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.