Normal human immunoglobulin G4 is bispecific: it has two different antigen-combining sites

The blocking activity of birch pollen-specific immunotherapy-induced IgG4 is not qualitatively superior to that of other IgG subclasses

Allergen-specific IgG antibodies induced by specific immunotherapy (SIT) interfere with the allergen-IgE interaction, and act as blocking antibodies in vitro. It has been hypothesised that IgG4, as opposed to other IgG subclasses, is particularly important in this function, which may play a role for the clinical efficacy of SIT. In this study, fractionated serum samples from 14 SIT-treated birch pollen allergic individuals enabled determination of the inhibitory capacity of IgG4 alone versus non-IgG4 IgG. Allergen-binding activities of IgG and the IgG-mediated inhibition of allergen binding to autologous IgE were detected using 125I-labelled rBet v 1.2801, a recombinant variant of the major allergen of Betula verrucosa pollen. Results show that IgG4-depletion resulted in equivalent reductions in binding and blocking activities. In contrast, a significant but less than two-fold higher relative blocking activity was found in the purified IgG4 fraction. There was no significant difference in the binding avidities (1/K(d)) measured in the two IgG fractions. Thus, it appears that SIT-induced specific IgG4 contributes to the IgG blocking of allergen binding to IgE in a simple quantitative manner and not by a particular intrinsic blocking activity.

Immunogenicity and efficacy of Cryptococcus neoformans capsular polysaccharide glucuronoxylomannan peptide mimotope-protein conjugates in human immunoglobulin transgenic mice

Peptide mimotopes of capsular polysaccharides have been proposed as antigens for vaccines against encapsulated pathogens. In this study, we determined the antibody response to and efficacy of P13, a peptide mimetic of the Cryptococcus neoformans capsular polysaccharide glucuronoxylomannan (GXM), in mice that produce human antibodies. P13 was conjugated to tetanus toxoid (TT) or diphtheria toxoid (DT) and administered subcutaneously in Alhydrogel with or without CpG to mice transgenic for human immunoglobulin loci (XenoMouse mice) and expressing either immunoglobulin G2 (IgG2) (G2 mice) or IgG4 (G4 mice). Mice were vaccinated and revaccinated two or three times. The serum antibody responses of the mice to GXM and P13 and antibody idiotype expression were analyzed by an enzyme-linked immunosorbent assay. The results showed that both P13-TT and P13-DT were antigenic, inducing a mimetic response to P13 in both G2 and G4 mice, and immunogenic, inducing a mimotope response including VH3 (idiotype)-positive antibodies to GXM in G2 but not G4 mice. CpG led to higher titers of IgG to P13 and GXM in P13-TT-vaccinated G2 mice. C. neoformans challenge of P13-protein conjugate-vaccinated and control G2 mice induced anamnestic IgG- and VH3-positive responses to GXM and was associated with a significantly decreased risk of death and a prolongation of survival in P13-DT-vaccinated mice compared to phosphate-buffered saline-treated or protein carrier-vaccinated mice. These findings reveal that P13 elicited a human antibody response with VH3 expression in human immunoglobulin transgenic mice that has been observed for human antibodies to GXM and support the concept that peptide mimotope-based vaccines may hold promise for the treatment of C. neoformans infections.

IgG4 breaking the rules

Immunoglobulin G4 (IgG4) antibodies have been known for some time to be functionally monovalent. Recently, the structural basis for this monovalency has been elucidated: the in vivo exchange of IgG half-molecules (one H-plus one L-chain) among IgG4. This process results in bispecific antibodies that in most situations will behave as functionally monovalent antibodies. The structural basis for the abnormal behaviour of IgG4 seems to be largely the result of a single amino acid change relative to human IgG1: the change of a proline in core hinge of IgG1 to serine. This results in a marked shift in the equilibrium between interchain disulphide bridges and intrachain disulphide bridges, which for IgG4 results in 25-75% absence of a covalent interaction between the H-chains. Because of strong non-covalent interactions between the CH3 domains (and possibly also between the CH1 domain and the trans-CH2 domain) IgG4 is a stable four-chain molecule and does not easily exchange half-molecules under standard physiological conditions in vitro. We postulate that the exchange is catalysed in vivo by protein disulphide isomerase (PDI) and/or FcRn (the major histocompatibility complex (MHC)-related Fc receptor) during transit of IgG4 in the endosomal pathway in endothelial cells. Because IgG4 is predominantly expressed under conditions of chronic antigen exposure, the biological relevance of this exchange of half-molecules is that it generates antibodies that are unable to form large immune complexes and therefore have a low potential for inducing immune inflammation. In contrast to monovalent immunoglobulin fragments, these scrambled immunoglobulins have a normal half-life. The significance of the ensuing bispecificity needs further evaluation, because this will be relevant only in situations where high IgG4 responses are found to two unrelated antigens that happen to be present in the body at the same time and place. In this context the significance of IgG4 autoreactivity might have to be re-evaluated. The main function of IgG4, however, is presumably to interfere with immune inflammation induced by complement-fixing antibodies, or, in the case of helminth infection or allergy, by IgE antibodies.

Sensitisation, asthma, and a modified Th2 response in children exposed to cat allergen: a population-based cross-sectional study

BACKGROUND

Although asthma is strongly associated with immediate hypersensitivity to indoor allergens, several studies have suggested that a cat in the house can decrease the risk of asthma. We investigated the immune response to cat and mite allergens, and asthma among children with a wide range of allergen exposure.

METHODS

We did a population-based cross-sectional study of children (aged 12-14 years), some of whom had symptoms of asthma and bronchial hyper-reactivity. Antibodies to mite (Der f 1) and cat (Fel d 1) allergens measured by isotype (IgG and IgG4) specific radioimmunoprecipitation assays were compared with sensitisation and allergen concentrations in house dust.

FINDINGS

226 children were recruited, 47 of whom had symptoms of asthma and bronchial hyper-reactivity. Increasing exposure to mite was associated with increased prevalence of sensitisation and IgG antibody to Der f 1. By contrast, the highest exposure to cat was associated with decreased sensitisation, but a higher prevalence of IgG antibody to Fel d 1. Thus, among children with high exposure, the odds of sensitisation to mite rather than cat was 4.0 (99% CI 1.49-10.00). Furthermore, 31 of 76 children with 23 microg Fel d 1 at home, who were not sensitised to cat allergen had >125 units of IgG antibody to Fel d 1. Antibodies to Fel d 1 of the IgG4 isotype were strongly correlated with IgG antibody in both allergic and non-allergic children (r=0.84 and r=0.66, respectively). Sensitisation to mite or cat allergens was the strongest independent risk factor for asthma (p<0.001).

INTERPRETATION

Exposure to cat allergen can produce an IgG and IgG4 antibody response without sensitisation or risk of asthma. This modified T-helper-2 cell response should be regarded as a form of tolerance and may be the correct objective of immunotherapy. The results may also explain the observation that animals in the house can decrease the risk of asthma.

The inter-heavy chain disulfide bonds of IgG4 are in equilibrium with intra-chain disulfide bonds

Unlike other immunoglobulin G (IgG) subclasses, IgG4 antibodies in plasma have been reported to be functionally monovalent. In a previous paper, we showed that the apparent monovalency of circulating IgG4 antibodies is caused by asymmetry of plasma IgG4-a large fraction has two antigen-binding sites resulting in bispecificity. We postulated that the generation of bispecific antibodies was caused by a post-secretion mechanism, involving the exchange of IgG4 half-molecules (i.e. one heavy and one light chain). This hypothesis was based on the observed instability of the inter-heavy chain disulfide bonds of IgG4. To investigate this instability, we constructed IgG4 mutants and analyzed the covalent interaction between the heavy chains by sodium dodecyl sulfate-poly acrylamide gel electrophoresis (SDS-PAGE) under non-reducing conditions. The mutation to serine of one of the hinge cysteines involved in the inter-heavy chain bond formation, Cys226, resulted in a more stable rather than a more labile inter-heavy chain linkage. Moreover, we confirmed that mutating the IgG4 hinge sequence Cys-Pro-Ser-Cys to the IgG1 hinge sequence Cys-Pro-Pro-Cys also markedly stabilizes the covalent interaction between the heavy-chains. These two observations suggested an explanation for the observed instability of the inter-heavy chain disulfide bonds: the formation of an alternative, intra-chain cystine. Obviously, this intra-chain cystine cannot be formed in the mutant where Cys226 is replaced by Ser, and cannot easily be formed in the mutant with the IgG1 hinge sequence (Cys-Pro-Pro-Cys) due to the restricted torsional freedom of prolines. We, therefore, postulate that the lack of a covalent heavy-chain interaction in a subpopulation of IgG4 reflects an equilibrium between inter- and intra-chain cystines. Based upon the published structure of the IgG4-related hinge-deleted IgG1 myeloma protein Mcg, we propose a model for the two forms of IgG4 and for the half-molecule exchange reaction, which might result in the formation of bispecific IgG4 antibodies.