Building Tolerance : T cell epitopes as a treatment for cow's milk allergy

Abstract

As there is no curative treatment for cow’s milk-allergic patients, patients are advised to avoid allergen exposure to prevent allergic symptoms. Because many products contain cow’s milk protein, diet choices are limited and there is a risk for accidental exposure. Prevention and/or treatment strategies are thus highly desirable. Allergen-specific immunotherapy, which is available for inhalation allergens, has been investigated as a therapy for food-allergy. However, these studies indicated that the risk/benefit ratio of this therapy, in which patients are exposed to increasing doses of allergens, is too high. One of the features of immunotherapy is an increase in allergen-specific IgG antibodies. These antibodies can inhibit allergen-IgE complex formation and thereby IgE-facilitated antigen presentation by B cells. However, cow’s milk- and peanut-allergic patients have naturally high allergen-specific IgG levels and are still allergic. We showed that these IgG antibodies are involved in allergen-antibody complex formation. The presence of IgG antibodies in the complexes induced complement activation and thereby influenced the receptors involved in complex binding to B cells. This may result in altered antigen presentation or allergen transport to lymph nodes. A disadvantage of immunotherapy is that exposure to allergens not only induces tolerance but can also induce allergic symptoms. T cells are important for tolerance induction. Therefore, using peptides that are too small to induce mast cell activation but contain intact T cell epitopes may be a safe alternative for conventional immunotherapy. Whether hydrolysates of whey, a major allergen in cow’s milk, could be used for tolerance induction was analysed by determining the time point at which these hydrolysates are still able to induce T cell proliferation but are unable to cross-link IgE and thus induce allergic symptoms. A clearcut time point could not be determined. Moreover, increasing the time of hydrolysis reduced both the allergenicity and immunogenicity of the whey hydrolysates for most patients but not all patients. This suggests that whey hydrolysates may be helpful for tolerance induction, but may not be suitable for all patients. T cell epitopes were determined in the major whey proteins, alpha-lactalbumin and beta-lactoglobulin, using human cow’s milk-specific T cell lines and 18-amino-acid-long synthetic peptides spanning the proteins. Whereas for beta-lactoglobulin several T cell epitopes were found, no major epitopes could be identified for alpha-lactalbumin. In general, alpha-lactalbumin was less immunogenic than other major cow’s milk allergens. Interestingly, prophylactic oral treatment with synthetic peptides containing T cell epitopes of beta-lactoglobulin was shown to reduce the acute allergic response in a mouse model for cow’s milk allergy. This effect may involve regulatory dendritic and T cells. Moreover, a specific mixture of non-digestible oligosaccharides may increase the tolerance-inducing effect of the peptides. Also in a curative setting, these peptides reduced the allergic response, although the results were variable and seemed to be influenced by the degree of sensitization. Further optimization of the therapy is thus necessary. In summary, this thesis shows that synthetic peptides and partial hydrolysates of whey may be useful tools to induce tolerance to cow’s milk when applied in preventive or curative treatment