Murine models for evaluating the allergenicity of novel proteins and foods(1)
The guidance from the Food and Agriculture Organization (FAO) and the World Health Organization (WHO) suggested developing in vivo models for assessing the allergenicity potential. Mouse models are favorable since they possess certain advantages compared to other species. Their small size and their short breeding cycle are certainly important. More specifically, their immunology is well-characterized. In particular, IgE is considered to be a critical biological marker of allergenicity of proteins and several strains are inbred high IgE responders. Moreover, the sequence of events resulting in IgE in mice can be briefly described as antigen presentation by dendritic cells (DC) to T helper (Th) cells, Th2-type cytokine secretion and isotype class switching to from IgG to IgE in B lymphocytes.
The presence of adjuvant plays also an important role by stimulating the immune system. IgE levels, for example, were found to be higher in a mouse model of food allergy when alum was used as an adjuvant while IgG1 antibody levels did not differ with or without alum. The mucosal adjuvant CT induced Th2 cells and IL-4 secretion in mice and caused biased results in a few models. Lipopolysaccharides (LPS) or endotoxins are important compounds with regard to the immunity status. For most strains of animals, LPS may alter immune responses through Toll-like receptor 4. It is possible that the different re- sponses to an allergen are a result of varying levels of endotoxins.
IgE and IgG1 are the only isotypes that potentially cause the anaphylactic reactions in mice. IgE is the most commonly used marker of allergenicity. In animal models, total or specific IgE is very often measured by ELISA. A strong correlation between total and allergen-specific IgE responses in different mice species has been shown during both primary and recall immune responses.
Th2 cells play a role both during the sensitization and during the effector phases of the allergic inflammatory response. IL-4 is the main cytokine released from Th2 cells and it is also critical for the development of the allergic reaction together with IL-13. Another Th2-type cytokine, IL-5 is also often used as an allergy marker. Mast cell granule neutral proteases in peripheral blood could be used as a specific marker of mast cell activation after an allergic response.
This state of non-responsiveness of the immune system to soluble antigen depends on the dose, exposure frequency and the nature of the antigen. Therefore, use of an adjuvant e.g. cholera toxin (CT) is fairly common when this route of exposure is chosen. In an oral anaphylaxis model, IgE antibodies to the major peanut (Arachis hypogea) allergens Ara h 1 and Ara h 2 were detected in the serum of female C3H/HeJ mice. Moreover, regulatory T lymphocytes (CD4+CD25+ T cells) play an important role in the control of tolerance induction and regulation of the intensity of allergic responses to peanuts. The importance of strain selection was strongly emphasized in an oral sensitization model. In particular, C3H/HeJ mouse strain has a propensity for high IgE production compared to BALB/c. Intraperitoneal (i.p.) sensitization has been shown to be a reliable, specific and sensitive method for inducing IgE antibodies compared to oral exposure.