5.1 The role of IgG and IgA in preventing sensitisation in early life
Maternal IgA and IgG antibodies from breast milk or transferred over the placenta during pregnancy, play an important role in the development of allergy in the offspring (summarized in Figure 3). During the third trimester of pregnancy, IgG immunoglobulins are transferred from the placenta into the serum of the fetus using the non-classical neonatal Fc Receptor (FcRN). These IgG antibodies are thought to be important for providing protection to infants from infectious disease22,72. Maternal IgG to airborne allergens (i.e. House dust mite, Birch pollen, cat) and food allergens (egg, cow milk) were also found to be transferred in utero in birth cohorts73,74. High levels of cord blood IgG antibodies to cat and birch, but not to food allergens, were associated with less atopic symptoms in the children during the first eight years of life21,73 Maternal allergen immunotherapy has also resulted in the induction of allergen-specific IgG in the serum of the offspring, further confirming they are passively transferred across the placenta into the fetus75,76. However, a review of five studies of allergen-specific immunotherapy during pregnancy did not show any clear evidence of allergy reduction in the offspring77 .
In addition to IgG, also maternal IgE can be transported over the placenta via FcRN, resulting in IgE binding to already competent mast cells in the fetus.78
Several studies have reported that following birth, mothers continue to transfer IgG in addition to secretory IgA to their offspring through breast milk22,79. Antibodies to both airborne and food allergens have been detected in human milk74,80,81. Maternal allergen-specific IgG can be detected in children’s serum up to 6 months of age, and the specificity to the allergen in plasma, breast milk and cord blood is quite similar21. It is noteworthy that infants of mothers with high concentrations of allergen-specific IgG in serum and breastmilk did not show sensitisation to the allergen at five years. More importantly, sensitised children had mothers with low concentrations of allergen-specific IgG21.
For four decades, rodent experiments have explored the impact of in utero and of milk transfer of IgG to offspring on allergy sensitisation and their mechanisms of action82. Neutralisation of the allergen and allergen-specific modulation of B and T cell regulatory properties of maternal IgG antibodies have been described82. In addition to possible immune regulation induced by the sole presence of maternal IgG, maternally derived immune complexes made of allergen bound to IgG may also be critical for regulation of long-term allergy susceptibility. Allergen-IgG immune complexes have been detected in cord blood83,84 and human milk81,84. There is strong evidence from rodent experiments that allergen-IgG immune complexes in breast milk are very potent in eliciting an immune response in offspring. Oral exposure to OVA-IgG immune complexes through breast milk resulted in the induction of OVA-specific Forkhead box protein P3 (FOXP3) regulatory T cells (Tregs) responsible for prolonged tolerance to OVA in offspring subsequently leading to respiratory and food allergy prevention20,81 . This appeared to result from a protected transport of OVA across the gut barrier and an enhanced presentation by dendritic cells, both depending on the use of the neonatal Fc Receptor (FcRn).
A recent report analysed the influence of maternal immune status on the induction of protection against cow milk allergic sensitisation upon β−lactoglobulin (β−LG) transfer through breast milk. Using two different protocols for maternal immunisation, the study showed that the levels of antibodies in breast milk positively correlated with the inhibition of allergic sensitisation in offspring and no protection was induced by the antigen transfer only85. Similarly, maternal exposure to peanut during breastfeeding inhibited allergic response to peanut in offspring only when mothers had been immunised but not if naïve to peanut86,87. However, allergen transfer to offspring in the presence of maternal antibodies does not systematically result in tolerance induction, as shown for House dust mite (HDM) allergen88. A study in mice showed that mice nursed by HDM-exposed mothers exposed developed a gut immunity imbalance associated with the expansion of Th2 cells and a refractory state to oral tolerance. Importantly, when neutralising HDM protease activity, this deleterious effect on gut immune ontogeny in offspring was abolished89. This observation highlights the importance of the biological properties of the allergen itself, as in the case of HDM, the proteolytic activity of the allergens was responsible for immune priming89.
In addition to human breast milk, allergen-specific IgG (bIgG) has been detected in cow’s milk90. It is not clear if allergen-specific IgG is complexed to allergens in the milk. However, after oral ingestion, the IgG can theoretically bind to allergens that are swallowed, and thereby play a role in tolerisation to the allergen, as has also been noted in epidemiological studies on the consumption of raw milk and allergies91-93.
In addition to allergen-specific IgG, there is some evidence that allergen-specific IgA in breast milk is associated with protection as shown for infants’ cow’s milk allergy94-97. The total levels of IgA in breast milk are inversely associated with AD in early life98. A recent a study reported that in mice maternal milk IgA might play an important role in establishing a gut regulatory T cell setpoint in offspring gut and thereby tuning gut immune responses and inflammatory disease susceptibility99.
5.2 Induction and function of allergen-specific IgG and IgA by allergen immunotherapy
Allergen immunotherapy involves the repeated administration of allergens or allergen products to IgE-sensitised allergic individuals to induce long-term tolerance on subsequent exposure to the offending allergen(s)100It is indicated in patients with symptoms on exposure to relevant allergens and failure to respond to regular use of anti-allergic drugs. AIT has been shown to be effective for allergic rhinoconjunctivitis, allergic asthma and anaphylaxis due to venom of stinging insects. AIT traditionally involves subcutaneous injections of allergen extracts weekly then monthly for 3 years. Daily administration via the sublingual route has been shown to be an effective and safer alternative 101. Strategies to improve efficacy, reduce side effects and enable shorter more convenient immunotherapy protocols are desirable102. These have included alternative routes (eg epicutaneous, intralymphatic) use of short T cell peptides, medium chain length hydrolysed or synthetic peptides, combination products of allergen with Toll-like receptor agonists or biologics and recombinant major allergen mixtures or hypoallergenic variants. So far these strategies have failed to deliver outcomes over and above currently available products103.
Allergen immunotherapy has been shown to be accompanied by increases in allergen-specific antibodies. Cooke originally identified passive transfer of a serum factor that provided protective immunity to ragweed following successful ragweed immunotherapy25 This was subsequently shown to reside within the immunoglobulin IgG fraction, long before IgE was discovered.
For pollen AIT, an initial transient rise in specific IgE is followed by blunting of seasonal IgE increases and a gradual decline over several years104. Both SCIT and SLIT result in increases in allergen-specific IgG1/IgG4, and specific IgA1/IgA2105. These antibodies increase at 2-6 months and are detectable both in blood and in local target organ secretions, for example in nasal fluid 106. Whereas SCIT induces largely IgG responses, a recent head-to-head trial showed that SLIT induces preferential allergen-specific27 IgA1/2.
A major advance has been the availability of recombinant major and minor allergenic components that enable an accurate molecular diagnosis. There is a strong case that measurements of allergen-specific antibodies to standardised whole extracts could be supplemented by molecular diagnosis using individual allergen molecules to discriminate between antibodies binding to allergens and non-allergenic extract components107,108. Whether standardised allergen extracts will be replaced or supplemented by tailor-made recombinant mixtures/ hypoallergenic variants based on individual molecular profiles remains to be tested.
IgG4 and other human IgG subclasses are similar in structure but have differences in binding to accessory molecules and receptors, altering their functionality. IgG4, in particular, induced following chronic antigen responses co-exist as two isomers diverging in their disulfide bonds of hinge cysteines. There is clear evidence that in vivo , half-molecules of IgG4 can recombine randomly with other half-molecules of IgG4, resulting in monovalent-bispecific antibodies109,110. As a consequence, IgG4 is unable to efficiently cross-link target allergen and form immune complexes. It is unable to bind with both Fab arms to a multivalent antigen, leading to a lower avidity. IgG4 has low affinity for activating FcγR whilst retaining high affinity for the FcγRIIb. These characteristics enable IgG4 to be an efficient inhibitor of IgE-dependent reactions without untoward inflammation associated with igG immune complex formation and complement activation.
Allergen-specific IgA2 and polymeric IgA2 has also been shown to be elevated following grass pollen SCIT. Polymeric IgA2 was purified from post-immunotherapy serum and used to passively sensitize autologous monocytes. Subsequent cross-linking in vitro of IgA on monocytes by antigen or anti-IgA resulted in IL-10 production, supporting an alternative role for IgA antibodies in inducing tolerance following AIT111.
Immunoreactive IgG and IgA antibodies are elevated after AIT but have correlated poorly with the clinical response to treatment. This may be explained in part by responses to non-allergenic proteins or to irrelevant minor or cross-reactive allergens and this can be addressed by measuring major allergen components105,112. However, at least as relevant, immunoreactive antibodies relate largely to allergen exposure during AIT and may have no bearing on the affinity and/or avidity of these antibodies in blocking the formation of allergen-IgE complexes and hence blocking IgE responses. This highlights the importance of using functional antibody assays to supplement immunoreactive IgG and IgA assays.
Allergen-specific IgG4 (and likely other antibody isotypes) compete with IgE for allergen and prevents the formation of allergen-IgE complexes from binding to FcεRI on effector cells (mast cells, basophils and dendritic cells) and to FcεRII (CD23) on B cells (Figure 2). van Neerven originally demonstrated that serum obtained after birch pollen immunotherapy inhibited IgE-facilitated allergen presentation by B cells to an allergen-specific T cell clone, with decreased specific clonal T cell proliferation and cytokine production113. This was confirmed by further studies of birch immunotherapy114,115. confirmed increases in serum IgG-associated blocking activity for IgE-FAB in grass pollen immunotherapy116. That persisted for years after discontinuation along with clinical benefit (and by affinity chromatography showed that the inhibitory factor resided largely but not exclusively within the IgG4 fraction117. Recent data supports a putative role for allergen-specific IgG2118as a blocking factor for IgE-mediated reactions. Shamji validated the IgE-FAB assay and showed that serum IgE-FAB increased in a time- and dose-dependent fashion after grass pollen AIT119,120and correlated more closely with clinical response than accompanying elevated IgG4 levels This raised the possibility for IgE-FAB inhibition to predict individual responses to AIT105. Such blocking antibodies could also prevent captured allergen from stimulating IgE-producing cells thereby reducing boosts of IgE production caused by allergen exposure70,121,122.
The functional role of serum blocking antibodies after AIT has also been illustrated by inhibition of IgE-mediated basophil activation (Figure 2). After grass pollen AIT, post-immunotherapy serum inhibited basophil histamine release with a time-course that paralleled inhibition of IgE-FAB and correlated with inhibition of the immediate skin response to grass pollen at 8-16 weeks123. This was also shown using Bet v 1-specific IgG1 and IgG4 antibodies after birch pollen AIT124 In a murine model this inhibitory effect of IgG was mediated via the FcγRIIB receptor. However, an125tibodies directed against FcγRII did not prevent serum IgG-mediated inhibition of basophil activation following birch AIT, implying that direct competition with IgE for allergen rather than activation of FcγRII-mediated inhibition of downstream IgE receptor signalling pathways was responsible125.
During grass pollen AIT inhibition of basophil activation has been shown by suppression of surface CD63 expression and by increases in intracellular Diamine Oxidase as detected by whole blood flow cytometry126. Suppression of basophil activation has also been shown for birch pollen immunotherapy (REF), as well as following venom immunotherapy127.
The therapeutic potential of blocking antibodies following AIT is highlighted by a recent study of passive immunotherapy in cat allergic individuals who received a single dose of two synthetic anti-Fel d 1 specific IgG4 antibodies that resulted in inhibition of the nasal response to a standardized cat whole allergen extract that persisted for twelve weeks128,129.