EDITORIAL COMMENTS Nurrition Vol. 12, Nos. 1 l/12, 1996

Contrast Between Immunologic and Nutritional Properties of Food Proteins

Food sensitivity-an adverse, harmful reaction to a food or food ingredient-affects many children and adults from time to time; reactions range in severity from trivial to life-threaten- ing and can be attributed to any of a number of disease mecha- nisms-toxic, biochemical, pharmacologic, immunologic, psy- chological.‘.* Food sensitivities are a matter of concern, not only to the affected individuals and the medical profession, but also, in light of questions about food safety, to nutritionists, the food industry, and government.

Food allergy is a form of food sensitivity caused by an abnormal immunologic reaction to one or more antigens of a food. The term is most commonly used to describe reactions that involve IgE antibody and mast cells, and thus occur pre- dominantly in atopic individuals who may also suffer from atopic eczema, asthma, rhinitis, hay fever, or allergic urticaria. Celiac disease is a quite different form of immune-mediated food sensitivity; there is good evidence that its pathology results from activation of gluten-sensitive T cells in the gut mucosa.

ANTIGEN RECOGNITION BY T CELLS AND B CELLS

T and B lymphocytes recognize and combine with different regions of antigenic protein molecules. Antigens are three-di- mensional structures and there are usually many peptide regions of the native molecule to which antibodies can bind. Sometimes these are concentrated in particular “immunodominant” re- gions on the outside of the molecule. Digestion of a food protein in the gut may also reveal new antigenic determinants that have been hidden deep within the native molecule. The key point to note is that antibody and B cells recognize antigens, in solution or on cell surfaces, in their native conformation.

On the other hand, it has been known for many years that T cells recognize both native and denatured antigen, indicating that it is primary sequences of peptides that are important for induction of cellular immunity and for other T cell functions. The T cell receptor sees only a small fragment of the original antigen, which has been internalized by antigen-presenting cells, partially degraded by proteolytic enzymes, and then car- ried back to be presented at the cell surface in physical associa- tion with a major histocompatibility complex (MHC) molecule. The optimum size of peptides for antigen presentation is 8-24 amino acids, a size that fits well in the groove of MHC mole- cules. Although different MHC molecules can interact with the same antigen, they will present it to T cells in slightly different ways. Thus, an individual’s genetic make-up as well as environ- mental factors will determine which polypeptide sequences from a single protein his or her T cells can recognize, which types of T cells are preferentially stimulated, and, ultimately, whether the immune response generated is appropriate or harm- ful-IgE and cell-mediated immune responses to foods being examples of the harmful type of response.

PROTEIN ANTIGENS AND IgE ANTIBODIES

The majority of allergens (i.e., substances that provoke the formation of IgE antibodies) are proteins or peptides, and there

is abundant evidence from animal experiments and human clini- cal studies that exquisitely small amounts of material can induce an IgE antibody response (often described as sensitization) when injected into, or when inhaled or eaten by an atopic person. Allergic infants have been reported to react on first known exposure to a food such as milk, fish, or peanut, and it is quite possible that sensitization to a food eaten by the mother can occur. In other words, intact antigens cross the mother’s gut mucosa and pass via the placenta into the tissues of the fetus, or are secreted into the mother’s breast milk and cross the infant’s gut mucosa to gain access to the infant’s immune apparatus. The foods most often implicated in food allergy are milk (mainly in infants), eggs, fish, peanuts, other nuts, shell- fish, soya. and wheat.

Allergic reactions may occur after only a small amount of a substance has been ingested by a previously sensitized person, and are completely unrelated to any physiologic or nutritional properties of the food. The central event is mast cell degranulation, which occurs when a divalent antigen bridges two IgE antibody molecules on the surface of a sensitized cell. This results in local or systemic release of chemical mediators including histamine, which have a variety of clinical effects including urticaria, other skin rashes, rhinitis, asthma pulmonary edema, vomiting, diarrhea, angio-edema, and the potentially fatal reaction of anaphylaxis. In most affected individuals this IgE-mediated reaction predominates, but, particularly in infants, there may also be IgG-mediated im- mune-complex and T cell-mediated delayed-type hypersensitivity reactions, leading to more chronic illnesses such as food-sensitive enteropathy with malabsorption, and food-sensitive colitis with bloody diarrhea.

Anaphylaxis may develop within minutes, and effects vary in severity from mild pruritus (itching) and urticaria to shock and death. Classically, anaphylaxis has been recognized as resulting from exposure to drugs (e.g., penicillin, streptokinase, insulin, tetanus antitoxin), contrast material used for x-rays and other imaging tests, insect stings, venomous reptile bites, and skin expo- sure to many plants and chemicals. Only recently has the frequent occurrence of food anaphylaxis also been recognized.

In 1988, Yunginger and colleagues’ reported on seven fatal cases of food-induced anaphylaxis. These workers had been contacted by coroners and medical examiners in the United States, because they were able to test postmortem blood for IgE antibodies. The patients they described were aged 11 to 43. Five were known to have had asthma, but the medical histories of the other two were not available. The foods respon- sible were peanut in four cases, and one each of crab, cod, and pecan. In all cases the person was aware of his or her food allergy; six had eaten the food inadvertently and the remaining patient ate a crab salad when he was drunk. None of the seven had been immediately treated with adrenaline.

In 1992, Sampson and colleagues4 described six fatal and seven near-fatal cases of food-induced anaphylaxis occurring in children and adolescents, seen in a 14-mo period in hospitals

Nutrition 12:817-823, 1996 OElsevier Science Inc. 1996 Printed in the USA. All rights reserved ELSEVIER 0899.9007/96/$15.00

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of three American cities. Again, all suffered from asthma; in all cases it was known that the child was allergic to the food, and the child had unknowingly ingested it. In the fatal cases the foods responsible were peanut (three cases), cashew (two cases), and egg (one case). In the nonfatal cases the foods were milk and hazelnut (two cases each), and one each of peanut, brazil nut, and walnut. All but one of the patients who survived had received an injection of adrenaline within 30 min of allergen ingestion, whereas, although the six patients who died all had symptoms within 3-30 min, only two had received adrenaline in the first hour.

Patients with any form of food allergy need to be constantly vigilant about all foods they eat. There will be many foods that they eat regularly and therefore know to be safe for them. However, there is a small but definite risk that they will react to any food that is not a regular constituent of their diet, or which they have not eaten for some time.

There appears to have been a striking rise in the incidence of peanut allergy recently, including several fatal cases in the United Kingdom, and studies of the immunologic properties of peanut proteins are being actively pursued by several research groups. Their results are giving considerable insight into ap- proaches that are likely to prove fruitful in studies of the dis- ease-provoking proteins in other forms of food allergy.” Blood samples from patients known to have severe allergic reactions to peanuts contain antibodies of the IgE type, which can react with more than 30 different peanut proteins in Western blots. Some are of low molecular weight and have not yet been fully characterized. One important allergen has been identified as an acidic glycoprotein of 20-30 kDa molecular weight. Two other relevant allergens are also glycoproteins, Ara h-1 (molecular weight 63 Kd) and Ara h-11 (molecular weight 17 KD) . These proteins are heat-stable and therefore will be unaffected by the roasting process or cooking.

Many people with allergic tendencies have IgE antibodies to a wide range of harmless environmental substances and foods to which they do not react clinically. Thus, positive antibody tests cannot be used alone to diagnose allergy. Clinical sensitiv- ity (in some patients shown by double-blind placebo-controlled challenges) is more relevant. This also means that, even in the well-characterized disease of peanut allergy, it cannot be assumed that all the proteins that are targets of serum IgE antibodies are actually disease-provoking; some of the antibod- ies may be epiphenomena. The final diagnosis will be based on careful correlations between serology and clinical sensitivities, backed up by in-vitro tests, e.g., using the patient’s own blood basophils.

INDUCTION OF T CELL-MEDIATED IMMUNITY-THE EXAMPLE OF GLUTEN

The fact that certain cereal proteins are toxic to celiac pa- tients has been recognized for 40 y. For most of that time, laboratory and clinical investigators have had to face major logistic problems with the preparation and handling of these large, complex molecules, which are insoluble in biological fluids. There is an enormous literature on competing methods for purification, separation, digestion, and, of course, describing various derivatives of wheat gluten/gliadin. When these prepa- rations have been used for immunologic research, the results have often been difficult to interpret, since the measured proper- ties of a harmless, non-immunogenic substance will have been

EDITORIAL COMMENTS

profoundly affected by even a trace of contamination by immu- nogenic material.

Today, the methods of structural biochemistry and molecu- lar medicine are being used to describe the precise structure of individual gliadin molecules; to prepare, by synthesis rather than by degradation, gliadin peptides for immunogenicity test- ing; and, by applying the approaches used for other proteins, to examine the precise interactions, at atomic level, between gliadin antigens and antibodies or T cell receptors.6

RELEVANCE OF ANIMAL DATA TO HUMAN FOOD SENSITIVITIES AND ALLERGY

In immune-mediated food intolerance (food allergy/food hypersensitivity) there are harmful clinical reactions to the pro- voking substance, abnormal immune reactivities to the food or food ingredient, and a plausible mechanistic link between the immune response and pathology. The examples above illustrate some of the sophisticated and highly productive new lines of investigation that are now feasible. They also support the view of most immunologists that the terms “hypoallergenic” and “hypoantigenic” are essentially meaningless. The annual dose of pollen inhaled by a person with hay fever amounts to only about 1 bg. In a highly sensitized individual, exposure to a few dozen molecules of a relevant food antigen may trigger an allergic reaction. Inadvertant ingestion of half a peanut has been fatal. If a food contains peptides with a length of eight or more amino acids, imunogenicity may exist.

Milk-protein antigens recognized by the highly avid IgG antibodies in hyperimmune rabbit sera are most unlikely to correspond to the antigens that will induce IgE antibodies or T cell activation in a milk-allergic infant. I caution against overinterpretation, in relation to human health, of data ap- pearing in one article in this issue on immunogenicity of hy- drolyzed milk proteins.7 The parallel data on nutritional ade- quacy of intact and hydrolyzed protein diets are, however, reas- suring and of considerable practical relevance.

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ANNE FERGUSON Professor of Gastroenterology

University of Edinburgh Edinburgh, UK

REFERENCES

David TJ. Food and food additive intolerance in childhood. Oxford: Blackwell Scientific Publications, 1993 Metcalfe DD, Sampson HA, Simon RA, eds. Food allergy. Adverse reactions to foods and food additives. Oxford: Blackwell Scientific Publications. 1993 Yunginger jW, et al. Fatal food-induced anaphylaxis. JAMA 1988; 260: 1450 Sampson HA, Mendelson L, Rosen JP. Fatal and near-fatal anaphy- lactic reactions to food in children and adolescents. N Engl J Med 1992; 327:380 Burks AW, Cockrell G, Stanley JS, et al. Recombinant peanut aller- gen Ara hI expression and IgE binding in patients with peanut hyper- sensitivity. J Clin Invest 1995;96:1715 Ferguson A. Coeliac disease research and clinical practice-main- taining momentum into the twenty-first century. In: Howdle PD. ed. Clinical Gastroenterology, Coeliac Disease. London: Bailliere Tindall, 1995:395 Cezard JP, Zarrabian S, de Week AL, et al. Antigenicity and nutri- tional value of selected milk proteins and their hydrolysate in grow- ing rats. Nutrition 1996; 12:788