CTLA-4 signaling regulates the intensity of hypersensitivity responses to food antigens, but is not decisive in the induction of sensitization

Alteration of Immunoregulatory Patterns and Survival Advantage of Key Cell Types in Food Allergic Children

All allergic responses to food indicate the failure of immunological tolerance, but it is unclear why cow's milk and egg (CME) allergies resolve more readily than reactivity to peanuts (PN). We sought to identify differences between PN and CME allergies through constitutive immune status and responses to cognate and non-cognate food antigens. Children with confirmed allergy to CME (n = 6) and PN (n = 18) and non-allergic (NA) (n = 8) controls were studied. Constitutive secretion of cytokines was tested in plasma and unstimulated mononuclear cell (PBMNC) cultures. Blood dendritic cell (DC) subsets were analyzed alongside changes in phenotypes and soluble molecules in allergen-stimulated MNC cultures with or without cytokine neutralization. We observed that in allergic children, constitutively high plasma levels IL-1β, IL-2, IL-4, IL-5 and IL-10 but less IL-12p70 than in non-allergic children was accompanied by the spontaneous secretion of sCD23, IL-1β, IL-2, IL-4, IL-5, IL-10, IL-12p70, IFN-γ and TNF-α in MNC cultures. Furthermore, blood DC subset counts differed in food allergy. Antigen-presenting cell phenotypic abnormalities were accompanied by higher B and T cell percentages with more Bcl-2 within CD69+ subsets. Cells were generally refractory to antigenic stimulation in vitro, but IL-4 neutralization led to CD152 downregulation by CD4+ T cells from PN allergic children responding to PN allergens. Canonical discriminant analyses segregated non-allergic and allergic children by their cytokine secretion patterns, revealing differences and areas of overlap between PN and CME allergies. Despite an absence of recent allergen exposure, indication of in vivo activation, in vitro responses independent of challenging antigen and the presence of unusual costimulatory molecules suggest dysregulated immunity in food allergy. Most importantly, higher Bcl-2 content within key effector cells implies survival advantage with the potential to mount abnormal responses that may give rise to the manifestations of allergy. Here, we put forward the hypothesis that the lack of apoptosis of key immune cell types might be central to the development of food allergic reactions.

Targeting type 2 immunity and the future of food allergy treatment

IgE-mediated food allergy affects 6-8% of the population in the United States. Type 2 immune responses are central to the pathogenesis of food allergy, but type 2 CD4+ T cell responses have been found to be heterogeneous in food allergy suggesting a division of labor between Tfh13 and peTH2 cells in promotion of IgE class switching, modulation of intestinal barrier function, and regulation of mast cell expansion. Oral immunotherapy for the treatment of food allergy incompletely targets subsets of type 2 immunity in a transient manner, but new therapeutics targeting different levels of type 2 immunity are in current or planned trials for food allergy. These new treatments and the basis for their use are the focus of this review.

Oral tolerance and oral immunotherapy for food allergy: Evidence for common mechanisms?

Food allergies affect up to 10% of the US population, can be life-threatening, and have a significant negative impact on quality of life. Delayed dietary introduction of foods in childhood can hinder the induction of oral tolerance, an active regulatory response to foods that prevents the development of food allergy. Some children outgrow their food allergies naturally, while many others have persistent, lifelong food allergy for which there are few therapeutic options. Oral immunotherapy (OIT) is a therapeutic approach of giving increasing amounts of food to attempt to desensitize the allergic individual. In this review, we focus on the immune mechanisms common to oral tolerance and response to oral immunotherapy, with the objective of determining whether true tolerance can be achieved after food allergy has been established.

A Mouse Model of the “LEAP” Study Reveals a Role for CTLA-4 in Preventing Peanut Allergy Induced by Environmental Peanut Exposure

BACKGROUND

A human study, Learning Early About Peanut Allergy (LEAP), showed that early introduction of peanut products decreases the prevalence of peanut allergy among children. However, the immunologic mechanisms mediating the protective effects of consuming peanut products are not well understood.

OBJECTIVE

The objective was to develop a mouse model that simulates the LEAP study and investigate the underlying mechanisms for the study observations.

METHODS

Adult naive BALB/c mice were fed a commercial peanut butter product (Skippy) or buffer control and concomitantly exposed to peanut flour through the airway or skin to mimic environmental exposure. The animals were analyzed for anaphylactic reaction and by molecular and immunologic approaches.

RESULTS

After exposure to peanut flour through the airway or skin, naive mice developed peanut allergy, as demonstrated by acute and systemic anaphylaxis in response to challenge with peanut extract. Ingestion of Skippy, however, nearly abolished the increase in peanut-specific IgE and IgG and protected animals from developing anaphylaxis. Skippy-fed mice showed reduced numbers of T follicular helper (Tfh) cells and germinal center B cells in their draining lymph nodes, and single-cell RNA sequencing revealed a CD4⁺ T-cell population expressing cytotoxic T lymphocyte-associated protein 4 (CTLA-4) in these animals. Critically, blocking CTLA-4 with antibody increased levels of peanut-specific antibodies and reversed the protective effects of Skippy.

CONCLUSION

Ingestion of a peanut product protects mice from peanut allergy induced by environmental exposure to peanuts, and the CTLA-4 pathway, which regulates Tfh cell responses, likely plays a pivotal role in this protection.

The bile acid-activated retinoic acid response in dendritic cells is involved in food allergen sensitization

BACKGROUND

Alteration of commensal microbiota is highly correlated with the prevalence of allergic reactions to food in the gastrointestinal tract. The mechanisms by which microbiota modulate food allergen sensitization in the mucosal site are not fully understood.

METHODS

We generate DCs specific knockout of retinoic acid receptor α (Rara) gene mice (DC KO Rara) to evaluate food sensitization. The bile acid-activated retinoic acid response was evaluated by flow cytometry, real-time RT-PCR and Illumina transcriptome sequencing. The global effect of Abx treatment on BA profiles in the mucosal lymph tissue mLN in mice was examined by UPLC-MS analysis.

RESULTS

In this study, we demonstrate that depletion of commensal gut bacteria leads to enhanced retinoic acid (RA) signaling in mucosal dendritic cells (DCs). RA signaling in DCs is required for the production of food allergen-specific IgE and IgG1. Antibiotics induced an enlarged bile acid (BA) pool, and dysregulated BA profiles contributed to enhanced RA signaling in mucosal DCs. BA-activated RA signaling promoted DC upregulation of interferon I signature, RA signature, OX40L, and PDL2, which may lead to T helper 2 differentiation of CD4+ T cells. BA-activated RA signaling involved the farnesoid X receptor and RA receptor α (RARa) interaction. Depletion of bile acid reduces food allergen specific IgE and IgG1 levels in mice.

CONCLUSION

Our research unveils a mechanism of food sensitization modulated by BA-RA signaling in DCs, which suggests a potential new approach for the intervention of food allergic reactions.

Mechanisms of Wheat Allergenicity in Mice: Comparison of Adjuvant-Free vs. Alum-Adjuvant Models

Wheat protein is considered a major type of food allergen in many countries including the USA. The mechanisms of allergenicity of wheat proteins are not well understood at present. Both adjuvant-based and adjuvant-free mouse models are reported for this food allergy. However, it is unclear whether the mechanisms underlying wheat allergenicity in these two types of models are similar or different. Therefore, we compared the molecular mechanisms in a novel adjuvant-free (AF) model vs. a conventional alum-adjuvant (AA) model of wheat allergy using salt-soluble wheat protein (SSWP). In the AF model, Balb/cJ mice were sensitized with SSWP via skin exposure. In the AA model, mice were sensitized by an intraperitoneal injection of SSWP with alum. In both models, allergic reactions were elicited using an identical protocol. Robust IgE as well as mucosal mast cell protein-1 responses were elicited similarly in both models. However, an analysis of the spleen immune markers identified strikingly different molecular activation patterns in these two models. Furthermore, a number of immune markers associated with intrinsic allergenicity were also identified in both models. Since the AF model uses skin exposure without an adjuvant, the mechanisms in the AF model may more closely simulate the human wheat allergenicity mechanisms from skin exposure in occupational settings such as in the baking industry.

Non-digestible oligosaccharides scFOS/lcFOS facilitate safe subcutaneous immunotherapy for peanut allergy

BACKGROUND

Improving the safety of subcutaneous immunotherapy (SCIT) for food allergy is necessary to reduce side effects and achieve long-term tolerance. We determined the effect of dietary supplementation with 1% non-digestible short- and long-chain fructo-oligosaccharides (scFOS/lcFOS) on safety and efficacy of SCIT using a peanut allergy mouse model.

METHODS

After sensitization, mice received a scFOS/lcFOS or control diet for the rest of the study. To study safety of SCIT, mice were dosed with a single subcutaneous injection of peanut extract (PE) or PBS. To study efficacy, mice were dosed subcutaneously (SCIT, 3 times/week) with PE or PBS for 3 weeks. Hereafter, acute allergic skin responses, anaphylactic shock symptoms and body temperature were assessed. To study the mechanism in vitro, the human IgE receptor (FcεRI)-transfected rat mast cell (RBL) line was sensitized with an oligoclonal pool of chimeric human (chu)IgE antibodies against bovine β-lactoglobulin (BLG) and incubated with the oligosaccharides before exposure to BLG to assess direct the effect on degranulation.

RESULTS

scFOS/lcFOS reduced anaphylaxis caused by a single PE SCIT dose. scFOS/lcFOS alone also reduced the acute allergic skin response. Moreover, scFOS/lcFOS supplementation resulted in lower MMCP-1 levels in serum after PE SCIT dose compared to control diet, while antibody levels were not affected by the diet. In vitro incubation with scFOS/lcFOS at 0.5% suppressed the degranulation of IgE-sensitized RBL cells. However, dietary supplementation with scFOS/lcFOS did not improve the efficacy of SCIT.

CONCLUSIONS

We show that scFOS/lcFOS diet improves the safety of SCIT, as evidenced by lower anaphylactic responses without compromising the efficacy in a mouse model for peanut allergy. This effect is likely to result from the suppression of mast cell effector function.

Mouse strain differences in response to oral immunotherapy for peanut allergy

Background

Promising therapies for food allergy are emerging, mostly based on animal experimentation. However, different mouse strains are used, which may make it hard to compare experiments. The current study investigated whether the immunological differences between C3H/HeOuJ (C3H) and BALB/c mice lead to differences in efficacy of peanut‐specific immunotherapy.

Methods

After sensitization using peanut extract (PE), C3H and BALB/c mice received oral immunotherapy (OIT) by intragastric dosing for three weeks. Hereafter, mice were exposed to PE via the intradermal, intragastric and intraperitoneal route, to determine allergic outcomes. Furthermore, PE‐specific antibody and cytokine production were determined and the number of various immune cells at different time points during the study were measured.

Results

OIT protected C3H mice against anaphylaxis, whereas no anaphylaxis was seen in BALB/c mice. In contrast, OIT induced an increase in MMCP‐1 levels in BALB/c mice but not in C3H mice. No effect of OIT on the acute allergic skin response was observed in either strain. Specific antibody responses showed similar patterns in both strains for IgA and IgG1. IgE levels were a tenfold higher in BALB/c mice and after the intragastric challenge (day 70) OIT‐treated BALB/c mice showed induced IgE levels. Moreover, in C3H mice IgG2a levels were higher and increased in response to OIT and challenges. After the final challenge, but not at other timepoints MLN‐derived lymphocytes from OIT‐treated BALB/c mice produced less IL‐13 and IL‐5 compared to control‐treated mice, whereas no differences were seen in case of C3H mice.

Conclusions

Taken together, these results show that the C3H strain is more suitable to study clinical outcomes of OIT, whereas the BALB/c strain is more optimal to study T cell responses.

Dietary Supplementation with Nondigestible Oligosaccharides Reduces Allergic Symptoms and Supports Low Dose Oral Immunotherapy in a Peanut Allergy Mouse Model

SCOPE

A major downside of oral immunotherapy (OIT) for food allergy is the risk of severe side effects. Non-digestible short- and long-chain fructo-oligosaccharides (scFOS/lcFOS) reduce allergy development in murine models. Therefore, it is hypothesized that scFOS/lcFOS can also support the efficacy of OIT in a peanut allergy model.

METHODS AND RESULTS

After sensitization to peanut extract (PE) using cholera toxin, C3H/HeOuJ mice are fed a 1% scFOS/lcFOS or control diet and receive OIT (1.5 or 15 mg PE). Hereafter, mice are exposed to PE via different routes to determine the safety and efficacy of treatment in clinical outcomes, PE-specific antibody production, and numbers of various immune cells. scFOS/lcFOS increases short-chain fatty acid levels in the caecum and reduce the acute allergic skin response and drop in body temperature after PE exposure. Interestingly, 15 mg and 1.5 mg OIT with scFOS/lcFOS induce protection against anaphylaxis, whereas 1.5 mg OIT alone does not. OIT, with or without scFOS/lcFOS, induces PE-specific immunoglobulin (Ig) IgG and IgA levels and increases CD103+ dendritic cells in the mesenteric lymph nodes.

CONCLUSIONS

scFOS/lcFOS and scFOS/lcFOS combined with low dose OIT are able to protect against a peanut-allergic anaphylactic response.

Hepatic Mitochondrial Dysfunction and Immune Response in a Murine Model of Peanut Allergy

Background: Evidence suggests a relevant role for liver and mitochondrial dysfunction in allergic disease. However, the role of hepatic mitochondrial function in food allergy is largely unknown. We aimed to investigate hepatic mitochondrial dysfunction in a murine model of peanut allergy. Methods: Three-week-old C3H/HeOuJ mice were sensitized by the oral route with peanut-extract (PNT). We investigated: 1. the occurrence of effective sensitization to PNT by analysing acute allergic skin response, anaphylactic symptoms score, body temperature, serum mucosal mast cell protease-1 (mMCP-1) and anti-PNT immunoglobulin E (IgE) levels; 2. hepatic involvement by analysing interleukin (IL)-4, IL-5, IL-13, IL-10 and IFN-γ mRNA expression; 3. hepatic mitochondrial oxidation rates and efficiency by polarography, and hydrogen peroxide (H₂O₂) yield, aconitase and superoxide dysmutase activities by spectrophotometry. Results: Sensitization to PNT was demonstrated by acute allergic skin response, anaphylactic symptoms score, body temperature decrease, serum mMCP-1 and anti-peanut IgE levels. Liver involvement was demonstrated by a significant increase of hepatic Th2 cytokines (IL-4, IL-5 and IL-13) mRNA expression. Mitochondrial dysfunction was demonstrated by lower state 3 respiration rate in the presence of succinate, decreased fatty acid oxidation in the presence of palmitoyl-carnitine, increased yield of ROS proven by the inactivation of aconitase enzyme and higher H₂O₂ mitochondrial release. Conclusions: We provide evidence of hepatic mitochondrial dysfunction in a murine model of peanut allergy. These data could open the way to the identification of new mitochondrial targets for innovative preventive and therapeutic strategies against food allergy.

Indole-3-carbinol, a plant nutrient and AhR-Ligand precursor, supports oral tolerance against OVA and improves peanut allergy symptoms in mice

In general, dietary antigens are tolerated by the gut associated immune system. Impairment of this so-called oral tolerance is a serious health risk. We have previously shown that activation of the ligand-dependent transcription factor aryl hydrocarbon receptor (AhR) by the environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) affects both oral tolerance and food allergy. In this study, we determine whether a common plant-derived, dietary AhR-ligand modulates oral tolerance as well. We therefore fed mice with indole-3-carbinole (I3C), an AhR ligand that is abundant in cruciferous plants. We show that several I3C metabolites were detectable in the serum after feeding, including the high-affinity ligand 3,3´-diindolylmethane (DIM). I3C feeding robustly induced the AhR-target gene CYP4501A1 in the intestine; I3C feeding also induced the aldh1 gene, whose product catalyzes the formation of retinoic acid (RA), an inducer of regulatory T cells. We then measured parameters indicating oral tolerance and severity of peanut-induced food allergy. In contrast to the tolerance-breaking effect of TCDD, feeding mice with chow containing 2 g/kg I3C lowered the serum anti-ovalbumin IgG1 response in an experimental oral tolerance protocol. Moreover, I3C feeding attenuated symptoms of peanut allergy. In conclusion, the dietary compound I3C can positively influence a vital immune function of the gut.

Epicutaneous immunotherapy for food allergy as a novel pathway for oral tolerance induction

Epicutaneous immunotherapy is a developing technique, aiming at desensitizing patients with food allergy with less risks that oral ingestion or injection could generate. Several clinical trials have been performed and are currently running, in milk and peanut allergy, assessing the safety of the technique and its efficacy. Preclinical models indicate a major role in the mechanisms of desensitization, for example, Tregs and epigenetic modifications.

Increased intake of vegetable oil rich in n-6 PUFA enhances allergic symptoms and prevents oral tolerance induction in whey-allergic mice

Increased intake of vegetable oils rich in n-6 PUFA, including soyabean oil, has been associated with an increase in allergic disease. The present study aimed to determine the effect of an increasing dose of dietary vegetable oil on allergic outcomes in mice. To study this, mice received a 7 v. 10 % soyabean oil diet before and during oral sensitisation with whey or whey hyperimmune serum transfer. Another group of mice received partial whey hydrolysate (pWH) while being fed the diets before oral sensitisation. The acute allergic skin response, serum Ig level, mouse mast cell protease-1 (mMCP-1) concentration and/or splenic T-cell percentages were determined upon whey challenge. When the diets were provided before and during oral sensitisation, the acute allergic skin response was increased in mice fed the 10 % soyabean oil diet compared with the 7 % soyabean oil diet. Whey IgE and IgG1 levels remained unaltered, whereas mMCP-1 levels increased in mice fed the 10 % soyabean oil diet. Furthermore, allergic symptoms were increased in naive mice fed the 10 % soyabean oil diet and sensitised with whey hyperimmune serum. In addition to enhancing the mast cell response, the 10 % soyabean oil diet increased the percentage of activated Th1 and Th2 cells as well as increased the ratios of Th2:regulatory T cells and Th2:Th1 when compared with the 7 % soyabean oil diet. Oral tolerance induction by pWH was abrogated in mice fed the 10 % soyabean oil diet compared with those fed the 7 % soyabean oil diet during pretreatment with pWH. In conclusion, increased intake of soyabean oil rich in n-6 PUFA suppresses tolerance induction by pWH and enhances the severity of the allergic effector response in whey-allergic mice. Dietary vegetable oils rich in n-6 PUFA may enhance the susceptibility to develop or sustain food allergy.

The regulatory T cells induction by epicutaneous immunotherapy is sustained and mediates long-term protection from eosinophilic disorders in peanut-sensitized mice

BACKGROUND

Allergen-specific immunotherapy favours immune deviation from a Th2 to a Th1 response and increases the number of regulatory T cells (Tregs). Epicutaneous immunotherapy (EPIT) of sensitized mice decreases the clinical and the allergen-specific Th2 responses and increases local and peripheral Foxp3(+) Tregs.

OBJECTIVE

To investigate the role of Tregs in EPIT and characterize their phenotype and maintenance following EPIT.

METHODS

Tregs were investigated using in vivo depletion or adoptive transfer into BALB/c mice. Tregs were depleted using anti-CD25 antibody injection during EPIT, and allergen-specific responses were compared with Sham, EPIT alone and naïve mice. To demonstrate that Tregs can mediate protection by their own, and to study their maintenance following the end of EPIT, CD25(+) CD4(+) Tregs isolated from mice just after or 8 weeks after EPIT were transferred into peanut-sensitized mice. Foxp3-IRES-mRFP mice were transferred with EPIT-induced Tregs to analyse the induction of host Tregs.

RESULTS

The anti-CD25 antibody injection to EPIT mice abrogated the induction of Tregs in spleen and the expression of Foxp3 in oesophagus. This resulted in levels of peanut-induced eosinophilic infiltration in oesophagus similar to Sham and significantly higher than EPIT. Whereas the transfer of Tregs from Sham-treated mice demonstrated no effect, the transfer of Tregs isolated just after EPIT prevented peanut-induced eosinophil infiltration and eotaxin expression and induced Foxp3 in oesophagus. The transfer of Tregs isolated 8 weeks after EPIT suppressed allergen-specific responses as efficiently as did Tregs isolated just after EPIT and increased spleen Foxp3(+) CD25(+) CD4(+) cells similarly. The use of reporter mice demonstrated an increase in host Tregs.

CONCLUSIONS

These results confirm the Tregs-mediated mechanism of EPIT and demonstrate the persistence of efficient Tregs during a long period of time after treatment cessation. This suggests that EPIT induces long-term tolerance in peanut-sensitized mice.

DHA-rich tuna oil effectively suppresses allergic symptoms in mice allergic to whey or peanut

BACKGROUND

Supplementation with long-chain n-3 polyunsaturated fatty acids (LCPUFAs) has been found to reduce the development of allergic disease.

OBJECTIVE

The aim of this study was to compare the effectiveness of fish oil diets rich in eicosapentaenoic acid (20:5n-3; EPA) or docosahexaenoic acid (22:6n-3; DHA) in suppressing food allergic symptoms.

METHODS

Mice were fed a control diet (10% soybean oil) or fish oil diet rich in EPA (4% soybean oil + 6% EPA oil containing 28.8% EPA and 13.7% DHA) or DHA (4% soybean oil + 6% DHA oil containing 7% EPA and 27.8% DHA), starting 14 d before and for 5 wk during oral sensitization with peanut extract (PE) or whey. Acute allergic skin responses, serum immunoglobulins (Igs), and mucosal mast cell protease-1 (mmcp-1) were assessed. Hyperimmune serum was transferred to naive recipient mice fed the different diets.

RESULTS

The DHA diet effectively reduced the acute allergic skin response compared with the control or EPA diet in PE-allergic mice (control, 159 ± 15, or EPA, 129 ± 8, vs. DHA, 78 ± 7 μm; P < 0.0001 or P < 0.05, respectively). In contrast, both the DHA and EPA diets reduced the allergic skin response in whey allergic mice (control, 169 ± 9, vs. DHA, 91 ± 13, or EPA, 106 ± 14 μm; P < 0.001 or P < 0.01, respectively); however, only the DHA diet reduced mmcp-1 and whey-specific IgE and IgG1. The DHA and EPA diets also reduced the acute skin response in passively immunized mice.

CONCLUSIONS

The DHA-rich fish oil diet reduced allergic sensitization to whey and allergic symptoms in both PE- and whey-allergic mice. These data suggest that DHA-rich fish oil is useful as an intervention to prevent or treat food allergy symptoms.

Commensal bacteria protect against food allergen sensitization

Environmentally induced alterations in the commensal microbiota have been implicated in the increasing prevalence of food allergy. We show here that sensitization to a food allergen is increased in mice that have been treated with antibiotics or are devoid of a commensal microbiota. By selectively colonizing gnotobiotic mice, we demonstrate that the allergy-protective capacity is conferred by a Clostridia-containing microbiota. Microarray analysis of intestinal epithelial cells from gnotobiotic mice revealed a previously unidentified mechanism by which Clostridia regulate innate lymphoid cell function and intestinal epithelial permeability to protect against allergen sensitization. Our findings will inform the development of novel approaches to prevent or treat food allergy based on modulating the composition of the intestinal microbiota.

Mucosal immunology of food allergy

Food allergies are increasing in prevalence at a higher rate than can be explained by genetic factors, suggesting a role for as yet unidentified environmental factors. In this review, we summarize the state of knowledge about the healthy immune response to antigens in the diet and the basis of immune deviation that results in immunoglobulin E (IgE) sensitization and allergic reactivity to foods. The intestinal epithelium forms the interface between the external environment and the mucosal immune system, and emerging data suggest that the interaction between intestinal epithelial cells and mucosal dendritic cells is of particular importance in determining the outcome of immune responses to dietary antigens. Exposure to food allergens through non-oral routes, in particular through the skin, is increasingly recognized as a potentially important factor in the increasing rate of food allergy. There are many open questions on the role of environmental factors, such as dietary factors and microbiota, in the development of food allergy, but data suggest that both have an important modulatory effect on the mucosal immune system. Finally, we discuss recent developments in our understanding of immune mechanisms of clinical manifestations of food allergy. New experimental tools, particularly in the field of genomics and the microbiome, are likely to shed light on factors responsible for the growing clinical problem of food allergy.

The aryl hydrocarbon receptor and food allergy

The immune system is important for protection against pathogens and malignant cells. However, malfunction of the immune system can also result in detrimental auto-immune diseases, inflammatory diseases, cancers and allergies. The aryl hydrocarbon receptor (AhR), present in numerous tissues and cell subsets, including cells of the immune system, plays an important role in the functioning of the immune system. Activation of the AhR is for example associated with various effects on dendritic cells (DCs), regulatory T cells and the Th1/Th2 cell balance. These cells play a major role in the development of food allergy. Food allergy is an increasing health problem in both humans and animals. Despite the knowledge in risk factors and cellular mechanisms for food allergy, no approved treatments are available yet. Recently, it has been shown that activation of the AhR by dioxin-like compounds suppresses allergic sensitization by suppressing the absolute number of precursor and effector T cells, by preserving CD4(+)CD25(+)Foxp3(+) Treg cells and by affecting DCs and their interaction with effector T cells. Future research should elucidate whether and how AhR activation can be used to interfere in food allergic responses in humans and in animals. This may lead to new prevention strategies and therapeutic possibilities for food allergy.

Can we produce true tolerance in patients with food allergy?

Immune tolerance is defined as nonresponsiveness of the adaptive immune system to antigens. Immune mechanisms preventing inappropriate immune reactivity to innocuous antigens include deletion of reactive lymphocytes and generation of regulatory T (Treg) cells. The normal response to food antigens is the generation of antigen-specific Treg cells. In patients with food allergy, the dominant immune response is a T(H)2-skewed T-cell response and the generation of food-specific IgE antibodies from B cells. It is not known whether a failure of the Treg cell response is behind this inappropriate immune response, but interventions that boost the Treg cell response, such as mucosal immunotherapy, might lead to a restoration of immune tolerance to foods. Tolerance has been notoriously difficult to restore in animal disease models, but limited data from human trials suggest that tolerance (sustained nonresponsiveness) can be re-established in a subset of patients. Furthermore, studies on the natural history of food allergy indicate that spontaneous development of tolerance to foods over time is not uncommon. The current challenge is to understand the mechanisms responsible for restoration of natural or induced tolerance so that interventions can be developed to more successfully induce tolerance in the majority of patients with food allergy.

Activation of the aryl hydrocarbon receptor reduces the number of precursor and effector T cells, but preserves thymic CD4+CD25+Foxp3+ regulatory T cells

Aryl hydrocarbon receptor (AhR) activation suppresses immune responses, including allergic sensitization, by increasing the percentage of regulatory (Treg) cells. Furthermore, AhR activation is known to affect thymic precursor T cells. However, the effect of AhR activation on intrathymic CD4+CD25+Foxp3+ Treg cells is unknown. Therefore, we investigated the effect of AhR activation on the percentage and number of CD4+CD25+Foxp3+ Treg cells during allergic sensitization in relevant immunological organs. C3H/HeOuJ mice were treated on day 0 with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and subsequently sensitized to peanut. On day 8, mice were sacrificed and thymus, spleen and mesenteric lymph nodes (MLN) were isolated. TCDD treatment decreased the number of CD4-CD8-, CD4+CD8+, CD4+CD8- and CD4-CD8+ precursor T cells, but not the number of thymic CD4+CD25+Foxp3+ Treg cells. TCDD treatment increased the number of splenic CD4+CD25+Foxp3+ Treg cells and decreased Th1, Th2 and cytotoxic T cells in the spleen. This appeared to be independent of allergic sensitization. In MLN, TCDD treatment suppressed the increase of the number of CD4+CD25+Foxp3+ Treg cells, Th1, Th2 and cytotoxic T cells induced by peanut sensitization. Together, TCDD treatment preserves thymic CD4+CD25+Foxp3+ Treg cells and decreases peripheral T helper and cytotoxic T cells. This effect of TCDD may contribute to the increased influence of CD4+CD25+Foxp3+ Treg cells on immune mediated responses and to the understanding of how AhR activation modulates immune mediated diseases, including food allergy.

Molecular mechanisms of IgE mediated food allergy

The purpose of this review is to collate current knowledge and recent advances in molecular mechanism behind the immediate type hypersensitivity of foods. Food allergy is a growing concern of human health in developed as well as developing countries now days. Food allergic reactions are mostly IgE mediated and also known as immediate type hypersensitivity or type I reaction. This review encompasses a wide range of molecular events during IgE mediated reactions like primary exposure of allergens, processing of allergens by antigen presenting cells, role of transcription factors like GATA-3, STAT-6, NF-AT, c-maf, c-kit and NF-κB, Treg cells, toll like receptors, cytokines and chemokines, class switch to IgE, FcεR1 receptor, priming of IgE on mast cells or basophils, signaling events followed by secondary exposure of allergens, degranulation and release of mediators like leukotrienes, histamines, prostaglandins, β-hexosaminidase and ultimately anaphylaxis. This review may be helpful to beginners as well as experts working in the field of allergy and immunology because of the stepwise explanations of molecular mechanisms involved in IgE mediated reactions.

Non-dioxin-like AhR ligands in a mouse peanut allergy model

Recently, we have shown that AhR activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) suppresses sensitization to peanut at least in part by inducing a functional shift toward CD4(+)CD25(+)Foxp3(+) T cells. Next to TCDD, numerous other AhR ligands have been described. In this study, we investigated the effect of three structurally different non-dioxin-like AhR ligands, e.g., 6-formylindolo[3,2-b]carbazole (FICZ), β-naphthoflavone (β-NF), and 6-methyl-1,3,8-trichlorodibenzofuran (6-MCDF), on peanut sensitization. Female C57BL/6 mice were sensitized by administering peanut extract (PE) by gavage in the presence of cholera toxin. Before and during peanut sensitization, mice were treated with FICZ, β-NF, or 6-MCDF. AhR gene transcription in duodenum and liver was investigated on day 5, even as the effect of these AhR ligands on CD4(+)CD25(+)Foxp3(+) T(reg) cells in spleen and mesenteric lymph nodes (MLNs). Mice treated with TCDD were included as a positive control. Furthermore, the murine reporter cell line H1G1.1c3 (CAFLUX) was used to investigate the possible role of metabolism of TCDD, FICZ, β-NF, and 6-MCDF on AhR activation in vitro. TCDD, but not FICZ, β-NF, and 6-MCDF, suppressed sensitization to peanut (measured by PE-specific IgE, IgG1, IgG2a and PE-induced interleukin (IL)-5, IL-10, IL-13, IL-17a, IL-22, and interferon-γ). In addition, FICZ, β-NF, and 6-MCDF treatments less effectively induced AhR gene transcription (measured by gene expression of AhR, AhRR, CYP1A1, CYP1A2, CYP1B1) compared with TCDD-treated mice. Furthermore, FICZ, β-NF and 6-MCDF did not increase the percentage of CD4(+)CD25(+)Foxp3(+) T(reg) cells in spleen and mesenteric lymph nodes compared with PE-sensitized mice, in contrast to TCDD. Inhibition of metabolism in vitro increased AhR activation. Together, these data shows that TCDD, but not FICZ, β-NF, and 6-MCDF suppresses sensitization to peanut. Differences in metabolism, AhR binding and subsequent gene transcription might explain these findings and warrant further studies to investigate the role of the AhR in food allergic responses.

Contribution of classic and alternative effector pathways in peanut-induced anaphylactic responses

Food allergy affects approximately 5% of children and is the leading cause of hospitalization for anaphylactic reactions in westernized countries. However, the pathways of anaphylaxis in food allergy are still relatively unknown. We investigated the effector pathways of allergic and anaphylactic responses of different strains of mice in a clinical relevant model of peanut allergy. C3H/HeOuJ, C57BL/6 and BALB/c mice were sensitized by intragastric peanut extract and challenged by intragastric or intraperitoneal injection of peanut. Peanut-specific T cell responses, IgE, IgG1 and IgG2a and mucosal mast cell degranulation were induced to different extent in C3H/HeOuJ, C57BL/6 and BALB/c mice. Interestingly, anaphylactic symptoms after systemic challenge were highest in C3H/HeOuJ followed by C57BL/6 but were absent in BALB/c mice. Mechanistic studies showed that the food allergic systemic anaphylaxis was dependent on platelets, FcRγ and mast cells, and partially dependent on platelet activating factor and monocytes/macrophages, depending on mouse strain. These data demonstrate that in three mouse strains, components of the classic and alternative anaphylactic cascade are differently expressed, leading to differential outcomes in parameters of allergic disease and food induced systemic anaphylaxis.

Mechanisms of allergic sensitization to foods: bypassing immune tolerance pathways

The default response of the mucosal immune system to antigens derived from food is one of active immune tolerance carried out by regulatory T cells and induced by dendritic cells residing in the intestinal mucosa. This tolerance response must be inhibited or bypassed to generate allergic sensitization in experimental food allergy and this has been achieved by 3 main approaches: genetic modifications, experimental adjuvants, and bypassing oral tolerance by administering the antigen through alternative routes. This article discusses the implications of these approaches for understanding the mechanisms of sensitization to food allergens in human disease.

Activation of the aryl hydrocarbon receptor suppresses sensitization in a mouse peanut allergy model

Food allergy is an increasing health problem in Western countries. Previously, it has been shown that the intensity of food allergic reactions can be regulated by regulatory T (T(reg)) cells. In addition, it has been shown that activation of the aryl hydrocarbon receptor (AhR) regulates T-cell responses by induction of T(reg) cells. Therefore, we hypothesized that activation of the AhR pathway can suppress development of food allergic responses through the induction of T(reg) cells. This was investigated by using a mouse model for peanut allergy. C3H/HeOuJ mice (AhR(b)(-2)) were sensitized to peanut by administering peanut extract (PE) by gavage in the presence of cholera toxin and were treated with the prototypical AhR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (0.6, 1.7, 5, and 15 μg/kg body weight) on days 3 and 11 orally. The functional role of CD4(+)CD25(+)Foxp3(+) T(reg) cells was investigated by depleting these cells with anti-CD25 mAb during sensitization to PE. TCDD treatment dose dependently suppressed sensitization to peanut (PE-specific IgE, IgG1, and IgG2a and PE-induced IL-5, IL-10, and IL-13, respectively). The percentage, but not the number, of CD4(+)CD25(+)Foxp3(+) T(reg) cells dose dependently increased by AhR activation in both spleen and mesenteric lymph nodes. Depletion of CD4(+)CD25(+)Foxp3(+) T(reg) cells markedly reversed the suppressive effect of TCDD on PE-specific antibody levels and PE-induced IL-5, IL-10, and IL-13 cytokine production. Present data demonstrate for the first time that activation of the AhR by TCDD suppressed the development of Th2-mediated food allergic responses. A functional shift within the CD4(+) cell population toward CD4(+)CD25(+)Foxp3(+) T(reg) cells appeared to underlie this effect. This suggests that the AhR pathway might provide potential therapeutic targets to treat food allergic diseases.

Regulation by intestinal γδ T cells during establishment of food allergic sensitization in mice

BACKGROUND

Food allergy affects approximately 5% of children and is the leading cause of hospitalization for anaphylactic reactions in westernized countries. The mucosal adjuvant cholera toxin induces allergic sensitization to co-administered proteins in mice, while feeding the protein alone induces oral tolerance. Intestinal γδ T cells could be of importance in the induction of oral tolerance. This study aims to investigate whether γδ T cells have functional relevance in food allergic sensitization.

METHODS

Changes in γδ T cells on days 1, 2, 3, and 7 after initiation of food allergy were evaluated using flowcytometry. Furthermore, the anti-γδ T-cell receptor (TCR) antibody UC7 was used to block the γδ TCR in mice in vivo, followed by sensitization to peanut. After 4 weeks, peanut-specific antibodies in serum and cytokine production in spleen were measured.

RESULTS

Induction of food allergy resulted in a profound decrease in the percentage of γδ T cells in intestinal tissues and Peyer's Patches, but not in mesenteric lymph nodes or spleen. This decrease could be detected from days 1 to 2 after the initiation of food allergy and the number of γδ T cells returned to normal on day 7. Blockade of the γδ TCR resulted in elevated food allergic responses upon sensitization with peanut characterized by increased IgE and Th2 cytokine production in splenocytes.

CONCLUSION

These results demonstrate a unique regulatory role of γδ T cells, suggesting that targeting γδ T cells in the intestine may contribute to strategies to prevent and possibly treat food allergy.

Allergen-specific antibody and cytokine responses, mast cell reactivity and intestinal permeability upon oral challenge of sensitized and tolerized mice

BACKGROUND

Food allergy has reached an epidemic level in westernized countries and although central mechanisms have been described, the variability associated with genetic diversity underscores the still unresolved complexity of these disorders.

OBJECTIVE

To develop models of food allergy and oral tolerance, both strictly induced by the intestinal route, and to compare antigen-specific responses.

METHODS

BALB/c mice were mucosally sensitized to ovalbumin (OVA) in the presence of the mucosal adjuvant cholera toxin, or tolerized by intra-gastric administrations of OVA alone. Antibody titres and cytokines were determined by ELISA, and allergic status was determined through several physiologic parameters including decline in temperature, diarrhoea, mast cell degranulation and intestinal permeability.

RESULTS

OVA-specific antibodies (IgE, IgGs and IgA in serum and feces) were produced in sensitized mice exclusively. Upon intra-gastric challenge with OVA, sensitized mice developed anaphylactic reactions associated with a decline of temperature, diarrhoea, degranulation of mast cells, which were only moderately recruited in the small intestine, and increased intestinal permeability. Cytokines produced by immune cells from sensitized mice included T-helper type 2 cytokines (IL-5, IL-13), but also IL-10, IFN-gamma and IL-17. In contrast, all markers of allergy were totally absent in tolerized animals, and yet the latter were protected from subsequent sensitization, demonstrating that oral tolerance took place efficiently.

CONCLUSION

This work allows for the first time an appropriate comparison between sensitized and tolerized BALB/c mice towards OVA. It highlights important differences from other models of allergy, and thus questions some of the generally accepted notions of allergic reactions, such as the protective role of IFN-gamma, the importance of antigen-specific secretory IgA and the role of mucosal mast cells in intestinal anaphylaxis. In addition, it suggests that IL-17 might be an effector cytokine in food allergy. Finally, it demonstrates that intestinal permeability towards the allergen is increased during challenge.

The serum D-xylose test as a useful tool to identify malabsorption in rats with antigen specific gut inflammatory reaction

The inappropriate immune response to foods, such as peanut, wheat and milk may be the basis in the pathogenesis of enteropathies like coeliac and Crohn disease, which present small intestinal malabsorption. A number of recent studies have utilized d-xylose absorption as an investigative tool to study small intestinal function in a variety of clinical settings. Thus, the aim of this experimental study was to evaluate the intestinal absorption of D-xylose in an antigen-specific gut inflammatory reaction rat model. Animals of the experimental group were inoculated with peanut protein extract before their exposure to a challenge diet containing exclusively peanut seeds to induce the gut inflammatory reaction caused by peanut allergy. Our results show that systemic inoculation with peanut protein extract renders significantly higher antibody titres (5.085 +/- 0.126 units) (P < 0.0001) than control rats (0.905 +/- 0.053 units) and that the antibody titres correlate positively to an inflammatory alteration of the gut morphology (P < 0.0001). Animals pertaining to the experimental group showed an intestinal absorption of D-xylose lower than control rats (P < 0.0001). We also observed that D-xylose absorption correlates negatively with IgG titres and positively with morphometric parameters (Pearson correlation). In conclusion, the use of serum D-xylose test was useful to identify the presence of small intestinal malabsorption in our antigen specific gut inflammatory reaction rat model.

Immunophysiology of experimental food allergy

Animal models of food allergy have been used to identify mechanisms involved in the development of sensitization to food proteins as well as immunologic mechanisms of adverse reactions to allergen reexposure. To counteract the normal tolerant responses to antigen generated in the gastrointestinal tract, investigators have used mucosal adjuvants or manipulated the mucosal barrier, taken advantage of endogenous adjuvanticity of some food allergens, or bypassed the oral route and sensitized through the skin. Site of antigen uptake in the gastrointestinal tract is a critical factor in both sensitization and anaphylaxis, and antigen uptake can be facilitated by immunoglobulin-E (IgE)-antigen complexes binding to CD23 on the epithelial cell surface. Studies on systemic anaphylaxis or local gastrointestinal manifestations of food allergy in mice have highlighted the contribution of IgE, mast cells, and pathogenic Th2 lymphocytes in experimental food allergy.

Impairing oral tolerance promotes allergy and anaphylaxis: a new murine food allergy model

BACKGROUND

Food allergy is a disorder in which antigenic food proteins elicit immune responses. Animal models of food allergy have several limitations that influence their utility, including failure to recapitulate several key immunologic hallmarks. Consequently, little is known regarding the pathogenesis and mechanisms leading to food allergy. Staphylococcus aureus-derived enterotoxins, a common cause of food contamination, are associated with antigen responses in atopic dermatitis.

OBJECTIVE

We hypothesized that S aureus-derived enterotoxins might influence the development of food allergy. We examined the influence of administration of staphylococcal enterotoxin B (SEB) with food allergens on immunologic responses and compared these responses with those elicited by a cholera toxin-driven food allergy model.

METHODS

Oral administration of ovalbumin or whole peanut extract with or without SEB was performed once weekly. After 8 weeks, mice were challenged with oral antigen alone, and the physiologic and immunologic responses to antigen were studied.

RESULTS

SEB administered with antigen resulted in immune responses to the antigen. Responses were highly T(H)2 polarized, and oral challenge with antigen triggered anaphylaxis and local and systemic mast cell degranulation. SEB-driven sensitization induced eosinophilia in the blood and intestinal tissues not observed with cholera toxin sensitization. SEB impaired tolerance specifically by impairing expression of TGF-beta and regulatory T cells, and tolerance was restored with high-dose antigen.

CONCLUSIONS

We demonstrate a new model of food allergy to oral antigen in common laboratory strains of mice that recapitulates many features of clinical food allergy that are not seen in other models. We demonstrate that SEB impairs oral tolerance and permits allergic responses.

Local secretion of anti-CTLA-4 enhances the therapeutic efficacy of a cancer immunotherapy with reduced evidence of systemic autoimmunity

Promising anti-tumor responses have been observed in the clinic using monoclonal antibodies (mAbs) that block immune checkpoints. One concern with these therapeutic agents remains the potential induction of immune breakthrough events (IBEs) resulting from the disruption of T cell homeostasis or the breaking of tolerance to self antigens. As an approach to maintaining anti-tumor responses but decreasing the likelihood of these events, the local expression of a mAb in combination with a GM-CSF-secreting cancer immunotherapy was evaluated. Using anti-cytotoxic T lymphocyte antigen (CTLA)-4 as a model antibody to test this hypothesis, tumor cell lines were generated that expressed the full-length mAb in addition to GM-CSF. Evaluation of these cell lines in two therapeutic tumor models revealed that local, cell-mediated delivery of anti-CTLA-4 from a GM-CSF-secreting tumor cell immunotherapy activated potent anti-tumor responses and prolonged overall survival at significantly lower serum mAb levels in the host. Furthermore, lowering the systemic exposure of the host to the immune modulatory mAb correlated with reduced evidence of systemic autoimmunity. This approach has broad utility for the delivery of mAbs or proteins locally from cellular immunotherapies to minimize IBEs while retaining the potent therapeutic effects of such combination treatments.

T(H)2 adjuvants: implications for food allergy

A persistent question for immunologists studying allergic disease has been to define the characteristics of a molecule that make it allergenic. There has been substantial progress elucidating mechanisms of innate priming of T(H)2 immunity in the past several years. These accumulating data demonstrate that T(H)2 immunity is actively induced by an array of molecules, many of which were first discovered in the context of antihelminthic immune responses. Similar intrinsic or associated activities are now known to account for the T(H)2 immunogenicity of some allergens, and may prove to play a role for many more. In this review, we discuss what has been discovered regarding molecules that induce innate immune activation and the pathways that promote T(H)2-polarized immune responses generally, and specifically what role these mechanisms may play in food allergy from models of food allergy and the study of T(H)2 gastrointestinal adjuvants.

Role for CTLA-4 but not CD25+T cells during Schistosoma mansoni infection of mice

Schistosoma mansoni infection of mice increases the frequency of cells that are CD4+ CD25+ in the acute (4 and 8 weeks) and chronic (16 week) stages of infection. Depletion of > 85% of CD25+ cells in the acute or chronic stages of schistosome infection caused no overt changes in morbidity or immunological responses. The absence of effect in mice with CD25+ cells depleted may be due to the preferential expression of IL-4 and IL-10, two cytokines that are protective in schistosome infection, on CD25- CD4+ cells. We also assessed infection-induced changes of other regulatory markers, GITR, CD103 and CTLA-4 on CD4+ cells. We identified a marked expansion of CTLA-4+ population on CD25- CD4+ cells in acute and chronic infection. Blocking of CTLA-4 during acute, but not chronic infection, caused significant weight loss and altered the type 2 cytokine response of mice, with increased IL-4 and IL-5 production associated with significantly more Th2 cells and eosinophils in the liver granuloma. This study illustrates the complexity of regulation of T cells in schistosome infection and highlights a specific role for CTLA-4+, but not CD25+ cells, in the regulation of Th2 responses in helminth infection.

Immunology of anaphylaxis: lessons from murine models

The use of animal models is increasing, and mice are suitable animals to use in exploring systemic anaphylaxis based on the similarity between human and mouse immune systems. Two anaphylaxis pathways have been characterized in mice, which may help in understanding some of the discrepancies found in humans. In addition, cytokine studies are raising new concepts that may put together some of the puzzling mechanisms described in this disease. Finally, the study of the mechanisms that parasitic infections use to evade the human immune system and the scene in which a parasite induces clinical anaphylaxis are opening new insights in the immunology pathways and new strategies to fight against this exclusive disease.

The CD28/CTLA-4-B7 Signaling Pathway Is Involved in Both Allergic Sensitization and Tolerance Induction to Orally Administered Peanut Proteins

Dendritic cells are believed to play an essential role in regulating the balance between immunogenic and tolerogenic responses to mucosal Ags by controlling T cell differentiation and activation via costimulatory and coinhibitory signals. The CD28/CTLA-4-CD80/CD86 signaling pathway appears to be one of the most important regulators of T cell responses but its exact role in responses to orally administered proteins remains to be elucidated. In the present study, the involvement of the CD28/CTLA-4-CD80/CD86 costimulatory pathway in the induction of allergic sensitization and oral tolerance to peanut proteins was investigated. In both an established C3H/HeOuJ mouse model of peanut hypersensitivity and an oral tolerance model to peanut, CD28/CTLA-4-CD80/CD86 interactions were blocked using the fusion protein CTLA-4Ig. To examine the relative contribution of CD80- and CD86-mediated costimulation in these models, anti-CD80 and anti-CD86 blocking Abs were used. In the hypersensitivity model, CTLA-4Ig treatment prevented the development of peanut extract-induced cytokine responses, peanut extract-specific IgG1, IgG2a, and IgE production and peanut extract-induced challenge responses. Blocking of CD80 reduced, whereas anti-CD86 treatment completely inhibited, the induction of peanut extract-specific IgE. Normal tolerance induction to peanut extract was found following CTLA-4Ig, anti-CD86, or anti-CD80 plus anti-CD86 treatment, whereas blockade of CD80 impaired the induction of oral tolerance. We show that CD28/CTLA-4-CD80/CD86 signaling is essential for the development of allergic responses to peanut and that CD86 interaction is most important in inducing peanut extract-specific IgE responses. Additionally, our data suggest that CD80 but not CD86 interaction with CTLA-4 is crucial for the induction of low dose tolerance to peanut.

CD4+CD25+T cells regulate the intensity of hypersensitivity responses to peanut, but are not decisive in the induction of oral sensitization

BACKGROUND

Naturally occurring CD4+CD25+ regulatory T cells (Tregs) play a critical role in the maintenance of self-tolerance and it has been suggested that these Tregs may also be involved in preventing allergic disease.

OBJECTIVE

The precise role of CD4+CD25+ T cells in the regulation of allergic responses to mucosal antigens remains to be elucidated. In the present study, it was investigated whether CD4+CD25+ T cells are involved in the induction of oral tolerance and whether they play a role in controlling hypersensitivity responses to food proteins.

METHODS

CD4+CD25+ T cells were depleted with PC61 mAb before the induction of low dose oral tolerance to peanut extract (PE). In addition, CD4+CD25+ T cell depletion was performed during sensitization or before oral challenge, using a C3H/HeOuJ mouse model of allergic sensitization to peanut.

RESULTS

Oral tolerance to PE could not be induced in CD4+CD25+ T cell-depleted mice. However, CD4+CD25+ T cell depletion during long-term exposure to PE alone did not result in allergic sensitization. In sensitized mice, anti-CD25 treatment during oral exposure resulted in higher levels of PE-specific IgE and increased mast cell degranulation upon an oral challenge. In contrast, anti-CD25 treatment of PE-sensitized mice before oral challenges did not affect the level of mast cell degranulation.

CONCLUSION

These results indicate that CD4+CD25+ Tregs are involved in maintaining tolerance to oral antigens and regulate the intensity of an IgE-mediated food hypersensitivity response, but are not crucial in preventing sensitization. Accordingly, CD4+CD25+ Tregs may represent a potential tool for the treatment of food allergic disorders.

Immune privilege in the gut: the establishment and maintenance of non-responsiveness to dietary antigens and commensal flora

Immune privilege in the gut is the result of a complex interplay between the gut microbiome, gut luminal antigens, and the intestinal epithelial barrier. Composed of both physical and immunochemical components, the intestinal barrier secretes immunoregulatory mediators that promote the generation of tolerogenic antigen-presenting cells, phagocytic innate immune cells characterized by 'inflammatory anergy', and regulatory cells of the adaptive immune system. Innate immune cells mediate controlled transepithelial transport of luminal antigens as far as the mesenteric lymph nodes, where the intestinal and peripheral immune systems intersect. This promotes the generation of adaptive regulatory lymphocytes that actively suppress effector cell responses against gut luminal antigens and flora. The net result is the generation of tolerance to dietary antigens and the maintenance of gut homeostasis. Dysregulation of this complex immunoregulatory network leads to diseases such as food allergy and inflammatory bowel disease. Future therapies for these diseases will likely involve the functional restoration of the barrier and regulatory cell functions at the epithelial/luminal interface.

Initiating mechanisms of food allergy: Oral tolerance versus allergic sensitization

Immediately after birth the mucosa of the gastrointestinal tract, which represents the greatest body surface area exposed to the outside environment, is confronted with a large variety of foreign antigens. The immune system of the intestine now has to meet the task of discriminating between pathogens and harmless antigens, such as food proteins and commensal bacteria, and to respond accordingly. This important job is fulfilled by cells of the gut-associated lymphoid tissue, the largest immunologic organ in the body. Despite the large extent of food antigen exposure, only a small percentage of individuals experience adverse immunologic reactions to food. This is due to the fact that the normal immune response to dietary proteins is associated with the induction of oral tolerance, which refers to a state of active inhibition of immune responses to an antigen by means of prior exposure to that antigen via the oral route. Abrogation of oral tolerance or failure to induce oral tolerance may result in the development of food hypersensitivity. In the present review, factors that may play a role in the outcome of oral tolerance versus sensitization to food proteins are discussed.

Current developments in peanut allergy

PURPOSE OF REVIEW

Peanut allergy is among the most serious, life-threatening food sensitivities, and recent studies indicate increasing prevalence, particularly among children. Our objective is to highlight recent advances in the immunology and treatment of peanut allergy.

RECENT FINDINGS

Peanut sensitization may be both a Th1- and Th2-driven process, and cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) may play a role in regulating the response intensity. Preliminary work shows that the food matrix is important in the immune response to peanut and that purified peanut allergens may have little intrinsic stimulatory capacity. Studies characterizing peanut allergens have revealed Ara h 1 and Ara h 2 as the most potent allergens, but Ara h 3 may be more allergenic than previously thought. There appears to be a relationship between the diversity of IgE-binding patterns and the severity of clinical symptoms. Multiple novel approaches to treatment are being investigated, which include traditional Chinese medicine, various forms of modified immunotherapy and the use of adjuvants in modified immunotherapy.

SUMMARY

By understanding the immunologic response to peanut and the roles of the major peanut allergens, it may be possible to predict those at risk for severe reactions, prevent peanut sensitization and effectively treat those already sensitized.

Blockade of CTLA-4 promotes airway inflammation in naive mice exposed to aerosolized allergen but fails to prevent inhalation tolerance

In subjects not developing allergy, inhalation of nonpathogenic protein antigens causes no harm and is associated with tolerance induction. Repeated exposure to aerosolized ovalbumin (OVA) likewise does not evoke airway inflammation and induces inhalation tolerance in experimental animals. The present study explored the role of the inhibitory T-cell receptor CTLA-4, in preventing inflammation and in establishing inhalation tolerance in response to a protein antigen. Naive BALB/c mice were injected intraperitoneally with anti-CTLA-4 monoclonal antibody or control immunoglobulin G (IgG) and exposed daily to aerosolized saline or OVA over 10 or 20 consecutive days. OVA-specific IgE levels and the inflammatory response in airway tissues were assessed 2 days after last exposure. The OVA-specific IgE response was also evaluated in mice subjected to a subsequent immunogenic OVA challenge 18 days after last aerosol exposure. Additional mice were made tolerant by 10 days of OVA aerosol exposure and were then subjected to an immunogenic OVA challenge combined with CTLA-4 blockade or control IgG treatment. Repeated inhalation of aerosolized OVA alone did not cause a pulmonary inflammatory response. In contrast, 10 days of OVA exposure combined with blockade of CTLA-4 led to development of eosinophilic lung infiltrates, BAL fluid eosinophilia, goblet cell hyperplasia and increased OVA-specific IgE. By 20 days of OVA exposure and blockade of CTLA-4, the inflammatory response remained. Mice exposed to aerosolized OVA for 10 days exhibited greatly reduced OVA-specific IgE responses to subsequent immunogenic OVA challenge. Blockade of CTLA-4 during the period of OVA aerosol exposure did not prevent this suppression of the OVA-specific IgE response. Neither did blockade of CTLA-4 during immunogenic OVA challenge alter the allergen-specific IgE response. Our results indicate that in vivo blockade of CTLA-4 modulates the initial immune response to a protein antigen allowing the development of allergen-induced airway inflammation in naive mice. However, this initial exaggerated immune response is followed by the induction of inhalation tolerance, demonstrating that CTLA-4 signalling is not decisive in this process. Our findings also show that once inhalation tolerance is established it may not be disrupted by blockade of CTLA-4.

Genetic and environmental risk factors for the development of food allergy

PURPOSE OF REVIEW

To review recent clinical and experimental studies of genetic and environmental risk factors for the development of food allergy.

RECENT FINDINGS

It may be true, although it is yet to be shown, that food allergies in early childhood are becoming more common and that the causes are the same as for later-developing respiratory allergies. The mother not only transfers 50% of her genes to her baby, but she is also the exclusive environment during gestation and continues to be a major environmental factor while breast-feeding her infant. Non-genetic maternal influences increasing the likelihood of food allergy include Caesarian section and high maternal age. Allergy to sesame seems to be increasing in children. This is possibly a consequence of increased use in processed foods. The search for dietary risk factors is not limited to allergenic foods, but may include other nutrients, for example excessive intake of vitamins. Two meta-analyses have seriously questioned the use of special infant formulas for allergy prevention. Novel prevention strategies, such as probiotic bacteria, have yet to be documented further.

SUMMARY

The causes of food allergy are still unknown and no particular genes associated particularly with food allergy have been identified, although there is a strong association in general between genetic susceptibility to food allergy and that to IgE-mediated allergy. There are still no measures for general recommendation in order to prevent food allergy and no genes have been linked conclusively to disease. Further research concentrating on food allergy is obviously needed.

The effect of the food matrix on in vivo immune responses to purified peanut allergens

There is little knowledge about the factors that determine the allergenicity of food proteins. One aspect that remains to be elucidated is the effect of the food matrix on immune responses to food proteins. To study the intrinsic immunogenicity of allergens and the influence of the food matrix, purified peanut allergens (Ara h 1, Ara h 2, Ara h 3, or Ara h 6) and a whole peanut extract (PE) were tested in the popliteal lymph node assay (PLNA) and in an oral model of peanut hypersensitivity. In the PLNA, peanut proteins were injected into the hind footpad of BALB/c mice; in the oral exposure experiments C3H/HeOuJ mice were gavaged weekly with PE or allergens in the presence of cholera toxin (CT). Upon footpad injection, none of the allergens induced significant immune activation. In contrast, PE induced an increase in cell number, cytokine production, and activation of antigen-presenting cells. Furthermore, the presence of a food matrix enhanced the immune response to the individual allergens. Oral exposure to the purified allergens in the presence of CT induced specific IgE responses, irrespective of the presence of a food matrix. These results suggest that purified peanut allergens possess little intrinsic immune-stimulating capacity in contrast to a whole PE. Moreover, the data indicate that the food matrix can influence responses to individual proteins and, therefore, the food matrix must be taken into account when developing models for allergenic potential assessment.