Abstract
In times of “Precision Medicine” it is fundamental to identify the individual disease phenotype in order to provide an individualized therapy for every patient. This concept is also becoming increasingly important for the treatment of allergic diseases. Thanks to the biological engineering of recombinant and native allergens for the assessment of allergen-specific IgE antibodies, it is now possible to easily obtain the individual sensitization profile of a patient. This allows the allergist to precisely identify the primary elicitor of an IgE response and, based on this knowledge, to choose the best treatment option. Several studies have observed the longitudinal evolution of sensitization profiles and identified a phenomenon termed “molecular spreading,” which describes a broadening of the recognized allergen spectrum from a source over time. Additionally, the identification of marker proteins, which can trigger an IgE response or correlate with an increased risk for certain clinical symptoms, helps to establish an individual risk profile. This information may not only affect the decision-making concerning immunotherapy, but also opens up avenues for future investigations with regard to prevention strategies. We provide here an overview on the role of individual sensitization patterns and their predictive value.
Molecular Allergology: New Pathways in Allergy Diagnosis
It is long since no secret that allergic diseases are continuously gaining importance in the industrialized world. While many scientists are trying to get to the bottom of these numbers, the situation for a multitude of practitioners is aggravating as they see themselves confronted not only by affected patients demanding treatment, but also with anxious parents seeking recommendations for effective prevention strategies for their children. In order to provide useful information for the doctors to pass on to their patients, research is being performed restlessly, often with changing results. Nevertheless, all research has one least common denominator: in order to prevent a disease and provide proper treatment, it is fundamental to understand the underlying biological and immunological mechanisms from scratch, including those taking place before and finally leading to the onset of the first symptoms.
Since the discovery of IgE antibodies by Ishizaka, Bennich and Johannson in the late sixties, serological tests have become an integral part of the allergological work-up, combined with the clinical history and in-vivo methods like skin tests or standardized allergen provocation. While skin tests can also be performed with raw allergens (e.g., prick-to-prick testing), allergenic extracts served as a base for serological assessments during the last decades. As these reagents are sometimes poorly standardized and their composition may vary significantly between preparations from different manufacturers, the precision of results left space for improvement. This gap is now progressively being covered by new diagnostic options based on advanced molecular and structural biology. Thanks to fast-placed technological advances, it became possible not only to identify and purify an increasing number of allergenic molecules with their isoforms, but also to produce them in large quantities through sequencing and cloning. This new opportunity goes along with the need for accurate diagnostic tools in times of “Precision Medicine” which requires the detailed knowledge of the patient's disease phenotype in order to provide individualized treatment options. Molecular singleplex and multiplex assays do not only provide detailed information on the patient's sensitization profile, but also on possible cross-reactivity. They also enable us to observe the longitudinal evolution of complex IgE and IgG repertoires in birth cohort studies, which is fundamental for the perception of pre-clinical immunological phenomena. Once these are understood in depth, more efficient long-term therapies, as well as prophylactic measures could possibly be identified. Of course, humoral responses are only one part of this complex journey, to which the following paragraphs will draw a roadmap.
The Origins: Allergen-Specific IgG-Responses
In order to evaluate a child's risk of developing an allergic disease as early as possible, ideally already in a pre-clinical stage, it is fundamentally important to have a clear idea of the immunological processes distinguishing the non-atopic from atopic individuals. As IgE antibodies appear during the first year of life and their quantity is minor, especially when compared to IgG levels, early observations should not be limited to allergen-specific IgE antibodies. The production of allergen-specific IgG has been broadly studied (), especially in relation to its protective effect as a “blocking antibody” () and its role in immunotherapy (). An analysis of two birth cohorts from Great Britain and Australia showed that IgG, not IgG4, specific to the cat allergen Fel d 1 was able to alter the cat-specific IgE and childhood wheezing association (). In this study, children with increasing IgG1 levels had a lower risk of developing symptoms. In order to get a broader overview of the early, non-challenged, natural IgG response, we analyzed the sera of 148 atopic as well as non-atopic children toward a broad panel of 91 allergenic molecules from various sources (). The results obtained at the age of 2 years showed that almost 100% of the children, independently from their atopic status, produced IgG antibodies toward allergenic molecules from foods of animal sources, such as cow's milk and egg. This prevalence, as well as the antibody concentrations, were considerably lower for vegetable foodborne allergens, and lowest for airborne allergens. Parallely, for these two allergen groups, a clear difference in terms of prevalence could be observed among atopic and non-atopic individuals with higher prevalences among the atopic children (Figure 1). Interestingly, antibodies of the IgG4 subclass could be observed only infrequently (<5% of the responses), suggesting that the measured antibodies mainly belong to the IgG1 subclass.
Figure 1
These results indicate that the route and dose of exposure are of prominent importance for the evoked immune response. In order to analyse the effect of different ways of exposure to almost identical allergenic molecules, we examined the natural evolution of allergen-specific IgG antibodies toward the birch-pollen related PR.10 molecules throughout childhood (
The chronological interrelation between the IgG and IgE responses further leads to the hypothesis that both isotypes may have been produced by the same B cell subset passing through an indirect class switch. This hypothesis has also been recently supported by immunoglobulin heavy chain mutational lineage data giving evidence that the primary IgE source in human subjects is the secondary isotype switching of mutated IgG1-expressing B cells (
It has to be mentioned though, that independently from the subclass, the assessment of IgG antibodies has no diagnostic value so far, especially not in food allergy. Further research needs to be performed in order to evaluate its diagnostic and/or predictive value.
The Initiator: Characteristics of the Early IgE-Response
Once a patient is seeking the opinion of a specialized allergist, he or she is usually already suffering from a significant loss of life quality due to his/her allergic symptoms. The serological assessment of IgE antibodies at this time point then typically shows a strong response toward one or, more likely, already various allergen sources. This information is usually obtained through tests based on allergen extracts. These do provide significant information and are a substantial diagnostic tool, however, especially in poly-sensitized patients, they cannot always reliably indicate the eliciting source primarily responsible for the clinical symptoms (
As mentioned before, IgE antibodies are mostly already present in the blood once a patient visits the allergist. But when did this sensitization begin? How long was the pre-clinical phase and what could he observe if the allergist had seen his patient before the onset of symptoms? In order to answer these questions, birth cohorts are an essential pool of resources, which has been used, to assess the longitudinal evolution of IgE responses toward grass pollen molecules in childhood. Their analysis of 177 grass-allergic children revealed a clear structure in the observed sensitization profiles (
Individual Sensitization Patterns and the Phenomenon of “Molecular Spreading”
After the identification of initiator molecules, the subsequent course of sensitization is of fundamental interest. An analysis of the individual IgE profiles among 176 grass-pollen-allergic children in Italy resulted in the identification of 39 different sensitization patterns (Figure 2). This broad heterogeneity shows that every immune system reacts individually to a stimulus, which in this case is assumed equal for all participants, as they have been entirely recruited within the city Rome (
Figure 2

Profiles of IgE sensitization to 8 P pratense molecules in 176 children with an IgE reaction to P pratense and complete dataset. The Allergen Profile Codification System (APCS) code and the absolute and cumulative frequencies are shown. The profiles are ordered by decreasing frequency, and the point at which the arbitrary threshold of 80% of the patient population has been reached is marked in red [reprinted from (
Strikingly, many subjects had produced IgE toward various allergenic grass pollen molecules already up to several years before the onset of disease. An analysis of IgE profiles before and after the beginning of clinical symptoms additionally showed that not only the complexity, but also the levels of the IgE responses are significantly lower before the first occurrence of symptoms (Figure 3). This sequential evolution of an immune (IgE-) response toward different, non-cross-reactive molecules of the same antigen/allergen source has been termed “molecular spreading” underlining the importance of an initiator molecule as the trigger of this phenomenon. A recent study among 59 grass pollen allergic children from Greece further suggested the usefulness of component-resolved diagnosis (CRD) as an indicator for disease severity as the presence of IgE antibodies to the Phleum pratense molecules Phl p 1, Phl p 2, Phl p 5, and Phl p 6 were associated significantly with moderate to severe symptoms (
Figure 3

IgE to P pratense by time from onset of grass-related seasonal allergic rhinitis. (A) Bars show the prevalence of IgE sensitization (≥0.35 kUA/L) to P pratense (extract) in children whose sera were available at each point in time. The number of tested children is indicated over each bar. (B) Bars show the geometric mean levels of IgE antibodies to P pratense (extract) at each point in time in IgE antibody–positive sera. The numbers of tested children is indicated over each bar. Lines show the average number of all 8 (triangles) or only 7 (squares; Phl p 6 excluded) allergenic molecules of P pratense recognized by IgE antibodies at each point in time in IgE antibody–positive sera [reprinted from (
Focusing on the concept of a “molecular spreading” in house dust mite allergy, the sera of 722 subjects from the MAS cohort were analyzed for IgE to 12 recombinant house dust mite (Dermatophagoides pteronyssinus) allergens (
Figure 4

Trajectories of IgE sensitization in mite-sensitized subjects (n = 129). Evolution of the IgE responses to 12 D pteronyssinus allergen molecules according to the A, AB, or ABC classification in participants sensitized at 2 or more follow-up points is shown. The round, rhombus, and rectangular boxes represent the initial, intermediate, and final sensitization stages, respectively. Numbers (percentages) refer to participants, and areas are proportional to their frequency [reprinted from (
Earlier, Custovic et al. tested the sera of 235 children of the Manchester Asthma and Allergy Study birth cohort for specific IgE toward grass pollen and mite allergens (
Predictive Value of Specific IgE-responses to Allergenic Molecules
The presence of allergen-specific IgE antibodies toward airborne allergens has been lately verified as a predictive marker for the development of asthma throughout childhood and adolescence (
Going back to the above mentioned stratification system of sensitization patterns in house dust mite allergy, we showed that subjects within the broadest sensitization stage (i.e., A->B->C) had a significantly elevated risk of mite-related allergic rhinitis and asthma, or both, than not sensitized participants. The authors could identify various factors leading to a broadening of the IgE repertoire. Especially an early onset of sensitization, a history of parental hay fever, and higher levels of exposure to mites could be identified as parameters associated with a more complex, polymolecular IgE sensitization pattern. Furthermore, this study also discovered certain marker molecules, as IgE to Der p 1 and/or Der p 23 at the age of 5 or younger predicted a higher risk of developing asthma at school age (i.e., 6–20 years).
In birch pollen allergy, increased risk of incidence and persistence of seasonal allergic rhinitis up to the age of 16 years was shown to correlate with increasing levels of Bet v 1-specific IgE or increasing numbers of IgE-reactive PR-10 proteins at the age of 4 years. This observation has been made among 764 children from the Barn/Children Allergi/Allergy Milieu Stockholm Epidemiologic (BAMSE) study birth cohort who were examined at the age of 4, 8, and 16 years. Additionally to these predictive results, some general observations on the evolution of IgE responses toward birch-related allergens were made. Confirming our previous observations (
An analysis of the sera of 779 children selected from the same cohort has shown that IgE to the major cat and dog allergen molecules Fel d 1 (cat) and Can f 1 (dog), allows a significantly better prediction of cat and dog allergy (cross-sectionally and longitudinally) than IgE assessed with cat and dog extract. The same could be observed for a polysensitization to either cat or dog allergen molecules, underlining the advantage of CRD in allergy diagnostics.
Implications for Immunotherapy
The expanded information obtained through molecular allergy diagnostics raises new questions, not only concerning the decision for or against an immunotherapy, but also on the correct timing for its application as well as the composition of therapeutic agents (
An important factor influencing the efficacy of any therapy is its suitability for the disease to be treated. As described previously, our theoretical exercise showed that in only 4% of the 176 examined grass-pollen allergic patients, the sensitization profile matched with the composition of a standardized immunotherapy solution (
Another option to increase the success rates of immunotherapy is a good timing. Unfortunately, most of the treatments are currently being initiated only years after the first onset of symptoms and often after a significant loss of life quality. Many patients and attending doctors first observe the natural course of clinical symptoms and especially the former hope for their symptoms to dissolve, without potentially painful treatment with subcutaneous injections. Unfortunately only very few of the clinical manifest allergies tend to dissolve naturally. On the contrary, many mono-sensitized children—especially those with parents affected by hay fever (
Future Perspectives—Immunoprophylaxis
As mentioned previously in this chapter, a “molecular spreading” phenomenon has been described for various airborne allergens with the same final outcome: patients with a broad spectrum of IgE to many molecules of one source are more likely to suffer from more severe clinical symptoms than those exhibiting a less complex sensitization profile. Reasoning about potential starting points to prevent this late clinical phase ends in two to three options for treatment (Figure 5). Currently, the most common option is to start an allergen-specific immunotherapy once symptoms have aggravated and cannot be properly controlled by symptom-relieving drugs (antihistamines, corticosteroids) anymore. The therapeutic agents used for this treatment usually consist of allergen extracts or standardized mixtures of molecules including the most important major allergens. Unfortunately, this treatment strategy bears the risk of over- or under-treatment and an individually tailored therapy may be more effective but is not yet established. Another approach is preventing the aggravation of symptoms by intervening at an early clinical stage, right after the onset of symptoms. At this stage, the molecular spreading process has not yet fully evolved and sensitization profiles are mostly simple. Consequently, such a customized SIT containing exclusively the targeted molecules might prevent a further spreading of the IgE response. Taking this theoretic reasoning even one step further, even the pre-clinical phase may be of interest for an early intervention. It has been shown that healthy pre-school children with IgE to grass pollen are likely to develop a seasonal allergic rhinitis triggered by grass pollen at school age, especially when their parents suffer from hay fever (
Figure 5

Molecular spreading of the IgE response to Timothy grass and potential implications for allergen-specific immunologic intervention in a child with seasonal allergic rhinitis to grass pollen (SARg). Molecular spreading of the IgE response to Phleum pratense and implications for allergen-specific immunological intervention in one child with hay fever (case from the MAS birth cohort) [reprinted with permission from (
A recent meta-analysis of 32 studies, however, could not depict a statistically significant reduction of the risk to develop a first allergic disease by early allergen-specific immunotherapy (AIT) (
This means that, albeit the disease prevention is not possible, its onset could be delayed and its symptoms moderated, maybe even prevented with regard to asthma.
Statements
Author contributions
Both authors have made a substantial, direct and intellectual contribution to the work, and approved it for publication.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
sensitization profile, molecular spreading, allergen-specific IgG, component-resolved diagnostics, prediction
Citation
Dramburg S and Matricardi PM (2019) Molecular Diagnosis of Allergy: The Pediatric Perspective. Front. Pediatr. 7:369. doi: 10.3389/fped.2019.00369
Received
01 June 2019
Accepted
27 August 2019
Published
24 September 2019
Volume
7 - 2019
Edited by
Amelia Licari, University of Pavia, Italy
Reviewed by
Gian Luigi Marseglia, San Matteo Hospital Foundation (IRCCS), Italy; Oskar A. Haas, St. Anna Children's Cancer Research Institute (CCRI), Austria; Sara Manti, University of Catania, Italy
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© 2019 Dramburg and Matricardi.
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*Correspondence: Paolo Maria Matricardi paolo.matricardi@charite.de
This article was submitted to Pediatric Immunology, a section of the journal Frontiers in Pediatrics
†These authors have contributed equally to this work
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