SOURCE OF BET-V-1 LOADED INHALABLE PARTICLES FROM BIRCH REVEALED

Allergenic proteins present in pollen grains, when inhaled, interact w ith the airways to cause an attack of asthma in susceptible humans. In one system, grass pollen grains rupture osmotically in rainfall, rele asing allergen-containing inhalable particles into the atmosphere. In contrast, birch tree pollen grains do not rupture under these conditio ns, yet the major allergen, Bet v 1, has been detected in the atmosphe re in inhalable particles of unknown origin. It is possible that Bet v 1 may diffuse from intact settled pollen grains and the allergenic ma terial may again become airborne, interacting with settled fine partic les from other sources prior to resuspension. This study investigates the mechanism for the release of birch pollen allergen-containing inha lable particles from pollen grains. We propose the hypothesis that (1) airborne birch pollen grains settle on nearby leaf surfaces; (2) then , following light rainfall, the grains germinate and, (3) later, polle n tubes burst, releasing inhalable particles carrying Bet v 1 into the atmospheric aerosol. We used microscopic analyses of pollen behaviour following anther opening, a Burkard volumetric trap for pollen counts and a high volume air sampler with a two-stage cascade impactor for q uantitative immunochemical analyses of Bet v 1. On dry days of high bi rch pollen count (48 grains/m(3), 1.5 ng/m(3) of Bet v 1), we found th at the surfaces of birch leaves became coated with pollen. This ''poll en rain'' is a source of secondary emission of allergens into the atmo sphere. We observed that following light rainfall (<1 mm per day), abo ut 80% of the birch pollen grains germinated, producing pollen tubes, especially in the sticky surface secretions of leaf glands. These poll en tubes may grow up to 300 mu m in length prior to rupturing, each re leasing about 400 starch granules coated with allergen molecules that may, after drying, be dispersed into the aerosol. On these days follow ing light rainfall, the highest atmospheric levels of Bet v 1 (1.18 ng /m(3)) are associated with inhalable particles. Following heavy rainfa ll, both pollen and inhalable particles are washed from the atmosphere . Immunoprinting studies show that Bet v 1 is associated with starch g ranules rather than the smaller orbicules. Bet v 1 is present in the a tmosphere in large particles, i.e. in particular pollen grains and in inhalable particles, i.e. in particular starch granules.