The biochemical basis and clinical evidence of food allergy due to lipid transfer proteins: a comprehensive review

Plant lipid transfer proteins (LTPs) are ubiquitous proteins that are found in divergent plant species. Although the exact function of LTPs is not fully understood, LTPs are conserved across a broad range of plant species. Because LTPs share structural features, there is an increased probability for significant allergic cross-reactivity. The molecular features of LTPs also decrease the probability of degradation due to cooking or digestion, thereby increasing the probability of systemic absorption and severe allergic reactions. LTP allergy, unlike other forms of anaphylaxis, tends to occur more frequently in areas of lower latitude. The geographic distribution of LTP allergy, along with evidence of increased sensitization after respiratory exposure, has led to the hypothesis that LTP-related food allergy may be secondary to sensitization via the respiratory route. Clinical reactions associated with LTPs have broad clinical phenotypes and can be severe in nature. Life-threatening clinical reactions have been associated with ingestion of a multitude of plant products. Component-resolved diagnosis has played a significant role in research applications for LTP allergy. In the future, component-resolved diagnosis may play a significant role in day-to-day clinical care. Also, quantitative analysis of LTPs in foodstuffs may allow for the identification and/or production of low-LTP foods, thereby decreasing the risk to patients with LTP allergy. Furthermore, sublingual immunotherapy may provide a therapeutic option for patients with LTP allergy.

Wheat lipid transfer protein is a major allergen associated with baker's asthma

BACKGROUND

Baker's asthma is a frequent occupational allergic disorder mainly caused by inhalation of cereal flours. Lipid transfer proteins (LTPs) constitute a family of plant food panallergens, but their role as inhalant and wheat allergens is still unclear.

OBJECTIVE

We sought to explore the involvement of wheat LTPs in baker's asthma caused by wheat flour sensitization.

METHODS

Forty patients with occupational asthma caused by wheat flour inhalation were studied. Wheat LTP, Tri a 14, was purified by using a 2-step chromatographic protocol and characterized by N-terminal amino acid sequencing and 3-dimensional modeling. Its reactivity was confirmed by means of IgE immunodetection, ELISA and ELISA-inhibition assays, and skin prick tests.

RESULTS

Specific IgE to Tri a 14 was found in 60% of 40 individual sera from patients with baker's asthma, and the purified allergen elicited positive skin prick test reactions in 62% of 24 of these patients. Tri a 14 and peach LTP, Pru p 3, showed a sequence identity of 45%, but the low cross-reactivity between both allergens detected in several individual sera reflected great differences in their 3-dimensional IgE-binding regions.

CONCLUSIONS

Wheat LTP is a major inhalant allergen associated with baker's asthma caused by wheat flour sensitization. Poor cross-reactivity with its peach homolog was found in some patients.

CLINICAL IMPLICATIONS

LTPs can be considered relevant inhalant allergens linked to respiratory disorders. LTP from wheat (Tri a 14) can be used as a helpful tool for the diagnosis of baker's asthma.

Detection of Some Safe Plant-Derived Foods for LTP-Allergic Patients

BACKGROUND

Lipid transfer protein (LTP) is a widely cross-reacting plant pan-allergen. Adverse reactions to Rosaceae, tree nuts, peanut, beer, maize, mustard, asparagus, grapes, mulberry, cabbage, dates, orange, fig, kiwi, lupine, fennel, celery, tomato, eggplant, lettuce, chestnut and pineapple have been recorded.

OBJECTIVE

To detect vegetable foods to be regarded as safe for LTP-allergic patients.

METHODS

Tolerance/intolerance to a large spectrum of vegetable foods other than Rosaceae, tree nuts and peanut was assessed by interview in 49 subjects monosensitized to LTP and in three distinct groups of controls monosensitized to Bet v 1 (n = 24) or Bet v 2 (n = 18), or sensitized to both LTP and birch pollen (n = 16), all with a history of vegetable food allergy. Patients and controls underwent skin prick test (SPT) with a large spectrum of vegetable foods. The absence of IgE reactivity to foods that were negative in both clinical history and SPT was confirmed by immunoblot analysis and their clinical tolerance was finally assessed by open oral challenge (50 g per food).

RESULTS

All patients reported tolerance and showed negative SPT to carrot, potato, banana and melon; these foods scored positive in SPT and elicited clinical symptoms in a significant proportion of patients from all three control groups. All patients tolerated these four foods on oral challenge. Immunoblot analysis confirmed the lack of IgE reactivity to these foods by LTP-allergic patients.

CONCLUSION

Carrot, potato, banana and melon seem safe for LTP-allergic patients. This finding may be helpful for a better management of allergy to LTP.

Plant non-specific lipid transfer proteins: an interface between plant defence and human allergy

Plant non-specific LTPs (lipid transfer proteins) form a protein family of basic polypeptides of 9 kDa ubiquitously distributed throughout the plant kingdom. The members of this family are located extracellularly, usually associated with plant cell walls, and possess a broad lipid-binding specificity closely related to their three-dimensional structure. The nsLTP fold is characterized by a compact domain composed of 4 alpha-helices, firmly held by a network of 4 conserved disulphide bridges. This fold presents a large internal tunnel-like cavity, which can accommodate different types of lipids. nsLTPs are involved in plant defence mechanisms against phytopathogenic bacteria and fungi, and, possibly, in the assembly of hydrophobic protective layers of surface polymers, such as cutin. In addition, several members of the nsLTP family have been identified as relevant allergens in plant foods and pollens. Their high resistance to both heat treatment and digestive proteolytic attack has been related with the induction by these allergens of severe symptoms in many patients. Therefore, they are probably primary sensitizers by the oral route. nsLTP sensitization shows an unexpected pattern throughout Europe, with a high prevalence in the Mediterranean area, but a low incidence in Northern and Central European countries.

Structural, biological, and evolutionary relationships of plant food allergens sensitizing via the gastrointestinal tract

The recently completed genome sequence of the model plant species Arabidopsis has been estimated to encode over 25,000 proteins, which, on the basis of their function, can be classified into structural and metabolic (the vast majority of plant proteins), protective proteins, which defend a plant against invasion by pathogens or feeding by pests, and storage proteins, which proved a nutrient store to support germination in seeds. It is now clear that almost all plant food allergens are either protective or storage proteins. It is also becoming evident that those proteins that trigger the development of an allergic response through the gastrointestinal tract belong primarily to two large protein superfamilies: (1) The cereal prolamin superfamily, comprising three major groups of plant food allergens, the 2S albumins, lipid transfer proteins, and cereal alpha-amylase/trypsin inhibitors, which have related structures, and are stable to thermal processing and proteolysis. They include major allergens from Brazil nut, peanuts, fruits, such as peaches, and cereals, such as rice and wheat; (2) The cupin superfamily, comprising the major globulin storage proteins from a number of plant species. The globulins have been found to be allergens in plant foods, such as peanuts, soya bean, and walnut; (3) The cyteine protease C1 family, comprising the papain-like proteases from microbes, plants, and animals. This family contains two notable allergens that sensitize via the GI tract, namely actinidin from kiwi fruit and the soybean allergen, Gly m Bd 30k/P34. This study describes the properties, structures, and evolutionary relationships of these protein families, the allergens that belong to them, and discusses them in relation to the role protein structure may play in determining protein allergenicity.

Relationship between peach lipid transfer protein specific IgE levels and hypersensitivity to non-Rosaceae vegetable foods in patients allergic to lipid transfer protein

BACKGROUND

Lipid transfer protein (LTP), the major allergen in Rosaceae in geographic areas where the prevalence of birch pollen allergy is low, is a widely cross-reacting pan-allergen, but the pattern of cross-reactivity to plant-derived foods botanically unrelated to Rosaceae shows much variability.

OBJECTIVE

To examine the relationship between peach LTP specific IgE levels and cross-reactivity to several non-Rosaceae, plant-derived foods.

METHODS

IgE specific for peach LTP was measured by enzyme-linked immunosorbent assay in serum samples from 40 patients with Rosaceae allergy monosensitized to LTP. Patients were considered monosensitized to this protein in the absence of sensitization to other cross-reacting, plant-derived foods as shown by negative skin prick test (SPT) results with both birch and mugwort pollen. SPTs with commercial extracts of walnut, hazelnut, peanut, celery, maize, rice, tomato, orange, and onion were performed to detect possible immunologic cross-reactivity to these foods.

RESULTS

Patients with negative SPT results with non-Rosaceae foods showed significantly lower levels of IgE to peach LTP than patients showing skin reactivity to one or more non-Rosaceae foods (P < .001). A significant difference in specific IgE to peach LTP between patients with positive or negative SPT results was observed with each individual food (P < .001 in all cases). The level of IgE to peach LTP was strongly related to the number of positive SPT results with non-Rosaceae foods (r = 0.78; P < .001). Increasing levels of IgE to peach LTP were associated with skin reactivity to nuts (29/40 [72%]), peanut (27/40 [67%]), maize (16/39 [41%]), rice (14/39 [36%]), onion (13/37 [35%]), orange (9/32 [28%]), celery (11/40 [27%]), and tomato (8/39 [20%]).

CONCLUSIONS

This study suggests that all allergenic determinants in LTP from vegetable foods other than peach cross-react with peach LTP determinants, whereas only some peach LTP epitopes cross-react with allergenic determinants on botanically unrelated, plant-derived foods. The high levels of IgE to peach LTP seem to reflect the presence of IgE targeting common allergenic determinants of LTP, causing cross-reactivity to botanically unrelated, vegetable foods. In LTP-allergic patients, increasing levels of IgE to peach LTP are paralleled by an increasing number of foods other than Rosaceae positive on SPT that cause clinical symptoms.