production and progression with mixed Th1/Th2 responses and production of IgG1 and IgG2a antibodies. Resistance is associated with a modulation of the anti-tumor response towards the Th1 pole in both systems. We conclude that the immune responses against these two tumors are in accord with our hypothesis, and argue that this is likely to be true of many human and murine tumors. The correlation of IgG isotype of anti-tumor antibody with the Th1/Th2 nature of the anti-tumor response readily allows one to longitudinally monitor the changing nature of the anti-tumor response. We suggest that such monitoring can guide immunotherapy to maximize the effectiveness of the host's immune response against cancer.

Enhanced glucocorticoid receptor signaling in T cells impacts thymocyte apoptosis and adaptive immune responses

To study the effect of enhanced glucocorticoid signaling on T cells, we generated transgenic rats overexpressing a mutant glucocorticoid receptor with increased ligand affinity in the thymus. We found that this caused massive thymocyte apoptosis at physiological hormone levels, which could be reversed by adrenalectomy. Due to homeostatic proliferation, a considerable number of mature T lymphocytes accumulated in the periphery, responding normally to costimulation but exhibiting a perturbed T-cell repertoire. Furthermore, the transgenic rats showed increased resistance to experimental autoimmune encephalomyelitis, which manifests in a delayed onset and milder disease course, impaired leukocyte infiltration into the central nervous system and a distinct cytokine profile. In contrast, the ability of the transgenic rats to mount an allergic airway response to ovalbumin was not compromised, although isotype switching of antigen-specific immunoglobulins was altered. Collectively, our findings suggest that endogenous glucocorticoids impact T-cell development and favor the selection of Th2- over Th1-dominated adaptive immune responses.

Evolution of the Immunoglobulin Heavy Chain Class Switch Recombination Mechanism

To mount an optimum immune response, mature B lymphocytes can change the class of expressed antibody from IgM to IgG, IgA, or IgE through a recombination/deletion process termed immunoglobulin heavy chain (IgH) class switch recombination (CSR). CSR requires the activation-induced cytidine deaminase (AID), which has been shown to employ single-stranded DNA as a substrate in vitro. IgH CSR occurs within and requires large, repetitive sequences, termed S regions, which are parts of germ line transcription units (termed "C(H) genes") that are composed of promoters, S regions, and individual IgH constant region exons. CSR requires and is directed by germ line transcription of participating C(H) genes prior to CSR. AID deamination of cytidines in S regions appears to lead to S region double-stranded breaks (DSBs) required to initiate CSR. Joining of two broken S regions to complete CSR exploits the activities of general DNA DSB repair mechanisms. In this chapter, we discuss our current knowledge of the function of S regions, germ line transcription, AID, and DNA repair in CSR. We present a model for CSR in which transcription through S regions provides DNA substrates on which AID can generate DSB-inducing lesions. We also discuss how phosphorylation of AID may mediate interactions with cofactors that facilitate access to transcribed S regions during CSR and transcribed variable regions during the related process of somatic hypermutation (SHM). Finally, in the context of this CSR model, we further discuss current findings that suggest synapsis and joining of S region DSBs during CSR have evolved to exploit general mechanisms that function to join widely separated chromosomal DSBs.

Transient inhibition of Th1-type cytokine production by CD4 T cells in hepatitis B core antigen immunized mice is mediated by regulatory T cells

The non-cytopathic hepatitis B virus (HBV) can induce chronic infections characterized by weak and limited T cell responses against the virus. The factors contributing to the failure to clear HBV and subsequent development of chronic HBV infections are not clearly understood, but a strong interferon-gamma (IFN-gamma) response by CD4+ T cells against the nucleocapsid hepatitis B core antigen (HBcAg) of the virus appears to be important for viral clearance. The present study documents depressed numbers of CD4+ T cells secreting IFN-gamma and interleukin-2 (IL-2) in enzyme-linked immunospot assay (ELISPOT) assays restimulated for 24 hr with antigen following both primary and secondary immunizations of mice with recombinant hepatitis B core antigen (rHBcAg). The kinetics of these responses showed that the depression occurred following a peak response and lasted approximately 2 weeks before returning to the previous peak levels. The depression was abrogated by depletion of CD25+ cells prior to culture in the ELISPOT assay, suggesting inhibition by regulatory T cells. This inhibition of IFN-gamma and IL-2 production was also reversed by in vitro restimulation of the test cells for 48 hr rather than 24 hr in the assay. No such transient, reversible inhibition was detected in the production of IL-5, a Th2-type cytokine. The inhibition in cytokine production did not appear to correlate with the number of antibody-secreting cells or the isotypes produced. This delay by regulatory T cells of Th1-type cytokine production could contribute to viral persistence in chronic HBV infection by interfering with the critical role IFN-gamma plays in protection against viral infections.

Stat1-dependent synergistic activation of T-bet for IgG2a production during early stage of B cell activation

During the adaptive phase of an immune response, naive B cells receive multiple signals to become activated. Among them are the engagement of the B cell Ag receptor and stimulation by cytokines. Specifically for an anti-microbial response, the recognition of viral or bacterial Ags by the BCR and the stimulation of IFN-gamma result in the predominant production of IgG2a. The T-bet protein has been shown to be required for class switching to IgG2a. In this report we further investigated the regulation of T-bet gene expression during the early stage of B cell activation. We show that there is a striking synergistic activation of T-bet in primary B cells when both the BCR and IFN-gamma signaling pathways are activated. The synergistic activation of T-bet correlates with a 100% increase in the number of B cells that produce IgG2a. This transcription synergy on T-bet is transient in the first 24 h of B cell activation. Furthermore, we demonstrate that the synergistic activation of T-bet is dependent on Stat1 and that Stat1 is required for the IgG2a germline transcription and the production of IgG2a in response to the simultaneous signaling of BCR and IFN-gamma. Finally, we show that Stat1 directly regulates the expression of T-bet by binding to the T-bet promoter. These results reveal the mechanism of regulation of T-bet expression and uncover a novel physiological function of Stat1 for B cell activation.

Evolution of isotype switching

This review discusses evolution of the process of Ig heavy chain class switching, relating it to the first appearance of somatic hypermutation (SHM) of variable region genes. First, we discuss recent findings on the mechanism of class switch recombination (CSR) in mice and humans, and then review the mechanisms of expression of Ig heavy chain isotypes from fishes to mammals. Importantly, activation-induced cytidine deaminase (AID), which is essential for CSR and somatic hypermutation, is found in fishes. Although at least some fishes are likely to undergo SHM, CSR is highly unlikely to occur in this group. We discuss the first appearance of CSR in amphibians and how it differs in birds and mammals.

Transcription of Ig Germline Genes in Single Human B Cells and the Role of Cytokines in Isotype Determination

We have developed a critical test of the chromatin accessibility model of Ig isotype determination in which local unfolding of chromatin higher order structure (chromatin accessibility) in the region of specific germline genes in the H chain locus determines the Ab class to be expressed in the B cell. We show that multiple germline genes are constitutively transcribed in the majority of naive human B cells in a population. Thus, because chromatin in its higher order structure cannot be transcribed, the entire Ig H chain locus must be unfolded in naive B cells. We have also established that IL-4 and anti-CD40 act by enhancing transcription in the majority of cells, rather than by activating transcription in more of the cells. Transcriptional activity in the human H chain locus rules out the perturbation of chromatin higher order structure as a factor in isotype determination. We have also found that the levels of germline gene transcription cannot fully account for the levels of secretion of the different Ig isotypes, and that secretion of IgE, in particular, is suppressed relative to that of IgG.

Differential activation of signal transducer and activator of transcription 6 in B cells from allergic children and their non-allergic siblings

BACKGROUND

The germline (GL) epsilon promoter is regulated by IL-4 and is essential for class switching to IgE. IL-4-induced gene expression is largely mediated through activation of latent transcription factor STAT6 (signal transducer and activator of transcription 6).

OBJECTIVE

We investigated whether increased levels of IgE in allergic individuals may be associated with alteration in the level or activation of STAT6 and subsequent increase in GL epsilon promoter activity.

METHODS

Electrophoretic mobility shift assay and Western blotting assays were used to investigate the level of expression and activation of STAT6 in Epstein-Barr virus (EBV)-transformed B cell lines from children with birch pollen allergy and their non-allergic siblings. The activity of the GL epsilon promoter was tested in a transient transfection assay.

RESULTS

STAT6 was expressed at the same level in all B cell lines tested. In two out of five sibling pairs STAT6 was activated by IL-4 more efficiently in the allergic individuals but in the three other pairs the opposite was found. In transient transfections, no difference in IL-4-induced GL epsilon promoter function was detected, although basal promoter activity varied between allergic and healthy siblings in two out of five pairs.

CONCLUSIONS

We demonstrate for the first time that upon IL-4 signalling STAT6 transcription factor activation differs in B cells from different individuals. Although we did not find any association between STAT6 activation and allergy, we do not exclude a possibility that stronger activation of this transcription factor is associated with an expression of allergic phenotype.

The role of regulatory T cells in allergy

Atopic diseases are characterized by Th2 and IgE responses to common environmental and food antigens. In vivo, IgE production depends on interactions between allergen-specific B lymphocytes and Th2 lymphocytes. IgE levels are extremely low in normal individuals, suggesting that IgE production is under strong regulation. One of the reasons behind the lack of atopy in healthy individuals is the activity of regulatory T cells, which prevent naïve T helper cell precursors from acquiring a differentiated Th2 phenotype. In addition to naturally occurring regulatory T cells, atopy can be prevented by allergen-specific tolerant/regulatory cells induced through mucosal stimulation, and by mechanisms that directly suppress Iepsilon sterile transcript production on activated B lymphocytes. This article reviews the recent progress on thymic-derived as well as peripherally induced regulatory T cells as they relate to atopy. The latter discussion also includes regulatory T cells that arise through immunotherapy.

Selective regulation of mature IgG1 transcription by CD86 and beta 2-adrenergic receptor stimulation

Stimulation of CD86 and the beta(2)-adrenergic receptor (beta(2)AR) on a B cell, either alone or together, is known to increase the level of IgG1 protein produced by a CD40 ligand/IL-4-activated B cell. It is also known that the mechanism by which CD40 and IL-4R stimulation on a B cell increases the level of IgG1 protein is by increasing germline gamma 1 transcription, IgG1 class switching, and mature IgG1 transcription, while the molecular mechanism responsible for mediating the CD86- and beta(2)AR-induced effect remains unknown. In the present study using real-time PCR we show that the level of mature IgG1 transcription increases in CD40 ligand/IL-4-activated B cells following stimulation of either CD86 and/or beta(2)AR, and that this increase reflects the increase in IgG1 protein. Furthermore, we show that the CD86- and/or beta(2)AR-induced increase in mature IgG1 transcript is due to an increase in the rate of mature IgG1 transcription, as determined by nuclear run-on analysis. This effect is additive when both receptors are stimulated and is lost when B cells from CD86- and beta(2)AR-deficient mice are used. In contrast, the level of germline gamma 1 transcription, the stability of mature IgG1 transcript, the number of IgG1-positive B cells, and the number of IgG1-secreting B cells did not change. These results provide the first evidence that CD86 and/or beta(2)AR stimulation on a CD40 ligand/IL-4-activated B cell increases the level of IgG1 protein produced per cell by increasing the rate of mature IgG1 transcription.

Prostaglandin E2 and cAMP promote B lymphocyte class switching to IgG1

Prostaglandins of the E series (PGE) have traditionally been considered as suppressive for immune responses; however, recent data suggest that PGE channels the immune response towards a T helper 2 type response and production of selected immunoglobulin isotypes. Herein, we present data showing that PGE(2) and other agents that induce intracellular rises in cAMP significantly increased B lymphocyte IgG1 production (up to sevenfold). PGE(2) acted on small resting B cells and on uncommitted B cells expressing high levels of surface IgM to increase the number of cells secreting IgG1. PGE(2) even increased IgG1 synthesis by purified B cells in the absence of exogenous IL-4. Finally, PGE(2) synergized with IL-4 to induce germline gamma1 transcripts through the switch region. This transcription is required for isotype switching. These data support the hypothesis that PGE(2) acts on uncommitted resting B cells at the level of germline gamma1 transcription to promote class switching to IgG1. PGE(2) is an important regulator of the immune response, shifting the balance towards a T helper type 2 response, directing selection of the isotypes produced, and promoting memory cell formation.

Type 1 and type 2 responses in regulation of Ig isotype expression in cattle

Regulation of humoral immune responses is multifactorial involving appropriate activation, costimulation and the presence of specific soluble factors. Polarized type 1 or type 2 humoral responses in the laboratory mouse have been linked to expression of specific cytokines and thus can be used to provide insight into the type of response generated by infection. For example, IFN-gamma has been linked to IgG2a and IgG3 production, IL-4 to IgG1 and IgE production and TGF-beta to IgA production. Unlike the laboratory mouse, generally housed under defined conditions, highly skewed isotype expression patterns generally occur in cattle in chronic infections. A few examples of polarized responses have been noted in chronic experimental or naturally occurring infections including F. hepatica, M. paratuberculosis, C. parvum and B. abortus. In vitro studies using purified bovine B cells and various forms of costimulation and cytokines have demonstrated that isotype responses can be polarized under certain experimental conditions in vitro. That is, IgG1 expression is positively regulated by IL-4 and IgG2 expression is positively regulated by IFN-gamma. Other as yet unidentified factors may play pivotal roles in regulating humoral immune responses in large ruminant species in vivo. This possibility is best exemplified by recent studies using DNA vaccines in cattle that have been demonstrated in the mouse to be generally polarizing to a type 1 response. Surprisingly, studies in cattle using plasmid DNA as vaccination material show an almost exclusive IgG1 response. Based on a number of studies using T cell clones and various biological assays, it is clear that the classical roles of many cytokines in the laboratory mouse do not extrapolate entirely or at all to cattle. Thus, the design of adjuvants and immune modulators should be based on studies done in cattle or using bovine cells. Based on studies to date, several "holes" in the cytokine repertoire exist and these roles may be assumed by unique factors or activities of other known cytokines.

Myb proteins repress human Ig epsilon germline transcription by inhibiting STAT6-dependent promoter activation

Cytokine-dependent induction of correctly spliced germline (GL) transcripts is required to target the appropriate switch region for class switch recombination. GL transcription is linked to the cell cycle and the number of cell divisions through mechanisms that have not been defined. The human proximal epsilon GL promoter contains an IL-4 responsive element (IL-4RE) that binds STAT6 and is sufficient to confer IL-4 inducibility to a heterologous promoter in transient transfection studies. We show herein that the IL-4RE contains a novel Myb binding motif that overlaps the 3' end of the STAT6 palindrome. EMSA analysis showed binding to the IL-4RE of endogenous Myb proteins expressed in BL-2 B cells and Jurkat T cells. However, double occupancy of a probe spanning both STAT6 and Myb binding motifs could not be detected. Thus, binding of either factor may prevent protein/DNA interactions at the other site, raising the possibility that Myb binding may interfere with STAT6-dependent activation of the IL-4RE. Indeed, cotransfection of A-Myb or c-Myb expression vectors in HEK293 and BL-2 cells suppressed STAT6-dependent transcription from a reporter construct containing four copies of the IL-4RE cloned upstream of a minimal thymidine kinase promoter. Most importantly, overexpression of A-Myb was sufficient to suppress IL-4-induced endogenous epsilon GL transcription in BL-2 cells. Our results indicate that Myb proteins, which are known to act as cell cycle sensors, may play an important mechanistic role in the in vivo regulation of epsilon GL transcription in human B cells.

Nippocystatin, a cysteine protease inhibitor from Nippostrongylus brasiliensis, inhibits antigen processing and modulates antigen-specific immune response

During infection, parasites evade the host immune system by modulating or exploiting the immune system; e.g., they suppress expression of major histocompatibility complex class II molecules or secrete cytokine-like molecules. However, it is not clear whether helminths disturb the immune responses of their hosts by controlling the antigen-processing pathways of the hosts. In this study, we identified a new cysteine protease inhibitor, nippocystatin, derived from excretory-secretory (ES) products of an intestinal nematode, Nippostrongylus brasiliensis. Nippocystatin, which belongs to cystatin family 2, consists of 144 amino acids and is secreted as a 14-kDa mature form. In vivo treatment of ovalbumin (OVA)-immunized mice with recombinant nippocystatin (rNbCys) profoundly suppressed OVA-specific proliferation of splenocytes but not non-antigen-specific proliferation of splenocytes. OVA-specific cytokine production was also greatly suppressed in rNbCys-treated mice. Although the serum levels of both OVA-specific immunoglobulin G1 (IgG1) and IgG2a were not affected by rNbCys treatment, OVA-specific IgE was preferentially downregulated in rNbCys-treated mice. In vitro rNbCys inhibited processing of OVA by lysosomal cysteine proteases from the spleens of mice. Mice with anti-nippocystatin antibodies became partially resistant to infection with N. brasiliensis. Based on these findings, N. brasiliensis appears to skillfully evade host immune systems by secreting nippocystatin, which modulates antigen processing in antigen-presenting cells of hosts.

Differential regulation of mouse germline Ig gamma 1 and epsilon promoters by IL-4 and CD40

Before Ig class switching, RNA transcription through the specific S regions undergoing recombination is induced by cytokines and other activators that induce and direct switching. The resulting germline (GL) transcripts are essential for switch recombination. To understand the differential regulation of mouse IgG1 and IgE, we compared the promoters for GL gamma1 and epsilon transcripts. We addressed the question of why the promoter that regulates GL epsilon transcription is more responsive to IL-4 than the gamma1 promoter and also why GL epsilon transcription is more dependent on IL-4 than is gamma1 transcription. We found that the IL-4-responsive region of the GL epsilon promoter is more inducible than that of the gamma1 promoter, although each promoter contains a binding site for the IL-4-inducible transcription factor Stat6, located immediately adjacent to a binding site for a basic region leucine zipper (bZip) family protein. However, the arrangement and sequences of the sites differ between the epsilon and gamma1 promoters. The GL epsilon promoter binds Stat6 with a 10-fold higher affinity than does the gamma1 promoter. Furthermore, the bZip elements of the two promoters bind different transcription factors, as the GL epsilon promoter binds and is activated by AP-1, whereas the gamma1 promoter binds and is activated by activating transcription factor 2. C/EBPbeta and C/EBPgamma also bind the gamma1 bZip element, although they inhibit rather than activate transcription. However, inhibition of promoter activity by C/EBPbeta does not require the bZip element and may instead occur via inhibiting the activity of NF-kappaB.

Th1/Th2-Balancing immunomodulating activity of gel-forming (1-->3)-beta-glucans from fungi

The immunomodulating effects of various gel-forming (1-->3)-beta-glucans, grifolan (GRN), SSG, sonifilan (SPG) and alkaline-treated SPG (SPG-OH), on balancing helper T cell activity were examined in a murine model. Plasma from mice that were injected with GRN or SPG-OH and trinitrophenyl ovalbumin (TNP-OVA) contained TNP-specific antibodies of both IgG1 and IgG2a isotypes. Administration of SSG and TNP-OVA significantly augmented the synthesis of IgG2a antibodies, while the synthesis of IgG1 was reduced. However, SPG did not enhance the antibody response. In the culture supernatants of splenocytes obtained from GRN- or SPG-OH-administered mice, high levels of IgGI and low levels of IgG2a and IFN gamma were detected. In contrast, high levels of IgG2a and IFN gamma and low levels of IgGI were detected in the case of administration of SSG. Furthermore, it was shown by intracellular cytokine staining that the proportion of IFN gamma+CD4+ double-positive cells among the CD4+ cells from mice administered SSG was most strongly increased by addition of PMA and A23187. On the other hand, the expression of IL-12 p40 mRNA was more markedly elevated in splenocytes after combined administration of TNP-OVA plus SSG than after administration of TNP-OVA alone. The highest IFN gamma production was observed when adherent cells of mice administered TNP-OVA and SSG were cultured with TNP-primed lymphocytes. This effect of administration of SSG on IFN-y production was completely inhibited by addition of anti-IL-12 mAb. In conclusion, our study showed that beta-glucans have various effects on the Th1 or Th2-dependent antibody subclasses, in particular, SSG induces the development of Th1 cells via the IL-12 pathway.

A DNase I hypersensitive site near the murine gamma1 switch region contributes to insertion site independence of transgenes and modulates the amount of transcripts induced by CD40 ligation

Several cis-acting elements regulate the expression of germline transcripts of heavy chain constant region genes and their subsequent switch recombination. To study such elements in the murine gamma1 gene, we have utilized a transgenic approach. In this study we focused on a DNase I hypersensitive site (termed 'Site II') that lies about 2 kb 3' of the gamma1 promoter region and I exon, just 5' to the gamma1 switch region. We have reported that gamma1 transgenes with Site II display the characteristics of a locus control region (LCR) in that they are insertion site independent and copy number dependent. For the present study we prepared six lines of transgenic mice that have the promoter region and I exon, but lack Site II. Expression of RNA from gamma1 transgenes that lack Site II is not correlated with transgene copy number; expression is insertion site dependent. This result indicates that DNase hypersensitive Site II is an important part of the LCR-like elements in the murine gamma1 gene. RNA expression from the gamma1 transgenes that lack Site II is inducible by IL-4 and by CD40 ligation. However, the induction of transgenic RNA expression by CD40 ligation is greater than expected, suggesting that elements within Site II participate in negative regulation of the amount of germline transcripts after CD40 ligation.

Cutting Edge: IFN-γ Regulated Germline Transcripts Are Expressed from γ2a Transgenes Independently of the Heavy Chain 3′ Enhancers

Several results indicate that transcriptional enhancers lying 3′ of the Cα gene regulate RNA expression and switch recombination of heavy chain genes. To investigate this regulation we prepared transgenic mice with a 10.5-kb transgene that included the germline form of the murine γ2α gene, including promoter, I, S, and C regions. RNA was expressed from these γ2a transgenes with correct IFN-γ regulation, in spite of the fact that they lacked the 3′ enhancers. This RNA expression was independent of insertion site and dependent on copy number, indicating that the γ2a gene includes locus control region-like elements. Addition of either a cassette containing 3′ enhancer DNase I hypersensitive sites 1, 2, 3B, and 4 or the intronic μ enhancer increased transcription from the γ2a transgene by ∼75-fold in B cells. However, this increased transcription was not responsive to IFN-γ treatment of the transgenic B cells.

Switching to IgG3, IgG2b, and IgA Is Division Linked and Independent, Revealing a Stochastic Framework for Describing Differentiation

LPS was used to induce switching of B cells to IgG3 and, in the presence of TGF-β, to IgG2b and IgA. Switching to all three isotypes increased with division number according to a consistent relationship that was independent of time in culture. The mode of activation altered the relationship with division, as CD40 ligand increased switching to IgA and decreased switching to IgG2b and IgG3 when measured per division. This division-linked switching behavior could be described by Gaussian probability distributions centered around a mean division number. The divisions at which switching to IgG3 and IgG2b occurred overlapped, raising the possibility that the two switching mechanisms were linked. However, when IgG3+ and IgG3− B cells were sorted and placed back in culture, they switched to IgG2b at an equivalent rate, indicating that alternative switching decisions were made independently within a single cell. As a consequence, isotype switching could be predicted at the population level by standard probability laws. Therefore, division number provides a framework for a stochastic description of differentiation that may be widely applicable.

Integrating Signals from IFN-γ and IL-4 by B Cells: Positive and Negative Effects on CD40 Ligand-Induced Proliferation, Survival, and Division-Linked Isotype Switching to IgG1, IgE, and IgG2a

IL-4 and IFN-γ each have potent effects on B cell responses as well as strong mutual antagonism. Here we have examined the quantitative effects of these cytokines on CD40 ligand-induced B cell proliferation, cell survival, and division-linked isotype switching. Both IL-4 (strongly) and IFN-γ (weakly) enhanced the number of B cells found in culture by reducing the average time cells take to enter the first division cycle and by promoting B cell survival. When added in combination, the net effect of IL-4 and IFN-γ on time to division and survival was a response intermediate between that of the two cytokines alone, indicating a partial antagonism of IL-4 by IFN-γ. By modulating both time to division and cell survival, these small effects of IFN-γ are amplified and give rise to large reductions in cell number in the presence of IL-4. At higher concentrations, IFN-γ had minor inhibitory effects on IL-4-induced isotype switching to IgG1 and greater effects on IgE. A reciprocal relation was observed between the ability to inhibit IgE at late cell divisions vs induction of IgG2a. In contrast, IL-4 did not prevent switching to IgG2a induced by IFN-γ alone. Therefore, antagonism between IFN-γ and IL-4 is observed at multiple levels and over different concentration ranges, resulting in complex net outcomes. The evolutionary significance of this complexity is discussed.

Switch recombination and germ-line transcription are division-regulated events in B lymphocytes

Treatment of resting murine B lymphocytes with CD40 ligand (CD40L) and IL-4 induces proliferation and a switch in immunoglobulin (Ig) isotype surface expression from IgM and IgD to IgG1 and IgE. Using a fluorescent dye to enable cell sorting according to cell division cycle number, we have examined molecular events associated with B cell differentiation, namely, germ-line transcription and DNA recombination. Digestion-circularisation polymerase chain reaction experiments showed that DNA recombination leading to isotype switching from IgM to IgG1 surface expression is division-dependent and was first detected after B cells had divided three times. Similarly, DNA rearrangement involving the IgE switch region was detectable only after five division cycles. These division cycle numbers correlate with the numbers of divisions required before surface expression of the switched isotype [P.D. Hodgkin, J.-H. Lee, A.B. Lyons, J. Exp. Med. 184 (1996) 277-281]. RT-PCR analyses also revealed that germ-line transcripts for both IgG1 and IgE increased with division number suggesting a threshold expression level may be required for recombination to occur.

Impaired immune responses and B-cell proliferation in mice lacking the Id3 gene

B-lymphocyte activation and proliferation induced by the B-cell receptor (BCR) signals are important steps in the initiation of humoral immune responses. How the BCR signals are translated by nuclear transcription factors into cell cycle progression is poorly understood. Id3 is an immediate-early gene responding to growth and mitogenic signals in many cell types including B cells. The primary function of the Id3 protein has been defined as that of inhibitor of basic-helix-loop-helix (bHLH) transcription factors. The interaction between Id3 and bHLH proteins, many of which are essential for cellular differentiation, has been proposed as a key regulatory event leading to cellular proliferation instead of differentiation. To further investigate the role of Id3 in tissue and embryo development and the mechanism of Id3-mediated growth regulation, we generated and analyzed Id3-deficient mice. While these mice display no overt abnormality in tissue and embryo development, their humoral immunity is compromised. The amounts of immunoglobulins produced in Id3-deficient mice immunized with a T-cell-dependent antigen and a type 2 T-cell-independent antigen are attenuated and severely impaired, respectively. Further analysis of lymphocytes isolated from Id3-deficient mice reveals a B-cell defect in their proliferation response to BCR cross-linking but not to lipopolysaccharide or a combination of BCR cross-linking and interleukin-4. Analyses of cultured lymphocytes also suggest involvement of Id3 in cytokine production in T cells and isotype switching in B cells. Finally, the proliferation defect in Id3-deficient B cells can be rescued by ectopic expression of Id1, a homologue of Id3. Taken together, these results define a necessary and specific role for Id3 in mediating signals from BCR to cell cycle progression during humoral immune responses.

Switch Recombination in a Transfected Plasmid Occurs Preferentially in a B Cell Line That Undergoes Switch Recombination of Its Chromosomal Ig Heavy Chain Genes

Ab class switching is induced upon B cell activation in vivo by immunization or infection or in vitro by treatment with mitogens, e.g. LPS, and results in the expression of different heavy chain constant region (CH) genes without a change in the Ab variable region. This DNA recombination event allows Abs to alter their biological activity while maintaining their antigenic specificity. Little is known about the molecular mechanism of switch recombination. To attempt to develop an assay for enzymes, DNA binding proteins, and DNA sequences that mediate switch recombination, we have constructed a plasmid DNA substrate that will undergo switch recombination upon stable transfection into the surface IgM+ B cell line (I.29μ), a cell line capable of undergoing switch recombination of its endogenous genes. We demonstrate that recombination occurs between the two switch regions of the plasmid, as assayed by PCRs across the integrated plasmid switch regions, followed by Southern blot hybridization. Nucleotide sequence analysis of the PCR products confirmed the occurrence of Sμ-Sα recombination in the plasmid. Recombination of the plasmid in I.29μ cells does not require treatment with inducers of switch recombination, suggesting that recombinase activity is constitutive in I.29μ cells. Recombination does not require high levels of transcription across the switch regions of the plasmid. Fewer recombination events are detected in four different B and T cell lines that do not undergo switch recombination of their endogenous genes.

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.

IL-5 Induces IgG1 Isotype Switch Recombination in Mouse CD38-Activated sIgD-Positive B Lymphocytes

Mouse B cells express CD38, whose ligation by anti-CD38 Ab induces their proliferation and protection from apoptosis. We previously showed that stimulation of mouse splenic B cells with IL-5 together with CS/2, an anti-mouse CD38 mAb, induces production of IgG1 and IgM. Here we examined the role of IL-5 and CS/2 in the expression of germline γ1 transcripts and the generation of reciprocal products forming DNA circles as byproducts of μ-γ1 switch recombination. By itself, CS/2 induced significant expression of germline γ1 transcripts in splenic naive B cells, whereas IL-5 neither induced nor enhanced germline γ1 expression. Increased cellular content of reciprocal product, which is characteristic of μ-γ1 recombination, was not observed after culturing B cells with CS/2, but increased reciprocal product, along with high levels of lgG1 secretion, was found when B cells were cultured with CS/2 plus IL-5. Although IL-4 did not, by itself, induce μ-γ1 recombination in B cells stimulated with CS/2, in conjunction with CS/2 plus IL-5, IL-4 dramatically enhanced sterile γ1 transcription and IgG1 production. These results demonstrate that CD38 ligation induces only germline γ1 transcription and that IL-5 promotes both μ-γ1 switch recombination and lgG1 secretion in an IL-4-independent manner.

Ialpha exon-replacement mice synthesize a spliced HPRT-C(alpha) transcript which may explain their ability to switch to IgA. Inhibition of switching to IgG in these mice

Antibody class switching is regulated by transcription of unrearranged C(H) genes to produce germline (GL) transcripts which direct the choice of isotype and are required for switching. However, their role is unknown. GL transcripts are initiated at the I exons located upstream of each switch region. Although deletion of the I exon by gene targeting prevents switch recombination to that CH gene, the Ialpha exon can be replaced by an entirely different DNA segment, a minigene driven by the phosphoglycerate kinase (PGK) promoter and encoding hypoxanthine phosphoribosyl transferase (HPRT), oriented in the sense direction, without reducing antibody class switching to IgA. To understand why HPRT substitution of the Ialpha exon does not disrupt switch recombination, we have analyzed the structure of the transcript from the targeted allele in these mice. We identify a spliced transcript in which the HPRT exons are spliced to the C(alpha) gene segments, resulting in a structure similar to normal GL transcripts. The abundance of this transcript is similar to that of the normal alpha GL RNA. We also demonstrate that switching to the four IgG subclasses in B cells from these mice is reduced in comparison to wild-type mice. We discuss the possibility that the strong PGK promoter inserted at the Ig alpha locus may interfere with interaction of the promoters for gamma GL transcripts with the 3' IgH enhancer.

Target specificity of immunoglobulin class switch recombination is not determined by nucleotide sequences of S regions

We describe a model system for class switch recombination (CSR) using CH12F3-2 cells transfected with a DNA construct containing two S sequences transcribed by different promoters and separated by a viral thymidine kinase (TK) gene. Recombination observed using this system shares key properties with physiological CSR: deletion of DNA between two S regions, requirement for cytokine stimulation, and nonhomologous and no consensus breakpoint sequences. Studies on transfectants with variants of this construct led us to the following conclusions: (1) two S sequences are required for CSR; (2) isotype specificity of recombination is not determined by nucleotide sequences of S regions; (3) S sequences are not strand-specific; and (4) induction of recombination activity requires cytokine stimulation.