ReviewLocalization, release and bioavailability of pollen allergens: the influence of environmental factors
Introduction
Allergic diseases represent a major health problem of most modern societies 1., 2., 3.. According to the European Allergy White Paper [1], the overall prevalence of seasonal allergic rhinitis in Europe is approximately 15%. Current asthma prevalence rates vary from 2.5%–10.0% and the prevalence of allergic atopic dermatitis ranges from 9%–24%. Atopic diseases are still increasing, especially in regions with low prevalence rates to date—for example in the former East Germany [4]. The reason, however, remains still to be elucidated although a variety of explanations are under scientific discussion.
Allergies are among those few diseases in which environmental factors of both natural and anthropogenic origin have been identified as a cause of disease development, as well as of elicitation and aggravation of symptoms. Naturally occurring allergens from indoor and outdoor environments—that is, house-dust-mite, animal dander and pollen allergens—are in the very first line of substances inducing sensitization among young children from atopic parents [5]. Urban air pollution, especially pollutants due to traffic emissions, exhibits adjuvant activities in allergen-specific IgE production [6] and has been shown to increase prevalence rates of both sensitization 7., 8•. and symptoms of allergic rhinitis in atopic children [8•].
It is commonly believed that allergic sensitization starts when an allergen gets into contact with antigen-presenting cells at sites of mucosal or skin epithelium. For elicitation of allergic symptoms and maintenance of the disease, the repeated contact of an allergen with the individual is the absolute prerequisite. This contact occurs via the air through inhalation, via the gastrointestinal tract through ingestion and also through direct contact on the skin surface or percutaneously via injections. Most IgE-inducing allergens are environmental substances of animal or plant origin and are proteins or glycoproteins with a molecular weight >10kDa 9., 10.. These allergens are taken up by dendritic cells, processed and presented on their surface to T lymphocytes, which then—via the development of a Th2-cytokine secretion pattern—induce B cells to secret IgE antibodies. This process—during which no symptoms of allergic disease are manifest—is called ‘sensitization’. Repeated allergen exposure of sensitized individuals leads then to the elicitation of allergic symptoms of disease such as wheal-and-flare reactions or eczema in the skin, rhinoconjunctivitis, sneezing or airway bronchoconstriction.
Some allergens are more potent in inducing IgE formation than others; the difference in this allergenic potency is, however, not well understood at a molecular level. Some allergens have enzyme activity, others do not 9., 10., 11., 12•.. Allergens from birch or grass pollen have a rather high allergenic potency, whereas allergies against pine pollen are extremely rare in Central Europe, although there is a tremendous amount of pine pollen in the outdoor air during spring and early summer.
Exposure to inhalant allergens occurs through the air. Allergens are, however, not freely available but are an integral constituent of plants or animals—the allergen carriers —and have to be bioavailable in order to provoke an allergy. In the outdoor and indoor environment, allergen carriers are mainly represented by pollen, mold spores, dust mites and animal epithelia. The most important allergen carriers in the outdoor air is pollen—with a diameter between 15 μm and 60 μm—from anemophilic plants such as trees, grasses and weeds. Pollen counts and pollen-count forecasts are available in most countries and document the amount and nature of different pollen species over the year in the respective areas [13].
During the past ten years knowledge about the molecular biology of allergens and its implications on the patho-mechanism of allergic inflammation, as well as on the development of new therapeutic strategies, has been greatly advanced. This review covers the increasingly interdisciplinary information available to date on allergen localization and release from its carriers and on factors influencing allergen bioavailability in the development of IgE-mediated allergic diseases.
Section snippets
Allergens as integral constituents of pollen
The most frequent aeroallergens derive from pollen. Among them, grass pollen represents the biggest allergen source. Since grass pollen is able to evoke IgE-mediated allergic reactions within seconds after contact with the mucosa, pollen allergens must be extremely water soluble and readily available. Allergens are non-self proteins with antigenic properties and molecular weights of >10kDa. They are not only able to elicit an IgE response but also trigger mediator release from mast cells or
The localization and visualization of grass pollen allergens
Pollen grains are considered as the multicellular male gametophyte in both angiosperms and gymnosperms. Upon contact with the receptive surface, the stigma, pollen germinates by tube development [18]. Of allergological relevance are the anemophilous pollen from gymnosperms that is wind-dispersed and transported over wide areas. Grass pollen, for example, can be transported over about 175km at an average wind-speed of 10 m/second. They sediment in motionless air about 3.1 cm/second [18].
Allergen liberation from pollen grains and the bioavailability of aeroallergens
Allergen liberation from sites within the pollen is a pre-requisite for its function in sensitized individuals. In a dry atmosphere, pollen is very stable and can contain their allergens over centuries [18]. There are at least three inducers of allergen liberation: first, a high relative air humidity; second, thunderstorms and heavy rain; and third, air pollutants. Under humid conditions, allergens are released from pollen grains in the process that occurs under the physiological conditions of
The influence of air pollutants on allergen bioavailability and on other allergy-relevant factors from pollen
By the use of fluidized bed reactors, the influence of both gaseous and particulate air pollutants upon allergen release from pollen can be studied in a dose-, time- and humidity-dependent fashion 26., 46.. Exposure of P. pratense pollen to high concentrations of SO2 induces a significant reduction of liberation of a major allergen, Phl p 5 [26], thus leading to the assumption that the bioavailabilty of major grass pollen allergens might be grossly reduced in regions polluted with SO2 as
Conclusions
Pollen allergens are integral pollen constituents. They are primarily localized within the pollen grain in specific patterns of distribution and have to be released during a process of activation in order to become bioavailable. Allergen liberation does not depend on cell death but is a temperature-, time- and pH-dependent active mechanism.
Environmental factors—such as humidity, thunderstorms and air pollutants—have been shown to influence allergen liberation in a complex manner. Among gaseous
References and recommended reading
Papers of particular interest, published within the annual period of review,have been highlighted as:
• of special interest
•• of outstanding interest
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