High-affinity B cell receptor ligation by cognate antigen induces cytokine-independent isotype switching

Antigen-specific IgG subclass composition in recipient mice can indicate the degree of red blood cell alloimmunization as well as discern between primary and secondary immunization

BACKGROUND

Despite the vast antigen disparity between donor and recipient red blood cells (RBCs), only 2%-6% of transfusion patients mount an alloantibody response. Recently, RBC antigen density has been proposed as one of the factors that can influence alloimmunization, however, there has been no characterization of the role of antigen density along with RBC dose in primary and secondary immunization.

STUDY DESIGN AND METHODS

To generate RBCs that express distinct antigen copy numbers, different quantities of hen egg lysozyme (HEL) were coupled to murine RBCs. The HEL-RBCs were subsequently transfused into recipient mice at different RBC doses and their HEL-specific IgM, IgG, and IgG subclass response was evaluated.

RESULTS

Productive immune responses could be generated through a high copy number antigen transfused at low RBC doses or a low copy number transfused at high RBC doses. Further, primary but submaximal humoral immunization predominantly induced the IgG2b and IgG3 subclasses. In contrast, a maximal primary immunization or a secondary immunization induced all four IgG subclasses.

DISCUSSION

Our results confirm the existence of an antigen threshold for productive immune responses but indicate that a high antigen copy number alone might not be enough to induce a response, but rather a combination of both antigen copy number and cell dosage may determine the outcome of immunization. Further, this study provides a proof of concept that the IgG subclass composition can be an indicator of the level of RBC alloimmunization as well as discern between primary and secondary immunization at least in this murine model.

Multimericity Amplifies the Synergy of BCR and TLR4 for B Cell Activation and Antibody Class Switching

Sustained signaling through the B cell antigen receptor (BCR) is thought to occur only when antigen(s) crosslink or disperse multiple BCR units, such as by multimeric antigens found on the surfaces of viruses or bacteria. B cell-intrinsic Toll-like receptor (TLR) signaling synergizes with the BCR to induce and shape antibody production, hallmarked by immunoglobulin (Ig) class switch recombination (CSR) of constant heavy chains from IgM/IgD to IgG, IgA or IgE isotypes, and somatic hypermutation (SHM) of variable heavy and light chains. Full B cell differentiation is essential for protective immunity, where class switched high affinity antibodies neutralize present pathogens, memory B cells are held in reserve for future encounters, and activated B cells also serve as semi-professional APCs for T cells. But the rules that fine-tune B cell differentiation remain partially understood, despite their being essential for naturally acquired immunity and for guiding vaccine development. To address this in part, we have developed a cell culture system using splenic B cells from naive mice stimulated with several biotinylated ligands and antibodies crosslinked by streptavidin reagents. In particular, biotinylated lipopolysaccharide (LPS), a Toll-like receptor 4 (TLR4) agonist, and biotinylated anti-IgM were pre-assembled (multimerized) using streptavidin, or immobilized on nanoparticles coated with streptavidin, and used to active B cells in this precisely controlled, high throughput assay. Using B cell proliferation and Ig class switching as metrics for successful B cell activation, we show that the stimuli are both synergistic and dose-dependent. Crucially, the multimerized immunoconjugates are most active over a narrow concentration range. These data suggest that multimericity is an essential requirement for B cell BCR/TLRs ligands, and clarify basic rules for B cell activation. Such studies highlight the importance in determining the choice of single vs multimeric formats of antigen and PAMP agonists during vaccine design and development.

Global Investigation of Immune Repertoire Suggests Kawasaki Disease Has Infectious Cause

BACKGROUND

Kawasaki disease (KD) severely threatens young children's health worldwide. The pathogenic mechanism of KD has not yet been solved, so there is still debate over whether KD is an infectious disease or an autoimmune disease.Methods and Results:To solve this problem, an immune repertoire analysis of KD was conducted. We collected blood cell RNA samples and prepared them into amplicons with iRepertoire kits. The amplicons were sequenced and analyzed with the iRepertoire pipeline. We first identified KD-specific VJ and VDJ forms that had the potential to serve as biomarkers of KD. In addition, the KD-specific VDJ forms were contributed mostly by immunoglobulin G. The D50 value analysis showed that B-cell diversity in KD is decreased, suggesting unique immunoglobulins are produced in KD. Moreover, V, D and J segment usage in IgA, IgG and IgM was consistent with previous KD studies. Further comparison showed no difference in CDR3 peptide length between KD and fever controls (subjects with fever but not diagnosed as KD), indicting KD had B-cell selection phenomenon that has a non-autoimmune pattern. The comparison of amino acid usage of the CDR3 region demonstrated a preference for hydrophilic amino acids in KD.

CONCLUSIONS

The results of D50 value, VDJ usage and CDR3 peptide length analyses suggested the characteristics of infectious disease for KD.

Relation of HLA-DRB1 to IgG4 autoantibody and cytokine production in muscle-specific tyrosine kinase myasthenia gravis (MuSK-MG)

A small subset of myasthenia gravis (MG) patients develop autoantibodies against muscle-specific kinase (MuSK), which are predominantly of the immunoglobulin (Ig)G4 isotype. MuSK-MG is strongly associated with HLA-DRB1*14, HLA-DRB1*16 and HLA-DQB1*05. In this study, the possible effects of these HLA associations on MuSK IgG autoantibody or cytokine production were investigated. Samples from 80 MG patients with MuSK antibodies were studied. The disease-associated HLA types were screened in the DNA samples. The IgG1, IgG2, IgG3 and IgG4 titres of the MuSK antibodies and the levels of interleukin (IL)-4, IL-6, IL-17A and IL-10 were measured in the sera. Comparisons were made among the groups with or without HLA-DRB1*14, HLA-DRB1*16 or HLA-DQB1*05. The IgG4 titres of the MuSK antibodies were higher than those of the IgG1, IgG2 and IgG3 isotypes among the whole group of patients. DRB1*14 (+) DRB1*16 (-) patients had higher levels of IgG4 antibodies than those of DRB1*14 (-) DRB1*16 (+) patients. DRB1*14 (+) DRB1*16 (+) patients also had higher levels of IgG4 antibodies than those of DRB1*14 (-) DRB1*16 (+) and DRB1*14 (-) DRB1*16 (-) patients. Higher IL-10 and lower IL-17A levels were measured in DRB1*14 (+) DRB1*16 (-) patients than in DRB1*14 (-) DRB1*16 (-) patients. The higher IgG4 titres of MuSK autoantibodies in patients carrying HLA-DRB1*14 than those in the other patients suggest a role for HLA in the production of the antibodies. The differences in IL-10 and IL-17A support the role of DRB1 in the etiopathogenesis of this autoimmune response.

Genetic regulation of antibody responsiveness to immunization in substrains of BALB/c mice

Antibody‐mediated immunity is highly protective against disease. The majority of current vaccines confer protection through humoral immunity, but there is high variability in responsiveness across populations. Identifying immune mechanisms that mediate low antibody responsiveness may provide potential strategies to boost vaccine efficacy. Here, we report diverse antibody responsiveness to unadjuvanted as well as adjuvanted immunization in substrains of BALB/c mice, resulting in high and low antibody response phenotypes. Furthermore, these antibody phenotypes were not affected by changes in environmental factors such as the gut microbiota composition. Antigen‐specific B cells following immunization had a marked difference in capability to class switch, resulting in perturbed IgG isotype antibody production. In vitro, a B‐cell intrinsic defect in the regulation of class‐switch recombination was identified in mice with low IgG antibody production. Whole genome sequencing identified polymorphisms associated with the magnitude of antibody produced, and we propose candidate genes that may regulate isotype class‐switching capability. This study highlights that mice sourced from different vendors can have significantly altered humoral immune response profiles, and provides a resource to interrogate genetic regulators of antibody responsiveness. Together these results further our understanding of immune heterogeneity and suggest additional research on the genetic influences of adjuvanted vaccine strategies is warranted for enhancing vaccine efficacy.

Deceleration of glycometabolism impedes IgG-producing B-cell-mediated tumor elimination by targeting SATB1

B lymphocytes, known as antibody producers, mediate tumor cell destruction in the manner of antibody-dependent cell-mediated cytotoxicity; however, their anti-tumor function seems to be weakened during tumorigenesis, while the underlying mechanisms remain unclear. In this study, we found that IgG mediated anti-tumor effects, but IgG-producing B cells decreased in various tumors. Considering the underlying mechanism, glycometabolism was noteworthy. We found that tumor-infiltrating B cells were glucose-starved and accompanied by a deceleration of glycometabolism. Both inhibition of glycometabolism and deprivation of glucose through tumor cells, or glucose-free treatment, reduced the differentiation of B cells into IgG-producing cells. In this process, special AT-rich sequence-binding protein-1 (SATB1) was significantly silenced in B cells. Down-regulating SATB1 by inhibiting glycometabolism or RNA interference reduced the binding of signal transducer and activator of transcription 6 (STAT6) to the promoter of germline Cγ gene, subsequently resulting in fewer B cells producing IgG. Our findings provide the first evidence that glycometabolic inhibition by tumorigenesis suppresses differentiation of B cells into IgG-producing cells, and altering glycometabolism may be promising in improving the anti-tumor effect of B cells.

IgG subclass co-expression brings harmony to the quartet model of murine IgG function

A model of murine IgG function is presented in which the co-expression of the IgG subclasses is a central feature, class switching occurs before the commencement of somatic hypermutation, and there is little switching between subclasses. It is named the quartet model to emphasize the harmony that comes from the simultaneous presence of the four subclasses. In this model, IgG3 and IgG2b antibodies are particularly important early in the response, when T-cell help may be limiting. IgG3 initiates inflammation through complement fixation, whereas IgG2b provides early FcγR-mediated effector functions. As T-cell help strengthens, IgG2a antibodies increase the power of the response, whereas IgG1 production helps limit the inflammatory drive and limits immunopathology. The model highlights the fact that murine IgG subclasses function quite differently to human IgG subclasses. This allows them to serve the special immunological needs of a species that is vulnerable because of its small size.

High affinity IgM(+) memory B cells are generated through a germinal center-dependent pathway

During a T cell-dependent immune response, B cells undergo clonal expansion and selection and the induction of isotype switching and somatic hypermutation (SHM). Although somatically mutated IgM(+) memory B cells have been reported, it has not been established whether they are really high affinity B cells. We tracked (4-hydroxy-3-nitrophenyl) acetyl hapten-specific GC B cells from normal immunized mice based on affinity of their B cell receptor (BCR) and performed BCR sequence analysis. SHM was evident by day 7 postimmunization and increased with time, such that high affinity IgM(+) as well as IgG(+) memory B cells continued to be generated up to day 42. In contrast, class-switch recombination (CSR) was almost completed by day 7 and then the ratio of IgG1(+)/IgM(+) GC B cells remained unchanged. Together these findings suggest that IgM(+) B cells undergo SHM in the GC to generate high affinity IgM(+) memory cells and that this process continues even after CSR is accomplished.

IgH chain class switch recombination: mechanism and regulation

IgH class switching occurs rapidly after activation of mature naive B cells, resulting in a switch from expression of IgM and IgD to expression of IgG, IgE, or IgA; this switch improves the ability of Abs to remove the pathogen that induces the humoral immune response. Class switching occurs by a deletional recombination between two switch regions, each of which is associated with a H chain constant region gene. Class switch recombination (CSR) is instigated by activation-induced cytidine deaminase, which converts cytosines in switch regions to uracils. The uracils are subsequently removed by two DNA-repair pathways, resulting in mutations, single-strand DNA breaks, and the double-strand breaks required for CSR. We discuss several aspects of CSR, including how CSR is induced, CSR in B cell progenitors, the roles of transcription and chromosomal looping in CSR, and the roles of certain DNA-repair enzymes in CSR.

B cell Rab7 mediates induction of activation-induced cytidine deaminase expression and class-switching in T-dependent and T-independent antibody responses

Class switch DNA recombination (CSR) is central to the maturation of the Ab response because it diversifies Ab effector functions. Like somatic hypermutation, CSR requires activation-induced cytidine deaminase (AID), whose expression is restricted to B cells, as induced by CD40 engagement or dual TLR-BCR engagement (primary CSR-inducing stimuli). By constructing conditional knockout Igh(+/C)γ(1-cre)Rab7(fl/fl) mice, we identified a B cell-intrinsic role for Rab7, a small GTPase involved in intracellular membrane functions, in mediating AID induction and CSR. Igh(+/C)γ(1-cre)Rab7(fl/fl) mice displayed normal B and T cell development and were deficient in Rab7 only in B cells undergoing Igh(C)γ(1-cre) Iγ1-Sγ1-Cγ1-cre transcription, as induced--like Igh germline Iγ1-Sγ1-Cγ1 and Iε-Sε-Cε transcription--by IL-4 in conjunction with a primary CSR-inducing stimulus. These mice could not mount T-independent or T-dependent class-switched IgG1 or IgE responses while maintaining normal IgM levels. Igh(+/C)γ(1-cre)Rab7(fl/fl) B cells showed, in vivo and in vitro, normal proliferation and survival, normal Blimp-1 expression and plasma cell differentiation, as well as intact activation of the noncanonical NF-κB, p38 kinase, and ERK1/2 kinase pathways. They, however, were defective in AID expression and CSR in vivo and in vitro, as induced by CD40 engagement or dual TLR1/2-, TLR4-, TLR7-, or TLR9-BCR engagement. In Igh(+/C)γ(1-cre)Rab7(fl/fl) B cells, CSR was rescued by enforced AID expression. These findings, together with our demonstration that Rab7-mediated canonical NF-κB activation, as critical to AID induction, outline a novel role of Rab7 in signaling pathways that lead to AID expression and CSR, likely by promoting assembly of signaling complexes along intracellular membranes.

Quantal and graded stimulation of B lymphocytes as alternative strategies for regulating adaptive immune responses

Lymphocytes undergo a typical response pattern following stimulation in vivo: they proliferate, differentiate to effector cells, cease dividing and predominantly die, leaving a small proportion of long-lived memory and effector cells. This pattern results from cell-intrinsic processes following activation and the influence of external regulation. Here we apply quantitative methods to study B-cell responses in vitro. Our results reveal that B cells stimulated through two Toll-like receptors (TLRs) require minimal external direction to undergo the basic pattern typical of immunity. Altering the stimulus strength regulates the outcome in a quantal manner by varying the number of cells that participate in the response. In contrast, the T-cell-dependent CD40 activation signal induces a response where division times and differentiation rates vary in relation to stimulus strength. These studies offer insight into how the adaptive antibody response may have evolved from simple autonomous response patterns to the highly regulable state that is now observed in mammals.

Cholera toxin B subunit acts as a potent systemic adjuvant for HIV-1 DNA vaccination intramuscularly in mice

Cholera toxin B subunit (CTB) was investigated as a classical mucosal adjuvant that can increase vaccine immunogenicity. In this study, we found out the in vitro efficacy of cholera toxin B subunit (CTB) in activating mice bone marrow-derived dendritic cells (BMDCs) through Toll-like receptor signaling pathways. In vitro RNA and transcriptional level profiling arrays revealed that CTB guides high levels of Th1 and Th2 type cytokines, inflammatory cytokines, and chemokines. Based on the robustness of these profiling results, we examined the induction of HIV Env-specific immunity by CTB co-inoculated with HIV Env DNA vaccine intramuscularly in vivo. CTB enhanced HIV-Env specific cellular immune responses in Env-specific IFN-γ ELISPOT, compared with DNA vaccine alone. Moreover, CTB induced high levels of Env specific humoral response and promoted antibody maturation after the third round of vaccination. This combination immunization strategy induced a Th2-type bias response which is indicative of a high ratio of IgG1/IgG2a. This study reports that CTB as a classical mucosal adjuvant could enhance HIV-1 DNA-based vaccine immunogenicity intramuscularly; therefore, these findings suggest that CTB could serve as an effective candidate adjuvant for DNA vaccination.

Wiskott-Aldrich Syndrome protein deficiency perturbs the homeostasis of B-cell compartment in humans

Wiskott–Aldrich Syndrome protein (WASp) regulates the cytoskeleton in hematopoietic cells and mutations in its gene cause the Wiskott–Aldrich Syndrome (WAS), a primary immunodeficiency with microthrombocytopenia, eczema and a higher susceptibility to develop tumors. Autoimmune manifestations, frequently observed in WAS patients, are associated with an increased risk of mortality and still represent an unsolved aspect of the disease. B cells play a crucial role both in immune competence and self-tolerance and defects in their development and function result in immunodeficiency and/or autoimmunity. We performed a phenotypical and molecular analysis of central and peripheral B-cell compartments in WAS pediatric patients. We found a decreased proportion of immature B cells in the bone marrow correlating with an increased presence of transitional B cells in the periphery. These results could be explained by the defective migratory response of WAS B cells to SDF-1α, essential for the retention of immature B cells in the BM. In the periphery, we observed an unusual expansion of CD21^low B-cell population and increased plasma BAFF levels that may contribute to the high susceptibility to develop autoimmune manifestations in WAS patients. WAS memory B cells were characterized by a reduced in vivo proliferation, decreased somatic hypermutation and preferential usage of IGHV4-34, an immunoglobulin gene commonly found in autoreactive B cells.

In conclusion, our findings demonstrate that WASp-deficiency perturbs B-cell homeostasis thus adding a new layer of immune dysregulation concurring to the increased susceptibility to develop autoimmunity in WAS patients.

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WASp-deficiency affects both central and peripheral B-cell development.

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An early egress of immature B cells leads to an increase of transitional B cells in periphery.

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Reduced maturation status of WAS memory B cells.

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Altered selection of both protective and autoreactive Ig gene families in WAS patients.

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Potentially autoreactive CD21^low B cells are expanded in WAS patients.