Allergy diagnosis in children IgE-sensitized to peanut : clinical and immunological evaluation

Background: Peanut allergy is often life-long and affects quality of life since accidental ingestion may lead to severe or even fatal reactions. Sensitization to peanut can be due to genuine peanut allergy or to cross-sensitization due to birch pollen. Peanut allergy diagnosis is usually based on clinical history, skin prick test (SPT) and presence of IgE-antibodies (IgE-ab) to peanut but these tests often need to be confirmed with an oral food challenge which may cause severe allergic reactions. Measurements of IgE-ab to specific proteins in an allergen source (component resolved diagnostics [CRD]) and basophil allergen threshold sensitivity (CD-sens) may be valuable tools for diagnosis of peanut allergy. Important allergen proteins in peanut are the storage proteins: Ara h 1, Ara h 2 and Ara h 3 and the PR-10 protein [birch-homologue] Ara h 8. Aim: The aim of this thesis was to evaluate different diagnostic methods in children IgE-sensitized to peanut with a suspected peanut allergy. Method: Paper I investigated if it is possible to predict the outcome of double-blind placebo-controlled food challenge (DBPCFC) with peanut by measuring CDsens to peanut and Ara h 2 as well as IgE-ab to peanut components (Ara h 1, Ara h 2, Ara h 3, Ara h 8 or Ara h 9) (n=38). In Paper II, the reproducibility of DBFCFC and CD-sens were investigated. Twenty-seven children underwent DBPCFC followed by a single-blinded food challenge with peanut, and CD-sens was measured before the two first peanut challenges. Paper III reports a birch pollen allergic child with cross-sensitization to peanut who had a severe reaction after eating a large amount of peanuts. The fourth paper investigated the outcome of a peanut challenge in relation to IgG4-ab (n=58). Paper V studied 20 birch pollen allergic children cross-sensitized to peanut in relation to CD-sens to peanut and Ara h 8. Results: In Paper I, 25 children had a positive DBPCFC and 92% of the children were positive in CD-sens. The remaining two children were low responders and could not be evaluated. Children with positive DBPCFC reactions had significantly higher levels of IgE-ab to peanut, Ara h 1, Ara h 2 and Ara h 3 than children with negative reactions. All children negative in CD-sens to peanut and Ara h 2 were also negative in challenge. In paper II, 14/27 children were positive at both active challenges but not placebo. Only three of these children reacted consistently at the same dose with the same severity score. All children with a positive or a negative CD-sens at the first challenge were also CD-sens positive/negative at the second challenge. Paper III revealed that the girl with birch pollen allergy who reacted with anaphylaxis after peanut ingestion was mono-sensitized to Ara h 8. Paper IV showed that children positive at peanut challenge had significantly higher levels of IgG4-ab to peanut and Ara h 2 than children negative at the challenge. The peanut and Ara h 2 IgG4/IgE-ab ratios were significantly higher in children who tolerated peanut than in allergic children. In Paper V, all children passed peanut challenge without any objective symptoms, but five experienced subjective symptoms from the oral cavity. CDsens to peanut was negative in 19/20 children but 17/20 were positive in CDsens to Ara h 8. Conclusion: CD-sens is a promising diagnostic method with good reproducibility in the diagnosis of peanut allergy and may exclude a peanut allergy. IgE-ab to the peanut storage proteins (Ara h 1, Ara h 2 and Ara h 3) seem to confirm a genuine peanut allergy. A peanut challenge can discriminate between positive and negative reactions but does not predict the severity of an allergic reaction. Birchpollen allergic children IgE-sensitized to peanut and Ara h 8 but not to Ara h 1, Ara h 2 and Ara h 3 have basophils sensitized with IgE-ab to Ara h 8 which can be activated by Ara h 8 proteins and initiate allergic inflammation. Children IgEsensitization to peanut who nonetheless tolerate peanuts are characterized by low levels of IgG4-antibodies to peanut and Ara h 2 but relatively high IgG4/IgE antibody ratios

Oral peanut challenge identifies an allergy but the peanut allergen threshold sensitivity is not reproducible.

BACKGROUND: Double-blind placebo-controlled food challenge, DBPCFC, the gold standard for diagnosing food allergy, is time-consuming and potentially dangerous. A basophil allergen threshold sensitivity test, CD-sens, has shown promising results as a diagnostic tool in food allergy. OBJECTIVES: To evaluate the reproducibility of oral peanut challenge and compare the outcome to CD-sens in peanut-sensitized children. METHODS: Twenty-seven children (4-19 years) underwent a DBPCFC followed by a single-blind oral food-challenge. The peanut challenges (1 mg to 5 g) were evaluated by severity scoring. Blood samples were drawn for CD-sens before the two first challenges. RESULTS: Thirteen children (48%) did not react at any of the challenges. Fourteen reacted at both peanut challenges but not to placebo. Only two of these children reacted at the same threshold dose and with the same severity score. All other children scored differently or reacted at different doses. For children with a positive challenge the geometric mean of the ratio of the doses was 1.834 (p = 0.307) and the arithmetic mean of the difference between the severity scores was 0.143 (p = 0.952). No association was obtained between the two peanut challenges regarding severity score (r(s) = 0.11, p = 0.71) or threshold dose (r(s) = 0.35, p = 0.22). Among the children positive in peanut challenge, 12 were positive in CD-sens. Two were low-responders and could not be evaluated. Geometric mean of the ratio of CD-sens values in children with a positive challenge was 1.035 (p = 0.505) but unlike for the severity score and the threshold dose the association between the two CD-sens values was strong (r(s) = 0.94, P<0.001). CONCLUSIONS: For a positive/negative test the reproducibility is 100% for both peanut challenge and CD-sens. However, a comparison of the degree of allergen threshold sensitivity between the two tests is not possible since the threshold dose and severity scoring is not reproducible

Severity scores according to Astier [<b>20</b>] in the same child at the two peanut challenges.

<p>Severity scores according to Astier <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0053465#pone.0053465-Astier1" target="_blank">[<b>20</b>]</a> in the same child at the two peanut challenges.</p

Peanut CD-sens values in the same child at the two peanut challenges.

<p>Peanut CD-sens values in the same child at the two peanut challenges.</p

The differences in severity score (challenge 2–challenge 1) in each child with positive peanut challenges, presented as a Bland-Altman plot.

<p>The arithmetic mean of the difference between the severity scores was 0.143.</p

Threshold doses of peanut in the same child at the two peanut challenges.

<p>The amount peanut eaten were divided into 5 dose steps: Dose 1 = 0.001 g; Dose 2 = 0.01 g; Dose 3 = 0.1 g; Dose 4 = 1 g; Dose 5 = 3.6–5 g.</p

Symptom score according to Astier [20] to evaluate clinical reactions in DBPCFC.

<p>Symptom score according to Astier <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0053465#pone.0053465-Astier1" target="_blank">[20]</a> to evaluate clinical reactions in DBPCFC.</p

The differences in the ratio for peanut CD-sens values (challenge 2/challenge 1) in each child with positive peanut challenges, presented as a Bland-Altman plot with logarithmically transformed data.

<p>Geometric mean of the ratio of CD-sens values was 1.035.</p