Insect bite hypersensitivity (IBH) or summer eczema (SE) is a dermatitis of horses, caused by IgE mediated hypersensitivity reaction, to Culicoides spp. (biting midges) not indigenous to Iceland (Schaffartzik et al., 2012).
All breeds of horses can be affected, but Icelandic horses born in Iceland and exported to the continent are more strongly affected than most other breeds (Bjornsdottir et al., 2006, Torsteinsdóttir et.al 2018). Since the year 2000 there has been an ongoing collaborative Summer Eczema project between Keldur and the University of Berne, Switzerland.
The aim of the summer eczema project is to find, and characterize, the allergens causing IBH and analyse the immune response that leads to the disease and the final aim is development of immunotherapy against insect bite hypersensitivity.
IBH is a Th2 type, IgE mediated allergy with an imbalance between Th1, Th2 and Treg (Hamza et al., 2007; Hamza et al., 2010; Hamza et al., 2008; Heimann et al., 2011). It is therefore important to use Th1/Treg focusing adjuvants or agents to induce and/or rebalance the Th1:Th2:Treg responses and restraint the inflammatory mechanisms. The route of injection has also to be considered and we have shown that injecting directly into the lymph nodes with the appropriate adjuvant is a promising option (Jonsdottir et al., 2015; Jonsdottir et al., 2016). Vaccination with purified allergens in Alum/monophosphoryl lipid A (MPL), but not in Alum alone, resulted in a Th1/Treg focused immune response. The IgG antibodies induced efficiently blocked the binding of IgE from IBH horses to the allergens (Jonsdottir et al., 2016). It has been demonstrated that allergen-specific IgG antibodies that compete with IgE for binding to the allergens are one of the major mechanisms in successful immunotherapy.
The intralymphatic vaccination with purified allergens in Alum/MPL was reinforced in more horses and compared to subcutaneous vaccination a more practical injection route. (Stefánsdóttir et al., manuscript in preparation).
In order to test whether the vaccination protects the horses from developing insect bite hypersensitivity a challenge experiment has to be performed where the vaccine is tested under real conditions.
Twenty-seven horses have been vaccinated at Keldur intralymphatic three times with 4 week interval with nine purified allergens in Alum/MPL. The horses were transported to Switzerland 16th of March 2020. They will be kept in Culicoides areas and be naturally exposed to the midges for 3 summers. No protection is allowed. The horses will be monitored, blood samples taken on a regular basis and clinical examination performed to look for evidence of IBH supervised by Dr Eliane Marti Vetsuisse Faculty, University of Bern. For more information see Horses of Iceland.
Unvaccinated control horses were not exported at the same time as it has be shown that at least 50% of horses exported from Iceland to Culicoides infested areas and not protected develop IBH (Björnsdóttir et al., 2006; Torsteinsdottir et al., 2018).
Non-injection routes of allergen specific immunotherapy are becoming more and more applicable such as through mucosal surfaces of the mouth. The sublingual route has been established and marketed for humans as a valid non-invasive alternative to the subcutaneous route (Moingeon & Mascarell, 2012). Three recombinant allergens have been expressed in barley in collaboration with ORF Genetics (Magnusdottir et al., 2013). Allergens have been expressed in barley and shown to be comparable with E. coli and insect cell produced allergens in immunoassays (Jonsdottir et al., 2018). A method to treat horses with recombinant barley flour via the oral mucosa was developed. Treated healthy horses developed an IgG response able to partly block IgE-binding to the allergen. (Jonsdottir et al., 2017). Our collaborators at Cornell University, Ithaca, New York own a group of Icelandic horses many of which are affected with IBH. Having all the horses located in the same environment as compared to horses from different owners spread over multiple locations provides a unique opportunity to perform an experimental barley treatment. Orf Genetics is presently expressing four additional allergens suitable for the affected horse in Ithaca. They will hopefully be orally treated with recombinant barley 2020.
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