The cholinergic tone as a modulator of Food Allergy

In healthy individuals, the physiological response towards dietary antigens in the gastrointestinal tract is one of immunological non-responsiveness and referred to as oral tolerance. Regrettably, in certain individuals with genetic or environmental predisposition (altered microbiota, viral or bacterial infection, chemicals additives or pollution), a break in oral tolerance occurs resulting in sensitization to allergens. Continues re-exposure to these allergens will induce and potentiate an aberrant type 2 immune response which will ultimately result into the typical symptoms of food allergy. Food allergy has been on the rise in westernized countries for several decades now, with numbers reportedly reaching up to approximately 10%. Despite considerable advances in recent years, the pathophysiology of food allergy still remains incompletely understood, largely explaining why currently no effective treatment is available. While the exposure to allergenic foods only results in mild symptoms in some patients, others will experience potentially life-threatening symptoms like anaphylaxis. Given the serious risks it poses to the public health and its rising prevalence, there is a growing need to devise new treatment approaches for food allergy. It is clear that the intestinal immune system's ability to discriminate and appropriately react towards danger or non-danger signals is crucial to prevent aberrant immune reactions against harmless antigens. In recent years, the vagus nerve has emerged as an important modulator of the intestinal immune system. It has the ability to fine-tune local inflammatory responses via the so-called "inflammatory reflex" which is also referred to as the cholinergic anti-inflammatory pathway (CAIP). Evidence for this concept in the gastrointestinal tract was provided by showing that vagus nerve stimulation (VNS) reduced surgery-induced inflammation of the intestinal muscle layer. This effect was mediated through the activation of enteric cholinergic neurons (cholinergic tone) interacting with muscular CX3CR1hi macrophages via nicotinic acetylcholine receptors (α7nAChR). Moreover, it was shown that mice with a reduced cholinergic tone fail to develop oral tolerance, clearly indicating the importance of the cholinergic tone in inducing and preserving oral tolerance. Considering that food allergy is typically associated with a break in oral tolerance, it is tempting to speculate that alteration of the cholinergic tone may affect the induction and course of this disease. Therefore, in this thesis, we aimed to investigate a possible role for cholinergic modulation in an experimental model of food allergy. Murine models of food allergy have been around for more than a decade now and have provided many valuable insights into allergic inflammation mechanisms. However, current studies failed to provide us with sufficient insight into the underlying inflammatory response of food allergy. Therefore, in chapter 3, we aimed to get a better characterization of the local intestinal inflammatory response in an experimental model of food allergy.Our study revealed a central role for mast cells in the pathology of food allergy. Mast cells were already known for their role as effector cells releasing harmful mediators resulting into the typical allergic symptoms. However, we found that mast cells were also acting as immune modulators, orchestrating IL-4 signaling and amplifying the underlying immune response. In response to allergen challenge, mast cells were identified as a major source of IL-4, a key cytokine indispensable for the development of allergic responses. Moreover, we found that mast cells were able to increase the activation state of other IL-4 competent cells, such as eosinophils and type 2 innate lymphocytes. In light of these results, our study confirms that mast cells should be considered as an important therapeutic target in food allergy patients. In chapter 4, we investigated the potential role of cholinergic modulation in our experimental model of food allergy.To this end, we evaluated the impact of both a reduced and increased vagal input by means of vagatomy (VGX) and VNS, respectively. Considering the importance of the vagus nerve in the development of oral tolerance, we hypothesized that loss of vagal input could increase the susceptibility and severity of food allergy. Surprisingly, the effect of VGX was rather limited with no statistical differences between VGX and control mice. These results imply that the basal vagal tone is probably redundant under these inflammatory conditions. However, this does not necessarily exclude the therapeutic potential of VNS. Hence, our next step was to evaluate whether potentiation of the anti-inflammatory properties of the vagus nerve through VNS could decrease susceptibility and severity of food allergy. We were able to confirm this hypothesis and showed that application of VNS was able to efficiently dampen the inflammatory response and improve disease outcome. Interestingly, VNS had a significant impact on mast cells as indicated by their reduced numbers and activation. In addition, we found that VNS affected the phagocytosis of allergen by certain immune cells. When the vagus nerve was stimulated, more CX3CR1hi macrophages were phagocytosing allergen at the cost of eosinophils. This is particularly interesting as both macrophages and eosinophils are able to present these antigens, each provoking a different immune response. Macrophages typically promote oral tolerance, while eosinophils are known for promoting type 2 responses. Taken together, our results have uncovered an anti-inflammatory effect of VNS in food allergy. We propose that dampening of mast cells could represent one of the major mechanisms by which VNS ameliorates food allergy. Further investigation into the underlying mechanism might point towards new treatment options for this immune mediated disorder. In conclusion, we have demonstrated the therapeutic potential of VNS in food allergy. Importantly, this is the first evidence that the CAIP has a role in modulating type 2 mediated immune disorders, as it was previously only investigated in disorders characterized by innate an Th1/Th17 immune responses, therefore expanding the applicability of VNS as a therapeutic tool to an even wider range of immune disorders. Moreover, our findings further highlight the intricate relationship that exists between the immune system and the nervous system, and the possibility to modulate this relationship to our advantage. Further studies unravelling the exact underlying molecular mechanism and identifying the target cells will be crucial to provide important insights for the development of novel therapeutic strategies that will not only benefit food allergy but also other inflammatory conditions.status: publishe