Discussion
Although PFA is a widespread allergic disorder, the only available
disease-modifying therapy for pollen allergy has a rather limited effect
on the associated food allergy.25–27 Therefore in
this study, human monocyte-derived IL-10 DC were investigated with
regard to their potential to induce allergen-specific (birch) and
cross-reactive (hazelnut) tolerance in birch pollen allergic patients
with associated hazelnut allergy in vitro and in vivo . We
found that IL-10 DC induce Bet-specific iTreg which show a regulatory
phenotype and strong suppressive capacities to inhibit allergen-specific
and cross-reactive immune responses in vitro . In addition,
Bet-specific iTreg were able to ameliorate allergic symptoms in
vivo in a humanized mouse model of allergic intestinal and airway
inflammation.
PFA results from highly conserved protein structures of pollen (e.g.
Bet) and food allergens (e.g. Cor), which was shown to facilitate
cross-reactions on IgE level and in T cell
clones.12,53,54 In a previous study, we confirmed the
data of cross-reactivity between pollen and food allergens in primary T
cells directly obtained from patients with allergies to birch pollen and
associated food allergens.12 Here, we focused on the
induction of allergen-specific and cross-reactive iTreg to modulate the
primary and secondary allergic immune response in patients suffering
from PFA.
Bet-specifically stimulated iTreg but not non-specific iTreg underwent
vigorous proliferation towards Bet- and Cor-induced restimulation,
suggesting activation as prerequisite for suppressive activity. In this
context, Pellerin et al. investigated peanut-specific Tr1 cells
induced in vitro by IL-10 DC from allergic
subjects.55 They found a highly proliferative
phenotype with TH2-cytokine profile upon peanut-specific
restimulation and, in contrast to our data, suggested a functional
impairment of the peanut-specific Tr1 subset.55 Our
experiments revealed that IL-10 DC-induced Bet-specific iTreg did have
the ability to suppress allergen-specific responder T cell
proliferation, displayed an activated and suppressive phenotype, even
though they were highly proliferative. This discrepancy might be due to
(1) different protocols for IL-10 DC culture and Treg generation, (2)
different allergen-specific immune responses and/or (3) lack of
functional assays in the study by Pellerin et
al .55 Anergy has been initially described as a
fundamental characteristic of functional Treg, but this idea has hence
been revised: although breaking Treg anergy can be accompanied by loss
of suppressive function, this is not always the
case.56 In fact, it was shown that proliferating Treg
can suppress T cell responses in vivo 56,57 and
Treg that have been stimulated to proliferate can even display an
enhanced suppressive capacity.57–59
One very crucial aspect of therapeutic tolerance induction is the
allergen-specificity. We are therefore thrilled to report that
Bet-specific iTreg showed significantly greater abilities to suppress
allergen-specific responder T cell proliferation than non-specific
iTreg, as was seen in in vitro suppressor assays from up to 80%
of allergic donors. These results were strongly supported by the T cell
cytokine profile in suppressor assays. Here, we found that Bet-specific
iTreg significantly decreased levels of the TH2 cytokine
IL-13, which was not achieved with non-specific iTreg. IL-13 is an
IgE-promoting TH2 cytokine, which contributes to airway
inflammation and food-induced anaphylaxis in asthma and type 1
allergies.60,61 In line with these data, amounts of
the immunosuppressive cytokine IL-10 were significantly increased in the
presence of Bet-specifically primed iTreg, which was not the case for
non-specific iTreg. Accordingly, IL-10 is well known for its suppressive
function in regulatory T cell activity, and particularly in control of
TH2-driven allergic diseases.62 As the
described cytokine shift was observed for both, Bet- and Cor-specific
responder T cells after coculture with Bet-stimulated iTreg - but not
with non-specific iTreg0- these data underlined the
induction of an allergen-specific (birch) and cross-reactive tolerance
(hazelnut) through IL-10 DC-induced Bet-specific iTreg priming.
In addition, Bet-specific iTreg were able to ameliorate asthmatic and
intestinal allergic symptoms provoked by challenge with birch extract
and reduced birch-specific IgE in allergic mice in vivo . These
combined pieces of evidence strongly suggest an allergen-specific
tolerance induction in vitro and in vivo by iTreg which
have been stimulated by allergen-loaded tolerogenic IL-10 DC.
Intriguingly, Bet-specific iTreg also reduced the allergic gut
inflammation and hazelnut-specific IgE levels in vivo after
challenge with the hazelnut extract in mice engrafted with PBMC from
birch pollen allergic patients with associated hazelnut allergy,
facilitating cross-reactive tolerance.
Aiming to replace general immunosuppressive therapies, tolerogenic DC
(tolDC) have been applied as antigen-specific immune-suppressors in
numerous phase 1 clinical trials for multiple sclerosis, type I
diabetes, rheumatoid arthritis and organ
transplantation.32,34,63–65 In all studies, tolDC had
negligible adverse effects and did not worsen disease symptoms. Clinical
outcomes were only investigated in a few trials so far but preliminary
evidence for antigen-specific tolerance induction was found.
In a comparative study by Boks et al. IL-10 modulation for human
tolDC generation was identified as the protocol most suited for tolDC
vaccination.66 We developed a protocol for IL-10 DC
that resulted in a subpopulation of tolerogenic
CD83highCCR7+ IL-10 DC that exhibit
a high migratory activity, stability to pro-inflammatory stimuli and
profound capacity to induce iTreg with a strong suppressive
function.35 In our current study, we did show that
human IL-10 DC through priming of allergen-stimulated iTreg are able to
induce specific- and cross-reactive tolerance in vitro andin vivo and, therefore are promising candidates to modulate
pollen as well as associated food allergies.
Combined with previous findings by us and other
groups,35,51,66–68 our study results might further
support the development of DC-based tolerance-inducing therapies for
allergic and autoimmune diseases.