Intérêt des allergènes recombinants dans le diagnostic de l’allergie alimentaire

To date, neither in vitro nor in vivo tests can establish with reliability the diagnosis of food allergy. The availability of recombinant allergens (RA) has led to improvement in the standardization of allergenic extracts and enrichment of natural extracts, resulting in more sensitive screening tests. These biotechnological advances facilitate the diagnostic approach which now rests on an individual reaction profile (component resolved diagnosis) with well-characterized allergens classified on a molecular basis. Development of diagnostic tests using RA or peptides expressing some distinctive epitopes of interest may improve the prediction of severe and/or persistent food allergies and guide the choice of the therapeutic measures that follow.     

Allergies croisées : de la théorie à la pratique

The notion of allergic cross-reactions must from now on be regarded by allergists as an indispensable element of the diagnostic and therapeutic approach. Indeed, knowledge of proteins and of mechanisms involved in this type of phenomenon is of interest to physicians not only in with regard to pathophysiology but also as an aid in interpretation of certain clinical history data and the results of skin or biological tests. Moreover, it makes the management of patients more rational, and probably in therapy of the future when recombinant allergens will be more widely used.     

Allergènes rares de l’environnement intérieur

Allergens rarely present in the indoor environment are not sufficiently recognized. The prevalence of sensitization to these allergens depends on the geographic and climatic characteristics of the habitat as well as on the lifestyle of the inhabitants. Their sources are diverse: allergens of animal (mites, other insects, mammals, etc.) or vegetal origin. Sensitization to rare allergens should be established by a well-documented clinical history, by immunological tests and by the effects of avoidance. A general review of rare allergens and/or some new airborne allergens present in the indoor environment will be presented.     

Histoire de l'allergie alimentaire : des précurseurs à l'histoire contemporaine

Food allergies are an important public health problem. Relatively unheard of during the 1970–1980 s, they had almost always been associated with the same allergens (cow milk, chicken eggs, and fish). Since then, they have become more frequent and more varied, involving about 4% of the general population of all ages; the prevalence may even be 5–6% or more among children. They are also becoming more and more severe, and interfering more in daily living. Beginning in the 1980 s, the story of food allergy has been marked by the explosion of the occurrence of peanut allergy, the prevalence of which has at least doubled over the past five years. Another important tendency has been an increase in the frequency of allergy to shelled fruits (e.g. exotic nuts) and to certain plant allergens (e.g. sesame, buckwheat and wheat). There has also been an increase in the number of near-fatal and fatal cases of food-related anaphylaxis, justifying the establishment of a monitoring network. The workup of food allergies has become more standardized, allowing a definitive diagnosis to be more easily established, thus justifying avoidance of the responsible substance. Indeed, until recently, in the absence of effective preventive measures and a good risk-benefit ratio, the prevention of food allergies depended on avoidance of the responsible food(s), which recommendation is nevertheless often not followed as prescribed. Other preventive measures (antihistamines, corticosteroids, adrenalin auto-injectors) are then adopted to avoid recurrences, which can be considered as evidence of treatment failure. Recent years have been marked by standardization of preventive measures based on the Project of Individual Care and by the establishment of educational protocols. The future may see the development of specific immunotherapy (until now, difficult and dangerous), modification of food allergens, and treatments that block IgE-dependant allergic reactions. For example, the preliminary results of a clinical trial of sublingual immunotherapy with hazelnut appear to be promising. Considering the difficulty of diagnosis and prevention, the social and psychological repercussions of food allergies are considerable for both children and their families.     

Intérêt des allergènes recombinants pour la prise en charge de patients allergiques : cas cliniques

Allergen extracts used in diagnosis by clinicians are manufactured from crude source materials. These extracts contain a number of allergenic proteins and their composition can vary. The employment of molecular biological methods in the field of allergy has led to the production of recombinant allergens from a variety of sources. Their use has allowed us to improve our understanding of allergenic proteins, to define families of allergens, to explain allergenic cross-reactions, and to show that pollen sensitivity profiles vary as a function of the geographic region in which individuals live. As a result, in the 21st century reasoning on the molecular scale should be included in patient management. In 2005, the clinician has at his disposal the level of specific IgE with 27 recombinant allergens from 7 different allergenic sources. The use of these materials can, in certain cases, allow us to have a more precise diagnosis and to orient the therapeutic decision more accurately. Three observations made in our consultation service are described to illustrate the contribution of these new diagnostic methods.     

Les allergènes recombinants : leur apport à l’allergologie en 2006

Advances in molecular biology techniques have led to the production of recombinant allergens, about thirty of them now being available for measurements (DIAGNOSIS?) in vitro. These recombinant allergens correspond to a precise molecular variant of a natural allergen, and their biological activity has to be evaluated in comparison with the corresponding natural allergen. The advantages of recombinant allergens are essentially the creation of allergenic preparations having constant pharmaceutic properties, which allows determination of specific IgE directed against different molecular components of an allergenic source, for example, pollen, mites, etc. The main consequences of these biological advances are the following: evaluation of sensitivities to allergen molecules in different populations (molecular epidemiology), improvement of extracts used for diagnosis by selection of the most pertinent allergenic sources and in quantifying their major allergen content, definition of the spectrum of recognition of specific IgE vis-à-vis different molecular components (spectrotype), quantitative evaluation of IgE responses, establishing the molecular basis of cross-reactions between different inhaled allergens, between different food allergens, and between inhaled allergens and food allergens. As regards allergy practice, this new diagnostic tool can lead to better interpretation of polysensitivities, observed by skin tests and in vitro tests. Some examples of particular clinical cases associated with specific sensitivities vis-à-vis certain recombinant allergens will be presented.     

Allergènes alimentaires croisant avec les allergènes des pollens

The range of pollen-food cross-reactions has increased over the past decade, the clinical pictures are more clear with respect to the allergens concerned, and the molecular basis for some of them have been determined. Diagnostic methods include skin tests, assays for specific IgE, and open and double blind oral provocation tests. Investigation of allergic cross-reactions, first based solely on immunological inhibition techniques using natural allergen extracts, have benefited from molecular biology. Many allergens homologous with pollen allergens have been sequenced and the three dimensional structure of Pru av 1 has been determined, allowing studies on a sub-molecular scale. Allergen cross-reactions between pollen and food, for which the clinical relevance is well established, involve allergens of the Bet v 1 and Bet v 2 families. These allergens are present in numerous edible fruits and vegetables. Identity with the Bet v 1 sequence varies from 38 to 67%, being closest for the profilins (70-80%). Cross sensitization with Bet v 1 and profilins, although particularly frequent, may be silent clinically. Other candidate molecules involved in cross-reactions between pollen and food allergens are Bet v 6, a minor birch allergen, the lipotransferases found in some of the compositae, and the 1,3-β-glucanases corresponding to a major olive allergen. The significance of the detection of specific IgE directed against carbohydrate determinants remains to be investigated. A major problem for the clinician is the absence of clinical significance of cross-reactivity that has been demonstrated in vitro and in vivo.     

Axes de recherche en allergologie alimentaire : hypoallergénicité et vaccins

The frequency of food allergy in the pediatric population (8%), as well as the worrying increase of prevalence of severe anaphylaxis boost the research for means of prevention and for therapeutics alternative to the sole eviction of foods. Oral desensitization and sublingual immunotherapy, being the main part of the present clinical research are not in the scope of this review. Future trends of research focus on hypoallergenicity and vaccines. The definition of hypoallergenicity is limited to a lesser reactivity because of a lesser binding of specific IgE to modified food allergens, since the conditions of the immunogenicity leading to sensitization remain unknown. Different ways for patients, alimentary industry and agronomical research are detailed: heating and cooking, enzymatic and chemical treatment of natural foods, physical treatments (texturization, ultrafiltration,…), screening of natural varieties in order to characterize some of them with a lower level or an absence of major allergens. Bioengineering of plants with a reduced level of major allergens, and site-directed mutagenesis on B epitopes could be helpful for a safer nutrition and vaccines. Possible molecular forms aimed at vaccines are considered: recombinant natural allergens, modified recombinant allergens by dimerisation, site-directed mutagenesis, fusion with other molecules, long peptides,… Associated considerations are the choice of adjuvants promoting a Th1 response, as well as vectors for the expression of recombinant food allergens: bacteria, probiotic ones, or poorly allergenic plants. Mucosal vaccines could be especially interesting for food allergens in order to add specific mechanisms of tolerance arising in the intestinal mucosa to the reorientation towards a Th1 and TREG response. Plasmidic DNA vaccines and anti-IgE vaccines are an object of research without any application in the near future. Therapeutic vaccines for food allergens might be substituted to oral desensitization and could be applied first to peanut allergy and to cross allergy between pollens and fruit or vegetable linked to panallergens. Prophylactic vaccines might be a second step for atopic infants, insofar as more knowledge could be obtained of mechanisms and enhancing factors of oral tolerance to food allergens and the “opportunity window” for the establishment of oral tolerance.     

Facteurs de risque de l’allergie alimentaire sévère

Asthma is the main risk factor for severe, potentially fatal food allergy reactions. Peanut and tree nuts are the main foods involved in these reactions. Consequently, any asthmatic individual with a food allergy must be controlled perfectly by daily treatment. Other risk factors for severe allergic reactions are parallel medications (especially aspirin, beta-blockers and angiotensin enzyme inhibitors), physical effort, food consumption outside the home, mastocytosis, hidden allergens, and a combination of such associated factors. Better patient education and, in addition, risk prevention, particularly away from home, in restaurants and at school, should reduce the frequency of severe food allergic reactions. Collection of data concerning food allergies will allow better identification of the individuals at risk and development of targeted preventive measures.     

Faut-il évaluer et doser la charge allergénique environnementale ?

Assessment of the indoor allergen load encountered by patients with respiratory allergy symptoms is essential in order to obtain objective evidence of the allergens present in the indoor environment. It is equally essential to justify application of efforts to eliminate the responsible allergen sources (mites, domestic animals, molds, etc.). The methods used to measure indoor allergens can be grouped into (1) quantitative immunoassays (ELISA using monoclonal antibodies specific for purified native allergens or recombinant allergens) and (2) home-based tests. The latter include the Acarex test (semiquantitative assay for guanine in mite feces) and semiquantitative immunoassays. Quantitative tests are used mainly for epidemiologic studies whereas the results of home-based tests are considered in the management of allergic patients.     

Étiquetage des aliments allergéniques : ce qui change

Here we review the new rules concerning labelling of foods, in particular, allergenic foods. These rules indicate that all the ingredients must be clearly indicated on the label and especially that no exemption to this rule can be made when it is a question of known allergens. The list of the most common foods/ingredients has been established and it is periodically updated on the basis of the most up-to-date scientific and clinical information. It is important to note that not only are these foods covered by these rules but also any products that can be derived from them by technical means are covered. In order that the ingredients listed on the label would be useful and pertinent to allergic individuals, case-by-case exceptions might be allowed on the basis of adequate justification. As a result, a temporary exemption has been granted to about 20 well-characterized derivative products for which it has been established that the transformation process that had been used (for example, extensive refining) had resulted in a loss of their potential allergen and that the risk of provoking an allergic reaction under the proposed conditions of use would be very low. Based on confirmation by appropriate studies (notably clinical studies), the temporary labelling exemptions allowed until November 2007 could become permanent.     

Devenir des allergènes dans le tube digestif

Mechanisms by which food allergens sensitize atopic individuals still remain unclear. However, most of them are thought to sensitize via the gastrointestinal tract. Due to the very acidic conditions in the stomach and the intense proteolysis occurring in the stomach and in the intestine, only small amounts of intact or immunologically active proteins are taken up by the gut mucosa. This suggested that food allergens are, at least partially, resistant to gastro-duodenal digestion in order to be able to sensitize the mucosal immune system. As a result, several in vitro models have been developed to evaluate the stability of potential allergens to digestion. Indeed, resistance to digestion is part of the premarketing allergenicity assessment of newly expressed proteins in genetically modified crops. However, some food allergens are rapidly and extensively degraded during digestion, whereas some other food proteins that are resistant to digestion are not allergenic. It has been shown that degradation products, i.e. peptide fragments of various sizes, produced during the digestion of a protein may keep (part of) the allergenicity of the native protein. In addition, other factors may interact as to make a food protein an allergen, such as the structure and composition of the food matrix, technologic processing including cooking of the whole food that contains the allergens. Some studies also suggested that the biological property of a protein to be an allergen can influence its mode and route of transport across the intestinal epithelial barrier, which may have a profound impact on the immune responses thus generated. It is noteworthy that a pregastric absorption also occurs, i.e. in the oral cavity, which explains the occurrence of symptoms few minutes after ingestion of food allergens.     

Les allergènes du grain de blé

Wheat is a major dietary cereal, which can cause respiratory, contact and food allergies. Wheat grain contains a large panel of proteins with very particular structures and functions. This review will consider diverse wheat grain proteins and summarize recent work on wheat grain allergens.     

Les recombinants des panallergènes polliniques : application à l’interprétation des polysensibilisations

Multiple pollen sensitization is frequently encountered in patients who present with seasonal allergic respiratory symptoms. This regularly leads the allergist to question the significance of skin and laboratory test results because such results make it difficult to differentiate between cross-reactions and co-sensitization. Assays for specific IgE with recombinant panallergens, such as those in the profilin or polcalcin families, the results of which are now available in clinical practice, may lead to a better interpretation of such cases.     

L'allergie au tournesol et à son huile : rôles du contact,de l'ingestion et de l'inhalation

We report the case of a 5-years old girl with food allergy to sunflower oil. Sensitivity to sunflower oil and seeds was demonstrated by skin tests whereas assays of serum for sunflower-specific IgE were negative. An oral challenge test with 54 ml of sunflower oil was positive. The patient also reacted to cutaneous contact and inhalation of sunflower seeds. Depending on the route of contact –– skin contact, ingestion or inhalation –– sunflower oil and seeds can trigger symptoms such as urticaria, erythema, vomiting, dyspnea or fatigue.     

Les modèles expérimentaux d’eczéma de contact

Knowledge of the pathophysiology of Allergic Contact Dermatitis (ACD) derives chiefly from the murine experimental model of Contact Hypersensitivity referred to as the Mouse Ear Swelling Test (MEST). ACD is mediated by activation of CD8+ cytotoxic T cells specific for haptens in contact with the skin, and down-regulated by CD4+ T cells which limit the magnitude of the skin inflammation and participate to its resolution. Although the MEST is a major tool in the fields of immunodermatology and fundamental immunology, it is not sensitive enough to detect the sensitizing properties of common weak allergens and therefore is not used anymore in immunotoxicological studies. The MEST has been replaced by the Local lymph node Assay (LLNA), which brought tangible ethical benefits, but is endowed with low sensitivity and specificity. Recently, we reported that mice deficient in CD4+ T regulatory cells, but not normal mice, could develop a typical ACD reaction to weak haptens involved in human ACD to fragrances (Hexylcinnamaldehyde, Eugenol, Hydroxycitronellal). From this in vivo results, we developed an in vitro assay, referred to as the Weak Hapten Interferon Secreting T LymphocytE test (Whistle), able to detect the sensitizing properties of chemicals in contact with the skin.     

Allergies cyprès-pêche : allergie croisée ou simple coïncidence ?

Recently, a cross reactivity between cypress pollens and peach has been published. In this paper, we have tried to confirm this possibility in 33 patients suffering from cypress pollen allergy. The analysis of the collected patients data lead to the following classification of them into two groups: nine patients (27.3%) had a sensitivity to peach (four patients were allergic to peach and five patients were only sensitised to this fruit but could eat it without experiencing any symptoms), the second group of 24 patients were allergic to cypress pollens, but not sensitive to peach. These results clearly confirm that the cypress-peach syndrome exists together with the apple-birch syndrome. Other analysis on these patients will enable to characterise the cross-reacting allergens.     

Recombinant Allergen Immunotherapy: Clinical Evidence of Efficacy—A Review

Recombinant allergens for immunotherapy aim to overcome the problems of natural extracts as they can be produced in unlimited amounts with exact physiochemical and immunological properties. These can be modified to have more favourable characteristics including reduced IgE reactivity or enhanced immunogenicity. Different types of recombinant allergens have been evaluated in clinical phase II and III trials whilst others are currently under development. In this review, we identified double-blind, placebo-controlled randomised clinical trials assessing the efficacy and safety of various recombinant allergen preparations. The majority of studies have up to now focused on cat, grass, birch, ragweed and bee venom allergens. Some studies have shown some of these preparations to be effective and well tolerated. However, there are still outstanding issues regarding optimum doses, minimising side effects and long-term effects.     

Vaccination with DNA encoding human T-cell epitopes suppresses Der p induced allergic responses in mice.

The apparent complexity of allergen-specific T-cell response in terms of epitope usage in humans is a potential barrier to peptide-based immunotherapy for allergy. A knowledge of cross-reacting T-cell epitopes of common allergens might have an impact on the development of vaccines for immunotherapy. We examined the efficiency of vaccinating with plasmid DNA coding only human T-cell epitopes on the suppression of allergic reactions in mice. BALB/c mice that received an injection of mixed naked DNA plasmids encoding the five classes of human T-cell epitopes on Der p 1 and Der p 2 produced a significant reduction in total and Der p-specific immunoglobulin E (IgE) synthesis. In Der p specific-IgG2a antibody responses, vaccinated mice showed more prominent responses than controls. Higher levels of interferon-gamma, a Th1 cytokine associated with the suppression of IgE production, were found in the sera of vaccinated mice. Histologic studies showed a marked reduction in the infiltration of inflammatory cells in the lung tissues of vaccinated mice vs. controls. These results suggest that vaccination with DNA encoding human T-cell epitopes effectively inhibits allergic responses in mice and might induce cross-regulation on helper T-cell level in vivo.