A novel ovine model to investigate the innate cellular response to food allergens

Food allergy is a growing epidemic, with the highest prevalence rates being reported amongst developed countries. Animal models have been variably successful in determining the mechanisms involved in the allergic pathway, predicting possible triggers and testing novel treatments. This thesis outlines the development and characterisation of an outbred sheep model that displays similar differential susceptibility to atopy when sensitised to food allergens as the human population. Sheep sensitised with peanut (PN) allergens displayed allergen specific-IgE responses and also hypersensitivity wheal reactions to allergens post sensitisation. Of greater significance, was that PN-allergic sheep displayed strong reactivity to two of the main PN allergens, Ara h 1 and Ara h 2, which also feature prominently in the human allergic state. Additionally, of the sheep that displayed high PN-IgE, 80% also displayed an immediate hypersensitivity reaction. This model is also likely to be extremely useful to study the basis underlying atopic predisposition, using technologies and experimental procedures not possible in humans. The innate immune system is usually the first line of defence against infectious agents, however in the case of allergy it inappropriately responds to harmless allergens. It is likely this aberrant response contributes to the establishment of allergic outcomes by over or inappropriate stimulation of adaptive immune cells. We aimed to determine which innate immune cells were recruited to the site of allergen administration and subsequently trafficked to the draining lymph node where the adaptive immune system is first engaged. Whether any of these cells may be involved in the skewing of the adaptive system towards an allergic type-2 response, was a key focus of this research. To determine the trafficking of cells within the lymphatic compartment we injected sheep with PN and ovalbumin (OVA) antigens and compared the immune responses between these allergens to gain further insights into food allergy. By directly cannulating ovine lymphatic vessels, we characterised the innate immune response of skin-draining afferent lymph in sheep following the injection of food allergens. Additionally we also observed the impact of using the adjuvant aluminium hydroxide (AlOH) on cellular recruitment and antigen trafficking. The injection of food allergens caused a dynamic change in cell populations with an immediate inflammatory response at the site of injection, including an early influx of monocytes and neutrophils. At later timepoints there was an increase in antigen uptake and transport by key antigen presenting cells including dendritic cells. Despite the food allergens having differential IgE mediated responses, in our cannulation model they displayed similar innate immune cell recruitment and antigen trafficking, with the presence of AlOH adjuvant enhancing both cellular recruitment and antigen uptake. Of great interest was the observation that sheep with higher levels of steady-state DCs in afferent lymph showed increased monocytic recruitment and reduced PN-specific IgE following sensitisation. DCs from afferent lymph that had ingested PN antigen increased the expression of monocyte chemoattractant mRNA. This suggests that quantitative differences in the DC population may determine the strength of the initial inflammatory response, which in turn may determine the nature of the antigen-specific adaptive response. This correlation was significant for the PN allergen, however not for OVA allergen due to the very low rate of IgE positive sheep following sensitisation with OVA. The work presented in thesis describes for the first time the development of a novel large animal model for the investigation of immune responses to food allergens. Studies using this model provided key insights into how the innate immune system transports food allergens to the lymph node following injection, illustrating cellular migration and antigen uptake both in real-time and in vivo. Together these studies outlined a possible role for monocytes and DCs in establishing the atopy phenotype and supported that our new large animal model has great potential for further investigations on food allergen-associated immune mechanisms.