Dr. Bettina Wagner
The term ‘allergy' is commonly used to refer to several types of diseases that are caused by different forms of hypersensitivity reactions. Hypersensitivity immune responses directed against harmless antigens subsequently causing damage to host tissues. Depending on the immune mechanisms involved, hypersensitivity reactions are clustered in four types (I-IV). Type I hypersensitivities are mediated by a particular class of antibody known as Immunoglobulin E, or IgE. We will use the term ‘allergy' in this proposal to refer to type I hypersensitivities.
In the horse, only a few epidemiological studies about allergy exist in selected breeds. This reflects in our opinion the current difficulties to diagnose allergy rather than its absence. Most allergies are diagnosed by clinical signs only. The most common diagnostic test to identify allergic responses is intradermal skin testing with allergens, which is, however, mainly performed in larger clinics or universities, but rare in the field.
Several diseases have been associated with IgE mediated allergy in the horse including skin hypersensitivities, urticaria, chronic obstructive pulmonary disease (recurrent airway obstruction) and head-shaking. The currently best characterized disease regarding etiology and pathogeneses is a skin hypersensitivity, called summer eczema (Figure 1). Although the disease has been described often in Icelandic horses, it occurs in several horse breeds, including Thoroughbreds, Warmbloods, Draft horses, Quarter horses and ponies. Summer eczema has been found all over the world including North America and is induced by biting midges of Culicoides species.
Figure 1: Horses affected with summer eczema, an allergic reaction induced by midges of Culicoides spp . injecting the allergen during the bite. Initial clinical signs are massive discomfort, extensive scratching and hair break, typically at the mane and tail, but also at other sides of the skin. This is followed by loss of hair and skin irritations, which are often complicated by secondary bacterial infections.
Although clinical signs and intradermal reactivity suggested to allergic pathogeneses in most of the diseases mentioned above, no direct proof existed that IgE mediates allergy in the horse. In the Zweig grant that preceded this proposal, we found the first direct evidence that IgE mediates allergy in horses. This was performed by transfer of allergic reactions form allergic to healthy horses via IgE (Prausnitz-K ü stner reaction). In addition, we tested for equine immunoglobulins which can mediate skin reactions by intradermal injection of anti-isotype antibodies. Skin reactivity was observed after injection of anti-IgE confirming the results that IgE mediates allergy in horses. Unexpectedly, a reaction was also found after anti-IgG(T) injection. Antibodies to horse IgGa or IgGb induced none or only very weak skin reaction. The finding that IgG(T) can mediate skin reactivity is novel. The data were presented during the Zweig Memorial Fund Poster Session on June 11 th , 2005 . A manuscript describing these results is in preparation. Erin Morgan, who is a senior at Cornell University worked on a honor's thesis in this project. Her honor's thesis will include the ELISA results to determine allergen-specific IgE in serum of allergic horses and their comparison with skin testing results. These experiments are still ongoing and will be finished within the next three months.
In contrast to many infectious diseases allergy develops slowly. Before clinical signs of allergy become obvious, many immunological processes occur without any signs of disease. The crucial early event in the development of allergy is the generation of IgE producing long-living B lymphocytes (plasma cells). Allergic individuals produce IgE antibodies against harmless environmental antigens, called allergens. Allergen-specific plasma cells rest in lymphatic organs and can secrete large amounts of IgE antibodies over several years and in the complete absence of allergen. The IgE antibodies are subsequently bound to surfaces of mast cells by high affinity IgE receptors. The arming of mast cells with IgE is called ‘sensitization' (Figure 2). The sensitized mast cells are silent and do not cause any signs of allergy. They can sit in tissues for a long time and are prepared to react immediately. As soon as the allergen binds to receptor-bound IgE antibodies, stimulation of mast cells and release of inflammatory mediators occurs within minutes and results quickly in clinical signs of allergy (e.g. bee sting or drug hypersensitivities, certain food allergies, rhinitis and asthma). Because of these characteristics of IgE production and mast cell sensitization allergic reactions can occur immediately in the presence of allergen even in individuals which were asymptomatic over years in the absence of allergen.
Figure 2: Model of mast cell sensitization and degranulation in skin hypersensitivity of the horse. The model is based on our previous findings that both IgE and IgG(T) can induce immediate skin reactions. Mast cell sensitization with allergen-specific antibodies precedes allergy sometimes for several years. The allergen, here originated from midges and injected with the bites, crosslinks the antibodies on the mast cell surface. This stimulates immediate degranulation. The release of inflammatory mediators in the tissue is responsible for the clinical allergic signs. The causal event leading to allergy is the formation of allergen-specific IgE, and in the horse may be also of IgG(T) antibodies.
Current treatments of allergic diseases include allergen avoidance, anti-allergic medications, and immunotherapy for specific allergens, also known as hypo- or desensitization. The current treatments reduce the clinical symptoms but do not cure the allergic disease. Anti-allergic medications need to be applied over the entire time the allergen is present and often cause side-effects. Some success was reported by hyposensitization in the horse. However, this is a non-standardized injection of crude allergens with almost no knowledge about the mechanisms involved and can also cause an increase of the disease. A causal therapy would require the inactivation of allergen specific long-living IgE secreting plasma cells which is difficult to perform. Another way to avoid allergy could be preventive vaccination before clinical allergy develops and which induces protective antibodies rather than allergy mediating IgE.
Our finding that anti-IgG(T) antibodies induced a skin reaction resulted in the hypothesis that IgG(T) as well as IgE can sensitize mast cells and mediate allergy in horses (Figure 2). We further hypothesize that IgG(T) could influence the early development of allergy by sensitizing mast cells and initiating the allergic response in the absence of IgE. The hypotheses would be investigated in this proposal by two specific aims:
Aim 1 will test if IgG(T) can mediate allergy in the horse. We would induce a Prausnitz-K ü stner reaction using IgG(T) of allergic horses and compare the skin reactivity induced by IgG(T) and IgE.
Aim 2 will test if IgG(T) precedes IgE in arming of mast cells and when the sensitization with IgE takes place. We have recently found that horses do not produce IgE for their first year of life. In contrast, foals do start to produce IgG(T) already a few weeks after birth. In this aim we would test whether IgG(T) is a useful cellular marker for sensitization in young horses between 6 months and 3 years of age. We would also compare the immune mechanisms mediated by IgE and IgG(T) over this period of time.
The goal and expected outcome of the project are to contribute to an advanced understanding of the mechanisms leading to allergy in horses and to identify a time window for preventive treatments of sensitized horses before allergen-specific IgE production and clinical allergy develop.