Grantee Research Project Results

Food allergy, defined as an adverse immunological response to food, affects more than 11 million Americans, and severe anaphylaxis triggered by reactions to foods are responsible for 150 deaths and more than 30,000 emergency room visits in the United States per year. Other food allergy-associated symptoms include vomiting, diarrhea, dehydration, abdominal distention, weight loss, oral ulcers, tingling and/or angioedema of the lips and palate, rapid onset nausea, reflux issues, as well as increased risk of atopic dermatitis. Despite this impact on health care expenditures, relatively little is known about the risk factors, disease pathogenesis, or the cellular and molecular processes involved in generating food allergy. Furthermore, food allergy has no current treatments or therapies and patients are left with the only options being strict food avoidance and injectable epinephrine for treatment of responses during accidental exposure. The incidence of food allergy is increasing at an alarming rate, and so research into pathogenesis and treatment is absolutely paramount. 

 

Ligation of receptors for the Fc portion of the IgG molecule (Fcγ) is one of the key downstream effector functions of IgG. Dr. Sperling’s group has recently demonstrated that signaling via the activating Fcγ receptor, FcγRIII, in conjunction with TLR stimulation modulated cytokine production from bone marrow derived dendritic cells (DC) and augmented their ability to promote Th2 responses in vitro and in an airway inflammation model for asthma. The augmentation of Th2 immunity was distinct and independent of the well-established role of FcγRIII in augmenting antigen presentation. Thus, our studies revealed a novel and specific role for FcγRIII signaling in the regulation of T helper 2 cell responses. 

 

The lack of adequate animal models has been a key obstacle in the ability to pursue the mechanisms and potential treatments for this disease. Current models for food allergy have failed to adequately recapitulate the physiological and immunological disease characteristics in humans and have required supraphysiological doses of allergen. Dr. Bryce’s group recently has developed an improved murine model that more accurately mirrors the physical and immunological characteristics of food allergic patients. This occurs at an antigen dose that easily can equate to normal dietary quantities and via the oral route. In addition, this model works in standard laboratory strains of mice, which allows investigation of specific gene products through use of genetically modified strains. Finally, this animal model development is a breakthrough for research into the allergenicity of pesticide proteins in food. Thus, utilizing Dr. Bryce’s novel murine model of food allergies, the overall objective of our project is to determine the role of the activating Fcγ receptor, FcγRIII, in the development of allergic sensitization in the gut of proteins such as those found in pesticides and its role in the effector phase of allergic symptoms. Further, we aimed to 1) determine the Fcγ receptor involved in the decreased allergic response in mice given direct administration of allergen-specific IgG, 2) determine the mechanism by which the decrease is mediated in the mice given direct administration of allergen-specific IgG, and 3) determine whether regulatory T cells (Tregs) are responsible for the loss of gut tolerance to food allergens in our model.

 

Mechanisms regulating the enhanced Th2 response in food allergy currently are unclear. During this project, we undertook two major studies. First, our previously published data showed that engaging the activating Fcγ receptor III on DCs by IgG immune complexes (IC) was necessary for Th2 inflammation in a mouse model of asthma. Therefore, we hypothesized that activating FcγRs also plays a role in Th2 sensitization that leads to food allergy. To test this, we used a food allergy mouse model whereby allergen-specific Th2 responses are elicited by the food allergen OVA co-administered with the superantigen Staphylococcus enterotoxin B (SEB). After an 8-week sensitization, systemic Th2 responses upon oral challenge at week 9 with OVA confirmed our earlier study in asthma that global host expression of FcγRIII was required for Th2 sensitization, as symptom scores, blood eosinophils, histamine, OVA-specific IgE/IgG1, and IL-4 from ex vivo OVA-restimulated mesenteric lymph node (MLN) cells were significantly decreased in FcγRIII-/- mice compared to WT. However, because signaling through FcγRIII requires IC, we surprisingly observed that local oral administration of OVA+SEB in the presence of IC (OVA-IC+SEB) during sensitization actually inhibited Th2 responses as compared to OVA+SEB alone. Inhibition was dependent on the inhibitory Fcγ receptor IIb because FcγRIIb-/- mice were resistant to inhibition by OVA-IC+SEB. Production of IL-21, an IgE-suppressing cytokine, was necessary to inhibit Th2 sensitization, as IL-21R-/- mice also were resistant. Therefore, our data suggest that oral administration of allergen-specific immune complexes inhibits Th2 sensitization, and this is dependent on the FcγRIIb inhibitory receptor through an IL-21-mediated mechanism. 

 

Our second study demonstrated that the co-administration of SEB and low doses of food antigen impairs oral tolerance barrier to promote sensitization to the antigen and anaphylaxis upon challenge. Critically, Tregs from sensitized mice exhibit a loss of function in their inability to effectively suppress CD25-T cell proliferation. We hypothesize that this loss of function could promote sensitization processes and permit mast cell degranulation. The adoptive transfer of functional Tregs rescues mice from anaphylaxis, suggesting that Tregs provide a basal level of control over mast cell function in the absence of antigen-specificity.