Should digestion assays be used to estimate persistence of potential allergens in tests for safety of novel food proteins?

Simulated gastric fluid assay for estimating the digestibility of newly expressed proteins in GE crops: Missteps in development and interpretation

Digestive stability of a food protein in simulated gastric fluid (SGF) continues to be considered a risk factor for allergy, even though the current science does not support this belief. Methodological shortcomings of the adaption of the SGF assay for use with purified proteins has been cited as a reason to discount results that do not conform to this belief. Missteps in conducting and interpreting the results of SGF assays are reviewed here. However, these methodological shortcomings do not invalidate the conclusion that allergenic proteins are not systematically more stable to digestion than non-allergens. The growing evidence for the dual allergen exposure hypothesis, whereby sensitization to food allergens is primarily caused by dermal and inhalation exposure to food dust, and tolerization against food allergy is primarily induced by gut exposure in food, likely explains why the digestive stability of a protein is not a risk factor for allergenicity.

Erroneous Belief that Digestive Stability Predicts Allergenicity May Lead to Greater Risk for Novel Food Proteins

There continues to be an erroneous belief that allergens (especially food allergens) are more resistant to gastrointestinal digestion than non-allergens. Government regulations based on this erroneous belief may result in technology developers altering the amino acid sequences of digestively stable native proteins to create digestively unstable modified versions for expression in genetically engineered crops. However, an investigation where a known stable allergen was modified to make it more digestible eliminated the protein’s ability to tolerize against allergy in a mouse model, which is consistent with the dual allergen exposure hypothesis. Thus, the false belief that digestive stability increases the allergenic risk of novel food proteins (e.g., such as expressed in genetically engineered crops) could, in some cases, lead to introduction of digestively unstable modified protein versions with greater sensitization risk. However, it is noteworthy that developers have historically been very effective at preventing allergens from being introduced into crops based on the other components of the weight-of-evidence assessment of allergenic risk such that no newly expressed protein in any commercialized genetically engineered crop has ever been documented to cause allergy in anyone.

Presence of small resistant peptides from new in vitro digestion assays detected by liquid chromatography tandem mass spectrometry: An implication of allergenicity prediction of novel proteins?

The susceptibility of newly expressed proteins to digestion by gastrointestinal proteases (e.g., pepsin) has long been regarded as one of the important endpoints in the weight-of-evidence (WOE) approach to assess the allergenic risk of genetically modified (GM) crops. The European Food Safety Authority (EFSA) has suggested that current digestion study protocols used for this assessment should be modified to more accurately reflect the diverse physiological conditions encountered in human populations and that the post-digestion analysis should include analytical methods to detect small peptide digestion products.The susceptibility of two allergens (beta-lactoglobin (β-Lg) and alpha-lactalbumin (α-La)) and two non-allergens (hemoglobin (Hb) and phosphofructokinase (PFK)) to proteolytic degradation was investigated under two pepsin digestion conditions (optimal pepsin digestion condition: pH 1.2, 10 U pepsin/μg test protein; sub-optimal pepsin digestion condition: pH 5.0, 1 U pepsin/10 mg test protein), followed by 34.5 U trypsin/mg test protein and 0.4 U chymotrypsin/mg test protein digestion in the absence or presence of bile salts. All samples were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) in conjunction with Coomassie Blue staining and, in parallel, liquid chromatography tandem mass spectrometry (LC-MS) detection. The results provide following insights: 1) LC-MS methodology does provide the detection of small peptides; 2) Peptides are detected in both allergens and non-allergens from all digestion conditions; 3) No clear differences among the peptides detected from allergen and non-allergens; 4) The differences observed in SDS-PAGE between the optimal and sub-optimal pepsin digestion conditions are expected and align with kinetics and properties of the specific enzymes; 5) The new methodology with new digestion conditions and LC-MS detection does not provide any differentiating information for prediction whether a protein is an allergen. The classic pepsin resistance assay remains the most useful assessment of the potential exposure of an intact newly expressed protein as part of product safety assessment within a WOE approach.

Microbial transglutaminase alters the immunogenic potential and cross-reactivity of horse and cow milk proteins

Horse milk is a valuable raw material and a very attractive alternative for scientific research to address the issue of cow milk (CM) allergy due to its protein profile. A decrease in immunoreactive properties can be achieved by thermal, enzymatic, and hydrolytic processing. Therefore, the aim of this study was to explore the possibility of reducing the immunoreactivity of horse milk proteins by microbial transglutaminase (TG) polymerization. To determine how TG linking alters immunoreactivity under simulated digestion of the examined milk, analyses were performed before, during, and after digestion. The dose-dependent (1, 10, and 100 U) effects of microbial TG on horse and cow milk were analyzed. A consecutive 3-stage digestion was simulated with salivary, gastric, and intestinal fluids. The effects of digestion were analyzed by SDS-PAGE, particle size analysis, and size-exclusion chromatography. Immunoreactivity was assessed using competitive ELISA (β-lactoglobulin and α-casein) and immunodot (sera from 7 patients aged 3 to 13 years who are allergic to CM proteins). Horse milk contained almost half of the amount of total proteins in CM. The dose 1 U/g of total milk protein changed the immunoreactivity of both cow and horse milk. With increasing TG doses, α-casein immunoreactivity increased, and β-lactoglobulin decreased. After total digestion, horse milk was characterized by 2.4-fold lower average IgE and 4.8-fold lower IgG reactivity than CM. We found that TG alters the IgE and IgG reactivity of CM after in vitro digestion. Horse milk was less reactive to IgE and IgG than was CM, with animal and patient sera. The effect of TG on immunoreactivity depends on enzyme quantity and milk protein type. The diet based on modified horse milk proteins could be an alternative for some patients with CM protein allergy; however, confirmation through clinical trials is needed.

Allergenic sensitization versus elicitation risk criteria for novel food proteins

The value of criteria used in the weight-of-evidence assessment of allergenic risk of genetically modified (GM) crops has been debated. This debate may originate, in part, from not specifying if the criteria are intended to contribute to the assessment of sensitization risk or elicitation risk. Here, this distinction is explicitly discussed in the context of exposure and hazard. GM crops with structural relationships with known allergens or sourced from an organism known to cause allergy (hazard) are screened for IgE-antibody reactivity using serum from sensitized individuals. If IgE reactivity is observed, the GM crop is not developed. While digestive and heat stability impact exposure and thus the elicitation risk to sensitized individuals, these attributes are not interpretable relative to sensitization risk. For novel food proteins with no identified hazard, heat stability cannot be validly assessed because relevant IgE antibodies are not available. Likewise, the uncertain and sometime non-monotonic dose relationship between oral exposure to allergens and sensitization makes digestive stability a poor predictor of sensitization risk. It is hoped that by explicitly distinguishing between sensitization risk and elicitation risk, some of the debate surrounding the weight-of evidence criteria for predicting the allergenic risk of GM crops can be resolved.

Allergenic potential of novel proteins - What can we learn from animal production?

Currently, risk assessment of the allergenic potential of novel proteins relies heavily on evaluating protein digestibility under normal conditions based on the theory that allergens are more resistant to gastrointestinal digestion than non-allergens. There is also proposed guidance for expanded in vitro digestibility assay conditions to include vulnerable sub-populations. One of the underlying rationales for the expanded guidance is that current in vitro assays do not accurately replicate the range of physiological conditions. Animal scientists have long sought to predict protein and amino acid digestibility for precision nutrition. Monogastric production animals, especially swine, have gastrointestinal systems similar to humans, and evaluating potential allergen digestibility in this context may be beneficial. Currently, there is no compelling evidence that the mechanisms sometimes postulated to be associated with allergenic sensitization, e.g. antacid modification of stomach pH, are valid among production animals. Furthermore, examples are provided where non-biologically representative assays are better at predicting protein and amino acid digestibility compared with those designed to mimic in vivo conditions. Greater emphasis should be made to align in vitro assessments with in vivo data.

Simulated gastrointestinal digestion reduces the allergic reactivity of shrimp extract proteins and tropomyosin

Shrimp are highly allergenic foods. Current management are limited to the avoidance of foods. Therefore, there is an unmet need for a safe and effective therapy using modified allergens. This study focuses on assessing the potential for modification of the allergenicity of shrimp proteins following heat treatment or simulated gastric digestion. Shrimp proteins do not reduce their IgE reactivity after heat treatment but it is reduced by simulated gastric digestion in a time- and dose-dependent manner. Tropomyosin in shrimp extract is worse digested than purified tropomyosin. After 60 min of 10 U/μg pepsin digestion, a strong inhibition was produced in the in vivo skin reactivity of shrimp extracts and in activation of basophils from allergic patients. Immunisation experiments performed in rabbits demonstrated that digested boiled shrimp extract is able to induce IgG antibodies that block the IgE binding to the untreated boiled shrimp extract in shrimp-allergic patients. Building on our observations, digestion treatment could be an effective method for reducing shrimp allergenicity while maintaining the immunogenicity.

Clinico-Immunological Analysis of Eggplant (Solanum melongena) Allergy Indicates Preponderance of Allergens in the Peel

Background

Eggplant (Solanum melongena L.) is known to cause food allergy in some Asian countries but detailed studies on eggplant allergy are lacking.

Objective

The objective is to investigate sensitization to different parts of eggplant fruit, and detection of the allergens.

Methods

Six eggplant-allergic subjects were assessed for sensitization to eggplant (peel/pulp, and raw/cooked) by skin prick test, allergen-specific IgE, and immunoblots. Allergens were analyzed for glycoprotein nature by staining/lectinoblots, and in vitro stability in simulated gastric fluid.

Results

All the eggplant-sensitized subjects showed positive skin prick test with peel, pulp, raw, and cooked eggplant extracts; allergen-specific IgE to all these was positive. Raw eggplant contains 5 allergens in the range 36-71 kD. Most allergens are localized in the eggplant peel (9 allergens; 26-71 kD range) than the pulp (3 allergens; 52-71 kD); among these, the 26, 28, 36, and 71 kD allergens seem to be heat-stable. The 43, 45, 64, and 71 kD allergens are detected as glycoproteins; the 26, 64, and 71 kD allergens are stable displaying retention of IgE-binding ability in simulated gastric fluid digestion.

Conclusions

Eggplant is a multiallergenic vegetable in the context of presence of allergens in all edible parts of eggplant having preponderance in the peel.

Comparative study of in vitro digestibility of major allergen, tropomyosin and other proteins between Grass prawn (Penaeus monodon) and Pacific white shrimp (Litopenaeus vannamei)

BACKGROUND

Stability in simulated gastric fluid is supposed to be an important parameter for the estimation of food allergenicity. In the present study, the digestive stability of allergenic protein tropomyosin (TM) and other food proteins from Grass prawn and Pacific white shrimp in simulated gastric fluid (SGF) and simulated intestinal fluid (SIF) digestion assay system was investigated and comparatively studied by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), western blotting, and inhibition enzyme-linked immunosorbent assay (ELISA).

RESULTS

In the SGF system, proteins such as actin and myosin heavy chain (MHC) were rapidly degraded within a short period of time, while TM was relatively resistant to pepsin digestion. In the SIF system, MHC was also easily decomposed, while TM and actin were resistant to digestion. Western blotting using a specific polyclonal antibody against TM indicated that the degradation pattern of shrimp TM by SGF and SIF was almost unaffected by the presence of other myofibrillar proteins. Further study by IgE immunoblotting and inhibition ELISA using sera from crustacean-allergic patients indicated that IgE binding of TM was decreased.

CONCLUSION

Proteinase digestion is effective in reducing IgE binding of shrimp TM. It is also of interest to notice that Pacific white shrimp TM had higher digestion stability than Grass prawn TM. However, Pacific white shrimp TM revealed enhanced IgE binding over that of TM from Grass prawn and thus it is possibly more allergenic.

Value of eight-amino-acid matches in predicting the allergenicity status of proteins: an empirical bioinformatic investigation

The use of biotechnological techniques to introduce novel proteins into food crops (transgenic or GM crops) has motivated investigation into the properties of proteins that favor their potential to elicit allergic reactions. As part of the allergenicity assessment, bioinformatic approaches are used to compare the amino-acid sequence of candidate proteins with sequences in a database of known allergens to predict potential cross reactivity between novel food proteins and proteins to which people have become sensitized. Two criteria commonly used for these queries are searches over 80-amino-acid stretches for >35% identity, and searches for 8-amino-acid contiguous matches. We investigated the added value provided by the 8-amino-acid criterion over that provided by the >35%-identity-over-80-amino-acid criterion, by identifying allergens pairs that only met the former criterion, but not the latter criterion. We found that the allergen-sequence pairs only sharing 8-amino-acid identity, but not >35% identity over 80 amino acids, were unlikely to be cross reactive allergens. Thus, the common search for 8-amino-acid identity between novel proteins and known allergens appears to be of little additional value in assessing the potential allergenicity of novel proteins.