The prevalence of allergic diseases has increased rapidly over the past few decades in developed countries.1,2 Asthma, rhinoconjunctivitis, and eczema were systematically evaluated in approximately 1.2 million children in 98 countries in the International Study of Asthma and Allergies in Childhood (ISAAC)3 and a Portuguese epidemiological study in adults revealed a prevalence of 16.8% of asthma and 33.6% of rhinitis in central Portugal.4 Inhalant allergens play a crucial role in the development of these diseases, with house dust mites (HDM) considered the most important allergen source worldwide.5,6 HDM, especially Dermatophagoides pteronyssinus, are considered an important source for allergen sensitization and are major risk factors for allergic respiratory diseases in genetically predisposed patients.7 These allergens are divided into groups according to their biochemical composition, homology, and molecular weight. Presently, 20 HDM allergens for D. pteronyssinus have been registered by the World Health Organization and International Union of Immunological Societies Allergen Nomenclature Sub-committee (http://allergen.org/), many of these represent digestive enzymes since HDM feces are the major source of allergen exposure.

Some studies report a dominant prevalence of IgE antibodies against group 1 (Der p 1) and group 2 (Der p 2) HDM allergens in Europe.8–11 In Portugal, data on the role of Der p 1 and Der p 2 are still scarce.

The group 1 (Der p 1, 25kDa cystein proteases) allergens are found in high concentrations in fecal pellets, while those of group 2 (Der p 2, 14kDa) are mostly located in components of the mite body.7,12 Der p 1 and Der p 2 are considered major allergens of D. pteronyssinus, as they are recognized in more than 80% of mite-sensitive patients.11,13

Allergen-specific immunotherapy (sIT) is an important weapon in the treatment of respiratory allergy in selected cases, and provided that the patient has a hypersensitivity reaction mediated by IgE. The procedure requires a gradual administration of increasing amounts of an allergen to which the patient is sensitive, allowing the modulation of the untoward immune response to that allergen and alleviating allergic symptoms. It is the only treatment that improves symptoms, reduces the requirement of pharmacological therapy and modifies the natural history of allergic disease.14–16 The therapeutic effect may be long-lasting and prevents the progression from mild to severe forms of allergic disease.16–18

Since currently marketed vaccines are standardized only for the Der p 1 and Der p 2 allergens, the authors aim to understand the local epidemiology and to clarify if the molecular assay of these two major allergens is advantageous in deciding and/or modifying the decision to initiate sIT in children sensitized to D. pteronyssinus and with clinical indication, similarly to what is described in the literature.

The clinical files of 279 patients were included in the study. One hundred eighty-one (64.9%) were males and 98 (35.1%) were females. Age varied between 4 and 17 years, with a mean age of 9.55 (Table 1). One hundred and ninety-nine children (71.3%) had asthma, 245 (87.8%) rhinitis and 165 (59.1%) of the total had asthma and rhinitis, simultaneously. Some of the children had other atopic clinic associated: conjunctivitis (42.7%), atopic eczema (17.2%) and food allergy (7.9%), with fish, seafood, egg and milk as main responsible foods. A family history of atopy was registered in 202 (72.4%) patients, 54 (19.4%) with a maternal and/or paternal history of asthma. A chi-square test of independence was performed to examine the relation between clinical files and family history of atopy, the relation between asthma (χ2 (1,N=279)=2.124, p=0.145) or rhinitis (χ2 (1,N=279)=3.572, p=0.059) and family history of atopy was non-significant. The test was also performed to examine the relation between clinical files and risk factors such as term and preterm gestation, type of delivery, exclusive breast milk, house features, humidity and contact with animals. The results had no relation with statistical significance (Table 2).

Eosinophils ≥4% was present in 144 children and 158 had increased total IgE values for age. The positive values for Der p 1 and Der p 2 are shown in Table 3.

Der p 1 and Der p 2 was <0.35kUA/L in 29 (10%) patients and positive values of Der p 1, Der p 2 or the larger of the two can be seen in Table 4. In children with rhinitis without asthma, a higher positivity of Der p 2 (71.3%) was observed, compared with Der p 1 (30%), without statistical significance. The positivity of Der p 1 was slightly higher in children with a family history of atopy (77.2%) compared to those without atopy in family (63.6%). No difference was observed in Der p 2 (Table 1).

The variables age, wheal size in the skin prick-test, Der p 1, Der p 2, total IgE levels and eosinophils do not follow a normal distribution (p<0.001).

The existence of a correlation between the serological measurements was assessed by computing the corresponding Spearman coefficient and a strong positive correlation was found between the variables Der p 1 and Der p 2 (ρ=0.621, p<0.001). A positive weak correlation was found between the variables Der p 1/Der p 2 and wheal size in the skin prick-test (ρ=0.324, p<0.001; ρ=0.252, p<0.001), between the variables Der p 1/Der p 2 and eosinophils levels (ρ=0.288, p<0.001; ρ=0.371, p<0.001). A positive moderate correlation was found between the variables Der p 1/Der p 2 and total IgE levels (ρ=0.346, p<0.001; ρ=0.449, p<0.001) (Fig. 1).

Figure 1.

Correlations between the sIgE levels of HDM extract and single-component Der p 1 and Der p 2 with other serum levels. A) Der p 1 and Der p 2 IgE levels. B) Sum of serum IgE Der p 1 and Der p 2 to wheal size in the skin prick-test. C) Sum of serum IgE Der p 1 and Der p 2 to eosinophils levels. D) Sum of serum IgE Der p 1 and Der p 2 to total IgE levels.

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No statistically meaningful correlation was found between the wheal size in the skin prick-test and eosinophils levels (ρ=−0.144, p=0.051) or total IgE levels (ρ=−0.055, p=0.478), nor between Der p 1/Derp 2 and age (ρ=−0.052, p=0.388/0.379).

In Fig. 2 we can see the ROC curves illustrating the value of wheal size in the skin prick-test, eosinophils and total IgE levels in predicting positive Der p 1/Der p 2. The risk was estimated by the area under the ROC curve, calculated individually for each parameter and for association of the three variables. The associated analysis of skin prick-test, eosinophils and total IgE levels reveled significant predictive value, with AUC=0.653. Individually, the observed wheal size and eosinophils have significant predictive value which is greater than total IgE levels, with predicted ability of AUC=0.673 and 0.641 versus AUC=0.598. However, all the parameters would be considered to be “poor” at separating positive Der p 1 and Der p 2 levels from negative levels. Using the Youden index, we calculated the optimal cut-off point for the meaningful wheal size (4.5mm), with a sensibility of 80.5% and a specificity of 50% and the same was undertaken for the relative quantity of eosinophils which was 5.55% with a sensibility of 49.8% and a specificity of 82.6%. The predictive ability of IgE total levels was not significant.

Figure 2.

ROC curves for ability of wheal size in the skin prick-test (A), eosinophils (B) and total IgE levels (C) in predicting positive Der p1/Der p2 serum levels. The risk was estimated by area under ROC curve (AUC)=0.653.

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HDM is a major perennial allergen source and a significant cause of allergic rhinitis and allergic asthma. Sensitization to mite allergens in the first years of life has a significant clinical effect on lung function in pediatric populations, due to wheezing and associates with poorer clinical outcomes in respiratory health in the long term.24 In the same way, a high percentage of patients with a family history of atopy, as can be observed in our sample (72.4%, particularly the children with asthmatic parents), are more prone to suffer from persistent asthma in adulthood.25,26

Prevalence data for HDM allergen sensitization vary from 65 to 130 million persons in the general population worldwide to as many as 50% among asthmatic patients.27 The two-key species of HDM in allergy research are D. pteronyssinus and D. farinae. The Dermatophagoides genus is probably the most researched of all the HDMs, although species dominance varies geographically, suggesting specialized adaptation.27 In Portugal, the high prevalence of acarin fauna is evident in all regions of the country and the importance of the D. pteronyssinus is notorious. According to the acarological map that was performed in our country, D. pteronyssinus is the most prevalent mite in the northern interior area of the country. That is the same area where our study was performed, with a frequency of occurrence of 79.5%. D. pteronyssinus and D. farinae are highly cross-reactive and in some geographical areas D. farinae is more prevalent. It is not the case of our region, because accordingly to the acarological map, the frequency of occurrence of D. farinae is low, about 6.8%.28 In our study, 73.5% and 76.3% of Der p-positive patients were sensitized to Der p 1 and Der p 2, respectively. The rate of Der p sensitization is comparable to rates reported for European cohorts (80%).24

Although at least 20 HDM allergen components have been identified and sequenced, several studies have demonstrated that clinically Der p 1 and Der p 2 are the most important HDM allergens,24,29,30 and this was also observed in our Der p-sensitized individuals. Ten per cent of the tested patients (29/279) were negative to both Der p 1 and Der p 2. Therefore, the present study shows that Der p 1 and Der p 2 are the major allergenic HDM components in our group of Portuguese patients.

The high correlation between serum eosinophil levels and total IgE levels against Der p 1 and Der p 2, further supports the dominant role of both allergens in the human IgE response against HDM in Portugal. The observed wheal size and eosinophils have significant predictive ability to separate positive Der p 1 and Der p 2 levels from negative levels and the optimal cut-off point of 4.5mm of wheal size displays a sensibility of 80.5% and a specificity of 50% and 5.55% eosinophils with a sensibility of 49.8% and a specificity of 82.6% (Fig. 2).

Allergen immunotherapy shows some potential for being disease-modifying by inducing beneficial immunological changes and by completely or partially abrogating the inception of asthma and new allergen sensitizations. It appears to improve clinical symptoms (nasal, ocular, and respiratory), to reduce rescue medication needs, to improve lung function indices and, more importantly, it has the potential to develop intrinsic disease-modifying effects such that the allergic inception of asthma and acquisition of new sensitizations can be completely or partially abrogated.31 In our sample, about 71.3% of the patients were asthmatic, the vast majority had rhinitis (87.8%) and only 28.7% had rhinitis exclusively. The advantages of treatment institution can be relevant, but since vaccines currently marketed for D. pteronyssinus are only standardized for Der p 1 and Der p 2, this means that for our sample only 89.6% of the children were eligible for HDM IT. Knowledge of the molecular allergen profile contributes to the characterization of the patient's risk profile, because it describes the primary sensitization. It also helps in the selection of patients for sIT with allergens and optimizes the choice of immunotherapy composition.

Recently, Der p 23, found in the chitinous membrane of the mite fecal ball, has been discovered to bind IgE with titers similar to Der p 1 and 2 and have high activity in basophil degranulation tests. The titers showed a strong concordance with binding to Der p 1 and 2. But some subjects have high IgE binding to Der p 23 without binding to Der p 1 or 2.11,32 According to these recent findings, it will be important to study in our sample the 10% of patients with Der p 1 and Der p 2 negative values and understand if they have positive Der p 23 values. At the time of this study, Der p 23 was not commercially available, so this will be object of study in the future.

In summary, the present study confirmed that Der p 1 and Der p 2 are major HDM allergen components in our pediatric population of a hospital in the North of Portugal since 73.5% and 76.3% of Dermotophagoide pteronissimus positive patients were sensitized to Der p 1 and Der p 2, respectively. The study also showed that the Der p 1 and Der p 2 assay was negative in 10.4% (29/279) patients. Something similar has been already demonstrated for other relevant aeroallergens.33,34 This, together with our results, proves that the study of allergenic sources by both skin prick-tests or specific IgE constitutes an incomplete approach in order to a right prescription of sIT. As vaccines for D. pteronyssinus that are currently marketed are standardized only for Der p 1 and Der p 2 allergens, the authors conclude that there may be an advantage in determining Der p 1/Der p 2 levels in patients with a positive prick-test and clinical indication for sIT prior to deciding initiation of immunotherapy or not, since 10% of children with clinical features of mite house dust allergy will not benefit with this treatment, with all the personal and economic efforts involved.

The authors declare that they do not to have any conflict of interest.