There are no formal testing guidelines for testing the allergenic potential of test chemicals. Moreover, there are no validated animal models for assessing immediate hypersensitivity. Industrial and academic scientists have developed animal models used to screen for potential allergenicity. These animal models will be discussed in later chapters.
Manufacturers of biotechnology-derived plants and the FDA have devised a strategy assessing the allergic potential of transgenic plants and foodstuffs derived from these plant types (Metcalfe et al., 1996).
There are two courses of action depending on whether the source gene introduced into foods originated in a plant known to be a source of allergens.
Proteins from Source Plants with Known Allergic Potential
If the gene originates in a plant with known allergens, the FDA, as part of its policy on foods derived from new plant varieties, has mandated a series of tests designed to assess the allergic potential of the gene product. To test for the presence of known source allergens in the new transgenic plant (e.g., peanut allergens transfected into soybeans), ELISAs can be performed using pooled IgE from allergic patients with a documented sensitivity to source plant allergens. A positive reaction in the assay gives strong evidence that a source allergen has been transfected into the new plant variety. Consequently, the FDA requires labeling of the product to indicate the presence of the allergen from the source plant.
Additional tests must be undertaken to ascertain whether normal food processing would eliminate the allergenic potential of the protein. For example, if derived oils are the final consumer products and the allergen is a protein, there is little possibility of exposure to the protein allergen. Similarly, normal food processing (e.g., heating or freezing) may destroy the allergenicity.
If the gene product is negative in the in vitro assays, additional tests would be undertaken to confirm the lack of allergenic potential. Subjects sensitive to the source protein would be skin tested with an extract from the new transgenic plant variety using an in vivo prick or scratch test. If positive skin tests are demonstrable, the FDA requires labeling as described previously. If the gene product is negative in both the in vitro and in vivo skin test assays, an in vivo challenge test is used to confirm nonallergenicity. Sensitive and nonsensitive patients would be challenged with the food in tightly controlled, doubleblind, placebo-controlled food challenges (DBPCFCs) (M. Bernstein et al., 1982). When no adverse reactions are observed in the testing protocols, it is assumed that the new plant variety does not express endogenous allergens present in the source plant.
The DBPCFCs are necessary to confirm food allergic reactions but they are not without risk to the patient. Double-blind testing is necessary because of the known discrepancy between subjective observations (e.g., migraine headaches and intestinal symptoms) and the challenge tests (Anonymous, 1997). In addition, the risk of anaphylactic reactions is an inherent danger in testing skin-test-positive patients.
To properly interpret the data from challenge tests, careful consideration should be given to technical and confounding factors (Bindslev-Jenson et al., 1994). Obviously, the form (e.g., raw, cooked, or freeze-dried) and the amount of food necessary to evoke symptoms are critical. The identification and delineation of additive or synergistic factors that influence the data are also important. Select trigger reactions induced by drugs, alcohol, exercise, heat or cold temperatures may be necessary to elicit food allergic reactions in challenge tests.
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