Direct and sequential switching from mu to epsilon in patients with Schistosoma mansoni infection and atopic dermatitis

Human B-cell isotype switching origins of IgE

BACKGROUND

B cells expressing IgE contribute to immunity against parasites and venoms and are the source of antigen specificity in allergic patients, yet the developmental pathways producing these B cells in human subjects remain a subject of debate. Much of our knowledge of IgE lineage development derives from model studies in mice rather than from human subjects.

OBJECTIVE

We evaluate models for isotype switching to IgE in human subjects using immunoglobulin heavy chain (IGH) mutational lineage data.

METHODS

We analyzed IGH repertoires in 9 allergic and 24 healthy adults using high-throughput DNA sequencing of 15,843,270 IGH rearrangements to identify clonal lineages of B cells containing members expressing IgE. Somatic mutations in IGH inherited from common ancestors within the clonal lineage are used to infer the relationships between B cells.

RESULTS

Data from 613,641 multi-isotype B-cell clonal lineages, of which 592 include an IgE member, are consistent with indirect switching to IgE from IgG- or IgA-expressing lineage members in human subjects. We also find that these inferred isotype switching frequencies are similar in healthy and allergic subjects.

CONCLUSIONS

We found evidence that secondary isotype switching of mutated IgG1-expressing B cells is the primary source of IgE in human subjects, with lesser contributions from precursors expressing other switched isotypes and rarely IgM or IgD, suggesting that IgE is derived from previously antigen-experienced B cells rather than naive B cells that typically express low-affinity unmutated antibodies. These data provide a basis from which to evaluate allergen-specific human antibody repertoires in healthy and diseased subjects.

IgG4 and IgE transcripts in childhood allergic asthma reflect divergent antigen-driven selection

The physiologic function of the "odd" Ab IgG4 remains enigmatic. IgG4 mediates immunotolerance, as, for example, during specific immunotherapy of allergies, but it mediates tissue damage in autoimmune pemphigus vulgaris and "IgG4-related disease." Approximately half of the circulating IgG4 molecules are bispecific owing to their unique ability to exchange half-molecules. Better understanding of the interrelation between IgG4 and IgE repertoires may yield insight into the pathogenesis of allergies and into potential novel therapies that modulate IgG4 responses. We aimed to compare the selective forces that forge the IgG4 and IgE repertoires in allergic asthma. Using an IgG4-specific RT-PCR, we amplified, cloned, and sequenced IgG4 H chain transcripts of PBMCs from 10 children with allergic asthma. We obtained 558 functional IgG4 sequences, of which 286 were unique. Compared with previously published unique IgE transcripts from the same blood samples, the somatic mutation rate was significantly enhanced in IgG4 transcripts (62 versus 83%; p < 0.001), whereas fewer IgG4 sequences displayed statistical evidence of Ag-driven selection (p < 0.001). On average, the hypervariable CDRH3 region was four nucleotides shorter in IgG4 than in IgE transcripts (p < 0.001). IgG4 transcripts in the circulation of children with allergic asthma reflect some characteristics of classical Ag-driven B2 immune responses but display less indication of Ag selection than do IgE transcripts. Although allergen-specific IgG4 can block IgE-mediated allergen presentation and degranulation of mast cells, key factors that influence the Ag-binding properties of the Ab differ between the overall repertoires of circulating IgG4- and IgE-expressing cells.

The production and regulation of IgE by the immune system

IgE not only provides protective immunity against helminth parasites but can also mediate the type I hypersensitivity reactions that contribute to the pathogenesis of allergic diseases such as asthma, allergic rhinitis and atopic dermatitis. Despite the importance of IgE in immune biology and allergic pathogenesis, the cells and the pathways that produce and regulate IgE are poorly understood. In this Review, we summarize recent advances in our understanding of the production and the regulation of IgE in vivo, as revealed by studies in mice, and we discuss how these findings compare to what is known about human IgE biology.

The distinctive germinal center phase of IgE+ B lymphocytes limits their contribution to the classical memory response

Direct class switching to IgE generates IgE⁺ GC cells that are highly apoptotic and do not contribute to the memory compartment, while sequential switching through an IgG⁺ intermediate results in the generation of long-lived IgE⁺ plasma cells.

The mechanisms involved in the maintenance of memory IgE responses are poorly understood, and the role played by germinal center (GC) IgE⁺ cells in memory responses is particularly unclear. IgE⁺ B cell differentiation is characterized by a transient GC phase, a bias toward the plasma cell (PC) fate, and dependence on sequential switching for the production of high-affinity IgE. We show here that IgE⁺ GC B cells are unfit to undergo the conventional GC differentiation program due to impaired B cell receptor function and increased apoptosis. IgE⁺ GC cells fail to populate the GC light zone and are unable to contribute to the memory and long-lived PC compartments. Furthermore, we demonstrate that direct and sequential switching are linked to distinct B cell differentiation fates: direct switching generates IgE⁺ GC cells, whereas sequential switching gives rise to IgE⁺ PCs. We propose a comprehensive model for the generation and memory of IgE responses.

The role of the T follicular helper cells in allergic disease

T follicular helper (Tfh) cells were discovered just over a decade ago as germinal centre T cells that help B cells make antibodies. Included in this role is affinity maturation and isotype switching. It is here that their functions become less clear. Tfh cells principally produce IL-21 which inhibits class switching to IgE. Recent studies have questioned whether the germinal centre is the main site of IgE class switching or IgE affinity maturation. In this review, I will examine the evidence that these cells are responsible for regulating IgE class switching and the relationship between Tfh cells and T helper 2 (Th2) effector cells.

Immature B cells preferentially switch to IgE with increased direct Sμ to Sε recombination

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Immunoglobulin heavy chain (IgH) class-switch recombination (CSR) replaces initially expressed Cμ (IgM) constant regions (C_H) exons with downstream C_H exons. Stimulation of B cells with anti-CD40 plus interleukin-4 induces CSR from Cμ to Cγ1 (IgG1) and Cε (IgE), the latter of which contributes to the pathogenesis of atopic diseases. Although Cε CSR can occur directly from Cμ, most mature peripheral B cells undergo CSR to Cε indirectly, namely from Cμ to Cγ1, and subsequently to Cε. Physiological mechanisms that influence CSR to Cγ1 versus Cε are incompletely understood. In this study, we report a role for B cell developmental maturity in IgE CSR. Based in part on a novel flow cytometric IgE CSR assay, we show that immature B cells preferentially switch to IgE versus IgG1 through a mechanism involving increased direct CSR from Cμ to Cε. Our findings suggest that IgE dysregulation in certain immunodeficiencies may be related to impaired B cell maturation.

The IgE repertoire in PBMCs of atopic patients is characterized by individual rearrangements without variable region of the heavy immunoglobulin chain bias

BACKGROUND

Patients with atopic diseases are characterized by high levels of specific IgE production. However, little is known about the composition of their B-cell repertoires.

OBJECTIVES

We sought to analyze the complete PBMC-derived IgE repertoire and to compare clonal expansions between different patients.

METHODS

We have analyzed the IgE-bearing B-cell receptor repertoire in highly atopic patients (>1000 IU/mL) using quantitative RT-PCR, complementarity determining region 3 spectratyping, and sequence analysis. Three representative patients were additionally followed during anti-IgE therapy.

RESULTS

Atopic patients exhibited 100 to 1000 times more IgE-specific transcripts than control individuals. These patients used a variable region of the heavy immunoglobulin chain (VH) epsilon repertoire highly similar to their IgM and IgG repertoires, with preference of VH3b, VH4, VH3a, and VH1 segments. Each patient harbored individual clonal expansions, most probably as correlation of allergen-specific IgE production. Common expansions within the complementary determining region 3 shared by several individuals with similar sensitization patterns were found in spectratyping analysis. However, these antigen-driven expansions showed differences on the sequence level. In omalizumab-treated patients the clinical improvement was paralleled by a clear increase in the ratio of IgG/IgE transcripts.

CONCLUSION

The IgE repertoire in atopic patients follows the VH use patterns seen for other immunoglobulins and seems to preferentially recruit individual rearrangements rather than public expansions.

CLINICAL IMPLICATIONS

The detailed analysis of the IgE B-cell repertoire is highly suitable to follow changes in IgE uses during different therapy modalities.

Characterization of VHepsilon gene expressed in PBL from children with atopic diseases: detection of homologous VH1-69 derived transcripts from three unrelated patients

To elucidate the molecular background of IgE production in early infancy, we analyzed the nucleotide sequences of 36 VH-Cepsilon transcripts expressed in PBL from three infants with allergic diseases. We detected transcripts derived from VH1, VH3, VH4 gene family members, and no bias was observed in the usage of particular VH gene family. However, some VH members, VH1; 1-46, 1-69, VH3; 3-11, 3-21, VH4; 4-39, 4-59 were frequently seen and thus notable. VH4 gene was dominant in one patient with severe atopic dermatitis and food allergy, suggesting the involvement of this gene in pathogenesis of the disease. Even a limited number of clones were analyzed, we also found highly homologous VH1-69 derived sequences from all the three patients, which share the same somatic mutations or polymorphic variations in complementarity-determining region (CDR) 1, and 2 with the same CDR3 (D-JH) sequences including the junctions. These findings might suggest that a rather limited VH gene might be rearranged for specific IgE in early life.

The biology of IGE and the basis of allergic disease

Allergic individuals exposed to minute quantities of allergen experience an immediate response. Immediate hypersensitivity reflects the permanent sensitization of mucosal mast cells by allergen-specific IgE antibodies bound to their high-affinity receptors (FcepsilonRI). A combination of factors contributes to such long-lasting sensitization of the mast cells. They include the homing of mast cells to mucosal tissues, the local synthesis of IgE, the induction of FcepsilonRI expression on mast cells by IgE, the consequent downregulation of FcgammaR (through an insufficiency of the common gamma-chains), and the exceptionally slow dissociation of IgE from FcepsilonRI. To understand the mechanism of the immediate hypersensitivity phenomenon, we need explanations of why IgE antibodies are synthesized in preference to IgG in mucosal tissues and why the IgE is so tenaciously retained on mast cell-surface receptors. There is now compelling evidence that the microenvironment of mucosal tissues of allergic disease favors class switching to IgE; and the exceptionally high affinity of IgE for FcepsilonRI can now be interpreted in terms of the recently determined crystal structures of IgE-FcepsilonRI and IgG-FcgammaR complexes. The rate of local IgE synthesis can easily compensate for the rate of the antibody dissociation from its receptors on mucosal mast cells. Effective mechanisms ensure that allergic reactions are confined to mucosal tissues, thereby minimizing the risk of systemic anaphylaxis.

Evolution of IgM, IgE and IgG(1-4 )antibody responses in early childhood monitored with recombinant allergen components: implications for class switch mechanisms

The formation of IgE antibodies against environmental allergens represents the hallmark of type I allergy. Data from in vitro cultured cells and experimental animal models provide controversial evidence for isotype switching from IgM to IgE production via sequential as well as non-sequential (i.e. direct) class switch. We analyzed the evolution of IgE responses in 11 children developing birch pollen and/or grass pollen allergy during the first 7 years of life using purified recombinant allergen molecules (major birch pollen allergen, Bet v 1; major timothy grass pollen allergens, Phl p 1, Phl p 2, Phl p 5). Demographic, clinical and serological data indicated a postnatal sensitization to pollen allergens. A parallel development of IgG(1-4) and IgE responses to recombinant allergen molecules compatible with a strictly sequential class switch to IgE was observed only in one child. The only partly synchronized and dissociated development of allergen-specific antibody responses found in all other cases can be best explained by a partly sequential class switch involving few switch stations or, more likely, by direct class switching. Kinetics and courses of allergen-specific antibody responses (IgM, IgG(1-4), IgE) during the first years of life suggest that, once established, allergen-specific IgE responses are driven by antigen contact rather than by cytokines controlling class switch to IgE.

IgG subclass switch capacity is low in switched and in IgM-only, but high in IgD+IgM+, post-germinal center (CD27+) human B cells

Recent studies have shown that in humans the germinal center reactions produce three types of V(D)J mutated B cells in similar proportions, i.e. Ig-switched, IgD-IgM+ (IgM-only) and IgD+IgM+ cells, and that together they form the CD27+ compartment of recirculating B cells. We investigated the Ig isotype switch capacity of these cells. Peripheral blood B subsets were sorted and IgG subclass secretion in presence or absence of IL-4 was compared in B cell assays which lead to Ig secretion in all (coculture with EL-4 thymoma cells) or only in CD27+ (CD40L stimulation) B cells. Already switched IgG+ B cells showed no significant sequential switch and IgM-only cells also had a low switch capacity, but IgD+CD27+ switched as much as IgD+CD27- B cells to all IgG subclasses. Thus, in switched B cells some alterations compromising further switch options occur frequently; IgM-only cells may result from aborted switch. However, IgD+CD27+ human B cells, extensively V(D)J mutated and "naive" regarding switch, build up a repertoire of B cells combining (1) novel cross-reactive specificities, (2) increased differentiation capacity (including after T-independent stimulation by Staphylococcus aureus Cowan I) and (3) the capacity to produce appropriate isotypes when they respond to novel pathogens.

Immunoglobulin production by cultured human lymphocytes

The synthesis and secretion of immunoglobulin induced in cultured B-lineage cells is of interest for several reasons: (i) analysing the B-cell repertoire, (ii) recall of immunological activity retained in the circulating lymphocyte population, and (iii) study of factors needed for clonal expansion, immunoglobulin class switching, IgV-region mutation and maturation of cells to Ig secretion. Methods available are outlined and alternative procedures for cell separation and purification, helper cell provision and Ab/Ig assay systems are discussed. The aim is to provide practical guidance for those who intend to begin work in what is a vitally important, but experimentally difficult, area. There are a bewildering number of methods described in innumerable publications, old and new. The review provides a personal assessment of the present state of knowledge and prospects for improvements when all the new observations relating to cell-cell interactions and cytokines are integrated into existing technologies. The survey is chiefly concerned with physiologically based procedures, but artificial auxiliary methods are also briefly mentioned.

Allergenicity of major cow's milk and peanut proteins determined by IgE and IgG immunoblotting

BACKGROUND

Specific IgG antibodies are frequently observed in food-allergic patients. However, the allergen-fraction specificity of IgG antibodies in relation to IgE antibodies is not well defined. Our aim was to determine the IgE and IgG antibody profile to major cow's milk and peanut-antigen fractions in food-allergic patients and tolerant individuals.

METHODS

Sera were collected from 10 patients allergic to cow's milk and 10 patients allergic to peanuts, as well as from 20 control subjects. Cow's milk and peanut proteins were migrated on SDS-PAGE and immunoblotted for IgE, IgG, and IgG4 antibodies. Food-specific IgE concentrations were measured by CAP System FEIA, and IgG and IgG4 concentrations by ELISA.

RESULTS

In food-allergic children, similar fraction-specific IgE, IgG, and IgG4 antibody-binding profiles to the major cow's milk or peanut antigens were found. In nonallergics, the presence of fraction-specific IgG antibodies was mostly dependent on regular ingestion of the food. The presence of specific antibody on immunoblots correlated with their quantitative measurement. The mean value for specific IgE in cow's milk-allergic patients was 450 +/- 1,326 IU/ml, and 337 +/- 423 IU/ml in peanut-allergic patients. Specific IgG antibody values in milk-allergic patients were not different (median OD 1.5, range 0.3-2.3) from controls (median OD 1, range 0.2-1.8). However, in peanut-allergic patients, IgG concentrations were significantly higher than in controls (OD 1.2 [0.5-1.3] vs 0.5 [0.3-0.7]; P< 0.01).

CONCLUSIONS

Similar fraction-specific IgE and IgG antibody profiles in allergic individuals suggest a common switching trigger involving both isotypes. Intrinsic allergenicity might explain identical IgG antibody fraction specificity in nonallergics and in allergics. The presence of IgG antibodies in nonallergics was related to regular ingestion of the food.

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.

Targeting of human switch recombination breakpoints: implications for the mechanism of mu-gamma isotype switching

We recently characterized the allelic variants of the human Sgamma4 region which makes it possible to accurately identity and map Smu-Sgamma4 fragments from in vivo switched B cells. Twenty-six fragments were identified and a comparison was made with all previously published Smu-Sgamma sequences ( n = 82). Switch recombination outside the region flanking the Sgamma repeat sequence is a rare event in vivo and differences previously observed in patterns between in vitro and in vivo switched B cells appear to be artefactual and due to constraints of the methods used. Furthermore, internal deletions in the switch regions are common, but do not appear to be involved in isotype stabilization. A slight preference for switching to the B (SNIP) site was observed, suggesting a limited importance of both the B and A (SNAP) in the switching process. Mutations can be identified on either one or both sides of the switch junction, showing involvement of an error-prone process, and the pattern of mutations/substitutions at and around the junctions shows non-random nucleotide replacements by the enzyme(s) involved which may help in its future identification.

In vitro production of IgG4 by peripheral blood mononuclear cells (PBMC): the contribution of committed B cells

IgG4 and IgE isotypes contribute marginally to the pool of circulating antibodies in healthy individuals, but are elevated during atopic diseases and particularly upon helminth infections. To examine whether the high levels of these isotypes in circulation are reflected in a higher capacity of PBMC to produce IgG4 and IgE, we examined cells from patients infected with filarial nematodes that exhibit high levels of IgG4 and IgE. Indeed, IgG4 production by PBMC correlated strongly with plasma levels of IgG4 (r=0.534, P=0.002), but such correlation was not found for IgE. The replacement of CD19+ cells from PBMC by IgD+ cells abrogated the high capacity of PBMC to make IgG4. This indicates that an altered B cell compartment accounts for the high IgG4-producing capacity of the PBMC. The high production of IgG4 in vitro was not dependent on IL-4 and IL-13, as neutralizing antibodies to these cytokines did not inhibit IgG4. However, IgE release by PBMC was dependent on IL-4 and IL-13. Antifilarial IgG4 was detected in culture supernatants from filarial patients and its production was independent of IL-4 and IL-13. These results demonstrate that in individuals with elevated IgG4. the B cell compartment in PBMC carries cells that are already committed to IgG4 production and are independent of IL-4 and IL-13.

Clonally related IgE and IgG4 transcripts in blood lymphocytes of patients with asthma reveal differing patterns of somatic mutation

Isotype switching to IgE contributes to atopic asthma, therefore strategies to divert this process to alternative isotypes could have therapeutic relevance. It is known that patients with allergic disease have serum IgE and IgG4 antibodies with similar specificities, and that cytokines such as IL-4 mediate switching to both of these isotypes. Availability of variable region gene analysis has allowed us to probe isotype variants at the single-cell level. An earlier report described identification in a single atopic patient of short transcripts with a common complementarity-determining region "clonal signature" in combination with C mu, C gamma4 and C epsilon. We have extended this analysis, and have identified V(H)-Cgamma4 transcripts with clear clonal relationship to IgE-derived sequences in blood lymphocytes from three of four patients with atopic asthma. No other IgG subclasses were detected, confirming the link between IgE and IgG4. Full sequences were obtained from each clonally related isotype in all patients, and showed extensive somatic mutation. As previously found for IgE, the IgG4 isotypes had evident intraclonal variation. There were shared mutations between isotypes, but also many differences, indicative of separate cell populations with divergent mutational histories. These findings indicate that, in atopic patients, an individual B cell commonly switches to either IgE or IgG4. Cells producing each isotype then co-exist in the recirculating pool, and the balance between them may influence the disease process.